THE PRINCIPLES OF
BIOLOGY

BY

HERBERT SPENCER

IN TIVO VOLUMES
VOL. II

REVISED AND ENLARGED EDITION
1899

NEW YORK

D. APPLETON AND COMPANY

1901

Copyright, 1867, 1899,
By D. APPLETON AND COMPANY.

S7
l9ol

o.2^

PKEFACE

TO THE REVISED AND ENLARGED EDITION
OF VOL. II.

To the statements made in the preface to the first volume
of this revised edition, there must here be added a few hav-
ing special reference to this second volume.

One of them is that the revision has not been carried
out in quite the same way, but in a way somewhat less com-
plete. When reviewing the first volume a friendly critic,
Prof. Lloyd Morgan, said : —

" But though the intellectual weight has also been augmented, it
is an open question whether it would not have been wiser to leave
intact a treatise, &c. . . . relegating corrections and additions to
notes and appendices."

I think that Prof. Morgan is right. Though at the close
of the preface to volume I, I wrote : — " in all sections not
marked as new, the essential ideas set forth are the same as
they were in the original edition of 1864," yet the reader
who has not read this statement, or does not bear it in mind,
will suppose that all or most of the enunciated conceptions
are of recent date, whereas only a small part of them are.
I have therefore decided to follow, in this second volume, a
course somewhat like that suggested by Prof. Morgan-
somewhat like, I say, because in sundry cases the amend-
ments could not be satisfactorily made by appended notes.

vi PREFACE TO THE REVISED EDITION.

But there has been a further reason for this change of
method. An invalid who is nearly eighty cannot with pru-
dence enter upon work which will take long to complete.
Hence I have thought it better to make the needful altera-
tions and additions in ways requiring relatively moderate
time and labour.

The additions made to this volume are less numerous and
less important than those made to the first volume. A new
chapter ending Part V, on " The Integration of the Or-
ganic World," serves to round off the general theory of
Evolution in its application to living things. Beyond a new
section (§ 289a) and the various foot-notes, serving chiefly
the purpose of elucidation, there are notes of some signifi-
cance appended to Chapters I, III, IY, and V, in Part IY,
Chapters Y and VIII, in Part V, and Chapters IX, X, and
XII in Part VI. Moreover there are three further appen-
dices, D3, F, and G, which have, I think, considerable sig-
nificance as serving to make clearer some of the views
expressed in the body of the work.

Turning from the additions to the revisions, I have to
say that the aid needed for bringing up to date the contents
of this volume, has been given me by the gentlemen who
gave me like aid in revising the first volume : omitting
Prof. Perkin, within whose province none of the contents
of this volume fall. Plant-Morphology and Plant-Physi-
ology have been overseen by Mr. A. G. Tansley. Criti-
cisms upon parts dealing with Animal Morphology I owe
to Mr. J. T. Cunningham and Prof. E. W. MacBride.
And the statements included under Animal Physiology
have been checked by Mr. W. B. Hardy.

PREFACE TO THE REVISED EDITION. vii

For reasons like those named in the preface to the first
volume, I have not submitted the proofs of this revised
second volume to these gentlemen : a fact which it is need-
ful to name, since one or other of them might else be held
responsible for some error which is not his but mine. It is
the more requisite to say this because while, in respect of
matters of fact, I have, save in one or two cases, accepted
their corrections as not to be questioned, I have not always
done this in respect of matters of inference, but in sundry
places have adhered to my own interpretations.

Perhaps I may be excused for expressing some satisfac-
tion that I have not been obliged to relinquish the views set
forth in 1864-7. The hypothesis of physiological units — or,
as I would now call them, constitutional units—has been
adopted by several zoologists under modified forms. So far
as I am aware, the alleged general law of organic symmetry
has not called forth any manifestations of dissent. The
suggested theory of vertebrate structure appears to have
become current ; and from the investigations of the late
Prof. Cope, has received verification. The conclusions
drawn in Part YI on " The Laws of Multiplication," have
not, I believe, been controverted. And though only some
works on botany have given currency to the doctrine set
forth in Appendix C, " On Circulation and the Formation
of Wood in Plants," yet I have met with no attempt to dis-
prove it. The only views contested by certain of the gen-
tlemen above named, are those concerning the origin of
the two great phsenogamic types of plants, and the origin
of the annulose type of animals. I have not, however, —
perhaps because of natural bias — found myself compelled

viii PREFACE TO THE REVISED EDITION.

to surrender these views. My reasons for adhering to them
will be found in notes to the ends of Chapters III and IV
in Fart IV, and in Appendix D9.

On now finally leaving biological studies, it remains only
to say that I am glad I have survived long enough to give
this work its finished form.

Brighton,

October, 1899.

PREFACE TO VOL. II.

The proof sheets of this volume, like those of the last
volume, have been looked through by Dr. Hooker and
Prof. Huxley ; and I have, as before, to thank them for
their valuable criticisms, and for the trouble they have
taken in checking the numerous statements of fact on
which the arguments proceed. The consciousness that
their many duties render time extremely precious to them,
makes me feel how heavy is my obligation.

Part IY., with which this volume commences, contains
numerous figures. Nearly one half of them are repeti-
tions, mostly altered in scale and simplified in execution, of
figures, or parts of figures, contained in the wTorks of vari-
ous Botanists and Zoologists. Among the authors whom I
have laid under contribution, I may name Berkeley, Car-
penter, Cuvier, Green, Harvey, Hooker, Huxley, Milne-
Edwards, Ralfs, Smith. The remaining figures, numbering
150, are from original sketches and diagrams.

The successive instalments which compose this volume,
were issued to the Subscribers at the following dates : — No.
13 (pp. 1—80) in January, 1865 ; No. 14 (pp. 81—160) in
June, 1865 ; No. 15 (pp. 161—240) in December, 1865 ; No.
16 (pp. 241—320) in June, 1866 ; No. IT (pp. 321—400) in
November, 1866 ; and No. 18 (pp. 401—566) in March, 1867.

London, March 23rd, 1867.

CONTENTS OF VOL. II.

PART IV.— MORPHOLOGICAL DEVELOPMENT.

CHAP. PAGE

I. — The problems of morphology 3

II. — The morphological composition of plants ... 17

III. — The morphological composition of plants — Continued . 37

IV. — The morphological composition of animals ... 85

V. — The morphological composition of animals — Continued 111

VI. — Morphological differentiation in plants . . . 128

VII. — The general shapes of plants 134

VIII. — The shapes of branches 145

IX.— The shapes of leaves 152

X.— The shapes of flowers 161

XL — The shapes of vegetal cells 175

XII. — Changes of shape otherwise caused .... 178

XIII. — Morphological differentiation in animals . . . 183

XIV. — The general shapes of animals 186

XV. — The shapes of vertebrate skeletons .... 209

XVI. — The shapes of animal-cells 228

XVII.— Summary of morphological development . . . .231

PART V.— PHYSIOLOGICAL DEVELOPMENT.

I. — The problems of physiology 239

II. — Differentiations between the outer and inner tis-
sues OF PLANTS 244

III. — Differentiations among the outer tissues of plants . 251
IV. — Differentiations among the inner tissues of plants . 272

xii CONTENTS.

CHAP. PAGK

V. — Physiological integration in plants .... 292
VI. — Differentiations between the outer and inner tissues

OF ANIMALS 299

VII. — Differentiations among the outer tissues of animals . 309

VIII. — Differentiations among the inner tissues of animals . 323

IX. — Physiological integration in animals .... 373

X. — Summary of physiological development .... 384

X\ — The integration of the organic world .... 396

PAET VI.— LAWS OF MULTIPLICATION.

I. — The factors 411

II. — A PRIORI principle , .417

III. — Obverse a PRIORI principle 424

IV. — Difficulties of inductive verification .... 432

V. — Antagonism between growth and asexual genesis . 439

VI. — Antagonism between growth and sexual genesis . . 448
VII. — The antagonism between development and genesis,

ASEXUAL AND SEXUAL 461

VIII. — Antagonism between expenditure and genesis . . 467

IX. — Coincidence between high nutrition and genesis . . 475

X. — Specialities of these relations 486

XL — Interpretation and qualification 497

XII. — Multiplication of the human race 506 v

XIII. — Human population in the future 522

APPENDICES.

A. — Substitution of axial for foliar organs in plants . . 541
B. — A criticism on Prof. Owen's theory of the vertebrate

skeleton 548

C. — On circulation and the formation of wood in plants . . 567

D. — On the origin of the vertebrate type 599

D2. — The annulose type 602

E. — The shapes and arrangements of flowers .... 608

P. — Physiological (or constitutional) units .... 612

C— The inheritance of functionally-caused modifications . 618

PART IV.

MORPHOLOGICAL DEVELOPMENT

47

CHAPTER I.

THE PROBLEMS OF MORPHOLOGY.

§ 175. The division of Morphology from Physiology, is
one which may be tolerably-well preserved so long as we do
not carry our inquiries beyond the empirical generalizations
of their respective phenomena; but it is one which becomes
in great measure nominal, when the phenomena are to be
rationally interpreted. It would be possible, after analyzing
our Solar System, to set down certain general truths respect-
ing the sizes and distances of its primary and secondary
members, omitting all mention of their motions ; and it would
be possible to set down certain other general truths respect-
ing their motions, without specifying their dimensions or
positions, further than as greater or less, nearer or more re-
mote. But on seeking to account for these general truths,
arrived at by induction, we find ourselves obliged to consider
simultaneously the relative sizes and places of the masses,
and the relative amounts and directions of their motions.
Similarly with organisms. Though we may frame sundry
comprehensive propositions respecting the arrangements of
their organs, considered as so many inert parts; and though
we may establish several wide conclusions respecting the sepa-
rate and combined actions of their organs, without knowing
anything definite respecting the forms and positions of these
organs; yet we cannot reach such a rationale of the facts as

3

4 MORPHOLOGICAL DEVELOPMENT.

the hypothesis of Evolution aims at, without contemplating
structures and functions in their mutual relations. Every-
where structures in great measure determine functions; and
everywhere functions are incessantly modifying structures.
In Nature the two are inseparable co-operators; and Science
can give no true interpretation of Nature without keeping
their co-operation constantly in view. An account of organic
evolution, in its more special aspects, must be essentially an
account of the inter-actions of structures and functions, as
perpetually altered by changes of conditions.

Hence, when treating apart Morphological Development
and Physiological Development, all we can do is to direct our
attention mainly to the one or to the other, as the case may
be. In dealing with the facts of structure, we must consider
the facts of function only in such general way as is needful
to explain the facts of structure; and conversely when deal-
ing with the facts of function.

§ 176. The problems of Morphology fall into two distinct
classes, answering respectively to the two leading aspects of
Evolution. In things which evolve there go on two processes
— increase of mass and increase of structure. Increase of
mass is primary, and in simple evolution takes place almost
alone. Increase of structure is secondary, accompanying or
following increase of mass with more or less regularity, wher-
ever evolution rises above that form which small inorganic
bodies, such as crystals, present to us. As the fundamental
antagonism between Dissolution and Evolution consists in this,
that while the one is an integration of motion and dis-
integration of matter, the other is an integration of matter
and disintegration of motion; and as this integration of mat-
ter accompanying disintegration of motion, is a necessary
antecedent to the differentiation of the matter so integrated;
it follows that questions concerning the mode in which the
parts are united into a whole, must be dealt with before

THE PROBLEMS OF MORPHOLOGY. 5

questions concerning the mode in which these parts become
modified.*

This is not obviously a morphological question. But an
illustration or two will make it manifest that fundamental
differences may be produced between aggregates by differences
in the degrees of composition of the increments : the ultimate
units of the increments being the same. Thus an accumula-
tion of things of a given kind may be made by adding one at
a time. Or the things may be tied up into bundles of ten,
and the tens placed together. Or the tens may be united
into hundreds, and a pile of hundreds formed. Such unlike-
nesses in the structures of masses are habitually seen in our
mercantile transactions. Articles which the consumer re-
cognizes as single, the retailer keeps wrapped up in dozens,
the wholesaler sends in gross, and the manufacturer supplies
in packages of a hundred gross. That is, they severally in-
crease their stocks by units of simple, of compound, and of
doubly-compound kinds. Similarly result those differences of
morphological composition which we have first to consider.
An organism consists of units. These units may be aggre-
gated into a mass by the addition of unit to unit. Or they
may be united into groups, and the groups joined together.
Or these groups of groups may be so combined as to form a
doubly-compound aggregate. Hence there arises respecting
each organic form the question — is its composition of the
first, second, third, or fourth order? — does it exhibit units of
a singly-compounded kind only, or are these consolidated
into units of a doubly-compounded kind, or a triply-com-
pounded kind? And if it displays double or triple composi-

* It seems needful here to say, that allusion is made in this paragraph to a
proposition respecting the ultimate natures of Evolution and Dissolution,
which is contained in an essay on The Classification of the Sciences, pub-
lished in March, 1864. When the opportunity comes, I hope to make the
definition there arrived at, the basis of a re-organization of the second part of
First Principles: giving to that work a higher development, and a greater
cohesion, than it at present possesses. [The intention here indicated was
<uly carried out in 1867.]

6 MORPHOLOGICAL DEVELOPMENT.

tion, the homologies of its different parts become problems.
Under the disguises induced by the consolidation of primary,
secondary, and tertiary units, it has to be ascertained which
answer to which, in their degrees of composition.

Such questions are more intricate than they at first ap-
pear ; since, besides the obscurities caused by progressive inte-
gration, and those due to accompanying modifications of form,
further obscurities result from the variable growths of units
of the different orders. Just as an army may be augmented
by recruiting each company, without increasing the number
of companies; or may be augmented by making up the full
complement of companies in each regiment, while the num-
ber of regiments remains the same; or may be augmented
by putting more regiments into each division, other things
being unchanged; or may be augmented by adding to the
number of its divisions without altering the components of
each division; or may be augmented by two or three of these
processes at once; so, in organisms, increase of mass may
result from additions of units of the first order, or those of the
second order, or those of still higher orders ; or it may be due
to simultaneous additions to units of several orders. And
this last mode of integration being the general mode, puts
difficulties in the way of analysis. Just as the structure of
an army would be made less easy to understand if companies
often outgrew regiments, or regiments became larger than
brigades; so these questions of morphological composition
are complicated by the indeterminate sizes of the units of
each kind: relatively-simple units frequently becoming more
bulky than relatively-compound units.

§ 177. The morphological problems of the second class
are those having for their subject-matter the changes of shape
which accompany changes of aggregation. The most general
questions respecting the structure of an organism, having been
answered when it is ascertained of what units it is composed
as a whole, and in its several parts; there come the more

1

THE PROBLEMS OF MORPHOLOGY. 7

special questions concerning its form — form in the ordinary
sense. After the contrasts caused by variations in the process
of integration, we have to consider the contrasts caused by
variations in the process of differentiation. To speak speci-
fically— the shape of the organism as a whole, irrespective
of its composition, has to be accounted for. Reasons have
to be found for the unlikeness between its general outlines
and the general outlines of allied organisms. And there
have to be answered kindred inquiries respecting the propor-
tions of its component parts : — Why, among such of these as
are homologous with one another, have there arisen the
differences that exist? And how have there been produced
the contrasts between them and the homologous parts of
organisms of the same type ?

Very numerous are the heterogeneities of form presenting
themselves for interpretation under these heads. The ulti-
mate morphological units combined in any group, may be dif-
ferentiated individually, or collectively, or both : each of them
may undergo changes of shape; or some of them may be
changed and others not; or the group may be rendered mul-
tiform by the greater growth of some of its units than of
others. Similarly with the compound units arising by union
of these simple units. Aggregates of the second order may
be made relatively complex in form, by inequalities in the
rates of multiplication of their component units in diverse
directions; and among a number of such aggregates, numer-
ous unlikenesses may be constituted by differences in their
degrees of growth, and by differences in their modes of growth.
Manifestly, at each higher stage of composition the possible
sources of divergence are multiplied still further.

That facts of this order can be accounted for in detail is
not to be expected — the data are wanting. All that we may
hope to do is to ascertain their general laws. How this is to
be attempted we will now consider.

§ 178. The task before us is to trace throughout these

8 MORPHOLOGICAL DEVELOPMENT.

phenomena the process of evolution; and to show how, as
displayed in them, it conforms to those first principles which
evolution in general conforms to. Two sets of factors have
to be taken into account. Let us look at them.

The factors of the first class are those which tend directly
to change an organic aggregate, in common with every other
aggregate, from that more simple form which is not in equi-
librium with incident forces, to that more complex form which
is in equilibrium with them. We have to mark how, in corre-
spondence with the universal law that the uniform lapses into
the multiform, and the less multiform into the more multi-
form, the parts of each organism are ever becoming further
differentiated; and we have to trace the varying relations to
incident forces by which further differentiations are entailed.
We have to observe, too, how each primary modification of
structure, induced by an altered distribution of forces, becomes
a parent of secondary modifications — how, through the neces-
sary multiplication of effects, change of form in one part
brings about changes of form in other parts. And then we
have also to note the metamorphoses constantly being induced
by the process of segregation — by the gradual union of like
parts exposed to like forces, and the gradual separation of like
parts exposed to unlike forces. The factors of the second

class which we have to keep in view throughout our interpret-
ations, are the formative tendencies of organisms themselves
— the proclivities inherited by them from antecedent organ-
isms, and which past processes of evolution have bequeathed,
We have seen it to be inferable from various orders of fact-ks
(§§ 65, 84, 97-97</), that organisms are built up of certain
highly-complex molecules, which we distinguished as physio-
logical units [or constitutional units as they might otherwise
be called] — each kind of organism being built up of units pe-
culiar to itself. We recognized in these units, powers of ar-
ranging themselves into the forms of the organisms to which
they belong; analogous to the powers which the molecules of
inorganic substances have of aggregating into specific crystal-

!

THE PROBLEMS OF MORPHOLOGY. 9

line forms. We have consequently to regard this proclivity of
the physiological units, as producing, during the development
of any organism, a combination of internal forces that expend
themselves in working out a structure in equilibrium with
the forces to which ancestral organisms were exposed; but
not in equilibrium with the forces to which the existing organ-
ism is exposed, if the environment has been changed. Hence
the problem in all cases is, to ascertain the resultant of inter-
nal organizing forces, tending to reproduce the ancestral form,
and external modifying forces, tending to cause deviations
from that form. Moreover, we have to take into account,

not only the characters of immediately-preceding ancestors,
but also those of their ancestors, and ancestors of all degrees
of remoteness. Setting out with rudimentary types, we have
to consider how, in each successive stage of evolution, the
structures acquired during previous stages have been ob-
scured by further integrations and further differentiations;
or, conversely, how the lineaments of primitive organisms
have all along continued to manifest themselves under the
superposed modifications.

§179. Two ways of carrying on the inquiry suggest them-
selves. We may go through the several great groups of
organisms, with the view of reaching, by comparison of parts,
certain general truths respecting the homologies, the forms,
and the relations of their parts; and then, having dealt with
the phenomena inductively, may retrace our steps with the
view of deductively interpreting the general truths reached.
Or, instead of thus separating the two investigations, we
may carry them on hand in hand — first establishing each
general truth empirically, and then proceeding to the ra-
tionale of it. This last method will, I think, conduce to
both brevity and clearness. Let us now thus deal with the
first class of morphological problems.

[Note. — In preparation for treating of morphological de-

10 MORPHOLOGICAL DEVELOPMENT.

velopment, sundry other general considerations should have
been included in the foregoing chapter when originally pub-
lished. This seems the most appropriate place for now nam-
ing them. Some were implicitly contained in the first vol-
ume, but it will be well definitely to state these, as well as
the others not yet implied.

Interpretation of the forms of organisms and the forms
of their parts, must depend mainly on the conclusions pre-
viously drawn respecting their phylogeny; and the drawing
of such conclusions must be guided by recognition of the
various factors of Evolution, as well as by recognition of
certain extremely general results of Evolution and certain
concomitants of Evolution.

A primary one among these is that no existing species can
exhibit more than approximately the ancestral structure of
any other existing species. As all ancestors have disappeared,
so, in a greater or less degree, the traits, specific, generic, or
ordinal, which distinguished the earlier of them have disap-
peared. Setting out with the familiar symbol, a tree, let us
regard its peripheral twigs as representing extant species;
let us assume that the interior of the tree is filled up with
some supporting substance, leaving only the ends of the
living twigs projecting; and let us suppose the trunk, main
branches, secondary branches, tertiary branches, &c, have
decayed away. Then if we take these decayed parts to stand
for the divergent and re-divergent lines of evolution which
are represented by fossils in the Earth's crust, it will be
manifest, first, that no one of the living superficial twigs (or
species) exhibits the ancestral organization whence any other
of the living superficial twigs (or species) has been developed;
it will be manifest, second, that the generic structure in-
herited by any existing species must be a structure out of
which came sundry allied species — the fork, as it were, at
which adjacent twigs diverged; and third, that the ancestor
of an order must, in like manner, be sought at some point
deeper down in the symbolic tree — a place of divergence of

THE PROBLEMS OF MORPHOLOGY. 11

the sub-branches representing allied genera. Similarly with
the ancestral types of classes, still deeper down in the tree or
further back in time. So that phylogeny becomes more and
more speculative as its questions become more and more
radical. And the difficulty is made greater by the deficiency
of palaxmtological evidence.

One obvious corollary is that an ancestral type from which
sundry allied types now existing diverged, was, speaking
generally, simpler than these; since the divergent types be-
came different by the superposing of modifications, adding
to their complexities. There is a further reason for inferring
that the least specialized member of any group is more like
Hit1 remote ancestor than any of the others; for every adap-
tation stands in the way of subsequent re-adaptations: it
presents a greater amount of structure to be undone. To get
some idea of the ancestral type where no extant member of
the group is manifestly simpler than the rest, the method
must be to take all its extant members and, after letting
their differences mutually cancel, observe what remains com-
mon to them all.

But there are difficulties standing in the way of phylogeny,
and consequently of morphology, much greater than these.
Returning to our symbolic tree, it is clear that it would be
far from easy to say of any one twig which extinct sub-
branch, branch, and main branch it belonged to, even sup-
posing that the growths of all parts had been uniformly out-
wards. Immensely more perplexing, then, must be the
affiliation if various of the branches, sub-branches, &c, have
sent out backward-growing shoots which have come to the
surface only after prolonged retrograde courses, and if other
branches have sent shoots into regions occupied by alien
branches — shoots bearing twigs which come to the surface
along with those to which they are but remotely allied. The
problems of origin and of structure which organisms present,
are met by both of the difficulties thus symbolized.

One of them arises from the prevalence of retrograde

12 MORPHOLOGICAL DEVELOPMENT.

metamorphoses. Throughout the animal world these ai
variously displayed by parasites, multitudinous in their
kinds; for most of them belong to types much higher in
organization. Changed habits and consequent changed struc-
tures have so transferred them that only by study of their
embryonic stages can their kinships be made out. And these
retrograde metamorphoses, conspicuous among parasites, have,
in the course of evolution, affected some members of all
groups; for in all groups the struggle for existence has com-
pelled some to adopt careers less trying but less profitable.

Not only by forcing on many kinds of organisms simpler
ways of living, and consequent degeneracy, has the universal
competition caused obscuring transformations. It has done
this also by tempting many other kinds of organisms
to adopt ways of life not simpler than before but merely
different. Pressure continually prompts every type to in-
trude on other types' spheres of activity; and so causes it to
assume certain structural characters of the types whose
spheres it invades, masking its previous characters. Modifi-
cations hence arising have, in the great mass of cases, been
superposed one on another time after time. The aquatic
animal becomes through several transitions a land-animal,
and then the land-animal through other transitions becomes
now an aerial animal like the bat and now an aquatic animal
like the whale. Certain kinds of birds furnish extreme
illustrations. There was the change from the fish to the
water-breathing amphibian and then to the air-breathing
amphibian; thence to the reptile living on the Earth's sur-
face; thence to the flying reptile and the bird; then came
the diving birds, joining with their aerial life a life passed
partly in the water; and finally came a type like the
penguin, in which the power of flight has been lost and the
water has again become the almost exclusive medium, except
for breathing. Of course the mouldings and re-mouldings of
structure resulting from these successive unlike modes of
life, in many cases put great difficulties in the way of ascer-

THE PROBLEMS OF MORPHOLOGY. 13

taining which are the original corresponding parts. Some
parts have become abnormally large; others have dwindled
or disappeared ; and the relative positions of parts have often
been greatly changed. A bat's wing and a bird's wing are
analogous organs, but their frameworks are but partially
homologous. While in the bird the terminal parts of the
fore-limb do little towards supporting the wing, in the bat
the wing is mainly supported by enormously-developed termi-
nal parts.

The effects of the struggle to survive, which here prompts
a simpler life with resulting degeneracy and there a different
life with resulting new developments, are far from being the
only causes of morphological obscurations. Fulfilment of
certain highly general requirements gives certain common
traits to plants of widely divergent classes; and fulfilment of
certain other highly general requirements gives certain
common traits to animals of widely divergent classes. It was
remarked in the first volume (§ 54/) that the cardinal distinc-
tion between the characters of plants and animals arises from
the fact that while the chief food of plants is universally
present the food of animals is scattered. Here it has to be
added that to utilize the universally distributed food the
ordinary plant needs the aid of light, and has to acquire
structures enabling it to get that aid; while the ordinary
animal, to utilize the scattered food, must acquire the struc-
tures needful for locomotion. Let us contemplate separately
the traits hence resulting in the vegetal world and the traits
hence resulting in the animal world.

The familiar plantain meets the requirement by growing
stiff leaves enabling it to press down the competing grasses
around which would else shade it; but the great majority of
ordinary plants meet the requirement by raising themselves
into the air. Hence the need for a stem, and hence the fact
that plants of widely unlike natures similarly form stems
which, in achieving strength enough to support the foliage
and resist the wind, acquire certain adaptive structures hav-

14 MORPHOLOGICAL DEVELOPMENT.

ing a general similarity. Here from the edge of a pool is
a reed, and here from the adjacent copse is a hemlock: the
one having grown tall in escaping the shade of its com-
panions and the other in escaping the shade of the surround-
ing brushwood. On being cut across each discloses a tube,
and each exhibits septa dividing this tube into chambers.
In either case by the tubular structure is gained the greatest
strength with the least ihaterial; but there is no morpho-
logical kinship between the tubes nor between the septa.
Still more marked is the simulation of homology by analogy
in another plant which the adjacent ditch may furnish — the
common Horsetail. In this, again, we see an elongated ver-
tical-growing part, raising the foliage into the air; and, as
before, this is tubular and divided by septa. A type utterly
alien from the other two has, by survival of the fittest, been
similarly moulded to meet mechanical needs.

Passing now to the obscurations in the animal world
caused by alterations favouring locomotion, we note first that
the locomotive power is at the outset very slight. Among
many orders of Protozoa, as also among many low types of
Metazoa, vibratile cilia are the most general agents of loco-
motion — necessarily feeble locomotion. Eegarded in the
mass, the Cwlenterata, when not stationary like the Hydra or
higher types in the hydroid stage, usually possess only such
small self-mobility as the slow rhythmical contractions of
their umbrella-disks effect, or else such as is effected by bands
of cilia or of vibratile plates, as in the Beroe. Even among
these low tpes of Metazoa, however, in which ordinarily the
radial structure is conspicuous, or but slightly obscured by
an ovoid form as in the Ctenophora, we find, in the Cestus
veneris, extreme obscuration caused by an elongation which
facilitates movement through the water; alike by the actions
of its vibratile plates and by its undulations, which simulate
those of sundry higher animals.

And here we come upon the essential fact to be recognized.
Elongation favours locomotion in various ways that are

THE PROBLEMS OF MORPHOLOGY. 15

severally taken advantage of by different types of creatures.
(1) To a given mass of moving matter the resistance of the
medium decreases along with decrease in the area of its
transverse section, and this implies increase of length : a given
force will move the lengthened mass along with greater
facility. (2) Reaching a certain point the elongated form
enables an animal to progress by undulations, as in the
water fish do, and even some ccelenterates and turbellarians
do, and as on land snakes do: lateral resistances serving in
either case as fulcra. (3) Lengthening of the body serves
otherwise to aid locomotion in the creeping or burrowing
worm, which, utilizing the statical resistance of its hinder
part thrusts onwards its fore part, and then, holding fast its
fore part by the aid of minute setce, draws the hinder part
after it. But elongation, doubly advantageous at first, while
the body is itself the chief instrument of locomotion, gradually
loses its advantageousness as special instruments of locomo-
tion are developed. (4) This we see in that locomotive
action effected by limbs, which, many and small in the lower
Arthropoda and becoming few and larger in the higher, at
length give great activity to a shortened and consolidated
body: a stage reached only through stages of decreasing
elongation accompanying increase of limb-power. (5) In
the Vertebrata locomotion by undulations comes, along cer-
tain lines of evolution, to be replaced by that limb-locomo-
tion which accompanies the rise from water-life to land-life :
the evolution of Amphibians exhibiting the transition. (6)
Further, we see among mammals that as limbs become effi-
cient the elongated body ceases to be itself instrumental in
locomotion, but that still some elongation remains a charac-
teristic. (7) Finally, where limb-locomotion reaches its high-
est degree, as in birds, elongation disappears.

These classes of familiar facts I have recalled to show
that, in the course of evolution, achievement by plants of the
all-essential elevation into the air and by animals of the
all-essential power of movement have developed this trait

I

16 MORPHOLOGICAL DEVELOPMENT.

of elongation in various types; and that in each kingdom
acquisition of the common trait has had a tendency now to
obscure morphological equivalence, and now to give the ap-
pearance of kinship where there is none. A further pur-
pose has been to prepare the way for a question hereafter
to be discussed — whether, in the various types of either king-
dom, the elongation is effected in the same ways or in dif-
ferent ways. We shall have to ask whether the vertically
growing part is always, like that of Lessonia, a simple in-
dividual, or whether, as possibly in Phsenogams, it is a united
series of individuals; and similarly whether the elongated
body is always single, like that of a mollusc, or whether, as
possibly in annulose animals, it is a series of united in-
dividuals.]

CHAPTER II.

THE MORPHOLOGICAL COMPOSITION" OF PLANTS.

§ 180. Evolution implies insensible modifications and
gradual transitions, which render definition difficult — which
make it impossible to separate absolutely the phases of
organization from one another. And this indefiniteness of
distinction, to be expected a priori, we are compelled to
recognize a posteriori, the moment we begin to group morpho-
logical phenomena into general propositions. Thus, on
inquiring what is the morphological unit, whether of plants
or of animals, we find that the facts refuse to be included in
any rigid formula. The doctrine that all organisms are built
up of cells, or that cells are the elements out of which every
tissue is developed, is but approximately true. There are
living forms of which cellular structure cannot be asserted ;
and in living forms that are for the most part cellular, there
are nevertheless certain portions which are not produced by
the metamorphosis of cells. Supposing that clay were the
only material available for building, the proposition that all
houses are built of bricks, would bear about the same relation
to the truth, as does the proposition that all organisms are
composed of cells. This generalization respecting houses
would be open to two criticisms: — first, that certain houses
of a primitive kind are formed, not of bricks, but out of
unmoulded clay; and second, that though other houses con-
sist mainly of bricks, yet their chimney-pots, drain-pipes, and
48 17

18 MORPHOLOGICAL DEVELOPMENT.

ridge-tiles, do not result from combination or metamorphosis
of bricks, but are made directly out of the original clay.
And of like natures are the criticisms which must be passed
on the generalization, that cells are the morphological units
of organisms. To continue the simile, the truth turns out to
be, that the primitive clay or protoplasm out of which
organisms are built, may be moulded either directly, or
with various degrees of indirectness, into organic structures.
The physiological units which we are obliged to assume as
the components of this protoplasm, must, as we have seen,
be the possessors of those proclivities which result in
the structural arrangements of the organism. The assump-
tion of such structural arrangements may go on, and in
many cases does go on, by the shortest route; without the
passage through what we call metamorphoses. But where
such structural arrangements are reached by a circuitous
route, the first stage is the formation of these small aggre-
gates which, under the name of cells, are currently regarded
as morphological units.

The rationale of these truths appears to be furnished by
the hypothesis of evolution. We set out with molecules
some degrees higher in complexity than those molecules of
nitrogenous colloidal substance into which organic matter is
resolvable; and we regard these very much more complex
molecules as having the implied greater instability, greater
sensitiveness to surrounding influences, and consequent
greater mobility of form. Such being the primitive physio-
logical units, organic evolution must begin with the formation
of a minute aggregate of them — an aggregate showing vitality
by a higher degree of that readiness to change its form of
aggregation which colloidal matter in general displays; and
by its ability to unite the nitrogenous molecules it meets
with, into complex molecules like those of which it is com-
posed. Obviously, the earliest forms must have been minute;
since, in the absence of any but diffused organic matter, no
form but a minute one could find nutriment. Obviously, too,

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 19

it must have been structureless; since, as differentiations are
producible only by the unlike actions of incident forces, there
could have been no differentiations before such forces had had
time to work. Hence, distinctions of parts like those required
to constitute a cell were necessarily absent at first. And we
need not therefore be surprised to find, as we do find, specks
of protoplasm manifesting life, and yet showing no signs of
organization. A further stage of evolution is

reached when the imperfectly integrated molecules forming
one of these minute aggregates, become more coherent; at
the same time as they pass into a state of heterogeneity,
gradually increasing in its definiteness. That is to say, we
may look for the assumption by them, of some distinctions of
parts, such as we find in cells and in what are called uni-
cellular organisms. They cannot retain their primordial
uniformity ; and while in a few cases they may depart from it
but slightly, they will, in the great majority of cases, acquire
a decided multiformity: there will result the comparatively
integrated and comparatively differentiated Protophyta and
Protozoa. The production of minute aggregates

of physiological units being the first step, and the passage
of such minute aggregates into more consolidated and more
complex forms being the second step, it must naturally
happen that all higher organic types, subsequently arising by
further integrations and differentiations, will everywhere bear
the impress of this earliest phase of evolution. From the
law of heredity, considered as extending to the entire suc-
cession of living things during the Earth's past history, it
follows that since the formation of these small, simple organ-
isms must have preceded the formation of larger and more
complex organisms, the larger and more complex organisms
must inherit their essential characters. We may anticipate
that the multiplication and combination of these minute
aggregates or cells, will be conspicuous in the early develop-
mental stages of plants and animals; and that throughout
all subsequent stages, cell-production and cell-differentiation

20 MORPHOLOGICAL DEVELOPMENT.

will be dominant characteristics. The physiological units
peculiar to each higher species will, speaking generally,
pass through this form of aggregation on their way towards
the final arrangement they are to assume; because those
primordial physiological units from which they are remotely
descended, aggregated into this form. And yet, just as in
other cases we found reasons for inferring (§ 131) that the
traits of ancestral organization may, under certain conditions,
be partially or wholly obliterated, and the ultimate structure
assumed without passing through them; so, here, it is to be
inferred that the process of cell-formation may, in some cases,
be passed over. Thus the hypothesis of evolution

prepares us for those two radical modifications of the cell-
doctrine which the facts oblige us to make. It leads us to
expect that as structureless portions of protoplasm must have
preceded cells in the process of general evolution; so, in the
special evolution of each higher organism, there will be an
habitual production of cells out of structureless blastema.
And it leads us to expect that though, generally, the physio-
logical units composing a structureless blastema, will display
their inherited proclivities by cell-development and meta-
morphosis; there will nevertheless occur cases in which the
tissue to be formed, is formed by direct transformation of the
blastema.*

* Let me here refer those who are interested in this question, to Prof.
Huxley's criticism on the cell-doctrine, published in the Medico- Chirurgical
Review in 1853.

A critic who thinks the above statements are "rather misleading" ad-
mits that the lowest types of organisms yield them support, saying that
"there are certainly masses of protoplasm containing many nuclei, but no
trace of cellular structure, in both animals and plants. Such non-cellular
masses may exist during development and later become separated up into
cells, but there are certain low organisms in which such masses exist in the
adult state. They are called by some botanists non- cellular, by others
multi-nucleate cells. Clearly the difference lies in the criteria of a cell.
There are also some Protozoa, and the Bacteria, in which no nucleus has
certainly been demonstrated. But it is usual to consider the bodies of such
organisms as cells nevertheless, and it is supposed that such cells represent

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 21

Interpreting the facts in this manner, we may recognize
that large amount of truth which the cell-doctrine contains,
without committing ourselves to the errors involved by a
sweeping assertion of it. We are enabled to understand how
it happens that organic structures are usually cellular in their
composition, at the same time that they are not universally
so. We are shown that while we may properly continue to
regard the cell as the morphological unit, we must constantly
bear in mind that it is such only in a qualified sense.

§ 181. These aggregates of the lowest order, each formed
of physiological units united into a group that is structurally
single and cannot be divided without destruction of its
individuality, may, as above implied, exist as independent
organisms. The assumption to which we are committed by
the hypothesis of evolution, that such so-called uni-cellular
plants were at first the only kinds of plants, is in harmony
with the fact that habitats not occupied by plants of higher
orders, commonly contain these protophytes in great abund-
ance and great variety. The various species of Pleurococ-
cacecs, of Desmidiacece, and Diatomacece, supply examples of
morphological units living and propagating separately, under
numerous modifications of form and structure. Figures 1, 2,
and 3, represent a few of the commonest types.

Mostly, simple plants are too small to be individually

a stage of development in which the nucleus has not yet been evolved, though
the chemical substance ' nuclein ' has been formed in some of them."

Perhaps it will be most correct to say that, excluding the minute, non-
nucleated organisms, all the higher organisms — Metazoa and Metaphyta — are
composed throughout of cells, or of tissues originally cellular, or of materials
which have in the course of development been derived from cells. It must,
however, be borne in mind that, according to sundry leading biologists, cells
in the strict sense are not the immediate products either of the primitive
fissions or of subsequent fissions ; but that the multiplying so-called cells are
nucleated masses of protoplasm which remain connected by strands of proto-
plasm, and which acquire limiting membranes by a secondary process. So
that, in the view of Mr. Adam Sedgwick and others, the substance of an
organism is in fact a continuous mass of vacuolated protoplasm.

22

MORPHOLOGICAL DEVELOPMENT.

visible without the microscope. But, in some cases, these
vegetal aggregates of the first order grow to appreciable

sizes. In the mycelium of some fungi, we have single cells
developed into long branched filaments, or ramified tubules,
that are of considerable lengths. An analogous structure char-
acterizes certain tribes of Algce, of which C odium adherens,
Fig. 4, may serve as an example. In Botrydium, another
alga, Fig. 5, we have a structure which is described as simu-
lating a higher plant, with root, stem, bud, and fruit, all
produced by the branching of a single cell. And among

fungi the genus Mucor, Fig. 6, furnishes an example of
allied kind.* Here, though the size attained is much greater
than that of many organisms which are morphologically
compound, we are compelled to consider the morphological
composition as simple; since the whole can no more be
separated into minor wholes, than can the branched vascular

* In further illustration, Mr. Tansley names the fact that in the genus
Caulerpa we have extremely complicated forms often of considerable size
produced in the same way. The various species simulate very perfectly the
members of different groups among the higher plants, such as Horse-tails,
Mosses, Cactuses, Conifers and the like.

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 23

system of an animal. In these cases we have considerable
bulk attained, not by a number of aggregates of the first
order being united into an aggregate of the second order, but
by the continuous growth of an aggregate of the first order. '

§ 182. The transition to higher forms begins in a very
unobtrusive manner. Among these aggregates of the first
order, an approach towards that union by which aggregates
of the second order are produced, is indicated by mere juxta-
position. Protophytes multiply rapidly; and their rapid
multiplication sometimes causes crowding. When, instead
of floating free in the water, they form a thin film on a moist
surface, or are imbedded in a common matrix of mucilage;
the mechanical obstacles to dispersion result in a kind of
feeble integration, vaguely shadowing forth a combined
group. Somewhat more definite combination is shown us by
such plants as Palmella botryoides. Here the members of a
family of cells, arising by the spontaneous fission of a parent-
cell, remain united by slender threads of that jelly-like sub-
stance which envelops their surfaces. In some Diatomacece
several individuals, instead of completely separating, hold
together by their angles; and in other Diatomacece, as the
Bacillaria, a variable number of units cohere so slightly, that
they are continually moving in relation to one another.

This formation of aggregates of the second order, faintly
indicated in feeble and variable unions* like the above, may
be traced through phases of increasing permanence and de-
finiteness, as well as increasing extent. In the yeast-plant,
Fig. 7, we have cells which may exist singly, or joined into
groups of several; and which have their shapes scarcely at
all modified by their connexion. Among the Desmidiacew, it
happens in many cases that the two individuals produced by
division of a parent-individual, part as soon as they are fully
formed; but in other cases, instead of parting they compose
a group of two. Allied kinds show us how, by subsequent
fissions of the adherent individuals and their progeny, there

I

24 MORPHOLOGICAL DEVELOPMENT.

result longer groups ; and in some species, a continuous thread
of them is thus produced. Figs. 8, 9, 11, exhibit these
several stages' Fig. 10 represents a Scenedesmus in which

'CD 'CXannffmx

*»*

the individuation of the group is manifest. Instead of linear
aggregation, many protophytes illustrate central aggregation;
as shown in Figs. 12, 13, 14, 15. Other instances are fur-
nished by such forms as the Gonium pectorale, Fig. 16 (a
being the front view, and b the edge view), and the Sarcina
ventriculi, Fig. 17. Further, we have that spherical mode
of aggregation of which the Volvox globator furnishes a
familiar instance.

Thus far, however, the individuality of the secondary ag-
gregate is feebly pronounced: not simply in the sense that
it is small; but also in the sense that the individualities of
the primary aggregates are very little subordinated. But on
seeking further, we find transitions towards forms in which
the compound individuality is more dominant, while the
simple individualities are more obscured. Obscuration

of one kind accompanies mere increase of size in the second-
ary aggregate. In proportion to the greater number of the
morphological units held together in one mass, becomes their
relative insignificance as individuals. We see this in the
irregularly-spreading lichens that form patches on rocks;
and in such creeping fungi as grow in films or laminae on
decaying wood and the bark of trees. In these cases, how-
ever, the integration of the component cells is of an almost

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 25

mechanical kind. The aggregate of them is scarcely more
individuated than a lump of inorganic matter : as witness the
way in which the lichen extends its curved edges in this or
that direction, as the surface favours; or the way in which
the fungus grows round and imbeds the shoots and leaves
that lie in its way, just as so much plastic clay might do.
Though here, in the augmentation of mass, we see a progress
towards the evolution of a higher type, we have as yet none
of that definiteness required to constitute a compound unit,
or true aggregate of the second order. Another kind

of obscuration of the morphological units, is brought about
by their more complete coalescence into the form of some
structure made by their union. This is well exemplified
among the Confervoidece and Conjugate/?. In Fig. 18, there

are represented the stages of a growing Mougeotia genuflexa,
in which this merging of the simple individualities into the
compound individuality, is shown in the history of a single
plant; and in Figs. 19, 20, 21, 22, 23, are represented a
series of species from this group, and that of Cladophora* in
which we see a progressing integration. While, in the lower
types, the primitive spheroidal forms of the cells are scarcely
altered, in the higher types the cells are so fused together
as to constitute cylinders divided by septa. Here, however,

* It may be objected that in Cladophora the separate compartments of
the thallus severally contain many nuclei, making it doubtful whether they
descend from uni-nucleate cells. If, however, they do not they simply illus-
trate another form of integration.

26

MORPHOLOGICAL DEVELOPMENT.

the indefiniteness is still great. There are no specific limits
to the length of any thread thus produced, and there is none
of that differentiation of parts required to give a decided in-
dividuality to the whole.

To constitute something like a true aggregate of the second
order, capable of serving as a compound unit that may be
combined with others like itself into still higher aggregates,
there must exist both mass and definiteness.

§ 183. An approach towards plants which unite these
characters, may be traced in such forms as Bangia ciliaris,
Fig. 24. The multiplication of cells here takes place, not in
24r a longitudinal direction only, but also in

a transverse direction; and the transverse
multiplication being greater towards the
middle of the frond, there results a differ-
ence between the middle and the two ex-
tremities— a character which, in a feeble
way, unites all the parts into a whole.
Even this slight individuation is, however,
very indefinitely marked; since, as shown
by the figures, the lateral multiplication
of cells does not go on in a precise manner.
From some such type as this there ap-
pear to arise, through slight differences in
the modes of growth, two closely-allied
groups of plants, having individualities
somewhat more pronounced. If, while
the cells multiply longitudinally, their lateral multiplication
goes on in one direction only, there results a flat surface, as
in the genus Ulva (Sea-lettuce) or in the upper part of the
thallus of E nter omorplia Linza, Fig. 25 ; or where the lateral
multiplication is less uniform in its rate, in types like
Fig. 26. But where the lateral multiplication occurs in two
directions transverse to one another, a hollow frond may be
produced — sometimes irregularly spheroidal, and sometimes

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 27

irregularly tubular; as in Enteromorpha intestinalis, Fig. 27.
And often, as in Enteromorpha compressa, Fig. 28, and other

species, this tubular frond becomes branched. Figs. 29 and
30 are magnified portions of such fronds, showing the
simple cellular aggregation which allies them with the pre-
ceding forms.

In the common Fuel of our coasts, other and somewhat
higher stages of this integration are displayed. We have
fronds preserving something like constant breadths and
dividing dichotomously with approximate regularity. Though
the subdivisions so produced are not to be regarded as
separate fronds, but only as extensions of one frond, they
foreshadow a higher degree of composition; and by the com-
paratively methodic way in which they are united, give to
the aggregate a more definite, as well as a more complex, in-
dividuality. Many of the higher lichens exhibit an
analogous advance. While in the lowest lichens, the different
parts of the thallus are held together only by being all
attached to the supporting surface, in the higher lichens the
thallus is so far integrated that it can support itself by
attachment to such surface at one point only. And then, in
still more developed kinds, we find the thallus assuming a

28 MORPHOLOGICAL DEVELOPMENT.

dichotomously-branched form, and so gaining a more specific
character as well as greater size.

Where, as in types like these, the morphological units
show an inherent tendency to arrange themselves in a manner
which is so far constant as to give characteristic proportions,
we may say that there is a recognizable compound individual-
ity. Considering the Thallophytes which grow in this way
apart from their kinships,- and wholly with reference to their
morphological composition, Ave might not inaptly describe
them as pseudo-foliar.

§ 184. Another mode in which aggregation is so carried
on as to produce a compound individuality of considerable
definiteness, is variously displayed among other families of
Algce. When the cells, instead of multiplying longitudinally
alone, and instead of all multiplying laterally as well as
longitudinally, multiply laterally only at particular places,
they produce branched structures.

Indications of this mode of aggregation occur among the
Confervoidece, as shown in Figs. 22, 23. Though, in some of
the more developed Algce which exhibit the ramified arrange-
ment in a higher degree, the component cells are, like those
of the lower Algce, united together end to end, in such way
as but little to obscure their separate forms, as in Cladophora
Hutchinsice, Fig. 31; they nevertheless evince greater sub-
ordination to the whole of which they are parts, by arranging
themselves more methodically. Still further pronounced
becomes the compound individuality when, while the com-
ponent cells of the branches unite completely into jointed
cylinders, the component cells of the stem form an axis dis-
tinguished by its relative thickness and complexity. Such
types of structures are indicated by Figs. 32, 33 — figures
representing small portions of plants which are quite tree-
like in their entire outlines. On examining Figs. 34, 35, 36,
which show the structures of the stems in these types, it
will be seen, too, that the component cells in becoming more

THE MORPHOLOGICAL COMPOSITION OF PLANTS. %)

coherent, have undergone changes of form which obscure
their individualities more than before. Not only are they

much elongated, but they are so compressed as to be pris-
matic rather than cylindrical. This structure, besides dis-
playing integration of the morphological units carried on in
two directions instead of one; and besides displaying this
higher integration in the greater merging of the individuali-
ties of the morphological units in the general individuality;
also displays it in the more pronounced subordination of the
branches and branchlets to the main stem. This differentia-
tion and consolidation of the stem, brings all the secondary
growths into more marked dependence; and so renders the
individuality of the aggregate more decided.

We might not inappropriately call this type of structure
pseud-axial. It simulates that of the higher plants in cer-
tain superficial characters. We see in it a primary axis along
which development may continue indefinitely, and from
which there bud out, laterally, secondary axes of like nature,
bearing like tertiary axes; and this is a mode of growth
with which Phaenogams make us familiar.

§ 185. Some of the larger Algce supply examples of an
integration still more advanced; not simply inasmuch as
they unite much greater numbers of morphological units

30 MORPHOLOGICAL DEVELOPMENT.

into continuous masses, but also inasmuch as they combine
the pseudo-foliar structure with the pseud-axial structure.
Our own shores furnish an instance of this in the common
Laminaria; and certain gigantic Laminar iacece of the Ant-
arctic seas, furnish yet better instances. In Necrocystis the
germ develops a very long slender stem, which eventually
expands into a large bladder-like or cylindrical air-vessel;
and the surface of this bears numerous leaf-shaped expan-
sions. Another kind, Lessonia fuscescens, Fig. 37, shows us a
massive stem growing up through water
many feet deep — a stem which, bifurcating
as it approaches the surface, flattens out the
ends of its subdivisions into fronds like
ribands. These, however, are not true foliar
appendages, since they are merely expanded
continuations of the stem. In Egregia
branches of the thallus not only take the
form of leaves, but these are differentiated
into several categories in accordance with a
division of labour. In any of these Lamin-
ariacea the whole plant, great as may be its
size, and made up though it seems to be of
many groups of morphological units, united
into a compound group by their marked subordination to a
connecting mass, is nevertheless a single thallus, which is
added to by intercalary growth at the "transition-place," at
the junction of the stem-like and leaf-like portions. The
aggregate is still an aggregate of the second order.

But among certain of the highest Alga, we do find some-
thing more than this union of the pseud-axial with the
pseudo-foliar structure. In addition to pseud-axes of com-
parative complexity; and in addition to pseudo-folia that
are like leaves, not only in their general shapes but in hav-
ing mid-ribs and even veins; there are the beginnings of
a higher stage of integration. Figs. 38, 39, and 40, show
some of the steps. In Rhodymenia palmata, Fig. 38, the

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 31

parent-frond is comparatively irregular in form, and without
a mid-rib; and along with this very imperfect integration,

we see that the secondary fronds growing from the edges are
distributed very much at random, and are by no means
specific in their shapes. A considerable advance is displayed
by Phyllophora rub ens, Fig. 39. Here the frond, primary,
secondary, or tertiary, betrays some approach towards regu-
larity in both form and size; by which, as also by its
partially-developed mid-rib, there is established a more
marked individuality; and at the same time, the growth of
the secondary fronds no longer occurs anywhere on the edge,
in the same plane as the parent-frond, but from the surface
at specific places. Delesseria sanguined, Fig. 40, illustrates a
much more definite arrangement of the same kind. The
fronds of this plant, quite regularly shaped, have their parts
decidedly subordinated to the whole; and from their mid-
ribs grow other fronds which are just like them. Each of
these fronds is an organized group of those morphological
units which we distinguish as aggregates of the first order.
And in this case, two or more such aggregates of the second

32 MORPHOLOGICAL DEVELOPMENT.

order, well individuated by their forms and structures, are
united together; and the plant composed of them is thus
rendered, in so far, an aggregate of the third order.

Just noting that in certain of the most-developed Algce, as
the Sargassum, or common gulf-weed, this tertiary degree of
composition is far more completely displayed, so as to pro-
duce among Thallophytes a type of structure closely simulat-
ing that of the higher plants, let us now pass to the considera-
tion of these higher plants.

§ 186. Having the surface of the soil for a support and the
air for a medium, terrestrial plants are mechanically circum-
stanced in a manner widely different from that in which
aquatic plants are circumstanced. Instead of being buoyed
up by a surrounding fluid of specific gravity equal to their
own, they have to erect themselves into a rare fluid which
yields no appreciable support. Further, they are dis-
similarly conditioned in having two sources of nutriment in
place of one. Unlike the Algce, which derive all the mate-
rials for their tissues from the water bathing their entire
surfaces, and use their roots only for attachment, most of the
plants which cover the Earth's surface, absorb part of their
food through their imbedded roots and part through their
exposed leaves. These two marked unlikenesses in the rela-
tions to surrounding conditions, profoundly affect the respec-
tive modes of growth. We must duly bear them in mind
while studying the further advance of composition.

The class of plants to which we now turn — that of the
Archegoniatce — is nearly related by its lower members to the
classes above dealt with : so much so, that some of the inferior
liverworts are quite licheniform, and are often mistaken for
lichens. Passing over these, let us recommence our synthesis
with such members of the class as repeat those indications of
progress towards a higher composition, which we have just ob-
served among the more-developed Algce. The Jungerman-
niacecB furnish us with a series of types, clearly indicating the

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 33

transition from an aggregate of the second order to an aggre-
gate of the third order. Figs. 41, and 42, indicate the struc-
ture among the lowest of this group. Here there is but an in-
complete development of the second order of aggregate. The

frond grows as irregularly as the thallus of a lichen: it is 'in-
definite in size and outline, spreading hither or thither as
the conditions favour. Moreover, it lacks the differentiations
required to subordinate its parts to the whole : it is uniformly
cellular, having neither mid-rib nor veins; and it puts out
rootlets indifferently from all parts of its under-surface. In
Fig. 43, Pellia epiphylla, we have an advance on this type.
There is here, as shown in the transverse section, Fig. 44, a
thickening of the frond along its central portion, producing
something like an approach towards a mid-rib; and from
this the rootlets are chiefly given off. The outline, too, is
much less irregular; whence results greater distinctness of
the individuality. A further step is displayed in Metzgeria
furcata, Fig. 45. The frond of this plant, comparatively well
integrated by the distribution of its substance around a
decided mid-rib, and by its comparatively-definite outlines,
produces secondary fronds. There is what is called prolifer-
ous growth; and occasionally, as shown in Fig. 46, represent-
ing an enlarged portion, the growth is doubly-proliferous. In
these cases, however, the tertiary aggregate, so far as it is
formed, is but very feebly integrated; and its integration is
but temporary. For not only do these younger fronds that
bud out from the mid-ribs of older fronds, develop rootlets of
their own ; but as soon as they are well grown and adequately
rooted, they dissolve their connexions with the parent-fronds,
49

34 MORPHOLOGICAL DEVELOPMENT.

and become quite independent. From these transi-

tional forms we pass, in the higher Jungermanniacece, to

50$

forms composed of many fronds that are permanently united
by a continuous stem. A more-developed aggregate of the
third order is thus produced. But though, along with in-
creased deflniteness in the secondary aggregates, there is here
an integration of them so extensive and so regular, that they
are visibly subordinated to the whole they form; yet the
subordination is really very incomplete. In some instances,
as in Radula complanata, Fig. 47, the leaflets develop roots
from their under surfaces, just as the primitive frond does;
and in the majority of the group, as in J. capitata, Fig. 48,
roots are given off all along the connecting stem, at the spots
where the leaflets or frondlets join it: the result being that
though the connected frondlets form a physical whole, they
do not form, in any decided manner, a physiological whole;
since successive portions of the united series, carry on their
functions independently of the rest. Finally, the

most developed members of the group, whether lineally de-
scended from the less developed or from an early type com-
mon to the two, present us with tertiary aggregates which are
physiologically as well as physically integrated.* Not lying

* The great mass of early ancestral types — plant and animal — consisting of
soft tissues, have left no remains whatever, and we have no reason to suppose
that those which left remains fell within the direct ancestral lines of any
existing forms. Contrariwise, we have reason to suppose that they fell with-
in lines of evolution out of which the lines ending in existing forms diverged.
We must therefore infer that the difficulties of affiliation which arise if we

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 35

prone like the kinds thus far described, but growing erect,
the stem and attached leaflets become dependent upon a
single root or group of roots; and being so prevented from
carrying on their functions separately, are made members of
a compound individual : there arises a definitely-established
aggregate of the third degree of composition.

The facts as arranged in the above order are suggestive.
M mute aggregates, or cells, the grouping of which we traced
in § 182, showed us analogous phases of indefinite union,
which appeared to lead the way towards definite union. We
see here among compound aggregates, as we saw there
among simple aggregates, the establishment of a specific
form, and a size that falls within moderate limits of varia-
tion. This passage from less definite extension to more
definite extension, seems in the one case, as the other, to be
accompanied by the result, that growth exceeding a certain
rate, ends in the formation of a new aggregate, rather than
an enlargement of the old. And on the higher stage, as on
the lower, this process, irregularly carried out in the simpler
types, produces in them unions that are but temporary ; while
in the more-developed types, it proceeds in a systematic way,
and ends in the production of a permanent aggregate that is
doubly compound.

contemplate divergent types now existing, would not arise if we had before
us all the early intermediate types. The Mammalia differ in sundry respects
from all other kinds of Vertebrata — Fishes, Reptiles, Birds; and if the
absence of hair, mammse, and two occipital condyles, in these other verte-
brates were taken to imply a fundamental distinction, it might, in the absence
of any known fossil links, be inferred that the Mammalia belonged to a sepa-
rate phylum. But these differences are not held to negative the assumed
relationship. Similarly among plants. We must not reject an hypothesis
respecting a certain supposed type, because the existing types it must have
been akin to present traits which it could not have had. We are justified in
assuming, within limits, a hypothetical type, unlike existing types in traits of
some importance. Hence results the answer to a criticism passed on the
above argument, that it implies relations between the undeveloped and devel-
oped forms of the Jungermanniaccce such as the facts do not show us. This
objection is met on remembering that the types in which the supposed transi-
tion took place disappeared myriads of years ago.

36 MORPHOLOGICAL DEVELOPMENT.

Must we then conclude that as cells, or morphological
units, are integrated into a unit of a higher order, which we
call a thallus or frond ; so, by the integration of fronds, there
is evolved a structure such as the above-delineated species
possess? Whether this is the interpretation to be given of
these plants, we shall best see when considering whether it is
the interpretation to be given of plants which rank above
them. Thus far we have dealt only with the Cryptogamia.
We have now to deal with the Phanerogamia or Phaenogamia.

CHAPTER III.

THE MORPHOLOGICAL COMPOSITION OF PLANTS,
CONTINUED.

§ 187. That advanced composition arrived at in the
Arehegoniatw, is carried still further in the Flowering Plants.
In these most-elevated vegetal forms, aggregation of the third
order is always distinctly displayed; and aggregates of the
fourth, fifth, sixth, &c, orders are very common.

Our inquiry into the morphology of these flowering plants,
may be advantageously commenced by studying the develop-
ment of simple leaves into compound leaves. It is easy to
trace the transition, as well as the conditions under which it
occurs; and tracing it will prepare us for understanding
how, and when, metamorphoses still greater in degree take
place.

§ 188. If we examine a branch of the common bramble,
when in flower or afterwards, we shall not unfrequently find
a simple or undivided leaf, at the insertion of one of the
lateral flower-bearing axes, composing the terminal cluster
of flowers. Sometimes this leaf is partially lobed ; sometimes
cleft into three small leaflets. Lower down on the shoot, if
it be a lateral one, occur larger leaves, composed of three
leaflets; and in some of these, two of the leaflets may be
lobed more or less deeply. On the main stem the leaves,
usually still larger, will be found to have five leaflets. Sup-
posing the plant to be a well-grown one, it will furnish all

37

38

MORPHOLOGICAL DEVELOPMENT.

gradations between the simple, very small leaf, and the large
composite leaf, containing sometimes even seven leaflets.
Figs. 50 to 64, represent leading stages of the transition.

£4 55

What determines this transition? Observation shows that
the quintuple leaves occur where the materials for growth
are supplied in greatest abundance; that the leaves become

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 39

less and less compound, in proportion to their remoteness
from the main currents of sap; and that where an entire
absence of divisions or lobes is observed, it is on leaves within
the flower-bunch : at the place, that is, where the forces which
cause growth are nearly equilibrated by the forces which
oppose growth; and where, as a consequence, gamogenesis is
about to be set in (§ 78). Additional evidence that the degree
of nutrition determines the degree of composition of the leaf,
is furnished by the relative sizes of the leaves. Not only, on
the average, is the quintuple leaf much larger in its total area
than the triple leaf; but the component leaflets of the one,
are usually much larger than those of the other. The like
contrasts are still more marked between triple leaves and
simple leaves. This connection of decreasing size with de-
creasing composition, is conspicuous in the series of figures:
the differences shown being not nearly so great as may be
frequently observed. Confirmation may be drawn from the
fact that when the leading shoot is broken or arrested in its
growth, the shoots it gives off (provided they are given off
after the injury), and into which its checked currents of sap
are thrown, produce leaves of five leaflets where ordinarily
leaves of three leaflets occur. Of course incidental circum-
stances, as variations in the amounts of sunshine, or of rain,
or of matter supplied to the roots, are ever producing changes
in the state of the plant as a whole; and by thus affecting
the nutrition of its leaf-buds at the times of their formation,
cause irregularities in the relations of size and composition
above described. But taking these causes into account, it is
abundantly manifest that a leaf-bud of the bramble will
develop into a simple leaf or into a leaf compounded in
different degrees, according to the quantity of assimilable
matter brought to it at the time when the rudiments of its
structure are being fixed. And on studying the habits of
other plants — on observing how annuals that have compound
leaves usually bear simple leaves at the outset, when the
assimilating surface is but small; and how, when compound-

40

MORPHOLOGICAL DEVELOPMENT.

leaved plants in full growth bear simple leaves in the midst
of compound ones, the relative smallness of such simple
leaves shows that the buds from which they arose were ill-
supplied with sap; it will cease to be doubted that a foliar
organ may be metamorphosed into a group of foliar organs,
if furnished, at the right time, with a quantity of matter
greater than can be readily organized round a single centre of
growth. An examination of the transitions through which a
compound leaf passes into a doubly-compound leaf, as seen
in the various intermediate forms of leaflets in Fig. 65, will
further enforce this conclusion.

Here we may advantageously note, too, how in such cases
the leaf-stalk undergoes concomitant changes of structure.
In the bramble-leaves above described, it becomes compound
simultaneously with the leaf — the veins become mid-ribs while
the mid-ribs become petioles. Moreover, the secondary stalks,
and still more the main stalks, bear thorns similar in their

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 41

shapes, and approaching in their sizes, to those on the stem;
besides simulating the stem in colour and texture. In the
petioles of large compound leaves, like those of the com-
mon Heracleum, we see still more distinctly both internal
and external approximations in character to axes. Nor are
there wanting plants whose large, though simple, leaves, are
held out far from the stems by foot-stalks that are, near the
ends, sometimes so like axes that the transverse sections of
the two are indistinguishable ; as instance the Calla palustris.

One other fact respecting the modifications which leaves
undergo, should be set down. Not only may leaf-stalks
assume to a great degree the characters of stems, when they
have to discharge the functions of stems, by supporting many
leaves or very large leaves; but they may assume the charac-
ters of leaves, when they have to undertake the functions
of leaves. The Australian Acacias furnish a remarkable
illustration of this. Acacias elsewhere found bear pinnate
leaves; but the majority of those found in Australia bear
what appear to be simple leaves. It turns out, however, that
these are merely leaf-stalks flattened out into foliar shapes:
the laminae of the leaves being undeveloped. And the proof
is that in young plants, showing their kinships by their
embryonic characters, these leaf-like petioles bear true leaflets
at their ends. A metamorphosis of like kind occurs in Oxalis
bupleurifolia, Fig. 66. The fact most deserving of notice,
however, is that these leaf-
stalks, in usurping the gen-
eral aspects and functions
of leaf-blades, have, to some
extent, also usurped their
structures : though their venation is not like that of the leaf-
blades they replace, yet they have veins, and in some cases
mid-ribs.

Eeduced to their most general expression, the truths above
shadowed forth are these: — That group of morphological
units, or cells, which we see integrated into the compound

42 MORPHOLOGICAL DEVELOPMENT.

unit called a leaf, has, in each higher plant, a typical form,
due to the special arrangement of these cells around a mid-
rib and veins. If the multiplication of morphological units,
at the time when the leaf -bud is taking on its main outlines,
exceeds a certain limit, these units begin to arrange them-
selves round secondary centres, or lines of growth, in such
ways as to repeat, in part or wholly, the typical form: the
larger veins become transformed into imperfect mid-ribs of
partially independent leaves; or into complete mid-ribs of
quite separate leaves. And as there goes on this transition
from a single aggregate of cells to a group of such aggregates,
there simultaneously arises, by similarly insensible steps, a
distinct structure which supports the several aggregates thus
produced, and unites them into a compound aggregate. These
phenomena should be carefully studied; since they give us a
key to more involved phenomena.*

§ 189. Thus far we have dealt with leaves ordinarily so
called: briefly indicating the homologies between the parts
of the simple and the compound. Let us now turn to the
homologies among foliar organs in general. These have been

* There is much force in the criticism passed on the above paragraph, and
by implication on some preceding paragraphs, that though in plants which
tend to produce compound leaves the production is largely dependent on the
supply of nutriment, yet the unqualified statement of this relation as a gen-
eral one, is negatived by the existence of plants which bear only simple
leaves, however much high nutrition causes growth. But mostly valid though
this objection is, it is probably not universally valid. I am led to say this by
what occasionally occurs in flowers. The flowering stem of the Hyacinth is
single; but I have seen a cultivated Hyacinth in which one of the flowers
had developed into a lateral spike. Still more striking evidence was once
supplied to me by Agrimony. All samples of this plant previously seen had
single flowering spikes, but some years ago I met with one, extremely luxuri-
ant, in which some flowers of the primitive spike were replaced by lateral
spikes ; and I am not sure that some of these, again, did not bear lateral
spikes. Now if in plants which, in probably millions of cases, have their
flowering stems single, excessive nutrition changes certain of their flowers into
new spikes, it is a reasonable supposition that in like manner plants which
are thought invariably to bear only single leaves, will, under kindred condi-
tions, bear compound leaves.

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 43

made familiar to readers of natural history by popularized
outlines of The Metamorphosis of Plants — a title, by the
way, which is far too extensive; since the phenomena treated
of under it, form but a small portion of those it properly
includes.

Passing over certain vague anticipations which have been
quoted from ancient writers, and noting only that some
clearer recognitions were reached by Joachim Jung, a Ham-
burg professor, in the middle of the 17th century; we come
to the Theoria Generationis, which Wolff published in 1759,
and in which he gives definite forms to the conceptions that
have since become current. Specifying the views of Wolff,
Dr. Masters writes : — " After speaking of the homologous
nature of the leaves, the sepals and petals, an homology
consequent on their similarity of structure and identity of
origin, he goes on to state that the ' pericarp is manifestly
composed of several leaves, as in the calyx, with this differ-
ence only, that the leaves which are merely placed in close
contact in the calyx, are here united together ' ; a view which
he corroborates by referring to the manner in which many
capsules open and separate ' into their leaves.' The seeds,
too, he looks upon as consisting of leaves in close combina-
tion. His reasons for considering the petals and stamens as
homologous with leaves, are based upon the same facts as
those which led Linnaeus, and, many years afterwards, Goethe,
to the same conclusion. ' In a word/ says Wolff, ' we see
nothing in the whole plant, whose parts at first sight differ
so remarkably from each other, but leaves and stem, to which
latter the root is referrible.' " It appears that Wolff, too,
enunciated the now-accepted interpretation of compound
fruits : basing it on the same evidence as that since assigned.
In the essay of Goethe, published thirty years after, these
relations among the parts of flowering plants were traced out
in greater detail, but not in so radical a way ; for Goethe did
not, as did Wolff, verify his hypothesis by dissecting buds in
their early stages of development. Goethe appears to have

44 MORPHOLOGICAL DEVELOPMENT.

arrived at his conclusions independently. But that they
were original with him, and that he gave a more variously-
illustrated exposition of them than had been given by Wolff,
does not entitle him to anything beyond a secondary place,
among those who have established this important generaliza-
tion.

Were it not that these pages may be read by some to
whom Biology, in all its divisions, is a new subject of study, it
would be needless to name the evidence on which this now-
familiar generalization rests. For the information of such
it will suffice to say, that the fundamental kinship existing
among all the foliar organs of a flowering plant, is shown by
the transitional forms which may be traced between them,
and by the occasional assumption of one another's forms.
" Floral leaves, or bracts, are frequently only to be distin-
guished from ordinary leaves by their position at the base of
the flower; at other times the bracts gradually assume more
and more of the appearance of the sepals." The sepals, or
divisions of the calyx, are not unlike undeveloped leaves:
sometimes assuming quite the structure of leaves. In other
cases, they acquire partially or wholly the colours of the
petals — as, indeed, the bracts and uppermost stem-leaves
occasionally do. Similarly, the petals show their alliances to
the foliar organs lower down on the axis, and to those higher
up on the axis. On the one hand, they may develop into
ordinary leaves that are green and veined; and, on the other
hand, as so commonly seen in double flowers, they may bear
anthers on their edges. All varieties of gradation into
neighbouring foliar organs may be witnessed in stamens.
Flattened and tinted in various degrees, they pass insensibly
into petals, and through them prove their homology with
leaves; into which, indeed, they are transformed in flowers
that become wholly foliaceous. The style, too, is occasionally
changed into petals or into green leaflets; and even the
ovules are now and then seen to take on leaf-like forms.
Thus we have clear evidence that in Phaenogams, all the

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 45

appendages of the axis are homologues : they are all modified
leaves.

Wolff established, and Goethe further illustrated, another
general law of structure in flowering plants. Each leaf
commonly contains in its axil a bud, similar in structure to
the terminal bud. This axillary bud may remain unde-
veloped; or it may develop into a lateral shoot like the
main shoot; or it may develop into a flower. If a shoot
bearing lateral flowers be examined, it will be found that the
internode, or space which separates each leaf with its axillary
flower from the leaf and axillary flower above it, becomes
gradually less towards the upper end of the shoot. In some
plants, as in the fox-glove, the internodes constitute a
regularly-diminishing series. In other plants, the series they
form suddenly begins to diminish so rapidly, as to bring the
flowers into a short spike : instance the common orchis. And
again, by still more sudden dwarfing of the internodes, the
flowers are brought into a cluster; as they are in the cow-
slip. On contemplating a clover flower, in which this
clustering has been carried so far as to produce a com-
pact head; and on considering what must happen if, by a
further arrest of axial development, the foot-stalks of the
florets disappear; it will be seen that there must result a
crowd of flowers, seated close together on the end of the axis.
And if, at the same time, the internodes of the upper stem-
leaves also remain undeveloped, these stem-leaves will be
grouped into a common involucre: we shall have a composite
flower, such as the thistle. Hence, to modifications in the
developments of foliar organs, have to be added modifications
in the developments of axial organs. Comparisons disclose
the gradations through which axes, like their appendages,
pass into all varieties of size, proportion, and structure. And
we learn that the occurrence of these two kinds of metamor-
phosis, in all conceivable degrees and combinations, furnishes
us with a proximate interpretation of morphological com-
positfon in Phsenogams.

46 MORPHOLOGICAL DEVELOPMENT.

I say a proximate interpretation, because there remain'
be solved certain deeper problems; one of which at once
presents itself to be dealt with under the present head.
Leaves, petals, stamens, &c, being shown to be homologous
foliar organs; and the part to which they are attached,
proving to be an indefinitely-extended axis of growth, or
axial organ; we are met by the questions, — What is a foliar
organ ? and What is an axial organ ? The morphological com-
position of a Phsenogam is undetermined, so long as we can-
not say to what lower structures leaves and shoots are homo-
logous; and how this integration of them originates. To
these questions let us now address ourselves.

§ 190-1. Already, in § 78, reference has been made to the
occasional development of foliar organs into axial organs:
the special case there described being that of a fox-glove, in
which some of the sepals were replaced by flower-buds.
The observation of these and some analogous monstrosities,
raising the suspicion that the distinction between foliar
organs and axial organs is not absolute, led me to examine
into the matter*; and the result has been the deepening of
this suspicion into a conviction. Part of the evidence is given
in Appendix A.

Some time after having reached this conviction, I found on
looking into the literature of the subject, that analogous ir-
regularities had suggested to other observers, beliefs similarly
at variance with the current morphological creed. Diffi-
culties in satisfactorily defining these two elements, have
served to shake this creed in some minds. To others, the
strange leaf-like developments which axes undergo in certain
plants, have afforded reasons for doubting the constancy of
this distinction which vegetal morphologists usually draw.
And those not otherwise rendered sceptical, have been made
to hesitate by such cases as that of the Nepaul-barley, in
which the glume, a foliar organ, becomes developed into an
axis and bears flowers. In his essay — " Vegetable Morph-

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 47

ology : its History and Present Condition," * whence I have
already quoted, Dr. Masters indicates sundry of the grounds
for thinking that there is no impassable demarcation between
leaf and stem. Among other difficulties which meet us if we
assume that the distinction is absolute, one is implied by this
question : — " What shall we say to cases such as those
afforded by the leaves of Guarea and Tricliilia, where the
leaves after a time assume the condition of branches and de-
velop young leaflets from their free extremities, a process less
perfectly seen in some of the pinnate-leaved kinds of Berberis
or Mahonia, to be found in almost every shrubbery ? "

A class of facts on which it will be desirable for us here to
dwell a moment, before proceeding to deal with the matter
deductively, is presented by the Cactacece. In this remark-
able group of plants, deviating in such varied ways from the
ordinary phaenogamic type, we find many highly instructive
modifications of form and structure. By contemplating the
changes here displayed within the limits of a single order,
we shall greatly widen our conception of the possibilities of
metamorphosis in the vegetal kingdom, taken as a whole.
Two different, but similarly-significant, truths are illustrated.
First, we are shown how, of these two components of a
flowering plant, commonly regarded as primordially distin-
guished, one may assume, throughout numerous species, the
functions, and to a great degree the appearance, of the other.
Second, we are shown how, in the same individual, there
may occur a re-metamorphosis: the usurped function and
appearance being maintained in one part of the plant, while
in another part there is a return to the ordinary appearance
and function. We will consider these two truths sepa-
rately. Some of the Euphorbiacece, which simulate
Cactuses, show us the stages through which such abnormal
structures are arrived at. In Euphorbia splendens, the lateral
axes are considerably swollen at their distal ends, so as often
to be club-shaped : still, however, being covered with bark

* See British and Foreign Medico- Chirurgical Review for January, 1862.

48 MORPHOLOGICAL DEVELOPMENT.

of the ordinary colour, and still bearing leaves. But
kindred plants, as Euphorbia neriifolia, this swelling of the
lateral axes is carried to a far greater extent; and, at the
same time, a green colour and a fleshy consistence have been
acquired: the typical relations nevertheless being still shown
by the few leaves that grow out of these soft and swollen
axes. In the Cactacece, which are thus resembled by plants
not otherwise allied to them, we have indications of a
parallel transformation. Some kinds, not commonly brought
to England, bear leaves; but in the species most familiar to
us, the leaves are undeveloped and the axes assume their
functions. Passing over the many varieties of form and
combination which these green succulent growths display, we
have to note that in some genera, as in Phyllocactus, they
become flattened out into foliaceous shapes, having mid-ribs
and something approaching to veins. So that here, and in
the genus Epiphyllum, which has this character still more
marked, the plant appears to be composed of fleshy leaves
growing one upon another. And then, in Rhipsalis, the
same parts are so leaf-like, that an uncritical observer
would regard them as leaves. These which are axial organs
in their homologies, have become foliar organs in their
analogies. When, instead of comparing these

strangely-modified axes in different genera of Cactuses, we
compare them in the same individual, we meet with transfor-
mations no less striking. Where a tree-like form is pro-
duced by the growth of these foliaceous shoots, one on another ;
and where, as a consequence, the first-formed of them become
the main stem that acts as support to secondary and tertiary
stems; they lose their green, succulent character, acquire
bark, and become woody. In resuming the functions of axes
they resume the structures of axes, from which they had devi-
ated. In Fig. 71 are shown some of the leaf -like axes of
Rhipsalis rhombm in their young state ; while Fig. 72 repre-
sents the oldest portion of the same plant, in which the foli-
aceous characters are quite obliterated, and there has resulted

THE MORPHOLOGICAL COMPOSITION OP PLANTS. 49

an ordinary stem-structure. One further fact is to

be noted. At the same time that their leaf-like appearances
are lost, the axes also lose
their separate individuali-
ties. As they become stem-
like, they also become inte-
grated; and they do this so
effectually that their origi-
nal points of junction, at
first so strongly marked, are effaced, and a consolidated trunk
is produced.

Joined with the facts previously specified, these facts
help us to conceive how, in the evolution of flowering plants
in general, the morphological components that were once
distinct, may become extremely disguised. We may ration-
ally expect that during so long a course of modification,
much greater changes of form, and much more decided fusions
of parts, have taken place. Seeing how, in an individual
plant, the single leaves pass into compound leaves, by the
development of their veins into mid-ribs while their petioles
begin to simulate axes; and seeing that leaves ordinarily
exhibiting definitely-limited developments, occasionally pro-
duce other leaves from their edges ; we are led to suspect the
possibility of still greater changes in foliar organs. When,
further, we find that within the limits of one natural order,
petioles usurp the functions and appearances of leaves, at the
same time that in other orders, as in Ruscus, lateral axes so
simulate leaves that their axial nature would by most not be
suspected, did they not bear flowers on their mid-ribs or
edges; and when, among Cactuses, we perceive that such
metamorphoses and re-metamorphoses take place with great
facility; our suspicion that the morphological elements of
Phaenogams admit of profound transformations, is deepened.
And then, on discovering how frequent are the monstrosities
which do not seem satisfactorily explicable without admitting
the development of foliar organs into axial organs ; we become
50

50 MORPHOLOGICAL DEVELOPMENT.

ready to entertain the hypothesis that during the evolution
of the phaenogamic type, the distinction between leaves and
axes has arisen by degrees.

With our preconceptions loosened by such facts, and
carrying with us the general idea which such facts suggest,
let us now consider in what way the typical structure of a
flowering plant may be interpreted.

§ 192. To proceed methodically, we must seek a clue to
the structures of Phanerogams, in the structures of those
inferior plants that approach to them — Archegoniatce. The
various divisions of this class present, along with sundry
characters which ally them with Thallophytes, other charac-
ters by which the phsenogamic structure is shadowed forth.
While some of the inferior Hepaticce or Liverworts, severally
consist of little more than a thallus-like frond, among the
higher members of this group, and still more among the
Mosses and Ferns, we find a distinctly marked stem.* Some
Archegoniates (or rather Ehizoids) have foliar expansions
that are indefinite in their forms; and some have quite
definitely-shaped leaves. Roots are possessed by all the
more developed genera of the class; but there are other
genera, as Sphagnum, which have no roots. Here the
fronds are formed of only a single layer of cells; and
there a double layer gives them a higher character — a differ-

* Schleiden, who chooses to regard as an axis that which Mr. Berkeley,
with more obvious truth, calls a mid-rib, says: — "The flat stem of the Liver-
worts presents many varieties, consisting frequently of one simple layer of
thin-walled cells, or it exhibits in its axis the elements of the ordinary stem."
This passage exemplifies the wholly gratuitous hypotheses wh'ch men will
sometimes espouse, to escape hypotheses they dislike. Schleiden, with the
positiveness characteristic of him, asserts the primordial distinction between
axial organs and foliar organs. In the higher Archegoniates he sees an
undeniable stem. In the lower Archegoniates, clearly allied to them by
their fructification, there is no structure having the remotest resemblance
to a stem. But to save his hypothesis, Schleiden calls that " a flat stem,"
which is obviously a structure in which stem and leaf are not differ-
entiated, lie is the more to be blamed for this unphilosophical assumption,
since he is merciless in his strictures on the unphilosophical assumptions of
other botanists.

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 51

ence exhibited between closely allied genera of one group, the
Mosses. Equally varied are the developments of the foliar-
organs in their detailed structures: now being without mid-
ribs or veins; now having mid-ribs but no veins; now having
both mid-ribs and veins. Nor must we omit the similarly-sig-
nificant circumstance, that whereas in the lower Archegoniates
the reproductive elements are immersed here and there in the
thallus-like frond, they are, in the higher orders, seated in
well-specialized and quite distinct fructifying organs, having
analogies with the flowers of Phaenogams. Thus, many facts
imply that if the Phaenogamic type is to be analyzed at all,
we must look among the Archegoniates for its morphological
components, and the manner of their integration.

Already we have seen among the lower Cryptogamia, how,
as they became integrated and definitely limited, aggregates
acquire the habit of budding out other aggregates, on reach-
ing certain stages of growth. Cells produce other cells
endogenously or exogenously; and fronds give origin to
other fronds from their edges or surfaces. We have seen, too,
that the new aggregates so produced, whether of the first
order or the second order, may either separate or remain
connected. Fissiparously-multiplying cells in some cases
part company, while in other cases they unite into threads or
laminae or masses; and fronds originating proliferously from
other fronds, sometimes when mature disconnect themselves
from their parents, and sometimes continue attached to them.
Whether they do or do not part, is clearly determined by
their nutrition. If the conditions are such that they can
severally thrive better by separating after a certain develop-
ment is reached, it will become their habit then to separate;
since natural selection will favour the propagation of those
which separate most nearly at that time. If, conversely, it
profits the species for the cells or fronds to continue longer
attached, which it can only do if their growths and subse-
quent powers of multiplication are thereby increased, it must
happen, through the continual survival of the fittest, that

52 MORPHOLOGICAL DEVELOPMENT.

longer attachment will become an established characteristic;
and, by persistence in this process, permanent attachment
will result when permanent attachment is advantageous.
That disunion is really a consequence of relative innutrition,
and union a consequence of relative nutrition, is clear a
posteriori. On the one hand, the separation of the new indi-
viduals, whether in germs or as developed aggregates, is a
dissolving away of the connecting substance ; and this implies
that the connecting substance has ceased to perform its
function as a channel of nutriment. On the other hand,
where, as we see among Phamogams, there is about to take
place a separation of new individuals in the shape of germs,
at the point where the nutrition is the lowest, a sudden
increase of nutrition will cause the impending separation to
be arrested; and the fructifying elements, reverting towards
the ordinary form, thereupon develop in connexion with the
parent. Turning to the Archegoniates, we find among

them many indications of this transition from discontinuous
development to continuous development. Thus the Liverworts
give origin to new plants by cells which they throw off from
their surfaces; as, indeed, we have seen that much higher
plants do. " According to Bischoff," says Schleiden, " both
the cells of the stem (Jungermannia [now Lophocolea] biden-
tata) and those of the leaves {J. exsecta) separate themselves
as propagative cells from the plant, and isolated cells shoot
out and develop while still connected with the parent plant
into small cellular bodies (Metzgeria furcata), which separate
from the plant, and grow into new plants, as in Mnium andro-
gynum among the Mosses." Now in the way above explained,
these propagative cells and proliferous buds, may continue
developing in connexion with the parent to various degrees
before separating; or the buds which are about to become
fructifying organs may similarly, under increased nutrition,
develop into young fronds. As Sir W. Hooker says of the
male fructification in Metzgeria furcata, — " It has the appear-
ance of being a young shoot or innovation (for in colour

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 53

and texture I can perceive no difference) rolled up into
a spherical figure." On finding in this same plant, that
sometimes the proliferously-produced frond buds out from
itself another frond before separating from the parent,
as shown in Fig. 46, it becomes clear
that this long-continued connexion may
readily pass into permanent connexion.
And when we see how, even among Phae-
nogams, buds may either detach them-
selves as bulbils, or remain attached and
become shoots; we can scarcely doubt
that among inferior plants, less definite in
their modes of organization, such transi-
tions must continually occur.

Let us suppose, then, that Fig. 73 is
the frond of some primitive Archegoniate,
similar in general characters to Pellia
epiphylla, Fig. 43; bearing, like it, the
fructifying buds on its upper surface,
and having a slightly-marked mid-rib and
rootlets. And suppose that, as shown,
a secondary frond is proliferously pro-
duced from the mid-rib, and continues
attached to it. Evidently the ordinary dis-
continuous development, can thus become
a continuous development, only on con-
dition that there is an adequate supply,
to the secondary frond, of such materials
as are furnished by. the rootlets: the re-
maining materials being obtainable by
itself from the air. Hence, that portion of
the mid-rib lying between the secondary
frond and the chief rootlets, having its function increased,
will increase in bulk. An additional consequence will be a
greater concentration of the rootlets — there will be extra
growth of those which are most serviceably placed. Observe,

54 MORPHOLOGICAL DEVELOPMENT.

next, that the structure so arising is likely to be main-
tained. Such a variation implying, as it does, circumstances
especially favourable to the growth of the plant, will give
to the plant extra chances of leaving descendants; since
the area of frond supported by a given area of the soil,
being greater than in other individuals, there may be a
greater production of spores. And then, among the more
numerous descendants thus secured by it, the variation will
give advantages to those in which it recurs. Such a mode
of growth having, in this manner, become established, let
us ask what is next likely to result. If it becomes the
habit of the primary frond to bear a secondary frond from its
mid-rib, this secondary frond, composed of physiological
units of the same kind, will inherit the habit; and supposing
that the supply of mineral matters obtained by the rootlets
suffices for the full development of the secondary frond, there
is a likelihood that the growth from it of a tertiary frond,
will become an habitual characteristic of the variety. Along
with the establishment of such a tertiary frond, as shown in
Fig. 74, there must arise a further development of mid-rib
in the primary frond, as well as in the secondary frond — a
development which must bring with it a greater integration
of the two; while, simultaneously, extra growth will take
place in such of the rootlets as are most directly connected
with this main channel of circulation. Without further ex-
planation it will be seen, on inspecting Figs. 75 and 76,
that there may in this manner result an integrated series of
fronds, placed alternately on opposite sides of a connecting
vascular structure. That this connecting vascular structure
will, as shown in the figures, become more distinct from the
foliar surfaces as these multiply, is no unwarranted assump-
tion ; for we have seen in compound-leaved plants, how, under
analogous conditions, mid-ribs become developed into sepa-
rate supporting parts, which acquire some of the characters
of axes while assuming their functions. And now

mark how clearly the structure thus built up by integration

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 55

of proliferously-growing fronds, corresponds with the struc-
ture of the more developed Jungermanniacece. Each of the
fronds successively produced, repeating the characters of its
parent, will bear roots; and will bear them in homologous
places, as shown. Further, the united mid-ribs having but
very little rigidity, will be unable to maintain an erect posi-
tion. Hence there will result the recumbent, continuously-
rooted stem, which these types exhibit : an embryo phaenogam
having the weakness of an embryo.*

A natural concomitant of the mode of growth here de-
scribed, is that the stem, while it increases longitudinally,
increases scarcely at all transversely: hence the old name
Acrogens. Clearly the transverse development of a stem is
the correlative, partly of its function as a channel of circula-
tion, and partly of its function as a mechanical support.
That an axis may lift its attached leaves into the air, implies
thickness and solidity proportionate to the mass of such
leaves; and an increase of its sap-vessels, also proportionate
to the mass of such leaves, is necessitated when the roots are
all at one end and the leaves at the other. But in the
generality of Acrogens, these conditions, under which arises
the necessity for transverse growth of the axis, are absent
wholly or in great part. The stem habitually creeps below
the surface, or lies prone upon the surface; and where it
grows in a vertical or inclined direction, does this by attach-
ing itself to a vertical or inclined object. Moreover, throwing
out rootlets, as it mostly does, at intervals throughout its
length, it is not called upon in any considerable degree, to
transfer nutritive materials from one of its ends to the other.

* To this interpretation it is objected that "the more developed Junger-
manniacece" do not appear to have arisen from the lower forms of Junger-
manniaccce — that is to say, from such lower forms as are now existing. It
may, however, be contended that this fact does not exclude the interpretation
given ; since the higher forms may well have been evolved, not f om any of
the lower forms we now know, but from lower forms which have become
extinct. This, indeed, is the implication of the evolutionary process as
pointed out in the note to Chap. I. If then we assume some early type of
intermediate structure, the explanation may not improbably hold.

56 MORPHOLOGICAL DEVELOPMENT.

Hence this peculiarity which gives their name to the Aerogel
now called Archegoniates, is a natural accompaniment of tl
low degree of specialization reached in them. And that it
an incidental and not a necessary peculiarity, is demonstratec
by two converse facts. On the one hand, in those higher
Acrogens which, like the tree-ferns, lift large masses of
foliage into the air, there is just as decided a transverse ex-
pansion of the axis as in dicotyledonous trees. On the other
hand, in those Dicotyledons which, like the common Dodder,
gain support and nutriment from the surfaces over which
they creep, there is no more lateral expansion of the axis
than is habitual among Acrogens or Archegoniates. Con-
cluding, as we are thus fully justified in doing, that the
lateral expansion accompanying longitudinal extension, which
is a general characteristic of Phanerogams as distinguished
from Archegoniates, is nothing more than a concomitant of
their usually- vertical growth ; * let us now go on to consider
how vertical growth originates, and what are the structural
changes it involves.

§ 193. Plants depend for their prosperity mainly on air
and light : they dwindle where they are smothered, and thrive
where they can expand their leaves into free space and sun-
shine. Those kinds which assume prone positions, conse-
quently labour under disadvantages in being habitually inter-
fered with by one another — they are mutually shaded and

* I am indebted to Dr. Hooker for pointing out further facts supporting
this view. In his Flora Antarctica, he describes the genus Lessonia (see
Fig. 37), and especially L. ovata, as having a mode of growth simulating that
of the dicotyledonous trees, not only in general form but in internal struc-
ture. The tall vertical stem thickens as it grows, by the periodical addition
of layers to its periphery. That even Thallophytes should thus, under
certain conditions, present a transversely-increasing axis, shows that there
is nothing absolutely characteristic of Phanerogams in their habit of
stem-thickening. Mr. Tansley gives me further verification by the state-
ment that "it is also now certain that members of the Equisetinem and
Zycopodinece, as well as some Ferns which flourished in Carboniferous times,
had secondary thickening in their stems quite comparable to that of modern
Dicotyledonous trees."

THE MORPHOLOGICAL COMPOSITION OF PLANTS, 57

mutually injured. Such of them, however, as happen, by
variations in mode of growth, to rise higher than others,
are more likely to flourish and leave offspring than others.
That is to say, natural selection will favour the more upright-
growing forms. Individuals with structures which lift them
above the rest, are the fittest for the conditions; and by the
continual survival of the fittest, such structures must become
established. There are two essentially-different ways in
which the integrated series of fronds above described, may be
modified so as to acquire the stiffness needful for maintaining
perpendicularity. We will consider them separately.

A thin layer of substance gains greatly in power of resist-
ing a transverse strain, if it is bent round so as to form a
tube : witness the difference between the pliability of a sheet
of paper when outspread, and the rigidity of the same sheet
of paper when rolled up. Engineers constantly recognize
this truth, in devising appliances by which the greatest
strength shall be obtained at the smallest cost of material;
and among organisms, we see that natural selection habit-
ually establishes structures conforming to the same principle,
wherever lightness and stiffness are to be combined. The
cylindrical bones of mammals and birds, and the hollow
shafts of feathers, are examples. The lower plants, too, fur-
nish cases where the strength
needful for maintaining an
upright position, is acquired
by this rolling up of a flat
thallus or frond. In Fig. 77
we have an Alga which ap-
proaches towards a tubular
distribution of substance ;
and which has a consequent
rigidity. Sundry common
forms of lichen, having the
thallus folded into a branched tube, still more decidedly dis-
play the connexion between this structural arrangement

58

MORPHOLOGICAL DEVELOPMENT.

and this mechanical advantage. And from the particular
class of plants we are here dealing with — the Archegoniates —
a type is shown in Fig. 78, Riella helicophylla, similarly char-
acterized by a thin frond that is made stiff enough to stand,
by an incurving which, though it does not produce a hollow
cylinder, produces a kindred form. If, then, as we have
seen, natural selection or survival of the fittest will favour
such among these recumbent Archegoniates as are enabled,
by variations in their structures, to maintain raised postures ;
it will favour the formation of fronds that curve round upon
themselves, and curve round upon the fronds growing out
of them. What, now, will be the result should such a
modification take place in- the group of proliferous fronds
represented in Fig. 76? Clearly, the result will be a
structure like that shown in Fig. 79. And if this inrolling
becomes more complete, a form like Jungermannia cordifolia,

represented in Fig. 80, will
be produced.

When the successive
fronds are thus folded round
so completely that their
opposite edges meet, these
opposite edges will be apt to
unite: not that they will
grow together after being
formed, but that they will
develop in connexion; or,
in botanical language, will become " adnate." That foliar
surfaces which, in their embryonic state, are in close contact,
often join into one, is a familiar fact. It is habitually so
with sepals or divisions of the calyx. In all campanulate
flowers it is so with petals. And in some tribes of plants
it is so with stamens. We are therefore well warranted in
inferring that, under the conditions above described, the suc-
cessive fronds or leaflets will, by union of their remote edges,
first at their points of origin and afterwards higher up,

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 59,

form sheaths inserted one within another, and including the
axis. This incurving of the successive fronds,

ending in the formation of sheaths, may be accompanied by
different sets of modifications. Supposing Fig. 81 to be a
transverse section of such type (a being the mid-rib, and
b the expansion of an older frond; while c is a younger frond
proliferously developed within it), there may begin two di-
vergent kinds of changes, leading to two contrasted struc-
tures. If, while frond continues to grow out of frond, the
series of united mid-ribs continues to be the channel of circu-
lation between the uppermost fronds and the roots — if, as a
consequence, the compound mid-rib, or rudimentary axis, con-
tinues to increase in size laterally; there will arise the series
of transitional forms represented by the transverse sections
82, 83, 84, 85; ending in the production of a solid axis,

everywhere wrapped round by the foliar surface of the
frond, as an outer layer or sheath. But if, on the other
hand, circumstances favour a form of plant which maintains
its uprightness at the smallest cost of substance — if the
vascular bundles of each succeeding mid-rib, instead of re-
maining concentrated, become distributed all round the tube

60

MORPHOLOGICAL DEVELOPMENT.

formed by the infolded frond; then the structure eventually
reached, through the transitional forms 86, 87, 88, 89, will
be a hollow cylinder.* And now observe how the

two structures thus produced, correspond with two kinds of
Monocotyledons. Fig. 90 represents a species of Dendrobium,
in which we see clearly how each leaf is but a continuation
of the external layer of a solid axis — a sheath such as would
result from the infolded edges of a frond becoming adnate;
and on examining how the sheath of each leaf includes the
one above it, and how the successive sheaths include the axis,
it will be manifest that the relations of parts are just such
as exist in the united series of fronds shown in Fig. 79 — the
successive nodes answering to the successive points of origin
of the fronds. Conversely, the stem of a grass, Fig. 91, dis-

plays just such relations of parts, as would result from the de-
velopment of the type shown in Fig. 79, if instead of the mid-
ribs thickening into a solid axis, the matter composing them
became evenly distributed round the foliar surfaces, at the
same time that the incurved edges of the foliar surfaces
united. The arrangements of the tubular axis and its ap-
pendages, thus resulting, are still more instructive than those
* See note at the end of the chapter.

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 61

of the solid axis. For while, even more clearly than in the
Dendrobium, we see at the point b, a continuity of structure
between the substance of the axis below the node, and the
substance of the sheath above the node: we see that this
sheath, instead of having its edges united as in Dendrobium,
has them simply overlapping, so as to form an incomplete
hollow cylinder which may be taken off and unrolled;
and we see that were the overlapping edges of this sheath
united all the way from the node a to the node b, it would
constitute a tubular axis, like that which precedes it or like
that which it includes. And then, giving an unexpected
conclusiveness to the argument, it turns out that in one
family of grasses, the overlapping edges of the sheaths do
unite : thus furnishing us with a demonstration that tubular
structures are produced by the incurving and joining of
foliar surfaces; and that so, hollow axes may be interpreted
as above, without making any assumption unwarranted by
fact. One further correspondence between the type

thus ideally constructed, and the monocotyledonous type,
must be noted. If, as already pointed out, the transverse
growth of an axis arises when the axis comes to be a channel
of circulation between all the roots at one of its extremities
and all the leaves at the other; and if this lateral bulging
must increase as fast as the quantity of foliage to be brought
in communication with the roots increases — especially if such
foliage has at the same time to be raised high above the
earth's surface; what must happen to a plant constructed in
the manner just described ? The elder fronds or foliar organs,
ensheathing the younger ones, as well as the incipient
axis serving as a bond of union, are at first of such circum-
ference only as suffices to inclose these undeveloped parts.
What, then, will take place when the inclosed parts grow —
when the axis thickens while it elongates? Evidently the
earliest-formed sheaths, not being large enough for the
swelling axis, must burst; and evidently each of the later-
formed sheaths must, in -its turn, do the like. There must

62 MORPHOLOGICAL DEVELOPMENT.

result a gradual exfoliation of the successive sheaths, like
that indicated as beginning in the above figure of Dendro-
bium; which, at a, shows the bud of the undeveloped parts
just visible above the enwrapping sheaths, while at b, and c,
it shows the older sheaths in process of being split open.
That is to say, there must result the mode of growth which
helped to give the name Endogens to this class.

The other way in which an integrated series of fronds
may acquire the rigidity needful for maintaining an erect
position, has next to be considered. If the successive fronds
do not acquire such habit of curling as may be taken ad-
vantage of by natural selection, so as to produce the requisite
stiffness; then, the only way in which the requisite stiffness
appears producible, is by the thickening and hardening of
the fused series of mid-ribs. The incipient axis will not, in
this case, be inclosed by the rolled-up fronds; but will con-
tinue exposed. Survival of the fittest will favour the genesis
of a type, in which those portions of the successive mid-ribs
that enter into the continuous bond, become more bulky than
the disengaged portions of the mid-ribs: the individuals
which thrive and have the best chances of leaving offspring,
being, by the hypothesis, individuals having axes stiff
enough to raise their foliage above that of their fellows.
At the same time, under the same influences, there will tend
to result an elongation of those portions of the mid-ribs,
which become parts of the incipient axis; seeing that it will
profit the plant to have its leaves so far removed from one
another, as to prevent mutual interferences. Hence, from the
recumbent type there will evolve, by indirect equilibration
(§ 167), such modifications as are shown in Figs. 92, 93, 94;
the first of which is a slight advance on the ideal type
represented in Fig. 76, arising in the way described ; and
the others of which are actual plants — Haplomitrium Hookeri,
and Plagiochila decipiens. Thus the higher Archegoniates
show us how, along with an assumption of the upright atti-
tude, there does go on, as we see there must go on, a separa-

THE MORPnOLOGICAL COMPOSITION OF PLANTS. 63

Hon of the leaf-producing parts from the root-producing
parts; a greater development of that connecting portion of
the successive fronds, by which they are kept in communica-

tion with the roots, and raised above the ground; and a con-
sequent increased differentiation of such connecting portion
from the parts attached to it. And this lateral bulging of
the axis, directly or indirectly consequent on its functions as
a support and a channel, being here unrestrained by the
early-formed fronds folded round it, goes on without the
bursting of these. Hence arises a leading character of what
is called exogenous growth — a growth which is, however, still
habitually accompanied by exfoliation, in flasks, of the outer-
most layers, continually being cracked and split by the accu-
mulation of layers within them. And now if we ex-
amine plants of the exogenous type, we find among them many
. displaying the stages of this metamorphosis. In Fig. 95, is
shown a form in which the continuity of the axis with the
mid-rib of the leaf, is manifest — a continuity that is con-
spicuous in the common thistle. Here the foliar expansion,
running some distance down the axis, makes the included
portion of the axis a part of its mid-rib; just as in the ideal
types above drawn. By the greater growth of the internodes,

64 MORPHOLOGICAL DEVELOPMENT.

which are very variable, not only in different plants but in
the same plant, there results a modification like that de-
lineated in Fig. 96. And then, in such forms as Fig. 97, there
is shown the arrangement that arises when, by more rapid
development of the proximal end of the mid-rib, the distal

part of the foliar surface is separated from the part which
embraces the axis: the wings of the mid-rib still serving,
however, to connect the two portions of the foliar surface.
Such a separation is, as pointed out in § 188, an habitual
occurrence; and in some compound leaves, an actual tearing
of the inter-venous tissue is caused by extra growth of the
mid-rib. Modifications like this, and the further one in Fig.
98, we may expect to be established by survival of the fittest,
among those plants which produce considerable masses of
leaves; since the development of mid-ribs into footstalks, by
throwing the leaves further away from the axes, will diminish
the shading of the leaves, one by another. And then, among
plants of bushy growth, in which the assimilating surfaces
become still more liable to intercept one another's light,
natural selection will continue to give an advantage to those
which carry their assimilating surfaces at the ends of the
petioles, and do not develop assimilating surfaces close to the
axis, where they are most shaded. Whence will result a dis-
appearance of the stipules and the foliar fringes of the mid-
ribs; ending in the production of the ordinary stalked leaf,
Fig. 99, which is characteristic of trees. Meanwhile, the axis
thickens in proportion to the number of leaves it has to
carry, and to put in communication with the roots; and so

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 65

there comes to be a more marked contrast between it and the
petioles, severally carrying a leaf each.*

§ 194. When, in the course of the process above sketched
out, there has arisen such community of nutrition among the
fronds thus integrated into a series, that the younger ones
are aided by materials which the older ones have elaborated;
the younger fronds will begin to show, at earlier and earlier
periods of development, the structures about to originate
from them. Abundant nutrition will abbreviate the intervals
between the successive prolifications ; so that eventually,
while each frond is yet imperfectly formed, the rudiment of
the next will begin to show itself. All embryology justifies
this inference. The analogies it furnishes lead us to expect
that when this serial arrangement becomes organic, the
growing part of the series will show the general relations of
the forthcoming parts, while they are very small and un-
specialized. What will in such case be the appearances they
assume? We shall have no difficulty in perceiving what it
will be, if we take a form like that shown in Fig. 92, and
dwarf its several parts at the same time that we generalize
them. Figs. 100, 101, 102, and 103, will show the result;

and in Fig. 104, which is the bud of a dicotyledon, we see
how clear is the morphological correspondence: a being the
rudiment of a foliar organ beginning to take shape ; b being
the almost formless rudiment of the next foliar organ; and

* Since this paragraph was put in type [this refers to the first edition],
I have observed that in some varieties of Cineraria, as probably in other
plants, a single individual furnishes all these forms of leaves — all gradations
between unstipulated leaves on long petioles, and leaves that embrace the
axis. It may be added that the distribution of these various forms is quite
in harmony with the rationale above given.
51

66

MORPHOLOGICAL DEVELOPMENT.

c being the quite-undifferentiated part whence the rudiments
of subsequent foliar organs are to arise.

And now we are prepared for entering on a still-remaining
question respecting the structure of Phaenogams — what is the
origin of axillary buds? As the synthesis at present stands,
it does not account for these; but on looking a little more
closely into the matter, we shall find that the axillary buds
are interpretable in the same manner as the terminal buds.
So to interpret them, however, we must return to that pro-
cess of proliferous growth with which we set out, for the pur-
pose of observing some facts not before named. Delesseria
hypoglossum, Fig. 105, represents a seaweed of the same genus
as one outlined in Fig. 40; but of a species in which pro-
liferous growth is carried much further. Here, not only does
the primary frond bud out many secondary fronds from its
mid-rib; but most of the secondary fronds similarly bud out
several tertiary fronds; and even by some of the tertiary

fronds, this proliflcation is repeated. Besides being shown
that the budding out of several fronds from one frond, may
become habitual; we are also shown that it may become a

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 67

habit inherited by the fronds so produced, and also by the
fronds they produce: the manifestation of the tendency
being probably limited only by failure of nutrition. That
under fit conditions an analogous mode of growth will occur
in fronds of the acrogenic type, like those we set out with,
is shown by the case of Metzgeria furcata, Figs. 45, 46, in
which such compound prolification is partially displayed.
Let us suppose, then, that the frond a, Fig. 106, produces
not only a single secondary frond b, but also another such
secondary frond b'. Let us suppose, further, that the frond
b is in like manner doubly proliferous: producing both c
and c'. Lastly, let us suppose that in the second frond br
which a produces, as well as in the second frond c' which b
produces, the doubly-proliferous habit is manifested. If,
now, this habit grows organic — if it becomes, as it naturally
will become, the characteristic of a plant of luxuriant growth,
the unfolding parts of which can be fed by the unfolded
parts; it will happen with each lateral series, as with the
main series, that its successive components will begin to
show themselves at earlier and earlier stages of development.
And in the same way that, by dwarfing and generalizing
the original series, we arrive at a structure like that of the
terminal bud; by dwarfing and generalizing a lateral series,
as shown in Figs. 107 — 110, we arrive at a structure answer-
ing in nature and position to the axillary bud.

JO? 103 109

Facts confirming these interpretations are afforded by the
structure and distribution of buds. The phasnogamic axis in
its primordial form, being an integrated series of folia; and
the development of that part by which thgse folia are
held together at considerable distances from one another,
taking place afterwards; it is inferable from the general

68 MORPHOLOGICAL DEVELOPMENT.

principles of embryology, that in its rudimentary stages, the
phsenogamic shoot will have its foliar parts more clearly
marked out than its axial parts. This we see in every bud.
Every bud consists of the rudiments of leaves packed to-
gether without any appreciable internodal spaces; and the
internodal spaces begin to increase with rapidity, only when
the foliar organs have been considerably developed. More-
over, where nutrition falls short, and arrest of development
takes place — that is, where a flower is formed — the inter-
nodes remain undeveloped: the unfolding ceases before
the later-acquired characters of the phaenogamic shoot
are assumed. Lastly, as the hypothesis leads us to expect,
axillary buds make their appearances later than the foliar
organs which they accompany; and where, as at the ends of
shoots, these foliar organs show failure of chlorophyll, the
axillary buds are not produced at all. That these are in-
ferable traits of structure, will be manifest on inspecting
Figs. 106 — 110; and on observing, first, that the doubly-
proliferous tendency of which the axillary bud is a result,
implies abundant nutrition; and on observing, next, that the
original place of secondary prolification, is such that the foliar
surface on which it occurs, must grow to some extent before
the bud appears.

On thus looking at the matter — on contemplating afresh
the ideal type shown in Fig. 106, and noting how, by the
conditions of the case, the secondary prolifications must cease
before that primary prolification which produces the main
axis; we are enabled to reconcile all the phenomena of axil-
lary gemmation. We see harmony among the several facts —
first, that the axillary bud becomes a lateral, leaf-bearing
axis if there is abundant material for growth; second, that
its development is arrested, or it becomes a flower-bearing
axis, if the supply of sap is but moderate; third, that it is
absent when, the nutrition is failing. We are no longer
committed to the gratuitous assumption that, in the phagno-
gamic type, there must exist an axillary bud to each foliar

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 69

organ; but we are led to conclude, a priori, that which we
find, a posteriori, that axillary buds are as normally absent
in flowers as they are normally present lower down the axis.
And then, to complete the argument, we are prepared for the
corollary that axillary prolification may naturally arise even
at the ends of axes, should the failing nutrition which
causes the dwarfing of the foliar organs to form a flower, be
suddenly changed into such high nutrition as to transform
the components of the flower into appendages that are green,
if not otherwise leaf-like — a condition under which only, this
phenomenon is proved to occur.

§ 195. One more question presents itself, when we con-
trast the early stages of development in the two classes of
Phaenogams; and a further answer, supplied by the hypo-
thesis, gives to the hypothesis a further probability. It is
characteristic of a monocotyledon, to have a single seed-leaf
or cotyledon; and it is characteristic of a dicotyledon, to
have at least two cotyledons, if not more than two. That is
to say, the monocotyledonous mode of germination every-
where coexists with the endogenous mode of growth; and
along with the exogenous mode of growth, there always goes
either a dicotyledonous or polycotyledonous germination.
Why is this? Such correlations cannot be accidental —
cannot be meaningless. A true theory of the phamogamic
types in their origin and divergence, should account for the
connexion of these traits. Let us see whether the foregoing
theory does this.

The higher plants, like the higher animals, bequeath to
their offspring more or less of nutriment and structure.
Superior organisms of either kingdom do not, as do all
inferior organisms, cast off their progeny in the shape of
minute portions of protoplasm, unorganized and without
stocks of material for them to organize; but they either
deposit along with the germs they cast off, certain quantities
of albuminoid substance to be appropriated by them while they

YO MORPHOLOGICAL DEVELOPMENT.

develop themselves, or else they continue to supply such
substance while the germs partially develop themselves before
their detachment. Among plants this constitutes one dis-
tinction between seeds and spores. Every seed contains a
store of food to serve the young plant during the first stages
of its independent life; and usually, too, before the seed is
detached, the young plant is so far advanced in structure,
that it bears to the attached stock of nutriment much the
same relation that the young fish bears to the appended yelk-
bag at the time of leaving the egg. Sometimes, indeed, the
development of chlorophyll gives the seed-leaves a bright
green, while the seed is still contained in the parent-
pod. This early organization of the phsenogam
must be supposed rudely to indicate the type out of which
the phamogamic type arose. On the foregoing hypothesis,
the seed-leaves therefore represent the primordial fronds;
which, indeed, they simulate in their simple, cellular, un-
veined structures. And the question here to be asked is —
do the different relations of the parts in .young monocotyledons
and dicotyledons correspond with the different relations of
the primordial fronds, implied by the endogenous and the
exogenous modes of growth? We shall find that they do.

Starting, as before, with the proliferous form shown in
Fig. Ill, it is clear that if the strength required for main-
taining the vertical attitude, is obtained by the rolling up of
the fronds, the primary frond will more and more conceal the
secondary frond within it. At the same time, the secondary
frond must continue to be dependent on the first for its nutri-
tion; and, being produced within the first, must be prevented
by defective supply of light and air, from ever becoming syn-
chronous in its development with the first. Hence, this
infolding which leads to the endogenous mode of growth,
implies that there must always continue such pre-eminence
of the first-formed frond or its representative, as to make the
germination monocotyledonous. Figs. Ill to 115, show the
transitional forms that would result from the infolding of

THE MOHPHOLOGICAL COMPOSITION OF PLANTS. 71

the fronds. In Fig. 116 (a vertical section of the form repre-
sented in Fig. 115) are exhibited the relations of the succes-
sive fronds to each other. The modified relations that would
result, if the nutrition of the embryo admitted of anticipatory
development of the successive fronds, are shown in Fig. 117.
And how readily the structure may pass into that of the
monocotyledonous germ, will be seen on inspecting Fig. 118;

W[jf

which is a vertical section of an actual monocotyledon at an
early stage — the incomplete lines at the left of its root, indi-
cating its connexion with the seed.* Contrariwise,

* Since these figures were put on the block, it has occurred to me that the
relations would be still clearer, were the primary frond represented as not
taking part in these processes of modification, which have been described as
giving rise to the erect form ; as, indeed, the rooting of its under surface
will prevent it from doing in any considerable degree. In such case, each of
the Figs. Ill to 117, should have a horizontal rooted frond at its base,
homologous with the pro-embryo among Acrogens. This primary frond
would then more manifestly stand in the same relation to the rest, as the

72 MORPHOLOGICAL DEVELOPMENT.

where the strength required for maintaining an upright atl
tude is not obtained by the rolling up of the fronds, but
the strengthening of the continuous mid-rib, the secoi
frond, so far from being less favourably circumstanced thai
the first, becomes in some respects even more favourably
circumstanced: being above the other, it gets a greater share
of light, and it is less restricted by surrounding obstacles.
There is nothing, therefore, to prevent it from rapidly gaining
an equality with the first. And if we assume, as the truths of
embryology entitle us to do, an increasing tendency towards
anticipation in the development of subsequent fronds — if
we assume that here, as in other cases, structures which were
originally produced in succession will, if the nutrition allows
and no mechanical dependence hinders, come to be produced
simultaneously; there is nothing to prevent the passage
of the type represented in Fig. Ill, into that represented
in Fig. 122. Or rather, there is everything to facilitate it;
seeing that natural selection will continually favour the pro-
duction of a form in which the second frond grows in such
way as not to shade the first, and in such way as allows the
axis readily to assume a vertical position.

Thus, then, is interpretable the universal connexion be-
tween monocotyledonous germination and endogenous growth ;
as well as the similarly-universal connexion between exoge-
nous growth and the development of two or more cotyledons.
That it explains these fundamental relations, adds very
greatly to the probability of the hypothesis.

§ 196. While we are in this manner enabled to discern
the kinship that exists between the higher vegetal types
themselves, as well as between them and the lower types; we

cotyledon does to the plumule — both by position, and as a supplier of nutri-
ment. Fig. 117a, which I am enabled to add, shows that this would com-
plete the interpretation. Of the dicotyledonous series, it is needful to add no
further explanation than that the difference in habit of growth, will permit
the second frond to root itself as well as the first ; and so to become an addi-
tional source of nutriment, similarly circumstanced to the first and equal with it.

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 73

art1 at the same time supplied with a rationale of those truths
which vegetal morphologists have established. Those homo-
logies which Wolff indicated in their chief outlines and
Goethe followed out in detail, have a new meaning given to
them when we regard the phaenogamic axis as having been
evolved in the way described. Forming the modified con-
ception which we are here led. to do, respecting the units of
which a flowering plant is composed, we are no longer left
without an answer to the question — What is an axis? And
we are helped to understand the naturalness of those cor-
respondences which the successive members of each shoot
display. Let us glance at the facts from our present stand-
point.

The unit of composition of a Phaenogam, is such portion of
a shoot as answers to one of the primordial fronds. This
portion is neither one of the foliar appendages nor one of the
internodes; but it consists of a foliar appendage together
with the preceding internode, including the axillary bud
where this is developed. The parts intercepted by the dotted
lines in Fig. 123, constitute such a segment; and the true
homology is between this and any other foliar organ with the
portion of the axis below it. And now observe how, when we
take this for the unit of composition, the metamorphoses
which the phaenogamic axis displays, are inferable from known
laws of development. Embryology teaches us that arrest

of development shows itself first in the absence of those parts
that have arisen latest in the course of evolution; that if
defect of nutrition causes an earlier arrest, parts that are of
more ancient origin abort; and that the part alone produced
when the supply of materials fails near the outset, is the prim-
ordial part. We must infer, therefore, that in each seg-
ment of a Phaenogam, the foliar organ, which answers to the
primordial frond, will be the most constant element; and
that the internode and the axillary bud, will be successively
less constant. This we find. Along with a smaller size of
foliar surface implying lower nutrition, it is usual to see a

74 MORPHOLOGICAL DEVELOPMENT.

much-diminished internode and a less-pronounced axillary
bud, as in Fig. 124. On approaching the flower, the
axillary bud disappears; and the segment is reduced to a
small foliar surface, with an internode which is in most

^%

71

J2& 1ZS J26 JZ? /2Z J2Q

ty

7

cases very short if not absent, as in 125 and 126. In the
flower itself, axillary buds and internodes are both want-
ing: there remains only a foliar surface (127), which,
though often larger than the immediately preceding foliar
surface, shows failing nutrition by absence of chlorophyll.
And then, in the quite terminal organs of fructification (129),
we have the foliar part itself reduced to a mere rudiment.
Though these progressive degenerations are by no means
regular, being in many cases varied by adaptations to par-
ticular requirements, yet it cannot, I think, be questioned,
that the general relations are as described, and that they are
such as the hypothesis leads us to expect. Nor are

we without a kindred explanation of certain remaining traits
of foliar organs in their least-developed forms. Petals,
stamens, pistils, &c, besides reminding us of the primordial
fronds by their diminished sizes, and by the want of those
several supplementary parts which the preceding segments
possess, also remind us of them by their histological charac-
ters: they consist of simple cellular tissue, scarcely at all
differentiated. The fructifying cells, too, which here make
their appearance, are borne in ways like those in which the
lower Acrogens bear them — at the edge of the frond, or at
the end of a peduncle, or immersed in the general substance ;
as in Figs. 128 and 129. Nay, it might even be said that

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 75

the colours assumed by these terminal folia, call to mind the
plants out of which we conclude that Phaenogams have been
evolved ; for it is said of the fronds of the Jungermanniacece,
that, "though under certain circumstances of a pure green,
they are inclined to be shaded with red, purple, chocolate, or
other tints."

As thus understood, then, the homologies among the parts
of the phaenogamic axis are interpretable, not as due to a
needless adhesion to some typical form or fulfilment of a pre-
determined plan; but as the inevitable consequences of the
mode in which the phaenogamic axis originates.

§ 197. And now it remains only to observe, in confirma-
tion of the foregoing synthesis, that it at once explains for us
various irregularities. When we see leaves sometimes pro-
ducing leaflets from their edges or extremities, we recognize
in the anomaly a resumption of an original mode of growth :
fronds frequently do this. When we learn that a flowering
plant, as the Drosera intermedia, has been known to develop
a young plant from the surface of one of its leaves, we are at
once reminded of the proliferous growths and fructifying
organs in the Liverworts. The occasional production of bul-
bils by Phaenogams, ceases to be so surprising when we find
it to be habitual among the inferior Acrogens, and when we
see that it is but a repetition, on a higher stage, of that self-
detachment which is common among proliferously-produced
fronds. Nor are we any longer without a solution of that
transformation of foliar organs into axial organs, which not
uncommonly takes place. How this last irregularity of de-
velopment is to be accounted for, we will here pause a moment
to consider. Let us first glance at our data.

The form of every organism, we have seen, must depend
on the structures of its physiological [or constitutional]
units. Any group of such units will tend to arrange itself
into the complete organism, if uncontrolled and placed in fit
conditions. Hence the development of fertilized germs; and

76 MORPHOLOGICAL DEVELOPMENT.

hence the development of those self-detached cells which
characterize some plants. Conversely, physiological units
which form a small group involved in a larger group, and are
subject to all the forces of the larger group, will become sub-
ordinate in their structural arrangements to the larger group
— will be co-ordinated into a part of the major whole, in-
stead of co-ordinating themselves into a minor whole. This
antithesis will be clearly understood on remembering how,
on the one hand, a small detached part of a hydra soon
moulds itself into the shape of an entire hydra; and how,
on the other hand, the cellular mass that buds out in place
of a lobster's lost claw, gradually assumes the form of a claw
— has its parts so moulded as to complete the structure of
the organism: a result which we cannot but ascribe to the
forces which the rest of the organism exerts upon it. Con-
sequently, among plants, we may expect that whether any
portion of protoplasm moulds itself into the typical form
around an axis of its own, or is moulded into a part subor-
dinate to another axis, will depend on the relative mass of
its physiological units — the accumulation of them that has
taken place before the assumption of any structural arrange-
ment. A few illustrations will make clear the validity of
this inference. In the compound leaf, Fig. 65, the several
lateral growths a, b, c, d, are manifestly homologous; and
on comparing a number of such leaves together, it will be
seen that one of these lateral growths may assume any de-
gree of complexity, according to the degree of its nutrition.
Every fern-leaf exemplifies the same general truth still bet-
ter. Whether each sub-frond remains an undeveloped wing
of the main frond, or whether it organizes itself into a group
of frondlets borne by a secondary rib, or whether, going
further, as it often does, it gives rise to tertiary ribs bear-
ing frondlets, is determined by the supply of materials for
growth; since such higher developments are most marked
at points where the nutrition is greatest; namely, next the
stem. But the clearest evidence is afforded among the Algce,

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 77

which, not drawing nutriment from roots, have their parts
much less mutually dependent; and are therefore capable of
showing more clearly, how any part may remain an append-
age or may become the parent of append-
ages, according to circumstances. In the
annexed Fig. 130, representing a branch of
Ptilota plumosa, we see how a wing grows
into a wing-bearing branch if its nutrition
passes a certain point. This form, so strik-
ingly like that of the feathery crystallizations /jo
of many inorganic substances, implies that,
as in such crystallizations, the simplicity
or complexity of structure at any place
depends on the quantity of matter that has to
be arranged at that place in a given time.*

Hence, then, we are not without an interpretation of those
over-developments which the phamogamic axis occasionally
undergoes. Fig. 104, represents the phamogamic bud in its
rudimentary state. The lateral process b, which ordinarily
becomes a foliar appendage, differs very little from the
terminal process c, which is to become an axis — differs
mainly in having, at this period when its form is being
determined, a smaller bulk. If while thus undifferentiated,
its nutrition remains inferior to that of the terminal process,
it becomes moulded into a part that is subordinate to the
general axis. But if, as sometimes happens, there is supplied
to it such an abundance of the materials needful for growth,
that it becomes as large as the terminal process; then we

* How the element of time modifies the result, is shown by the familiar
fact that crystals rapidly formed are small, and become relatively large when
left to form more slowly. If the quantity of molecules contained in a solution
is relatively great, so that the mutual polarities of the molecules crowded
together in every place throughout the solution are intense, there arises a
crystalline aggregation around local axes ; whereas, in proportion as the local
action of molecules on one another is rendered less intense by their wider
dispersion, they become relatively more subordinate to the forces exerted on
them by the larger aggregates of molecules that are at greater distances, and
thus are left to arrange themselves round fewer axes into larger crystals.

78 MORPHOLOGICAL DEVELOPMENT.

may naturally expect it to begin moulding itself round
axis of its own: a foliar organ will be replaced by an axial
organ. And this result will be especially liable to occur,
when the growth of the axis has been previously undergoing
that arrest which leads to the formation of a flower; that is
when, from defect of materials, the terminal process has
almost ceased to increase, and when some concurrence of
favourable causes brings a sudden access of sap which reaches
the lateral processes before it reaches the terminal process.*

§ 198. The general conclusion to which these various lines
of evidence converge, is, then, that the shoot of a flowering
plant is an aggregate of the third degree of composition.
Taking as aggregates of the first order, those small portions
of protoplasm which ordinarily assume the forms under
which they are known as cells ; and considering as aggregates
of the second order, those assemblages of such cells which,
in the lower cryptogams, compose the various kinds of thal-
lus; then that structure, common to the higher cryptogams
and to phaenogams, in which we find a series of such groups
of cells bound up into a continuous whole, must be regarded
as an aggregate of the third order. The inference drawn
from analysis, and verified by a synthesis which corresponds
in a remarkable manner with the facts, is that those com-
pound parts which, in Monocotyledons and Dicotyledons are
called axes, have really arisen by integration of such simple
parts as in lower plants are called fronds. Here, on a higher

* It is objected that these transformations should be much commoner than
they are, were they caused solely by the variations of nutrition described.
The reply is that they are comparatively rare in uncultivated plants, where
such variations are not frequent. The occurrence of them is chiefly among
cultivated plants which, being artificially manured, are specially liable to
immense accessions of nutriment, caused now by sudden supplies of fertilizing
matters, and now by sudden arrival of the roots at such matters already
deposited in the soil. It is to these great changes of nutrition, especially apt
to take place in gardens, that these monstrosities are ascribed ; and it seems
to me that they are as frequent as may be expected.

THE MORPHOLOGICAL COMPOSITION OF PLANTS. ?9

level, appears to have taken place a repetition of the process
already observed on lower levels. The formation of those
small groups of physiological units which compose the lowest
protophytes, is itself a process of integration; and the con-
solidation of such groups into definitely-circumscribed and
coherent cells or morphological units, is a completing of the
process. In those coalescences by which many such cells
are joined into threads, and discs, and solid or fiattened-
out masses, we see these morphological units aggregating
into units of a compound kind: the different phases of the
transition being exemplified by groups of various sizes,
various degrees of cohesion, and various degrees of definite-
ness. And now we find evidences of a like process on a
larger scale: the compound groups are again compounded.
Moreover, as before, there are not wanting types of organi-
zation by which the stages of this higher integration are
shadowed forth. From fronds that occasionally produce
other fronds from their surfaces, we pass to those that
habitually produce them; from those that do so in an in-
definite manner, to those that do so in a definite manner;
and from those that do so singly, to those that do so doubly
and triply through successive generations of fronds. Even
within the limits of a sub-class, we find gradations between
fronds irregularly proliferous, and groups of such fronds
united into a regular series.

ISTor does the process end here. The flowering plant is
rarely uniaxial — it is nearly always multiaxial. From its
primary shoot there grow out secondary shoots of like kind.
Though occasionally among Phsenogams, and frequently
among the higher Cryptogams, the germs of new axes detach
themselves under the form of bulbils, and develop separately
instead of in connexion with the parent axis; yet in most
Phasnogams the germ of each new axis maintains its con-
nexion with the parent axis : whence results a group of axes
— an aggregate of the fourth order. Every tree, by the pro-

80 MORPHOLOGICAL DEVELOPMENT.

duction of branch out of branch, shows us this integratioi
repeated over and over again; forming an aggregate having a
degree of composition too complex to be any longer defined.

[Note. — A criticism passed on the general argument set
forth in the foregoing sections, runs as follows : — " I have
already pointed out that the process of evolution by which
you believe the Liverworts with a distinct axis and append-
ages to have been produced from the thalloid forms is not
founded on sound evidence either in comparative morphology
or development. But even if we admit that such an inte-
gration of a proliferously-pcoduced colony might have given
rise to the leafy Jungermanniacece, there are even more
weighty objections to the supposition that the same process
produced the shoot structures of the flowering plants. In the
first place the flowering plant-body is not homologous with
the liverwort plant-body, since they represent different genera-
tions. The liverwort plant-body or gametophyte, i.e., the
generation bearing sexual organs, is homologous with the
prothallus of ferns and other Pteridophytes, and in the
Flowering Plants with reduced structures contained within
the spores (embryo-sac and pollen-grain) but still giving rise
to sexual cells. The liverwort spore-capsule and its accessory
parts (in fact everything produced from the fertilized egg) is
homologous with the sporogonium of the mosses, and, as most
botanists think, with the leafy plant-body of Pteridophytes
and Phanerogams. This generation is called the sporophyte
and from the spores which it produces are developed the
gametophytes of the next generation. These generalizations
were first established by Hofmeister, and all subsequent
work has tended to establish them more firmly. The
only doubtful question is (and the doubt is mainly, I think,
peculiar to myself, certainly not being shared by the majority
of botanists) whether the sporophyte of Mosses and Liver-
forts is really homologous with that of Pteridophytes and

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 81

Phanerogams, whether it may not rather be regarded as a
parallel development along another line of descent from the
Green Algae.

" Hence we must look for the origin of the shoot-structure
of flowering plants in the sporophytes of the Pteridophytes,
from which group there is no reason to doubt that the
phanerogams have arisen in descent. The various groups of
Pteridophytes vary much in the organization of these shoot-
systems, as a mental glance at the types exhibited by the
Ferns, Horse-tails, Club-mosses, Ophioglossaccce, and the iso-
lated Isoetes will convince you at once. It may be that
some of these groups are independent in descent, i.e., that the
Pteridophyta are polyphyletic, and the current hypothesis
with regard to the phanerogams is that they have arisen by
two, if not three, separate lines of descent from different
groups of Pteridophytes (this is indicated in the classificatory
diagram on p. 377 of vol. I). I should not, however, care to
pin my faith to these or to any such lines of ancestry. Still
I think we must look for the ancestors of the Flowering
Plants among the Pteridophytes, and the latter always
have a good distinction between axis and appendages. The
problem of the evolution of these differentiated sporophytic
shoots is undoubtedly the great outstanding problem of
morphology. Various attempts have been made to solve it,
of which probably the most important is the theory of Profs.
Bown and Campbell, who derive the Pteridophytes from some
Liverwort like Anthoceros, but the sporophyte of course
from the sporophytic portion of the plant (not much more
than a spore-capsule), the prothallus of the Fern representing
the vegetative thallus of Anthoceros. I am not wholly con-
vinced by these undoubtedly ingenious hypotheses, in support
of which an immense amount of facts have been collected;
but my position would, I know, simply ' put us to ignorance
again ' on this question.

"I have discussed this at some length in order to bring
out clearly the immense difficulty of constructing a well-
52

82 MORPHOLOGICAL DEVELOPMENT.

I

grounded theory of the origin of the differentiated shoot-
system of the higher plant. I confess I don't think it can
be done at all with the materials at present at onr disposal.
Of course it is just possible to suppose that some ancestral
sporophyte had the structure of a proliferous thalloid liver-
wort gametophyte, and that from it was evolved the phanero-
gamic shoot in the ways you suggest. This gives us abso-
lutely no clue, however, to any Pteridophytic shoot, which
ought to be intermediate (more or less) between the hypo-
thetical ancestor and the Phanerogam, and is furthermore,
as far as I can see, not supported by an atom of evidence of
any kind. It is true that your theory fits in well with the
phenomena exhibited by phanerogamic shoots themselves,
but this fact you will see must lose much of its significance
if the hypothesis lacks foundation.

" With regard to your method of explaining the funda-
mental characters of ' Exogens ' and ' Endogens/ this of
course is part of the same hypothesis; but I may point out
that since Yon Mohl and Sanio, between 1855 and 1865,
showed (1) that the growth at the stem apex of a mono-
cotyledon was not endogenous, and (2) that the ' thickening
ring J near the apex of a dicotyledon was not to be confused,
as had been done up till then, with the ring of secondary
meristem or true cambium, which arose lower down, and only
in woody or practically woody stem, the terms ' Exogen ' and
' Endogen ' have necessarily fallen into disuse, since they
imply a false conception of what happens. Both monocotyl-
edons and dicotyledons have a i thickening ring/ which
gives rise to the primary vascular cylinder of the stem.
When the stem is of considerable thickness, as in Palms, &c,
it grows by the active cell-division of its outer layers, so that
both classes are f exogenous ' in this sense ; while the addition
of a centrifugal zone of secondary wood is confined to certain
Dicotyledons (Trees, shrubs, &c).

" The distinction between the embryos, moreover, is not
absolute. The single cotyledon is usually terminal in mono-

THE MORPHOLOGICAL COMPOSITION OF PLANTS. 83

cotyledons, but not always (Dioscoracece have lateral cotyl-
edons), but the plumule may push through it (Grasses) or
make its exit sideways (Palms), or be formed at the side
(Alisma) ; and Dicotyledons very similarly.

" The occurrence of completely sheathing leaves in grasses
is perhaps correlated with the absence of cambium, but
grasses are an aberrant type among monocotyledons, and
secondary thickening is only found in very few genera of
this class, so that the correlation is, so to speak, negative and
indirect It is clear that the greater part of the dis-
cussion will have to be re-written."

For the reasons assigned in the preface I cannot undertake
to re-write the discussion, as suggested. It must stand for
what it is worth. All I can do is here to include along with
it the foregoing criticisms.

I may, however, indicate the line of defence I should take
were I to go again into the matter. The objections are based
on the structure of existing Liverworts and Phaenogams. But
I have already referred to the probability — or, indeed, the
certainty — that in conformity with the general principle set
forth in the note to Chapter I, we must conclude that the
early types of Liverworts out of which the Phagnogams are
supposed to have evolved, as well as the early types of
Phgenogams in which the stages of evolution were presented,
no longer exist. We must infer that forms simpler than
any now known, and more intermediate in their traits, were
the forms concerned; and if so, it may be held that the
incongruities with the hypothesis which are presented by
existing forms, do not negative it. The scepticism my critic
himself expresses respecting the current interpretation is a
partial justification of this view. Moreover, his admission
that the theory set forth " fits in well with the phenomena
exhibited by phanerogamic shoots," must, I think, be regarded
as weighty evidence. On the Evolution-hypothesis we are
obliged to suppose that the Monocotyledons and Dicotyledons
respectively arose by integration of fronds; and if to the

84 MORPHOLOGICAL DEVELOPMENT.

question after what manner the integration took place, thei
is an hypothesis which renders it comprehensible, and agrees
both with the structures of the two kinds of shoots and the
structures of the two kinds of seeds, as well as with various
of the other phenomena the two types present, it has strong
claims for acceptance.

Eeconsideration suggests the following remarks.

1. Alternation of generations is a means of furthering
multiplication. To be effective each member of either genera-
tion must be a self-supporting centre of growth or diffusion
or both. Hence if, as in the Liverworts, one of the so-called
alternating generations is not independent, but a permanent
growth on the other — a parasite — it is a misuse of words
to call the arrangement Alternation of generations. (Since
this was written I have found that Sir Edward Fry takes the
same view. He approvingly quotes Professor Bower, who
says that " the alternation of generations is not an accurate
statement of facts or a useful analogy.")

2. The alternating of sexual and non-sexual processes is
not fundamentally distinctive; for, as shown by sundry
Archegoniates, it is an inconstant trait, and as shown by
Klebe' experiments on Vaucheria, the conditions may be
varied so as to determine its occurrence or non-occurrence.
Nay, the same individual may reproduce in either way.

3. Still more significant is the fact that in some of the
marine Thallophytes, there is a process like that which in a
moss or a fern is considered an alternation of generations,
whereas in others, as the Brown Wrack (Fucus), each genera-
tion is sexual. Thus the presence or absence of this mode of
genesis cannot be a cardinal distinction.

4. With these facts before us, it is not only a reasonable
supposition but a highly probable supposition, that there have
existed plants of the Liverwort type in which the so-called
alternation of generations did not take place. If so, nearly all
the foregoing objections to my hypothesis fall to the ground.]

CHAPTER IV.

THE MORPHOLOGICAL COMPOSITION OF ANIMALS.

§ 199. What was said in § 180, respecting the ultimate
structure of organisms, holds more manifestly of animals
than of plants. That throughout the vegetal kingdom the
cell is the morphological unit, is a proposition admitting of a
better defence, than the proposition that the cell is the mor-
phological unit throughout the animal kingdom. The quali-
fications with which, as we saw, the cell-doctrine must be
taken, are qualifications thrust upon us more especially by
the facts which zoologists have brought to light. -It is among
the Protozoa that there occur numerous cases of vital activity
displayed by specks of protoplasm; and from the minute
anatomy of all creatures above these, are drawn the numer-
ous proofs that non-cellular tissues may arise by direct meta-
morphosis of mixed colloidal substances.*

* Since this paragraph was published in 1865, much has been learned con-
cerning cell-structure, as is shown in Chapter VIA of Part I. While some
assert that there exist portions of living protoplasm without nuclei, others
assert that a nucleus is in every case present, and that where it does not exist
in a definite aggregated form it exists in a dispersed form. As remarked in
the chapter named, " the evidence is somewhat strained to justify this dogma."
Words are taken in their non-natural senses, if one which connotes an indi-
vidualized body is applied to the widely-diffused components of such a body ;
and this perverting of proper meanings leads to obscuration of what may
perhaps be an essential truth. As argued in the chapter named (§§ *74e, 74/*),
nuclear matter is, as shown by its chemical character, an extremely unstable
substance, the molecular changes of which, perpetually going on, initiate

85

86 MORPHOLOGICAL DEVELOPMENT.

Our survey of morphological composition throughout tl
animal kingdom, must therefore begin with those undiffer-
entiated aggregates of physiological units [or constitutional
units], out of which are formed what we call, with consider-
able license, morphological units.

§ 200. In that division of the Protozoa distinguished as
Rhizopoda, are presented, under various modifications, these
minute portions of living organic matter, so little differenti-
ated, if not positively undifferentiated, that animal individu-
ality can scarcely be claimed for them. Figs. 131, 132, and
133, represent certain nearly-allied types of these — Amoeba,

i

13/ 138

133

Actinophrys, and Lieberkiihnia. The viscid jelly or sarcode,
comparable in its physical properties to white of egg, out of
which one of these creatures is mainly formed, shows us in
various ways, the feebleness with which the component
physiological units are integrated — shows us this by its very
slight cohesion, by the extreme indefiniteness and mutability
of its form, and by the absence of a limiting membrane.
It is no longer held even by unqualified adherents of the
cell-doctrine that the Amceba has an investment. Its outer
surface, compared to the film which forms on the surface of
paste, does not prevent the taking of solid particles into the
mass of the body, and does not, in such kindred forms as
Fig. 133, prevent the pseudopodia from coalescing when they
meet. Hence it cannot properly have the name of a cell-wall.
A considerable portion of the body, however, in Difflugia,

shocks, producing changes all around. In the earlier stages of cell-evolution
this unstable substance is dispersed throughout the cytoplasm ; whereas in
the more advanced stages it is gathered together in one mass. If so, instead
of saying there is a dispersed nucleus we should say there are the materials
of a nucleus not yet integrated.

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 87

Fig. 134, has a denser coating formed of agglutinated foreign
particles; so that the protrusion of the pseudopodia is limited
to one part of it. And in the solitary Foraminifera, like
Gromia, the sarcode is covered over most of its surface by a
delicate calcareous shell, pierced with minute holes, through
which the slender pseudopodia are thrust. The

Gregarina exhibits an advance in integration, and a conse-
quent greater definiteness. Figs. 135 and 136, exemplifying
this type, show the complete membrane in which the sub-
stance of the creature is contained. Here there has arisen
what may be properly called a cell: under its solitary ?orm
this animal is truly unicellular. Its embryology has con-
siderable significance. After passing through a certain qui-
escent, " encysted " state, its interior breaks up into sn. all
portions, which, after their exit, assume forms like that of
the Amoeba; and from this young condition in which they
are undifferentiated, they pass into that adult condition in
which they have limiting membranes. If this development
of the individual Gregarina typifies the mode of evolution of
the species, it yields further support to the belief, that frag-
ments of sarcode existed earlier than any of the structures
which are called cells. Among aggregates of the first

order, there are some much more highly developed. These
are the Infusoria, constituting the most numerous of the
Protozoa, in species as in individuals. Figs. 137, 138, and
139, are examples. In them we find, along with greater
definiteness, a considerable heterogeneity. The sarcode of
which the body consists, has an indurated outer layer, bearing
cilia and sometimes spines; there is an opening serving as
mouth, a permanent oesophagus, and a cavity or cavities,
temporarily formed in the interior of the sarcode, to serve as
one or more stomachs; and there is a comparatively specific
arrangement of these and various minor parts.

Thus in the animal kingdom, as in the vegetal kingdom,
there exists a class of minute forms having this peculiarity,
that no one of them is separable into a number of visible

88

MORPHOLOGICAL DEVELOPMENT.

components homologous with one another — no one of them
can be resolved into minor individualities. Its proximate
units are those physiological units of which we conclude every
organism consists. The aggregate is an aggregate of the first
order.

§ 201. Among plants are found types indicating a transi-
tion from aggregates of the first order to aggregates of the
second order; and among animals we find analogous types.
But the stages of progressing integration are not here so dis-
tinct- The reason probably is, that the simplest animals,
having individualities much less marked than those of the
simplest plants, do not afford us the same facilities for obser-
vation. In proportion as the limits of the minor individuali-
tips are indefinite, the formation of major individualities out
of them, naturally leaves less conspicuous traces.

Be this as it may, however, in such types of Protozoa as
the compound Radiolaria, we find that though there is reason
to regard the aggregate as an aggregate of the second order,
yet its divisibility into minor individualities like those just
described, is less manifest. Fig. 140 representing Sphcerozoum

punctatum, one of the group, illustrates this. The sceptic-
ally-minded may perhaps doubt whether we can regard the
" cellaeform bodies " contained in it, as the morphological
units of the animal. The jelly-like mass in which they are
imbedded, is but indefinitely divisible into portions having
each a cell or nucleus for its centre.* Among the

* This statement seems at variance with the figure ; but the figure is very
inaccurate. Its inaccuracy curiously illustrates the vitiation of evidence.
When I saw the drawing on the block, I pointed out to the draughtsman,

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 89

Foraminifera, we find only indefinite evidence of the coal-
escence of aggregates of the first order, into aggregates of the
second order. There are solitary Foraminifers, allied to the
creature represented in Fig. 134. Certain ideal types of
combination among them, are shown in Fig. 141. And
setting out from these, we may ascend in various directions
to kinds compounded to an immense variety of degrees in an
immense variety of ways. In all of them, however, the
separability of the major individuality into minor indivi-
dualities, is very incomplete. The portion of sarcode con-
tained in one of these calcareous chambers, gives origin to
an external bud; and this presently becomes covered, like
its parent, with calcareous matter: the position in which
each successive chamber is so produced, determining the
form of the compound shell. But the portions of sarcode
thus budded out one from another, do not become distinctly
individualized. Fig. 142, representing the living net-work
Which remains when the shell of an Orbitolite has been dis-
solved, shows the continuity that exists among the occupants
of its aggregated chambers.* In the compound

Infusoria, the component units remain quite distinct. Being,
as aggregates of the first order, much more definitely or-
ganized, their union into aggregates of the second order does

that he had made the surrounding curves much more obviously related to
the contained bodies, than they were in the original (in Dr. Carpenter's For-
aminifera) ; and having looked on while he in great measure remedied this
defect, thought no further care was needed. Now, however, on seeing the
figure in the printer's proof, I find that the engraver, swayed by the same
supposition as the draughtsman that such a relation was meant to be shown,
has made his lmes represent it still more decidedly than those of the draughts-
man before they were corrected. Thus, vague linear representations, like
vague verbal ones, are apt to grow more definite when repeated. Hypothesis
warps perceptions as it warps thoughts.

* Though the subdivision into chambers of the shell does not correspond
to the subdivision into cell-units it may still be held that since in the solitary
types the subdivision of the nucleus is followed by formation of new indi-
viduals which separate, and since in the compound types the subdivision of
the nucleus is followed by growth and formation of new chambers, the com-
pound type must be regarded as an aggregate of the second order.

90 MORPHOLOGICAL DEVELOPMENT.

not destroy their original individualities. Among the Vorti-
cellce, of which two kinds are delineated in Figs. 144 and
145, there are various illustrations of this: the members of
the community being sometimes appended to a single stem;
sometimes attached by long separate stems to a common
base; and sometimes massed together.

Thus far, these aggregates of the second order exhibit but
indefinite individualities. The integration is physical; but
not physiological. Though, in the Polycytlwria, there is a
shape that has some symmetry; and though, in the Fora-
minifera, the formation of successive chambers proceeds in
such methodic ways as to produce quite-regular and toler-
ably-specific shells ; yet no more in these than in the Sponges
or the compound Vorticellce, do we find such co-ordination as
gives the whole a life predominating over the lives of its
parts. We have not yet reached an aggregate of the second
order, so individuated as to be capable of serving as a unit in
still higher combinations. But in the class Ccelenterata, this
advance is displayed. The common Hydra, habitually taken
as the type of the lowest division of this class, has specialized
parts performing mutually-subservient functions, and thus
exhibiting a total life distinct from the lives of the units.
Fig. 146 represents one of these creatures in its contracted
state and in its expanded state; while Fig. 147 is a diagram

showing the wall of this crea-
ture's sac-like body as seen in
section under the microscope:
a and b being the outer and
inner cellular layers; while be-
tween them is the " mesogloea "
or " structureless lamella," the
supporting or skeletal layer. But this lowly-organized
tissue of the Hydra, illustrates a phase of integration
in which the lives of the minor aggregates are only par-
tially-subordinated to the life of the major aggregate
formed by them. For a Hydra's substance is separable

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 91

into Amcsba-likc portions, capable of moving about inde-
pendently. If we bear in mind how analogous are the
extreme extensibility and contractility of a Hydra's body
and tentacles, to the properties displayed by the sarcode
among IMiizopods; we may infer that probably the move-
ments and other actions of a Hydra, are due to the half-
independent co-operation of the Amoeba-\ike individuals
composing it.

§ 202. A truth which we before saw among plants, we
here see repeated among animals — the truth that as soon as
the integration of aggregates of the first order into aggregates
of the second order, produces compound wholes so specific in
their shapes and sizes, and so mutually dependent in their
parts, as to have distinct individualities ; there simultaneously
arises the tendency in them to produce, by gemmation, other
such aggregates of the second order. The approach towards
definite limitation in an organism, is, by implication, an ap-
proach towards a state in which growth passing a certain
point, results, not in the increase of the old individual, but
in the formation of a new individual. Thus it happens that
the common polype buds out other polypes, some of which
very shortly do the like, as shown in Fig. 148: a process
paralleled by the fronds of
sundry Algce, and by those
of the lower Jungermanni-
acece. And just as, among
these last plants, the pro-
liferously-produced fronds,
after growing to certain
sizes and developing root-
lets, detach themselves from their parent fronds; so among
these animals, separation of the young ones from the bodies
of their parents ensues when they have acquired tolerably
complete organizations.

