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A THEORY OF

DEVELOPMENT AND HEREDITY

GORI) 44

UEIVE RED EY

OF

DEVELOPMENT AND HEREDITY

BY

HENRY B. ORR, Pu.D.

-PROFESSOR AT THE TULANE UNIvERsITY OF LOUISIANA

“Biology must put forward a conception of life applicable
to all its stages, from the organic process of nutrition in its
simplest forms to the most ideal process of feeling or
thought.” — HaraALD H6FFDING, Outlines of Psychology,
p. 25.

New Work
MACMILLAN AND CO.

AND LONDON
1893
Ali rights reserved

Wh

ijatens
YVICEENT LU
YEARE LA

.

A. 5330

CopyRIGHT, 1893,

By MACMILLAN AND CO.

Norwood Jress :
J. S. Cushing & Co.— Berwick & Smith.
Boston, Mass., U.S.A.

PREFACE.

In presenting in so small a volume a subject of
such broad scope, and of such great importance as
must pertain to a theory of development and hered-
ity,-I have been actuated by several reasons. The
theory is based upon the broadest generalisations of
natural science, and accordingly it rests upon facts
which may be grouped together in great classes. It
is necessary to mention only one fact of each class
in order to suggest a host of similar familiar facts.
As the facts and generalisations are so well known,
I have thought it sufficient to give them briefly and
only a few examples in each class. At this time,
after the critical and minute investigation to which
biological phenomena have been subjected for the
last thirty years, it is not to be expected that we
should suddenly discover some new and recondite
fact, far removed from ordinary observation, which
should revolutionise all our views in regard to biolog-
ical phenomena. In announcing, therefore, a theory
of development and heredity, I do not pretend to
have made any such discovery. But I believe that,

v

vi PREFACE.

by a critical review of the facts of biology in the
light of the great conclusions derived from the allied
sciences of physics and psychology, we may obtain a
view of the great phenomena of life which shall bring
into harmony a more extensive range of facts, and
explain intelligibly relations which have hitherto
been hidden. In the following pages I have at-
tempted to bring together a number of facts and
conclusions which have not generally been supposed
to be related. The success which has attended the
effort to find in this way explanations for phenomena
which were before inexplicable, has convinced me
that the theory which has forced itself upon me is
a real advance in the right direction. Rather than
attempt to perfect the theory before publishing it, I
desire to put it in a form that it may be discussed,
so that what is good in the theory may become of
general use; and, in adapting anything for general
use, many heads are better than one. Then, too, the
experimental tests of the theory can be carried out
by many better than by one,—if the theory shall
fortunately arouse so much interest.

CONTENTS.

CHAPTER I.

Evolution. — Limits of Natural Selection. — Statement of
the Problem to be Solved .

CHAPTER II.

Source of Organic Energy. — Constitution of Matter. —
Action of Forces upon Living Matter

CHAPTER III.

Proof of the Action of Forces of the Environment upon
Organisms. — Effect of Changes of Environment

CHAPTER IV.

Action of the Nervous System intermediate between the
Forces of the Environment and the Ultimate Effects
in the Organism. — Universality of Nervous Activity,
or the Nervous Property in Living Matter

CHAPTER V.

Nervous Activity and also Development dependent on
Association and Repetition
vii

PAGE

25

41

59

79

viii CONTENTS.

CHAPTER VI. .

PAGE
Theory of Development and Heredity illustrated among
Protozoa. : z 5 A : : . . 102

CHAPTER VII.

Application of the Theory to Multicellular Organisms, and
the Relation of the Germ-cells to the Body : . 120

CHAPTER VIII.

The Inherited Impulse of Development, or Tendency of
Growth : ‘ ‘ : . ; : » 135

CHAPTER IX.

Illustration of the Method of Growth and Development,

and of the Origin of Variations . , 3 ‘ . 146
CHAPTER X.

Correlation of Growth : : ‘i F : . - Vil
CHAPTER XI.

Dimorphism and Polymorphism in Species . 5 - 183

CHAPTER XII.

Degeneration and Laws of Variation . ‘ ‘ - 198

CONTENTS. ix

CHAPTER XIII.

PAGE
Embryonic Organs. — Metamorphosis and Alternation of
Generations ‘ ; : i ‘ : . 207

CHAPTER XIV.