There is reason to think that the parallel holds still fur-

92

MORPHOLOGICAL DEVELOPMENT.

ther. Within the limits of the Jungermanniacece, we found
that while some genera exhibit this discontinuous develop-
ment, other genera exhibit a development that is similar to
it in all essential respects, save that it is continuous. And
here within the limits of the Hydrozoa, we find, along with
this genus in which the gemmiparous individuals are pre-
sently cast off, other genera in which they are not cast off,
but form a permanent aggregate of the third order. Figs.
149 and 150, exemplify these compound Hydrozoa — one of
them showing this mode of growth so carried out as to pro-
duce a single axis; and the other showing how, by repeti-
tions of the process, lateral axes are produced. Integrations
characterizing certain higher genera
of the Hydrozoa which swim or float
instead of being fixed, are indicated
by Figs. 151 and 152: the first of
them representing the type of a
group in which the polypes growing
from an axis, or ccenosarc, are drawn
through the water by the rhythmi-
cal contractions of the organs from
which they hang; and the second of
them representing a Physalia the
component polypes of which are
united into a cluster, attached to an
air-vessel.

A parallel series of illustrations might be drawn from that
second division of the Cwlenterata, known as the Actinozoa.
Here, too, we have a group of species — the Sea-anemones —
the individuals of which, are solitary. Here, too, we have
agamogenetic multiplication : occasionally by gemmation, but
more frequently by that modified process called spontaneous
fission. And here, too, we have compound forms resulting
from the arrest of this spontaneous fission before it is com-
plete. To give examples is needless; since they would but
show, in more varied ways, the truth already made sufn-

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 93

ciently clear, that the compound Coslenterata are aggregates
of the third order, produced by integration of aggregates of
the second order such as we have in the Hydra. As before,
it is manifest that on the hypothesis of evolution, these
higher integrations will insensibly arise, if the separation of
the ge nnniparous polypes is longer and longer postponed;
and that an increasing postponement will result by survival
of the fittest, if it profits the group of individuals to remain
united instead of dispersing.*

§ 203. The like relations exist, and imply that the like
processes have been gone through, among those more highly-
organized animals called Polyzoa and Tunicata. We have
solitary individuals, and we have variously-integrated groups
of individuals: the chief difference between the evidence
here furnished, and that furnished in the last case, being the
absence of a type obviously linking the solitary state with
the aggregated state.

This integration of aggregates of the second order, is car-
ried on among the Polyzoa in divers ways, and with different
degrees of completeness. The little patches of minute cells,
shown as magnified in Fig. 153, so common on the fronds of
sea-weeds and the surfaces of rocks at low- water mark, display
little beyond mechanical combination. The adjacent indi-

154:

* A critic says the question is u what are the forces internal or external
which produce union or separation." A proximate reply is — degree of nutri.
tion. As in a plant new individuals or rudiments of them are cast off where
nutrition is failing, so in a compound animal. The connecting part dwindles
if it ceases to carry nutriment.

94 MORPHOLOGICAL DEVELOPMENT.

viduals, though severally originated by gemmation from the
same germ, have but little physiological dependence. In
kindred kinds, however, as shown in Figs. 154 and 155, one
of which is a magnified portion of the other, the integration
is somewhat greater: the co-operation of the united indi-
viduals being shown in the production of those tubular
branches which form their common support, and establish
among them a more decided community of nutrition.

Among the Ascidians this general law of morphological
composition is once more displayed. Each of these creatures
subsists on the nutritive particles contained in the water
which it draws in through one orifice and sends out through
another; and it may thus subsist either alone, or in con-
nexion with others that are in some cases loosely aggregated
and in other cases closely aggregated. Fig. 156, Phallusia

mentula, is one of the solitary forms. A type in which the
individuals are united by a stolon that gives origin to them
by successive buds, is shown in Perophora, Fig. 157. Among
the Botryllidce, of which one kind is drawn on a small scale
in Fig. 159, and a portion of the same on a larger scale in
Fig. 158, there is a combination of the 'individuals into an-
nular clusters, which are themselves imbedded in a common
gelatinous matrix. And in this group there are integrations
even a stage higher, in which several such clusters of clusters
grow from a single base. Here the compounding and re-
compounding appears to be carried further than anywhere
else in the animal kingdom.

Thus far, however, among these aggregates of the third
order, we see what we before saw among the simpler aggre-
gates of the second order — we see that the component indi-
vidualities are but to a very small extent subordinated to

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 95

the individuality made up of them. In nearly all the forms
indicated, the mutual dependence of the united animals is so
Blight, that they are more fitly comparable to societies, of
which the members co-operate in securing certain common
benefits. There is scarcely any specialization of functions
among them. Only in the last type described do we see a
number of individuals so completely combined as to simulate
a single individual. And even here, though there appears to
be an intimate community of nutrition, there is no physio-
logical integration beyond that implied in several mouths and
stomachs having a common vent.*

§ 204. We come now to an extremely interesting question.
Does there exist in other sub-kingdoms composition of the
third degree, analogous to that which we have found so
prevalent among the Ccelenterata and the Polyzoa and Tuni-
cataf The question is not whether elsewhere there are
tertiary aggregates produced by the branching or clustering
of secondary aggregates, in ways like those above traced;
but whether elsewhere there are aggregates which, though
otherwise unlike in the arrangement of their parts, never-
theless consist of parts so similar to one another -that we
may suspect them to be united secondary aggregates. The
various compound types above described, in which the united
animals maintain their individualities so distinctly that the
individuality of the aggregate remains vague, are constructed
in such ways that the united animals carry on their several
activities with scarcely any mutual hindrance. The members
of a branched Hydrozoon, such as is shown in Fig. 149 or
Fig. 150, are so placed that they can all spread their tentacles
and catch their prey as well as though separately attached to
stones or weeds. Packed side by side on a flat surface or

* It has been pointed out that I have here understated the evidence of
physiological integration. An instance of it among Hydrozoa is shown in
Fig. 151, but by a strange oversight I have forgotten to name the various
cases furnished by the Siphonophora in which the individual polypes of a com-
pound aggregate are greatly specialized in adaptation to different functions.

96 MORPHOLOGICAL DEVELOPMENT.

forming a tree-like assemblage, the associated individuals
among the Polyzoa are not unequally conditioned: or if one
has some advantage over another in a particular case, the
mode of growth and the relations to surrounding objects are
so irregular as to prevent this advantage re-appearing with
constancy in successive generations. Similarly with the
Ascidians growing from a stolon or those forming an annular
cluster: each of them is as well placed as every other for
drawing in the currents of sea-water from which it selects its
food. In these cases the mode of aggregation does not expose
the united individuals to multiform circumstances; and
therefore is not calculated to produce among them any
structural multiformity. For the same reason no marked
physiological division of labour arises among them; and
consequently no combination close enough to disguise their
several individualities. But under converse conditions we
may expect converse results. If there is a mode of integration
which necessarily subjects the united individuals to unlike
sets of incident forces, and does this with complete uniformity
from generation to generation, it is to be inferred that the
united individuals will become unlike. They will severally
assume 'such different functions as their different positions
enable them respectively to carry on with the greatest
advantage to the assemblage. This heterogeneity of function
arising among them, will be followed by heterogeneity of
structure; as also by that closer combination which the
better enables them to utilize one another's functions. And
hence, while the originally-like individuals are rendered
unlike, they will have their homologies further obscured by
their progressing fusion into an aggregate individual of a
higher order.

These converse conditions are in nearly all cases fulfilled
where the successive individuals arising by continuous devel-
opment are so budded-off as to form a linear series. I say
in nearly all cases, because there are some types in which
the associated individuals, though joined in single file, are

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 97

not thereby rendered very unlike in their relations to the
environment; and therefore do not become differentiated and
integrated to any considerable extent. I refer to such Asci-
dians as the Salpidce. These creatures float passively in the
sea, attached together in strings. Being placed side by side
and having mouths and vents that open laterally, each of
them is as well circumstanced as its neighbours for absorb-
ing and emitting the surrounding water; nor have the in-
dividuals at the two extremities any marked advantages
over the rest in these respects. Hence in this type, and in
the allied type Pyrosoma, which has its component indivi-
duals built into a hollow cylinder, linear aggregation may
exist without the minor individualities becoming obscured
and the major individuality marked: the conditions under
which a differentiation and integration of the component
individuals may be expected, are not fulfilled. But where
the chain of individuals produced by gemmation, is either
habitually fixed to some solid body by one of its extremities
or moves actively through the water or over submerged
stones and weeds, the several members of the chain become
differently conditioned in the way above described; and may
therefore be expected to become unlike while they become
united. A clear idea of the contrast between these two
linear arrangements and their two diverse results, will be
obtained by considering what happens to a row of soldiers,
when changed from the ordinary position of a single rank
to the position of Indian file. So long as the men stand
shoulder to shoulder, they are severally able to use their
weapons in like ways with like efficiency; and could, if
called on, similarly perform various manual processes directly
or indirectly conducive to their welfare. But when, on the
word of command " right face," they so place themselves
that each has one of his neighbours before him and another
behind him, nearly all of them become incapacitated for
fighting and for many other actions. They can walk or run
one after another, so as to produce movement of the file in
53

98 MORPHOLOGICAL DEVELOPMENT.

the direction of its length; but if the file has to oppose an
enemy or remove an obstacle lying in the line of its march,
the front man is the only one able to use his weapons or
hands to much purpose. And manifestly such an arrange-
ment could become advantageous only if the front man pos-
sessed powers peculiarly adapted to his position, while those
behind him facilitated his actions by carrying supplies, &c.
This simile, grotesque as it seems, serves to convey better
perhaps than any other could do, a clear idea of the relations
that must arise in a chain of individuals arising by gemma-
tion, and continuing permanently united end to end. Such
a chain can arise only on condition that combination is more
advantageous than separation; and for it to be more advan-
tageous, the anterior members of the series must become
adapted to functions facilitated by their positions, while the
posterior members become adapted to functions which their
positions permit. Hence, direct or indirect equilibration or
both, must tend continually to establish types in which the
connected individuals are more and more unlike one another,
at the same time that their several individualities are more
and more disguised by the integration consequent on their
mutual dependence.

Such being the anticipations warranted by the general laws
of evolution, we have now to inquire whether there are any
animals which fulfil them. Very little search suffices; for
structures of the kind to be expected are abundant. In that
great division of the animal kingdom at one time called An-
nulosa, but now grouped into Annelida and Arthropoda, we
find a variety of types having the looked-for characters. Let
us contemplate some of them.

§ 205. An adult Chaetopod is composed of segments which
repeat one another in their details as well as in their general
shapes. Dissecting one of the lower orders, such as is
shown in Fig. 160, proves that the successive segments, be-
sides having like locomotive appendages, like branchiae, and

THE MORPHOLOGICAL COMPOSITION OP ANIMALS. 99

sometimes even like pairs of eyes, also have like internal
organs. Each has its enlargement of the alimentary canal;
each its contractile dilatation of the great blood-vessel; each
its portion of the double nervous cord, with ganglia when

/6/

wSmmm

these exist ; each its branches from the nervous and vascular
trunks answering to those of its neighbours; each its simi-
larly answering set of muscles; each its pair of openings
through the body-wall; and so on throughout, even to the
organs of reproduction. That is to say, every segment is in
great measure a physiological whole — every segment con-
tains most of the organs essential to individual life and mul-
tiplication: such essential organs as it does not contain,
being those which its position as one in the midst of a chain,
prevents it from having or needing. If we

ask what is the meaning of these homologies, no adequate
answer is supplied by any current hypothesis. That this
" vegetative repetition " is carried out to fulfil a prede-
termined plan, was shown to be quite an untenable notion
(§§ 133, 134). On the one hand, we found nothing satis-
factory in the conception of a Creator who prescribed to him-
self a certain unit of composition for all creatures of a par-
ticular class, and then displayed his ingenuity in building up
a great variety of forms without departing from the " arche-
typal idea." On the other hand, examination made it mani-
fest that even were such a conception worthy of being enter-
tained, it would have to be relinquished; since in each class
there are numerous deviations from the supposed " archetypal
idea." Still less can these traits of structure be accounted

100 MORPHOLOGICAL DEVELOPMENT.

for ideologically. That certain organs of nutrition and re-
spiration and locomotion are repeated in each segment of a
dorsibranchiate annelid, may be regarded as functionally ad-
vantageous for a creature following its mode of life. But
why should there be a hundred or even two hundred pairs of
ovaries? This is an arrangement at variance with that
physiological division of labour which every organism pro-
fits by — is a less advantageous arrangement than might have
been adopted. That is to say, the hypothesis of a designed
adaptation fails to explain the facts. Contrariwise,

these structural traits are just such as might naturally be
looked for, if these annulose forms have arisen by the in-
tegration of simpler forms. Among the various compound
animals already glanced at, it is very general for the united
individuals to repeat one another in all their parts — repro-
ductive organs included. Hence if, instead of a clustered or
branched integration, such as the Ccelenterata, Polyzoa and
Tunicata exhibit, there occurs a longitudinal integration; we
may expect that the united individuals will habitually indi-
cate their original independence by severally bearing germ-
producing or sperm-producing organs.

The reasons for believing one of these creatures to be an
aggregate of the third order, are greatly strengthened when
we turn from the adult structure to the mode of develop-
ment. Among the Dorsibranchiata and Tubicolce, the em-
bryo leaves the egg in the shape of a ciliated gemmule, not
much more differentiated than that of a polype. As shown
in Fig. 162, it is a nearly globular mass; and its interior
consists of untransformed cells. The first appreciable change
is an elongation and a simultaneous commencement of seg-
mentation. The segments multiply by a modified gemma-
tion, which takes place from the hinder end of the penultimate
segment. And considerable progress in marking out these
divisions is made before the internal organization begins.
Figs. 163, 164, 165, represent some of these early stages. In
annelids of other orders, the embryo assumes the segmented

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 101

form while still in the egg. But it does this in just the
same manner as before. Indeed, the essential identity of the
two modes of development is shown by the fact that the seg-
mentation within the egg is only partially carried out: in

765

all these types the segments continue to increase in number
for some time after hatching. Now this process is as

like that by which compound animals in general are formed,
as the different conditions of the case permit. When new
individuals are budded-out laterally, their unfolding is not
hindered — there is nothing to disguise either the process or
the product. But gemmae produced one from another in the
same straight line, and remaining connected, restrict one
another's developments; and that the resulting segments are
so many gemmiparously-produced individuals, is necessarily
less obvious.

§ 206. Evidence remains wdiich adds very greatly to the
weight of that already assigned. Thus far we have studied
only the individual segmented animal ; considering what may
be inferred from its mode of evolution and final organization.
We have now to study segmented animals in general. Com-
parison of different groups of them and of kinds within each
group, will disclose various phases of progressive integration
of the nature to be anticipated.

Among the simpler Platyhelminthes, as in some kinds of
Planaria, transverse fission occurs. A portion of a Planaria
separated by spontaneous constriction, becomes an inde-

102 MORPHOLOGICAL DEVELOPMENT.

pendent individual. Sir J. G. Dalyell found that in some
cases numerous fragments artificially separated, grew into
perfect animals.* In these creatures which thus remind us
of the lowest Hydrozoa in their powers of agamogenetic
multiplication, the individuals produced one from another
do not continue connected. As the young ones laterally
budded-off by the Hydra separate when complete, so do the
young ones longitudinally budded-off by the Planaria.
Fig. 166 indicates this. But there are allied types which
show us a more or less persistent union of homologous parts,
or individuals, similarly arising by longitudinal gemmation, f
The cestoid Entozoa furnish illustrations. Without dwelling
on the fact that each segment of a Taenia, like each separate
Planaria, is an independent hermaphrodite; and without
specifying the sundry common structural traits which add
probability to the suspicion that there is some kinship be-
tween the individuals of the one order and the segments of
the other; it will suffice to point out that the two types are
so far allied as to demand their union under the same sub-
class title. And recognizing this kinship, we see significance
in the fact that in the one case the longitudinally-produced
gemmae separate as complete individuals, and in the other
continue united as segments in smaller or larger numbers
and for shorter or longer periods. In Tarnia echino coccus,

* Kecently Mr. T. H. Morgan has made elaborate experiments which
show that Planaria Maculata may be cut into many pieces from various
parts and of various shapes — even a slice out of the side — and each, if not
too small, will produce a perfect animal.

f Since this was written in 1865 there has come to light evidence more
completely to the point than any at that time known. In the subdivision of
Platyhelminthes known as Turbcllaria, there are some, the Microstoma da
which, by a process of segmentation form "chains of 4, then 8, then 16, and
sometimes even 32 individuals." "Each forms a mouth [lateral] and for
some time the chain persists, but the individuals ultimately become sexually
matured and then separate." (Shipley, Zoology of the Invertebrata, p. 92.)
Here it should be remarked that the lateral mouths enable the members of
a string to feed separately, and that nutrition not being interfered with they
doubtless gain some advantage by temporary maintenance of their union —
probably in creeping.

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 103

represented in Fig. 167, we have a species in which the
number of segments thus united does not exceed four. In
Echinobothrium typus there are eight or ten ; and in cestoids
generally they are numerous.* A considerable

hiatus occurs between this phase of integration and the next
higher phase which we meet with; but it is not greater
than the hiatus between the types of the Platyhelminthes and
the Chwtopoda, which present the two phases. Though it is

doubtful whether separation of single segments occurs among
the Annelida,] yet very often we find strings of segments,

* I find that the reasons for regarding the segment of a Tcenia as answering
to an individual of the second order of aggregation, are much stronger than
I supposed when writing the above. Van Beneden says : — " Le Proglottis
(segment) ayant acquis tout son developpement, se detache ordinairement de
la colonie et continue encore a croitre dans l'intestin du meme animal ; il
change meme souvent de forme et semble doue' d'une nouvelle vie ; ses angles
s'effacent, tout le corps s'arrondit, et il nage comme une Planaire au milieu
des muscosites intestinales."

f Though this was doubtful in 1865 it is no longer doubtful. In an indi-
vidual Ctenodri/us movostylus, which multiplies by dividing and subdividing
itself, " parts arise which are destitute of both head and anus and at times
consist of only a single segment." In another species, C. partialis, there is
separation into many segments ; and each segment before separating forms
a budding zone out of which other segments are afterwards produced, com-
pleting the animal (Korschelt and Heider, Embryology, i, 301-2).

104 MORPHOLOGICAL DEVELOPMENT.

arising by repeated longitudinal budding, which after reach-
ing certain lengths undergo spontaneous fission: in some
cases doing this so as to form two or more similar strings
of segments constituting independent individuals; and in
other cases doing it so that the segments spontaneously
separated are but a small part of the string. Thus a Syllis,
Fig. 168, after reaching a certain length, begins to trans-
form itself into two individuals : one of the posterior segments
develops into an imperfect head, and simultaneously narrows
its connexion with the preceding segments, from which it
eventually separates. Still more remarkable is the extent to
which this process is carried in certain kindred types; which
exhibit to us several individuals thus being simultaneously
formed out of groups of segments. Fig. 169, copied (omit-
ting the appendages) from one contained in a memoir
by M. Milne-Edwards, represents six worms of different
ages in course of development: the terminal one being the
eldest, the one having the greatest number of segments,
and the one that will first detach itself; and the success-
ively anterior ones, with their successively smaller numbers
of segments, being successively less advanced towards fitness
for separation and independence. Here among groups of
segments we see repeated what in the previous cases occurs
with single segments. And then in other annelids we find that
the string of segments arising by gemmation from a single
germ becomes a permanently united whole: the tendency to
any more complete fission than that which marks out the seg-
ments, being lost; or, in other words, the integration having
become relatively complete. Leaving out of sight the

question of alliance among the types above grouped together,
that which it here concerns us to notice is, that longitudinal
gemmation does go on; that it is displayed in that primitive
form in which the gemmae separate as soon as produced ; that
we have types in which such gemmae hang together in
groups of four, or in groups of eight and ten, from which
however the gemmae successively separate as individuals;

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 105

that among higher types we have long strings of similarly-
formed gemmae which do not become individually independ-
ent, but separate into organized groups; and that from
these we advance to forms in which all the gemmae remain
parts of a .single individual. One other significant

fact must be added. There are cases in which annelids
multiply by lateral gemmation.* That the longitudinally-
produced gemmae which compose an annelid, should thus
have, one of them or several of them, the power of laterally
budding-off gemmae, from which other annelids arise, gives
further support to the hypothesis that, primordially, the seg-
ments were independent individuals. And it suggests this
belief the more strongly because, in certain types of Ccelen-
terata, we see that longitudinal and lateral gemmation do
occur together, where the longitudinally-united gemmae are
demonstrably independent individuals.

§ 207. Though it seems next to impossible that we shall
ever be able to find a type such as that which is here sup-
posed to be the unit of composition of the annulose type,
since we must assume such a type to have been long since
extinct, yet the foregoing evidence goes far towards showing
that an annulose animal is an aggregate of the third order.
This repetition of segments, sometimes numbering several
hundreds, like one another in all their organs even down to
those of reproduction, while it is otherwise unaccountable, is
fully accounted for if these segments are homologous with
the separate individuals of some lower type. The gemma-
tion by which these segments are produced, is as similar as
the conditions allow, to the gemmation by which compound

* In place of those originally here instanced about which there are dis-
putes, I may give an undoubted one described by Mcintosh, the Si/Uis
ramosa, a species of chaetopod living in hexactinellid sponges from the Ara-
fura Sea, which branches laterally repeatedly so as to extend in all directions
through the canals of the sponge. In most cases the buds terminate in oval
segments with two long cirri each. But male and female buds were found,
provided each with a head, and containing ovaries and testes. Sometimes
these sexual buds had become separate from the branched stock.

106 MORPHOLOGICAL DEVELOPMENT.

animals in general are produced. As among plants, and as
among demonstrably-compound animals, we see that the only
thing required for the formation of a permanent chain of
gemmiparously-produced individuals, is that by remaining
associated such individuals will have advantages greater than
are to be gained by separation. Further, comparisons of
the annuloid and lower annulose forms, disclose a number
of those transitional phases of integration which the hypo-
thesis leads us to expect. And, lastly, the differences among
these united individuals or successive segments, are not
greater than the differences in their positions and functions
explain — not greater than such differences are known to pro-
duce among other united individuals: witness sundry com-
pound Hydrozoa.

Indirect evidence of much weight has still to be givei
Thus far we have considered only the less-developed Annu-
losa. The more integrated and more differentiated types of
the class remain. If in them we find a carrying further of
the processes by which the lower types are here supposed to
have been evolved, we shall have additional reason for be-
lieving them to have been so evolved. If we find that in
these superior orders, the individualities of the united seg-
ments are much less pronounced than in the inferior, we
shall have grounds for suspecting that in the inferior the
individualities of the segments are less pronounced than in
those lost forms which initiated the annulose sub-kingdom.

[Note. — Partly from the wish to incorporate further evi-
dence, and partly from the wish to present the evidence, old
and new, in a more effective order, I decide here to recast the
foregoing exposition.

Significant traits of development are exhibited in common
by two groups otherwise unallied — certain of the Platyhel-
minthes and certain of the lower Annulosa. Of the Platyhel-
minthes the ordinary type is an unsegmented creature: a

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 107

Planarian or a Trematode exemplifying it. Among the free
forms, as in some Planarians, there occurs transverse fission,
and prompt separation of the segments; while among some
other free forms, as the Microstomida, the two segments first
produced, themselves become segmented while still adherent,
and this process is repeated until a string is formed. Another
group of the Platyhelminthes, the Cestoid Entozoa, exhibit
analogous processes. There are unsegmented forms, as the
Caryophyllaeus, and there are forms in which the segments,
now few now many, adhere together in chains; the terminal
members of which, however, eventually separate, and having
before separation approached the trematode structure, become
independent individuals which grow, creep about, and con-
tinue the race. In both of these types the condition under
which the gemmiparously-produced members remain con-
nected, is that they shall be able to feed individually: in
the one case by lateral mouths, in the other case by absorp-
tion through the integument. It is further observable that
in both cases separation of the component individuals occurs
at sexual maturity, when advantage in nutrition has ceased
to be the dominant need and dispersion of the species has
taken its place in degree of importance. Among

Annelids, higher though they are in type, we find parallel-
isms. Usually in its first stage an annelid is unsegmented,
but as fast as it elongates lines of segmentation indent its
surface. This segmentation proceeds in various ways, and
the segments exhibit various degrees of dependence. In
some low types, spontaneous fission goes on to the extent of
producing single segments, each of which has such vitality
that it buds out anterior and posterior parts at its two ends.
Thus alike in the simple form which exists before segmenta-
tion and in the form exhibited by a detached segment, we
have a unit analogous to each of the units which are joined
together in certain free Turbellaria and in the Cestoids : the
difference being that in the Annelids the sexually-mature
units do not individually disunite. But though there does

108 MORPHOLOGICAL DEVELOPMENT.

not take place separation of single completed segments, there
takes place separation of groups of segments, which are
either sexually mature at the time they drop off or presently
become so. And the groups of segments which have become
sexually mature before they drop off, have simultaneously
acquired swimming organs and developed eyes, enabling them
to spread and diffuse the species. Sundry biologists recognize
a parallelism between that detachment of developed segments
which goes on in the cestoid Entozoa, and that which goes on
in the Scyphomedusce. The successively detached members
of the strobila are sexually-matured or maturing individuals
which, as medusae, are fitted for swimming about, multiply-
ing, and reaching other habitats; while each detached pro-
glottis of the cestoid is, by the nature of its medium, limited
to creeping about. Clearly this fissiparous process in such
Annelids as the Syllidce, which has similarly been compared
to the strobilization of the Scyphomedusce, differs simply in
the respect that single segments are not adapted for locomo-
tion, and it therefore profits the species to separate in groups.
All these facts and analogies point to the conclusion that the
remote ancestor of the Annelids was an unsegmented crea-
ture homologous with each of the segments of an existing
Annelid.

This conclusion is supported by other kinds of evidence
here to be added. The larvae of Annelids are very various
but amid their differences there is a recognizable type. " The
Trochophore is the typical larval form of the Annelid stem " :
a trochophore being a curious spheroidal ciliated structure
suggestive of ccelenterate affinities. And this unsegmented
larva, representing the remote ancestor from which the many
Annelid types diverged, is similar to the larvae of the Rotifera
and the Mollusca : a trochophore is common to all these great
classes. Moreover since, among the Rhizota (a sub-class of
the Rotiferw), there is a species, Trochosphcera, solitary and
free-swimming, resembling in form and structure a trocho-
phore, though it is not a larva but an adult, we get further

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 109

evidence that there was a primitive creature of this general
character, of which the trochophores of Mollusca, Rot if era,
and Annelida are divergent modifications, and which was
unsegmented : the implication being that the segmentation
or the Annelida was superinduced. That this segmentation
resulted from gemmation is implied by what are called poly-
trochal larvee. These " sometimes appear as a stage succeed-
ing other larval types. Thus those of Arenicola marina arise
from larvae which at first were monotrochal, later became
telotrochal, and finally, by the appearance of new ciliated
rings between those already present, assumed the stage of
polytrochal larvag. . . . This condition warrants the
assumption that the segmented forms are to be looked upon
as the younger, the unsegmented, on the other hand, as the
phylogenetically older." (Korschelt and Heider, i, 278.) And
that the above-described rings of cilia mark off segments is
shown by the case of Ophryotrocha puerilis, which " remains,
as it were, in a larval condition, since the segments retain
their ciliation throughout life." {lb., 277.) Yet one more
significant fact must be named. In early stages of develop-
ment each segment of an archiannelidan has ccelomic spaces
separate from those of neighbouring segments, but in the
adult the septa "generally break down either partially or
completely, so that the perivisceral cavity becomes a con-
tinuous space from end to end of the animal." (Sedgwick,
Text Boole, 449.) While this fact is congruous with the
hypothesis here maintained, it is incongruous with the hypo-
thesis that the annelid was originally an elongated creature
which afterwards became segmented; since in that case the
implication would be that the ccelomic septa, not arising from
recapitulation of an ancestral structure, but originated by the
process of segmentation, were first superfluously formed and
then destroyed.

Various lines of evidence thus converge to the conclusion
that an annulose animal is an aggregate of the third order.

In June, 1865, when No. 14 of my serial containing the

110

MORPHOLOGICAL DEVELOPMENT.

foregoing chapter was issued, I supposed myself to be alone
in holding this belief respecting the annulose type, and long
continued to suppose so. Over thirty years later, however, in
M. Edmond Perrier's work, La Philosophie Zoologique avant
Darwin, I found mention of a lecture delivered by M. Lacaze-
Duthiers at the Ecole ISTormale Superieure in Paris, and re-
ported in the Revue des Cours Scientifiques for January 28,
1865, in which he enunciated a like belief. Judging, how-
ever, by the account of this lecture which M. Perrier gives
(he was present), it appears that M. Lacaze-Duthiers simply
contended that this view of the annulose structure as arising
by union of once-independent units, is suggested by certain
a priori considerations. There is no indication that he
assigned any of the classes of facts above given, which go to
show that it has thus arisen.

For further facts and arguments concerning the genesis
of the annulose type, see Appendix D 2.]

CHAPTER V.

THE MORPHOLOGICAL COMPOSITION OF ANIMALS,
CONTINUED.

§ 208. Insects, Arachnids, Crustaceans, and Myriapods,
are all members of that higher division of the Annulosa *
called Articulata or now more generally Arthropoda. Though
in these creatures the formation of segments may be inter-
preted as a disguised gemmation; and though, in some of
them, the number of segments increases by this modified bud-
ding after leaving the eggf as it does among the Annelids;
yet the process is not nearly so dominant: the segments are
usually much less numerous than we find them in the types
last considered. In most cases, too, the segments are in a
greater degree differentiated one from another, at the same
time that they are severally more differentiated within them-
selves. Nor is there any instance of spontaneous fission
taking place in the series of segments composing an articu-
late animal. On the contrary, the integration, always great
enough permanently to unite the segments, is frequently
carried so far as to hide very completely the individualities
of some or many of them; and occasionally, as among the
Acari, the consolidation, or the arrest of segmentation, is so

* The name Annulosa, once used to embrace the Annelida and Arthro-
poda, has of late ceased to be used. It seems to me better than Appen-
diculata, both as being more obviously descriptive and as being more
exclusive.

Ill

112

MORPHOLOGICAL DEVELOPMENT.

decided as to leave scarcely a trace of the articulate struc-
ture: the type being in these cases indicated chiefly by the
presence of those characteristically-formed limbs, which give
the alternative name Arthropoda to all the higher Annulosa.
Omitting the parasitic orders, which, as in other cases, are
aberrant members of their sub-kingdom, comparisons between
the different orders prove that the higher are strongly dis-
tinguished from the lower, by the much greater degree in
which the individuality of the tertiary aggregate dominates
over the individualities of those secondary aggregates called
segments or " somites," of which it is composed. The suc-
cessive Figs. 170 — 176, representing (without their limbs) a

Julus, a Scolopendra, an isopodous Crustacean, and four
kinds of decapodous Crustaceans, ending with a Crab, will
convey at a glance an idea of the way in which that greater
size and heterogeneity reached by the higher types, is accom-
panied by an integration which, in the extreme cases, nearly
obliterates all traces of composite structure. In the Crab
the posterior segments, usually folded underneath the shell,
alone preserve their primitive distinctness. So completely
confluent are the rest, that it seems absurd to say that a
Crab's carapace is composed of as many segments as there are
pairs of limbs, foot-jaws, and antennas attached to it; and
were it not that during early stages of the Crab's develop-

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. U3

ment the segmentation is faintly marked, the assertion might
be considered illegitimate.

That all articulate animals are thus composed from end to
end of homologous segments, is, however, an accepted doc-
trine among naturalists. It is a doctrine that rests on careful
observation of three classes of facts — the correspondences
of parts in the successive " somites " of an adult articulate
animal; the still more marked correspondences of such parts
as they exist in the embryonic or larval articulate animal;
and the maintenance of such correspondences in some types,
which are absent in types otherwise near akin to them.
The nature of the conclusion which these evidences unite in
supporting, will best be shown by the annexed copies from
the lecture-diagrams of Prof. Huxley; exhibiting the typical
structures of a Myriapod, an Insect, a Spider, and a Crust-
acean, with their relations to a common plan, as interpreted
by him.

JnSPCt eyf.'^/

A/titter *79

/so

Treating of these homologies, Prof. Huxley says " that a

striking uniformity of composition is to be found in the heads

of, at any rate, the more highly organized members of these

four classes; and that, typically, the head of a Crustacean,

an Arachnid, a Myriapod, or an Insect, is composed of six
54

114 MORPHOLOGICAL DEVELOPMENT.

somites (or segments corresponding with those of the body)
and their appendages, the latter being modified so as to serve
the purpose of sensory and manducatory organs." *

Thus even in the higher Arthropoda, the much greater con-
solidation and much greater heterogeneity do not obliterate
all evidence of the fact, that the organism is an aggregate of
the third order. Comparisons show that it is divisible into a
number of proximate units, each of which is akin in certain
fundamental traits to its neighbours, and each of which is an
aggregate of the second order, in so far as it is an organized
combination of those aggregates of the first order which we
call morphological units or cells. And that these segments
or somites, which make up an annulose animal, were origin-
ally aggregates of the second order having independent in-
dividualities, is an hypothesis which gathers further support
from the contrast between the higher and the lower Arthro-
pods, as well as from the contrast between the Arthropods

* The fusion of the segments forming the Arthropod head and the
extreme changes, or perhaps in some cases disappearances, of their appen-
dages, put great difficulties in the way of identification ; so that there are
differences of opinion respecting the number of included segments. Prof.
MacBride writes: — "It is highly probable that a primary head (praeoral
lobe or praestomium) has been derived from annelid ancestors, but the
secondary fusion of body-segments with this head, in other words the forma-
tion of a secondary head, has gone on independently in the different classes
of the phylum Arthropoda, viz., Arachnida, Crustacea, and Tracheata
(including Insects and Myriapods). Judged by the number of appendages
(which gives an inferior limit) the head of a malacostracous Crustacean
consists of praestomium and 8 segments ; the head of an insect of praesto-
mium and 4 segments; the head of a Myriapod of praestomium and 3
segments; and the head of an Arachnid of praestomium and 3 segments."
Again, the comment of Mr. J. T. Cunningham is : — " According to Claus
and most modern authorities there are only 5 segments in the head of an
Arthropod, the eyes not counting as appendages ; and further it should be
noted that the second pair of antennae are wanting in Insects."

Of course difference of opinion respecting the number of somites in the
head involves difference of opinion respecting the number constituting the
entire body, which, in the higher Arthropods, is said by some to be 19 and
by others 20. But those who thus differ in detail, agree in regarding all
the segments of head and body as homologous, and this is the essential point
with which we are here concerned.

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 115

in general and the Annelids. For if that masking of the
individualities of the segments which we find distinguishes
the higher forms from the lower, has been going on from the
beginning, as we may fairly assume; it is to be inferred that
the individualities of the segments in the lower forms, were
originally more marked than they now are. Eeversing those
processes of change by which the most developed Annulosa
have arisen from the least developed; and applying in
thought this reversed process to the least developed, as they
were described in the last Chapter; we are brought to the
conception of attached segments that are all completely alike,
and have their individualities in no appreciable degree sub-
ordinated to that of the chain they compose. From which
there is but one step to the conception of gemmiparously-
produced individuals which severally part one from another
as soon as they are formed.

§ 209. We must now return to a junction whence we
diverged some time ago. As before explained under the
head of Classification, organisms do not admit of uni-serial
arrangement, either in general or in detail; but everywhere
form groups within groups. Hence, having traced the phases
of morphological composition up to the highest forms in any
sub-kingdom, we find ourselves at the extremity of a great
branch, from which there is no access to another great branch,
except by going back to some place of bifurcation low down
in the tree.

There exist such similarities of shape and structure be-
tween the larval forms of low Molluscs and those of Annelids
and Kotifers, as to show that there was an early type common
to them all; and its probable characters, suggested by com-
parison, seem to imply that it had arisen from some ccelen-
terate type, intermediate between the Cnidaria and the Cteno-
phora. But there is this noteworthy difference between the
molluscan larva and the allied larvae, that it gives origin to
only one animal and not to a group of animals, united or

116 MORPHOLOGICAL DEVELOPMENT.

disunited. No true Mollusc multiplies by gemmation, either
continuous or discontinuous; but the product of every ferti-
lized germ is a single individual.

It is a significant fact that here, where for the first time
we have homogenesis holding throughout an entire sub-king-
dom, we have also throughout an entire sub-kingdom no case
in which the organism is divisible into two, three, or more,
like parts. There is neither any such clustering or branch-
ing as a ccelenterate or molluscoid animal usually displays;
nor is there any trace of that segmentation which charac-
terizes the Annulosa. Among these animals in which no
single egg produces several individuals, no individual is
separable into several homologous divisions. This connexion
will be seen to have a probable meaning, on remembering
that it is the converse of the connexion which obtains among
the Annulosa, considered as a group.

A Mollusc, then, is an aggregate of the second order. Not
only in the adult animal is there no sign of a multiplicity of
like parts that have become obscured by integration; but
there is no sign of such multiplicity in the embryo. And this
unity is just as conspicuous in the lowest Lamellibranch as in
the highest Cephalopod.

It may be well to note, however, more especially because
it illustrates a danger of misinterpretation presently to be
guarded against, that there are certain Molluscs which simu-
late the segmented structure. Externally a Chiton, Fig. 188,

appears to be made up of
divisions substantially like
those of the creature Fig.
189 ; and one who judged
only by externals, would say
that the creature Fig. 190
differs as much from the
creature Fig. 189, as this
does from the preceding one. But the truth is, that while
190 and 189 are closely-allied types, 189 differs from 188

THE M0RPI10L0GICAL COMPOSITION OF ANIMALS. 117

much more widely than a man does from a fish. And the
radical distinction between them is this: — Whereas in the
Crustacean the segmentation is carried transversely through
the whole mass of the body, so as to render the body more
or less clearly divisible into a series of parts which are simi-
larly composed; in the Mollusc the segmentation is limited
to the shell carried on its upper surface, and leaves its
body as completely undivided as is that of a common slug.*
Were the body cut through at each of the divisions, the
section of it attached to each portion of the shell would be
unlike all the other sections. Here the segmentation has a
purely functional derivation — is adaptive instead of genetic.
The similarly-formed and similarly-placed parts, are not
homologous in the same sense as are the appendages of a
phsenogamie axis or the limbs of an insect.

§ 210. In studying the remaining and highest sub-king-
dom of animals, it is important to recognize this radical dif-
ference in meaning between that likeness of parts which is
produced by likeness of modifying forces, and that likeness
of parts which is due to primordial identity of origin. On
our recognition of this difference depends the view we take
of certain doctrines that have long been dominant, and have
still a wide currency.

Among the Vertebrata, as among the Mollusca, homogene-
sis is universal. The two sub-kingdoms are like one another
and unlike the remaining sub-kingdoms in this, that in all
the types they severally include, a single fertilized ovum pro-
duces only a single individual. It is true that as the eggs of
certain gasteropods occasionally exhibit spontaneous fission

* Prof. MacBride corrects this statement by saying that " The ctenidia or
gills (which in Mollusca generally are represented only by a single pair) are
here represented by a large number of pairs ; they do not, however, correspond
in either number or position to the shell plates." It may, I think, be con-
tended that if these had any morphological significance, they would not differ
in arrangement from the shell plates, and would not be limited to this special
type of Mollusc.

118 MORPHOLOGICAL DEVELOPMENT.

of the vitelline mass, which may or may not result in
formation of two individuals; so among vertebrate animals
we now and then meet with double monsters, which appear
to imply such a spontaneous fission imperfectly carried out.
But these anomalies serve to render conspicuous the fact,
that in both these sub-kingdoms the normal process is the
integration of the whole germ-mass into a single organism,
which at no phase of its development displays any tendency
to separate into two or more parts.

Equally as throughout the Mollusca, there holds through-
out the Vertebrata the correlative fact, that not even in its low-
est any more than in its highest types, is the body divisible
into homologous segments. The vertebrate animal, under its
simplest as under its most complex form, is like the mollusc-
ous animal in this, that you cannot cut it into transverse
slices, each of which contains a digestive organ, a respiratory
organ, a reproductive organ, &c. The organs of the least-
developed fish as well as those of the most-developed
mammal, form but a single physiological whole; and they
show not the remotest trace of having ever been divisible
into two or more physiological wholes. That segmentation
which the vertebrate animal usually exhibits throughout
part of its organization; is the same in origin and meaning
as the segmentation of a Chitons shell ; and no more implies
in the vertebrate animal a composite structure, than do the
successive pairs of branchiae of the Doto, or the transverse
rows of branchiae in the Eolis, imply composite structure in
the molluscous animal. To some this will seem a very ques-
tionable proposition; and had we no evidence beyond that
which adult vertebrate animals of developed types supply, it
would be a proposition not easy to substantiate. But abundant
support for it is to be found in the structure of the vertebrate
embryo, and in the comparative morphology of the Vertebrata
in general.

Embryologists teach us that the primordial relations of
parts are most clearly displayed in the early stages of evo-

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 119

lution; and that they generally become partially or com-
pletely disguised in its later stages. Hence, were the verte-
brate animal on the same level as the annulose animal in
degree of composition — did it similarly consist of segments
which are homologous in the sense that they are the prox-
imate units of composition; we ought to find this funda-
mental fact most strongly marked at the outset. As in
the annelid-embryo the first conspicuous change is the
elongation and division into segments, by constrictions that
encircle the whole body; and as in the arthropod embryo
the blastoderm becomes marked out transversely into pieces
which extend themselves round the yelk before the internal
organization has made any appreciable progress; so in the
embryo of every vertebrate animal, had it an analogous com-
position, the first decided change should be a segmentation
implicating the entire mass. But it is not so. Sundry im-
portant differentiations occur before any divisions begin to
show themselves. There is the defining of that elongated,
elevated area with its longitudinal groove, which becomes the
seat of subsequent changes; there is the formation of the
notochord lying beneath this groove; there is the growth
upwards of the boundaries of the groove into the dorsal
laminae, which rapidly develop and fold over in the region of
the head. Eathke, as quoted and indorsed by Prof. Huxley,
describes the subsequent changes as follows : — " The gelatin-
ous investing mass, which, at first, seems only to constitute
a band to the right and to the left of the notochord forms
around it, in the further course of development, a sheath,
which ends in a point posteriorly. Anteriorly, it sends out
two processes which underlie the lateral parts of the skull,
but very soon coalesce for a longer or shorter distance.
Posteriorly, the sheath projects but little beyond the noto-
chord ; but, anteriorly,, for a considerable distance, as far as the
infundibulum. It sends upwards two plates, which embrace
the future central parts of the nervous system laterally, prob-
ably throughout their entire length." That is to say, in the

120 MORPHOLOGICAL DEVELOPMENT.

Vertebrata the first step is the marking out on the blastodei
of an integrated structure, within which segments subse-
quently appear. When these do appear, they are for some
time limited to the middle region of the spinal axis; and no
more then than ever after, do they implicate the general
mass of the body in their transverse divisions. On the
contrary, before vertebral segmentation has made much pro-
gress, the rudiments of the vascular system are laid down in
a manner showing no trace of any primordial correspondence
of its parts with the divisions of the axis. Equally

at variance with the belief that the vertebrate animal is
essentially a series of homologous parts, is the heterogeneity
which exists among these parts on their first appearance.
Though in the head of an adult articulate animal there is
little sign of divisibility into segments like those of the
body; yet such segments, with their appropriate ganglia and
appendages, are easily identifiable in the articulate embryo.
But in the Vertebrata this antithesis is reversed. At the
time when segmentation has become decided in the dorsal
region of the spine, there is no trace of segments in the parts
which are to form the skull — nothing whatever to suggest that
the skull is being formed out of divisions homologous with
vertebrae.* And minute observation no more discloses any
such homology than does general appearance. " Remak,"
says Prof. Huxley, "has more fully proved than any other
observer, the segmentation into ' urwirbel/ or proto-vertebraa,
which is characteristic of the vertebral column, stops at
the occipital margin of the skull — the base of which, before
ossification, presents no trace of that segmentation which
occurs throughout the vertebral column."

Consider next the evidence supplied by comparative mor-
phology. In preceding sections (§§ 206, 208,) it has been

* Though it is alleged that at a later stage the posterior part of the skull
is formed by fusion of divisions which are assumed to represent vertebrae, yet
it is admitted that the anterior part of the skull never shows any signs of
such division. Moreover in both parts the bones show no trace of primitive
segmentation.

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 121

shown that among annulose animals, the divisibility into
homologous parts is most clearly demonstrable in the lowest
fcypes. Though in decapodous Crustaceans, in Insects, in
Arachnids, there is difficulty in identifying some or many of
the component somites; and though, when identified, they
display only partial correspondences; yet on descending to
Annelids, the composition of the entire body out of such
somites becomes conspicuous, and the homology between each
somite and its neighbours is shown by the repetition of one
another's structural details, as well as by their common
gemmiparous origin: indeed, in some cases we have the
homology directly demonstrated by seeing a somite of the
body transformed into a head. If, then, a vertebrate animal
had a segmental composition of kindred nature, we ought to
find it most clearly marked in the lowest Vertebrata and
most disguised in the highest V ertebrata. But here, as before,
the fact is just the reverse. Among the Vertebrata of
developed type, such segmentation as really exists remains
conspicuous — is but little obscured even in parts of the spinal
column formed out of integrated vertebrae. Whereas in the
undeveloped vertebrate type, segmentation is scarcely at all
traceable.* The Ampliioxus, Fig. 191, is not only without

ossified vertebrae; not only is it without cartilaginous repre-
sentatives of them; but it is even without anything like
distinct membranous divisions. The spinal column exists
as a continuous notochord: the only signs of incipient seg-
mentation being given by its membranous sheath, in the
upper part of which " quadrate masses of somewhat denser
* See note at the end of the chapter.

122 MORPHOLOGICAL DEVELOPMENT.

tissue seem faintly to represent neural spines." Moreover,
throughout sundry groups of fishes and amphibians, the
segmentation remains very imperfect: only certain peri-
pheral appendages of the vertebrae becoming defined and
solidified, while in place of the bodies of the vertebras there
still continues the undivided notochord. Thus, instead of
being morphologically composed of vertebral segments, the
vertebrate animal in its primitive form is entirely without
vertebral segments; and vertebral segments begin to appear
only as we advance towards developed forms. Once

more, evidence equally adverse to the current hypothesis
meets us on observing that the differences between the parts
supposed to be homologous, are as great at first as at last.
Did the vertebrate animal primordially consist of homo-
logous segments from snout to tail; then the segments said
to compose the skull ought, in the lowest Vertebrata, to show
themselves much more like the remaining segments than
they do in the highest Vertebrata. But they do not. Fishes
have crania made up of bones that are no more clearly
arrangeable into segments like vertebras, than are the cranial
bones of the highest mammal. Nay, indeed, the case is
much stronger. The simplest fish possessing a skeleton,
has a cranium composed of cartilage that is not segmented
at all !

Besides being inconsistent with the leading truths of
Embryology and Comparative Morphology, the hypothesis of
Goethe and Oken is inconsistent with itself. The facts
brought forward to show that there exists an archetypal
vertebra, and that the vertebrate animal is composed of
archetypal vertebras arranged in a series, and severally modi-
fied to fit their positions — these facts, I say, so far from
proving as much, suffice, when impartially considered, to dis-
prove it. No assigned, nor any conceivable, attribute of the
supposed archetypal vertebra is uniformly maintained. The
parts composing it are constant neither in their number, nor
in their relative positions, nor in their modes of ossification,

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 123

nor in the scparateness of their several individualities when
present. There is no fixity of any one element, or con-
nexion, or mode of development, which justifies even a
suspicion that vertebras are modelled after an ideal pattern.
To substantiate these assertions here would require too much
space, and an amount of technical detail wearisome to the
general reader. The warrant for them will be found in a
criticism on the osteological works of Prof. Owen, originally
published in the British and Foreign Medico-Chirurgical Re-
view for Oct. 1858. This criticism I add in the Appendices,
for the convenience of those who may wish to study the
question more fully. ( See Appendix B. )

Everything, then, goes to show that the segmental compo-
sition which characterises the apparatus of external relation
in most Vertebrata, is not primordial or genetic, but function-
ally determined or adaptive. Our inference must be that the
vertebrate animal is an aggregate of the second order, in
which a relatively superficial segmentation has been pro-
duced by mechanical intercourse with the environment. We
shall hereafter see that this conception leads us to a con-
sistent interpretation of the facts — shows us why there has
arisen such unity in variety as exists in every vertebral column,
and why this unity in variety is displayed under countless
modifications in different skeletons.*

§ 211. On glancing back at the facts brought together in
these two chapters, we see it to be probable that there has gone
on among animals a process like that which we saw reason
to think has gone on among plants. Minute aggregates of
those physiological units which compose living protoplasm,

* A qualifying fact should be named. When the production of vertebral
segments has become constitutionally established, so that there is an innate
tendency to form them, there arises a liability to form supernumerary ones ;
and this, from time to time recurring, may lengthen the series, as in the
body of a snake or the neck of a swan. This qualification, however, affects
equally the hypothesis of an ideal type and the hypothesis of mechanical
genesis.

124: MORPHOLOGICAL DEVELOPMENT.

exist as Protozoa: some of them incoherent, indefinite, and
almost homogeneous, and others of them more coherent, de-
finite, and heterogeneous. By union of these nucleated parti-
cles of sarcode, are produced various indefinite aggregates of
the second order — Sponges, Polycytharia, Foraminifers, &c. ;
in which the compound individuality is scarcely enough
marked to subordinate the primitive individualities. But in
other types, as in Hydra, the lives of the morphological
units are in a considerable degree, though not wholly, merged
in the life of the integrated body they form. As the primary
aggregate, when it passes a certain size, undergoes fission or
gemmation; so does the secondary aggregate. And as on
the lower stage so on the higher, we see cases in which the
gemmiparously-produced individuals part as soon as formed,
and other cases in which they continue united, though in
great measure independent. This massing of secondary aggre-
gates into tertiary aggregates, is variously carried on among
the Ilydrozoa, the Actinozoa, the Polyzoa, and the Tunicata.
In most of the types so produced, the component individu-
alities are very little subordinated to the individuality of the
composite mass — there is only physical unity and not physio-
logical unity; but in certain of the oceanic Ilydrozoa, the
individuals are so far differentiated and combined as very
much to mask them. Forms showing us clearly the transi-
tion to well-developed individuals of the third order, are not
to be found. Nevertheless, in the great sub-kingdom Annu-
losa, there are traits of structure, development, and mode of
multiplication, which go far to show that its members are
such individuals of the third order; and in the relations to
external conditions involved by the mode of union, we find
an adequate cause for that obscuration of the secondary indi-
vidualities which we must suppose has taken place. The
two other great sub-divisions, Mollusca and Vertebrata,
between the lower members of which there are suggestive
points of community, present us only with aggregates of the
second order, that have in many cases become very large and

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 125

very complex. We find in them no trace of the union of
gemmiparously-produced individuals. Neither the molluscous
nor the vertebrate animal shows the faintest trace of a seg-
mentation affecting the totality of its structure; and we see
good grounds for concluding that such segmentation as ex-
ceptionally occurs in the one and usually occurs in the other,
is superinduced.

[Note : — A critic calls in question the statement on p. 121
respecting the Amphioxus. At the outset, however, he
admits that in the Amphioxus "the central nervous system
and the notochord are not segmented." In the Annelid,
however, the central nervous system is segmented, and there
is segmentation of the part which, as a supporting structure,
is analogous to the notochord in respect of function — the
outer part which represents the exo-skeleton in contrast to
the endo-skeleton. He goes on to say that "the gut is not
involved [in the segmentation] and exhibits in Amphioxus
just as it does in worms differentiations entirely independent
of the segmentation of the mesoblast." Part of this state-
ment is, I think, not congruous with all the facts. In Proto-
drilus, one of the lowest of the Archiannelida, " the intestine
is moniliform, there being a constriction between each seg-
ment " and the next. (Shipley.) Complete segmentation of
the intestine is obviously impossible, since, were the canal
divided into portions by septa, no food could pass. But the
fact that the gut has these successive expansions and con-
strictions, corresponding to the successive segments, and
giving to each segment a partially-separate stomach, shows
that segmentation has gone as far as consists with the
carrying on of the lives of the segments. No such partial
segmentation exists in the Amphioxus. Thus, then, three
fundamental structures — the directive structure, the sup-
porting structure, and the alimentary structure — are respec-
tively simple in the lowest vertebrate and segmented, or

126 MORPHOLOGICAL DEVELOPMENT.

partially segmented, in the lowest Annelid. Again, while it
is said that the gill-clefts exhibit segmentation, it is admitted
that this has no relevance to any constitutional segmenta-
tion: "they are segmented on a plan of their own" irre-
spective of other organs. Another allegation is that the
ovaries of Amphioxus are segmented. Their segmentation,
however, like that of the gills, is isolated, and may be con-
sidered as illustrating those repetitions of like parts seen in
supernumerary vertebrae in various creatures — a repetition
which becomes habitual if the resulting structure is advan-
tageous to the species. On the statement that while the
Amphioxus has no rudiments of a renal system the Elasmo-
branch embryo has such rudiments, which are as distinctly
segmented as the nephridia of a worm, two comments may
be made. The first is that if in these Vertebrates the
nephridia bear a relation to the general structure like that
which they do in Annelids, then one would expect to find
the segmental arrangement shown in the lowest type, as in
Annelids, rather than in a type considerably advanced in
development. Should it be replied that in the Amphioxus
an excretory system had not yet arisen, though one is re-
quired for the higher organization of an Elasmobranch, then
the answer may be that since the segmental arrangement in
the Elasmobranch corresponds with that of the myotomes, it
has no reference to any primordial segmentation, since the
myotomes have been functionally generated. The second
comment is that whereas the nephridia of the Annelid have
independent external openings, the nephridia in the Elasmo-
branch have not. These discharge their secretions into cer-
tain general tubes of exit common to them all; showing that
each of them, instead of being a member of a partially inde-
pendent structure, is united with others in subordination to
a general structure. That is to say, the segmentations are
far from being parallel in their essential natures. The asser-
tion accompanying these criticisms, that there is "no differ-
ence in principle between the segmentation of Amphioxus

THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 127

and Annelid " is difficult to reconcile with the visible con-
trast between the two. Whatever local segmentations there
are in an Amphioxus appear to me quite unlike "in prin-
ciple " to those which an Annelid exhibits. Could its
portion of gut be duly supplied with nutriment, the segment
of a low Annelid could carry on its vital functions independ-
ently. In the parts of the Amphioxus we see nothing
approaching to this. Cut it into transverse sections and no
one of them contains anything like the assemblage of struc-
tures required for living. The Amphioxus is a physiological
whole, and in that respect differs radically from the Annelid,
each segment of which is in chief measure a physiological
whole. No occurrence of local segmentation in the Am-
phioxus can obliterate this fundamental contrast.

An accompanying contrast tells the same story. On as-
cending from the lowest to the highest annulose types we
see a progressing integration, morphological and physiologi-
cal; so that whereas in a low annelid the successive parts
are in large measure independent in their structures and in
their lives, in a high arthropod, as a crab, most of the parts
have lost their individualities and have become merged in a
consolidated organism with a single life. Quite otherwise is
it in the vertebrate series. Its lowest member is at the very
outset a complete morphological and physiological whole, and
the formation of those serial parts which some think analo-
gous to the serial parts of an Annelid, begins at a later stage
and becomes gradually pronounced. That is to say, the course
of transformation is reversed.]

CHAPTER VI.

MORPHOLOGICAL DIFFERENTIATION IN PLANTS.

§ 212. While, in the course of their evolution, plants and
animals have displayed progressive integrations, there have
at the same time gone on progressive differentiations of the
resulting aggregates, both as wholes and in their parts.
These differentiations and the interpretations of them, form
the second class of morphological problems.

We commence as before with plants. We have to con-
sider, first, the several kinds of modification in shape they
have undergone; and, second, the relations between these
kinds of modification and their factors. Let us glance at the
leading questions that have to be answered.

§ 213. Irrespective of their degrees of composition, plants
may, and do, become changed in their general forms. Are
their changes capable of being formulated? The inquiry
which meets us at the outset is — does a plant's shape admit
of being expressed in any universal terms? — terms that
remain the same for all genera, orders, and classes.

After plants considered as wholes, have to be considered
their proximate components, which vary with their degrees
of composition, and in the highest plants are what we call
branches. Is there any law traceable among the contrasted
shapes of different branches in the same plant? Do the rela-
tive developments of parts in the same branch conform to
any law? And are these laws, if they exist, allied with one
128

MORPHOLOGICAL DIFFERENTIATION IN PLANTS. 129

another and with that to which the shape of the whole plant
conforms ?

Descending to the components of these components, which
in developed plants we distinguish as leaves, there meet us
kindred questions respecting their relative sizes, their rela-
tive shapes, and their shapes as compared with those of
foliar organs in general. Of their morphological differentia-
tions, also, it has to be asked whether they exemplify any
truth that is exemplified by the entire plant and by its larger
parts.

Then, a step lower, we come down to those morphological
units of which leaves and fronds consist; and concerning
these arise parallel inquiries touching their divergences from
one another and from cells in general.

The problems thus put together in several groups cannot
of course be rigorously separated. Evolution presupposes
transitions which make all such classings more or less con-
ventional; and adherence to them must be subordinate to
the needs of the occasion.

§ 214. In studying the causes of the morphological differ-
entiations thus divided out and prospectively generalized,
we shall have to bear in mind several orders of forces which
it will be well briefly to specify.

Growth tends inevitably to initiate Changes in the shape
of any aggregate, by altering both the amounts of the inci-
dent forces and the forces which the parts exert on one
another. With the mechanical actions this is obvious.
Matter that is sensibly plastic cannot be increased in mass
without undergoing a change in its proportions, consequent
on the diminished ratio of its cohesive force to the force of
gravitation. With the physiological actions it is equally
obvious. Increase of size, other things equal, alters the rela-
tions of the parts to the material and dynamical factors of
nutrition ; and by so affecting differently the nutrition of
different parts, initiates further changes of proportions.
55

130 MORPHOLOGICAL DEVELOPMENT.

In plants of the third order it is thus with the proximate
components: they are subject to mutual influences that
are unlike one another and are continually changing. The
earlier-formed units become mechanical supporters of the
later-formed units, and so experience modifying forces from
which the later-formed units are exempt. Further, these
elder units simultaneously begin to serve as channels through
which materials are carried to and from the younger units —
another cause of differentiation that goes on increasing in in-
tensity. Once more, there arise ever-strengthening contrasts
between the amounts of light which fall upon the youngest or
outermost units and the eldest or innermost units; whence
result structural contrasts of yet another kind. Evidently,
then, along with the progressive integration of cells into
fronds, of fronds into axes, and of axes into plants still more
composite, there come into play sundry causes of differen-
tiation which act on the whole and on each of its parts,
whatever their grade. The forces to be overcome, the forces
to be utilized, and the matters to be appropriated, do not
remain the same in their proportions and modes of action for
any two members of the aggregate: be they members of the
first, second, third, or any other order.

§ 215. Nor are these the only kinds and causes of hetero-
geneity which we 'have to consider. Beyond the more
general changes produced in the relative sizes and shapes of
plants and their parts by progressive aggregation, there are
the more particular changes determined by the more par-
ticular conditions.

Plants as wholes assume unlike attitudes towards their
environments; they have many ways of articulating their
parts with one another; they have many ways of adjusting
their parts towards surrounding agencies. These are causes
of special differentiations additional to those general differen-
tiations that result from increase of mass and increase of com-
position. In each part considered individually, there arises

MORPHOLOGICAL DIFFERENTIATION IN PLANTS. 131

a characteristic shape consequent on that relative position
towards external and internal forces, which the mode of
growth entails. Every member of the aggregate presents
itself in a more or less peculiar way towards the light, towards
the air, and towards its point of support; and according to
the relative homogeneity or heterogeneity in the incidence of
the agencies thus brought to bear on it, will be the relative
homogeneity or heterogeneity of its shape.

§ 216. Before passing from this a priori view of the
morphological differentiations which necessarily accompany
morphological integrations, to an a posteriori view of them, it
seems needful to specify the meanings of certain descriptive
terms we shall have to employ.

Taking for our broadest division among forms, the regular
and the irregular, we may divide the latter into those which
are wholly irregular and those which, being but partially
irregular, suggest some regular form to which they approach.
By slightly straining the difference between them, two cur-
rent words may be conveniently used to describe these sub-
divisions. The entirely irregular forms we may class as
asymmetrical — literally as forms without any equalities of
dimensions. The forms which approximate towards regu-
larity without reaching it, we may distinguish as unsymmetri-
cal: a word which, though it asserts inequality of dimensions,
has been associated by use rather with such slight inequality
as constitutes an observable departure from equality.

Of the regular forms there are several classes, differing in
the number of directions in which equality of dimensions is
repeated. Hence results the need for names by which sym-
metry of several kinds may be expressed.

The most regular of figures is the sphere: its dimensions
are the same from centre to surface in all directions; and if
cut by any plane through the centre, the separated parts are
equal and similar. This is a kind of symmetry which stands
alone, and will be hereafter spoken of as spherical symmetry.

132 MORPHOLOGICAL DEVELOPMENT.

When a sphere passes into a spheroid, either prolate or
©Mate, there remains but one set of planes that will divide it
into halves, which are in all respects alike; namely, the
planes in which its axis lies, or which have its axis for their
line of intersection. Prolate and oblate spheroids may
severally pass into various forms without losing this pro-
perty. The prolate spheroid may become egg-shaped or pyri-
form, and it will still continue capable of being divided into
two equal and similar parts by any plane cutting it down
its axis; nor will the making of constrictions deprive it of
this property. Similarly with the oblate spheroid. The
transition from a slight oblateness, like that of an orange,
to an oblateness reducing it nearly to a flat disc, does not
alter its divisibility into like halves by every plane passing
through its axis. And clearly the moulding of any such
flattened oblate spheroid into the shape of a plate, leaves it
as before, symmetrically divisible by all planes at right
angles to its surface and passing through its centre. This
species of symmetry is called radial symmetry. It is familiar-
ly exemplified in such flowers as the daisy, the tulip, and the
dahlia.

From spherical symmetry, in which we have an infinite
number of axes through each of which may pass an infinite
number of planes severally dividing the aggregate into equal
and similar parts; and from radial symmetry, in which we
have a single axis through which may pass an infinite number
of planes severally dividing the aggregate into equal and
similar parts; we now turn to bilateral symmetry, in which
the divisibility into equal and similar parts becomes much
restricted. Noting, for the sake of completeness, that there is
a sextuple bilateralness in the cube and its derivative fori
which admit of division into equal and similar parts by planes
passing through the three diagonal axes and by planes passing
through the three axes that join the centres of the surfaces,
let us limit our attention to the three kinds of bilateralness
which here concern us. The first of these is triple

MORPHOLOGICAL DIFFERENTIATION IN PLANTS. 133

bilateral symmetry. This is the symmetry of a figure having
three axes, at right angles to one another, through each of
which there passes a single plane that divides the aggregate
into corresponding halves. A common brick will serve as an
example; and of objects not quite so simple, the most familiar
is that modern kind of spectacle-case which is open at both
ends. This may be divided into corresponding halves along
its longitudinal axis by cutting it through in the direction
of its thickness, or by cutting it through in the direction of
its breadth; or it may be divided into corresponding halves
by cutting it across the middle. Of objects which

illustrate double bilateral symmetry, may be named one of
those boats built for moving with equal facility in either
direction, and therefore made alike at stem and stern. Ob-
viously such a boat is separable into equal and similar parts
by a vertical plane passing through stem and stern; and it is
also separable into equal and similar parts by a vertical plane
cutting it amidships. To exemplify single bilateral

symmetry it needs but to turn to the ordinary boat of which
the two ends are unlike. Here there remains but the one
plane passing vertically through stem and stern, on the oppo-
site sides of which the parts are symmetrically disposed.

These several kinds of symmetry as placed in the foregoing
order, imply increasing heterogeneity. The greatest uni-
formity in shape is shown by the divisibility into like parts
in an infinite number of infinite series of ways; and the
greatest degree of multiformity consistent with any regularity,
is shown by the divisibility into like parts in only a single
way. Hence, in tracing up organic evolution as displayed in
morphological differentiations, we may expect to pass from
the one extreme of spherical symmetry, to the other extreme
of single bilateral symmetry. This expectation we shall find
to be completely fulfilled.

CHAPTER VII.

i

THE GENERAL SHAPES OF PLANTS.

§ 217. Among protophytes those exemplified by Pleuro-
coccus vulgaris are by general consent considered the simplest.
As shown in Fig. 1, they are globular cells presenting no
obvious differentiation save that between inner and outer
parts. Their uniformity of figure coexists with a mode of
life involving the uniform exposure of all their sides to
incident forces. For though each individual may have its
external parts differently related to environing agencies, yet
the new individuals produced by spontaneous fission, whether
they part company or whether they form clusters and are
made polyhedral by mutual pressure, have no means of main-
taining parallel relations of position among their parts.
On the contrary, the indefiniteness of the attitudes into
which successive generations fall, must prevent the rise of
any unlikeness between one portion of the surface and
another. Spherical symmetry continues because, on the
average of cases, incident forces are equal in all directions.

Other orders of Protophyta have much more special
forms, along with much more special attitudes: their ho-
mologous parts maintaining, from generation to generation,
unlike relations to incident forces. The Desmidiacece anc
Diatomacece, of which Figs. 2 and 3 show examples, severally
include genera characterized by triple bilateral symmetry.
A Navicula is divisible into corresponding halves by a trans-
134

THE GENERAL SHAPES OF PLANTS. 135

verse plane and by two longitudinal planes — one cutting its
valves at right angles and the other passing between its
valves. The like is true
of those numerous trans- q a
versely-constricted forms of (g) vJs/
Desmidiacece, exemplified by (&k

the second of the individuals V_^

represented in Fig. 2. If now we ask how a Navicula is
related to its environment, we see that its mode of life ex-
poses it to three different sets of forces: each set being
resolvable into two equal and opposite sets. A Navicula
moves in the direction of its length, with either end. foremost.
Hence, on the average, its ends are subject to like actions
from the agencies to which its motions subject it. Further,
either end while moving exposes its right and left sides to
amounts of influence which in the long run must be equal.
If, then, the two ends are not only like one another, but have
corresponding right and left sides, the symmetrical distribu-
tion of parts answers to the symmetrical distribution of
forces. Passing to the two edges and the two flat surfaces,
we similarly find a clue to their likenesses and differences in
their respective relations to the things around them. These
locomotive protophytes move through the entangled masses
of fragments and fibres produced by decaying organisms and
confervoid growths. The interstices in such matted accu-
mulations are nearly all of them much longer in one dimen-
sion than in the rest — form crevices rather than regular
meshes. Hence, a small organism will have much greater
facility of insinuating itself through this debris, in which it
finds nutriment, if its transverse section is flattened instead
of square or circular. And while we see how, by survival of
the fittest, a flattened form is likely to be acquired by
diatoms having this habit; we also see that likeness will be
maintained between the two flat surfaces and between the
two edges. For, on the average, the relations of the two flat
surfaces to the sides of the openings through which the

136

MORPHOLOGICAL DEVELOPMENT.

diatom passes, will be alike; and so, too, on the average,
will be the relations of the two edges. In desmids

of the type exemplified by the second individual in Fig. 2,
a kindred equalization of dimensions is otherwise insured.
There is nothing to keep one of the two surfaces uppermost
rather than the other; and hence, in the long succession of
individuals, the two surfaces are sure to be similarly exposed
to light and agencies in general. When to this is added the
fact that spontaneous fission occurs transversely in a constant
way, it becomes manifest that the two ends, while they are
maintained in conditions like one another, are maintained in
conditions unlike those of the two edges. Here then, as
before, triple bilateral symmetry in form, coexists with a triple
bilateral symmetry in the average distribution of actions.

Still confining our attention to aggregates of the fir
order, let us next note what results when the two ends are
permanently subject to different conditions. The fixed
unicellular plants, of which examples are given in Figs. 4,
5, and 6, severally illustrate the contrast in shape arising

between the part that is applied to the supporting surfac
and the part that extends into the surrounding medium.
These two parts which are the most unlike in their relations
to incident forces, are the most unlike in the forms. Observe,
next, that the part which lifts itself into the water or air, is
more or less decidedly radial. Each outward growing tubule
of Codium adhwrens, Fig. 4, has its parts disposed with some
regularity around its axis; the upper stem and spore-vessel

THE GENERAL SHAPES OP PLANTS. 137

of Botrydium, Fig. 5, display a lateral growth that is approxi-
mately equal in every direction; and the stems of the
Mucor, Fig. 6, shoot up with an approach to evenness on all
sides. Plants of this low type are naturally very variable
in their modes of growth : each individual being greatly modi-
fied in form by its special circumstances. But they neverthe-
less show us a general likeness between parts exposed to like
forces, as well as a general unlikeness between parts exposed
to unlike forces.

Eespecting the forms of these aggregates of the first order,
it has only to be added that they are asymmetrical where
there is total irregularity in the incidence of forces. We
have an example in the indefinitely contorted and branched
shape of a fungus-cell, growing as a mycelium among the
particles of soil or through the interstices of organic tissue.

§ 218. Ee-illustrations of the general truths which the
forms of these vegetal aggregates of the first order display,
are furnished by vegetal aggregates of the second order. The
equalities and inequalities of growth in different directions,
prove to be similarly related to the equalities and inequalities
of environing actions in different directions.

Of spherical symmetry an instance occurs in Eudorina
elegans. The ciliated cells are here so united as to produce a
small, mulberry-shaped, hollow ball which, being similarly
conditioned on all sides, shows no unlikenesses of structure.
An allied form, however, Volvox globator, presents a highly
instructive, though very trifling, modification. It is not
absolutely homogeneous in its structure and is not absolutely
homogeneous in its motions. The waving cilia of its compo-
nent cells have fallen into such slight heterogeneities of
action as to cause rotation in a constant direction; and
along with a fixed axis of rotation there has arisen a fixed
axis of progression. A concomitant fact is that the cells
of the colony exhibit an appreciable differentiation in relation
to the fixed axis. There is an incipient divergence from

138

MORPHOLOGICAL DEVELOPMENT.

spherical uniformity along with this slight divergence from
uniformity of conditions.

Vegetal aggregates of the second order are usually fixed
locomotion is exceptional. Fixity implies that the surface
of attachment is differently circumstanced from the free sur-
face. Hence we may expect to find, as we do find, that
among these rooted aggregates of the second order, as among
those of the first order, the primary contrast of shape is
between the adherent part and the loose part. Sea-weeds
variously exemplify this. In some the fronds are very
irregular and in some tolerably regular; in some the form is
pseudo-foliar and in some pseud-axial; but differing though
they do in these respects, they agree in having the end
which is attached to a solid body unlike the other end. The
same truth is seen in such secondary aggregates as the com-
mon Agarics, or rather in their immensely-developed organs
of fructification. A puff-ball, Fig. 192, presents no other
obvious unlikeness of parts than that between its under and
upper surfaces. So too with the stalked kinds that frequent
our woods and pastures. In the types which Figs. 193,
194, 195, delineate, the unlikenesses between the rooted
ends and the expanded ends, as well as between the under
and upper surfaces of the expanded ends, are obviously
related to this fundamental contrast of conditions. Nor is
this relation less clearly displayed in the sessile fungi which
grow out from the sides of trees, as shown at a, b, Fig. 196.

fS3

4' <?

That which is common to this and the preceding types, is the
contrast between the attached end and the free end.

THE GENERAL SHAPES OF PLANTS. I39

From what these forms have in common, let us turn to
that which they have not in common, and observe the causes
of the want of community. A puff-ball shows us in the
simplest way, the likeness of parts accompanying likeness of
conditions, along with the unlikeness of parts accompanying
unlikeness of conditions. For while, if we cut vertically
through its centre, we find a difference between top and
bottom, if we cut horizontally through its centre, we find no
differences among its several sides. Being, on the average of
cases, similarly related to the environment all round, it
remains the same all round. The radial symmetry of the
mushroom and other vertically-growing fungi, illustrates
this connexion of cause and effect still better. But now
mark what happens in the group of Agaricus noli-tangere>
shown in Fig. 195. Radially symmetrical as is the type, and
radially symmetrical as are those centrally-placed individuals
which are equally crowded all round, we see that the peri-
pheral individuals, dissimilarly circumstanced on their outer
sides and on their sides next the group, have partially
changed their radial symmetry into bilateral symmetry. It
is no longer possible to make two corresponding halves by
any vertical plane cutting down through the pileus and the
stem; but there is only one vertical plane that will thus pro-
duce corresponding halves — the plane on the opposite sides
of which the relations to the environment are alike. And
then mark that the divergence from all-sided symmetry
towards two-sided symmetry, here caused in the individual
by special circumstances, is characteristic of the race where
the habits of the race constantly involve two-sidedness of
conditions. Besides being exemplified by such comparatively
undifferentiated types as certain Poly port, Fig. 196, a, 0, this
truth is exemplified by members of the genus just named.
In Agaricus liorizontalis, Fig. 196, c, we have a departure
from radial symmetry that is conspicuous only in the form
of the stem. A more decided bilateralness exists in A. sub-
palmatus, shown in elevation at d and in section at d'. And

140 MORPHOLOGICAL DEVELOPMENT.

Lentinus flab elli for mis, of which e and e' are different views,
exhibits complete bilateralness — a bilateralness in which
there is the greatest likeness of the parts that are most simi-
larly conditioned, and the greatest unlikeness of the parts
that are most dissimilarly conditioned.

Among plants of the second order of composition, it will
suffice to note one further class of facts which are the con-
verse of the foregoing and have the same implications. These
are the facts showing that along with habitual irregularity in
the relations to external forces, there is habitual irregularity
in the mode of growth. Besides finding such facts among
Thallophytes, as in the tubers of underground fungi and in
the creeping films of sessile lichens, which severally show us
variations of proportions obviously caused by variations in
the amounts of the influences on their different sides, we also,
among Archegoniates of inferior types, find irregularities of
form along with irregularities in environing actions. The
fronds of the Marchantiacece or such Jungermanniacece as are
shown in Figs. 41, 42, 43, illustrate the way in which each
lowly-organized aggregate of the second order, not individuated
by the mutual dependence of its parts, has its form deter-
mined by the balance of facilities and resistances which each
side of the frond meets with as it spreads.

§ 219. Among plants displaying integration of the third
degree, and among plants still further compounded, these
same truths are equally manifest. In the forms of such plants
we see primary contrasts and secondary contrasts which, no
less clearly than the foregoing, are related to contrasts of
conditions.

That flowering plants from the daisy up to the oak, have
in common the fundamental unlikeness between the upward
growing part and the downward growing part; and that
this most marked unlikeness corresponds with the most
marked unlikeness between the two parts of their environ-
ment, soil and air; are facts too conspicuous to be named

THE GENERAL SHAPES OF PLANTS. 141

were they not important items in the argument. More
instructive perhaps, because less familiar, is the fact that we
miss this extreme contrast in flowering plants which have not
their higher and lower portions exposed to conditions thus
extremely contrasted. A parasite like the Dodder, growing
in entangled masses upon other plants, from which it sucks
the juices, is not thus divisible into two strongly-distinguished
halves.

Leaving out of consideration the difference between the
supporting part and the supported part in phamogams, and
looking at the supported part only, we observe between its
form and the habitual incidence of forces, a relation like that
which we observed in the simpler plants. Phsenogams that
are practically if not literally uniaxial, and those which de-
velop their lateral axes only in the shape of axillary flowers,
when uninterfered with commonly send up vertical stems
round which the leaves and flowers are disposed with a more
or less decided radial symmetry. Gardens and fields supply
us with such instances as the Tulip and the Orchis; and, on
a larger scale, the Palms and the Aloes are fertile in ex-
amples. The exceptions, too, are instructive. Besides the
individual divergences arising from special interferences, there
are to be traced general divergences where the habits of the
plants expose them to general interferences in anything
approaching to constant ways. Plants which, like the Fox-
glove, have spikes of flowers that are borne on flexible foot-
stalks, have their flowers habitually bent round to one face of
the stem: an unlikeness of distribution probably caused by
unlikeness in the relation to the Sun's rays. The wild Hya-
cinth, too, with stem so flexible that its upper part droops,
shows us how a consequent difference in the action of gravity
on the flowers, causes them to deviate from their typically-
radial arrangement towards a bilateral arrangement.

Much more conspicuous are these general and special rela-
tions of form to general and special actions in the environ-
ment, among phasnogams that are multiaxial. That when

142 MORPHOLOGICAL DEVELOPMENT.

standing alone, and in places where the winds do not injure
them nor adjacent things shade them, shrubs and trees develop
with tolerable evenness on all sides, is an obvious truth.
Equally obvious is the truth that, when growing together in a
wood, and mutually interfered with on all sides, trees still
show obscurely radial distributions of parts; though, under
such conditions, they have tall taper stems with branches
directed upwards — a difference of shape clearly due to the
different incidence of forces. And almost equally obvious is
the truth, that a tree of this same kind growing at the edge
of the wood, has its outer branches well developed and its
inner branches comparatively ill-developed. Fig. 197, which

t98 V 1S>9

inaccurately represents this difference, will serve to make it
manifest that while one of the peripheral trees can be cut
into something like two similar halves by a vertical plane
directed towards the centre of the wood — a plane on each side
of which the conditions are alike — it cannot be cut into simi-
lar halves by any other plane. A like divergence from an
indefinitely-radial symmetry towards an indefinitely-bilateral
symmetry, occurs in trees that have their conditions made
bilateral by growing on inclined surfaces. Two of the common
forms observable in such cases are given in Fig. 198. Here
there is divisibility into parts that are tolerably similar, by
a vertical plane running directly down the hill; but not by
any other plane. Then, further, there is the bilateralness,
similar in general meaning though differently caused, often
seen in trees exposed to strong prevailing winds. Almost

THE GENERAL SHAPES OF PLANTS. 143

every sea-coast has abundant examples of stunted trees which,
like the one shown in Fig. 199, have been made to deviate
from their ordinary equal growth on all sides of a vertical
axis, to a growth that is equal only on the opposite sides of a
vertical plane directed towards the wind's eye.

From among vegetal aggregates of the third order, we have
now only to add examples of the entirely asymmetrical form
which accompanies an entirely irregular distribution of inci-
dent forces. Creeping plants furnish such examples. They
show, both when climbing up vertical or inclined surfaces and
when trailing on the ground, that their branches grow hither
and thither as the balance of forces aids or opposes; and the
general outline is without symmetry of any kind, because
the environing influences have no kind of regularity in their
arrangement.

§ 220. Along with some unfamiliar facts, I have here set
down facts which are so familiar as to seem scarcely worth
noting. It is because these facts have become meaningless
to perceptions deadened by infinite repetitions of them, that
it is needful here to point out their meanings. Not alone for
its intrinsic importance has the unlikeness between the
attached ends and the free ends been traced among plants of
all degrees of integration. Nor is it simply because of the
significance they have in themselves, that instances have
been given of those varieties of symmetry and asymmetry
which the free ends of plants equally display : bewthey plants
of the first, second, third, or any higher order. Neither has
the only other purpose been that of showing how, in the radial
symmetry of some vegetal aggregates and the single bilateral
symmetry of others, there are traceable the same ultimate
principles as in the spherical symmetry and triple bilateral
symmetry of certain minute plants first described. But the
main object has been to present, under their simplest aspects,
those general laws of morphological differentiation which are
fulfilled by the component parts of each plant.

144 MORPHOLOGICAL DEVELOPMENT.

If organic form is determined by the distribution of forces,
and the approach in every case towards an equilibrium of
inner actions with outer actions; then this relation between
forms and forces must hold alike in the organism as a whole
in its proximate units, and in its units of lower orders. For-
mulas which express the shapes of entire plants in terms of
surrounding conditions, must be formulas which also express
the shapes of their several parts in terms of surrounding
conditions. If, therefore, we find that a plant as a whole is
radially symmetrical or bilaterally symmetrical or asymme-
trical, according as the incident forces affect it equally on all
sides of an axis, or affect it equally only on the opposite sides
of one plane, or affect it equally in no two directions; then,
we may expect that, in like manner, each member of a plant
will display radial symmetry where environing influences are
alike along many radii, bilateral symmetry where there is
bilateralness of environing influences, and unsymmetry or
asymmetry where there is partial or entire departure from a
balance of surrounding actions.

To show that this expectation is borne out by the facts,
will be the object of the following four chapters. Let us
begin with the largest parts into which plants are divisible;
and proceed to the successively smaller parts.

CHAPTER VIII.

THE SHAPES OF BRANCHES.

200

§221. Aggregates of the first order supply a few examples
of forms ramified in an approximately-regular manner, under
conditions which subject their parts to approximately-regu-
lar distributions of forces. Some unicellular Algce, becoming
elaborately branched, assume very much the aspects of small
trees; and show us in their branches analogous relations of
forms to forces. Bryopsis plumosa may
be instanced. Fig. 200 represents the
end of one of its lateral ramifications,
above and beneath which come others of
like characters. Here it will be seen that
the attached and free ends differ; that
the two sides are much alike; and that they are unlike the
upper and under surfaces, which resemble one another. The
more highly developed members of the same group of Algce,
the Siphonece, show a marked radial symmetry coexisting
with very elaborate branching, e.g., Neo-
meris, Cymopolia, and others.

§ 222. Fig. 201 shows us how, in an
aggregate of the second order, each proxi-
mate component is modified by its rela-
tions to the rest; just as we before saw
a whole fungus of the same type modified
56

146

MORPHOLOGICAL DEVELOPMENT.

by its relations to environing objects. If a branch of the
fungus here figured, be compared with one of the fungi
clustered together in Fig. 195, or, still better, with one of the
laterally-growing fungi shown in Fig. 196, there will be per-
ceived a kindred transition from radial to bilateral symmetry,
occurring under kindred conditions. The portion of the
pileus next to the side of attachment is undeveloped in this
branched form as in the simpler form ; and in the one case as
in the other, the stem is modified towards the side of attach-
ment. A division into similar halves, which, as shown in
Fig. 196 e, might be made of the whole fungus by a vertical
plane passing through the centre of the pileus and the axis
of the supporting body, might here be made of the branch,
by a vertical plane passing through the centre of its pileus
and the axis of the main stem. Among aggregates of this
order, the Algce furnish cases of kindred nature. In the
branches of Lessonia, Fig. 37, may be observed a substantially-
similar relationship. As their inner parts are less developed
than their outer parts, while their two sides are developed in
approximately equal degrees, they are rendered bilateral.

§ 223. These few cases introduce us to the more familiar
but more complex cases which plants of the third degree of

202

aggregation present. At a, b, c, Fig. 202, are sketched three
homologous parts of the same tree: a being the leading

THE SHAPES OF BRANCHES. 147

shoot ; b a lateral branch near the top, and c a lateral branch
lower down. There is here a double exemplification. While
the branch a, as a whole, has its branchlets arranged with
tolerable regularity all round, in correspondence with its
equal exposure on all sides, each branchlet shows by its
curve as much bilateral symmetry as its simple form permits.
The branch b, dissimilarly circumstanced on the side next
the main stem and on the side away from it, has an approxi-
mate bilateralness as a whole, while the bilateralness of its
branchlets varies with their respective positions. And in the
branch c, having its parts still more differently conditioned,
these traits of structure are still more marked. Extremely
strong contrasts of this kind occur in trees having very
regular modes of growth. The uppermost branches of a
Spruce-fir have radially-arranged branchlets: each of them,
if growing vigorously, repeats the type of the leading shoot,
as shown in Fig. 203, a, b. But if we examine branches
lower and lower down the tree, we find the vertically-growing
branchlets bear a less and less ratio to the horizontally-
growing ones; until, towards the bottom, the radial arrange-
ment has wholly merged into the bilateral. Shaded and
confined by the branches above them, these eldest branches
develop their offshoots in those directions where there is
most space and light: becoming finally quite flattened and
fan-shaped, as shown at Fig. 203, c. And on remembering
that each of these eldest branches, when first it diverged
from the main stem, was radial, we see not only that between
the upper and lower branches does this contrast in structure
hold, but also that each branch is transformed from the
radial to the bilateral by the progressive change in its en-
vironment. Other forces besides those which aid or
hinder growth, conspire to produce this two-sided character
in lateral branches. The annexed Fig. 204, sketched from
an example of the Pinus Coulterii at Kew, shows very clearly
how, by mere gravitation, the once radially-arranged branch-
lets may be so bent as to produce in the branch as a whole a

148 MORPHOLOGICAL DEVELOPMENT.

decided bilateralness. A full-grown Araucaria, too, exhibits
in its lower branches modifications similarly caused; and in
each of such branches there may be remarked the further
fact, that its upward-bending termination has a partially-
modified radialness, at the same time that its drooping lateral
branchlets give to the part nearer the trunk a completely
bilateral character.

Now in these few instances, typical of countless instances
which might be given, we see, as we saw in the case of
the fungi, that the same thing is true of the parts in their
relations to the whole and to one another, which is true of
the whole in its relations to the environment at large. Entire
trees become bilateral instead of radial, when exposed to
forces that are equal only on opposite sides of one plane ; and
in their branches, parallel changes of form occur under
parallel changes of conditions.

§ 224. There remains to be said something respecting th
distribution of leaves. How a branch carries its leaves
constitutes one of its characters as a branch, and is to be
considered apart from the characters of the leaves them-
selves. The principles hitherto illustrated we shall here find
illustrated still further.

The leading shoot and all the upper twigs of a fir-tree,
have their pin-shaped leaves evenly distributed all round, or
placed radially ; * but as we descend we find them beginning
to assume a bilateral distribution ; and on the lower, horizon-
tally-growing branches, their distribution is quite bilateral, f
Between the Irish and English kinds of Yew, there is a con-
trast of like significance. The branches of the one, shooting
up as they do almost vertically, are clothed with leaves

* Here and throughout, the word radial is applied equally to the spiral
and the whorled structures. These, as being alike on all sides, are similarly
distinguished from arrangements that are alike on two sides only.

f It should be added that this change of distribution is not due to
change in the relative positions of the insertions of the leaves but to their
twistings.

.

THE SHAPES OF BRANCHES. 149

all round; while those of the other, which spread laterally,
bear their leaves on the two sides. In trees with better-
developed leaves, the same principle is more or less manifest
in proportion as the leaves are more or less enabled by their
structures to maintain fixed positions. Where the foot-stalks
are long and slender, and where, consequently, each leaf,
according to its weight, the flexibility and twist of its foot-
stalk, and the direction of the branch it grows from, falls
into some indefinite attitude, the relations are obscured. But
where the foot-stalks are stiff, as in the Laurel, it will be
found, as before, that from the topmost and upward-growing
branches the leaves diverge on all sides; while the under-
most branches, growing out from the shade of those above,
have their leaves so turned as to bring them into rows hori-
zontally spread out on the two sides of each branch.

A kindred truth, having like implications, comes into view
when we observe the relative sizes of leaves on the same
branch, where their sizes differ. 20s

Fig. 205 represents a branch of a
Horse-chestnut, taken from the
lowermost fringe of the tree, where
the light has been to a great extent
intercepted from all but the most
protruded parts. Beyond the fact
that the leaves become by appro-
priate growths of their foot-stalks
bilaterally distributed on this droop-
ing branch, instead of being distributed symmetrically all
round, as on one of the ascending shoots, we have here to
note the fact that there is unequal development on the upper
and lower sides. Each of the compound leaves acquires a
foot-stalk and leaflets that are large in proportion to the
supply of light; and hence, as we descend towards the bot-
tom of the tree, the clusters of leaves display increasing
contrasts. How marked these contrasts become will be seen
on comparing a and b, which form one pair of leaves that

150 MORPHOLOGICAL DEVELOPMENT.

are normally equal, or c and d, which form another pair noi
mally equal.

Let us not omit to note, while we have this case before us,
the proof it affords that these differences of development are
in a considerable degree determined by the different con-
ditions of the parts after they have been unfolded. Though
those inequalities of dimensions whence the differentiations
of form result, may be in many cases largely due to the
inequalities in the circumstances of the parts while in the
bud (which are, however, representative of inequalities in
ancestral circumstances) ; yet these are clearly not the sole
causes of the unlikenesses which eventually arise. This bi-
lateralness resulting from the unequal sizes of the leaves,
must be considered as due to the differential actions that
come into play after the leaves have assumed their typical
structures.

§ 225. How, in the arrangement of their twigs and leaves,
branches tend to lapse from forms that are approximately
symmetrical to forms that are quite asymmetrical, need not
be demonstrated: it is sufficiently conspicuous. But it may
be well to point out how the tendency to do this further
enforces our argument. The comparatively regular budding-
out of secondary axes and tertiary axes, does not usually
produce an aggregate which maintains its regularity, for
the simple reason that many of the axes abort. Terminal
buds are some of them destroyed by birds; others are bur-
rowed into by insects; others are nipped by frost; others
are broken off or injured during gales of wind. The envi-
ronment of each branch and its branchlets is thus ever
being varied on all sides: here, space being left vacant by
the death of some shoot that would ordinarily have occupied
it; and there, space being trenched on by the lateral growth
of some adjacent branch that has had its main axis broken.
Hence the asymmetry, or heterogeneity of form, assumed
by the branch, is caused by the asymmetrical distribution

THE SHAPES OP BRANCHES. 151

of incident forces — a result and a cause which go on ever
complicating.

§ 226. One conspicuous trait in the shapes of branches
has still to be named. Their proximal or attached ends
differ from their distal or free ends, in the same way that
the lower ends of trees differ from their upper ends. This
fact, like the fact to which it is here paralleled, has had its
significance obscured by its extreme familiarity. But it
shows in a striking way how the most differently-conditioned
parts become the most strongly contrasted in their struc-
tures. A phaenogamic axis is made up of homologous seg-
ments, marked off from one another by the nodes; and a
compound branch consists of groups of such segments. The
earliest-formed segments, alike of the tree and of each
branch, serve as mechanical supports and channels for sap to
the successive generations of segments that grow out of
them; and become more and more shaded by their progeny
as these increase. Hence the progressively-increasing con-
trasts which, while mainly due to the unlikenesses of bulk
accompanying differences of age, are in part due to the un-
likenesses of structure which differences of relation to the
environment have caused.

§ 227. Thus, then, it is with the proximate parts of plants
as it is with plants as wholes. The radial symmetry, the
bilateral symmetry, and the asymmetry, which branches dis-
play in different trees, in different parts of the same tree, and
at different stages of their own growths, prove to be all con-
sequent on the ways in which they stand towards the entire
plexus of surrounding actions. The principle that the
growths are unequal in proportion as the relations of parts
to the environment are unequal, serves to explain all the
leading traits of structure.

CHAPTEE IX.

THE SHAPES OP LEAVES.

§ 228. Next in the descending order of composition conn
compound leaves. The relative sizes and distributions oi
their leaflets, as affecting their forms as wholes, have to be
considered in their relations to conditions. Figs. 206, 207,
represent leaves of the common Oxalis and of the Marsilea,
in which radial symmetry is as completely displayed as the
small number of leaflets permits. This equal development
of the leaflets on all sides, occurs where the foot-stalks, grow-
ing up vertically from creeping or underground stems, are
so long that the leaves either do not interfere with one
another or do it in an inconstant way: the leaflets are not
differently conditioned on different sides, as they are where
the foot-stalks grow out in the ordinary manner. How un-
likeness of position influences the leaflets is clearly shown in
a Clover-leaf, Fig. 208, which deviates from the Oxalis-leaf
but slightly towards bilateralness, as it deviates from it but
slightly in the attitude of its petiole; which is a little in-
clined away from the others borne by the same procumbent
axis. A familiar example of an almost radial symmetry
along with almost equal relations to surrounding conditions,
occurs in the root-leaves of the Lupin, Fig. 209 b. Here
though we have lateral divergence from a vertical axis, yet
the long foot-stalks preserve nearly erect positions, and
carry their leaves to such distances from the axis, that the
development of the leaflets on the side next it is not much
152

THE SHAPES OF LEAVES.

153

hindered. Still the interference of the leaves with one
another is, on the average, somewhat greater on the proximal
side than on the distal side; and hence the interior leaflets
are rather less than the exterior leaflets. In further proof of
which influence, let it be added that, as shown in the figure,
at a, the leaves growing out of the flowering-stem deviate
towards the two-sided form more decidedly. Two-sidedness
is much greater where there is a greater relative proximity
of the inner leaflets to the axis, or where the foot-stalk
approaches towards a horizontal position. The Horse-chest-
nut, Fig. 205, already instanced as showing how the arrange-
ments and sizes of leaflets are determined by the incidence of
forces, serves also to show how the incidence of forces deter-

mines the relative sizes and arrangements of leaflets. Fig.
210, which shows a leaf of the Bombax, further illustrates
this relation of structure to conditions.

Compound leaves that are completely bilateral, present us
with modifications of form exemplifying the same general
truth in another way. In them the proximal and distal
parts have none of that resemblance which we see in those
intermediate forms just described. The portion next the axis
and the portion furthest from* the axis are entirely different ;
and the only likeness is between the wings or leaflets on
opposite sides of the main foot-stalk or mid-rib. On turning
back to Fig. 65, it will be seen that the compound leaf there

154 MORPHOLOGICAL DEVELOPMENT.

drawn to exemplify another truth, serves also to exemplify this
truth: the homologous parts a, b, c, d, while they are unlike
one another, are, in their main proportions, severally like
the parts with which they are paired. And here let us not
overlook a characteristic which is less conspicuous but not
less significant. Each of the lateral wings has winglets
that are larger on the one side than on the other; and in
each case the two sides are dissimilarly conditioned. Even
in the several components of each wing may be traced a like
divergence from symmetry, along with a like inequality in
the relations to the rest: the proximal half of each leaflet
is habitually larger than the distal half. In the leaves of
the Bramble, previously figured, kindred facts are presented.
How far such differences of development are due to the posi-
tions of the parts in the bud; how far the respective
spaces available for the parts when unfolded affect them;
and how far the parts are rendered unlike by unlikenesses
in their relations to light; it is difficult to say. Probably
these several factors operate in all varieties of proportion.
That the habitual shading of some parts by others largely
aids in causing these divergences from symmetry, is very
instructively shown by the compound leaves of the Cow-
parsnip. Fig. 211 represents one of these. While the leaf as a

whole is bilaterally symmetrical, each of the wings has an un-
s}onmetrical bilateralness : the side next the axis being larger
than the remoter side. How does this happen? Fig. 212,
which is a diagrammatic section down the mid-rib of the
leaf, showing its inclined attitude and the positions of the

THE SHAPES OF LEAVES. 155

wings a, b, c, will make the cause clear. As the wings
overlap, like the bars of a Venetian blind, each intercepts
some light from the one below it; and the one below it
thus suffers more on its distal side than on its proximal side.
Hence the smaller development of the distal side. That this
is the cause is further shown by the proportion that is main-
tained between the degree of obscuration and the degree of
non-development; for this unlikeness is greater between the
two sides a and a', than between b and b' or c and c', at the
same time that the interference is greater in the lower wings
than in the upper. Of course in this case and in the kindred
cases hereafter similarly interpreted, it is not meant that
this differentiation is consequent solely, or even chiefty, on
the differential actions experienced by the individual plant.
Though there is good reason to believe that the rate of growth
in each part of each leaf is affected by the incidence of light,
yet contrasts so marked and so systematic as these are not
explicable without taking into account the inheritance of
modifications either functionally caused or caused by spon-
taneous variation. Clearly, the tendency will be towards
the preservation of a plant which distributes its chlorophyll
in the most advantageous way ; and hence there will always be
a gravitation towards a form in which shaded parts of leaves
are undeveloped.

§ 229. From compound leaves to simple ones, we find
transitions in leaves of which the divisions are partial in-
stead of total; and in these we see, with equal clearness, the
relations between forms and positions that have been traced
thus far. Fig. 213 is the leaf of a Winter-aconite in which,
round a vertical petiole, there is a radial distribution of half-
separated leaflets. The Cecropia-le&i, Fig. 214, shows us a
two-sided development of the parts beginning to modify,
but not obliterating, the all-sided arrangement; and this
mixed symmetry occurs under conditions that are interme-
diate. A more marked degree of the same relation is pre-

156

MORPHOLOGICAL DEVELOPMENT.

sented in the leaf of the Lady's Mantle, Fig. 215. And
then in the Sycamore and the Vine, we have a cleft type of

leaf in which a decided bilateralness of form co-exists with a
decided bilateralness of conditions.

The quite simple leaves to which we now descend, exhibit,
very distinctly, a parallel series of facts. Where they grow
up on long and completely-independent foot-stalks, without
definite subordination to some central vertical axis, the
leaves of water-plants are symmetrically peltate. Of this
the sacred Indian-bean, Fig. 216, furnishes an example. Here
there is only a trace of bilateralness in the venation of the
leaf, corresponding to the very small difference of the con-
ditions oh the proximal and distal sides. In the Victoria
regia, Fig. 217, the foot-stalks, though radiating almost
horizontally from a centre, are so long as to keep the leaves
quite remote from one another; and in it each leaf is almost
symmetrically peltate, with a bilateralness indicated only by
a seam over the line of the foot-stalk. The leaves of the
Nymphcea, Fig. 218, more closely clustered, and having less

21<?

217

218

room transversely than longitudinally, exhibit a marke(
advance to the two-sided form; not only in the excess of
the length over the breadth, but in the existence of a cleft,

THE SHAPES OF LEAVES.

157

where in the Victoria regia there is merely a seam. Among
land-plants similar forms are found under analogous condi-
tions. The common Hydrocotyle, Fig. 219, which sends

up direct from its roots a few almost upright leaf-stalks, has
these surmounted by peltate leaves; which leaves, however,
diverge slightly from radial symmetry in correspondence with
the slight contrast of circumstances which their grouping in-
volves. Another case is supplied by the Nasturtium, Fig.
220, which combines the characters — a creeping stem, long
leaf-stalks growing up at right angles to it, and unsymme-
trically peltate leaves, of which the least dimension is, on
the average, towards the stem. But perhaps the most
striking illustration is that furnished by the Cotyledon umbi-
licus, Fig. 221, in which different kinds of symmetry occur
in the leaves of the same plant, along with differences in their
relations to conditions. The root-leaves, a, growing up on
vertical petioles before the flower-stalk makes its appearance,
are symmetrically peltate; while the leaves which subse-
quently grow out of the flower-stalk, o, are at the bottom
transitionally bilateral, and higher up completely bilateral.

That the bilateral form of leaf is the ordinary form,
corresponds with the fact that, ordinarily, the circum-
stances of the leaf are different in the direction of the plant's
axis from what they are in the opposite direction, while

158

MORPHOLOGICAL DEVELOPMENT.

transversely the circumstances are alike. It is needless t(
give diagrams to illustrate this extremely familiar truth.
Whether they are broad or long, oval or heart-shaped, pointec
or obtuse, the leaves of most trees and plants will be remem-
bered by all as having the ends by which they are attached
unlike the free ends, while the two sides are alike. And it will
also be remembered that these equalities and inequalities of
development correspond with the equalities and inequalities
in the incidence of forces.

§ 230. A confirmation that is interesting and important,
is furnished by the cases in which leaves present unsymme-
trical forms in positions where their parts are unsymmetri-
cally related to the environment. A considerable deviation
from bilateral symmetry may be seen in a leaf which habitu-
ally so carries itself, that the half on the one side of the mid-rib
is more shaded than the other half. The drooping branches of
the Lime, delineated in Fig. 222, show us leaves so arranged

and so modified. On examining their attitudes and their
relations one to another, it will be found that each leaf is so
inclined that the half of it next the shoot grows over the
shoot and gets plenty of light ; while the other half so hangs
down that it comes a good deal into the shade of the pre-
ceding leaf. The result is that having leaves which fall into
these positions, the species profits by a large development of
the exposed halves; and by survival of the fittest, acting
along with the direct effect of extra exposure, this modifi-
cation becomes established. How unquestionable is the
connexion between the relative positions of the halves and
their relative developments, will be admitted on observing a

THE SHAPES OF LEAVES. 159

converse case. Fig. 223 represents a shoot of Strobilanthes
glomeratus. Here the leaves are so set on the stem that the
inner half of each leaf is shaded by the subsequently-formed
leaf, while its outer half is not thus shaded ; and here we find
the inner half less developed than the outer half. But the
most conclusive evidence of this relation between unsymme-
trical form and unsymmetrical distribution of surrounding
forces, is supplied by the genus Begonia; for in it we have
a manifest proportion between the degree of the alleged
effect and the degree of the alleged cause. These plants
produce their leaves in pairs, in such ways that the connate
leaves interfere with one another, much or little according
as the foot-stalks are short or long; and the result is a cor-
relative divergence from symmetry. In Begonia nelumbii-
folia, which has petioles so long that the connate leaves are not
kept close together, there is but little deviation from a bilater-
ally-peltate form; whereas, accompanying the comparatively
marked and constant proximity in B. pruinata, Fig. 224, we
see a more decidedly unsymmetrical shape; and in B.
mahringii, Fig. 225, the modification thus caused is pushed
so far as to destroy the peltate structure.*

§ 231. Again, then, we are taught the same truth. Here,
as before, we see that homologous units of any order become

* We may note that some of these leaves, as those of the Lime, furnish
indications of the ratio which exists between the effects of individual circum-
stances and those of typical tendencies. On the one hand, the leaves borne
by these drooping branches of the Lime are with hardly an exception unsym-
metrical more or less decidedly, even in positions where the causes of unsym-
metry are not in action : a fact showing us the repetition of the type irrespec-
tive of the conditions. On the other hand, the degree of deviation from
symmetry is extremely variable, even on the same shoot : a fact proving that
the circumstances of the individual leaf are influential in modifying its form.
But the most striking evidence of this direct modification is afforded by the
suckers of the Lime. Growing, as these do, in approximately upright atti-
tudes, the leaves they bear do not stand to one another in the way above
described, and the causes of unsymmetry are not in action ; and here, though
there is a general leaning to the unsymmetrical form, a large proportion of the
leaves become quite symmetrical.

160 MORPHOLOGICAL DEVELOPMENT.

differentiated in proportion as their relations to incident
forces become different. And here, as before, we see that ii
each unit, considered by itself, the differences of dimension
are greatest in those directions in which the parts are most
differently conditioned; while there are no differences be-
tween the dimensions of the parts that are not differently
conditioned.*

* It was by an observation on the forms of leaves, that I was first led to
the views set forth in the preceding and succeeding chapters on the mor-
phological differentiation of plants and animals. In the year 1851, during
a country ramble in which the structures of plants had been a topic of con-
versation with a friend — Mr. G. H. Lewes — I happened to pick up the leaf
of a buttercup, and, drawing it by its foot- stalk through my fingers so as to
thrust together its deeply-cleft divisions, observed that its palmate and almost
radial form was changed into a bilateral one ; and that were the divisions tc
grow together in this new position, an ordinary bilateral leaf would result.
Joining this observation with the familiar fact that leaves, in common with
the larger members of plants, habitually turn themselves to the light, it
occurred to me that a natural change in the circumstances of the leaf might
readily cause such a modification of form as that which I had produced arti-
ficially. If, as they often do with plants, soil and climate were greatly to
change the habit of the buttercup, making it branched and shrub-like ; and if
these palmate leaves were thus much overshadowed by one another ; would
not the inner segments of the leaves grow towards the periphery of the plant
where the light was greatest, and so change the palmate form into a more
decidedly bilateral form ? Immediately I began to look round for evidence of
the relation between the forms of leaves and the general characters of the
plants they belong to ; and soon found some signs of connexion. Certain
anomalies, or seeming anomalies, however, prevented me from then pursuing
the inquiry much further. But consideration cleared up these difficulties;
and the idea afterwards widened into the general doctrine here elaborated.
Occupation with other things prevented me from giving expression to this
general doctrine until Jan. 1859; when I published an outline of it in the
Medico- Chirurgical Review.

CHAPTER X.

THE SHAPES OF FLOWERS.

§ 232. Following an order like that of preceding chap-
ters, let us first note a few typical facts respecting the forms of
clusters of flowers, apart from the forms of the flowers them-
selves. Two kindred kinds of Leguminosce serve to show how
the members of clusters are distributed in an all-sided manner
or in a two-sided manner, according as the circumstances
are alike on all sides or alike on only two sides. In Ilippo-
crepis, represented in Fig. 226, the flowers growing at the end
of a vertical stem, are arranged
round it in radial symmetry.
Contrariwise in Melilotus, Fig.
227, where the axillary stem
bearing the flowers is so
placed in relation to the main
stem, that its outer and inner
faces are differently condi-
tioned, the flowers are all on
the outer face: the cluster is
bilaterally symmetrical, since
it may be cut into approx-
imately equal and similar
groups by a vertical plane passing through the main axis.

Plants of this same tribe furnish clusters of intermediate
characters having intermediate conditions. Among these,
as among the clusters which other types present, may be
57 161

162 MORPHOLOGICAL DEVELOPMENT.

found some in which conformity to the general law is not
obvious. The discussion of these apparent anomalies would
carry us too much out of our course. A clue to the expla-
nation of them will, I believe, be found in the explanation
presently to be given of certain kindred anomalies in the
forms of individual flowers.

§ 233. The radially-symmetrical form is common to all
individual flowers that have vertical axes. In plants which
are practically if not literally uniaxial, and bear their flowers
at the ends of upright stalks, so that the faces open hori-
zontally, the petals are disposed in an all-sided way. Cro-
cuses, Tulips, and Poppies are familiar examples of this
structure occurring under these conditions. A Ranunculus
flower, Fig. 228, will serve as a typical
Z*f_ /( one. Similarly, flowers which have

peduncles flexible enough to let them
hang directly downwards, and are not
laterally incommoded, are also radial;
as in the Fuchsia, Fig. 229, as in Cycla-
men, Hyacinth, &c. These relations of
form to position are, I believe, uniform. Though some flowers
carried at the ends of upright or downright stems have
oblique shapes, it is only when they have inclined axes or
are not equally conditioned all round. No solitary flower
having an axis habitually vertical, presents a bilateral form.
This is as we should expect; since flowers which open out
their faces horizontally, whether facing upwards or down-
wards, are, on the average, similarly affected on all sides.

At first it seems that flowers thus placed should alone be
radial; but further consideration discloses conditions under
which this type of symmetry may exist in flowers otherwise
placed. Remembering that the radial form is the primitive
form — that, morphologically speaking, it results from the
contraction into a whorl, of parts that are originally arranged
in the same spiral succession as the leaves; we must expect

THE SHAPES OP FLOWERS. 163

it to continue wherever there are no forces tending to change
it. What now must be the forces tending to change it?
They must be forces which do not simply affect differently
the different parts of an individual flower. They must be
forces which affect in like contrasted ways the homologous
parts of other individual flowers, both on the same plant and
on surrounding plants of the same species. A permanent
modification can be expected only in cases where, by inherit-
ance, the effects of the modifying causes accumulate. That
they may accumulate the flowers must keep themselves so
to la ted to the environment, that the homologous parts may,
generation after generation, be subjected to like differentiating
forces. Hence, among a plant's flowers which maintain no
uniformity in the relations of their parts to surrounding in-
fluences, the radial form will continue. Let us glance at the
several causes which entail this variability. When

flowers are borne on many branches, which have all inclina-
tions from the vertical to the horizontal — as are the flowers
of the Apple, the Plum, the Hawthorn — they are placed in
countless different attitudes. Consequently, any spontaneous
variation in shape which might be advantageous were the
attitude constant, is not likely to be advantageous; and any
functionally-produced modification in one flower, is likely to
be neutralized in offspring by some opposite functionally-pro-
duced modification in another flower. It is quite compre-
hensible, therefore, that irregularly-branched plants should
thus preserve their laterally-borne flowers from under-
going permanent devia-
tions from their primi-
tive radial symmetry.
Fig. 230, representing a
blossoming twig of the
Blackthorn, illustrates
this. Again, upright
panicles, such as those of the Saxifrage exemplified in
Fig. 231, and irregular terminal groups of flowers other-

164

MORPHOLOGICAL DEVELOPMENT.

wise named, furnish conditions under which there is simi-
larly an absence of determinate relations between tl
parts of the flowers and the incident forces; and hence
absence of bilateralness. This inconstancy of reh

tive position is produced in various other ways — by extreme
flexibility of the stems, as in the Blue-bell; by the ten-
dency of the peduncles to curl to a greater or less extent
in diverse directions, as in Pyrola; by special twistings ol
the peduncles, differing in degree in different individuals,
as in Convolvulus; by unusual laxity of the petals, as
Ly thrum. Elsewhere the like general result arises from
progressive change of attitude, as in Myosotis, the stem oi
which as it unfolds causes each flower to undergo a transitioi
from an upward position of the mouth to a lateral position
or as in most Cruciferce, where the like effect follows from
altered direction of the peduncle.

There are, however, certain seemingly-anomalous cases
where radial symmetry is maintained by laterally-placed
flowers, which keep their parts in relative positions that are
tolerably constant. The explanation of these exceptions is
not manifest. It is only when we take into account certain
incident actions liable to be left unremembered, that we find
a probable solution. It will be most convenient to postpone
the consideration of these cases until we have reached the
general rule to which they are exceptions.

§ 234. Transitions varying in degree from the radial to-
wards the bilateral, are common in flowers that are borne a1
the ends of branches or axes which are inclined in tolerabb
constant ways. We may see this in sundry garden flowei
#33 r^Y such as Petunia, or such

Isoloma and Achimenes,
shown in Figs. 232 and 233.
If these plants be examined,
it will be perceived that the
mode of growth makes the flower unfold in a partially one-

THE SHAPES OF FLOWERS. 165

sided position; that its parts of attachment have rigidity
sufficient to prevent this attitude from being very mrvch in-
terfered with; and that though the individual flowers vary
somewhat in their attitudes, they do not vary to the extent of
neutralizing the differentiating conditions — there remains an
average divergence from a horizontal unfolding of the flower,
to account for its divergence from radial symmetry.

We pass insensibly from forms like these, to forms having
bilateral symmetry strongly pronounced. Some such forms
occur among flowers that grow at the ends of upright stems;
as in Pinguicula, and in the Violet tribe. But this happens
only where, in successive generations, the flower unfolds its
parts sideways in constant relative positions. And in the
immense majority of flowers having well-marked two-sided
forms, the habitual exposure of the different parts to different
sets of forces, is effectually secured by the mode of placing.
As illustrations, I may name the genera — Orchis, Utricularia,
Salvia, Salix, Delphinium, Mentha, Teucrium, Ajuga, Ballota,
Galeopsis, Lamium, Stachys, Nepeta, Marrubium, Calamintha,
Melittis, Prunella, Scutellaria, Bartsia, Euphrasia, Rhinan-
thus, Melampyrum, Pedicularis, Linaria, Digitalis, Oro-
banche, Fumaria, &c; to which may be added all the Grasses
and all the Papilionacew. In most of these cases the flowers,
being sessile on the sides of upright stems, are kept in quite
fixed attitudes ; and in the other cases the peduncles are very
short, or else stiff enough to secure general uniformity in the
positions. A few of the more marked types are shown in
Figs. 234 to 241.

S&s

234 23S 2,30 237 ^S 2>3$ 240 241

Very instructive evidences here meet us. Sometimes within
the limits of one genus we find radial flowers, bilateral
flowers, and flowers of intermediate characters. The genus

166

MORPHOLOGICAL DEVELOPMENT.

Begonia may be instanced. In B. rigida the flowers, various
in their attitudes, are in their more conspicuous characters
radial: though there is a certain bilateralness in the calyx,
the five petals are symmetrically disposed all round. B.
Wageneriana furnishes two forms of flowers. On the same in-
dividual plant may be. found radial flowers like Fig. 242, and
others, like Fig. 243, which are merging into the bilateral.
More decided is the bilateralness in B. albo-coccinea, Fig. 244;
and still more in B. nitida, Fig. 245. While in B. heraclei-

242

243

244

245

2,46

folia, Fig. 246, the change reaches its extreme by the dis-
appearance of the lateral petals. On examining the modes of
growth in these several species, they will be seen to explain
these changes in the manner alleged. Even

more conclusive are the nearly-allied transformations occur-
ring in artificially-produced varieties of the same species.
Gloxinia may be named in illustration. In Fig. 247 is repre-
sented one of the ordinary forms, which shows us bilateralness
of shape along with a mode of growth that renders the condi-
tions alike on the two sides while different above and below.
247 24L^ But in 0. erecta, Fig. 248, we

have the flower assuming ai
upright attitude, and at the
same time assuming the radial
type. This is not to be inter-
preted as a production of ra-
dial symmetry out of bilateral symmetry, under the action of
the appropriate conditions. It is rather to be taken as a case
of what is termed " peloria " — a reversion to the primitive
radial type, from which the bilateral modification had been
derived. The significant inference to be drawn from it is,

I

THE SHAPES OF FLOWERS. 167

that this primitive radial type had an upright attitude; and
that the derivation of a bilateral type from it, occurred along
with the assumption of an inclined attitude.

We come now to a group of cases above referred to, in
which radial symmetry continues to co-exist with that con-
stant lateral attitude ordinarily accompanied by the two-
sided form. Two examples will suffice: one a very large
flower, the Hollyhock, and the other a very small flower, the
Agrimony. Why does the radial form here remain unchanged ?
and how does its continuance consist with the alleged general
law?

Until quite recently I have been unable to find any prob-
able answers to these questions. When the difficulty first
presented itself, I could think of no other possible cause for
the anomaly, than that the parts of the Hollyhock-flower,
unfolding spirally as they do, might have different degrees
of spiral twist in different flowers, and might thus not be
unfolded in sufficiently-constant positions. But this seemed
a questionable interpretation; and one which did not ob-
viously apply to the case of the Agrimony. It was only on
inquiring what are the special causes of modifications in the
forms of flowers, that a more feasible explanation suggested
itself; and this would probably never have suggested itself,
had not Mr. Darwin's investigations into the fertilization of
Orchids led me to take into account an unnoticed agency.

The actions which affect the forms of leaves, affect much
less decidedly the forms of flowers; and the forms of flowers
are influenced by actions which do not influence the forms of
leaves. Partly through the direct action of incident forces
and partly through the indirect action of natural selection,
leaves get their parts distributed in ways that most facilitate
their assimilative functions, under the circumstances in which
they are placed ; and their several types of symmetry are thus
explicable. But in flowers, the petals and fructifying organs
of which do not contain chlorophyll, the tendency to grow
most where the supply of light is greatest, is less decided, if

168 MORPHOLOGICAL DEVELOPMENT.

not absent; and a shape otherwise determined is hence less
liable to alter in consequence of altered relations to sun and
air. Gravity, too, must be comparatively ineffective in caus-
ing modifications : the smaller sizes of the parts, as well as
their modes of attachment, giving them greater relative
rigidity. Not, indeed, that these incident forces of the inor-
**9 KH^ ganic world are here quite inoperative. Fig.

249, representing a species of Campanula,
shows that the developments of individual
flowers are somewhat modified by the rela-
tions of their parts to general conditions. But
the fact to be observed is, that the extreme
transformations which flowers undergo are
not likely to be thus caused: some further
cause must be sought. And if we bear in
mind the functions of flowers, we shall find in
their adaptations to these functions, under conditions that are
extremely varied, an adequate cause for the different types
of symmetry, as well as for the exceptions to them. Flowers
are parts in which fertilization is effected; and the active
agents of this fertilization are insects — bees, moths, butter-
flies, &c. Mr. Darwin has shown in many cases, that the
forms and positions of the essential organs of fructification,
are such as to facilitate the actions of insects in trans-
ferring pollen from the anthers of one flower to the pistil of
another — an arrangement produced by natural selection.
And here we shall find reason for concluding, that the forms
and positions of those subsidiary parts which give their
shapes to flowers, similarly arise by the survival of indi-
viduals which have the subsidiary parts so adjusted as to aid
this fertilizing process — the deviations from radial symmetry
being among such adjustments. The reasoning is as fol-
lows. So long as the axis of a flower is vertical and
the conditions are similar all round, a bee or butterfly alight-
ing on it, will be as likely to come from one side as from
another; and hence, hindrance rather than facilitation would

THE SHAPES OF FLOWERS. 169

result if the several sides of the flower did not afford it equally
free access. In like manner, flowers which are distributed
over a plant in such ways that their discs open out on
planes of all directions and inclinations, will have no tend-
ency to lose their radial symmetry; since, on the average,
no part of the periphery is differently related to insect-
agency from any other part. But flowers so fixed as to
open out sideways in tolerably-constant attitudes, have
their petals differently related to insect-agency. A bee or
butterfly coming to a laterally-growing flower, does not settle
on it in one way as readily as in another; but almost of
necessity settles with the axis of its body inclined upwards
towards the stem of the plant. Hence the side-petals of a
flower so fixed, habitually stand to the alighting insect in
relations different from those in which the upper and lower
petals stand; and the upper and lower petals differ from one
another in their relations to it. If, then, there so arises an
habitual attitude of the insect towards the petals, there is
likely to be some arrangement of the petals that will be
most convenient to the insect — will most facilitate its entrance
into the flower. Thus we see in many cases, that a long
undermost petal or lip, by enabling the insect to settle in
such way as to bring its head opposite to the opening of the
tube, aids its fertilizing agency. But whatever be the special
modifications of the corolla which facilitate the actions of
the particular insects concerned, all of them will conduce to
bilateral symmetry; since they will be alike for the two sides
but unlike for the top and bottom. And now we

are prepared for understanding the exceptions. Flowers
growing sideways can become thus adapted by survival of
the fittest, only if they are of such sizes and structures that
insect-agency can affect them in the way described. But
in the plants named above, this condition is not fulfilled. A
Hollyhock-flower is so open, as well as so large, that its petals
are n$t in any appreciable degree differently related to the
insects which visit it. On the other hand, the flower of the

170 MORPHOLOGICAL DEVELOPMENT.

Agrimony is so small, that unless visited by insects of a
corresponding size which settle as bees and butterflies settle,
its parts will not be affected in the alleged manner. That
all anomalies of this kind can at once be satisfactorily ex-
plained, is scarcely to be expected : the circumstances of each
case have to be studied. But it seems not improbable that
they are due to causes of the kind indicated.*

§ 235. We have already glanced at clusters of flowers for
the purpose of considering their shapes as clusters. We must
now return to them to observe the modifications undergone
by their component flowers. Among these occur illustrations
of great significance.

An example of transition from the radial to the bilateral
form in clustered flowers of the same species, is furnished by
the cultivated Geraniums, called by florists Pelargoniums.
Some of these, bearing somewhat small terminal clusters of
flowers, which are closely packed together with their faces
almost upwards, have radially-symmetrical flowers. But
among other varieties having terminal clusters of which the
members are mutually thrust on one side by crowding, the
flowers depart very considerably from the radial shape

* It is objected to the above interpretation that " many flowers of sizes
intermediate between the Hollyhock and the Agrimony are radially sym-
metrical and yet grow sideways. I may mention various Ziliacece, e.g. Chlo-
rophytum, Eitcomis, Muscari, Anthericvm. Sagittaria, also, has many of its
flowers in this position. Further, if the higher insects alight on flowers in
a definite way. as they do, the parts of the flower must bear different rela-
tions to the visiting insect, however large, so that flowers unvisited ought
all to be zygomorphic." My reply is that in the sense which here concerns
us, the different petals of the Hollyhock -flower do not bear different rela-
tions to the visiting insect ; since, practically, the upper and lateral petals
bear no physical relations at all : in so far as the visiting bee is concerned
they are non-existent. The argument implies that change in the form of a
flower from the radial to the bilateral is likely to take place only when the
contact-relations of the petals to the visiting insect, are such as to make some
forms facilitate its action more than others ; and the large petals of the
Hollyhock cannot facilitate its action at all. In respect of the LiHacece
instanced, it is needful to inquire whether the structures are such that this
alleged cause of bilateral symmetry can come into play.

THE SHAPES OF FLOWERS. 171

towards the trilateral shape. A like result occurs under like
conditions in Rhododendrons and Azaleas. The Verbena, too,
furnishes an illustration of radial flowers rendered slightly
two-sided by the slight two-sidedness of their relations to
other flowers in the cluster. And among the Cruciferce a
kindred case occurs in the cultivated Candytuft.

Evidence of a somewhat different kind is offered us by
clustered flowers in which the peripheral members of the
clusters differ from the central members; and this evidence
is especially significant where we find allied species that do
not exhibit the deviation, at the same time that they do not
fulfil the conditions under which it may be expected. Thus,
in Scabiosa succisa, Fig. 250, which bears its numerous small
flowers in a hemispherical knob, the component flowers,
similarly circumstanced, are all equal and all radial; but in
Scabiosa arvensis, Fig. 251, in which the numerous small
flowers form a flattened disk
only the confined central ones
are radial : round the edge the

flowers are much larger and
conspicuously bilateral.

But the most remarkable
and most conclusive proofs of these relations between forms
and positions, are those given by the clustered flowers called
Umbellifera?. In some cases, as where the component flowers
have all plenty of room, or where the surface of the umbel is
more or less globular, the modifications are not conspicuous;
but where, as mViburnum,Cha?rophyttum, Anthriscus, Torilis,
Caucalis, Daucus, Tordylium, &c, we have flowers clustered
in such ways as to be differently conditioned, we find a num-
ber of modifications that are marked and varied in propor-
tion as the differences of conditions are marked and varied.
In Cheer ophyllum, where the flowers of each umbellule are
closely placed so as to form a flat surface, but where the
umbellules are wide apart and form a dispersed umbel, the
umbellules do not differ from one another; though among the

172

MORPHOLOGICAL DEVELOPMENT.

flowers of each umbellule there are decided differences: the
central flowers being small and radial, while the peripheral
ones are large and bilateral. But in other genera, where not
only the flowers of each nmbelhile but also the umbellules
themselves, are closely clustered into a flat surface, the umbel-
lules themselves become contrasted; and many remarkable
secondary modifications arise. In an umbel of Heracleum,
for instance, there are to be noted the facts; — first, that the
external umbellules are larger than the internal ones;
second, that in each umbellule the central flowers are less
developed than the peripheral ones; third, that this greater
development of the peripheral flowers is most marked in the
outer umbellules ; fourth, that it is most marked on the outer
sides of the outer umbellules; fifth, that while the interior
flowers of each umbellule are radial, the exterior ones are
bilateral; sixth, that this bilateralness is most marked in
the peripheral flowers of the peripheral umbellules; seventh,
that the flowers on the outer sides of these peripheral
umbellules are those in which the bilateralness reaches a
maximum; and eighth, that where the outer umbellules

touch one another, the flow-
ers, being unsymmetrically
placed, are unsymmetrically
bilateral.* The like modifi-
cations are displayed, though
not in so clearly-traceable a
way, in an umbel of Tordy-
lium, Fig. 252. Considering
how obviously these various
forms are related to the vari-
ous conditions, we should be
scarcely able, even in the

* I had intended here to insert a figure exhibiting these differences ; but
as the Cow-parsnip does not flower till July, and as I can find no drawing
of the umbel which adequately represents its details, I am obliged to take
another instance.

THE SHAPES OF FLOWERS. 173

absence of all other facts, to resist the conclusion that the
differences in the conditions are the causes of the differences
in the forms.

Composite flowers furnish evidence so nearly allied to that
which clustered flowers furnish, that we may fitly glance
at them under the same head. Such a common type of
this order as the Sun-flower, exempli-
fies the extremely marked difference
which arises in many of these plants
between the closely-packed internal
florets, each similarly circumstanced on
all sides, and the external florets, not
similarly circumstanced on all sides.
In Fig. 253, representing the inner and
outer florets of a Daisy, the contrast is
marked between the small radial corolla of the one and the
larger bilateral corolla of the other. In many cases, how-
ever, this contrast is less marked: the inner florets also
having their outward-growing prolongations — a difference
possibly related to some difference in the habits of the insects
that fertilize them. Nevertheless, these composite flowers
which have inner florets with strap-shaped corollas out-
wardly directed, equally conform to the general principle;
both in the radial arrangement of the assemblage of florets,
and in the bilateral shape of each floret; which has its
parts alike on the two sides of a line passing from the centre
of the assemblage to the circumference. Certain

other members of this order fulfil the law somewhat differ-
ently. In Centaurea, for instance, the inner florets are small
and vertical in direction, while the outer florets are large and
lateral in direction. And here may be remarked, in passing,
a clear indication of the effect which great flexibility of the
petals has in preventing a flower from losing its original
radiate form ; for while in G. cyanus, the large outward-grow-
ing florets, having short, stiff divisions of the corolla, are
decidedly bilateral, in C. scabiosa, where the divisions of the

174 MORPHOLOGICAL DEVELOPMENT.

corolla are long and flexible, the radial form is scarcely at
all modified. On bearing in mind the probable relations of
the forms to insect-agency, the meaning of this difference
will not be difficult to understand.*

§ 236. In extremely-varied ways there are thus re-illus-
trated among flowers, the general laws of form which leaves
and branches and entire plants disclose to us. Composed as
each cluster of flowers is of individuals that are originally
similar; and composed as each flower is of homologous foliar
organs; we see both that the like flowers become unlike and
the like parts of each flower become unlike, where the posi-
tions involve unlike incidence of forces. The symmetry
remains radial where the conditions are equal all round;
shows deviation towards two-sidedness where there is slight
two-sidedness of conditions; becomes decidedly bilateral
where the conditions are decidedly bilateral; and passes into
an unsymmetrical form where the relations to the environ-
ment are unsymmetrical.

* It has been pointed out to me that " the extreme development of the
corolla so often found in the outer flowers or on the outer side of the outer
flowers in closely packed inflorescences, associated as it often is with disap-
pearance of stamens or carpels or both, is usually put down to specialization
of these outer flowers for attractive purposes. Since the whole inflorescence
is increased in conspicuousness by such a modification, it is supposed that
natural selection favoured those plants which sacrificed a portion of their
seed-bearing capacity for the supposed greater advantage of securing more
insect visits." But granting this interpretation, it may still be held that
increase of attractiveness due to increase of area must be achieved by florets
at the periphery, and that their ability to achieve it depends on their having
an outer, unoccupied, space which the inner florets have not; so that, thougt
in a more indirect way, their different development is determined by different
exposure to conditions.

CHAPTEK XL

THE SHAPES OF VEGETAL CELLS.

§ 237. We come now to aggregates of the lowest order.
Already something has been said (§217) concerning the
forms of those morphological units which exist as indepen-
dent plants. But it is here requisite briefly to note the
modifications undergone by them where they become compo-
nents of larger plants.

Of the numerous cell-forms which are found in the tissues
of the higher plants, it will suffice to give, in Fig. 254, re-
presenting a section of
a leaf, a single example.
In this it will be seen
that the cells forming
the upper and lower sur-
faces, a and b, have dif-
ferences of shape related d ;^U^-^
to differences in the inci-
dence of forces: they are
more or less flattened in

relation to the environment. The underneath cells at c,
form a class which, similarly exposed to light at their
outer ends, and, as we may assume, largely developed in
adjustment to their active assimilative functions, are, by
mutual pressure, made to grow more in the direction of their
lengths than in the direction of their breadths. Then on
the other side we see that the cells d, next above the outer
layer, while approximately similar, become more and more
dissimilar as they diverge from the surface, and are quite

175

176

MORPHOLOGICAL DEVELOPMENT.

irregular in the interior e, where there is no definiteness in
the conditions to which they are exposed. Thus the diver-
gences of these cells from primordial sphericity are such as
correspond with unlikenesses in their circumstances. And
throughout the more complex modifications which the cells
of other tissues exhibit, the like correspondences hold.

Among plants of a lower order of aggregation, we have
already seen how cells become metamorphosed as they become
integrated into masses having definite organizations. The
higher Algce, exemplified in Figs. 32, 34, 35, show this very

clearly. Here the departure from
the simple cell-form to the form
of an elongated prism, is mani-
festly subordinated to the con-
trasts in the relations of the
parts. And it is interesting to ob-
serve how, in one of the branches
of Fig. 32, we pass from the small,
almost-spherical cells which ter-
minate the branchlets, to the
large, much-modified cells which
join the main stem, through gra-
dations obviously related in their changed forms to the
altered actions their positions expose them to.

More simply, but quite as conclusively, do the inferior

Algce, of which Figs. 19 — 23 are examples, show us how

J9

cells pass from their original spherical symmetry into radial
symmetry, as they pass from a state in which they are simi-

THE SHAPES OF VEGETAL CELLS.

177

larly-conditioned on all sides, to a state in which two of their
opposite sides or ends are conditioned in ways that are like
one another, but unlike the ways in which all other sides are
conditioned.

Still more instructive are the morphological differentia-
tions of those protophytes in which the first steps towards a
higher degree of integration are shown. In Fig. 10, represent-
ing one of the transitional forms of Desmidiacece, it is to be
noted that besides the difference between the transverse and
longitudinal dimensions, which the component units display
in common, the two end-units differ from the rest : they have

«ffi*:

y3&

appendages which the rest have not. Once more, where the
integration is carried on in such ways as to produce not strings
but clusters, there arise contrasts and correspondences just
such as might be looked for. All the four members of the
group shown in Fig. 12, are similarly conditioned; and each
of them has a bilateral shape answering to its bilateral rela-
tions. In Fig. 14 we have a number of similarly-bilateral
individuals on the circumference, including a central in-
dividual differing from the rest by having the bilateral
character nearly obliterated. And then, in Fig. 15, we have
two central components of the group, deviating more deci-
dedly from those that surround them.*

* One of my critics writes : — " This chapter might of course be enormously
extended, not only as in the preceding ones by citation of quite similar cases,
but by the introduction of fresh groups of cases."

58

CHAPTER XII.

CHANGES OF SHAPE OTHERWISE CAUSED.

§ 238. Besides the more special causes of modification in
the shapes of plants and of their parts, certain more general
causes must be briefly noticed. These may be described as
consequences of variations in the total quantities of the
matters and forces furnished to plants by their environments.
Some of the changes of form so produced are displayed by
plants as wholes, and others only by their parts. We will
glance at them in this order.

&7S-

2&S

§ 239. It is a familiar fact that luxuriant shoots have
relatively-long internodes; and, conversely, that a shoot
dwarfed from lack of sap, has its nodes closely clustered: a
concomitant result being that the lateral axes, where these
are developed, become in the one case far apart and in the
other case near together. Fig. 255 represents a branch to
the parts of which the longer and
shorter internodes so resulting give
differential characters. A whole tree
being in many cases simultaneously
thus affected by states of the earth or
the air, all parts of it may have such
variations impressed on them; and,
indeed, such variations, following more
or less regularly the changes of the
seasons, give to many trees manifest

CHANGES OP SHAPE OTHERWISE CAUSED. 179

traits of structure. In Fig. 256, a shoot of Phyllocactus
crenatus, we have an interesting example of a variation
essentially of the same nature, little as it appears to be so.
For each of the lateral indentations is here the seat of an
axillary bud; and these we see are separated by internodes
which, becoming broader as they become longer, and narrower
as they become shorter, produce changes of form that corre-
spond with changes in the luxuriance of growth.

To complete the statement it must be added that these
variations of nutrition often determine the development or
non-development of lateral axes; and by so doing cause still
more marked structural differences. The Fox-glove may be
named as a plant which illustrates this truth.*

§ 240. From the morphological differentiations caused by
unlikenesses of nutrition felt by the whole plant, we pass
now to those which are thus caused in some of its parts and
not in others. Among such are the contrasts between
flowering axes, and the axes that bear leaves only. It has
already been shown in § 78, that the belief expressed by
Wolff in a direct connexion between fructification and innu-
trition, is justified inductively by many facts of many kinds.
Deductively too, in § 79, we saw reason to conclude that such
a relation would be established by survival of the fittest;
seeing that it would profit a species for its members to begin
sending off migrating germs from the ends of those axes
which innutrition prevented from further agamogenetic mul-
tiplication. Once more, when considering the nature of the
phaenogamic axis, we found support for this belief in the fact

* Natural selection may have operated in establishing a constitutional
tendency to other sudden abridgments. Mr. Tansley alleges that this is a
part-cause of the varying distribution of leaves. He says : — " I have myself
made some observations on the length of internodes in the Beech, and am
satisfied that it follows quite other laws, connected with the suitable dis-
position of the leaves on the branch. Although I have not had the oppor-
tunity of following up this line of work so as in any way to generalize the
results, I suspect that ' indirect equilibration ' is a widespread cause of such
variation."

180 MORPHOLOGICAL DEVELOPMENT.

that the components of a flower exhibit a reversion to that
type from which the phasnogamic type has probably arisen —
a reversion which the laws of embryology would lead us to
look for where innutrition had arrested development.

Hence, then, we may properly count those deviations of
structure which constitute inflorescence, as among the mor-
phological differentiations produced by local innutrition. I do
not mean that the detailed modifications which the essential
and subservient organs of fructification display, are thus
accounted for: we have seen reason to think them otherwise
caused. But I mean that the morphological characters which
distinguish gamogenetic axes in general from agamogenetic
axes, such as non-development of the internodes and dwarf-
ing of the foliar organs, are primarily results of failure in
the supply of some material required for further growth.*

§ 241. Another trait which has to be noticed under this
head, is the spiral, or rather the helical, arrangement of
parts. The successive nodes of a phaenogam habitually bear
their appendages in ways implying more or less twist in the
substance of the axis ; and in climbing plants the twist is such

* It is but just to the memory of Wolff, here to point out that he was
immensely in advance of Goethe in his rationale of these metamorphoses.
Whatever greater elaboration Goethe gave to the theory considered as an
induction, seems to me more than counter-balanced by the irrationality of his
deductive interpretation ; which unites mediaeval physiology with Platonic
philosophy. A dominant idea with him is that leaves exist for the purpose of
carrying off crude juices— that "as long as there are crude juices to be carried
off, the plant must be provided with organs competent to effect the task " ;
that while " the less pure fluids are got rid of, purer ones are introduced "
and that " if nourishment is withheld, that operation of nature (flowering) is
facilitated and hastened; the organs of the nodes (leaves) become more
refined in texture, the action of the purified juices becomes stronger, and the
transformation of parts having now become possible, takes place without
delay." This being the proximate explanation, the ultimate explanation is,
that Nature wishes to form flowers — that when a plant flowers it " attains the
end prescribed to it by nature " ; and that so " Nature at length attains her
object." Instead of vitiating his induction by a teleology that is as unwar-
ranted in its assigned object as in its assigned means, Wolff ascribes the
phenomena to a cause which, whether sufficient or not, is strictly scientific in

CHANGES OF SHAPE OTHERWISE CAUSED. 181

as to produce a corkscrew shape. This structure is ascrib-
able to differences of interstitial nutrition. Take a shoot
which is growing vertically. It is clear that if the molecules
are added with perfect equality on all sides, there will be no
tendency towards any kind of lateral deviation; and the
successively-produced parts will be perpendicularly over one
another. But any inequality in the rate of growth on the
different sides of the shoot, will destroy this straightness in
the lines of growth. If the greatest and least rates of mole-
cular increase happen to be on opposite sides, the shoot must
assume a curve of single curvature; but in every other case
of unequal molecular increase, a curve of double curvature
must result. Now it is a corollary from the instability of the
homogeneous, that the rates of growth on all sides of a shoot
can never be exactly alike; and it is also to be inferred from
the same general law, that the greatest and least rates of
growth will not occur on exactly opposite sides of the shoot,
at the same time that equal rates of growth are preserved by
the two other sides. Hence, there must almost inevitably
arise more or less of twist; and the appendages of the inter-
nodes will so be prevented from occurring perpendicularly
one over another.

A deviation of this kind, necessarily initiated by physical
causes in conformity with the general laws of evolution, is
likely to be made regular and decided by natural selection.
For under ordinary circumstances, a plant profits by having
its axis so twisted as to bring the appended leaves into posi-
tions which prevent them from shading one another. And,
manifestly, modifications in the forms, sizes, and insertions of
the leaves, may, under the same agency, lead to adapted
modifications of the twist. We must therefore ascribe this
common characteristic of phasnogams, primarily to local differ-
ences of nutrition, and secondarily to survival of the fittest.

its character. Variation of nutrition is unquestionably a "true cause" of
variation in plant-structure. We have here no imaginary action of a fictitious
agency ; but an ascertained action of a known agency.

182

MORPHOLOGICAL DEVELOPMENT.

It is proper to add that there are some Monocotyledons,
as Ravenala madagascariensis, in which this character does
not occur. What conditions of existence they are that here
hold this natural tendency in check, it is not easy to see.*

* The Natural History Review for July, 1865, contained an article on
the doctrine of morphological composition set forth in the foregoing Chaps.
I. to III. In this article, which unites exposition and criticism in a way that
is unhappily not common with reviewers, it is suggested that the spiral struc-
ture may be caused by natural selection. When this article appeared, the
foregoing five pages were standing over in type, as surplus from No. 14, issued
in June, 1865.

CHAPTEE XIII.

MORPHOLOGICAL DIFFERENTIATION IN ANIMALS.

§ 242. The general considerations which preluded our in-
quiry into the shapes of plants and their parts, equally serve,
so far as they go, to prelude an inquiry into the shapes of
animals and their parts. Among animals, as among plants,
the formation of aggregates greater in bulk or higher in de-
gree of composition, or both, is accompanied by changes of
form in the aggregates as wholes as well as by changes of
form in their parts; and the processes of morphological
differentiation conform to the same general laws in the one
kingdom as in the other.

It is needless to recapitulate the several kinds of modifi-
cation to be explained, and the several factors that co-
operate in working them. In so far as these are common
to plants and animals, the preceding chapters have suf-
ficiently familiarized them. Nor is it needful to specify
afresh the several types of symmetry and their descriptive
names; for what is true of them in the one case is true of
them in the other. There is, however, one new and all-
important factor which we shall have now to take into
account; and about this a few preliminary remarks are
requisite.

§ 243. This new factor is motion — motion of the organism
in relation to surrounding objects, or of the parts of the

183

184 MORPHOLOGICAL DEVELOPMENT.

organism in relation to one another, or both. Though there
are plants, especially of the simpler kinds, which move, and
though a few of the simpler animals do not move ; yet move-
ments are so exceptional and unobtrusive in the one kin^
dom, while they are so general and conspicuous in the other,
that the broad distinction commonly made is well warranted.
What, among plants, is an inappreciable cause of morpho-
logical differentiation, becomes, among animals, the chief
cause of morphological differentiation.

Booted animals or animals otherwise fixed, of course pre-
sent traits of structure nearest akin to those we have lately
been studying. The motions of parts in relation to one another
and to the environment, being governed by the mode of aggre-
gation and mode of fixing, we are presented with morpho-
logical differentiations similar in their general characters to
those of plants, and showing us parallel kinds of symmetry
under parallel conditions. But animals which move from
place to place are subject to an additional class of actions
and reactions. These actions and reactions affect them in
various ways according to their various modes of movement.
Let us glance at the several leading relations between shape
and motion which we may expect to find.

If an organism advances through a homogeneous medium
with one end always foremost, that end, being exposed to
forces unlike those to which the other end is exposed, may
be expected to become unlike it; and supposing this to be
the only constant contrast of conditions, we may expect an
equal distribution of the parts round the axis of movement —
a radial symmetry. If, in addition to this habitual

attitude of the ends, one surface of the body is always upper-
most and another always lowermost, there arise between the
top and bottom dissimilarities of conditions, while the two
sides remain similarly conditioned. Hence it is inferable
that such an organism will be divisible into similar halves
by a vertical plane passing through its axis of motion — will
have a bilateral symmetry. We may presume that this

MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. 185

symmetry will deviate but little from double bilateralness
where the upper and under parts are not exposed to strongly-
contrasted influences; while we may rationally look for
single bilateral symmetry of a decided kind, in creatures
having dorsal and ventral parts conversant with very unlike
regions of the environment: as in all cases where the move-
ment is over a solid surface. If the movement,
though over a solid surface, is not constant in direction, but
takes place as often on one side as on another, radial sym-
metry may be again looked for; and if the motions are still
more variously directed — if they are not limited to approxi-
mately-plane surfaces, but extend to surfaces that are dis-
tributed all around with a regular irregularity — an approach
of the radial towards the spherical symmetry is to be antici-
pated. Where the habits are such that the inter-
course between the organism and its environment, does not
involve an average equality of actions and reactions on any
two or more sides, there may be expected either total irregu-
larity or some divergence from regularity.

The like general relations between forms and incident
forces are inferable in the component parts of animals, as
well as in the animals as wholes. It is needless, however, to
occupy space by descriptions of these. Let us now pass to
the facts, and see how they confirm, a posteriori, the conclu-
sions here reached a priori.

CHAPTEE XIV.

THE GENERAL SHAPES OF ANIMALS.

§ 244. Certain of the Protozoa are quite indefinite in their
shapes, and quite inconstant in those indefinite shapes which
they have — the relations of their parts are indeterminate
both in space and time. In one of the simpler Khizopods, at
least during the active stage of its existence, no permanent
distinction of inside and outside is established; and hence
there can arise no established correspondence between the
shape of the-^ outside and the distribution of environing
actions. But when the relation of inner and outer becomes
fixed, either over part of the mass or over the whole of it, we
have kinds of symmetry that correspond with the habitual
incidence of forces. An Amoeba in becoming encysted,
passes from an indefinite, ever-changing form into a spherical
form; and the order of symmetry which it thus assumes, is
in harmony with the average equality of the actions on all
its sides. In Difflugia, Fig. 134, and still better in Arcella,
we have an indefinitely-radial symmetry occurring where the
conditions are different above and below but alike all around.
Among the Gregarinida the spherical symmetry and sym-
metry passing from that into the radial, are such as appear
to be congruous with the simple circumstances of these
creatures in the intestines of insects. But the relations of
these lowest types to their environments are comparatively
so indeterminate, and our knowledge of their actions so
186

THE GENERAL SHAPES OF ANIMALS. 187

scanty, that little beyond negative evidence can be expected
from the study of them.

ml /j/ ua \j

The like may be said of the Infusoria. These are more or
less irregular. In some cases, where the line of movement
through the water is tolerably definite and constant, we have
a form that is approximately radial — externally at least.
But usually, as shown in Figs. 137, 138, 139, there is either
an unsymmetrical or an asymmetrical shape. And when one
of these creatures is watched under the microscope, the con-
gruity of this shape with the incidence of forces is manifest.
For the movements are conspicuously varied and indetermi-
nate— movements which do not expose any two or more sides
of the mass to approximately equal sets of actions.*

§ 245. Among aggregates of the second order, as among
aggregates of the first order, we find that of those possessing
any definite shapes the lowest are spherical or spheroidal.
Such are some of the Radiolaria, as Collozoum inerme. These
bodies which float passively in the sea, and present in turn
all their sides to the same influences, have their parts dis-
posed with approximate regularity round a centre — approxi-
mate, because in the absence of locomotion a slight irregu-
larity of growth, almost certain to take place, may cause a
fixed attitude and a resulting deviation from spherical sym-
metry. The best cases in illustration of the truth here
named, are furnished by rotating and locomotive organisms
respecting which there is a dispute whether they are animal
or vegetal — the Volvocinece. These, already instanced under

* A verifying comment on this paragraph runs as follows : — " In the
Hypotricha Infusoria, which creep over solid surfaces, there is a differen-
tiation between ventral and dorsal surface and an approach to bilateral sym-
metry. The ventral surface is provided with movable cilia, the dorsal with
immobile setae."

188 MORPHOLOGICAL DEVELOPMENT.

the one head in § 218, may here be instanced afresh under
the other. Further, among these secondary aggregates in
which the units, only physically integrated, have not had their
individualities merged into an individuality of a higher
order, must be named the compound Infusoria. The cluster
of VorUcellce in Fig. 144, will sufficiently exemplify them;
and the striking resemblance borne by its individuals to
those of a radially-arranged cluster of flowers, will show how,
under analogous conditions, the general principles of mor-
phological differentiation are similarly illustrated in the two
kingdoms. *

§ 246. Eadial symmetry is usual in low aggregates of
the second order which have their parts sufficiently differen-
tiated and integrated to give individualities to them as wholes.
The Ccelenterata offer numerous examples of this. Solitary
polypes — hydroid or helianthoid — mostly stationary, and
when they move, moving with any side foremost, do not by
locomotion subject their bodies to habitual contrasts of con-
ditions. Seated with their mouths upwards or downwards,
or else at all degrees of inclination, the individuals of a
species taken together, are subject to no mechanical actions
affecting some parts of their discs more than other parts.
And this indeterminateness of attitude similarly prevents
their relations to prey from being such as subject some of
their prehensile organs to forces unlike those to which the
rest are subject. The fixed end is differently conditioned
from the free end, and the two are therefore different; but
around the axis running from the fixed to the free end the
conditions are alike in all directions, and the form therefore
is radial. Again, among many of the simple free-

swimming Hydrozoa, the same general truth is exemplified
under other circumstances. In a common Medusa, advanc-
ing through the water by the rhythmical contractions of its
disc, the mechanical reactions are the same on all sides; and
as, from accidental causes, every part of the edge of the disc

THE GENERAL SHAPES OF ANIMALS.

189

comes uppermost in its turn, no part is permanently affected
in a different way from the rest. Hence the radial form con-
tinues.

In others of this same group, however, there occur forms
which show us an incipient bilateralness ; and help us to see
how a more decided bilateralness may arise. Sundry of the
Medusidce are proliferous, giving -origin to gemmae from the
body of the central polypite or from certain points on the
edge of the disc; and this budding, unless it occurs equally
on all sides, which it does not and is unlikely to do, must
tend to destroy the balance of the disc, and to make its
attitude less changeable. In other cases the growth of a
large process [a much-developed tentacle] from the edge of
the disc on one side, as in Steenstrupia, Fig. 257, constitutes
a similar modification, and a cause of further modification.
The animal is no longer divisible into any two quite similar
halves, except those formed by a plane passing through the
process ; and unless the process is of the same specific gravity
as the disc, it must tend towards either the lowest or the
highest point, and must so serve to increase the bilateralness,
by keeping the two sides of the disc similarly conditioned
while the top and bottom are differently conditioned. Fig. 258
represents the underside of another Medusa, in which a more
decided bilateralness is produced by the presence of two such
processes. Among the

simple free-swimming Acti-
nozoa, occur like deviations
from radial symmetry, along
with like motions through the
water in bilateral attitudes.
Of this a Cydippe is a familiar
example. Though radial in
some of its characters, as in
the distribution of its meridi-
onal bands of locomotive paddles with their accompanying
canals, this creature has a two-sided distribution of tentacles

357

190 MORPHOLOGICAL DEVELOPMENT.

and various other parts, corresponding with its two-sidec
attitude in moving through the water. And in other genen
of this group, as in Cestum, Eurhamphcea, and Callianira,
that almost equal distribution of parts which characterize
the Beroe is quite lost.

Here seems a fit place to meet the objection which some
may feel to this and other such illustrations, that they amount
very much to physical truisms. If the parts of a Medusa are
disposed in radial symmetry round the axis of motion through
the water, there will of course be no means of maintaining
one part of its edge uppermost more than another; and the
equality of conditions may be ascribed to the radiateness, as
much as the radiateness to the equality of conditions. Con-
versely, when the parts are not radially arranged around the
axis of motion, they must gravitate towards some one atti-
tude, implying a balance on the two sides of a vertical plane
— a bilateralness ; and the two-sided conditions so necessi-
tated, may be as much ascribed to the bilateralness as the
bilateralness to the two-sided conditions. Doubt-

less the form and the conditions are, in the way alleged,
necessary correlates; and in so far as it asserts this, the ob-
jection harmonizes with the argument. To the difficulty
which it at the same time raises by the implied question —
Why make the form the result of the conditions, rather than
the conditions the result of the form? the reply is this: —
The radial type, both as being the least differentiated type
and as being the most obviously related to lower types, must
be taken as antecedent to the bilateral type. The indi-
vidual variations which incidental circumstances produce in
the radial type, will not cause divergence of a species from
the radial type, unless such variations give advantages to the
individuals displaying them ; which there is no reason to sup-
pose they will always do. Those occasional deviations from
the radial type, which the law of the instability of the homo-
geneous warrants us in expecting to take place, will, however,
in some cases be beneficial; and will then be likely to estab-

THE GENERAL SHAPES OP ANIMALS. 191

lish themselves. Such deviations must tend to destroy the
original indefiniteness and variability of attitude — must cause
gravitation towards an habitual attitude. And gravitation
towards an habitual attitude having once commenced, will
continually increase, where increase of it is not negatived by
adverse agencies : each further degree of bilateralness render-
ing more decided the actions that conduce to bilateralness. If
this reply be thought insufficient, it may be enforced by the
further one, that as, among plants, the incident forces are the
antecedents and the forms the consequents (changes of forces
being in many cases visibly followed by changes of forms) we
are warranted in concluding that the like order of cause and
effect holds among animals.*

§ 247. Keeping to the same type but passing to a higher
degree of composition, we meet more complex and varied
illustrations of the same general laws. In the compound
* Criticisms on the above passage have shown the need for naming sun-
dry complications. These complications chiefly, if not wholly, arise from
changes in modes of life — changes from the locomotive to the stationary, and
from the stationary to the locomotive. Referring to my statement that (ignor-
ing the spherical) the radial type is the lowest and must be taken as ante-
cedent to the bilateral type, it is alleged that all existing "radial animals
above Protozoa are probably derived from free swimming, bilaterally-sym-
metrical animals." If this is intended to include the planulse of the hydroid
polyps, then it seems rather a straining of the evidence. These locomotive
embryos, described as severally having the structure of a gastrula with a
closed mouth, can be said to show bilateralness only because the first two ten-
tacle^ make their appearance on opposite sides of the mouth — a bilateralness
which lasts only till two other tentacles make their appearance in a plane at
right angles, so giving the radial structure. I think the criticism applies only
to cases furnished by Echinoderms. The larvae of these creatures have bilat-
erally-symmetrical structures, which they retain as long as they swim about
and which such of them as fix themselves lose by becoming similarly related
to conditions all round : the radial structure being retained by those types
which, becoming subsequently detached, move about miscellaneously. But, as
happens in some of the Sea-urchins and still more among the Holothurians,
the structure is again made bilaterally-symmetrical by a locomotive life pur-
sued with one end foremost. Should it be contended that the conditions and
the forms are reciprocally influential — that either may initiate the other, it
still remains unquestionable that ordinarily the conditions are the antecedents,
as is so abundantly shown by plants.

192 MORPHOLOGICAL DEVELOPMENT.

Calenterata, presenting clusters of individuals which ai
severally homologous with the solitary individuals last deall
with, we have to note both the shapes of the individuals thi
united, and the shapes of the aggregates made up of them.

Such of the fixed Hydrozoa an&Actinozoa as form branched
societies, continue radial; both because their varied attitudes
do not expose them to appreciable differences in their rela-
tions to those surrounding actions which chiefly concern
them (the actions of prey), and because such differences, even
if they were appreciable, would be so averaged in their effects
on the dissimilarly-placed members of each group as to be
neutralized in the race. Among the tree-
like coral-polypedoms, as well as in such
ramified assemblages of simpler polypes
as are shown in Figs. 149, 150, we have,
indeed, cases in many respects parallel
to the cases of scattered flowers (§ 233),
which though placed laterally remain
radial, because no differentiating agency
can act uniformly on all of them. Meanwhile, in the

groups which these united individuals compose, we see the
shapes of plants further simulated under a further parallelism
of conditions. The attached ends differ from the free ends
as they do in plants; and the regular or irregular branches
obviously stand to environing actions in relations analogous
to those in which the branches of plants stand.

The members of those compound Ccelenterata which move
through the water by their own actions, in attitudes that are
approximately constant, show us a more or less distinct two-
sidedness. Diphyes, Fig. 259, furnishes an example. Each
of the largely-developed and modified polypites forming its
swimming sacs is bilateral, in correspondence with the bi-
lateralness of its conditions; and in each of the appended
polypites the insertion of the solitary tentacle produces a
kindred divergence from the primitive radial type. The

aggregate, too, which here very much subordinates its mem-

THE GENERAL SHAPES OF ANIMALS.

193

bers, exhibits the same conformity of structure to circum-
stances. It admits of symmetrical bisection by a plane pass-
ing through its two contractile sacs, or nectocalyces, but not

by any other plane; and the plane which thus symmetrically
bisects it, is the vertical plane on the two sides of which its
parts are similarly conditioned as it propels itself through
the water.

Another group of the oceanic Hydrozoa, the Physoplioridce,
furnishes interesting evidence — not so much in respect of the
forms of the united individuals, which we may pass over, as
in respect of the forms of the aggregates. Some of these
are without swimming organs, and have their parts sus-
pended from air-vessels which habitually float on the surface
of the water. Hence the distribution of their parts is asym-
metrical. The Physalia, Fig. 152, is an example. Here the
relations of the integrated group of
individuals to the environment are in-
definite; and there is thus no agency
tending to change that comparatively
irregular mode of growth which is pro-
bably derived from a primordial type
of the branched Hydrozoa.

So various are the modes of union
among the compound Ccelenterata, that
it is out of the question to deal with
them all. Even did space permit, it
would be impracticable for any one but
a professed naturalist, to trace through-

59 J&

194 MORPHOLOGICAL DEVELOPMENT.

out this group the relations between shapes and conditions of
existence. The above must be taken simply as a few of the
most significant and easily-interpretable cases.

§ 248. In the sub-kingdoms Polyzoa and Tunicata we meet
with examples not wholly unlike the foregoing. Among the
types assembled under these names there are simple indivi-
duals or aggregates of the second order, and societies or
tertiary aggregates produced by their union. The relations
of forms to forces have to be traced in both.

Solitary Ascidians, fixed or floating, carry on an inactive
and indefinite converse with the actions in the environment.
Without power to move about vivaciously, and unable to
catch any prey but that contained in the currents of water
they absorb and expel, these creatures are not exposed to
sets of forces which are equal on two or more sides ; and their
shapes consequently remain vague. Though internally their
parts have a partially-symmetrical arrangement, due to their
derivation, yet they are substantially unsymmetrical in that
part of the body which is concerned with the environment.
Fig. 156 is an example.* Among the composite

Ascidians, floating and fixed, the shape of the aggregate,
partly determined by the habitual mode of gemmation and
partly by the surrounding conditions in each case, is in
great measure indefinite. We can say no more about it than
that it is not obviously at variance with the laws alleged.

Evidence of a more positive kind occurs among those com-
pound Molluscoida which are most like the compound
Ccclenterata in their modes of union — the Polyzoa. Many of
these form groups that are more or less irregular — spread-
ing as films over solid surfaces, combining into sea-weed-
like fronds, budding out from creeping stolons, or growing
up into tree-shaped societies; and besides aggregating

* Should it be proved that the Ascidian is a degraded vertebrate, then the
argument will be strengthened ; since loss of bilateral symmetry has gone
along with change to asymmetrical conditions.

THE GENERAL SHAPES OF ANIMALS. 195

irregularly they are irregularly placed on surfaces inclined
in all directions. Merely noting that this asymmetrical
distribution of the united individuals is explained by the
absence of definiteness in the relations of the aggregate to
incident forces, it concerns us chiefly to observe that the
united individuals severally exemplify the same truth as do
similarly-united individuals among the Ccelenterata. Averag-
ing the members of each society, the ciliated tentacles they
protrude are similarly related to prey on all sides; and
therefore remain the same on all sides. This distribution of
tentacles is not, however, without exception. Among the
fresh-water Polyzoa there are some genera, as Plumatella and
Crystatella, in which the arrangement of these parts is very
decidedly bilateral. Some species of them show us such
relations of the individuals to one another and to their sur-
face of attachment, as give a clue to the modification; but
in other species the meaning of this deviation from the
radial type is not obvious.

§ 249. In the Platyhelminthes good examples of the con-
nexions between forms and forces occur. The Planaria
exemplifies the single bilateral symmetry which, even in
very inferior forms, accompanies the habit of moving in one
direction over a solid surface. Humbly organized as are
these creatures and their allies the Nemertidce, we see in
them, just as clearly as in the highest animals, that where
the movements subject the body to different forces at its two
ends, different forces on its under and upper surfaces, and
like forces along its two sides, there arises a corresponding
form, unlike at its extremities, unlike above and below, but
having its two sides alike.

tThe Echinodermata furnish us with instructive illustra-
tions— instructive because among types that are nearly allied,
cc

e meet with wide deviations of form answering to marked
contrasts in the relations to the environment. The facts fall
into four groups. The Crinoidea, once so abundant

196 MORPHOLOGICAL DEVELOPMENT.

and now so rare, present a radial symmetry answering to
an incidence of forces that are equal on all sides. In the
general attitudes of their parts towards surrounding actions,
they are like uniaxial plants or like polypes; and show, as
those do, marked differences between the attached ends and
the free ends, along with even distributions of parts all round
their axes. In the Ophiuridea, and in the Star-

fishes, we have radial symmetry co-existing with very differ-
ent habits; but habits which nevertheless account for the
maintenance of the form. Holding on to rocks and weeds
by its simple or branched arms, or by the suckers borne on
the under surface of its rays, one of these creatures moves
about not always with one side foremost, but with any side
foremost. Consequently, averaging its movements, its arms
or rays are equally affected, and therefore remain the same
on all sides. On watching the ways of the com-

mon Sea-urchin, we are similarly furnished with an ex-
planation of its spherical, or rather its spheroidal, figure.
Here the habit is not to move over any one approximately-
flat surface; but the habit is to hold on by several surfaces
on different sides at the same time. Frequenting crevices
and the interstices among stones and weeds, the Sea-urchin
protrudes the suckers arranged in meridional bands over its
shell, laying hold of objects now on this side and now on that,
now above and now below: the result being that it does not
move in all directions over one plane but in all directions
through space. Hence the approach in general form towards
spherical symmetry — an approach which is, however, re-
strained by the relations of the parts to the mouth and vent :
the conditions not being exactly the same at the two poles as
at other parts of the surface. Still more significant is

that deviation from this shape which occurs among such of
the Echinidea as have habitats of a different kind, and con-
sequently, different habits. The genera Echinocyamus, Spa-
tangus, Brissus, and Ampliidotus, diverge markedly towards
a bilateral structure. These creatures are found not on rocky

THE GENERAL SHAPES OF ANIMALS. 197

shores but on flat sea-bottoms, and some of them only on
bottoms of sand or mud. Here, there is none of that distri-
bution of surfaces on all sides which makes the spheroidal
form congruous with the conditions. Having to move about
over an approximately-horizontal plane, any deviation of
structure arising accidentally which leads to one side being
kept always foremost, will be an advantage: greater fitness
to function becoming possible in proportion as function
becomes fixed. Survival of the fittest will therefore tend to
establish, under such conditions, a form that keeps the same
part in advance — a form in which, consequently, the original
radial symmetry diverges more and more towards bilateral
symmetry.

§ 250. Very definite and comparatively uniform, are the
relations between shapes and circumstances among the
Annulosa: including under that title the Annelida and the
Arthropoda. The agreements and the disagreements are
equally instructive.

At one time or other of its life, if not throughout its life,
every annulose animal is locomotive; and its temporary or
permanent locomotion, being carried on with one end habitu-
ally foremost and one surface habitually uppermost, it fulfils
those conditions under which bilateral symmetry arises.
Accordingly, bilateral symmetry is traceable throughout the
whole of this sub-kingdom. Traceable, we must say,
because, though it is extremely conspicuous in the immense
majority of annulose t}rpes, it is to a considerable extent
obscured where obscuration is to be expected. The embryos
of the Tubicolce, after swimming about a while, settle down
and build themselves tubes, from which they protrude
their heads; and in them, or in some of them, the bilateral
symmetry is disguised by the -development of head-append-
ages in an all-sided manner. The tentacles of Terebella are
distributed much in the same way as those of a polype. The
breathing organs in Sabella unispira, Fig. 260, do not corre-

198

MORPHOLOGICAL DEVELOPMENT.

spond on opposite sides of a median plane. Even here, how-
ever, the body retains its primitive bilateralness ; and it is
further to be remarked that this loss of bilateralness in the
external appendages, does not occur where the relations to
external conditions continue bilateral: witness the Serpula,
Fig. 261, which has its respiratory tufts arranged in a two-

sided way, under the two-sided conditions involved by the
habitual position of its tube.

The community of symmetry among the higher Annulosa,
has an unobserved significance. That Flies, Beetles, Lob-
sters, Centipedes, Spiders, Mites, have in common the cha-
racters, that the end which moves in advance differs from
the hinder end, that the upper surface differs from the under
surface, and that the two sides are alike, is a truth received
as a matter of course. After all that has been said above,
however, it will be seen to have a meaning not to be over-
looked ; since it supplies a million-fold illustration of the laws
which have been set forth. It is needless to give diagrams.
Every reader can call to mind the unity indicated.

While, however, annulose animals repeat so uniformly
these traits of structure, there are certain other traits in
which they are variously contrasted; and their contrasts
have to be here noted, as serving further to build up the
general argument. In them we see the stages through which

THE GENERAL SHAPES OF ANIMALS.

199

bilateral symmetry becomes gradually more marked, as the
conditions it responds to become more decided. A

common Earth-worm may be instanced as a member of
this sub-kingdom that is among the least-conspicuously
bilateral. Though internally its parts have a two-sided
arrangement; and though the positions of its orifices give it
an external two-sidedness, at the same time that they estab-
lish a difference between the two ends; yet its two-sidedness
is not strongly-marked. The form deviates but little from
what we have distinguished as triple bilateral symmetry: if
the creature is cut across the middle, the head and tail ends
are very much alike; if cut in two along its axis by a hori-
zontal plane, the under and upper halves are very much
alike, externally if not internally ; and if cut in two along its
axis by a vertical plane, the two sides are quite alike.
Figs. 263 and 264 will make this clear. Such creatures

as the Julus and the Centipede, may be taken as showing
a transition to double bilateral symmetry. Besides being
divisible into exactly similar halves by a vertical plane pass-
ing through its axis, one of these animals may be bisected
transversely into parts that differ only slightly ; but if cut in

267

D

Jte&

468

^

"^mm&

<?ye

two by a horizontal plane passing through its axis, the under
and upper halves are decidedly unlike. Figs. 265, 266,
exhibit these traits. Among the isopodous crustaceans,

the departure from these low types of symmetry is more

200 MORPHOLOGICAL DEVELOPMENT.

marked. As shown in Figs. 267 and 268, the contrast
between the upper and under parts is greater, and the head
and tail ends differ more obviously. In all the higher

Arthropoda, the unlikeness between the front half and the
hind half has become conspicuous. There is in them single
bilateral symmetry of so pronounced a kind, that no other
resemblance is suggested than that between the two sides.
By Figs. 269 and 270, representing a decapodous crustacean
divided longitudinally and transversely, this truth is made
manifest. On calling to mind the habits of the

creatures here drawn and described, it will be seen that
they explain these forms. The incidence of forces is the
same all around the Earth-worm as it burrows through the
compact ground. The Centipede, creeping amid loose soil or
debris or beneath stones, insinuates itself between solid sur-
faces— the interstices being mostly greater in one dimension
than in others. And all the higher Annulosa, moving about
as they do over exposed objects, have their dorsal and ventral
parts as dissimilarly acted upon as are their two ends.

One other fact only respecting annulose animals needs to
be noticed under this head — the fact, namely, that they
become unsymmetrical where their parts are unsymmetric-
ally related to the environment. The common Hermit-crab
serves as an instance. Here, in addition to the unlikeness of
the two sides implied by that curvature of the body which
fits the creature to the shell it inhabits, there is an unlikeness
due to the greater development of the limbs, and especially
the claws, on the outer side. As in the embryo of the
Hermit-crab the two sides are alike ; and as both the embryo
and the ancestor lived in such a way, being free,
that the conditions were alike on the two sides ;
and as the embryo may be taken to repre-
sent the type from which the Hermit-crab has
been derived; we have in this case evidence
that a symmetrically-bilateral form has been
zii moulded into an unsymmetrically-bilateral

THE GENERAL SHAPES OF ANIMALS. 201

form, by the action of unsymmetrically-bilateral conditions.
A further illustration is supplied by Bopyrus, Fig. 271 : a
parasite which lives in the branchial chamber of prawns, and
whose habits similarly account for its distorted shape.

§ 251. Among the Mollusca we find more varied relations
between shapes and circumstances. Some of these relations
are highly instructive.

Mollusks of one order, the Pteropoda, swim in the sea
much in the same way that butterflies fly in
the air, and have shapes not altogether unlike
those of butterflies. Fig. 272 represents one
of these creatures. That its bilaterally-sym-
metrical shape harmonizes with its bilater-
ally-symmetrical conditions is sufficiently
obvious. tt m

Among the Lamellibranchiata, we have
diverse forms accompanying diverse modes of life. Such
of them as frequently move about, like the fresh-water
Mussel, have their two valves and the contained parts
alike on the opposite sides of a vertical plane: they are
bilaterally symmetrical in conformity with their mode of
movement. The marine Mussel, too, though habitually
fixed, and though not usually so fixed that its two valves are
similarly conditioned, still retains that bilateral symmetry
which is characteristic of the order; and it does this because
in the species considered as a whole, the two valves are not
dissimilarly conditioned. If the positions of the various
individuals are averaged, it will be seen that the differen-
tiating actions neutralize one another. In certain
other fixed Lamellibranchs, however, there is a considerable
deviation from bilateral symmetry; and it is a deviation of
the kind to be anticipated under the circumstances. Where
one valve is always downwards, or next to the surface of
attachment, while the other valve is always upwards, or next
to the environing water, we may expect to find the two

202 MORPHOLOGICAL DEVELOPMENT.

valves become unlike. This we do find: witness the Oyster.
In the Oyster, too, we see a further irregularity. There is a
great indefiniteness of outline, both in the shell and in the
animal — an indefiniteness made manifest by comparing dif-
ferent individuals. We have but to remember that growing
clustered together, as Oysters do, they must interfere with
one another in various ways and degrees, to see how the
indeterminateness of form and the variety of form are
accounted for.

Among the Gasteropods modifications of a more definite
kind occur. " In all Mollusks," says Professor Huxley,
" the axis of the body is at first straight, and its parts are
arranged symmetrically with regard to a longitudinal vertical
plane, just as in a vertebrate or an articulate embryo." In
some Gasteropods, as the Chiton, this bilateral symmetry
is retained — the relations of the body to surrounding actions
not being such as to disturb it. But in those more numerous
types which have spiral shells, there is a marked deviation
from bilateral symmetry, as might be expected. " This
asymmetrical over-development never affects the head or
foot of the mollusk": only those parts which, by inclosure
in a shell, are protected from environing actions, lose their
bilateralness ; while the external parts, subjected by the
movements of the creatures to bilateral conditions, remain
bilateral. Here, however, a difficulty meets us. Why is it
that the naked Gasteropods, such as our common slugs,
deviate from bilateral symmetry, though their modes of
movement are those along with which complete bilateral
symmetry usually occurs? The reply is that their devia-
tions from bilateral symmetry are probably inherited, and
that they are maintained in such parts of their organization
as are not exposed to bilaterally-symmetrical conditions.
There is reason to believe that the naked Gasteropods are
descended from Gasteropods which had shells: the evidence
being that the naked Gasteropods have shells during the
early stages of their development, and that some of them

THE GENERAL SHAPES OP ANIMALS. 203

retain rudimentary shells throughout life. Now the shelled
Gasteropods deviate from bilateral symmetry in the disposi-
tion of both the alimentary system and the reproductive
system. The naked Gasteropods, in losing their shells, have
lost that immense one-sided development of the alimentary
system which fitted them to their shells, and have acquired
that bilateral symmetry of external figure which fits them
for their habits of locomotion; but the reproductive system
remains one-sided, because, in respect to it, the relations to
external conditions remain one-sided.

The Cephalopods show us bilaterally-symmetrical external
forms along with habits of movement through the water in
two-sided attitudes. At the same time, in the radial distri-
bution of the arms, enabling one of these creatures to take
an all-sided grasp of its prey, we see how readily upon one
kind of symmetry there may be partially developed another
kind of symmetry, where the relations to conditions favour it.

§ 252. The Vertebrata illustrate afresh the truths which
we have already traced among the Annulosa. Flying through
the air, swimming through the water, and running over the
earth as vertebrate animals do, in common with annulose
animals, they are, in common with annulose animals, different
at their anterior and posterior ends, different at their dorsal
and ventral surfaces, but alike along their two sides. This
single bilateral symmetry remains constant under the ex-
tremest modifications of form. Among fish we see it alike
in the horizontally-flattened Skate, in the vertically-flattened
Bream, in the almost spherical Diodon, and in the greatly-
elongated Syngnathus. Among reptiles the Turtle, the Snake,
and the Crocodile all display it. And under the countless
modifications of structure displayed by birds and mammals,
it remains conspicuous.

A less obvious fact which it concerns us to note among the
Vertebrata, parallel to one which we noted among the An-
nulosa, is that whereas the lower vertebrate forms deviate

204

MORPHOLOGICAL DEVELOPMENT.

but little from triple bilateral symmetry, the deviation be-
comes great as we ascend. Figs. 273 and 274 show how,
besides being divisible into similar halves by a vertical plane
passing through its axis, a Fish is divisible into halves that
are not very dissimilar by a horizontal plane passing through
its axis, and also into other not very dissimilar halves by a
plane cutting it transversely. If, as shown in Figs. 275 and
276, analogous sections be made of a superior Eeptile, the
divided parts differ more decidedly. When a Mammal and a

Bird are treated in the same way, as shown in Figs. 277,
278, and Figs. 279, 280, the parts marked off by the dividing
planes are unlike in far greater degrees. On considering

THE GENERAL SHAPES OF ANIMALS. 205

the mechanical converse between organisms of these several
types and their environments — on remembering that the
fish habitually moves through a homogeneous medium of
nearly the same specific gravity as itself, that the terrestrial
reptile either crawls on the surface or raises itself very in-
completely above it, that the more active mammal, having
its supporting parts more fully developed, thereby has the
under half of its body made more different from the upper
half, and that the bird is subject by its mode of life to yet
another set of actions and reactions; we shall see that these
facts are quite congruous with the general doctrine, and
furnish further support to it.

One other significant piece of evidence must be named.
Among the Annulosa we found unsymmetrical bilateralness
in creatures having habits exposing them 'to unlike conditions
on their two sides ; and among the Vertebrata we find parallel
cases. They are presented by the Pleuronectidce — the order
of distorted flat fishes to which the Sole and the Flounder
belong. On the hypothesis of evolution, we must conclude
that fishes of this order have arisen from an ordinary bila-
terally-S3rmmetrical type of fish, which, feeding at the bottom
of the sea, gained some advantage by placing itself with one
of its sides downwards, instead of maintaining the vertical
attitude. Besides the general reason there are special
reasons for concluding this. In the first place, the young
Sole or Flounder is bilaterally symmetrical — has its eyes on
opposite sides of its head and swims in the usual way. In
the second place, the metamorphosis which produces the un-
symmetrical structure sometimes does not take place — there
are abnormal Flounders that swim vertically, like other fishes.
In the third place, the transition from the symmetrical struc-
ture to the unsymmetrical structure may be traced. Almost
incredible though it seems, one of the eyes is transferred
from the under-side of the head to the upper-side: the
transfer being effected by a distorted development of the
cranial bones — atrophy of some and hypertrophy of others,

206 MORPHOLOGICAL DEVELOPMENT.

along with a general twist. This metamorphosis furnishes
several remarkable illustrations of the way in which forms
become moulded into harmony with incident forces. For
besides the divergence from bilateral symmetry involved by
presence of both eyes upon the upper side, there is a further
divergence from bilateral symmetry involved by differentiation
of the two sides in respect to the contours of their surfaces
and the sizes of their fins. And then, what is still more
significant, there is a near approach to likeness between the
halves that were originally unlike, but are, under the new
circumstances, exposed to like conditions. The body is
divisible into similarly-shaped parts by a plane cutting it
along the side from head to tail: "the dorsal and ventral
instead of the lateral halves become symmetrical in outline
and are equipoised."

§ 253. Thus, little as there seems in common between the
shapes of plants and the shapes of animals, we yet find, on
analysis, that the same general truths are displayed by
both. The one ultimate principle that in any organism equal
amounts of growth take place in those directions in which
the incident forces are equal, serves as a key to the phenomena
of morphological differentiation. By it we are furnished
with interpretations of those likenesses and unlikenesses of
parts, which are exhibited in the several kinds of symmetry;
and when we take into account inherited effects, wrought
under ancestral conditions contrasted in various ways with
present conditions, we are enabled to comprehend, in a gen-
eral way, the actions by which animals have been moulded
into the shapes they possess.

To fill up the outline of the argument, so as to make it
correspond throughout with the argument respecting vegetal
forms, it would be proper here to devote a chapter to the
differentiations of those homologous segments out of which
animals of certain types are composed. Though, among most
animals of the third degree of composition, such as the

THE GENERAL SHAPES OF ANIMALS. 207

rooted Hydrozoa, the Polyzoa, and the Ascidioida, the united
individuals are not reduced to the condition of segments of a
composite individual, and do not display any marked differ-
entiations; yet there are some animals in which such
subordinations, and consequent heterogeneities, occur. The
oceanic Hydrozoa form one group of them ; and we have seen
reason to conclude that the Annulosa form another group.
It is not worth while, however, to occupy space in detailiug
these unlikenesses of homologous segments, and seeking
specific explanations of them. Among the oceanic Hydrozoa
they are extremely varied; and the habits and derivations of
these creatures are so little known, that there are no ade-
quate data for interpreting the forms of the parts in terms
of their relations to the environment. Conversely, among
the Annulosa those differentiations of the homologous seg-
ments which accompany their progressing integration, have
so much in common, and have general causes which are so
obvious, that it is needless to deal with them at any length.
They are all explicable as due to the exposure of different
parts of the chain of segments to different sets of actions and
reactions : the most general contrast being that between the
anterior segments and the posterior segments, answering to
the most general contrast of conditions to which annulose
animals subject their segments; and the more special con-
trasts answering to the contrasts of conditions entailed by
their more special habits.

Were an exhaustive treatment of the subject practicable,
there should here, also, come a chapter devoted to the in-
ternal structures of animals — meaning, more especially, the
shapes and arrangements of the viscera. The relations
between forms and forces among these inclosed parts are,
however, mostly too obscure to allow of interpretation.
Protected as the viscera are in great measure from the inci-
dence of external forces, we are not likely to find much
correspondence between their distribution and the distribu-
tion of external forces. In this case the influences, partly

208 MORPHOLOGICAL DEVELOPMENT.

mechanical, partly physiological, which the organs exercise
on one another, become the chief causes of their changes of
figure and arrangement; and these influences are complex
and indefinite. One general fact may, indeed, be noted — the
fact, namely, that the divergence towards asymmetry which
generally characterizes the viscera, is marked among those
of them which are most removed from mechanical converse
with the environment, but not so marked among those of
them which are less removed from such converse. Thus
while, throughout the Vertebrata, the alimentary system,
with the exception of its two extremities, is asymmetrically
arranged, the respiratory system, which occupies one end of
the body, generally deviates but little from bilateral sym-
metry, and the reproductive system, partly occupying the
other end of the body, is in the main bilaterally symmetrical :
such deviation from bilateral symmetry as occurs, being
found in its most interiorly-placed parts, the ovaries. Just
indicating these facts as having a certain significance, it will
be best to leave this part of the subject as too involved for
detailed treatment.

Internal structures of one class, however, not included
among the viscera, admit of general interpretation — struc-
tures which, though internal, are brought into tolerably-
direct relations with environing forces, and are therefore
subordinate in their forms to the distribution of those forces.
These internal structures it will be desirable to deal with
at some length; both because they furnish important illustra-
tions enforcing the general argument, and because an inter-
pretation of them which we have seen reason to reject, can-
not be rejected without raising the demand for some other
interpretation.

CHAPTER XV.

THE SHAPES OF VERTEBRATE SKELETONS.

§ 254. When an elongated mass of any substance is
transversely strained, different parts of the mass are ex-
posed to forces of opposite kinds. If, for example, a bar
of metal or wood is supported at its two ends, as shown in
Fig. 281, and has to bear a weight on its centre, its lower

zei

part is thrown into a state of tension, while its upper part is
thrown into a state of compression. As will be manifest to
any one who observes what happens on breaking a stick
across his knee, the greatest degree of tension falls on the
fibres forming the convex surface, while the fibres forming
the concave surface are subject to the greatest degree of
compression. Between these extremes the fibres at different
.depths are subject to different forces. Progressing upwards
from the under surface of the bar shown in Fig. 281, the
tension of the fibres becomes less; and progressing down-
wards from the upper surface, the compression of the fibres
becomes less; until, at a certain distance between the two
surfaces, there is a place at which the fibres are neither ex-
tended nor compressed. This, shown by the dotted line in
60 209

210 MORPHOLOGICAL DEVELOPMENT.

the figure, is called in mechanical language the " neutral
axis." It varies in position with the nature of the substance
strained : being, in common pine- wood, at a distance of about
five-eighths of the depth from the upper surface, or three-
eighths from the under surface. Clearly, if such a piece of
wood, instead of being subject to a downward force, is secured
at its ends .and subject to an upward force, the distribution
of the compressions and tensions will be reversed, and the
neutral axis will be nearest to the upper surface. Fig. 282
represents these opposite attitudes of the bar and the changed

ASH

f

position of its neutral axis: the arrow indicating the direc-
tion of the force producing the upward bend, and the faint
dotted line a, showing the previous position of the neutral axis.
Between the two neutral axes will be seen a central space;
and it is obvious that when the bar has its strain from time
to time reversed, the repeated changes of its molecular con-
dition must affect the central space in a way different from
that in which they affect the two outer spaces. Fig. 283 is a
diagram conveying some idea of these contrasts in molecular
condition. If A B C D be the middle part of a bar thus
treated, while G- H and K L are the alternating neutral
axes ; then the forces to which the bar is in each case subject,
may be readily shown. Supposing the deflecting force to
be acting in the direction of the arrow E, then the tensions
to which the fibres between G and F are exposed, will be
represented by a series of lines increasing in length as the
distance from G increases; so that the triangle G F M, will
express the amount and distribution of all the molecular
tensions. But the molecular compressions throughout the
space from G to E, must balance the molecular tensions;
and hence, if the triangle G E N" be made equal to the tri-

THE SHAPES OF VERTEBRATE SKELETONS.

211

angle G F M, the parallel lines of which it is composed (here
dotted for the sake of distinction) will express the amount
and distribution of the compressions between E and G.

Similarly, when the deflecting force is in the direction of the
arrow F, the compressions and tensions will be quantitatively
symbolized by the triangles K F 0, and K E P. And
thus the several spaces occupied by full lines and by dotted
lines and by the two together, will represent the different
actions to which different parts of the transverse section are
subject by alternating transverse strains. Here, then, it is
made manifest to the eye that the central space between G
and K, is differently conditioned from the spaces above and
below it; and that the difference of condition is sharply
marked off. The fibres forming the outer surface C D, are
subject to violent tensions and violent compressions. Pro-
gressing inwards the tensions and compressions decrease —
the tensions the more rapidly. As we approach the point G,
the tensions to which the fibres are alternately subject, bear
smaller and smaller ratios to the compressions, and disappear
at the point G. Thence to the centre occur compressions
only, of alternating intensities, becoming at the centre small

212 MORPHOLOGICAL DEVELOPMENT.

and equal ; and from the centre we advance, through a reverse
series of changes, to the other side.

Thus it is demonstrable that any substance in which the
power of resisting compression is unequal to the power of
resisting tension, cannot be subject to alternating transverse
strains, without having a central portion differentiated in its
conditions from the outer portions, and consequently dif-
ferentiated in its structure. This conclusion may easily be
verified by experiment. If something having a certain
toughness but not difficult to break, as a thick piece of sheet
lead, be bent from side to side till it is broken, the surface of
fracture will exhibit an unlikeness of texture between the
inner and outer parts.

§ 255. And now for the application of this seemingly-
irrelevant truth. Though it has no obvious connection with
the interpretation of vertebral structure, we shall soon see
that it fundamentally concerns us.

The simplest type of vertebrate animal, the fish, has a
mode of locomotion which involves alternating transverse
strains. It is not, indeed, subjected to alternating transverse
strains by some outer agency, as in the case we have been
investigating: it subjects itself to them. But though the
strains are here internally produced instead of externally
produced, the case is not therefore removed into a wholly

94 different category. For sup-
mm^. posing Fig. 284 to represent

^-- -""" the outline of a fish when

bent on one side (the dotted lines representing its outline
when the bend is reversed), it is clear that part of the sub-
stance forming the convex half must be in a state of tension.
This state of tension implies the existence in the other half
of some counter-balancing compression. And between the
two there must be a neutral axis. The way in which this
conclusion is reconcilable with the fact that there is tension
somewhere in the concave side of a fish, since the curve is

THE SHAPES OF VERTEBRATE SKELETONS. 213

caused by muscular contractions on the concave side, will be
made clear by the rude illustration which a bow supplies.
A bow may be bent by a thrust against its middle (the two
ends being held back), or it may be bent by contracting
a string that unites its ends; but the distributions of me-
chanical forces within the wood of the bow, though not quite
alike in the two cases, will be very similar. Now while the
muscular action on the concave side of a fish differs from that
represented by the tightened string of a bow, the difference
is not such as to destroy the applicability of the illustration:
the parallel holds so far as this, that within that portion of
the fish's body which is passively bent by the contracting
muscles, there must be, as in a strung bow, a part in com-
pression, a part in tension, and an intermediate part which
is neutral.

After thus seeing that even in the developed fish with
its complex locomotive apparatus, this law of the transverse
strain holds in a qualified way, we shall understand how
much more it must hold in any form that may be supposed
to initiate the vertebrate type — a form devoid of that
segmentation by which the vertebrate type is1 more or less
characterized. We shall see that assuming a rudimentary
animal, still simpler than the Amphioxus, to have a feeble
power of moving itself through the water by the undulations
of its body, or some part of its body, there will necessarily
come into play certain reactions which must affect the median
portion of the undulating mass in a way unlike that in
which they affect its lateral portions. And if there exists in
this median portion a tissue which keeps its place with any
constancy, we may expect that the differential conditions
produced in it by the transverse strain, will initiate a dif-
ferentiation. It is true that the distribution of the viscera
in the Amphioxus, Fig. 191, and in the type from which we
may suppose it to have -arisen, is such as to interfere with this
process. It is also true that the actions and reactions de-
scribed would not of themselves give to the median portion

214

MORPHOLOGICAL DEVELOPMENT.

a cylindrical shape, like that of the cartilaginous rod running
along the back of the Amphioxus. But what we have here
to note in the first place is, that these habitual alternate

flexions have a tendency to mark off from the outer parts
an unlike inner part, which may be seized hold of, main-
tained, and further modified, by natural selection, should
any advantage thereby result. And we have to note in the
second place, that an advantage is likely to result. The
contractions cannot be effective in producing undulations,
unless the general shape of the body is maintained. External
muscular fibres unopposed by an internal resistant mass,
would cause collapse of the body. To meet the require-
ments there must be a means of maintaining longitudinal
rigidity without preventing bends from side to side ; and such
a means is presented by a structure initiated as described.
In brief, whether we have or have not the actual cause, we
have here at any rate " a true cause." Though there are
difficulties in tracing out the process in a definite way, it
may at least be said that the mechanical genesis of this rudi-
mentary vertebrate axis is quite conceivable. And even the
difficulties may, I think, be more fully met than at first
sight seems possible.

What is to be said of the other leading trait which the
simplest vertebrate animal has in common with all higher
vertebrate animals — the segmentation of its lateral muscular
masses? Is this, too, explicable on the mechanical hypo-
thesis? Have we, in the alternating transverse strains, a
cause for the fact that while the rudimentary vertebrate axis

THE SHAPES OF VERTEBRATE SKELETONS. 215

is without any divisions, there are definite divisions of the
substance forming the animal's sides? I think we have. A
glance at the distribution of forces under the transverse
strain, as represented in the foregoing diagrams, will show
how much more severe is the strain on the outer parts than
on the inner parts ; and how, consequently, any modifications
of structure eventually necessitated, will arise peripherally
before they arise centrally. The perception of this may be
enforced by a simple experiment. Take a stick of sealing-
wax and warm it slowly and moderately before the fire, so as
to give it a little flexibility. Then bend it gently until it is
curved into a semi-circle. On the convex surface small
cracks will be seen, and on the concave surface wrinkles;
while between the two the substance remains undistorted.
If the bend be reversed and re-reversed, time after time,
these cracks and wrinkles will become fissures which gradu-
ally deepen. But now, if changes of this class, entailed by
alternating transverse strains, commence superficially, as they
manifestly must; there arise the further questions — What
will be the special modifications produced under these special
conditions? and through what stages will these modifica-
tions progress ? Every one has literally at hand an example
of the way in which a flexible external layer that is now
extended and now compressed, by the bending of the mass it
covers, becomes creased; and a glance at the palms and the
fingers will show that the creases are near one another
where the skin is thin, and far apart where the skin is thick.
Between this familiar case and the case of the rhinoceros-
hide, in which there are but a few large folds, various grada-
tions may be traced. Now the like must happen with the
increasing layers of contractile fibres forming the sides of
the muscular tunic in such a type as that supposed. The
bendings will produce in them small wrinkles while they are
thin, but more decided and comparatively distant fissures
as they become thick. Fig. 289, which is a horizontal longi-
tudinal section, shows how these thickening layers will

216 MORPHOLOGICAL DEVELOPMENT.

adjust themselves on the convex and the concave surfaces,
supposing the fibres of which they are composed to be ob-
lique, as their function requires; and it
is not difficult to see that when once
definite divisions have been established,
they will advance inwards as the layers
develop; and will so produce a series of
muscular bundles. Here then we have
something like the myocommata [or myotomes as now called]
which are traceable in the Amphioxus, and are conspicuous
in all superior fishes.

§ 256. These are highly speculative conceptions. I have
ventured to present them with the view of implying that
the hypothesis of the mechanical genesis of vertebrate struc-
ture is not wholly at fault when applied to the most rudi-
mentary vertebrate animal. Lest it should be alleged that
the question is begged if we set out with a type which, like
the Amphioxus, already displays segmentation throughout
its muscular system, it seemed needful to indicate conceiv-
able modes in which there may have been mechanically pro-
duced those leading traits that distinguish the Amphioxus.
All I intend to suggest is that mechanical actions have been
at work, and that probably they have operated in the manner
alleged : so preparing the way for natural selection.

But now let us return to the region of established fact, and
consider whether such actions and reactions as we actually
witness, are adequate causes of those observed differentiations
and integrations which distinguish the more-developed verte-
brate animals. Let us see whether the theory of mechanical
genesis affords us a deductive interpretation of the inductive
generalizations.

Before proceeding, we must note a process of functional
adaptation which here co-operates with natural selection.
I refer to the usual formation of denser tissues at those
parts of an organism which are exposed to the greatest

THE SHAPES OF VERTEBRATE SKELETONS. 21?

strains — either compressions or tensions. Instances of hard-
ening under compression are made familiar to us by the
skin. We have the general contrast between the soft skin
covering the body at large, and the indurated skin covering
the inner surfaces of the hands and the soles of the feet.
We have the fact that even within these areas the parts
on which the pressure is habitually greatest have the skin
always thickest; and that in each person special points
exposed to special pressures become specially dense — often
as dense as horn. Further, we have the converse fact that
the skin of little-used hands becomes abnormally thin — even
losing, in places, that ribbed structure which distinguishes
skin subject to rough usage. Of increased density directly
following increased tension, the skeletons, whether of men
or animals, furnish abundant evidence. Anatomists easily
discriminate between the bones of a strong man and those of
a weak man, by the greater development of those ridges and
crests to which the muscles are attached; and naturalists, on
comparing the remains of domesticated animals with those
of wild animals of the same species, find kindred differences.
The first of these facts shows unmistakably the immediate
effect of function on structure, and by obvious alliance with
it the second may be held to do the same: both implying
that the deposit of dense substance capable of great resist-
ance, constantly takes place at points where the tension is
excessive.

Taking into account, then, this adaptive process, con-
tinually aided by the survival of individuals in which it
has taken place most rapidly, we may expect, on tracing up
the evolution of the vertebrate axis, to find that as the mus-
cular power becomes greater there arise larger and harder
masses of tissue, serving the muscles as points d'appui; and
that these arise first in those places where the strains are
greatest. Now this is just what we do find. The myocom-
mata are so placed that their actions are likely to affect first
that upper coat of the notochord, where there are found

218 MORPHOLOGICAL DEVELOPMENT.

" quadrate masses of somewhat denser tissue," which " seem
faintly to represent neural spines," even in the Amphioxus.
It is by the development of the neural spines, and after them
of the haemal spines, that the segments of the vertebral
column are first marked out ; and under the increasing strains
of more-developed myocommata, it is just these peripheral
appendages of the vertebral segments that must be most
subject to the forces which cause the formation of denser
tissue. It follows from the mechanical hypothesis that as
the muscular segmentation must begin externally and pro-
gress inwards, so, too, must the vertebral segmentation.
Besides thus finding reason for the fact that in fishes with
wholly cartilaginous skeletons, the vertebral segments are
indicated by these processes, while yet the notochord is
unsegmented; we find a like reason for the fact that the
transition from the less-dense cartilaginous skeleton to the
more-dense osseous skeleton, pursues a parallel course. In
the existing Lepidosiren, which by uniting certain piscine and
amphibian characters betrays its close alliance with primitive
types, the axial part of the vertebral column is unossified,
while there is ossification of the peripheral parts. Similarly
with numerous genera of fishes classed as palaeozoic. The
fossil remains of them show that while the neural and haemal
spines consisted of bone, the central parts of the vertebrae
were not bony. It may in some cases be noted, too, both in
extant and in fossil forms, that while the ossification is com-
plete at the outer extremities of the spines it is incomplete
at their inner extremities — thus similarly implying centri-
petal development.

§ 257. After these explanations the process of eventual
segmentation in the spinal axis itself, will be readily under-
stood. The original cartilaginous rod has to maintain longi-
tudinal rigidity while permitting lateral flexion. As fast as
it becomes definitely marked out, it will begin to concentrate
within itself a great part of those pressures and tensions

THE SHAPES OF VERTEBRATE SKELETONS. 219

caused by transverse strains. As already said, it must be
acted upon much in the same manner as a bow, though it is
bent by forces acting in a more indirect way ; and like a bow,
it must, at each bend, have the substance of its convex side
extended and the substance of its concave side compressed.
So long as the vertebrate animal is small or inert, such a
cartilaginous rod may have sufficient strength to withstand
the muscular strains; but, other things equal, the evolution
of an animal that is large, or active, or both, implies mus-
cular strains which must tend to cause modification in such a
cartilaginous rod. The results of greater bulk and of greater
vivacity may be best dealt with separately. As the

animal increases in size, the rod will grow both longer and
thicker. On looking back at the diagrams of forces caused
by transverse strains, it will be seen that as the rod grows
thicker, its outer parts must be exposed to more severe ten-
sions and pressures if the degree of bend is the same. It is
doubtless true that when the fish, advancing by lateral
undulations, becomes longer, the curvature assumed by the
body at each movement becomes less; and that from this
cause the outer parts of the notochord are, other things
equal, less strained — the two changes thus partially neutrali-
zing one another. But other things are not equal. For
while, supposing the shape of the body to remain constant,
the force exerted in moving the body increases as the cubes
of its dimensions, the sectional area of the notochord, on
which fall the reactions of this exerted force, increases only
as the squares of the dimensions: whence results a greater
stress upon its substance. This, however, will not be very
decided where there is no considerable activity. It is clear
that augmenting bulk, taken alone, involves but a moderate
residuary increase of strain on each portion of the notochord ;
and this is probably the reason why it is possible for a large
sluggish fish like the Sturgeon, to retain the notochordal
structure. But now, passing to the effects of greater

activity, a like dynamical inquiry at once shows us how rapid-

220 MORPHOLOGICAL DEVELOPMENT.

ly the violence of the actions and reactions rises as the move-
ments become more vivacious. In the first place, the resist-
ance of a medium such as water increases as the square of
the velocity of the body moving through it ; so that to main-
torn double the speed, a fish has to expend four times the
energy. But the fish has to do more than this — it has to
initiate this speed, or to impress on its mass the force implied
by this speed. Now the vis viva of a moving body varies as
the square of the velocity; whence it follows that the energy
required to generate that vis viva is measured by the square
of the velocity it produces. Consequently, did the fish put
itself in motion instantaneously, the expenditure of energy in
generating its own vis viva and simultaneously overcoming
the resistance of the water, would vary as the fourth power
of the velocity. But the fish cannot put itself in motion
instantaneously — it must do it by increments; and thus it
results that the amounts of the forces expended to give itself
different velocities must be represented by some series of
numbers falling between the squares and the fourth powers
of those velocities. Were the increments slowly accumulated,
the ratios of increasing effort would but little exceed the ratios
of the squares; but whoever observes the sudden, convulsive
action with which an alarmed fish darts out of a shallow into
deep water, will see that the velocity is rapidly generated,
and that therefore the ratios of increasing effort probably
exceed the ratios of the squares very considerably. At any
rate it will be clear that the efforts made by fishes in rushing
upon prey or escaping enemies (and it is these extreme efforts
which here concern us) must, as fishes become more active,
rapidly exalt the strains to be borne by their motor organs;
and that of these strains, those which fall upon the noto-
chord must be exalted in proportion to the rest. Thus the
development of locomotive power, which survival of the
fittest must tend in most cases to favour, involves such in-
crease of stress on the primitive cartilaginous rod as will
tend, other things equal, to cause its modification.

THE SHAPES OF VERTEBRATE SKELETONS. 221

What must its modification be ? Considering the compli-
cation of the influences at work, conspiring, as above indi-
cated, in various ways and degrees, we cannot expect to do
more than form an idea of its average character. The nature
of the changes which the notochord is likely to undergo, where
greater bulk is accompanied by higher activity, is rudely
indicated by Figs. 291, 292, and 293. The successively

thicker lines represent the successively greater strains to
which the outer layers of tissue are exposed ; and the widen-
ing inter-spaces represent the greater extensions which they
have to bear when they become convex, or else the greater
gaps that must be formed in them. Had these outer layers
to undergo extension only, as on the convex side, continued
natural selection might result in the formation of a tissue
elastic enough to admit of the requisite stretching. But at
each alternate bend these outer layers, becoming concave,
are subject to increased compression — a compression which
they cannot withstand if they have become simply more
extensible. To withstand this greater compression they must
become harder as well as more extensible. How are these
two requirements to be reconciled? If, as facts warrant
us in supposing, a formation of denser substance occurs at
those parts of the notochord where the strain is greatest;
it is clear that this formation cannot so go on as to produce
a continuous mass : the perpetual flexions must prevent this.
If matter that will not yield at each bend, is deposited while
the bendings are continually taking place, the bendings will
maintain certain places of discontinuity in the deposit —
places at which the whole of the stretching consequent on
each bend will be concentrated. And thus the tendency will
be to form segments of hard tissue capable of great resistance

222 MORPHOLOGICAL DEVELOPMENT.

to compression, with intervals filled by elastic tissue capable
of great resistance to extension — a vertebral column.

And now observe how the progress of ossification is just
such as conforms to this view. That centripetal develop-
ment of segments which holds of the vertebrate animal as a
whole, as, if caused by transverse strains, it ought to do, and
which holds of the vertebral column as a whole, as it ought
to do, holds also of the central axis. On the mechanical
hypothesis, the outer surface of the notochord should be the
first part to undergo induration, and that division into seg-
ments which must accompany induration. And accordingly,
in a vertebral column of which the axis is beginning to
ossify, the centrums consist of bony rings inclosing a still-
continuous rod of cartilage.

§ 258. Sundry other general facts disclosed by the com-
parative morphology of the Vertebrata, supply further con-
firmation. Let us take first the structure of the skull.

On considering the arrangement of the muscular flakes, or
myocommata, in any ordinary fish which comes to table — an
arrangement already sketched out in the Amphioxus — it is
not difficult to see that that portion of the body out of which
the head of the vertebrate animal becomes developed, is a
portion which cannot subject itself to bendings in the same
degree as the rest of the body. The muscles developed there
must be comparatively short, and much interfered with by
the pre-existing orifices. Hence the cephalic part will not
partake in any considerable degree of the lateral undula-
tions ; and there will not tend to arise in it any such distinct
segmentation as arises elsewhere. We have here, then, an
explanation of the fact, that from the beginning the develop-
ment of the head follows a course unlike that of the spinal
column; and of the fact that the segmentation, so far as il
can be traced in the head, is most readily to be traced in the
occipital region and becomes lost in the region of the face.
For if, as we have seen, the segmentation consequent on

THE SHAPES OP VERTEBRATE SKELETONS. 223

mechanical actions and reactions must progress from without
inwards, affecting last of all the axis; and if, as we have
seen, the region of the head is so circumstanced that the
causes of segmentation act but feebly even on its periphery;
then that terminal portion of the primitive notochord which
is included in the head, having to undergo no lateral land-
ings, may ossify without division into segments.

Of other incidental evidences supplied by comparative
morphology, let me next refer to the supernumerary bones,
which the theory of Goethe and Oken as elaborated by Prof.
Owen, has to get rid of by gratuitous suppositions. In many
fishes, for example, there are what have been called inter-
neural spines and inter-haemal spines. These cannot by any
ingenuity be affiliated upon the archetypal vertebra, and
they are therefore arbitrarily rejected as bones belonging to
the exo-skeleton ; though in shape and texture they are
similar to the spines between which they are placed. On the
hypothesis of evolution, however, these additional bones are
accounted for as arising under actions like those that gave
origin to the bones adjacent to them. And similarly with
such bones as those called sesamoid; together with others
too numerous to name.

§ 259. Of course the foregoing synthesis is to be taken
simply as an adumbration of the process by which the verte-
brate structure may have arisen through the continued
actions of known agencies. The motive for attempting it has
been two-fold. Having, as before said, given reasons for con-
cluding that the segments of a vertebrate animal are not
homologous in the same sense as are those of an annulose
animal, it seemed needful to do something towards showing
how they are otherwise to be accounted for ; and having here,
for our general subject, the likenesses and differences among
the parts of organisms, as determined by incident forces, it
seemed out of the question to pass by the problem presented
by the vertebrate skeleton.

224 MORPHOLOGICAL DEVELOPMENT.

Leaving out all that is hypothetical, the general argument
may be briefly presented thus : — The evolution from the
simplest known vertebrate animal of a powerful and active
vertebrate animal, implies the development of a stronger
internal fulcrum. The internal fulcrum cannot be made
stronger without becoming more dense. And it cannot be-
come more dense while retaining its lateral flexibility, with-
out becoming divided into segments. Further, in conformity
w&h the general principles thus far traced, these segments
must be alike in proportion as the forces to which they are
exposed are alike, and unlike in proportion as these forces
are unlike; and so there necessarily results that unity in
variety by which the vertebral column is from the beginning
characterized. Once more, we see that the explanation ex-
tends to those innumerable and more marked divergences
from homogeneity, which vertebras undergo in the various
higher animals. Thus, the production of vertebras, the pro-
duction of likenesses among vertebras, the production of un-
likenesses among vertebras, and the production of unlike-
nesses among vertebral columns, are interpretable as parts of
one general process, and as harmonizing with one general
principle.

Whether sufficient or insufficient, the explanation here
given assigns causes of known kinds producing effects such
as they are known to produce. It does not, as a solution of
one mystery, offer another mystery of which no solution is
to be asked. It does not allege a Platonic 184a, or fictitious
entity, which explains the vertebrate skeleton by absorbing
into itself all the inexplicability. On the contrary, it assumes
nothing beyond agencies by which structures in general are
moulded — agencies by which these particular structures are,
indeed, notoriously modifiable. An ascertained cause of cer-
tain traits in vertebras and other bones, it extends to all
other traits of vertebras; and at the same time assimilates
the morphological phenomena they present to much wider
classes of morphological phenomena.

THE SHAPES OP VERTEBRATE SKELETONS. 225

[Note. — The theory set forth in the foregoing chapter, is
an elaboration of one suggested at the close of a criticism
of Prof. Owen's Archetype and Homologies of the Vertebrate
Skeleton, already referred to in § 210 as having been pub-
lished in the Medico-Chirurgical Review for October, 1858.
It is now reproduced in Appendix B. Since the issue of this
elaborated exposition, in No. 15 of my serial in December,
1865, verifications of it have from time to time been published.
In his work The Primary Factors of Organic Evolution,
Prof. Cope of Philadelphia writes : —

" Mr. Herbert Spencer has endeavoured to account for the
origin of the segmentation of muscles into myotomes, and
the division of the sheath of the notochord into vertebrae,
by supposing it to be due to the lateral swimming movements
of the fishes, which first exhibit these structures. With this
view various later authors have agreed, and I have offered
some additional evidence of the soundness of this position
with respect to the vertebral axis of Batrachia, and the
origin of limb articulations. It is true that the origin of
segmentation in the vertebral column of the true fishes and
the Batrachia turns out to have been less simple in its pro-
cess than was suggested by Mr. Spencer, but his general
principle holds good, now that paleontology has cleared up
the subject" (pp. 367-8).

An allusion in the foregoing extract is made by Prof. Cope
to certain observations set forth in his work entitled The
Origin of the Fittest. On pp. 305-6 of it will be found the
following sentences : —

" Now, all the Permian land-animals, reptiles and batra-
chians, retain this notochord with the elements of osseous
vertebrae, in a greater or less degree of completeness. There
are some in South Africa, I believe, in which the ossification
has come clear through the notochord; but they are few.
. . . There is something to be said as to the condition of
the column from a mechanical standpoint, and it is this:
that the chorda exists, with its osseous elements disposed
61

226 MORPHOLOGICAL DEVELOPMENT.

about it; and in the Permian batrachians, equally related to
salamanders and frogs, these osseous elements are arranged in
the sheath or skin of the chorda ; and they are in the form of
regular concave segments, very much like such segments as
you can take from the skin of an orange — but parts of a
cylinder, and having greater or less dimensions according to
the group or species. Now, the point of divergence of these
segments is on the side of the column. The contacts are
placed on the side of the column where the segments separ-
ate— the upper segments rising and the lower segments
coming downward. To the upper segments are attached the
arches and their articulations, and the lower segments are
like the segments of a cylinder. If you take a flexible
cylinder, and cover it with a more or less inflexible skin or
sheath, and bend that cylinder sidewise, you of course will
find that the wrinkles or fractures of that part of the surface
will take place along the line of the shortest curve, which
is on the side; and, as a matter of fact, you have breaks
of very much the character of the segments of the Permian
Batrachia. ... In the cylinder bending both ways, of
course the shortest line of curve is right at the centre of the
side of that cylinder, and the longest curve is of course at
the summit and base, and the shortest curve will be the point
of fracture. And that is exactly what I presume has
happened in the case of the construction of the segments of
the sheath of the vertebral column, by the lateral motion of
the animal in swimming, and which has been the actual cause
of the disposition of the osseous material in its form. . . .
That is the state of the vertebral column of many of the
Vertebrata of the Permian period."

In his essay on " The Mechanical Causes of the Develop-
ment of the Hard Parts of the Mammalia," published in the
American Journal of Morphology (Vol. Ill), Prof. Cope has
carried the interpretation further, by showing that in kindred
ways the genesis of articulations and limb-bones may be ex-

THE SHAPES OF VERTEBRATE SKELETONS. 227

plained. On p. 163 he enunciates the general principle of
his interpretation as follows : —

"It cannot have been otherwise than that, since the
motions of animals continued during the evolution of their
hard parts, these hard parts grew in exact adaptation to
these movements. Thus at the points of greatest flexure
joints would be formed, and between these joints the deposit
would be continuous."

Evidently if osseous structures are produced by deposits
of calcareous matters in pre-existing cartilaginous structures,
or other structures of flexible materials, the deposits must be
so carried on that while dense resistant masses are produced
these must admit of such free movements as the creature's
life necessitates, and must so form adapted joints.

Let it be understood, however, that the hypothesis set
forth in the foregoing chapter and extended by Prof. Cope,
which serves to interpret a large part of the phenomena
of osseous structures in the Vertebrata, does not serve to
interpret them all. While the formation of hard parts has
been in large measure initiated and regulated by tensions and
pressures, there are hard parts the formation of which cannot
be thus explained. The bones of the skull are the most
obvious instances. These are apparently referable to no
other cause than the survival of the fittest — the survival of
individual animals in which greater density of the brain-
covering yielded better protection against external injuries.
Without enumerating other instances which might be given,
it will suffice to recognize the truth that natural selection of
favourable variations and the inheritance of functionally-
produced changes have all along co-operated: each of them
in some cases acting alone, but in other cases both acting
together.]

CHAPTEE XVI.

THE SHAPES OP ANIMAL-CELLS.

§ 260. Among animals as among plants, the laws of mor-
phological differentiation must be conformed to by the mor-
phological units, as well as by the larger parts and by the
wholes formed of them. It remains here to point out that
the conformity is traceable where the conditions are simple.

In the shapes assumed by those rapidly-multiplying cells
out of which each animal is developed, there is a conspicuous
23* subordination to the surrounding actions.

Fig. 294 represents the cellular embryonic
mass that arises by repeated spontaneous
fissions. In it we see how the cells, origin-
ally spherical, are changed by pressure
against one another and against the limit-
ing membrane; and how their likenesses
and unlikenesses are determined by the likenesses and un-
likenesses of the forces to which they are exposed. This fact
may be thought scarcely worth pointing out. But it is
worth pointing out, because what is here so obvious a con-
sequence of mechanical actions, is in other cases a conse-
quence of actions composite in their kinds and involved in
their distribution. Just as the equalities and inequalities of
dimensions among aggregated cells, are here caused by the
equalities and inequalities among their mutual pressures in
different directions ; so, though less manifestly, the equalities

THE SHAPES OP ANIMAL CELLS.

229

and inequalities of dimensions among other aggregated cells,
are caused by the equalities and inequalities of the osmotic,
chemical, thermal, and other forces besides the mechanical,
to which their different positions subject them.

§ 261. This we shall readily see on observing the ordinary
structures of limiting membranes, internal and external.
In Fig. 295, is shown a 29s

much-magnified section
of a papilla from the
gum. The cells of which
it is composed originate
in its deeper part; and
are at first approximately
spherical. Those of them
which, as they develop, are thrust outwards by the new
cells that continually take their places, have their shapes
gradually changed. As they grow and successively advance
to replace the superficial cells, when these exfoliate, they
become exposed to forces which are more and more different
in the direction of the surface from what they are in lateral
directions; and their dimensions gradually assume corre-
sponding differences.

Another species of limiting membrane, called cylinder-
epithelium, is represented
in Fig. 296. Though its
mode of development is
such as to render the
shapes of its cells quite
unlike those of pavement-
epithelium, as the above-described kind is sometimes called,
its cells equally exemplify the same general truth. For the
chief contrast which each of them presents, is the contrast
between its dimension at right angles to the surface of the
membrane, and its dimension parallel to that surface.

It is needless for our present purpose to examine further

230

MORPHOLOGICAL DEVELOPMENT.

the evidence furnished by Histology; nor, indeed, would
further examination of this evidence be likely to yield
definite results. In the cases given above we have marked
differences among the incident forces; and therefore have a
chance of finding, as we do find, relations between these and
differences of form. But the cells composing masses of
tissue are severally subject to forces which are indeterminate ;
and therefore the interpretation of their shapes is imprac-
ticable. It must suffice to observe that so far as the facts go
they are congruous with the hypothesis.

CHAPTER XVII.

SUMMARY OF MORPHOLOGICAL DEVELOPMENT.

«

§ 262. That any formula should be capable of expressing
a common character in the shapes of things so unlike as a
tree and a cow, a flower and a centipede, is a remarkable
fact ; and is a fact which affords strong prima facie evidence
of truth. For in proportion to the diversity and multiplicity
of the cases to which any statement applies, is the probability
that it sets forth the essential relations. Those connexions
which remain constant under all varieties of manifestation,
are most likely to be the causal connexions.

Still higher will appear the likelihood of an alleged law of
organic form possessing so great a comprehensiveness, when
we remember that on the hypothesis of Evolution, there must
exist between all organisms and their environments, certain
congruities expressible in terms of their actions and reactions.
The forces being, on this hypothesis, the causes of the forms,
it is inferable, a priori, that the forms must admit of generali-
zation in terms of the forces ; and hence, such a generalization
arrived at a posteriori, gains the further probability due to
fulfilment of anticipation.

Nearer yet to certainty seems the conclusion thus reached,
on finding that it does but assert in their special manifesta-
tions, the laws of Evolution in general — the laws of that
universal re-distribution of matter and motion which hold

231

232 MORPHOLOGICAL DEVELOPMENT.

throughout the totality of things, as well as in each of its
parts.

It will be useful to glance back over the various minor
inferences arrived at, and contemplate them in their ensemble
from these higher points of view.

§ 263. That process of integration which every plant dis-
plays during its life, we found reason to think has gone on
during the life of the vegetal kingdom as a whole. Proto-
plasm into cells, cells into folia, folia into axes, axes into
branched combinations — such, in brief, are the stages passed
through by every shrub; and such appear to have been the
stages through which plants of successively-higher kinds
have been evolved from lower kinds. Even among certain
groups of plants now existing, we find aggregates of the first
order passing through various gradations into aggregates of
the second order — here forming small, incoherent, indefinite
assemblages, and there forming large, definite, coherent
fronds. Similar transitions are traceable through which
these integrated aggregates of the second order pass into
aggregates of the third order: in one species the unions of
parent-fronds with the fronds that bud out from them, being
temporary, and in another species such unions being longer
continued; until, in species still higher, by a gemmation
which is habitual and regular, there is produced a definitely-
integrated aggregate of the third order — an axis bearing
fronds or leaves. And even between this type and a type
further compounded, a link occurs in the plants which cast
off, in the shape of bulbils, some of the young axes they
produce. As among plants, so among animals. A

like spontaneous fission of cells ends here in separation, there
in partial aggregation, while elsewhere, by closer combina-
tion of the multiplying units, there arises a coherent and
tolerably definite individual of the second order. By the
budding of individuals of the second order, there are in some
cases produced other separate individuals like them; in some

SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 233

cases temporary aggregates of such like individuals; and in
other cases permanent aggregates of them: certain of which
become so definitely integrated that the individualities of
their component members are almost lost in a tertiary indi-
viduality.

Along with this progressive integration there has gone on
a progressive differentiation. Vegetal units of whatever
order, originally homogeneous, have become heterogeneous
while they have become united. Spherical cells' aggregating
into threads, into laminae, into masses, and into special tis-
sues, lose their sphericity; and instead of remaining all
alike assume innumerable unlikenesses — from uniformity
pass into multiformity. Fronds combining to form axes,
severally acquire definite differences between their attached
ends and their free ends; while they also diverge from one
another in their shapes at different parts of the axes they
compose. And axes, uniting into aggregates of a still higher
order, become contrasted in their sizes, curvatures, and the
arrangements of their appendages. Similarly among

animals. Those components of them which, with a certain
license, we class as morphological units, while losing their
minor individualities in the major individualities formed of
them, grow definitely unlike as they grow definitely com-
bined. And where the aggregates so produced become, by
coalescence, segments of aggregates of a still higher order,
they, too, diverge from one another in their shapes.

The morphological differentiation which thus goes hand
in hand with morphological integration, is clearly what the
perpetually-complicating conditions would lead us to antici-
pate. Every addition of a new unit to an aggregate of such
units, must affect the circumstances of the other units in all
varieties of ways and degrees, according to their relative
positions — must alter the distribution of mechanical strains
throughout the mass, must modify the process of nutrition,
must affect the relations of neighbouring parts to surround-
ing diffused actions; that is, must initiate a changed inci-

234 MORPHOLOGICAL DEVELOPMENT.

dence of forces tending ever to produce changed structural
arrangements.

§ 264. This broad statement of the correspondence be-
tween the general facts of Morphological Development and
the principles of Evolution at large, may be reduced to
statements of a much more specific kind. The phenomena
of symmetry and unsymmetry and asymmetry, which we
have traced out among organic forms, are demonstrably in
harmony with those laws of the re-distribution of matter and
motion to Which Evolution conforms. Besides the myriad-
fold illustrations of the instability of the homogeneous,
afforded by these aggregates of units of each order, which,
at first alike, lapse gradually into unlikeness; and besides
the myriad-fold illustrations of the multiplication of effects,
which these ever-complicating differentiations exhibit to us;
we have also myriad-fold illustrations of the definite equali-
ties and inequalities of structures, produced by definite equali-
ties and inequalities of forces.

The proposition arrived at when dealing with the causes
of Evolution, " that in the actions and reactions of force and
matter, an unlikeness in either of the factors necessitates an
unlikeness in the effects; and that in the absence of unlike-
ness in either of the factors the effects must be alike " (First
Principles, § 169), is a proposition which implies all these
particular likenesses and unlikenesses of parts which we
have been tracing. For have we not everywhere seen that
the strongest contrasts are between the parts that are most
contrasted in their conditions; while the most similar parts
are those most-similarly conditioned? In every plant the
leading difference is between the attached end and the free
end; in every branch it is the same; in every leaf it is
the same. And in every plant the leading likenesses are
those between the two sides of the branch, the two sides of
the leaf, and the two sides of the flower, where these parts
are two-sided in their conditions ; or between all sides of the

SUMMARY OF MORPHOLOGICAL DEVELOPMENT. 235

branch, all sides of the leaf, and all sides of the flower, where
these parts are similarly conditioned on all sides. So, too, is it
with animals which move about. The most marked contrasts
they present are those between the part in advance and the
part behind, and between the upper part and the under part ;
while there is complete correspondence between the two sides.
Externally the likenesses and differences among limbs, and
internally the likenesses and differences among vertebrae, are
expressible in terms of this same law.

And here, indeed, we may see clearly that these truths are
corollaries from that ultimate truth to which all phenomena
of Evolution are referable. It is an inevitable deduction
from the persistence of force, that organic forms which have
been progressively evolved, must present just those funda-
mental traits of form which we find them present. It cannot
but be that during the intercourse between an organism and
its environment, equal forces acting under equal conditions
must produce equal effects; for to say otherwise is, by im-
plication, to say that some force can produce more or less
than its equivalent effect, which is to deny the persistence of
force. Hence those parts of an organism which are, by its
habits of life, exposed to like amounts and like combina-
tions of actions and reactions, must develop alike; while
unlikenesses of development must as unavoidably follow
unlikenesses among these agencies. And this being so, all
the specialities of symmetry and unsymmetry and asymmetry
which we have traced, are necessary consequences.

PART V.

PHYSIOLOGICAL DEVELOPMENT

CHAPTER I.

THE PROBLEMS OF PHYSIOLOGY.

§ 265. The questions to be treated under the above title
are widely different from those which it ordinarily expresses.
We have no alternative, however, but to use Physiology in
a sense co-extensive with that in which we have used
Morphology. We must here consider the facts of function
in a manner parallel to that in which we have, in the fore-
going Part, considered the facts of structure. As, hitherto,
we have concerned ourselves with those most general pheno-
mena of organic form which, holding irrespective of class
and -order and sub-kingdom, illustrate the processes of
integration and differentiation characterizing Evolution at
large; so, now, we have to concern ourselves with the evi-
dences of those differentiations and integrations of organic
functions which have simultaneously arisen, and which
similarly transcend the limits of zoological and botanical
divisions. How heterogeneities of action have progressed
along with heterogeneities of structure — that is the inquiry
before us; and obviously, in pursuing it, all the specialities
with which Physiology usually deals can serve us only as
materials.

Before entering on the study of Morphological Develop-
ment, it was pointed out that while facts of structure may
be empirically generalized apart from facts of function, they
cannot be rationally interpreted apart; and throughout the

239

240 PHYSIOLOGICAL DEVELOPMENT.

foregoing pages this truth has been made abundantly mani-
fest. Here we are obliged to recognize the inter-dependence
still more distinctly; for the phenomena of function cannot
even be conceived without direct and perpetual consciousness
of the phenomena of structure. Though the subject-matter
of Physiology is as broadly distinguished from the subject-
matter of Morphology as motion is from matter ; yet, just as
the laws of motion cannot be known apart from some matter
moved, so there can be no knowledge of function without a
knowledge of some structure as performing function.

Much more than this is obvious. The study of functions,
considered from our present point of view as arising by
Evolution, must be carried on mainly by the study of the
correlative structures. Doubtless, by experimenting on the
organisms which are growing and moving around us, we may
ascertain the connexions existing among certain of their
actions, while we have little or no knowledge of the special
parts concerned in those actions. In a living animal that
can be conveniently kept under observation, we may learn
the way in which conspicuous functions vary together — how
the rate of a man's pulse increases with the amount of
muscular exertion he is undergoing; or how a horse's
rapidity of breathing is in part dependent on his speed.
But though observations of 'this order are indispensable —
though by accumulation and comparison of such observations
we learn which parts perform which functions — though such
observations, prosecuted so as to disclose the actions of all
parts under all circumstances, constitute, when properly
generalized and co-ordinated, what is commonly understood
as Physiology; yet such observations help us but a little
way towards learning how functions came to be established
and specialized. We have next to no power of tracing up
the genesis of a function considered purely as a function — no
opportunity of observing the progressively-increasing quan-
tities of a given action that have arisen in any order of
organisms. In nearly all cases we are able only to show

THE PROBLEMS OF PHYSIOLOGY. 241

the greater growth of the part which we have found performs
the action, and to infer that greater action of the part has
accompanied greater growth of it. 'The tracing out of
Physiological Development, then, becomes substantially a
tracing out of the development of the organs by which the
functions are known to be discharged — the differentiation
and integration of the functions being presumed to have
progressed hand in hand with the differentiation and integra-
tion of the organs. Between the inquiry pursued in Part IV,
and the inquiry to be pursued in this Part, the contrast is
that, in the first place, facts of structure are now to be used
to interpret facts of function, instead of conversely; and, in
the second place, the facts of structure to be so used are not
those of conspicuous shape so much as those of minute texture
and chemical composition.

§ 266. The problems of Physiology, in the wide sense
above described, are, like the problems of Morphology, to be
considered as problems to which answers must be given in
terms of incident forces. On the hypothesis of Evolution
these specializations of tissues and accompanying concentra-
tions of functions, must, like the specializations of shape in
an organism and its component divisions, be due to the ac-
tions and reactions which its intercourse with the environment
involves; and the task before us is to explain how they are
wrought — how they are to be comprehended as results of
such actions and reactions.

Or, to define these problems still more specifically : — Those
extremely unstable substances composing the protoplasm
of which organisms are mainly built, have to be traced
through the various modifications in their properties and
powers, that are entailed on them by changes of relation to
agencies of all kinds. Those organic colloids which pass from
liquid to solid and from soluble to insoluble on the slightest
molecular disturbance — those albumenoid matters which, as
we see in clotted blood or the coagulable lymph poured

242 PHYSIOLOGICAL DEVELOPMENT.

out on abraded surfaces and causing adhesion between
inflamed membranes, assume new forms with the greatest
readiness — are to have their metamorphoses studied in con-
nexion with the influences at work. Those compounds which,
as we see in the quickly-acquired brownness of a bitten apple
or in the dark stains produced by the milky juice of a Dande-
lion, immediately begin to alter when the surrounding actions
alter, are to be everywhere considered as undergoing modifi-
cations by modified conditions. Organic bodies, consisting
of substances that, as I here purposely remind the reader,
are prone beyond all others to change when the incident
forces are changed, we must contemplate as in all their parts
differently changed in response to the different changes of
the incident forces. And then we have to regard the con-
comitant differentiations of their reactions as being concomi-
tant differentiations of their functions.

Here, as before, we must take into account two classes
of factors. We have to bear in mind the inherited results of
actions to which antecedent organisms were exposed, and to
join with these the results of present actions. Each organism
is to be considered as presenting a moving equilibrium of
functions, and a correlative arrangement of structures, pro-
duced by the aggregate of actions and reactions that have
taken place between all ancestral organisms and their envi-
ronments. The tendency in each organism to repeat this
adjusted arrangement of functions and structures, must be
regarded as from time to time interfered with by actions to
which its inherited equilibrium is not adjusted — actions t
which, therefore, its equilibrium has to be re-adjusted. An
in studying physiological development we have in all cases
to contemplate the progressing compromise between the old
and the new, ending in a restored balance or adaptation.

Manifestly our data are so scanty that nothing more
than very general and approximate interpretations of this
kind are possible. If the hypothesis of Evolution fur-
nishes us with a rude conception of the way in which the

i

THE PROBLEMS OF PHYSIOLOGY. 243

more conspicuous and important differentiations of functions
have arisen, it is as much as can be expected.

§ 267. It will be best, for brevity and clearness, to deal
with these physiological problems as we dealt with the
morphological ones — to carry on the inductive statement and
the deductive interpretation hand-in-hand: so disposing of
each general truth before passing to the next. Treating
separately vegetal organisms and animal organisms, we will
in each kingdom consider: — first, the physiological differen-
tiations and accompanying changes of structure which arise
between outer tissues and inner tissues; next, those which
arise between different parts of the outer tissues ; and, finally,
those which arise between different parts of the inner tissues.
What little has to be said concerning physiological integra-
tion must come last. For though, in tracing up Morpho-
logical Evolution, we have to study those processes of inte-
gration by which organic aggregates are formed, before
studying the differentiations that arise among their parts;
we must, contrariwise, in tracing up Physiological Evolution,
study the genesis of the different functions before we study
the interdependence that eventually arises among them and
constitutes physiological unity.

CHAPTER II.

DIFFERENTIATIONS BETWEEN THE OUTER AND INNER
TISSUES OF PLANTS.

§ 268. The simplest plant presents a contrast between its
peripheral substance and its central substance. In each pro-
tophyte, be it a spherical cell or a branched tube, or such
a more-specialized form as a Desmid, a marked unlikeness
exists between the limiting layer and that which it limits.
These vegetal aggregates of the first order may differ widely
from one another in the natures of their outer coats and in
the natures of their contents. As in the Palmella-form of
one of the lower Algoe, there may exist a clothing of jelly;
or, as in Diatom, the walls may take the form of silicious
valves variously sculptured. The contained matter may be
partly or wholly here green, there red, and in other cases
brown. But amid all these diversities there is this one
uniformity — a strong distinction between the parts in contact
with the environment and the parts not in contact with the
environment.

When we remember that this trait is one which these
simple living bodies have in common with bodies that are
not living — when we remember that each inorganic mass
eventually has its outer part more or less differentiated froi
its inner part, here by oxidation, there by drying, and else
where by the actions of light, of moisture, of frost; we cai
scarcely resist the conclusion that, in the one case as in the
other, the contrast is due to the unlike actions to which the
344

THE OUTER AND INNER TISSUES OF PLANTS. 245

parts are subject. Given an originally-homogeneous portion
of protoplasm, and it follows from the general laws of Evolu-
tion (First Principles, §§ 149 — 155), first, that it must lose
its homogeneity, and, second, that the leading dissimilarities
must arise between the parts most-dissimilarly conditioned —
that is, between the outside and the inside. The exterior
must bear amounts and kinds of force unlike the amounts
and kinds which the interior bears ; and from the persistence
of force it follows inevitably that unlike effects must be
wrought on them — they must be differentiated.

What is the limit towards which the differentiation tends ?
We have seen that the re-distribution of matter and motion
whence, under certain conditions, evolution results, can
never cease until equilibrium is reached — proximately a
moving equilibrium, and finally a complete equilibrium (First
Principles, §§170 — 175). Hence, the differentiation must
go on until it establishes such differences in the parts as
shall balance the differences in the forces acting on them.
When dealing with equilibration in general, we saw that this
process is what is called adaptation (First Principles, § 173) ;
and, in this work, we saw that by it the totality of func-
tions of an organism is brought into correspondence with the
totality of actions affecting it (§§ 159 — 163). Manifestly in
this case, as in all others, either death or adjustment must
eventually result. A force falling on one of these minute
aggregates of protoplasm, must expend itself in working its
equivalent of change. If this force is such that in expend-
ing itself it disturbs beyond rectification the balance of the
organic processes, then the aggregate is disintegrated or de-
composed. But if it does not overthrow that moving equi-
librium constituting the life of the aggregate, then the
aggregate continues in that modified form produced by the
expenditure of the force. Thus, by direct equilibration, con-
tinually furthered by indirect equilibration, there must arise
this distinction between the outer part adapted to meet outer
forces, and the inner part adapted to meet inner forces. And

246 PHYSIOLOGICAL DEVELOPMENT.

their respective actions, as thus meeting outer and inner
forces, must be what we call their respective functions.

§ 269. Aggregates of the second order exhibit parallel
traits, admitting of parallel interpretations. Integrated
masses of cells or units homologous with protophytes,
habitually show us contrasts between the characters of the
superficial tissues and the central tissues. Such among these
aggregates of the second order as have their component units
arranged into threads or lamina?, single or double, cannot, of
course, furnish contrasts of this kind; for all their units are
as much external as internal. We must turn to the more or
less massive forms.

Of these, among Fungi, the common Puff-ball is a good
example — good because it presents this fundamental differen-
tiation but little complicated by others. In it we have a
cortical layer of interwoven hyphae obviously unlike the
mass of spores which it incloses. So far as the unlikeness
between external and internal parts is concerned, we see here
a relation analogous to that existing in the simple cell; and
we see in it a similar meaning: there is a physiological
differentiation corresponding to the difference in the incidence
of forces.

Under various forms the Algce show just the same rela-
tion. Where, as in Codium Bursa, we have the ramified
tubular branches of the thallus aggregated into a hollow
globular mass, the outer and inner surfaces are contrasted
both in colour and structure, though the tubules composing
the two surfaces are continuous with one another. In Rivu-
laria, again, we see the like, both in the radial arrangement
of the imbedded threads and in the difference of colour
between the exterior of the imbedding jelly and its interior.
The more-developed Algce of all kinds repeat the antithesis.
In branched stems, when they consist of more than single
rows of cells, the outer cells become unlike the inner, as shown
in Fig. 35. Such types as Chrysymenia rosea show us this

THE OUTER AND INNER TISSUES OF PLANTS. 247

im likeness very conspicuously. And it holds even with rib-
bon-shaped fronds. Wherever one of these is composed of
three, four, or more layers, as in Laminaria and Punctaria,
the cells of the external layers are strongly distinguished
from those of the internal layers, both by their comparative
smallness and by their deep colour.

§ 270. The higher plants variously display the like
fundamental distinction between outer and inner tissues.
Each leaf, thin as it is, exemplifies this differentiation of the
parts immediately in contact with the environment from the
parts not in immediate contact with the environment. Its
epidermal cells, forming a protecting envelope, diverge physi-
cally and chemically from the mesophyll cells, which carry
on the more active functions. And the contrast may be
observed to establish itself in the course of development. At
first the component cells of the leaf are all alike; and this
unlikeness between the cells of the outer and inner layers,
arises simultaneously with the rise of differences in their con-
ditions— differences that have acted on all ancestral leaves
as they act on the individual leaf.

An unlikeness more marked in kind but similar in mean-
ing, exists between the bark of every branch and the tissues
it clothes. The phaenogamic axis, especially when it under-
goes what is known as secondary thickening, is commonly
characterized by an outer zone of cells (the cork layer) differ-
ing from the inner layers in character and function, as it
differs from them in position. Subject as this outer layer is
to the unmitigated actions of forces around — to abrasions, to
extremes of heat and cold, to evaporation and soaking with
water — its units have to be brought into equilibrium with
these more violent actions, and have acquired molecular
constitutions more stable than those of the interior cells.
That is to say, the forces which differentiate the cortical part
from the rest are the forces which it has to resist, and from
which it passively protects the parts within. How

248 PHYSIOLOGICAL DEVELOPMENT.

clearly this heterogeneity of structure and function is conse-
quent upon intercourse with the environment, every tree
and shrub shows. The young shoots, alike of annuals and
perennials, are quite green and soft at their extremities.
Among plants of short lives, there is usually but a slight
development of bark or none at all : such traces of it as the
surface of the axis acquires being seen only at its lowermost
or oldest portion. In long-lived plants, however, this forma-
tion of a tough opaque coating takes place more rapidly ; and
shows us distinctly the connexion between the degree of
differentiation and the length of exposure. For, in a growing
twig, we see that the bark, invisible at the bud, thickens by
insensible gradations as we go downwards to the junction of
the twig with the branch ; and we come to still thicker parts
of it as we descend along the branch towards the main stem.
Moreover, on examining main stems we find that while in
some trees the bark, cracked by expansion of the wood, drops
off in flakes, leaving exposed patches of the inner tissue which
presently become green and finally develop new bark; in
other trees the exfoliated flakes continue adherent, and in the
course of years form a rugged fissured coat: so producing
a still more marked contrast between outside and in-
side. Of course the establishment of this hetero-
geneity is furthered by natural selection, which, where a
protective covering is needed, gives an advantage to those
individuals in which it has become strongest. But that this
divergence of structure commences as a direct adaptation, is
clearly shown by other facts than the foregoing. There is
the fact that many of the plants which in our gardens
develop bark with considerable rapidity, do not develop it
with the same rapidity in a greenhouse. And there is the
fact that plants which, in some climates, have their stems
covered only by thin semi-transparent layers, acquire thick
opaque layers when taken to other climates.

Just noting, for the sake of completeness, that in the
roots of the higher plants there arises a contrast between

THE OUTER AND INNER TISSUES OF PLANTS. 249

outer and inner parts, parallel to the one we have traced in
their branches, let me draw attention to another differentia-
tion of the same ultimate nature, which the higher plants
exhibit to us — a differentiation which, familiar though it is,
gains a new meaning by association with those named above,
and makes their meaning still more manifest. Each great
plant shows it. When, by the budding of axes out of axes,
there is produced one of those highly-compounded Phasnogams
which we call a tree, the central part of the aggregate be-
comes functionally and structurally unlike the peripheral
part. On looking into a large tree, or even a small one
which has thick foliage, like the Laurel, we see that the in-
ternal branches are almost or quite bare of leaves, while the
leaf-clad branches form an external stratum; and all our
experience unites in proving that this contrast- arises by
degrees, as fast as the growth of the tree entails a contrast be-
tween the conditions to which inner and outer branches are
exposed. Now when, in these most-composite aggregates,
we see a differentiation between peripheral and central parts
demonstrably caused by a difference in the relations of these
parts to environing forces, we get support for the conclusion
otherwise reached, that there is a parallel cause for the paral-
lel differentiations exhibited by all aggregates of lower orders
— branches, leaves, cells.

§ 271. Before leaving this most general physiological
differentiation, it may be well to say something respecting
certain secondary unlikenesses which usually arise between
interior and exterior. For the contrast is not, as might be
supposed from the foregoing descriptions, a simple contrast :
it is a compound contrast. The outer structure itself is
usually divisible into concentric structures. This is equally
true of a protophyte and of a phaenogamic axis. Between
the centre of an independent vegetal cell and its surface,
there are at least two layers; and the bark coating the sub-
stance of a shoot, besides being itself compound, includes

250 PHYSIOLOGICAL DEVELOPMENT,

another tissue lying between it and the wood. What is the
physical interpretation of these facts ?

When a mass of something we distinguish as inert matter
is exposed to external agencies capable of working changes in
it — when it is chemically acted upon, or when, being dry, it
is allowed to soak, or when, being wet, it is allowed to dry —
the changes set up progress in an equable way from the
surface towards the centre. At any time during the process
(supposing no other action supervenes) the modification
wrought, first completed at the outside, either gradually
diminishes as we approach the centre, or ceases suddenly at
a certain distance from the centre. But now suppose that
the mass, instead of being inert, is the seat of active changes
^suppose that it is a portion of complex colloidal substance,
permeable by light and by fluids capable of affecting its
unstable molecules — suppose that its interior is a source of
forces continually liberated and diffusing themselves out-
wards. Is it not likely that while at the centre the action
of the internally-liberated forces will dominate, and while at
the surface the action of the environing forces will dominate,
there will be between the two a certain place at which their
actions balance? May we not expect that this will be the
place where the most unstable matter exists — the place out-
side of which the matter becomes relatively stable in the
face of external forces, and inside of which the matter be-
comes relatively stable in the face of internal forces? And
must we not conclude that though part of the adjustment is
due to indirect equilibration, the initiation of it is due to
direct equilibration ?

But we are here chiefly concerned with the more general
interpretation, which is independent of any such speculation
as the foregoing. These contrasted tissues and the contrasted
functions they severally perform are, beyond question, sub-
ordinated to the relations of outside and inside. And the
evidence makes it tolerably clear that the unlike actions or
forces involved by the relations of outside and inside, deter-
mine these contrasts — partly directly and partly indirectly.

CHAPTER III.

DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS.

§ 272. The motionless protococcoid forms of lower Algce,
which do not permanently expose any parts of their surfaces
to actions unlike those which other parts are exposed to, have
no parts of their surfaces unlike the rest in function and
composition. This is what the hypothesis prepares us for.
If physiological differentiations are determined by differences
in the incidence of forces, then there will be no such differ-
entiations where there are no such differences. Contrariwise,
it is to be expected that the most conspicuous unlikeness of
function and minute structure will arise between the most-
dissimilarly circumstanced parts of the surface. We find
that they do. The upper end and the lower end, or, more
strictly speaking, the free end and the attached end, habitu-
ally present the strongest physiological contrasts.

Even aggregates of the first order illustrate this truth.
Such so-called unicellular plants as those delineated in
Figs. 4, 5, and 6, show us, on comparing the contents of
their fixed ends and their loose ends, that different processes
are going on in them, and that different functions are
being performed by their limiting membranes. C aider pa
prolifera, which " consists of a little creeping stem with
roots below and leaves above," originating " in the growth
of a body which may be regarded as an individual cell,"
supplies a still-better example. Among aggre-

251

252 PHYSIOLOGICAL DEVELOPMENT.

gates of the second order a like connexion is displayed in
more various modes but with equal consistency. As before,
the Puff-ball served to exemplify the primary physiological
differentiation of outer parts from inner parts; so, here, it
supplies a simple illustration of the way in which the
differentiated outer part is re-differentiated, in correspon-
dence with the chief contrast in its relations to the environ-
ment. The only marked unlikeness which the cortical layer
of the Puff-ball presents, is that between the portion next
the ground and the opposite portion. The better-developed
Fungi exhibit a more decided heterogeneity of parallel kind.
Such incrusting Algae as Ralfsia verrucosa furnish a kin-
dred contrast; and in the higher Algce it is uniformly
repeated. Phsenogams display this physiologi-

cal differentiation very conspicuously. That earth and air
are unlike portions of the environment, subjecting roots and
leaves to unlike physical forces, which entail on them unlike
reactions, and that the unlike functions and structures of
their respective surfaces are fitted to these unlike physical
forces, are familiar facts which it would be needless here
to name, were it not that they must be counted as coming
within a wider group of facts.

Is this unlikeness between the outer tissues of the attached
ends and those of the free ends in plants, determined by
their converse with the unlike parts of the environment?
That they result from an equilibration partly arising in
the individual and partly arising by the survival of indivi-
duals in which it has been carried furthest, is inferable
a priori; and this a priori argument may be adequately
enforced by arguments of the inductive order. A few
typical ones must here suffice. The gemmules

of the Marchantia are little disc-shaped masses of cells
composed of two or more layers. Their sides being alike,
there is nothing to determine which side falls lowermost
when one of them is detached. Whichever side falls lower-
most, however, presently begins to send out rootlets, while

THE OUTER TISSUES OF PLANTS. 253

the uppermost side begins to assume those characters which
distinguish the face of the frond. When this differentiation
has commenced, the tendency to its complete establishment
becomes more and more decided; as is proved by the fact
that if the positions of the surfaces be altered, the gemmule
bends itself so as to re-adjust them: the change towards
equilibrium with environing forces having been once set up,
there is acquired, as it were, an increasing momentum which
resists any counter-change. But the evidence shows that
at the outset, the relations to earth and air alone deter-
mine the differentiation of the under surface from the
upper. The experiences of the gardener, multi-

plying his plants by cuttings and layers, constitute another
class of evidences not to be omitted: they are commonplace
but instructive examples of physiological differentiation.
While circumstanced as it usually is, the meristematic tissue
of each branch in a Phaenogam continues to perform its
ordinary function — regularly producing on its outer side the
cortical substances, and on its inner side the vascular and
woody tissues. But change the conditions to those which
the underground part of the plant is exposed to, and there
begins another differentiation resulting in underground struc-
tures. Contact with water often suffices alone to produce
this result, as in the branches of some trees when they droop
into a pool, or as occasionally with a cutting placed in a
bottle of water; and when the light is excluded by im-
bedding the end of the cutting, or the middle of the still-
attached branch, in the earth, this production of tissues
adapted to the function of absorbing moisture and mineral
constituents proceeds still more readily. With such cases
may be grouped those in which this development of under-
ground organs by an above-ground tissue, is not excep-
tional but habitual. Creeping plants furnish good illus-
trations. From the shoots of the Ground-Ivy, rootlets are
put out into the soil in a manner differing but little from
that in which they are put out by an imbedded layer; save

254 PHYSIOLOGICAL DEVELOPMENT.

that the process follows naturally-induced conditions instead
of following artificially-induced conditions. But in the
common Ivy which, instead of running along the surface
of the earth, runs up inclined or vertical surfaces, we see the
process interestingly modified without being essentially
changed. The rootlets, here differentiated by their con-
ditions into organs of attachment much more than organs of
absorption, still develop on that side of the shoot next the
supporting surface, and do not develop where the shoot,
growing away from the tree or wall, is surrounded equally
on all sides by light and air: thus showing, undeniably,
that the production of the rootlets is determined by the
differential incidence of forces. Though survival of the
fittest doubtless furthered this transition yet it clearly did
not initiate it. That greenness which may be

observed in these Ivy-branch rootlets while they are quite
young, soft, and unshaded, introduces us to facts which are
the converse of the foregoing facts ; and proves that the parts
ordinarily imbedded in the soil and adapted to its actions,
acquire, often in very marked degrees, the superficial
structures of the aerial parts, when they are exposed to light
and air. This may be witnessed in Maize, which, when
luxuriant, sends out from its nodes near the ground, clusters
of roots that are thick, succulent, and of the same colour as
the leaves. Examples more familiar to us in England occur
in every field of turnips. On noting how green is the un-
covered part of a turnip-root, and how manifestly the
area over which the greenness extends varies with the area
exposed to light, as well as with the degree of the exposure,
it will be seen that beyond question, root-tissue assumes
to a considerable extent the appearances and function of
leaf-tissue, when subject to the same agencies. Let us not
forget, too, that where exposed roots do not approach in
superficial character towards leaves, they approach in
superficial character towards stems: becoming clothed
with a thick, fissured bark, like that of the trunk and

THE OUTER TISSUES OF PLANTS. 255

branches. But the most conclusive evidence is

furnished by the actual substitutions of surface-structures
and functions, that occur in aerial organs which have taken
to growing permanently under ground, and in under-ground
organs which have taken to growing permanently in the
air. On the one hand, there is the rhizome exemplified by
Ginger — a stem which, instead of shooting up vertically,
runs horizontally below the surface of the soil, and assumes
the character of a root, alike in colour, texture, and
production of rootlets; and there is that kind of swollen
under-ground axis, bearing axillary buds, which the Potato
exemplifies — a structure which, though homologically an
axis, simulates a tuberous root in surface-character, and
when exposed to the air, manifests no greater readiness to
develop chlorophyll than a tuberous root does. On the other
hand, there are the aerial roots of certain Orchids which,
habitually green at their tips, continue green throughout
their whole lengths when kept moist; which have become
leaf-like not only by this development of chlorophyll, but
also by the acquirement of stomata; and which do not bury
themselves in the soil when they have the opportunity.*
Thus we have aerial organs so completely changed to fit
under-ground actions, that they will not resume aerial func-
tions; and under-ground organs so completely changed to
fit aerial actions, that they will not resume under-ground
functions.

That the physiological differentiation between the part of
a plant's surface which is exposed to light and air and the
part which is exposed to darkness and moisture and solid

* A critical comment made on this sentence runs as follows : — " The
aerial roots of most epiphytic orchids contain chlorophyll in their cortex
throughout their length, but the cortex being covered by a l velamen ' of air-
containing cells which break up and reflect incident light, the green colour
is not visible through this opaque coat. When moistened the cells of the
velamen take up water and the green colour immediately shows through.
Such roots do not however possess stomata. The roots of certain species of
Angrozcum, however, contain the whole of the assimilating tissue of the
Dlant."

256 PHYSIOLOGICAL DEVELOPMENT.

matter, is primarily due to the unlike actions of these unlike
parts of the environment, is, then, clearly implied by observed
facts — more clearly, indeed, than was to be expected. Con-
sidering how strong must be the inherited tendency of a plant
to assume those special characters, physiological as well as
morphological, which have resulted from an enormous accu-
mulation of antecedent actions, it may be even thought
surprising that this tendency can be counteracted to so great
an extent by changed conditions. Such a degree of modifi-
ability becomes comprehensible only when we remember
how little a plant's functions are integrated, and how much,
therefore, the functions going on in each part may be altered
without having to overcome the momentum of the functions
throughout the whole plant. But this modifiability being as
great as it is, we can have no difficulty in understanding
how, by the cumulative aid of natural selection, this primary
differentiation of the surface in plants has become what we
see it.

§ 273. We will leave now these contrasts between the free
surfaces of plants and their attached or imbedded surfaces,
and turn our attention to the secondary contrasts existing
between different parts of their free surfaces. Were a full
statement of the evidence practicable, it would be proper
here to dwell on that which is furnished by the inferior
classes. It might be pointed out in detail that where, as
among the Algce, the free surfaces are not dissimilarly con-
ditioned, there is no systematic differentiation of them — that
the frond of an Viva, the ribbon-shaped divisions of a
Laminaria, and the dichotomous expansions of the Fuci
which clothe the rocks between tide-marks, are alike on both
sides; because, swayed about in all directions as they are by
the waves and tides, their sides are equally affected. Con-
versely, from the Fungi might be drawn abundant proof that
even among Thallophytes, unlikenesses arise between different
parts of the free surfaces when their circumstances are unlike.

THE OUTER TISSUES OF PLANTS. 257

In such laterally-growing kinds as are shown in Fig. 196b,
the honey-combed under surface and the smooth leathery
upper surface, have their contrasts related to contrasted con-
ditions; and in the adjacently-figured Agarics, and other
stalked genera, the pileus exhibits a parallel difference, ex-
plicable in a parallel way. But passing over Cryptogams it
must suffice if we examine more at length these traits as they
are displayed by Phamogams. Let us first note the dis-
similarities between the outer tissues of stems and leaves.

That these dissimilarities arose by degrees, as fast as the
units of which the phasnogamic axis is composed became
integrated, is a conclusion in harmony with the truth that in
every shoot of every plant, they are at first slight and become
gradually marked. Already, in briefly tracing the contrasts
between the outer and inner tissues of plants, some facts
have been named showing, by implication, how the cessation
of the leaf-function in axes is due to that change of condi-
tions entailed by the discharge of other functions. Here
we have to consider more closely facts of this class, together
with others immediately to the point. On pulling

off from a stem of grass the successive sheaths of its leaves,
the more-inclosed parts of which are of a fainter green than
the outer parts, it will be found that the tubular axis even-
tually reached is of a still fainter green; but when the axis
eventually shoots up into a flowering stem, its exposed part
acquires as bright a green as the leaves. In other Mono-
cotyledons, the leaf-sheaths of which are successively burst
and exfoliated by the swelling axis, it may be observed that
where the dead sheaths do not much obstruct the light and
air, the surface of the axis underneath is full of chlorophyll.
Dendrobium is an example. But when the dead sheaths
accumulate into an opaque envelope, the chlorophyll is ab-
sent, and also, we may infer, the function which its presence
habitually implies. Carrying with us this evidence, we shall
recognize a like relation in Dicotyledons. While its outer

layer remains tolerably transparent, an exogenous stem or
63

258 PHYSIOLOGICAL DEVELOPMENT.

branch continues to show, by the formation of chlorophyll,
that it shares in the duties of the leaves; but in proportion
as a bark which the light cannot penetrate is produced by
the adherent flakes of dead skin, or by the actual deposit of
a protective substance, the differentiation of duties becomes
more decided. Cactuses and Euphorbias supply

us with converse facts having the same implication. The
succulent axes so strangely combined in these plants, main-
tain for a long time the translucency of their outer layers
and their greenness ; and they so efficiently perform the offices
of leaves that leaves are not produced. In some cases, axes
that are not succulent participate largely in the leaf -function,
or entirely usurp it — still, however, by fulfilling the same
essential conditions. Occasionally, as in Statice brassiccefolia,
stems become fringed; and the fringes they bear assume,
along with the thinness of leaves, their darker green and
general aspect. In the genus Ruscus, the flattened axis
simulates so closely the leaf-structure, that were it not for the
flower borne on its midrib, or edge, its axial nature would
hardly be suspected. And let us not omit to note that where
axes usurp the characters of leaves, in their attitudes as well
as in their shapes and thicknesses, there are contrasts between
their under and upper surfaces, answering to the contrasts
between the relations of these surfaces to the light. Of this
Ruscus androgynus furnishes a striking example. In it the
difference which the unaided eye perceives is much less con-
spicuous than that disclosed by the microscope; for I find
that while the face of the pseudo-leaf has no stomata, the back
is abundantly supplied with them. One more illustration
must be added. Equally for the morphological and physio-
logical truths which it enforces, the Muhleribechia platyclada
is one of the most instructive of plants. In it the simulation
of forms and usurpation of functions, are carried out in
much more marvellous way than among the Cactacece.
Imagine a growth resembling in outline a very long willow-
leaf, but without a midrib, and having its two surfaces alike.

THE OUTER TISSUES OF PLANTS. 259

Imagine that across this thin, green, semi-transparent struc-
ture, there are from ten to thirty divisions, which prove to be
the successive nodes of an axis. Imagine that along the
edges of this leaf-shaped aggregate of internodes, there arise
axillary buds, some of which unfold into flowers, and others
of which shoot up vertically into growths like the one which
bears them. Imagine a whole plant thus seemingly composed
of jointed willow-leaves growing from one another's edges,
and some conception will be formed of the Muhlenbeckia.
The two facts which have meaning for us here are — first, that
the performance of leaf-functions by these axes goes along
with the assumption of a leaf-like translucency ; and, second,
that these flattened axes, retaining their upright attitudes,
and therefore keeping their two faces similarly conditioned,
have these two faces alike in colour and texture.

That physiological differentiation of the surface which
arises in Phaenogams between axial organs and foliar organs,
is thus traceable with tolerable clearness to those differences
between their conditions which integration has entailed —
partly in the way above described and partly in other ways
still to be named. By its relative position, as being shaded
by the leaves, the axis is less-favourably circumstanced for
performing those assimilative actions effected by the aid of
light. Further, that relatively-small ratio of surface to mass
in the axis, which is necessitated by its functions as a support
and a channel for circulation, prevents it from taking in,
with the same facility as the leaves, those surrounding gases
from which matter is to be assimilated. Both these special
causes, however, in common with that previously assigned,
fall within the general cause. And in the fact that where
the differential conditions do not exist, the physiological
differentiation does not arise, or is obliterated, we have clear
proof that it is determined by unlikenesses in the relations
of the parts to the environment.

§ 274. From this most general contrast between aerial

260 PHYSIOLOGICAL DEVELOPMENT.

surface-tissues — those of axes and those of folia — we pass
now to the more special contrasts of like kind existing in
folia themselves. Leaves present us with superficial differen-
tiations of structure and function; and we have to consider
the relations between these and the environing forces.

Over the whole surface of every phaanogamic leaf, as over
the fronds of the Pteridophyta, there extends a simple or
compound epidermal layer, formed of cells that are closely
united at their edges and devoid (in the Flowering Plants)
of that granular colouring matter (chlorophyll) contained in
the layers of cells they inclose: the result being that the
membrane formed of them is comparatively transparent.
On the submerged leaves of aquatic Phaenogams, this outer
layer is thin, delicate, and permeable by water; but on
leaves exposed to the air, and especially on their upper sur-
faces, is comparatively strong, dense, often smooth and
impermeable by water: being thus fitted to prevent the
rapid escape of the contained juices by evaporation. Simi-
larly, while the leaves of terrestrial plants which live in tem-
perate climates, usually have comparatively thin coats thus
composed, in climates that are both hot and dry, leaves are
commonly clothed with a very thick cuticle. Nor is this all.
The outside of an aerial leaf differs from that of a submerged
leaf by containing a deposit of waxy substance. Whether
this be exuded by the exposed surfaces of the cells, as some
contend, or whether it is deposited within the cells, as thought
by others, matters not in so far as the general result is con-
cerned. In either case a waterproof coating is formed at the
outermost sides of these outermost cells ; and in many cases
produces that polish by which the upper surface of the leaf is
more or less distinguished from the under surface. This

external pellicle presents us with another contrast of allied
meaning. On the upper surfaces of leaves subject to the
direct action of the sun's rays, there are either few or none
of those minute openings, or stomata, through which gases
can enter or escape ; but on the under surfaces these stomata

THE OUTER TISSUES OF PLANTS. 261

are abundant : a distribution which, while permitting free
absorption of the needful carbonic acid, puts a check on the
exit of watery vapour. Two general exceptions to this ar-
rangement may be noted. Leaves that float on the water
have all their stomata on their upper sides, and leaves that
are Bubmerged have no stomata — modifications obviously ap-
propriate to the conditions. What is to be said
respecting the genesis of these differentiations? For the
last there seems no direct cause: its cause must be indirect.
The unlike actions to which the upper and under surfaces of
leaves are subject, have no apparent tendency to produce
unlikeness in the number of their breathing holes. Here
the natural selection of spontaneous variations furnishes the
only feasible explanation. For the first, however, there is a
possible cause in the immediate actions of incident forces,
which survival of the fittest continually furthers.

The fluid exhaling through the walls of the cells next the
air, will be likely to leave behind suspended substances on
their outer surfaces. On remembering the pellicle which is apt
to form on thick solutions or emulsions as they dry, and how
this pellicle as it grows retards the further drying, it will be
perceived that the deposit of waxy matter next to the outer
surfaces of the cuticular cells in leaves, is not improbably
initiated by the evaporation which it eventually checks.
Should it be so, there results a very simple case of equilibra-
tion. Where the loss of water is too great, this waxy pellicle
left behind by the escaping water will protect most those in-
dividuals of the species in which it is thickest or densest ; and
by inheritance and continual survival of the fittest, there will
be established in the species that thickness of the layer which
brings the evaporation to a balance with the supply of water.

Another superficial differentiation, still more familiar, has
to be noted. Every child soon learns to distinguish by its
colour the upper side of a leaf from its under side, if the leaf
is one that has grown in such way as to establish the rela-
tions of upper and under. The upper surfaces of leaves are

262 PHYSIOLOGICAL DEVELOPMENT.

habitually of a deeper green than the under. Microscopic
examination shows that this deeper green results from the
closer clustering of those parenchyma-cells full of chlorophyll
that are in some way concerned with the assimilative actions ;
while beneath them are more numerous intercellular passages
communicating with those openings or stomata through
which is absorbed the needful air. Now when it is remem-
bered that the formation of chlorophyll is clearly traceable to
the action of light — when it is remembered that leaves are pale
where they are much shaded and colourless when developed
in the dark, as in the heart of a Cabbage — when it is remem-
bered that succulent axes and petioles, like those of Sea-kale
and Celery, remain white while the light is kept from them
and become green when exposed; it cannot be questioned
that this greater production of chlorophyll next to the upper
surface of a leaf, is directly consequent on the greater
amount of light received. Here, as in so many other cases,
we must regard the differentiation as in part due to direct
equilibration and in part to indirect equilibration. Familiar
facts compel us to conclude that from the beginning, each
individual foliar organ has undergone a certain immediate
adaptation of its surfaces to the incidence of light; that
when there arose a mode of growth which exposed the leaves
of successive generations in similar ways, this immediately-
produced adaptation, ever tending to be transmitted, was
furthered by the survival of individuals inheriting it in the
greatest degree; and that so there was gradually established
that difference between the two surfaces which each leaf dis-
plays before it unfolds to the light, but which becomes more
marked when it has unfolded.*

* The current doctrine that chlorophyll is the special substance concerned
in vegetal assimilation, either as an agent or as an incidental product, must
be taken with considerable qualification. Besides the fact that among the
Algce there are many red and brown kinds which thrive ; and besides the
fact that among the lower Archegoniates there are species which are purple or
chocolate-coloured ; there is the fact that Phsenogams are not all green. We
have the Copper-Beech, we have the black-purple Coleus VerschafieUii, and
we have the red variety of Cabbage, which seems to flourish as well as the

THE OUTER TISSUES OF PLANTS. 263

From the ordinary cases let us now pass to the exceptional
cases. We will look first at those in which the two faces of
the leaves differ but little, or not at all — their circumstances
being similar or equal. Leaves that grow in approximately-
upright attitudes, and attitudes which do not maintain the
relative positions of the two surfaces with constancy, may be
expected to display an unusual likeness between the two
surfaces; and among tliem we see it. The Grasses may be
named as a group exemplifying this relation; and if, instead
of comparing them as a group with other groups, we compare
those dwarf kinds of them which spread out their leaves
horizontally, with the large aspiring kinds, such as Arundo,
we trace a like antithesis: in the one the contrast of upper
and under is very obvious, while in the other it is scarcely to
be detected. Leaves of various other Monocotyledons that
grow in a similar way, similarly show us a near approach to
uniformity of the two surfaces; as instance the genus Clivia
and the thinner-leaved kinds of Yucca. Where the con-
trast of upper and under is greatly diminished by the assump-
tion of a rounded or cylindrical form, instead of a flattened
form, the same thing happens. The genus Kleinia furnishes
illustrations. It may be remarked, too, that even within
the limits of this genus there are instructive variations; for
while in Kleinia ficoides the leaves, shaped like pea-pods,
are broadest in a vertical direction, and have their lateral
surfaces alike in conditions and structure, in other species
the leaves, broader horizontally than vertically, exhibit
unlikeness between the upper and under sides. Equally
to the point is the evidence furnished by vertically-growing
leaves that are cylindrical, as those of Sanseviera cylindrical
or as those of the Rush-tribe: the similarly-placed surface
has all around a similar character. Of kindred meaning,

other varieties. Chlorophyll, then, must be regarded simply as the most
general of the colouring matters found in those parts of plants in which
assimilation is being effected by the agency of light. Though it is always
present alonq icith the red and brown pigments, yet there is much evidence to
show that these are the actual assimilative pigments.

204 PHYSIOLOGICAL DEVELOPMENT.

and still more conclusive, are the cases in which the under
side of the leaf, being more exposed to light than the upper
side, usurps the character and function of the upper side.
If a common Flag be pulled to pieces, it will be seen that
what answers to the face in other leaves, forms merely the
inside of the sheath including the younger leaves, and is
obliterated higher up. The two surfaces of the blade answer
to the two under halves of a leaf that has been, as it were,
folded together lengthways, with the two halves of its upper
surface in contact. And here, in default of an upper surface,
the under surface acquires its character and discharges its
function. A like substitution occurs in Aristea corymbosa;
and there are some of the Orchids, as Lockhartia, which dis-
play it in a very obvious way.

When joined with the foregoing evidence, the evidence
which another kind of substitution supplies is of great
weight. I refer to that which occurs in the Australian
Acacias, already instanced as throwing light on morpho-
logical changes. In these plants the leaves properly so called
are undeveloped, and the footstalks, flattened out into folia-
ceous shapes, acquire veins and midribs, and so far simulate
leaves as ordinarily to be taken for them : a fact in itself of
much physiological significance. But that which it concerns
us especially to note, is the absence of distinction between
the two faces of these phyllodes, as they are named, and
the cause of its absence. These transformed petioles do not
flatten themselves out horizontally, so as to acquire under
and upper sides, as most true leaves do; but they flatten
themselves out vertically: the result being that their two
sides are similarly circumstanced with respect to light and
other agencies; and there is consequently nothing to cause
their differentiation. And then we find an analogous case
where differential conditions arise, and where some differ-
entiation results. In Oxalis bupleuri folia, Fig. 66, there is a
similar flattening out of the petiole into a pseudo-leaf; but
in it the flattening takes place in the same plane as the leaf,

THE OUTER TISSUES OF PLANTS. 265

so as to produce an under and an upper surface; and here
the two surfaces of the pseudo-leaf are slightly unlike — in
contour if in nothing else.

§ 275. We now come to such physiological differentiations
among the outer tissues of plants, as are displayed in the
contrasts between foliar organs on the same axis, or on
different axes — contrasts between the seed-leaves and the
leaves subsequently formed, between submerged and aerial
leaves in certain aquatic plants, between leaves and bracts,
and between bracts and sepals. To deal even briefly with
these implies information which even a professed botanist
would have to increase by special inquiries, before attempting
interpretations. Here it must suffice to say something
respecting those marked unlikenesses existing between the
tissues of the more characteristic parts of flowers, and the
tissues of the homologous foliar organs.

It was pointed out in § 196, that the terminal folia of a
phasnogamic axis have sundry characters in common with such
fronds as those out of which we concluded that the phaeno-
gamic axis has arisen by integration — common character's of
a kind to be expected. In their simple cellular composition,
comparative want of chlorophyll, and deficiency of vascular
structures, the undeveloped ends of leaf-shoots and the de-
veloped ends of flower-shoots, approach to the fronds of the
simpler Archegoniates. We also noted between them another
resemblance. It is said of the Jungermanniacece, that
" though under certain circumstances of a pure green, thev
are inclined to be shaded with red, purple, chocolate, or other
tints;" and answering to this we have the facts that such
colours commonly occur in the terminal folia of a phaeno-
gamic axis, when arrest of its development leads to the
formation of a flower, and that very frequently they are
visible at the ends of leaf-axes. In the unfolding parts of
shoots, more or less of red, or copper-colour, or chocolate-
colour, may generally be seen: often, indeed, it charac-

%Q6 PHYSIOLOGICAL DEVELOPMENT.

terizes the leaves for some time after they are unfoldt
Occasionally the traces of it are permanent; and, as i]
the scarlet terminal leaves of Poinsettia pulcherrima, we st
that it may become, and continue, extremely conspicuous.
The question, then, now to be asked is — has this colouring
by which the immature part of the phaenogamic axis is cha-
racterized, anything to do with the colouring of flowers?
Has this difference between undeveloped folia and folia that
are further developed, been increased by natural selection
where an advantage accrued from it, until it has ended in
the strong contrast we now see? I think we may not irra-
tionally infer that this has happened.

Facts, very numerous and varied, united to warrant us in
concluding that gamogenesis commences where the forces
which conduce to growth are nearly equilibrated by the forces
which resist growth (§ 78) ; and the induction that in plants,
fertilized germs are. produced at places where there is an
approach towards this balance, we found to be in harmony
with the deduction that an advantage to the species must be
gained by sending off migrating progeny from points where
nutrition is failing. Other things equal, failure of nutrition
may be expected in parts which have the most remote or most
indirect access to the materials furnished by the roots —
materials which have to be carried great distances by a very
imperfect apparatus. The ends of lateral axes are therefore
the probable points of fructification, in aggregates of the
third order that have taken to growing vertically. But if
these points at which nutrition is failing, are also the points
at which the colours inherited from lower types are likely to
recur in more marked degrees than elsewhere; then we may
infer that the organs of fructification will not unfrequently
co-exist with such colours at the ends of such axes. How
may the resulting contrast between the older fronds and the
fronds next the germ-producing organs be increased? If
uninterfered with it would be likely to diminish. These
traits inherited from remote ancestry might be expected

THE OUTER TISSUES OF PLANTS. 2G7

slowly to fade away. How, then, is the intensification of
them to be explained ?

If a contrast of the kind described favours the propagation
of a race in which it exists, it will be maintained and
increased; and if we take into account an agency of which
Mr. Darwin has shown the great importance — the agency of
insects — we shall have little difficulty in understanding how
such a contrast may facilitate propagation. We cannot, of
course, here assume the agency of insects so specialized in
their habits as Bees and Butterflies; for their specialized
habits imply the pre-existence of the contrast to be explained.
But there is an insect-agency of a more general kind which
may be fairly counted upon as coming into action. Various
small Flies and Beetles wander over the surfaces of plants in
search of food. It is a legitimate assumption that they will
frequent most those parts in which they find most food, or
food most to their liking — especially if at the same time
they gain the advantage of concealment. Now the ends of
axes, formed of young, soft, and closely-packed folia, are the
parts which more than any others offer these several advan-
tages. They afford shelter from, enemies; they frequently
contain exuded juices; and when they do not, their tissues
are so tender as to be easily pierced in search of the sap.
If, then, from the first, as at present, these ends of axes
have been favourite haunts of small insects; and if, where
the closely-clustered folia contained the generative organs,
the insects frequenting them occasionally carried adherent
fructifying cells from one plant to another, and so aided
fertilization; it would follow that anything which made
such terminal clusters more attractive to such insects, or
more conspicuous to them, or both, would further the multi-
plication of the race, and would so be continually increased
by the extra multiplication of individuals in which it was
greatest. Here we find the clue. This contrast of colour
between the folia next to the fructifying parts and all other
folia, must constantly have facilitated insect-agency; sup-

268 PHYSIOLOGICAL DEVELOPMENT.

posing the insects to have had the power of distinguishing
between colours. That Bees and Butterflies have this power
is manifest. They may be watched flying from flower to
flower, disregarding all other parts of the plants. And if
the less-specialized insects possessed some degree of such
discrimination, then the initial contrasts of colour above
described would be maintained and increased. Let such a
connexion be once established, and it must tend to become
more decided. Insects most able to discern the parts of
plants which afford what they seek, will be those most likely
to survive and leave offspring. Plants presenting most of
the desired food, and showing most clearly where it lies, will
have their fertilization and multiplication furthered in the
greatest degree. And so the mutual adaptation will become
ever closer; while it is rendered at the same time more
varied by the special requirements of the insects and of
the plants in each locality, under each change of con-
ditions. Of course, the genesis of the sweet
secretions and the" odours of flowers, has a parallel interpre-
tation. The simultaneous production of honey, or some
kindred substance, is implied above; since, unless a bait
co-existed with the colour, the colour would not attract
insects, and would not be maintained and intensified by
natural selection. Gums, and resins, and balsams, are familiar
products of plants; apparently, in many cases, excreted as
useless matters from various parts of their surfaces. These
substances, admitting of wide variations in quality, as they
do, afford opportunities for the action of natural selection
wherever any of them, attractive to insects, happen t*o be
produced near the organs of fructification. And this action
of natural selection once set up, may lead to the establish-
ment of a local excretion, to the production of an excretion
more and more attractive, and to the disposal of the organ
containing it in such a way as most to facilitate the carry-
ing away of pollen. Similarly and simultaneously with
odours. Odours, like colours, draw insects to flowers. After

THE OUTER TISSUES OF PLANTS. 269

observing how Bees come swarming into a house where
honey is largely exposed, or how Wasps find their way into
a shop containing much ripe fruit, it cannot be questioned
that insects are to a considerable extent guided by scent.
Being thus sensitive to the aromatic substances which flowers
exhale, they may, when the flowers are in large masses, be
attracted by them from distances at which the flowers them-
selves are invisible. And manifestly,, the flowers which so
attract them from the greatest distances, increasing thereby
their chances of efficient fertilization, will be most likely to
perpetuate themselves. That is to say, survival of the fittest
must tend to produce perfumes that are both more powerful
and more attractive.

These physiological differentiations, then, which mark off
the foliar organs constituting flowers from other foliar organs,
are the consequences of indirect equilibration. They are not
due to the immediate actions of unlike incident forces on
the parts of the individual plant; but they are due to the
actions of such unlike incident forces on the aggregate of
individuals, generation after generation.*

§ 276. The unity of interpretation which we here find for
phenomena of such various orders, could hardly be found

* This seems as fit a place as any for noting the fact, that the greater part
of what we call beauty in the organic world, is in some way dependent on
the sexual relation. It is not only so with the colours and odours of flowers.
It is so, too, with the brilliant plumage of birds ; and it is probable that the
colours of the more conspicuous insects are in part similarly determined. The
remarkable circumstance is, that these characteristics, which have originated
by furthering the production of the best offspring, while they are naturally
those which render the organisms possessing them attractive to one another,
directly or indirectly, should also be those which are so generally attractive
to us — those without which the fields and woods would lose half their charm.
It is interesting, too, to observe how the conception of human beauty is in a
considerable degree thus originated. And the trite observation that the
element of beauty which grows out of the sexual relation is so predominant
in aesthetic products — in music, in the drama, in fiction, in poetry — gains a
new meaning when we see how deep down in organic nature this connexion
extends.

270 PHYSIOLOGICAL DEVELOPMENT.

were the phenomena otherwise caused. That the stronger
and the feebler contrasts among the different parts of the
outer tissues in plants, should so constantly occur along with
stronger and feebler contrasts among the incident forces, is
in itself weighty evidence that unlike outer actions have
caused unlike inner actions, and correspondingly-unlike
structures; either by changing the functional equilibrium in
the individual, or by changing it in the race, or by both.

Even in the absence of more direct proof, there would be
great significance in the marked differences that habitually
exist between the exposed and imbedded parts of plants, be-
tween the stems and the leaves, and between the upper and
under surfaces of the leaves. The significance of these differ-
ences is increased when we discover that they vary in degree
as the differences in the conditions vary in degree. Still
greater becomes the force of the evidence on finding that
these strongly-contrasted parts may, when placed in one
another's conditions, and kept in them from generation to
generation, permanently assume one another's functions, and,
in a great degree, one another's structures. Even more con-
clusive yet is the argument rendered, by the discovery that,
where these substitutions of function and structure take place,
the superinduced modifications differ in different circum-
stances ; just as the original modifications do. The fact that
a flattened stem simulating a vertically-growing leaf has its
two surfaces alike, while when it simulates a horizontally-
growing leaf its upper and under surfaces differ, is a fact
which, standing alone, might prove little, but proves much
when joined with all the other evidence. And its profound
meaning becomes the more obvious on discovering that the
same thing happens with petioles when they usurp leaf-
functions.

Finally, when we remember how rapidly analogous modi-
fications of function and structure arise in the superficial
tissues of individual plants, the general inference can scarcely
be resisted. When we meet with so striking a case as that

THE OUTER TISSUES OF PLANTS. 271

of the Begonia-leaf, a fragment of which stuck in the ground
produces roots from its under surface and leaves from its
upper surface — when we see that though, in this case, the
typical structure of the plant presently begins to control the
organizing process, yet the initial differentiations are set up
by the differential actions of the environment ; the presump-
tion becomes extremely strong that the heterogeneities of
surface which we have considered, result, as alleged, directly
or indirectly from heterogeneities in the incident forces.

CHAPTEE IV.

DIFFERENTIATIONS AMONG THE INNER TISSUES OF
PLANTS,*

§ 277. In passing from plants formed of threads or thin
laminae, to plants having some massiveness, we find that after
the external and internal parts have become distinguished
from one another, there arise distinctions among the internal
parts themselves, as well as among the external parts
themselves: the primarily-differentiated parts are both re-
differentiated.

From types of very low organisation illustrations of this
may be drawn. In the thinner kinds of Laminaria there
exists but the single contrast between the outer layer of cells
and an inner layer; but in larger species of the same genus,
as L. digitata, there are three unlike layers on each side of a
central layer differing from them — augmentation of bulk is
accompanied by multiplication of concentric internal struc-
tures, having their unlikenesses obviously related to unlike-
nesses in their conditions. In Furcellaria and various Algce
of similarly swollen forms, the like relation may be traced.

Just indicating the generality of this contrast, but not

* Students of vegetal physiology, familiar with the controversies respecting
sundry points dealt with in this chapter, will probably be surprised to find
taken for granted in it, propositions which they have habitually regarded as
open to doubt. Hence it seems needful to say that the conclusions here set
forth, have resulted from investigations undertaken for the purpose of form-
ing opinions on several unsettled questions whwh I had to treat, but which I
could find in books no adequate data for treating. The details of these
investigations, and the entire argument of which this chapter is partly an
abstract, will be found in Appendix C.
272

THE INNER TISSUES OF PLANTS. 273

attempting to seek in these lower types for any more specific
interpretation of it, let us pass to the higher types. The
argument will be amply enforced by the evidence obtained
from them. We will look first at the conditions which they
have to fulfil; and then at the ways in which the functions
and structures adapting them to these conditions arise.

§ 278. A terrestrial plant that grows vertically needs no
marked modification of its internal tissues, so long as the
height it reaches is very small. As we before saw, the spiral
or cylindrical rolling up of a simple cellular frond, or the
more bulky growth of a simple cellular axis, may give the
requisite strength; and the requisite circulation may be car-
ried on through the unchanged cellular tissue. But in pro-
portion as the height to be attained and the mass to be
supported increase, the supporting part must acquire greater
bulk or greater density, or both ; and some modification that
shall facilitate the transfer of nutritive liquids must take
place. Hence, in the inner tissues of plants we may expect
to find that structural changes answering to these require-
ments become marked, as the growth of the aerial part
becomes great. Facts correspond with these expectations.

Among the humbler Cormophytes, which creep over or
raise themselves but little above, the surfaces they flourish
upon, there is scarcely any internal differentiation: the
vascular and woody structures, if not in all cases absolutely
unrepresented, are rarely and very feebly indicated. But
among the higher types — the Ferns and Lycopodiums —
which raise their fronds to considerable heights, there are
vascular bundles and hard tissues like wood; and by the
Tree-Ferns massive axes are developed. That the relation
which thus shows itself among Cryptogams is habitual among
Phamogams, scarcely needs saying.

Phaenogams, however, are not universally thus charac-
terized in a decided way. Besides the comparative want of
woody tissue in flowering plants of humble growth, and
64

274 PHYSIOLOGICAL DEVELOPMENT.

besides the paucity of vessels in ordinary water-plants, thei
are cases of much more marked divergence from this typical
internal structure. These exceptional cases occur undei
exceptional conditions, and are highly instructive. They are
of two kinds. One group of them is furnished by

certain plants which are parasitic on the exposed roots of
trees — parasitic not partially, as the Mistletoe, but to the
extent of subsisting wholly on the sap they absorb. Fungus-
like in colour and texture, and having scales for leaves, these
Balanophorce and Rafflesiacece are recognizable as Phsenogams
by scarcely any other traits than their fructifications. Along
with their aborted leaves and absence of chlorophyll, there
is a great degradation of those internal tissues by which
Phamogams are commonly distinguished. Though Dr. [now
Sir J.] Hooker has shown that they are not, as some botanists
thought, devoid of spiral vessels; yet, as shown by the mis-
take previously made in classifying them, their appliances for
circulation are rudimentary. And this trait goes along with a
greatly-simplified distribution of nutriment. In the absence
of leaves there can be but little down-current of sap, such
as leaves usually supply to roots: there cannot be much
beyond an upward current of the absorbed juices. The

other cases occur where circulation is arrested or checked in
a different way ; namely, in plants that are wholly submerged.
These are the Podostemaceoe. Clothing as they do the sub-
merged rocks, their roots play the part of rhizomes, being-
attached to the substratum by hairs and other processes, and
having the leaf-bearing and flower-bearing shoots on their
surfaces. The latter spread out more or less horizontally and
are also fixed to the substratum in the same manner as the
roots. Observe then the connexion of facts. One of these
Podostemaceoe needs no internal stiffening substance, for it
exists in a medium of its own specific gravity; and being in
a position to absorb water over its entire surface, it has no
need for a circulation of crude sap — nor, indeed, in the
absence of evaporation from any part of its surface, could

THE INNER TISSUES OP PLANTS. 275

any active circulation take place. Here, accordingly, the
tracheal and mechanical elements are undeveloped. Though
spiral vessels are not entirely absent, yet they are so rare as
to do no more than verify the inference of phasnogamic rela-
tionship drawn from the flowers.

The method of agreement, the method of difference, and
the method of concomitant variations, thus unite in proving
a direct relation between the demand for support and circu-
lation, and the existence of these vascular woody bundles
which the higher plants habitually possess. The question
which we have to consider is — Under what influences are
these structures, answering to these requirements, developed ?
How are these internal differentiations caused ? The inquiry
may be conveniently divided. Though the supporting tissues
and the tissues concerned in the circulation of liquids are
closely connected, and indeed entangled, with one another,
we may fitly deal with them apart. Let us take first the
supporting tissue.

§ 279. Many common-place facts indicate that the me-
chanical strains to which upright growing plants are exposed,
themselves cause increase of the dense deposits by which such
plants are enabled to resist such strains. There is the fact
that the massiveness of a tree-trunk varies according to the
stress habitually put upon it. If the contrast between the
slender stem of a tree growing in a wood and the bulky stem
of a kindred tree growing in the fields, be ascribed to differ-
ence of nutrition rather than difference of exposure to winds j
there is still the fact that a tree trained against a wall has a
less bulky stem than a tree of the same kind growing un-
supported ; and that between the long weak branches of the
one and the stiff ones of the other there are decided contrasts.
If it be objected that a tree so trained and branches so borne
have relatively less foliage, and that therefore these unlike-
nesses also are due to unlikenesses of general nutrition, which
may in part be true; there are still such cases as those of

276 PHYSIOLOGICAL DEVELOPMENT.

garden plants, which when held up by tying them to sticks
have weaker stems than when they are unpropped, and sink
down if their props are taken away. Again, there is the
evidence supplied by roots. Though the contrast between
the feeble roots of a sheltered tree and the strong roots of
an exposed tree, may, like the contrast of their stems, be
mainly due to difference of nutrition, and therefore supplies
but doubtful evidence, we get tolerably clear evidence' where
trees growing on inclined rocky surfaces, send into crevices
that afford little moisture or nutriment, roots which never-
theless become thick where they are so directed as to bear
great strains. Suspicion thus raised is strengthened

into conviction by special evidences occurring in the places
where they are to be expected. The Cactuses, with their
succulent growths that pass into woody growths slowly and
irregularly, give us the opportunity of tracing the conditions
under which the wood is formed. Good examples occur in the
genus Cereus, and especially in forms like C. crenulatus.
Here, from a massive vertically-growing rod of fleshy tissue,
two inches or more in diameter, there grow at intervals lateral
rods similarly bulky, which, quickly curving themselves, take
vertical directions. One of these heavy branches puts great
strains on its own substance and that of the stem at their
point of junction; and here both of them become brown and
hard, while they continue green and succulent all around.
Such differentiations may be traced internally before they
are visible on the surface. If a joint of an Opuntia be sliced
through longitudinally, the greater resistance to the knife
all around the narrow neck, indicates there a larger deposit
of lignin than elsewhere; and a section of the tissue placed
under the microscope, exhibits at the narrowest part a con-
centration of the woody and vascular bundles. Clear
evidence of another kind has been noted by Mr. Darwin, in the
organs of attachment of climbing plants. Speaking of Sola-
rium jasminoides he says : — " When the flexible petiole of a
half- or a quarter-grown leaf has clasped any object, in three

THE INNER TISSUES OF PLANTS. 277

or four days it increases much in thickness, and after several
weeks becomes wonderfully hard and rigid; so that I could
hardly remove one from its support. On comparing a thin
transverse slice of this petiole with one from the next or
older leaf beneath, which had not clasped anything, its
diameter was found to be fully doubled,' and its structure
greatly changed. . . . This clasped petiole had actually
become thicker than the stem close beneath; and this was
chiefly due to the greater thickness of the ring of wood,
which presented, both in transverse and longitudinal sections,
a closely similar structure in the petiole and axis. The
assumption by a petiole of this structure is a singular
morphological fact; but it is a still more singular physio-
logical fact that so great a change should have been induced
by the mere act of clasping a support."

If there is a direct relation between mechanical stress and
the formation of wood, it ought to explain for us the internal
distribution of the wood. Let us see whether it does this.

When seeking in mechanical actions and reactions the
cause of that indurated structure which forms the verte-
brate axis (§§ 254-7), it was pointed out that in a transverse-
ly-strained mass, the greatest pressures and tensions are
thrown on the molecules of the concave and convex surfaces.
Hence, supposing the transversely-strained mass to be a cylin-
der, bent backwards and forwards not in one plane but now
in this plane and now in that, its peripheral layers will be
those on which the greatest stress falls. An ordinary dicoty-
ledonous axis is such a cylinder so strained. The main-
tenance of its attitude either as a lateral shoot or a vertical
shoot, implies subjection to the bendings caused by its own
weight and by the ever-varying wind. These bendings
imply tensions and pressures falling most severely first on
one side of its outer layers and then on another. And if the
dense substance able to resist these tensions and pressures is
deposited most where they are greatest, we ought to find it
taking the shape of a cylindrical casing. This is just what

278 PHYSIOLOGICAL DEVELOPMENT.

we do find. On cutting across a shoot in course of formation,
we see its central space either unoccupied or occupied only
by soft tissue. That the layer of hard tissue surrounding
this is not the outermost layer, is true: there lies beyond it
the cambium layer, from which it is formed, the phloem,
and the cortex. But outside of the soft phloem there is
frequently another layer of dense tissue now known as the
pericyclic fibres, having frequently a tenacity greater even
than that of the wood — a layer which, while it protects the
cambium and offers additional resistance to the transverse
strain, admits of being fissured as fast as the cylinder of
wood thickens. That is to say, the deposit of resisting sub-
stance is as completely peripheral as the exogenous mode of
growth permits. So, too, in general arrangement is it with
the ordinary monocotyledonous stem. Different as is here
the internal structure, there yet holds the same general dis-
tribution of tissues, answering to the same mechanical con-
ditions. The vascular woody bundles, more abundant towards
the outside of the stem than near the centre, produce a harder
casing surrounding a softer core. In the supporting

structures of leaves we find significant deviations from this
arrangement. While axes are on the average exposed to
equal strains on all sides, most leaves, spreading out their
surfaces horizontally, have their petioles subject to strains
that are not alike in all directions; and in them the hard
tissue is differently arranged. Its transverse section is
not ring-shaped but crescent-shaped: the two horns being
directed towards the upper surface of the petiole. That this
arrangement is one which answers to the mechanical con-
ditions, is not easy to demonstrate : we must satisfy ourselves
by noting that here, where the distribution of forces is dif-
ferent, the distribution of resisting tissue is different. And
then, showing conclusively the connexion between these differ-
ences, we have the fact that in petioles growing vertically
and supporting peltate leaves — petioles which are therefore

THE INNER TISSUES OF PLANTS. 2Y9

subject to equal transverse strains on all sides — the vascular
bundles are arranged cylindrically, as in axes.

Such, then, are some of the reasons for concluding that the
development of the supporting tissue in plants, is caused by
the incident forces which this tissue has to resist. The
individuals in which this direct balancing of inner and outer
actions progresses most favourably, are those which, other
things equal, are most likely to prosper; and, by habitual
survival of the fittest, there is established a systematic and
constant distribution of a deposit adapted to the circum-
stances of each type.

§ 280. The function of circulation may now be dealt with.
We have to consider here by what structures this is dis-
charged; and what connexion exists between the demand
for them and the genesis of them.

The contrast between the rates at which a dye passes
through, simple cellular tissue and cellular tissue of which the
units have been elongated, indicates one of the structural
changes required to facilitate circulation. If placed with its
cut surface in a coloured liquid, the parenchyma of a potato
or the medullary mass of a cabbage-stalk, will absorb the
liquid with extreme slowness ; but if the stalk of a fungus be
similarly placed, the liquid runs up it, and especially up its
loose central substance, very quickly. On comparing the
tissues which thus behave so differently, we find that whereas
in the one case the component cells, packed close together,
have deviated from their primitive sphericity only as much as
mutual pressure necessitates, in the other case they are drawn
out into long tubules with narrow spaces among them — the
greatest dimensions of the tubules and the spaces being in the
direction which the dye takes so rapidly. That which we
should infer, then, from the laws of capillary action, is
experimentally shown : liquid moving through tissues follows
the lines in which the elements of the tissues are most

PHYSIOLOGICAL DEVELOPMENT.

elongated. It does this for two reasons. That narrowing of
the cells and intercellular spaces which accompanies their
elongation, facilitates capillarity; and at the same time fewer
of the septa formed by the joined ends of the cells have to be
passed through in a given distance. Hence the

general fact that the establishment of a rudimentary vascular
system, is the formation of bundles of cells lengthened in the
direction which the liquid is to take. This we see very
obviously among the lower Cormophytes. In one of the
lichen-like Liverworts, the veins which, branching through
its frond, serve as communications with its scattered rootlets,
are formed of cells longer than those composing the general
tissue of the frond : the lengths of these cells corresponding
in their directions with the lengths of the veins. So, too, is
it with the midribs of such fronds as assume more definite
shapes; and so, too, is it with the creeping stems which
unite many such fronds. That is to say, the current which
sets towards the growing part from the part which supplies
certain materials for growth, sets through a portion of the
tissues composed of units that are longer in the line of the
current than at right angles to that line. The like is true

of Phasnogams. Omitting all other characteristics of those
parts of them through which chiefly the currents of sap
flow, we find the uniform fact to be that they consist of cells
and intercellular spaces distinguished from others by their
lengths. It is thus with veins, and midribs, and petioles;
and if we wish proof that it is thus with stems, we have but
to observe the course taken by a coloured solution into which
a stem is inserted.

What is the original cause of this differentiation? Is it
possible that this modification of cell-structure which favours
the transfer of liquid towards each place of demand, is itself
caused by the current which the demand sets up ? Does the
stream make its own channel? There are various reasons
for thinking that it does. In the first place, the simplest and
earliest channels, such as we see in the Liverworts, do not

THE INNER TISSUES OF PLANTS. 281

develop in any systematic way, but branch out irregularly,
following everywhere the irregular lobes of the fronds as
these spread ; and on examining under a magnifier the places
at which the veins are lost in the cellular tissue, it will be
seen that the cells are there slightly longer than those
around: suggesting that the lengthening of them which
produces an extension of the veins, takes place as fast as
the growth of the tissue beyond causes a current to pass
through them. In the second place, a disappearance of the
granular contents of these cells accompanies their union
into a vein — a result which the transmission of a current
may not improbably bring about. But be the special causes
of this differentiation what they may, the evidence favours
very much the conclusion that the general cause is the
setting up of a current towards a place where the sap is
being consumed. In the histological development

of the higher plants we find confirmation. The more

finished distributing canals in Phaenogams are formed of cells
previously lengthened. At parts of which the typical struc-
ture is fixed, and the development direct, this fact is not easy
to trace; the cells rapidly take their elongated structures in
anticipation of their pre-determined functions. But in places
where new vessels are required in adaptation to a modifying
growth, we may clearly trace this succession. The swelling
root of a turnip, continually having its vascular system
further developed, and the component vessels lengthened as
well as multiplied, gives us an opportunity of watching the
process. In it we see that the reticulated cells which unite
to form ducts, arise in the midst of bundles of cells that have
previously become elongated, and that they arise by trans-
formation of such elongated cells ; and we also see that these
bundles of elongated cells have an arrangement suggestive
of their formation by passing currents.

'Are there grounds for thinking that these further trans-
formations by which strings of elongated cells pass into
vessels lined with spiral, annular, reticulated, or other

282 PHYSIOLOGICAL DEVELOPMENT.

frameworks, are also in any way determined by the current
of sap carried ? There are some such grounds.

As just indicated, the only places where we may look
for evidence with any rational hope of finding it, are
places where some local requirement for vessels has arisen
in consequence of some local development which the type
does not involve. In these cases we find such evidence.
Good illustrations occur in those genera of the Cactacece,
which simulate leaves, like Epiphyllum and Phyllocactus.
A branch of one of these is outlined in Fig. 256. As before
explained this is a flattened axis; and the notches along
its edges are the seats of the axillary buds. Most of these
axillary buds are arrested; but occasionally one of them
grows. Now if, taking an Epiphyllum-shoot which bears a
lateral shoot, we compare the parts of it that are near the
aborted axillary buds with the part that is near the de-
veloped axillary bud, we find a conspicuous difference. In
the neighbourhood of an aborted axillary bud there is no
external sign of any internal differentiation; and on hold-
ing up the branch against the light, the uniform trans-
lucency shows that there is no greater amount of dense
tissue near it than in other parts of the succulent mass.
But where an axillary bud has developed, a prominent rounded
ridge joins the midrib of the lateral branch with the midrib
of the parent branch. In the midst of this rounded ridge
an opaque core may be seen. And on cutting through it, this
opaque core proves to be full of vascular bundles imbedded
in woody deposits. Clearly, these clusters of vessels imply
transformations of the tissues, caused by the passage of in-
creased currents of sap. The vessels were not there when
the axillary bud was formed; they would not have de-
veloped had the axillary bud proved abortive; but they
arise as fast as growth of the axillary bud draws the sap
along the lines in which they lie. Verification is obtained
by examining the internal structures. If longitudinal
sections be made through a growing bud of Opuntia or

THE INKER TISSUES OF PLANTS. 283

Cereus, it will be found that the vessels in course of forma-
tion converge towards the point of growth, as they would
do if the sap-currents determined their formation ; that they
are most developed near their place of convergence, which
they would be if so produced; and that their terminations
in the tissue of the parent shoot are partially-formed lines
of irregular elongated cells, like those out of which the ves-
sels of a leaf or bud are developed.

Concluding, then, that sap-vessels arise along the lines of
least resistance, through which currents are drawn or forced,
the question to be asked is — What physical process produces
them ? Their component cells, united end to end more or less
irregularly in ways determined by their original positions,
form a channel much more permeable, both longitudinally
and laterally, than the tissue around. How is this greater
permeability caused? The idea, first propounded

I believe by Wolff, that the adjoined ends of the cells are
perforated or destroyed by the passing current, is one for
which much is to be said. Whether these septa are dissolved
by the liquids they transmit, or whether they are burst by
those sudden gushes which, as we shall hereafter see, must
frequently take place along these canals, need not be dis-
cussed : it is sufficient for us that the septa do, in many cases,
disappear, leaving internal ridges showing their positions;
and, in other cases, become extremely porous. Though it is
manifest that this is not the process of vascular development
in tissues that unfold after pre-determined types, since, in
these, the dehiscences or perforations of septa occur before
such direct actions can have come into play; yet it is still
possible that the disappearances of septa which now arise by
repetition of the type were established in the type by such
direct actions. Be this as it may, however, a

simultaneous change undergone by these longitudinally-
united cells must be otherwise caused. Frame-works are
formed in them — frame-works which, closely fitting their
inner surfaces, may consist either of successive rings, or con-

284 PHYSIOLOGICAL DEVELOPMENT.

tinuous spiral threads, or networks, or structures between
spirals and networks, or networks with openings so far
diminished that the cells containing them are distinguished
as fenestrated. Their differences omitted, however, these
structures have the common character that, while support-
ing the coats of the vessels, they also give special facilities
for the passage of liquids, both through the sides of the
transformed cells and through their united ends, where these
are not destroyed.

To attempt any physical interpretation of this change is
scarcely safe: the conditions are so complex. There are
reasons for suspecting, however, that it arises from a vacuo-
lation of the substance deposited on the cell-wall. If rapidly
deposited, as it is likely to be along lines where sap is freely
supplied, this may, in passing from the state of a soluble
colloid to that of an insoluble colloid, so contract as to leave
uncovered spaces on the cell-membrane; and this change,
originally consequent on a physico-chemical action, may be
so maintained and utilized by natural selection, as to result
in structures of definite kinds, regularly formed in growing
parts in anticipation of functions to be afterwards discharged.
But, without alleging any special cause for this metamor-
phosis, we may reasonably conclude that it is in some way
consequent upon the carrying of sap. If we examine tissues
such as that in the interior of a growing turnip that has not
yet become stringy, we may, in the first place, find bundles
of elongated cells not having yet developed in them those
fenestrated or reticulated structures by which the ducts are
eventually characterized. Along the centres of adjacent
bundles we may find incomplete lines of such cells — some
that are partially or wholly transformed, with some between
them that are not transformed. In other bundles, completed
chains of such transformed cells are visible. And then, in
still older bundles, there are several complete chains running
side by side. All which facts imply a metamorphosis of the

THE INNER TISSUES OF PLANTS. 285

»

elongated cells, indirectly caused by the continued action of
the currents carried.

§ 281. Here, however, presents itself a further problem.
Taking it as manifest that there is a typical distribution of
supporting tissue adapted to meet the mechanical strains a
plant is exposed to by its typical mode of growth, and also
that there goes on special adaptation of the supporting tissue
to the special strains the individual plant has to bear; and
taking it as tolerably evident that the sap-channels are
originally determined by the passage of currents along lines
of least resistance ; there still remains the ultimate question —
Through what physical actions are established these general
and special adjustments of supporting tissue to the strains
borne, and these distributions of nutritive liquid required to
make possible such adjustments? Clearly, if the external
actions produce internal reactions ; and if this play of actions
and reactions results in a balancing of the strains by the
resistances; we may rationally suspect that the incident
forces are directly conducive to the structural changes by
which they are met. Let us consider how they must work.