Origin and Significance of Sex . i : . : + 226

CHAPTER XV.

Conditions of Development. — Death. — Conclusion . 237

DEVELOPMENT AND HEREDITY.

CHAPTER I.

EVOLUTION. — LIMITS OF NATURAL SELECTION, —
STATEMENT OF THE PROBLEM TO BE SOLVED.

THE present state of the theory of organic evolu-
tion may best be understood by a brief reference to
its history. Lamarck, the first to propose a theory
of evolution similar in scope to the present theory,
believed that effort, or the striving of an animal
after a certain thing, influenced its growth toward
the more ready attainment of that end. Thus use
of organs caused their further growth and develop-
ment, and disuse caused their degeneration. Every
change of environment necessitated a change in
the manner of activity, thus entailing changes in
the organism, which were cumulatively transmitted
to each succeeding generation, until finally two
widely separated generations in the same line of
descent bore no resemblance to each other. In

this way Lamarck supposed the diversity of animals
1

2 DEVELOPMENT AND HEREDITY.

to have been caused. He applied his conception
inaccurately, and it was generally found to be too
limited or too indefinite to be intelligibly applicable
to the known facts. It is worthy of remark, however,
that Lamarck’s theory was one which attempted
to explain the origin of variations, and though,
as such, it has been abandoned, yet it has never
been supplanted. Darwin based his theory on the
fact of variation, supposing it to be universal, and
not at first attempting to account for it further than
to point. out that in some unknown way it was made
more pronounced by a change of environment. The
great strength of Darwin’s theory was “xatural
selection”; that is, the universal struggle for exist-
ence, and the survival of those individuals whose
“fortuitous ’’ variations made them fittest for their
environment. Later in his study of variations,
Darwin adopted to some extent a modification of
Lamarck’s view of the origin of variations. Thus
the use and disuse of organs was admitted as a
factor of evolution, and it was thought there was
some ground for believing that all variation was
due in some way to changing environment. Pro-
fessor W. K. Brooks embodied this view in his
mechanical theory of heredity,! and the idea obtained
largely among other students of the subject, that

1 Heredity, \V. IX. Brooks, Baltimore, 1883.

LIMITS OF NATURAL SELECTION. 3

there was a causal relation between environment
and useful variations. That this causal relation
existed to a limited degree has been generally recog-
nised in the accepted theory of the growth or
degeneration of organs through use or disuse, the
effects of which were supposed to be inherited.
Also, the acquired habits of animals were supposed
to become the instincts of their later posterity.
The reaction of an animal to the influences of its
environment was thus supposed to produce: certain
changes in the animal, — changes which were called
acquired characters, as distinguished from hereditary
characters, and these acquired characters were
supposed to be inherited by the posterity of the
animal, and thus become hereditary characters.
This theory suggests the only clue to a relation
between environment and useful variations and
development. The biologists who have adhered to
these views have been dubbed the “ Neo-Lamarck-
ian School,” while differing from them are others
bearing the name of ‘‘ Neo-Darwinian School.”

In these two schools we have represented the
tendencies of the two conceptions of evolution which
have existed since Darwin’s time. The Neo-Dar-
winists have held that natural selection—the
potency of which was universally recognised — was
alone sufficient to account for all the phenomena

4 DEVELOPMENT AND HEREDITY.

of evolution. According to them, in the severe
struggle for existence, every slightest variation must
tell; no matter how slight, it must be either useful
or detrimental to the individual. Even the further
degeneration of an already useless and degenerate
organ is held to be advantageous as a saving of
energy, which will help the individual in the struggle
for existence. So acute is the action of natural
selection that each slight degeneration of the eye
of cave-dwelling animals is a useful variation — an
economy of energy —which is acted upon by nat-
ural selection. Also the degeneration of parasites
in the flesh of their hosts— where food is super-
abundant, and where warfare with fellow-parasites
does not occur—is also brought about by natural
selection for the purpose of saving energy, even
though energy in the shape of food be so abundant.
According to this view, while it is not denied that
environment may cause variation, it is not thought
that environment can in any way determine the
variation for the advantage of the _ individual,
which means that it is equally liable to be detri-
mental: and thus the whole matter of organic
evolution resolves itself into the one factor of
natural selection. Indeed, Professor Weismann !
has expressed the opinion that the keenest action

1 Essays upon Heredity. Authorised trans., Oxford, 1889.

LIMITS OF NATURAL SELECTION. 5

of natural selection is necessary to keep animals
from degenerating —thus apparently implying that
variations are even more likely to be detrimental
than useful.