When any part of a plant is bent by the wind, the tissues
on its convex surface are subject to longitudinal tension, and
these extended outer layers compress the layers beneath
them. Such of the vessels or canals in these subjacent layers
as contain sap, must have some of this sap expelled. Part of
it will be squeezed through the more or less porous walls of
the canals into the surrounding tissue, thus supplying it with
assimilable materials; while part of it, and probably the
larger part, will be thrust along the canals longitudinally
upwards and downwards.* When the branch or twig or
leaf-stalk recoils, these vessels, relieved from pressure, expand
to their original diameters. As they expand, the sap rushes
back into them from above and below. In whichever of
these directions least has been expelled by the compression,

286 PHYSIOLOGICAL DEVELOPMENT.

from that direction most must return during the dilation;
seeing that the force which more efficiently resisted the
thrusting back of the sap is the same force which urges it
into the expanded vessels again, when they are relieved from
pressure. At the next bend of the part a further portion of
sap will be squeezed out, and a further portion thrust for-
wards along the vessels. This rude pumping process thus
serves for propelling the sap to heights which it could not
reach by capillary action, at the same time that it incident-
ally serves to feed the parts in which it takes place. It
strengthens them, too, just in proportion to the stress to be
borne; since the more severe and the more repeated the
strains, the greater must be the exudation of sap from the
vessels or ducts into the surrounding tissue, and the greater
the thickening of this tissue by secondary deposits. By

this same action the movement of the sap is determined
either upwards or downwards, according to the conditions.
While the leaves are active and evaporation is going on from
them, these oscillations of the branches and petioles urge
forward the sap into them; because so long as the vessels of
the leaves are being emptied, the sap in the compressed
vessels of the oscillating parts will meet with less resistance
in the direction of the leaves than in the opposite direction.
But when evaporation ceases at night, this will no longer be
the case. The sap drawn to the oscillating parts, to supply
the place of the exuded sap, must come from the directions
of least resistance. A slight breeze will bring it back from
the leaves into the gently-swaying twigs, a stronger breeze
into the bending branches, a gale into the strained stem and
roots — roots in which longitudinal tension produces, in an-
other way, the same effects that transverse tension does in
the branches.

Two possible misinterpretations must be guarded against.
It is not to be supposed that this force-pump action causes
movement of the sap towards one point rather than another :
it is simply an aid to its movement. From the stock of sap

THE INNER TISSUES OF PLANTS. 287

distributed through the plant, more or less is everywhere
being abstracted — here by evaporation, here by the unfold-
ing of the parts into their typical shapes, here by both.
The result is a tension on the contained liquid columns, which
is greatest now in this direction and now in that. This
tension it is which must be regarded as the force that de-
termines the current upwards or downwards; and all which
the mechanical actions do is to facilitate the transfer to the
places of greatest demand. Hence it happens that in a plant
prevented from oscillating, but having a typical tendency to
assume a certain height and bulk, the demands set up by its
unfolding parts will still cause currents; and there will still
be alternate ascents and descents, according as the varying
conditions change the direction of greatest demand — the
only difference being that, in the absence of oscillations, the
growth will be less vigorous. Similarly, it must not

be supposed that mechanical actions are here alleged to be
the sole causes of wood-formation in the individual plant.
The tendency of the individual plant to form wood at places
where wood has been habitually formed by ancestral plants,
is manifestly a cause, and, indeed, the chief cause. In this,
as in all other cases, inherited structures repeat themselves
irrespective of the circumstances of the individual: absence
of the appropriate conditions resulting simply in imperfect
repetition of the structures. Hence the fact that in trained
trees and hothouse shrubs, dense substance is still largely
deposited; though not so largely as where the normal me-
chanical strains have acted. Hence, too, the fact, that in
such plants as the Elephant's-foot or the Welwitschia mira-
bilis, which for untold generations can have undergone no
oscillations, there is an extensive formation of wood (though
not to any considerable height above the ground), in repeti-
tion of an ancestral type : natural selection having here main-
tained the habit as securing some other advantage than that
of support.

Still, it must be borne in mind that though intermittent

288 PHYSIOLOGICAL DEVELOPMENT.

mechanical strains cannot be assigned as the direct causes
these internal differentiations in plants that are artificially
sheltered or supported, they are assignable as the indirect
causes; since the inherited structures, repeated apart from
such strains, are themselves interpretable as accumulated
results of such strains acting on successive generations of
ancestral plants. This will become clear on combining the
several threads of the argument and bringing it to a close,
which we may now do.

§ 282. To put the co-operative actions in their actual
order, would require us to consider them as working on in-
dividuals small modifications that become conspicuous and
definite only by inheritance and gradual increase; but it will
aid our comprehension without leading us into error, if we
suppose the whole process resumed in a single continuously-
existing plant.

As the plant erects the integrated series of fronds whose
united parts form its rudimentary axis, the increasing area
of frond-surface exposed to the sun's rays entails an increas-
ing draught upon the liquids contained in the rudimentary
axis. The currents of sap so produced, once established along
certain lines of cells that offer least resistance, render them
by their continuous passage more and more permeable. This
establishment of channels is aided by the wind. Each bend
produced by it while yet the tissue is undifferentiated,
squeezes towards the place of growth and evaporation the
liquids that are passing by osmose from cell to cell; and
when the lines of movement become defined, each bend helps,
by forcing the liquid along these lines, to remove obstructions
and make continuous canals. As fast as this transfer of sap
is facilitated, so fast is the plant enabled further to raise it-
self, and add to its assimilating surfaces ; and so fast do the
transverse strains, becoming greater, give more efficient aid.
The canals thus formed can be neither in the centre of the
rudimentary axis nor at its surface: for at neither of these

THE INNER TISSUES OF PLANTS. 289

places can the transverse strains produce any considerable
compressions. They must arise along a tract between the
outside of the axis and its core — a tract along which there
occur the severest squeezes between the stretched outer layers
and the internal mass. Just that distribution which we find,
is the distribution which these mechanical actions tend to
establish.

As the plant gains in height, and as the mass of its foliage
accumulates, the strains thrown upon its axis, and especially
the lower part of its axis, rapidly increase. Supposing the
forms to remain similar, the strains must increase in the ratio
of the cubes of the dimensions; or even in a somewhat higher
ratio. One consequence must be that the compressions to
which the vessels at the lower part of the incipient stem are
subject, become greater as fast as the height to which the sap
has to be raised becomes greater; and another consequence
must be that the local exudation of sap produced by the
pressure is proportionately augmented. Hence the materials
for interstitial nutrition being there supplied more abun-
dantly, we may expect thickening of the surrounding tissues
to show itself there first: in other words, wood will be
formed round the vessels of the lower part of the incipient
stem. The resulting greater ability of this lower part of the
stem to bear strains, renders possible an increase of height;
and while after an increase of height the lowest part be-
comes still further strained, and still further thickens, the
part above it, exposed to like actions, undergoes a like
thickening. This induration, while it spreads upwards, also
spreads outwards. As fast as the rude cylinder of dense
matter formed in this way, begins to inclose the original
vessels, it begins to play the part of a resistant mass, which
more and more prevents the contained vessels from being
squeezed; while between it and the outer layers the greatest
compression occurs at each bend. Thus at the same time
that the original vessels become useless, the peripheral cells
of the developing wood become those which have their liquid
65

290 PHYSIOLOGICAL DEVELOPMENT.

contents squeezed out longitudinally and laterally with in-
creasing force; and, consequently, amid them are formed
new sap-channels, from which there is the most active local
exudation, producing the greatest deposit of dense matter.

Thus fusing together, as it were, the individualities of
successive generations of plants, and recognizing as all-
important that facilitation of the process which natural
selection has all along given, we are enabled to interpret the
chief internal differentiations of plants as consequent on an
equilibration between inner and outer forces. Here, indeed,
we see illustrated in a way more than usually easy to follow,
the eventual balancing of outer actions by inner reactions.
The relation between the demand for liquid and the formation
of channels that supply liquid, as well as that between the
incidence of strains and the deposit of substance which resists
strains, are among the clearest special examples of the general
truth that the moving equilibrium of an organism, if not
overthrown by an incident force, must eventually be adjusted
to it.

The processes here traced out are, of course, not to be
taken as the only differentiating processes to which the inner
tissues of plants have been subject. Besides the chief changes
we have considered, various less conspicuous changes have
taken place. These must be passed over as arising in ways
too involved to admit of specific interpretations; even sup-
posing them to have been produced by causes of the kind
assigned. But the probability, or rather indeed the certainty,
is that some of them have not been so produced. Here, as
in nearly all other cases, indirect equilibration has worked in
aid of direct equilibration ; and in many cases indirect equili-
bration has been the sole agency. Besides ascribing to
natural selection the rise of various internal modifications
of other classes than those above treated, we must ascribe
some even of these to natural selection. It is so with the
dense deposits which form thorns and the shells of nuts :
these cannot have resulted from any inner reactions imme-

THE INNER TISSUES OF PLANTS. 291

diately called forth by outer actions ; but must have resulted
mediately through the effects of such outer actions on the
species. Let it be understood, therefore, that the differ-
entiations to which the foregoing interpretation applies, are
only those most conspicuous ones which are directly related
to the most conspicuous incident forces. They must be taken
as instances on the strength of which we may conclude that
other internal differentiations have had a natural genesis,
though in ways that we cannot trace.

CHAPTER V.

PHYSIOLOGICAL INTEGRATION IN PLANTS.

§ 283. A good deal has been implied on this topic in the
preceding chapters. Here, however, we must for a brief
space turn our attention immediately to it.

Plants do not display integration in such distinct and
multiplied ways as do animals. But its advance may be
traced both directly and indirectly — directly in the increas-
ing co-ordination of actions, and indirectly in the effect of
this upon the powers and habits.

Let us group the facts under these heads: ascending in
both cases from the lower to the higher types.

§ 284. The inferior Algce, along with little unlikeness of
parts, show us little mutual dependence of parts. Having
surfaces similarly circumstanced everywhere, much physio-
logical division of labour cannot arise; and therefore there
cannot be much physiological unity. Among the superior
Algce, however, the differentiation between the attached part
and the free part is accompanied by some integration. There
is evidently a certain transfer of materials, which is doubtless
facilitated by the elongated forms of the cells in the stem,
and probably leads to the formation of dense tissue at the
places of greatest strain, in a way akin to that recently ex-
plained in other cases. And where there is this co-ordina-
tion of actions, the parts are so far mutually dependent that
292

PHYSIOLOGICAL INTEGRATION IN PLANTS. 293

each dies if detached from the other. That though the
organization is so low neither part can reproduce the other
and survive by so doing, is probably due to the circumstance
that neither part contains any considerable stock of untrans-
formed protoplasm, out of which new tissues may be pro-
duced.

Fungi and Lichens present no very significant advances
of integration. We will therefore pass at once to the
Archegoniates. In those of them which, either as single
fronds or strings of fronds, spread over surfaces, and which,
rooting themselves as they spread, do not need that each part
should receive aid from remote parts, there is no developed
vascular system serving to facilitate transfer of nutriment:
the parts being little differentiated there is but little integra-
tion. But along with assumption of the upright attitude and
the accompanying specializations, producing vessels for dis-
tributing sap and hard tissue for giving mechanical support,
there arises a decided physiological division of labour ; render-
ing the aerial part dependent on the imbedded part and the
imbedded part dependent on the aerial part. Here, in-
deed, as elsewhere, these concomitant changes are but two
aspects of the same change. Always the gain of power to dis-
charge a special function involves a loss of power to perform
other functions; and always, therefore, increased mutual de-
pendence constituting physiological integration, must keep
pace with that increased fitting of particular parts to particu-
lar duties which constitutes physiological differentiation.

Making a great advance among the Archegoniates, this
physiological integration reaches its climax among Phaeno-
gams. In them we see interdependence throughout
masses that are immense. Along with specialized appli-
ances for support and transfer, we find an exchange of aid at
great distances. We see roots giving the vast aerial growth
a hold tenacious enough to withstand violent winds, and
supplying water enough even during periods of drought; we
see a stem and branches of corresponding strength for up-

294 PHYSIOLOGICAL DEVELOPMENT.

holding the assimilating organs under ordinary and extraor-
dinary strains; and in these assimilating organs we see
elaborate appliances for yielding to the stem and roots the
materials enabling them to fulfil their offices. As a con-
sequence of which greater integration accompanying the
greater differentiation, there is ability to maintain life over
an immense period under marked vicissitudes.

Even more conspicuously exemplified in Phaenogams, is
that physiological integration which holds together the func-
tions not of the individual only but of the species as a whole.
The organs of reproduction, both in their relations to other
parts of the individual bearing them and in their relations to
corresponding parts of other individuals, show us a kind of
integration conducing to the better preservation of the race;
as those already specified conduce to the better preservation of
the individual. In the first place, this greater co-ordination
of functions just described, itself enables Phaenogams to be-
queath to the germs they cast off, stores of nutriment, pro-
tective envelopes, and more or less of organization : so giving
them greater chances of rooting themselves. In the second
place, certain differentiations among the parts of fructifica-
tion, the meaning of which Mr. Darwin has so admirably ex-
plained, give to the individuals of the species a kind of inte-
gration that makes possible a mutual aid in the production of
vigorous offspring. And it is interesting to observe how, in
that dimorphism by which in some cases this mutual aid is
made more efficient, the greater degree of integration is
dependent on the greater degree of differentiation — not simply
differentiation of the fructifying organs from other parts of
the plant bearing them, but differentiation of these fructify-
ing organs from the homologous organs of neighbouring indi-
viduals of the same race. Another form of this
co-ordination of functions which conduces to the maintenance
of the species, may be here named — partly for its intrinsic
interest. I refer to the strange processes of multiplication
occurring in the genus Bryophyllum. It is well known that

PHYSIOLOGICAL INTEGRATION IN PLANTS. 295

the succulent leaves of B. calycinum, borne on foot-stalks
so brittle that they are easily snapped by the wind, send
forth from their edges when they fall to the ground, buds
which root themselves and grow into independent plants. The
correlation here obviously furthering the preservation of the
race, is more definitely established in another species of the
genus — B. proliferum. This plant, shooting up to a consider-
able height, and having a stem containing but little woody
fibre, habitually breaks near the bottom while still in flower;
and is thus generally prevented from ripening its seeds. The
multiplication is, however, secured in another way. Before
the stem is broken young plants have budded out from the
pedicels of the flowers, and have grown to considerable lengths ;
and on the fall of the parent they forthwith commence their
separate lives. Here natural selection has established a
remarkable kind of co-ordination between a special habit of.
growth and decay, and a special habit of proliferation.

§ 285. The advance of physiological integration among
plants as we ascend to the higher types, is implied by their
greater constancy of structure, as well as by the stricter limi-
tations of their habitats and modes of life. " Complexity of
structure is generally accompanied with a greater tendency
to permanence in form," says Dr. [now Sir J.] Hooker; or,
conversely, " the least complex are also the most variable."
This is the second aspect under which we have to contem-
plate the facts.

The differences between the simpler Algce and Fungi are
so feebly marked that botanists have had great difficulty in
framing definitions of these classes. This structural indefi-
niteness is accompanied by functional indefiniteness. Algce,
which are mostly aquatic, include many small forms that
frequent the damp places preferred by Fungi. Among Fungi,
there are kinds which lead submerged lives like the Algce.
Besides this indistinctness of the classes, there is great varia-
bility in the shapes and modes of life of their species — a vari-

296 PHYSIOLOGICAL DEVELOPMENT.

ability so great that what were at first taken to be different
species, or different genera, or even different orders, have
proved to be merely varieties of one species. So inconstant
in structure are the Algce that Schleiden quotes with approval
the opinion of Kutzing, that " there are no species but merely
forms of Algce:" an opinion which though now rejected
sufficiently implies extreme indefiniteness. In all which
facts we see that these lowest types of plants, little differ-
entiated, are also but little integrated.

Archegoniates present a like relation between the small
specialization of functions which constitutes physiological
differentiation, and the small combination of functions which
constitutes physiological integration. " Mosses," says Mr.
Berkeley, " are no less variable than other cryptogams, and
are therefore frequently very difficult to distinguish. iSTot
•only will the same species exhibit great diversity in the size,
mode of branching, form and nervation of the leaves, but the
characters of even the peristome itself are not constant."
And concerning the classification of the remaining group,
Filicales, he says : — " Not only is there great difficulty in
arranging ferns satisfactorily, but it is even more difficult to
determine the limits of species."

After this vagueness of separation as well as inconstancy
of structure and habit among the lower plants, the stability
of structure and habit and divisibility of groups among the
higher plants, appear relatively marked. Though Phaenogams
are much more variable than most botanists have until
lately allowed, yet the definitions of species and genera
may be made with far greater precision, and the forms are
far less capable of change, than among Cryptogams. And
this comparative fixity of type, implying, as it does, a closer
combination of the component functions, we see to be the
accompaniment of the greater differentiation of those func-
tions and of the structures performing them. That these
characters are correlatives is further shown by the fact that
the higher plants are more restricted in their habitats than

PHYSIOLOGICAL INTEGRATION IN PLANTS. 297

the lower plants, both in space and time. " The much
narrower del imitation in area of animals than plants," says
Sir J. Hooker, " and greater restriction of Faunas than
Floras, should lead us to anticipate that plant-types are, geo-
logically speaking, more ancient and permanent than the
higher animal types are, and so I believe them to be, and I
would extend the doctrine even to plants of highly complex
structure." " Those classes and orders which are the least
complex in organization are the most widely distributed."

§ 286. Thus that which the general doctrine of evolution
leads us to anticipate, we find implied by the facts. The
physiological division of labour among parts, can go on only
in proportion to the mutual dependence of parts; and the
mutual dependence of parts can progress only as fast as there
arise structures by which the parts are efficiently combined,
and the mutual utilization of their actions made easy.

To say definitely by what process is brought about this
co-ordination of functions which accompanies their specializa-
tion, is hardly practicable. Direct and indirect equilibration
doubtless co-operate in establishing it. We may see, for
example, that every increase of fitness for function produced
in the aerial part of a plant by light, as well as every increase
of fitness for function. produced in its imbedded part by the
direct action of the moist earth, must conduce to an increased
current of the liquid evaporated from the one and supplied
by the other — must serve, therefore, to aid the formation of
sap-channels in the ways already described; that is — must
serve to develop the structures through which mutual aid
of the parts is given: the additional differentiation tends
immediately to bring about the additional integration. Con-
trariwise, it is obvious that an inter-dependence such as we
see between the secretion of honey and the fertilization of
germs, or between the deposit of albumen in the cotyledons
of an embryo-plant and its subsequent striking root, is a
kind of integration in the actions of the individual or of the

298 PHYSIOLOGICAL DEVELOPMENT.

species, which no differentiation has a direct tendency to
initiate. Hence we must regard the total results as due to a
plexus of influences acting simultaneously on the individual
and on the species : some chiefly affecting the one and some
chiefly affecting the other.

[Note. — In Nature for June 11, 1896, Dr. Maxwell Mas-
ters, in an essay on " Plant Breeding," names an instructive
fact concerning the production of varieties by selection of
slightly divergent forms. He says : —

" To the untrained eye, the primordial differences noted are
often very slight; even the botanist, unless his attention be
specially directed to the matter, fails to see minute differences
which are perceptible enough to the raiser or his workmen.
Nor must it be thought that these variations, difficult as they
are to recognise in the beginning, are unimportant. On the
contrary, they are interesting, physiologically, as the potential
origin of new species, and very often they are commercially
valuable also. These apparently trifling morphological dif-
ferences are often associated with physiological variations
which render some varieties, say of wheat, much better
enabled to resist mildew and disease generally than others.
Some, again, prove to be better adapted for certain soils or
for some climates than others ; some are less liable to injury
from predatory birds than others, and so on."

Thus we are shown that, to a much greater degree than
might be supposed, minute changes of forms and functions
in one part of a plant are correlated with changes of forms
and functions throughout it. The inter-dependence — that is
to say, the physiological integration — is very close at the
same time that it is very complex.

Here while naming these facts in illustration of physio-
logical integration in plants I name them because they
illustrate an important truth bearing upon the general ques-
tion of heredity which I have dealt with in Appendix G-, and
to which I now especially draw attention.]

CHAPTER VI.

DIFFERENTIATIONS BETWEEN THE OUTER AND INNER
TISSUES OF ANIMALS.

§287. What was said respecting the primary physiological
differentiation in plants, applies with little beyond change of
terms to animals. Among Protozoa, as among Protophyta,
the first definite contrast of parts is that between outside
and inside. The speck of jelly or sarcode which appears
to constitute the simplest animal, proves, on closer examina-
tion, to be a mass of substance containing a nucleus — a
periplast in the midst of which there is a minute endoplast,
consisting of a spherical membrane and its contents.

This parallel, only just traceable among these Rhizopods,
which are perpetually changing the distribution of their outer
substance, becomes at once marked in those higher Protozoa
which have fixed shapes, and maintain constant relations
between their surfaces and their environments. Indeed the
Rhizopods themselves, on passing into a state of quiescence
in which the relations of outer and inner parts are fixed,
become encysted: there is formed a hardened outer coat
different from the matter which it contains. And what is
here a temporary character answering to a temporary definite-
ness of conditions, is in the Infusoria a constant character,
answering to definite conditions that are constant. Each of
these minute creatures, though not coated by a distinct
membrane, has an outer layer of excreted substance forming
a delicate cuticle.

§ 288. The early establishment of this primary contrast of

300 PHYSIOLOGICAL DEVELOPMENT.

tissues answering to this primary contrast of conditions, is
no less conspicuous in aggregates of the second order. The
feebly-integrated units of a Sponge, with individualities so
little merged in that of the whole they form that most of
them still retain their separate activities, nevertheless show
us, in the unlikeness that arises between the outermost layer
and the contained mass, the effect of converse with unlike
conditions. This outermost layer is composed of units some-
what flattened and united into a continuous membrane — a
kind of rudimentary "skin.

Secondary aggregates in which the lives of the units are
more subordinate to the life of the whole, carry this dis-
tinction further. The leading physiological trait of every
ccelenterate animal is the divisibility of its substance into
endoderm and ectoderm — the part next the food and the
part next the environment. Fig. 147 (§ 201), representing a
portion of the body-wall of a Hydra seen in section, gives
some idea of this fundamental differentiation. The creature
consists of a simple sac, the cavity of which is in communi-
cation with the surrounding water; and hence the unlike-
ness between the outer and inner layers has not become
great. The essential contrast is that between the differen-
tiated parts of what was originally the same part — a uniform
membrane composed of juxtaposed cells.

For here, indeed, we are shown unmistakably how the
primary contrast of structures follows upon the primary con-
trast of conditions. The ordinary form from which low
types of the Metazoa set out, is a hollow sphere formed of
cells packed side by side — a blastula, as it is called : all these
cells being similarly exposed to the environment. The
blastula presently changes into what is called a gastrula — a
form resulting from the introversion of one of the sides of
the blastula. If there be taken a small ball of vulcanized
india-rubber, say an inch or more in diameter, and having a
hole in it through which the air may escape, and if one side
of it be thrust inwards so as to produce a cup, and if the

THE OUTER AND INNER TISSUES OF ANIMALS. 301

wide opening of the cup be supposed to contract, thus
becoming a narrow opening, there will result something like
the gastrula form. Manifestly that part of the original layer
which has become internal is differently conditioned from
the rest which remains external: the one continuing to hold
converse with the forces of the environment, while the other
begins to hold converse with the nutritive matters taken
into the sac-formed chamber — the archenteron or primitive
stomach. Interesting evidence of the primitive externality
of the digestive cavity is yielded by the fact that whereas
the blastula consisted of ciliated cells, and whereas the cilia-
tion persists throughout life on the outer layer, or parts of it,
in sundry low types — even in some Chaetopods — it persists
also on the alimentary tract of sundry low types : not only
in the Hydra but commonly in Nemertines, in some Platy-
helminthes, and even in some leeches.

Besides being enabled thus to understand how an aggre-
gate of Amoeba- form units, originally consisting of a single
layer, may pass into an aggregate consisting of a double
layer; we may also understand under what influences the
transition takes place. If the habit which some of the
primary aggregates have, of wrapping themselves round
masses of nutriment, is followed by a secondary aggregate,
there will naturally arise just that re-differentiation which
the Hydra shows us.

§ 289. This account of the primary differentiation carries
us only half-way towards a true conception of the distinction
between outer and inner tissues. Though, using words in
their current senses, this introverted part of the primitive
layer has become internal in contrast with the remainder,
which continues external, yet this introverted part has not
become internal in the strict physiological sense. For it
remains subject to the actions of those environing matters
which are taken in as food: such environing matters, when
they happen to be moving prey, acting upon it much as they

302 PHYSIOLOGICAL DEVELOPMENT.

might act upon the exterior. So that this introverted part
has a quasi-externality. It has not the same absolute in-
ternality as have those parts which never come in contact
with products of the outer world. Here we must briefly
recognize the distinction between these parts and the parts
thus far considered.

Eeverting to our symbol, the india-rubber ball, it will be
seen that the introversion may be so complete that the cavity
is obliterated, with the result that the internal surfaces of
the outer and inner layers come in contact. This is the
state reached in the simplest ccelenterate animal, the Hydra:
there being in it nothing more than a thin structureless
lamella between the ectoderm and endoderm, as shown in
Fig. 147. This lamella represents all that there is of strictly
internal tissues. But the introversion, instead of bringing
the inner surfaces of the ball into contact, may be so far in-
complete as to leave a space, and in various creatures and
embryos of others, symbolized by this arrangement, this space
becomes occupied by a tissue formed from one or other or
both of the two primary tissues — the mesoblast or meso-
derm. This intermediate layer, sometimes, as in the Medusa,
growing into a mass of jelly serving as a fulcrum for the
creature's contractions, or, as in the Sponge, giving a passive
basis to the active tissues, becomes in higher animals the
layer out of which the structures that support the body and
move it about, as well as those that distribute prepared
nutriment, are developed. From it arise the bones, the
muscles, and the vascular system — the masses of differen-
tiated tissue which are truly internal and occupy what is
called the body-cavity or peri-visceral space.

In the higher types of animals this space comes to be
partially occupied by a structure that may be described as a
cavity within a cavity — the ccelom. Most zoologists regard
this as arising by a re-introversion of the archenteron or
primary alimentary sac. It is easily to be perceived that
after the introversion which produces this digestive cavity, the

THE OUTER AND INNER TISSUES OF ANIMALS. 303

wall of the cavity may be again introverted in such way as to
intrude into the peri-visceral space. The ccelom thus formed
is subsequently shut off. Becoming included among the more
truly internal structures, and in part giving origin to certain
lining membranes, it has for its chief function the formation
of organs for the excretion and emission of nitrogenous waste
and of the generative products : some portions of it retaining,
as a consequence, indirect connexions with the environment
and characters usually accompanying such connexions.

Here we are not concerned with further details : the aim
being simply to indicate the way in which out of the original
layer, wholly external, there arise, by primary and secondary
introversions, and the formation of intermediate membranes
and spaces, the chief contrasts between outer and inner tis-
sues, and how there simultaneously go on the differentia-
tions accompanying different conditions.

§ 289a. Another all-important differentiation between
outer tissues and inner tissues has now to be set forth — that
by which the nervous system becomes established and dis-
tinguished. Strangely enough, like the one above described,
it is sequent upon an introversion: the nervous system is
primarily a skin-structure and develops by the infolding of
this skin-structure.

In creatures possessing the earliest rudiments of nerves
these exist in certain superficial cells. Each has a small
tubular orifice from which projects a minute hair, and each
has on its under side processes running into the tissue
below, and serving, as it seems, to conduct impressions from
the projecting hair when it is disturbed by contacts with
foreign bodies. A plexus of fibres bringing the inner pro-
cesses of such cells into communication arises, and forms
something like a nervous layer capable of propagating im-
pulses in all directions. At a subsequent stage some of the
superficial cells, ceasing to be themselves the recipients of
external stimuli, sink inwards and become ganglion-cells con-

304: PHYSIOLOGICAL DEVELOPMENT.

nected with, the nervous plexus — agents, as we must suppose,
for the reception, multiplication, and diffusion of the impulses
received from the outer cells.

As thus far developed, the nervous structure is one fitted
only for a vague stimulation of dispersed contractile fibres,
causing movements of an undirected kind. A concentration
of these superficial nervous structures is a probable prelim-
inary to the next change — an all-important change. For a
part of the surface begins to sink inwards, forming, in the
Vertebrata, a groove ; and from the lining cells of this groove,
which presently closes over, the central parts of the nervous
system arise: definite nerves having meantime, as we may
suppose, been developed out of the indefinite nervous plexus.

Neglecting what there is in this of a speculative nature, it
is sufficient for the present purpose to recognize the un-
doubted fact that the nervous system is developed from the
ectoderm, and that, originally external, it is made internal by
a process of sinking in or by a process of definite introversion.

§ 290. Whether direct equilibration or indirect equilibra-
tion has had the greater share in producing these fundamental
contrasts between the inner and outer tissues of animals,
must be left undecided. The two causes have all along co-
operated— modification of the individual accumulated by
inheritance predominating in some cases, and in other cases
modification of the race by survival of the incidentally fittest.
On the other hand, the action of the medium on the organism
cannot fail to change its surface more than its centre, and so
differentiate the two; while, on the other hand, the surfaces
of organisms inhabiting the same medium display extreme
unlikenesses which cannot be due to the immediate actions
of their medium. Let us dwell a moment on the antithesis.

We have abundant evidence that animal protoplasm is
rapidly modified by light, heat, air, water, and the salts
contained in water — coagulated, turned from soluble into in-
soluble, partially changed into isomeric compounds, or other-

THE OUTER AND INNER TISSUES OF ANIMALS. 305

wise chemically altered. Immediate metamorphoses of this
kind are often obviously produced in ova by changes of their
media. At the outset, therefore, before yet there existed
any such differentiation as that which now usually arises by
inheritance, these environing agencies must have tended to
originate a protective envelope. For a modification produced
by them on the superficial part of the protoplasm, must
either have been a decomposition or else the formation of a
compound which remained stable under their subsequent
action. There would be generated an outer layer of substance
that was so molecularly immobile as to be incapable of further
metamorphoses, while it would shield the contained proto-
plasm from that too-great action of external forces which, by
rapidly changing the unstable equilibrium of its molecules
into a relatively stable equilibrium, would arrest development.
Evidently organic evolution, whether individual or general,
must always and everywhere have been subordinate to these
physical necessities. Though natural selection, beginning
with minute portions of protoplasm, must all along have
tended to establish a molecular composition apt to undergo
this differentiation of surface from centre to the most favour-
able extent, yet it must all along have done so while con-
trolled by this process of direct equilibration.

Contrariwise, the many and great unlikenesses among the
dermal structures of creatures inhabiting the same element,
cannot be ascribed to any such cause. The contrasts between
naked and shelled Gastropods; between marine Worms and
Crustaceans, between soft-skinned. Fishes and Fishes in
armour like the Pterichthys, must have been produced entirely
by natural selection. Environing forces are, as before, the
ultimate causes; but the forces are now not so nluch those
exercised by the medium as those exercised by the other in-
habitants of the medium; and they do not act by modifying
the surface of the individual, but by killing off individuals
whose surfaces are least fitted to the requirements: thus
slowly affecting the species. Still the dermal skeleton bristling
6(?

306 PHYSIOLOGICAL DEVELOPMENT.

with spines, which protects the Diodon or the Cyclichthys
from enemies it could not escape, comes within the general
formula of an outer tissue differentiated from inner tissues
by the outer actions to which the creature is exposed: the
differentiation having gone on until there is equilibrium
between the destructive forces to be met and the protective
forces which meet them.

If we venture to apportion the respective shares which
mediate and immediate actions have had in differentiating
outer from inner tissues, we shall probably not be far wrong
in ascribing that part of the result which is alike in all
animals, mainly to the direct actions of their media, while
we ascribe the multitudinous unlikenesses of the results in
various animals, partly to the indirect actions of the media,
and partly to the indirect actions of other animals by which
the media are inhabited. That is to say, while assigning the
specialities of the differentiations to the specialities of con-
verse with the agencies in the environment, most of them
organic, we may assign to the constant and universal con-
verse with its inorganic agencies, the universal characteristic
of tegumentary structures — their growth outwards from a
layer lying below the surface which continually produces new
substance to replace the substance worn away or cast off.

Here let me add a piece of evidence which strengthens
the general argument, at the same time that it justifies this
apportionment. When ulceration has gone deep enough to
destroy the tegumentary structures, these are never repro-
duced. The puckered surface formed where an ulcer heals,
or where a serious burn has destroyed the skin, consists of
modified connective tissue, which, as the healing goes on,
spreads inwards from the edges of the ulcer: some of it,
perhaps, growing from the portions of connective tissue that
dip down between the muscular bundles. This connective
tissue is normally covered by the epidermis and thus sheltered
from environing actions. What has happened to it ? It has
now become the outermost layer. And how does it comport

THE OUTER AND INNER TISSUES OF ANIMALS. 307

itself under its new conditions? It produces a superficial
substance which plays the part of the epidermis and grows
outwardly. For since the surface, subject to friction and
exfoliation, has to be continually renewed, there must be a
continual reproduction of an outermost layer from a layer
beneath. That is to say, the contact of this deep-seated tissue
with outer agencies, produces in it some approach towards
that character which we find universally characterizes outer-
tissue. But while we see under this exposure to the con-
ditions common to all integument, a tendency to assume
the structure common to all integument, we see no tendency
to assume any of the specialities of tegumentary structure:
no rudiments of glands or hair sacs make their appearance.

Analogous conclusions may be drawn respecting the pro-
cesses of differentiation by which from the outer layer
nervous tissue and finally a nervous system are evolved.
Here, also, both direct and indirect equilibration appear to
have operated. Two reasons may be assigned for the belief
that the transformation of certain superficial cells into sensi-
tive cells was initiated by exposure to external stimuli. The
first is that, extremely unstable as protoplasm is, disturb-
ances received by the outer side of a specially-exposed cell
could scarcely fail to cause changes passing through it
towards the interior mass of the body, and that perpetual
repetition of such changes would tend to generate channels
of easy transmission through the protoplasm. The second
reason is that, if we do not assume this process of initiation
but assume that survival of the fittest was the sole agency,
then no reason can be assigned why the nervous system
should not have been at the outset formed internally instead
of being initiated externally and then transferred to *the in-
terior: the roundabout process would be inexplicable. At
the same time the production of a central nervous system by
introversion of superficial sensitive cells cannot be ascribed
to the differentiating effects of external stimuli, but must be
ascribed to natural selection. No perpetual repetition of

308 PHYSIOLOGICAL DEVELOPMENT.

outer disturbances would cause the sinking inwards, and
covering up, of the specially-sensitive area and the plexus
below it. But it is manifest that since these nervous struc-
tures, at once all-important and easily injured, would be
safer if removed from the surface, survival of the fittest, con-
tinually preserving those in which they were more deeply
seated, would tend to produce an arrangement in which all
parts but the actual receivers of external stimuli became
internal.

Hence, contemplating generally these two fundamental
differentiations of inner from outer tissues, we may conclude
that though their first stages resulted from direct equilibra-
tion, their subsequent and higher stages resulted from in-
direct equilibration.

CHAPTER VII.

DIFFERENTIATIONS AMONG THE OUTER TISSUES OF
ANIMALS.

§ 291. The outer tissues of animals, originally homo-
geneous over their whole surfaces, pass into a heterogeneity
which fits their respective parts to their respective conditions.
So numerous and varied are the implied differentiations, that
it is impracticable here to deal with them all even in outline.
To trace them up through classes of animals of increasing
degrees of aggregation, would carry us into undue detail.

Did space permit, it would be possible to point out among
the Protozoa, various cases analogous to that of the Arcella;
which may be described as like a microscopic Limpet, having
a sarcode body of which the upper surface has become horny,
while the lower surface with its protruding pseudopodia,
retains the primitive jelly-like character. That differentia-
tions of this kind have been gradually established among
these minute creatures through the unlike relations of their
parts to the environment, is an inference supported by a
form which, while the rest of the body has a scarcely dis-
tinguishable coating, " agrees with Arcella and Difflugia in
having the pseudopodia protrusible from one extremity only
of the body."

Many parallel specializations of surface among aggregates
of the second order might be instanced from the Cailenterata.
In the Hydra, the ectoderm presents over its whole area no
conspicuous unlikenesses ; but there usually exist in the
hydroid polypes of superior types, decided contrasts between

310 PHYSIOLOGICAL DEVELOPMENT.

the higher and lower parts. While the higher parts retain
their original characters, the lower parts excrete hard outer
layers yielding support and protection. Various stages of
the differentiation might be followed. " In Hydractinia"
says Prof. Green, this horny layer " becomes elevated at in-
tervals to form numerous rough processes or spines, while
over the general surface of the ectoderm its presence is
almost imperceptible." In other types, as in Cordylophora,
it spreads part way up the animaFs sides, ending indefinitely.
In Bimeria it " extends itself so as to enclose the entire body
of each polypite, leaving bare only the mouth and tips of the
tentacles." While in Campanularia it has become a partially-
detached outer cell, into which the creature can retract its
exposed parts.

But it is as needless as it would be wearisome to trace
through the several sub-kingdoms the rise of these multiform
contrasts, with the view of seeking interpretations of them.
It will suffice if we take a few groups of the illustrations
furnished by the higher animals.

§ 292. We may begin with those modifications of surface
which subserve respiration. Though we ordinarily think of
respiration as the quite special function of a quite special
organ, yet originally it is not so. Little-developed animals
part with their carbonic acid and absorb oxygen, through the
general surface of the body. Even in the lower types of the
higher classes, the general surface of the body aids largely in
aerating the blood ; and the parts which discharge the greater
part of this function are substantially nothing more than
slightly altered and extended portions of the skin.

Such differentiations, marked in various degrees, are to be
seen among Mollusca. In the Pteropoda the only modification
which appears to facilitate respiration, is the minute vascu-
larity of one part of the skin. Higher types possess special
skin-developments. The Doris has appendages developed
into elaborately-branched forms — small trees of blood-vessels

THE OUTER TISSUES OF ANIMALS. 311

covered by slightly-changed dermal tissues. And these
arborescent branchiae are gathered together into a single
cluster. Thus there is evidence that large external respira-
tory organs have arisen by degrees from simple skin: as,
indeed, they do arise during the development of each indi-
vidual having them. Just as gradually as in the embryo
a simple bud on the integument, with its contained vascular
loop, passes by secondary buddings into a tree-like growth
penetrated everywhere by dividing and sub-dividing blood-
vessels; so gradually has there probably proceeded the
differentiation which has turned part of the outer surface
into an organ for excreting carbonic acid and absorbing
oxygen.

Certain inferior vertebrate animals present us with a like
metamorphosis of tissues. These are the Amphibia. The
branchiae here developed from the skin, are covered with cel-
lular epidermis, not much thinner than that covering the rest
of the body. Like it they have their surfaces speckled with
pigment-cells; and are not even conspicuous by their extra
vascularity — where they are temporary at least. They facili-
tate the exchange of gases in scarcely any other way than by
affording a larger area of contact with the water, and inter-
posing a rather thinner layer of tissue between the water
and the blood-vessels. Those very simple branchiae of the
larval Amphibia that have them but for a short time,
graduate into the more complex ones of those that have them
for a long time or permanently ; showing, as before, the small
stages by which this heterogeneity of surface accompanying
heterogeneity of function may arise.

In what way are such differentiations established ? Main-
ly, no doubt, by natural selection ; but also to some degree, I
think, by the inheritance of direct adaptations. That a por-
tion of the integument at which aeration is favoured by local
conditions, should thereby be led to grow into a larger
surface of aeration, appears improbable. Survival of those
individuals which happen to have this portion of the integu-

312 PHYSIOLOGICAL DEVELOPMENT.

ment somewhat more developed, seems here the only likely
cause.

§ 293. Among the conspicuous modifications by which the
originally-uniform outer layer is rendered multiform, are the
protective structures. Let us look first at the few cases in
which the formation of these is ascribable mainly to direct
equilibration.

Already reference has been more than once made to those
thickenings that occur where the skin is exposed to unusual
pressure and friction. Are these adaptations inheritable?
and may they, by accumulation through many generations,
produce permanent dermal structures fitted to permanent or
frequently-recurring stress? Take, for instance, the callosi-
ties on the knuckles of the Gorilla, which are adapted to
its habit of partially supporting itself on its closed hands
when moving along the ground. Shall we suppose that these
defensive thickenings are produced afresh in each individual
by the direct actions; or that they are inherited modifica-
tions caused by such direct actions; or that they are wholly
due to the natural selection of spontaneous variations ? The
last supposition does not seem a probable one. Such thicken-
ings, if spontaneous, would be no more likely to occur on the
knuckles than on any other of the hundred equal areas form-
ing the skin-surface at large; and the chances against their
simultaneous occurrence on all eight knuckles would be in-
calculable. Moreover, the implication would be that those
slight extra thicknesses of skin on the knuckles, with which
we must suppose the selection to have commenced, were so
advantageous as to cause survivals of the individuals having
them, in presence of other superiorities possessed by other
individuals. Then that survivals so caused, if they ever
occurred at all, should have occurred with the frequency
requisite to establish and increase the variation, is hardly
supposable. And if we reject, as also unlikely, the repro-
duction of these callosities de novo in each individual (for

i

THE OtJTER TISSUES OF ANIMALS. 313

this would imply that after a thousand generations each
young gorilla began with knuckles having skin no thicker
than elsewhere), there remains only the inference that they
have arisen by the transmission and accumulation of func-
tional adaptations. Another case which seems
interpretable only in an analogous way, is that of the spurs
that are developed on the wings of certain birds — on those
of the Chaja screamer for example. These are weapons of
offence and defence. It is a familiar fact that some birds
strike with their wings, often giving severe blows ; and in the
birds named, the blows are made more formidable by the
horny, dagger-shaped growths standing out from those points
on the wings which deliver them. Are these spurs directly
or indirectly adaptive? To conclude that natural selection
of spontaneous variations has caused them, is to conclude
that, without any local stimulus, thickenings of the skin
occurred symmetrically on the two wings at the places
required; that such thickenings, so localized, happened to
arise in birds given to using their wings in fight; and that
on their first appearance the thickenings were decided
enough to give appreciable advantages to the individuals dis-
tinguished by them — advantages in bearing the reactions of
the blows if not in inflicting the blows. But to conclude this
is, I think, to conclude against probability. Contrariwise,
if we assume that the thickening of the epidermis produced
by habitual rough usage is inheritable, the development of
these structures presents no difficulty. The points of impact
would become indurated in wings used for striking with
unusual frequency. The callosities of surface thus generated,
rendering the parts less sensitive, would enable the bird in
which they arose to give, without injury to itself, more
violent blows and a greater number of them: so, in some
cases, helping it to conquer and multiply. Among its descend-
ants, inheriting the modification and the accompanying habit,
the thickening would be further increased in the same way:
survival of the fittest tending ever to accelerate the process.

314 PHYSIOLOGICAL DEVELOPMENT.

Presently the horny nodes so formed, hitherto defensive only
in their effects, would, by their prominence, become offensive
— would make the blows given more hurtful. And now
natural selection, aiding more actively, would mould the
nodes into spurs: the individuals in which the nodes were
most pointed would be apt to survive and propagate ; and the
pointedness generation after generation thus increased, would
end in the well-adapted shape we see.

But if in these cases the differentiations which fit par-
ticular parts of the outer tissues to bear rough usage are
caused mainly by the direct balancing of external actions by
internal reactions, then we may suspect that the like is true
of other modifications that occur where special strains and
abrasions have to be met. Possibly it is true of sundry parts
that are formed of hardened epidermis, such as the nails,
claws, hoofs, and hollow horns of Mammals ; " all of which,"
says Prof. Huxley, " are constructed on essentially the same
plan, being diverticula of the whole integument, the outer
layer of whose ecderon has undergone horny metamorphosis."
Leaving open, however, the question what tegumentary
structures are due to direct equilibration, furthered and con-
trolled by indirect equilibration, it is tolerably clear that
direct equilibration has been one of the factors.

§ 294. Dermal structures of another class are developed
mainly, if not wholly, by the actions of external causes
on species rather than on individuals. These are the
various kinds of clothing — hairs, feathers, quills, scales,
scutes. Though it is no longer thought as at one time that
all these various tegumentary structures are homologous with
one another, yet it is unquestionable that sundry of the more
conspicuous ones are. Those which are extremely unlike
may be seen linked together by a long series of graduated
forms. A retrograde metamorphosis from feathers to ap-
pendages that are almost scale-like, is well seen in the coat
of the Penguin. There is manifest a transition from the

THE OUTER TISSUES OP ANIMALS. 315

bird-like covering to the fish-like covering — a transition so
gradual that no place can be found where an appreciable
break occurs; and if the scale-like appendages are not
truly scales yet they exemplify an extreme metamorphosis.
Less striking, perhaps, but scarcely less significant, are
the modifications through which we pass from feathers to
hairs, on the surfaces of the Ostrich and the Cassowary.
The skin of the Porcupine shows us hairs and quills united
by a series of intermediate structures, differing from one
another inappreciably. Even more remarkable are certain
other alliances of dermal structures. " It may be taken as
certain, I think," says Prof. Huxley, " that the scales, plates,
and spines of all fishes are homologous organs ; nor as less so
that the tegumentary spines of the Plagiostomes are homo-
logous with their teeth, and thence with the teeth of all
vertebrata."

Further details concerning these tegumentary structures
are not needful for present purposes, and are indeed but in-
directly relevant to the subject of physiological development.
Here they are of interest to us only by involving the general
question — What physical influences have brought them into
existence? Still with a view to definite presentation of the
problem, it will be well to contemplate the mode of de-
velopment common to the most familiar of them.

Suppose a small pit to be formed on the previously flat
skin; and suppose that the growth and casting off of horny
cells which goes on over the skin in general, continues to
go on at the usual rate over the depressed surface of
this pit. Clearly the quantity of horny matter produced
within this hollow, will be greater than that produced on a
level portion of the skin subtending an equal area of the
animal's outside. Suppose such a pit to be deepened
until it becomes a small sac. If the exfoliation goes on as
before, the result will be that the horny matter, expelled, as
it must be, through the mouth of the sac, which now bears
a small proportion to the internal surface of the sac, will be

316 PHYSIOLOGICAL DEVELOPMENT.

large in quantity compared with that exfoliated from a
portion of the skin equal in area to the mouth of the sac:
there will be a conspicuous thrusting forth of horny matter.
Suppose once more that the sac, instead of remaining simple,
has its bottom pushed up into its interior, like the bottom of
a wine-bottle — the introversion being carried so far that the
introverted part reaches nearly to the external opening, and
leaves scarcely any space between the introverted part and
the walls of the sac. It is easy to see that the exfoliation
continuing from the surface of the introverted part, as well as
from the inside of the sac generally, the horny matter cast
off will form a double layer; and will come out of the sac
in the shape of a tube having within its lower end the intro-
verted part, as the core on which it is moulded, and from the
apex of which is cast off the substance filling, less densely,
its interior. The structure resulting will be what we know
as a hair. Manifestly by progressive enlargement of the sac,
and further complication of that introverted part on which
the excreted substance is moulded, the protruding growth may
be rendered larger and more involved, as we see it in quills
and feathers. So that insensible steps, thus indicated in
principle, carry us from the exfoliation of epidermis by a flat
surface, to the exfoliation of it by a hollow simple sac, an
introverted sac, and a sac further complicated ; each of which
produces its modified kind of tegumentary appendage.

But now, after contemplating this typical illustration,
we return to the general question. What are the agencies
which have been operative in developing these skin-
structures ? Indirect equilibration must have worked almost
alone in producing them. No direct incidence of forces can
have developed the enamelled armour of the Lepidosteus or
the tesselated plates of the Glyptodon and its modern allies.
Survival of the fittest must here and in multitudinous other
cases be regarded as the sole cause.

§ 295. Among many other differentiations of the outer

THE OUTER TISSUES OP ANIMALS. 31?

tissues, the most worthy to be noticed in the space that re-
mains, are those by which organs of sense are formed. We
will begin with the simplest and most closely allied to the
foregoing.

Every hair that is not too long or flexible to convey to its
rooted end a strain put upon its free end, is a rudimentary
tactual organ; as may be readily proved by touching one of
those growing on the back of the hand. If, then, a creature
has certain hairs so placed that they are habitually touched
by the objects with which it deals, or amid which it moves,
an advantage is likely to accrue if these hairs are modified
in a way that enables them the better to transmit the im-
pressions derived. Such modified hairs we have in the
vibrissa, or, as they are commonly called, the " whiskers "
possessed by Cats and feline animals generally, as well as by
Seals and many Eodents. These hairs are long enough to
reach objects at considerable distances; they are so stiff that
forces applied to their free ends, cause movements of their
imbedded ends ; and the sacs containing their imbedded ends
being well covered with nerve-fibres, these developed hairs
serve as instruments of exploration. By constant use of them
the animal learns to judge of the relative positions of objects
past which, or towards which, it is moving. When stealthily
approaching prey or stealthily escaping enemies, such aids to
perception are obviously important: indeed their importance
has been proved by the diminished power of self-guidance in
the dark, that results from cutting them off. These, then, are
dermal appendages originally serving the purpose of cloth-
ing, but afterwards differentiated into sense-organs.

That eyes are essentially dermal structures seems scarcely
conceivable. Yet an examination of their rudimentary types,
and of their genesis in creatures that have them well deve-
loped, shows us that they really arise by successive modifica-
tions of the double layer composing the integument. They
make their first appearance among the simpler animals as
specks of pigment, covered by portions of epidermis slightly

318 PHYSIOLOGICAL DEVELOPMENT.

convex and a little more transparent than that around it.
Here their fundamental community of structure with the
skin is easy to trace ; and the formation of them by differen-
tiation of it presents no difficulty. Not so far
in advance of these as much to obscure the relationship, are
the eyes which the Crustaceans possess. In every fish-
monger's shop we may see that the eyes of a Lobster are
carried on pedicles ; and when the Lobster casts its shell, the
outer coat of each eye, being continuous with the epidermis
of its pedicle, is thrown off along with the rest of the exo-
skeleton. Beneath the transparent epidermic layer, there
exists a group of eyes of the kind which we see in an
insect ; and these, according to a high authority, are inclosed
in the dermal system. Describing the arrangement of the
parts, M. Milne Edwards writes : — " But the most remarkable
circumstance is, that the large cavity within which the whoh
of these parallel columns, every one of which is itself a per-
fect eye, are contained, is closed posteriorly by a membrane,
which appears to be neither more nor less than the middle
tegumentary membrane, pierced for the passage of the optic
nerve; so that the ocular chamber at large results from
the separation at a point of the two external layers of the
general envelope." Thus too is it, in the main,
even with the highly-developed eyes of the Vertebrata.
" The three pairs of sensory organs appertaining to the
higher senses," says Prof. Huxley — " the nasal sacs, the eyes,
and the ears — arise as simple ccecal involutions of the ex-
ternal integument of the head of the embryo. That such
is the case, so far as the olfactory sacs are concerned, is
obvious, and it is not difficult to observe that the lens and
the anterior chamber of the eye are produced in a perfectly
similar manner. It is not so easy to see that the labyrinth
of the ear arises in this way, as the sac resulting from the
involution of the integument is small, and remains open but
a very short time. But I have so frequently verified
Huschke's and Remak's statement that it does so arise, that

THE OUTER TISSUES OF ANIMALS. 319

I entertain no doubt whatever of the fact. The outer ends
of the olfactory sacs remain open, but those of the ocular
auditory sacs rapidly close up, and shut off their contents
from all direct communication with the exterior." That is
to say, the eye considered as an optical apparatus is pro-
duced by metamorphoses of the skin: the only parts of it
not thus produced, being the membranes lying between the
sclerotic and the vitreous humour, including those retinal
structures formed in them. All is tegumentary save that
which has to appreciate the impressions which the modified
integument concentrates upon it.

Thus, as Prof. Huxley has somewhere pointed out, there
is a substantial parallelism between all the sensory organs in
their modes of development; as there is, too, between their
modes of action. A vibrissa may be taken as their common
type. Increased impressibility by an external stimulus,
requires an increased peripheral expansion of the nervous
system on which the stimulus may fall; and this is secured
by an introversion of the integument, forming a sac on the
walls of which a nerve may ramify. That the more extended
sensory area thus constituted may be acted upon, there
requires some apparatus conveying to it from without the
appropriate stimulus; and in the case of the vibrissa, this
apparatus is the epidermic growth which, under the form of
a hair, protrudes from the sac. And that the greatest
sensitiveness may be obtained, the external action must be
exaggerated or multiplied by the apparatus which conveys it
to the recipient nerve ; as, in the case of the vibrissa, it is by
the development of a hair into an elastic lever, that trans-
forms the slight force acting through considerable space on
its exposed end, into a greater force acting through a smaller
space at its rooted end. Similarly with the organs of the
higher senses. In a rudimentary eye, the slightly modified
sense cell has but a rudimentary nerve to take cognizance of
the impression; and to concentrate the impression upon it,
there is nothing beyond a thickening of the epidermis into a

320 PHYSIOLOGICAL DEVELOPMENT.

lens-shape. But the developed eye shows us a terminatk
of the nerve greatly expanded and divided to receive the
external stimulus. It shows us an introverted portion of the
integument containing the apparatus by which the external
stimulus is conveyed to the recipient nerve. The structure
developed in this sac not only conveys the stimulus, but also,
like its homologue, concentrates it; and in the one case as
in the other, the structure which does this is an epidermic
growth from the bottom of the sac. Even with the ear it is
the same. Again we have an introverted portion of the inte-
gument, on the walls of which the nerve is distributed in
the primitive ear. The otolithes contained in the sac thus
formed, are bodies which are set in motion by the vibrations
of the surrounding water, and convey these vibrations in
an exaggerated form to the nerves. And though it is not
alleged that these otolithes are developed from the epidermic
lining of the chamber, yet as, if not so developed, they are
concretions from the contents of an epidermic sac, they must
still be regarded as epidermic products.

Whether these differentiations are due wholly to indirect
equilibration, or whether direct equilibration has had a share
in working them, are questions that must be left open.
Possibly a short hair so placed on a mammal's face as to be
very often touched, may, by conveying excitations to the
nerves and vessels at its root, cause extra growth of the
bulb and its appendages, and so the development of a vibrissa
may be furthered. Possibly, too, the light itself, to which
the tissues of some inferior animals are everywhere sensitive,
may aid in setting up certain of the modifications by which
the nervous parts of visual organs are formed : producing, as
it must, the most powerful effects at those points on the sur-
face which the movements of the animal expose to the greatest
and most frequent contrasts of light and shade; and propa-
gating from those points currents of molecular change
through the organism. But it seems clear that the complexities

THE OUTER TISSUES OF ANIMALS. 321

of the sensory organs are not thus explicable. They must
have arisen by the natural selection of favourable variations.

§ 296. A group of facts, serving to elucidate those put
together in the several foregoing sections, has to be added.
I have reserved this group to the last, partly because it is
transitional — links the differentiations of the literally outer
tissues with those of the truly inner tissues. Though physi-
cally internal, the mucous coat of the alimentary canal has
a quasi-extem&lity from a physiological point of view. As
was pointed out in the last chapter, the skin and the assimi-
lating surface have this in common, that they come in direct
contact with matters not belonging to the organism; and
we saw that along with this community of relation to alien
substances, there is a certain community of structure and
development. The like holds with the linings of all internal
cavities and canals that have external openings.

The transition from the literally outer tissues to those
tissues which are intermediate between them and the truly
inner tissues, is visible at all the orifices of the body; where
skin and mucous membrane are continuous, and the one
passes insensibly into the other. This visible continuity is
associated not simply with a great degree of morphological
continuity, but also with a great degree of physiological con-
tinuity. That is to say, these literally outer and quasi-outer
layers are capable of rapidly assuming one another's struc-
tures and functions when subject to one another's conditions.
Mucous surfaces, normally kept covered, become skin-like if
exposed to the air; but resume more or less fully their
normal characters when restored to their normal positions.
These are truths familiar to pathologists. They continually
meet with proofs that permanent eversion of the mucous
membrane, even where it is by prolapse of a part deeply
seated within the body, is followed by an adaptation eventu-
ally almost complete: originally moist, tender to the touch,

and irritated by the air, the surface gradually becomes covered
67

322 PHYSIOLOGICAL DEVELOPMENT.

with a thick, dry cuticle; and is then scarcely more sensitive
than ordinary integument.

Whether this equilibration between new outer forces and
reactive inner forces, which is thus directly produced in in-
dividuals, is similarly produced in races, must remain as a
question not to be answered in a positive way. On the one
hand, we have the fact that among the higher animals there
are cases of quasi-outer tissues which are in one species
habitually ensheathed, while in another species they are not
ensheathed; and that these two tissues, though unquestion-
ably homologous, differ as much as skin and mucous mem-
brane differ. On the other hand, there are certain analogous
changes of surface, as on the abdomen of the Hermit-Crab,
which give warrant to the supposition that survival of the
fittest is the chief agent in establishing such differentiations ;
since the abdomen of a Hermit-Crab, bathed by water within
the shell it occupies, is not exposed to physical conditions
that directly tend to differentiate its surface from the surface
of the thorax. But though in cases like this last, we must
assign the result to the natural selection of variations arising
incidentally; we may, I think, legitimately assign the result
to the immediate action of changed conditions where, as in
cases like the first, we see these producing in the individual,
effects of the kinds observed in the race.

However this may be, the force of the general argument
remains the same. In these exchanges of structure and
function between the outer and quasi-outer tissues, we get
undeniable proof that they are easily differentiable. And
seeing this, we are enabled the more clearly to see how there
have, in course of time, arisen those extreme and multi-
tudinous differentiations of the outer tissues which have been
glanced at.

CHAPTER VIII.

DIFFERENTIATIONS AMONG THE INNER TISSUES OF
ANIMALS.

§ 297. The change from the outside of the lips to their
inside, introduces us to a new series of interesting and in-
structive facts, joining on to those with which the last chapter
closed. They concern the differentiations of those coats of
the alimentary canal which, as we have seen, are physiologi-
cally outer, though physically inner.

These coats are greatly modified at different parts; and
their modifications vary greatly in different animals. In
the lower types, where they compose a simple tube running
from end to end of the body, they are almost uniform in their
histological characters; but on ascending from these types,
we find them presenting an increasing variety of minute
structures between their two ends. The argument will be
adequately enforced if we limit ourselves to the leading
modifications they display in some of the higher animals.

The successive parts of the alimentary canal are so placed
with respect to its contents, that the physical and chemical
changes undergone by its contents while passing from one
end to the other, inevitably tend to transform its originally
homogeneous surface into a heterogeneous surface. Clearly,
the effect produced on the food at any part of the canal by
trituration, by adding a secretion, or by absorbing its nutri-
tive matters, implies the delivery of the food into the next
part of the canal in a state more or less unlike its previous

323

324 PHYSIOLOGICAL DEVELOPMENT.

states — implies that the surface with which it now comes ii
contact is differently affected by it from the preceding sur-
faces— implies, that is, a differentiating action. To use con-
crete language ; — food that is broken down in the mouth aci
on the oesophagus and stomach in a way unlike that which
it would have done had it been swallowed whole; the masti-
cated food, to which certain solvents or ferments are added,
becomes to the intestine a different substance from that which
it must have otherwise been; and the altered food, resolved
by these additions into its proximate principles, cannot have
those proximate principles absorbed in the next part of the
intestine, without the remoter parts being affected as they
would not have been in the absence of absorption. It is true
that in developed alimentary canals, such as the reasoning
here tacitly assumes, these marked successive differentiations
of the food are themselves the results of pre-established
differentiations in the successive parts of the canal. But it is
also true that actions and reactions like those here so definite-
ly marked, must go on indefinitely in an undeveloped alimen-
tary canal. If the food is changed at all in the course of its
transit, which it must be if the creature is to live by it, then it
cannot but act dissimilarly on the successive tracts of the
alimentary canal, and cannot but be dissimilarly reacted on
by them. Inevitably, therefore, the uniformity of the surface
must lapse into greater or less multiformity: the differentia-
tion of each part tending ever to initiate differentiations oJ
other parts.

Not, indeed, that the implied process of direct equilibra-
tion can be regarded as the sole process. Indirect equilibra-
tion aids ; and, doubtless, there are some of the modifications
which only indirect equilibration can accomplish. But we
have here one unquestionable cause — a cause that is known
to work in individuals, changes of the kind alleged. Where,
for instance, cancerous disease of the oesophagus so narrows
the passage into the stomach as to prevent easy descent
of the food, the oesophagus above the obstruction becomes

THE INNER TISSUES OP ANIMALS. 325

enlarged into a kind of pouch; and the inner surface of this
pouch begins to secrete juices that produce in the food a kind
of rude digestion. Again, stricture of the intestine, when it
arises gradually, is followed by hypertrophy of the muscular
coat of the intestine above the constricted part : the ordinary
peristaltic movements being insufficient to force the food
forwards, and the lodged food serving as a constant stimulus
to contraction, the muscular fibres, habitually more exercised,
become more bulky. The deduction from general principles
being thus inductively enforced, we cannot, I think, resist
the conclusion that the direct actions and reactions between
the food and the alimentary canal have been largely instru-
mental in establishing the contrasts among its parts. And
we shall hold this view with the more confidence on observ-
ing how satisfactorily, in pursuance of it, we are enabled to
explain one of the most striking of these differentiations,
which we will take as a type of the class.

The gizzard of a bird is an expanded portion of the alimen-
tary canal, specially fitted to give the food that trituration
which the toothless mouth of a bird cannot give. Besides
having a greatly-developed muscular coat, this grinding-
chamber is lined with a thick, hard cuticle, capable of bear-
ing the friction of the pebbles swallowed to serve as grind-
stones. This differentiation of the mucous coat into a ridged
and tubercled layer of horny matter — a differentiation which,
in the analogous organs of certain Mollusca, is carried to the
extent of producing from this membrane cartilaginous plates,
and even teeth — varies in birds of different kinds, according
to their food. It is moderate in birds that feed on flesh and
fish, and extreme in granivorous birds and others that live
on hard substances. How does this immense modification of
the alimentary canal originate? In the stomach

of a mammal, the macerating and solvent actions are united
with that triturating action which finishes what the teeth
have mainly done; but in the bird, unable to masticate, these
internal functions are specialized, and while the crop is the

326 PHYSIOLOGICAL DEVELOPMENT.

macerating chamber, the gizzard becomes a chamber adapted
to triturate more effectually. This adaptation requires simply
an exaggeration of certain structures and actions which
characterize stomachs in general, and, in a less degree,
alimentary canals throughout their whole lengths. The
massive muscles of the gizzard are simply extreme develop-
ments of the muscular tunic, which is already considerably
developed over the stomach, and incloses also the oesophagus
and the intestine. The indurated lining of the gizzard,
thickened into horny buttons at the places of severest pres-
sure, is nothing more than a greatly strengthened and
modified epithelium. And the grinding action of the gizzard
is but a specialized form of that rhythmical contraction by
which an ordinary stomach kneads the contained food, and
which in the oesophagus effects the act of swallowing, while
in the intestine it becomes the peristaltic motion. Allied as
the gizzard thus clearly is in structure and action to the
stomach and alimentary canal in general; and capable of
being gradually differentiated from a stomach where a grow-
ing habit of swallowing food unmasticated entails more
trituration to be performed before the food passes the pylorus ;
the question is — Does this change of structure arise by direct
adaptation? There is warrant for the belief that it does.
Besides such collateral evidence as that mucous membrane
becomes horny on the toothless gums of old people, when
subject to continual rough usage, and that the muscular coat
of the intestine thickens where unusual activity is demanded
of it, we have the direct evidence of experiment. Hunter
habituated a sea-gull to feed on grain, and found that the
lining of its gizzard became hardened, while the gizzard-
muscles doubled in thickness. A like change in the diet of
a kite was followed by like results. Clearly, if differentiations
so produced in the individuals of a race under changed habits,
are in any degree inheritable, a structure like a gizzard will
originate through the direct actions and reactions between
the food and the alimentary canal.

THE INNER TISSUES OF ANIMALS. 327

Another case — a very interesting one, somewhat allied to
this — is presented by the ruminating animals. Here several
dilatations of the alimentary canal precede the true stomach;
and in them large quantities of unmasticated food are stored,
to be afterwards returned to the mouth and masticated at
leisure. What conditions have made this specialization
advantageous? and by what process has it been established?
To both these questions the facts indicate answers which are
not unsatisfactory. Creatures that obtain their

food very irregularly — now having more than they can con-
sume, and now being for long periods without any — must,
in the first place, be apt, when very hungry, to eat to the
extreme limits of their capacities; and must, in the second
place, profit by peculiarities which enable them to compensate
themselves for long fasts, past and future. A perch which,
when its stomach is full of young frogs, goes on filling its
oesophagus also; or a trout which, rising to the fisherman's
fly, proves when taken off the hook to be full of worms and
insect-larvae up to the very mouth, gains by its ability to take
in such unusual supplies of food when it meets with them —
obviously thrives better than it would do could it never eat
more than a stomachful. That this ability to feed greatly in
excess of immediate requirement, is one that varies in indi-
viduals of the same race, we see in the marked contrast
between our own powers in this respect, and. the powers of
uncivilized men; whose fasting and gorging are to us so
astonishing. Carrying with us these considerations, we shall
not be surprised at finding dilatations of the oesophagus in
vultures and eagles, which get their prey at long intervals
in large masses ; and we may naturally look for them, too, in
birds like pigeons, which, coming in flocks upon occasional
supplies of grain, individually profit by devouring the
greatest quantity in a given time. Now where the trituration
of the food is, as in these cases, carried on in a lower part of
the alimentary canal, nothing further is required than the
storing-chamber; but for a mammal, having its grinding

328

PHYSIOLOGICAL DEVELOPMENT.

apparatus in its mouth, to gain by the habit of hurriedly
swallowing unmasticated food, it must also have the habit oi
regurgitating the food for subsequent mastication. This
correlation of habits with their answering structures, may,
as we shall see, arise in a very simple way. The

starting point of the explanation is a familiar fact — the fact
that indigestion, often resulting from excess of food, is apt
to cause that reversed peristaltic action known as vomiting.
From this we pass to the fact, also within the experience of
most persons, that during slight indigestion the stomach
sometimes quietly regurgitates a small part of its contents as
far as the back of the mouth — giving an unpleasant acquaint-
ance with the taste of the gastric juices. Exceptional facts of
the same class help the argument a step further. " There are
certain individuals who are capable of returning, at will, a
greater or smaller portion of the contents of the digesting
stomach into the cavity of the mouth. ... In some of these
cases, the expulsion of the food has required a violent effort.
In the majority it has been easily evoked or suppressed.
While in others, it has been almost uncontrollable; or its
non-occurrence at the habitual time has been followed by a
painful feeling of fulness, or by the act of vomiting."
Here we have a certain physiological action, occasionally
happening in most persons and in some developed into
a habit more or less pronounced: indigestion being the
habitual antecedent. Suppose, then, that gregarious

animals, living on innutritive food such as grass, are subject
to a like physiological action, and are capable of like varia-
tions in the degree of it. What will naturally happen?
They wander in herds, now over places where food is scarce
and now coming to places where it is abundant. Some mas-
ticate their food completely before swallowing it, while some
masticate it incompletely. If an oasis, presently bared by
their grazing, has not supplied to the whole herd a full meal,
then the individuals which masticate completely will have
had less than those which masticate incompletely — will not

THE INNER TISSUES OF ANIMALS. 329

have had enough. Those which masticate incompletely and
distend their stomachs with food difficult to digest, will be
liable to these regurgitations ; but if they re-masticate what
is thus returned to the mouth (and we know that animals
often eat again what they have vomited), then the extra
quantity of food taken, eventually made digestible, will yield
them more nourishment than is obtained by those which
masticate completely at first. The habit initiated in this
natural way, and aiding survival when food is scarce, will
be apt to cause modifications of the alimentary canal.
We know that dilatations of canals readily arise under
habitual distensions. We know that canals habitually
distended become gradually more tolerant of the contained
masses that at first irritated them. And we know that
there commonly take place adaptive modifications of their
surfaces. Hence if a habit of this kind and the structural
changes resulting from it, are in any degree inheritable, it is
clear that, increasing in successive generations, both imme-
diately by the cumulative effect of repetitions and mediately
by survival of the individuals in which they are most decided,
they may go on until they end in the peculiarities which
Ruminants display.

§ 298. There are structures belonging to the same group
which cannot, however, be accounted for in this way. They
are the organs that secrete special products facilitating diges-
tion— the liver, pancreas, and various smaller glands. All
these appendages of the alimentary canal, large and inde-
pendent as some of them seem, really arise by differentia-
tions from its coats. The primordial liver consists of nothing
more than bile-cells scattered along a tract of the intestinal
surface. Accumulation of these bile-cells is accompanied by
increased growth of the surface which bears them — a growth
which at first takes the form of a cul-de-sac, having an outside
that projects from the intestine into the peri-visceral cavity.
As the mass of bile-cells becomes greater, there arise se-

330 PHYSIOLOGICAL DEVELOPMENT.

condary lateral cavities opening into the primary one, and
through it into the intestine; until, eventually, these cavities
with their coatings of bile-cells, become ramifying ducts dis-
tributed through the solid mass we know as a liver. How is
this differentiation caused ?

Before attempting any answer to this question, it is
requisite to inquire the nature of bile. Is that which the
liver throws into the intestines a waste product of the organic
actions ? or is it a secretion aiding digestion ? or is it a mix-
ture of these? Modern investigations imply that it is mosl
likely the last. The liver is found to have a compound func-
tion. Bernard has proved to the satisfaction of physiologists,
that there goes on in it a formation of glycogen — a substance
which is transformed into sugar before it leaves the liver and
is afterwards carried away by the blood to eventually dis-
appear in the active organs, chiefly the muscles. It is also
shown, experimentally, that there are generated in the liver
certain biliary acids; and by the aid either of these or of
some other compounds, it is clear that bile renders certain
materials more absorbable. Its effect on fat is demonstrable
out of the body; and the greatly diminished absorption of
fat from the food when the discharge of bile into the
intestine is prevented, is probably one of the causes of that
pining away which results. But while recognizing the fact
that the bile consists in part of a solvent, or solvents, aid-
ing digestion, there is abundant evidence that one element
of it is an effete product; and probably this is the prima]
element. The yellow-green substance called biliverdine in
herbivora and bilirubin in man and carnivora, which gives
its colour to bile, is a product the greater part of which is
normally cast out from the system continually, as is shown
by the contrast between the normal and abnormal colours
of fascal matters, and as is still more strikingly shown b]
the effects on the system when there is a stoppage of the
excretion, and an attack of jaundice. Hence we are war-
ranted in classing biliverdine as a waste product, and we

THE INNER TISSUES OF ANIMALS. 331

may fairly infer that the excretion of it is the original
function of the liver.

One further preliminary is requisite. We must for a
moment return to those physico-chemical data set down in
the first chapter of this work (§§ 7 — 8). We there saw that
the complex and large-atomed colloids which mainly compose
living organic matter,, have extremely little molecular mo-
bility; and, consequently, extremely little power of diffusing
themselves. Whereas we saw not only that those absorbed
matters, gaseous and liquid, which further the decomposition
of living organic matter, have very high diffusibilities, but
also that the products of the decomposition are much more
diffusible than the components of living organic matter.
And we saw that, as a consequence of this, the tissues give
ready entrance to the substances which decompose them, and
ready exit to the substances into which they are decomposed.
Hence it follows that, under its initial form, uncomplicated
by nervous and other agencies, the escape of effete matters
from the organism, is a physical action parallel to that which
goes on among mixed colloids and crystalloids that are dead
or even inorganic. Excretion is a specialized form of this
spontaneous action ; and we have to inquire how the special-
ization arises.

Two causes conspire to establish it. The first is that these
products of decomposition are diffusible in widely different
degrees. While the carbonic acid and water permeate the
tissues with ease in all directions, and escape more or less
from the exposed surfaces, urea, and other waste substances
incapable of being vaporized, cannot escape thus readily.
The second is that the different parts of the body, being
subject to different physical conditions, are from the outset
sure severally to favour the exit of these various products of
decomposition in various degrees. How these causes must
have co-operated in localizing the excretions, we shall see on
remembering how they now co-operate in localizing the sepa-
ration of morbid materials. The characteristic substances of

332 PHYSIOLOGICAL DEVELOPMENT.

gout and rheumatism have their habitual places of deposit.
Tuberculous matter, though it may be present in various
organs, gravitates towards some much more than towards
others. Certain products of disease are habitually got rid of
by the skin, instead of collecting internally. Mostly, these
have special parts of the skin which they affect rather than
the rest; and there are those which, by breaking out syi
metrically on the two sides of the body, show how definitely
the places of their excretion are determined by certaii
favouring conditions, which corresponding parts may be pre
sumed to furnish in equal degrees. Further, it is to
observed of these morbid substances circulating in the blood,
that having once commenced segregating at particular places,
they tend to continue segregating at those places. Assum-
ing, then, as we may fairly do, that this localization of
excretion, which we see continually commencing afresh with
morbid matters, has always gone on with the matters pro-
duced by the waste of the tissues, let us take a further step,
and ask how localizations become fixed. Other things equal,
that which from its physical conditions is a place of least
resistance to the exit of an effete product, will tend to become
established as the place of excretion; since the rapid exit
of an effete product will profit the organism. Other things
equal, a place at which the excreted matter produces least
detrimental effect will become the established place. If at
any point the excreted matter produces a beneficial effect,
then, other things equal, survival of the fittest will determine
it to this point. And if facility of escape anywhere goes
along with utilization of the escaping substance, then, other
things equal, the excretion will be there localized still more
decisively by survival of the fittest.

Such being the conditions of the problem, let us ask what
will happen with the lining membrane of the alimentary
canal. This, physiologically considered, is an external sur-
face; and matters thrown off from it make their way out of
the body. It is also a surface along which is moving the food

THE INNER TISSUES OP ANTMALS. 333

to be digested. Now, among the various waste products
continually escaping from the living tissues, some of the
more complex ones, not very stable in composition, are likely,
if added to the food, to set up changes in it. Such changes
may either aid or hinder the preparation of the food for
absorption. If an effete matter, making its exit through the
wall of the intestine, hinders the digestive process, the en-
feeblement and disappearance of individuals in which this
happens, will prevent the intestine from becoming the esta-
blished place for its exit. While if it aids the digestive
process, the intestine will, for converse reasons, become more
and more the place to which its exit is limited. Equally
manifest is it that if there is one part of this alimentary
canal at which, more than at any other part, the favourable
effect results, this will become the place of excretion.

Thus, then, reverting to the case in question, we may
understand how a product to be cast out, such as biliverdine,
if it either directly or indirectly serves a useful purpose,
when poured into a particular part of the intestine, may lead
to the formation of a patch of excreting cells on its wall;
and once this place of excretion having been established, the
development of a liver is simply a question of time and
natural selection.

§ 299. A differentiation of another order occurring in the
alimentary canal, is that by which a part of it is developed
into a lateral chamber or chambers, through which carbonic
acid exhales and oxygen is absorbed. Comparative anatomy
and embryology unite in showing that a lung is formed, just
as a liver or other appendage of the alimentary canal is
formed, by the growth of a hollow bud into the peri-visceral
cavity, or space between the alimentary canal and the wall of
the body. The interior of this bud is simply a cul-de-sac of
the alimentary canal, with the mucous lining of which its
own mucous lining is continuous. And the development of
this cul-de-sac into an air-chamber, simple or compound, is

334 PHYSIOLOGICAL DEVELOPMENT.

merely a great extension of area in the internal surface of
the cul-de-sac, along with that specialization which fits it
for excreting and absorbing substances different from those
which other parts of the mucous surface excrete and
absorb. These lateral air-chambers, universal

among the higher Vertebrata and very general among the
lower, and everywhere attached to the alimentary canal
between the mouth and the stomach, have not in all cases the
respiratory function. In most fishes that have them they
are what we know as swim-bladders. In some fishes the
cavities of these swim-bladders are completely shut off from
the alimentary canal : nevertheless showing, by the communi-
cations which they have with it during the embryonic stages,
that they are originally diverticula from it. In other fishes
there is a permanent ductus pneumaticus, uniting the cavity
of the swim-bladder with that of the gullet: the function,
however, being still not respiratory in an appreciable degree,
if at all. But in certain still extant representatives of the
sauroid fishes, as the Lepidosteus, the air-bladder is " divided
into two sacs that possess a cellular structure," and "the
trachea which proceeds from it opens high-up in the throat,
and is surrounded with a glottis." In the Amphibia the
corresponding organs are chambers over the surfaces of which
there are saccular depressions, indicating a transition towards
the air-cells characterizing lungs; and accompanying this
advance we see, as in the common Triton, the habit of coming
up to the surface and taking down a fresh supply of air in
place of that discharged.

How are the internal air-chambers, respiratory or noi
respiratory, developed? Upwards from the amphibian stage,
in which they are partially refilled at long intervals, there is
no difficulty in understanding how, by infinitesimal steps,
they pass into complex and ever-moving lungs. But
how is the differentiation that produces them initiated?
How comes a portion of the internal surface to be specialized
for converse with a medium to which it is not naturally

THE INNER TISSUES OF ANIMALS. 335

exposed? The problem appears a difficult one; but there is
a not unsatisfactory solution of it.

When many gold-fish are kept in a small aquarium, as
with thoughtless cruelty they frequently are, they swim
close to the surface, so as to breathe that water which is from
instant to instant absorbing fresh oxygen. In doing this
they often put their mouths partly above the surface, so that
in closing them they take in bubbles of air; and sometimes
they may be seen to continue doing this — the relief due to
the slight extra aeration of blood so secured, being the
stimulus to continue. Air thus taken in may be detained.
If a fish that has taken in a bubble turns its head down-
wards, the bubble will ascend to the back of its mouth, and
there lodge; and coming within reach of the contractions of
the oesophagus, it may be swallowed. If, then, among fish
thus naturally led upon occasion to take in air-bubbles, there
are any having slight differences in the alimentary canal that
facilitate lodgment of the air, or slight nervous differences
such as in human beings cause an accidental action to be-
come " a trick," it must happen that if an advantage accrues
from the habitual detention of air-bubbles, those individuals
most apt to detain them will, other things equal, be more
likely than the rest to survive; and by the survival of
descendants inheriting their peculiarities in the greatest de-
grees, and increasing them, an established structure and an
established habit may arise. And that they do in some
way arise we have proof. The common Loach swallows air,
which it afterwards discharges loaded with carbonic acid.

From air thus swallowed the advantages that may be
derived are of two kinds. In the first place, the fish is made
specifically lighter, and the muscular effort needed to keep it
from sinking is diminished — or, indeed, if the bubble is of
the right size, is altogether saved. The contrast between the
movements of a Goby, which, after swimming up towards the
surface, falls rapidly to the bottom on ceasing its exertions,
and the movements of a Trout, which remains suspended just

336 PHYSIOLOGICAL DEVELOPMENT.

balancing itself by slight undulations of its fins, shows hoi
great an economy results from an internal float, to fishes
which seek their food in mid-water or at the surface. Hence
the habit of swallowing air having been initiated in the way
described, we see why natural selection will, in certain fishes,
aid modifications of the alimentary canal favouring its
lodgment — modifications constituting air-sacs. In

the second place, while from air thus lodged in air-sacs thus
developed, the advantage will be that of notation only if the
air is infrequently changed or never changed, the advantage
will be that of supplementary respiration if the air-sacs are
from time to time partially emptied and refilled. The re-
quirements of the animal will determine which of the two
functions predominates. Let us glance at the different sets
of conditions under which these divergent modifications may
be expected to arise.

The respiratory development is not likely to take place in
fishes that inhabit seas or rivers in which the supply of
aerated water never fails : there is no obvious reason why
the established branchial respiration should be replaced by a
pulmonic respiration. Indeed, if a fish's branchial respiration
is adequate to its needs, a loss would result from the effort of
coming to the surface for air; especially during those first
stages of pulmonic development when the extra aeration
achieved was but small. Hence in fishes so circumstanced,
the air-chambers arising in the way described would naturally
become specialized mainly or wholly into floats. Their con-
tained air being infrequently changed, no advantage would
arise from the development of vascular plexuses over theii
surfaces ; nothing would be gained by keeping open the com-
munication between them and the alimentary canal; and
there might thus eventually result closed chambers the
gaseous contents of which, instead of being obtained from
without, were secreted from their walls, as gases often are
from mucous membranes. Contrariwise, aquatic

vertebrates in which the swallowing of air-bubbles, becoming

THE INNER TISSUES OF ANIMALS. 337

habitual, had led to the formation of sacs that lodged the
bubbles; and which continued to inhabit waters not always
supplying them with sufficient oxygen, might be expected to
have the sacs further developed, and the practice of chang-
ing the contained air made regular, if either of two advan-
tages resulted — either the advantage of .being able to live in
old habitats that had become untenable without this modifi-
cation, or the advantage of being able to occupy new habitats.
Now it is just where these advantages are gained that we
see the pulmonic respiration coming in aid of the branchial
respiration, and in various degrees replacing it. Shallow
waters are liable to three changes which conspire to make
this supplementary respiration beneficial. The summer's sun
heats them, and raising the temperatures of the animals they
contain, accelerates the circulation in these animals, exalts
their functional activities, increases the production of car-
bonic acid, and thus makes aeration of the blood more need-
ful than usual. Meanwhile the heated water, instead of
yielding to the highly carbonized blood brought to the
branchiae the usual quantity of oxygen, yields less than
usual ; for as the heat of the water increases, the quantity of
air it contains diminishes. And this greater demand for
oxygen joined with smaller supply, pushed to an extreme
where the water is nearly all evaporated, is at last still more
intensely felt in consequence of the excess of carbonic acid
discharged by the numerous creatures congregated in the
muddy puddles that remain. Here, then, it is, that the habit
of taking in air-bubbles is likely to become established, and
the organs for utilizing them developed; and here it is, ac-
cordingly, that we find all stages of the transition to aerial
respiration. The Loach before-mentioned, which swallows
air, frequents small waters liable to be considerably warmed.
The Amphipnous Cuchia, an anomalous eel-shaped fish, which
has vascular air-sacs opening out at the back of the mouth,
" is generally found lurking in holes and crevices, on the
muddy banks of marshes or slow-moving rivers " ; and
68

338 PHYSIOLOGICAL DEVELOPMENT. •

though its air-sacs are not morphological equivalents of those
above described, yet they equally well illustrate the relation
between such organs and the environing condition. Still
more significant is the fact that the Lepidosiren, or " mud-
fish" as it is called from its habits, though it is a true fish
nevertheless has lungs. But it is among the Amphibia that
we see most conspicuously this relation between the develop-
ment of air-breathing organs, and the peculiarities of the
habitats. Pools, more or less dissipated annually, and so
rendered uninhabitable by most fishes, are very generally
peopled by these transitional types. Just as we see, too,
that in various climates and in various kinds of shallow
waters, the supplementary aerial respiration is needful in dif-
ferent degrees; so do we find among the Amphibia many
stages in the substitution of the one respiration for the other.
The facts, then, are such as give to the hypothesis a vrai-
semblance greater than could have been expected.

The relative effects of direct and indirect equilibration in
establishing this further heterogeneity, must, as in many
other cases, remain undecided. The habit of taking in bub-
bles is scarcely interpretable as a result of spontaneous varia-
tion: we must regard it as arising accidentally during the
effort to obtain the most aerated water; as being persevered
in because of the relief obtained ; and as growing by repetition
into a tendency bequeathed to offspring, and by them, or
some of them, increased and transmitted. The formation of
the first slight modifications of the alimentary canal favour-
ing the lodgment of bubbles, is not to be thus explained. Some
favourable variation in the shape of the passage must here
have been the initial step. But the gradual increase of this
structural modification by the survival of individuals in
which it is carried furthest, will, I think, be all along aided
by immediate adaptation. The part of the alimentary canal
previously kept from the air, but now habitually in contact
with the air, must be in some degree modified by the action
of the air; and the directly-produced modification, increasing

THE INNER TISSUES OF ANIMALS. 339

in the individual and in successive individuals, cannot cease
until there is a complete balance between the actions of the
changed agency and the changed tissue.

§ 300. We c§me now to differentiations among the truly
inner tissues — the tissues which have direct converse neither
with the environment nor with the foreign substances taken
into the organism from the environment. These, speaking
broadly, are the tissues which lie between the double layer
forming the integument with its appendages, and the double
layer forming the alimentary canal with its diverticula. We
will take first the differentiation which produces the vascular
system.

Certain forces producing and aiding distribution of liquids
in animals, come into play before any vascular system exists ;
and continue to further circulation after the development of
a vascular system. The first of these is osmotic exchange,
acting locally and having an indirect general action; the
second is local variation of pressure, which movement of the
body throws on the tissues and their contained liquids. A
few words are needed in elucidation of each. If in

any creature, however simple, different changes are going on
in parts that are differently conditioned — if, as in a Hydra,
one surface is exposed to the surrounding medium while the
other surface is exposed to dissolved food; then between the
unlike liquids which the dissimilarly-placed parts contain,
osmotic currents must arise; and a movement of liquid
through the intermediate tissue must go on as long as an
unlikeness between the liquids is kept up. This primary
cause of re-distribution remains one of the causes of re-dis-
tribution in every more-developed organism: the passage of
matters into and out of the capillaries is everywhere thus
set up. And obviously in producing these local currents,
osmose must also indirectly produce general currents, or aid
them if otherwise produced. In the absence of a pumping
organ, this force is probably an important aid to that

340 PHYSIOLOGICAL DEVELOPMENT.

movement of the nutritive liquids which the functions set
up. How the second cause — the changes of internal

pressure which an animal's movements produce — furthers
circulation, will be sufficiently manifest. That parts which
are bent or strained necessarily have their contained vessels
squeezed, has been shown (§ 281) ; and whether the bend or
strain is caused, as in a plant, by an external force, or, as
usually in an animal, by an internal force, there must be a
thrusting of liquids towards places of least resistance — com-
monly places of greatest consumption. This which in animals
without hearts is a main agent of circulation, continues to
further it very considerably even among the highest animals.
In these the effect becomes as it were systematized. The valve
in the veins necessitate perpetual propulsions towards the
heart.

Even in such simple types as the Hydrozoa, cavities in th*
tissues faintly indicate a structure which facilitates the trans-
fer of nutritive matters. These cavities become reservoirs
filled with the plasma that slowly oozes through the substance
of the body ; and every movement of the animal, accompanied
as it must be by changed pressures and tensions on these
reservoirs, tends here to fill them and there to squeeze out
their contents in that or the other direction — possibly aiding
to produce, by union of several cavities, those lacunas or
irregular canals which the body in some cases presents.

Irregular canals of this kind, not lined with any mem-
branes but being simply cavities running through the flesh,
mainly constitute the vascular system in Polyzoa and Brach-
iopoda and some Mollusca. Though the central parts of a
Vascular system are rudely developed, yet its peripheral parts
consist of sinuses permeating the tissues. The higher orders
of Mollusca have a more developed system of vessels or
arteries, which run into the substance of the body and end
in lacunae or simple fissures. This ending in lacunae takes
place at various distances from the vascular centre. In some
genera the arterial structure is carried to the periphery of
the blood-system, while in others it stops short midway.

THE INNER TISSUES OF ANIMALS. 341

Throughout most orders of the Mollusca the back current of
blood continues to be carried by channels of the original
kind: there are no true veins, but the blood having been
delivered into the tissues, finds its way back to the peri-
visceral cavity through inosculating sinuses. Among the
Cephalopods, however, the afferent blood-canals, as well as
the efferent ones, acquire distinct walls. On putting

together these facts, we may conceive pretty clearly the
stages of vascular development. From the original reservoir
of nutritive liquid between the alimentary canal and the
wall of the body, a portion partially shut off becomes a con-
tractile vessel; and by its actions there is produced a more
rapid transfer of the nutritive liquid than was originally
produced by the motions of the animal. Clearly, the exten-
sion of this contractile tube anc^ the development from it of
branches running hither and thither into the tissues, must,
by defining the channels of blood throughout a part of its
course, render its distribution more regular and active. As
fast as this centrifugal growth advances, so fast are the effer-
ent currents of blood, prevented from escaping laterally,
obliged to move from the centre towards the circumference;
and so fast also does the less-developed set of channels become,
of necessity, occupied by afferent currents. When, by a paral-
lel increase of definiteness, the lacunas and irregular sinuses
through which the afferent currents pass, become trans-
formed into veins, the accompanying disappearance of all
stagnant or slow-moving collections of blood, implies a fur-
ther improvement in the circulation.

By what agency is effected this differentiation of a definite
vascular system? No sufficient reply is obvious. The
genesis of the primordial heart is not comprehensible as a
result of direct equilibration, and we cannot readily see our
way to it as a result of indirect equilibration ; for it is diffi-
cult to imagine what favourable variation natural selection
could have seized hold of to produce such a structure. A
contractile tube that aided the distribution of nutritive

342 PHYSIOLOGICAL DEVELOPMENT.

liquid, having been once established, survival of the fittest
would suffice for its gradual extension and its successive
modifications. But what were the early stages of the con-
tractile tube, while it was yet not sufficiently formed to help
circulation, and while it must nevertheless have had some
advantage without which no selective process could go on?
The question seems insoluble. To another part of

the question, however, an answer may be ventured. If we
ask the origin of these ramifying channels which, first
appearing as simple lacunaa, eventually become vessels
having definite walls, a reply admitting of considerable
justification, is, that the currents of nutritive liquid forced
and drawn hither and thither through the tissues, themselves
initiate these channels. We know that streams running
over and through solid and quasi-solid inorganic matter,
tend to excavate definite courses. We saw reason for con-
cluding that the development of sap-channels in plants
conforms to this general principle. May we not then
suspect that the nutritive liquid contained in the tissue
of a simple animal, made to ooze now in this direction and
now in that by the changes of pressure which the animal's
movements cause, comes to have certain lines along which it
is thrust backwards and forwards more than along other
lines; and must by repeated passings make these more and
more permeable until they become lacunas? Such actions
will inevitably go on; and such actions appear competent to
produce some, at least, of the observed effects. The leading
facts which indicate that this is a part cause of vascular
development are these.

Growths normally recurring in certain places at certain
intervals, are accompanied by local formations of blood-ves-
sels. The periodic maturation of ova among the Mammalia
supplies an instance. Through the stroma of an ovarium are
distributed innumerable minute vesicles, which, in their early
stages, are microscopic. Of these, severally contained in their
minute ovi-sacs, any one may develop: the determining

THE INNER TISSUES OF ANIMALS. 343

cause being probably some slight excess of nutrition. When
the development is becoming rapid, the capillaries of the
neighbouring stroma increase and form a plexus on the walls
of the ovi-sac. Now since there is no typical distribution of
the developing ova; and since the increase of an ovum to
a certain size precedes the increase of vascularity round it;
we can scarcely help concluding that the setting up of cur-
rents towards the point of growth determines the forma-
tion of the blood-vessels. It may be that having once com-
menced, this local vascular structure completes itself in a
typical manner; but it seems clear that this greater develop-
ment of blood-vessels around the growing ovum is initiated
by the draught towards it. Abnormal growths show

still better this relation of cause and effect. The false mem-
branes sometimes found in the bronchial tubes in inflam-
matory diseases, may perhaps fairly be held abnormal in but
a partial sense: it may be said that their vascular systems
are formed after the type of the membranes to which they are
akin. But this can scarcely be said of the morbid growths
classed as malignant. The blood-vessels in an encephaloid
cancer, are led to enlarge and ramify, often to an immense
extent, by the unfolding of the morbid mass to which they
carry blood. Alien as is the structure as a whole to the type
of the organism; and alien in great measure as is its tissue
to the tissue on which it is seated; it nevertheless happens
that the growth of the alien tissue and accompanying ab-
straction of materials from the blood-vessels, determine a
corresponding growth of these blood-vessels. Unless, then,
we say that there is a providentially-created type of vascular
structure for each kind of morbid growth (and even this
would not much help us, since the vascular structure has
no constancy within the limits of each kind), we are com-
pelled to admit that in some way or other the currents of
blood are here directly instrumental in forming their own
channels. One more piece of evidence, before cited

as exemplifying adaptation (§ 67), may be called to mind.

344 PHYSIOLOGICAL DEVELOPMENT.

When any main channel for blood, leading to or from a
certain part of the body, has been rendered impervious,
others among the channels leading to or from this same part,
enlarge to the extent requisite for fulfilling the extra func-
tion that falls upon them : the enlargement being caused, as
we must infer, by the increase of the currents carried.

Here, then, are facts warranting inductively the deduction
above drawn. It is true that we are left in the dark respecting
the complexities of the process. How the channels for blood
come to have limiting membranes, and many of them mus-
cular coats, the hypothesis does not help us to say. But the
evidence assigned goes far to warrant the belief that vascular
development is initiated by direct equilibration; though in-
direct equilibration may have had the larger share in establish-
ing the structures which distinguish finished vascular systems.

§ 301. Of the inner tissues which remain let us next take
bone. In what manner is differentiated this dense substance
serving in most cases for internal support?

When considering the vertebrate skeleton under its
morphological aspect (§ 256), it was pointed out that the
formation of dense tissues, internal as well as external, is, in
some cases at least, brought about by the mechanical forces
to be resisted. Through what process it is brought about we
could not then stay to inquire: this question being not
morphological but physiological. Answers to some kindred
questions have since been attempted. Certain actions to
which the internal dense tissues of plants may be ascribed,
have been indicated; and more recently, analogous actions
have been assigned as causes of some external dense tissues
of animals. We have now to ask whether actions of the
same nature have produced these internal dense tissues o\
animals.

The problem is an involved one. Bones have more than
one stage. They are membranous or cartilaginous before they
become osseous; and their successive component substances

THE INNER TISSUES OF ANIMALS. 345

so far differ that the effects of mechanical actions upon them
differ. And having to deal with transitional states in which
bone is formed of mixed tissues, having unlike physical
properties and unlike minute structures, the effects of
strains become too complicated to follow with precision.
Anything in the way of interpretation must therefore be
regarded as tentative. If analysis and comparison show that
the phenomena are not inconsistent with the hypothesis of
mechanical genesis, it is as much as can be expected. Let us
first observe more nearly the mechanical conditions to which
bones are subject.

The endo-skeleton of a mammal with the muscles and
ligaments holding it together, may be rudely compared to a
structure built up of struts and ties; of which, speaking
generally, the struts bear the pressures and the ties bear the
tensions. The framework of an ordinary iron roof will give
an idea of the functions of these two elements, and of the
mechanical characters required by them. Such a framework
consists partly of pieces which have each to bear a thrust in
the direction of its length, and partly of pieces which have
each to bear a pull in the direction of its length; and these
struts and ties are differently formed to adapt them to these
different strains. Further, it should be remarked that though
the rigidity of the framework depends on the ties which are
flexible, as much as on the struts which are stiff, yet the ties
help to give the rigidity simply by so holding the struts in
position that they cannot escape from the thrusts which fall
on them. Now the like relation holds with a difference
among the bones and muscles : the difference being that here
the ties admit of being lengthened or shortened and the
struts of being moved about upon their joints. The mecha-
nical relations are not altered by this, however. The actions
are of essentially the same kind in an animal that is stand-
ing, or keeping itself in a strained attitude, as in one that
is changing its attitude — the same in so far that we have
in each a set of flexible parts that are pulling and a set of

346 PHYSIOLOGICAL DEVELOPMENT.

rigid parts that are resisting. It needs but to remember the
sudden collapse and fall which take place when the muscles
are paralyzed, or to remember the inability of a bare skeleton
to support itself, to see that the struts without the ties can-
not suffice. And we have but to think of the formless mass
into which a man would sink when deprived of his bones, to
see that the ties without the struts cannot suffice. To trace
the way in which a particular bone has its particular thrust
thrown upon it, may not always be practicable. Though it
is easy to perceive how a flexor or extensor of the arm causes
by its tension a reactive pressure along the line of the
humerus, and is enabled to produce its effect only by the
rigidity of the humerus ; yet it is not so easy to perceive how
such bones as those of a horse's pelvis are similarly acted
upon. Still, as the weight of the hind quarters has to be
transferred from the back to the feet, and must be so trans-
ferred through the bones, it is manifest that though these
bones form a very crooked line, the weight must produce a
pressure along the axis of each: the muscles and ligaments
concerned serving here, as in other cases, so to hold the
bones that they bear the pressure instead of being displaced
by it. Not forgetting that many processes of the bones have
to bear tensions, we may then say that generally, though by
no means universally, bones are internal dense masses that
have to bear pressures — pressures which in the cylindrical
bones become longitudinal thrusts. Leaving out exceptional
cases, let us consider bones as masses thus circumstanced.

When giving reasons for the belief that the vertebrate
skeleton is mechanically originated, one of the facts put ii
evidence was, that in the vertebrate series the transition from
the cartilaginous to the osseous spine begins peripherally
(§ 257) : each vertebra being at first a ring of bone sur-
rounding a mass of cartilage. And it was pointed out that
this peripheral ossification is ossification at the region of
greatest pressures. Now it is not vertebrae only that follow
this course of development. In a cylindrical bone, though

THE INNER TISSUES OF ANIMALS. 347

it is differently circumstanced, the places of commencing ossi-
fication are still the places on which the severest stress falls.
Let us consider how such a bone that has to bear a longitu-
dinal pressure is mechanically affected. If the end
of a walking-cane be thrust with force against the ground, the
cane bends ; and partially resuming its straightness when re-
lieved, again bends, usually towards the same side, when the
thrust is renewed. A bend so caused acts on the fibres of the
cane in nearly the same way as does a bend caused by sup-
porting the cane horizontally at its two ends and suspending
a weight from its middle. In either case the fibres on the
convex side are extended and the fibres on the concave side
compressed. Kindred actions occur in a rod that is so thick
as not to yield visibly under the force applied. In the absence
of complete homogeneity of its substance, complete symmetry
in its form, and an application of a force exactly along its
axis, there must be some lateral deflection; and therefore
some distribution of tensions and pressures of the kind indi-
cated. And then, as the fact which here specially concerns us,
we have to note that the strongest tensions and pressures are
borne by the outer layers of fibres. Now the shaft of a long
bone, subject to mechanical actions of this kind, similarly has
its outer layer most strained. In this layer, therefore, on the
mechanical hypothesis, ossification should commence, and here
it does commence — commences, too, midway between the ends,
where the bends produce on the superficial parts their most
intense effects. But we have not in this place simply
to observe that ossification commences at the places of greatest
stress, but to ask what causes it to do this. Can we trace the
physical actions which set up this deposit of dense tissue ? It
is, I think, possible to indicate a "true cause" that is at work;
though whether it is a sufficient cause may be questioned.
We concluded that in certain other cases, the formation of
dense tissue indirectly results from the alternate squeezing
and relaxation of the vessels running through the part ; and
the inquiry now to be made is, whether, in developing bone,

348 PHYSIOLOGICAL DEVELOPMENT.

the same actions go on in such ways as to produce the ob-
served effects. At the outset we are met by what seems a
fatal difficulty — cartilage is. a non-vascular tissue : this sub-
stance of which unossified bones consist is not permeated
by minute canals carrying nutritive liquid, and cannot,
therefore, be a seat of actions such as those assigned.
This apparent difficulty, however, furnishes a confirmation.
For cartilage that is wholly without permeating canals does
not ossify: ossification takes place only at those parts of
it into which the canals penetrate. Hence, we get ad-
ditional reason for suspecting that bone-formation is due
to the alleged cause; since it occurs where mechanical
strains can produce the actions described, but does not occur
where mechanical strains cannot produce them. Let us
consider more closely what the several factors are. It will
suffice for the argument if we commence with the external
vascular layer as already existing, and consider what
will take place in it. Cartilage is elastic — is some-

what extensible, and spreads out laterally under pressure,
but resumes its form when relieved. How, then, will the
minute channels traversing it in all directions be affected at
the places where it is strained by a bend? Those on the
convex side will be laterally squeezed, in the same way that
we saw the sap-vessels on the convex side of a bent branch
are squeezed; and as exudation of the sap into the adjacent
prosenchyma will be caused in the one case, so, in the other,
there will be caused exudation of serum into the adjacent
cartilage: extra nutrition and increase of strength resulting
in both cases. The parallel ceases here, however. In 'the
shoot of a plant, bent in various directions by the wind, the
side which was lately compressed is now extended; and
hence that squeezing of the sap-vessels which results from
extension, suffices to feed and harden the tissue on all sides
of the shoot. But it is not so with a bone. Having yielded
on one side under longitudinal pressure, and resumed as
nearly as may be its previous shape when the pressure is

THE INNER TISSUES OF ANIMALS. 349

taken off, the bone yields again towards the same side when
again longitudinally pressed. Hence the substance of its
concave side, never rendered convex by a bend in the oppo-
site direction, would not receive any extra nutrition did no
other action come into play. But if we consider how inter-
mittent pressures must act on cartilage, we shall see that
there will result extra nutrition of the concave side also.
Squeeze between two pieces of glass a thin bit of caoutchouc
which has a hole through it. While the caoutchouc spreads
out away from the centre, it also spreads inwards, so as
partially to close the hole. Everywhere its molecules move
away in directions of least resistance; and for those near
the hole, the direction of least resistance is towards the hole.
Let this hole stand for the transverse section of one of the
minute canals or channels passing through cartilage, and it
will be manifest that on the side of the unossified bone made
concave in the way described, the compressed cartilage will
squeeze the canals traversing it; and, in the absence of
perfect homogeneity in the cartilage, the squeeze will cause
extra exudation from the canals into the cartilage. Thus
every additional strain will give to the cartilage it falls upon,
an additional supply of the materials for growth. So that
presently the side which, by yielding more than any other,
proves itself to be the weakest, will cease to be the weakest.
What further will happen? Some other side will yield a
little — the bends will take place in some other plane; and
the portions of cartilage on which repeated tensions and
pressures now fall will be strengthened. Thus the rate of
nutrition, greatest at the place where the bending is greatest,
and changing as the incidence of forces changes, will bring
about at every point a balance between the resistances and
the strains. Thus, too, there will be determined that peri-
pheral induration which we see in bones so circumstanced.
As in a shoot we saw that the woody deposit takes place
towards the outside of the cylinder, where, according to the
hypothesis, it ought to take place; so, here, we see that the

350 PHYSIOLOGICAL DEVELOPMENT.

excess of exudation and hardening, occurring where the
strains are most intense, will form a cylinder having a dense
outside and a porous or hollow inside. These pro-

cesses will be essentially the same in bones subject to more
complex mechanical actions, such as sundry of the flat bones
and others that serve as internal fulcra. Be the strains
transverse or longitudinal, be they torsion strains or mixed
strains, the outer parts of the bone will be more affected by
them than its inner parts. They will therefore tend every-
where to produce resisting masses having outer parts more
dense than their inner parts. And by causing most growth
where they are most intense, they will call out reactive forces
adequate to balance them. There are doubtless

obstacles in the way of this interpretation. It may be said
that the forces acting on the outer layers in the manner
described, would compress the canals too little to produce
the alleged effects ; and if evenly distributed along the whole
lengths of the layers, they would probably do so. But it'
needs only to bend a flexible mass and observe the tendency,
to form creases on the concave surface, to feel assured that
along the surface of an ossifying bone, the yielding of the
tissue when bent will not be uniform. In the absence of
complete homogeneity, the interstitial yielding will take
place at some points more than others, and at one point
above all others. When, at the weakest point — the centre
of commencing ossification — an extra amount of deposit has
been caused, it will cease to be the weakest; and adjacent
points, now the weakest, will become the places of yielding
and induration. It may be further objected that the hypo-
thesis is incompatible with the persistence of cartilage for so
long a time between the epiphysis of bones and the bony
masses which they terminate. But there is the reply that
the places occupied by this cartilage being places at which
the bone lengthens, the non-ossification is in part apparent
only — it is rather that new cartilage is formed as fast as the
pre-existing cartilage ossifies; and there is the further reply

THE INNER TISSUES OF ANIMALS. 351

that the slowness of the ultimate ossification of this part, is
due to its non-vascularity, and to mechanical conditions
which are unfavourable to its acquirement of vascularity.
Once more, there is the demurrer that in the epiphyses ossifi-
cation does not begin at the surface but within the mass of
the cartilage. Explanation of this implies ability to follow
out the mechanical actions in a resilient substance which, like
india-rubber, admits of being distorted in all ways^y pressure
and recovering its form, and it seems impossible to say how
the more superficial and more deep-seated canals traversing
it will be respectively affected.

Of course it is not meant that this osseous development
by direct equilibration takes place in the individual. Though
it is a corollary from the argument that in each individual
the process must be furthered and modified by the particular
actions to which the particular bones are exposed; yet the
leading traits of structure assumed by the bones are assumed
in conformity with the inherited type. This, however, is no
difficulty. The type itself is to be regarded as the accumu-
lated result of such modifications, transmitted and increased
from generation to generation. The actions above described
as taking place in the bone of an individual, must be under-
stood as producing their total effect little by little in the
corresponding bones of a long series of individuals. Even if
but a small modification can be so wrought in the individual,
yet if such modification, or a part of it, is inheritable, we
may readily understand how, in the course of geologic epochs,
the observed structures may arise in the assigned way.

Here may fitly be added a strong confirmation. If we find
cases where individual bones, subject in exceptional degrees
to the actions described, present in exceptional amounts the
modifications attributed to them, we are greatly helped in
understanding how there may be produced in the race that
aggregate of modifications which the hypothesis implies.
Such cases occur in ricketty children. I am indebted to Mr.
Busk for pointing out these abnormal formations of dense
tissue, that are not apparently explicable as results of

352 PHYSIOLOGICAL DEVELOPMENT.

mechanical actions and re-actions. It was only on tracing
out the processes here at work, that there suggested itself the
specific interpretation of the normal process, as above set
forth. When, from constitutional defect, bones do

not ossify with due rapidity, and are meanwhile subject to
the ordinary strains, they become distorted. Remembering
how a mass which has been made to yield in any direction
by a force it cannot withstand, is some little time before it
recovers completely its previous form, and usually, indeed,
undergoes what is called a " permanent set ; " it is inferable
that when a bone is repeatedly bent at the same time that
the liquid contained in its canals is poor in the materials
for forming dense tissue, there will not take place a propor-
tionate strengthening of the parts most strained; and these
parts will give way. This happens in rickets. But this
having happened, there goes on what, in teleological language,
we call a remedial process. Supposing the bone to be one
commonly affected — a femur; and supposing a permanent
bend to have been caused in it by the weight of the body;
the subsequent result is an unusual deposition of cartilagin-
ous and osseous matter on the concave side of the bone. If
the bone is represented by a strung bow, then the deposit
occurs at the part represented by the space between the bow
and the string. And thus occurring where its resistance is
most effective, it increases until the approximately-straight
piece of bone formed within the arc, has become strong enough
to bear the pressure without appreciably yielding. Now

this direct adaptation, seeming so like a special provision,
and furnishing so remarkable an instance' of what, in medical
but unscientific language, is called the vis medicatrix naturae,
is simply a result of the above-described mechanical actions
and re-actions, going on under the exceptional conditions.
Each time such a bent bone is subject to a force which again
bends it, the severest compression falls on the substance of
its concave side. Each time, then, the canals running
through this part of its substance are violently squeezed —

THE INNER TISSUES OF ANIMALS. 353

far more squeezed than they or any other of the canals
would have been, had the bone remained straight. Hence,
on every repetition of the strain, these canals near the con-
cave surface have their contents forced out in more than
normal abundance. The materials for the formation of tissue
are supplied in quantity greater than can be assimilated by
the tissue already formed; and from the excess of exuded
plasma, new tissue arises.* A layer of organizable material
accumulates between the concave surface and the peri-
osteum; in this, according to 'the ordinary course of tissue-
growth, new vessels appear; and the added layer presently
assumes the histological character of the layer from which
it has grown. What next happens? This added layer,
further from the neutral axis than that which has thrown
it out, is now the most severely compressed, and its vessels
are the most severely squeezed. The place of greatest exuda-
tion and most rapid deposit of matter, is therefore transferred
to this new layer ; and at the same time that active nutrition
increases its density, the excess of organizable material
forms another layer external to it: the successive layers so
added, encroaching on the space between the concave surface
of the bone and the chord of its arc. What limits

the encroachment on this space? — what stops the process of
filling it up? The answer to this question will be manifest
when observing that there comes into play a cause which
gradually diminishes the forces falling on each new layer.
For the transverse sectional area is step by step increased;
and an increase of the area over which the weight borne is
distributed, implies a relatively smaller pressure upon each
part of it. Further, as the transverse dimensions of the bone
increase, the materials composing its convex and concave
layers, becoming further from the neutral axis, become better

* To this implied inference it is objected that " excess of nutritive mate-
rial does not necessarily lead to correspondingly increased growth." My reply
is that a concomitant factor is activity of the tissue, and that in its absence
growth is not to be expected.
69

354 PHYSIOLOGICAL DEVELOPMENT.

placed for resisting the strains to be borne. So that both by
the increased quantity of dense matter and by its mechanically
more-advantageous position, the bendings of the bone are
progressively decreased. But as they are decreased, each
new layer formed on the concave surface has its substance
and its vessels less compressed; and the resulting growth
and induration are rendered less rapid. Evidently, then, the
additions, slowly diminishing, will eventually cease; and this
will happen when the bone no longer bends. That is to say,
the thickening of the bone will reach its limit when there is
equilibrium between the incident forces and the forces which
resist them. Here, indeed, we may trace with great clearness
the process of direct equilibration — may see how an unusual
force, falling on the moving equilibrium of an organism and
not overthrowing it, goes on working modifications until the
re-action balances the action.

That, however, which now chiefly concerns us, is to note
how this marked adaptation supports the general argument.
Unquestionably bone is in this case formed under the influ-
ence of mechanical stress, and formed just where it most
effectually meets the stress. This result, not otherwise
explained, is explained by the hypothesis above set forth.
And when we see that this special deposit of bone is ac-
counted for by actions like those to which bone-formation in
general is ascribed, the probability that these are the actions
at work becomes very great.*

* In recent years (since 1890) Prof. Wilhelm Roux, in essays on func-
tional adaptation, has set forth some views akin to the foregoing in respect
to the general belief they imply, though differing in respect of the physio-
logical processes he indicates. The following relevant passage has been
translated for me from an article of his in the Real-Encyclopadie der ge-
sammlen Heilkunde: — "A more complete theory of functional adaptation
by the author is founded on the assumption that the ' functional ' stimulus,
or ' the act of exercising the function ' (in muscles and glands), and espe-
cially, in the case of bones, the concussion and tension caused by stress and
strain, exert a ' trophic ' stimulus on the cells, in consequence of which, and
along with an increased absorption of nutriment, they grow and eventually
increase (or the osteoblasts at the point of greater stimulus form more bone) ;
while, conversely, with continued inactivity, by absence of these stimuli the

THE INNER TISSUES OP ANIMALS. 355

Of course it is not alleged that osseous structures arise in
this way alone. The bones of the skull and various dermal
bones cannot be thus interpreted. Here the natural selec-
tion of favourable variations appears the only assignable
cause — the equilibration is indirect. We know that ossific
deposits now and then occur in tissues where they are not
usually found; and such deposits, originally abnormal, if
they occurred in places where advantages arose from them,
might readily be established and increased by survival of the
fittest. Especially might we expect this to happen when a
constitutional tendency to form bone had been established by
actions of the kind described; for it is a familiar fact that
differentiated types of tissue, having once become elements
of an organism, are apt occasionally to arise in unusual
places, and there to repeat all their peculiar histological
characters. And this may possibly be the reason why the
bones of the skull, though not exposed to forces such as
those which produce, in other bones, dense outer layers in-
cluding less dense interiors, nevertheless repeat this general
trait of bony structure. While, however, it is beyond doubt
that some bones are not due to the direct influence of
mechanical stress, we may, I think, conclude that mechanical
stress initiates bone-formation.

§ 302. What is the origin of nerve ? In what way do its
properties stand related to the properties of that protoplasm
whence the tissues in general arise? and in what way is it
differentiated from protoplasm simultaneously with the other
tissues? These are profoundly interesting questions; but
questions to which positive answer's cannot be expected.
All that can be done is to indicate answers which seem
feasible.

That the property specially displayed by nerve, is a pro-
nourishment of the cell declines so that the waste is insufficiently replaced (or
otherwise that the bone-substance gradually loses its power of resistance to
the osteoblasts formed as a result of inactivity ").

356 PHYSIOLOGICAL DEVELOPMENT.

perty which protoplasm possesses in a lower degree, is mani-
fest. The sarcode of a Rhizopod and the substance of an
unimpregnated ovum, exhibit movements that imply a pro-
pagation of stimulus from one part of the mass to another.
We have not far to seek for a probable origin of this pheno-
menon. There is good reason for ascribing it to the extreme
instability of the organic colloids of which protoplasm con-
sists. These, in common with colloids in general, assume dif-
ferent isomeric forms with great facility ; and they display not
simply isomerism but polymerism. Further, this readiness
to undergo molecular re-arrangement, habitually shows itself
in colloids by the rapid propagation of the re-arrangement
from part to part. As Prof. Graham has shown, matter in
this state often " pectizes " almost instantaneously — a touch
will transform an entire mass. That is to say, the change of
molecular state once set up at one end, spreads to the other
end — there is a progress of a stimulus to change; and this is
what we see in a nerve. So much being understood, let us
re-state the case more completely.

Molecular change, implying as it does motion of molecules,
communicates motion to adjacent molecules; be they of the
same kind or of a different kind. If the adjacent molecules,
either of the same kind or of a different kind, be stable in
composition, a temporary increase of oscillation in them as
wholes, or in their parts, may be the only result ; but if they
are unstable there are apt to arise changes of arrangement
among them, or among their parts, of more or less permanent
kinds. Especially is this so with the complex molecules
which form colloidal matter, and with the organic colloids
above all. Hence it is1 to be inferred that a molecular dis-
turbance in any part of a living animal, set up by either an
external or internal agency, will almost certainly disturb and
change some of the surrounding colloids not originally im-
plicated— will diffuse a wave of change towards other parts
of the organism: a wave which will, in the absence of per-
fect homogeneity, travel further in some directions than in

THE INNER TISSUES OP ANIMALS. 35?

others. Let us ask next what will determine the

differences of distance travelled in different directions. Ob-
viously any molecular agitation spreading from a centre, will
go furthest along routes that offer least resistance. What
routes will these be ? Those along which there lie most mole-
cules that are easily changed by the diffused molecular motion,
and which yet do not take up much molecular motion in as-
suming their new states. Molecules which are tolerably stable
will not readily propagate the agitation ; for they will absorb
it in the increase of their own oscillations, instead of passing
it on. Molecules which are unstable but which, in assuming
isomeric forms, absorb motion, will not readily propagate it;
since it will disappear in working the changes in them. But
unstable molecules which, in being isomerically transformed,
do not absorb motion, and still more those which, in being
so transformed, give out motion, will readily propagate any
molecular agitation ; since they will pass on the impulse either
undiminished, or increased, to adjacent molecules. If

then we assume, as we are not only warranted in doing but
are obliged to do, that protoplasm contains two or more
colloids, either mingled or feebly combined (since it cannot
consist of simple albumen or fibrin or casein, or any allied
proximate principle) ; it may be concluded that any mole-
cular agitation set up by what we call a stimulus, will diffuse
itself further along some lines than along others, if the com-
ponents of the protoplasm are not quite homogeneously dis-
persed, and if some of them are isomerically transformed
more easily, or with less expenditure of motion, than
others; and it will especially travel along spaces occupied
chiefly by those molecules which give out molecular mo-
tion during their metamorphoses, if there should be any
such. But now let us ask what structural effects

will be wrought along a tract traversed by this wave of
molecular disturbance. As is shown by those transforma-
tions which so rapidly propagate themselves through colloids,
molecules that have undergone a certain change of form,

358 PHYSIOLOGICAL DEVELOPMENT.

are apt to communicate a like change of form to ad-
jacent molecules of the same kind — the impact of each
overthrow is passed on and produces another overthrow.
Probably the proneness towards isochronism of molecular
movements necessitates this. If any molecule has had
its components re-arranged, and their oscillations conse-
quently altered, there result movements not concordant with
the movements in adjacent untransformed molecules, but
which, impressing themselves on the parts of such untrans-
formed molecules, tend to generate in them concordant move-
ments— tend, that is, to produce the re-arrangements involved
by these concordant movements. Is this action limited to
strictly isomeric substances? or may it extend to substances
that are closely allied? If along with the molecules of a
compound colloid there are mingled those of some kindred
colloid; or if with the molecules of this compound colloid
there are mingled the components out of which other such
molecules may be formed; then there arises the question —
does the same influence which tends to propagate the iso-
meric transformations, tend also to form new molecules of
the same kind out of the adjacent components? There is
reason to suspect that it does. Already when treating of the
nutrition of parts (§64), it was pointed out that we are
obliged to recognize a power possessed by each tissue to build
up, out of the materials brought to it, molecules of the same
type as those of which it is formed. This building up of like
molecules seems explicable as caused by the tendency of the
new components which the blood supplies, to acquire move-
ments isochronous with those of the like components in the
tissue; which they can do only by uniting into like com-
pound molecules. Necessarily they must gravitate towards
a state of equilibrium; such state of equilibrium — moving
equilibrium of course — must be one in which they oscillate
in the same times with neighbouring molecules; and so
to oscillate they must fall into groups identical with the
groups around them. If this be a general principle of

THE INNER TISSUES OF ANIMALS. 359

tissue-growth and repair, we may conclude that it will apply
in the case before us. A wave of molecular disturbance
passing along a tract of mingled colloids closely allied in com-
position, and isomerically transforming the molecules of one
of them, will be apt at the same time to form some new mole-
cules of the same type, at any place where there exist the
proximate components, either uncombined or feebly combined
in some not very different way. And this will be most likely
to occur where the molecules of the colloid that are under-
going the isomeric change, predominate, but have scattered
through them the other molecules out of which they may be
formed, either by composition or modification. That is to
say, a wave of molecular disturbance diffused from a centre,
and travelling furthest along a line where lie most molecules
that can be isomerically transformed with facility, will be
likely at the same time to further differentiate this line, and
make it more characterized than before by the easy-trans-
formability of its molecules. One additional step,

and the interpretation is reached. Analogy shows it to be
not improbable that these organic colloids, isomerically trans-
formed by slight molecular impact or increase of molecular
motion, will some of them resume their previous molecular
structures after the disturbance has passed. We know that
what are stable molecular arrangements under one degree of
molecular agitation, are not stable under another degree ; and
there is evidence that re-arrangements of an inconspicuous
kind are occasionally brought about by very slight changes
of molecular agitation. Water supplies a clear case. Prof.
Graham infers that water undergoes a molecular re-arrange-
ment at about 32° — that ice has a colloid form as well as a
crystalloid form, dependent on temperature. Send through
it an extra wave of the molecular agitation we call heat, and
its molecules aggregate in one way. Let the wave die away,
and its molecules resume their previous mode of aggregation.
And obviously such transformations may be repeated back-
wards and forwards within narrow limits of temperature.

360 PHYSIOLOGICAL DEVELOPMENT.

Now among the extremely unstable organic colloids, such a
phenomenon is far more likely to happen. Suppose, then, that
the nerve-colloid is one of which the molecules are changed in
form by a passing wave of extra agitation, but resume their
previous form when the wave has passed: the previous form
being the most stable under the conditions which then recur.
What follows ? It follows that these molecules will be ready
again to undergo isomeric transformation when there again
occurs the stimulus ; will, as before, propagate the transforma-
tion most along the tract where such molecules are most
abundant ; will, as before, tend to form new molecules of their
own type; will, as before, make the line along which they lie
one of easier transfer for the molecular agitation. Every
repetition will help to increase, to integrate, to define more
completely, the course of the escaping molecular motion —
extending its remoter part while it makes its nearer part
more permeable — will help, that is, to form a line of dis-
charge, a line for conducting impressions, a nerve.

Such seems to me a not unfair series of deductions from
the known habitudes of colloids in general and the organic
colloids in particular. And I think that the implied nature
and properties of nerve correspond better with the observed,
phenomena than do the nature and properties implied by
other hypotheses. Of course the speculation as it here stands
is but tentative, and leaves much unexplained. It gives no
obvious reply to the questions — what causes the formation of
nerves in directions adapted to the needs? what determines
their appropriate connexions ? — questions, however, to which,
when we come to deal with physiological integration, we may
find not unsatisfactory answers. Moreover it says nothing
about the genesis of ganglia. A ganglion, it is clear, must
consist of a colloidal matter equally unstable, or still more
unstable, which, when disturbed, falls into some different
molecular arrangement, perhaps chemically simpler, and gives
out in so doing a large amount of molecular motion — serves
as a reservoir of molecular motion which may be suddenly

THE INNER TISSUES OP ANIMALS. £61

discharged along an efferent nerve or nerves, when excite-
ment of an afferent nerve has disengaged it. How such a
structure as this results, the hypothesis does not show. But
admitting these shortcomings it may still be held that we
are, in I lie way pointed out, enabled to form some idea of the
actions by which nervous tissue is differentiated. •

§ 303. A speculation akin to, and continuous with, the last,
is suggested by an inquiry into the origin of muscular tissue.
Contractility as well as irritability is a property of protoplasm
or Barcode : and, as before suggested (§ 22), is not improbably
due to isomeric change in one or more of its component col-
loids. It is a feasible supposition that of the several isomeric
changes simultaneously set up among these component col-
loids, some may be accompanied by change of bulk and some
not. Clearly the isomeric change undergone by the colloid
which we suppose to form nerve, must be one not accompanied
by appreciable change of bulk; since change of bulk implies
" internal work," as physicists term it, and therefore ex-
penditure of force. Conversely, the colloid out of which
muscle originates, may be one that readily passes into an
isomeric state in which it occupies less space: the molecular
disturbance causing this contraction being communicated to
it from adjacent portions of nerve-substance that are mole-
cularly disturbed; or being otherwise communicated to it
by direct mechanical or chemical stimuli : as happens where
nerves do not exist, or where their influence has been cut
off. This interpretation seems, indeed, to be directly at
variance with the fact that muscle does not diminish in bulk
during contraction but merely changes its shape. That which
we see take place with the muscle as a whole, is said also to
take place with each fibre — while it shortens it also broadens.
There is, however, a possible solution of this difficulty. A
contracting colloid yields up its water; and the contracted
colloid plus the free water, may have the same bulk as before
though the colloid has less. If it be replied that in this

362 PHYSIOLOGICAL DEVELOPMENT.

case the water should become visible between the substanc
of the fibre and its sarcolemma or sheath, it may be rejoinec
that this is not necessary — it may be deposited interstitial^
Possibly the striated structure is one that facilitates i1
exudation and subsequent re-absorption; and to this may be
due the superiority of striated muscle in rapidity of contrac-
tion. Granting the speculative character of this
interpretation, let us see how far it agrees with the facts. If
the actions are as here supposed, the contracted or more inte-
grated state of the muscular colloid will be that which it
tends continually to assume — that into which it has an in-
creasing aptitude to pass when artificial paralysis has been
produced, as shown by Dr. Norris — that into which it lapses
completely in rigor mortis. The sensible motion generated
by the contraction can arise only from the transformation
of insensible motion. This insensible motion suddenly
yielded up by a contracting mass, implies the fall of its
component molecules into more stable arrangements. And
there can be no such fall unless the previous arrangement is
unstable. From this point of view, too, it is possible
to see how the hydro-carbons and carbo-hydrates consumed
in muscular action, may produce their effects. For these
non-nitrogenous elements of food, when consumed in the
tissues, give out large amounts of molecular motion. They
do this in presence of the muscular colloids which have lost
molecular motion during their fall in the stable or contracted
state. From the molecular motion they give out, may be
restored the molecular motion lost by the contracted colloids ;
and these contracted colloids may thus have their molecules
raised to that unstable state from which, again falling, they
can again generate mechanical motion.

This conception of the nature and mode of action of mus-
cle, while it is suggested by known properties of colloidal
matter and conforms to the recent conclusions of organic
chemistry and molecular physics, establishes a comprehensible
relation between the vital actions of the lower and the higher

THE INNER TISSUES OP ANIMALS. 363

animals. If we contemplate the movements of cilia, of a
Rhizopod's pseudopodia, of a Polype's body, or of the long
pendant tentacles of a Medusa, we shall see great congruity
between them and this hypothesis. Bearing in mind that the
contractile substance of developed muscle is affected not by
nervous influence only, but, where nervous influence is de-
stroyed, is made to contract by mechanical disturbance and
chemical action, we may infer that it does not differ intrin-
sically from the primordial contractile substance which, in
the lowest animals, changes its bulk under other stimuli than
the nervous. We shall see significance in the fact ascer-
tained by Dr. Ransom, that various agents which excite
and arrest nervo-muscular movements in developed animals,
excite and arrest the protoplasmic movements in ova. We
shall understand how tissues not yet differentiated into mus-
cle and nerve, have this joint irritability and contractility;
how muscle and nerve may arise by the segregation of their
mingled colloids, the one of which, not appreciably altering
its bulk during isomeric change, readily propagates molecular
disturbance, while the other, contracting when isomerically
changed, less readily passes on the molecular disturbance;
and how, by this differentiation and integration of the con-
ducting and the contracting colloids, the one ramifying
through the other, it becomes possible for a whole mass to
contract suddenly, instead of contracting gradually, as it does
when undifferentiated.

The question remaining to be asked is — What causes the
specialization of contractile substance? — What causes the
growth of colloid masses which monopolize this contractility,
and leave kindred colloids to monopolize other properties?
Has natural selection gradually localized and increased the
primordial muscular substance? or has the frequent recur-
rence of irritations and consequent contractions at particular
parts done it? We have, I think, reason to conclude that
direct equilibration rather than indirect equilibration has been
chiefly operative. The reasoning that was used in the case

364 PHYSIOLOGICAL DEVELOPMENT.

of nerve applies equally in the case of muscle. A portion
of undifferentiated tissue containing a predominance of the
colloid that contracts in changing, will, during each change,
tend to form new molecules of its own type from the other
colloids diffused through it : the tendency of these entangled
colloids to fall into unity with those around them, will be
aided by every shock of isomeric transformation. Hence, re-
peated contractions will further the growth of the contracting
mass, and advance its differentiation and integration. If,

too, we remember that the muscular colloid is made to con-
tract by mechanical disturbance, and that among mechanical
disturbances one which will most readily affect it simulta-
neously throughout its mass is caused by stretching, we shall
be considerably helped towards understanding how the con-
tractile tissues are developed. If extension of a muscular
colloid previously at rest, produces in it that molecular dis-
turbance which leads to isomeric change and decrease of
bulk, then there is no difficulty in explaining the movements
of cilia; the formation of a contractile layer in the vascular
sjrstem becomes comprehensible; each dilatation of a blood-
vessel caused by a gush of blood, will be followed by a con-
striction; the heart will pulsate violently in proportion as
it is violently distended; arteries will develop in power as
the stress upon them becomes greater; and we shall simi-
larly have an explanation of the increased muscularity of
the alimentary canal which is brought about by increased
distension of it.

That the production of contractile tissue in certain locali-
ties, is due to the more frequent excitement in those localities
of the contractility possessed by undifferentiated tissue in
general, is a view harmonizing with traits which the diffe-
rentiated contractile tissue exhibits. These are the rela-
tions between muscular exercise, muscular power, and mus-
cular structure; and it is the more needful for us here to
notice them because of certain anomalies they present,
which, at first sight, seem inconsistent with the belief that

THE INNER TISSUES OF ANIMALS. 365

the functionally-determined modifications of muscle are in-
heritable.

Muscles disagree greatly in their tints: all gradations
between white and deep red being observable. Contrasts are
visible between the muscles of different animals, between the
muscles of the same animal at different ages, and between
different muscles of the same animal at the same age.
We will glance at the facts under these heads : noting under
each of them the connexion which here chiefly concerns
us — that between the activity of muscle and its depth
of colour. The cold-blooded Vertebrata are, taken

as a group, distinguished from the warm-blooded by the
whiteness of their flesh; and they are also distinguished by
their comparative inertness. Though a fish or a reptile can
exert considerable force for a short time, it is not capable
of prolonged exertion. Birds and mammals show greater
endurance along with the darker-coloured muscles. If among
birds themselves or mammals themselves we make compari-
sons, we meet with kindred contrasts — especially between
wild and domestic creatures of allied kinds. Barn-door fowls
are lighter-fleshed than most untamed gallinaceous birds;
and among these last the pheasant, moving about but little,
is lighter-fleshed than the partridge and the grouse which are
more nomadic. The muscles of the sheep are not on the
average so dark as those of the deer; and it is said that
the flesh of the wild-boar is darker than that of the pig.
Perhaps, however, the contrast between the hare and the
rabbit affords, among familiar animals, the best example of
the alleged relation: the dark-fleshed hare having no retreat
and making wide excursions, while the white-fleshed rabbit,
passing a great part of its time in its burrow, rarely wanders
far from home. The parallel contrast between young

and old animals has a parallel meaning. Veal is much
whiter than beef, and lamb is of lighter colour than mutton.
Though at first sight these facts may not seem to furnish
confirmatory evidence, since lambs in their play appear to

366 PHYSIOLOGICAL DEVELOPMENT.

expend more muscular force than their sedate dams ; yet the
meaning of the contrast is really as alleged. For in conse-
quence of the law that the strains which animals have to
overcome, increase as the cubes of the dimensions, while
their powers of overcoming them increase only as the squares
(§46), the movements of an adult animal cost much more
in muscular effort than do those of a young animal: the
result being that the sheep and the cow exercise their muscles
more vigorously in their quiet movements, than the lamb
and the calf in their lively movements. It may be added as
significant, that the domestic animal in which no very marked
darkening of the flesh takes place along with increasing
age, namely the pig, is one which, ordinarily kept in a sty,
leads so quiescent a life that the assigned cause of darkening
does not come into action. . But perhaps the most

conclusive evidences are the contrasts which exist between
the active and inactive muscles of the same animal. Between
the leg-muscles of fowls and their pectoral muscles, the differ-
ence of colour is familiar; and we know that fowls exercise
their leg-muscles much more than the muscles which move
their wings. Similarly in the turkey, in the guinea fowl, in
the pheasant. And then, adding much to the force of this
evidence, we see that in partridges and grouse, which belong
to the same order as our domestic fowls but use their wings
as constantly as their legs, little or no difference is visible
between the colour of these two groups of muscles. Special
contrasts like these do not, however, exhaust the proofs; for
there is a still more significant general contrast. The
muscle of the heart, which is the most active of all muscles,
is the darkest of all muscles.

The connexion of phenomena thus shown in so many ways,
implies that the bulk of a muscle is by no means the sole
measure of the quantity of force it can evolve. It would seem
that, other things equal, the depth of colour varies with the
constancy of action ; while, other things equal, the bulk varies
with the amount of force that has to be put forth upon occa-

THE INNER TISSUES OP ANIMALS. 367

sion. These of course are approximate relations. More
correctly we may say that the actions of pale muscles are
either relatively feeble though frequent (as in the massive
flanks of a fish), or relatively infrequent though strong (as in
the pectoral muscles of a common fowl) ; while the actions of
dark muscles are both frequent and strong. Some such dif-
ferentiation may be anticipated by inference from the respec-
tive physiological requirements. A muscle which has upon
occasion to evolve considerable force, but which has thereafter
a long period of rest during which repair may restore it to
efficiency, requires neither a large reserve of the contractile
substance that is in some way deteriorated by action, nor
highly-developed appliances for bringing it nutritive mate-
rials and removing effete products. Where, contrariwise, an
exerted muscle which has undergone much molecular change
in evolving much mechanical force, has soon again to evolve
much mechanical force, and so on continually ; it is clear that
either the quantity of contractile substance present must be
great, or the apparatus for nutrition and depuration must
be very efficient, or both. Hence we may look for marked
unlikenesses of minute structure between muscles which are
markedly contrasted in activity. And we may suspect that
these conspicuous contrasts of colour between active and
inactive muscles, are due to these implied differences of
minute structure: partly differences between the numbers
of blood-vessels and partly differences between the quantities
or qualities of sarcous matter.

Here, then, we have a key to the apparent anomaly above
hinted at — the maintenance of bulk by certain muscles which
have been rendered comparatively inactive by changed habits
of life. That the pectoral muscles of those domestic birds
which fly but little, have not dwindled to any great extent,
has been thought a fact at variance with the conclusion that
functionally-produced adaptations are inheritable. It has
been argued that if parts which are exercised increase, not
only in the individual but in the race, while parts which

368 PHYSIOLOGICAL DEVELOPMENT.

become less active decrease; then a notable difference of size
should exist between the muscles used for flight in birds that
fly much, and those in birds of an allied kind that fly little.
But, as we here see, this is not the true implication. The
change in such cases must be chiefly in vascularity and abun-
dance of contractile substance; and cannot be, to any great
extent, in bulk. For a bird to fly at all, its pectoral muscles,
bones of attachment, and all accompanying appliances, must
be kept up to a certain level of power. If the parts dwindle
much, the creature will be unable to lift itself from the
ground. Bearing in mind that the force which a bird ex-
pends to sustain itself in the air during each successive instant
of a short flight is, other things equal, the same as it ex-
pends in each successive instant of a long flight, we shall see
that the muscles employed in the two cases must have some-
thing like equal intensities of contractile power ; and that the
structural differences between them must have relation mainly
to the lengths of time during which they can continue to re-
peat contractions of like intensity. That is to say, while the
power of flight is retained at all, the muscles and" bones can-
not greatly dwindle ; but the dwindling, in birds whose flights
are short or infrequent or both, will be in the reserve stock
of the substance that is incapacitated by action, or in the
appliances that keep the apparatus in repair, or in both.
Only where, as in the struthious birds, the habit of flight is
lost, can we expect atrophy of all the parts concerned in
flight ; and here we find it.

Are such differentiations among the muscles functionally
produced? or are they produced by the natural selection of
variations distinguished as spontaneous? We have, I think,
good grounds for concluding that they are functionally pro-
duced. We know that in individual men and animals, the
power of sustained action in muscles is rapidly adaptable to
the amount of sustained action required. We know that
being "out of condition/' is usually less shown by the inability
to put out a violent effort than by the inability to continue

THE INNER TISSUES OF ANIMALS. 369

making violent efforts ; and we know that the result of train-
ing for prize-fights and races, is more shown in the prolonga-
tion of energy than in the intensification of energy. At the
same time, experience has taught us that the structural change
which accompanies this functional change, is not so much a
change in the bulk of the muscles as a change in their inter-
nal state : instead of being soft and flabby they become hard.
We have inductive proof, then, that exercise of a muscle causes
some interstitial growth along with the power of more sus-
tained action; and there can be no doubt that the one is a
condition to the other. What is this interstitial growth?
There is reason to suspect that it is in part an increased
deposit of the sarcous substance and in part a development of
blood-vessels. Microscopic observation tends to confirm the
conclusions before drawn, that repetition of contractions fur-
thers the formation of the matter which contracts, and that
greater draughts of blood determine greater vascularity.
And if the contrasts of molecular structure and the contrasts
of vascularity, directly caused in muscles by contrasts in their
activities, are to any degree inheritable; there results an
explanation of those constitutional differences in the colours
and textures of muscles, which accompany constitutional
differences in their degrees of activity.

It may be added that if we are warranted in so ascribing
.the differentiations of muscles from one another to direct
equilibration, then we have the more reason for thinking
that the differentiation of muscles in general from other
structures is also due to direct equilibration. That unlike-
nesses between parts of the contractile tissues having unlike
functions, are caused by the unlikenesses of their functions,
renders it the more probable that the unlikenesses between
contractile tissue and other tissues, have been caused by ana-
logous unlikenesses.

§ 304. These interpretations, which have already occupied

too large a space, must here be closed. Of course out of
70

370 PHYSIOLOGICAL DEVELOPMENT.

phenomena so multitudinous and varied, it has been imprs
ticable to deal with any but the most important; and it has
been practicable to deal with these only in a general way.
Much, however, as remains to be explained, I think the possi-
bility of tracing, in so many cases, the actions to which these
internal differentiations may rationally be ascribed, makes it
likely that the remaining internal differentiations are due to
kindred actions. We find evidence that, in more cases
than seemed probable, these actions produce their effects
directly on the individual; and that the unlikenesses are
produced by accumulation of such effects from generation to
generation. While for all the other unlikenesses, we have,
as an adequate cause, the indirect effects wrought by the sur-
vival, generation after generation, of the individuals in which
favourable variations have occurred — variations such as those
of which human anatomy furnishes endless instances. Thus
accounting for so much, we may not unreasonably presume
that these co-operative processes of direct and indirect equili-
bration will account for what remains.

[Note. — After having dismissed this revised chapter as
done with, and sent it to the printer, further thought con-
cerning those differentiations which produce bone, has re-
minded me of a fact of extreme and varied significance
named in the first volume. I refer to the formation of
adaptive structures round the ends of dislocated bones, and
to the formation of " false joints."

These are ontogenetic changes of which phylogeny yields
no explanation. They do not repeat the traits of ancestral
organisms, and they cannot be ascribed to either of the
recognized evolutionary factors. If a humerus be broken
across and, failing to set, presently comes to have its two
loose ends so modified as in a measure to simulate the parts
of a normal joint — the ends becoming smooth, covered with
periosteum and supplied with fibrous tissue, and attached
by ligaments in such ways as to allow of restrained move-

THE INNER TISSUES OF ANIMALS. 371

ments — it is impossible to think that natural selection has
had anything to do with the power of adjustment thus
shown. No survival of individuals in which adaptations
of this kind, now in one place and now in another, were
better and better effected, could account for acquirement of
the ability. Nor can it be supposed that the ability might
result from a functionally-produced habit; since it is
scarcely conceivable that the number of cases in which indi-
viduals profited by it (at first a little and gradually more)
could be such (even did they survive) as to affect the
constitution of the species. Both of the alleged causes of
structural modifications are out of court. It is manifest,
too, that the foregoing hypothesis respecting bone-formation
yields us not the slightest help.

But on carefully considering the facts, certain phenomena
of profound meaning may strike us. Here, in a part of the
body where no such tissues ordinarily exist and to which no
such structures are ordinarily appropriate, there arise tissues
and structures adapted to the physical circumstances imposed
on that part. Out of what do these abnormal but appropriate
tissues arise? The substances around — osseous, cartilagin-
ous, membranous — consist of differentiated elements too far
specialized to allow of transformation. These new tissues,
then, must originate from the undifferentiated protoplasm
pervading the part. The units of this protoplasm, subject to
the actions proper to an articulation, begin to assume the ap-
propriate histological traits — are determined by local stimuli
to form tissues ordinarily associated with such stimuli. What
is the inevitable implication? These units — physiological or
constitutional, as we may call them — must have possessed
latent potentialities of falling into these special arrangements
under stress of such conditions. At one point there arises
periosteum and at another ligamentous tissue, while for the
shaping of the ends of the bones — here into a rude hinged
form and there into a rude ball-and-socket form, according to
the habitual movements — there goes on some appropriate de-

372 PHYSIOLOGICAL DEVELOPMENT.

posit of bone. Hence we must conclude that in the units
protoplasm which have not yet been organized into special
tissues, there resides the ability to take on one or other type
of histological structure according to circumstances; and,
further, that there resides in each of them the still more
marvellous ability to cooperate with kindred units dispersed
around in developing that arrangement of the parts required
to constitute a " false joint/' So that while these units
have a general proclivity towards the structure of the
organism as a whole, they have also proclivities towards
structures proper to the local conditions into which they
fall. There is latent in each unit the constitution of the
entire organism and by implication the constitution of every
organ; and each unit while cooperating with the aggregate is
ready to take part in that particular arrangement proper to
the position it has fallen into. If the reader will refer back
to §§ 97 d, 97 e, in which it is shown that each member of a
human society possesses a combination of potentialities like
these, he will be the better enabled to believe that this thing
may he so while he is unable to conceive how it is so.

And here, indeed, let it be pointed out how completely
irrelevant is the test of conceivableness as applied to these
ultimate physiological actions. For as here, from the un-
united ends of the broken bone, there presently arises a rude
joint with fit membranes, ligaments, and even synovial fluid,
though we are absolutely unable to imagine the process by
which the adjacent tissues produce this structure; so there
may be from an organ enlarged by function, such reactive
effect upon the system at large as eventually to influence the
reproductive cells, though we may be absolutely unable to
imagine how this can be done.]

CHAPTEK IX.

PHYSIOLOGICAL INTEGRATION IN ANIMALS.

§ 305. Physiological differentiation and physiological
integration, are correlatives that vary together. We have but
to recollect the familiar parallel between the division of labour
in a society and the physiological division of labour, to see
that as fast as the kinds of work performed by the com-
ponent parts of an organism become more numerous, and as
fast as each part becomes more restricted to its own work, so
fast must the parts have their actions combined in such ways
that no one can go on without the rest and the rest cannot
go on without each one.

Here our inquiry must be, how the relationship of these
two processes is established — what causes the integration to
advance pari passu with the differentiation. Though it is
manifest, a priori, that the mutual dependence of functions
must be proportionate to the specialization of functions ; yet
it remains to find the mode in which the increasing co-ordi-
nation is determined.

Already, among the Inductions of Biology, this relation
between differentiation and integration has been specified
and illustrated (§59). Before dealing with it deductively,
a few further examples, grouped so as to exhibit its several
aspects, will be advantageous.

§ 306. If the lowly-organized Planaria has its body broken
up and its gullet detached, this will, for a while, continue

373

374 PHYSIOLOGICAL DEVELOPMENT.

to perform its function when called upon, just as though it
were in its place: a fragment of the creature's own body
placed in the gullet, will be propelled through it, or swal-
lowed by it. But, as the seeming strangeness of this fact
implies, we find no such independent actions of analogous
parts in the higher animals. Again, a piece cut out of the
disc of a Medusa continues with great persistence repeating
those rhythmical contractions which we see in the disc as a
whole; and thus proves to us that the contractile function
in each portion of the disc, is in great measure independent.
But it is not so with the locomotive organs of more differen-
tiated types. When separated from the rest these lose their
powers of movement. The only member of a vertebrate
animal which continues to act after detachment, is the heart ;
and the heart has motor powers complete within itself.

Where there is this small dependence of each part upon
the whole, there is but small dependence of the whole upon
each part. The longer time which it takes for the arrest of
a function to produce death in a less-differentiated animal
than in a more-differentiated animal, may be illustrated by
the case of respiration. Suffocation in a man speedily
causes resistance to the passage of the blood through the
capillaries, followed by congestion and stoppage of the heart :
great disturbance throughout the system results in a few
seconds, and in a minute or two all the functions cease.
But in a frog, with its undeveloped respiratory organ, and a
skin through which a considerable aeration of the blood is
carried on, breathing may be suspended for a long time with-
out injury. Doubtless this difference is proximately due to
the greater functional activity in the one case than in the
other, and the more pressing need for discharging the pro-
duced carbon dioxide; but the greater functional activity being
itself made possible by the higher specialization of functions,
this remains the primary cause of the greater dependence of
the other functions on respiration, where the respiratory
apparatus has become highly specialized. Here

PHYSIOLOGICAL INTEGRATION IN ANIMALS. 375

indeed, we see the relation under another aspect. This more
rapid rhythm of the functions which increased heterogeneity
of structure makes possible, is itself a means of integrating
the functions. Watch, when it is running down, a compli-
cated machine of which the parts are not accurately adjusted,
or are so worn as to be somewhat loose. There will be
observed certain irregularities of movement just before it
comes to rest — certain of the parts which stop first, are
again made to move a little by the continued movement of
the rest, and then become themselves, in turn, the causes of
renewed motion in other parts which have ceased to move.
That is to say, while the connected rhythmical changes of
the machine are quick, their actions and reactions on one
another are regular — all the motions are well integrated; but
as the velocity diminishes irregularities arise — the motions
become somewhat disintegrated. Similarly with organic
functions : increase of their rapidity involves increase of a
joint momentum which controls each and co-ordinates all.
Thus if we compare a snake with a mammal, we see that
its functions are not tied together so closely. The mammal,
and especially the superior mammal, requires food with con-
siderable regularity; keeps up a respiration which varies
within but moderate limits ; and has periods of activity and
rest that alternate evenly and frequently. But the snake,
taking food at long intervals, may have these intervals
greatly extended without fatal results; its dormant and its
active states recur less uniformly; and its rate' of respiration
varies within much wider limits — now being scarcely per-
ceptible and now, as you may prove by exciting it, becoming
conspicuous. So that here, where the rhythms are very slow,
they are individually less regular, and are united into a less
regular compound rhythm — are less integrated.

Perhaps the clearest general idea of the co-ordination of
functions that accompanies their specialization, is obtained
by observing the slowness with which a little-differentiated
animal responds to a stimulus applied to one of its parts,

376 PHYSIOLOGICAL DEVELOPMENT.

and the rapidity with which such a local stimulus is re
sponded to by a more-differentiated animal. A sea-anemone
and a fly will serve for the comparison. A tentacle of a ses
anemone, when touched, slowly contracts; and if the toucl
has been rude, the contraction presently extends to the other
tentacles and eventually to the entire body: the stimulus to
movement is gradually diffused throughout the organism.
But if you touch a fly, or rather if you come near enough
to threaten a touch, the entire apparatus of flight is instantly
brought into combined action. Whence arises this contrast?
The one creature has but faintly specialized contractile
organs, and fibres for conveying impressions. The other has
definite muscles and nerves and a co-ordinating centre. The
parts of the' little-differentiated sea-anemone have their
functions so feebly co-ordinated, that one may be strongly
affected for some time before any effect is felt by another at
a distance from it ; but in the much-differentiated fly, various
remote parts instantly have changes propagated to them from
the affected part, and by their united actions thus set up, the
whole organism adjusts' itself so as to avoid the danger.

These few added illustrations will make the nature of this
general relation sufficiently clear. Let us now pass to the
interpretation of it.

§ 307. If a Hydra is cut in two, the nutritive liquids
diffused through its substance cannot escape rapidly, since
there are no open channels for them; and hence the con-
ditions of the parts at a distance from the cut is but little
affected. But where, as in the more-differentiated animals,
the nutritive liquid is contained in vessels which have con-
tinuous communications, cutting the body in two, or cutting
off any considerable portion of it, is followed by escape of
the liquid from these vessels to a large extent; and this
affects the nutrition and efficiency of organs remote from
the place of injury. Then where, as^ in further-developed
creatures, there exists an apparatus for propelling the blood

PHYSIOLOGICAL INTEGRATION IN ANIMALS. 377

through 11 use ramifying channels, injury of a single one
will cause a loss of blood that quickly prostrates the entire
organism. Hence the rise of a completely-differentiated vas-
cular system, is the rise of a system which integrates all
members of the body, by making each dependent on the in-
tegrity of the vascular system, and therefore on the integrity
of each member through which it ramifies. In

another mode, too, the establishment of a distributing appa-
ratus produces a physiological union that is great in propor-
tion as this distributing apparatus is efficient. As fast as it
assumes a function unlike the rest, each part of an animal
modifies the blood in a way more or less unlike the rest, both
by the materials it abstracts and by the products it adds;
and hence the more differentiated the vascular system be-
comes, the more does it integrate all parts by making each
of them feel the qualitative modification of the blood which
every other has produced. This is simply and conspicuously
exemplified by the lungs. In the absence of a vascular
system, or in the absence of one that is well marked off
from the imbedding tissues, the nutritive plasma or the crude
blood, gets what small aeration it can, only by coming near
the creature's outer surface, or those inner surfaces which are
bathed by water. But where there have been formed definite
channels branching throughout the body, and particularly
where there exist specialized organs for pumping the blood
through these channels, it manifestly becomes possible for
the aeration to be carried on in one part peculiarly modified
to further it, while all other parts have the aerated blood
brought to them. And how greatly the differentiation of the
vascular system thus becomes a means of integrating the
various organs, is shown by the fatal result that follows when
the current of aerated blood is interrupted.

Here, indeed, it becomes obvious both that certain physio-
logical differentiations make possible certain physiological
integrations; and that, conversely, these integrations make
possible other differentiations. Besides the waste products

378 PHYSIOLOGICAL DEVELOPMENT.

which escape through the lungs, there are waste products
which escape through the skin, the kidneys, the liver. The
blood has separated from it in each of these structures, the
particular product which this structure has become adapted
to separate ; leaving the other products to be separated by the
other adapted structures. How have these special adapta-
tions been made possible? By union of the organs as
recipients of one circulating mass of blood. While there is no
efficient apparatus for transfer of materials through the body,
the waste products of each part have to make their escape
locally; and the local channels of escape must be competent
to take off indifferently all the waste products. But it
becomes practicable and advantageous for the differently-
localized excreting structures to become fitted to separate
different waste products, as soon as the common circulation
through them grows so efficient that the product left unex-
creted by one is quickly carried to another better fitted to
excrete it. So that the integration of them through a
common vascular system, is the condition under which only
they can become differentiated. Perhaps the clearest

idea of the way in which differentiation leads to integration,
and how, again, increased integration makes possible still
further differentiation, will be obtained by contemplating the
analogous dependence in the social organism. While it has
no roads, a country cannot have its industries much special-
ized: each locality must produce, as best it can, the various
commodities it consumes, so long as it has no facilities for
barter with other localities. But the localities being unlike in
their natural fitnesses for the various industries, there tends
ever to arise some exchange of the commodities they can
respectively produce with least labour. This exchange leads
to the formation of channels of communication. The cur-
rents of commodities once set up, make their foot-paths and
horse-tracks more permeable; and as fast as the resistance
to exchange becomes less, the currents of commodities
become greater. Each locality takes more of the products

PHYSIOLOGICAL INTEGRATION IN ANIMALS. 379

of adjacent ones, and each locality devotes itself more to the
particular industry for which it is naturally best fitted: the
functional integration makes possible a further functional
differentiation. This further functional differentiation reacts.
The greater demand for the special product of each locality,
excites improvements in production — leads to the use of
methods which both cheapen and perfect the commodity.
Hence results a still more active exchange; a still clearer
opening of the channels of communication; a still closer
mutual dependence. Yet another influence comes into play.
As fast as the intercourse, at first only between neighbouring
localities, makes for itself better roads — as fast as rivers are
bridged and marshes made easily passable, the resistance to
distribution becomes so far diminished, that the things grown
or made in each district can be profitably carried to a greater
distance; and as the economical integration is thus extended
over a wider area, the economical differentiation is again
increased; since each district, having a larger market for its
commodity, is led to devote itself more exclusively to pro-
ducing this commodity. These actions and reactions con-
tinue until the various localities, becoming greatly developed
and highly specialized in their industries, are at the same
time functionally integrated by a network of roads, and
finally railways, along which rapidly circulate the currents
severally sent out and received by the localities. And it will
be manifest that in individual organisms a like correlative
progress must have been caused in an analogous way.

§ 308. Another and higher form of physiological integra-
tion in animals, is that which the nervous system effects.
Each part as it becomes specialized, begins to act upon the
rest not only indirectly through the matters it takes from
and adds to the blood, but also directly through the molecular
disturbances it sets up and diffuses. Whether nerves them-
selves are differentiated by the molecular disturbances thus
propagated in certain directions, or whether they are other-

380 PHYSIOLOGICAL DEVELOPMENT.

wise differentiated, it must equally happen that as fast
they become channels along which molecular disturbances
travel, the parts they connect become physiologically inte-
grated, in so far that a change in one initiates a change in
the other. We may dimly perceive that if portions of what
was originally a uniform mass having a common function,
undertake subdivisions of the function, the molecular
changes going on in them will be in some way complemen-
tary to one another : that peculiar form of molecular motion
which the one has lost in becoming specialized, the other has
gained in becoming specialized. And if the molecular motion
that was common to the two portions while they were undiffer-
entiated, becomes divided into two complementary kinds of
molecular motion; then between these portions there will be
a contrast of molecular motions such that whatever is plus
in the one will be minus in the other ; and hence there will be
a special tendency towards a restoration of the molecular equi-
librium between the two: the molecular motion continually
propagated away from either will have its line of least resist-
ance in the direction of the other. If, as argued
in the last chapter, repeated restorations of molecular equili-
brium, always following the line of least resistance, tend ever
to make it a line of diminished resistance; then, in propor-
tion as any parts become more physiologically integrated by
the establishment of this channel for the easy transmission
of molecular motion between them, they may become more
physiologically differentiated. The contrast between their
molecular motions leads to the line of discharge; the line of
discharge, once formed, permits a greater contrast of their
molecular motions to arise; thereupon the quantities of
molecular motion transferred to restore equilibrium, being
increased, the channel of transfer is made more permeable;
and its further permeability, so caused, renders possible a still
more marked unlikeness of action between the parts. Thus
the differentiation and the integration progress hand in hand
as before. How the same principle holds through-

PHYSIOLOGICAL INTEGRATION IN ANIMALS. 381

out the higher stages of nervous development, can be seen
only still more vaguely. Nevertheless, it is comprehensible
that as functions become further divided, there will arise the
need for sub-connexions along which there may take place
secondary equilibrations subordinate to the main ones. It is
manifest, too, that whereas the differentiation of functions
proceeds, not necessarily by division into two, but often by
division into several, and usually in such ways as not to leave
any two functions that are just complementary to one an-
other, the restorations of equilibrium cannot be so simple as
above supposed. And especially when we bear in mind that
many differentiated functions, as those of the senses, cannot
be held complementary to any other functions in particular;
it becomes manifest that the equilibrations that have to be
made in an organism of much heterogeneity, are extremely
complex, and do not take place between each organ and some
other, but between each organ and all the others. The pecu-
liarity of the molecular motion propagated from each organ,
has to be neutralized by some counter-peculiarity in the
average of the molecular motions with which it is brought
into relation. All the variously-modified molecular motions
from the various parts, must have their pluses and minuses
mutually cancelled: if not locally, then at some centre to
which each unbalanced motion travels until it meets with
some opposite unbalanced motion to destroy it. Still, involved
as these actions must become, it is possible to see how the
general principle illustrated by the simple case above sup-
posed, will continue to hold. For always the molecular
motion proceeding from any one differentiated part, will
travel most readily towards that place where a molecular
motion most complementary to it in kind exists — no matter
whether this complementary molecular motion be that pro-
ceeding from any one other organ, or the resultant of the
molecular motions proceeding from many other organs. So
that the tendency will be for each channel of communication
or nerve, to unite itself with some centre or ganglion, where it

382 PHYSIOLOGICAL DEVELOPMENT.

comes into relation with other nerves. And if there be any
parts of its peculiar molecular motion uncancelled by the
molecular motions it meets at this centre ; or if, as will prob-
ably happen, the average molecular motion which it there
unites to produce, differs from the average molecular motion
elsewhere; then, as before, there will arise a discharge along
another channel or nerve to another centre or ganglion,
where the residuary difference may be cancelled by the
differences it meets; or whence it may be still further
propagated till it is so cancelled. Thus there will be a ten-
dency to a general nervous integration keeping pace with
the differentiation.

Of course this must be taken as nothing more than the
indication of initial tendencies — not as an hypothesis suffi-
cient to account for all the facts. It leaves out of sight the
origin and functions of ganglia, considered as something more
than nerve-junctions. Were there only these lines of easy
transmission of molecular disturbance, a change set up in
one organ could never do more than produce its equivalent
of change in some other or others; and there could be none
of that large amount of motion initiated by a small sensation,
which we habitually see. The facts show, unmistakably, that
the slight disturbance communicated to a ganglion, causes an
overthrow of that highly-unstable nervous matter contained
in it, and a discharge from it of the greatly-increased quantity
of molecular motion so generated. This, however, is beyond
our immediate topic. All we have here to note is the inter-
dependence and unification of functions that naturally follow
the differentiation of them.

§ 309. Something might be added concerning the further
class of integrations by which organisms are constituted
mechanically-coherent wholes. Carrying further certain of
the arguments contained in the last chapter, it might be
not unreasonably inferred that the binding together of parts
by bones, muscles, and ligaments, is a secondary result of

PHYSIOLOGICAL INTEGRATION IN ANIMALS. 383

those same actions by which bones, muscles, and ligaments
are specialized. But adequate treatment of this division of
the subject is at present scarcely possible.

What little of fact and inference has been above set down,
will, however, serve to make comprehensible the general truths
respecting which, in their main outlines, there can be no
question. Beginning with the feebly-differentiated sponge,
of which the integration is also so feeble that cutting off a
piece interferes in no appreciable degree with the activity
and growth of the rest, it is undeniable that the advance is
through stages in which the multiplication of unlike parts
having unlike actions, is accompanied by an increasing inter-
dependence of the parts and their actions; until we come to
structures like our own, in which a slight change initiated in
one part will instantly and powerfully affect all other parts —
will convulse an immense number of muscles, send a wave of
contraction through all the blood-vessels, awaken a crowd of
ideas with an accompanying gush of emotions, affect the
action of the lungs, of the stomach, and of all the secreting
organs. And while it is a manifest necessity that along with
this subdivision of functions which the higher organisms show
us, there must be this close co-ordination of them, the fore-
going paragraphs suggest how this necessary correlation is
brought about. For a great part of the physiological union
that accompanies the physiological specialization, there ap-
pears to be a sufficient cause in the process of direct equili-
bration; and indirect equilibration may be fairly presumed a
sufficient cause for that which remains.

CHAPTER X.

SUMMARY OF PHYSIOLOGICAL DEVELOPMENT.

§ 310. Intercourse between each part and the particular
conditions to which it is exposed, either habitually in the
individual or occasionally in the race, thus appears to be the
origin of physiological development ; as we found it to be the
origin of morphological development. The unlikenesses of
form that arise among members of an aggregate that were
originally alike, we traced to unlikenesses in the incident
forces. And in the foregoing chapters we have traced to
unlikenesses in the incident forces, those unlikenesses of
minute structure and chemical composition that simulta-
neously arise among the parts.

In summing up the special truths illustrative of this
general truth, it will be proper here to contemplate more
especially their dependence on first principles. Dealing with
biological phenomena as phenomena of evolution, we have to
interpret not only the increasing morphological heterogeneity
of organisms, but also their increasing physiological hetero-
geneity, in terms of the re-distribution of matter and motion.
While we make our rapid re-survey of the facts, let us then
more particularly observe how they are subordinate to the
universal course of this re-distribution.

§ 311. The instability of the homogeneous, or, strictly
speaking, the inevitable lapse of the more homogeneous into
the less homogeneous, which we before saw endlessly exem-
384

SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 385

plified by the morphological differentiations of the parts of
organisms, we have here seen afresh exemplified in ways also
countless, by the physiological differentiations of their parts.
And in the one case as in the other, this change from uni-
formity to multiformity in organic aggregates, is caused, as
it is in all inorganic aggregates, by the necessary exposure
of their component parts to actions unlike in kind or quan-
tity or both. General proof of this is furnished by the order
in which the differences appear. If parts are rendered
physiologically heterogeneous by the heterogeneity of the in-
cident forces, then the earliest contrasts should be between
parts that are the most strongly contrasted in their relations
to incident forces; the next earliest contrasts should occur
where there are the next strongest contrasts in these relations ;
and so on. It turns out that they do so.

Everywhere the differentiation of outside from inside
comes first. In the simplest plants the unlikeness of the
cell-wall to the cell-contents is the conspicuous trait of
structure. The contrasts seen in the simplest animals are
of the same kind: the film that covers a Ehizopod and the
more indurated coat of an Infusorian, are more unlike the
contained sarcode than the other parts of this are from one
another; and the tendency during the life of the animal is
for the unlikeness to become greater. What is true

of Protophyta and Protozoa, is true of the germs of all organ-
isms up to the highest : the differentiation of outer from inner
is the first step. When the protoplasm of an Alga-cell has
broken up into the clusters of granules which are eventually
to become spores, each of these quickly acquires a mem-
branous coating; constituting an unlikeness between surface
and centre. Similarly with the ovule of every higher plant :
the mass of cells forming it, early exhibits an outside layer of
cells distinguished from the cells within. With animal-germs
it is the same. Be it in a ciliated gemmule, be it in the
unfertilized ova of Aphides and of the Cecidomyia, or be it in

true ova, the primary differentiation conforms to the rela-
71

386 PHYSIOLOGICAL DEVELOPMENT.

tions of exterior and interior. If we turn to adult

organisms, vegetal or animal, we see that whether they do or
do not display other contrasts of parts, they always display
this contrast. Though otherwise almost homogeneous, such
Fungi as the puff-ball, or, among Algce, all which have a
thallus of any thickness, present marked differences between
those of their cells which are in immediate contact with the
environment and those which are not. Such differences they
present in common with every higher plant; which, here in
the shape of bark and there in the shape of cuticle, has an
envelope inclosing it even up to its petals and stamens. In
like manner among animals, there is always either a true
skin or an outer coat analogous to one. Wherever aggregates
of the first order have united into aggregates of the second
and third orders — wherever they have become the morpho-
logical units of such higher aggregates — the outermost of
them have grown unlike those lying within. ' Even the
Sponge is not without a layer that may by analogy be called
dermal.

This lapse of the relatively homogeneous into the rela-
tively heterogeneous, first showing itself, as on the hypothesis
of evolution it must do, by the rise of an unlikeness between
outside and inside, goes on next to show itself, as we infer
that it must do, by the establishment of secondary contrasts
among the outer parts answering to secondary contrasts
among the forces falling on them. So long as the whole sur-
face of a plant remains similarly related to the environment,
as in a Protococcus, it remains uniform; but when there come
to be an attached surface and a free surface, these, being sub-
ject to unlike actions, are rendered unlike. This is visible
even in a unicellular Alga when it becomes fixed; it is
shown in the distinction between the under and upper parts
of ordinary Fungi; and we see it in the universal difference
between the imbedded ends and the exposed ends of the
higher plants. And then among the less marked contrasts
of surface answering to the less marked contrasts in the inci-

SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 387

dent forces, come those between the upper and under sides of
leaves; which, as we have seen, vary in degree as the con-
trasts of forces vary in degree, and disappear where these con-
trasts disappear. Equally clear proof is furnished
by animals, that the original uniformity of surface lapses into
multiformity, in proportion as the actions of the environment
upon the surface become multiform. In a Worm, burrowing
through damp soil which acts equally on all its sides, or in a
Taenia, uniformly bathed by the contents of the intestine it
inhabits, the parts of the integument do not appreciably
differ from one another; but in creatures not surrounded by
the same agencies, as those that crawl and those that have
their bodies partially inclosed, there are unlikenesses of in-
tegument corresponding to unlikenesses of the conditions. A
snail's foot has an under surface not uniform with the
exposed surface of its body, and this again is not uniform
with the protected surface. Among articulate animals there
is usually a distinction between the ventral and the dorsal
aspects; and in those of the Arthropoda which subject their
anterior and posterior ends to different environing agencies,
as do the ant-lion and the hermit-crab, these become super-
ficially differentiated. Analogous general contrasts
occur among the Vertebrata. Fishes, though their outsides are
uniformly bathed by water, have their backs more exposed
to light than their bellies, and the two are commonly distinct
in colour. When it is not the back and belly which are thus
dissimilarly conditioned, but the sides, as in the Pleuronectidce,
then it is the sides which become contrasted; and there may
be significance in the fact that those abnormal individuals of
this order which revert to the ancestral undistorted type, and
swim vertical^, have the two sides alike. In such higher
vertebrates as reptiles, we see repeated this differentiation
of the upper and under surfaces : especially in those of them
which, like snakes, expose these surfaces to the most diverse
actions. Even in birds and mammals which usually, by
raising the under surface considerably above the ground,

388 PHYSIOLOGICAL DEVELOPMENT.

greatly diminish the contrast between its conditions and tl
conditions to which the "upper surface is subject, there still
remains some unlikeness of clothing answering to the remain-
ing unlikeness between the conditions. Thus, with-
out by any means saying that all such differentiations are
directly caused by differences in the actions of incident forces,
which, as before shown (§ 294), they cannot be, it is clear
that many of them are so caused. It is clear that parts of
the surface exposed to very unlike environing agencies, become
very unlike; and this is all that needs to be shown.

Complex as are the transformations of the inner parts
of organisms from the relatively homogeneous into the rela-
tively heterogeneous, we still see among them a conformity
to the same general order. In both plants and animals the
earlier internal differentiations answer to the stronger con-
trasts of conditions. Plants, absorbing all their
nutriment through their outer surfaces, are internally modi-
fied mainly by the transfer of materials and by mechanical
stress. Such of them as do not raise their fronds above the
surface, have their inner tissues subject to no marked con-
trasts save those caused by currents of sap; and the lines
of lengthened and otherwise changed cells which are formed
where these currents run, and are most conspicuous where
these currents must obviously be the strongest, are the only
decided differentiations of the interior. But where, as in
the higher Cryptogams and in Phsenogams, the leaves are
upheld, and the supporting stem is transversely bent by
the wind, the inner tissues, subject to different amounts of
mechanical strain, differentiate accordingly: the deposit of
dense substance commences in that region where the sap-
containing cells and canals suffer the greatest intermittent
compressions. Animals, or at least such of them
as take food into their interiors, are subject to forces of
another class tending to destroy their original homogeneity.
Food is a foreign substance which acts on the interior as an
environing object which touches it acts on the exterior — is

SUMMARY OP PHYSIOLOGICAL DEVELOPMENT. 389

literally a portion of the environment which, when swal-
lowed, becomes a cause of internal differentiations as the rest
of the environment continues a cause of external differentia-
tions. How essentially parallel are the two sets of actions
and reactions, we have seen implied by the primordial identity
of the endoderm and ectoderm in simple animals, and of the
skin and mucous membrane in complex animals (§§288, 289).
Here we have further to observe that as food is the original
source of internal differentiations, these may be expected to
show themselves first where the influence of the food is
greatest; and to appear later in proportion as the parts are
more removed from the influence of the food. They do this.
In animals of low type, the coats of the alimentary cavity or
canal are more differentiated than the tissue which lies be-
tween the alimentary canal and the wall of the body. This tis-
sue in the higher Cwlenterata, is a feebly-organized parenchyma
traversed by canals lined with simple ciliated cells ; and in the
lower Mollusca the structures bounding the perivisceral cavity
and its ramifying sinuses, are similarly imperfect. Further,
it is observable that the differentiation of this perivisceral
sac and its sinuses into a vascular system, proceeds centri-
fugally from the region where the absorbed nutriment enters
the mass of circulating liquid, and where this liquid is quali-
tatively more unlike the tissues than it is at the remoter
parts of the body.

Physiological development, then, is initiated by that in-
stability of the homogeneous which we have seen to be every-
where a cause of evolution (First Principles, §§ 149 — 155).
That the passage from comparative uniformity of composi-
tion and minute structure to comparative multiformity, is set
up in organic aggregates, as in all other aggregates, by the
necessary unlikenesses of the actions to which the parts are
subject, is shown by the universal rise of the primary differen-
tiation into the parts that are universally most contrasted in
their circumstances, and by the rise of secondary differen-

390 PHYSIOLOGICAL DEVELOPMENT.

tiations obviously related in their order to secondary contrasl
of conditions.

§ 312. How physiological development has all along beei
aided by the multiplication of effects — how each differen-
tiation has ever tended to become the parent of new differen-
tiations, we have had, incidentally, various illustrations,
us here review the working of this cause.

Among plants we see it in the production of progressively-
multiplying heterogeneities of tissue by progressive increase
of bulk. The integration of fronds into axes and of axes into
groups of axes, sets up unlikenesses of action among the
integrated units, followed by unlikenesses of minute struc-
ture. Each gust transversely strains the various parts of the
stem in various degrees, and longitudinally strains in various
degrees the roots; and while there is inequality of stress at
every place in stem and branch, so, at every place in stem
and branch, the outer layers and the successively inner layers
are severally extended and compressed to unequal amounts,
and have unequal modifications wrought in them. Let the
tree add to its periphery another generation of the units
composing it, and immediately the mechanical strains on the
supporting parts are all changed in different degrees, initiat-
ing new differences internally. Externally, too, new differ-
ences are initiated. Shaded by the leaf-bearing outer stratum
of shoots, the inner structures cease to bear leaves, or to put
out shoots which bear leaves ; and instead of that green cover-
ing which they originally had, become covered with bark of
increasing thickness. Manifestly, then, the larger integration
of units that are originally simple and uniform, entails
physiological changes of various orders, varying in their
degrees at all parts of the aggregate. Each branch which,
favourably circumstanced, flourishes more than its neigh-
bours, becomes a cause of physiological differentiations, not
only in its neighbours from which it abstracts sap and pres-

SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 391

eiilly turns from leaf-bearers into fruit-bearers, but also in
the remoter parts.

That among animals physiological development is fur-
thered by the multiplication of effects, we have lately seen
proved by the many changes in other organs, which the
growth or modification of each excreting and secreting organ
initiates. By the abstracted as well as by the added mate-
rials, it alters the quality of the blood passing through all
members of the body; or by the liquid it pours into the
alimentary canal, it acts on the food, and through it on the
blood, and through it on the system as a whole: an addi-
tional differentiation in one part thus setting up additional
differentiations in many other parts; from each of which,
again, secondary differentiating forces reverberate through
the organism. Or, to take an influence of another order, we
have seen how the modified mechanical action of any member
not only modifies that member, but becomes, by its reactions,
a cause of secondary modifications — how, for example, the
burrowing habits of the common mole, leading to an almost
exclusive use of the fore limbs, have entailed a dwindling
of the hind limbs, and a concomitant dwindling of the
pelvis, which, becoming too small for the passage of the
young, has initiated still more anomalous modifications.

So that throughout physiological development, as in evo-
lution at large, the multiplication of effects has been a factor
constantly at work, and working more actively as the develop-
ment has advanced. The secondary changes wrought by
each primary change, have necessarily become more numerous
in proportion as organisms have become more complex. And
every increased multiplication of effects, further differen-
tiating the organism and, by consequence, further integrating
it, has prepared the way for still higher differentiations and
integrations similarly caused.

§ 313. The general truth next to be resumed, is that these
processes have for their limit a state of equilibrium — proxi-

392 PHYSIOLOGICAL DEVELOPMENT.

mately a moving equilibrium and ultimately a complete
equilibrium. The changes we have contemplated are but the
concomitants of a progressing equilibration. In every aggre-
gate which we call living, as well as in all other aggregates,
the instability of the homogeneous is but another name for
the absence of balance between the incident forces and the
forces which the aggregate opposes to them ; and the passage
into heterogeneity is the passage towards a state of balance.
And to say that in every aggregate, organic or other, there
goes on a multiplication of effects, is but to say that one part
which has a fresh force impressed on it, must go on changing
and communicating secondary changes, until the whole of the
impressed force has been used up in generating equivalent
reactive forces.

The principle that whatever new action an organism is
subject to, must either overthrow the moving equilibrium of
its functions and cause the sudden equilibration called death,
or else must progressively alter the organic rhythms until,
by the establishment of a new reaction balancing the new
action a new moving equilibrium is produced, applies as
much to each member of an organism as to the organism in
its totality. Any force falling on any part not adapted to
bear it, must either cause local destruction of tissue, or must,
without destroying the tissue, continue to change it until it
can change it no further ; that is — until the modified reaction
of the part has become equal to the modified action. What-
ever the nature of the force this must happen. If it is a
mechanical force, then the immediate effect is some distortion
of the part — a distortion having for its limit that attitude
in which the resistance of the structures to further change of
position, balances the force tending to produce the further
change ; and the ultimate effect, supposing the force to be con-
tinuous or recurrent, is such a permanent alteration of form,
or alteration of structure, or both, as establishes a permanent
balance. If the force is physico-chemical, or chemical, the
general result is still the same: the component molecules of

SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 393

the tissue must have their molecular arrangements changed,
and the change in their molecular arrangements must go on
until their molecular motions are so re-adjusted as to equili-
brate the molecular motions of the new physico-chemical or
chemical agent. In other words, the organic matter com-
posing the part, if it continues to be organic matter at all,
must assume that molecular composition which enables it to
bear, or as we say adapts it to, the incident forces.

Nor is it less certain that throughout the organism as a
whole, equilibration is alike the proximate limit of the changes
wrought by each action, as well as the ultimate limit of the
changes wrought by any recurrent actions or continuous
action. The movements every instant going on, are move-
ments towards a new state of equilibrium. Eaising a limb
causes a simultaneous shifting of the centre of gravity, and
such altered tensions and pressures throughout the body as
re-adjust the disturbed balance. Passage of liquid into or
out of a tissue, implies some excess of force in one direction
there at work ; and ceases only when the force so diminishes or
the counter-forces so increase that the excess disappears. A
nervous discharge is reflected and re-reflected from part to
part, until it has all been used up in the re-arrangements pro-
duced— equilibrated by the reactions called out. And what
is thus obviously true of every normal change, is equally true
of every abnormal change — every disturbance of the estab-
lished rhythm of the functions. If such disturbance is a
single one, the perturbations set up by it, reverberating
throughout the system, leave its moving equilibrium slightly
altered. If the disturbance is repeated or persistent, its suc-
cessive effects accumulate until they have produced a new
moving equilibrium adjusted to the new force.

Each re-balancing of actions, having for its necessary con-
comitant a modification of tissues, it is an obvious corollary
that organisms subjected to successive changes of conditions,
must undergo successive differentiations and re-differentia-
tions. Direct equilibration in organisms, with all its accom-

394 PHYSIOLOGICAL DEVELOPMENT.

pairing structural alterations, is as certain as is that uni-
versal progress towards equilibrium of which it forms part.
And just as certain is that indirect equilibration in organisms
to which the remaining large class of differentiations is due.
The development of favourable variations by the killing of
individuals in which they do not occur or are least marked,
is, as before, a balancing between certain local structures and
the forces they are exposed to ; and is no less inevitable than
the other.

§ 314. In all which universal laws, we find ourselves again
brought down to the persistence of force, as the deepest
knowable cause of those modifications which constitute
physiological development; as it is the deepest knowable
cause of all other evolution. Here, as elsewhere, the per-
petual lapse from less to greater heterogeneity, the perpetual
begetting of secondary modifications by each primary modi-
fication, and the perpetual approach to a temporary balance
on the way towards a final balance, are necessary implica-
tions of the ultimate fact that force cannot disappear but can
only change its form.

It is an unquestionable deduction from the persistence of
force, that in every individual organism each new incident
force must work its equivalent of change; and that where it
is a constant or recurrent force, the limit of the change it
works must be an adaptation of structure such as opposes to
the new outer force an equal inner force. The only thing
open to question is, whether such re-adjustment is inherit-
able; and further consideration will, I think, show, that to
say it is not inheritable is indirectly to say that force does
not persist. If all parts of an organism have their func-
tions co-ordinated into a moving equilibrium, such that every
part perpetually influences all other parts, and cannot be
changed without initiating changes in all other parts — if the
limit of change is the establishment of a complete harmony
among the movements, molecular and other, of all parts ; then

SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 395

among other parts that are modified, molecularly or other-
wise, must be those which cast off the germs of new organ-
isms. The molecules of their produced germs must tend
ever to conform the motions of their components, and there-
fore the arrangements of their components, to the molecular
forces of the organism as a whole; and if this aggregate
of molecular forces be modified in its distribution by a local
change of structure, the molecules of the germs must be
gradually changed in the motions and arrangements of their
components, until they are re-adjusted to the aggregate of
molecular forces.

CHAPTER X\

THE INTEGRATION OF THE ORGANIC WORLD.

§ 314a. That from the beginning of life there has beei
an ever-increasing heterogeneity in the Earth's Flora and
Fauna, is a truth recognized by all biologists who accept the
doctrine of evolution. In discussing the origin of species
Mr. Darwin and others have been mainly occupied in ex-
plaining the genesis of now this and now that form of
organism, considered as a member of one or other series, and
regarded as becoming differentiated from its allies. But by
implication, if not avowedly, there has been simultaneously
accepted the belief that the forms continually produced by
divergences and re-divergences, have constituted an assem-
blage increasingly multiform in its included kinds. And
this, which we are shown by the process of organic evolution
as followed out in its details, is a corollary from the doctrine
of evolution at large, as was pointed out in § 159 of First
Principles.

Meanwhile there has been little if any recognition of an
accompanying change, no less fundamental. In the general
transformation which constitutes Evolution, differentiation
and integration advance hand in hand; so that along with
the production of unlike parts there progresses the union of
these unlike parts into a whole. Examples of various kinds
before given will recur to the reader, and an addition to them
has just been set forth in the chapter on " Physiological
Integration." One more example, world-wide in its read
has still to be named.
396

THE INTEGRATION OF THE ORGANIC WORLD. 397

For here it remains to point out that along with the in-
creasing multiplication of types of organisms covering the
Earth's surface, there has been ever going on an increasing
mutual dependence of them — an increasing integration of
the entire aggregate of living things.

Many facts which are obvious and many which are quite
familiar will be named as evidence. But I must be excused
for reminding the reader of things that he knows and things
that he may easily observe, since, unless the evidence, trite
as it may be, is gathered together and properly marshalled,
the generalization enunciated will not be thought valid.

§ 314&. Respecting the physiological characters of the
earliest forms there is an assumption from which no escape
seems possible — the assumption that they united animal and
vegetal characters. Even among existing microscopic types
of the lowest classes, there is such community of plant-
traits and animal-traits that doubts respecting their proper
places in one or the other kingdom are continually raised
— doubts, too, whether, if regarded as vegetal, they are to be
grouped as algoid or fungoid.

Here, however, without entering on moot questions, we
may draw the a priori conclusion that these earliest living
things were double-natured, in so far that they must have
had the ability to assimilate from the inorganic world all the
materials of which protoplasm consists — must therefore,
along with the power of appropriating carbon from its gase-
ous compound, also have had the power of appropriating ni-
trogen, either from one of its combined oxides or directly from
the air with which water is more or less charged. For before
organic substances existed there could have been none but
inorganic sources from which nitrogen could be obtained.

This conclusion concerns us only because it implies
homogeneity of nature in these primordial forms of life.
There could not at first have existed among these minutest
of Protozoa even such vague distinctions as are now presented

398 PHYSIOLOGICAL DEVELOPMENT.

in a shadowy way by their modern representatives. And the
implication is that during the period throughout which thes
smallest, lowest, and simplest living things alone existed,
there could have been, in the absence of kinds, no mutual
dependence.

Since, among various of the lowest types now known to
us, the same individual exhibits a life which is now pre-
dominantly vegetal and now predominantly animal, w(
cannot err in assuming that there eventually took pla(
differentiations of this original plant-animal type into types
permanently unlike: some in which the traits were moi
markedly vegetal and others in which they were more
markedly animal. As fast as this differentiation arose,
there came the beginnings of cooperation between the pre
dominantly vegetal types which by the aid of light formee
organic matter from the inorganic world, and the predomi-
nantly animal types which, in chief measure, utilized the
matter so formed. Evidently with the rise of such a dif-
ferentiation came an incipient mutual dependence. If to
the implied algoid type and the animal type there be added
the fungoid type, somewhat intermediate in character, which
in a large proportion of cases lives on the decaying remnants
of the other two, we are furnished with a rude conception of
the primary differentiations and the accompanying vague
mutual dependences.

Speculation aside, it suffices to say that early in the histoi
of life there must have arisen the distinction between Pro-
tozoa and Protophyta, and that this distinction foreshadowe
that widest contrast which the higher organic world presents
— the contrast between plants and animals. It is needless
to do more than name the mutual dependence between these
two great divisions. That, as being respectively decomposers
of carbon dioxide and exhalers of carbon dioxide, they act
reciprocally, as also in some measure by interchange of nitre
genous matters; and that the implied general cooperatioi
serves in an indirect way to unite their lives, and in thai

THE INTEGRATION OF THE ORGANIC WORLD. 399

sense to integrate the two kingdoms ; needs not to be insisted
upon. Further complications of the mutual dependence will
be mentioned by and by. For the present it suffices to
recognize this division of organic functions as the first which
arose and as continuing to be that fundamental one which
more than all others binds organisms at large together.

§ 314c. It will be thought by many readers that in speak-
ing of the contrasted vital activities of plants and animals as
constituting a "division of organic functions/' I am straining
words beyond their meanings ; since the conception of organic
functions postulates an organized whole in which they exist,
and plants and animals constitute no such organized whole.
But there is at hand an unexpected defence for this concep-
tion— a defence not forthcoming a generation ago, but which
now all biologists will recognize as relevant. I refer to the
phenomena of symbiosis. These present various cases in
which the plant-function and the animal-function are carried
on in the same body, — cases in which the cooperation is not
between separate vegetal organisms which accumulate nutri-
tive matters and separate animal organisms which consume
them, but is a cooperation between vegetal elements and.
animal elements forming parts of the same organism.

As introductory to examples of these must first, however,
be named an example of such cooperation between the two
great classes of vegetal organisms — the fungoid and the al-
goid. Incredible as the statement once seemed, it is a state-
ment now accepted, that what we know as lichens, and used to
consider as plants forming a certain low class, are now found
to be not plants in the ordinary sense at all, but compound
growths formed Of minute algge and minute fungi, carrying
on their lives together: the algoa furnishing to the fungi
certain constituents they need but cannot directly obtain,
and the fungi profiting by certain materials they obtain from
the algae, either while living or while individually decaying.
Whence it would seem that after the microscopic vegetal

400 PHYSIOLOGICAL DEVELOPMENT.

type had become in a large degree differentiated into two
main types, in adaptation to different conditions of life, and
had acquired appropriate specialities of nature, there grew up
this communistic arrangement between certain of them, en-
abling each to benefit by the powers which the other had
acquired: evidently an exchange of services, a physiological
division of labour, a mutual dependence of functions analo-
gous to that which exists between functions in an ordinal
plant or animal.

Not differing in principle but only in application, is th*
symbiosis above referred to as existing between Protophyia
and many Protozoa, as well as between such Protophyta
and the lowest kinds of Metazoa. A recent statement that
certain amoeba?, made green by contained chlorophyll, coi
tinue to grow and multiply after they have consumed whal
nutritive matter may be at hand, is in harmony with various
facts alleged of other Protozoa — various other kinds of Ehizo-
pods, various Heliozoa, numerous ciliated and flagellated
Infusoria. Among Metazoa the like association occurs in
one of the sponges, in the Hydra viridis, in various turbel-
larians, in a rotifer, and even in two molluscs. In these
cases the partnership between the vegetal cells and the
animal cells (existing either as units or as an organized group
such as a polype), is a partnership which, as before, profits
each of the partners — an inference supported by the fact th*
Metazoa containing these algoid cells usually place them-
selves where the light falls upon them, and can therefore
further the production of the carbo-hydrates which even!
ually become useful to the animal-cells, while these in som<
way reciprocate the benefit.

Here, then, we have exchange of services between ass(
ciated plant-elements and animal-elements — a performance
by them of different organic functions for the benefit of the
aggregate which they unite to form. Hence, when these
vegetal elements and animal elements are separately em-
bodied in plants and animals, which profit by one another,

THE INTEGRATION OF THE ORGANIC WORLD. 401

we may still properly regard their respective lives as mutually-
dependent organic functions, as said in the preceding section.
We are enabled the better to see how the Earth's Flora and
Fauna, which are respectively accumulators of motion and
expenders of motion, form mutually-dependent parts of a
whole, and are in that sense integrated. And we shall be
prepared to see how all other relations between organisms
which make them subservient one to another, similarly con-
stitute elements in a general integration of the organic world.

§ 314d. Another form of mutual dependence and conse-
quently of integration is conspicuous — that which accom-
panied the progressive increase of size in organisms of the
higher classes. We have but to contemplate the possibilities
to see that life must necessarily have commenced with minute
forms, and that the progress to larger ones must have been
by small steps.

For had creatures of appreciable sizes been the first to
exist they would inevitably have disappeared from lack of
food. Having no resource but to devour one another, they
would quickly have brought life to an end. There must have
been smaller types serving as prey for larger ones before
these could continue to exist and to multiply: microbes
affording food to infusoria, infusoria affording food to such
sized creatures as the Entomostraca, these again supplying
food to small fishes, such as loch-trout, and these last yielding
to larger fishes masses sufficiently great for their needs : each
higher grade requiring lower grades of appropriate bulk. It
needs but to ask what would become of tigers if th.ere were
no mammals larger than mice, to see that the animal world is
a linked assemblage, of which the connected members stand
within certain ratios of mass ; and that during the evolution
of higher and larger types the linking of grades has become
closer.

That among plants considered as an aggregate relations
of like kind, though far less distinct ones, have all along
72

402 PHYSIOLOGICAL DEVELOPMENT.

been growing may be reasonably concluded. In a world
peopled only by microscopic types there could not have
existed the conditions needful for large trees. Gradual dis-
integration of rock-surfaces, partly effected by physical
agencies and partly by low forms of plants, had to prepare
the way for superior plants. The production of sufficient
soil by mineralogical decay as well as by the decay of
organisms, plant and animal, may be regarded as having
been a preliminary to larger plant-growth; and though at
present the dependence is far less close than that among
animals, yet the benefits yielded to metaphytes by the de-
composing actions carried on by protophytes, as well as those
carried on by microbes permeating the soil, imply a con-
tinued general interdependence throughout the aggregate of
plant-forms, apart from more special interdependences. And
then along with this indebtedness of the greater plants to
the smaller during the process of evolution, there must be
named that indebtedness of plant-life to animal-life which
Mr. Darwin has shown in his book on the agency of worms
as producers of mould.

§ 314e. Services of one to another, and consequent unions,
of more special kinds are infinitely varied, alike within each
kingdom and between the two kingdoms. I refer to those
seen in parasitism, commensalism, and other forms of asso-
ciation. While they do not conduce to unions of the kind
thus far considered, these nevertheless constitute innumer-
able links whereby the lives of organisms, plant and animal,
are tied together; sometimes for the advantage of both but
in most cases for the benefit of one to the injury of the other.

Among plants the degrees of dependence are various. Un-
able to raise themselves into the air and light, some climb,
like the ivy, by modified rootlets, or spirally coil themselves,
or hang by tendrils. Others there are which gradually
strangle the trees they embrace, or which, like lichens in
damp climates, festooning the smaller trees, by and by cause

THE INTEGRATION OF THE ORGANIC WORLD. 403

their decay. Of higher types of epiphytes which use trees
only to gain elevation, the orchids may be instanced. And
then we have plants which, like the mistletoe, fix themselves
on the bark of their hosts, utilizing them partly for purposes
of elevation and partly by appropriation of their juices. After
these may be named those extreme cases in which the para-
sitic plants, ceasing to have any chlorophyll-bearing leaves,
live wholly on the juices of the invaded plants. At home
the common dodder, and in the tropics the Rafflesiacece,
belong to this group. There must be added the numerous
forms of minute fungi which in like manner thrive at the
expense of the plants they infest. In all these cases the
interdependence is one-sided, though, as we shall presently
see, while detrimental to one of the two concerned, it is not
always detrimental to the organic world as a whole.

That utilization of one by another among animals which
causes immediate death, is familiar enough in the relations
between carnivores and herbivores. Almost as familiar are
those seen in parasitism. Less familiar are those seen in
commensalism ; and the least familiar are those which show
us exchange of services. Among these last — the mutually-
beneficial relations — that between the crocodile and the bird
which picks parasites out of its teeth is a striking one; and
no less so is that of the pique-gouffe, an African bird which
pierces the tumour on a buffalo's back that incloses a para-
site. Then of another kind we have the connexion between
aphides and ants: the one profiting by being carried to
better pastures and the other by increased saccharine excre-
tion. Next comes the class of messmates, the connexions
between some of which are relatively innocent, as witness
the Sea-anemone which settles itself on the shell occupied
by a Hermit-crab, or as witness the Remora fixed on a
shark's skin. Less innocent is the relation under which one
of the two seizes a share of the food obtained by the other,
like the annelid which insinuates itself between the Hermit-
crab and the whelk-shell it inhabits, or like the small fishes

404 PHYSIOLOGICAL DEVELOPMENT.

inhabiting certain Medusce, or those which nestle in the
branchial sac of the Lophius. After these may be named the
less injurious forms of parasites proper — those which, dis-
tinguished as Epizoa, fix themselves on the skins of their
hosts, permanently or temporarily, such as, of the one kind,
the Lerncea on fishes, and of the other kind the Tick on
mammals and birds. Then there come the other class of
parasites, most of them highly injurious, distinguished as
Entozoa, living within the bodies of their hosts, now in parts
of their alimentary canals, now on other of their mucous
surfaces, and now in various of their organs : these last two
groups being so numerous in their kinds that there are
commonly more species than one proper to each larger animal.
One stage further in the complication meets us in the para-
sites upon parasites.

But now the general fact, to which these brief indications
are introductory, is that the use made of one organism by
another has been ever widening and becoming more involved.
Among plants utilization of the larger by the smaller — of
trees by epiphytes and parasites — must have arisen since the
times when the larger came into existence — times relatively
late in the course of organic evolution. Moreover most of
the plants which utilize others, either by climbing up them or
settling themselves high up on their stems or sucking their
juices, are phsenogams, and the plants they utilize are also ]
phasnogams; so that these innumerable interdependences
must have been established since the phsenogamic type has
become so predominant in respect of both size and kind.
Similarly among animals. Though there are many parasites
belonging, like the Trematodes, to very low classes, there are
many which belong to the Arthropoda, and, being degraded
forms of that class, must have come into existence after
Arthropods of considerable structure had been evolved.
Again, a large part of the animals infested by Epizoa and
Entozoa are vertebrates — many of the highest types ; and as
these are relatively modern all this parasitism must be of

THE INTEGRATION OF THE ORGANIC WORLD. 405

late date. So, too, of much commensalism and many
mutually-beneficial associations. The reciprocal services of
ants and aphides must have originated since the Hymenoptera
and Ilemiptera became established types, and since the days
when certain insects of the ant-type had become social, and
since the days when aphides had become degraded members
of their order: both dates being relatively recent. And still
more recent must have been the commensalism between the
ants and the many species of other insects which inhabit
their nests.

Leaving out relations of the kinds just named, it seems
that down from those between carnivores and their prey to
those between lice and their hosts, such relations profit one
of the two species concerned and injure the other, and that
there the matter ends. But it does not end there; for that
multiplication of effects to which people are usually blind,
brings about changes which, as hinted above, though inju-
rious to the individual are beneficial to the species, and
which, when not beneficial to the species, are often beneficial
to the aggregate of species.

Even where animals of one class live by devouring animals
of another class, we see, on looking beyond the immediate
results, certain remote results that are advantageous. In
the first place the process is one by which inferior individuals
— the least agile, swift, strong, or sagacious — are picked out
and prevented from leaving posterity and lowering the
average quality of their kind. At the same time individuals
made feeble by injury or old age, are among those to be
killed and saved from suffering prolonged pains : the evils of
death by disease and starvation being thus limited to the pre-
datory animals, relatively small in their numbers. Mean-
while a check is put on undue multiplication. Where a tract
of country has been overrun by rabbits, weasels, thriving on
the abundant supply of food, presently become numerous
enough to bring the population of rabbits within moderate
limits ; and by doing this benefit not only all those kinds of

406 PHYSIOLOGICAL DEVELOPMENT.

plants which are being eaten down, and all those other ai
mals which live on such plants, but also the rabbits them-
selves; since, increasing beyond the means of subsistence,
large part of them would, if not killed, die of hunger,
tween aphides and lady-birds we see a connexion of like
nature: great increase of the first yielding abundant food to
larvae of the second, ending after a season or so in swarms of
lady-birds, and consequently of their larvae, whereby the
aphides, immensely diminished, cease so greatly to injure
various plants and the animals dependent on them. Even
minute parasites, by the evils they inflict on one species,
profit others: instance the enormous destruction of flies
which a microscopic fungus caused a few years ago — a
destruction which relieved not only man but all the animals
which flies irritate: often so much as to hinder them from
feeding. Such instances remind us how numerous are the
bonds by which the lives of organisms are tied together.

§ 314/. I have reserved to the last the clearest and most
striking illustration of this progressing integration through-
out the organic world. I refer to the mutually-beneficial
relations established between plants and animals through the
agency of flowers and insects.

Everyone nowadays has been made familiar with the pre
cess of plant-fertilization, and knows that (leaving out
consideration plants fertilized by wind-borne pollen) the
ability to bear seed depends largely on the aid given by bees
butterflies, and moths. The exchange of services has beei
growing ever more various and complicated during long past
periods. We have the acquirement by flowers of bright
colours serving to guide these insects to places where honey
is to be found ; and we have their perfumes, also serving for
guidance. Then we have the many different arrangements,
often complicated, by which the visiting insects are obliged
to carry away pollen and dust with it the stigmas of flowers
on which they subsequently settle: thus effecting cross-

THE INTEGRATION OF THE ORGANIC WORLD. 407

fertilization. Pari passu have gone on insect-developments
made possible by these arrangements and furthering them.
Especially must be named the modification of certain Hymen-
optera into honey-storing bees: the implication being that
the entire economy established by these social insects has
been sequent on the growth of this system of reciprocal
benefits. And then, just instancing the dependence between
a particular flower having a long tubular corolla, and a par-
ticular moth having an appropriately long proboscis, it
suffices to say that innumerable specialities of this general
relation everywhere multiply the links by which the vegetal
world and the animal world are here connected. That the
effects of the connections tell largely on the prosperity of
both, is suggested by some instances Mr. Darwin gives, and
by a statement recently made in the United States, by Dr. L.
0. Howard, that the greater fostering of bees would much
increase certain of the crops.

But now observe the broad fact to which these few details
concerning plant-fertilization are introductory. All these
general and special relations between plants and animals
have arisen since the phaenogamic type came into existence —
have, indeed, arisen since the higher members of that type,
the Angiosperms, have appeared; for the Gymnosperms do
not play any part in this intercommunion. But so far as we
can judge of present results of geologic explorations, there
were no Angiosperms during the Eozoic and Paleozoic
periods. So that this class of connexions between animals
and vegetals must have been established since carboniferous
times — a period long, indeed, but far shorter than that which
organic evolution at large has occupied.

§ 314#. I have but just touched on some salient parts of a
subject, immense in extent and extremely involved, which it
would take a volume to set forth adequately. Enough has
been said, however, to indicate the truth which* it is the
purpose of the chapter to bring into view and emphasize —

408 PHYSIOLOGICAL DEVELOPMENT.

the truth that both of the two great laws of evolution are
exemplified in the organic world as a whole, as they are
exemplified in every organism, and in all other things.

The reader has long since become familiar with the gene-
ralization that while Evolution is a change from the homo-
geneous to the heterogeneous, it is also a change from the
incoherent to the coherent; and this change from the inco-
herent to the coherent has been above exhibited as going on
even throughout that vast assemblage of organisms, plant
and animal, which cover the Earth's surface. In what we
are obliged to conceive as the earliest stage, when the most
minute types of life alone existed, the aggregate of living
things was at once homogeneous and incoherent. In the
course of epochs immeasurable in duration, this uniform
aggregate of beings has been becoming more multiform.
And now we see that instead of forms of life everywhere
without the slightest union caused by mutual dependence,
there have slowly arisen forms of life among which mutual
dependences have entailed vital connexions correspondingly
marked. Along with progressing differentiation there has
ever been progressing integration. So that we may recog-
nize something like a growing life of the entire aggregate of
organisms in addition to the lives of individual organisms —
an exchange of services among parts enhancing the life of
the whole.

In this final generalization the law of Evolution is mani-
fested under its most transcendental form.

PART VI.

LAWS OF MULTIPLICATION

CHAPTER I.

THE FACTORS.*

§ 315. If organisms have been evolved, their respective
powers of multiplication must have been determined by-
natural causes. Grant that the countless specialities of
structure and function in plants and animals, have arisen
from the actions and reactions between them and their
environments, continued from generation to generation; and
it follows that from these actions and reactions have also
arisen those countless degrees of fertility which we see
among them. As in all other respects an adaptation of each
species to its conditions of existence is directly or indirectly
brought about; so must there be directly or indirectly
brought about an adaptation of its reproductive activity to
its conditions of existence.

We may expect to find, too, that permanent and temporary

* An outline of the doctrine set forth in the following chapters, was
originally published in the Westminster Review for April, 1852, under the
title — A Theory of Population deduced from the General Law of Animal
Fertility; and was shortly afterwards republished with a prefatory note
stating that it must be accepted as a sketch which I hoped at some future
time to elaborate. In now revising and completing it, I have omitted a
non-essential part of the argument, while I have expanded the remainder
by adding to the number of facts put in evidence, by meeting objections
which want of space before obliged me to pass over, and by drawing various
secondary conclusions. The original paper, with omissions, will be found in
Appendix A to Volume I of this work.

411

412 LAWS OF MULTIPLICATION.

differences of fertility have the same general interpretation.
If the small variations of structure 'and function that arise
within the limits of each species, are due to actions like those
which, by their long-accumulating effects, have produced the,
immense contrasts between the various types; we may con-
clude that, similarly, the actions to which changes in the
rate of multiplication of each species are due, also produce,
in great periods of time, the enormous differences between
the rates of multiplication of different species.

Before inquiring in what ways the rapidities of increase
are adjusted to the requirements, both temporary and perma-
nent, it will be needful to look at the factors. Let us set
down first those which belong to the environment, and then
those which belong to the organism.

§ 316. Every living aggregate being one of which the
inner actions are adjusted to balance outer actions, it follows
that the maintenance of its moving equilibrium depends on
its exposure to the right amounts of these actions. Its
moving equilibrium may be overturned if one of these actions
is either too great or too small in amount ; and it may be so
overturned either by excess or defect of some inorganic
agency in its environment, or by excess or defect of some
organic agency.

Thus a plant, constitutionally fitted to a certain warmth
and humidity, is killed by extremes of temperature, as well
as by extremes of drought and moisture. It may dwindle
away from want of soil, or die from the presence of too great
or too small a quantity of some mineral substance which the
soil supplies to it. In like manner, every animal can main-
tain the balance of its functions so long only as the environ-
ment adds to or deducts from its heat at rates not exceeding
definite limits. Water, too, must be accessible in amounl
sufficient to compensate loss. If the parched air is rapidly
abstracting its liquid which there is no pool or river to
restore, its functions cease; and if it is an aquatic creature,

THE FACTORS. 413

drought may kill it either by drying up its medium or by
giving it a medium inadequately aerated. Thus each organ-
ism, adjusted to a certain average in the actions of its
inorganic environment, or rather, we should say, adjusted to
certain moderate deviations from this average, is destroyed
by extreme deviations. So, too, is it with the environ-

ing organic agencies. Among plants, only the parasitic kinds
and those united by symbiosis (as well as a few innocent
" lodgers " ) depend for their individual preservation on the
presence of certain other organisms (though the presence of
certain other organisms is needful to most plants for the pre-
servation of the race by aiding fertilization). Here, for the
continuance of individual life, particular organisms must
be absent or not very numerous — beasts that browse, cater-
pillars that devour leaves, aphides that suck juices. Among
animals, however, the maintenance of the functional balance
is both positively and negatively dependent on the amounts
of surrounding organic agents. There must be an accessible
sufficiency of the plants or animals serving for food; and of
organisms that are predatory or parasitic or otherwise detri-
mental, the number must not pass a certain limit.

This dependence of the moving equilibrium in every indi-
vidual organism on an adjustment of its forces to the forces
of the environment, and the overthrow of this equilibrium
by failure of the adjustment, is comprehensive of all cases.
At first sight it does not seem to include what we call natural
death; but only death by violence, or starvation, or cold, or
drought. But in reality natural death, no less than every
other kind of death, is caused by the failure to meet some
outer action by a proportionate inner action. The apparent
difference is due to the fact that in old age, when the
quantity of force evolved in the organism gradually dimi-
nishes, the momentum of the functions becomes step by step
less, and the variations of the external forces relatively
greater; until there finally comes an occasion when some
quite moderate deviation from that average to which the

w

414 LAWS OF MULTIPLICATION.

feeble moving equilibrium is adjusted, produces in it a fatal
perturbation.

§ 317. The individuals of every species being thus de-
pendent on certain environing actions ; and severally having
their moving equilibria sooner or later overthrown by one or
other of these environing actions; we have next to consider
in what ways the environing actions are so met as to prevent
extinction of the species. There are two essentially different
ways. There may be in each individual a small or great
ability to adjust itself to variations of the agencies around
it and to a small or great number of such varying agencies
— there may be little or much power of preserving the
balance of the functions. And there may be much or little
\V power of producing new individuals to replace those whose
moving equilibria have been overthrown. A few facts must
be set down to enforce these abstract statements.

There are both active and passive adaptations by which
organisms are enabled to survive adverse influences. Plants
show us but few active adaptations : that of the Pitcher-plant
and those of the reproductive parts of some flowers (which do
not, however, conduce to self-preservation) are exceptional
instances. But plants have various passive adaptations; as
thorns, stinging hairs, poisonous and acrid juices, repugnant
odours, and the woolliness or toughness that makes theii
leaves uneatable. Animals exhibit far more numerous

adjustments, both passive and active. In some cases they
survive desiccation, they hybernate, they acquire thicker
clothing, and so are fitted to bear unfavourable inorganic
actions; and they are in many cases fitted passively to meet
the adverse actions of other organisms, by bearing spines
armour or shells, by simulating neighbouring objects in colour
or form or both, by emitting disagreeable odours, or by having
disgusting tastes. In still more numerous ways they actively
contend with unfavourable conditions. Against the seasons
they guard by storing up food, by secreting themselves in

.

THE FACTORS. 415

crevices, or by forming burrows and nests. They save them-
selves from enemies by developed powers of locomotion,
taking the shape of swiftness or agility or aptitude for
changing their media; by their strength either alone or
aided by weapons; lastly by their intelligence, without
which, indeed, their other superiorities would avail them
little. And then these various active powers serving for
defence, become, in other cases, the powers that enable ani-
mals to aggress, and to preserve their lives by the success
of their aggressions.

The second process by which extinction is prevented — the
formation of new individuals to replace the individuals
destroyed — is carried on, as described in the chapter on
" Genesis," by two methods, the sexual and the asexual.
Plants multiply by spontaneous fission, by gemmation, by
proliferation, and by the evolution of young ones from de-
tached cells and scales and leaves; and they also multiply
by the casting off of spores and sporangia and seeds. In like
manner among animals, there are varied kinds of agamo-
genesis, from spontaneous fission up to parthenogenesis, all of
them conducing to rapid increase of numbers; and we have
the more familiar process of gamogenesis, also carried on
in a great variety of ways. This formation of new

individuals to replace the old, is, however, inadequately con-
ceived if we contemplate only the number born or detached
on each occasion. There are four factors, all variable, on
which the rate of multiplication depends. The first is the
age at which reproduction commences; the second is the
frequency with which broods are produced; the third is the
number contained in each brood; and the fourth is the
length of time during which the bringing forth of broods
continues. There must be taken into account a further
element — the amount of aid given by the parent to each
germ in the shape of stored-up nutriment, continuous feed-
ing, warmth, protection, &c. : on which amount of aid, vary-
ing between immensely wide limits, depends the number of

416 LAWS OF MULTIPLICATION.

the new individuals that survive long enough to replace the
old, and perform the same reproductive process.

Thus, regarding every living organism as having a moving
equilibrium dependent on environing forces, but ever liable
to be overthrown by irregularities in those forces, and always
so overthrown sooner or later; we see that each species of
organism can be maintained only by the generation of new
individuals with a certain rapidity, and by helping them
more or less fully to establish their moving equilibria.

§ 318. Such are the factors with which we are here con-
cerned. I have presented them in abstract shapes for the
purpose of showing how they are expressible in general
terms of force — how they stand related to the ultimate laws
of redistribution of matter and motion.

For the purposes of the argument now to follow, we may,
however, conveniently deal with these factors under a more
familiar guise. Ignoring their other aspects, we may class
the factors which affect each race of organisms as forming
two conflicting sets. On the one hand, by what we call
natural death, by enemies, by lack of food, by atmospheric
changes, &c, the race is constantly being destroyed. On the
other hand, partly by the endurance, the strength, the swift-
ness, and the sagacity of its members, and partly by their
fertility, it is constantly being maintained. These conflicting
sets of factors may be generalized as — the forces destructive
of race and the forces preservative of race. So generalizing
them, let us ask what are the necessary implications.

CHAPTER II.

A PRIORI PRINCIPLE.

§ 319. The number of a species must at any time be either
decreasing or stationary or increasing. If, generation after
generation, its members die faster than others are born, the
species must dwindle and finally disappear. If its rate of
multiplication is equal to its rate of mortality, there can be
no numerical change in it. And if the deductions by death
are fewer than the additions by birth, the species must
become more abundant. These we may safely set down as

necessities. The forces destructive of race must be either

«

greater than the forces preservative of race, or equal to them,
or less than them; and there cannot but result these effects
on number.

We are here concerned only with races that continue to
exist; and may therefore leave out of consideration those
in which the destructive forces, remaining permanently in
excess of the preservative forces, cause extinction. Prac-
tically, too, we may exclude the stationary condition ; for the
chances are infinity to one against the maintenance of a per-
manent equality between the births and the deaths. Hence,
our inquiry resolves itself into this : — In races that con-
tinue to exist, what laws of numerical variation result from
these variable conflicting forces, which are respectively de-
structive of race and preservative of race?

§ 320. Clearly if the forces destructive of race, when once
73 417

418 LAWS OF MULTIPLICATION.

in excess, had nothing to prevent them from remaining in
excess, the race would disappear; and clearly if the forces
preservative of race, when once in excess, had nothing to
prevent them from remaining in excess, the race would go on
increasing to infinity. In the absence of any compensating
actions, the only possible avoidance of these opposite extremes
would be an unstable equilibrium between the conflicting
forces, resulting in a perfectly constant number of the
species : a state which we know does not exist, and against
the existence of which the probabilities are, as already said,
infinite. It follows, then, that as in every continuously-
existing species, neither of the two conflicting sets of forces
remains permanently in excess; there must be some way of
stopping that excess of the one or the other which is ever
occurring.

How is this done? Should any one allege, in conformity
with the old method of interpretation, that there is in each
case a providential interposition to rectify the disturbed
balance, he commits himself to the supposition that of the
millions of species inhabiting the Earth,* each one is yearly
regulated in its degree of fertility by a miracle; since in no
two years do the forces which foster, or the forces which
check, each species, remain the same; and therefore, in no
two years is there required the same fertility to balance
the mortality. Few if any will say that God continually
alters the reproductive activity of every parasitic fungus and
every Tape-worm or Trichina, so as to prevent its extinction
or undue multiplication; which they must say if they adopt
the hypothesis of supernatural adjustment. And in the
absence of this hypothesis there remains only one other.
The alternative possibility is, that the balance of the pre-
servative and destructive forces is self-sustaining — is of the
kind distinguished as a stable equilibrium: an equilibrium
such that any excess of one of the forces at work, itself
generates, by the deviation it produces, certain counter-forces
which eventually out-balance it, and initiate an opposite devi-

A PRIORI PRINCIPLE. 419

ation. Let us consider how, in the case before us, such a
stable equilibrium must be constituted.

§ 321. When a season favourable to it, or a diminution of
creatures detrimental to it, causes any species to become
more numerous than usual, an immediate increase of certain
destructive influences takes place. If it be a plant, the sup-'
posed greater abundance itself implies fuller occupation of
the places available for growth — an occupation which, leav-
ing fewer such places as the multiplication goes on, becomes
a check on further multiplication — itself causes a greater
mortality of seeds that fail to root themselves. And after-
wards, in addition to this passive resistance to continued
increase, there comes an active resistance : the creatures which
thrive at the expense of the species — the larvae, the birds, the
herbivores — increase too. If it be an animal that has grown
more numerous, then, unless by some exceptional coincidence
a simultaneous and proportionate addition to the animals or
plants serving for food has occurred, there must result a
relative scarcity of food. Enemies, too, be they beasts of
prey or be they parasites, must quickly begin to multiply.
(^Hence, each kind of organism, previously existing in some-
thing like its normal number, cannot have its number raised
without a rise of the destructive forces, negative and positive,
quickly commencing^ • Both negative and posi-

tive destructive forces must augment until this increase of
the species is arrested. The competition for places on which
to grow, if the species be vegetal, or for food if it be animal,
must become more intense as the over-peopling of the habitat
progresses; until there is reached the limit at which the
mortality equals the reproduction. And as, at the same
time, enemies will multiply with a rapidity which soon
brings them abreast of the augmented supply of prey, the
positive restraint they exert will help to bring about an
earlier arrest of the expansion than pressure of population
alone would cause. One more inference mav be

420 LAWS OF MULTIPLICATION.

drawn. Had the species to meet no repressing influen
save that negative one of relatively-diminished space o
relatively-diminished food-supply, the cause leading to it
increase might carry it up to the limit set by this, and then
leave it: its enlarged number might be permanent. Bu
the positive repressing influence that has been called int<
existence, will prevent this. For the increase of enemies
commencing, as it must, after the increase of the species,
and advancing in geometrical progression until it is itself
checked in like manner, will end in an excess of enemies.
Whereupon must result a mortality of the species greate
than its multiplication — a decrease which will continu
until its habitat is under-peopled, its unduly-numerous
enemies decimated by starvation, and the destroying agencies
reduced to a minimum. Whence will follow another in-
crease.

Thus, as before indicated (First Prin. §§ 85, 173), there is
here, as wherever antagonistic forces are in action, an alter-
nate predominance of each, causing a rhythmical movement
— a rhythmical movement which constitutes a moving equili-
brium in those cases where the forces are not dissipated with
appreciable rapidity, or are re-supplied as fast as they are
dissipated. While, therefore, on the one hand, we see that
the continued existence of a species necessarily implies some
action by which the destructive and preservative forces a
self-adjusted ; we see, on the other hand, that such an action
is an inevitable consequence of the universal process of
equilibration.

ne

§ 322. Is this the sole equilibration which must exist ?
Clearly not. The temporary compensating adjustments of
multiplication to mortality in each species, are but intro-
ductory to the permanent compensating adjustments of mul-
tiplication to mortality among species in general. The above
reasoning would hold just as it now does, were all species
equally prolific and all equally short-lived. It yields no

A PRIORI PRINCIPLE. 421

answer to the inquiries — why do their fertilities differ so
enormously, or why do their mortalities differ so enormously?
and how is the general fertility adapted to the general mor-
tality in each? The balancing process we have contemplated
can go on only within moderate limits — must fail entirely in
the absence of a due proportion between the ordinary birth-
rate and the ordinary death-rate. If the reproduction of
mice proceeded as slowly as the reproduction of men, mice
would be extinct before a new generation could arise: even
did their natural lives extend to fifteen or sixteen years, it
would still be extremely improbable that any would for so
long survive all the dangers they are exposed to., Con-
versely, did oxen propagate as fast as infusoria, the race
would die of starvation in a week. Hence, the minor adjust-
ment of varying multiplication to varying mortality in each
species, implies some major adjustment of average multiplica-
tion to average mortality. What must this adjustment be?

We have already seen that the forces preservative of race ju.
are two — ability in each member of the race to preserve
itself, and ability to produce other members — power to main-
tain individual life, and power to generate the species.
These must vary inversely. When, from lowness of organi-
zation, the ability to contend with external dangers is small,
there must be great fertility to compensate for the conse-
quent mortality; otherwise the race must die out. When,
on the contrary, high endowments give much capacity of
self-preservation, a correspondingly-low degree of fertility is
requisite. Given the dangers to be met as a constant quan-
tity; then as the ability to meet them must be a constant
quantity too; and as this is made up of the two factors,
power to maintain individual life and power to multiply,
these cannot do other than vary inversely: one must de-
crease as the other increases.

It needs but to conceive the results of nonconformity to
this law, to see that every species must either conform to it
or cease to exist. Suppose, first, a species whose individuals,

422 LAWS OF MULTIPLICATION.

having but small self-preservative powers, are rapidly cU
stroyed, to be at the same time without reproductive powei
proportionately great. The defect of fertility, if extreme
will result in the death of one generation before another hi
grown up. If less extreme, it will entail a scarcity sucl
that in the next generation sexual congress will be too infn
quent to maintain even the small number which remains ; an<
the race will dwindle with increasing rapidity. If still less
extreme, the consequent degree of sparseness, while not so
great as to prevent an adequate number of procreative unions,
will be so great as to render special food abundant and
special enemies few — will thus diminish the destructive
forces so much that the self-preservative forces will become
relatively great: so great, relatively, that when combined
with the small ability to propagate the species, they will
suffice to balance the small destructive forces. Suppose,
next, a species whose individuals have high powers of self-
preservation, while they have powers of multiplication much
beyond what is needful. The excess of fertility, if extreme,
will cause sudden extinction of the species by starvation.
If less extreme, it must produce a permanent increase in
the number of the species; and this, followed by intenser
competition for food and augmented number of enemies, will
involve such an increase of the dangers to individual life,
that the great self-preserving powers of the individuals will
not be more than sufficient to cope with them. That is to
say, if the fertility is relatively too great, then the ability to
maintain individual life inevitably becomes smaller, relatively
to the requirements; and the inverse proportion is thus
established.

So that when, from comparing the different states of the
same species, we go on to compare the states of different
species, we see that there is an analogous adjustment — ana-
logous in the sense that great mortality is associated with
great multiplication, and small mortality with small multi-
plication. And we see that the unlikeness of the cases con-

A PRIORI PRINCIPLE. 423

sists merely in this, that what is a temporary relation in the
one is a permanent relation in the other.

§ 323. For the moment it does not concern us to inquire
what is the origin of this permanent relation. That which
we have now to note, is simply that in some way or other
there must be established an inverse proportion between the
power to sustain individual life and the power to produce
new individuals. .Whether or not this permanent relation
is self-adjusting in long periods of time, as the temporary
relation is self-adjusting in short periods of time, is a
separate question. The purpose of this chapter is fulfilled by
showing that such a permanent relation must exist.

But having recognized the a priori principle that in races
which continuously survive, the forces destructive of race
must be equilibrated by the forces preservative of race; and
that, supposing these are constant, there must be an inverse
proportion between self-preservation and race-preservation;
we may go on to inquire how this relation, necessary in
theory, arises in fact. Leaving out the untenable hypothesis
of a supernatural pre-adjustment, we have to ask in what
way an adjustment comes about as a result of Evolution.
Is it due to the survival of varieties in which the proportion
of fertility to mortality happens to be the best? Or is the
fertility adapted to the mortality in a more direct way? To
these questions let us now address ourselves.

CHAPTER III.

OBVERSE A PRIORI PRINCIPLE.

§ 324. When dealing with its phenomena inductively, we
saw that however it may be carried on, Genesis " is a process
of negative or positive disintegration ; and is thus essentially
opposed to that process of integration which is the primary
process in individual evolution." (§ 76.) Each new indivi-
dual, whether separated as a germ or in some more-developed
form, is a deduction from the mass of a pre-existing indivi-
dual or of two pre-existing individuals. Whatever nutritive
matter is stored up along with the germ, if it be deposited in
the shape of an egg, is so much nutritive matter lost to the
parent. No drop of blood can be absorbed by the foetus, nor
any draught of milk sucked by the young when born, without
taking from the mother tissue-forming and force-evolving
materials to an equivalent amount. And all subsequent sup-
plies given to progeny, if they are nurtured, involve, to a
parent or parents, so much waste in exertion which does not
bring its return in assimilated food.

Conversely, the continued aggregation of materials into
one organism, renders impossible the formation of other or-
ganisms out of those materials. As much assimilated food
as is united into a single whole, is so much assimilated food
withheld from a plurality of wholes which might else have
been produced. Given the absorbed nutriment as a constant
quantity, and the longer the building of it up into a con-
crete shape goes on, the longer must be postponed any build-
424

OBVERSE A PRIORI \ PRINCIPLE. 425

ing of it up into discrete shapes. And, similarly, the larger
the proportion of matter consumed in the functional actions
of parents, the smaller must be the proportion of matter
which can remain to establish and support the functional
actions of offspring.

Though the necessity of these universal relations is toler-
ably obvious as thus stated generally, it will be useful to
dwell for a brief space on their leading aspects.

§ 325. That disintegration which constitutes genesis, may
be such as to disperse entirely the aggregate which integra-
tion has previously produced — the parent may dissolve wholly
into progeny. This dissolution of each aggregate into two
or many aggregates, may occur at very short intervals, in
which case the bulk attained can be but extremely small; or
it may occur at longer intervals, in which case a larger bulk
may be attained.

Instead of quickly losing its own individuality in the
individualities of its offspring, each member of the race may,
after growing for a time, have portions of its substance begin
to develop into the parental shape and presently detach
themselves; and the parent, maintaining its own identity,
may continue indefinitely so to produce young ones. But
clearly, the earlier it commences doing this, and the more
rapidly it does it, the sooner must the increase of its own
bulk be stopped.

Or again, growth and development continuing for a long
period without any deduction of materials, an individual of
considerable size and organization may result; and then the
abstraction of substance for the formation of new individuals,
or rather the eggs of them, may be so great that as soon as
the eggs are laid the parent dies of exhaustion — dies, that is,
from an excessive loss of the nutritive matters needed for its
own activities.*

* I was here thinking only of the cases which are general among insects,
but it seems that vertebrate animals, too, furnish cases. Mr. Cunningham
writes : — " There is a curious instance of this in the Conger : the female

426 LAWS OF, MULTIPLICATION.

Once more, the deduction of materials for the propagation
of the species may be postponed long enough to allow of great
bulk and complex structure being attained. The procreative
subtraction then setting in, while it checks and presently
stops growth, may be so moderate as to leave vital capital
sufficient to carry on the activities of the parent; may go
on as long as parental vigour suffices to furnish, without fatal
result, the materials needed to produce young ones ; and may
cease when such a surplus cannot be supplied, leaving the
parental life to continue.

§ 326. The opposite side of this antagonism has also
several aspects. Progress of organic evolution may be shown
in increased bulk, in increased structure, in increased amount
or variety of action, or in combinations of these; and under
any of its forms this carrying higher of each individuality,
implies a correlative retardation in the establishment of new
individualities.

Other things equal, every normal addition to the bulk of
an organism is an augmentation of its life.* Besides being an
advance in integration, it implies a greater total of activities
gone through in the assimilation of materials ; and it implies,
thereafter, a greater total of the vital changes taking place
from moment to moment in all parts of the enlarged mass.
Moreover, while increased size is thus, in so far, the expres-
sion of increased life, it is also, where the organism is active,
the expression of increased ability to maintain life — increased
strength. Aggregation of substance is almost the only mode
in which self-preserving power is shown among the lowest
types; and even among the highest, sustaining the body in
its integrity is that in which self-preservation fundamentally
consists — is the end which the widest intelligence is indi-

grows to 6 or 7 feet long and a weight of 60 lbs. and upwards and then
ceases to feed for 6 months while the eggs develop, and when the eggs are
shed dies."

* I say " normal " for the purpose of excluding not only morbid growths
but excess of fat.

OBVERSE A PRIORI PRINCIPLE. 427

rectly made to subserve. While, on the one hand, the in-
crease of tissue constituting growth is conservative both in
essence and in result; on the other hand, decrease of tissue,
either from injury, disease, or old age, is in both essence and
result the reverse. And if so, every addition to individual
life thus implied, necessarily delays or diminishes the casting
off of matter to form new individuals.

Other things equal, too, a greater degree of organization
involves a smaller degree of that disorganization shown by
the separation of reproductive gemma? and germs. Detach-
ment of a living portion or portions from what was previously
a living whole, is a ceasing of co-ordination ; and is therefore
essentially at variance with that establishment of greater co-
ordination which is achieved by structural development. In
the extreme cases where a living mass is continually dividing
and subdividing, it is manifest that there cannot arise much
physiological division of labour; since progress towards
mutual dependence of parts is prevented by the parts be-
coming independent. Contrariwise, it is equally clear that
in proportion as the physiological division of labour is carried
far, the separative process must be localized in some com-
paratively small portion of the organism, where it may go
on without affecting the general structure — must become
relatively subordinate. The advance that is shown by
greater heterogeneity, must be a hindrance to multiplication
in another way. For organization entails cost. That trans-
fer and transformation of materials implied by differentia-
tion, can be effected only by expenditure of force; and this
supposes consumption of digested and absorbed food, which
might otherwise have gone to make new organisms, or the
germs of them. Hence, that individual evolution which
consists in progressive differentiation, as well as that which
consists in progressive integration, necessarily diminishes
that species of dissolution, general or local, which propaga-
tion of the race exhibits.

In active organisms we have yet a further opposition

428 LAWS OF MULTIPLICATION.

between self-maintenance and maintenance of the race. All
motion, sensible and insensible, generated by an animal for
the preservation of its life, is motion liberated from decom-
posed nutriment — nutriment which, if not thus decomposed,
would have been available for reproduction; or rather —
might have been replaced by nutriment fitted for reproductive
purposes, absorbed from other kinds of food. Hence, in pro-
portion as the activities increase — in proportion as, by its
more varied, complex, rapid, and vigorous actions, an animal
gains power to support itself and to cope with surrounding
dangers, it must lose power to propagate.

§ 327. How may this antagonism be best expressed in a
brief way? If self-preservation displayed itself in the
highest organisms, as it does in the lowest, in little else but
continuous growth ; and if race-preservation consisted always,
as it does often, of nothing beyond detachment of portions
from the parental mass; then the antagonism would be,
throughout, the obviously-necessary one of integration and
disintegration. Maintenance of the individual and propaga-
tion of the species, being respectively aggregative and sepa-
rative, it would be as self-evident that they vary inversely,
as it is self-evident that addition and subtraction undo one
another. But though the simplest types show us the opposi-
tion of self-maintenance and race-maintenance almost wholly
under this form; and though higher types, up to the most
complex, exhibit it to a great extent under this form ; yet, as
we have just seen, this is not its only form. The total
material monopolized by the individual and withheld from
the race, must be stated as the quantity united to form its
fabric, plus the quantity expended in differentiating its
fabric, plus the quantity expended in its self-conserving
actions. Similarly, the total material devoted to the race at
the expense of the individual, includes that which is directly
subtracted from the parent in the shape of egg or foetus, plus
that which is directly subtracted in the shape of milk, plus

OBVERSE A PRIORI PRINCIPLE. 429

that which is indirectly subtracted in the shape of matter
consumed in exertions for fostering the young. Hence
this inverse variation is not expressible in simple terms of
aggregation and separation. As we advance to more highly-
evolved organisms, the total cost of an individual becomes
very much greater than is implied by the amount of tissue
composing it. So, too, the total cost of producing each new
individual becomes very much greater than that of its mere
substance. And it is between these two total costs that the
antagonism exists.

We may, indeed, reduce the antagonism to a form com-
prehensive of all cases, if we consider it as existing between
the sums of the forces, latent and active, used for the two
purposes. The molecules which make up a plant or animal,
have been formed by the absorption of forces directly or
indirectly derived from the Sun; and hence the quantity of
matter raised to the form called organic, which a plant or
animal presents, is equivalent to a certain amount of force.
Another amount of force is expressed by the totality of its
differentiations. A further amount of force is that dissipated
in its actions. And in these three amounts added together,
we have the whole expense of the individual life. So, too,
the whole expense of establishing each new individual in-
cludes— first the forces latent in the substance composing
it when born or hatched; second the forces latent in the
prepared nutriment afterwards supplied ; and third the forces
expended in feeding and protecting it. These two sets of
forces being taken from a common fund, it is manifest that
either set can increase only by decrease of the other. If, of
the force which the parent obtains from the environment,
much is consumed in its own life, little remains to be con-
sumed in producing other lives; and, conversely, if there is
a great consumption in producing other lives, it can only be
where comparatively little is reserved for parental life.

Hence, then, Individuation and Genesis are necessarily
antagonistic. Grouping under the word Individuation all

430 LAWS OF MULTIPLICATION.

processes by which individual life is completed and main-
tained; and enlarging the meaning of the word Genesis so
as to include all processes aiding the formation and perfecting
of new individuals; we see that the two are fundamentally
opposed. Assuming other things to remain the same —
assuming that environing conditions as to climate, food,
enemies, &c, continue constant; then, inevitably, evei
higher degree of individual evolution is followed by a lowei
degree of race-multiplication, and vice versa. Progress
bulk, complexity, or activity, involves retrogress in fertility
and progress in fertility involves retrogress in bulk, com-
plexity, or activity.

This statement needs a slight qualification. For reasons
to be hereafter assigned, the relation described is never com-
pletely maintained; and in the small departure from it, we
shall find a remarkable self-acting tendency to further the
supremacy of the most-developed types. Here, however, this
hint must suffice : explanation would carry us too far out of
our line of argument. For the present it will not lead us
astray if we regard this inverse variation of Individuation
and Genesis as exact.

§ 328. Thus, then, the condition which each race must
fulfil if it is to survive, is a condition which, in the nature of
things, it ever tends to fulfil. In the last chapter we saw
that a species cannot be maintained unless the power to
preserve individual life and the power to propagate other
individuals vary inversely. And here we have seen that,
irrespective of an end to be subserved, these powers cannot
do other than vary inversely. On the one hand, given
certain totality of destroying forces with which the species
has to contend; and in proportion as its members have
severally but small ability to resist these forces, it is requisite
that they should have great ability to form new individuals,
and vice versa. On the other hand, given the quantity of
force, absorbed as food or otherwise, which the species can

OBVERSE A PRIORI PRINCIPLE. 431

use to counterbalance these destroying forces ; and in propor-
tion as much of it is expended in preserving the individual,
little of it can be reserved for producing new individuals,
and vice versa. There is thus complete accordance between
the requirements considered under each aspect. The two
necessities correspond.

We might rest on these deductions and their several corol-
laries. Without going further we might with safety assert
the general truths that, other things equal, advancing evolu-
tion must be accompanied by declining fertility; and that, in
the highest types, fertility must still further decrease if evolu-
tion still further increases. We might be sure that if, other
things equal, the relations between an organism and its en-
vironment become so changed as permanently to diminish
the difficulties of self-preservation, there will be a permanent
increase in the rate of multiplication; and, conversely, that
a decrease of fertility will result where altered circumstances
make self-preservation more laborious.

But we need not content ourselves with these a priori
inferences. If they are true, there must be an agreement
between them and the observed facts. Let us see how far
such an agreement is traceable.

CHAPTER IV.

DIFFICULTIES OF INDUCTIVE VERIFICATION".

§ 329. Were all species subject to the same kinds and
amounts of destructive forces, it would be easy, by comparing
different species, to test the inverse variation of Individuation
and Genesis. Or if either the power of self-preservation or
the power of multiplication were constant, there would be
little difficulty in seeing how the other changed as the
destroying forces changed. But comparisons are nearly
always partially vitiated by some want of parity. Each
factor, besides being variable as a whole, is compounded of
factors that are severally variable. Not simply is the sum
of the 'forces destructive of race different in every case; and
not simply are both sets of forces preservative of race unlike
in their totalities in every case ; but each is made up of actions
that bear such changing proportions to one another as to
prevent any positive estimation of its amount.

Before dealing with the facts as well as we can, it will be
best to glance at the chief difficulties; so that we may see
the kind of verification which is alone possible.

§ 330. Either absolutely, or relatively to any species, every
environment differs more or less from every other.

There are the unlikenesses of media — air, water, earth,
organic matter; severally involving special resistances to
movement, and special losses of heat. There are the con-
432

DIFFICULTIES OF INDUCTIVE VERIFICATION. 433

trasts of climate : here great expenditure for the maintenance
of temperature is needed, and there very little; in one
zone an organism is supplied with abundant light all the
year round, and in another only for a few months; this
region yields an almost unfailing supply of water, while that
entails the exertion of travelling many miles every night for a
draught.

Permanent differences in the natures and distributions of
aliment greatly interfere with the comparisons. The Swal-
low goes through more exertion than the Sparrow in securing
a given weight of food; but then their foods are dissimilar
in nutritive qualities. There is a want of parallelism between
the circumstances of those herbivores which live where the
plains are annually covered for a time with rich herbage, but
afterwards become parched up, and of those inhabiting more
temperate regions. Insects whose larvae feed on an abundant
plant, as do several of the genus Vanessa on the Nettle, have
practically an environment very unlike that of insects such
as Deilephila Euphorbia, whose larvae feed on a comparatively
rare plant — the Sea-Spurge.

Again, comparisons between creatures otherwise akin in
their constitutions and circumstances, are hindered by ine-
qualities in their relations to enemies. Two animals, of
which one is predatory and has no foes but parasites while
the other is much pursued, cannot properly be contrasted
with a view to determining the influence of size or com-
plexity.

Without multiplying instances, it will be clear enough
then that the aggregate of destructive actions, positive and
negative, which each species has to contend with, is so
^indefinable in the amounts and kinds of its components,
that nothing beyond a vague idea of its relative total can be
formed.

§ 331. Besides these immense variations in the outer
actions to be counter-balanced, there are immense variations

74

434 J^AWS OF MULTIPLICATION.

in the inner actions required to counter-balance them. Even
were species similarly conditioned, self-preservation would
require of them extremely unlike expenditures of force.

The cost of locomotion increases in a greater ratio than
the size. In virtue of the law that the weights of animals in-
crease as the cubes of their dimensions, while their powers of
bearing strains increase only as the squares of their dimen-
sions (§46), preservation of its various attitudes requires a
large animal to consume more substance in proportion to its
weight, than it requires a small animal to consume ; and there
results, other things equal, a difficulty of self-maintenance
which augments in a more rapid ratio than the bulk. Nor
must we overlook the further complication, that among
aquatic creatures the variation of resistance of the medium
tends to produce an opposite effect.

Again, the heat-consumption is a changing element in the
total expense of self-preservation. Creatures which have tem-
peratures scarcely above that of the air or water, may, other
things equal, accumulate more surplus nutriment than crea-
tures which have to keep their bodies warm spite of the con-
tinual loss by radiation and conduction. This difference of
cost is modified by the presence or absence of natural cloth-
ing; and it is also modified by unlikenesses of size. Here
the bulky animals have the advantage : small masses cooling
more rapidly than large ones.

Dissimilarities of attack and defence are also causes of
variation in the outlay for self-maintenance. A creature that
has to hunt, as compared with another that gets a sufficiency
of prey by lying in wait, or a creature that escapes by speed
as compared with another that escapes by concealment,
obviously leads a life that is physiologically more costly.
Animals which protect themselves passively, as the Hedge-hog
by its spines or as the Skunk and the Musk-rat by their in-
tolerable odours, are relatively economical; and have the
more vital capital for other purposes.

Amplification is needless. These instances will show that

DIFFICULTIES OF INDUCTIVE VERIFICATION. 435

anything beyond very general conceptions of the individual
expenditures in different cases, cannot be reached.

§ 332. Still more entangled are we among qualifying con-
siderations when we contrast species in their powers of multi-
plication. The total cost of Genesis admits of even less
definite estimation than does the total cost of Individua-
tion. I do not refer merely to the truth that the degree of
fertility depends on four factors — the age of commencing
reproduction, the number in each brood, the frequency of the
broods, and the time during which broods continue to be
repeated. There are many further obstacles in the way of
comparisons.

Were all multiplication carried on sexually, the problem
would be less involved; but there are many kinds of asexual
multiplication alternating with the sexual. This asexual
multiplication is in some cases perpetual instead of occa-
sional; and often has more forms than one in the same
species. The result is that we have to compare what is here
a periodic process with what is elsewhere a cyclical process
partly continuous and partly periodic: the calculation of
fertility in this last case being next to impossible.

We have to avoid being misled by the assumption that the
cost of Genesis is measured by the number of young produced,
instead of being measured, as it is, by the weight of nutri-
ment abstracted to form the young, plus the weight con-
sumed in caring for them. This total weight may be very
diversely apportioned. In contrast to the Cod with its
millions of small ova spawned without protection, we may
put the Hippocampus, or the Pipe-fish, with its few relatively-
large ova carried about by the male in a caudal pouch, or
seated in hemispherical pits in its skin; or we may put the
still more remarkable genus Arius, and especially Arius
Boakeii — a fish some six or seven inches long, which produces
ten or a dozen eggs 5 — 10 mm. in diameter, that are carried
by the male in his mouth till they are hatched. Here though

436 LAWS OF MULTIPLICATION.

the degrees of fertility, if measured by the numbers of
fertilized germs deposited, are extremely unlike, they are
less unlike if measured by the numbers of young which are
hatched and survive long enough to take care of themselves ;
nor will the tax on the parent-Cod seem so immensely dif-
ferent from that on the parent-Arms, if the masses of the
ova, instead of their numbers, are compared. Again,

while sometimes the parental loss is little else but the matter
deducted to form eggs, &c, at other times it takes the
shape of a small direct deduction joined with a large indirect
outlay. The Mason-wasp furnishes a typical instance. In
journeyings hither and thither to fetch bit by bit the
materials for building a cell; in putting together these
materials, as well as in secreting glutinous matter to act as
cement; and then, afterwards, in the labour of seeking for,
and carrying, the small caterpillars with which it fills up the
cell to serve its larva with food when it emerges from the
egg; the Mason- wasp expends more substance than is con-
tained in the egg itself. And this supplementary expenditure
is manifestly so great that but few eggs can be housed and
provisioned.

Estimates of the cost of Genesis are further complicated
by variations in the ratio borne by the two sexes. Among
Fishes the mass of milt approaches in size the mass of spawn ;
but among higher Vertebrata the substance lost by the one
sex in the shape of sperm-cells is small compared with that
lost by the other sex in the shape of albumen stored-up in
the eggs, or blood supplied to the foetus, or milk given to the
young. Then there come the differences of indirect tax
on males and females. While, frequently, the fostering of
the young devolves entirely on the female, occasionally the
male undertakes it wholly or in part. After building a
nest, the male Stickleback guards the eggs till they are
hatched; as does also the great Silurus giants for some forty
days, during which he takes no food. And then, among most
birds, we have the male occupied in feeding the female during

DIFFICULTIES OF INDUCTIVE VERIFICATION. 437

incubation, and the young afterwards. Evidently all these
differences affect the proportion between the total cost of re-
production and the total cost of individuation.

Whether the species is monogamous or polygamous, and
whether there are marked differences of size or of structure
between males and females, are also questions not to be over-
looked. If there are many females to one male, the total
quantity of assimilated matter devoted by each generation to
the production of a new generation, is greater than if there
is a male to each female. Similarly, where the requirements
are such that small males will suffice, the larger quantity of
food left for the females makes possible a greater surplus
available for reproduction. 'Another cause has a like effect.
Where the habits of the race render it needless that both
sexes should have developed powers of locomotion — where,
as in the Glow-worm and sundry Lepidoptera, the female is
wingless while the male has wings — the cost of Individuation
not being so great for the species as a whole, there arises a
greater reserve for Genesis: the matter which would other-
wise have gone to the production of wings and the using of
them, may go to the production of ova.

Other complications, as those which we see in Bees and
Ants, might be dwelt on; but the foregoing will amply serve
the intended purpose.

§ 333. To ascertain by comparison of cases whether Indi-
viduation and Genesis vary inversely, is thus an undertaking
so beset with difficulties, that we might despair of any satis-
factory results, were not the relation too marked a one to be
hidden even by all these complexities. Species are so ex-
tremely contrasted in their degrees of evolution, and so
extremely contrasted in their rates of multiplication, that the
law of relation between these traits becomes unmistakable
when the evidence is looked at in its ensemble. This we shall
soon find on ranging in order a number of typical cases.

In doing this it will be convenient to neglect, temporarily,

438 LAWS OF MULTIPLICATION.

all unlikenesses among the circumstances in which organ-
isms are placed. At the outset, we will turn our attention
wholly to the antagonism displayed between the integrative
process which results in individual evolution and the disinte-
grative process which results in multiplication of individuals ;
and this we will consider first as we see it under the several
forms of agamogenesis, and then as we see it under the
several forms of gamogenesis. We will next look at the
antagonism between propagation and that evolution which is
shown by increased complexity. And then we will consider
the remaining phase of the antagonism, as it exists between
the degree of fertility and the degree of evolution expressed
by activity.

Afterwards, passing to the varying relations between
organisms and their environments, we will note how relative
increase in the supply of food, or relative decrease in the
quantity of force expended by the individual, entails relative
increase in the quantity of force devoted to multiplication,
and vice versa.

Certain minor qualifications, together with sundry impor-
tant corollaries, may then be entered upon.

CHAPTER V.

ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS.

§ 334. When illustrating, in Part IV, the morphological
composition of plants and animals, there were set down in
groups, numerous facts which we have here to look at from
another point of view. Then we saw how, by union of small
simple aggregates, there are produced large compound aggre-
gates. Now we have to observe the reactive effect of this
process on the relative numbers of the aggregates. Our pre-
sent subject is the antagonism of Individuation and Genesis
as seen under its simplest form, in the self-evident truth that
the same quantity of matter may be divided into many small
wholes or few large wholes ; but that number negatives large-
ness and largeness negatives number.

In setting down some examples we may conveniently
adopt the same arrangement as before. We will look at the
facts as they are presented by vegetal aggregates of the first
order, of the second order, and of the third order; and then
as they are presented by animal aggregates of the same three
orders.

§ 335. The ordinary unicellular plants are at once micro-
scopic and enormously prolific. The often cited Sphcerella
nivalis, which shows its immense powers of multiplication by
reddening wide tracts of snow in a single night, does this by
developing in its cavity a brood of young cells, which, being

439

440 LAWS OF MULTIPLICATION.

presently set free by the bursting of the parent-cell, severally
grow and quickly repeat the process. The like occurs amon|
sundry of those kindred forms of minute Algce which, fy
their enormous numbers, sometimes suddenly change pools t(
an opaque green. So, too, the Desmidiacece often multiply s(
greatly as to colour the water; and among the Diatomacec
the rate of genesis by self-division, " is something really ex-
traordinary. So soon as a frustule is divided into two, eacl
of the latter at once proceeds with the act of self-division ; s<
that, to use Professor Smith's approximative calculation oi
the possible rapidity of multiplication, supposing the process
to occupy, in any single instance, twenty-four hours, ' we
should have, as the progeny of a single frustule, the amazing
number of one thousand millions in a single month/ " In
these cases the multiplication is so carried on that the parent
is lost in the offspring — the old individuality disappears either
in the swarms of zoospores it dissolves into, or in the two
or four new individualities simultaneously produced by fis-
sion. Vegetal aggregates of the first order, have, how-
ever, a form of agamogenesis in which the parent individual-
ity is not lost : the young cells arise from the old cells by ex-
ternal gemmation. This process, too, repeated as it is at
short intervals, results in immense fertility. The Yeast-
fungus, which in a few hours thus propagates itself through-
out a large vat of wort, offers a familiar example.

In certain compound forms that must be classed as plants
of the second order of aggregation, though very minute ones,
self-division similarly increases the numbers at high rates.
The Sarcina ventriculi, a parasitic plant which infests the
stomach and swarms afresh as fast as previous swarms are
vomited, shows us a spontaneous fission of clusters of cells.
An allied mode of increase occurs in Gonium perforate: each
cell of the cluster resolving itself into a secondary cluster,
and the secondary clusters then separating. " Supposing,
which is very probable, that a young Gonium after twenty-
four hours is capable of development by fission, it follows

GROWTH AND ASEXUAL GENESIS 441

thai under favourable conditions a single colony may on the
second day develop 16, on the third 256, on the fourth 4,096,
and at the end of a week 268,435,456 other organisms like
itself." In the Volvocinece this continual dissolution of a
primary compound individual into secondary compound indi-
viduals, is carried on endogenously, and on a modified system:
some only of the component cells giving origin to young
colonies, and the parent bursting to liberate them. The
numbers arising by this method also, are sometimes so great
as to tint large bodies of water. More fully estab-

lished and organized aggregates of the second order, such as
the higher Thallophytes and the lower Archegoniates, do not
sacrifice their individualities by fission; but nevertheless, by
the kindred process of gemmation, are continually hindered
in the increase of their individualities. The gemmae called
tetraspores are cast off in great numbers by the marine Algce.
Among those simple Jungermanniacece which consist of single
fronds, the young ones that bud out grow for a time in con-
nexion with their parents, send rootlets from their under
sides into the soil, and presently separate themselves — a
habit which augments the number of individuals in propor-
tion as it checks their growths.

Plants of the third order of composition, arising by arrest
of this separation, exhibit a further corresponding decrease
in the abundance of the aggregates formed. Archegoniates of
inferior types, in which the axes produced by integration of
fronds are but small and feeble, are characterized by the
habit of throwing off bulbils — bud-shaped axes which, falling
and taking root, add to the number of distinct individuals.
This agamic multiplication, very general among the Mosses
and their kindred, and not uncommon under a modified form
in such higher types as the Ferns, many of which produce
young ones from the surfaces of their fronds, becomes very
unusual among Phamogams. The detachment of bulbils,
though not unknown among them, is exceptional. And while
it is true that some flowering plants, as the Strawberry,

442 LAWS OF MULTIPLICATION.

multiply by a process allied to gemmation, yet this is not
characteristic of the class. A leading trait of these highest
groups, to which the largest members of the vegetal king-
dom belong, is that agamogenesis has so far ceased that it
does not usually originate independent plants. Though the
axes which, budding one out of another, compose a tree,
are the equivalents of asexually-produced individuals; yet
the asexual production of them stops short of separation.
These vast integrations arise where spontaneous disintegra-
tion, and the multiplication effected by it, have come to an
end.

Thus, not forgetting that certain Phaenogams, as Begonia
phyllomaniaca, revert to quite primitive modes of increase,
we may hold, it as beyond question that while among the
most minute plants asexual multiplication is universal, and
produces enormous numbers in short periods, it becomes step
by step more restricted in range and frequency as we ad-
vance to large and compound plants; and disappears so
generally from the highest and largest, that its occurrence is
regarded as anomalous.

§ 336. Parallel examples furnished by animals make clear
the purely quantitative nature of this relation under its origi-
nal form. Among the Protozoa, as among the Protophyta,
there occurs that process by which the individuality of the
parent is wholly lost in producing offspring — the breaking
up of the parental mass into a number of germs. Some of
the Infusoria, as for instance those of the genus Kolpoda
and several allied genera, become encysted and subsequently
break up into young ones. The more familiar mode

of increase among these animal-aggregates of the first order,
by fission, though it sacrifices the parent individuality by
merging it in the individualities of the two produced, sacri-
fices it less completely than does the dissolution into a great
number of germs. Occurring, however, as this fission does,
very frequently, and being completed, in some cases that

GROWTH AND ASEXUAL GENESIS. 443

have been observed, in the course of half-an-hour, it results
in immensely-rapid multiplication. If all its offspring sur-
vive, and continue dividing themselves, a single Paramcecium
is said to be capable of thus originating 268 millions in the
course of a month.* Nor is this the greatest known rate
of increase. Another animalcule, visible only under a high
magnifying power, "is calculated to generate 170 billions
in four days." f And these enormous powers of propagation
are accompanied by a minuteness so extreme, that of some
species one drop of water would contain as many individuals
as there are human beings on the Earth ! Even if we allow
a large margin for exaggeration in these estimates, it is
beyond question that among these smallest of animals the
rate of asexual multiplication is immensely the greatest;
and this suffices for the purposes of argument.

Of animal aggregates belonging to the second order, that
multiply asexually with rapidity, the familiar Polypes
furnish conspicuous examples. By gemmation in most cases,
in other cases by fission, and in some cases by both, the
agamogenesis is carried on among these tribes. As shown in
Fig. 148, the budding of young ones from the parent Hydra
is carried on so actively, that before the oldest of them is
cast off half-a-dozen or more others have reached various
stages of growth; and even while still attached, the first-
formed of the group have commenced budding out from their
sides a second generation of young ones. In the Hydra tuba

* To meet a possible criticism it should bo remarked that this calcula-
tion assumes that the power of asexual reproduction is not exhausted by
the end of the month. It has been found that " the successive fissions of
Paramcecium cannot continue indefinitely. After some hundreds of genera-
tions the products of fission are small, have no mouth, and die unless before
this they have been allowed to conjugate with individuals of another brood."
It may, however, be fairly taken for granted that " some hundreds of genera-
tions " would take longer than a month.

f Even this number is far exceeded. Dr. Edward Klein, in a lecture he
gave at the Royal Institution on June 2, 1898, asserted that 246 bacteria in
a cubic centimetre of nutritive liquid would multiply 'to 20,000,000 in the
course of twenty-four hours : a rate which, at the end of the third day, would
give, as the offspring of one individual, 537,367,797,000,000.

444 LAWS Of MULTIPLICATION.

this gemmiparous multiplication is from time to time inter-
rupted by a transverse splitting-up of the body into segments,
which successively separate and swim away: the result of
the two processes being that, in the course of a season, there
are produced from a single germ great numbers of young
Medusas, which are the adult or sexual forms of the species.
Eespecting ccelenterate animals of this degree of composi-
tion, it may be added that when we ascend to the larger
kinds we find asexual genesis far less active. Though com-
parisons are interfered with by differences of structure and
mode of life, yet the contrasts are too striking to have their
meanings much obscured. If, for instance, we take a solitary
Actinozoon and a solitary Hydrozoon, we see that the rela-
tively-great bulk of the first, goes along with a relatively-
slow agamogenesis. The common Sea-anemones are but
occasionally observed to undergo self-division : multiplication
by budding being in some cases largely followed, but their
numbers are not rapidly increased by either process. A
higher class of secondary aggregates exemplifies the same
general truth with a difference. In the smaller members the
agamogenesis is incomplete, and in the larger it disappears.
The gemmation of the minute Polyzoa, though it does not
end in the separation of the young individuals, habitually
goes to the extent of producing families of partially-inde-
pendent individuals ; but their near ally, the Phoronis, which
immensely exceeds them in size, is solitary and not gemmi-
parous. So, too, is it with the Ascidioida. And then among
the true Mollusca, which are relatively large, no such thing
is known as fission or gemmation.

Take next the Annulosa, including under this title the
Annelida and Arthropoda. When treating of morphological
composition, reasons were given for the belief that the annu-
lose animal is an aggregate of the third order, the segments
of which, produced one from another by gemmation, ori-
ginally became separate; but by progressive integration, or
arrested disintegration, there resulted a type in which many

GROWTH AND ASEXUAL GENESIS. 445

such segments were permanently united (§§ 205-7 and note
to § 207). Part of the evidence there assigned, is evidence to
be here repeated in illustration of the direct antagonism of
Growth and Asexual-Genesis. We saw how, among the
lower Annelids, the string of segments produced by gemma-
tion presently divides transversely into two strings; and
how, in some cases, this resolution of the elongating string
of segments into groups that are to form separate individuals,
goes on so actively that as many as six groups are found in
different stages of progress to ultimate independence — a fact
implying a high rate of fissiparous multiplication.* Then we
saw that, in the superior annulose types, distinguished in the
mass by including the larger species, fission does not occur.
The higher Annelids do not propagate in this way; there is
no known case of new individuals being so formed among
the Myriapoda; nor do the Crustaceans afford us a single
instance of this primordial mode of increase. It is,

indeed, true that while articulate animals never multiply
a sexually after this simplest method, and while they are
characterized in the mass by the cessation of agamogenesis of
every kind, there nevertheless occur in a few of their small
species, those higher forms of agamogenesis known as parthe-
nogenesis and pseudo-parthenogenesis; and that by these
some of them multiply very rapidly. Hereafter we shall
find, in the interpretation of these anomalies, further support
for the general doctrine.

To the above evidence has to be added that which the
Vertebrata present. This may be very briefly summed up.
On the one hand this class, whether looked at in the aggre-
gate or in its particular species, immensely exceeds all other
classes in the sizes of its individuals ; and on the other hand,
agamogenesis under any form is absolutely unknown in it.
If it be said that budding occurs among the Tunicata which,

* It has since been shown that in Myrianida fasciata as many as 29 at-
tached groups exist. See Cambridge Natural History, Vol. II, Worms, Roti-
fers and Polyzoa, p. 280.

446 ^aws of multiplication.

under the common title of Chordata, are included in the same
phylum with the Vertebrata, then it may firstly be replie
that those types which have no vertebra cannot properly
called Vertebrata. and secondly that if, as being Chordata,
they must be recognized, then the exception which they pn
sent further illustrates the truth that agamogenetic multi-
plication occurs only in creatures small in size, or low in
structure, or both.

§ 337. Such are a few leading facts serving to show how
deduction is inductively verified, in so far as the antagonism
between Growth and Asexual Genesis is concerned. Ii
whatever way we explain this opposition of the integrative
and disintegrative processes, the facts and their implications
remain the same. Indeed we need not commit ourselves to
any hypothesis respecting the physical causation. It suffices
to recognize the results under their most general aspects.
We cannot help admitting there are at work these two anta-
gonist tendencies to aggregation and separation; and we
cannot help admitting that the proportion between the aggre-
gative and separative tendencies, must in each case determine
the relation between increase in bulk of the individual and
increase of the race in number.

The antithesis is as manifest a posteriori as it is necessary
a priori. While the minutest organisms multiply asexually
in their billions; while the Infusoria thus multiply in their
millions; while the small compound types next above them
thus multiply in their thousands; while larger and more
compound types thus multiply in their hundreds and their
tens; the largest types do not thus multiply at all. Con-
versely, those which do not multiply asexually at all, are a
billion or a million times the size of those which thus mul-
tiply with greatest rapidity; and are a thousand times, or a
hundred times, or ten times the size of those which thus
multiply with less and less rapidity. Without saying that
this inverse proportion is regular, which, as we shall here-

GROWTH AND ASEXUAL GENESIS. 447

after see, it cannot be, we maj unhesitatingly assert its
average truth. That the smallest organisms habitually re-
produce asexually with immense rapidity; that the largest
organisms never reproduce at all in this manner; and that
between these extremes there is a general decrease of asexual
reproduction along with an increase of bulk; are proposi-
tions which admit of no dispute.

CHAPTER VI.

ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS.

§ 338. In so far as it is a process of separation, sexual
genesis is like asexual genesis ; and is therefore, equally with
asexual genesis, opposed to that aggregation which results in
growth. Whether deduction is made from one parent or
from two, whether it is made from any part of the body
indifferently or from a specialized part, or whether it is made
directly or indirectly, it remains in any case a deduction;
and in proportion as it is great, or frequent, or both, it must
restrain the increase of the individual.

Here we have to group together the leading illustrations
of this truth. We will take them in the same order as
before.

§ 339. The lowest vegetal forms, or rather, we may say,
those forms which we cannot class as either distinctly vegetz
or distinctly animal, show us a process of sexual multiplica-
tion that differs much less from the asexual process than ii
the higher forms. The common character which distinguish^
sexual from asexual genesis, is that the mass of protoplasi
whence a new generation is to arise, has been produced by the
union of two portions of matter which were before more wide-
ly separated. I use this general expression because, among
the simplest Algce, this is not invariably matter supplied
by different individuals: certain Diatomacece exhibit within
a single cell, the formation of a sporangium by a drawing
448

GROWTH AND SEXUAL GENESIS. 449

together of the opposite halves of the endochrome into a
ball. Mostly, however, sporangia are products of conjuga-
tion. The protoplasmic contents of two cells unite to form
the germ-mass or zygote ; and these conjugating cells may be
either entirely independent, as in many Desmidiacece and in
the gametes of many Confervoidece; or they may be two of
the adjacent cells forming a thread, as in some Cotijugatece
and the gametes of Confervoidece; or they may be cells
belonging to adjacent threads, as in other Cotijugatece. But
whether it is originated by a single parent-cell, or by two
parent-cells, the zygote, after remaining quiescent until there
recur the fit conditions for growth, either breaks up into a
multitude of spores, each of which produces an individual
that usually multiplies asexually, or germinates directly to
produce one new individual; and the fact here to be noted
is, that as the entire contents of the parent-cells unite to
form the zygote, their individualities are lost in the germs of
a new generation. In these minute simple types, sexual
propagation just as completely sacrifices the life of the
parent or parents, as does that form of asexual propagation
in which the protoplasm resolves itself directly into zoo-
spores. And in the one case as in the other, this sacrifice
is the concomitant of a prodigious fertility. Slightly

in advance of this, but still showing us an almost equal loss
of parental life in the lives of offspring, is the process seen in
such unicellular Algce as Botrydium, and in minute Fungi of
the same degree of composition. These exhibit a relatively-
enormous development of the spore-producing part, and an
almost entire absorption of the parental substance into it.
As evidence of the resulting powers of multiplication, we
have but to remember that the spread of mould over stale
food, the rapid destruction of crops by mildew, and other
kindred occurrences, are made possible by the incalculably
numerous spores thus generated and universally dispersed.

Plants a degree higher in composition supply a parallel
series of illustrations. We have among the larger Fungi, in
75

450 LAWS OF MULTIPLICATION.

which the reproductive apparatus is relatively so enormous as
to constitute the ostensible plant, a similar subordination of
the individual to the race, and a similarly-immense fertility.
Thus, as quoted by Dr. Carpenter, Fries says — " in a single in-
dividual of Reticularia maxima, I have counted (calculated?)
10,000,000 sporules." It needs but to note the clouds of
particles, so minute as to look like smoke, which ripe puff-
balls give off when they are burst, and then to remember
that each particle is a potential fungus, to be impressed with
the almost inconceivable powers of propagation which these
plants possess. The Lichens, too, furnish examples.

Though they are nothing like so prolific as the Fungi (the
difference yielding, as we shall hereafter see, further support
to the general argument), yet there is a great production of
germs, and a proportionate sacrifice of the parental indi-
viduality. Considerable areas of the thallus develop into the
fruit-bodies characteristic of the various fungi which, com-
bined with algce, form the different lichens (various members
of the Ascomycetes and the Basidiomycetes) . From these are
produced great numbers of ascospores or basidiospores, as the
case may be. Very many lichens also reproduce themselves
by means of Soredia, i.e., little masses of algal cells closely
wrapped in a weft of fungal hyphse. Some con-

trasts presented by the higher Algce may also be named as
exemplifying the inverse proportion between the size of the
individual and the extent of the generative structures. While-
in the smaller kinds relatively large portions of the fronds are
transformed into reproductive elements, in the larger kinds
these portions are relatively small: instance the Macrocystis
pyrifera, a gigantic sea-weed which sometimes attains a
length of 1,500 feet, of which Dr. Carpenter remarks —
" This development of the nutritive surface takes place at
the expense of the fructifying apparatus, which is here quite
subordinate."

When we turn to vegetal aggregates of the third order oi
composition, facts having the same meaning are conspicuous

GROWTH AND SEXUAL GENESIS. 451

On the average these higher plants are far larger than
plants of a lower degree of composition; and on the average
their rates of sexual reproduction are far less. Similarly if,
among Archegoniates and Phamogams, we compare the
smaller types with the larger, we find them proportionately
more prolific. This is not manifest if we simply calculate
the number of seeds ripened by an individual in a single
season; but it becomes manifest if we take into account the
further factor which here complicates the result — the age at
which sexual genesis commences. The smaller Phaenogams
are mostly either annuals, or perennials that die down
annually; and seeding as they do annually before their
deaths, or the deaths of their reproductive parts, it results
that in the course of a year each gives origin to a multitude
of potential plants, of which every one may the next year, if
preserved, give origin to an equal multitude. Supposing but
a hundred offspring to be produced the first year, ten
thousand may be produced in the second year, a million in
the third, a hundred millions in the fourth. Meanwhile,
what has been the possible multiplication of a large Phseno-
gam? While its small congener has been seeding and dying,
and leaving multitudinous progeny to seed and die, it has
simply been growing; and may so continue to grow for ten
or a dozen years without bearing fruit. Before a Cocoa-nut
tree has ripened its first cluster of nuts, the descendants of
a wheat plant, supposing them all to survive and multiply,
will have become numerous enough to occupy the whole
surface of the Earth. So that though, when it begins to
bear, a tree may annually shed as many seeds as a herb, yet
in consequence of this delay in bearing, its fertility is incom-
parably less; and its relatively-small fertility becomes still
further reduced where, as in Lodoicea callipyge, the seeds
take two years from the date of fertilization to the date of
germination.

§ 340. Some observers state that in certain Protozoa there

452 LAWS OF MULTIPLICATION.

occurs a process of conjugation akin to that which th(
Protophyta exhibit — a coalescence of the substance of two
individuals to form a germ-mass. This has been alleged
more especially of Actinophrys. If this statement should
be proved true,* then of the minute forms that appear to be
more animal than vegetal in their characters, some have a
mode of sexual multiplication by which the parents are
sacrificed bodily in the production of a new generation.

Among small animal aggregates of the second order, the
first to be considered are of course the Ccelenterata. A Hydra
occasionally devotes a large part of its substance to sexual
genesis. In the walls of its body groups of ova, or sperma-
tozoa, or both, take their rise; and develop into masses
greatly distorting the creature's form, and leaving it much
diminished when they escape. Here, however, gamogenesis is
obviously supplementary to agamogenesis — the immensely
rapid multiplication by budding continues as long as food is
abundant and warmth sufficient, and is replaced by gamo-
genesis only at the close of the season. A better example

* To this passage Prof. MacBride appends the remark : — " This is quite
proven now, and the statement as it stands is quite correct ; but far better
and more minutely worked out cases are to be found amongst the Infusoria.
In Paramecium for example, there are normally present a large macro-
nucleus and a small micronucleus lying alongside of it. When two indi-
viduals adhere preparatory to conjugation, the macronucleus breaks up into
fragments which are absorbed : the micronucleus — which has some time pre-
viously divided into two — begins to break up further and eventually forms
eight bodies ; all of these except one disappear ; this last piece then divides
into two ; of these two one represents a male genital cell, for it passes over
into the body of the other Paramecium and fuses with one of the two corre-
sponding nuclei there ; thus each of the two individuals which adhere
fertilizes the other. The two individuals then separate and the nucleus
(result of fusion of male and female nuclei) in each divides into four. Of
these, two move to one end of the animal and two to the other. The
animal then divides into two transversely — each of the products thus hav-
ing two nuclei which form the micro- and macro-nucleus of it. Thus it
appears that the function of sexual union is simply to give increased vigour
to all the vital processes including fission. Since as mentioned above (p.
443) if it is prevented, the products of fission are eventually unable to feed
themselves."

GROWTH AND SEXUAL GENESIS. 453

of the relation between small size and active gamogenesis
among low types of the Metazoa is supplied by the Rotifera.
Microscopic as these are, they have a great rate of sexual
increase. According to Ehrenberg, HydaHna senta " is
capable of a four-fold propagation every twenty-four or
thirty hours, bringing forth in this time four ova, which grow
from the embryo to maturity, and exclude their fertile ova
in the same period. The same individual, producing in ten
days forty eggs, developed with the rapidity above cited, this
rate, raised to the tenth power, gives one million of indi-
viduals from one parent, on the eleventh day four millions,
and on the twelfth day sixteen millions, and so on."
Ehrenberg, however, characterized by Huxley as " the
greatest looker and the worst observer," is not a safe autho-
rity, and it is better to state the estimate of Ludwig Plate,
who says that Hydatina lays fifty eggs in two to three weeks
— a number which, multiplying in the manner described,
will yield in the time named a much smaller total though still
an enormous total.

The Annulosa, including among them the inferior types,
have habits and conditions of life so various that only the
broadest contrasts can be instanced in support of the pro-
position before us. The differences of organization and
activity greatly complicate the inverse variation of fertility
and bulk. Bearing in mind, however, that the rate of multi-
plication depends much less on the number of each brood
than on the quickness with which maturity is reached and a
new generation commenced, it will be obvious that though
Annelids, relatively enormous in size, produce great numbers
of ova, yet as they do this at comparatively long inter-
vals, their rates of increase fall immensely below that just
instanced in the Eotifers. And when at the other extreme
we come to the large articulate animals, such as the Crab
and the Lobster, the further diminution of fertility is seen in
the still longer delay which occurs before each new generation
begins to reproduce.

454 LAWS OP MULTIPLICATION.

Perhaps the best examples are supplied by vertebrate
animals, and especially those that are most familiar to us.
Comparisons between Fishes are unsatisfactory, because of
our ignorance of their histories. In some cases Fishes equal
in bulk produce widely different numbers of eggs; as the
Cod which spawns millions at once, and the Salmon by
which nothing like so great a number is spawned. But then
the eggs are very unlike in size ; and if the ovaria of the two
fishes be compared, the difference between their masses is
comparatively moderate. There are, indeed, contrasts which
seem at variance with the alleged relation; as that between
the Cod and the Stickleback which, though so much smaller,
produces fewer ova. The Stickleback's ova, however, are
relatively large ; and their total bulk bears as great a ratio to
the bulk of the Stickleback as does the bulk of the Cod's ova
to that of the Cod. Moreover if, as is not improbable, the
reproductive age is arrived at earlier by the Stickleback than
by the Cod, the fertility of the species may be greater not-
withstanding the smaller number produced by each indi-
vidual. Evidence which admits of being tolerably
well disentangled is furnished by Birds. They differ but
little in their grades of organization; and the habits of life
throughout extensive groups of them are so similar, that
comparisons may be fairly made. It is true that, as here-
after to be shown, the differences of expenditure which
differences of bulk entail, have doubtless much to do with
the differences of fertility. But we may set down under the
present head some of those cases in which the activity, being
relatively slight, does not greatly interfere with the relation
we are considering; and may note that among such birds
having similarly slight activities, the small produce more eggs
than the large, and eggs that bear in their total mass a
greater ratio to the mass of the parent. Consider, for ex-
ample, the gallinaceous birds; which are like one another
and unlike birds of most other groups in flying comparatively
little. Taking first the wild members of this order, which

GROWTH AND SEXUAL GENESIS. 455

rarely breed more than once in a season, we find that the
Pheasant has from 10 to 14 eggs, the Black-cock from 6 to
10, the Grouse 8 to 14, the Partridge 12 to 20, the Quail
still more, sometimes reaching two broods of 7 to 12 in each.
Here the only exception to the relation between decreasing
bulk and increasing number of eggs, occurs in the cases of
the Pheasant and the Black-cock; and it is to be remem-
bered, in explanation, that the Pheasant is constitutionally
adapted to a warmer region, is better fed — often artificially —
and leads a less active life. If we pass to domesticated
genera of the same order, we meet with parallel differences.
From the numbers of eggs laid, little can be inferred; for
under the favourable conditions artificially maintained, the
laying is carried on indefinitely. But though in the sizes of
their broods the Turkey and the Fowl do not greatly differ,
the Fowl begins breeding at a much earlier age than the
Turkey, and produces broods more frequently: a consider-
ably higher rate of multiplication being the result. Now
these contrasts among domestic creatures which are similarly
conditioned, and closely-allied by constitution, may be
held to show, more clearly than most other contrasts, the
inverse variation between bulk and sexual genesis; since
here the cost of activity is diminished to a comparatively
small amount. There is little expenditure in flight — some-
times almost none; and the expenditure in walking about
is not great: there is more of standing than of actual
movement. It is true thai, voung Turkeys commence
their existence as larger masses than chickens; but it is
tolerably manifest that the total weight of the eggs laid
by a Turkey during each season, bears a less ratio to the
Turkey's weight, than the total weight of the eggs which a
Hen lays during each season, bears to the Hen's weight;
and this is the fairest way of making the comparison. The
comparison so made shows a greater difference than appears
likely to be due to the different costs of locomotion; con-
sidering the inertness of the creatures. Eemembering that

456 LAWS OP MULTIPLICATION.

the assimilating surface increases only as the squares of the
dimensions, while the mass of the fabric to be built up by the
absorbed nutriment increases as the cubes of the dimensions,
it will be seen that the expense of growth becomes relatively
greater with each increment of size; and that hence, of two
similar creatures commencing life with different sizes, the
larger one in reaching its superior adult bulk, will do this at
a more than proportionate expense; and so will either be
delayed in commencing its reproduction, or will have a
diminished reserve for reproduction, or both. Other orders
of Birds, active in their habits, show more markedly the con-
nexion between augmenting mass and declining fertility.
But in them the increasing cost of locomotion becomes an
important, and probably the most important, factor. The
evidence they furnish will therefore come better under
another head. Contrasts among Mammals, like

those which Birds present, have their meanings obscured by
inequalities of the expenditures for motion. The smaller
fertility which habitually accompanies greater bulk, must
in all cases be partly ascribed to this. Still, it may be
well if we briefly note, for as much as they are worth,
the broader contrasts. While a large Mammal bears but
a single young one at a time, is several years before it com-
mences doing this, and then repeats the reproduction at long
intervals; we find, as we descend to the smaller members of
the class, a very early commencement of breeding, an increas-
ing number at a birth, reaching in small Eodents to 10 or
even more, and a much more frequent recurrence of broods:
the combined result being a relatively prodigious fertility.
If a specific comparison be desired between Mammals that
are similar in constitution, in food, in conditions of life, and
all other things but size, the Deer-tribe supplies it. While
the large Eed-deer has but one at a birth, the small Eoe-deei
has frequently two at a birth.*

§ 341. The antagonism between growth and sexual gene-
* A passage translated for me from the German may be here given in

GROWTH AND SEXUAL GENESIS. 457

sis, visible in these general contrasts, may also be traced in the
history of each plant and animal. So familiar is the fact
that sexual genesis does not occur early in life, and in all
organisms which expend much begins only when the limit of
size is nearly reached, that we do not sufficiently note its
significance. It is a general physiological truth, however,
that while the building-up of the individual is going on
rapidly, the reproductive organs remain imperfectly deve-
loped and inactive; and that the commencement of repro-
duction at once indicates a declining rate of growth, and
becomes a cause of arresting growth. As was shown in § 78,
the exceptions to this rule are found where the limit of
growth is indefinite; either because the organism expends
little or nothing in action, or expends in action so moderate
an amount that the supply of nutriment is never equilibrated
by its expenditure.

We will pass over the inferior plants and, limiting
ourselves to Phamogams, will not dwell on the less con-
spicuous evidence with the smaller types present. A few
cases such as gardens supply will serve. All know that
a Pear-tree increases in size for years before it begins to
bear; and that, producing but few pears at first, it is long
before it fruits abundantly. A young Mulberry-tree, branch-
ing out luxuriantly season after season, but covered with
nothing but leaves, at length blossoms sparingly and sets
some small and imperfect berries, vvhich it drops while they
are green; and it makes these futile attempts time after

verification. Dr. Dionys Hellin in an essay on the origin of Multiparity
and Twin-births, refers to the thesis above set forth, and says that "the
fact that it is generally women of small growth who bear twins is in com-
plete agreement with it." He adds that " Puech is right in his opinion that
twin pregnancies are a direct result of relatively large ovaries (i.e., in com-
parison with the whole body). He has observed that for the same size of
body the ovarium of a pluriparous animal is always of greater volume than
that of a uniparous animal .... a sow has ovaries as large as a cow's ;
but while the latter bears only one calf [at a time], the sow brings forth
6 — 15 at each litter. Even in animals of the same species but belonging to
different races these relations may be verified," e.g., Barbary sheep and ordi-
nary sheep.

45S

LAWS OF MULTIPLICATION.

time before it succeeds in ripening any seeds. But these
multi-axial plants, or aggregates of individuals some of
which continue to grow while others become arrested and
transformed into seed-bearers, show us the relation less de-
finitely than certain plants that are substantially, if not
literally, uni-axial. Of these the Cocoa-nut may be in-
stanced. For some years it goes on shooting up without
making any sign of becoming fertile. About the sixth year
it flowers; but the flowers wither without result. In the
seventh year it flowers and produces a few nuts; but these
prove abortive and drop. In the eighth year it ripens a
moderate number of nuts; and afterwards increases the
number until, in the tenth year, it comes into .full bearing.
Meanwhile, from the time of its first flowering its growth
begins to diminish, and goes on diminishing till the tenth
year, when it ceases. Here we see the antagonism between
growth and sexual genesis under both its aspects — see a
struggle between self-evolution and race-evolution, in which
the first for a time overcomes the last, and the last ultimately
overcomes the first. The continued aggrandizement of the
parent-individual makes abortive for two seasons the tendency
to produce new individuals; and the tendency to produce
new individuals, becoming more decided, stops any further
aggrandizement of the parent individual.

Parallel illustrations occur in the animal kingdom. The
eggs laid by a pullet are relatively small and few. Similarly,
it is alleged that, as a general rule, "a bitch has fewer
puppies at first, than afterwards." According to Burdach,
as quoted by Dr. Duncan, "the elk, the bear, &c, have at
first only a single young one, then they come to have most
frequently two, and at last again only one. The young
hamster produces only from three to six young ones, while
that of a more advanced age produces from eight to sixteen.
The same is true of the pig." It is remarked by Buffon that
when a sow of less than a year old has young, the number of
the litter is small, and its members are feeble and even im-

GftOWTS AND SEXUAL GENESIS. 459

perfect. Here we have evidence that in animals growth
checks sexual genesis. And then, on the other hand, we
.have evidence that sexual genesis checks growth. It is well
known to breeders that if a filly is allowed to bear a foal,
she is thereby prevented from reaching her proper size. And
a like loss of perfection as an individual, is suffered by a
cow which breeds too early. It may be added, as a converse
fact, that castrated animals, as capons and notably cats,
often become larger than their unmutilated associates.

§ 342. Notwithstanding the way in which the inverse
variation of growth and sexual genesis is complicated with
other relations, its existence is, I think, sufficiently mani-
fest. Individually, many of the foregoing instances are open
to criticism, and have to be taken with qualifications; but
when looked at in the mass their meaning is beyond doubt.
Comparisons between the largest with the smallest types,
whether vegetal or animal, yield results which are unmis-
takable. On the one hand, remembering the fact that dur-
ing its centuries of life an Oak does not produce as many
acorns as a Fungus does spores in a single night, we see that
the Fungus has a fertility exceeding that of the Oak in a
degree literally beyond our powers of calculation or imagina-
tion. On the other hand when, taking a microscopic pro-
tophyte which has billions of descendants in a few days,
we ask how many such would be required to build up the
forest tree which is years before it drops a seed, we are met by
a parallel difficulty in conceiving the number, if not in setting
it down. Similarly, if from the minute and prodigiously-
fertile Eotifer we turn to the Elephant, which approaches
thirty years before it bears a solitary young one, we find the
connexions between small size and great fertility and between
great size and small fertility, too intensely marked to be
much disguised by the perturbing relations that have been
indicated. Finally, as this induction, reached by a survey of
organisms in general, is verified by observations on the rela-

460

LAWS OF MULTIPLICATION.

tion between decreasing growth and commencing reproduc-
tion in individual organisms, we may, I think, consider the
alleged antagonism as proved.*

* When, after having held for some years the general doctrine elaborat
in these chapters, I agreed, early in 1852, to prepare an outline of it for the
Westminster Review, I consulted, among other works, the just-issued third
edition of Dr. Carpenter's Principles of Physiology, General and Compara-
tive— seeking in it for facts illustrating the different degrees of fertility of
different organisms, I met with a passage, quoted above in § 339, which
seemed tacitly to assert that individual aggrandizement is at variance with
the propagation of the race ; but nowhere found a distinct enunciation of
this truth. I did not then read the Chapter entitled " General View of the
Functions," which held out no promise of such evidence as I was looking for.
But on since referring to this chapter, I discovered in it the definite state-
ment that — " there is a certain degree of antagonism between the Nutritive
and Reproductive functions, the one being executed at the expense of the
other. The reproductive apparatus derives the materials of its operations
through the nutritive system, and is entirely dependent upon it for the con-
tinuance of its function. If, therefore, it be in a state of excessive activity,
it will necessarily draw off from the individual fabric some portion of the
aliment destined for its maintenance. It may be universally observed that,
when the nutritive functions are particularly active in supporting the indi-
vidual, the reproductive system is in a corresponding degree undeveloped, —
and vice versd." P. 592.

CHAPTER VII.

THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS,
ASEXUAL AND SEXUAL.

§ 343. By Development, as here to be dealt with apart
from Growth, is meant increase of structure as distinguished
from increase of mass. As was pointed out in § 50, this is
the biological definition of the word. In the following sec-
tions, then, we have to note how complexity of organization
is hindered by reproductive activity, and conversely.

This relation partially coincides with that which we have
just contemplated ; for, as was shown in § 44, degree of
growth is to a considerable extent dependent on degree of
organization. But while the antagonism to be illustrated in
this chapter is much entangled with that illustrated in the
last chapter, it may be so far separated as to be identified as
an additional antagonism.

Besides the direct opposition between that continual dis-
integration which rapid genesis implies, and the fulfilment of
that pre-requisite to extensive organization — the formation of
an extensive aggregate, there is an indirect opposition which
we may recognize under several aspects. The change

from homogeneity to heterogeneity takes time; and time
taken in transforming a relatively-structureless mass into a
developed individual, delays the period of reproduction. Usu-
ally this time is merged in that taken for growth ; but certain
cases of metamorphosis show us the one separate from the
other. An insect, passing from its lowly-organized cater-

461

462 LAWS OF MULTIPLICATION.

pillar-stage into that of chrysalis, is afterwards a week,
fortnight, or a longer period in completing its structure : th(
re-commencement of genesis being by so much postponed,
and the rate of multiplication therefore diminished. Further,
that re-arrangement of substance which development implies,
entails expenditure. The chrysalis loses weight in the course
of its transformation; and that its loss is not loss of watei
only, may be inferred from the fact that it respires, and that
respiration indicates consumption. Clearly the matter con-
sumed is, other things equal, a deduction from the surplus
which may go to reproduction. Yet again, the more

widely and completely an organic mass becomes differentiated,
the smaller is the portion of it which retains the relatively-
undifferentiated state that admits of being moulded into new
individuals, or the germs of them. Protoplasm which has
become specialized tissue cannot be generalized afresh, and
afterwards transformed into something else; and hence th(
progress of structure in an organism, by diminishing the
unstructured part, diminishes the amount available for mak-
ing offspring.

It is true that higher structure, like greater growth, may
insure to a species advantages which eventually further its
multiplication — may give it access to larger supplies of food,
or enable it to obtain food more economically; and we shall
hereafter see how the inverse variation we are considering is
thus qualified. But here we are concerned only with the
necessary and direct effects; not with those that are con-
tingent and remote. These necessary and direct effects we
will now look at as exemplified.

§ 344. Speaking generally, the simpler plants propagate
both sexually and asexually; and, speaking comparatively,
the complex plants propagate only sexually: their asexual
propagation is usually incomplete — produces a united aggre-
gate of individuals instead of' numerous distinct individuals.
The Protophytes that perpetually subdivide, the merely-

DEVELOPMENT AND GENESIS. 463

cellular Algce that shed their tetraspores, the Archegoniates
that spontaneously separate their fronds or drop their gem-
ma?, show us an extra mode of multiplication which, among
flowering plants, is exceptional. This extra mode of multipli-
cation among these simpler plants, is made easy by their low
development. Tetraspores arise only where the frond con-
sists of untransformed cells; gemma? bud out and drop off
only where the tissue is comparatively homogeneous.

Should it be said that this is but another aspect of the
antagonism already set forth, since these undeveloped forms
are also the smaller forms; the reply is that though in part
true this is not wholly true. Various marine Algce which
propagate asexually, are larger than some Phaenogams which
do not thus propagate. The objection that difference of
medium vitiates this comparison, is met by the fact that it is
the same among land-plants themselves. Sundry of the low-
ly-organized Liverworts which are habitually gemmiparous,
exceed in size many flowering plants. And the Ferns show
us agamic multiplication occurring in plants which, while
they are inferior in complexity of structure, are superior in
bulk to numbers of annual Monocotyledons and Dicotyledons.

§ 345. In the ability of the lowly-organized substance of a
Sponge to transform itself into mu]titudes of gemmules, we
have an instance of this same direct relation in the animal
kingdom. Moreover, the instance yields very distinct proof
of an antagonism between development and genesis, inde-
pendent of the antagonism between growth and genesis;
for the Sponge which thus multiplies itself asexually, as well
as sexually, is far larger than hosts of more complex animals
which do not multiply asexually.

Once again may be cited the creature so often brought in
evidence, the Hydra, as showing us how rapidity of agamic
propagation is associated with inferiority of structure. Its
power to produce young ones from nearly all parts of its
body, is due to the comparative homogeneity of its body. In

464 LAWS OF MULTIPLICATION.

kindred but more-organized types, the gemmiparity is
greatly restricted, or disappears. Among the free-swimming
Hydrozoa, multiplication by budding, when it occurs at all,
occurs only at special places. That increase of structure
apart from increase of size, is here a cause of declining agamo-
genesis, we may see in the contrast between the simple Hydra
and the compound Hydroids. These last, along with more-
differentiated tissues, show us a gemmation which does not
go on all over the body of each polype, and much of it does
not end in separation.

It is, however, among the Annulosa that progressing
organization is most conspicuously operative in diminishing
agamogenesis. The segments or " somites " composing an
animal belonging to this class, are primordially alike; and,
as before argued (§§ 205-7), are probably the homologues of
what were originally independent individuals. The progress
from the lower to the higher types of the class, is at once a
progress towards types in which the strings of segments cease
to undergo subdivision, and towards types in which the seg-
ments, no longer alike in their structures and functions, have
become physiologically integrated or mutually dependent.
Already this group of cases has been named as illustrating
the antagonism between growth and asexual genesis; but it
is proper also to name it here, since, on the one hand, "the
greater size due to the ceasing of fission, is made possible
only by the specialization of parts and the development of a
co-ordinating apparatus to combine their actions, and since,
on the other hand, specialization and co-ordination can ad-
vance only in proportion as fission ceases.

§ 346. The inverse variation of development and sexual
genesis is by no means easy to follow. One or two facts indi-
cative of it may, however, be named.

Phaenogams that have but little supporting tissue may
fairly be classed as structurally inferior to those having stems
with a bulky and complex woody system; for these imply

DEVELOPMENT AND GENESIS. 465

additional differentiations, and constitute wider departures
from the primitive type of vegetal tissue. That the con-
comitant of this higher organization is a slower gamogenesis,
scarcely needs pointing out. While the herbaceous annual
is blossoming and ripening seed, the young tree is transform-
ing its originally-succulent axis into dense fibrous substance;
and year by year the young tree expends in doing the like,
nutriment which successive generations of the annual expend
in fruit. Here the inverse relation is between sexual repro-
duction and complexity, and not between sexual reproduction
and bulk, seeing that besides seeding, the annual often grows
to a size greater than that reached by the young infertile tree
in several years.

Proof of the antagonism between complexity and gamo-
genesis in animals, is still more difficult to disentangle.
Perhaps the evidence most to the point is furnished by the
contrast between Man and certain other Mammals approach-
ing him in mass. To compare him with the domestic
Sheep which, though not very unlike in size, is relatively
prolific, is objectionable because of the relative inactivity of
Sheep ; and this, too, may be alleged as a reason why the Ox,
though far more bulky, is also far more fertile, than Man.
Further, against a comparison with the Horse which, while
both larger and more prolific, is tolerably active, it may be
urged that in his case, and the cases of herbivorous creatures
generally, the small exertion required to procure food, joined
with the great ratio borne by the alimentary organs to the
organs they have to build up and repair, vitiates the result.
We may, however, fairly draw a parallel between Man and a
large carnivore. The Lion, superior in size, and perhaps
equal in activity, has a digestive system not proportionately
greater; and yet has a higher rate of multiplication than
Man. Here the only decided want of parity, besides that of
organization, is that of food. Possibly a carnivore gains an
advantage in having a surplus nutriment consisting almost
wholly of those nitrogenous materials from which the bodies
76

±6Q LAWS OF MULTIPLICATION.

of young ones are mainly formed. But, allowing for all other
differences, it appears not improbable that the smallness of
human fertility compared with the fertility of large feline
animals, is due to the greater complexity of the human
organization — more especially the organization of the nervous
system. Taking degree of nervous organization as the chief
correlative of mental capacity ; and remembering the physio-
logical cost of that slow evolution whereby high mental
capacity is reached; we may suspect that nervous organiza-
tion is very expensive: the inference being that bringing it
up to the level it1 reaches in Man, whose digestive system, by
no means large, has at the same time to supply materials for
general growth and daily waste, involves a great retardation
of maturity and sexual genesis.

CHAPTER VIII.

ANTAGONISM BETWEEN EXPENDITURE AND GENESIS.

§ 347. Under this head we have to set down no evidence
derived from the vegetal kingdom. Plants are not expenders
of force in such degrees as to affect the general relations with
which we are dealing. They have not to maintain a heat
above that of their environment, nor have they to generate
motion; and hence consumption for these two purposes does
not diminish the stock of material which serves on the one
hand for growth and on the other hand for propagation.

It will be well, too, if we pass over the lower animals:
especially those aquatic ones which, being nearly of the
same temperature as the water, and nearly of the same
specific gravity, lose but little in evolving motion, sensible
and insensible. A further reason for excluding from con-
sideration these inferior types, is that we do not know enough
of their rates of genesis to permit of our making, with any
satisfaction, those involved comparisons here to be entered
upon.

The facts on which we must mainly depend are those to be
gathered from terrestrial animals, and chiefly from those
higher classes of them which are at the same time great
expenders and have rates of multiplication about which our
knowledge is tolerably definite. We will restrict ourselves,
then, to the evidence which Birds and Mammals supply.

§ 348. Satisfactory proof that loss of substance in the

467

468 LAWS OF MULTIPLICATION.

maintenance of heat diminishes the rapidity of propagation,
is difficult to obtain. It is, indeed, obvious that the warm-
blooded Vertebrata are less prolific than the cold-blooded;
but then they are at the same time more vivacious. Simi-
larly, between Mammals and Birds (which are the warmer-
blooded of the two) there is, other things equal, a parallel,
though much smaller, difference; but here, too, the unlike-
nesses of muscular action complicate the evidence. Again,
the annual return of generative activity has an average cor-
respondence with the annual return of a warmer season,
which, did it stand alone, might be taken as evidence that a
diminished cost of heat-maintenance leads to such a surplus
as makes reproduction possible. But then, this periodic rise
of temperature is habitually accompanied by an increase in
the quantity of food — a factor of equal or greater importance.
We must be content, therefore, with such few special facts as
admit of being disentangled.

Certain of these we are introduced to by the general rela-
tion last named — the habitual recurrence of genesis with the
recurrence of spring. For in some cases a domesticated crea-
ture has its supplies of food almost equalized ; and hence the
effect of varying nutrition may be in great part eliminated
from the comparison. The common Fowl yields an illustra-
tion. It is fed through the cold months, but nevertheless, in
mid-winter, it either wholly leaves off laying or lays very
sparingly. And then we have the further evidence that if it
lays sparingly, it does so only on condition that the heat, as
well as the food, is artificially maintained. Hens lay in cold
weather only when they are kept warm. To which fact may
be added the kindred one that "when pigeons receive arti-
ficial heat, they not only continue to hatch longer in autumn,
but will recommence in spring sooner than they would other-
wise do." An analogous piece of evidence is that, in
winter, inadequately sheltered Cows either cease to give milk
or give it in diminished quantity. For though giving milk
is not the same thing as bearing a young one, yet, as milk

EXPENDITURE AND GENESIS. 469

is part of the material from which a young one is built up,
it is part of the outlay for reproductive purposes, and diminu-
tion of it is a loss of reproductive power. Indeed the case
aptly illustrates, under another aspect, the struggle between
self-preservation and race-preservation. Maintenance of the
cow's life depends on maintenance of its heat; and main-
tenance of its heat may entail such reduction in the supply
of milk as to cause the death of the calf.

Evidence derived from the habits of the same or allied
genera in different climates, may naturally be looked for ; but
it is difficult to get, and it can scarcely be expected that the
remaining conditions of existence will be so far similar as to
allow of a fair comparison being made. The only illustrative
facts I have met with which seem noteworthy, are some named
by Mr. Gould in his work on The Birds of Australia. He
says : — " I must not omit to mention, too, the extraordinary
fecundity which prevails in Australia, many of its smaller
birds breeding three or four times in a season; but laying
fewer eggs in the early spring when insect life is less de-
veloped, and a greater number later in the season, when the
supply of insect food has become more abundant. I have
also some reason to believe that the young of many species
breed during the first season, for among others, I frequently
found one section of the Honey-eaters (the Melithrepti) sit-
ting upon eggs while still clothed in the brown dress of imma-
turity; and we know that such is the case with the intro-
duced Gallinacece (or poultry) three or four generations of
which have been often produced in the course of a year."
Though here Mr. Gould refers only to variation in the
quantity of food as a cause of variation in the rate of multi-
plication, may we not suspect that warmth is a part-cause
of the high rate which he describes as general ?

§ 349. Of the inverse variation between activity and
genesis, we get clear proof. Let us begin with that which
Birds furnish.

470 LAWS OF MULTIPLICATION.

First we have the average contrast, already hinted, between
the fertility of Birds and the fertility of Mammals. Compar-
ing the large with the large and the small With the small, we
see that creatures which continually go through the muscular
exertion of sustaining themselves in the air and propelling
themselves rapidly through it, are less prolific than creatures
of equal weights which go through the smaller exertion of
moving about over solid surfaces. Predatory Birds have
fewer young ones than predatory Mammals of approximately
the same sizes. If we compare Rooks with Rats, or Finches
with Mice, we find like differences. And these differences are
greater -than at first appears. For whereas among Mammals
a mother is able, unaided, to bear and suckle and rear half-
way to maturity, a brood that probably weighs more in pro-
portion than does the brood of a Bird; a Bird, or at least a
Bird that flies much, is unable to do this. Both parents have
to help; and this indicates that the margin for reproduction
in each adult individual is smaller.

Among Birds themselves occur contrasts which may be
next considered. In the Raptorial class, various species of
which, differing in their sizes, are similarly active in their
habits, we see that the small are more prolific than the large.
The Golden Eagle has usually 2 eggs: sometimes 3, some-
times only 1. As we descend to the Kites and -Falcons, the
number is 2 or 3, and 3 or 4. And when we come to the
Sparrow-Hawk, 3 to 5 is the specified number. Similarly
among the Owls : while the Great Eagle-Owl has 2 or 3
eggs, the comparatively small Common Owl has 4 or 5. As
before hinted, it is impossible to say what proportions of these
differences are due to unlikenesses of bulk merely, and what
proportions are due to unlikenesses in the costs of locomo-
tion. But we may fairly assume that the unlikenesses in the
costs of locomotion are here the more important factors.
Weights varying as the cubes of the dimensions, while the
surfaces of digestive systems vary as the squares, the expense
of flight increases more rapidly than does the ability to take

EXPENDITURE AND GENESIS. 471

in nutriment; and as motion through the air requires more
effort than motion on the ground, this geometrical progression
tells more rapidly on Birds than on Mammals. Be this as it
may, however, these contrasts support the argument; as do
various others which may be set down. The Finch-family, for
example, have broods averaging about 5 in number, and have
commonly 2 broods in the season; while in the Crow-family
the number of the brood is on the average less, and there is
but one brood in the season. And then on descending to such
small birds as the Wrens and the Tits, we have 8, 10, 12 to
15 eggs, and sometimes two broods in the year. One of the
best illustrations is furnished by the Swallow-tribe, through-
out which there is little or no difference in mode of life or in
food. The Sand-Martin, much the least of them, has 4 to 6
eggs and two broods ; the Swallow, somewhat larger, has 4 or
5; and the Swift (similar in habits though unrelated), larger
still, has but 2. Here we see a lower fertility associated
in part with greater size, but associated still more con-
spicuously with greater expenditure. For the difference of
fertility is more than proportionate to the difference of bulk,
as shown in other cases ; and for this greater difference there
is the reason, that the Swift has to support not only the cost
of propelling its larger mass through the air, but also the
cost of propelling it at a higher velocity.

Omitting much evidence of like nature, let us note that
disclosed by comparisons of certain groups of birds with
other groups. " Skulkers " is the descriptive title applied to
the Water-Kail, the Corn-Crake, and their allies, which evade
enemies by concealment — consequently expending but little
in locomotion. These birds have relatively large broods — 6
to 11, 8 to 12, &c. Not less instructive are the contrasts be-
tween the Gallinaceous Birds and other Birds of like sizes but
more active habits. The Partridge and the Wood-Pigeon are
about equal in bulk and have much the same food. Yet while
the one has from 12 to 20 young ones, the other has but 2
young ones twice a-year : its annual reproduction is less than

472 LAWS OF MULTIPLICATION.

one-third. It may be said that the ability of the Partridge
to bring up so large a brood, is due to that habit of its tribe
which one of its names, " Scrapers," describes ; and to the
accompanying habit of the young, which begin to get their
own living as soon as they are hatched : so saving the parents'
labour. Conversely, it may be said that the inability of
Pigeons to rear more than 2 at a time, is caused by the neces-
sity of fetching everything they eat. But the alleged relation
holds nevertheless. On the one hand, a great part of the food
which the Partridge chicks pick up, is food which, in their
absence, the mother would have picked up. Though each chick
costs her far less than a young Pigeon costs its parents, yet
the whole of her chicks cost her a great deal in the shape of
abstinence — an abstinence she can bear because she has to fly
but little. On the other hand, the Pigeon's habit of laying
and hatching but two eggs, must not be referred to any fore-
seen necessity of going through so much labour in supporting
the young, but to a constitutional tendency established by
such labour. This is proved by the curious fact that when
domesticated, and saved from such labour by artificial feeding,
Pigeons, says Macgillivray (quoting Aitkin), " are frequently
seen sitting on eggs long before the former brood is able to
leave the nest, so that the parent bird has at the same time
young birds and eggs to take care of."