In connection with this belief in the sole efficacy
of natural selection, the old theory of the inheritance
of acquired characters has been abandoned, and in
its place has been substituted Professor Weismann’s
theory of heredity, according to which the inheri-
tance of acquired characters is absolutely impossible.
This theory may be briefly stated as follows: In
the germ-cells of animals there is a certain amount of
“germ-plasm,” a substance of very complex physico-
chemical composition, of which one part gives rise
to the development of the incipient animal and is
thereby used up; but the other part remains in
the body of the developing animal, unchanged in
composition but increasing in quantity. This un-
changed part then forms the germ-plasm from which
the next succeeding generation develops and receives
its share of the unchanged germ-plasm. The same
process is repeated in all generations, one part of
the germ-plasm being used up to produce the bodies
of each generation, the other part of the germ-
plasm remaining forever unchanged. As all the
generations arise from the same germ-plasm, they
are therefore alike, with this exception (which is

6 DEVELOPMENT AND HEREDITY.

omitted above for simplicity’s sake), that each ani-
mal arises from the fusion of the germ-plasms of
two individuals, and this fused germ-plasm from
which the animal grows and which he transmits,
is slightly different from either original germ-plasm
—being a cross between the two. Therefore the
animal differs slightly from either parent. This is
the origin of variation. The union of the two
germ-plasms determines all the characters of the
animal which is to arise from it, and all the ten-
dencies which he will transmit to his offspring.
Whatever happens to the animal during his life-
time cannot change these tendencies; for they
are inherent in the germ-plasm, and the composi-
tion of the germ-plasm became fixed by the union
of the two germ-plasms of the parents. Thus the
transmission of acquired characters is impossible.
The fundamental truths on which this theory is
based are too well known to need repetition. The
theory itself is at once seen to involve the most
extreme phase of the natural selection theory, dis-
claiming everything like a causal relation between
environment and favourable variation; and necessi-
tating the action of natural selection on the most
minute variations, as, for instance, those necessary
to produce the human eye. There is another
remarkable part of the theory of variation which-

LIMITS OF NATURAL SELECTION. 7

Professor Weismann sets forth. It seems that
among the Protozoa the single cells were changed
by the influences of different environment, and
when each cell divided itself into two or more parts,
the changes were naturally propagated in the dif-
ferent parts. Thus arose a diversity among the
Protozoa. When some of the Protozoa became
Metazoa, this diversity continued and expressed
itself as differences in the germ-plasms of the higher
group. From this point onward variation depended
alone on the mixture of two different germ-plasms,
because the germ-cells were hidden from the influ-
ence of the environment, and only the body-cells
were exposed to it.

This theory of heredity increases the difficulty
in explaining evolution, as it necessitates an expla-
nation by natural selection alone: in addition, the
theory seems to have its own weak points. In the
first place, our knowledge of the laws of cross-
breeding makes it seem certain that when other
conditions remain unchanged, the general result of
crossing two dissimilar animals is an animal inter-
mediate in form between the two parents. If this
be a general law, then the original differences of
the Metazoa would tend to disappear, for each
generation would contain only forms intermediate
to the forms of the preceding generation, Thus

8 DEVELOPMENT AND HEREDITY.

variations would become ever smaller and smaller
and eventually disappear. If the Metazoa separated
into groups, the same process would occur in each
group. Artificial selection has proven that where
only similar animals are allowed to interbreed, the
stock is kept pure and variations are not likely to
occur.

Another objection to this theory of heredity —
and it seems to me insuperable —lies in the sup-
position that the germ-plasm may exist in the
body, undoubtedly a living part of it, and still be
no more affected by the changes which pass over
the body, than if it were enclosed in an hermeti-
cally sealed vial. This idea seems to be based on
a peculiar assumption in regard to the individuality
of a cell, as though the neighbouring cells of the
same organism were as distinct from each other
physiologically as they are morphologically ; or that
the cell walls are such firm and impermeable bar-
riers that the molecular condition of one cell
might be changed without affecting its neighbour.
We shall have occasion to consider this subject in
a later chapter.

In accordance with this theory of variation and
heredity, Professor Weismann and his school deny
that the environment causes variations of growth for
the advantage of the organism. They believe in an

LIMITS OF NATURAL SELECTION. 9

indefinite variation, that is, a variation without rela-
tion to the needs of the organism or the demands
of the environment; and consequently they believe
in a natural selection which takes note of the
minutest deviation of growth. To test this idea,
let us consider for a moment the millions of genera-
tions which compose the line of ancestry of the
mammals, and the inconceivable number of millions
of variations that have taken place along the line.
If the struggle for existence be so severe, and natural
selection so relentlessly active and acute as it is
claimed to be, then all these variations must have
been useful, or the mammal’s ancestors would have
died — perhaps as the Devonian fishes or Silurian
invertebrates. Each one of this inconceivable num-
ber of variations must have occurred, not only at the
right period of geological time, but also at the right
moment in the life of the individual in whom it
first appeared: some of them occurred in embryonic
stages, some in adult stages; many of them were
complex and correlated, that is, advantageous only
when accompanied by certain other variations. In
each case there could have been only one chance
for a useful variation against countless chances for
detrimental variations: for an organism is a delicate
piece of machinery, and while there is, perhaps, one
point where a slight change will make it more per-

10 DEVELOPMENT AND HEREDITY.

fect, there are a thousand points where an equally
slight change would make it less perfect. When
we endeavour to unite the conception of this vast
uninterrupted series of useful variations with the
idea that the detrimental variations are just as likely
to occur as the useful ones, we discover that the
two ideas are incompatible. Especially is this the
case when we recognise the fact that it has not
been a single individual nor a single pair which has
survived in each generation, but a species or variety,
composed probably of as many individuals as we
find to-day in a species or variety. In fact, we
would expect it nearly always to have been what
Darwin called a “dominant species.’ When we
remember the many cases of wholesale destruction
of animals,—for instance, the killing of countless
fishes by a sudden change in the temperature of
the ocean, the killing of birds and insects by cold
and storms of wind and rain, and the killing of
myriads of organisms of all kinds by circumstances
over which they could have no control, and from
which no mere individual variation could save them,
—we are led by these facts to doubt that natural
selection acts with such mathematical certainty and
accuracy in accumulating slight individual variations.
It seems more plausible to suppose, since organisms
are generally adapted to their environment, and

LIMITS OF NATURAL SELECTION. il

progressively adapt themselves to changes in the
environment, that therefore environment must, in
some way, be the cause of their variation, and must
determine what that variation shall be! Even could
the theory of the mixture of germ-plasms be shown to
be the only means of causing variations, we would
find that we had made but a single step in advance,
and were again confronted by the same problem as
to why variations occur along certain lines tending
toward the better adaptation of the species.

There is another point of view, however, from
which this whole subject may be regarded. In a
mechanical theory of evolution, natural selection
cannot be regarded as a cause of evolution. It is

1«Tt is plain that the useful additions which have constituted
certain genera, families, orders, etc., what they are, must have been
produced as a consequence of the existence of a need for them; or,
on the other hand, being created first, they must have sought for use
and found it. But what are the relative chances of truth for these
two propositions? In the second case, admitting evolution as proved,
we perceive that an almost infinite chance exists against any usual
amount of variation as observed, producing a structure which shall
be fit to survive, in consequence of its superior adaptation to external
circumstances. It would be incredible that a blind or undirected
variation should not fail, in a vast majority of instances, to produce a
single case of the beautiful adaptation to means and ends which we
see so abundantly around us. The amount of attempt, failure, and
consequent destruction would be preposterously large, and in nowise
consistent with the facts of teleology as we behold them.’’ — PROFESSOR
E. D. Cops, Origin of the Fittest.