§ 350. Made to illustrate the effect of activity on fertility,
most comparisons among Mammals are objectionable: other
circumstances are not equal. A few, however, escape this
criticism.

One is that between the Hare and the Eabbit. These are
closely-allied species of the same genus, similar in their diet
but unlike in their expenditures for locomotion. The rela-
tively-inert Eabbit has 6 young ones in a litter, and four
litters a-year; while the relatively-active Hare has but 2 to
5 in a litter. This is not all. The Eabbit begins to breed
at six months old ; but a year elapses before the Hare begins

EXPENDITURE AND GENESIS. 473

to breed. These two factors compounded, result in a dif-
ference of fertility far greater than can be ascribed to un-
likeness of the two creatures in size.

Perhaps the most striking piece of evidence which Mam-
mals furnish, is the extreme infertility of our common Bat.
The Cheiroptera and the Rodentia are not very dissimilar in
their internal structures. Diversity of constitution, therefore,
cannot vitiate the comparison between Bats and Mice, which
are about the same in size. Though their diets differ, the
difference is in favour of the Bat : its food being exclusively
animal while that of the Mouse is mainly vegetal. What
now are their respective rates of genesis? The Mouse has
several litters in a year of 5 to 7 in each; while the Bat pro-
duces only one at a time. Whether the Bat repeats its one
more frequently than the Mouse repeats its 7 is not stated;
but it is quite certain that even if it does so (an absurd
supposition), the more frequent repetition cannot be such
as to raise its fertility to anything like that of the Mouse.
And this relatively-low rate of multiplication we may fairly
ascribe to its relatively-high rate of expenditure.

Here let us note, in passing, an interesting example of the
way in which a species which has no specially-great power of
self-preservation, while its power of multiplication is ex-
tremely small, nevertheless avoids extinction because it has
to meet an unusually-small total of race-destroying forces.
Leaving out parasites, the only enemy of the Bat is the Owl ;
and the Owl is sparingly distributed.

§ 351. These general evidences may be enforced by some
special evidences. We have few opportunities of observing
how, within the same species, variations of expenditure are
related to variations of fertility. But a fact or two showing
the connexion may be named.

Doctor Duncan quotes a statement to the point respecting
the breeding of dogs. Already in § 341 1 have extracted a part
of this statement, to the effect that before her growth is com-

474 LAWS OF MULTIPLICATION.

plete, a bitch bears at a birth fewer puppies than when she
becomes full-grown. An accompanying allegation is, that
her declining vigour is shown by a decrease in the number of
puppies contained in a litter, " ending in one or two." And
then it is further alleged that, " as regards the amount of
work a dog has to perform, so will the decline be rapid or
gradual ; and hence, if a bitch is worked hard year after year,
she will fail rapidly, and the diminution of her puppies will
be accordingly; but if worked moderately and well kept, she
will fail gradually, and the diminution will be less rapid."

In this place, more fitly than elsewhere, may be added a
fact of like implication, though of a different order. Of
course whether excessive expenditure be in the continual re-
pairs of nervo-muscular tissues or in replacing other tissues,
the reactive effects, if not quite the same, will be similar —
there will be a decrease of the surplus available for genesis.
If, then, in any animals there from time to time occur unusual
outlays for self-maintenance, we may expect the periods of
such outlays to be periods of diminished or arrested repro-
duction. That they are so the moulting of birds shows us.
When hens begin to moult they cease to lay. While they
are expending so much in producing new clothing, they have
nothing to expend for producing eggs.

CHAPTER IX.

COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS.

§ 352. Under this head may be grouped various facts
which, in another way, tell the same tale as those contained
in the last chapter. The evidence there put together went
to show that increased cost of self-maintenance entailed de-
creased power of propagation. The evidence to be set down
here, will go to show that power of propagation is augmented
by making self-maintenance unusually easy. For into this
may be translated the effect of abundant food.

To put the proposition more specifically — we have seen
that after individual growth, development, and daily con-
sumption, have been provided for, the surplus nutriment
measures the rate of multiplication. This surplus may be
raised in amount by such changes in the environment as
bring a larger supply of the materials or forces on which
both parental life and the lives of offspring depend. Be
there, or be there not, any expenditure, a higher nutrition
will make possible a greater propagation. We may expect
this to hold both of agamogenesis and of gamogenesis; and
we shall find that it does so.

§ 353. On multi-axial plants, the primary effect of surplus
nutriment is a production of large and numerous leaf -shoots.
How this asexual multiplication results from excessive nutri-
tion, is well shown when the leading axis, or a chief branch,
is broken off towards its extremity. The axillary buds below

475

476 LAWS OF MULTIPLICATION.

the breakage quickly swell and burst into lateral shoots,
which often put forth secondary shoots: two generations of
agamic individuals arise where there probably would have
been none but for the local abundance of sap, no longer
drawn off. In like manner the abnormal agamogenesis which
we have in proliferous flowers, is habitually accompanied by
a general luxuriance, implying an unusual plethora.

No less conclusive is the evidence furnished by agamo-
genesis in animals. Sir John Dalyell, speaking of Hydra
tuba, and of the period before strobilization commences, says
— " It is singular how much propagation is promoted by
abundant sustenance." This Polype goes on budding-out
young polypes from its sides, with a rapidity proportionate
to the supply of materials. So, too, is it with the

agamic reproduction of the Aphis. As cited by Professor
Huxley, Kyber " states that he raised viviparous broods of
both this species (Aphis Dianthi) and A. Rosa? for four con-
secutive years, without any intervention of males or ovi-
parous females, and that the energy of the power of agamic
reproduction was at the end of that period undiminished.
The rapidity of the agamic proliferation throughout the
whole period was directly proportional to the amount of
warmth and food supplied."

In these cases the relation is not appreciably complicated
by expenditure. The parent having reached its limit of
growth, the absorbed food goes to asexual multiplication:
scarcely any being deducted for the maintenance of parental
life.

§ 354. The sexual multiplication of organisms under
changed conditions, undergoes variations conforming to a
parallel law. Cultivated plants and domesticated animals
yield us proof of this.

Facts showing that in cultivated plants sexual genesis in-
creases with nutrition, are obscured by facts showing that a
less rapid asexual genesis, and an incipient sexual genesis,

NUTRITION AND GENESIS. 477

accompany the fall from a high to a moderate nutrition. The
confounding of these two relations has led to mistaken infer-
ences. When treating of Genesis inductively, we reached the
generalization that " the products of a fertilized germ go
on accumulating by simple growth, so long as the forces
whence growth results are greatly in excess of the antagonist
forces; but that when diminution of the one set of forces, oj
increase of the other, causes a considerable decline in this ex-
cess, and an approach towards equilibrium, fertilized germs
are again produced.'' (§ 78.) It was pointed out that this
holds of organisms which multiply by heterogenesis, as well
as those which multiply by homogenesis. And plants were
referred to as illustrating, both generally and locally, the
decline of agamic multiplication and commencement of gamic
multiplication, along with a lessening rate of nutrition. Now
the many cases which are given of fruitfulness caused in trees
by depletion, are really cases of this change from agamo-
genesis to gamogenesis; and simply go to prove that what
would naturally arise when decreased peripheral growth had
followed increased size, may be brought about artificially by
diminishing the supply of materials for growth. Cramping
its roots in a pot, or cutting them, or ringing its branches,
will make a tree bear very early: bringing about a prema-
ture establishment of that relative innutrition which would
have spontaneously arisen in course of time. Such facts
by no means show that in plants sexual genesis increases as
nutrition diminishes. When it has once set in, sexual
genesis is scanty or imperfect unless nutrition is good.
Though the starved plant may blossom, yet many of its
blossoms will fail; and such seeds as it produces will be ill-
furnished with those enveloping structures and that store of
albumen, &c, needed to give good chances of successful ger-
mination— the number of surviving offspring will be dimin-
ished. Were it otherwise, the manuring of fields which are to
bear seed-crops, would be not simply useless but injurious.
Were it otherwise, dunging the roots of a fruit-tree would in

478 LAWS OF MULTIPLICATION.

all cases be impolitic; instead of being impolitic only where
the growth of sexless axes is still luxuriant. Were it other-
wise, a tree which has borne a heavy crop should, by the
consequent depletion, be led to bear a still heavier crop next
year; whereas it is apt to be wholly or partially barren next
year — has to recover a state of tolerably-high nutrition before
its sexual genesis again becomes large.

But the best illustrations are yielded by animals — those
animals at least in which we have, besides an increased sup-
ply of nutriment, a diminished expenditure. Two classes of
comparisons, alike in their implications, may be made —
comparisons between tame and wild animals of the same
species or genus, and comparisons between tame animals of
the same species differently treated.

To begin with Birds, let us first contrast the farm-yard
Gallinacece with their kindred of the fields and woods. Not-
withstanding their greater size, which, other things equal,
should be accompanied by smaller fertility, the domesticated
kinds have more numerous offspring than the wild kinds. A
Turkey has a dozen in a brood, while a Pheasant has from 6
to 10. Twice or thrice in a season, a Hen rears as many
chickens as a Partridge rears once in a season. Anserine
birds show us parallel differences. The Tame Goose sits on
13 to 18 eggs and often sits a second time; but the Wild
Goose sits on 5, 6, or 7, and these are noted as considerably
smaller. It is the same with Ducks. The domesticated
variety lays and hatches twice as many eggs as the wild
variety. And the like holds of Pigeons. After remarking
of the Columba livia that " in spring when they have plenty
of corn to pick from the newly-sown fields, they begin to
get fat and pair; and again in harvest, when the corn is cut
down," Macgillivray goes on to say that " the same pair when
tamed generally breed four times " in the year. That

between different poultry-yards inequalities of fertility are
caused by inequalities in the supplies of food, is a familiar
truth. High feeding shows its effects not only in the con-

NUTRITION AND GENESIS. 479

tinuous laying, but also in the sizes of the eggs. Among
directions given for obtaining eggs from pullets late in the
year, it is especially insisted on that they shall have a
generous diet. Respecting Pigeons Macgillivray writes: —
" that their breeding depends much on their having plenty of
food to fatten them, seems, I think, evident from the circum-
stance that, when tamed, which they easily are, they are
observed to breed in every month of the year. I do not mean
that the same pair will breed every month; but some in the
flock, if well fed, will breed at any season." There may

be added a fact of like meaning which partially-domesticated
birds yield. The Sparrow is one of the Finch tribe that has
taken to the neighbourhood of houses; and by its boldness
secures food not available to its congeners. The result is
that it has several broods in a season, while its field-haunting
kindred have none of them more than two broods, and some
have only one.

Equally clear proof that abundant nutriment raises the
rate of multiplication, occurs among Mammals. Compare the
litters of the Dog with the litters of the Wolf and the Fox.
Whereas those of the one range in number from 6 to 14, those
of the others contain respectively 5 or 6 or occasionally 7, and
4 or 5 or rarely 6. Again, the Wild Cat has 4 or 5 kittens ;
but the tame Cat has 5 or 6 kittens 2 or 3 times a-year.
So, too, is it with the Weasel tribe. The Stoat has 5 young
ones once a-year. The Ferret has 2 litters yearly, each con-
taining from 6 to 9 ; and this notwithstanding that it is the
larger of the two. Perhaps the most striking contrast is that
between the wild and tame varieties of the Pig. While the
one produces, according to its age, from 4 to 8 or 10 young
ones once a year, the other produces sometimes as many as 17
in a litter; or, in other cases, will bring up 5 litters of 10 each
in two years — a rate of reproduction which is unparalleled
in animals of as large a size.* And let us not omit to note
that this excessive fertility occurs where there is the

* The climate, the locality, and the kind of food, are of course all factors ;

480 LAWS OF MULTIPLICATION.

greatest inactivity — where there is plenty to eat and nothing
to do. There is no less distinct evidence that amonj

domesticated Mammals themselves, the well-fed individual
are more prolific than the ill-fed individuals. On the high
and comparatively-infertile Cotswolds, it is unusual for
ewes to have twins; but they very commonly have twins
in the adjacent rich valley of the Severn. Similarly, among
the barren hills of the west of Scotland, two lambs will be
borne by about one ewe in twenty; whereas in England,
something like one ewe in three will bear two lambs. Nay,
in rich pastures, twins are more frequent than single births ;
and it occasionally happens that, after a genial autumn and
consequent good grazing, a flock of ewes will next spring
yield double their number of lambs — the triplets balancing
the uniparae. So direct is this relation, that I have heard a
farmer assert his ability to foretell, from the high, medium,
or low, condition of an ewe in the autumn, whether she will
next spring bear two, or one, or none.

§ 355. An objection must here be met. Many facts may
be brought to prove that fatness is not accompanied by ferti-
lity but by barrenness ; and the inference drawn is that high
feeding is unfavourable to genesis. The premiss may be
admitted while the conclusion is denied.

There is a distinction between what may be called normal
plethora, and an abnormal plethora, liable to be confounded
with it. The one is a mark of constitutional wealth ; but the
other is a mark of constitutional poverty. Normal plethora
is a superfluity of materials both for the building up oJ

and hence, probably, the differences between the statements of different at
thorities concerning these several cases. Prof. MacBride writes : —

"According to Flower (Mammals, Living and Extinct) the Ferret is a
domesticated variety of the common polecat, which has 3 to 8 young. Dar-
win (Animals and Plants) says that the wild sow often breeds twice a year
and produces a litter of 4 to 8, and sometimes even 12. The domestic sow
breeds twice and would breed oftener if permitted, and if any good at all pro-
duces 8 in litter."

NUTRITION AND GENESIS. 481

tissue and the evolution of force; and this is the plethora
which we have found to be associated with unusual fecundity.
Abnormal plethora which, as truly alleged, is accompanied
by infecundity, is a superfluity of force-evolving materials
joined with either a positive or a relative deficiency of tissue-
forming materials: the increased bulk indicating this state,
being really the bulk of so much inert or dead matter. Note,
first, a few of the facts which show us that obesity implies
physiological impoverishment.

Neither in brutes nor men does it ordinarily occur either
in youth or in that early maturity during which the vigour
is the greatest and the digestion the best: it does not
habitually accompany the highest power of taking up nutri-
tive materials. When fatness arises in the prime of life,
whether from peculiarity of food or other circumstance, it is
not the sign of an increased total vitality. On the contrary,
if great muscular action has to be gone through, the fat
must be got rid of; either, as in a man, by training, or as in
a horse that has grown bulky while out at grass, by putting
him on such more nutritive diet as oats. The frequency

of senile fatness, both in domesticated creatures and in our-
selves, has a similar implication. Whether we consider the
smaller ability of those who display it to withstand large
demands on their powers, or whether we consider the com-
paratively-inferior digestion common among them, we see
that the increased size indicates, not an abundance of mate-
rials which the organism requires, but an abundance of
materials which it does not require. Of like meaning

is the fact that women who have had several children, and
animals after they have gone on bearing young for some
time, frequently become fat; and lose their fecundity as
they do this. In such cases the fatness is not to be taken
as the cause of the infecundity; but the constitutional
exhaustion which the previous production of offspring has
left, shows itself at once in the failing fecundity and the
commencing fatness. There is yet another kind of evi-

77

482 LAWS OF MULTIPLICATION.

dence. Obesity not uncommonly sets in after the system has
been subject to debilitating influences. Often a serious illness
is followed by a corpulence to which there was previously
no tendency. And the prolonged administration of mercury,
constitutionally injurious as it is, sometimes produces a like
effect.

Closer inquiry verifies the conclusion to which these facts
point. The microscope shows that along with the increase of
bulk common in advanced life, there goes on what is called
" fatty degeneration : " oil-globules are deposited where there
should be particles of flesh — or rather, we may say, the
hydrocarbonaceous molecules locally produced by decom-
position of the nitrogenous molecules, have not been replaced
by other nitrogenous molecules, as they should have been.
This fatty degeneration is, indeed, a kind of local death.
For so regarding it we have not simply the reason that an
active substance has its place occupied by an inert substance ;
but we have the further reason that the flesh of dead bodies,
under certain conditions, is transformed into a fatty matter
called adipocere.

The infertility that accompanies fatness in domestic
animals has, however, other causes than that declining con-
stitutional vigour which the fatness commonly indicates.
Being artificially fed, these animals cannot always obtain
what their systems need. That which is given to them is
given expressly because of its fattening quality. And since
the capacity of the digestive apparatus remains the same,
the absorption of fat-producing materials in excess, implies
defect in the absorption of materials from which the
tissues are formed, and out of which young ones are built
up. Moreover, this special feeding with a view

to rapid and early fattening, continued as it is through
generations, and accompanied as it is by a selection of indi-
viduals and varieties which fatten most readily, tends to
establish a modified constitution, more fitted for producing
fat and correspondingly-less fitted for producing flesh — a

NUTRITION AND GENESIS. 483

constitution which, from this relatively-deficient absorption
of nitrogenous matters, is likely to become infertile; as, in-
deed, these varieties often do become. * Hence, no con-
clusions respecting the effects of high nutrition, properly so
called, can be drawn from cases of this kind. The cases are,
in truth, of a kind which could not exist but for human
agency. Under natural conditions no animal would diet
itself in the way required to produce such results. And if it
did its race would quickly disappear.*

There is yet another mode in which accumulation of fat
diminishes fertility. Even supposing it unaccompanied by
a smaller absorption of nitrogenous materials, it is still a
cause of lessening the surplus of nitrogenous materials. For
the repair of the motor tissues becomes more costly. Fat
stored-up is weight to be carried. A creature loaded with
inert matter must, other things equal, consume a greater
amount of tissue-forming substances for keeping its loco-
motive apparatus in order ; and thus expending more for self-
maintenance can expend less for race-maintenance. Abnormal
plethora is thus antagonistic to reproduction in a double way.
It ordinarily implies a smaller absorption of tissue-forming
matters, and an increased demand on the diminished supply.
Hence fertility decreases in a geometrical progression.

The counter-conclusion drawn from facts of this class is,
then, due to a misconception of their nature — a misconception

* It is worth while inquiring whether unfitness of the food given to them,
is not the chief cause of that sterility which, as Mr. Darwin says, " is the
great bar to the domestication of animals." He remarks that " when animals
and plants are removed from their natural conditions, they are extremely
liable to have their reproductive systems seriously affected." Possibly the
relative or absolute arrest of genesis, is less due to a direct effect on the
reproductive system, than to a changed nutrition of which the reproductive
system most clearly shows the results. The matters required for forming an
embryo are in a greater proportion nitrogenous than are the matters required
for maintaining an adult. Hence, an animal forced to live on insufficiently-
nitrogenized food, may have its surplus for reproduction cut off, but still have
a sufficiency to keep its own tissues in repair, and appear to be in good health
— meanwhile increasing in bulk from excess of the non-nitrogenous matters
it eats.

484: LAWS OF MULTIPLICATION.

arising partly from the circumstance that the increase of bulk
produced by fat is somewhat like the increase of bulk which
growth of tissues causes, and partly from the circumstance
that abundance of good food normally produces a certain
quantity of fat, which, within narrow limits, is a valuable
store of force-evolving material. When, however, we limit
the phrase high nutrition to its proper meaning — an abun-
dance of, and due proportion among, all the substances which
the organism needs — we find that, other things equal, fertility
always increases as nutrition increases. And we see that these
apparently-exceptional cases, are cases which really show us
the same thing ; since they are cases of relative innutrition.

[Note. — By a strange oversight when writing this chapter
in the first edition — an oversight I was on the eve of repeat-
ing in this present edition — I omitted to bring forward the
familiar and all-important evidence furnished by the varia-
tions of genesis which ordinarily accompany the alternations
of the seasons. These variations, in multitudinous creatures
of all types, show unmistakably that reproduction begins at
those times of the year when greater warmth and larger
supplies of food render maintenance of individual life
relatively easy, and when there is therefore a surplus avail-
able for producing new individuals. Conversely, along
with the decrease of heat and the relative deficiency of food
which make it comparatively difficult in winter to maintain
individual life, there ceases to be the power of producing
other lives: the reproductive organs become quiescent and
often dwindle. With this general fact is associated a special
fact. Though among wild animals — birds, mammals, and
others — breeding ceases when Nature no longer supplies
abundant food and warmth; in domesticated mammals and
birds, artificially supplied with food and warmth, the breed-
ing season is greatly extended and often made continuous,
as, under the same conditions, it is in Man himself.

Evidence yielded by the vegetal world is less conspicuous,

NUTRITION AND GENESIS. 485

for the reason that the cold which arrests reproductive
activity also arrests individual activity: growth of the indi-
vidual and multiplication of the race vary simultaneously
with variations in the seasons. Still there are some familiar
facts showing that the external conditions which favour
nutrition also bring about reproduction. Early in the year
we are supplied with flowers from regions warmer than our
own, and by and by there come to our markets fruits and
vegetables from the south of France, the Channel Islands,
and even from the Scilly Isles, which are much in advance
of those furnished by the gardens of our own colder regions :
reproduction commences earlier where the light and heat
furthering nutrition are greater. And then there is a
kindred meaning in the not unfrequent occurrence of a sec-
ond flowering and even of a second fruiting in warm, bright
and prolonged autumns. Here the abnormal recommence-
ment of reproduction is determined by an abnormal increase
of nutrition.]

CHAPTER X.

SPECIALITIES OF THESE RELATIONS.

§ 356. Tests of the general doctrines set forth in preced-
ing chapters, are afforded by organisms having modes of life
which diverge widely from ordinary modes. Here, as else-
where, aberrant cases yield crucial proofs.

If certain organisms are so circumstanced that highly-
nutritive matter is supplied to them without stint, and they
have nothing to do but absorb it, we may infer that their
powers of propagation will be enormous.

If there are classes of creatures which expend very little
for self-support in comparison with allied creatures, a rela-
tively-extreme prolificness may be expected of them.

Or if, again, we find species presenting the peculiarity
that while some of their individuals have much to do and
little to eat, others of their individuals have much to eat and
little to do, we may look for great fertility in these last and
comparative infertility or barrenness in the first.

These several anticipations we shall find completely
verified.

§357. Plants which, like the Rafflesiacece, carry their para-
sitism to the extent of living on the juices they absorb from
other plants, exhibit one of these relations in the vegetal
kingdom. In them the organs for self-support being need-
less, are rudimentary; and the parts directly or indirectly
concerned in the production and distribution of germs, con-
stitute the mass of the organism. That small ratio which
486

SPECIALITIES OF THESE RELATIONS. 487

the race-preserving structures bear to the self -preserving
structures in ordinary Phaenogams, is, in these Phaenogams,
inverted. A like relation occurs in the common Dodder.

There may be added a kindred piece of evidence which the
Fungi present. Those of them which grow on living plants,
repeat the above connection completely; and those of them
which, though not parasitic, nevertheless subsist on organized
materials previously elaborated by other plants, substantially
repeat it. The spore-producing part is relatively enormous;
and the fertility is far greater than that of Cryptogams of
like sizes, which have to form for themselves the organic
compounds of which they and their germs consist.

§ 358. The same lesson is taught us by animal-parasites.
Along with the decreased cost of Individuation, they similarly
show us an increased expenditure for Genesis ; and they show
us this in the most striking manner where the deviation from
ordinary conditions of life is the greatest.

Take, among the Epizoa, such an instance as Chondracan-
thus gibbosus. Belonging to the Entomostraca, both males
and females of this species are, in their early days, similar to
their allies ; and the males, practically parasitic, though they
become greatly degraded, continue throughout life to show
by their segmentation and other external traits their original
nature. The female, however, having fixed herself where
she can suck the juices of her host, the Lophius, grows to
twelve times the length of the male and probably a thousand
times its bulk, and becomes utterly transformed by loss of
the organs of animal life and enormous development of the
organs of reproduction. " No heart is discoverable, and the
nervous system and organs of sense (if any) are equally un-
distinguishable. The interspace between the alimentary
canal and the walls of the body is almost wholly occupied
by the ovarium." * And then beyond this there are ap-
pended ovi-sacs twice the length of the body. So that the

* Huxley, Anatomy of Invertebrated Animals, p. 274.

488 LAWS OF MULTIPLICATION.

germ-producing organs and their contents, eventually
acquire a total bulk many times that of all the other organs
put together. Numerous species of this type and habit,
repeat this relation between a life of inaction with high
feeding, and an enormous rate of genesis. Parasites belong-
ing to another great division of the animal kingdom, the
Platyhelminthes, supply an example of an epizoon in which
the rate of multiplication is made great not so much by
immense development of the egg-producing organs as by
the rapidity with which generations succeed one another — a
rapidity such that each generation partially develops the
next before it is itself anything like ready for independent
life. This is the Gyrodactylus elegans, of which it is said
that " its most remarkable feature is that it is viviparous,
and its embryos before they leave the body of their mother
have already developed their embryos inside them; and the
latter may contain their embryos, so that four generations
may be included under the cuticle of the sexually mature
animal." *

Entozoa yield us many examples of this causal relation,
raised to a still higher degree. The Gordius, or Hair-worm,
is a creature which, finding its way when young into the
body of an insect which is afterwards swallowed by a fish,
there grows rapidly, and then emerging to breed, lays as
many as 8,000,000 eggs in less than a day. Similarly with
those larger types infesting the higher animals. It has been
calculated by Dr. Eschricht, as quoted by Professor Owen,
that there are "64,000,000 of ova in the mature female
'Ascaris lumbricoides." Very many of the Entozoa belong to
the Platyhelminthes, and among them occur examples of fer-
tility caused not only by great numbers of ova, but by rapid
succession of partially-developed individuals and also exam-
ples of fertility caused by production of ova almost exceeding
numeration. Among the first the Liver-fluke may be named.
Of the half-million eggs it produces each yields a free-swim-

* Shipley, Zoology of Invertebrata, p. 112.

SPECIALITIES OP THESE RELATIONS. 489

ming ciliated embryo, and any one of these, which finds its
way into a water-snail, becomes a sporocyst — a bag, presently
occupied exclusively by masses of cells: each mass by and
by becoming a Redia, which makes its way out. Like all
its fellows which develop in succession, this, with the excep-
tion of a small space occupied by the stomach, devotes the
whole of its interior partly to the formation of other Redice
(which presently escape and become similarly transformed),
and partly to the development of Cercarice, into which the
internal substance of all the Redice is eventually transformed :
Cercarice which, escaping from the host, become agents for
infecting other creatures. So that each ovum thus gives
rise to a number of forms which severally subserve multi-
plication in different ways. Of the other division of Platy-
helminthes referred to as carrying on its multiplication by
production of ova only, the commonest of the Cestoidea
furnishes the best example. Immersed as a Tape-worm is in
nutritive liquid, which it absorbs through its integument, it
requires no digestive apparatus. The room which one would
occupy, and the materials it would use up, are therefore avail-
able for germ-producing organs, which nearly fill each seg-
ment : each segment, sexually complete in itself, is little else
than an enormous reproductive system, with just enough of
other structures to bind it together. Eemembering that the
Tape-worm, retaining its hold, continues to bud-out such
segments as fast as the fully-developed ones are cast off, and
goes on doing this as long as the infested individual lives;
we see that here, where there is no expenditure, where the
cost of individuation is reduced to the greatest extent while
the nutrition is the highest possible, the degree of fertility
reaches its extreme. These Entozoa yield us further

interesting evidence. Of their various species, most if not
all undergo passive migration from animal to animal before
they become mature. Usually, the form assumed in the body
of the first host is devoid of all that part in which the repro-
ductive structures take their rise; and this part grows and

490 ^AWS OF MULTIPLICATION.

develops reproductive structures, only in some predatory
animal to which its first host falls a sacrifice. Occasionally,
however, the egg gives origin to the sexual form in the
animal that originally swallowed it, but the development
remains incomplete — there is no sexual genesis, no formation
of eggs in the rudimentary segments. That these may
become fertile it is needful, as before, for the containing
animal to be devoured; so that the imperfect Tape- worm
may find its way into the intestine of a higher animal.
Thus the Bothriocephalus solidus, found in the abdominal
cavity of the Stickleback, is barren while it remains there;
but if the Stickleback be eaten by a Water-fowl, the re-
productive system of the transferred Bothriocephalus (then
known as B. nodosus) becomes developed and active. So, too,
a kind of Tape-worm which remains infertile while in the
intestine of a Mouse, becomes fertile in the intestine of a Cat
that devours the mouse. May we not regard these facts as
again showing the dependence of fertility on nutrition?
Barrenness here accompanies conditions unfavourable to the
absorption of nutriment ; and it gives way to fecundity where
nutriment is large in quantity and superior in quality.

§ 359. Extremely significant are those cases of partial
reversion to primitive forms of genesis, which occur under
special conditions in some of the higher Annulosa. I refer
to the pseudo-parthenogenesis and metagenesis in Insects.

Under what conditions do the Aphides exhibit this strange
deviation from the habits of their order? Why among them
should imperfect females produce, agamically, others like
themselves, generation after generation, with great rapidity?
There is the obvious explanation that they get plenty of
easily-assimilated food without exertion. Piercing the tender
coats of young shoots, they sit and suck — appropriating the
nitrogenous elements of the sap and ejecting its saccha-
rine matter as " honey dew." Along with a sluggishness
strongly contrasted with the activity of most insects — along

SPECIALITIES OP THESE RELATIONS. 491

with a very low rate of consumption and a correlative degra-
dation of structure; we have here a retrogression to asexual
genesis, and a greatly-increased rate of multiplication.

The recently discovered instance of internal metagenesis
in the maggots of certain Flies has a like meaning. In-
credible as it at first seemed to naturalists, it is now proved
that the Cecydomia-larva develops in its interior a brood of
larvae of like structure with itself. In this case, as in the
last, abundant food is combined with low expenditure. These
larvae are found in such habitats as the refuse of beet-root-
sugar, factories — masses of nitrogenous debris remaining after
the extraction of the saccharine matter. Each larva has a
practically-unlimited supply of sustenance imbedding it on
all sides.*

It is true that some other maggots, as those of the Flesh-
fly, are similarly, or still better, circumstanced; and, it may
be said, ought therefore to have the same habit. But this
does not necessarily follow. Survival of the fittest will
determine whether such specially-favourable conditions result
in aggrandizement of the individual or in multiplication
of the race. And in the case of the Flesh-fly there is a
reason why greater individuation rather than more rapid
genesis will occur. For a decomposing animal body lasts so
short a time, that were Flesh-fly larvae to multiply agamically,
the second generation would die from the disappearance of
their food. Hence individuals in which the excessive
nutrition led to internal metagenesis, would leave no pos-
terity, and natural selection would establish the variety in

* I am told that " Wagner, who described the larva, found that it bored
into the bark of trees. It attacks also the wheat plant, and is a most
destructive parasite." Apparently this statement is at variance with the
foregoing inference. It is clear, however, that since these heaps of nitro-
genous refuse in which it has been found are artificial and recent, they can-
not be its natural habitats ; and it seems not improbable that these larvae,
suddenly supplied with a more nutritive food in unlimited amount, may have
as a consequence acquired this habit of agamogenetic multiplication which
did not characterize the species under its natural conditions and relatively low
nutrition.

492 LAWS OF MULTIPLICATION.

which greater growth resulted. All which the argument
requires is that when such reversion to agamogenesis does
take place, it shall be where the food is unusually abundant
and the expenditure unusually small; and this the cases
instanced go to show.

§ 360. The physiological lesson taught us by Bees and
Ants, not quite harmonizing with the moral lesson they are
supposed to teach, is that highly-fed idleness is favourable to
fertility, and that excessive industry has barrenness for its
concomitant.

The egg of a Bee develops into a small barren female or
into a large fertile female, according to the supply of food
given to the larva hatched from it. We here see that the
germ-producing action is an overflow of the surplus remain-
ing after completion of the individual; and that the lower
feeding which the larva of a working Bee has, results in a
dwarfing of the adult and an arrested development of the
generative organs. Further, we have the fact that the con-
dition under which the perfect female, or mother-Bee, goes
on, unlike insects in general, laying eggs continuously, is
that she has plenty of food brought to her, is kept warm, and
goes through no considerable exertion. While, contrariwise,
it is to be noted that the infertility of the workers is asso-
ciated with the ceaseless labour of bringing materials for the
combs and building them, as well as the labour of feeding
the queen, the larva?, and themselves.

Ants also show us these relations, and they are shown in a
greatly exaggerated form by what are called white ants —
insects belonging to a quite different order. The contrast
in bulk between the fecund and infecund females is here
immensely greater. The mother-Ant has the reproductive
system so enormously developed, that the remainder of her
body is relatively insignificant. Entirely incapable of loco-
motion, she is unable to deposit her eggs in the places where
they are to be hatched ; so that they have to be carried away

SPECIALITIES OF THESE RELATIONS. 493

by the workers as fast as they are extruded. Her life is thus
reduced substantially to that of a parasite — an absorption of
abundant food supplied gratis, a total absence of expendi-
ture, and a consequent excessive rate of genesis. " The
queen-ant of the African Termites lays 80,000 eggs in twenty-
four hours."

§ 361. It may be needful to say that these exceptional
relations cannot be ascribed to the assigned causes acting
alone. The extreme fertility which, among parasites and
social insects, accompanies extremely high feeding and an
expenditure reduced nearly to zero, presupposes typical struc-
tures and tendencies of suitable kinds; and these are not
directly accounted for. On creatures otherwise organized,
unlimited supplies of food and total inactivity are not fol-
lowed by such results. There of course requires a consti-
tution fitted to the special conditions, and the evolution of
this cannot be due simply to plethora joined with rest. These
cases are given as illustrating the conditions under which
extreme exaltations of fertility become possible. Their mean-
ings, thus limited, are clear, and completely to the point. We
see in them that the devotion of nutriment to race-preserva-
tion, is carried furthest where the cost of self-preservation
is reduced to a minimum; and, conversely, that nothing is
devoted directly to race-preservation by individuals on which
falls an excessive expenditure for self-preservation and pre-
servation of other's offspring.

[Note. — Among specialities of these relations may be fitly
added here a very strange one, for a description of which I
am indebted to M. Charles Julin, Professor of Comparative
Anatomy in the University of Liege. In the Revue Generate
des Sciences for 30th August, 1894, in an account of certain
investigations of M. Giard, he describes what he calls "la
castration parasitaire" — a castration not of a literal kind

494 LAWS OF MULTIPLICATION.

but one effected by the arrest of development which follows
from the depletion caused by a parasite. The Sacculina is
an amazingly transformed type belonging to the Cirrhipedia
— a type without segments or appendages and without mouth
and alimentary canal. Fixing itself, during its early loco-
motive stage, under the abdomen of a decapodous crustacean,
and leaving behind its exo-skeleton, it makes its way into
the interior, and there becoming a mere bag containing the
reproductive organs, obtains the needful nutriment by de-
veloping what are practically roots and rootlets which run
everywhere among the viscera and absorb nutriment from
the surrounding tissues. Here we are concerned merely
with the effect produced upon the host by this physiological
robbery. This effect is to arrest the development not only
of the primary sexual organs devoted to the production of
germs, but also of those secondary sexual organs which char-
acterize the male. M. Julin writes : —

"II convient cependant de dire, pour etre plus exact, que, dans
les cas des Crabes infestes par des Sacculines, il n'y a pas, en realite,
apparition de caracteres femelles chez le sexe male, mais plutdt
absence de developpement des caracteres males. En fait, l'animal
reste a un stade jeune, non differencie sexuellement, tout en prenant
une taille plus considerable. Cela nous porte & attribuer les modi-
fications dont nous avons parle a un simple arret de developpement,
qui est plus sensible chez le male, parce que chez lui les caracteres
sexuels secondaires sont a l'etat normal plus developpes que chez la
femelle.

D'une maniere generate, nous croyons, avec M. Giard, qu'il faut
assimiler les modifications dues a la castration parasitaire & celles qui
sont le resultat de la progenese ou qui engendrent le dimorphisme
saisonnier.

II y a progenese lorsque, chez un animal, la reproduction sexuee
s'opere d'une facon plus ou moins precoce, c'est-a-dire lorsque les
produits sexuels (ceufs ou spermatozo'ides) se forment et murissent
avant que l'etre n'ait atteint son complet developpement. On peut
citer comme exemples les Axolotls et les larves de Tritons qui, les uns
normalement, les autres accidentellement, pondent en ayant encore
leurs branchies.

Tres souvent la progenese n'affecte qu'un seul sexe. Tantot, c'est

SPECIALITIES OF THESE RELATIONS. 495

le sexe femelle qui murit a l'etat larvaire comme chez les pucerons,
les Stylops, etc...Tant6t c'est le sexe male, comme chez la Bonellie, les
males compl^mentaires de Cirrip&des, les males pygmees des Rotiferes,
le male de l'Anguille, etc. D'autres fois, enfin, l'animal presente
successivement les deux sexes avec progenese pour l'un d'entre eux.
C'est ainsi qu'il y a progenese protandrique chez les Crustaces cymo-
thoadiens, et, parmi les Vertebres, chez les Myxines, qui, males dans
le jeune age, deviennent femelles en vieillissant et en achevant de
prendre leur developpement. Le cas des vieilles femelles de Galli-
naces a plumage et a instincts masculins semble etre, au contraire, un
exemple imparfait de progenhe pi'otogynique, puisque ces femelles ont
pondu lorsqu'elles avaient encore la livree des jeunes et qu'elles ont
continue plus tard leur developpement, et pr6sentent le caractere des
males sans que, cependant, Ton ait constate la production de sper-
matozoiides.

Dans les cas extremes de progenese femelle, la reproduction se fait
meme sans le concours de l'element male, revenant ainsi a la forme
agamique primordiale. Ces cas sont connus depuis longtemps sous le
nom de pedogenese. On les a observe chez les larves de Miastor, de
Chironomm et chez certains pucerons.

Chaque fois qu'il y a progenese dans un type determine^ on cons-
tate soit momentanement, soit d'une facon definitive, un arret de
croissance et de developpement: l'animal progenetique a, par suite,
l'aspect d'une larve sexuee, lorsqu'on le compare soit a l'autre sexe,
soit aux formes voisines, qui ne presentent pas le ph6nomene de la
progenese.

Cela est en parfaite harmonie avec le principe, si bien mis en
lumiere par Herbert Spencer, de Vantagonisme entre la genfoe et la
croissance et entre la genese et le developpement. Get antagonisme
s'explique facilement si l'on songe que les materiaux employes pour
la reproduction ne peuvent servir a l'accroissement de l'individu.
S'il est avantageux pour un organisme de se reproduire sans acquerir
des organes inutiles, la selection naturelle d6terminera bient6t une
progenese de plus en plus complete. Les animaux parasites, outre
qu'ils tirent de leur h6te une nourriture abondante, n'ont guere besoin
d'une foule d'organes qui servent a leurs congeneres libres dans la
vie de relation. Aussi voyons-nous qu'un tres grand nombre d'ani-
maux parasites sont progenetiques. Les males prog6netiques de la
Bonellie et des Cirripedes vivent en parasites dans leurs femelles.
Chez certains types, les pucerons, la progenese cesse des que, la
nourriture devenant moins abondante, un deplacement pourra &tre
necessaire.

496 LAWS OF MULTIPLICATION.

En resume, l'arret de developpement du a la progenese resulte
d'une derivation des principes nourriciers au detriment de l'animal
progenetique. Dans les exemples de castration parasitaire que nous
avons examines, le parasite joue, par rapport a son h6te, absolument
le meme rdle que la glande genitale d'un type progenetique. II
detourne, pour sa propre subsistance, une partie des principes qui
auraient servi au developpement de l'animal. Aussi les effets produits
sont-ils tout a fait de meme ordre."

A phenomenon so anomalous as this, explicable upon the
hypothesis set forth but not otherwise explicable, furnishes
striking verification.]

CHAPTER XI.

INTERPRETATION AND QUALIFICATION.

§ 362. Considering the difficulties of inductive verifica-
tion, we have, I think, as clear a correspondence between the
a priori and a posteriori conclusions, as can be expected. The
many factors co-operating to bring about the result in f>very
case, are so variable in their absolute and relative amounts,
that we can rarely disentangle the effect of each one, and
have usually to be content with qualified inferences. Though
in the mass organisms show us an unmistakable relation
between great size and small fertility, yet special compari-
sons among them are nearly always partially vitiated by
differences of structure, differences of nutrition, differences of
expenditure. Though it is beyond question that the more
complex organisms are the less prolific, yet as complexity
has a certain general connexion with bulk, and in animals
with expenditure, we cannot often identify its results as inde-
pendent of these. And, similarly, though the creatures which
waste much matter in producing motion, sensible and insen-
sible, have lower rates of multiplication than those which
waste less, yet, as the creatures which waste much are
generally larger and more complex, we are again met by an
obstacle which limits our comparisons, and compels us to
accept conclusions less definite than are desirable.

Such difficulties arise, however, only when we endeavour,
as in foregoing chapters, to prove the inverse variation
78 497

*

498 JjAWS of multiplication.

between Genesis and each separate element of Individuation
— growth, development, activity. We are scarcely at all
hampered by qualifications when, from contemplating these
special relations, we return to the general relation. The
antagonism between Individuation and Genesis is shown by
all the facts which have been grouped under each head. We
have seen that in ascending from the lowest to the highest
types, there is a decrease of fertility so great as to be abso-
lutely inconceivable, and even inexpressible by figures; and
whether the superiority of type consists in relative largeness,
in greater complexity, in higher activity, or in some or all of
these combined, matters not to the ultimate inference. The
broad fact, enough for us here, is that organisms in which
the integration and differentiation of matter and motion have
been carried furthest, are those in which the rate of multipli-
cation has fallen lowest. How much of the decline of repro-
ductive power is due to the greater integration of matter,
how much to its greater differentiation, how much to the
larger amounts of integrated and differentiated motions gene-
rated, it may be impossible to say; and it is not needful to
say. These are all elements of a higher degree of life, an
augmented ability to maintain the organic equilibrium amid
environing actions, an increased power of self-preservation;
and. we find their invariable accompaniment to be, a dimi-
nished expenditure of matter, or motion, or both, in race-
preservation.

In brief, then, examination of the evidence shows that
there does exist that relation which we inferred must exist.
Arguing from general data, we saw that for the maintenance
of a species, the ability to produce offspring must be great,
in proportion as the ability of the individuals to contend with
destroying forces is small; and conversely. Arguing from
other general data, we saw that, derived as the self-sustain-
ing and race-sustaining forces are from a common stock of
force, it necessarily happens that, other things equal, increase
of one involves decrease of the other. And then, turning

INTERPRETATION AND QUALIFICATION. 499

to special facts, we have found that this inverse variation is
clearly traceable throughout both the animal and vegetal
kingdoms. We may therefore set it down as a law, that
every higher degree of organic evolution, has for its concomi-
tant a lower degree of that peculiar organic dissolution which
is seen in the production of new organisms.

§ 363. Something remains to be said in reply to the in-
quiry— how is the ratio between Individuation and Genesis
established in each case ? This inquiry has been but partially
answered in the course of the foregoing argument.

Many specialities of the reproductive process are mani-
festly due to the natural selection of favourable variations.
Whether a creature lays a few large eggs or many small ones
equal in weight to the few large, is not determined by any
physiological necessity : here the only assignable cause is the
survival of varieties in which the matter devoted to repro-
duction happens to be divided into portions of such size and
number as most to favour multiplication. Whether in any
case there are frequent small broods or larger broods at
longer intervals, depends wholly on the constitutional pecu-
liarity that has arisen from the dying out of families in
which the sizes and intervals of the broods were least suited
to the conditions of life. Whether a species of animal pro-
duces many offspring of which it takes no care or a few of
which it takes much care — that is, whether its reproductive
surplus is laid out wholly in germs or partly in germs and
partly in labour on their behalf — must have been decided by
that moulding of constitution to conditions slowly effected
through the more frequent preservation of descendants from
those whose reproductive habits were best adapted to the
circumstances of the species. Given a certain surplus avail-
able for race-preservation, and it is clear that by indirect
equilibration only, can there be established the more or
less peculiar distribution of this surplus which we see in
each case. Obviously, too, survival of the fittest

500 LAWS OF MULTIPLICATION.

has a share in determining the proportion between the
amount of matter that goes to Individuation and the amount
that goes to Genesis. Whether the interests of the species
are most subserved by a higher evolution of the individual
joined with a diminished fertility, or by a lower evolution of
the individual joined with an increased fertility, are ques-
tions ever being experimentally answered. If the more-
developed and less-prolific variety has a greater number of
survivors, it becomes established and predominant. If, con-
trariwise, the conditions of life being simple, the larger or
more-organized individuals gain nothing by their greater size
or better organization; then the greater fertility of the less
evolved ones, will insure to their descendants an increasing
predominance.

But direct equilibration all along maintains the limits
within which indirect equilibration thus works. The
necessary antagonism we have traced, rigidly restricts the
changes that natural selection can produce, under given con-
ditions, in either direction. A greater demand for Individua-
tion, be it a demand caused by some spontaneous variation or
by an adaptive increase of structure and function, inevitably
diminishes the supply for Genesis; and natural selection
cannot, other things remaining the same, restore the rate of
Genesis while the higher Individuation is maintained. Con-
versely, survival of the fittest, acting on a species that has,
by spontaneous variation or otherwise, become more prolific,
cannot again raise its lowered Individuation, so long as every-
thing else continues constant.

#

§ 364. Here, however, a qualification must be made. It
was parenthetically remarked in § 327, that the inverse varia-
tion between Individuation and Genesis is not exact ; and it
was hinted that a slight modification of statement would be
requisite at a more advanced stage of the argument. We
have now reached the proper place for specifying this modi-
fication.

INTERPRETATION AND QUALIFICATION. 501

Each increment of evolution entails a decrement of repro-
duction which is not accurately proportionate, but somewhat
less than proportionate. The gain in the one direction is not
wholly cancelled by a loss in the other direction, but only
partially cancelled : leaving a margin of profit to the species.
Though augmented power of self-maintenance habitually
necessitates diminished power of race-propagation, yet the
product of the two factors is greater than before ; so that the
forces preservative of race become, thereafter, in excess of the
forces destructive of race, and the race spreads. We shall
soon see why this happens.

Every advance in evolution implies an economy. That any
increase in bulk, or structure, or activity, may become estab-
lished, the life of the organism must be to some extent
facilitated by the change — the cost of self-support must be,
on the average, reduced. If the greater complexity, or the
larger size, or the more agile movement, entails on the in-
dividual an outlay that is not repaid in food more-easily
obtained, or danger more-easily escaped; then the individual
will be at a relative disadvantage, and its diminished posterity
will disappear. If the extra outlay is but just made good
by the extra advantage, the modified individual will not sur-
vive longer, or leave more descendants, than the unmodified
individuals. Consequently, it is only when the expense of
greater individuation is out-balanced by a subsequent saving,
that it can tend to subserve the preservation of the indi-
vidual, and, by implication, the preservation of the race.
The vital capital invested in the alteration must bring a
more than equivalent return. A few instances

will show that, whether the change results from direct
equilibration or from indirect equilibration, this must happen.
Suppose a creature takes to performing some act in an un-
usual way — leaps where ordinarily its kindred crawl, eludes
pursuit by diving instead of, like others of its kind, by swim-
ming along the surface, escapes by doubling instead of by
speed. Clearly, perseverance in the modified habit will, other

502

LAWS OF MULTIPLICATION.

things equal, imply that it takes less effort. The creature's
sensations will ever prompt desistance from the more labori-
ous course; and hence a congenital habit is not likely to be
diverged from unless an economy of force is achieved by the
divergence. Assuming, then, that the new method has no
advantage over the old in directly diminishing the chances
of death, the establishment of it, and of the structural
complications involved, nevertheless implies a physiological
gain. Suppose, again, that an animal takes to some
abundant food previously refused by its kind. It is likely to
persist only if the comparative ease in obtaining this food,
more than compensates for any want of adaptation to its
digestive organs; so that superposed modifications of the
digestive organs are likely to arise only when an average
economy results. What now must be the influence

on the creature's system as a whole ? Diminished expenditure
in any direction, or increased nutrition however effected,
will leave a greater surplus of materials. The animal will be
physiological richer. Part of its augmented wealth will go
towards its own greater individuation — its size, or its
strength, or both, will increase; while another part will go
towards more active genesis. Just as a state of plethora
directly produced enhances fertility; so will such a state
indirectly produced.

In another way, the same thing must result from those
additions to bulk or complexity or activity that are due to
survival of the fittest. Any change which prolongs individual
life will, other things remaining the same, further the pro-
duction of offspring. Even when it is not, like the foregoing,
a means of economizing the forces of the individual, still, if it
increases the chances of escaping destruction, it increases the
chances of leaving posterity. Any further degree of evolu-
tion, therefore, will be established only where the cost of
it is more than repaid: part of the gain being shown in the
lengthened life of the individual, and part in the greater
production of other individuals.

INTERPRETATION AND QUALIFICATION. 503

We have here the solution of various minor anomalies by
which the inverse variation of Individuation and Genesis is
obscured. Take as an instance the fertility of the Blackbird
as compared with that of the Linnet. Both birds lay five eggs,
and both usually have two broods. Yet the Blackbird is far
the larger of the two, and ought, according to the general
law, to be much less prolific. What causes this noncon-
formity? We shall find an answer in their respective foods
and habits. Except during the time that it is rearing its
young, the Linnet collects only vegetal food — lives during
the winter on the seeds it finds in the fields, or, when hard
pressed, picks up around farms; and to obtain this spare
diet is continually flying about. The result, if it survives the
frost and snow, is a considerable depletion; and it recovers
its condition only after some length of spring weather. The
Blackbird, on the other hand, is omnivorous. While it eats
$ grain and fruit when they come in its way, it depends largely
on animal food. It cuts to pieces and devours the dew-worms
which, morning and evening, it finds on the surface of a lawn,
and, even discovering where they are, unearths them; it
swallows slugs, and breaking snail-shells, either with its beak
or by hammering them against stones, tears out their tenants ;
and it eats beetles and larvas. Thus the strength of the
Blackbird opens to it a store of good food, much of which is
inaccessible to so small and weak a bird as a Linnet — a store
especially helpful to it during the cold months, when the
hybernating snails in hedge-bottoms yield it abundant pro-
vision. The result is that the Blackbird is ready to breed
very early in spring, and is able during the summer to rear
a second, and sometimes even a third, brood. Here, then, a
higher degree of Individuation secures advantages so great,
as to much more than compensate its cost. It is not that the
decline of Genesis is less than proportionate to the increase of
Individuation, but there is no decline at all. Com-

parison of the Bat with the Mouse yields a parallel result.
Though they differ greatly in size, yet the one is as prolific

504

LAWS OF MULTIPLICATION.

as the other. This absence of difference cannot be ascribed
to their unlike degrees of activity. We must seek its cause
in some facility of living secured to the Rat by its greater
intelligence, greater power and courage, greater ability to
utilize what it finds. The Eat is notoriously cunning ; and its
cunning gives success to its foraging expeditions. It is not,
like the Mouse, limited mainly to vegetal food; but while it
eats grain and beans like the Mouse, it also eats flesh and
carrion, devours young poultry and eggs. The result is that,
without a proportionate increase of expenditure, it gets a far
larger supply of nourishment than the Mouse; and relative
excess of nourishment makes possible a larger size without
a smaller rate of multiplication. How clearly this is the
cause, we see in the contrast between the common Eat and
the Water-Eat. While the common Eat has ordinarily
several broods a-year of from 10 to 12 each, the Water-Eat,
though somewhat smaller, has but 5 or 6 in a brood, and but
one brood, or sometimes two broods, a-year. But the Water-
Eat lives on vegetal food, and it lacks all that its bold, saga-
cious, omnivorous congener gains from the warmth as well as
the abundance which men's habitations yield.

The inverse variation of Individuation and Genesis is,
therefore, but approximate. Eecognizing the truth that
every increment of evolution which is appropriate to the
circumstances of an organism, brings an advantage somewhat
in excess of its cost ; we see the general law, as more strictly
stated, to be that Genesis decreases not quite so fast as
Individuation increases. Whether the greater Individuation
takes the form of a larger bulk and accompanying access of
strength; whether it be shown in higher speed or agility;
whether it consists in a modification of structure which facili-
tates some habitual movement, or in a visceral change that
helps to utilize better the absorbed aliment; the ultimate
effect is identical. There is either a more economical per-
formance of the same actions, internal or external, or there
is a securing of greater advantages by modified actions, which

INTERPRETATION AND QUALIFICATION. 505

cost no more, or have an increased cost less than the in-
creased gain. In any case the result is a greater surplus of
vital capital, part of which goes to the aggrandizement of
the individual, and part to the formation of new individuals.
While the higher tide of nutritive matters, everywhere filling
the parent-organism, adds to its power of self-maintenance, it
also causes a reproductive overflow larger than before.

Hence every type which is best adapted to its conditions,
(and this on the average means every higher type), has a rate
of multiplication that insures a tendency to predominate.
Survival of the fittest, acting alon*e, is ever replacing in-
ferior species by superior species. But beyond the longer
survival, and therefore greater chance of leaving offspring,
which superiority gives, we see here another way in which
the spread of the superior is insured. Though the more-
evolved organism is the less fertile absolutely, it is the more
fertile relatively.

/

CHAPTER XII.

MULTIPLICATION" OF THE HUMAN" EACE.

f

§ 365. The relative fertility of Man considered as a
species, and those changes in Man's fertility which occur under
changed conditions, must conform to the laws which we have
traced thus far. As a matter of course, the inverse variation
between Individuation and Genesis holds of him as of all
other organized beings. His extremely low rate of multipli-
cation— far below that of all terrestrial Mammals except the
Elephant, (which though otherwise less evolved is, in extent
of integration, more evolved) — we shall recognize as the
necessary concomitant of his much higher evolution. And
the causes of increase or decrease in his fertility, special or
general, temporary or permanent, we shall expect to find in
those changes of bulk, of structure, or of expenditure, which
we have in all other cases seen associated with such effects.

In the absence of detailed proof that these parallelisms
exist, it might suffice to contemplate the several communities
between the reproductive function in human beings and other
beings. I do not refer simply to the fact that genesis pro-
ceeds in a similar manner; but I refer to the similarity of
the relation between the generative function and the func-
tions which have for their joint end the preservation of the
individual. In Man, as in other creatures that expend much,
genesis commences only when growth and development are
declining in rapidity and approaching their termination.
Among the higher organisms in general, the reproductive
506

MULTIPLICATION OP THE HUMAK RACE. 507

activity, continuing during the prime of life, ceases when the
vigour declines, leaving a closing period of infertility ; and in
like manner among ourselves, barrenness supervenes when
middle age brings the surplus vitality to an end. So, too,
it is found that in Man, as in beings of lower orders, there is
a period at which fecundity culminates. In § 341, facts were
cited showing that at the commencement of the reproductive
period, animals bear fewer offspring than afterwards; and
that towards the close of the reproductive period, there is a
decrease in the number produced. In like manner it is shown
by the tables of Dr. Duncan's recent work, that the fecundity
of women increases up to the age of about 25 years, and
continuing high with but slight diminution till after 30,
then gradually wanes. It is the same with the sizes and
weights of offspring. Infants born of women from 25 to 29
years of age, are both longer and heavier than infants born
of younger or older women ; and this difference has the same
implication as the greater total weight of the offspring pro-
duced at a birth, during the most fecund age of a pluriparous
animal. Once more, there is the fact that a too-early bearing
of young produces on a woman the same injurious effects as
on an inferior creature — an arrest of growth and an enfeeble-
ment of constitution.

Considering these general and special parallelisms, we
might safely infer that variations of human fertility conform
to the same laws as do variations of fertility in general.
But it is not needful to content ourselves with an implication.
Evidence is assignable that what causes increase or decrease
of genesis in other creatures, causes increase or decrease of
genesis in Man. It is true that, even more than hitherto, our
reasonings are beset by difficulties. So numerous are the
inequalities in the conditions, that but few unobjectionable
comparisons can be made. The human races differ consider-
ably in their sizes, and notably in their degrees of cerebral
development. The countries they inhabit entail on them
widely different consumptions of matter for maintenance of

508 LAWS OF MULTIPLICATION.

temperature. Both in their qualities and quantities the
foods they live on are unlike; and the supply is here regular
and there very irregular. Their expenditures in bodily action
are extremely unequal; and even still more unequal are
their expenditures in mental action. Hence the factors,
varying so much in their amounts and combinations, can
scarcely ever have their respective effects identified. Never-
theless there are a few comparisons the results of which may
withstand criticism.

§ 366. The increase of fertility caused by a nutrition that
is greatly in excess of the expenditure, is to be detected by
contrasting populations of the same race, or allied races,
one of which obtains good and abundant sustenance much
more easily than the other. Three cases may here be set
down.

The traveller Barrow, describing the Cape-Boers, says : —
" Unwilling to work and unable to think/' ..." indulging
to excess in the gratification of every sensual appetite, the
African peasant grows to an unwieldy size ; " and respecting
the other sex, he adds — " the women of the African peasantry
lead a life of the most listless inactivity." Then, after illus-
trating these statements, he goes on to note "the prolific
tendency of all the African peasantry. Six or seven children
in a family are considered as very few; from a dozen to
twenty are not uncommon." The native races of

this region yield evidence to the same effect. Speaking of
the cruelly-used Hottentots (he is writing a century ago),
who, while they are poor and ill-fed, have to do all the work
for the idle Boers, Barrow says that they " seldom have more
than two or three children; and many of the women are
barren." This unusual infertility stands in remarkable con-
trast with the unusual fertility of the Kaffirs, of whom he
afterwards gives an account. Eich in cattle, leading easy
lives, and living almost exclusively on animal food (chiefly
milk with occasional flesh), these people were then reputed

MULTIPLICATION OF THE HUMAN RACE. 509

to have a very high rate of multiplication. Barrow writes : —
" They are said to be exceedingly prolific ; that twins are
almost as frequent as single births, and that it is no un-
common thing for a woman to have three at a time." Pro-
bably both these statements are in excess of the truth; but
there is room for large discounts without destroying the
extreme difference. A third instance is that of the

French Canadians. "Nous sommes terribles pour les en-
f ants I " observed one of them to Prof. Johnston, who tells us
that the man who said this " was one of fourteen children —
was himself the father of fourteen, and assured me that from
eight to sixteen was the usual number of the farmers'
families. He even named one or two women who had
brought their husbands five-and-twenty, and threatened ' le
vingt-sixieme pour le pretre/ " From these large families,
joined with the early marriages and low rate of mortality, it
results that, by natural increase, "there are added to the
French-Canadian population of Lower Canada four persons
for every one that is added to the population of England."
Now these French-Canadians are described by Prof. Johnston
as home-loving, contented, unenterprising; and as living in
a region where " land and subsistence are easily obtained."
Very moderate industry brings to them liberal supplies of
necessaries ; and they pass a considerable portion of the year
in idleness. Hence the cost of Individuation being much
reduced, the rate of Genesis is much increased. That this
uncommon fertility is not due to any direct influence of the
locality, is implied by the fact that along with the " restless,
discontented, striving, burning energy of their Saxon neigh-
bours," no such rate of multiplication is observed; while
further south, where the physical circumstances are more
favourable if anything, the Anglo-Saxons, leading lives of
excessive activity, have a fertility below the average. And
that the peculiarity is not a direct effect of race, is proved by
the fact that in Europe, the rural French are certainly not
more prolific than the rural English.

510 LAWS OF MULTIPLICATION.

To every reader there will probably occur the seemingly-
adverse evidence furnished by the Irish; who, though not
well fed, multiply fast. Part of this more rapid increase is
due to the earlier marriages common among them, and con-
sequent quicker succession of generations — a factor which,
as we have seen, has a larger effect than any other on the
rate of multiplication. Part of it is due to the greater
generality of marriage — to the comparative smallness of the
number who die without having had the opportunity of pro-
ducing offspring. The effects of these causes having been
deducted, we may doubt whether the Irish, individually con-
sidered, would be found more prolific than the English.
Perhaps, however, it will be said that, considering their diet,
they ought to be less prolific. This is by no means obvious.
It is not simply a question of nutriment absorbed. It is a
question of how much remains after the expenditure in self-
maintenance. Now a notorious peculiarity in the life of the
Irish peasant is, that he obtains a return of food which is
large in proportion to his outlay in labour. The cultivation
of his potatoe-ground occupies each cottager but a small part
of the year ; and the domestic economy of his wife is not of a
kind to entail on her much daily exertion. Consequently the
crop, tolerably abundant in quantity though innutritive in
quality, possibly suffices to meet the comparatively-low ex-
penditure, and to leave a good surplus for genesis — perhaps
a greater surplus than remains to the males and females of
the English peasantry, who, though fed on better food, are
harder worked.

We conclude, then, that in the human race, as in all other
races, such absolute or relative abundance of nutriment as
leaves a large excess after defraying the cost of carrying on
parental life, is accompanied by a high rate of genesis.*

§ 367. Evidence of the converse truth, that relative in-

* This is exactly the reverse of Mr. Doubleday's doctrine ; which is that
throughout both the animal and vegetable kingdoms, "over-feeding checks
increase ; whilst, on the other hand, a limited or deficient nutriment stimu-

MULTIPLICATION OF THE HUMAN RACE. 511

crease of expenditure, leaving a diminished surplus, reduces
the degree of fertility, is not wanting. Some of it has been
set down for the sake of antithesis in the foregoing section.
Here may be grouped a few facts of a more special kind
having the same implication.

To prove that much bodily labour renders women less pro-
lific, requires more evidence than has at present been collected.
Nevertheless it may be noted that De Boismont in France and
Dr. Szukits in Austria, have shown by extensive statistical
comparisons, that the reproductive age is reached a year
later by women of the labouring class than by middle-class
women; and. while ascribing this delay in part to inferior

latcs and adds to it." Or, as he elsewhere says — " Be the range of the
natural power to increase in any species what it may, the plethoric state inva-
riably checks it, and the deplethoric state invariably develops it ; and this hap-
pens in the exact ratio of the intensity and completeness of each state, until
each state be carried so far as to bring about the actual death of the animal
or plant itself."

I have space here only to indicate the misinterpretations on which Mr.
Doubleday has based his argument.

In the first place, he has confounded normal plethora with what I have, in
§ 355, distinguished as abnormal plethora. The cases of infertility accom-
panying fatness, which he cites in proof that over-feeding checks increase, are
not cases of high nutrition properly so called ; but cases of such defective ab-
sorption or assimilation as constitutes low nutrition. In Chap. IX, abundant
proof was given that a truly plethoric state is an unusually fertile state. It
may be added that much of the evidence by which Mr. Doubleday seeks to
show that among men, highly-fed classes are infertile classes, may be out-
balanced by counter-evidence. Many years ago Mr. G. H. Lewes pointed this
out : extracting from a book on the peerage, the names of 16 peers who had,
at that time, 186 children ; giving an average of 11 '6 in a family.

Mr. Doubleday insists much on the support given to his theory by the
barrenness of very luxuriant plant", and the fruitfulness produced in plants
by depletion. Had he been aware that the change from barrenness to fruit-
fulness in plants, is a change from agamogenesis to gamogenesis — had it been
as well known at the time when he wrote as it is now, that a tree which goes
on putting out sexless shoots, is thus producing new individuals ; and that
when it begins to bear fruit, it simply begins to produce new individuals after
another manner — he would have perceived that facts of this class do not tell
in his favour.

In the law which Mr. Doubleday alleges, he sees a guarantee for the
maintenance of species. He argues that the plethoric state of the indivi-

512 LAWS OF MULTIPLICATION.

nutrition, we may suspect that it is in part due to greater
muscular expenditure. A kindred fact, admitting of a
kindred interpretation, may be added. Though the com-
paratively-low rate of increase in France is attributed to
other causes, yet, very possibly, one of its causes is the
greater proportion of hard work entailed on French women,
by the excessive abstraction of men for non-productive
occupations, military and civil. The higher rate of multipli-
cation in England than in continental countries generally, is
not improbably furthered by the easier lives which English
women lead.

That absolute or relative infertility is commonly produced
in women by mental labour carried to excess, is more clearly
shown. Though the regimen of upper-class girls is not what
it should be, yet, considering that their feeding is better than
that of girls belonging to the poorer classes, while, in most
other respects, their physical treatment is not worse, the

duals constituting any race of organisms, presupposes conditions so favour-
able to life that the race can be in no danger ; and that rapidity of multi-
plication becomes needless. Conversely, he argues that a deplethoric state
implies unfavourable conditions — implies, consequently, unusual mortality;
that is — implies a necessity for increased fertility to prevent the race from
dying out. It may be readily shown, however, that such an arrangement
would be the reverse of self-adjusting. Suppose a species, too numerous
for its food, to be in the resulting deplethoric state. It will, according to
Mr. Doubleday, become unusually fertile ; and the next generation will be
more numerous rather than less numerous. For, by the hypothesis, the un-
usual fertility due to the deplethoric state, is the cause of undue increase of
population. But if the next generation is more numerous while the supply
of food has not increased in proportion, then this next generation will be in
a still more deplethoric state, and will be still more fertile. Thus there will
go on an ever-increasing rate of multiplication, and an ever-decreasing share
of food, for each person, until the species disappears. Suppose, on the
other hand, the members of a species to be in an unusually plethoric state.
Their rate of multiplication, ordinarily sufficient to maintain their numbers,
will become insufficient to maintain their numbers. In the next generation,
therefore, there will be fewer to eat the already abundant food, which be-
coming relatively still more abundant, will render the fewer members of the
species still mor^ plethoric, and still less fertile, than their parents. And the
actions and reactions continuing, the species will presently die out from abso-
lute barrenness.

MULTIPLICATION OF THE HUMAK RACE. 513

deficiency of reproductive power among them may be reason-
ably attributed to the overtaxing of their brains — an over-
t axing which produces a serious reaction on the physique.
This diminution of reproductive power is not shown only by
the greater frequency of absolute sterility; nor is it shown
only in the earlier cessation of child-bearing; but it is also
shown in the very frequent inability of such women to suckle
their infants. In its full sense, the reproductive power means
the power to bear a well-developed infant and to supply that
infant with the natural food for the natural period. Most of
the flat-chested girls who survive their high-pressure educa-
tion, are incompetent to do this. Were their fertility mea-
sured by the number of children they could rear without
artificial aid, they would prove relatively very infertile.

The cost of reproduction to males being so much less
than it is to females, the antagonism between Genesis and
Individuation is not often shown in men by suppression of
generative power consequent on unusual expenditure in
bodily action. Nevertheless, there are indications that this
results in extreme cases. We read that the ancient athletes
rarely had children; and among such of their modern repre-
sentatives as acrobats, an allied relation of cause and effect
is alleged. Indirectly this truth, or rather its converse,
appears to have been ascertained by those who train men
for feats of strength — they find it needful to insist on con-
tinence.

Special proofs that in men great cerebral expenditure
diminishes or destroys generative power, are difficult to
obtain. It is, indeed, asserted that intense application to
mathematics, requiring as it does extreme concentration of
thought, is apt to have this result; and it is asserted, too,
that this result is produced by the excessive emotional ex-
citement of gambling. Then, again, it is a matter of common
remark how frequently men of unusual mental activity leave
no offspring. But facts of this kind admit of another inter-
pretation. The reaction of the brain on the body is so violent
79

514 LAWS OF MULTIPLICATION.

— the overtaxing of the nervous system is so apt to prostrate
the heart and derange the digestion; that the incapacities
caused in these cases, are probably often due more to con-
stitutional disturbance than to the direct deduction which
excessive action entails. Such instances harmonize with the
hypothesis; but how far they yield it positive support we
cannot say.

§ 368. An objection must here be guarded against. It is
likely to be urged that since the civilized races are, on the
average, larger than many of the uncivilized races ; and since
they are also somewhat more complex as well as more active ;
they ought, in conformity with the alleged general law, to
be less prolific. There is, however, no evidence to prove that
they are so : on the whole, they seem rather the reverse.

The reply is that were all other things equal, these
superior varieties of men should have inferior rates of in-
crease. But other things are not equal; and it is to the
inequality of other things that this apparent anomaly is
attributable. Already we have seen how much more fertile
domesticated animals are than their wild kindred; and the
causes of this greater fertility are also the causes of the
greater fertility, relative or absolute, which civilized men
exhibit when compared with savages.

There is the difference in amount of food. Australians,
Fuegians, and sundry races that might be named as having
low rates of multiplication, are obviously underfed. The
sketches of natives contained in the volumes of Livingstone,
Baker, and others, yield clear proofs of the extreme depletion
common among the uncivilized. In quality as well

as in quantity, their feeding is bad. Wild fruits, insects,
larvse, vermin, &c, which we refuse with disgust, often enter
largely into their dietary. Much of this inferior food they
eat uncooked; and they have not our elaborate appliances
for mechanically-preparing it, and rejecting its useless parts.
So that they live on matters of less nutritive value, which

MULTIPLICATION OF THE HUMAN RACE. 515

cost more both to masticate and to digest. Further,

to uncivilized men supplies of food come very irregularly.
Long periods of scarcity are divided by short periods of
abundance. And though by gorging when opportunity
occurs, something is done towards compensating for previous
fasting, yet the effects of prolonged starvation cannot be
neutralized by occasional enormous meals. Bearing in mind,'
too, that improvident as they are, savages often bestir them-
selves only under pressure of hunger, we may fairly consider
them as habitually ill-nourished — may see that even the
poorer classes of civilized men, making regular meals on food
separated from innutritive matters, easy to masticate and
digest, tolerably good in quality and adequate if not abundant
in quantity, are much better nourished.

Then, again, though a greater consumption in muscular
action appears to be undergone by civilized men than
by savages; and though it is probably true that among our
labouring people the daily repairs cost more; yet in many
cases there does not exist so much difference as we are apt
to suppose. The chase is very laborious; and great amounts
of exertion are gone through by the lowest races in seeking
and securing the odds and ends of wild food on which they
largely depend. We naturally assume that because bar-
barians are averse to regular labour, their muscular action
is less than our own. But this is not necessarily true. The
monotonous toil is what they cannot tolerate; and they may
be ready to go through as much or more exertion when
it is joined with excitement. If we remember that the
sportsman who gladly scrambles up and down rough hill-
sides all day after grouse or deer, would think himself hardly
used had he to spend as much effort and time in digging; we
shall see that a savage who is the reverse of industrious,
may nevertheless be subject to a muscular waste not very
different in amount from that undergone by the indus-
trious. When it is added that a larger physiolo-
gical expenditure is entailed on the uncivilized than on the

516

LAWS OF MULTIPLICATION.

civilized by the absence of good appliances for shelter and
protection — that in some cases they have to make good a
greater loss of heat, and in other cases suffer much wear from
irritating swarms of insects; we shall see that the total cost
of self -maintenance among them is probably in many cases
little less, and in some cases more, than it is among ourselves.
So that though, on the average, the civilized are probably
larger than the savage; and though they are, in their
nervous systems at least, somewhat more complex; and
though, other things equal, they ought to be the less
prolific; yet other things are so unequal as to make it
quite conformable to the general law that they should be
more prolific. In § 365 we observed how, among inferior
animals, higher evolution sometimes makes self-preservation
far easier, by opening the way to resources previously un-
available: so involving an undiminished, or even an in^
creased, rate of genesis. And similarly we may expect that
among races of men, those whose slight further develop-
ments have been followed by habits and arts which immensely
facilitate life, will not exhibit a lower degree of fertility, and
may even exhibit a higher.

§ 369. One more objection has to be met — a kindred ob-
jection to which there is a kindred reply. Cases may be
named of men conspicuous for activity, bodily and mental,
who were also noted, not for less generative power than usual,
but for more. As their superiorities indicate higher degrees
of evolution, it may be urged that such men should, accord-
ing to the theory, have lower degrees of reproductive activity.
The fact that here, along with increased powers of self-pre-
servation, there go increased powers of race-propagation,
seems irreconcilable with the general doctrine. Keconcilia-
tion is not difficult however.

The cases are analogous to some before named, in which
more abundant food simultaneously aggrandizes the indi-

MULTIPLICATION OF THE HUMAN RACE. 517

vidual and adds to the production of new individuals: the
difference between the cases being, that instead of a better
external supply of materials there is a better internal
utilization of materials. Creatures of the same species noto-
riously differ in goodness of constitution. Here there is some
visceral defect, showing itself in feebleness of all the func-
tions; while here some peculiarity of organic balance, some
high quality of tissue, some abundance or potency of the
digestive juices, gives to the system a perpetual high tide of
rich blood, which serves at once to enhance the vital activities
and to raise the power of propagation. Such variations,
however, are independent of changes in the proportion be-
tween Individuation and Genesis. This remains the same,
while both are increased or decreased by the increase or
decrease of the common stock of materials.

An illustration will best clear up any perplexity. Let us
say that the fuel burnt in the furnace of a locomotive steam-
engine, answers to the food which a man consumes. Let us
say that the produced steam expended in working the engine,
corresponds to that portion of absorbed nutriment which
carries on the man's functions and activities. And let us
say that the steam blowing off at the safety-valve,
answers to that portion of the absorbed nutriment which
goes to the propagation of the race. Such being the condi-
tions of the case, several kinds of variations are possible.
All other circumstances remaining the same, there may be
changes of proportion between the steam used for working
the engine and the steam that escapes by the safety-valve.
There may be a structural or organic change of proportion.
By enlarging the safety-valve or weakening its spring, while
the cylinders are reduced in size, there may be established a
constitutionally-small power of locomotion and a constitu-
tionally-large amount of escape-steam ; and inverse variations
so produced, will answer to the inverse variations between
Individuation and Genesis which different types of organisms

518

LAWS OF MULTIPLICATION.

show us. Again, there may be a functional change of pro-
portion. If the engine has to draw a considerable load, the
abstraction of steam by the cylinders greatly reduces the
discharge by the safety-valve; and if a high velocity is kept
up, the discharge from the safety-valve entirely ceases. Con-
versely, if the velocity is low, the escape-steam bears a large
ratio to the steam consumed by the motor apparatus ; and if
the engine becomes stationary the whole of the steam escapes
by the safety-valve. This inverse variation answers to that
which we have traced between Expenditure and Genesis, as
displayed in the contrasts between species of the same type
but unlike activities, and in the contrasts between active and
inactive individuals of the same species. But now beyond
these inverse variations between the quantities of consumed
steam and escape-steam, which are structurally and function-
ally caused, there are coincident variations, producible in both
by changes in the quantity of steam supplied — changes which
may be caused in several ways. In the first place, the fuel
thrown into the furnace may be increased or made better.
Other things equal, there will result a more active locomo-
tion as well as a greater escape ; and this will answer to that
simultaneous addition to its individual vigour and its repro-
ductive activity, caused in an animal by a larger quantity, or
a superior quality, of food. In the second place, the steam
generated may be economized. Loss by radiation from the
boiler may be lessened by a covering of non-conducting sub-
stances; and part of the steam thus prevented from con-
densing, will go to increase the working power of the engine,
while part will be added to the quantity blowing off. This
variation corresponds to that simultaneous addition to bodily
vigour and propagative power, which results in animals that
have to expend less in keeping up their temperatures. In
the third place, by improvement of the steam-generating
apparatus, more steam may be obtained from a given weight
of fuel. A better-formed evaporating surface, or boiler tubes
which conduct more rapidly, or an increased number of them

MULTIPLICATION OF THE HUMAN RACE. 519

may cause a larger absorption of heat from the burning mass
or the hot gases it gives off; and the extra steam generated
by this extra heat will, as before, augment both the motive
force and the emission through the safety-valve. And this
last case of coincident variation, is parallel to the case with
which we are here concerned — the augmentation of individual
expenditure and of reproductive energy, that may be caused
by a superiority of some organ on which the utilizing or
economizing of materials depends.

Manifestly, therefore, an increased expenditure for Gene-
sis, or an increased expenditure for Individuation, may arise
in one of two quite different ways — either by diminution of
the antagonistic expenditure, or by addition to the store which
supplies both expenditures; and confusion results from not
distinguishing between these. Given the ratio 4 to 20, as
expressive of the relative costs of Genesis and Individuation ;
then the expenditure for Genesis may be raised to 5 while the
expenditure for Individuation is raised to 25, without any
alteration of type, merely by favourable circumstances or
superiority of constitution. On the other hand, circumstances
remaining the same, the expenditure for . Genesis may be
raised from 4 to 5, by lowering the expenditure for Indi-
viduation from 20 to 19 : which change of ratio may be
either functional and temporary, or structural and per-
manent. And only when it is the last does it illustrate that
inverse variation between degree of evolution and degree of
procreative dissolution, which we have everywhere seen.

§ 370. There is no reason to suppose, then, that the laws
of multiplication which hold of other beings, do not hold of
the human being. On the contrary, there, are special facts
which unite with general implications to show that these
laws do hold of the human being. The absence of direct
evidence in some cases where it might be looked for, we find
fully explained when all the factors are taken into account.
And certain seemingly-adverse facts prove, on examination,

520

£AWS OF MULTIPLICATION.

to be facts belonging to a different category from that in
which they are placed, and harmonize with the rest when
rightly interpreted.

The conformity of human fertility to the laws of multipli-
cation in general, being granted, it remains to inquire what
effects must be caused by permanent changes in 'men's natures
and circumstances. Thus far we have observed, how, by their
exceptionally-high evolution and exceptionally-low fertility,
mankind display the inverse variation between Individuation
and Genesis, in one of its extremes. And we have also ob-
served how mankind, like other kinds, are functionally changed
in their rates of multiplication by changes of conditions. But
we have not observed how alteration of structure in Man
entails alteration of fertility. The influence of this factor is
so entangled with the influences of other factors which are
for the present more potent, that we cannot recognize it.
Here, if we proceed at all, we must proceed deductively.

[Note. — From among the publications of the American
Academy of Political and Social Science, there was sent to
me some years ago an essay entitled " The Significance of a
Decreasing Birth Bate" by (Miss) J. L. Brownell, Fellow in
Political Science, Bryn Mawr College. This essay contains
a number of elaborate comparisons drawn from the vital
statistics of the tenth* United States Census. The results
of these comparisons are thus summed up: —

" 1. Whether or not it be true that the means spoken of by Dr.
Billings, M. Dumont, M. Levasseur, and Dr. Edson has become an
important factor in the diminishing birth-rate of civilized countries,
it is evident that it is not the only factor, and that, quite apart from
voluntary prevention, there is a distinct problem to be investigated.
This is shown by the fact that the white and the colored birth-rate
vary together.

" 2. Mr. Spencer's generalization that the birth-rate diminishes as
the rate of individual evolution increases is confirmed by a comparison
of the birth-rates with the death-rates from nervous diseases, and also

MULTIPLICATION OF THE HUMAN RACE. 521

with the density of population, the values of agricultural and manu-
factured products, and the mortgage indebtedness."

Of course multitudinous differences of race, class, mode
of living, occupation, locality, make it difficult to draw posi-
tive inferences from the data ; but the inferences above drawn
are held to remain outstanding after allowing for all the
qualifying conditions.]

CHAPTER XIII.

HUMAN POPULATION IN THE FUTURE.

§ 371. Any further evolution in the most-highly evolved
of terrestrial beings, Man, must be of the same nature as
evolution in general. Structurally considered, it may consist
in greater integration, or greater differentiation, or both —
augmented bulk, or increased heterogeneity and definiteness,
or a combination of the two. Functionally considered, it
may consist in a larger sum of actions, or more multiplied
varieties of actions, or both — a larger amount of sensible and
insensible motion generated, or motions more numerous in
their kinds and more intricate and exact in their co-ordina-
tions, or motions that are greater alike in quantity, com-
plexity, and precision.

Expressing the change in terms of that more special
evolution displayed by organisms; we may say that it must
be one which further adapts the moving equilibrium of
organic actions. As was pointed out in First Principles,
§ 173, "the maintenance of such a moving equilibrium, re-
quires the habitual genesis of internal forces corresponding
in number, directions, and amounts to the external incident
forces — as many inner functions, single or combined, as there
are single or combined outer actions to be met." And it
was also pointed out that " the structural complexity accom-
panying functional equilibration, is definable as one in which
there are as many specialized parts as are capable, separately
and jointly, of counteracting the separate and joint forces
522

HUMAN POPULATION IN THE FUTURE. 523

amid which the organism exists." Clearly, then, since all
incompletenesses in Man as now constituted, are failures to
meet certain of the outer actions (mostly involved, remote,
irregular), to which he is exposed; every advance implies
additional co-ordinations of actions and accompanying com-
plexities of organization.

Or, to specialize still further this conception of future pro-
gress, we may consider it as an advance towards completion
of that continuous adjustment of internal to external rela-
tions, which Life shows us. In Part I. of this work, where
it was shown that the correspondence between inner and
outer actions which under its phenomenal aspect, we call
Life, is a particular kind of what, in terms of Evolution, we
called a moving equilibrium; it was shown that the degree
of life varies as the degree of correspondence. Greater evo-
lution or higher life implies, then, such modifications of
human nature as shall make more exact the existing corre-
spondences, or shall establish additional correspondences, or
both. Connexions of phenomena of a rare, distant, unobtru-
sive, or intricate kind, which we either suffer from or do not
take advantage of, have to be responded to by new connexions
of ideas, and acts properly combined and proportioned : there
must be increase of knowledge, or skill, or power, or of all
these. And to effect this more extensive, more varied, and
more accurate, co-ordination of actions, there must be organi-
zation of still greater heterogeneity and definiteness.

§ 372. Let us, before proceeding, consider in what par-
ticular ways this further evolution, this higher life, this
greater co-ordination of actions, may be expected to show
itself.

Will it be in strength ? Probably not to any considerable
degree. Mechanical appliances are fast supplanting brute
force, and doubtless will continue doing this. Though at
present civilized nations largely depend for self-preservation
on vigour of limb, and are likely to do so while wars con-

524

LAWS OF MULTIPLICATION.

tinue; yet that progressive adaptation to the social state
which must at last bring wars to an end, will leave the
amount of muscular power to adjust itself to the requirements
of a peaceful regime. Though, taking all things into account,
the muscular power then required may not be less than now,
there seems no reason why more should be required. .

Will it be swiftness or agility ? Probably not. In savages
these are important elements of the ability to maintain
life; but in civilized men they aid self-preservation in quite
minor degrees, and there seems no circumstance likely to
necessitate an increase of them. By games and gymnastic
competitions, such attributes may indeed be artificially in-
creased; but no artificial increase which does not bring a
proportionate advantage can be permanent; since, other
things equal, individuals and societies that devote the same
amounts of energy in ways which subserve life more effectu-
ally, must by and by predominate.

Will it be in mechanical skill, that is, in the better-
co-ordination of complex movements? Most likely in some
degree. Awkwardness is continually entailing injuries and
deaths. Moreover the complicated tools which civilization
brings into use, are constantly requiring greater delicacy of
manipulation. All the arts, industrial and aesthetic, as they
develop, imply a corresponding development of perceptive and
executive faculties in men : the two act and react.

Will it be in intelligence? Largely, no doubt. There is
ample room for advance in this direction, and ample demand
for it. Our lives are universally shortened by our ignorance.
In attaining complete knowledge of our own natures and of
the natures of surrounding things — in ascertaining the con-
ditions of existence to which' we must conform, and in dis-
covering means of conforming to them under all variations
of seasons and circumstances; we have abundant scope for
intellectual progress.

Wrill it be in morality, that is, in greater power of self-

HUMAN POPULATION IN THE FUTURE. 525

regulation ? Largely also : perhaps most largely. Eight con-
duct is usually come short of more from defect of will than
defect of knowledge. For the right co-ordination of those
complex actions which constitute human life in its civilized
form, there goes not only the pre-requisite — recognition of
the proper course; but the further pre-requisite — a due
impulse to pursue that course. On calling to mind our
daily failures to fulfil often-repeated resolutions, we shall
perceive that lack of the needful desire, rather than lack of
the needful insight, is the chief cause of faulty action. A
further endowment of those feelings which civilization is
developing in us — sentiments responding to the requirements
of the social state — emotive faculties that find their gratifi-
cations in the duties devolving on us — must be acquired
before the crimes, excesses, diseases, improvidences, dishones-
ties, and cruelties, that now so greatly diminish the duration
of life, can cease.

Thus, looking at the several possibilities, and asking
what direction this further evolution, this more complete
moving equilibrium, this better adjustment of inner to outer
relations, this more perfect co-ordination of actions, is likely
to take; we conclude that it must take mainly the direction
of a higher intellectual and emotional development.

§ 373. This conclusion we shall find equally forced on us
if we inquire for the causes which are to bring about such
results. <JTo more in the case of Man than in the case of
any other being, can we presume that evolution has taken
place, or will hereafter take place, spontaneously. In the
past, at present, and in the future, all modifications, func-
tional and organic, have been, are, and must be, immediately
or remotely consequent on surrounding conditions.'^ What,
then, are those changes in the environment to which, by
direct or indirect equilibration, the human organism has been
adjusting itself, is adjusting itself now, and will continue to

526

LAWS OF MULTIPLICATION.

adjust itself? And how do they necessitate a higher evolu-
tion of the organism ?

Civilization, everywhere having for its antecedent the in-
crease of population, and everywhere having for one of its
consequences a decrease of certain race-destroying forces, has
for a further consequence- an increase of certain other race-
destroying forces. Danger of death from predatory animals
lessens as men grow more numerous. Though, as they spread
over the Earth and divide into tribes, men become wild
beasts to one another, yet the danger of death from this
cause also diminishes as tribes coalesce into nations. But the
danger of death which does not diminish, is that produced by
augmentation of numbers itself — the danger from deficiency
of food. Supposing human nature to remain unchanged, the
mortality hence resulting would, on the average, rise as
human beings multiplied. If mortality, under such condi-
tions, does not rise, it must be because the supply of food
also augments; and this implies some change in human
habits wrought by stress of human needs. Here, then, is
the permanent cause of modification to which civilized men
are exposed. Though the intensity of its action is ever being
mitigated in one direction by greater production of food, it
is, in the other direction, ever being added to by the greater
production of individuals. Manifestly, the wants of their
redundant numbers constitute the only stimulus mankind
have to obtain more necessaries of life. Were not the demand
beyond the supply, there would be no motive to increase the
supply. And manifestly, this excess of demand over supply
is perennial: this pressure of population, of which it is the
index, cannot be eluded. Though by the emigration that
takes place when the pressure arrives at a certain intensity,
temporary relief is from time to time obtained; yet as, by
this process, all habitable countries must become peopled, it
follows that in the end the pressure, whatever it may then
be, must be borne in full.

This constant increase of people beyond the means of sub-

HUMAN POPULATION IN THE FUTURE. r,27

sistence causes, then, a never-ceasing requirement for skill,
intelligence, and self-control — involves, therefore, a constant
exercise of these and gradual growth of them. Every indus-
trial improvement is at once the product of a higher form
of humanity, and demands that higher form of humanity to
carry it into practice. The application of science to the arts,
is the bringing to bear greater intelligence for satisfying our
wants, and implies continued progress of that intelligence.
To get more produce from the acre, the farmer must study
chemistry, must adopt new mechanical appliances, and must,
by the multiplication of processes, cultivate both his own
powers and the powers of his labourers. To meet the
requirements of the market, the manufacturer is per-
petually improving his old machines and inventing new
ones; and by the premium of high wages incites artizans to
acquire greater skill. The daily-widening ramifications of
commerce entail on the merchant a need for more know-
ledge and more complex calculations; while the lessening
profits of the ship-owner force him to build more scientifi-
cally, to get captains of higher intelligence and better crews.
In all cases pressure of population is the original cause.
Were it not for the competition this entails, more thought
and energy would not daily be spent on the business of life;
and growth of mental power would not take place.
Difficulty in getting a living is alike the incentive to a
higher education of children, and to a more intense and
long-continued application in adults. In the mother it
prompts foresight, economy, and skilful house-keeping ; in the
father, laborious days and constant self-denial. Nothing but
necessity could make men submit to this discipline; and
nothing but this discipline could produce a continued pro-
gression.

In this case, as in many others, Nature secures each step
in advance by a succession of trials; which are perpetually
repeated, and cannot fail to be repeated, until success is
achieved. All mankind in turn subject themselves more or

528

LAWS OF MULTIPLICATION.

less to the discipline described; they either may or may not
advance under it; but, in the nature of things, only those
who do advance under it eventually survive. For, neces-
sarily, families and races whom this increasing difficulty of
getting a living which excess of ferJilifa*--^nt^rrrs, does not
stimulate to improvements in production — that is, to greater
mental activity — are on the high road to extinction; and
must ultimately be supplanted by those whom the pressure
does so stimulate. This truth we have recently seen exem-
plified in Ireland. And here, indeed, without further illus-
tration, it will be seen that premature death, under all. its
forms and from all its causes, cannot fail to work in the
same direction. For as those prematurely carried off must,
in the average of cases, be those in whom the power of self-
preservation is the least, it unavoidably follows that those
leftbehind to continue the race, must be those in whom the
power of self-preservation is the greatest — must be the select
of their generation. So that, whether the dangers to exist-
ence be of the kind produced by excess of fertility, or of any
other kind, it is clear that by the ceaseless exercise of the
faculties needed to contend with them, and by the death of
all men who fail to contend with them successfully, there is
ensured a constant progress towards a higher degree of skill,
intelligence, and self-regulation — a better co-ordination of
actions — a more complete life.*

§ 374. The proposition at which we have thus arrived is,
then, that excess of fertility, through the changes it is ever

* A good deal of this chapter retains its original form ; and the above
paragraph is reprinted verbatim from the Westminster Review for April,
1852, in which the views developed in the foregoing hundred pages were
first sketched out. This paragraph shows how near one may be to a great
generalization without seeing it. Though the struggle for life is the alleged
motive force ; though the process of natural selection is recognized ; and
though to it is ascribed a share in the evolution of a higher type ; yet the
conception is not that which Mr. Darwin has worked out with such wonder-
ful skill and knowledge. In the first place, natural selection is here de-
scribed only as furthering direct adaptation — only as aiding progress by the
preservation of individuals in whom functionally-produced modifications have

\

HUMAN P(X,(<JLATION IN THE FUTURE. 529

working in Man's environment, is itself the cause of Man's
further evolution ; and the obvious corollary here to be drawn
is, that Man's further evolution so brought about, itself
necessitates a decline in his fertility.

All future progress in civilization which the never-
ceasing pressure of population must produce, will be accom-
panied by 'an enhanced cost of Individuation, both in
structure and function; and more especially in nervous
structure and function. The peaceful struggle for existence
in societies ever growing more crowded and more compli-
cated, must have for its concomitant an increase of the great
nervous centres in mass, in complexit}^ in activity. That
larger body of emotion needed as a fountain of energy for
men who have to hold their places and rear their families
under the intensifying competition of social life, is, other
things equal, the correlative of larger brain. Those higher
feelings presupposed by the better self-regulation which, in
a better society, can alone enable the individual to leave a
persistent posterity, are, other things equal, the correlatives
of a more complex brain; as are also those more numerous,
more varied, more general, and more abstract ideas, which
must also become increasingly requisite for successful life as
society advances. And the genesis of this larger quantity of

gone on most favourably. In the second place, there is no trace of the idea
that natural selection may by co-operation with the cause assigned, or with
other causes, produce divergences of structure ; and of course, in the absence
of this idea, there is no implication that natural selection has anything to do
with the origin of species. And in the third place, the all-important factor
of variation — " spontaneous," or incidental as we may otherwise call it — is
wholly ignored. Though use and disuse are, I think, much more potent
causes of organic modification than Mr. Darwin supposes — though, while pur-
suing the inquiry in detail, I have been led to believe that direct equilibration
has played a more active part even than I had myself at one time thought ;
yet I hold Mr. Darwin to have shown beyond question, that a great part of
the facts — perhaps the greater part — are explicable only as resulting from
the survival of individuals^which have deviated in some indirectly-caused way
from the ancestral type. NThus, the above paragraph contains merely a pass-
ing recognition of the selective process ; and indicates no suspicion of the
enormous range of its effects, or of the conditions under which a large part
of its effects are produced,/
80

530

LAWS OF MULTIPLICATION.

feeling and thought, in a brain thus augmented in size and
developed in structure, is, other things equal, the correlative
of a greater wear of nervous tissue and greater consumption
of materials to repair it. So that both in original cost of con-
struction and in subsequent cost of working, the nervous
system must become a heavier tax on the organism. Already
the brain of the civilized man is larger by nearly thirty per
cent, than the brain of the savage. Already, too, it presents
an increased heterogeneity — especially in the distribution of
its convolutions. And further changes like these which have
taken place under the discipline of civilized life, we infer
will continue to take place. But everywhere and

always, evolution is antagonistic to procreative dissolution.
Whether it be in greater growth of the organs which sub-
serve self-maintenance, whether it be in their added com-
plexity of structure, or whether it be in their higher activity,
the abstraction of the required materials implies a dimi-
nished reserve of materials for race-maintenance. And we
have seen reason to believe that this antagonism between
Individuation and Genesis, becomes unusually marked where
the nervous system is concerned, because of the costliness of
nervous structure and function. In § 346 was pointed out
the apparent connexion between high cerebral development
and prolonged delay of sexual maturity; and in §§ 366, 367,
the evidence went to show that where exceptional fertility
exists there is sluggishness of mind, and that where there
has been during education excessive expenditure in mental
action, there frequently follows a complete or partial infer-
tility. Hence the particular kind of further evolution which
Man is hereafter to undergo, is one which, more than any
other, may be expected to cause a decline in his power of
reproduction.

The higher nervous development and greater expenditure
in nervous action, here described as indirectly brought about
by increase of numbers, and as thereafter becoming a check
on the increase of numbers, must not be taken to imply

HUMAN POPULATION IN THE FUTURE. 531

an intenser strain — a mentally-laborious life. The greater
emotional and intellectual power and activity above con-
templated, must' be understood as becoming, by small incre-
ments, organic, spontaneous, and pleasurable. As, even when
relieved from the pressure of necessity, large-brained Euro-
peans voluntarily enter on enterprises and activities which
the savage could not keep up even to satisfy urgent wants;
so, their still larger-brained descendants will, in a still higher
degree, find their gratifications in careers entailing still
greater mental expenditures. This enhanced demand for
materials to establish and carry on the psychical functions,
will be a constitutional demand. We must conceive the
type gradually so modified, that the more-developed nervous
system irresistibly draws off, for its normal and unforced
activities, a larger proportion of the common stock of nutri-
ment; and while so increasing the intensity, completeness,
and length of the individual life, necessarily diminishing the
reserve applicable to the setting up of new lives — no longer
required to be so numerous.

Though the working of this process will doubtless be
interfered with and modified in the future, as it has been
in the past, by the facilitations of living which civilization
brings; yet nothing beyond temporary interruptions can so
be caused. However much the industrial arts may be im-
proved, there must be a limit to the improvement; while,
with a rate of multiplication in excess of the rate of mor-
tality, population must continually tread on the heels of
production. So that though, during the earlier stages of
civilization, an increased amount of food may accrue from a
given amount of labour, there must come a time when this
relation will be reversed, and when every additional incre-
ment of food will be obtained by a more than proportionate
labour : the disproportion growing ever higher, and the dimi-
nution of the reproductive power becoming greater.

§ 375. There now remains but to inquire towards what

532 LAWS OF MULTIPLICATION.

limjt this progress tends. So long as the fertility of the
race is monTthan sufficient to balance the diminution by
deaths, population must continue to increase. So long as
population conEmues Ixflncrease, there must be pressure on
the means of subsistence. And so long as there is pressure
on the means of subsistence, further mental development
must go on, and further diminution of fertility must result;
provided that the actions and reactions which have been
described are not artificially interfered with. I append this
qualifying clause advisedly, and especially emphasize it, be-
cause these actions and reactions have been hitherto, and
are now, greatly interfered with by governments, and the
continuance of the interferences may retard, if not stop,
that further evolution which would else go on.

I refer to those hindrances to the survival of the fittest
which in earlier times resulted from the undiscriminating
charities of monasteries and in later times from the opera-
tion of Poor Laws. Of course if the competition which
increasing pressure of population entails, is prevented from
acting on a considerable part of the community, such part,
saved from the needed intellectual and moral stress, will not
undergo any further mental development; and must ever
tend to leave a posterity, and an increasing posterity, in
which none of that higher individuation which checks
genesis takes place. Suqh State-meddlings with the natural
play of actions and reactions produce a further evil equally
great or greater. For those who are not self-maintained, or
but partially self-maintained, are supplied with the means
they lack by the better members of the community; and
these better members have thus not only to support them-
selves and their offspring, but also to support or aid the
inferior members and their offspring. The under-working of
one part is accompanied by the over-working of the other
part — by a working which at each stage of progress exceeds
that which the normal conditions necessitate, and results
sometimes in illness, premature age, or death, or in lessened

HUMAN POPULATION IN THE FUTURE. 533

number of children, or in imperfect rearing of children: the
bad are fostered and the good are repressed.

It does not follow that the struggle for life and the sur-
vival of the fittest must be left to work out their effects
without mitigation. It is contended only that there shall
not be a forcible burdening of the superior for the support
of the inferior. Such aid to the inferior as the superior
voluntarily yield, kept as it will be within moderate limits,
may be given with benefit to both — relief to the one, moral
culture to the other. And aid willingly given (little to the
least worthy and more to the most worthy) will usually be
so given as not to further the increase of the unworthy.
For in proportion as the emotional nature becomes more
evolved, and there grows up a higher sense of parental re-
sponsibility, the begetting of children that cannot be properly
reared will be universally held intolerable. If, as we see,
public opinion in many places and times becomes coercive
enough to force men to fight duels, we can scarcely doubt that
at a higher stage of evolution it may become so coercive as to
prevent men from marrying improvidently. If the frowns
of their fellows can make men commit immoral acts, surely
they may make men refrain from immoral acts — especially
when the actors themselves feel that the threatened frowns
would be justified. Hence with a higher moral nature will
come a restriction on the multiplication of the inferior.

In brief, the sole requirement is that there shall be no
extensive suspension of that natural relation between merit
and benefit which constitutes justice. Holding, then, that
this all-essential condition will itself come to be recognized
and enforced by a more evolved humanity, let us consider
what is the goal towards which the restraint on genesis by
individuation progresses.

§ 375a. Supposing the Sun's light and heat, on which all
terrestrial life depends, to continue abundant for a period
long enough to allow the entire evolution we are contem-

534

LAWS OF MULTIPLICATION.

plating; there are still certain changes which must prevent
such complete adjustment of human nature to surrounding
conditions, as would permit the rate of multiplication to
become equal to the rate of mortality. As before pointed out
(§ 148), during an epoch of 21,000 years each hemisphere goes
through a cycle of temperate seasons and seasons extreme in
their heat and cold — variations which are themselves alter-
nately exaggerated and mitigated in the course of far longer
cycles; and we saw that these cause perpetual ebbings and
Sowings of species over different parts of the Earth's surface.
Further, by slow but inevitable geologic changes, especially
those of elevation and subsidence, the climate and physical
characters of every habitat are modified; while old habitats
are destroyed and new are formed. This, too, we noted as
a constant cause of migrations and of resulting alterations
of environment. Now though the human race differs from
other races in having a power of artificially counteracting
external changes, yet there are limits to this power; and,
even were there no limits, the changes could not fail to work
their effects indirectly, if not directly. If, as is thought
probable, these astronomic cycles entail recurrent glacial
periods in each hemisphere, then parts of the Earth which are
at one time thickly peopled, will at another time be almost
deserted, and vice versa. The geologically-caused alterations
of climate and surface, must produce further slow re-distri-
butions of population; and other currents of people, to and
from different regions, will be necessitated by the rise of suc-
cessive centres of higher civilization. Consequently, man-
kind cannot but continue to undergo changes of environ-
ment, physical and moral, analogous to those which they
have thus far been undergoing. Such changes may eventu-
ally become slower and less marked; but they can never
cease. And if they can never cease there can never arise a
perfect adaptation of human nature to its conditions of exist-
ence. To establish that complete correspondence between
inner and outer actions which constitutes the highest life and

HUMAN POPULATION IN THE FUTURE. 535

greatest power of self-preservation, there must be a prolonged
converse between the organism and circumstances which re-
main the same. If the external relations are being altered
while the internal relations are being adjusted to them, the
adjustment can never become exact. And in the absence of
exact adjustment, there cannot exist that theoretically-highest
power of self-preservation with which there would co-exist
the theoretically-lowest power of race-production.

Hence though the number of premature deaths may ulti-
mately become very small, it can never become so small as
to allow the average number of offspring from each pair to
fall so low as two. Some average number between two and
three may be inferred as the limit — a number, however, which
is not likely to be quite constant, but may be expected at
one time to increase somewhat and afterwards to decrease
somewhat, according as variations in physical and social con-
ditions lower or raise the cost of self-preservation.

To this qualification must be added a further qualifica-
tion. The foregoing argument tacitly assumes that the
causes described will continuously operate on all mankind;
whereas a survey of the facts makes it clear that some parts
only of the Earth's surface are capable of bearing high
types of civilization, and consequently high types of Man.
There must remain hereafter, as there are now, considerable
parts of its surface which can support only groups of nomads,
or other groups obliged by their habitats to lead simple
and inferior kinds of life. /Only by subjection to the disci-
pline we have been contemplating can there be produced the
fully-developed Man; and evidently in many parts of the
world this discipline will continue to be eluded. \ Not only
must we conclude that the varieties of our race now liv-
ing in desert regions and arctic climates will continue here-
after to do so, but we may conclude that always, as now, a
certain proportion of men who are born in civilized societies,
impatient of the stress which pressure of population puts
on them, will escape into unoccupied or sparsely-peopled

536 LAWS OF MULTIPLICATION.

regions, where they can lead unrestrained lives though lives
of hardship. Recognizing as we must the probability that
in common with all other things, humanity will continue to
differentiate and produce a more heterogeneous assemblage
of types, we must infer that only in some of the highest of
these will the antagonism of individuation and genesis have
the anticipated effects.

Restricting ourselves to these, then, we may conclude
that in the end, pressure of population and its accompany-
ing evils will almost disappear; and will leave a state of
things requiring from each individual little more than a
normal and pleasurable activity. Cessation in the decrease
of fertility implies cessation in the development of the
nervous system; and this implies a nervous system which
has become equal to all that is demanded of it — has not to do
more than is natural to it. But that exercise of faculties which
does not exceed what is natural, constitutes gratification.

The necessary antagonism of Individuation and Genesis,
not only, then, fulfils the a priori law of maintenance of
race, from the monad up to Man, but ensures final attainment
of the highest form of this maintenance — a form in which
the amount of life shall be the greatest possible and the
births and deaths the fewest possible. From the beginning
pressure of population has been the proximate cause of pro-
gress. It produced the original diffusion of the race. It
compelled men to abandon predatory habits and take to
agriculture. It led to the clearing of the Earth's surface.
It forced men into the social state; made social organiza-
tion inevitable; and has developed the social sentiments. It
has stimulated to progressive improvements in production,
and to increased skill and intelligence. It is daily thrusting
us into closer contact and more mutually-dependent relation-
ships. And after having caused, as it ultimately must, the
due peopling of the globe, and the raising of its habitable
parts into the highest state of culture — after having perfected
all processes for the satisfaction of human wants — after

HUMAN POPULATION IN THE FUTURE. 53?

having, at the same time, developed the intellect into com-
petence for its work, and the feelings into fitness for social
life — after having done all this, the pressure of population
must gradually approach to an end — an end, however^ which
for the reasons given it cannot absolutely reach.

§ 377. In closing the argument let us not overlook the
self-sufficingness of those universal processes by which the
results reached thus far have been wrought out, and which
may be expected to work out these future results.

Evolution under all its aspects, general and special, is an
advance towards equilibrium. We have seen that the theo-
retical limit towards which the integration and differentia-
tion of every aggregate advances, is a state of balance be-
tween all the forces to which its parts are subject, and the
forces which its parts oppose to them (First Prin. § 170).
And we have seen that organic evolution is a progress towards
a moving equilibrium completely adjusted to environing
actions.

It has been also pointed out that, in civilized Man, there is
going on a new class of equilibrations — those between his ac-
tions and the actions of the societies he forms (First Prin.
§ 175). Social restraints and requirements are ever altering
his activities and by consequence his nature ; and as fast as his
nature is altered, social restraints and requirements undergo
more or less re-adjustment. Here the organism and the con-
ditions are both modifiable; and by successive conciliations
of the two, there is effected a progress towards equilibrium.

More recently we have seen that in every species, there
establishes itself an equilibrium of an involved kind between
the total race-destroying forces and the total race-preserving
forces— an equilibrium which implies that where the ability
to maintain individual life is small, the ability to propagate
must be great, and vice versa. Whence it follows that the
evolution of a race more in equilibrium with the environ-
ment, is also the evolution of a race in which there is a cor-

538

LAWS OF MULTIPLICATION.

relative approach, towards equilibrium between the number
of new individuals produced and the number which survive
and propagate.

The final result to be observed is that in Man, all these
equilibrations between constitution and conditions, between
the structure of society and the nature of its members,
between fertility and mortality, advance simultaneously
towards a common climax. In approaching an equilibrium
between his nature and the ever-varying circumstances of his
inorganic environment, and in approaching an equilibria]
between his nature and all the requirements of the social
state, Man is at the same time approaching that lowest limit
of fertility at which the equilibrium of population is main-
tained by the addition of as many infants as there are sub-
tractions by death in old age. But in a universe of which all
parts are in motion and every part is consequently subject to
change of conditions, neither this equilibrium nor any other

equilibrium can become complete.

THE END.

APPENDICES.

APPENDIX A.

SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS.

I append here the evidences referred to in § 190. The most
numerous and striking I have met with among the Umbelliferce.
Monstrosities having the alleged implication, are frequent in the
common Cow-Parsnep — so frequent that they must be familiar to
botanists ; and wild Angelica supplies many over-developments of
like meaning. Omitting numerous cases of more or less significance,
I will limit myself to two.

One of them is that of a terminal umbel, in which nine of the
outer umbellules are variously transformed — here a single flower
being made monstrous by the development of some of its members
into buds ; there several such malformed flowers being associated
with rays that bear imperfect umbellules ; and elsewhere, flowers

e

being replaced by umbellules : some of which are perfect, and others
imperfect only in the shortness of the flower-stalks. The annexed
Fig. 69, representing in a somewhat conventionalized way, a part. of

541

542

APPENDIX A.

the dried specimen, will give an idea of this Angelica. At a is
shown a single flower partially changed ; in the umbellule marked
b, one of the rays bears a secondary umbellule ; and there may
be seen at c and d, several such over-developments.

But the most conclusive instance is that of a Cow-Parsnep, in
which a single terminal umbel, besides the transformations already
mentioned, exhibits higher degrees of such transformations.* The
components of this complex growth are ; — three central umbellules,
abnormal only in minor points ; one umbellule, external to these,
which is partially changed into an umbel ; one rather more out of the
centre, which is so far metamorphosed as to be more an umbel than
an umbellule : nine peripheral clusters formed by the development
of umbellules into umbels, some of which are partially compounded
still further. Examined in detail, these structures present the fol-
lowing facts : — 1. The innermost umbellule is normal, save in having
a peripheral flower of which one member (apparently a petal) is
transformed into a flower-bud. 2. The next umbellule, not quite so
central, has one of its peripheral flowers made monstrous by the
growth of a bud from the base of the calyx. 3. The third of the
central umbellules has two abnormal outer flowers. One of them
carries a flower-bud on its edge, in place of a foliar member.
The other is half flower and half umbellule : being composed of
three petals, three stamens, and five flower- buds growing where
the other petals and stamens should grow. 4. Outside of these
umbellules comes one of the mixed clusters. Its five central
flowers are normal. Surrounding these are several flowers trans-
formed in different degrees : one having a stamen partially changed
into a flower bud. And then, at the periphery of this mixed
cluster, come three complete umbellules and an incomplete one in
which some petals and stamens of the original flower remain.
5. A mixed cluster, in which the umbel-structure predominates,
stands next. Its three central flowers are normal. Surrounding
them are five flowers over-developed in various ways, like those
already described. And on its periphery are seven complete um-
bellules in place of flowers ; besides an incomplete umbellule that
contains traces of the original flower, one of them being a petal
imperfectly twisted up into a bud. 6. Of the nine external
clusters, in which the development of simple into compound um-
bels is most decided, nearly all present anomalies. Three of them
have each a central flower untransformed ; and in others, the central

* For the information of those who may wish to examine metamorphoses
of these kinds, I may here state that I have found nearly all the examples
described, in the neighbourhood of the sea — the last-named, on the shore of
Locheil, near Fort William. Whether it is that I have sought more dili-
gently for cases when in such localities, or whether it is that the sea-air
favours that excessive nutrition whence these transformations result, I am
unable to say.

SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS. 543

umbellule is composed of two, three, or four flowers. 7. But the
most remarkable fact is, that in sundry of these peripheral clusters,
resulting from the metamorphosis of simple umbels into compound
umbels, the like metamorphosis is carried a stage higher. Some of
the component rays, are themselves the bearers of compound umbels

instead of simple umbels. In Fig. 70, a portion of the dried speci-
men is represented. Two of the central umbellules are marked a
and b ; those marked c and d are mixed clusters; at e and /are
compound umbels replacing simple ones ; and g shows one of the
rays on which the over-development goes still further.

Does not this evidence, enforced as it is by much more of like
kind, go far to prove that foliar organs may be developed into axial
organs ? Even were not the transitional forms traceable, there would
still, I think, be no other legitimate interpretation of the facts last
detailed. The only way of eluding the conclusion here drawn, is by
assuming that where a cluster of flowers replaces a single flower, it
is because the axillary buds which hypothetically belong to the
several foliar organs of the flower, become developed into axes ; and
assuming this, is basing an hypothesis on another hypothesis that is
directly at variance with facts. The foliar organs of flowers do not
bear buds in their axils ; and it would never have been supposed
that such buds are typically present, had it not been for that
mistaken conception of " type " which has led to many other errors
in Biology. Goethe writes : " Now as we cannot realize the idea
of a leaf apart from the node out of which it springs, or of a node
without a bud, we may venture to infer," &c. See here an example
of a method of philosophizing not uncommon among the Germans.

544 APPENDIX A.

The method is this — Survey a portion of the facts, and draw from
them a general conception ; project this general conception back
into the objective world, as a mould in which Nature casts her
products ; expect to find it everywhere fulfilled ; and allege poten-
tial fulfilment where no actual fulfilment is visible.

If instead of imposing our ideal forms on Nature, we are con-
tent to generalize the facts as Nature presents them, we shall find
no warrant for the morphological doctrine above enunciated. The
only conception of type justified by the logic of science, is — that
correlation of parts which remains constant under all modifications
of the structure to be defined. To ascertain this, we must compare
all these modifications, and note what traits are common to them.
On doing so with the successive segments of a phsenogamic axis,
we are brought to a conclusion widely different from that of Goethe.
Axillary buds are almost universally absent from the cotyledons ;
they are habitually present in the axils of fully-developed leaves
higher up the axis ; they are often absent from leaves that are close
to the flower ; they are nearly always absent from the bracts ; absent
from the sepals ; absent from the petals ; absent from the stamens ;
absent from the carpels. Thus, out of eight leading forms which
folia assume, one has the axillary bud and seven are without it.
With these facts before us, it seems to me not difficult to " realize
the idea " " of a node without a bud." If we are not possessed
by a foregone conclusion, the evidence will lead us to infer, that
each node bears a foliar appendage and may bear an axillary bud.

Even, however, were it granted that the typical segment of a
Phasnogam includes an axillary bud, which must be regarded as
always potentially present, no legitimate counter-interpretation of
the montrosities above described could thence be drawn. If when
an umbellule is developed in place of a flower, the explanation is,
that its component rays are axillary to the foliar organs of the
flower superseded ; we may fairly require that these foliar organs to
which they are axillary, shall be shown. But there are none. In
the last specimen figured, the inner rays of each such umbellule are
without them ; most of the outer rays are also without them ; and
in one cluster, only a single ray has a bract at its point of origin.
There is a rejoinder ready, however : the foliar organs are said to
be suppressed. Though Goethe could not " realize the idea " " of
a node without a bud," those who accept his typical form appear to
find no difficulty in realizing the idea of an axillary bud without
anything to which it is axillary. But letting this pass, suppose we
ask what is the warrant for this assumed suppression. Axillary
buds normally occur where the nutrition is high enough to produce
fully-developed leaves ; and when axillary buds are demonstrably
present in flowers, they accompany foliar organs that are more leaf-
like than usual — always greener ii not always larger. That is to

SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS. 545

say, the normal and the abnormal axillary buds, are alike the con-
comitants of foliar organs coloured by that chlorophyll which
habitually favours foliar development. How, then, can it be sup-
posed that when, out of a flower there is developed a cluster of
flower-bearing rays, the implied excess of nutrition causes the foliar
organs to abort ? It is true that very generally in a branched in-
florescence, the bracts of the several flower-branches are very small
(their smallness being probably due to that defective supply of
certain chlorophyll-forming matters, which is the proximate cause
of flowering) ; and it is true that, under these conditions, a flower-
ing axis of considerable size, for the development of which chloro-
phyll is less needful, grows from the axil of a dwarfed leaf. But
the inference that the foliar organ may therefore be entirely sup-
pressed, seems to me irreconcilable with the fact, that the foliar
organ is always developed to some extent before the axillary bud
appears. Until it has been shown that in some cases a lateral bud
first appears, and a foliar organ afterwards grows out beneath it, to
form its axil, the conception of an axillary bud of which the foliar
organ is suppressed, will remain at variance with the established
truths of development.

The above originally formed a portion of § 190. I have trans-
ferred it to the Appendix, partly because it contains too much
detail to render it fit for the general argument, and partly because
the interpretations being open to some question, it seemed unde-
sirable to risk compromising that argument by including them.
The criticisms passed upon these interpretations have not, how-
ever, sufficed to convince me of their incorrectness. Unfortu-
nately, I have since had no opportunity of verifying the above
statements by microscopic examinations, as I had intended.

Though unable to enforce the inference drawn by further facts
more minutely looked into, I may add some arguments based on
facts that are well known. One of these is the fact that the so-
called axillary bud is not universally axillary — is not universally
seated in the angle made by the axis and an appended foliar organ.
In certain plants the axillary bud is placed far above the node,
half-way between it and the succeeding node. So that not only may
a segment of a phasnogamic axis be without the axillary bud, but
the axillary bud, when present, may be removed from that place in
which, according to Goethe, it necessarily exists. Another fact not
congruous with the current doctrine, is the common occurrence of
" adventitious " buds — the buds that are put out from roots and from
old stems or branches bare of leaves. The name under which they are
thus classed, is meant to imply that they may be left out of conside-
ration. Those, however, who have not got a theorv to save by
81

546 APPENDIX A.

putting anomalies out of sight, may be inclined to think that the
occurrence of buds where they are avowedly unconnected with
nodes, and are axillary to nothing, tells very much against the as-
sumption that every bud implies a node and a corresponding foliar
organ. And they may also see that the development of these ad-
ventitious buds at places where there is excess of nutritive mate-
rials, favours the view above set forth. For if a bud thus arises at
a place where it is not morphologically accounted for, simply because
there happens to be at that place an abundance of. unorganized pro-
toplasm ; then, clearly, it is likely that if the mass of protoplasm
from which a leaf would usually arise, is greatly increased in mass
by excess of nutrition, it may develop into an axis instead of a leaf.

Many years after this work was published, I discovered among
my papers a memorandum which unfortunately I had overlooked,
containing further evidence in support of the foregoing conclusion.
With the omission of an error concerning the species of plant, I
reproduce this memorandum just as it stood : —

" I found at Dieppe, July 1, 1860, in a garden near the sea a
sample of cultivated wild flower (I thought it was grown as an
ornamental flower) in which some of the single flowers of the
umbel were developed into groups of flowers thus : —

" In the case where the transformation was fully effected the
umbellule had six flowers, answering to the six petals of the
original flowers. In other cases the transformation was incom-
plete. There were instances where but two of the petals were
developed into flowers ; and the other petals remained unchanged.
Others in which three were developed ; and others where four
were developed. In some cases, too, the development of a petal
into a flower was imperfect, in the absence of the flower stalk —
the flowers were sessile in the place where the petals would
have been. In one case there was an imperfect flower sessile ;
another imperfect flower on a short stalk; and three perfect
flowers on long stalks.

SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS. 547

" I was in some doubt whether the petals or the stamens were
developed. In cases of imperfect transformation the petals at
the base of the umbellule seemed to stand in the position of
calyx or involucrum, giving the idea that the stamens were de-
veloped into flowers. But in the case where there were six flowers
developed there were no petals at the base.

" That it was a matter of extra nutrition was shown by this : —

" 1. That they were cultivated as garden flowers.

" 2. That where there was one perfectly developed umbellule,
it was the only one in the umbel.

" 3. That where there were three umbellules they were all im-
perfect.

" 4. That in this imperfect umbellule the perfect flowers were
on long stalks and the imperfect ones sessile.

" 5. That the umbellules were on stalks both longer and
thicker than those of single flowers."

[Concerning the foregoing argument at large an expert
writes : — " The abnormalities you describe certainly show that
an axis may arise abnormally in the place of a normal leaf-
structure, and every modern botanist would be in agreement
with you in your criticism of the older form of the doctrine of
axillary buds. I think we are largely emancipated from the
dextrous juggling with the arrangements and relations of organs
which used to pass current as morphology.

"You have quoted sufficient evidence in the text (§ 190) to
establish the conclusion that no sharp line can be drawn between
axes and leaf-structure ; and a very great deal more could be
added in the same sense. Petioles for instance, exist which the
most highly trained histological observer could not distinguish
from stems.

" But I must demur to the suggestion that the replacement of
one by the other is primarily a question of nutrition. We are
as ignorant as ever of the proximate cause of the production
of a leaf or a shoot at a certain spot in meristematic tissue."

To this last remark I had at first made only the reply that the
plants exhibiting the abnormalities were in all cases excessively
luxuriant in their growths ; but to this I am now able to add a
more definite reply. The expert from whom I have just quoted,
had read this appendix before there had been made to it the
above addition describing the flower from Dieppe ; and I was not
myself aware, until I came to read over this addition, what clear
evidence it contains that extra nutrition was the cause of these
transformations of foliar structures into axial structures ; but
the above paragraphs 1, 2, 3, 4, 5, contain different evidences
conspiring to prove this.]

APPENDIX B

A CRITICISM ON PROF. OWEN'S THEORY OF THE
VERTEBRATE SKELETON.

[From the British & Foreign Medico- Chirurgical Review for Oct., 1858.]

I. On the Archetype and Homologies of the Vertebrate Skeleton.
By Richard Owen, F.R.S. — London, 1848. pp. 172.

II. Principes <F Osteologie Comparee, ou Recherches sur V Arche-
type et les Homologies du Squelette Vertebre. Par Richard
Owen. — Paris.

Principles of Comparative Osteology ; or, Researches on the Arche-
type and the Homologies of the Vertebrate Skeleton. By Richard
Owen.

III. On the Nature of Limbs. A Discourse delivered on Friday,
February 9, at an Evening Meeting of the Royal Institution of
Great Britain. By Richard Owen, F.R.S. — London, 1849.

pp. 119.

Judging whether another proves his position is a widely different
thing from proving your own. To establish a general law requires
an extensive knowledge of the phenomena to be generalized ; but to
decide whether an alleged general law is established by the evidence
assigned, requires merely an adequate reasoning faculty. Especially
is such a decision easy where the premises do not warrant the con-
clusion. It may be dangerous for one who has but little previous
acquaintance with the facts, to say that a generalization is demon-
strated ; seeing that the argument may be one-sided : there may be
many facts unknown to him which disprove it. But it is not
dangerous to give a negative verdict when the alleged demonstra-
548

A CRITICISM ON PROF. OWEN'S THEORY. 549

tion is manifestly insufficient. If the data put before him do not
bear out the inference, it is competent for every logical reader to
say so.

From this stand-point, then, we venture to criticize some of
Professor Owen's osteological theories. For his knowledge of
comparative osteology we have the highest respect. We believe
that no living man has so wide and detailed an acquaintance with
the bony structure of the Vertebrata. Indeed, there probably has
never been any one whose information on the subject was so nearly
exhaustive. Moreover, we confess that nearly all we know of this
department of biology has been learnt from his lectures and writ-
ings. We pretend to no independent investigations, but merely to
such knowledge of the phenomena as he has furnished us with.
Our position, then, is such that, had Professor Owen simply enun-
ciated his generalizations, we should have accepted them on his
authority. But he has brought forward evidence to prove them.
By so doing he has tacitly appealed to the judgments of his readers
and hearers — has practically said, " Here are the facts ; do they
not warrant these conclusions ? " And all we propose to do, is to
consider whether the conclusions are warranted by the facts brought
forward.

Let us first limit the scope of our criticisms. On that division
of comparative osteology which deals with what Professor Owen
distinguishes as " special homologies," we do not propose to enter.
That the wing of a bird is framed upon bones essentially parallel to
those of a mammal's fore-limb ; that the cannon-bone of a horse's
leg answers to the middle metacarpal of the human hand ; that
various bones in the skull of a fish are homologous with bones in
the skull of a man — these and countless similar facts, we take to be
well established. It may be, indeed, that the doctrine of special
homologies is at present carried too far. It may be that, just as
the sweeping generalization at one time favoured, that the embryonic
phases of the higher animals represent the adult forms of lower
ones, has been found untrue in a literal sense, and is acceptable
only in a qualified sense ; so the sweeping generalization that the
skeletons of all vertebrate animals consist of homologous parts,
will have to undergo some modification. But that this generaliza-
tion is substantially true, all comparative anatomists agree.

The doctrine which we are here to consider, is quite a separate
one — that of " general homologies." The truth or falsity of this
may be decided on quite apart from that of the other. Whether
certain bones in one vertebrate animal's skeleton correspond with
certain bones in another's, or in every other's, is one question ; and
whether the skeleton of every vertebrate animal is divisible into a
series of segments, each of which is modelled after the same type,
is another question. While the first is answered in the affirmative,

550 APPENDIX B.

the last may be answered in the negative ; and we propose to give
reasons why it should be answered in the negative.

In so far as his theory of the skeleton is concerned, Professor
Owen is an avowed disciple of Plato. At the conclusion of his
Archetype and Homologies of the Vertebrate Skeleton, he quotes ap-
provingly the Platonic hypothesis of iSeai, " a sort of models, or
moulds in which matter is cast, and which regularly produce the
same number and diversity of species." The vertebrate form in
general (see diagram of the Archetypus), or else the form of each
kind of vertebrate animal (see p. 172, where this seems implied),
Professor Owen conceives to exist as an " idea " — an " arche-
typal exemplar on which it has pleased the Creator to frame
certain of his living creatures." Whether Professor Owen holds
that the typical vertebra also exists as an " idea," is not so
certain. From the title given to his figure of the " ideal typical
vertebra," it would seem that he does; and at p. 40 of his
Nature of Limbs, and indeed throughout his general argument, this
supposition is implied. But on the last two pages of the Archetype
and Homologies, it is distinctly alleged that " the repetition of simi-
lar segments in a vertebral column, and of similar elements in a
vertebral segment, is analogous to the repetition of similar crystals
as the result of polarizing force in the growth of an inorganic
body ; " it is pointed out that, " as we descend the scale of animal
life, the forms of the repeated parts of the skeleton approach more
and more to geometrical figures ; " and it is inferred that " the
Platonic iSea or specific organizing principle or force, would seem
to be in antagonism with the general polarizing force, and to sub-
due and mould it in subserviency to the exigencies of the resulting
specific form." If Professor Owen's doctrine is to be understood
as expressed in these closing paragraphs of his Archetype and Homo-
logies— if he considers that "the iSea" "which produces the diver-
sity of form belonging to living bodies of the same materials," is
met by the " counter-operation " of " the polarizing force pervading
all space," which produces " the similarity of forms, the repetition
of parts, the signs of unity of organization," and which is "sub-
dued" as we ascend " in the scale of being ; " then we may pass on
with the remark that the hypothesis is too cumbrous and involved
to have much vraisemblance. If, on the other hand, Professor Owen
holds, as every reader would suppose from the general tenor of his
reasonings, that not only does there exist an archetypal or ideal
vertebrate skeleton, but that there also exists an archetypal or
ideal vertebra; then he carries the Platonic hypothesis much
further than Plato does. Plato's argument, that before any species
of object was created it must have existed as an idea of the Creative
Intelligence, and that hence all objects of such species must be

A CRITICISM OK PROF. OWEN'S THEORY. 551

copies of this original idea, is tenable enough from the anthropo-
morphic point of view. But while those who, with Plato, think fit
to base their theory of creation upon the analogy of a carpenter
designing and making a table, must yield assent to Plato's inference,
they are by no means committed to Professor Owen's expansion of
it. To say that before creating a vertebrate animal, God must
have had the conception of one, does not involve saying that God
gratuitously bound himself to make a vertebrate animal out of seg-
ments all moulded after one pattern. As there is no conceivable
advantage in this alleged adhesion to a fundamental pattern — as,
for the fulfilment of the intended ends, it is not only needless, but
often, as Professor Owen argues, less appropriate than some other
construction would be (see Nature of Limbs, pp. 39, 40), to sup-
pose the creative processes thus regulated, is not a little startling.
Even those whose conceptions are so anthropomorphic as to think
they honour the Creator by calling him " the Great Artificer," will
scarcely ascribe to him a proceeding which, in a human artificer,
they would consider a not very worthy exercise of ingenuity.

But whichever of these alternatives Professor Owen contends
for — whether the typical vertebra is that more or less crystalline
figure which osseous matter ever tends to assume in spite of " the
ISea or organizing principle," or whether the typical vertebra is
itself an " iSia or organizing principle " — there is alike implied
the belief that the typical vertebra has an abstract existence apart
from actual vertebras. It is a form which, in every endoskeleton,
strives to embody itself in matter — a form which is potentially
present in each vertebra ; which is manifested in each vertebra
with more or less clearness ; but which, in consequence of antago-
nizing forces, is nowhere completely realized. Apart from the
philosophy of this hypothesis, let us here examine the evidence
which is thought to justify it.

And first as to the essential constituents of the " ideal typical
vertebra." Exclusive of "diverging appendages" which it "may
also support," " it consists in its typical completeness of the follow-
ing elements and parts " : — A centrum round which the rest are
arranged in a somewhat radiate manner; above it two neurapophyses
— converging as they ascend, and forming with the centrum a trian-
guloid space containing the neural axis ; aneural spine, surmounting
the two neurapophyses, and with them completing the neural arch ;
below the centrum two h&mapophyses and a haemal spine, forming a
haemal arch similar to the neural arch above, and enclosing the
haemal axis; two yleur apophyses radiating horizontally from the
sides of the centrum ; and two parapophyses diverging from the
centrum below the pleurapophyses. " These," says Professor
Owen, " being usually developed from distinct and independent

552 APPENDIX B.

centres, I have termed ' autogenous elements.' " The remaining
elements, which he classes as " exogenous," because they " shoot
out as continuations from some of the preceding elements," are
the diapophyses diverging from the upper part of the centrum as
the parapophyses do below, and the zygapophyses which grow out
of the distal ends of the neurapophyses and haemapophyses.

If, now, these are the constituents of the vertebrate segment " in
its typical completeness;" and if the vertebrate skeleton consists
of a succession of such segments ; we ought to have in these con-
stituents, representatives of all the elements of the vertebrate
skeleton — at any rate, all its essential elements. Are we then to
conclude that the " diverging appendages," which Professor Owen
regards as rudimental limbs, and from certain of which he considers
actual limbs to be developed, are typically less important than some
of the above-specified exogenous parts — say the zygapophyses ?

That the meaning of this question may be understood, it will be
needful briefly to state Professor Owen's theory of The Nature of
Limbs ; and such criticisms as we have to make on it must be in-
cluded in the parenthesis. In the first place, he aims to show that
the scapular and pelvic arches, giving insertion to the fore and hind
limbs respectively, are displaced and modified haemal arches,
originally belonging in the one case to the occipital vertebra, and in
the other case to some trunk- vertebra not specified. In support of
this assumption of displacement, carried in some cases to the extent
of twenty-seven vertebras, Professor Owen cites certain acknow-
ledged displacements which occur in the human skeleton to the ex-
tent of half a vertebra — a somewhat slender justification. But for
proof that such a displacement has taken place in the scapular arch,
he chiefly relies on the fact that in fishes, the pectoral fins, which
are the homologues of the fore-limbs, are directly articulated to
certain bones at the back of the head, which he alleges are parts
of the occipital vertebra. This appeal to the class of fishes is
avowedly made on the principle that these lowest of the Vertebrata
approach closest to archetypal regularity, and may therefore be
expected to show the original relations of the bones more nearly.
Simply noting the facts that Professor Owen does not give us any
transitional forms between the alleged normal position of the
scapular arch in fishes, and its extraordinary displacement in the
higher Vertebrata ; and that he makes no reference to the embryonic
phases of the higher Vertebrata, which might be expected to ex-
hibit the progressive displacement ; we go on to remark that, in
the case of the pelvic arch, he abandons his principle of appealing
to the lowest vertebrate forms for proof of the typical structure.
In fishes, the rudimentary pelvis, widely removed from the spinal
column, shows no signs of having belonged to any vertebra ; and
here Professor Owen instances the perennibranchiate Batrachia as

A CRITICISM ON PROF. OWEN'S THEORY. 553

exhibiting the typical structure : remarking that " mammals, birds,
and reptiles show the rule of connexion, and fishes the exception."
Thus in the case of the scapular arch, the evidence afforded by
fishes is held of great weight, because of their archetypal regularity ;
while in the case of the pelvic arch, their evidence is rejected as
exceptional. But now, having, as he considers, shown that these
bony frames to which the limbs are articulated are modified haemal
arches, Professor Owen points out that the haemal arches habitually
bear certain " diverging appendages ; " and he aims to show that
the " diverging appendages " of the scapular and pelvic arches re-
spectively, are developed into the fore and hind limbs. There are
several indirect ways in which we may test the probability of this
conclusion. If these diverging appendages are " rudimental limbs "
— "future possible or potential arms, legs, wings, or feet," we may
fairly expect them always to bear to the heemal arches a relation
such as the limbs do. But they by no means do this. " As the
vertebrae approach the tail, these appendages are often transferred
gradually from the pleurapophysis to the parapophysis, or even to
the centrum and neural arch." (Arch, and Horn., p. 93.) Again,
it might naturally be assumed that in the lowest vertebrate forms,
where the limbs are but little developed, they would most clearly
display their alliance with the appendages, or " rudimental limbs,"
by the similarity of their attachments. Instead of this, however,
Professor Owen's drawings show that whereas the appendages are
habitually attached to the pleurapophyses, the limbs, in their earliest
and lowest phase, alike in fishes and in the Lepidosiren, are articu-
lated to the haemapophyses. Most anomalous of all, however, is
the process of development. When we speak of one thing as being
developed out of another, we imply that the parts next to the germ
are the first to appear, and the most constant. In the evolution of
a tree out of a seed, there come at the outset the stem and the
radicle ; afterwards the branches and divergent roots ; and still
later the branchlets and rootlets ; the remotest parts being the latest
and most inconstant. If, then, a limb is developed out of a " di-
verging appendage " of the haemal arch, the earliest and most con-
stant bones should be the humerus and femur ; next in order of
time and constancy should come the coupled bones based on these ;
while the terminal groups of bones should be the last to make their
appearance, and the most liable to be absent. Yet, as Professor
Owen himself shows, the actual mode of development is the very re-
verse of this. At p. 16 of the Archetype and Homologies, he says : —

" The earlier stages in the development of all locomotive extremities are
permanently retained or represented in the paired fins of fishes. First the
essential part of the member, the hand or foot, appears : then the fore-arm or
leg, both much shortened, flattened, and expanded, as in all fins and all em-
bryonic rudiments of limbs : finally come the humeral and femoral segments ;
but this stage I have not found attained in any fish."

554 APPENDIX B.

That is to say, alike in ascending through the Vertebrata gene-
rally, and in tracing up the successive phases of a mammalian em-
bryo, the last-developed and least constant division of the limb, is
that basic one by which it articulates with the haemal arch. It
seems to us that, so far from proving his hypothesis, Professor
Owen's own facts tend to show that limbs do not belong to the
vertebrae at all ; that they make their first appearance peripherally ;
that their development is centripetal ; and that they become fixed
to such parts of the vertebrate axis as the requirements of the case
determine.

But now, ending here this digressive exposition and criticism,
and granting the position that limbs " are developments of costal
appendages," let us return to the question above put — Why are not
these appendages included as elements of the " ideal typical ver-
tebra ? " It cannot be because of their comparative inconstancy ;
for judging from the illustrative figures, they seem to be as con-
stant as the haemal spine, which is one of the so-called autogenous
elements : in the diagram of the Archetypus, the appendage is re-
presented as attached to every vertebrate segment of the head and
trunk, which the haemal spine is not. It cannot be from their com-
parative unimportance ; seeing that as potential limbs they are
essential parts of nearly all the Vertebrata — much more obviously
so than the diapophyses are. If, as Professor Owen argues, " the
divine mind which planned the archetype also foreknew all its
modifications ; " and if, among these modifications, the development
of limbs out of diverging appendages was one intended to charac-
terize all the higher Vertebrata ; then, surely, these diverging ap-
pendages must have been parts of the " ideal typical vertebra."
Or, if the " ideal typical vertebra " is to be understood as a crystal-
line form in antagonism with the organizing principle ; then why
should not the appendages be included among its various offshoots ?
We do not ask this question because of its intrinsic importance.
We ask it for the purpose of ascertaining Professor Owen's method
of determining what are true vertebral constituents. He presents
us with a diagram of the typical vertebra, in which are included
certain bones, and from which are excluded certain others. If re-
lative constancy is the criterion, then there arises the question —
What degree of constancy entitles a bone to be included ? If re-
lative importance is the criterion, there comes not only the question
— What degree of importance suffices ? but the further question
— How is importance to be measured ? If neither of these is the
criterion, then what is it ? And if there is no criterion, does it
not follow that the selection is arbitrary ?

This question serves to introduce a much wider one : — Has the
" ideal typical vertebra " any essential constituents at all ? It might

A CRITICISM ON PROF. OWEN'S THEORY. 555

naturally be supposed that though some bones are so rarely
developed as not to seem worth including, and though some that
are included are very apt to be absent, yet that certain others are
invariable : forming, as it were, the basis of the ideal type. Let
us see whether the facts bear out this supposition. In his " summary
of modifications of corporal vertebrae " (p. 96), Professor Owen
says — " The hcemal spine is much less constant as to its existence,
and is subject to a much greater range of variety, when present,
than its vertical homotype above, which completes the neural arch."
Again he says — " The hcemapophyses, as osseous elements of a
vertebra, are less constant than the pleurapophyses." And again —
" The pleurapophyses are less constant elements than the neurapo-
physes." And again — " Amongst air-breathing vertebrates the
pleurapophyses of the trunk segments are present only in those spe-
cies in which the septum of the heart's ventricle is complete and im-
perforate, and here they are exogenous and confined to the cervical
and anterior thoracic vertebras." And once more, both the neura-
pophyses and the neural spine " are absent under both histological
conditions, at the end of the tail in most air-breathing vertebrates,
where the segments are reduced to their central elements." That
is to say, of all the peripheral elements of the " ideal typical ver-
tebra," there is not one which is always present. It will be ex-
pected, however, that at any rate the centrum is constant : the
bone which " forms the axis of the vertebral column, and com-
monly the central bond of union of the peripheral elements of the
vertebrate (p. 97), is of course an invariable element. No : not
even this is essential.

" The centrums do not pass beyond the primitive stage of the notochord
(undivided column) in the existing lepidosiren, and they retained the like
rudimental state in every fish whose remains have been found in strata
earlier than the permian aera in Geology, though the number of vertebrae is
frequently indicated in Devonian and Silurian ichthyolites by the fossilized
neur- and htemapophyses and their spines " (p. 96).

Indeed, Professor Owen himself remarks that " the neurapo-
physes are more constant as osseous or cartilaginous elements of the
vertebrae than the centrums " (p. 97). Thus, then, it appears that
the several elements included in the " ideal typical vertebra " have
various degrees of constancy, and that no one of them is essential.
There is no one part of a vertebra which invariably answers to its
exemplar in the pattern-group. How does this fact consist with the
hypothesis ? If the Creator saw fit to make the vertebrate skeleton
out of a series of segments, all formed on essentially the same model
— if, for the maintenance of the type, one of these bony segments
is in many cases formed out of a coalesced group of pieces, where,
as Professor Owen argues, a single piece would have served as well
or better; then we ought to find this typical repetition of parts

556 APPENDIX B.

uniformly manifested. Without any change of shape, it would ob-
viously have been quite possible for every actual vertebra to have
contained all the parts of the ideal one — rudimentally where they
were not wanted. Even one of the terminal bones of a mammal's
tail might have been formed out of the nine autogenous pieces,
united by suture but admitting of identification. As, however,
there is no such uniform typical repetition of parts, it seems to us
that to account for the typical repetition which does occur, by sup-
posing the Creator to have fixed on a pattern- vertebra, is to ascribe
to him the inconsistency of forming a plan and then abandoning it.
If, on the other hand, Professor Owen means that the " ideal
typical vertebra " is a crystalline form in antagonism with " the
idea or organizing principle ; " then we might fairly expect to find
it most clearly displaying its crystalline character, and its full com-
plement of parts, in those places where the organizing principle
may be presumed to have " subdued " it to the smallest extent.
Yet in the Vertebrata generally, and even in Professor Owen's
Arcketypus, the vertebras of the tail, which must be considered as,
if anything, less under the influence of the organizing principle
than those of the trunk, do not manifest the ideal form more com-
pletely. On the contrary, as we approach the end of the tail, the
successive segments not only lose their remaining typical elements,
but become as uncrystalline-looking as can be conceived.

Supposing, however, that the assumption of suppressed or unde-
veloped elements be granted — supposing it to be consistent with
the hypothesis of an " ideal typical vertebra," that the constituent
parts may severally be absent in greater or less number, sometimes
leaving only a single bone to represent them all ; may it not be that
such parts as are present, show their respective typical natures by
some constant character : say their mode of ossification ?

To this question some parts of the Archetype and Homologies
seem to reply, " Yes ; " while others clearly answer, " No." Criticis-
ing the opinions of Geoffroy St. Hilaire and Cuvier, who agreed in
thinking that ossification from a separate centre was the test of a
separate bone, and that thus there were as many elementary bones
in the skeleton as there were centres of ossification, Professor Owen
points out that, according to this test, the human femur, which is
ossified from four centres, must be regarded as four bones ; while
the femur in birds and reptiles, which is ossified from a single
centre, must be regarded as a single bone. Yet, on the other hand,
he attaches weight to the fact that the skull of the human foetus
presents " the same ossific centres " as do those of the embryo kan-
garoo and the young bird. [Nature of Limbs, p. 40.) And at p.
104 of the Homologies, after giving a number of instances, he says —

" These and the like correspondences between the points of ossification of

A CRITICISM ON PROP. OWEN'S THEORY. 557

the human foetal skeleton, and the separate bones of the adult skeletons of
inferior animals, are pregnant with interest, and rank among the most striking
illustrations of unity of plan in the vertebrate organization."

It is true that on the following page he seeks to explain this
seeming contradiction by distinguishing

" between those centres of ossification that have homological relations, and
those that have teleological ones — i.e., between the separate points of ossifica-
tion of a human bone which typify vertebral elements, often permanently dis-
tinct bones in the lower animals ; and the separate points which, without such
signification, facilitate the progress of osteogeny, and have for their obvious
final cause the well-being of the growing animal."

But if there are thus centres of ossification which have homo-
logical meanings, and others which have not, there arises the ques-
tion— How are they always to be distinguished ? Evidently in-
dependent ossification ceases to be a homological test, if there are
independent ossifications that have nothing to do with the homo-
logies. And this becomes the more evident when we learn that
there are cases where neither a homological nor a teleological
meaning can be given. Among various modes of ossification of the
centrum, Professor Owen points out that " the body of the human
atlas is sometimes ossified from two, rarely from three, distinct
centres placed side by side " (p. 89) ; while at p. 87 he says : — " In
osseous fishes I find that the centrum is usually ossified from six
points." It is clear that this mode of ossification has here no homo-
logical signification ; and it would be difficult to give any teleo-
logical reason why the small centrum of a fish should have more
centres of ossification than the large centrum of a mammal. The
truth is, that as a criterion of the identity or individuality of a bone,
mode of ossification is quite untrustworthy. Though, in his " ideal
typical vertebra," Professor Owen delineates and classifies as sepa-
rate " autogenous " elements, those parts which are " usually
developed from distinct and independent centres ; " and though by
doing so he erects this characteristic into some sort of criterion ;
yet his own facts show it to be no criterion. The parapophyses
are classed among the autogenous elements ; yet they are auto-
genous in fishes alone, and in these only in the trunk vertebrae,
while in all air-breathing vertebrates they are, when present at all,
exogenous. The neurapophyses, again, " lose their primitive in-
dividuality by various kinds and degrees of confluence : " in the
tails of the higher Vertebrata they, in common with the neural
spine, become exogenous. Nay, even the centrum may lose its
autogenous character. Describing how, in some batrachians,
" the ossification of the centrum is completed by an extension of
bone from the bases of the neurapophyses, which effects also the
coalescence of these with the centrum," Professor Owen adds : —
"In Pelobates fuscus and Pelobates cultripes, Muller found the en-

558 APPENDIX B.

tire centrum ossified from this source, without any independent
points of ossification " (p. 88). That is to say, the centrum is in
these cases an exogenous process of the neurapophyses. We see,
then, that these so-called typical elements of vertebrae have no
constant developmental character by which they can be identified.
Not only are they undistinguishable by any specific test from other
bones not included as vertebral elements ; not only do they fail to
show their typical characters by their constant presence ; but,
when present, they exhibit no persistent marks of individuality.
The central element may be ossified from six, four, three, or two
points ; or it may have no separate point of ossification at all :
and similarly with various of the peripheral elements. The whole
group of bones forming the "ideal typical vertebra" may severally
have their one or more ossific centres ; or they may, as in a mam-
mal's tail, lose their individualities in a single bone ossified from
one or two points.

Another fact which seems very difficult to reconcile with the
hypothesis of an " ideal typical vertebra," is the not infrequent
presence of some of the typical elements in duplicate. Not only,
as we have seen, may they severally be absent, but they may seve-
rally be present in greater number than they should be. When
we see, in the ideal diagram, one centrum, two neurapophyses,
two pleurapophyses, two haemapophyses, one neural spine, and one
haemal spine, we naturally expect to find them always bearing to
each other these numerical relations. Though we may not be
greatly surprised by the absence of some of them, we are hardly
prepared to find others multiplied. Yet such cases are common.
Thus the neural spine " is double in the anterior vertebrae of some
fishes " (p. 98). Again, in the abdominal region of extinct saurians,
and in crocodiles, " the freely-suspended hsemapophyses are com-
pounded of two or more overlapping bony pieces" (p. 100). Yet
again, at p. 99, we read — "I have observed some of the expanded
pleurapophyses in the great Testudo elephantopus ossified from two
centres, and the resulting divisions continuing distinct, but united
by suture." Once more " the neurapophyses, which do not advance
beyond the cartilaginous stage in the sturgeon, consist in that fish
of two distinct pieces of cartilage ; and the anterior pleurapophyses
also consist of two or more cartilages, set end on end" (p. 91).
And elsewhere referring to this structure, he says : —

" Vegetative repetition of perivertebral parts not only manifests itself in
the composite neurapophyses and pleurapophyses, but in a small accessory
(interneural) cartilage, at the fore and back part of the base of the neura-
pophysis ; and by a similar (interhaemal) one at the fore and back part of
most of the parapophyses " (p. 8*7).

Thus the neural and haemal spines, the neurapophyses, the pleu-

A CRITICISM ON PROF. OWEN'S THEORY. 559

rapophyses, the haemapophyses, may severally consist of two or
more pieces. This is not all : the like is true even of the centrums.

u In Hrptanchus (Squalus cinereus) the vertebral centres are feebly and
vegetatively marked out by numerous slender rings of hard cartilage in the
notochordal capsule, the number of vertebrae being more definitely indicated
by the ncurapophyses and parapophyses. ... In the piked dog-fish
(Acanthias) and the spotted dog-fish (Scyllium) the vertebral centres coin-
cide in number with the neural arches " (p. 87).