12 DEVELOPMENT AND HEREDITY.

merely a negative factor, and can only nullify those
influences (the crossing of useful with detrimental
variations) which would retard or end the career
of development. Upon analysis, it is found that
the actual working of natural selection has been
merely to cut off without posterity all organisms
except the direct ancestors of the present generation.
It may do as a form of popular expression to say
that, because the less fit are killed, therefore the
fittest survive, but, in nature, organisms do not con-
tinue to live decause some others die. The causes of
life and death are totally distinct, and the death of
one organism does not cause the continued life and
development of others. It might, perhaps, be re-
garded as a secondary reason for their not being
other than they are, but it is in no true sense the
cause of their being what they are, any more
than the fact that the earth was not destroyed by
an astral collision a thousand years ago is the cause
of our living to-day in our present state of civilisa-
tion. The development and the civilisation we know
to be each the combined effects of a multitude of
real causes.

Natural selection merely prevents the retardation
of the more developed individuals, by preventing
their crossing with the less developed —the latter
being killed off. Such retardation has, however,

LIMITS OF NATURAL SELECTION. 13

occurred, so that evolution has been slow, and a
great number of individuals have progressed together,
kept abreast of each other by crossing. Marvellous
adaptations have been developed, but they were not
caused by natural selection. Natural selection, like
the stone walls of a labyrinth of lanes and avenues,
shows limits over which evolution cannot go; but
as to why evolution should sweep through all the
intricate paths open to its progress, and how it
effects this progress, natural selection offers no clue.
Natural selection is a barrier to progress in certain
directions, not a cause producing progress. The
cause of this progressive development we must seek
in the properties of living matter and in the forces
which act upon it. For instance, the properties of
living matter which are displayed in the phenomena
of heredity must surely affect in some way the
progressive development of organisms.

The present theories of organic development are
based chiefly upon evidence drawn from the corporeal
structure of plants and animals. Their anatomy
and the minute structure of their parts have been
analysed, compared, and classified, and in this man-
ner their genetic relationship has been made appar-
ent. While this method is sufficient to show their
relationship, and while observations of bre¢ding and
artificial and natural selection show that there is an

14 DEVELOPMENT AND HEREDITY.

evolution among animals and plants, yet there is
still a large class of phenomena in the nervous
activity and psychic life of organisms, which is barely
taken into account in summing up the conclusion
as to the essential nature of evolution. That this
latter class of phenomena should not contradict the
conclusions drawn from the bodily structure, was
apparently all that could be expected from it. But
the vast importance which intelligence, instinct, and
reflex nervous activity have for organisms, demands
for these phenomena a more careful study, —
especially in regard to the light they shed upon the
ultimate nature of living matter, and consequently
upon its developmental activity.

It is really just among the phenomena of this
class that we shall find the clearest proof of the
inheritance of acquired characters, as I shall en-
deavour to show.

From the results obtained by the science of
physiological psychology it is evident that what
we know as psychical development must have its
correlative physical development. Nor must we
forget that even in the higher realms of psychical
life these two terms merely express different views
of the same thing. It has been sometimes supposed
that psychic activity is the antecedent cause of
growth changes in the nervous matter of the brain,

LIMITS OF NATURAL SELECTION. 15

—that the psychic action moved the molecules.
But unless the fundamental conclusion of physiolog-
ical psychology be wrong, psychic action is simul-
taneous and co-existent with change in the material
particles of the brain. Thus we may consider the
one as indicative of the other.

According to this principle, an animal which for
a lifetime has been meeting with new experiences,
new conditions, and new demands upon its ingenuity,
and acquiring new habits, must have at the end
a more developed and more complicated nervous
structure than it had at the beginning of its life.
In such an animal there have arisen new nervous
co-ordinations, new actions are possible, its dis-
charges of nervous force are more concentrated and
efficient, and the purely reflex mechanical actions
are performed in a more perfect way. The ner-
vous mechanism of the animal has been improved,
and the brain, the material substratum of its psychic
life, has been changed in structure. Moreover, we
know that this development, viewed as a physical
change, must have occurred according to the laws
of matter in motion and the law of the conservation
of energy. The material change, the new arrange-
ment of the molecules or atoms, is the effect caused
by the action of the stimuli or forces of the en-
vironment upon the animal. Were these same