Is it not strange that the pattern vertebra should be so little
adhered to, that each of its single typical pieces may be trans-
formed into two or three ?

But there are still more startling departures from the alleged
type. The numerical relations of the elements vary not only in
this way, but in the opposite way. A given part may be present
not only in greater number than it should be, but also in less. In
the tails of homocercal fishes, the centrums " are rendered by cen-
tripetal shortening and bony confluence fewer in number than the
persistent, neural, and haemal arches of that part " — that is, there
is only a fraction of a centrum to each vertebra. Nay, even this
is not the most heteroclite structure. Paradoxical as it may seem,
there are cases in which the same vertebral element is, considered
under different aspects, at once in excess and defect. Speaking
of the haemal spine, Professor Owen says : —

" The horizontal extension of this vertebral element is sometimes accom-
panied by a median division, or in other words, it is ossified from two
lateral centres ; this is seen in the development of parts of the human
sternum ; the same vegetative character is constant in the broader thoracic
haemal spines of birds; though, sometimes, as e.g., in the struthionidae,
ossification extends from the same lateral centre lengthwise — i.e., forwards and
backwards, calcifying the connate cartilaginous homologues of halves of four
or five hcemal spines, before these finally coalesce with their fellows at the median
line" (p. 101).

So that the sternum of the ostrich, which according to the hypo-
thesis, should, in its cartilaginous stage, have consisted of four or
five transverse pieces, answering to the vertebral segments, and
should have been ossified from four or five centres, one to each
cartilaginous piece, shows not a trace of this structure ; but in-
stead, consists of two longitudinal pieces of cartilage, each ossified
from one centre, and finally coalescing on the median line. These
four or five haemal spines have at the same time doubled their in-
dividualities transversely, and entirely lost them longitudinally !

There still remains to be considered the test of relative position.
It might be held that, spite of all the foregoing anomalies, if the
typical parts of the vertebrae always stood towards each other in
the same relations — always preserved the same connexions, some-
thing like a case would be made out. Doubtless, relative position

560 APPENDIX B.

is an important point ; and it is one on which Professor Owen mani-
festly places great dependence. In his discussion of " moot cases
of special homology," it is the general test to which he appeals.
The typical natures of the alisphenoid, the mastoid, the orbito-
sphenoid, the prefrontal, the malar, the squamosal, &c, he deter-
mines almost wholly by reference to the adjacent nerve-perforations
and the articulations with neighbouring bones (see pp. 19 to 72) :
the general form of the argument being — This bone is to be classed
as such or such, because it is connected thus and thus with these
others, which are so and so. Moreover, by putting forth an " ideal
typical vertebra," consisting of a number of elements standing
towards each other in certain definite arrangement, this persistency
of relative position is manifestly alleged. The essential attribute
of this group of bones, considered as a typical group, is the con-
stancy in the connexions of its parts : change the connexions, and
the type is changed. But the constancy of relative position thus
tacitly asserted, and appealed to as a conclusive test in •' moot
cases of special homology," is clearly negatived by Professor
Owen's own facts. For instance, in the " ideal typical vertebra,"
the haemal arch is represented as formed by the two haemapophyses
and the haemal spine ; but at p. 91 we are told that

" The contracted haemal arch in the caudal region of the body may be
formed by different elements of the typical vertebra: e.g., by the para-
pophyses (fishes generally) ; by the pleurapophyses (lepidosiren) ; by both
parapophyses and pleurapophyses (Sudis, Lepidosteus), and by hsemapo-
physes, shortened and directly articulated with the centrums (reptiles and
mammals)."

And further, in the thorax of reptiles, birds, and mammals, " the
haemapophyses are removed from the centrum, and are articulated to
the distal ends of the pleurapophyses ; the bony hoop being com-
pleted by the intercalation of the haemal spine " (p. 82). So that
there are Jive different ways in which the haemal arch may be formed
— four modes of attachment of the parts different from that shown
in the typical diagram ! Nor is this all. The pleurapophyses " may
be quite detached from their proper segment, and suspended to the
haemal arch of another vertebra ; " as we have already seen, the
entire haemal arch may be detached and removed to a distance,
sometimes reaching the length of twenty-seven vertebrae ; and, even
more remarkable, the ventral fins of some fishes, which theoretically
belong to the pelvic arch, are so much advanced forward as to be
articulated to the scapular arch — " the ischium elongating to join
the coracoid." With these admissions it seems to us that relative
position and connexions cannot be appealed to as tests of homology,
nor as evidence of any original type of vertebra.

In no class of facts, then, do we find a good foundation for the
hypothesis of an " ideal typical vertebra." There is no one con-

A CRITICISM ON PROF. OWEN'S THEORY. 561

ceivable attribute of this archetypal form which is habitually realised
by actual vertebrae. The alleged group of true vertebral elements
is not distinguished in any specified way from bones not included in
it. Its members have various degrees of inconstancy ; are rarely
all present together ; and no one of them is essential. They are
severally developed in no uniform way : each of them may arise
either out of a separate piece of cartilage, or out of a piece con-
tinuous with that of some other element ; and each may be ossified
from many independent points, from one, or from none. Not only
may their respective individualities be lost by absence, or by con-
fluence with others ; but they may be doubled, or tripled, or halved,
or may be multiplied in one direction and lost in another. The en-
tire group of typical elements may coalesce into one simple bone
representing the whole vertebra ; and even, as in the terminal piece
of a bird's tail, half-a-dozen vertebrae, with all their many elements,
may become entirely lost in a single mass. Lastly, the respective
elements, when present, have no fixity of relative position : sundry
of them are found articulated to various others than those with
which they are typically connected ; they are frequently displaced
and attached to neighbouring vertebrae ; and they are even removed
to quite remote parts of the skeleton. It seems to us that if this
want of congruity with the facts does not disprove the hypothesis,
no such hypothesis admits of disproof.

Unsatisfactory as is the evidence in the case of the trunk and
tail vertebrae, to which we have hitherto confined ourselves, it is far
worse in the case of the alleged cranial vertebrae. The mere fact
that those who have contended for the vertebrate structure of the
skull, have differed so astonishingly in their special interpretations
of it, is enough to warrant great doubt as to the general truth of
their theory. From Professor Owen's history of the doctrine of
genera] homology, we gather that Dumeril wrote upon "latete
considered comme une vertebre;" that Kielmeyer, "instead of
calling the skull a vertebra, said each vertebra might be called a
skull ; " that Oken recognized in the skull three vertebrae and a
rudiment ; that Professor Owen himself makes out four vertebrae ;
that Goethe's idea, adopted and developed by Cams, was, that the
skull is composed of six vertebrae ; and that Geolfroy St. Hilaire
divided it into seven. Does not the fact that different comparative
anatomists have arranged the same group of bones into one, three,
four, six, and seven vertebral segments, show that the mode of de-
termination is arbitrary, and the conclusions arrived at fanciful ?
May we not properly entertain great doubts as to any one scheme
being more valid than the others ? And if out of these conflicting
schemes we are asked to accept one, ought we not to accept it only
on the production of some thoroughly conclusive proof — some
82

562 APPENDIX B.

rigorous test showing irrefragably that the others must be wrong
and this alone right ? Evidently where such contradictory opinions
have been formed by so many competent judges, we ought, before
deciding in favour of one of them, to have a clearness of demon-
stration much exceeding that required in any ordinary case. Let
us see whether Professor Owen supplies us with any such clear-
ness of demonstration.

To bring the first or occipital segment of the skull into corre-
spondence with the " ideal typical vertebra," Professor Owen argues,
in the case of the fish, that the parapophyses are displaced, and
wedged between the neurapophyses and the neural spine — removed
from the haemal arch and built into the upper part of the neural
arch. Further, he considers that the pleurapophyses are ideologi-
cally compound. And then, in all the higher vertebrata, he alleges
that the haemal arch is separated from its centrum, taken to a dis-
tance, and transformed into the scapular arch. Add to which, he
says that in mammals the displaced parapophyses are mere processes
of the neurapophyses (p. 133) : these vertebral elements, typically
belonging to the lower part of the centrum, and in nearly all cases
confluent with it, are not only removed to the far ends of elements
placed above the centrum, but have become exogenous parts of them!

Conformity of the second or parietal segment of the cranium with
the pattern-vertebra, is produced thus : — The petrosals are excluded
as being partially-ossified sense-capsules, not forming parts of the
true vertebral system, but belonging to the " splanchno-skeleton."
A centrum is artificially obtained by sawing in two the bone which
serves in common as centrum to this and the preceding segment; and
this though it is admitted that in fishes, where their individualities
ought to be best seen, these two hypothetical centrums are not
simply coalescent, but connate. Next, a similar arbitrary bisection
is made of certain elements of the haemal arches. And then, " the
principle of vegetative repetition is still more manifest in this arch
than in the occipital one :" each pleurapophysis is double ; each
haemapophysis is double ; and the haemal spine consists of six pieces !

The interpretation of the third and fourth segments being of
the same general character, need not be detailed. The only point
calling for remark being, that in addition to the above various
modes of getting over anomalies, we find certain bones referred to
the dermo-skeleton.

Now it seems to us, that even supposing no antagonist interpre-
tations had been given, an hypothesis reconcilable with the facts
only by the aid of so many questionable devices, could not be con-
sidered satisfactory ; and that when, as in this case, various com-
parative anatomists have contended for other interpretations, the
character of this one is certainly not of a kind to warrant the re-
jection of the others in its favour ; but rather of a kind to make

A CRITICISM ON PROF. OWEN'S THEORY. 563

us doubt the possibility of all such interpretations. The question
which naturally arises is, whether by proceeding after this fashion,
groups of bones might not be arranged into endless typical forms.
If, when a given element was not in its place, we were at liberty to
consider it as suppressed, or connate with some neighbouring element,
or removed to some more or less distant position ; — if, on finding a
bone in excess, we might consider it, now as part of the dermo-
skeleton, now as part of the splanchno-skeleton, now as transplanted
from its typical position, now as resulting from vegetative repetition,
and now as a bone ideologically compound (for these last two are
intrinsically different, though often used by Professor Owen as
equivalents) ; — if, in other cases, a bone might be regarded as
spurious (p. 91), or again as having usurped the place of another;
— if, we say, these various liberties were allowed us, we should
not despair of reconciling the facts with various diagrammatic
types besides that adopted by Professor Owen.

When, in 1851, we attended a course of Professor Owen's lec-
tures on Comparative Osteology, beginning though we did in the at-
titude of discipleship, our scepticism grew as we listened, and reached
its climax when we came to the skull ; the reduction of which to the
vertebrate structure, reminded us very much of the interpretation
of prophecy. The delivery, at the Royal Society, of the Croonian
Lecture for 1858, in which Prof essor Huxley, confirming the state-
ments of several German anatomists, has shown that the facts of
embryology do not countenance Professor Owen's views respecting
the formation of the cranium, has induced us to reconsider the verte-
bral theory as a whole. Closer examination of Professor Owen's
doctrines, as set forth in his works, has certainly not removed the
scepticism generated years ago by his lectures. On the contrary,
that scepticism has deepened into disbelief. And we venture to think
that the evidence above cited shows this disbelief to be warranted.

There remains the question — What general views are we to
take respecting the vertebrate structure ? If the hypothesis of an
" ideal typical vertebra " is not justified by the facts, how are we
to understand that degree of similarity which vertebras display ?

We believe the explanation is not far to seek. All that our
space will here allow, is a brief indication of what seems to us the
natural view of the matter.

Professor Owen, in common with other comparative anatomists,
regards the divergences of individual vertebras from the average
form, as due to adaptive modifications. If here one vertebral ele-
ment is largely developed, while elsewhere it is small — if now the
form, now the position, now the degree of coalescence, of a given
part varies ; it is that the local requirements have involved this
change. The entire teaching of comparative osteology implies that

564 APPENDIX B.

differences in the conditions of the respective vertebrae necessitate
differences in their structures.

Now, it seems to us that the first step towards a right concep-
tion of the phenomena, is to recognize this general law in its converse
application. If vertebras are unlike in proportion to the unlike-
ness of their circumstances, then, by implication, they will be like in
proportion to the likeness of their circumstances. While successive
segments of the same skeleton, and of different skeletons, are all in
some respects more or less differently acted on by incident forces,
and are therefore required to be more or less different ; they are
all, in other respects, similarly acted on by incident forces, and are
therefore required to be more or less similar. It is impossible to
deny that if differences in the. mechanical functions of the vertebrae
involve differences in their forms ; then, community in their mechani-
cal functions, must involve community in their forms. And as we
know that throughout the Vertebrata generally, and in each vertebrate
animal, the vertebras, amid all their varying circumstances, have a
certain community of function, it follows necessarily that they will
have a certain general resemblance — there will recur that average
shape which has suggested the notion of a pattern vertebra.

A glance at the facts at once shows their harmony with this
conclusion. In an eel or a snake, where the bodily actions are such
as to involve great homogeneity in the mechanical conditions of the
vertebras, the series of them is comparatively homogeneous. On the
contrary, in a mammal or a bird, where there is considerable hetero-
geneity in their circumstances, their similarity is no longer so great.
And if, instead of comparing the vertebral columns of different
animals, we compare the successive vertebrae of any one animal, we
recognize the same law. In the segments of an individual spine,
where is there the greatest divergence from the common mechanical
conditions ? and where may we therefore expect to find the widest
departure from the average form ? Obviously at the two extremities.
And accordingly it is at the two extremities that the ordinary
structure is lost.

Still clearer becomes the truth of this view, when we consider the
genesis of the vertebral column as displayed throughout the ascend-
ing grades of the Vertebrata. In its first embryonic stage, the spine
is an undivided column of flexible substance. In the early fishes,
while some of the peripheral elements of the vertebrae were marked
out, the central axis was still a continuous unossified cord. And
thus we have good reason for thinking that in the primitive verte-
brate animal, as in the existing Amphioxus, the notochord was per-
sistent. The production of a higher, more powerful, more active
creature of the same type, by whatever method it is conceived to
have taken place, involved a change in the notochordal structure.
Greater muscular endowments presupposed a firmer internal fulcrum

A CRITICISM ON PROF. OWEN'S THEORY. 565

— a less yielding central axis. On the other hand, for the central
axis to have become firmer while remaining continuous, would have
entailed a stiffness incompatible with the creature's movements.
Hence, increasing density of the central axis necessarily went hand
in hand with its segmentation : for strength, ossification was re-
quired ; for flexibility, division into parts. The production of ver-
tebras resulting thus, there obviously would arise among them a
general likeness, due to the similarity in their mechanical condi-
tions, and more especially the muscular forces bearing on them.
And then observe, lastly, that where, as in the head, the terminal
position and the less space for development of muscles, entailed
smaller lateral bendings, the segmentation would naturally be less
decided, less regular, and would be lost as we approached the
front of the head.

But, it may be replied, this hypothesis does not explain all the
facts. It does not tell us why a bone whose function in a given
animal requires it to be solid, is formed not of a single piece, but by
the coalescence of several pieces, which in other creatures are sepa-
rate ; it does not account for the frequent manifestations of unity
of plan in defiance of teleological requirements. This is quite true.
But it is not true, as Professor Owen argues respecting such cases,
that " if the principle of special adaptation fails to explain them, and
we reject the idea that these correspondences are manifestations of
some archetypal exemplar, on which it has pleased the Creator to
frame certain of his living creatures, there remains only the alterna-
tive that the organic atoms have concurred fortuitously to produce
such harmony." This is not the only alternative : there is another,
which Professor Owen has overlooked. It is a perfectly tenable
supposition that all higher vertebrate forms have arisen by the su-
perposing of adaptations upon adaptations. Either of the two anta-
gonist cosmogonies consists with this supposition. If, on the one
hand, we conceive species to have resulted from acts of special
creation ; then it is quite a fair assumption that to produce a higher
vertebrate animal, the Creator did not begin afresh, but took a
lower vertebrate animal, and so far modified its pre-existing parts
as to fit them for the new requirements ; in which case the original
structure would show itself through the superposed modifications.
If, on the other hand, we conceive species to have resulted by
gradual differentiations under the influence of changed conditions ;
then, it would manifestly follow that the higher, heterogeneous
forms, would bear traces of the lower and more homogeneous forms
from which they were evolved.

Thus, besides finding that the hypothesis of an " ideal typical
vertebra " is irreconcilable with the facts, we find that the facts are
interpretable without gratuitous assumptions. The average com-
munity of form which vertebras display, is explicable as resulting

566 APPENDIX B.

from natural causes. And those typical similarities which are
traceable under adaptive modifications, must obviously exist if,
throughout creation in general, there has gone on that continuous
superposing of modifications upon modifications which goes on in
every unfolding organism.

[I might with propriety have added to the foregoing criticisms,
the remark that Professor Owen has indirectly conferred a great
benefit by the elaborate investigations he has made with the view of
establishing his hypothesis. He has himself very conclusively
proved that the teleological interpretation is quite irreconcilable
with the facts. In gathering together evidence in support of his
own conception of archetypal forms, he has disclosed adverse
evidence which I think shows his conception to be untenable.
The result is that the field is left clear for the hypothesis of Evo-
lution as the only tenable one.]

APPENDIX C

[From the Transactions of the Linnean Society, yol. xxv.]

XV. On Circulation and the Formation of Wood in Plants. By
Herbert Spencer, Esq. Communicated by George Busk,
Esq., F.B.S., Sec. L.S.

Read March 1st, 1866.

Opinions respecting the functions of the vascular tissues in plants
appear to make but little progress towards agreement. The suppo-
sition that these vessels and strings of partially-united cells, lined
with spiral, annular, reticulated, or other frameworks, are carriers
of the plant- juices, is objected to on the ground that they often
contain air : as the presence of air arrests the movement of blood
through arteries and veins, its presence in the ducts of stems and
petioles is assumed to unfit them as channels for sap. On the
other hand, that these structures have a respiratory office, as some
have thought, is certainly not more tenable, since, if the presence
of air in them negatives the belief that their function is to dis-
tribute liquid, the presence of liquid in them equally negatives the
belief that their function is to distribute air. Nor can any better
defence be made for the hypothesis which I find propounded, that
these parts serve " to give strength to the parenchyma." Tubes
with fenestrated and reticulated internal skeletons have, indeed,
some power of supporting the tissue through which they pass ; but
tubes lined with spiral threads can yield extremely little support,
while tubes lined with annuli, or spirals alternating with annuli, can
yield no support whatever. Though all these types of internal
framework are more or less efficient for preveniing closure by
lateral pressure, they are some of them quite useless for holding
up the mass through which the vessels pass ; and the best of them
are for this purpose mechanically inferior to the simple cylinder.
The same quantity of matter made into a continuous tube would be
more effective in giving stiffness to the cellular tissue around it.

in the absence of any feasible alternative, the hypothesis that
these vessels are distributors of sap claims reconsideration. The
objections are not, I think, so serious as they seem. The habitual

5G7

568 APPENDIX C.

presence of air in the ducts that traverse wood, can scarcely be
held anomalous if when the wood is formed their function ceases.
The canals which ramify through a Stag's horn, contain air after
the Stag's horn is fully developed ; but it is not thereby rendered
doubtful whether it is the function of arteries to convey blood.
Again, that air should frequently be found even in the vessels of
petioles and leaves, will not appear remarkable when we call to
mind the conditions to which a leaf is subject. Evaporation is
going on from it. The thinner liquids pass by osmose out of the
vessels into the tissues containing the liquids thickened by evapora-
tion. And as the vessels are thus continually drained, a draught is
made upon the liquid contained in the stem and roots. Suppose
that this draught is unusually great, or suppose that around the
roots there exists no adequate supply of moisture. A state of
capillary tension must result — a tendency of the liquid to pass into
the leaves resisted below by liquid cohesion. Now, had the vessels
impermeable coats, only their upper extremities would under these
conditions be slowly emptied. But their coats, in common with all
the surrounding tissues, are permeable by air. Hence, under this
state of capillary tension, air will enter ; and as the upper ends of
the tubes, being both smaller in diameter and less porous than the
lower, will retain the liquids with greater tenacity, the air will
enter the wider and more porous tubes below — the ducts of the
stem and branches. Thus the entrance of air no more proves that
these ducts are not sap- carriers, than does the emptiness of tropical
river-beds in the dry season prove that they are not channels for
water. There is, however, a difficulty which seems more serious.
It is said that air, when present in these minute canals, must be a
great obstacle to the movement of sap through them. The investi-
gations of Jamin have shown that bubbles in a capillary tube resist
the passage of liquid, and that their resistance becomes very great
when the bubbles are numerous — reaching, in some experiments, as
much as three atmospheres. Nevertheless the inference that any
such resistance is offered by the air-bubbles in the vessels of a
plant, is, I think, an erroneous one. What happens in a capillary
tube having impervious sides, with which these experiments were
made, will by no means happen in a capillary tube having pervious
sides. Any pressure brought to bear on the column of liquid con-
tained in the porous duct of a plant, must quickly cause the expul-
sion of a contained air- bubble through the minute openings in the
coats of the duct. The greater molecular mobility of gases than
liquids, implies that air will pass out far more readily than sap.
Whilst, therefore, a slight tension on the column of sap will cause
it to part and the air to enter, a slight pressure upon it will force
out the air and reunite the divided parts of the column.

To obtain data for an opinion on this vexed question, I have

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 569

lately been experimenting on the absorption of dyes by plants. So
far as I can learn, experiments of this kind have most, if not all of
them, been made on stems, and, as it would seem from the results,
on stems so far developed as to contain all their characteristic
structures. The first experiments I made myself were on such
parts, and yielded evidence that served but little to elucidate
matters. It was only after trying like experiments with leaves of
different ages and different characters, and with undeveloped axes,
as well as with axes of special kinds, that comprehensible results
were reached ; and it then became manifest that the appearances
presented by ordinary stems when thus tested, are in a great degree
misleading. Let me briefly indicate the differences.

If an adult shoot of a tree or shrub be cut off, and have its lower
end placed in an alumed decoction of logwood or a dilute solution
of magenta,* the dye will, in the course of a few hours, ascend to a
distance varying according to the rate of evaporation from the
leaves. On making longitudinal sections of the part traversed by
it, the dye is found to have penetrated extensive tracts of the
woody tissue ; and on making transverse sections, the openings of
the ducts appear as empty spaces in the midst of a deeply-coloured
prosenchyma. It would thus seem that the liquid is carried up the
denser parts of the vascular bundles ; neglecting the cambium layer,
neglecting the central pith, and neglecting the spiral vessels of the
medullary sheath. Apparently the substance of the wrood has
afforded the readiest channel. When, howrever, we examine these
appearances critically, we find reasons for doubting this conclusion.
If a transverse section of the lower part, into which the dye passed
first and has remained longest, be compared with a transverse sec-
tion of the part which the dye has but just reached, a marked
difference is visible. In the one case the whole of the dense tissue
is stained ; in the other case it is not. This uneven distribution of
stain in the part which the dye has incompletely permeated is not
at random ; it admits of definite description. A tolerably regular
continuous ring of colour distinguishes the outer part of the wood
from the inner mass, implying a passage of liquid up the elongated
cells next the cambium layer. And the inner mass is coloured more
round the mouths of the pitted ducts than elsewhere : the dense
tissue is darkest close to the edges of these ducts ; the colour fades
away gradually on receding from their edges ; there is most colour
where there are several ducts together ; and the dense tissue which

* These two dyes have affinities for different components of the tissues,
and may be advantageously used in different cases. Magenta is rapidly
taken up by woody matter and other secondary deposits ; while logwood
colours the cell-membranes, and takes but reluctantly to the substances seized
by magenta. By trying both of them on the same structure, we may guard
ourselves against any error arising from selective combination.

570 APPENDIX C.

is fully dyed for some space, is that which lies between two or more
ducts. These are indications that while the layer of pitted cells
next the cambium has served as a channel for part of the liquid, the
rest has ascended the pitted ducts, and oozed out of these into the
prosenchyma around. And this conclusion is confirmed by the
contrast between the appearances of the lowest part of a shoot
under different conditions. For if, instead of allowing the dye
time for oozing through the prosenchyma, the end of the shoot be
just dipped into the dye and taken out again, we find, on making
transverse sections of the part into which the dye has been rapidly
taken up, that, though it has diffused to some distance round the
ducts, it has left tracts of wood between the ducts uncoloured — a
difference which would not exist had the ascent been through the
substance of the wood. Even still stronger is the confirmation
obtained by using one dye after another. If a shoot that has ab-
sorbed magenta for an hour be placed for five minutes in the log-
wood decoction, transverse sections of it taken at a short distance
from its end show the mouths of the ducts surrounded by dark
stains in the midst of the much wider red stains.

Based on these comparisons only, the inference pointed out has
little weight ; but its weight is increased by the results of experi-
ments on quite young shoots, and shoots that develope very little
wood. The behaviour of these corresponds perfectly with the ex-
pectation that a liquid will ascend capillary tubes in preference to
simple cellular tissue or tissue not differentiated into continuous
canals. The vascular bundles of the medullary sheath are here
the only channels which the coloured liquid takes. In sections of
the parts up to which the dye has but just reached, the spiral, fenes-
trated, scalariform, or other vessels contained in these bundles are
alone coloured, and lower down it is only after some hours that
such an exudation of dye takes place as suffices partially to colour
the other substances of the bundle. Further, it is to be noted that
at the terminations of shoots, where the vessels are but incompletely
formed out of irregularly- joined fibrous cells which still retain
their original shapes, the dye runs up the incipient vessels and
does not colour in the smallest degree the surrounding tissue.

Experiments with leaves bring out parallel facts. On placing in
a dye a petiole of an adult leaf of a tree, and putting it before the
fire to accelerate evaporation, the dye will be found to ascend the
midrib and veins at various rates, up even to a foot per hour. At
first it is confined to the vessels ; but by the time it has reached the
point of the leaf, it will commonly be seen that at the lower part it
has diffused itself into the sheaths of the vessels. In a quite young
leaf from the same shoot, we find a much more rigorous restriction
of the dye to the vessels. On making oblique sections of its petiole,
midrib, and veins, the vessels have the appearance of groups of

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 571

sharply defined coloured rods imbedded in the green prosenchyma ;
and this marked contrast continues with scarcely an appreciable
change after plenty of time has been allowed for exudation.

The facts thus grouped and thus contrasted seem, at first sight,
to imply that while they are young the coats of these ramifying
canals lined with spiral or allied structures are not readily perme-
able, but that, becoming porous as they grow old, they allow the
liquids they carry to escape with increasing facility ; and hence a
possible interpretation of the fact that, in the older parts, the stain-
ing of the tissue around the vessels is so rapid as to suggest that the
dye has ascended directly through this tissue, whereas in the younger
parts the reverse appearance necessitates the reverse conclusion.
But now, is this difference determined by difference of age, or is it
otherwise determined ? The evidence as presented in ordinary stems
and leaves shows us that the parts of the vascular system at which
there is a rapid escape of dye are not simply older parts, but are
parts where a deposit of woody matter is taking place. Is it, then,
that the increasing permeability of the ducts, instead of being
directly associated with their increasing age, is directly associated
with the increasing deposit of dense substance around them ?

To get proof that this last connexion is the true one, we have
but to take a class of cases in which wood is formed only to a small
extent. In such cases experiments show us a far more general and
continued limitation of the dye to the vessels. Ordinary herbs and
vegetables, when contrasted with shrubs and trees, illustrate this ;
as instance the petioles of Celery, or of the common Dock, and the
leaves of Cabbages or Turnips. And then in very succulent plants,
such as Bryophyllum calycinum, Kalancho'e rotundifolia, the various
species of Crassula, Cotyledon, Kleinia, and others of like habit, the
ducts of old and youngleaves alike retain the dye very persistently :
the concomitant in these cases being the small amount of prosen-
chyma around the ducts, or the small amount of deposit in it, or
both. More conclusive yet is the evidence which meets us when we
turn from very succulent leaves to very succulent axes. The tender
young shoots of Kleinia ante-euphorbium, or Euphorbia Mauritanica,
which for many inches of their lengths have scarcely any ligneous
fibres, show us scarcely any escape of the coloured liquid from the
vessels of the medullary sheath. So, too, is it with Stapelia
Bufonia, a plant of another order, having soft swollen axes. And
then we have a repetition of the like connexion of facts throughout
the Cactacea; : the most succulent showing us the smallest perme-
ability of the vessels. In two species of Rhipsalis, in two species of
Cereus, and in two species of Mammillaria, which I have tried, I
have found this so. Mammillaria gracilis may be named as ex-
emplifying the relation under its extreme form. Into one of these
small spheroidal masses, the dye ascends through the large bundles

572 APPENDIX C.

of spiral or annular ducts, or cells partially united into such ducts,
colouring them deeply, and leaving the feebly-marked sheath of
prosenchyma, together with the surrounding watery cellular tissue,
perfectly uncoloured.

The most conclusive evidence, however, is furnished by those
Cactacece in which the transition from succulent to dense tissue
takes place variably, according as local circumstances determine.
Opuntia yields good examples. If a piece of it including one of
the joints at which wood is beginning to form, be allowed to absorb
a coloured liquid, the liquid, running up the irregular bundles of
vessels and into many of their minute ramifications, is restricted to
these where they pass through the parenchyma forming the mass of
the stem ; but near the joints the hardened tissue around the vessels
is coloured. In one of these fleshy growths we get clear evidence
that the escape of the dye has no immediate dependence on the age
of the vessels, since, in parts of the stem that are alike in age, some
of the vessels retain their contents while others do not. Nay, we
even find that the younger vessels are more pervious than the older
ones, if round the younger ones there is a formation of wood.

Thus, then, is confirmed the inference before drawn, that in ordi-
nary stems the staining of the wood by an ascending coloured liquid
is due, not to the passage of the coloured liquid up the substance of
the wood, but to the permeability of its ducts and such of its pitted
cells as are united into irregular canals. And the facts showing
this, at the same time indicate with tolerable clearness the process
by which wood is formed. What in these cases is seen to take place
with a dye, may be fairly presumed to take place with sap. Where
the dye exudes but slowly, we may infer that the sap exudes but
slow]y ; and it is a fair inference that where the dye leaks rapidly out
of the vessels, the sap does the same. Inferring, thus, that where-
ever there is a considerable formation of wood there is a considerable
escape of the sap, we see in the one the result of the other. The
thickening of the prosenchyma is proportionate to the quantity of
nutritive liquid passing into it ; and this nutritive liquid passes
into it from the vessels, ducts, and irregular canals it surrounds.

But an objection is made to such experiments as the foregoing,
and to all the inferences drawn from them. It is said that portions
of plants cut off and thus treated, have their physiological actions
arrested, or so changed as may render the results misleading ; and it
is said that when detached snoots and leaves have their cut ends
placed in solutions, the open mouths of their vessels and ducts are
directly presented with the liquids to be absorbed, which does not
happen in their natural states. Further, making these objections
look serious, it is alleged that when solutions are absorbed through
the roots, quite different results are obtained : the absorbed matters
are found in the tissues and not in the vessels. Clearly, were the ex-

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 573

periments yielding these adverse results conducted in unobjection-
able ways, the conclusion implied by them would negative the con-
clusions above drawn. But these experiments are no less objection-
able than those to which they are opposed. Such mineral matters
as salts of iron, solutions of which have in some cases been supplied
to the roots for their absorption, are obviously so unlike the mat-
ters ordinarily absorbed, that they are likely to interfere fatally
with the physiological actions. If experiments of this kind are
made by immersing the roots in a dye, there is, besides the dif-
ficulty that the mineral mordant contained by the dye is injurious
to the plant, the further difficulty that the colouring matter, being
seized by the substances for which it has an affinity, is left behind
in the first layers of root tissues passed through, and that the
decolorized water passing up into the plant is not traceable. To
be conclusive, then, an experiment on absorption through roots
must be made with some solution which will not seriously inter-
fere with the plant's vital processes, and which will not have its
distinctive element left behind. To fulfil these requirements I
adopted the following method. Having imbedded a well-soaked
broad-bean in moist sand, contained in an inverted cone of card-
board with its apex cut off for the radicle to come through — having
placed this in a wide-mouthed dwarf bottle, partly filled with water,
so that the protruding radicle dipped into the water — and having
waited until the young bean had a shoot some three or more inches
high, and a cluster of secondary rootlets from an inch to an inch
and a-half long — I supplied for its absorption a simple decoction of
logwood, which, being a vegetal matter, was not likely to do it much
harm, and which, being without a mordant, would not leave its sus-
pended colour in the first tissues passed through. To avoid any
possible injury, I did not remove the plant from the bottle, but
slightly raising the cone out of its neck, I poured away the water
through the crevice and then poured in the logwood decoction ; so
that there could have been no broken end or abraded surface of a
rootlet through which the decoction might enter. Being prepared
with some chloride of tin as a mordant, I cut off, after some three
hours, one of the lowest leaves, expecting that the application of the
mordant to the cut surface would bring out the characteristic colour
if the logwood decoction had risen to that height. I got no re-
action, however. But after eight hours I found, on cutting off
another leaf, that the vessels of its petiole were made visible as dark
streaks by the colour with which they were charged — a colour differ-
ing, as was to be expected, from that of the logwood decoction,
which spontaneously changes even by simple exposure. It was then
too late in the day to pursue the observations ; but next morning
the vessels of the whole plant, as far as the petioles of its highest
unfolded leaves, were full of the colouring-matter; and on applying
chloride of tin to the cut surfaces, the vessels assumed that purplish

574 APPENDIX C.

red which this mordant produces when directly mixed with the log
wood decoction. Subsequently, when one of the cotyledons was cut
open by Prof. Oliver, to whom, in company with Dr. Hooker, I
snowed the specimen, we found that the whole of its vascular system
was filledwith thedecoction, which every where gavethe characteristic
reaction. And it became manifest that the liquid absorbed through
the rootlets, in the central vessels of which it was similarly traceable,
had part of it passed directly up the vessels of the axis, while part of
it had passed through other vessels into the cotyledon, out of which,
no doubt, the liquid ordinarily so carried returns charged with a
supply of the stored nutriment. I have since obtained a verification
by varying the method. Digging up some young plants (Marigolds
happened to afford the best choice) with large masses of soil round
them, placing them in water, so as gradually to detach the soil with-
out injuring the rootlets, planting them afresh in a flower-pot full
of washed sand, and then, after a few days, watering them with a
logwood decoction, I found, as before, that in less than twenty-
four hours the colouring-matter had run up into the vessels of the
leaves. Though the reaction produced by the mordant was not so
strong as before, it was marked enough to be quite unquestionable.

As these experiments were so conducted that there was no ac-
cess to the vessels except through the natural channels, and as the
vital actions of the plants were so little interfered with that at the
end of twenty-four hours they showed no traces of disturbance, I
think the results must be held conclusive.

Taking it, then, as a fact that in plants possessing them the vessels
and ducts are the channels through which sap is distributed, we come
now to the further question — What determines the varying permea-
bility of the walls of the vessels and ducts, and the consequent vary-
ing formation of wood ? To this question I believe the true reply is
— Theexposureof the parts to intermittent mechanical strains, actual
or potential, or both. By actual strains I of course mean those
which the plant experiences in the course of its individual life. By
potential strains I mean those which the form, attitude, and circum-
stances common to its kind involve, and which its inherited struc-
ture is adapted to meet. In plants with stems, petioles, and leaves,
having tolerably constant attitudes, the increasing porosity of the
tubes and consequent deposit of dense tissue takes place in anticipa-
tion of the strains to which the parts of the individual are liable, but
takes place at parts which have been habitually subject to such
strains in ancestral individuals. But though in such plants the
tendency to repeat that distribution of dense tissue caused by
mechanical actions on past generations, goes on irrespective of the
mechanical actions to which the developing individual is subject,
these direct actions, while they greatly aid the assumption of the
typical structure, are the sole causes of those deviations in the rela-

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 575

tive thickenings of parts which distinguish the individual from others
of its kind. And then, in certain irregularly growing plants, such as
Cactuses and Euphorbias, where the strains fall on parts that do
not correspond in successive individuals, we distinctly trace a direct
relation between the degrees of strain and the rates of these changes
which result in dense tissue. I will not occupy space in detailing
the evidence of this relation, which is conspicuous in the orders
named, butwill pass to thequestion — What arethephysicalprocesses
by which intermittent mechanical strains produce this deposit of
resistant substance at places where it is needed to meet the strains?
We have not to seek far for an answer. If a trunk, a bough, a
shoot, or a petiole, is bent by a gust of wind, the substance of its
convex side is subject to longitudinal tension: the substance of its
concave side being at the same time compressed. This is the pri-
mary mechanical effect. There is, however, a secondary mechani-
cal effect, which here chiefly concerns us. That bend by which the
tissues of the convex side are stretched, also produces lateral com-
pression of them. Buttoning on a tight glove and then closing the
hand, will make this necessity clear : the leather, while it is strained
along the backs of the fingers, presses with considerable force on the
knuckles. It is demonstrable that the tensions of the outer layer
of a mass made convex by bending, must, by composition of forces,
produce at every point a resultant at right angles to the layer be-
neath it ; that, similarly, the joint tensions of these two layers must
throw a pressure on the next deeper layer ; and so on. Hence, if
at some little distance beneath the surface of a stem, twig, or leaf-
stalk, there exist longitudinal tubes, these tubes must be squeezed
each time the side of the branch they are placed on becomes convex.
Modifying the illustration just drawn from the clenched hand will
make this clear. When, on forcibly grasping something, the skin is
drawn tightly over the back of the hand, the whitening of the
knuckles shows how the blood is expelled from the vessels below
the surface by the pressure of the tightened skin. If, then, the sap-
vessels must be thus compressed, what will happen to the liquid they
contain ? It will move away along the lines of least resistance.
Part, and probably the greater part, will escape lengthways from the
place of greatest pressure : some of it being expelled downwards,
and some of it upwards. But, at the same time, part of it will be
likely to ooze through the walls of the tubes. If these walls are so
perfect as to permit the passage of liquid only by osmose, it may
still be inferred that the osmose will increase under pressure ; and
probably, under recurrent pressure, the places at which the osmotic
current passes most readily will become more and more permeable,
until they eventually form pores. At any rate it is manifest that
where pores and slits exist, whether thus formed or formed in any
other way, the escape of sap into the adjacent tissue at each bend

576 APPENDIX C.

will become easy and rapid. What further must happen ? When
the branch or shoot recoils, the vessels on the side that was convex,
being relieved from pressure, will tend to resume their previous
diameters ; and will be helped to do this by the elasticity of the
surrounding tissue, as well as by those spiral, annular, and allied
structures which they contain. But this resumption of their previ-
ous diameters must cause an immediate rush of sap back into them.
Whence will it come ? Not to any considerable extent from the sur-
rounding tissues into which part of it has been squeezed, seeing that
the resistance to the return of liquid through small pores will be
greater than the resistance to its return along the vessels themselves.
Manifestly the sap which was thrust up and down the vessels from
the place of compression will return — the quantities returning from
above and from below varying, as we shall hereafter see, according
to circumstances. But this is not all. From some side a greater
quantity must come back than was sent away ; for the amount that
has escaped out of the tube into the prosenchyma has to be replaced.
Thus during the time when the side of the branch or twig becomes
concave, more sap returns from above or below than was expelled
upwards or downwards during the previous compression. The
refilled vessels, when the next bend renders their side convex, again
have part of their contents forced through their parietes, and are
again refilled in the same way. There is thus set up a draught of sap
to the place where these intermittent strains are going on, an exuda-
tion proportionate to the frequency and intensity of the strains,
and a proportionate nutrition or thickening of the wood- cells,
fitting them to resist the strains. A rude idea of this action may
be obtained by grasping in one hand a damp sponge, having its
lower end in water, while holding a piece of blotting-paper in
contact with its upper end, and then giving the sponge repeated
squeezes. At each squeeze some of the water will be sent into
the blotting-paper ; at each relaxation the sponge will refill from
below, to give another portion of its contents to the blotting-
paper when again squeezed.

But how does this explanation apply to roots ? If the formation
of wood is due to intermittent transverse strains, such as are pro-
duced in the aerial parts of upright plants by the wind, how does it
happen that woody matter is deposited in roots, where there are no
lateral oscillations, no transverse strains ? The answer is, that
longitudinal strains also are capable of causing the effects described.
It is true that perfectly straight fibres united into a bundle and pulled
lengthways would not exert on one another any lateral pressure, and
would not laterally compress any similarly-straight canals running
along with them. But if the fibres united into a bundle are variously
bent or twisted, they cannot be longitudinally strained without com-
pressing one another and structures imbedded in them. It needs

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 577

but to watch a wet rope drawn tight by a capstan, to see that an
action like that which squeezes the water out of its strands, will
squeeze the sap out of the vessels of a root into the surrounding
tissue, as often as the root is pulled by the swaying of the plant it
belongs to. Here, too, as before, the vessels will refill when the
pull intermits ; and so, in the roots as in the branches, this rude
pumping process will produce a growth of hard tissue proportion-
ate to the stress to be borne.

These conclusions are supported by the evidence which excep-
tional cases supply. If intermittent mechanical strains thus cause
the formation of wood where wood is found, then where it is not
found, there should be an absence of intermittent mechanical strains.
There is such an absence. Vascular plants characterized by little
or no deposit of dense substance, are those having vessels so con-
ditioned that no considerable pressures are borne by them. The
more succulent a petiole or leaf becomes, the more do the effects of
transverse strains fall on its outer layers of cells. Its mechanical sup-
port is chiefly derived from the ability of these minute vesicles, full
of liquid, to resist bursting and tearing under the compressions and
tensions they are exposed to. And just as fast as this change from
a thin leaf or foot- stalk to a thick one entails increasing stress on the
superficial tissue, so fast does it diminish the stress on the internally -
seated vascular tissue. The succulent leaf cannot be swayed about
by the wind as much as an ordinary leaf ; and such small bends as
can be given to it and its foot-stalk are prevented from affecting
in any considerable degree the tubes running through its interior.
Hence the retentiveness of the vessels in these fleshy leaves, as shown
by the small exudation of dye ; and hence the small thickening of
their surrounding prosenchyma by woody deposit. Still more con-
spicuously is this connexion of facts shown when, from the soft thick
leaves before named and such others as those of Echeveria, Rochea,
Pereskia, we turn to the thick leaves that have strong exo-skeletons.
Gasteria serves as an illustration. The leathery or horny skin here
evidently bears the entire weight of the leaf, and is so stiff as to pre-
vent any oscillation. Here, then, the vessels running inside are pro-
tected from all mechanical stress ; and accordingly we find that the
cells surrounding them are not appreciably thickened.

Equally clear, and more striking because more obviously excep-
tional, is the evidence given by succulent stems which are leafless.
Stapelia Buffonia, having soft procumbent axes not liable to be
bent backwards and forwards in any considerable degree by the
wind, has, ramifying through its tissue, vessels that allow but an ex-
tremely slow escape of dye and have unthickened sheaths. Such of
the Euphorbias as have acquired the fleshy character while retaining
the arborescentgro wth, WkzEuphorbia Canariensis, teach us the same
truth in another way. In them the formation of wood around the
83

578 APPENDIX C.

vessels is inconspicuous where the intermittent strains are but slight;
but it is conspicuous at those joints on which lateral oscillations of
the attached branches throw great extensions and compressions of
tissue. Throughout the Cactacece we find varied examples of the
alieged relation. Mammillaria furnishes a very marked one. The
substance of one of these globular masses, resting on the ground,
admits of no bending from side to side ; and accordingly its large
bundles of spiral and annular vessels, or partially-united cells, have
very feebly-marked sheaths not at all thickened. In such types as
Cereus and Opuntia we see, as in the Euphorbias, that where little
stress falls on the vessels, little deposit takes place around them ;
while there is much deposit where there is mu ch stress. Here let me
add a confirmation obtained since writing the above. After observ-
ing among the Cactuses the very manifest relation between strain
and the formation of wood, I inquired of Mr. Croucher, the intelli-
gent foreman of the Cactus-house at Kew, whether he found this
relation a constant one. He replied that he did, and that he had
frequently tested it by artificially subjecting parts of them to strains.
Neglecting at the time to inquire how he had done this, it afterwards
occurred to me that if he had so done it as to cause constant strains,
the observed result would not tell in favour of the foregoing inter-
pretation. Subsequently, however, I learned that he had produced
the strains by placing the plants in inclined attitudes — a method
which, by permitting oscillations of the strained joints, allowed the
strains to intermit. And then, making the proof conclusive, Mr.
Croucher volunteered the statement that where he had produced
constant strains by tying, no formation of wood took place.

Aberrant growths of another class display the same relations
of phenomena. Take first the underground stems, such as the
Potato and the Artichoke. The vessels which run through these,
slowly take up the dye without letting it pass to any considerable
extent into the surrounding tissues.* Only after an interval of
many hours does the prosenchyma become stained in some places.
Here, as before, an absence of rapid exudation accompanies an
absence of woody deposit ; and both these go along with the ab-
sence of intermittent strains. Take again the fleshy roots. The
Turnip, the Carrot, and the Beetroot, have vessels that retain very
persistently the coloured liquids they take up. And differing in this,
as these roots do, from ordinary roots, we see that they also differ
from them in not being woody, and in not being appreciably sub-

* Those who repeat these experiments must be prepared for great irregu-
larities in the rates of absorption. Succulent structures in general absorb
much more slowly than others, and sometimes will scarcely take up the dye
at all. The differences between different structures, and the same structure
at different times, probably depend on the degrees in which the tissues are
charged with liquid and the rates at which they are losing it by evaporation.

CIRCULATION AND FOEMATION OF WOOD IN PLANTS. 579

ject to the usual mechanical actions. In these cases, as in the
others, parts that ordinarily become dense, deviate from this typ-
ical character when they are not exposed to those forces which
produce dense tissue by increasing the extravasation of sap.

To complete the proof that such a relation exists, let me add the
results of some experiments on equal and similarly-developed parts,
kept respectively at rest and in motion. I have tested the effects on
large petioles, on herbaceous shoots, and on woody shoots. If two
such petioles as those of lihubarb, with their leaves attached, have
their cut ends inserted in bottles of dye, and the one be bent back-
wards and forwards while the other remains motionless, there arises,
after the lapse of an hour, scarcely any difference in the states of
their vessels: a certain proportion of these are in both cases charged
with the dye, and little exudation has been produced by the motion.
Here, however, it is to be observed that the causes of exudation are
scarcely operative ; the vascular bundles are distributed all through
the mass of the petiole, which is formed of soft watery tissue ; and
they are, therefore, not so circumstanced as to be effectually com-
pressed by the bends. In herbaceous stems, such as those of the
Jerusalem Artichoke and of the Foxglove, an effect scarcely more
decided is produced ; and here, too, when we seek a reason, we find
it in the non-fulfilment of the mechanical conditions; for the vascular
bundles are not so seated between a tough layer of bark and a solid
core as to be compressed at each bend. When, however, we come
to experiment upon woody shoots, we meet with conspicuous effects,
though by no means uniformly. In some cases oscillations produce
immense amounts of exudation — parallel transverse sections of the
compared shoots showing that where, in the one that has been at
rest, there are spots of colour round but a few pitted ducts, in the
one that has been kept in motion the substance of the wood is soaked
almost uniformly through with dye. In other cases, especially where
there is much undifferentiated tissue remaining, the exudation is not
very marked. The difference appears to depend on the quantity of
liquid contained in the shoot. If its substance is relatively dry, the
exudation is great ; but it is comparatively small if all the tissues
are fully charged with sap. This contrast of results is one which
contemplation of the mechanical actions will lead us to expect.

And now, with these facts to aid our interpretation, let us re-
turn to ordinary stems. If the upper end of a growing shoot, the
prosenchyma of which is but little thickened, be allowed to imbibe
the dye, the vessels of its medullary sheath alone become charged ;
and from them there takes place but a slow oozing. If a like ex-
periment be tried with a lower part of the shoot, where the wood in
course of formation has its inner boundary marked but not its outer
boundary, we find that the pitted ducts, and more especially the
inner ones, come into play. And then lower still, where the wood

580 APPENDIX C.

has its periphery defined and its histological characters decided,
the appearances show that the tissue forming its outer surface
begins to take a leading part in the transmission of liquid. What
now is the explanation of these changes, mechanically considered ?
In the young soft part of the shoot, as in all normal and abnormal
growths that have not formed wood, the channels for the passage of
sap are the spiral, annular, fenestrated, or reticulated vessels. These
vessels, here included in the bundles of the medullary sheath, are,
in common with the tissues around them, subject, by the bendings
of the shoot, to slight intermittent compressions, and, especially
the outermost of them, are thus forced to give the prosenchyma
an extra supply of nutritive liquid. The thickening of the pro-
senchyma, spreading laterally as well as outwards from each bundle
of the medullary sheath, goes on until it meets the thickenings that
spread from the other bundles ; and there is so formed an irregular
cylinder of hardened tissue, surrounding the medulla and the vas-
cular bundles of its sheath. As soon as this happens, these vascular
bundles become, to a considerable extent, shielded from the effects
of transverse strains, since the tensions and compressions chiefly
fall on the developing wood outside of them. Clearly, too, the
greatest stress must be felt by the outer layer of the developing
wood : being further removed from the neutral axis, it must be
subject to severer strains at each bend ; and lying between the
bark and the layer of wood first formed, it must be most exposed
to lateral compressions. Among the elongated cells of this outer
layer, some unite to form the pitted ducts. Being, as we see,
better circumstanced mechanically, they become greater carriers
of sap than the original vessels, and, in consequence of this, as well
as in consequence of their relative proximity, become the sources
of nutrition to the still more external layers of wood-cells. The
same causes and the same effects hold with each new indurated
coat deposited round the previously indurated coats.

This description may be thought to go far towards justifying the
current views respecting the course taken by the sap. But the
justification is more apparent than real. In the first place, the im-
plication here is that the sap-carrying function is at first discharged
entirely by the vessels of the medullary sheath, and that they cease
to discharge this function only as fast as they are relatively incapaci-
tated by their mechanical circumstances. And the second implica-
tion is, that it is not the wood itself, but the more or less continuous
canals formed in it, which are the subsequent sap-distributors. This,
though readily made clear by microscopic examination of the large
pitted ducts in a partially lignified shoot that has absorbed the dye,
is less manifestly true of the peripheral layer of sap-carrying tissue
finally formed. But it is really true here. For this layer, though
nominally a layer of wood, is practically a layer of inosculating

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 581

vessels. It is formed out of irregular Hues and networks of elon-
gated pitted cells, obliquely united by their ends. Examination of
them after absorption of a dye, shows that it is only along the con-
tinuous channels they unite to form that the current has passed.
But the essentially vascular character of this outer and latest-formed
layer of the alburnum is best seen in the fact that the vascular sys-
tems of new axes take their rise from it, and form with it continuous
canals. If a shoot of last year in which growth is recommencing, be
cut lengthways after it has imbibed a dye, clear proof is obtained
that the passage of the dye into a lateral bud takes place from this
outermost layer of pitted cells, and that the channels taken by the
dye through the new tissue are composed of cells that pass through
modified forms into the spiral vessels of the new medullary sheath.
This transition may be still more clearly traced in a terminal bud
that continues the line of last year's shoot. A longitudinal section
of this shows that the vessels of the new medullary sheath do not
obtain their sap from the vessels of last year's sheath (which, as
shown by the non-absorption of dye, have become inactive), but
that their supplies are obtained from those inosculating canals
formed out of last year's outermost layer of prosenchyma, and that
between the component cells of this and those of the new vascular
system there are all gradations of structure.*

* It may be added here that, on considering the mechanical actions that
must go on, we are enabled in some measure to understand both how such inos-
culating channels are initiated, and how the structures of their component
cells are explicable. What must happen to one of these elongated prosen-
chyma-cells if, in the course of its development, it is subject to intermittent
compressions ? Its squeezed-out liquid while partially escaping laterally,
will more largely escape upwards and downwards; and while repeated
lateral escape will tend to form lateral channels communicating with
laterally-adjacent cells, repeated longitudinal escape will tend to form
channels communicating with longitudinally-adjacent cells— so producing
continuous though irregular longitudinal canals. Meanwhile each cell into
and out of which the nutritive liquid is from time to time squeezed
through small openings in its walls, cannot thicken internally in an
even manner: deposition will be interfered with by the passage of the
currents through the pores. The rush to or from each pore will tend
to maintain a funnel-shaped depression in the deposit around ; and the
opening from cell to cell will so acquire just that shape which the microscope
shows up — two hollow cones with their apices meeting at the point where
the cell-membranes are in contact. Moreover, as confirming this inter-
pretation, it may be remarked that we are thus supplied with a reason
for the differences of shape between these passages from one pitted cell
to another, and the analogous passages that exist between cells other-
wise formed and otherwise conditioned. In the cells of the medulla, and
others which are but little exposed to compression, the passages are seve-
rally formed more like a tube with two trumpet-mouths, one in each cell.
This is just the form which might be expected where the nutritive fluid
passes from cell to cell in moderate currents, and not by the violent rushes
caused by intermittent pressures. Of course it is not meant that in each

582 APPENDIX C.

It is not the aim of the foregoing reasoning to show that mechani-
cal actions are the sole causes of the formation of dense tissue in
plants. Dense tissue is in many cases formed where no such causes
have come into play — as, for example, in thorns and in the shells of
nuts. Here the natural selection of variations can alone have ope-
rated. It is manifest, too, that even those supporting structures the
building up of which is above ascribed io intermittent strains, may,
in the individual plant of a species that ordinarily has them, be de-
veloped to a great extent when intermittent strains are prevented.
We see this in trees that are artificially supported by nailing to
walls ; and we also see a kindred fact in natural climbers. Though
in these cases the formation of wood is obviously less than it would
be were the stem and branches habitually moved about by the wind, it
nevertheless goes on. Clearly the tendency of the plant to repeat the
structure of its type (in the one case the structure of its species, and
in the other case that of the order from which it has diverged in be-
coming a climber) is here almost the sole cause of wood-formation.
But though in plants so circumstanced intermittent mechanical
strains have little or no direct share, it may still be true, and I believe
is true, that intermittent mechanical strains are the original cause ;
for, as before hinted, the typical structure which the individual thus
repeats irrespective of its own conditions, is interpretable as a typical
structure that is itself the product of these actions and reactions be-
tween the plant and its environment. Grant the inheritance of func-
tionally-produced modificatious ; grant that natural selection will
always co-operate in such way as to favour those individuals and
families in which function ally -produced modifications have pro-
gressed most advantageously ; and it will follow that this mechani-
cally-caused formation of dense substance, accumulating from gen-
eration to generation by the survival of the fittest, will result in an
organic habit of forming dense tissue at the required places. The
deposit arising from exudation at the places of greatest strain, re-
curring from generation to generation at the same places, will
come to be reproduced in anticipation of strain, and will continue
to be reproduced for a long time after a changed habit of the
species prevents the strain — eventually, however, decreasing, both
through functional inactivity and natural selection, to the point
at which it is in equilibrium with the requirement.

individual cell these structures are determined by these mechanical actions.
The facts clearly negative any such conclusion, showing us, as they in many
cases do, that these structures are assumed in advance of these mechanical
actions. The implication is, that such mechanical actions initiated modifi-
cations that have, with the aid of natural selection, been accumulated from
generation to generation ; until, in conformity with ordinary embryological
laws, the cells of the parts exposed to such actions assume these special
structures irrespective of the actions — the actions, however, still serving to
aid and complete the assumption of the inherited type.

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 583

Anotherside of the general question may now be considered. We
have seen how, by intermittent pressures on capillary vessels and
ducts and inosculating canals, there must be produced a draught of
sap towards I he point of compression to replace the sap squeezed out.
But we have still to inquire what will be the effect on the distribu-
tion of sap throughout the plant as a whole. It was concluded that
out of the compressed vessels the greater part of the liquid would
escape longitudinally — the longitudinal resistance to movement
being hast. In every case the probabilities are infinity to one against
the resistances being equal upwards and downwards. Always, then,
more sap will be expelled in one direction than in the other. But in
whichever direction least sap is expelled, from that same direction
most sap will return when the vessels are relieved from pressure — the
force which is powerful in arrestingthe back current in that direction
being the same force which is powerful in producing a forward cur-
rent. Ordinarily, the more abundant supply of liquid being from be-
low, there will result an upward current. At each bend a portion of
the contents will be squeezed out through the sides of the vessels — a
portion will be squeezed downwards, reversing the current ascending
from the roots, but soon stopped by its resistance ; while a larger por-
tion will be squeezed upwards towards the extremities of the vessels,
where consumption and loss are most rapid. At each recoil the ves-
sels will be replenished, chiefly by the repressed upward current ; and
at the next bend more of it will be thrust onwards than backwards.
Hence we have everywhere in action a kind of rude force-pump,
worked by the wind ; and we see how sap may thus be raised to a
height far beyond that to which it could be raised by capillary ac-
tion, aided by osmose and evaporation.

Thus far, however, the argument proceeds on the assumption that
there is liquid enough to replenish every time the vessels subject
to this process. But suppose the supply fails — suppose the roots
have exhausted the surrounding stock of moisture. Evidently the
vessels thus repeatedly having their contents squeezed out into the
surrounding tissue, cannot go on refilling themselves from other
vessels without tending to empty the vascular system. On the one
hand, evaporation from the leaves causing a draught on the capillary
tubes that end in them, continually generates a capillary tension up-
wards ; while, on the other hand, the vessels below, expanding after
their sap has been squeezed out, produce a tension both upwards
and downwards towards the point of loss. Were the limiting mem-
branes of the vessels impermeable, the movement of sap would, under
these conditions, soon be arrested. But these membranes are perme-
able ; and the surrounding tissues readily permit the passage of air.
This state of tension, then, will cause an entrance of air into the tubes;
the columns of liquid they contain will be interrupted by bubbles.
It seems, indeed, not improbable that this entrance of air may take

584 APPENDIX C.

place even when there is a good supply of liquid, if the mechanical
strains are so violent and the exudation so rapid that the currents
cannot refill the half-emptied vessels with sufficient rapidity. And
in this case the intruding air may possibly play the same part as
that contained in the air-chamber of a force-pump — tending, by
moderating the violence of the jets, and by equalizing the strains,
to prevent rupture of the apparatus. Of course when the supply
of liquid becomes adequate, and the strains not too violent, these
bubbles will be expelled as readily as they entered.

Here, as before, let me add the conclusive proof furnished by
a direct experiment. To ascertain the amount of this propulsive
action, I took from the same tree, a Laurel, two equal shoots, and
placing them in the same dye, subjected them to conditions that
were alike in all respects save that of motion : while one remained
at rest, the other was bent backwards and forwards, now by switch-
ing and now by straining with the fingers. After the lapse of an
hour, I found that the dye had ascended the oscillating shoot three
times as far as it had ascended the stationary shoot — this result
being an average from several trials. Similar trials brought out
similar effects in other structures. The various petioles and herba-
ceous shoots experimented upon for the purpose of ascertaining
the amount of exudation produced by transverse strains, showed
also the amount of longitudinal movement. It was observable
that the height ascended by the dye was in all cases greater where
there had been oscillation than where there had been rest — the
difference, however, being much less marked in succulent struc-
tures than in woody ones.

It need scarcely be said that this mechanical action is not here
assigned as the sole cause of circulation, but as a cause co-operating
with others, and helping others to produce effects that could not
otherwise be produced. Trees growing in conservatories afford us
abundant proof that sap is raised to considerable heights by other
forces. Though it is notorious that trees so circumstanced do not
thrive unless, through open sashes, they are frequently subject to
breezes sufficient to make their parts oscillate, yet there is evidently
a circulation that goes on without mechanical aid. The causes of
circulation are those actions only which disturb theliquidequilibrium
in a plant, by permanently abstracting water or sap from some part
of it; and of these the first is the absorption of materials for the for-
mation of new tissue in growing parts ; the second is the loss by
evaporation, mainly through adult leaves; and the third is the loss by
extravasation, through compressed vessels. Only so far as it pro-
duces this last, can mechanical strain be regarded as truly a cause of
circulation. All the other actions concerned must be classed as aids
to circulation— as facilitating that redistribution of liquid that con-
tinually restores the equilibrium continually disturbed ; and of these

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 585

capillary action may be named as the first, osmose as the second, and
the propulsive effect of mechanical strains as the third. The first
two of these aids are doubtless capable by themselves of producing
a large part of the observed result — more of the observed result than
is at first sight manifest ; for there is an important indirect effect
of osmotic action which appears to be overlooked. Osmose does not
aid circulation only by setting up, within the plant, exchange currents
between the more dense and the less dense solutions in different parts
of it ; but it aids circulation much more by producing distention
of the plant as a whole. In consequence of the average contrast in
density between the water outside of the plant and the sap inside of
it, the constant tendency is for the plant to absorb a quantity in excess
of its capacity, and so to produce distention and erection of its
tissues. It is because of this that the drooping plant raises itself
when watered ; for capillary action alone could only refill its tissues
without changing their attitudes. And it is because of this that
juicy plants with collapsible structures bleed so rapidly when cut, not
only from the cut surface of the rooted part, but from the cut sur-
face of the detached part — the elastic tissues tending to press out the
liquid which distends them. And manifestly if osmose serves thus
to maintain a state of distention throughout a plant, it indirectly fur-
thers circulation ; since immediately evaporation or growth at any
part, by abstracting liquid from the neighbouring tissues, begins to
diminish the liquid pressure within such tissues,the distended struc-
tures throughout the rest of the plant thrust their liquid contents to-
wards the place of diminished pressure. This, indeed, may very pos-
sibly be the most efficient of the agencies at work. Remembering
how great is the distention producible by osmotic absorption — great
enough to burst a bladder — it is clear that the force with which the
distended tissues of a plant urge forward the sap to places of con-
sumption, is probably very great. We must therefore regard the aid
which mechanical strains give as being one of several. Oscillations
help directly to restore any disturbed liquid equilibrium ; and they
also help indirectly, by facilitating the redistribution caused by capil-
lary action and the process just described ; but in the absence of
oscillations the equilibrium may still be restored, though less rapidly
and within narrower limits of distance.

One half of the problem of the circulation, however, has been left
out of sight. Thus far our inquiry has been, how the ascending cur-
rent of sap is produced. There remains the rationale of the descend-
ing current. What forces cause it, and through what tissues it takes
place, are questions to which no satisfactory answers have been
given. That the descent is due to gravitation, as some allege,
is difficult to conceive, since, as gravitation acts equally on all
liquid columns contained in the stem, it is not easy to see why
it should produce downward movements in some while permitting

586 APPENDIX C.

upward movements in others — unless, indeed, there existed descend-
ing tubes too wide to admit of much capillary action, which there
do not. Moreover, gravitation is clearly inadequate to cause cur-
rents towards the roots out of branches that droop to the ground.
Here the gravitation of the contained liquid columns must nearly
balance that of the connected columns in the stem, leaving no
appreciable force to cause motion. Nor does there seem much
probability in the assumption that the route of the descending sap
is through the cambium layer, since experiments on the absorption
of dyes prove that simple cellular tissue is a very bad conductor
of liquids: their movement through it does not take place with one-
fiftieth of the rapidity with which it takes place through vessels.*
Of course the defence for these hypotheses is, that there must be
a downward current, which must have a course and a cause ; and the
very natural assumption has been that the course and the cause must
be other than those which produce the ascending current. Never-
theless there is an alternative supposition to which the foregoing
considerations introduce us. It is quite possible for the same vascular
system to serve as a channel for movement in opposite directions
at different times. We have among animals well-known cases in
which the blood-vessels carry a current first in one direction and
then, after a brief pause, in the reverse direction. And there seems
an a priori probability that, lowly organized as they are, plants are
more likely to have distributing appliances of this imperfect kind
than to have two sets of channels for two simultaneous currents. If,
led by this suspicion, we inquire whether among the forces which
unite to produce movements of sap, there are any variations or inter-
missions capable of determining the currents in different directions,
we quickly discover that there are such, and that the hypothesis of
an alternating motion of the sap, now centrifugal and now centri-
petal, through the same vessels, has good warrant. What are the
several forces at work ? First may be set down that tendency
existing in every part of a plant to expand into its typical form, and
to absorb nutritive liquids in doing this. The resulting competition

* Some exceptions to this occur in plants that have retrograded in the
character of their tissues towards the simpler vegetal types. Certain very
succulent leaves, such as those of Sempervivum, in which the cellular tissue
is immensely developed in comparison with the vascular tissue, seem to
have resumed to a considerable extent what we must regard as the primitive
form of vegetal circulation — simple absorption from cell to cell. These,
when they have lost much of their water, will take up the dye to some dis-
tance through their general substance, or rather through its interstices, even
neglecting the vessels. At other times, in the same leaves, the vessels will
become charged while comparatively little absorption takes place through
the cellular tissue. Even in these exceptional cases, however, the movement
through cellular tissue is nothing like as fast as the movement through
vessels.

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 587

for sap will, other things being equal, cause currents towards the
most rapidly-growing parts— towards unfolding shoots and leaves,
but not towards adult leaves. Next we have evaporation, acting
more on the adult leaves than on those which are in the bud,
or but partially developed. This evaporation is both regularly
and irregularly intermittent. Depending chiefly on the action
of the sun, it is, in fine weather, greatly checked or wholly
arrested every evening; and in cloudy weather must be much
retarded during the day. Further, every hygrometric variation,
as well as every variation in the movement of the air, must
vary the evaporation. This chief action, therefore, which, by con-
tinually emptying the ends of the capillary tubes, makes upward
currents possible, is one which intermits every night, and every day
is strong or feeble as circumstances determine. Then, in the third
place, we have this rude pumping process above described, going
on with greater vigour when the wind is violent, and with less
vigour when it is gentle — drawing liquid towards different
parts according to their degrees of oscillation, and from diffe-
rent parts according as they can most readily furnish it. And
now let us ask what must result under changing conditions from
these variously-conflicting and conspiring forces. When a warm
sunshine, causing rapid evaporation, is emptying the vessels of the
leaves, the osmotic and capillary actions that refill them will be
continually aided by the pumping action of the swaying petioles,
twigs, and branches, provided their oscillations are moderate. Under
these conditions the current of sap, moving in the direction of least
resistance, will set towards the leaves. But what will happen when
the sun sets ? There is now nothing to determine currents either
upwards or downwards, except the relative rates of growth in the
parts and the relative demands set up by the oscillations ; and the
oscillations acting alone, will draw sap to the oscillating parts as
much from above as from below. If the resistance to be overcome
by a current setting back from the leaves is less than the resistance
to be overcome by a current setting up from the roots, then a
current will set back from the leaves. Now it is, I think, tolerably
manifest that in the swaying twigs and minor branches, less force
will be required to overcome the inertia of the short columns of
liquid between them and the leaves than to overcome the inertia of
the long columns between them and the roots. Hence during the
night, as also at other times when evaporation is not going on, the
sap will be drawn out of the leaves into the adjacent supporting
parts ; and their nutrition will be increased. If the wind is strong
enough to produce a swaying of the thicker branches, the back
current will extend to them also ; and a further strengthening will
result from their absorption of the elaborated sap. And when the
great branches and the stem are bent backwards and forwards by a

588 APPENDIX C.

gale, they too will share in the nutrition. It may at first sight seem
that these parts, being nearer to the roots than to the leaves, will
draw their supplies from the roots only. But the quantity which the
roots can furnish is insufficient to meet so great a demand. Under
the conditions described, the exudation of sap from the vessels will
be very great, and the draught of liquid required to refill them, not
satisfied by that which the root-fibres can take in, will extend to the
leaves. Thus sap will flow to the several parts according to their
respective degrees of activity — to the leaves while light and heat
enable them to discharge their functions, and back to the twigs,
branches, stem, and roots when these become active and the leaves
inactive, or when their activity dominates over that of the leaves.
And this distribution of nutriment, varying with the varying
activities of the parts, is just such a distribution as we know must
be required to keep up the organic balance.

To this explanation it may be objected that it does not account
for the downward current of sap in plants that are sheltered. The
stem and roots of a drawing-room Geranium display a thickening
which implies that nutritive matters have descended from the leaves,
although there are none of those oscillations by which the sap is said
to be drawn downwards as well as upwards. The reply is, that the
stem and roots tend to repeat their typical structures, and that the
absorption of sap for the formation of their respective dense tissues,
is here the force which determines the descent. Indeed it must be
borne in mind that the mechanical strains and the pumping process
which they keep up, as well as the distention caused by osmose, do
not in themselves produce a current either upwards or downwards :
they simply help to move the sap towards that place where there is
the most rapid abstraction of it — the place towards which its motion
is least resisted. Whether there is oscillation or whether there is
not, the physiological demands of the different parts of the plant
determine the direction of the current ; and all which the oscillations
and the distention do is to facilitate the supply of these demands.
Just as much, therefore, in a plant at rest as in a plant in motion,
the current will set downwards when the function of the leaves is
arrested, and when there is nothing to resist that abstraction of sap
caused by the tendency of the stem- and root-tissues to assume their
typical structures. To which admission, however, it must be added
that since this typical structure assumed, though imperfectly as-
sumed, by the hot-house plant, is itself interpretable as the in-
herited effect of external mechanical actions on its ancestors, we
may still consider the current set up by the assumption of the
typical structure to be indirectly due to such actions.

Interesting evidence of another order here demands notice. In
the course of experiments on the absorption of dyes by leaves, it
happened that in making sections parallel to the plane of a leaf, with

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 589

the view of separating its middle layer containing the vessels, I came
upon some structures that were new to me. These structures, where
they are present, form the terminations of the vascular system. They
are masses of irregular and imperfectly united fibrous cells, such as
those out of which vessels are developed ; and they are sometimes
slender, sometimes bulky — usually,however, being more or less club-
shaped. In transverse sections of leaves their distinctive characters
are not shown : they are taken for the smaller veins. It is only by
carefully slicing away the surface of a leaf until we come down
to that part which contains them, that we get any idea of their
nature. Fig. 1 represents a specimen taken from a leaf of Eu-
phorbia neriifolia. Occupying one of the interspaces of the ulti-
mate venous network, it consists of a spirally-lined duct or set of
ducts, which connects with the neighbouring vein a cluster of half-
reticulated, half -scalarif orin cells. These cells have projections,many
of them tapering, that insert themselves into the adjacent intercell-
ular spaces, thus producing an extensive surface of contact between
the organ and the imbedding tissues. A further trait is, that the en-
sheathing prosenchyma is either but little developed or wholly ab-
sent ; and consequently this expanded vascular structure, especially
at its end, comes immediately in contact with the tissues concerned
in assimilation. The leaf of Euphorbia neriifolia is a very fleshy
one ; and in it these organs are distributed through a compact,
though watery, cellular mass. But in any leaf of the ordinary type
which possesses them, they lie in the networkparenchyma composing
its lower layer ; and wherever they occur in this layer its cells unite
to enclose them. This arrangement is shown in fig. 2, representing
a sample from the Caoutchouc-leaf, as seen with the upper part of
its envelope removed ; and it is shown still more clearly in a sample
from the leaf of Panax Lessonii, fig. 3. Figures 4 and 5 represent,
without their sheaths, other such organs from the leaves of Panax
Lessonii and Clusia flava. Some relation seems to exist between
their forms and the thicknesses of the layers in which they lie.
Certain very thick leaves, such as those of Clusia flava, have them
less abundantly distributed than is usual, but more massive. Where
the parenchyma is developed not to so great an extreme, though
still largely, as in the leaves of Holly, Aucuba, Camellia, they are
not so bulky ; and in thinner leaves, like those of Privet, Elder,
&c, they become longer and less conspicuously club-shaped. Some
adaptations to their respective positions seem implied by these modi-
fications ; and we may naturally expect that in many thin leaves
these free ends, becoming still narrower, lose the distinctive and
suggestive characters possessed by those shown in the diagrams.
Relations of this kind are not regular, however. In various other
genera, members of which I have examined, as Rhus, Viburnum,
Griselinia, Brexia, Botryodendron, Pereskia, the variations in the

590 APPENDIX C.

bulk and form of these structures are not directly determined by
the spaces which the leaves allow : obviously there are other modi-
fying causes. It should be added that while these expanded free
extremities graduate into tapering free extremities, not differing
from ordinary vessels, they also pass insensibly into the ordinary
inosculations. Occasionally, along with numerous free endings,
there occur loops ; and from such loops there are transitions to
the ultimate meshes of the veins.

These organs are by no means common to all leaves. In many
that afford ample spaces for them they are not to be found. So far
as I have observed, they are absent from the thick leaves of plants
which form very little wood. In Semper vivum^ in Echeveria, in
Bryophyllum, they do not appear to exist ; and I have been unable
to discover them mKalancho'e rotundifolia, in Kleinia ante-euphor-
bium and ficoides, in the several species of Crassula, and in other
succulent plants. It may be added that they are not absolutely
confined to leaves, but occur in stems that have assumed the func-
tions of leaves. At least I have found, in the green parenchyma
of Opuntia, organs that are analogous though much more rudely
and irregularly formed. In other parts, too, that have usurped
the leaf -function, they occur, as in the phyllodes of the Australian
Acacias. These have them abundantly developed ; and it is interest-
ing to observe that here, where the two vertically-placed surfaces
of the flattened-out petiole are equally adapted to the assimilative
function, there exist two layers of these expanded vascular termina-
tions, one applied to the inner surface of each layer of parenchyma.

Considering the structures and positions of these organs, as well
as the natures of the plants possessing them, may we not form a
shrewd suspicion respecting their function ? Is it not probable that
they facilitate absorption of the juices carried back from the leaf for
the nutrition of the stem and roots ? They are admirably adapted
for performing this office. Their component fibrous cells, having
angles insinuated between the cells of the parenchyma, are shaped
just as they should be for taking up its contents; and the absence
of sheathing tissue between them and the parenchyma facilitates the
passage of the elaborated liquids. Moreover there is the fact that
they are allied to organs which obviously have absorbent functions.
I am indebted to Dr. Hooker for pointing out the figures of two
such organs in the " Icones Anatomicae " of Link. One of them is
from the end of a dicotyledonous root-fibre, and the other is from
the prothallus of a young Fern. In each case a cluster of fibrous
cells, seated at a place from which liquid has to be drawn, is con-
nected by vessels with the parts to which liquid has to be carried.
There can scarcely be a doubt, then, that in both cases absorption
is effected through them. I have met with another such organ,
more elaborately constructed, but evidently adapted to the same

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 591

office, in the common Turnip-root. As shown by the end view
and longitudinal section in figs. 6 and 7, this organ consists of
rings of fenestrated cells, arranged with varying degrees of regu-
larity into a funnel, ordinarily having its apex directed towards the
central mass of the Turnip, with which it has, in some cases at least,
a traceable connexion by a canal. Presenting as it does an external
porous surface terminating one of the branches of the vascular sys-
tem, each of these organs is well fitted for taking up with rapidity
the nutriment laid by in the Turnip-root, and used by the plant
when it sends up its flower-stalk. Nor does even this exhaust the
analogies. The cotyledons of the young bean, experimented upon
as before described, furnished other examples of such structures,
exactly in the places where, if they are absorbents, we might ex-
pect to find them. Amid the branchings and inosculations of the
vascular layer running through the mass of nutriment deposited in
each cotyledon, there are conspicuous free terminations that are club-
shaped, and prove to be composed, like those in leaves, of irregularly
formed and clustered fibrous cells ; and some of them, diverging
from the plane of the vascular layer, dip down into the mass of
starch and albumen which the young plant has to utilize, and
which these structures can have no other function but to take up.
Besides being so well fitted for absorption, and besides being
similar to organs which we cannot doubt are absorbents, these vas-
cular terminations in leaves afford us yet another evidence of their
functions. They are seated in a tissue so arranged as specially to
facilitate the abstraction of liquid. The centripetal movement of
the sap must be set up by a force that is comparatively feeble, since,
the parietes of the ducts being porous, air will enter if the tension
on the contained columns becomes considerable. Hence it is needful
that the exit of sap from the leaves should meet with very little resist-
ance. Now were it not for an adjustment presently to be described,
it would meet with great resistance, notwithstanding the peculiar
fitness of these organs to take it in. Liquid cannot be drawn out
of any closed cavity without producing a collapse of the cavity's
sides;" and if its sides are not readily collapsible, there must be a cor-
responding resistance to the abstraction of liquid from it. Clearly
the like must happen if the liquid is to be drawn out of a tissue
which cannot either diminish in bulk bodily or allow its components
individually to diminish in bulk. In an ordinary leaf, the upper
layer of parenchyma, formed as it is of closely-packed cells that are
without interspaces, and are everywhere held fast within their frame-
work of veins, can neither contract easily as a mass, nor allow its sep-
arate cells to do so. Quite otherwise is it with the network-paren-
chyma below. The long cells of this, united merely by their ends
and having their flexible sides surrounded by air, may severally have
their contents considerably increased and decreased without offering

592 APPENDIX C.

appreciable resistances; and the network-tissue which they form will,
at the same time, be capable of undergoing slightexpansions and con-
tractions of its thickness. In this layer occur these organs that are so
obviously fitted for absorption. Here we find them in direct commu-
nication with its system of collapsible cells. The probability appears
to be, that when the current sets into the leaf, it passes through the
vessels and their sheaths chiefly into the upper layer of cells (this
upper layer having a larger surface of contact with the veins than the
lower layer, and being the seat of more active processes) ; and that
the juices of the upper layer, enriched by the assimilated matters,
pass into the network-parenchyma, which serves as a reservoir from
which they are from time to time drawn for the nutrition of the rest
of the plant, when the actions determine the downward current.
Should it be asked what happens where the absorbents, instead of
being inserted in a network-parenchyma, are, as in the leaves of
Euphorbia neriifolia, inserted in a solid parenchyma, the reply is,
that such a parenchyma, though not furnished with systematically
arranged air-chambers, nevertheless contains air in its intercellular
spaces ; and that when there occurs a draught upon its contents,
the expansion of this air and the entrance of more from without,
quickly supply the place of the abstracted liquid.

If then, returning to the general argument, we conclude that
these expanded terminations of the vascular system in leaves are ab-
sorbent organs, we find a further confirmation of the views set forth
respecting the alternating movement of the sap along the same chan-
nels. These spongioles of the leaves, like the spongioles of the roots,
being appliances by which liquid is taken up to be carried into the
mass of the plant, we are obliged to regard the vessels that end in
these spongioles of the leaves as being the channels of the down
current whenever it is produced. If the elaborated sap is abstracted
from the leaves by these absorbents, then we have no alternative
but to suppose that, having entered the vascular system, the elab-
orated sap descends through it. And seeing how, by the help of
these special terminations, it becomes possible for the same vessels
to carry back a quality of sap unlike that which they bring up, we
are enabled to understand tolerably well how this rhythmical
movement produces a downward transfer of materials for growth.

The several lines of argument may now be brought together ;
and along with them may be woven up such evidences as remain.
Let me first point out the variety of questions to which the
hypothesis supplies answers.

It is required to account for the ascent of sap to a height beyond
that to which capillary action can raise it. This ascent is accounted
for by the propulsive action of transverse strains, joined with that of
osmotic distention. A cause has to be assigned for that rise of sap

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 593

which, in the spring, while yet there is no considerable evaporation
to aid it, goes on with a power which capillarity does not explain.
The co-operation of the same two agencies is assignable for this result
also.* The circumstance that vessels and ducts here contain sap and
there contain air, and at the same place contain at different seasons
now air and now sap is a fact calling for explanation. An explana-
tion is furnished by these mechanical actions which involve the- en-
trance or expulsion of air according to the supply of liquid. That
vessels and ducts which were originally active sap-carriers go com-
pletely out of use, and have their function discharged by other
vessels or ducts, is an anomaly that has to be solved. Again, we
are supplied with a solution : these deserted vessels and ducts are
those which, by the formation of dense tissue outside of them,
become so circumstanced that they cannot be compressed as they
originally were. A channel has to be found for the downward
current of sap, which, on any other hypothesis than the foregoing,
must be a channel separate from that taken by the upward
current ; and yet no good evidence of a separate channel has been
pointed out. Here, however, the difficulty disappears, since one
channel suffices for the current alternating upwards and downwards
according to the conditions. Moreover there has to be found a
force producing or facilitating the downward current, capable even
of drawing sap out of drooping branches ; and no such force is
forthcoming. The hypothesis set forth dispenses with this necessity ;
under the recurring change of conditions, the same distention and
oscillation which before raised the sap to the places of consumption,
now bring it down to the places of consumption. A physical
process has to be pointed out by which the material that forms
dense tissue is deposited at the places where it is wanted, rather
than at other places. This physical process the hypothesis in-
dicates. It is requisite to find an explanation of the fact that,
when plants ordinarily swayed about by the wind are grown indoors,
the formation of wood is so much diminished that they become ab-
normally slender. Of this an explanation is supplied. Yet a further

* It seems probable, however, that osmotic distention is here, especially,
the more important of the two factors. The rising of the sap in spring may
indirectly result, like the sprouting of the seed, from the transformation of
starch into sugar. During germination, this change of an oxy-hydro-carbon
from an insoluble into a soluble form, leads to rapid endosmose ; con-
sequently to great distention of the seed ; and therefore to a force which
thrusts the contained liquids into the plumule and radicle, and gives them
power to displace the soil in their way : it sets up an active internal move-
ment when neither evaporation nor the change which light produces can be
operative. And similarly, if, in the spring, the starch stored up in the roots
of a tree passes into the form of sugar, the unusual osmotic absorption that
arises will cause an unusual distention — a distention which, being resisted by
the tough bark of the roots and stem, will result in a powerful upward thrust
of the contained liquid.
84

594 APPENDIX C.

fact to be interpreted is, that in the same individual plant homol
ogous parts, which, according to the type of the plant, should be
equally woody, become much thicker one than another if subject
to greater mechanical stress. And of this too an interpretation is
similarly afforded.

Now the sufficiency of the assigned actions to account for so many
phenomena not otherwise explained, would be strong evidence that
the rationale is the true one, even were it of a purely hypothetical
kind. How strong, then, becomes the reason for believing it the
true one when we remember that the actions alleged demonstrably
go on in the way asserted. They are ever operating before our
eyes ; and that they produce the effects in question is a conclusion
deducible from mechanical principles, a conclusion established by
induction, and a conclusion verified by experiment. These three
orders of proof may be briefly summed up as follows.

That plants which have to raise themselves above the earth's sur-
face, and to withstand the actions of the wind, must have a power of
developing supporting structure, is an a priori conclusion which may
be safely drawn. It is an equally safe a priori conclusion, that if
the supporting structure, either as a whole or in any of its parts, has
to adapt itself to the particular strains which the individual plant
is subject to by its particular circumstances, there must be at work
some process by which the strength of the supporting structure is
everywhere brought into equilibrium with the forces it has to bear.
Though the typical distribution of supporting structure in each kind
of plant may be explained teleologically by those whom teleological
explanations satisfy ; and though otherwise this typical distribution
may be ascribed to natural selection acting apart from any directly
adaptive process ; yet it is manifest that those departures from the
typical distribution which fit the parts of each plant to their special
conditions are explicable neither teleologically nor by natural selec-
tion. We are, therefore, compelled to admit that, if in each plant
there goes on a balancing of the particular strains by the particular
strengths, there must be a physical or physico-chemical process by
which the adjustments of the two are effected. Meanwhile we are
equally compelled to admit, a priori, that the mechanical actions to
be resisted, themselves affect the internal tissues in such ways as to
further the increase of that dense substance by which they are re-
sisted. It is demonstrable that bending the petioles, shoots, and
stems must compress the vessels beneath their surfaces, and increase
the exudation of nutritive matters from them, and must do this act-
ively in proportion as the bends are great and frequent ; so that
while, on the one hand, it is a necessary deduction that, if the parts of
each plant are to be severally strengthened according to the several
strains, there must be some direct connexion between strains and
strengths, it is, on the other hand, a necessary deduction from

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 595

mechanical principles that the strains do act in such ways as to aid
the increase of the strengths. How a like correspondence between
two a priori arguments holds in the case of the circulation, needs
not to be shown in detail. It will suffice to remind the reader
that while the raising of sap to heights beyond the limit of cap-
illarity implies some force to effect it, we have in the osmotic
distention and the intermittent compressions caused by transverse
strains, forces which, under the conditions, cannot but tend to
effect it ; and similarly with the requirement for a downward
current, and the production of a downward current.

Among the inductive proofs we find a kindred agreement. Diffe-
rent individuals of the same species, and different parts of the same
individual, do strengthen in different degrees ; and there is a clearly
traceable connexion between their strengthenings and the intermit-
tent strains they are exposed to. This evidence, derived from contrasts
between growths on the same plant or on plants of the same type, is
enforced by evidence derived from contrasts between plants of diffe-
rent types. The deficiency of woody tissue which we see in plants
called succulent, is accompanied by a bulkiness of the parts which
prevents any considerable oscillations; and this character is also
habitually accompanied by a dwarfed growth. When, leaving these
relations as displayed externally, we examine them internally, we
find the facts uniting to show, by their agreements and differences,
that between the compression of the sap-canals and the production
of wood there is a direct relation. We have the facts, that in each
plant, and in every new part of each plant, the formation of sap-
canals precedes the formation of wood ; that the deposit of woody
matter, when it begins, takes place around these sap-canals, and
afterwards around the new sap-canals successively developed ; that
this formation of wood around the sap-canals takes place where the
coats of the canals are demonstrably permeable, and that the amount
of wood-formation is proportionate to the permeability. And then
that the permeability and extravasation of sap occur wherever, in
the individual or in the type, there are intermittent compressions,
is proved alike by ordinary cases and by exceptional cases. In
the one class of cases we see that the deposit of wood round the
vessels begins to take place when they come into positions that
subject them to intermittent compressions, while it ceases when
they become shielded from compressions. And in the other class
of cases, where, from the beginning, the vessels are shielded from
compression by surrounding fleshy tissue, there is a permanent
absence of wood-formation.

To which complete agreement between the deductive and induc-
tive inferences has to be added the direct proof supplied by experi-
ments. It is put beyond doubt by experiment that the liquids ab-
sorbed by plants are distributed to their different parts through their

596 APPENDIX C.

vessels — at first by the spiral or allied vessels originally developed,
and then by the better-placed duots formed later. By experiment
it is demonstrated that the intermittent compressions caused by os-
cillations urge the sap along the vessels and ducts. And it is also ex-
perimentallyprovedthat the same intermittentcompressionsproduce
exudation of sap from vessels and ducts into the surrounding tissue.
That the processes here described, acting through all past time,
have sufficed of themselves to develope the supporting and distribut-
ing structures of plants, is not alleged. What share the natural
selection of variationsdistinguishedas spontaneous, has had in estab-
lishing them, is a question which remains to be discussed. Whether
acting alone natural selection would have sufficed to evolve these
vascular and resisting tissues, I do not profess to say. That it has
been a co-operating cause, I take to be self-evident : it must all along
have furthered the action of any other cause, by preserving the in-
dividuals on which such other cause had acted most favourably.
Seeing, however, the conclusive proof which we have that another
cause has been in action — certainly on individuals, and, in all proba-
bility, by inheritance on races — we may most philosophically ascribe
the genesis of these internal structures to this cause, and regard
natural selection as having here played the part of an accelerator.

EXPLANATION OF PLATE.

Fig. 1. Absorbent organ from the leaf of Euphorbia neriifolia.
The cluster of fibrous cells forming one of the terminations of the
vascular system is here imbedded in a solid parenchyma.

Fig. 2. A structure of analogous kind from the leaf of Ficus
elastica. Here the expanded terminations of the vessels are im-
bedded in the network parenchyma, the cells of which unite to
form envelopes for them.

Fig. 3. Shows on a larger scale one of these absorbents from
the leaf of Panax Lessonii. In this figure is clearly seen the way
in which the cells of the network parenchyma unite into a closely-
fitting case for the spiral cells.

Fig. 4. Represents a much more massive absorbent from the
same leaf, the surrounding tissues being omitted.

Fig. 5. Similarly represents, without its sheath, an absorbent
from the leaf of Clusia flava.

Fig. 6. End view of an absorbent organ from the root of a
Turnip. It is taken from the outermost layer of vessels. Its
funnel-shaped interior is drawn as it presents itself when looked
at from the outside of this layer, its narrow end being directed
towards the centre of the Turnip.

Fig. 7. A longitudinal section through the axis of another such
organ, showing its annuli of reticulated cells when cut through.
The cellular tissue which fills the interior is supposed to be removed.

CIRCULATION AND FORMATION OF WOOD IN PLANTS. 597

Fig. 8. A less-developed absorbent, showing its approximate
connexion with a duct. In their simplest forms, these structures
consist of only two fenestrated cells, with their ends bent round
so as to meet. Such types occur in the central mass of the Turnip,

where the vascular system is relatively imperfect. Besides the
comparatively regular forms of these absorbents, there are forms
composed of amorphous masses of fenestrated cells. It should
be added that both the regular and irregular kinds are very vari-
able in their numbers : in some turnips they are abundant, and in
others scarcely to be found. Possibly their presence depends on

598 APPENDIX C.

the age of the Turnip. Judging from the period during which
my investigations were made, namely winter and early spring, I
suspect that they are developed only in preparation for sending
up the flower-stalk.

Let me add that experiments on circulation in plants made
during the state of inactivity, when it is to be presumed that the
vessels and tissues contain but little sap, are much more suc-
cessful than those made in the summer. It would seem that
when the tissues are fully charged with sap the taking up of
dyes is comparatively slow and the above-described effects are
not so easily demonstrable.

[An expert writes concerning this essay : — " I have not
attempted to annotate critically this paper. There is no doubt
that many of your conclusions are perfectly sound, particularly
those relating to the passage of crude sap through the cavities of
the elements of the wood, though the opinion that the actual
passage was through the walls very generally held till about 12
years ago."]

APPENDIX D.

ON THE ORIGIN OF THE VERTEBRATE TYPE.

[ When studying the development of the vertebrate skeleton, there
occurred to me the following idea respecting the possible origin of the
notochord. I was eventually led to omit the few pages of Appendix in
which I had expressed this idea, because it was unsupported by develop-
mental evidence. The developmental evidence recently discovered, how-
ever, has led Professor Haeckel and others to analogous views respecting
the affiliation of the Vertebrata on the Molluscoida. Having fortu-
nately preserved a proof of the suppressed pages, I am able now to
add them. With the omission of a superfluous paragraph, they are
reprinted verbatim from this proof, which dates back to the autumn
of 1865, at which time the chapter on " The Shapes of Vertebrate
Skeletons" was written. — December, 1869.]

The general argument contained in Chap. XVI. of Part IV., I
have thought it undesirable to implicate with any conception more
speculative than those essential to it ; and to avoid so implicating
it, I transfer to this place an hypothesis respecting the derivation
of the rudimentary vertebrate structure, which appears to me
worth considering.

Among those molluscoid animals with which the lowest verte-
brate animal has sundry traits in common, it very generally happens
that while the adult is stationary the larva is locomotive. The
locomotion of the larva is effected by the undulations of a tail. In
shape and movement one of these young Ascidians is not altogether
unlike a Tadpole. And as the tail of the Tadpole disappears
when its function comes to be fulfilled by limbs ; so the Ascidian
larva's tail disappears when fixation of the larva renders it useless.
This disappearance of the tail, however, is not without exception.
The Appendicularia is an Ascidian which retains its tail through-
out life ; and by its aid continues throughout life to swim about.
Now this tail of the Appendicularia has a very suggestive structure.
It is long, tapering to a point, and flattened. From end to end
there runs a mid-rib, which appears to be an imbedded gelatinous
rod, not unlike a notochord. Extending along the two sides of

600 APPENDIX D.

this mid-rib, are bundles of muscular fibres ; and its top bears a
gangliated nervous thread, giving off, at intervals, branches to the
muscular fibres. In the Appendicularia this tail, which is inserted
at the lower part of the back, is bent forwards, so as not to be
adapted for propelling the body of the animal head foremost ; but
the homologous tails of the larval Ascidians are directed backwards,
so as to produce forward movement. If we suppose a type like the
Appendicularia in the structure and insertion of its permanent tail,
but resembling the larval forms in the direction of its tail, it is, I
think, not difficult to see that functional adaptation joined with
natural selection, might readily produce a type approximating to
that whose origin we are considering. It is a fair assumption
that an habitually - locomotive creature would profit by in-
creased power of locomotion. This granted, it follows that
such further development of the tail-structures as might arise
from enhanced function, and such better distribution of them
as spontaneous variation might from time to time initiate,
would be perpetuated. What must be the accompanying changes ?
The more vigorous action of such an appendage implies a firmer
insertion into the body ; and this would be effected by the pro-
longation forwards of the central axis of the tail into the creature's
back. As fast as there progressed this fusion of the increasingly-
powerful tail with the body, the body would begin to partake of its
oscillations ; and at the same time that the resistant axis of the tail
advanced along the dorsal region, its accompanying muscular fibres
would spread over the sides of the body : gradually taking such
modified directions and insertions as their new conditions rendered
most advantageous. Without further explanation, those who
examine drawings of the structures described, will, I think,
see that in such a way a tail homologous with that of the
Appendicularia, would be likely, in the course of that de-
velopment required for its greater efficiency, gradually to
encroach on the body, until its mid-rib became the dorsal
axis, its gangliated nerve-thread the spinal chord, and its
muscular fibres the myocommata. Such a development of an
appendage into a dominant part of the organism, though at first
sight a startling supposition, is not without plenty of parallels :
instance the way in which the cerebral ganglia, originally mere
adjuncts of the spinal chord, eventually become the great centres of
the nervous system to which the spinal chord is quite subordinate ;
or instance the way in which the limbs, small and inconspicuous in
fishes, become, in Man, masses which, taken together, outweigh the
trunk. It may be added that these familiar cases have a further
appropriateness ; for they exhibit higher degrees of that same
increasing dominance of the organs of external relation, which the
hypothesis itself implies.

ON THE ORIGIN OF THE VERTEBRATE TYPE. 601

Of course, if the rudimentary vertebrate apparatus thus grew
into, and spread over, a molluscoid visceral system, the formation
of the notochord under the action of alternating transverse strains,
did not take place as suggested in § 255 ; but it does not therefore
follow that its differentiation from surrounding tissues was not
mechanically initiated in the way described. For what was said in
that section respecting the effects of lateral bendings of the body,
equally applies to lateral bendings of the tail ; and as fast as the
developing tail encroached on the body, the body would become
implicated in the transverse strains, and the differentiation would
advance forwards under the influences originally alleged. Obviously,
too, though the lateral muscular masses would in this case have a
different history ; yet the segmentation of them would be eventually
determined by the assigned causes. For as fast as the strata of
contractile fibres, developing somewhat in advance of the dorsal
axis, spread along the sides, they would come under the influence
of the alternate flexions ; and while, by survival of the fittest, their
parts became adjusted in direction, their segmentation would, as
before, accompany their increasing massiveness. The actions and
reactions due to lateral undulations would still, therefore, be the
causes of differentiation, with which natural selection would co-
operate. ,

APPENDIX D2.

THE ANNULOSE TYPE.

The production of a segmental structure by undulatory move-
ments, suggested in Appendix D, as also in B (first published in
1858) as explaining the vertebral column, has been recently
suggested by Prof. Korschelt as the cause of that segmentation of
the annulose type which gives the name to it. He espouses a —

" view which is based upon the assumption that at first an unsegmented,
elongated ancestral form was produced by terminal growth, whereupon the
entire body became separated at once into a large number of segments by a
rearrangement of the individual organs. This assumption is supported by
the consideration that with the lateral sinuous movement of the body, and
with the rigidity of the tissues caused by increasing differentiation, the
formation of alternating regions of greater and less motility was of con-
siderable advantage to the individual, and rendered possible a further elon-
gation of the body. The first cause for the appearance of metameric
segmentation would then be sought in the manner of locomotion and in
mechanical conditions. However, this latter view is not supported in any
way by embryology." {Embryology of Invertebrates, Part I, pp. 349-50.)

I venture to think the confession that this view " is not sup-
ported in any way by embryology " should be joined with the
confession that it is at variance with that abstract embryology
which comprehends the process of development in general. The
assumption that there took place "a rearrangement of the
individual organs " of u an unsegmented, elongated ancestral
form," in such wise that the organs, previously single, presently
became multiple, so that instead of one organ of each kind there
were substituted many organs of each kind, is inconsistent with
the general law of evolution, organic and other — implies not
integration but disintegration. Everywhere the advance is from
many like parts performing like functions to relatively few unlike
parts performing unlike functions. The higher forms of the annu-
lose type itself show this. Compare a myriapod and a crab. In the
one we have not only a great number of similar segments bearing
similar limbs, but we have in each segment a dilatation of the main
blood-vessel — a rudimentary heart — a swollen portion of the
nerve cord — a small ganglion — and so on ; whereas in the other,
602

THE ANNTJLOSE TYPE. 603

besides relatively few segments and few limbs (sundry of them
extremely unlike the rest) we have a vascular system concentrated
into a central heart with arteries and a concentrated nervous
system, such that the great ganglia in the integrated carapace
immensely subordinate the ganglia of the remaining segments;
and similarly with the other organs. Now unless it be denied
that these highest decapods have been evolved from low types
akin to myriapods in composition, it must be admitted that the
progress has been from a string of many like segments with
similar sets of organs to a group of relatively-few unlike segments
with dissimilar sets of organs. If so we cannot rationally deny
that the progress has been of this nature up from the lowest
annelid, instead of having been, as Prof. Korschelt's hypothesis
implies, of opposite nature at the beginning.

In a preceding passage a clear recognition of the normal
course of development occurs. In opposing the view set forth in
§§ 205-7 of this work, Prof. Korschelt says : —

" It seems scarcely favourable to this theory that the degree of inde-
pendence which the individual segments present is comparatively slight.
The most important organs (nervous system, body musculature, blood-vascu-
lar system) show themselves to be single fundaments of the entire body,
and are also developed as such even though they also exhibit evidences
of metamerism. Even the excretory canals may give up their segmental
isolation and become united to one another by means of longitudinal canals."
(lb. p. 348.)

On turning back to § 206, the reader will, I think, demur to the
assertion that the independence is " comparatively slight " ; seeing
that, as in Ctenodrilus, a single segment sometimes becomes sepa-
rate and reproduces other segments to form a new series. Instead
of admitting that "the most important organs" "show them-
selves to be single fundaments of the entire body," it may be
held, contrariwise, that their original independence in each seg-
ment is masked only to the degree involved by their cooperation
as parts of a compound organism. But chiefly I remark that
when it is said that " the excretory canals may give up their seg-
mental isolation and become united" by "longitudinal canals,"
there is a clear confession that the isolation of these organs was
original and their union superinduced — an implication that the
course of evolution is as I have described it, and at variance with
the course of evolution assumed by Prof. Korschelt.

Yet another incongruity is involved in his interpretation. He
writes : —

" Just as in the consideration of the tape-worm chain we were induced by
the comparison with unsegmented forms to refer the entire chain to an
unsegmented individual, and, on the other hand, to sec in the proglottis,
not a complete individual, but only the abstricted hinder portion of the
body of the Cestode, in the same manner, and with much more reason, we
adhere to the individuality of the Annelid body." (P. 349.)

604 APPENDIX D2.

And then on the preceding page, referring to the composition
the Annelid body, he says : — " The most natural comparisons are
those with the tapeworm chain and with the strobila of the
Scyphomedusse." Now since it is here assumed that the tape-
worm and the strobila are analogous in composition, it is implied
that the detached proglottis and the detached medusa are analo-
gous ; and hence if we are to regard the proglottis as " not a com-
plete individual but only the abstricted hinder portion of the body
of the Cestode," then we must similarly regard the medusa as not
a complete individual, but only the abstricted hinder portion of
the strobila. This commits us to the strange conclusion that
whereas individuality is ascribed to the original simple polyp,
and by-and-by to the partially-segmented strobila, though these
are without special senses and with only rudiments of muscular
and nervous systems, individuality is denied to the detached
medusa, which has organs of sense, a distinct nervo-muscular
system and a considerable power of locomotion, as well as a
generative system : traits which in other cases characterize
developed individuals. Here also, then, there seems to be an
inversion of the ordinary conception.

This conception of the proglottis and the medusa is, I see,
accepted by some as tenable. But if we accept it we must accept
also an analogous conception, which will I think be regarded as
untenable. It is that supplied by the Aphides. From an egg
proceeds a series of sexless and wingless females, and at the end
of the series there come winged males and females with resulting
gamic reproduction. If instead of forming a discrete series the
imperfect females formed a concrete series, the members of which
could individually feed without being detached from one another,
as the segments of a tapeworm can, the parallelism would be
complete ; and then, according to the view in question, we should
have to regard the perfect males and females eventually arising,
not as individuals but as terminal portions of the series, contain-
ing generative products and having wings for the dispersion of
them — locomotive egg-bearing segments of the chain. Whoever
espouses this view must hold either that the first imperfect female
of the series was the individual or that the entire string of them
constituted the individual (in conformity with a view once pro-
pounded by Prof. Huxley). But he must do more than this.
Since the Aphides have descended from some winged species of
the order ITemiptera, he must hold that among those remote
ancestors each particular fly, male or female, was an individual ;
but that when abundant food and inert life led to the partheno-
genetic habit, and to chains of sexless forms, the males and
females eventually produced at the end of each chain, though,
like their remote ancestors, possessed of procreative organs and
wings, are not individuals.

THE ANNULOSE TYPE. 605

[Some memoranda bearing on the question here discussed,
mislaid at the time when the chapter dealing with it was revised,
have been discovered in time for utilization in this appendix.]

One of my critics says : —

" You have overstated the case in your favour : the alimentary canaf does
not, as you suggest, show a segmentation corresponding to that of the other
organs "in Annelida. Either it is a simple uniform tube, or else its differ-
entiations (pharynx, oesophagus, crop, intestine) are quite independent of
the repetition of the somites."

In presence of statements made in works of authority, this objec-
tion greatly surprises me. I meet with the descriptive word
" moniliform " applied to the intestine in some Annelids, and
then in the Text-book of Claus, translated and edited by
Sedgwick, it is said, concerning the alimentary canal in the
Annelida : —

" This is followed by the gastric region of the gut, which occupies the
greatest portion of the length of the body, and is either regularly con-
stricted in correspondence with the segments, or possesses lateral diverti-
cula." (P. 365.)

And again on p. 369 it is said : —

"The intestine usually preserves the same structure in its entire length
and is divided by regular constrictions into a number of divisions or
chambers, which correspond to the segments and dilate again into lateral
diverticula and caeca."

The alimentary canal thus presents the segmental character as
clearly as consists with fulfilment of its function. If the suc-
cessive segments are cooperating units of a compound animal
having but one mouth, then, necessarily, the gut cannot be com-
pletely cut into parts, each answering to a segment, for there
could be, in that case, no passage for the food. If the portion of
the intestine belonging to each segment has a conspicuous dila-
tation, or has a caecum on each side, it exhibits the segmental
character as much as the physical requirements permit. So far
from being at variance with the hypothesis, its structure exhibits
a verification of it.

The next objection runs as follows: —

u Then, again, the ovaries and testes do not exhibit a corresponding seg-
mentation. When it is allowable to speak of ovary or testis at all as in
Lumbricus, we find that in the case of both organs we have at most two
pairs."

It seems to me that the distribution of the generative organs in
a comparatively-developed member of the Annelid type, is not the
question. We have to ask what it is in undeveloped members of
that type. Among them the repetition of generative parts is in
some cases just what the theory implies. Thus in Claus I read : —
11 In the marine Chcetopoda, the ova or spermatozoa originate on
the body-wall from cells of the peritoneal membrane, either in

606 APPENDIX D2.

the anterior segments alone or along the whole length of the
body." So that in these last cases there are, in all the segments,
parts from which arise generative products. The fact that these
parts are not definite ovaries and testes is irrelevant. Ovaries
and testes are developed generative structures, and in the order
of evolution are preceded by undeveloped ones ; and the fact that
these undeveloped ones are found in little-developed members of
the type conforms perfectly to the hypothesis. [I may remark in
passing that here is a good illustration of that process of evolu-
tion which, in the above speculation of Prof. Korschelt, is sup-
posed to be inverted : many dispersed, similar, and indefinite
parts, are integrated into a few localized and definite parts.]

In continuation the critic above quoted says : — " My position
is that the repetition of segments in an Annelid is a phenomenon
of the same nature as the repetition of hairs in a Mammal or of
scutes in a Reptile ", and he proceeds to give instances of repe-
titions of organs in other types, as of the reproductive struc-
tures and excretory system in the young Dog-fish or of the
ovaries in Amphioxus. These examples do not seem to me
relevant. No parallelism exists between the repetition of a par-
ticular organ in an animal, and the repetition of an entire cluster
of organs constituting a physiological whole. The repetitions of
the ovaries in Amphioxus and of the excretory system in a young
Dog-fish, occur without threatening to divide into similar parts
the entire organism. But the segmental repetitions in an annu-
lose creature implicate the structures at large, and would, if
pushed a little further, result in separate creatures. The segment
of a low Annelid contains alimentary, vascular, nervous, excre-
tory, reproductive, sensory and locomotive organs — all the organs
required for carrying on life, save certain organs of external
relation which its position excludes. When there is shown some
vertebrate animal, or proto-vertebrate animal, that is divisible
into parts each of which is in great measure physiologically inde-
pendent, I shall feel obliged to abandon my position.

While this appendix is in hand I have received from another
expert, whose view is in general agreement with my own, a letter
containing the following passage : —

" You will see that Dohrn's theory was the antithesis of your own view
of vertebrate structure, namely that the vertebra? were formed by the seg-
mentation, from mechanical causes of a body originally simple. This view
of yours has been confirmed by later researches, which have shown that
the most primitive forms allied to the Vertebrates, possessing the essential
organs, viz., gill-slits, notochord, and dorsal nerve cord, are not segmented
animals, like Annelids and Crustacea, but simple animals, having at most
three regions, not exactly corresponding to segments. These primitive
unsegmented forms are Ascidian tadpoles, Balanoglossus, and certain other
primitive forms. The embryology of Vertebrates also proves that they are

THE ANNULOSE TYPE. 607

originally simple and not segmented animals, especially the fact that there
is originally one pronephric duct or primitive kidney."

Nevertheless there survives a leaning towards the notion of a
segmental origin of the Vertebrata. But the repetitions of organs
named in support of this notion have, I think, no more relation
to the genesis of the vertebrate type than the multiplication of
vertebrae in a snake has relation to the genesis of the vertebral
column.

APPENDIX E.

THE SHAPES AND ARRANGEMENTS OF FLOWERS.

In Part IV., Chapter X., under the title of " The Shapes of
Flowers," I have, after describing their several kinds of symmetry,
as habitually related to their positions, made some remarks by way
of interpretation. The truth that flowers exhibit a radial symmetry
when they are so placed as to be equally affected all round by
incident forces, having been exemplified, and also the truth that
they assume a bilateral symmetry when they are so placed that
their two sides are conditioned in ways different from the ways in
which their upper and lower parts are conditioned ; I have gone
on to inquire (in § 234) by what causes such modifications of
form are produced. I have stated that, originally, I inclined to
ascribe them entirely to differences in the relations of the parts
to physical forces — light, heat, gravitation, etc. ; but that I found
sundry facts stood in the way of this interpretation. And I have
said that " Mr. Darwin's investigations into the fertilization of
Orchids led me to take into account an unnoticed agency." Con-
tinuing to recognize the physical forces as factors having some
influence, I have concluded that the most important factor is the
action of insects ; which, aiding most the fertilization of those
flowers which most facilitate their entrance, produce, in course of
generations, a form of flower specially adapted to the special
position.

Though still adhering to this interpretation, I have since found
reason to think that the original interpretation contains a larger
portion of truth than I supposed at the time when I was led thus to
revise it. While staying at Miirren, in Switzerland, in 1872, 1 ob-
served some modifications in a species of Gentian, which proved to me
that the action of incident physical forces on flowers is, in some cases,
very rapid and decided. The species furnishing this evidence was the
Gentiana Asclepiadea ; which If ound in a copse formed of bushes that
were here wide apart and there close together. In some places not
near to the bushes, the individuals of the species grew vertically ; in
other places, partially shaded, their inclined shoots curved in such
608

THE SHAPES AND ARRANGEMENTS OF FLOWERS. 609

directions as to get the most light ; and in other cases their shoots
were led to take directions almost or quite horizontal. That, along
with these modifications in the directions of their shoots, there went
adjustments in the attitudes of their leaves, was a fact not specially
worthy of remark ; for plants placed inside the windows of houses
habitually show us that leaves quickly bend themselves into atti-
tudes giving them the greatest amounts of light. But the fact
which attracted my attention was, that the flowers changed their
attitudes in an equally-marked manner. The radial distribution
passed into a bilateral distribution with the greatest readiness.
Comparison of the annexed figures will show the character of this
change.

Figure I. represents part of a vertically growing shoot. This
belonged to an individual growing unimpeded by bushes, and getting
light on all sides. Here it is observable that the pairs of leaves,
placed alternately in directions transverse to one another — one pair
pointing, say, north and south, and the next pair pointing east and
west — maintain, taking them in the aggregate, a radial distribution ;
and it is also observable that the alternate pairs of flowers are
similarly arranged.

Figure II. is a sketch from a shoot which leaned towards one
side, and of which the higher part, as it bent more and more,
got its upper side more and more diiferently conditioned from its
lower side. Here we find that not only the leaves, but also the
flowers, have adjusted themselves to the changed conditions. The
leaves of the lowest pair hang out in the normal way, on the op-
posite sides of the axis, so that a plane passing through their sur-
faces will cut the axis transversely ; and their two axillary flower-
buds, c and d, are similarly placed on opposite sides of the axis.
But at the other part of the shoot, we see both that the leaves have
adjusted themselves so that their planes, no longer cutting the axis
transversely, keep a fit adjustment with respect to the light ; and
also that the flowers, no longer on opposite sides of the axis, have
bent round to the upper side, as at a and b.

Figure III. shows us this re-arrangement carried still further.
The shoot it represents was growing in a direction nearly horizontal,
and therefore receiving the light only on one side. And here,
besides seeing that the leaves have so adjusted themselves that they
all lie in approximately the same plane, which is parallel to the axis
instead of transverse to it, we see that the two pairs of flower-buds
have both come round to the upper side of the axis. So that
in this shoot, the original radial symmetry in the arrangement
of leaves and flowers, is completely changed into a bilateral
symmetry.

These facts do not, it is true, prove any modification in the forms
85

THE SHAPES AND ARRANGEMENTS OF FLOWERS. 611

of the flowers themselves : they only prove modification in the
grouping of the flowers. But beyond snowing, as they do conclu-
sively, how readily a bilateral arrangement of flowers is producible
out of an arrangement that was not bilateral, by the action of light,
etc. ; they give increased probability to the belief that changes in the
shapes of flowers are producible by the same agencies. Doubtless
this change in the attitudes of the flower-buds is due to the action of
light on their calyces and peduncles more than to its action on their
unfolding corollas. But along with an action so decided on the growth
of these sheathing and supporting organs containing chlorophyl, it is
scarcely probable that there is no action on the growth of the petals,
containing other colouring matter; considering that in both cases
the development of the colouring matter depends on the action of
light, and considering also the effect of light on petals, familiarly
shown by their opening and closing. And if even but a small
effect is producible on the growth of the corolla, then it is to be
expected that light will be an agent in changing the form of the
corolla, when the attitude of the flower causes its parts to be dif-
ferently exposed. For a small effect on the individual flower will
become a great effect in the flowers of remote descendants ; pro-
vided the changed attitudes of the flowers preserve considerable
constancy throughout the succession of individuals.

Be this as it may, however, the facts I have here described,
which I doubt not other observers have seen paralleled in other
plants, are instructive, as showing how quickly certain metamor-
phoses are produced, and as implying the easy establishment of such
metamorphoses as permanent characters in a species, if the modify-
ing conditions become permanent. The changes of arrangement
I have pointed out, do not become permanent in this species because
its individuals are variously affected by the modifying forces : on
some they do not act at all, on some a little, on some much ; and
even on the same individual the different shoots are quite differently
affected. But if the habit of this plant were greatly changed — if,
for instance, by spreading into habitats yielding abundant nutri-
ment, the plant became very luxuriant, and, multiplying its branches,
grew shrub-like ; it is clear that, being shaded by one another, these
branches would be habitually circumstanced in a way like that which
we here see produces bilateralness in the distribution of the flowers,
if not in the flowers themselves ; and being thus permanently
affected, would become permanently bilateral. Accumulating by
inheritance, what is here only an individual peculiarity, would
become a peculiarity of the species — a specific character.

APPENDIX F.

PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS.

There has recently come before me a fact which has a signifi-
cant bearing on the hypothesis of Constitutional units : serving,
indeed, to give an apparently conclusive proof of its truth. Before
stating it, however, I may with advantage re-state the several
evidences already assigned in support of it.

1. First comes the a priori reason. These units in the germ of
an organism which cause development into a special structure,
cannot be chemical units — cannot be simply molecules of protei<
substance in one or other of its forms ; since these are not spech
to any type of creature but common to all creatures. Nor can
they be what we may call morphological units — the cells or proto-
plasts ; because in the early stages of development the cells of
one organism are indistinguishable from those of others, and
because were cells the units of composition there could be no
interpretation of what are called unicellular organisms — nothing
to account for the innumerable varieties of them. Hence, of
necessity, the structural elements of which each organism is built,
being neither proteid molecules nor cells, must be something
between them : probably some complex combination of different
isomeric forms of proteids.

2. That units of such natures are the essential components of
each species of organism, is shown by the fact that in low types of
creatures, little differentiated into special tissues, any considerable
portion of the body will, when separated, begin to assume the
structure proper to the species — a truth recently shown afresh by
Prof. T. H. Morgan's experiments on the regeneration of Planaria
maculata (already referred to in § 206) showing that various frag-
ments cut out develop into new individuals, and that when, being
too small they die before doing this, there is always an abortive
attempt to assume the specific structure.

3. This truth that a portion of undifferentiated tissue, if ade-
quate in quantity, assumes the structure of the type, illustrating

6X2

PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS. 613

as it does the proclivity of the constitutional units towards the
structure of the species, allies itself with the phenomena of both
■gamogenesis and gainogenesis. The first of these shows us how
a fissiparously-detached portion of the parental tissue takes on
the same form as the parent; and the second shows how those
small detached portions distinguished as sperm-cell and germ-cell
also, when united and supplied with the needful materials, do the
same thing.

4. But the set of phenomena following the union of sperm-cell
and germ-cell differ in a certain way from those which follow
when a gemma or other unfertilized portion of parental tissue is
detached. The incomprehensibleness of this difference as other-
wise contemplated, and the partial comprehensibleness of it when
joined with the hypothesis of physiological units, furnish a
further support for the hypothesis.

The familiar truth learnt by the tyro in algebra that an ap-
parent solution which contains the unknown quantity is no
solution, is a truth apt to be overlooked in other spheres than
the algebraic. An illustration is supplied by the answer once
given in Parliament to the question " What is an Archdeacon ? "
— " One who discharges archidiaconal functions." But science as
well as daily life furnishes examples. When it is said by Engel-
mann, Hensen, Hertwig, and Maupas that " the essential end of
sexuality is rejuvenescence, that is, the restoration of growth-
energy," we have another instance of an explanation which explains
nothing. What is the phenomenon to be explained ? That un-
folding of an organism from a germ which displays growth-energy.
And what is the explanation ? The giving of fresh growth-energy.
The unknown quantity " growth-energy " is contained in the
explanation proposed. There exists no conception of "juven-
escence " save that derived from observing developing plants and
animals; and if "re" be prefixed, no interpretation is thereby
given to the unexplained thing " juvenescence."

Coleridge somewhere comments on a source of fallacy which he
calls the " hypostasis of a relation " — the changing of a relation
into a thing. The plumber who tells you that water rises in a
pump " by suction " supplies an instance. Having assumed suction
to be an agent, he thinks that he understands how the piston does
its work. Some of the explanations given of fertilization supply
further instances. When it is said that sexual union has for its
end " to give increased vigour to all the vital processes," it is
tacitly implied that vigour is a something — a something which
can be given. But now, in the first place, it is only by the hypo-
stasis of a relation that we are led to think of vigour as a thing.
Vigour is a state — that state of a living body which enables it to

614 APPENDIX F.

give out much motion. What enables it to do this ? The presence
in it of abundant molecules containing much molecular motion
which can be transformed into molar motion : the transformation
being effected by the falling of these molecules into their simpler
and relatively-inert components, which are thereupon excreted.
Energy-containing matter is used up, and more energy or vigour
can be given only by supplying more such matter. How then
can the union of two nuclei — those of the sperm-cell and germ-
cell — give vigour ? Only an infinitesimal portion of vigour in the
sense above explained exists in either, and the union of them
leaves it still infinitesimal. And then, even supposing the vigour
to be an entity and to be appreciable in quantity, how could it
go on producing that immense -combination of physiological
actions seen in the unfolding of the germ into an organism ? and
how could it go on producing the physiological actions of an adult
organism during a whole century ?

May we not then say that these proposed explanations leave the
question where it was — are nominal solutions, not real solutions ?

5. But the hypothesis of constitutional units furnishes, if not a
satisfactory answer yet, something in the nature of an answer — a
true cause ; that is to say, a cause actually known to us as
operating in other cases. In § 92 it was pointed out that in
proportion as units are similar, there may be built up from them
an aggregate which is relatively stable, and that along with in-
creasing dissimilarity the stability of the aggregate decreases. It
was inferred that if a group of constitutional units belonging to
one individual which have become moulded into relatively exact
congruity with the organism and with one another by long co-
operation, are mingled with some belonging to another individual
which, differently circumstanced, has become somewhat different
in itself and in its units, then the mass formed by the union of
the two groups will be relatively unstable — relatively modifiable
by incident forces. Whereas in either organism, no longer per-
petually changed in the relations of its parts by growth, there is
an approach towards equilibrium between the whole and its com-
ponents, the components contributed by the two to form a
germ, being slightly unlike one another, will not form a group
in a state of equilibrium. The group they form will be capable
of easy change by incident forces ; and they will so be rendered
free to follow their proclivities towards the typical form of the
species. Inferring this we must also infer that so long as these
two sets of slightly different units are not exposed to any constant
forces tending to coerce them into the same form, there will con-
tinue to exist in the nuclei of all descendant cells this same rela-
tive instability and consequent plasticity.

PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS. 615

Such evidence as we have verifies this interpretation. There is
first the universal fact that development of the germ begins
when it is exposed to an incident force — heat — the undulations of
which, increasing the oscillations of the mixed units, give them
greater freedom to arrange themselves in conformity with their
type. We see this alike when spring warmth makes a seed
germinate and when the warmth of a sitting hen sets up organiza-
tion in her eggs. Heat frees the molecules of inorganic matter
from local restraints and, as we see in molten metal, lets them
yield to other forces ; and similarly in this organic matter, the
units are made free to follow their proclivities. Then, secondly,
there comes the evidence from comparisons between the effects of
mixing constitutional units differing in various degrees. Let the
cluster of mixed units be derived from animals that are ordinally
distinct. Nothing happens. The units each contributes tend to
arrange themselves after the parental type. Hence a conflict
between the tendencies towards two markedly unlike structures,
and no structure arises. Suppose the mixed units come from two
kindred species — say horse and ass. The structures which they
respectively tend to form, being in their main characters alike,
there is such cooperation as produces a working organism but an
organism in certain respects imperfect — a mule. Suppose, again,
the units come from two varieties of the same species. A perfect
organism results, and, as shown by Mr. Darwin when detailing
the effects of crossing, an unusually vigorous organism. The
units being more unlike than those belonging to the same variety,
the instability of the germ-plasm is unusually great, and the
transformations which constitute development and action become
unusually active. When, as in ordinary cases, the units are sup-
plied by members of the same variety who have not been made
very much alike by their antecedents, there follows the usual
amount of organic vigour. Coming now to the results of breed-
ing in-and-in — breeding between individuals whose constitutions
(i.e. constitutional units) have for generations been growing more
alike in the absence of crossing with other stirps — we see that
diminution of organic vigour is displayed : there is a decrease in
the rate of physiological change. Finally, on coming to a closer
relationship, as in marriages between cousins, in whom the
constitutional units are more than commonly alike, we see there
frequently follows either barrenness or the production of feeble
offspring.

All these facts, then, are congruous with the hypothesis that
the use of fertilization is the mixing of unlike units, and conse-
quent production of plasticity. Leaving out cases in which the
unlikenesses are so great as wholly to prevent cooperation among
the units, the degree of vigour, that is, the activity of physiologi-

616 APPENDIX F.

cal change, is great where the unlikeness is great and diminishes
with the approach towards likeness.

6. The existence of constitutional units seems otherwise
necessarily implied. I refer to the fact that no organism is a
homogeneous mean between its parents but consists of a mixture
of parts, some following one parent and some the other. Among
illustrations of this the most conspicuous are those yielded by
the variously -mixed colours of hair or feathers. Horses, cattle,
dogs, cats, hens, pigeons display these mixtures : colours in one
place like the mother and in another place like the father. As the
internal organs are invisible, and as visible organs have indefinite
shapes and graduate indefinitely into adjacent ones, the mixture
of traits is elsewhere less conspicuous ; but occasional marked
cases (especially in malformations) leave no doubt that it pervades
the entire organism.

This peculiarity of transmission seems necessarily to imply that
there are distinct units derived from the two parents, and that in
the course of development there is more or less segregation of
them — those of the one origin predominating so far in some
places as to give special likeness to one parent, and those derived
from the other doing the like in other places. All which inter-
pretation is impossible unless the hypothesis of constitutional
units be admitted.

7. I come at length to the special evidence referred to at the
outset. It is evidence of the same nature as that just assigned,
but carried to a higher stage. It is furnished not by the segre-
gation of traits derived from two parents of the same variety,
but is furnished by the segregation of traits derived from parents
of different varieties. In articles on " Bud Variations or Sports "
[Gardener'' s Chronicle, 1891) Dr. Masters gives various examples
of the separation or unmixing of ancestral constitutions. Mr.
Noble formed a hybrid between Clematis Jackmani and C. patens.
One of these varieties flowers in the autumn on new wood, while
the other flowers in the spring on old wood ; and the result is
that flowers of two kinds, quite unlike, are produced at different
parts of the year, and that by pruning so as to cut away one or
other set of shoots, the plant may be made to produce exclusively
for the time being one or other sort of Slower.

" Another very interesting case of unmixing, or, if it be preferred, of par-
tial mixture, is afforded by Neubert's Berberis. This is a hybrid between
the evergreen pinnate-leaved Mahonia and the deciduous simple-leaved
Berberis vulgaris, and it bears leaves some of which are intermediate in
appearance, while others are much like those of one or other of its parents.

11 A not uncommon illustration of a similar kind, is the production of a
Peach and a Nectarine on the same branch, and we have just learnt from

PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS. 617

Canon Ellacombe that some of the Berlin Hellebores show evidence of their
hybrid nature by occasionally producing foliage [and flowers ?] of the two
parents separately from the same root-stock.

" In addition to the cases given above, we may here cite a few more which
have come under our notice, such as a Chrysanthemum, half the florets of
which are of one colour, half of another. A hybrid Calanthe, showing a
similar piebald variation, is shown in Fig. 14. A very curious case was that
of the Narcissus received from Mr. Walker, and in which flowers of two
distinct varieties sprang from the same bulb. Grapes not uncommonly
show their crossed origin by presenting a striped appearance, one stripe
being of one colour, one of another, as may also be seen in the Orange,
Apple, Lemon, and Currant."

Thus, however the germ-plasm is constituted its essential com-
ponents cannot be all alike. Before there can be this dissociation
of ancestral characters, there must be in the germ-plasm different
elements capable of being dissociated. This single fact seems to
compel us to assume constitutional units.

APPENDIX G.

THE INHERITANCE OF FUNCTIONALLY-CAUSED
MODIFICATIONS.

In Part II, Chapter XA, I have confessed that the process by
which a structure changed by use or disuse affects the sperm-
cells or germ-cells whence arise descendants, is unimaginable :
without, however, inferring that therefore such a process does
not exist. With others it seems different. Some three years
ago the following expression of opinion came to me from a
zoological expert : —

" Many zoologists — most of us here at Cambridge — are intensely opposed
to the doctrine of the inheritability of acquired variations. Even assuming
that the developmental power of a germ is determined by its molecular
structure (and I for one would question this — Driesch and his school when
they find that they can squeeze a developing egg into all sorts of shapes
without altering the final result, that one blastomere in an egg which has
divided into 8 is still able to reproduce a whole embryo — question it also),
we still fail to conceive any means by which, for instance, a change in the
development of a muscle or nerve can effect a corresponding change in that
part of the germ which is destined to produce a corresponding part in the
descendant."

Here it will be observed that belief in the inheritance of
structural effects wrought by use and disuse, is rejected because
of inability " to conceive any means " by which the modifications
produced in an organ can effect a correlated modification in the
germ of a descendant : failure to conceive is the test. The im-
plication is that some alternative hypothesis is accepted because
the correlating of a variation in an organ with a corresponding
germ-variation is effected by a means which is conceivable. This
is the hypothesis of Weismann. Concerning its conceivability I
have, in the chapter just named, already written as follows : —

" If we follow Prof. Weismann we are led into an astounding supposition.
He admits that every variable part must have a special determinant, and
that this results in the assumption of over two hundred thousand for the
four wings of a butterfly. Let us ask what must happen in the case of a
peacock's feather. On looking at the eye near its end, we see that the
minute processes on the edge of each lateral thread must have been in some
way exactly adjusted, in colour and position, so as to fall into line with the
processes on adjacent threads : otherwise the symmetrical arrangement of
coloured rings would be impossible. Each of these processes, then, being an
618

FUNCTIONALLY-CAUSED MODIFICATIONS. 619

independent variable, must have had its particular determinant. Now there
are about 300 threads on the shaft of a large feather, and each of them
bears on the average 1,600 processes, making for the whole feather 480,000
of these processes. For one feather alone there must have been 480,000
determinants, and for the whole tail many millions. And these, along with the
determinants for the detailed parts of all the other feathers, and for the
variable components of all organs forming the body at large, must have been
contained in the microscopic head of a spermatozoon ! " [And each of them
must, throughout all the complex developmental processes, have preserved
the ability to find its way to the exact place where it was wanted !]

If ray Cambridge correspondent is able to conceive this process
implied by the hypothesis of Weismann, I can only say that he
has an enviable power of imagination.

But now comes the strange fact that an impossibility of thought
implied by Weismann's hypothesis does not cause rejection of it,
but yet is urged as a reason for rejecting an alternative hypo-
thesis which does not imply it. One objector cannot conceive
that " a change in the development of a muscle or nerve can
effect a corresponding change in that part of the germ which is
destined to produce a corresponding part in the descendant " ;
and another objector says it is " very hard to believe " that a
functionally-changed organ will so affect spermatozoa and ova
that " one particular part of them will be so altered that the
organisms which grow up from them will be able to present the
same modification on the application of a different stimulus." It
is tacitly assumed by both that, as in the hypothesis of Weis-
mann so in the counter-hypothesis, a particular part of the
germ-plasm gives origin to a particular part of the developed
organism. But nothing of the kind is implied. The nature of
the counter-hypothesis (at any rate as held by me) is entirely
misapprehended. Anyone who turns back to the chapters in the
first volume where the conception of physiological units (or con-
stitutional units) was set forth, or who re-reads the foregoing
appendix, will see that there is altogether excluded any idea of
correlation between certain parts of the germ and certain parts
of the resulting organism. The units are supposed to be all
alike, and during the progressive embryological changes local
groups of them are supposed to take on different forms and
structures under the combined forces, general and local, brought
to bear on them. This conception is necessitated by all the
evidence. The fact disclosed by the experiments of Driesch,
Wilson, and Chabry, that from fractions of an ovum structures
may be obtained like that obtained from the whole ovum, only
smaller, necessitates it. The fact that any sufficiently large
fragment of a polyp or planarian, no matter from what part of
the body taken, will develop into a complete polyp or planarian
necessitates it. The fact that from an undifferentiated portion

620 APPENDIX G.

of a plant, even so small as a scale, a complete plant may aris
necessitates it. And it is necessitated by the fact that among
plants, roots are produced by imbedded shoots and shoots by
roots, as well as by the fact that low animals, such as hydroids,
if deprived of both head and root, will develop a head from the
root part and a root from the head part, if their respective con-
ditions are inverted. All this evidence shows conclusively that
the component units of each species, whether existing in the germ
or in the developed organism, are, when not yet differentiated by
local conditions, all alike, and that the notion of special parts of
the germ-plasm correlated with special parts of the resulting or-
ganism, is entirely alien to the hypothesis.

" But how do the units of a modified organ affect the units of
the germ in such wise that these produce an inherited modifica-
tion of the organ ? " will be asked. This difficulty has been dealt
with in §§ 97 d, 97 e, where the analogy between the social organism
and the individual organism has been brought in aid : serving, if
not to furnish a conception, yet to furnish an adumbration. Re-
garding citizens as the units of an unfolding society, say a colony,
it was pointed out that the nature they inherit from a mother-
society gives them a proclivity towards a society of like struc-
ture, the traits of which are progressively assumed as the
colony grows sufficiently large to make them possible. At the
same time it was pointed out that while the influence of the
entire aggregate on the individuals is seen in this forming of
them into a society of the inherited type, the influences of local
circumstances, and of individuals on one another, in each group,
make them differentiate into appropriate social structures, taking
on fit occupations and industries : the implication being that in
virtue of their inherited natures they all have partial capacities
for the various activities they undertake ; so that an immigrant
clerk sets up a tavern, a compositor takes to carpentering, and a
university man rides after cattle or is employed on a sheep farm.
Evidence was given in that place, as in the above paragraph, that
the constitutional units of an organism similarly have all of them
potentialities for taking on this or that structure and mode of
action which local conditions determine. It was further argued
that as citizens are continually being remoulded by their society
into congruity with it, and, if circumstances change them, tend to
remould their society ; so in the individual organism, there is this
reciprocal action of the whole on the units and of the units on
the whole. Hence it was inferred that the modified units in any
modified part tend to diffuse modifications like their own through
the units at large : being aided by the circulation of protoplasm,
as suggested in §§ 54c? and 97/. And it was urged that, however
inconceivably complex such a process may be, yet it seems not

FUNCTIONALLY-CAUSED MODIFICATIONS. 621

incredible when we recognise the probability that an organism is
more or less permeable to undulations propagated by its mole-
cules : Rontgen rays giving warrant. If such units throughout the
tissues may take in and send out ettfereal waves which bring it
into rhythmical relations with others of its kind and tend to pro-
duce congruity, it becomes, if not conceivable still supposable,
that throughout the circulating protoplasm there goes on a con-
tinual harmonization of its components — a moulding of each by
all and of all by each. Should it be said that such a process is
too marvellous to be reasonably assumed, the reply is that it is
not more marvellous than heredity itself, which, were it not
familiar to us, would be thought incredible.

But as I have said in the place referred to — "At last then we
are obliged to admit that the actual organizing process transcends
conception. It is not enough to say that we cannot know it ; we
must say that we cannot even conceive it : " can only conceive
the possibility of a suggested interpretation.

Hence we have to rely upon evidences of other kinds. Among
these, some which I think dispose absolutely of the fashionable
hypothesis while they harmonize with the opposed hypothesis,
have now to be named. That their implication should not have
been generally recognized would have seemed to me incompre-
hensible were it not that I have myself only now observed this
implication. The facts are these : —

" Verlot mentions a gardener who could distinguish 150 kinds of camellia,
when not in flower ; and it has been positively asserted that the famous old
Dutch florist Voorhelm, who kept above 1,200 varieties of the hyacinth, was
hardly ever deceived in knowing each variety by the bulb alone. Hence
we must conclude that the bulbs of the hyacinth and the branches and
leaves of the camellia, though appearing to an unpractised eye absolutely
undistinguishable, yet really differ." (Darwin, Variation of Animals and
Plants, Jbc, vol. ii, p. 251.)

More recently testimony to like effect has been given by Dr.
Maxwell Masters, and has already been quoted by me in a note
to § 286 in illustration of another truth. He says concerning
such variations : —

" To the untrained eye, the primordial differences noted are often very
slight ; even the botanist, unless his attention be specially directed to the
matter, fails to see minute differences which are perceptible enough to the

raiser or his workmen These apparently trifling morphological

differences are often associated with physiological variations which render
some varieties, say of wheat, much better enabled to resist mildew and dis-
ease generally than others. Some, again, prove to be better adapted for
certain soils or for some climates than others ; some are less liable to injury
from predatory birds than others, and so on."

In his Vegetable Teratology, p. 493, Dr. Masters names another
fact having a like implication — the fact that among seedling

622 APPENDIX G.

stocks which have not yet flowered, those which will produce
double flowers are distinguishable. He says : —

" This separation of the single from the double-flowered plants, M. Chatie
tells us is not so difficult as might be supposed. The single stocks, he ex-
plains, have deep green leaves (glabrous in certain species), rounded at the
top, the heart being in the form of a shuttlecock, and the plant stout and
thick-set in its general aspect, while the plants yielding double flowers have
very long leaves of a light green colour, hairy and curled at the edges, the
heart consisting of whitish leaves, curved so that they enclose it com-
pletely."

What is the general truth implied ? Clearly that there exists
no such thing as an independent local variation. Some marked
change in the form or colour of a flower or a fruit draws atten-
tion ; and, being a change which interests the florist or gardener,
pecuniarily or otherwise, not only draws attention but usually
monopolizes attention : the natural impression produced* being that
this variation stands there by itself — is without relation to
variations elsewhere. But now it turns out that there are
concomitant variations all over the plant. Even in under-
ground bulbs certain appreciable differences go along with certain
conspicuous differences in the flowers. And if along with a
striking change in a flower which the florist contemplates, there
go changes all over the plant not obvious to careless observers
but visible to him, we must infer that there are everywhere
minute differences which even the florist cannot perceive: the
whole constitution of the plant has diverged in some measure
from the constitutions of kindred plants. Every local variation
implies a change pervading the entire organism, manifested in
concomitant variations everywhere else.

If so, what becomes of the hypothesis of determinants — the
hypothesis that there is a special element in the germ-plasm
which results in a special local modification in the adult organism ?
That there are no facts supporting it has been all along mani-
fest ; but now it is manifest that the facts directly contradict it.

At the same time it may be remarked that while the facts are
wholly incongruous with the hypothesis of determinants and its
accompanying elaborate speculation, they are not incongruous
with the alternative hypothesis. Impossible though it may be
to imagine the natures of those ultimate units peculiar to each
species, which have proclivities towards the particular form
of organization characterizing it, yet that a change of structure
arising in one part of the organism is accompanied by multitudi-
nous changes of structure in other parts of the organism, is not
only congruous with the belief that there exist such constitutional
units, but yields it distinct support. For if, as above argued, a
conspicuous local variation is not the result of any modification of
units special to the locality, but is the result of a modification of

FUNCTIONALLY-CAUSED MODIFICATIONS. 623

the units at large, then it must happen that such modification must
have its effects on all other parts of the organism ; so that there
cannot fail to result all those small concomitant variations above
indicated.

May we not also say that it becomes less incomprehensible
that structural changes caused by use and disuse are inherited ?
If, as we see, a local variation spontaneously arising is accompanied
by multitudinous other local variations, implying a necessary
correlation between each local variation and the general constitu-
tion of the organism ; then it may be argued that if a marked
change of function in an organ causes increase or decrease of it,
this general correlation implies that there must be a reciprocal re-
action between the part and the whole, tending to re-establish
their congruity. The constitution at large will in so far be
changed, and along with its change will go corresponding changes
in the sperm-cells and germ-cells.

Finally let me add, not another argument, but another fact of
observation, of the kind which opponents demand, but which,
when they are from time to time furnished, are severally pooh-
poohed as not enough. Each of them is spoken of as a solitary
fact and slighted as inadequate ; and when by-and-by another is
named, this is treated in the same way ; so that the facts which
if brought together would be recognized as sufficient are never
brought together. That to which I refer is set forth in a pamphlet
by M. Leo Errera, Professor at the University of Brussels,
entitled " Heredite d'un Caractere acquis chez un Champignon
pluricellulaire ; " being an account of experiments of Dr. Hunger,
at the Botanical Institute in Brussels. First enumerating various
instances of adaptations to climate, as those of plants which, fitted
to northern regions, preserve their constitutional rapidity of
growth and seeding when brought south, and do this for several
generations, he goes on to detail the culture-experiments of
M. Hunger, and sums up the results of these in the following
words : —

" On deduit de la que :

"1° Les conidies d' Aspergillus niger sont adaptees a la concentration du
milieu ou a vecu l'individu qui les porte ; cet effet est encore plus marque
apres deux generations passees dans un milieu donne (Exper. I et II) ;

" 2° II s'agit d'une veritable adaptation et non pas simplement d'un
accroissement de vigueur chez les conidies provenant des liquides concentres,
car ces memes conidies germent moins rapidement et donncnt des plantes
moins vigoureuses que les conidies normales lorsqu'on les seme de nouveau
sur le milieu-type: en s'adaptant aux liquides concentres, elles se sont
desadaptees du liquide normal (Exper. Ill) ;

" 3° Une generation passee sur le liquide normal n'cfface pas Tinfluence
d'une ou de deux generations anterieures passees sur une liquide plus con-
centre (Exper. IV).

"Tous ces resultats concordent: Us monlrent une legdre, mats incontest-
able transmission hereditaire de V adaptation au milieu,"

SUBJECT-INDEX.

(For this Index as it appeared in previous editions the Author is indebted to F.
Howard Collins, Esq., of Edgbaston, Birmingham. It has now been adjusted
to suit the present revised and enlarged edition.)

Acacia, foliar organs, II, 41, 264.

Acalephw: environment, I, 105;
water in, I, 173..

Acari: special creation and effects
of, I, 428; direct transformations,
I, 706; segmentation, II, 111.

Acorus calamus, agamic propaga-
tion, I, 642.

Acquired characters, inheritance
of: functionally-produced modi-
fications in plants and animals,
I, 307-13, 318, 526, 541, 562, 692-
5, II, 618-22; conceivability of,
on the hypothesis of physiologi-
cal units, I, 368-71, 695, II, 618-
22; diminution of jaw, I, 541-2,
693; current views on, I, 559-60;
cessation of selection, I, 560-3;
Eimer's theory of orthogenesis,
I, 560; species differentiation, I,
573; location of mammalian
testes, I, 573; tactual perceptive-
ness, I, 602-8, 633, 665, 666, 672-3,
692; blindness of cave-animals, I,
612-3, 647-9; coadaptation of co-
operative parts, I, 621, 663-5;
transmission of disease, I, 622-3;
hypothesis supported by telegony,
I, 624-8, 644-6, 649-50; views of
Darwin and neo-Darwinists, I,
t>30, 685, 690; why facts in sup-
port are meagre, I, 632; degrada-
tion of little toe, I, 652-3, 673;
neuter forms of social insects, I,
658-9, 663-4, 670, 675; degenerated
instinct in ants, I, 660-2; rudi-
mentary limbs of whale, I, 669,
692; importance of question, I,
672, 690; monstrous development
of honey-ants, I, 683-4; osteology
of Punjabis, I, 689; summary of
evidences in support, I, 692-5;

86

genesis of vertebrate skull, II,
227; false joints, II, 371, 372; con.
ceivability of rival hypotheses, II>
618-22; adaptation to environment
in Aspergillus, II, 623.

Acrogens, the term, II, 55-6. (See
Archegoniatcw.)

Actinophrys: a primary aggregate,
II, 76, genesis, II, 452.

Actinozoa: multiaxial development,
I, 166; waste and repair, I, 213,
219; differentiation, I, 391; para-
sitism, I, 397; integration, II, 92;
symmetry, II, 189, 192; growth
and genesis, II, 444.

Activity: the principle of, the es-
sential element in Life, I, 113,
114, 122; not inherent in living
matter, I, 120; nutrition and gene-
sis, resume, II, 497-9; and evolu-
tion, II, 501-4.

Adaptation: general truths, I, 227-
33, 233-5; botanical, I, 227; physio-
logical, I, 228-33; psychological,
I, 229, 230-3; structural, func-
tional, and interdependence, I,
235-9, 240-1, 318; social and or-
ganic stability, I, 240-2; resume, I,
242-3; to varied media, I, 479-81,
489, 556: multiplication of effects,
I, 512-3, 550; direct equilibration,
I, 522-3; natural selection and
equilibration, I, 530-5; non-adap-
tive specific characters, I, 565;
time required for effecting, I,
565-6; an obstacle to re-adapta-
tion, II, 11; of skin and skele-
ton, II, 215, 217; outer tissue, II,
312-4, 387; skin and mucous mem-
brane differentiation, II, 321-2;
389; vascular system, II, 343-4;
osseous, II, 352; muscular, II,

625

626

SUBJECT-INDEX.

368-9, 391; persistence of force
and physiological, II, 394; of re-
productive activity to conditions,
II, 411-6; vertebrae development,
II, 563-6. (See also Co-adapta-
tion.)

Africa, effect of climate on inhabi-
tants, I, 30.

Agamogenesis : alternation with
gamogenesis, I, 266-7, 272-3, 284-
94, 336, 592, II, 415; parallelism in
karyokinesis, I, 267-8; a process
of disintegration, I, 276-7; condi-
tions determining its continuance,

I, 284-94, 295-7, 330; physiological
units, I, 351, II, 613; spontane-
ous fission, I, 582, 584-7, 589-92,
595-6, 599; remarkable extent of,
under favourable conditions, I,
591-2, 640-1; in Actinozoa, II, 92;
in Hydrozoa, II, 102; in Annelida,

II, 103; innutrition, II, 179-80.
Agaricinw, II, 139, 257.

Agassiz, L. J. R., zoological classi-
fication, I, 380.

Aggregates, Animal and Plant (see
Morphology).

Agility, a vital attribute, I, 578.

Agrimony, floral symmetry, II, 42,
167, 170.

Air, in vegetal tissues, II, 567-8,
583, 591, 593.

" Air plants," I, 208.

Albumen: properties, I, 12; Lieber-
kuhn's formula, I, 13; diffusibil-
ity, I, 19; in organic tissues, I, 41.

Alcohols, properties, I, 10-12.

Algw: reproduction, and the dy-
namic element in life, I, 118-9;
multicentral development, I, 163,
164; axial development, I, 165;
locomotive powers of minute
forms, I, 196; uniform tissue and
function, I, 200, 586; gamogenesis,
I, 271, 279, 280, 283, II, 448, 449,
450; fertility, I, 582, II, 440, 441;
fission, I, 584, 585; unicellular
forms, II, 22; integration in Con-
fervoidcce and Conjugates, II, 25;
pseudo-foliar and axial develop-
ment, II, 28-33, 57; foliar devel-
opment, II, 76, 91; branch sym-
metry, II, 145; cell metamor-
phoses, II, 176; tissue differentia-

tion, II, 244, 246, 251, 252, 256,
272, 385-6; adaptation of repro-
ductive activity to conditions, II,
289; integration, II, 292; indefi-
niteness, II, 295; genesis and de-
velopment, II, 403.
Alimentary canal: metabolic pro-
cesses and agents, I, 68-9, 74;
structural traits, I, 192; progres-
sive development, I, 195; relation
to environment, I, 196; function,

I, 205; segmentation in annelids,

II, 125; differentiation, II, 301,
. 302, 321-2, 323-5, 389; specializa-
tions in birds, II, 325; in rumi-
nants, II, 327-9; differentiation of
liver, II, 329-33; muscularity, II,
364.

Allotropism: of organic constitu-
ents, I, 4, 9; muscular action, 1, 59.

Alloys, melting point of, I, 339.

Alternation of generations, mislead-
ing application of term, II, 84.
(See Agamogenesis and Gamogen-
esis.)

Amitosis, occurrence of, in morbid
tissues,. I, 264.

Ammonia: properties, I, 7, 9; nerve
stimulation, I, 55.

Amoeba: central development, I, 163;
a primary aggregate, II, 86; sym-
metry of encysted, II, 186; sym-
biosis, II, 400.

Amphibia: classification of, I, 392;
embryonic respiratory system, I,
457; structure and media, I, 483;
limb locomotion, II, 15; segmenta-
tion, II, 122, 225; outer tissues,
II, 311; respiration, II, 334, 338;
Owen on skeleton, II, 552, 557,
558.