16 DEVELOPMENT AND HEREDITY.

stimuli withheld from a growing animal, the same
development would fail to occur. In this case,
therefore, although we are unable to analyse the
details of the process, we have, without doubt, a
certain amount of development directly caused by
forces of the environment acting upon the animal.
But the amount of development which we can
ascribe to causes acting thus directly from the
environment, seems small when compared with that
which takes place during the embryonic period of
the life of an animal. At first sight, this latter
appears to be of an entirely different nature. It
seems to be spontaneous. But we know that in the
embryonic life, as well as in later life, the changes
in material structure are only effects caused
according to physical laws: therefore the nature
of embryonic development cannot differ from that
of the adult, except in the origin of the forces which
cause the activity. In each case, the movements of
the particles in building up the organism must
proceed according to physical and chemical laws.
We cannot follow the different steps of devel-
opment of the nervous system in the embryo by
observing the correlative psychical activity; but
when in the normal embryological development of
an animal we see marked physical changes taking
place in the brain, must we not infer that they

LIMITS OF NATURAL SELECTION. 17

have their orderly psychical correlatives? We see
the brain going through successive changes of
shape and structure until it arrives at such a
stage in the new-born animal that it is capable of
directing highly complex instinctive actions. At
what point in this succession of changes does the
psychical element appear? can we imagine it as
suddenly arising in the brain as the new-born creat-
ure draws its first breath? If instinct be correlated
with certain material structures of the brain, as
we have reason to believe, can we suppose that
those structures have been produced by a method
of development totally different from that of all
the other organs. The general course of develop-
ment of any organ is from the simple to the com-
plex; and the development of the organ in the
individual is a repetition of the development of the
organ in the race. Thus it seems probable that
a bird, which, at one period of its embryonic devel-
opment, with its gill-clefts and unjointed rudiment-
ary limbs, so strongly resembles a fish, should
also have passed through a stage of brain devel-
opment or a potential psychical condition some-
what similar to that stage at which the fishes have
remained. So far as may be judged by the appear-
ance of the brain, we may conclude that such a
stage has been passed through by the bird, for

18 DEVELOPMENT AND HEREDITY.

there is a period in the growth of the bird’s brain
when there are nervous fibres in the base of the
brain while it is roofed over merely by a cellular
membrane, —thus reproducing a condition of the
brain which is characteristic of the majority of
fishes throughout their lives. The embryonic devel-
opment of the vertebrate brain corresponds closely
to what the study of morphology has shown to be
its probable phylogenetic development.

But whether we can or cannot infer the psychical
development of the embryo from its physical develop-
ment, we can at least compare the psychical result
of the embryonic development with the psychical
result of the later development. And from the
similarity of the two results we can infer a similarity
In the processes which produced them. As the
result of the embryonic development we have in-
stinct; and as the result of the development which
takes place in later life we have that intelligence
which is derived from perception and experience.
There is such a close and constant interaction be-
tween instinct and intelligence, that the material
changes accompanying the two phases of psychic
activity must often form parts of an uninterrupted
series, as closely united as are in us the brain-
changes accompanying an unbroken train of thought.
An action may have an instinct for its main cause,

LIMITS OF NATURAL SELECTION. 19

and yet, throughout its whole performance, may
display intelligence in its details. It has been ob-
served that young birds do not build their nests as
perfectly as older birds. The later improvement in
building such a complicated structure as a bird’s
nest cannot be due wholly to the experience —in
the ordinary sense of the word — of an animal which
has not sufficient intelligence to loosen a slip-knot
tied around its leg. The old bird has, however,
acquired a greater general intelligence, which enables
its instinct to find more perfect expression. We
cannot doubt that it is instinct which determines the
general plan of a bird’s nest, its architecture and
the materials used; and yet the selection of a par-
ticular site for building, and the adaptation of the
structure tothe peculiar necessities of the location,
we must believe to depend on intelligence. For the
instincts of last year’s birds could not have been so
perfectly adapted to the requirements of this year’s
branches, as to account for the adaptation of nest to
branch, that we find existing. If the entire matter of
selecting a site and building a nest were dependent
purely on instinct, it would be impossible to conceive
how each pair of birds found the particular shape
of branch demanded by the separate instincts of each
one of them; and many birds would, for this reason,
die without posterity. We must believe that intelli-

20 DEVELOPMENT AND HEREDITY.

gence comes to the aid of instinct ; and thus instinct
and intelligence are most intimately connected.