Amphioxus: separation of segmenta-
tion spheres of egg, I, 691; em-
bryogeny, II, 121; local segmenta-
tion, II, 125-7, 605; genesis of
vertebrate axis, II, 213-6, 218,
222; development, II, 564.

Amphipnous cuchia, vascular air-
sacs, II, 337.

Anabas scandens, the climbing fish,
I, 480, 483.

Anacharis (see Eloidea).

Anaesthetics, diverse effects of, I,
55.

SUBJECT-INDEX.

627

AngrcEcum, assimilative function of
root, II, 255.

" Animal Spirits," vitalism and, I,
115.

Animals: nutrition and molecular
rearrangement, I, 3G-7; nitrogen-
ous character, I, 39-41; sensible
motion, I, 57; metabolism, I, (>2-
77; multiplication of energies, I,
75; contrasted traits of plants and,
I, 196; what is an individual? I,
240-7; solar influence, I, 500, 550;
geologic changes affecting, I, 501-

4, 549, 550, 556; interdependence
with plants, I, 504-6, 514, II, 398-
401; complexity of influences af-
fecting, I, 506; geographical iso-
lation and origin of species, I,
568-9; vital attributes, I, 577-9;
distribution and antiquity of
plant and animal types, II, 297;
mutual dependence of organisms
at large, II, 397-408; hypothetical
plant-animal type, II, 397; pro-
gressive increase of size, II, 401;
laws of multiplication, II, 411-6;
rhythm in numbers, II, 419; law
of weights and dimensions, II,
434.

Animals, domesticated: variation,

I, 324, 326, 560, 563, 693; inter-
breeding, I, 345-7, 354, II, 615;
pure and mixed breeds, I, 354,
625.

Annelida: phosphorescence, I, 50;
axial development, I, 165, 166; in-
tegration, I, 363; larval forms and
phylogeny, I, 447, II, 115; seg-
mental fission, I, 588-9; seg-
mentation, II, 98-101, 103-4, 602-

5, II, 107-9, 125-7; lateral gem-
mation, II, 105; embryogeny, II,
119; bilateral symmetry, II, 197-
200; genesis, II, 444, 453.

Annulosa: regeneration, I, 361-2;
distinctive traits, I, 392; origin of
type, II, 98-110, 602-6; unit of
composition, II, 105; application
of term, II, 111; vertebrate sym-
metry compared, II, 203-6; seg-
mental differentiation, II, 207-9;
unintegrated function in Planaria,

II, 373; development and genesis,
II, 464; nutrition and genesis, II,

490. (See also Annelida and
Arthropoda.)

Anthropomorphism, former preva-
lence of, I, 419.

Ants: utilization of aphids, I, 660-

I, II, 403, 405; nest-mates, II, 405;
castes in social species, I, 658-9,
670, 675; loss of self-feeding in-
stinct in Amazons, I, 660-1, 663-
4; monstrous development of Hon-
ey-ants, I, 683; bulk and fecun-
dity, II, 492. (See also Termites.)

Aphis: iudividuality, I, 249, 250; II,
603; parthenogenesis, I, 274-5,
289; fertility, I, 582, 640-1; II,
476, 490; utilized by ants, I, 660-
1; II, 403, 405; over-multiplica-
tion checked by lady-bird, II, 406.

Aquatic animals, large size attained
by, I, 156.

Arachnida: avoidance of danger, I,
92; oviparous homogenesis, I, 271;
segmentation, I, 469; II, 113, 314;
integration and homology, II, 111,
121; bilateral symmetry, II, 198.

Arcella: symmetry, II, 186; outer
tissue differentiation, II, 309.

Archcgoniatea? : morphological com-
position, II, 32-5; growth and de-
velopment, II, 50-6; tubular
structure, II, 58, 62; alternating
generation not distinctive, II, 84;
asymmetry and environment, II,
140; integration, II, 293, 296; in-
dividuation and genesis, II, 441,
451, 463.

Archenteron: primitive externality,

II, 301; formation of coelom, II,
302.

Archiannelida: segmentation, II,
125.

Arenicola marina: polytrochal lar-
vae, II, 109.

Arm: embryogeny of human, I, 169;
vicarious use of, I, 209.

Army, morphological analogy, II, 6.

Arteries (see Vascular System).

Arthropoda: uniaxial development,
I, 165; protoplasmic continuity, I,
190, 629; excursiveness, I, 481;
limb locomotion, II, 15; integra-
tion and homology, II, 111-4, 121;
bilateral symmetry, II, 197-200;
genesis, II, 445, 453.

628

SUBJECT-INDEX.

Ascidians: multlaxlal development,
I, 165, 166; functional differentia-
tion, I, 202; composite individual-
ity of Doliolum, I, 247; self-fer-
tilization, I, 342; integration, II,
94, 96, 97; symmetry, II, 194;
origin of vertebrate type, II, 194,
598, 605.

Ascomycetes, reproduction, II, 450.

Assimilation: compared with rea-
soning, I, 81-7; a trait of vitality,

I, 577.

Asteroidea, radial symmetry, II,
196.

Astronomy: growth of celestial
bodies, I, 135; Schleiden on indi-
viduality, I, 245; evolution, I,
432, 435; classification of stars, I,
444; rhythm of, and organic
change, I, 499-501, 557; law of
equilibration, I, 519-20; coopera-
tion of structure and function,

II, 3.

Atavism: occurrence of, I, 305-6
314; digital variation, I, 321-3.

Atoms: use of term, I, 6, 31; ethe-
real undulations and oscillations,

I, 31-5.

Australia: settler's usages, I, 364:
ratio of jaw to skull in natives, I,
541.

Axillary buds, origin and develop-
ment, II, 65-8.

Axis: " neutral " of mechanics, II,
210; genesis of vertebrate, II,
212-6, 224-7.

Bacteria: fission, I, 270; non-nu-
cleated, II, 20; rate of increase,

II, 443.

Baer, K. E. von: embryological for-
mula, I, 171, 172, 451, 453, 461,
466; zoological classification, I,
383; on animal transitions, I,
480.

Balanophorw, inner tissue, II, 274.

Bark: varied development, II, 247-
9; physiological differentiation,
II, 249-50, 258, 386.

Basidiomycetes, reproduction, II,
450.

Bat, infertility of, II, 473.

Bates, H. W., protective mimicry
of butterflies, I, 398.

Batrachia (see Amphibia).

Bean, vascular system, II, 573,
591.

Beaver, tail and co-adapted struc-
tures, I, 616.

Bees (see Insects).

Bcgoniacett: multiplication I, 224,
317, 442; individuality, I, 251; de-
velopment from scales, I, 282;
symmetry, II, 159, 166; develop-
ment, II, 271.

Berkeley, M. J., indefiniteness of
mosses and ferns, II, 296.

Bile, arrest of excretion, I, 209.

Bilirubin and biliverdine, function
of, II, 330, 333.

Biology: definition and divisions, I,
124-5; organic structural pheno-
mena, I, 125-7; also functional, I,
127-9; actions and reactions of
function and structure, I, 129-30;
genesis, I, 130-1; limited knowl-
edge of, I, 131; evolution, I, 432,
434; sociological analogies (see
Sociology).

Biophors, Weismann's germ-plasm
units (see Weismann).

Birds: flesh-eating and grain-eat-
ing contrasted, I, 68; growth and
expenditure of force, I, 142; size
of egg and adult, I, 144; limita-
tions on flight, I, 155; self-mobil-
ity, I, 175; temperature, I, 176;
functional and structural differ-
entiation, I, 201; food of starv-
ing pigeon, I, 215; viviparous-
ness, I, 271; heredity and pigeon
breeding, I, 305; atavism in
pigeon, I, 314; osseous varia-
tion in pigeon, I, 321; classifica-
tion, I, 392; migrations and
change of habits, I, 399, 402, 500;
distribution in time, I, 410; Dar-
win on petrels, I, 455; rudi-
mentary teeth, I, 457; vertebrae,
I, 471; II, 564; feather develop-
ment, I, 473; habits of water
ouzel, I, 485; egg shells and di-
rect equilibration, I, 526; bones of
waders and direct equilibration,
I, 527; fertility and nervous de-
velopment, I, 598; cellular con-
tinuity, I, 629; adaptation of
structure to environment, II, 12;

SUBJECT-INDEX.

629

sexual selection, II, 269; wing
spurs, II, 313; outer tissue differ-
entiation, II, 314-5, 387; ali-
mentary canal development, II,
828, 327; muscular colour and
activity, II, 305-9; nutrition, II,
433; cost of genesis, II, 436;
growth and genesis, II, 454, 458;
heat expenditure and genesis, II,
468-9, 474; activity and genesis,
II, 470-2, 474; contrasted mam-
malian fertility, II, 470; eggs of
wild and tame, II, 478; fertility
of blackbird and linnet compared,
II, 503; Owen on skeleton of, II,
559, 560, 561.

Bischoff, embryogeny of human
arm, I, 169.

Bison, modifications entailed by In-
creased weight of head, I, 512.

Blackbird, contrasted with linnet
in development, II, 503.

Blainville, de, definition of life, I,
79, 93.

Blastosphere, independence of cells
in Echinoderm larvae, I, 185.

Blastula, definition of life and for-
mation of, I, 112.

Blood: similarity of iron peroxide,
I, 17; metabolic processes, I, 69;
segregation of abnormal constitu-
ents, I, 180; protozoon life of cor-
puscles, I, 186-7; morbid changes,

I, 221, 701; assimilative power
and organic repair, I, 221-2; res-
piratory tissue differentiation, II,
310-1; pressure in mammals, II,
340. (See also Vascular System.)

Blow-fly, Weismann on nutrition

and genesis in, I, 678-9.
Boers, Cape, habits and fertility,

II, 508.

Boismont, A. B. de, on human fer-
tility, II, 511.

Bone: growth and function, I, 151;
adaptability, I, 230; II, 217-8;
function and weight, I, 308, 693;
mammalian cervical vertebrae, I,
394; evolution and vertebral col-
umn, I, 470-1; partial develop-
ment, I, 473; size of head as in-
fluencing, I, 512, 536-9; direct
equilibration and strength, I,
527; natural selection and co-

adaptations, I, 614-21, 674, 677;
rudimentary limbs of whale, I,
668, 685, 692; inheritance of ac-
quired modifications In Punjabis,

I, 689; skull development, II, 222;
theory of supernumerary, II, 223;
Cope on origin of vertebrate osse-
ous system, II, 225-7; differentia-
tion, II, 344-56; false joints, II,
370-2; Owen's theory of verte-
brate skeleton, II, 548-66.

Book-worm, food of, I, 77.

Born, G., experiments on frog-lar-
vee, I, 365.

Botany, biological classification, I,
124, 125. (See Plants.)

Bothriocephalus, development, II,
490.

Botryllidw: development, I, 166; in-
dependence of components, I, 247;
agamogenesis, I, 641.

Bower, Prof., on alternation of gen-
erations, II, 84.

Brachiopoda, rude vascular system,

II, 340.

Bradbury, J. B., on vaso-dilators, I,
55.

Brain: natural selection and mental
evolution, I, 553; analysis of sub-
stance, I, 596; weight in higher
animals, I, 598-9; size in civilized
and uncivilized, II, 530.

Branches (see Morphology).

Branchiw (see Respiratory System).

Brass, effect of antimony on, I, 121.

Bread, diamagnetism, I, 370.

Breeding: heredity, I, 304-5; in-
and-in, I, 344-7, 353; II, 615;
pure and mixed, I, 354, 625.

Bricks, changed equilibrium shown
by, I, 38, 42.

Brodie, T. G., cell chemistry, I,
260.

Brownell, Miss J. L., on birth-rate
in United States, II, 520.

Brown-Sequard, on inherited epi-
lepsy, I, 312, 624.

Bryophyllum, peculiar proliferation,
II, 295.

Bryophyta, large size attained by
some, I, 138.

Bryozoa, gemmation, I, 588.

Budding (see Gemmation).

Buds: development, I, 167-8; the-

630

SUBJECT-INDEX.

ories of heredity and cauline, I,
358-9, 360; axillary, II, 65-9; ef-
fects of nutrition, II, 73-4.
Butterfly: protective mimicry, I,
398; instance of tame, I, 684.

Cabbage, varieties of, I, 302.

Cactacew: foliar and axial develop-
ment, II, 47-9; differentiation in,
II, 258, 276, 282; vascular system,
II, 282; dye permeability and cir-
culation, II, 571, 572; wood for-
mation, II, 575, 577, 578, 580.

" Callus," budding from, I, 358,
359.

Camel, natural selection and hump
of, I, 534.

Canadians, French, fertility of, II,
509.

Cancer, the definition of life, I, 111;
oesophageal, II, 324; and vascular
system, II, 343.

Caoutchouc, leaf structure, II, 589.

Capillaries (see Vascular System).

Capillarity, and vegetal vascular
system, II, 279-80, 286, 568, 570,
585, 587, 592-6.

Carbohydrates: instability, I, 10;
the term " hydro-carbon," ib.;
molecular changes in, I, 42-3; or-
ganic transformation, I, 43, 48;
metabolic processes, I, 63-77,
262-3, II, 362.

Carbon: properties, I, 3-5, 20; com-
pounds, I, 6, 7, 9, 10-12, 13, 24-5.

Carbonic acid (carbon dioxide):
properties, I, 6, 7, 9; in animal
and plant functions, I, 62, 214,
II, 398; diffusibility, II, 331.

Carbonic oxide, properties, I, 6.

Carnivores: nitrogenous food, I, 47,
68; katabolic process, I, 71; re-
stricted environment, I, 396;
their beneficial effects on animal
life, II, 405-6.

Carpenter, W. B. : on functional
specialization, I, 208; reproduc-
tion of sea-weed, I, 582; vegetal
cell multiplication, I, 585; struc-
ture and multiplication of com-
pound organisms, I, 586-9; on
fundamental traits of sex, I, 595;
nutritive system of invertebrates,
I, 595; Macrocystis, II, 450; nutri-

tion and reproductive function,
II, 460.

Cartilage (see Bone).

Castration, effect of, on growth, II,
459.

" Castration parasitaire," Julin on,
II, 493-6.

Catalysis, and vital metamorphosis,
I, 39, 43.

Cattell, McKeen, on tactual percep-
tiveness, I, 666.

Caulcrpa, simulation of higher
plant-forms, II, 22.

Cave-animals, degeneration of eyes,
I, 309, 612-3, 614, 647-9, 693.

Cell, the: incomprehensibility of
forces at work in, I, 118; proto-
plasts and their traits, I, 181;
the cell-theory, I, 184, 252, II, 17-
21, 85; differentiation, I, 188-9,
194; the continuity of protoplasm,

I, 190-2, 194, 628-30, II, 21; its
structure, I, 253-5; function of
centrosome, I, 254-5, 257; struc-
ture and function of nucleus, I,
255-6, 258-9; karyokinesis, I, 257-
8; function of chromatin, I, 259-
65; fertilization and function of
polar bodies, I, 266-8; theories of
heredity based on theory, I, 356;
Weismann's differentiation into
reproductive and somatic, I, 622,
628-30, 633-44; nucleus absent or
dispersed, II, 20, 85; morphologi-
cal differentiation, II, 175-7; ani-
mal morphology, II, 228-30; mor-
phological summary, II, 233;
vegetal tissue differentiation, II,
249-50, 386; vascular develop-
ment, II, 279-84, 389.

Centipede, bilateral symmetry, II,

198-200.
Cephalopoda: bilateral symmetry,

II, 203; vascular system, II, 341.
Cercariw (see Distoma).

Cereus, tissue differentiation, II,

276, 283.
Cesalpino, I, 377.
Cestoda (see Entozoa).
Chcetopoda, segmentation, II, 98,

103, 605.
C'haja, wing spurs, II, 313.
Change, and definition of life, I,

81-90, 113.

SUBJECT-INDEX.

631

Charles, R. H., on inheritance of
acquired modifications in leg-
hones of Tunjahis, I, 689.

Chatie, on single and double stocks,
II, 622.

Chemistry: properties of organic
elements, I, 3-5, 20, 22; of dia-
tomic compounds, I, 7-10; tria-
tomic, I, 10-12; polyatomic, I,
12-13, 25; traits of evolution, I,
23-4; ethereal undulations and
atomic oscillation, I, 31-6; chemi-
cal affinity and organic change, I,
3G-7, 38-43; oxidation and genera-
tion of heat, I, 46-9, 60; genera-
tion of nerve force, I, 52, 60;
metabolism, I, 62-77; physiology
and organic, I, 127; flesh constitu-
ents, I, 154; composition of or-
ganisms and environment, I, 173;
organic development and differen-
tial assimilation, I, 179-80; chemi-
cal units, I, 225, II, 612; primi-
tive ideas of elements, I, 41 i;
evolution of organic compounds,

I, 696-701, 703.

Chestnut, leaf symmetry, II, 149,

153.
Chiton: simulation of segmentation,

II, 116, 118; symmetry, II, 202.
Chlorophyll: function, I, 65, II,

263; nutrition and absence of, II,
74; constitution, II, 262; symbi-
otic presence in animals, II, 400.

Chondracanthus gibbosus, enormous
development of reproductive sys-
tem, II, 487.

Chordata, affinities, I, 466.

Chromatin (see Cell).

Circle, the, and evolution hypoth-
esis, I, 433.

Circulation (see Vascular System).

Cirripedia: Darwin on retrograde
development, I, 458; remarkable
transformation in Sacculina, II,
494-5.

Civilization, human evolution and
genesis, II, 529-31.

Cladophora: integration, II, 25;
axial development, II, 28.

Classification: subjective concep-
tion, I, 78; two purposes of, I,
374; a gradual process, I, 375;
botanical, I, 377-80, 389-90; zo-

ological, I, 380-9; incomplete
equivalence of groups, I, 389,
445-6, 448, 555, 572; group at-
tributes, I, 390-3; the truths in-
terpreted, I, 393-4; ethnologic
and linguistic evolution, I, 441-6;
organic evolution, I, 443, 447,
555; differences in kind and de-
gree, I, 444-6; antecedent struc-
tural similarity, I, 447, 448-9;
Von Baer's formula, I, 451-4,
555; organic, not uniserial, II,
115.

Classification of the Sciences, The,
and evolution and dissolution, II,
5.

Claus, C, on segmentation in An-
nelids and Chsetopods, II, 605.

Clover: flower and axial develop-
ment, II, 45; symmetry, II, 152.

Co-adaptation of cooperative parts:
principles underlying, I, 234-5,
511-3, 514-5; slow operation of
the process, I, 236; sociological
analogy, I, 237^0; reversion un-
der original conditions, I, 240; the
analogy continued, ib. ; the case
of bison's head, I, 512; natural se-
lection an inadequate explana-
tion, I, 535, 614-21, 692; Romanes
on " cessation of selection " as
effecting, I, 560, 561-2; Weis-
mann's theories, I, 560-3, 663-5,
670, 674-5; natural selection and
economy of growth, I, 562; phy-
siological processes involved, I,
566-7; Wallace's argument from
artificial selection, I, 615; what
are cooperative parts? I, 616-7;
" intra-selection " examined, I,
676-8.

Coal, social effects of supply, I,
238-9, 241.

Cocoa-nut, growth and fertility, II,
457.

Coccospheres: vital problem pre-
sented by protective structures, I,
119; imbricated plates, I, 182.

Cockroach, ousting of European
species, I, 399.

Cod: ova of, II, 435; growth and
fertility, II, 454.

Codium: symmetry, II, 136; tissue
differentiation, II, 246.

632

SUBJECT-INDEX.

Coelenterata: rudimentary contrac-
tile organs, I, 58; vital changes
in polyp, I, 95; axial development,
I, 165, 16G; environment and
structure, I, 173; self-mobility, I,
175; II, 14, 15; functional differ-
entiation, I, 201, 391; inactivity
and waste, I, 213; reparative
power, I, 219, 224; individuality,

I, 246, 247, 250; heterogenesis, I,
273, 277, 296; negative disintegra-
tion in Hydrozoa, I, 276, 587; re-
productive tissue, I, 281; differen-
tiation in Hydrozoa, I, 391; classi-
ficatory value, I, 446; regenera-
tion of fragments, II, 90; integra-
tion, II, 90, 102, 105, 124; gemma-
tion, II, 91; tertiary aggregation,

II, 92, 95, 124; molluscan affin-
ities, II, 115; radial symmetry, II,
188; symmetry of compound, II,
192-3; segmental differentiation,
II, 207; physiological differentia-
tion in Hydra and analogy, II,
300; ciliation of blastula, II, 301;
tissue reduplication, II, 301-2,
389; outer tissue differentiation,
II, 309; osmosis in Hydra, II, 339;
vascular system in Hydra, II,
340, 376; functional co-ordination,
II, 376; symbiosis, II, 400; asex-
ual genesis, II, 443-4; growth and
sexual genesis, II, 452; develop-
ment and genesis, II, 462; nutri-
tion and genesis, II, 476.

Coelom, origin and function, II,
302-3.

Collins, F. Howard, jaws and teeth
of savages and civilized, I, 541.

Colloids: T. Graham on, I, 15-8;
diffusibility, I, 18-21; organic, I,
21, 25, 26; pliability and elas-
ticity, I, 27; capillary affinity, I,
28; isomerism, I, 59; instability,
I, 350; molecular mobility and dif-
fusibility, II, 331; instability of,
and nerve differentiation, II, 356-
61; and muscular tissue, II, 361^1.

Colonies, autogenous development
and parallel in heredity, I, 366-
8; II, 620.

Colour: sensation of, I, 54; phceno-
gamic, II, 75, 265-6; light and
vegetal, II, 261-2; floral fertiliza-

ection.

tion, II, 267-9; sexual selection,
II, 269; activity and muscular, II,
365-9; physiological units and
mixture of, in offspring, II, 616,
617.

Commensalism, organic integration
as displayed in, II, 402-4.

Composite: floral symmetry, II, 173.

Condor, weight of, I, 155.

Confervoidece, I, 279, 280; II, 25, 28,
449. (See Algw.)

Conjugatcw, II, 449. (See Algw.)

Conjugation, in Algw, I, 279; in Pro-
tozoa, I, 280; II, 452; can fission
persist without? I, 637; relation
to growth, II, 449.

Connective tissue, Hertwig's classi-
fication, I, 189.

Constitutional units, I, 369. (See
Physiological Units.)

Consumption, hereditary transmis-
sion, I, 307.

Co-ordination of actions (see Life).

Cope, E. D., on origin of vertebrate
structure, II, 225-7.

Cormophyta: slight internal differ-
entiation, II, 273; vascular sys-
tem, II, 280.

Corpuscula tactus, their function, I,
75.

Correspondence, use of word, I, 97.
(See Life.)

Cousin-marriages, I, 345, II, 615.

Cow: what prompts her to mumble
a bone? I, 120.

Cow-parsnip (see Heracleum).

Crab (see Crustacea).

Creation (see Special creation).

Crinoidea, symmetry, II, 195-6.

Crocodile, continuous growth, I,
154, 292.

Crookes, Sir W., hypothetical
chemical unit " protyle," I, 22, 23.

Cruciferw, floral symmetry, II, 164,
171.

Crustacea: locomotion of lobster, I,
175; regeneration of limbs, I, 224,
360, 589, II, 76; homogenesis, I,
271; genesis and nutrition in
Daphnidw, I, 290-1; growth and
genesis, I, 292; degeneration of
eye in cave-inhabiting, I, 309, 614,
648; hermit crab parasite, I, 397;
changes of media, I, 401, 481-2;

SUBJECT-INDEX.

633

retrograde development In cirri-
pedes, I, 458; segmentation, I,
408-9, II, 114; Darwin on jaws
and legs, I, 471; survival of cirri-
pedes, I, 517; integration and
homology, II, 111-4, 121, 603; bi-
lateral symmetry, II, 198-201;
eyes, II, 318; dermal structure of
hermit erab, II, 322, 387; fer-
tility, II, 453; nutrition and gene-
sis in parasitic species, II, 487;
" castration parasitaire," II,
4!t:; o.

Crystalloids: Prof. Graham on, I,
15-8; diffusibility, I, 18-21; or-
ganic, I, 21-2, 26.

Crystals: simulation of life in
11 storm glass," I, 96; growth, I,
135-7, 577; segregation, I, 179,
221, 223; equilibration, I, 337;
physiological units and polarity, I,
701-6; time and formation, II, 77.

Ctcnodrilus, segmental individual-
ity, II, 103, 603, 604.

Cube, bilateral symmetry, II, 132.

Cunningham, J. T., I, vi; II, vi;
on non-adaptive specific charac-
ters, I, 565; food of blow-fly lar-
vae, I, 678; arthropod segmenta-
tion, II, 114; egg-production of
Conger, II, 425.

Cuttle-fish, individuality of Hecto-
cotylus, I, 250.

Cuvier, zoological classification, I,
381.

Cyanogen, properties, I, 7, 9.

Cyclichthys, dermal structure, II,
306.

Daltell, Sir J., regeneration in
Dasychone, I, 361; propagation of
Hydra, II, 476.

Daphnidw, heterogenesis and nutri-
tion, I, 290-1.

Darwin, C: Origin of Species, I,
129, II, 528; natural selection and
function, I, 308-9, 693: atavism,
I, 314; osseous variations in pig-
eons, I, 321; plant variation and
domestication, I, 325; " spontane-
ous variation," I, 328, 697; floral
fertilization, I, 340, II, 1(58, 267,
407, 608; intercrossing and self-
fertilization, I, 344, 345; inter-

crossing I, 347, 611, 669; his the-
ory of pangenesis examined, I,
356-62, 370, 372; plant fertiliza-
tion and distribution, I, 397;
habits of birds, I, 400; distribu-
tion and natural barriers, I, 402,
476; disappearance and non-reap-
pearance of species, I, 406; dis-
tribution in time and space, I,
410; linguistic classification, I,
442; classification of organisms, I,
443; classification and descent, I,
448; on petrels, I, 455; suppres-
sion of organs, I, 457; develop-
ment of Cirrhipedia, I, 458; jaws
and legs of Crustacea, I, 471;
aborted organs, I, 474, 563; rela-
tions of species in Galapagos
archipelago, I, 478; opinions of E.
Darwin and Lamarck, I, 491; the
term " survival of the fittest," I,
530; indirect equilibration by
natural selection, I, 530-5; in-
heritance of acquired characters,
I, 535-42, 560, 630, 685, 690; Wal-
lace on natural selection in man,

I, 553; misleading connotations of
term " natural selection," I, 609,
695; caste gradations and jaws of
driver ants, I, 658; attachment of
climbing plants, II, 276-7; vegetal
fructification, II, 294; earth-
worm, II, 402; animal sterility
and domestication, II, 480, 483;
variation in hyacinth and camel-
lia, II, 621.

Darwin, Dr. E., modifiability of or-
ganisms, I, 490, 492-7.

Death: an arrest of vital corre-
spondence, I, 102; only limit to
vegetal growth, I, 153; cessation
of coordination of actions, I, 578,
579; Weismann's hypothesis, I,
636-8; physiological integration,

II, 374, 392; cause of natural, II,
413; relation to births, II, 417.

Definiteness: of vital change, I, 87-
90, 106, 109; developmental, I,
178; functional, I, 212; segrega-
tion of evolution, I, 514-6.

Definition, difficulties of, I, 78; II,
17.

Degeneracy, morphological obscura-
tions due to, II, 12, 13.

634

SUBJECT-INDEX.

I

Dendrobium (see Orchids).

Desmidiaccw: unicellular, II, 21;
linear and central aggregation, II,
23; natural selection and sym-
metry, II, 134, 135; morphological
differentiation, II, 177; tissue, II,
244; genesis, II, 440, 449.

Determinants, Weismann's germ-
plasm units (see Germ-plasm).

Development: an increase of struc-
ture, I, 162, II, 461; primarily
central, I, 162, 166; uni- and mul-
ti-central, I, 163-4, 166-7; axial,
I, 164, 167; uni- and multi-axial,

I, 165-6; a change to coherent
definite heterogeneity, I, 167-70,
179; Von Baer's formula, I, 171-2;
individual differentiation from
environment, I, 172-8; cell forma-
tion, I, 225; discontinuous, and
agamogenesis, I, 275; Prof. Hux-
ley's classification, I, 276; socio-
logical parallel to autogenous, I,
364-8, II, 620; retrograde, I, 457-
8: inequalities among co-operative
parts, I, 617; " heterochrony," I,
655; continuous and discontinuous
vegetal, II, 52; summary of
physiological, II, 384-90; nutri-
tion and genesis, resume, II, 497-
9; evolution, II, 501-5; commence-
ment of genesis, II, 506; of ver-
tebrate limbs, II, 553. (See also
Multiplication.)

Development Hypothesis, The, I, 417.
Dialects (see Language).
Dialysis, and diffusibility, I, 19, 20.
Diastase, decomposition of, I, 38,

40.
Diatomacece: tissue, II, 244; genesis,

II, 440, 448.

Diatomic compounds (see Chem-
istry).

Dicotyledons: growth, I, 139, 143,
II, 63-4, 69-72, 78, 82-3; uniaxial
development, I, 165; stem and
leaf functions, II, 257; mechani-
cal stress and wood formation, II,
277; growth and genesis, II, 451.

Differentiation (see Morphology and
Physiology).

Difflugia: primary aggregate, II, 86-
7; symmetry, II, 186; outer tissue
differentiation, II, 309.

Diffusion, of colloids and crystal-
loids, I, 18-20; II, 331.

Digestion: action of nitrogenous
compounds, I, 69; obesity, II,
480-4; fertility, II, 514.

Dimorphism: floral, I, 534; sexual,
in parasites, I, 315; social insects
(see Insects).

Dinosaurs, size of, I, 139.

Diphyes: individuality, I, 246; sym-
metry, II, 192.

Disease: segregation of blood con-
stituents, I, 179; changes in
blood from, I, 221, 701; heredity,

I, 306-7, 312-3, 622-3; belief in
supernatural origin, I, 419; para-
sitism and special creation, I,
427; morbid products as specific
characters, I, 567; telegony, I,
646; dermal structure, II, 306; in-
testinal muscular hypertrophy, II,
325; indigestion and alimentary
canal development, II, 328; jaun-
dice and bilirubin, II, 330; locali-
zation of excretion, II, 331; mem-
branes in inflammatory, II, 343;
osseous differentiation in rickets,

II, 352; fatty degeneration, II,
482.

Disintegration, physiological (see
Physiology).

Distoma: metagenesis, I, 273-4; dis-
integration of genesis, I, 276;
cycle of generations, II, 489.

Distribution: physical limits, I, 396;
organic environment, I, 396-8;
parasitic conditions, I, 397-8; sim-
ultaneity of agencies affecting,
I, 398; mutual encroachments of
species, I, 398-401, 477, 489; facts
disproving pre-adaptation to habi-
tats, I, 401-3, 411-2; of animals
and plants in time, I, 404-11, 412;
ousting of native species in New
Zealand, I, 477; local influences,
I, 477-9, 489; through varied
media, I, 479-85, 489, 556; past
and present organic forms, I,
485-9, 556; complex organization
and, II, 296-7.

Division of labour, physiological
(see Labour).

Dog: contrasted lives of tortoise
and, I, 103, 104; inherited habits,

SUBJECT-INDEX.

635

I, 309, 573; abnormal digits, I,
324; interbreeding of divergent
varieties, I, 666; decrease of jaw,

I, G15, 693; telegony, I, 645; con-
ditions affecting fertility, II, 474,
479.

Dohrn, theory of vertebrate struc-
ture, II, 606.

Doliolum, combination of individu-
alities, I, 247.

Domestication (see Animals).

Doubleday, E., on nutrition of gene-
sis, II, 510-2.

Driesch, separation of segmentation
spheres of Echinus ovum, I, 691;

II, 618.

Dropsy (see Disease). .

Droscra: individuality, I, 251; pro-
liferous growth, II, 75.

Du Bois-Reymond, E. H., elec-
tricity from muscles and nerves,

I, 50.

Dumas, antithesis of animals and

plants, I, iVl.
Dwarfs, Hindu family of, I, 310.

Ear, development of vertebrate, II,

318, 320.
Earth, climatic rhythm and organic

change, I, 499-501, 557.
Earth-worm : bilateral symmetry,

II, 199, 200; mould production, II,
402.

Echinococcus (see Entozoa).

Echinodermata: independence of
blastosphere cells, I, 185; proto-
plasmic continuity in embryos, I,
190; separation of segmentation
spheres of ovum, I, 691; II, 618;
symmetry, II, 191, 195-6.

Economy: of growth in natural se-
lection, I, 536, 562; a trait of or-
ganic evolution, II, 501, 504.

Ectoderm: functional differentia-
tion, I, 202, 203; functional vicari-
ousness, I, 209; reproductive func-
tion, I, 281.

Effects, Multiplication of: varia-
tion, I, 329-30, 333; organic evo-
lution, I, 511-4, 515, 517, 549,
557, II, 405-6; morphological de-
velopment, II, 7-9, 234; physio-
logical differentiation, II, 390-1,
392.

Eggs (see Embryology).

Eimer, T., theory of orthogenesis
I, 563-4.

Elasmobranchii: protoplasmic con
tinuity, I, 629; segmentation, II
126.

Electricity: genesis in organic mat
ter, I, 50-2, 60; muscular ac
tion, I, 59; incomprehensibility
I, 121.

Elephant: fertility, I, 583, 599, II
459, 506; cerebro-spinal system, I
598, 599.

Elk, Irish, horns and correlated
parts, I, 537, 674.

Eloidea canadensis: individuality, I,
248; enormous agamic multiplica-
tion, I, 642.

Elongation, and locomotion in ani-
mals, II, 15.

Embryology: as aiding biology, I,
125-6; simulated growth, I, 136;
initial and final organic bulks, I,
143, 158, 161; foetal flesh constitu-
ents, I, 154; human arm develop-
ment, I, 169; Von Baer's for-
mula, I, 170-2, 451^, 466: em-
bryonic heat, I, 177; spherical or-
ganic form, I, 177; unit-life in
multicellular organisms, I, 185-6;
functional differentiation, I, 203;
individuality, I, 246-7: unspecial-
ized reproductive tissue, I, 279-
83, 317; changes following im-
pregnation, I, 283-4: nutrition and
vegetal growth, I, 285-8, 295-7;
and animal growth, I, 289-94,
295-7; physiological units and
heredity, I, 317-9; variation and
parental functional condition, I,
324; uterine environment, I, 327-8;
physiological units and variation,
I, 330-4, 458; fertilized and un-
fertilized ova, I, 340-1; hermaph-
rodism, I, 341-2, 344; sociologi-
cal parallel, I, 366-8: evolution
hypothesis, I, 434, 436, 453, 454,
555; petrel development, I, 455;
substitution and suppression of
organs, I, 456-8, 466, 472-3; struc-
tural proclivities of physiological
units, I, 458; abridgment of
stages, I, 458-9, 464: disappear-
ance of intermediate forms, I,

636

SUBJECT-INDEX.

I

459-60, 463; pre-adaptation, I,
461-3; discrimination of species
in early stages, I, 461; anomalous
persistence of ancestral traits, I,
463-5; phylogeny, I, 466; egg-shell
function, I, 527; genesis of grades
in social insects, I, 654-6, 658-9,
679-80; determination of sex, I,
657; order of development quali-
fied by needs, I, 679; osteology of
Punjabis, I, 689; direct transfor-
mations and physiological units,

I, 706: transformation of blas-
tema, II, 20; arrest of growth and
innutrition, II, 73: development of
segmented animals, II, 100-2, 602;
adaptive vertebrate segmentation,

II, 118-23, 124, 223-4, 605-6; ani-
mal cell morphology, II, 228; pri-
mary differentiations of germinal
layers, II, 300-2; lung develop-
ment, II, 333-4; mammalian ova
maturation, II, 342-3: movements
of ova, II, 356, 363; modifications
in mole, II, 391; genesis and nu-
trition, II, 424, 425; fish ova, II,
435, 454; cost of genesis, II, 435-
6; number of birds' eggs, II, 454-
6, 478; heat and genesis, II, 468,
474: activity and genesis in birds,
II, 470-2, 474; vertebrate limb
development, II, 553; ossification
in vertebrates, II, 556; Owen's
vertebrate theory, II, 563; devel-
opment of vertebrae, II, 564. (See
also Multiplication.)

Embryology of conceptions, I, 451.

Emigrants, type of organization
among, I, 364, II, 620.

Endoderm: functional differentia-
tion, I, 202, 203; functional vicari-
ousness, I, 209.

Endogen, application of term, II,
62, 78, 82. (See Monocotyledons.)

Energy: evolution of, in animals, I,
71-7; organic growth and expen-
diture, I, 141; functional transfer,
I, 201-6; chromatin as the source
of, in karyokinesis, II, 261-5.
(See also Force.)

Entozoa: metagenesis, I, 273, 641;
self-fertilization, I, 342; distribu-
tion, I, 398; and special creation,
I, 428; fission in simple types, I,

584; nutrition and genesis, I, 641;
II, 488; direct transformation, I,
706; integration, II, 102; seg-
mentation, II, 107, 108; interde-
pendence and organic integration,
II, 404.

Environment: degree of life and
complexity of, I, 104-7; relation
to organic structure and func-
tion, I, 172-8; II, 12-5; adaptation
to varied media an evidence of
evolution, I, 479-81, 556; influence
of solar system, I, 500, 556; in-
herited adaptation to, II, 623.

Eolis, branchiae, II, 118.

Epidermis (see Skin).

Epilepsy: definition of life and
movements in, I, 112; heredity, I,
312.

Epithelium: ciliated, I, 187; Hert-
wig's classification, I, 189; repro-
ductive function, I, 280; " pave-
ment " and " cylinder," II, 229.

Epizoa: distribution, I, 398; special
creation and effects of, I, 428: in-
terdependence and organic inte-
gration, II, 404; nutrition and
genesis, II, 487.

Equilibration: variation and law of,
I, 326, 334; molecular arrange-
ment, I, 337-45; of organic
change, I, 348, 547, 557; direct
and indirect, I, 519-22, 573: adap-
tation by direct, I, 522-3, 551,
557; nutrition, defence, and fer-
tilization of plants, I, 523-5; di-
rect of animals, I, 525-8, 551,
557; natural selection and indi-
rect, I, 530-4, 552, 557; of natural
selection, I, 543-7, 552-3, 557;
increasing importance of direct, I,
553; of forces acting on species,
I, 571-2, II, 417-20; phenomena
not accounted for by, I, 573; tis-
sue differentiation, II, 245; gene-
sis of nervous system, II, 307-8;
functional, II, 391-4; laws of mul-
tiplication, II, 411-6; in human
and social evolution, II, 537. (See
also Acquired characters and
Natural selection.)

Errera, L., on inherited adaptation
to environment in Aspergillus, II,
623.

SUBJECT-INDEX.

637

Ethnology: heredity, I, 303-4, 310;
plasticity of mixed races, I, 354;
primitive ideas, I, 417; evolution
and classification, I, 441-3, 446;
natural selection, I, 553.

Euphorbiaccw: foliar and axial de-
velopment, II, 47-8; physiological
differentiation, II, 258; dye per-
meability and circulation, II, 571;
wood formation, II, 575, 577,
578; foliar vascular system, II,
589-92, 506.

Evaporation: organic change, I, 28;
vegetal circulation, II, 587.

Evolution: chemical elements and
compounds, I, 22-4, 67; primordial
form of living matter, I, 63-4,
181; II, 21-2; definitions of life, I,
107-10; growth the primary trait
of, I, 135; comprehends growth
and development, I, 162; illustra-
tions in development, I, 167-70,
178-9; progressive structural dif-
ferentiation, I, 181-4, 192-6, 211-
2; life before organization, I, 210;
heterogeneity of function, I, 211;
stability of species, I, 242, 515,
518; individuality, I, 247; cell-or-
ganization, I, 262; genesis, hered-
ity, and variation resulting from,
I, 354-5; period required for or-
ganic, I, 407, 565-6; contrasted
with special-creation hypothesis,
I, 415, 431-40; derivation of hy-
pothesis, I, 431, 439, 554; increas-
ing belief in, I, 431-3, 439; ex-
periences supporting conceivabil-
ity, I, 433-5, 439; direct evidence,
I, 435-7, 439; malevolence not im-
plied by, I, 437-9; evidence from
classification, I, 443, 444, 449,
466, 555; embryology, I, 451-3,
466; substitution and suppression
of organs, I, 456-8, 466, 472-3; in-
sect segmentation, I, 468-9; ver-
tebral column development, I,
470-2; rudimentary organs, I,
472-5; adaptation to varied media,
I, 479-85, 556; growth of the the-
ory of organic, I, 490-8; instabil-
ity of the homogeneous, a cause,
I, 509-11, 516, 550; multiplication
of effects, I, 511-14, 517-8, 550, II,
405; segregation, and heteroge-

neity and definiteness of, I, 514-8,
550; natural selection and general
doctrine of, I, 543-8, 557; factors
tabulated, I, 551; inductive evi-
dences summarized, I, 555-6; sur-
viving disbelief in France, I, 559;
current theories of organic, I,
559-74; Eimer's theory of ortho-
genesis, I, 563-4; Gulick on
monotypic and polytypic, I, 569;
phenomena unexplained by the-
ories, I, 573-4; inorganic and the
System of Philosophy, I, 696;
" spontaneous generation," I, 696-
701, 702; dissolution and problems
of morphology, II, 4-6; morphol-
ogy and formula, II, 7-9, 231-5;
difliculties of definition, II, 17;
cell-doctrine, II, 17-21, 85; unicel-
lular origin of plants, II, 21-2;
resume of plant morphology, II,
78-80; origin and differentiation
of phsenogamic type, II, 83;
physiological problems, II, 239-43;
tissue differentiation, II, 244-6,
385; integration of organic world,
II, 396, 406; race and individual
multiplication, II, 428-30; declin-
ing fertility and human, II, 431,
529-30; individuation, genesis,
and, II, 501-5; human life, pro-
spective, II, 522-5; forces influ-
encing human, II, 525-8; future
of population, II, 532-7; self-suf-
ficingness of, II, 537; vertebral,
II, 563-6.

Excretion: genesis of organs of, II,
303; localization of, II, 331-3.

Exogen, application of term, II, 82.
(See Dicotyledons.)

Expenditure (see Multiplication).

Eye, the: molecular transforma-
tions in visual process, I, 75-6;
progressive development, I, 195,
II, 317-9; waste and repair, I,
218; transmitted defects, I, 306,
311, 694; degeneration in cave-
animals, I, 309, 612-3, 614, 647-9,
693; late development in insects,
I, 658; migration in flat fishes, II,
205.

Fabre, J. H., nutrition and sex in
Osmia tricomis, I, 657.

638

SUBJECT-INDEX.

False joints, I, 230; theories of
heredity and, I, 362, 364, II, 371-2.

Fats, the: physical and chemical
properties, I, 10-12; non-nitrogen-
ous, I, 41; action of bile, II, 330.

Fatty degeneration, and failing vi-
tality, I, 41.

Feathers, development, I, 474, II,
314-6.

Feet, heredity and size, I, 311.

Ferments, changes and nitrogen-
ous character of, I, 38.

Ferns: foliar development and nu-
trition, II, 76; inner tissue differ-
entiation, II, 273; indefiniteness,
II, 296; genesis, II, 441, 463.

Fertility, the General Law of Ani-
mal, I, 577-601. (See Multiplica-
tion.)

Fertilization: unit-life of generative
elements, I, 185-6; the function of
chromatin, I, 260, 263-5; extru-
sion of polar bodies, I, 266-8; na-
ture and functions of generative
elements, I, 279-83, 317, 334, 342,
593-7; differentiation and varia-
tion effected by, I, 330-2; the es-
sential object of, I, 340-1, II, 614-
6; hermaphrodism and self-, I,
341-2; crossing and its effects, I,
343-7; isolation of species in re-
spect of, I, 570; floral (see Flow-
ers).

Ficus, foliar structure, II, 589, 596.

Fingers: embryogeny of human, I,
169; heredity and abnormal, I,
305, 314, 321-3; autogenous de-
velopment of supernumerary, I,
363; rudimentary, I, 473.

Fishes: sizes of ova and adult, I,
143-4; growth of pike, I, 154,
292; size and environment, I, 156;
temperature, I, 174; self-mobility,
I, 175; continuity of blastomeres,
I, 214, II, 327; genesis, I, 271, II,
435, 436; conditions affecting
genesis, I, 292-3, 583, 598, 599, II,
454; classification, I, 392; change
of media, I, 401, 480; distribution
in time, I, 408-9; climbing spe-
cies, I, 480, 482; migrations, I,
500; dermal structure, I, 526, II,
305-6, 315, 387; Cunningham on
non-adaptive specific characters,

1, 565; elongation and locomotion,

11, 15; segmentation, II, 122, 225;
bilateral symmetry, II, 203-5;
eyes of Pleuronectidce, II, 205;
genesis of vertebrate axis, II,
212-6, 218-21, 225; ossification of
paleozoic, II, 218; respiratory or-
gans, II, 334-8; activity and mus-
cular colour, II, 365-9; Owen on
skeleton, II, 552, 557, 558-60, 562,
564.

Fission (see Agamogenesis).

Flint, Austin, on telegony, I, 644.

Flounder, symmetry and eyes, II,
205.

Flower, Sir W., on ferret, II, 480.

Flowers: pollen propulsion in or-
chids, I, 57; nature of reproduc-
tive elements, I, 283; insect fer-
tilization, I, 340, 525, II, 168, 174,
267, 407; self- and mutual fertili-
zation, I, 342-5, 570; Darwin on
homologies, I, 472; direct equi-
libration and fertilization, I, 524-
5; dimorphism, I, 534; foliar ho-
mology of petals, II, 43-6; sym-
metry, II, 132, 161, 162-^, 170,
174, 608; fertilization and sym-
metry, II, 164-70; clusters and
components, II, 170-4; nutrition
and inflorescence, II, 179-80, 541-

2, 546-7; tissue differentiation, II,
265-9; separation of ancestral
traits in hybrids, II, 616-7.

Fly, beneficial parasitism, II, 406.

Food (see Nutrition).

Food-cavity, genesis and develop-
ment of, I, 188, 195.

Foraminifera: form, I, 173; primary
aggregate, II, 87, 124; progress-
ing integration, II, 89-90, 124.

Force: action on like and unlike
units, I, 5; expenditure and or-
ganic growth, I, 149-54, 161; func-
tional accumulation, transfer, and
expenditure, I, 198-9, 201-3, 391;
waste and expenditure, I, 214-5;
distribution during strain, II, 209-

12. (See also Energy, and Persist-
ence of Force.)

Fossils (see Palaeontology).

Foster, Sir M., on storage of glyco-
gen, I, 70, 74; increase of weight
in hybernating dormouse, I, 214.

SUBJECT-INDEX.

639

Fowls (see Gallinaccw).

Foxglove: abnormal development,

I, 287, II, 4G; floral distribution,

II, 141; nutrition and growth, II,
179.

France: surviving disbelief in or-
ganic evolution, I, 559; rate of
multiplication, II, 509, 512.

Frankland, Sir E., on isomerism of
protein, I, 700.

Fraser, Col. A. T., on family of
Hindu dwarfs, I, 316.

Fries, E., multiplication of Rcticu-
laria, I, 582, II, 450.

Frog: vitality of detached heart, I,
111; of larval fragments, I, 365.

Fry, Sir E., on alternation of gen-
erations, II, 84.

Fnci: cell multiplication, II, 27; un-
differentiated outer tissue, II, 256.

Function: as a basis of classifica-
tion, I, 124-9, 129-31; simultane-
ous progress of structure and, I,
197, 211; divisions of, I, 198-200,
391; correlative complexity of
structure, I, 200, 210-1; progres-
sive differentiations, I, 201-4; con-
comitant integration, I, 205-8;
specialization and vicariousness,
I, 208-10; formula of evolution, I,
211; diminished ability and over-
work, I, 215-6; growth and in-
creased, I, 228-33, 234-5; inter-
dependence of social and organic,

I, 237-9, 240-2; structure and
heredity, I, 306-13, 318-9 (see Ac-
quired Characters); aids natural
selection, I, 308; organic interde-
pendence, I, 318-9; parental con-
dition and variation, I, 324, 326;
variation and altered, I, 325-6,
333-4; as causing variation, I,
334-5; effect on physiological
units, I, 353-4, II, 620; zoological
classification, I, 391-3; multipli-
cation of effects, I, 512; law. of
equilibration, I, 519-22, 557; cor-
relation of changes in, I, 529;
structural effects of changing, I,
541-2; structural cooperation, II,
3, 217; vicarious vegetal, II, 270;
vicariousness and specialization,

II, 293; epidermic structure, II,
312-4, 387; structure and muscu-

lar, II, 369, 391; adaptive bone-
structures, II, 370-1; equilibra-
tion and adaptation, II, 392; per-
sistence of force and adaptation,
II, 394. (See also Physiology.)
Fungi: nitrogenous character, I,
40; development, I, 163, 164, 165;
conjugation, I, 279, II, 449; fis-
sion, I, 584, 585; integration, II,
24-5, 293; symmetry, II, 137^0,
146: puff-ball tissue, II, 246, 252,
386; tissue differentiation, II, 256;
inner tissue, II, 279; indefinite-
ness, II, 295; growth and genesis,
II, 459; nutrition and genesis, II,
487.

Gallinaccw: conditions affecting fer-
tility, II, 454-5, 469, 471; mascu-
line traits of old hens, II, 495.

Galls: definition of life and, I, III;
Hertwig on, I, 690.

Galton, F., on variation outside the
mean, I, 669.

Gamogenesis: homogenesis, I, 270,
271, 336; heterogenesis, I, 270,
336; independence of offspring, I,
278; reproductive tissue, I, 279-
84; vegetal nutrition, I, 285-8,
293; II, 39; animal nutrition, I,
289-94, 297; when and why does
it recur? I, 294-7, 336-40; effect
on species, I, 347-9; leaf forma-
tion, II, 39; alternating genera-
tion in liverworts, II, 80^4; mol-
luscan homogenesis, II, 116, 117-
8; vertebrate, II, 118; growth, II,
266. (See also Fertilization, and
Multiplication.)

Gasteropoda (see Mollusca).

Geddes and Thompson, on the de-
termination of sex, I, 657.

Gelatine, nutritive value of, I, 77.

Gemmation: and genesis, I, 272-6;
theories of heredity and, I, 361;
annulose, II, 100-5, 106.

Generalization, impossibility of per-
fect, I, 450.

Generation, and genesis: the words,
I, 269.

Genesis (see Multiplication).

Gcntiana: floral arrangement, II,
608-11.

Genus: indefinite value, I, 389, 446;

640

SUBJECT-INDEX.

Instability of homogeneous and
heterogeneity of, I, 509-11, 515,
517-8, 550, 557.

Geology: growth displayed in, I,
135. 136; distribution in time, I,
404-11, 412; special creation, I,
419, 426; evolution, I, 432, 437;
record congruous with evolution,
I, 485-9, 556; organic influence of
changes, I, 501-3, 549, 550, 557;
climatic influence of changes, I,
503; time required for organic
evolution, I, 565-6; rise of insect
and plant relations, II, 407; hu-
man evolution and changes, II,
534.

Geometry, evolution illustrated by,
I, 433-4.

Germ-cell: unspecialized nature, I,
279-83, 317; dissimilarity, I, 330,
332, 334, 342; equilibrium, I, 340.
(See also Fertilization.)

Germ-plasm, Weismann's theory of,

I, 357-8; inconsistent with plant
embryogeny, I, 359; regeneration
of lost limbs, I, 362; variations in
peacock's tail feather, I, 372, 695;

II, 618-9; alleged differentiation
of reproductive and somatic cells,
I, 622, 628-30, 633^4, 646; origin
of variations in neuter insects, I,
659, 663-5, 671, 675; correlated
variations in stag, I, 677; insuper-
able difficulties, I, 682; conceiva-
bility of hypothesis, I, 695; II,
619; correlated variations in culti-
vated plants, II, 621-2.

Ghost-theory, Vitalism and, I, 114.
Giraffe, co-adaptation of structures,

I, 615.
Gizzard, development of birds, II,

320.
Glass, molecular rearrangement,

I, 337, 352, 704.

Glove, strain analogy, II, 575.
Glycogen, in animal metabolism, I,

70, 72.
Goethe, J. W. von: foliar homology,

II, 43-4, 543, 544, archetypal hy-
pothesis, II, 122; vegetal fructifi-
cation and nutrition, II, 180; the-
ory of supernumerary bones, II,
223; on the skull, II, 561.

Gold, effect of bismuth on, I, 121.

Gorilla, callosities, II, 312

Gould, J., Birds of Australia, II,
469.

Gout (see Disease).

Grafting, Born's experiments with
frog larvae, I, 365.

Graham, T., properties of water, I,
9, II, 359; colloids and crystal-
loids, I, 15-8, II, 356; their dif-
fusibility, I, 18-20; sapid and in-
sipid substances, I, 53.

Graminw: foliar surfaces, II, 61,
263; floral symmetry, II, 165;
physiological differentiation, II,
257.

Graminivores, food contrasted with
that of carnivores, I, 68.

Grassi, on food-habits of Termites,
I, 686.

Gravity: its ultimate incomprehen-
sibility, I, 121; vegetal circula-
tion, II, 586. (See also Specific
Grav'ty.)

Gregarina: central development, I,
163; primary aggregate, II, 87;
symmetry, II, 186.

Grimaux, on artificial proteids, I,
39.

Growth: organic and inorganic, I,
135-7; simulation of, I, 136; limits-
to, I, 137, 155-7; structural com-
plexity, I, 138-40, 145-7, 161; nu-
trition, I, 140, 147-9, 161; expen-
diture of energy, I, 141-3, 161;
initial and final bulks, I, 143-4,
157-60, 161; final arrest of, I,
149-55, 639; where unceasing, I,
154; resume' with generalizations,
I, 161; defined, I, 162; II, 461;
increased function, I, 228-33, 234-
5; functional interdependence, I,
235-9, 240; nutrition and vegetal,
I, 293, 294-7, 336, II, 39; hetero-
genesis and animal nutrition, I,
289-93, 296, 336; homo- and hetero-
genesis and natural selection, I,
294-8; of acrogens, II, 56; cylin-
drical form of vegetal, II, 56-64;
endogenous, II, 60-2, 78; exogen-
ous, II, 63-4, 78; plant differentia-
tion, II, 129-131; tissue differen-
tiation, II, 370; formation of
adaptive bone-structures, II, 370-
2; progressive increase of size

SUBJECT-INDEX.

641

with evolution, II, 401-2; vegetal,
and asexual genesis, II, 439-42;
animal, and asexual genesis, II,
442-5; antagonistic to asexual
genesis, II, 446; vegetal and sex-
ual genesis, II, 448-51; animal
and sexual genesis, II, 452-6, 495;
antagonistic to soxual genesis, II,
457-8; nutrition and genesis, re-
sume, II, 497-9; evolution and,
II, 501-5; commencement of gen-
esis, II, 500; fertilization and
restoration of growth-energy, II,
613.

Gulick, T. : on monotypic and poly-
typic evolution, I, 569; physio-
logical selection, I, 569-71.

Gunpowder, nitrogenous instability,
I, 8, 43.

Gymnotus, electricity of, I, 51.

Gyrodactylus elcgans, rapid succes-
sion of generations, I, 641; II,
488.

Habit, change of, in plants, I, 308.

Haemal, term applied to female ele-
ment, I, 594-5.

Hairs: non-conductors of heat, I,
526; vegetal, and natural selec-
tion, I, 532; development, II, 314-
6; tactual organs, II, 317.

Hand: embryogeny, I, 169; heredity
and size of, I, 311; distribution of
veins, I, 364.

Hardy, W. B., I, vii; II, vi.

Hare: activity and muscular col-
our, II, 365; expenditure and
genesis, II, 472.

Hart, J. A., on " Parasol " ants, I,
687-8.

Haviland, G. D., collection of Ter-
mites, I, 687.

Haystack, chemical action in, I, 74.

Head, structural influence of size,
I, 512, 537.

Hearing: the sense of, I, 54; multi-
plying agencies, I, 75.

Heart (see Vascular System).

Heat: action on di- and tri-atomic
compounds, I, 7-8, 10-12, 23, 24;
on colloids and crystalloids, I, 26;
organic changes from evaporation,
I, 29; chemical decomposition by,
I, 33; organic oxidation, I, 46-9,
87

60; growth and organic, I, 152-3;
animal, vegetal, and environment,
I, 174-5, 177; alloy melting points,

I, 339; organic effects of rhythm
in terrestrial, I, 498, 557; effect
on physiological units, I, 705;
respiration in fishes, II, 337; ani-
mal preservation, II, 434; verte-
brate expenditure and genesis, II,
468-9, 474; insect genesis, II, 476;
seasonal variations and genesis,

II, 484-5; in germination, II, 615.
Hebrew idea of creation, I, 421.
Hectocotylus, individuality, I, 250.
Hellin, D., on multiparity and

twin-births, II, 457.

Hen, what prompts her to pick up
egg-shell fragments? I, 120.

Henslow, Rev. G., inheritance of
functionally-produced changes, I,
560.

Hepaticw: Schleiden on, II, 51, 52;
continuous and discontinuous de-
velopment, II, 52< phyletic homo-
logies, II, 80-4; meaning of so-
called alternating generation, II,
84; vascular system, II, 280; gene-
sis and development, II, 463.

Heredity: structural modification,
I, 232; function of cell-nucleus in,
258-59; general truths, I, 301-4;
transmission of congenital pecul-
iarities, I, 304-7; structure and
altered function, I, 307-13, 318-9
(see also Acquired Characters);
atavism, or recurrence of ances-
tral traits, I, 314; sex limitation,
I, 314-6; physiological units, re-
sume, I, 350-5; II, 612-6; Darwin's
and Weismann's theories exam-
ined, I, 356 et seq., 559-61; II, 622;
true theory must include plants,
I, 358; inadequacy of theory of
physiological units, I, 360-1; so-
ciological parallel, I, 366-8; natu-
ral selection (q. v.), I, 545-7, 553,
557; ethnology and natural selec-
tion, I, 553; unsolved problems, I,
573-4; mutilations, I, 631; ulti-
mate process incomprehensible, I,
695; cell doctrine, II, 19; physio-
logical development, II, 242; wood
formation, II, 287; tissue differ-
entiation, II, 304, 312-4; respira-

642

SUBJECT-INDEX.

tory system, II, 311; osseous dif-
ferentiation, II, 351; muscular
adaptation, II, 367; persistence of
force and physiological adapta-
tion, II, 394; vegetal vascular sys-
tem, II, 574, 582, 588, 596.

Hermaphrodism, I, 340-3.

Hertwig, O.: on Weismann's germ-
plasm theory, I, 690; cell charac-
ters, I, 691; meaning of fertiliza-
tion, II, 613.

Hertwig, R., classification of tis-
sues, I, 189.

Heterochrony of development, I,
655.

Heterogeneity: in chemical evolu-
tion, I, 23-4; of vital changes, I,
84-90; of development, I, 170, 178;
functional, I, 204-8, 211-2; of or-
ganic matter, I, 350-5; organic
and instability of homogeneous, I,
509-11, 517, 549, 557; segregation
accompanying, I, 514-6, 517-8,
550.

Heterogenesis: occurrence, I, 270,
272-5, 336; animal nutrition, I,
289-91, 295-7; natural selection, I,
295-8; heredity, I, 301.

Hindus: food, I, 68; dwarf family,

I, 316.

Histology (see Physiology).

Hofmeister, sporophytic generation
of Archegoniates, II, 80.

Hollyhock, floral symmetry, II, 167,
169, 170.

Homogeneous, instability of the:
variation, I, 330, 334, 342; evolu-
tion, I, 509-11, 517, 549, 557; mor-
phological development, II, 7-9,
234; direction of vegetal growth,

II, 181; radial symmetry, II, 190;
physiological differentiation, II,
384, 392.

Homogenesis (see Gamogenesis).

Homology, simulation of, by anal-
ogy, II, 14.

Hooker, Sir J. D., I, ix; European
plants in New Zealand, I, 477;
plant distribution, I, 479; adapta-
tion of plants to varied media, I,
484; plant growth, II, 56; Balano-
phorw and Rafjflesiacew, II, 274;
structural complexity, II, 295,
297; relative antiquity and dis-

tribution of plants and animals,
II, 297; bean vascular system, II,
574.

Hooker, Sir W., on fructification in
Jungermanniacew, II, 52.

Horns, natural selection and corre-
lated variation, I, 537, 567, 674,
677.

Horse: ancestral types, I, 409; fer-
tility, I, 598; weight of brain, I,
599; quagga markings, I, 624, 627.

Husbandry, co-ordination of actions
in, I, 96, 579.

Hutchinson, Sir J., hereditary
syphilis, I, 623.

Huxley, T. H., I, ix; " continuous "
and " discontinuous " develop-
ment, I, 164; classification of de-
velopment, I, 276; hermaphrodism,
I, 344; zoological classification, I,
383; on " Persistent Types," I,
408-9; ancestral equine types, ib. ;
segmentation of articulates, I,
468-9, II, 113; agamic multiplica-

m tion of Aphis and Entozoa, I, 640-

' 1; II, 476; cell doctrine, II, 21; ver-
tebrate embryo, II, 119, 120; mol-
luscan symmetry, II, 202; tegu-
mentary organs, II, 314, 315; ver-
tebrate sensory organs, II, 318,
319; Chotidracanthus, II, 487;
Owen's vertebrate theory, II, 563.

Hyacinth: lateral spike, II, 42;
symmetry, II, 141, 162.

Hybernation, waste and repair in,
I, 214-5.

Hybrids, separation of ancestral
traits in, II, 616-7.

Hydro-carbons: properties, I, 6-9;
the term carbo-hydrates (q. v.), I,
10.

Hydrochloric acid, in gastric juice,
I, 69.

Hydrogen: chemical and physical
properties, I, 3-5; compounds, I,
6, 8, 9, 10-12; 12-13.

Hydrozoa (see Coelentcrata).

Hymenoptera (see Insects).

Hypertrophy (see Disease).

Hypospadias, telegonic transmis-
sion, I, 646.

Hypostasis of a relation, exempli-
fied in explanations of fertiliza-
tion, II, 613.

SUBJECT-INDEX.

643

Ideas (see Psychology).

Individuality: the botanical, I, 244-
6; the zoological, I, 246-7; the fer-
tilized germ product, I, 248-9;
definition of life, I, 250.

Individuation: and genesis, I, 583-
4; II, 428-30, 499; total cost, II,
435-7; genesis and evolution, II,
501-5, 529, 530.

Infusoria: functional specialization,
I, 391; primary aggregate, II, 87;
asymmetry, II, 187, 188; differ-
entiation, II,. 299, 385; genesis, II,
442, 446, 452.

Injuries, repair of animal, I, 219,
222-4, 316, II, 102, 611.

Insanity, inherited, I, 314.

Insects: temperature, I, 47, 174;
phosphorescence, I, 49; self-mo-
bility, I, 175; parthenogenesis, I,
274-5, 277, 294, 592, 640; growth
and reproduction, I, 292; species
distribution determined by pres-
ence of, I, 396-7; eyes of cave-in-
habiting, I, 309, 612-3, 614, 647-9,
693; persistent types, I, 408; re-
trograde development, I, 458;
segmentation, I, 468-9; II, 114;
aborted organs, I, 474; East In-
dian distribution, I, 478; floral fer-
tilization, I, 525; II, 168-9, 406 7,
608; appliances for cleaning an-
tennae, I, 651; eyes, I, 658, II,
318; integration and homology, II,
111-3, 121; bilateral symmetry, II,
198; sexual selection, II, 269;
eyes, II, 318; environment, II,
433; cost of genesis, II, 436, 437;
development and genesis, II, 461;
nutrition and genesis, II, 476,
490-2.

Insects, Social, origin of caste-
gradations in, I, 654-65, 670, 674,
675, 678-84, 686-8.

Instability of the homogeneous (see
Homogeneous).

Instinct: organic evolution and co-
ordination of, in mason-wasp, I,
574; a vital attribute, I, 578; loss
of self-feeding, in Amazon ants, I,
660-1, 663-4.

Integration: in chemical evolution,

I, 23; morphological composition,

II, 4-6; arthropod, II, 111-4, 121;

physiological, in plants, II, 292-5,
295-8, 390; of organic world, II,
396-408; genesis, II, 424, 426-9.

Intelligence, a vital attribute, I,
579.

Internodes: varied development, II,
45; nutrition and length, II, 178-9.

Intestine (see Alimentary Canal).

Intra-selection, Roux's theory of, I,
562, 676-8.

Irish, nutrition and genesis, II, 510.

Iron: colloidal form of peroxide, I,
17, 20; molecular rearrangement,
I, 337, 704; vegetal absorption, II,
573.

Iron industry, interdependence of
social function, I, 237-41.

Isolation, and species differentia-
tion, I, 568-9.

Isomerism: of organic constituents,
I, 4, 9, 25; tri- and poly-atomic
compounds, I, 11, 13, 25; muscular
action, I, 59; organic evolution, I,
700, 703; differentiation of nerve-
tissue, II, 356-60, 361; of muscu-
lar tissue, II, 361-4.

Jackson, J. Hughlings, on inherit-
ance of nervous peculiarities, I,
313, 694.

Jaundice (see Disease).

Jaws, of uncivilized and civilized,
I, 541-2, 612, 693.

Johnson, G. Lindsey, on inherited
myopia, I, 694.

Jones, T. Rymer, on fission, I, 585,
590.

Julin, C, on " castration parasi-
taire " in Crustaceans, II, 493-6.

Jungermanniacew: morphology, II,
33-4; relations of high and low
types, II, 35, 55; continuous and
discontinuous development, II,
52-5, 92: tubular structure, II, 58,
62; proliferous growth, II, 67, 91;
colour, II, 75, 265: symmetry, II,
140; fertility and growth, II, 441.

Jussieu, A. de, plant classification,
I, 378.

Karyokinesis, I, 257, 259, 263-5.

Kerner, A., on cauline buds, I, 358;
plant-classification in Natural His-
tory of Plants, I, 378-9.

644

SUBJECT-INDEX.

Kidd, Ben;}., his acceptance of
Weismannism, I, 690.

Kitto, Dr., his visual memory and
deafness, I, 230.

Klebs, on Hydrodictyon, I, 288; Vau-
cheria, II, 84.

Klein, E., multiplication of Bac-
teria, II, 443.

Korschelt, E., annulose segmenta-
tion, II, 103, 601-3, 605; Arenicola
larvae, II, 109.

Labour, physiological division of,
I, 204, 207, 591, II, 373; its. mean-
ing and Weismann's fallacious in-
terpretation, I, 634-5.

Lacaze-Duthiers, on origin of annu-
lose type, II, 110.

Lamarck: zoological classification,

I, 382: opinions of E. Darwin
and, I, 491, 493-7; neo-Darwinists
and, I, 630-1.

Laminariacew: pseudo-foliar and
axial development, II, 30; tissue,

II, 247, 256, 272.

Language: and evolution, I, 442,
444, 446; perceptiveness of tongue-
tip, I, 607.

Lankester, Sir E. Ray, absence of
nucleus in Archerina, I, 183; di-
versity of Protozoa, ib.; zoological
classification, I, 387; blindness of
cave-animals, I, 647-8, 649.

Laugh, definition of life and, I, 112.

Laurel, leaves of, II, 149, 249.

Leaves: growth of shoot, I, 168; de-
velopment and aggregation, II,
37-42, 76; stem-like stalks, II, 41;
homologies, II, 42, 75-7, 83; nu-
trition and compound, II, 42;
foliar and axial development, II,
46-50, 541-7; " adnate," II, 58;
proliferous growth, II, 67, 91; nu-
trition and development, II, 76-8;
symmetry, and of branches, II,
148-50, 151; size and distribution
of leaflets, II, 152-5; transition
from compound to simple, II, 155-
8; unsymmetrical form, II, 158-
9; natural selection and distribu-
tion, II, 179; morphological sum-
mary, II, 234-5; tissue differentia-
tion, II, 247; distribution, II, 249;
outer tissues of stem and, II,

256-9, 270, 386; distribution of
stomata, II, 260-1; wax deposit
on, II, 260, 261; light and colour,
II, 261-2; superficial differentia-
tion, II, 263-5, 270, 387; abortive
in parasitic plants, II, 274; sub-
merged, in aquatic plants, II,
274-5; inner tissue differentiation,
II, 278, 388; vascular tissue dif-
ferentiation, II, 286, 288, 388; dye
absorption and circulation, II,
570^4, 577; vascular system, II,
588-92, 596; arrangement, II, 608-
11.

Lcpidoptera (see Insects).

Lcpidosiren: ossification, II, 218;
respiration, II, 338; skeleton, II,
553, 555, 560.

Lcpidosteus: armour, I, 526; air
bladder, II, 334.

Leroy-Beaulieu, Pierre, on Austra-
lian miners' usages, I, 364.

Lessonia: Hooker on growth, II, 56;
branch symmetry, II, 146.

Lewes, G. H., definition of life, I,
80.

Lichens: tissue, I, 586; cell multi-
plication, II, 27; Hooker on
growth, II, 56; tubular structure,
II, 57; integration, II, 293; dual
nature, II, 399; reproduction, II,
450.

Liebig, Baron, nitrogenous food
stuffs, I, 47-8.

Life: co-ordination of actions, I,
79, 89, 577-^80; defined by Schell-
ing, I, 78, 178; Richeraud, I, 79;
De Blainville, I, 79, 93; Lewes, I,
80; definition yielded by contrast-
ing most unlike kinds, I, 81-8;
changes showing, I, 91; vital ac-
tions and environment, I, 92-3;
resulting addition to conception,
I, 93, 326; Comte's definition, I,
93; correspondence of external
and internal relations, I, 93-6,
100; II, 523; continuous adjust-
ment of such relations, I, 99; com-
pleteness proportionate to corre-
spondence, I, 101-4, 109, 349;
length and complexity, I, 103;
complexity of environment and
degree of, I, 104-6; definitions of
evolution and, I, 107-10; deficien-

SUBJECT-INDEX.

645

cies of formula, I, 112-3; activity
the essential element, I, 113; hy-
pothesis of independent vital
principle examined, I, 114-7; dif-
ficulties of physico-chemical the-
ory, I, 117-20; ultimate incompre-
hensibility, I, 120-3, 373: validity
of conclusions reached, I, 123; is
organization produced by? I, 107;
precedes organization, I, 210;
definitions of individuality and, I,
250; effect of incident forces on,
I, 348-9, 355: length in individuals
and species, I, 422; equilibration
of, I, 547, 557; final formulation
of definition, I, 580; co-ordination
of actions and sexual differentia-
tion, I, 593; " absolute " com-
mencement of, I, 699, 702; inte-
gration and augmentation, II,
426; prospective human, II, 522-5.

Light: influence on organisms, I,
30-6, II, 433; nitrogenous plants,
I, 40; organic phosphorescence, I,
49; heliotropism, I, 92, II, 160;
effects on organic matter, I, 149;
plant adaptation, I, 227; rhyth-
mical variation of, and organic
life, I, 499, 557; vegetal influ-
ences, II, 130, 131, 147, 149, 158;
influence on flowers, II, 167-8,
608-11; vegetal tissue-differentia-
tion, II, 253-5, 258, 259; action on
leaves, II, 260-4 ; on plant vascular
system, II, 288, 297, 586; develop-
ment of sensory organs, II, 320.

Liliaccw, floral symmetry, II, 170.

Lime, leaf forms, II, 158, 159.

Lindley, J., plant classification, I,
377.

Linnaeus, C, classificatory system,
I, 377, 380.

Linnet, contrasted with blackbird
in development, II, 503.

Liver: metabolic processes, I, 69,
70; vitality of excised, I, 111; de-
velopment, II, 329-33.

Liver-fluke (see Distoma).

Liverworts (see Hepaticce).

Lizard, regeneration of lost tail, I,
360,

Locomotion (see Motion).

Logic, reasoning and definition of
life, I, 81-6.

Logwood, vegetal staining, II, 569-
74, 577-81, 584.

Longevity, and complexity of life,
I, 102-3.

Lubbock, Sir J.: on growth and
genesis in insects and crusta-
ceans, I, 292; aquatic flies, I, 400.

Lungs (see Respiratory System).

Lymphatic system: amoeboid cells,
I, 187; structural traits, I, 192,
193.

MacBridb, E. W., I, vi, II, vi;

zoological phyla, I, 386-7; arthro-
pod segmentation, II, 114; cteni-
dia of slug, II, 117; conjugation
of Paramecium, II, 452.

Macrocystis pyrifera, gigantic sea-
weed, I, 121.

Magenta, vegetal staining, II, 569-
74, 577-81, 584.

Magnetism: muscular action, I, 59;
incomprehensibility, I, 121.

Maillet, B. de, modifiability of or-
ganisms, I, 490, 496.

Mammalia: temperature and mole-
cular change, I, 30; nutrition and
growth, I, 141; expenditure of
force, I, 142, 156; flesh constitu-
ents, I, 154; temperature, I, 174,
177; self-mobility, I, 175; func-
tional and structural differentia-
tion, I, 201; heart-function, I, 208;
viviparous homogenesis, I, 271;
variation and uterine environ-
ment, I, 327; classification, I, 392;
cervical vertebra?, I, 394, II, 564;
aquatic types, I, 400; fossil re-
mains and rate of evolution, I,
407; ancient and modern forms
contrasted, I, 408-10; embryonic
respiratory system, I, 456; sup-
pression of teeth, I, 457; arrested
development, I, 473^; simulated
homologies, I, 485; natural selec-
tion and inactive parts, I, 534;
re-development of rudimentary
organs, I, 563; location of testes
and current theories, I, 573; fer-
tility and development, I, 583, II,
465; fertility and nervous develop-
ment, I, 598-9; locomotion and
elongated form, II, 15; symmetry,
11,204; tegumentary structure, II,

M6

SUBJECT-INDEX.

314; circulation, II, 340; vascular-
ity and ova-maturation, II, 342-3;
activity and muscular colour, II,
365-9; functional integration, II,
375; outer tissue differentiation,
II, 387; growth and genesis, II,
456, 459; comparative fertility,
II, 465, 470; heat expenditure, and
genesis, II, 467-9; activity and
fertility, II, 472; nutrition and
genesis, II, 479-80.
Man: effect of climate on vigour, I,
30; flesh and grain eaters com-
pared, I, 68; longevity and life, I,
103; complex environment, I, 105;
embryogeny of arm, I, 169; fer-
tility and conditions affecting it,

I, 300, 570, 583, II, 484, 506-21;
inheritance of functionally pro-
duced changes, I, 310-3, 541, 605,
608, 612, 652, 673, 689, 693-^;
heredity and sex, I, 315-6; func-
tion of bilirubin, I, 330; cousin-
marriages, I, 346, II, 615; primi-
tive notions, I, 417-9; inutility of
Appendix vermiformis, I, 474; di-
minution of jaw, I, 541, 612, 693;
co-ordination of actions greatest
in, I, 579; fundamental traits of
sex, I, 594-7; obesity, I, 594; sub-
stance and weight of brain, I,
596, 599; distribution of tactual
perceptiveness, I, 602-8, 665-6,
672-3, 692; telegony, I, 625, 644-5;
degradation of little toe, I, 652,
673; transmitted osteological pe-
culiarities of Punjabis, I, 689;
traits of twin-bearing women, II,
457; comparative mammalian fer-
tility, II, 465; future evolution,

II, 522-37. (See also Language
and Sociology.)

Manatee, nailless paddles, I, 473.

Manx cats, I, 303.

Marchantiacew: symmetry, II, 140;
outer tissue differentiation, II,
252.

Marmot, hybernation and waste, I,
214-5.

Marriage (see Multiplication).

Marsh, O. C, on telegony, I, 644.

Masters, M. T., on foliar homology,
II, 46-7; selection of inconspicu-
ous variations in plants, II, 298,

621; separation of ancestral con-
stitutions in plant hybrids, II,
616; single and double stocks, II,
622.

Matter, incomprehensibility of in-
teractions, I, 121-2.

Mechanics: tran verse strains, II,
209-12; genesis of vertebrate axis,
II, 212-6, 216-8, 224, 225-7; osse-
ous differentiation, II, 345-51; dis-
integrated motion, II, 375; anal-
ogy from locomotive, II, 517-9;
future human evolution, II, 524;
strain and vegetal structure, II,
574-88, 592-6.

Medusw: contractile functions, I,
58; II, 374; individuality, I, 248;
heterogenesis, I, 273; fertility, I,
582; stabilization, I, 592; sym-
metry, II, 188-91.

Mehnert, E., on feet of pentadae-
tyle vertebrates, I, 461.

Mensel's salt, temperature and iso-
merism, I, 77.

Metabolism: antithesis between
plants and animals, I, 62-3; evo-
lution-hypothesis and primordial,
I, 63-4; in plants, I, 64-7; ani-
mals, I, 67-77; nervo-muscular ac-
tivities, I, 71-7; summary, I, 77;
cell processes, I, 261.

Metals: remarkable interactions of
some, I, 121; melting of alloys, I,
339; atomic re-arrangement, I,
352.

Metamerism (see Segmentation).

Metazoa: cellular structure, I, 184,
194, II, 21; subordination of units,
I, 185-7; general characters of tis-
sues, I, 188-9; protoplasmic con-
tinuity, I, 190-2, 194, 628; genesis
of food cavity and visual organ,
I, 195; Weismann's differentiation
theory, I, 637-43.

Meteorology: non-vital changes
shown in, I, 82, 84; crystalliza-
tion of " storm glass," I, 96; spe-
cial creation, I, 419; rhythm in,
and organic change, I, 499-501,
557; variations due to geologic
change, I, 503.

Microstomida, segmental reproduc-
tion, II, 102.

Migration: of animal species, I,

SUBJECT-INDEX.

647

396-401, 411; solar influences, I,
500; part played by, In organic
evolution, I, 508; causes of, II,
533-4.

Milk, heat and supply of, II, 468.

Milne-Edwards, H., " physiological
division of labour," I, 204; Weis-
mann's erroneous application of
it, I, 634; on ocular structure, II,
318.

Mind (see Psychology).

Mitosis (see Karyokinesis).

Mobility, molar and molecular, I,
14; environment and self mobil-
ity, I, 177.

Mohl, on phsenogamic growth, II,
82.

Mole, modifications due to habits,
II, 391.

Molecules: mechanically considered,
I, 14; stability, I, 337-40; nerve-
differentiation, II, 355-61, 379-82.

Mollusca: axial development, I, 165;
genesis, I, 271, II, 444; her-
maphrodism, I, 341; classifieatory
traits, I, 392; distribution in
time, I, 405, 408, 410, 446-7;
trochophore and its relationships,

I, 447, II, 10S, 109, 115; devel-
opment, I, 460; amphibious and
terrestrial, I, 481; indirect equi-
libration, I, 534; secondary ag-
gregation, II, 115-7; symmetry,

II, 201-3; outer tissue, II, 310,
387; alimentary system, II, 325;
vascular system, II, 340-1.

Molluscoida, II, 598. (See Polyzoa
and Tunicata.)

Monocotyledons: growth, I, 138,
139, 143; uniaxial development, I,
165; cotyledonous germination and
endogenous growth, II, 59-62, 69-
72, 82-3, 181-2; absence of helical
phyllotaxy in Ravcnala, II, 182;
surface contrasts, II, 257; outer
leaf tissue, II, 263; wood forma-
tion, II, 278; growth and genesis,
II, 451.

Monstrosities, in plants, II, 78, 541,
546; vertebrate, II, 118.

Morgan, T. H., on regeneration of
Planaria, II, 102, 611.

Morphology: facts comprised in, I,
125-6; morphological units, I, 190-

2, 225; rudimentary organs, I,
472-5, 556; structural and func-
tional co-operation, II, 3, 239; in-
tegration, II, 4-6, 181-96; change
of shape, II, 6; formula of evolu-
tion, II, 7-9; as interpreted by
phylogeny, II, 10-6; evolution and
cell doctrine, II, 17-21.
Morphology, Animal: evolution and
segmentation of Articulata, I, 468-
9; vertebral column development,

I, 470; simulated homologies, II,
14-5; primary aggregates, II, 85-
8, 123-4; secondary, II, 88-91, 124;
tertiary, II, 91-3; integration and
independence of individuality, II,
93-9, 124; annulose segmentation,

II, 98-101, 106-10, 125-7, 602-7;
progressive annulose integration,
II, 100-5, 111-5, 121, 124, 223; un-
integrated molluscan form, II,
115-7; vertebrate segmentation
and integration, II, 117-23, 124-7,
223-4, 602, 606-7; motion and
symmetry, II, 183-5; symme-
try of primary and secondary ag-
gregates, II, 186, 187-91; of
compound Ccelenterata, II, 192-4;
simulation of plant shapes, II,
192; symmetry of Polyzoa and
Tunicata, II, 194; of Platyhel-
minthes and Echinoderms, II, 195-
7; of Annulosa, II, 197-201; of
molluscs, II, 201-3; of verte-
brates, II, 203-6, 208; similarity
of animal and plant, II, 206; cell-
shapes, II, 228-30; evolution and
generalizations summarized, II,
231-5. (See also Structure.)

Morphology, Vegetal: simulated
homologies, II, 13-4; unicellular
plants, II, 21; aggregation and in-
tegration, II, 22-6, 78-9; pseudo-
foliar development, II, 2(5-8;
pseud-axial, II, 28-9: pseudo-
foliar and axial, II, 30-2; com-
position of Archegoniates, II, 33-
5; leaf development and aggrega-
tion, II, 37-42, 75-8; foliar homo-
logies, II, 42-6, 75-8; foliar and
axial development, II, 46-50, 541-
7; growth and development of
Archegoniates, II, 50-6; of Phre-
not'ams, II, 56-64, 78-80; axil-

648

SUBJECT-INDEX.

lary bud development, II, 65-9;
phaenogamic modes of growth, II,
69-72; homologies, II, 73-5, 80-4;
development of foliar into axial
organs, II, 75-8; resume', II, 78-
80; criticisms and replies, II, 80-
4; can plant-shapes be formu-
lated? II, 128; growth and differ-
entiation, II, 129-31; kinds of
symmetry, II, 131-3; symmetry of
primary aggregates, II, 134-7; of
secondary, II, 137-40; tertiary, II,
140-3; symmetry and environing
influences, II, 143-4; symmetry of
branches, II, 145-8; leaf and
branch symmetry, II, 148-50;
phaenogamic unit homology, II,
151; size and distribution of leaf-
lets, II, 152-5; transition from
compound to simple leaves, II,
155-8; unsymmetrical leaf devel-
opment, II, 158-9; differentiation
of homologous units, II, 159-60;
floral symmetry, II, 161-74; cell-
differentiation and metamorpho-
sis, II, 175-7; nutrition and differ-
entiation, II, 178; and inflores-
cence, II, 179; helical growth of
phsenogams, II, 180-1; summary
of symmetry, II, 234; stress and
structure, II, 275-9, 388. (See also
Structure.)

Morton, Lord, quagga-marked foal,
I, 624.

Moser, impressions produced by
light on metals, I, 352.

Mosses: varied development, II, 50-
1, 52; homologies, II, 80, 81; in-
deflniteness, II, 296; multiplica-
tion, II, 441.

Moth, clothes, food of larva, I,
77.

Motion: organic, and environment,
I, 75-7, 175-8, 196; of animals and
waste, I, 214, 220; simulation of
locomotive structures, II, 15.

Motor organs, differentiation of, I,
262.

Mountains: climatic effects, I, 504;
growth of trees on, II, 142.

Mouse: fertility of, II, 421, 473;
tapeworm parasitism, II, 490;
compared with rat, II, 503-4.

Mucor, II, 22, 123.

Mucous membrane, differentiation,
II, 321-2, 389.

Multiplication: decline of fertility
with evolution, I, 103, II, 431;
vitalism, I, 116; phenomena classi-
fied, I, 130; the term " genesis,"

I, 269: processes classified, I, 270-
6, 336, 583; a process of disinte-
gration, I, 276; reproductive tis-
sue in gamogenesis, I, 278-84; nu-
trition and growth, I, 285-94,
295-7, 299; natural selection, I,
295-8; hermaphrodism, I, 340-4:
in-and-in breeding, I, 344-7; phy-
siological units, I, 350-5; law of
race-maintenance, I, 581; II, 420-
3, 430; effect of mental applica-
tion, I, 597, II, 511-4, 516-9, 530;
individuation antagonistic to, I,
598-600, II, 428-30, 435-7, 499,
501-5; checks put by carnivores
on, II, 405; four factors in rate
of, II, 416, 435; destructive and
preservative forces, II, 417-20;
rhythm of species, II, 419; nutri-
tion and disintegration of, II, 424,
425, 430; integration and genesis,

II, 426-8; influence of environ-
ment, II, 432-3; and variations of
expenditure, II, 433-5; growth
and asexual genesis, II, 439^46;
asexual and sexual distinguished,
II, 448; sexual genesis and
growth, II, 448-58, 495; and de-
velopment, II, 461-5; plant ex-
penditure, II, 467; animal expen-
diture, II, 468-72; nutrition in
plants, II, 475, 511; in animals, II,
476-84, 511; seasonal variations,
II, 484-5; nutrition, resume, II,
486, 497-9; nutrition and para-
sitic, II, 486-90; reversion to
agamogenesis, II, 490-2; human
fertility, II, 506-10; Doubleday on,
II, 510-2; civilized and uncivil-
ized, II, 514-6; human evolution
and decline in, II, 529-31; the
future of population, II, 532-7:
equilibration and evolution, II,
537.

Muscle: electrical contrasts, I, 50:
action of, I, 59; metabolism, I, 70,
71-4: definition of life and actidns
of, I, 112-3: growth and function,

SUBJECT-INDEX.

649

I, 151, 155; development, I, 170;
Hertwlg's classification of tissues,
I, 189; functional differentiation,

I, 203-4; waste and repair, 215-7;
modifiabllity and adaptability, I,
228-9, 230, 232; correlated varia-
tions, I, 536-9, 614-21, 676, 693;
resistance to strains, I, 639; ac-
tion on bones in Punjabis, I, 889:
differentiation, II, 361-9; activity
and colour, II, 365-9: integration,

II, 376, 382: equilibration in ac-
tion, II, 393; activity and fertility
in birds, II, 470-2; future human
evolution, II, 523; origin of ver-
tebrate type, II, 598-600.

Music: limited adaptability of voice
and ear, I, 231; inheritance of
faculty, I, 311-2, 694.

Mutilations, the question of their
inheritance, I, 631.

Mycctozoa, growth and reproduction,
I, 298-9.

Myocommata (myotomes), and ver-
tebrate skeleton, II, 216, 217-8,
222.

Myopia, inheritance of, I, 306, 694.

Myrianida fasciata, I, 361; II, 445.

Myriapoda: gemmation, I, 589; seg-
mentation, I, 590, II, 113, 114,
601; degenerated eyes of cave-in-
habiting, I, 649; integration and
homology, II, 111-4; genesis, II,
445.

Myxothallophyta, I, 378.

Nails, mammalian, I, 473.

Nais: regeneration of detached
parts, I, 219, 361.

Narcissus, separation of ancestral
traits in hybrids, II, 617.

Natural selection: structural modifi-
cation, I, 211; in cell processes, I,
263-4; multiplication, I, 295-8;
aided by function, I, 308-10; spe-
cial creation, I, 426-7; the term
" survival of the fittest," I, 530;
indirect equilibration, I, 530-5,
552-3, 557, 571; changes unex-
plained by, I, 535-42, 571, II, 371;
tendency to economy, I, 536, 562;
decrease of jaw, I, 541, 693; gen-
eral doctrine of evolution, I, 543-
8, 557; unceasing operation, I,

552; human races, I, 553; current
views, I, 559-60; panmixia and
cessation of selection, I, 560-3:
intra-selection, I, 562, 676-8;
Eimer's theory of orthogenesis, I,
564; Mr. Cunningham's criticism,
I, 565-6; location of mammalian
testes, I, 573; co-ordinated in-
stincts of mason-wasp, I, 574;
tactual perceptiveness, I, 603-8,
633, 646, 665, 671, 672, 692; errone-
ously identified with artificial se-
lection, I, 609, 695; reversed se-
lection, I, 611; blindness of cave-
animals, I, 613, 614, 647-8, 693; co-
adaptation of co-operative parts,
I, 614, 621, 663-5, 670, 674, 675,
689, 692; where operative, I, 632;
Weismann on conceivability of
process, I, 651; degeneration of
little toe, I, 652-3, 673; genesis of
caste gradations in social insects,
I, 654-60, 663, 670, 675, 684; self-
feeding instinct in ants, I, 660-2,
670; rudimentary organs, I, 667-9,
671, 692; horns of stag, I, 676-8,
692; musical faculty, I, 694; the
neo-Darwinian position reviewed,

I, 694-5; vegetal nutrition, II, 51-
2; upright vegetal growth, II, 56-
7; endogenous growth, II, 57-8;
exogenous, II, 64; Navieula sym-
metry, II, 135; foliar, II, 158;
foliar distribution, II, 167, 179;
floral fertilization and symmetry,

II, 168-70, 608-11; helical phseno-
gamic growth, II, 181; Echinodcr-
mata and bilateral symmetry, II,
197; vertebrate structure, II, 214-
20, 227; phsenogamic tissue differ-
entiation, II, 248: physiological
differentiation, II, 252, 256; root-
lets of ivy, II, 254; stomata and
foliar surfaces, II, 261, 262; floral
fertilization, II, 268-9; sexual se-
lection, II, 269; vegetal tissue dif-
ferentiation, II, 279; wood forma-
tion, II, 287-8, 290; animal tissue
differentiation, II, 304-8; evolu-
tion of nervous system, II, 307-8;
respiratory system, II, 311; der-
mal callosities, II, 312^; sensory
organ complexities, II, 321; skin
and mucous membrane differen-

650

SUBJECT-INDEX.

tiatlon, II, 322; localization of ex-
cretion, II, 333; respiratory or-
gans of fishes, II, 335-8; heart
and vascular system, II, 341, 344;
osseous differentiation, II, 355;
also muscular, II, 363, 368-9;
" false joints," II, 371; insect nu-
trition and genesis, II, 499; econo-
mics of evolution, II, 501-5; au-
thor's enunciation of survival of
the fittest in 1852, II, 528-9; evils
of interference with, in man, II,
532-3; vegetal tissue formation,
II, 582, 594-6; origin of verte-
brate type, II, 599.

Nature, more complex than sup-
posed, I, 252, 450.

Navicula, symmetry, II, 134-5.

" Nebular Hypothesis," I, 23.

Negation, inconceivability of, the
ultimate test of truth, I, 675.

Negroes, telegony in United States,
I, 644-5.

Nemertidce: continuing vitality of
pilidium, I, 250; bilateral sym-
metry, II, 195.

Neo-Darwinists, and Lamarck, I,
630; their position reviewed, I,
694-5.

Nerves: electrical phenomena, I, 51;
generation of nerve-force, I, 52-6,
60; corpuscula tactus, I, 75; Hert-
wig's classification of tissues, I,
189: structural traits, I, 192, 193;
environment and structure, I,
196; differentiation, I, 203; II,
355-61; vasomotor system, I, 206;
vicarious function, I, 209; activity
and waste, I, 216; adaptability, I,
229, 232, 236; parallelism in cell-
processes, I, 260-2; heredity, I,
313; effects of severance, I, 349;
relative development in men and
women, I, 594; analysis of brain
substance, I, 596; individuation
and development of, I, 598, 599,
600; distribution of tactual per-
ceptiveness, I, 603-8, 633, 646,
665-6, 671, 672, 692; alleged cost-
liness of tissue, I, 662; instinct
degeneration in ants, ib. ; " sensa-
tion areas," I, 666; segmentation
in Annelids, II, 125; ectodermal
derivation, II, 303^; cooperating

factors in evolution of, II, 307-8;
differentiation from muscle, II,
363. (See also Psychology.)

Nervousness, hereditary transmis-
sion, I, 307.

Neurine, I, 594, 597.

Neuter-insects (see Insects).

New Zealand: invasion of alien spe-
cies, I, 477; kinship of past and
present forms, I, 489.

Nitrogen: properties, I, 3-5, 20, 24;
compounds and their properties,

I, 6, 8, 9, 12-14, 25-6, 39, 41, 42-3,

II, 250; organic importance, I,
42-3; evolution of heat and oxida-
tion, I, 47; violent organic effects
of compounds, I, 54-5; function in
metabolism, I, 63-4, 66, 68-76;
presence in protoplasm, I, 66; ac-
tion in digestion, I, 69; fat accu-
mulation and fertility, II, 483.

Nitro-glycerine, violent effects of,
I, 55, 122.

Notochord: segmentation, II, 125,
218-22; formation, II, 217-8, 600;
in Permian vertebrates, II, 225.

Noumenon, life not manifested as,
I, 5S0.

Nuclein, II, 21.

Nucleus: central development, I,
163; in simple organisms, I, 183;
phenomena exhibited by, I, 255-
8; current hypotheses of function,
I, 258-9; properties and function
of chromatin, I, 259-65; fusion in
fertilization, I, 283-4; function in
unicellular reproduction, I, 595-6;
absence of, II, 20-1; diffused
form, II, 85; macro- and micro-
nucleus in conjugation, II, 452.

Nutrition: organic molecular re-ar-
rangement, I, 36; nitrogenous and
non-nitrogenous, I, 47-8, 68, 71-4,
77, II, 362; food assimilation and
reasoning, I, 81; needful for vital
change, I, 94; relation to growth,
I, 140, 143, 144, 147-9, 150, 157,
161; expenditure of energy, I,
157, 391; fluid, I, 208; vegetal
fructification, I, 267; II, 266;
vegetal growth and genesis, I,
293, 294-7, 336; animal growth
and genesis, I, 289-93, 295-7, 336;
conditions qualifying antagonism

SUBJECT-INDEX.

651

of growth and genesis, I, 299;
competition among parts of an or-
ganism for, I, 562, 566, 676; sex
differentiation, I, 594-5; cell-mul-
tiplication, I, 638; differentiation
of neuter insects, I, 655-60, 670,
674, 686-8; monstrous ant forms,

I, 683-4: leaf-development, II, 39,
42, 73-8; vegetal development, II,
51-2, 178, 276; axillary buds, II,
65-9, 73-4; effect on animal aggre-
gation, II, 93; internodes and in-
florescence, II, 178-80; helical
phsenogamic growth, II, 181; ac-
tion of bile, II, 330; osseous de-
velopment, II, 349, 353; genesis,

II, 419, 422, 427, 435-7, 452; par-
ental loss in feeding young, II,
424, 429; diverse sources, II,
433; Carpenter on reproduction
and, II, 460; animal development
and genesis, II, 465; expenditure
and genesis, II, 468; variations of
genesis, II, 475-80, 511; obesity
and genesis, II, 480-4, 511; gen-
eral doctrine of genesis, II, 486;
genesis and vegetal parasitism,
II, 486; also animal, II, 487-90,
495; insect genesis, II, 490-2:
genesis, resume, II, 497-9; and evo-
lution, II, 501-4; of blackbird
and linnet, II, 503; genesis in
human race, II, 508-10, 514-6;
Doubleday on, II, 510-2; future
human evolution, II, 526, 531; flo-
ral monstrosities, II, 542, 546, 547.

Obesity, nutrition and genesis, II,

480-4, 511.
Odoriferous glands, natural selec-
tion and, I, 534.
Odours: floral fertilization, II, 268-

9; animal protection, II, 434.
Offspring: parental loss entailed by

nurture, II, 424, 429; influence of

age on, II, 507.
Oken, L., archetypal hypothesis, II,

122; theory of supernumerary

bones, II, 223; on the skull, II,

561.
Oliver, F. W., classification of

plants, I, 378-9.
Ophryotrocha pucrilis, ciliation of

segments, II, 109.

Orchids: pollen propulsion, I, 57;
leaf formation in Dendrobium, II,
60-1; aerial roots and physiologi-
cal differentiation, II, 255, 257;
foliar surface, II, 264.

Organic matter: properties of ele-
ments, I, 3-5, 22; of compounds,
I, 5-13, 25; molar and molecular
mobility, I, 12-14; colloid and
crystalloid form, I, 15-8, 25;
their diffusibility, I, 18-21, 26; ex-
treme complexity, I, 21; laws of
evolution and genesis of, I, 22-4;
modifiability, I, 27, 44; capillarity
and osmosis, I, 28; effects of heat,
I, 29; of light, I, 30-4; nitrogen-
ous, I, 39^3; oxidation and evo-
lution of heat, I, 46, 60; genesis
of electricity, I, 50-2, 60; sensible
motions in, I, 59; transformations
and persistence of force, I, 61;
metabolism, 1, 62-77; artificial pro-
duction of compounds, I, 64: con-
trasted with inorganic matter, I,
107-8; incomprehensibility of vital
changes in, I, 122; instability, I,
149, 508; phosphorus in cell-or-
ganization, I, 260-1; heteroge-
neity, I, 350-5; " spontaneous gen-
eration " and evolution of, I, 696-
701; cell-doctrine and evolution
of, II, 17-21.

Organization (see Structure).

Ormerod, Dr., on sex and nutrition
in wasps, I, 656.

Orthogenesis, Eimer's theory of, I,
563-4.

Osmosis: organic effects, I, 28, 29;
in animals, I, 58; in vascular sys-
tem, II, 339; in vegetal tissue, II,
568, 575, 577, 585, 592-6.

Osteology (see Bone).

Ovum (see Egg and Fertilization).

Owen, Sir R. : metagenesis and par-
thenogenesis, I, 273-4; fossil
mammals, I, 410; human para-
sites, I, 427; continuous operation
of creative power, I, 492; fission
in Infusoria, I, 584, 585, 595-6;
parthenogenesis, I, 592; theory of
vertebrate skeleton, II, 123, 548-
66; , theory of supernumerary
bones, II, 223; Eschricht on
Ascaris, II, 488.

652

SUBJECT-INDEX.

Oxalis: radial symmetry, II, 152;
foliar surface, II, 264.

Oxen: comparison with sheep, I,
158, 160; cerebro-spinal system, I,
598.

Oxidation (see Oxygen).

Oxygen: properties, I, 3-5, 20, 22;
compounds, I, 6-7, 10-13, 22, 24-5;
a crystalloid, I, 21; combining
power and atomic weight, I, 33;
organic change from, I, 37; heat
generation, I, 46-9; phosphores-
cence, I, 49; nerve force depend-
ent on, I, 53; animal metabolism,
I, 72, 73; necessary to animal life,
I, 94-5, 577; activity and amount
inhaled, I, 214.

Packard, A. S., on eyes of cave-
animals, I, 648-9, 693.

Paget, Sir J., blood changes in
small-pox and scarlatina, I, 221,
701.

Palaeontology: distribution in time,
I, 404-11, 412; special creation, I,
425; congruity with evolution hy-
pothesis, I, 485-9, 556; relations
of present to extinct species, II,
10-11; scarcity of remains, II,
34-5; secondary thickening in
plants, II, 56; Cope on osteology
of Permian Vertebrates, II, 225-6.

Pangenesis, Darwin's theory of, I,
356, 357, 359, 360, 362, 372.

Panmixia, Weismann's hypothesis
of: its relation to Romanes* " ces-
sation Of selection," I, 560; al-
leged selective process denied, I,
561-3, 667, 685; distribution of

. tactual perceptiveness, I, 608;
rudimentary eyes of cava-fauna,
I, 612-3, 647; Romanes on pro-
cess, I, 649, 667; degeneration of
self-feeding instinct in Amazon-
ants, I, 660-2, 670; rudimentary
limbs of whale, I, 668-9, 685; a
pure speculation, I, 671; markings
on leg-bones of Punjabis, I, 689.

Paramcecium: parasite infesting, I,
427; reproduction, II, 443, 452.

Parasites: sexual dimorphism, I,
315; limits to distribution, I, 397;
special-creation and, I, 427-9, 438;
retrograde development, I, 457,

II, 12; aphis and ant, I, 660-1, II,
403, 405; as an integrating
agency, II, 402-4; its comparative
recency, II, 404; nutrition and
genesis in vegetal, II, 486; in ani-
mal, II, 487-90, 493; " castration
parasitaire " in crustaceans, II,
493-6.

Parasol Ants, origin of classes, I,
687-8.

Parthenogenesis: occurrence, I,
274-5; alternating with gamo-
genesis, I, 289-91; Owen on, I,
592; laws of multiplication, II,
415; in articulate animals, II, 445.

Pasteur, L., silk-worm diseases, I,
622-3.

Peacock: theories of heredity and
structure of tail feather, I, 372-3,
695, II, 618-9.

Pear, foreright shoots, I, 287.

Peloria: in gloxinia, II, 166; phseno-
gams, II, 180.

Penguin, dermal structure, II, 314.

Pepsin, I, 69.

Pericyclic fibres of monocotyledons,
II, 278.

Peripatus capensis, protoplasmic
continuity, I, 629.

Peri-visceral sac, function and dif-
ferentiation, I, 391.

Perkin, W. H., I, vi.

Persistence of force, corollaries
from: properties of compounds,
I, 3; organic transformation, I,
60; growth, I, 150; organic en-
ergy, I, 220; variation, I, 335;
genesis, heredity, and variation,

I, 354-5; morphological summary,

II, 235; vegetal tissue differentia-
tion, II, 245; physiological devel-
opment, II, 394.

Petals: foliar homology, II, 43-6;
" adnate," II, 58.

Petrels, Darwin on, I, 455.

Phsenogams: production of sperma-
tozoids, I, 186; morphological
composition, II, 37-79; leaf tran-
sitions, II, 37-42; foliar homolo-
gies, II, 42-9; origin of type, II,
49-84; vertical growth, II, 56-64;
axillary buds, II, 66; cotyledon-
ous germination and endogenous
growth, II, 69-72; axial homolo-

SUBJECT-INDEX.

653

gles, II, 73-5; irregular develop-
ment, II, 75-8; degree of com-
position, II, 78; reproductive ho-
mology, II, 80-4; uni- and multi-
axial symmetry, II, 141-3; unit of
composition, II, 151; helical
growth, II, 181; secondary thick-
ening, II, 247; tissue and leaf dif-
ferentiation, II, 247-9, 387; also
bark and cambium, II, 249-50,
386; also outer tissue, II, 252,
256-9, 270, 386-7; wax deposit on
leaves, II, 260-2; differentiation
of inner tissues, II, 273-5, 388;
vascular system development, II,
280-4, 388; integration, II, 293-5,
296, 390; insect-fertilization, II,
407; multiplication, II, 441, 442;
genesis and growth, II, 451, 457;
and development, II, 464; and nu-
trition, II, 476, 477, 511; substi-
tution of axial for foliar organs,
II, 541-7.

Phenomenon, life manifested as, I,
580.

Philology (see Language).

Phoronis, individuality, II, 444.

Phosphorescence, organic, I, 49.

Phosphorus: allotropic, I, 4; in cell
physiology, I, 259-62; cerebral ac-
tivity, I, 596-7; organic evolution,
I, 703.

Photogenes, visibility of, I, 218.

Phylogeny: as interpreting mor-
phology, II, 10-12; difficulties of
affiliation, II, 34-5. {See Embry-
ology and Evolution.)

Physiological Units: definition, I,
226; genesis, I, 280-1, 316; hered-
ity, I, 315-9; variation, I, 330,
331-2, 333, II, 619: stability, I,
340: II, 614; self-fertilization, I,
342-4, 353; interbreeding, I, 345,
353, II, 615: recapitulation of hy-
pothesis, I, 350-5; II, 612-7;
structural proclivities, I, 362, 364,
369-71, II, 613, 622; sociological
analogy, I, 364, II, 620; complex-
ity in organized types, I, 368-70;
re-named " constitutional units,"
I, 369; telegony, I, 650; " me-
chanical theory," I, 701-6; mor-
phological development, II, 7-9;
cell doctrine, II, 17-21; develop-

ment, II, 76; " false-joints," II,
371-2; dissociation of ancestral
traits in hybrids, II, 616-7; in-
heritance of acquired characters,
II, 618-23.

Physiological division of labour
(see Labour).

Physiological Selection, I, 569-71.

Physiology: and psychology, I, 127;
subdivisions, I, 128: vicarious
function, I, 208; primitive inter-
pretations, I, 417; multiplication
of effects exemplified, I, 512, II,
390; relations to morphology, II,
3, 239-41; evolutionary interpre-
tation of phenomena, II, 241-5,
384-95; ultimate inconceivability
of processes, II, 372; correlated
integration and differentiation, II,
373.

Physiology, Animal: metabolism, I,
67-77; vertebrate internal sym-
metry, II, 108; tissue differentia-
tion in Protozoa, II, 299, 385; pri-
mary tissue differentiation, II, 300-
2, 382, 389; natural selection and
tissue differentiation, II, 304-8;
outer tissue in Cmlenterata, II,
309-10; respiratory organs, II,
310-1, 333-8; differentiation of
animal epidermic tissue, II, 312-4,
387; development of tegumentary
organs, II, 314-6; of sensory, II,
317-20: inner and outer tissue
transition, II, 321-2, 389; aliment-
ary canal differentiation, II, 323-
5; gizzard development in birds,
II, 325; alimentary canal of rumi-
nants, II, 327-9; differentiation of
liver, II, 329-33; of animal vascu-
lar system, II, 339-44; of osseous
system, II, 344-55; of nerve tissue,
II, 355-61; of muscle, II, 361-9;
differentiation and integration, II,
373-6; in vascular system, II, 376-
9, 383; in nerves, II, 379-82; origin
of development, II, 384; differen-
tiation and instability of homo-
geneous, II, 384-9, 392; summary
of development, II, 384-94; mul-
tiplication of effects, II, 390-1,
392; equilibration, II, 391-4. (See
also Function.)

Physiology, Plant: metabolism, I,

654

SUBJECT-INDEX.

62-7; tissue differentiation in sec-
ondary aggregates, II, 246, 385;
in phsenogams, II, 247-9, 386; in
bark and cambium, II, 249-50,
386; in free and fixed surfaces, II,
251-6, 270, 386; outer stem and
leaf tissue, II, 256-9, 270, 386; su-
perficial differentiation in leaves,
II, 260-4, 270, 387; floral tissue
differentiation, II, 265-9; outer tis-
sue, risumi, II, 270: inner tissue
differentiation, II, 273-5, 388;
supporting tissue, II, 275-9, 285-S,
388; vascular system develop-
ment, II, 273-5, 279-84, 285-8,
388: inner tissue, summary, II,
288-91, 388; integration, II, 292-8;
differentiation and instability of
homogeneous, II, 384-9, 392; mul-
tiplication of effects, II, 390-1,
392; equilibration, II, 391-4; cir-
culation and wood formation, II,
564-97; dye permeability, II, 569-
74, 577-81, 584, 586. (See also
Function.)

Pickering, J. W., on artificial pro-
teids, I, 39.

Pig: colour of muscles, I, 365-6;
telegony, I, 627; fertility of do-
mestic and wild sow, II, 479-80.

Pigeons: food of starving, I, 215;
heredity and variation, I, 305,
321, 615; atavism, I, 314; fertility,
II, 471-2, 478.

Pike, unceasing growth, I, 154, 292.

Pique-gouffe, commensal relations
with buffalo, II, 403.

Plogiochila, evolution of stem, II,
62.

Planaria: integration, II, 101-2;
Morgan on regeneration, II, 102,
611; segmentation, II, 107; sym-
metry, II, 195: unintegrated func-
tion, II, 373.

Plants: influence of heat, I, 29;
effect of solar rays, I, 31-6, 500,
557; chemical composition, I, 40-
1; heat generation, I, 47; phos-
phorescence, I, 49; electricity, I,
51; sensible motion, I, 56-7, 58;
metabolism, I, 62-7, 70; vital
changes, I, 86, 87, 91, 94; simula-
tion by crystals, I, 96; vital ad-
justments, I, 102; length and com-

plexity of life, I, 103-4; biological
classification, I, 125; growth, I,
136, 138, 140, 143, 145-9, 153, 160-
1, II, 401-2; development, I, 163-
5, 167-70, 272; weight, tempera-
ture, and self- mobility, I, 174;
function, I, 174-8; structure, I,
194-6, II, 21; animal structure
contrasted, I, 196; function and
structure, I, 200; vicarious func-
tion, I, 208-9; waste and re-
pair, I, 213, 220; physiological
units, I, 225-6, 317, 360; adap-
tation, I, 227; what is an in-
dividual? I, 244-6, 250-1; gene-
sis, I, 270, 271, 272-3, 274, 276-8,
279-85; relation of nutrition to
growth and genesis, I, 284-9, 294,
295-300, 642, II, 39; ovule homo-
logues, I, 288; natural selection,
I, 294-8, 532, 533, II, 51; heredity,

I, 301-4, 308, 358-60; variation, I,
320, 323-4, 325-6; fertilization, I,
340-5; classification, I, 377-80,
389-90; distribution, I, 396-400,
401-3, 404-12, 478-9, 556; special
creation and parasitism, I, 428;
evolution hypothesis, I, 434, 443,
449-50; rudimentary organs, I,
474, 475, 556; varied media, I, 484,

II, 32; alien and native species in
New Zealand, I, 477; E. Darwin
and Lamarck on evolution of, I,
490-8; geologic changes affecting,
I, 501-3, 557; interdependence of
animals and, I, 504-6, 514, II,
398; complexity of influences af-
fecting, I, 506; direct equilibra-
tion, I, 523-5; indirect, I, 532,
533; seed distribution, I, 546;
wood development, II, 285-7, 289,
567-97; interdependence, II, 402-
3, 404; insect relations, II, 406-7;
adaptation and multiplication, II,
411-6; rhythm in numbers, II,
419; growth and asexual genesis,
11,439-42; growth and sexual gene-
sis, 11,448-51; expenditure, II, 467;
horticulture, nutrition, and gene-
sis, II, 477; tree development, II,
553; circulation and wood forma-
tion, II, 567-92; dye permeability
and circulation, II, 569-74, 577-81,
584, 586; resume on circulation

SUBJECT-INDEX.

655

and wood formation, II, 592-7.
(See also Multiplication, Mor-
phology, and Physiology.)

Plasmodium, dissolution of, I, 185.

Tlato, iSrfa of, II, 550.

Phityhelminthes: transverse fission,
II, 101; segmented and non-seg-
mented types, II, 102, 107; sym-
metry, II, 195, 197; multiplication
and growth, II, 488-9.

Plethora, fertility and, II, 480-4,
511.

Pleuroeoccaccw, unicellular form, II,
21, 134.

Plcuroneetidw: symmetry and loca-
tion of eyes, II, 205; outer tissue,
II, 387.

Plumatella: metagenesis, I, 277;
symmetry, II, 195.

Podostemaccw, undeveloped circula-
tory system, II, 274.

Polar bodies, hypothesis concerning
extrusion of, I, 266-8.

Polarity, organic, of physiological
units, I, 226, 315, 317, 332, 350-1,
701-6.

Polyatomic compounds (see Chem-
istry).

Polyclwtce, anomalous development
in Myrianida, I, 361.

Polycytliaria, integration, II, 90,
124.

Polygastrica, aggregation, I, 586.

Polymerism: of compounds, I, 9, 11,
25; nerve tissue, II, 356.

Polypori, symmetry and environ-
ment, II, 139.

Polyps (see Coelenterala).

Polyzoa: size, I, 140; multiaxial de-
velopment, I, 165; structural in-
definiteness, I, 173: functional dif-
ferentiation, I, 202; trochophoral
kinship, I, 447; integration, II,
93-4, 96, 124; symmetry, II, 194,
207; vascular system, II, 340;
gemmation, II, 444.

Poor Laws, and natural selection,
II, 532.

Population, A Theory of, I, 265, 577-
601, II, 411.

Potato: simulated growth, I, 136;
vicarious function of tuber, I, 209,
II, 255; sub-species, I, 302; dye
absorption, II, 279.

Preservation: fertility and self-, I,
581; II, 423, 430; nutrition, II,
493.

" Progress; its Law and Cause,"
theory of species differentiation,

I, 568.

Projectiles, factors in flight of, I,
450-1.

Proteids: metabolic function, I, 67,
68, 69, 72, 76; complexity of
molecule, I, 122.

Protein: evolution, I, 23, 24; isomer-
ism, I, 700, 703, 704.

Proteus, degeneration of eye, I, 613.

Protodrilus, intestine segmentation,

II, 125.

Protophyta: internal movements, I,
56; limit of growth, I, 138; devel-
opment, I, 164; structure, I, 173,
181-3; self-mobility, I, 175; indi-
viduality, I, 245; multiplication,
I, 270, 276, 279, 581, 584-5, II,
439, 462; genesis and nutrition, I,
295; unicellular, II, 21; central ag-
gregation, II, 24; symmetry, II,
134; tissues, II, 244, 249; primary
differentiation, II, 385; primordial
type, II, 398; symbiosis, II, 400.

Protoplasm: self-increasing func-
tion of primordial, I, 63-4; plant
metabolism, I, 65-7; complexity,
1,122,253-5; differentiatkm in sim-
ple organisms, I, 182-3; continuity
and inter-circulation, I, 190-2,
371, 629, II, 21, 620; " streaming,"

I, 253; structure, I, 253-5. (See
also Cell.)

Protozoa: inorganic components*, I,
17; locomotion, I, 58, 175, II, 14;
vital changes shown by, I, 94;
limitation of growth, I, 138; de-
velopment, I, 164; structure, I,
173, 181-3; incipient differentia-
tion, I, 198, 391, II, 299, 3D9; mul-
tiplication, I, 270, 276, 279, 280,
582,584; 11,442,451-2; genesis and
nutrition, I, 295; distribution, I,
396; parasites infesting, I, 427;
Weismann's hypothesis of immor-
tality, I, 637; " spontaneous gen-
eration," I, 697-701; non-nucle-
ated, II, 20; primary aggregate,

II, 86-7, 124; progressing integra-
tion, II, 89-91, 124; symmetry, II,

SUBJECT-INDEX.

186; primordial plant-animal type,
II, 397-8: symbiosis, II, 400,
Protyle, hypothetical chemical unit,

I, 22, 23.

Pseud-axial development, vegetal,

II, 28-9, 30.

Pseudo-foliar development, vegetal,
II, 26^8, 30.

Psychidw: parthenogenesis, I, 275;
sexual dimorphism, I, 683.

Psychology: reasoning and defini-
tion of life, I, 81-8; correspond-
ence shown by recognition, I, 95;
contrasted with physiology, I,
127; departments of, I, 127-8;
vicarious function, I, 209; waste
and repair in sensory organs, I,
217; sensory adaptability, I, 229,
231, 232; inheritance of sensory
defects, I, 306; musical talent, I,
311-2; intellectual progress and
special-creation hypothesis, 1,417;
special-creation a pseud-idea, I,
420, 429, 433, 554; legitimacy of
evolution-hypothesis, I, 433-5,
439, 554; embryology of ideas, I,
450, 457; persistent formative
power unrepresentable, I, 492; E.
Darwin's and Lamarck's theory of
desires, I, 494; natural selection
and brain evolution, I, 553; gene-
sis and cerebral activity, I, 594,
II, 512-4, 516-9, 530; heredity and
distribution of tactual perceptive-
ness, I, 602-8, 646, 665-6, 672,
692; inconceivability of the nega-
tion, I, 675; vitiation of evidence,
II, 88; repetition and perception,
II, 143; differentiation of sensory
organs, II, 317-20; differentiation
of nerve tissue, II, 355-61; func-
tional integration, II, 376; also in-
tegration, II, 380-2; equilibration
of nerve discharge, II, 393; human
fertility and nerve development,
II, 466, 532; future human evolu-
tion, II, 523-5, 527; human evolu-
tion and genesis, II, 529-31; future
mental development, II, 535; origin
of vertebrate type, II, 598-600.

Pteridophyta: size attained by, I,
138, 139; homologies, II, 80-1, 82;
frond surface differentiation, II,
260.

Pteropoda: bilateral symmetry, II,
201; dermal respiration, II, 310.

Ptyaline, metabolic function, I, 69.

Punjabis, inheritance of acquired
osteological peculiarities, I, 689.

Pyrosomidw: phosphorescence, I, 47;
integration, I, 588, II, 97.

Quagga, telegonic transmission of
markings to offspring of mare, I,
624, 627, 646.

Quills, development, II, 314-6.

Rabbit: activity and muscle col-
our, II, 365; over-running checked
by weasels, II, 405; expendi-
ture and genesis, II, 472.

Radial, definition, II, 148.

Radiolaria: unicentral development,

I, 163; secondary aggregation, II,
88; symmetry, II, 187.

Radula, development of roots from
leaflets, II, 34.

Rafflcsiaccce: homogenesis, I, 272;
tissue differentiation, II, 274; nu-
trition and genesis, II, 486.

Rat (see Rodentia).

Rathke, H., on vertebrate embryo,

II, 119.

Ray, J., plant classification, I, 378.

Reasoning, compared with assimila-
tion, I, 81-7.

Recapitulation, embryological, I,
453.

Regeneration (see Repair).

Rejuvenescence, and sexual fertili-
zation, I, 637; II, 613.

Remak, R., vertebrate embryo, II,
120.

Repair: continuity of, I, 216-9; ani-
mal injuries, I, 219, 222-4, II, 102,
611; deductive interpretation, I,
221-2; theories of heredity and
regenerative phenomena, I, 360-1.

Repetition of like parts, II, 126.

Reproduction (see Multiplication).

Rcptilia: growth and expenditure of
force, I, 142; sizes of ova and
adult, I, 144; longevity of croco-
dile, I, 154; temperature, I, 174;
waste, I, 214; distinctive charac-
ters, I, 392; distribution in time,
I, 409, 412; vertebral segmenta-
tion, I, 470; rudimentary limbs of

SUBJECT-INDEX.

657

snakes, I, 473; fertility and devel-
opment, I, 583, 598, 590; regenera-
tion, I, 189; elongated form, II,
1.1; supernumerary vertebrae, II,
123, 584; bilateral symmetry, II,
203, 204; Cope on segmentation in
extinct, II, 225, 2liti; activity and
muscular colour, II, 385; func-
tional Integration, II, 375; outer
tissue differentiation, II, 387;
Owen on skeleton, II, 500.

Resistance of media to locomotion,
II, 15.

Respiratory System: effect of light,

I, 31; organic re-arrangement, I,
37; cutaneous, I, 209; air-cells of
lungs, I, 254; embryonic branchiae
of salamander, I, 457; differen-
tiation, II, 310-1, 333-8; physio-
logical integration, II, 374-5, 382;
vascmar differentiation and inte-
gration, II, 377.

Retrograde metamorphoses, in ani-
mals, II, 12.

Retzius, G., superficial nerve-end-
ings, I, 6<!<i.

Reversed Selection, I, 611, 012.

Rhabdospheres, calcareous armour
and dynamic element in life, I,
119.

Rhizoids, foliar expansions, II, 50.

L'hizopoda: structure, I, 173; undif-
ferentiated function, I, 200; a pri-
mary aggregate, II, 86; symmetry,

II, 186; tissue differentiated, II,
299, 385; motion of sarcode, II,
356; symbiosis, II, 400.

Rhythm: astronomic and organic, I,
499, 557; law of equilibration, I,
520-1; in multiplication, II, 419.

Richeraud, Baron A., definition of
life, I, 79.

Riley, C. V., on telegony, I, 645;
Termites, I, 680, 681; pouch of
Honey-ants, I, 684.

Rodentia: incursions, I, 399; Ameri-
can types, I, 403; fertility and
development, I, 583, 599.

Rivinus, plant classification, I, 377.

Rokitansky, on false joints, I, 230.

Romanes, G. J.: on ''cessation of

selection," I, 500-2: isolation and

species differentiation, I, 588;

" physiological selection," I, 509-

88

71; panmixia, I, 649, 667; influence
of a previous sire on progeny, I,
649.

Rbntgen rays, I, 121, II, 621.

Roots: developed from leaflets, II,
34; physiological differentiation,
II, 253-5, 270; nutrition from
leaves, II, 274; size and function,
II, 276.

Rotiferw: latent vitality of desic-
cated, I, 117; trochopore, II, 108,
109; molluscan relationship, II,
115; fertility and size, II, 453, 459.

Roux, W. : "intra-selection," I, 676;
functional adaptation, II, 354.

Rudimentary organs: the definition
of life and, I, 112, natural selec-
tion and eyes of cave fauna, I,
309, 612-4, 647-9, 693; evolution
hypothesis, I, 472-5, 556; limbs of
whale, I, 608-9, 685, 093.

Ruminants, alimentary canal de-
velopment, II, 327-9.

Salamander, embryonic branchiae,

I, 457.
Salmonidev, reproduction and growth,

I, 291-3, II, 454.

Solpidw: heterogenesis, I, 272, 277;
integration, I, 588, II, 97.

Sap (see Vascular system).

Sarcina: central aggregation, II, 24;
fertility, II, 440.

Savage, Dr., on " Heredity and
Neurosis," I, 313.

Sccncdcsmus, individuation, II, 24.

Scent: natural selection and keen-
ness of, I, 610; floral fertilization,

II, 2G8-9; animal protection, II,
434.

Schelling, E. W. J. von, definition

of life, I, 78, 178.
Schleiden, J. M., on individuality,

I, 245; on liverworts, II, 50, 52;

algal indefiniteness, II, 293.
Science, complex revelations of, I,

252, 369, 450.
Scyphomtdusce, strobilization, II,

108.
Sea: changes and movements in, I,

83; life in, lower than terrestrial,

I, 104; distribution, I, 396, 517;

change of media caused by, I,

481; geologic influence, I, 502.

658

SUBJECT-INDEX.

Seals: nail-bearing toes, I, 473; vi-
brissa, II, 317.
Seasons: reproductive periodicity,

I, 299; variations of genesis with,

II, 484-5.

Sedgwick, Adam: on continuity of
protoplasm in animals, I, 190,
629, II, 21; zoological classifica-
tion, I, 387; discrimination of spe-
cies in embryonic stages, I, 461;
persistence of ancestral traits, I,
463-4; Arehiannelidan segmenta-
tion, II, 109.

Sedgwick, Wm. : heredity and sex,
I, 305, 314; telegonic transmission
of hypospadias, I, 646.

Seeds: nitrogenous, I, 40; tempera-
ture of germinating, I, 47, II, 615;
vitalism and latent vitality of, I,
116-7; variation in environment,
I, 327; natural selection among, I,
532.

Segmentation (metameric): special
creation hypothesis, I, 468-9;
Huxley on number of somites in
higher articulates, ib.; in annu-
lose animals, II, 98-110, 111-5,
601-5; simulated molluscan, II,
116; in vertebrates, II, 125-7, 225-7,
606-7; in elasmobranchs, II, 126.

Segregation: of growth, I, 136; of
like units, I, 179; organic repair,
I, 221; variation, I, 331, 334;
heterogeneity, and definiteness of
evolution, I, 514-6, 517-8; mor-
phological development, II, 7-9;
physiological units, II, 616.

Self-fertilization, animal and vege-
tal, I, 341-4, 353.

Senses, the (see Psychology).

Sex: in Ascidian colonies, I, 247;
limitation of heredity by, I, 314-6;
correlated traits, I, 371-2, 513; nu-
trition and determination of, in
social insects, I, 655-60, 678-84,
686-9; neural and haemal traits, I,
683; differentiation of organs, II,
303: castration and growth, II,
4o9; Julin on " castration parasi-
taire " in crustaceans, II, 493-6;
the object of fertilization, II, 613.
(See also Fertilization.)

Sexual Selection (see Natural Selec-
tion).

Sharp, D. : on insect somites, I, 469;
food habits of Termites, I, 686-7.

Sheep: contrasted with oxen, I,
158, 160; crossing of English and
French breeds, I, 625; nutrition
and genesis, II, 480.

Sherrington, Prof., on effects of
nerve severance, I, 349.

Ship-building, interdependence of
social functions, I, 237-9, 241.

Shipley, A. E.: segmentation of
Microstomida, II, 102; Protodrilus,
II, 125.

Silica, colloid and crystalloid, I, 16.

Silicic acid: properties, I, 16; isom-
erism, I, 59.

Silicon, allotropic, I, 4.

Silkworm disease, I, 622-3.

Simulation: of homology by anal-
ogy, II, 14, 485; of segmented
structure by molluscs, II, 116.

Siphonophora, specialization of com-
ponent polyps, II, 95.

Sirenia, simulated fish form, I, 485.

Size (see Growth).

Skeleton, vertebrate (see Verte-
brata).

Skin: respiratory function, I, 209;
adaptability, I, 228, II, 312-4,
387; transmitted peculiarities, I,
306; Wallace on distribution of
sensitiveness, I, 646-7; differen-
tiation, II, 215, 217, 304-7; tegu-
mentary development, II, 314-6,
387; differentiation of sensory or-
gans, II, 317-20; and mucous
membrane, II, 303-4, 321-2, 389.

11 Skin friction," and locomotion of
aquatic animals, I, 156.

Skull (see Vertebrata).

Sleep, repair favoured by, I, 216.

Small-pox, blood changes from, I,
221.

Smith, Prof. W., on fertility of
diatomacece, II. 440.

Smith, W. P., on telegony in calves
and foals, I, 645.

Smith, W. W., on habits of Tetra-
morium, I, 660.

Snakes (see Rrptilia).

" Social organism," author's essay
on, I, 363, 676.

Sociology: environment and degree
of life, I, 105-6; functional dif-

SUBJECT-INDfiX.

659

ferentiation, I, 204; division of
labour, I, 207, 363-4, 367; func-
tional interdependence, I, 237-9,
240-2; autogenous development of
units in colonies, I, 364, 367-8, II,
620; belief in social evolution, I,
432; natural selection, I, 553, II,
532; integration and differentia-
tion, II, 378-9; effects of popula-
tion, II, 535-6; equilibration, II,
537.

Soil, dependence of plant evolution
on, II, 402.

Solarium jasminoides, organs of at-
tachment, II, 276.

Solar system, autogenous develop-
ment illustrated by distribution
of forces in, I, 366.

Sole, symmetry and location of
eyes, II, 205.

boma-plasm, Weismann's theory of
differentiation from germ-plasm,
I, 357, 022, 628-30, 633-44.

Somites (see Segmentation).

Special creation: and evolution, I,
412, 415, 431; improbabilities, I,
418-9, 430, 439, 554; inconceiva-
bility, I, 420, 429, 431, 554; of in-
dividuals and species, I, 421-4;
the implication of beneficence, I,
425-9; summary, I, 429, 554; Von
Baer's formula, I, 451-6; verte-
brate skeleton, II, 551, 556, 565.

Species: adaptation and stability, I,
242; hereditary transmission, I,
301-4; variation in wild and culti-
vated, I, 323-5, 326, 693; ga mo-
genesis and life of, I, 347-9; phy-
siological units, I, 362, 364, 369-
71, 458, II, 613; indefiniteness, I,
389, 445, 572; special creation, I,
422-4; instability of homogeneous,
and differentiation of, I, 509-11,
515, 517-8, 550, 557; persistence
of, I, 516, 518, II, 10-11; natural
selection and equilibration, I,
543-8, 553, 557; non-adaptive char-
acters, I, 565; morbid products as
marks of, I, 567; migration and
isolation as causes of differentia-
tion, I, 568-9; increasing multi-
formity of aggregate, II, 396.

Specific gravity, of organisms and
environment, I, 174, 177.

Spermatozoa ) . „ i.„ -
Sperm-cell } (see Fertilization).

Sphere: tendency of units to form,

I, 15; the embryonic form, I, 177;
symmetry, II, 131.

Spheroid, symmetry, II, 132.

Spiders (see Arachnida).

Spine (see Vertebrata).

Sponge: structure and dynamic ele-
ment in life, I, 119; multicentral
development, I, 164; units and ag-
gregate, I, 185^ reproductive tis-
sue, I, 283; integration, I, 586; II,
90, 383; physiological differentia-
tion, II, 300, 386; development
and genesis, 11,463; analogy from,

II, 576.

Spontaneous generation: and hete-
rogenesis, I, 270; and evolution,
I, 696-701, 703.

Stag, horns and correlated struc-
tures, I, 567, 670, 676-7, 692.

Stamens, and foliar homology, II,
44.

Starches: properties, I, 11; trans-
formations, I, 66, 68, 69, 70, II,
593.

Star-fishes (see Asteroidea).

Statoblasts, of Plumatella, I, 277.

Steenstrup, on " Alternate Genera-
tion," I, 592.

Sterility (see Multiplication).

Stickleback: ova, II, 454; bothrio-
cephalus in, II, 490.

Stomach (see Alimentary canal).

Stomata, distribution, II, 260-1.

Straight line, and evolution hypoth-
esis, I, 433.

Strain: compression and tension of,

I, 151, II, 209-12; relation to
mass, I, 155-7; vegetal structure,

II, 574-88, 592-6; origin of ver-
tebrate type, II, 600.

Strawberry: multiaxial develop-
ment, I, 166; multiplication, II,
441.

Strength, a vital attribute, I, 578.

Structure: appliances for generat-
ing motion, I, 75-7; biological
classification, I, 125-7, 129; size
and organic, I, 137; growth and
complexity, I, 138, 145, 161; rela-
tion to environment, 1, 172-8, 195-6;
of unicellular organisms, I, 181-3;

660

SUBJECT-INDEX.

multicellular, I, 183-96; Hertwig's
classification of tissues, I, 189;
continuity of units, I, 190-2; sys-
tems of organs, I, 192; division
into universal and particular, I,
193-4; general truths, I, 194-5;
plant and animal, contrasted, I,
195-6; precedence of function or,
I, 197, 211; correlative complexity
of function and, I, 200, 211; pro-
gressive concomitant differentia-
tion, I, 201-4; physiological units,

I, 22o-6, 362, 364, 369-71, II, 613;
social and organic interdepend-
ence, I, 235-42; varied by func-
tion, I, 334, 535; II, 217 (see Ac-
quired Characters) ; zoological
classification, I, 390-2; equilibra-
tion, I, 521, 557; progress of, and
genesis, I, 590-1; II, 462; coopera-
tion with function, II, 3; evolu-
tion and increased, II, 4; retro-
grade metamorphosis, II, 12;
simulated homologies, II, 13-14;
earliest organic forms, II, 19;
cylindrical vegetal, II, 57-62; per-
manence and complexity, II, 295,
296; function and epidermic, II,
312-4, 387; and muscular, II, 369,
391; adaptation and equilibration,

II, 392; persistence of force and
physiological adaptation, II, 394;
evolution, II, 501-4. (See also
Morphology.)

Struggle, for nutriment among com-
ponents of an organism, I, 562,
676; for existence (see Natural
Selection).

Struthers, Sir J.: on heredity, I,
305, 314; digital variation, I, 321;
rudimentary limbs of whale, I,
668.

Strychnine, effects of, I, 54, 55.

Sturgeon, size of ova and adult, I,
144.

Sugars: properties, I, 10-11; trans-
formations, I, 38, 40, 06, 69, 70,
II, 593.

Suicide, hereditary tendency to, I,
307.

Sulphur: allotropic, I, 4, 59; organic
evolution, I, 703.

Sun (see Light).

Survival of the Fittest, the expres-

sion, I, 530, 610. (See Natural

Selection.)
Swan, vertebrae of neck, II, 123.
Swiftness, a vital attribute, I,

578.
Syllls ramosa, lateral branching, I,

166, 361, II, 105, 10S.
Symbiosis, II, 399, 400.
Symmetry (see Morphology).
Syphilis, hereditary transmission, I,

62b.

Tactual Perceptiveness, heredity
and the distribution of, I, 602-8,
633, 665, 666, 672, 692.

Tcenia (see Eniozoa).

Tansley, A. G., I, vi, II, vi; adapta-
tion of reproductive activity to
conditions in Algw, I, 288-9;
shapes of Caulcrpa, II, 22; stem-
thickening in extinct Thallo-
phytes, II, 56; natural selection
and leaf-distribution, II, 179.

Tape-worm (see E-ntozoa).

Taste, dependent on chemical ac-
tion, I, 54.

Teeth: hereditary transmission, I,
306; suppression of mammalian, I,
457; of uncivilized and civilized, I,
541, 693.

Tegumentary organs, origin of, I,
314-6.

Telegony, or the influence of a pre-
vious sire on offspring, I, 624-7,
644-6, 649-50.

Temperature (see Heat).

Tension (see Strain).

Termites: fertility, I, 583, II, 493;
late development of sexual or-
gans, I, 680; nutrition and differ-
entiation of forms, I, 681.

Tetramorium, utilization of aphides
by, I, 660-1.

Thallophyta: size, I, 138, 139; low
co-ordination of parts, I, 164;
pseudo-foliar, II, 28; "transition
place," II, 30; simulation of
higher types, II, 32; secondary
thickening in extinct species, II,
56; sexual and asexual genesis, II,
84. (See also Algce.)

Tickling, physiology of, I, 76.

Tide (see Sea).

Time, as a factor in growth, II, 77.

SUBJECT-INDEX.

661

Tissue, Hertwig's classification, I,
189. (See Physiology.)

Tongue, perceptiveness of tip, I,
600-8, 005, 072-3.

Tortoise: contrasted life of dog
and, I, 103-4; natural selection
and carapace, I, 534.

" Transcendental Physiology," I,
170.

Tree, as symbolizing phylogeny, I,
428, 452-3. (See Plants.)

Trcmatoda: agamogenesis, I, 277;
parasitism, I, 428; alternate gen-
eration, I, 592.

Trembley, A., on the polyp, I, 223.

Trichinosis, in Germany, I, 428.

Trochophore, phyletic relationships
shown by, I, 447, II, 10S-9.

Tubicolw: development, II, 100; bi-
lateral symmetry, II, 197.

Tunicata: gemmation, I, 588, II, 445;
alternate generation, I, 592; inte-
gration, II, 93-4; tertiary aggre-
gation, II, 124; symmetry, II,
194-5.

Tunny, size of ova and adult, I,
144.

Turbcllaria: segmentation, II, 102;
symbiosis, II, 400.

Turnip: chlorophyll in roots, I, 209,
II, 254; vascular system, II, 281,
284, 578, 591, 590.

Twins: similarity of, I, .324; traits
of women bearing, II, 457.

" Types, persistent," Huxley on, I,
408.

Ulcer, dermal structure, II, 306.

Ultimate Reality, incomprehensibil-
ity of, I, 120.

Ulva: cell multiplication, II, 20;
outer tissue, II, 250.

Umbelliferw: floral symmetry, II,
171; axial and foliar organs, II,
541-0.

United States: cases of telegony, I,
644-5; birth rate, II, 520.

Units: differentiation and dissimi-
larity, I, 20; " protyle," I, 22-3:
shapes in higher types, I, 104;
differential assimilation, I, 180;
primordial organic, I, 181; mor-
phological composition, I, 184-7,
194, 252, II, 5, 7-9, 21, 79, 85-0;

segregation and organic repair, I,
221-2, 222-0; chemical, morpho-
logical, and physiological, 1, 225-0;
II, 012; stability, I, 339; instabil-
ity and heterogeneity of organic,

I, 350; Darwin's gemmules, I,
350-60, 302, 372; Weissmann's
germ-plasm (q. v.) ib.; sociological
comparison, I, 303-8; specific pro-
clivities in embryogeny, I, 458;
phsenogamic, II, 73, 151; annulose,

II, 105; incident force and ho-
mologous, II, 159; morphological
summary, II, 233. (See also Phy-
siological units.)

" Universal Postulate," I, 075.
Unsymmetrical, definition, II, 131.
Urea, muscular energy and excre-
tion, I, 72.

Van Beneden, P. J., on Twnia, Hi
103.

Variation: digital, I, 331; effects of
parental conditions, I, 324; of al-
tered function, I, 325, 334, 693;
dissimilarity of initial conditions,

I, 327-32, 333; " spontaneous," I,
328, 513, C97, II, 529; persistence
of force, I, 335; physiological
units, I, 348-54, 300, 3G9, 371-3,

II, 014-7, 022-3; Weismann's
germ-plasm theory, I, 357-8,
372-3, 071, 077; II, 022; equilibra-
tion and vegetal, I, 523-5; Weis-
mann's panmixia theory, I, 501-3,
649, 667-9, 671, 685; reproductive
organs, I, 570; natural selection
and concomitant, I, 014-21, 053,
004, 074, 692; and disused organs,
I, 048, 008; plus and minus, I,
007, 085; Masters on correlated,
in plants, II, 298, 621-2; equili-
bration of favourable, II, 394.

Vascular System: effects of vegeto-
alkalies, I, 55; nutrition, I, 146,
148; embryonic development, I,
169; structural traits, I, 192, 193;
function, I, 199; of Ascidians, I,
202; functional differentiation and
integration, I, 205-0; organic re-
pair, I, 217, 221-2; effect of func-
tion, I, 229, 234-5, 230; equilibra-
tion, I, 535; community in com-
pound organisms, I, 588; develop-

SUBJECT-INDEX.

ment of vegetal, II, 273-5, 279-84,
285-8, 388; differentiation of, sum-
mary, II, 288-90, 388; differentia-
tion of animal, II, 339-44; osseous
development, II, 347-51; muscu-
larity, il, 3G4; muscular colour,
II, 365-9; heart-motor apparatus,
II, 374; differentiation and inte-
gration in animal, II, 376-9, 383;
wood formation, II, 567-92; re-
sume of wood formation, II, 592-7.

Vauchcria, reproduction, I, 279, 289.

Vegetative System, co-ordination of
actions in, I, 578.

Vegeto-alkalies, physiological ef-
fects of, I, 54-5.

Velocity, of moving bodies, II, 219-
20.

Vcrtebrata: size, I, 139; size at
birth and maturity, I, 144; axial
structure, I, 165; embryonic de-
velopment and self-mobility, I,
175; functional differentiation, I,
206, 591; reparative power, I, 219,
223, 589; homogenesis universal,

I, 271; distinctive traits, I, 392,

II, 35; distribution in time, I,
408; classificatory value, I, 446;
embryonic mammalian respiratory
system, I, 456; embryological pre-
adaptation, I, 461; evolution and
vertebral column, I, 470; rudimen-
tary organs, I, 473; evolution and
varied media, I, 479-85; size of
head and vertebrae, I, 512, 537;
segregation and evolution of
vertebrae, I, 515; fertility and
development, I, 583, 598-9; Weis-
mann on reproductive cells, I,
635; limb locomotion, II, 15; adap-
tive segmentation, II, 117-23,
125-7, 223, 602, 605-7; supernu-
merary vertebrae, II, 123; bilat-
eral symmetry, II, 203-6; internal
organic symmetry, II, 208; gene-
sis of rudimentary axis, II, 212-6;
natural selection and genesis of
structure, II, 216, 227; origin of
notochord, II, 216-8; spinal seg-
mentation, II, 218-22, 224; skull
development, II, 222, 227; resume
of axis development, II, 224;
Cope on author's theory, 11,225-7;
nerve differentiation, II, 304; sen-

sory organs, II, 318; air chambers,
II, 334; osseous differentiation, II,
344-55; activity and muscular col-
our, II, 365-9; beart-motor appa-
ratus, II, 374; cost of genesis, II,
436; agamogenesis unknown, II,
445; growth and genesis, II, 454;
heat expenditure and genesis, II,
468-9, 474; Owen, theory of skele-
ton, II, 548-66; evolution of verte-
brae, II, 563-6; origin of type, II,
598-600.

Vestiges of Creation, I, 491.

Vibrissas, function of, I, 75.

Vitalism, hypothesis examined, I,
114-7.

Vittadini, C, on silkworm disease,
I, 622-3.

Viviparous genesis, I, 271, 274-5,
278.

Voice, correlated sexual traits, I,
371-2.

Volcano, definition of life and, I,
85, 89.

Volvocincw: unicentral development,
I, 163; individuality, I, 245; disin-
tegration of genesis, I, 276, 587;
spherical aggregation, II, 24; sym-
.metry, II, 137, 187; fertility, II,
441.

Vomiting, alimentary canal devel-
opment, II, 328.

Vorticella: secondary aggregate, II,
90; symmetry, II, 188.

Wallace, A. R.: "The Origin of
the Human Races," I, 553; the ex-
pression " Survival of the Fit-
test," I, 530; his association of
natural with artificial selection, I,
609; co-adaptation in giraffe, I,
615; skin sensitiveness, I, 646.

Wasp: co-ordination of instincts in
Mason-, I, 574, 679-80; genesis of
worker, I, 654-7.

Waste, animal, I, 69, 213-5, 228; re-
lation to activity, I, 196, 220-1; in
plants, I, 213, 220.

Water: properties, I, 7, 9; colloidal
affinity for, I, 28; organic change
from, I, 29; organic need for, I,
147; proportion in mammalian
adult and foetus, I, 154; motion
through, I, 156; organic develop-

SUBJECT-INDEX.

663

ment and environment, I, 173,
177, 479; terrestrial organisms in-
habiting, I, 400; adaptation of or-
ganisms to change of media, I,
470-85; vegetal tissue differentia-
tion, II, 253; molecular rear-
rangement, II, 359; colloidal con-
traction, II, 3G1-2.

Water-weed, American, invasion of,
I, 399.

Watts, Dr., on The Principles of Bi-
ology, I, ix.

Wax, foliar deposit, II, 260-1.

Weber, on tactual discriminative-
ness, I, 602.

Weight: relation to environment of
organic, I, 174, 177; varying as
cube of dimensions, I, 151, II,
434, 470.

Weismann, Aug.: reproductive tis-
sue in Medusw, 1, 281; in Daphnidw,
I, 290; his theory of the differ-
entiated germ-plasm and its fun-
damental units, I, 357, 622-3,
628-30, 633-44, 646, II, 618-9, 622;
the alleged differentiation and
plant-phenomena, I, 359-60; and
regenerative processes, I, 360;
false joints, I, 362; implied com-
plexity of determinants, I, 370;
theory inadequate to explain cor-
relation of sexual traits, I, 372;
and variations in peacock's tail
feather, I, 372-3, 695, II, 618; his
view of natural selection as sole
factor in organic evolution, I, 559;
the doctrine of panmixia, I, 561-3,
612, 632, 649, 667-9, 671, 685,
689; arguments against inherit-
ance of acquired characters, I,
612-3, 651-65, 669-71; blindness of
cave-animals, I, 613; current ac-
ceptance of his views, I, 631,
690; cannot explain the process
of natural selection, I, 651; the
degradation of the little toe in
man, I, 652, 669, 673; caste gra-

dations of social insects, I, 654,
658-65, 670, 675, 678-84, 685; food-
seeking instinct in Amazon ants,
I, 660, 670; the co-adaptation of
co-operative parts, I, 663-4, 670,
674, 675, 676; tactual discrimina-
tiveness, I, 665, 672; intra-selec-
tion, I, 676-8; effect of nutrition
on fertility of blow-fly, I, 678-9.

Whale: weight of brain, I, 599;
rudimentary limbs, I, 668-9, 685,
693.

Wheat, adaptive variations, II, 298.

Whistling, definition of life and, I,
112.

White-Cooper, Mr., on inheritance
of abnormal vision, I, 306.

Willow, nutrition and growth, I,
294.

Wilson, E. B. : composition of chro-
matin, I, 260; separation of seg-
mentation spheres of Amphyoxus
ovum, I, 691.

Wind: and vegetal bilateral sym-
metry, II, 142; and inner vegetal
tissue differentiation, II, 275-9,
285, 288,^388; and proliferation of
Bryophyllum, II, 295; and vegetal
sap movement, II, 583, 584, 587;
resume, 592-6.

Wolff, C. : vegetal fructification and
nutrition, I, 283, II, 179-80; vege-
tal vascular system, II, 283.

Women (see Man).

Wood (see Plants).

Yeast: fermentation, I, 38; fertil-
ity, I, 581, II, 440; linear aggre-
gation, I, 587, II, 23.

Zebra marks in horses, I, 314.
Zoology, classification, I, 124-5,

380-9.
Zoophytes, structural indefinite-

ness, I, 173.
Zoospores, unit-life of, I, 185.
Zygote, of conjugating Algce, I, 283.

THE END.

cmumza o^u s . ArK 3 uian

QH' Spencer, Herbert

307 The principles of biology

S7

1901

v.2

Biological

& Medical

)

PLEASE DO NOT REMOVE
CARDS OR SLIPS FROM THIS POCKET

UNIVERSITY OF TORONTO LIBRARY