The observation of the nesting of birds shows
further how closely instinct resembles the result of
intelligent experience. The common American black-
bird arrives in the north in the cold season, before
the deciduous trees have put out their foliage, and
accordingly the blackbirds frequent the pines,
spruces, balsams, etc., enjoying their shelter for
nests, and their dry twigs for building material.
The robin arrives in the north during the wet and
muddy weeks of spring, and builds its nest largely
of mud. The bobolink comes when the meadow-
grass is high, and finds security for its nest under
the tall grasses. The yellow-bird arrives when the
willows are scattering their downy seeds, and uses
these and other cotton-like material for its nest.
The several methods of the nesting habits of these
birds are so invariable, that they may, without hesita-
tion, be classed as instinctive, and yet we can hardly
conceive of this remarkable adaptation to circum-
stances as having originated in any other way than
by experience and intelligent choice of that which was
the most suitable. If it were due to an undirected
variation, we ought to find some trace of such varia-
tion; but we do not hear of such cases as blackbirds,
robins, and bobolinks failing to rear their young

LIMITS OF NATURAL SELECTION. 21

because the first two nested on the ground, and the
last ina pine tree. It seems to be very seldom the
case that birds lose their young by mistakes in
location or manner of building, such as could have
been guarded against by the foresight of such intelli-
gence as a bird may be supposed to possess. Yet
it is exactly such odd and unintelligible mistakes
which are demanded by the theory of undirected
variation and selection, in order to produce the nest-
ing instincts. On the other hand, in the change of
the nesting habits of the chimney-martins and barn-
swallows, from hollow trees and rocks, to chim-
neys and barns, we find two cases of change of
instinct which there is no ground for assuming to
be the result of the general destruction of a hypo-
thetical number of birds which refused to make the
change. That the kea-bird of New Zealand has
learned to dig the kidney fat out of living sheep
since the introduction of sheep into that country,
is another wonderful instance of change of instinct
hardly to be accounted for by means of natural
selection, but rather as the result of intelligent
experience.

If we could conceive of a species as having one
long uninterrupted mental existence, in which gen-
eration after generation of individuals replace each
other in some way without interrupting the mental

22 DEVELOPMENT AND HEREDITY.

development of the species, as the material parti-
cles of the brain replace each other without inter-
rupting the mental unity of the individual, then the
difficulty as to the origin of instinct would van-
ish, and instead of what we call the instincts of
the present generation we would recognise only
the accumulated experience of past generations of
ancestors, naturally adapted and made use of by
the intelligence which acquired it.

We know that the material structures of the
brain, which are the correlates of instinct, are repro-
duced by the power of heredity. The question arises,
can we conceive of animal reproduction as reproduc-
ing those material changes of the brain which are
produced by contact with environment and which
are the physical presentments of what we know as
experience? The general tendency of living things
to reproduce themselves not only in general form,
but also in minute details, is too well known to need
a repetition of its proofs; and since instinct and
intelligence are so closely interwoven in the psy-
chic life of animals, why should not the structural
changes correlated with intelligent action be repro-
duced by the process of inheritance as well as those
material conditions and changes correlated with
instinct? For, as we have seen, we have reason to
believe that the embryonic development of instinct

LIMITS OF NATURAL SELECTION. 23

in the individual is a gradual process, and the acqui-
sition of experience is likewise a gradual process,—
occurring later in the same organ.

Viewing just the plain facts as they stand before
us, so striking is their relation to one another that we
cannot escape the conviction that an instinct is an
inherited idea—an inherited plan of action. We
see in it the inheritance of knowledge which has
been acquired by ancestors. From the physical
point of view we see in it the inheritance of struct-
ural changes which were caused in the nervous sys-
tem by the action of the forces of the environment
upon each individual ancestor. This seems to be
a well-defined case of the inheritance of acquired
characters. We cannot deny that the idea or knowl-
edge has been acquired, neither can we deny that it
has been inherited. To refuse this plain inference
and then to appeal to natural selection as explanation
of the evolution of instincts, is to argue wide of the
mark; for we must believe that knowledge is the
direct result of an intelligent creature’s contact with
its environment, and not the result of any fortu-
itous happenings or undirected variations inside the
animal.

The problem to be solved is: how are changes
brought about in the structure of organisms, and how
are these changes transmitted to succeeding genera-

24 DEVELOPMENT AND HEREDITY.

tions? In attempting a solution of this problem, we
must inquire how matter is affected by the forces
which act upon it, and especially how living matter
is affected by the forces of its environment. We
must study the nature of living matter and the
source and transformations of its energy. Finally,
we must consider the natural operation of this pecu-
liar form of matter under the action of the forces
of the environment.

CHAPTER II.

SOURCE OF ORGANIC ENERGY. — CONSTITUTION OF
MATTER. — ACTION OF FORCES UPON LIVING MAT-
TER.

In considering the problems of living matter, it
must never be forgotten that we are studying but
a subdivision of science, and that the limited range
of phenomena which we have before us forms but
a portion arbitrarily selected from the great body
of natural phenomena. From this great body of
phenomena have been drawn certain fundamental
generalisations which by a vast preponderance of
evidence are shown to be universally applicable as
laws. We must therefore believe that biological
phenomena, as well as all others, are governed by
these laws, unless it can be definitely proven that
biological phenomena are excepted from their general
application.

Biology is bordered on one side by the science of
physics, and on the other side by psychology; so

that the laws discovered in these two branches of
25

26 DEVELOPMENT AND HEREDITY.

science cannot be neglected in biological research.
The law of the conservation of energy which is fun-
damental in the science of physics, when analysed in
detail in its application to the phenomena of living
matter, has a most extensive import and profound
significance. It means that whatever forces of growth
and development may be displayed by living organ-
isms, these forces must have their sufficient physical
causes. We can only regard them as transforma-
tions of the general sum of energy. That any con-
ceivable part of the energy now displayed by living
matter in its various forms should have originally
resided in the ancestral forms of life, is unimagin-
able. We have no ground to suppose that proto-
plasm in its lowest form, and at its first appearance
on the earth, possessed any powers except those
which are found to be its universal properties to-day,
— just as the amceba now does not contain in itself
the forces necessary to produce a mammal. So long
as nourishment was supplied, and the proper chem-
ical changes kept up in this original living matter,
so long would life continue ; but all changes in shape,
and all changes in the method of activity, must have
been the result of additional force from without.
Therefore if we are to account for the various forces
displayed by living organisms, either in their indi-
vidual activity and growth or in their racial devel-

SOURCE OF ORGANIC ENERGY. 27

opment, and at the same time regard the law of the
conservation of energy, then we must believe that
those forces have their origin in the environment.
The form of organic matter, z.¢. the shape of an
organism, is dependent on the forces from the out-
side acting on the organic matter, meaning by
forces from the outside every transfer of energy to
the organism, as by heat, light, food, blows, pres-
sure, gravity, etc.

It is a distinguishing characteristic of living matter
that it undergoes perpetual changes. The changes
in living matter are of two kinds: first, chemical
changes in the composition and constitution of the
molecules ; and, second, changes consisting in the mo-
tions of molecules and masses, such as heat, contrac-
tion of parts, circulation, locomotion, etc. The chemical
changes follow the laws of chemistry which univer-
sally prevail when different elements and compounds
are brought into contact. The changes of the mo-
tion of molecules and masses must, in like manner,
follow the laws of dynamics. These latter changes
are of an exceedingly complex character. They
include the acquiring of nutrition, its transportation
to different parts of the body, and deposition at cer-
tain points, and all the phenomena of growth and
the successive changes of growth which are classi-
fied under the head of development. Each of these

28 DEVELOPMENT AND HEREDITY.

motions of matter requires a certain definite amount
of energy to bring it about; and we find a corre-
sponding source of energy in the chemical changes
which convert the potential energy of nutriment into
kinetic energy. But it is evident that this simple
liberation of energy by chemical action is not of
itself sufficient to account for the ensuing wonder-
fully complex motions of the parts of the organism.
To guide masses of matter through the devious
courses necessary to build up the body of a living
organism, it is necessary not only to have a sufficient
energy of motion, but at certain definite points along
the line that motion must meet with a resistance
sufficient to deflect and guide the mass to its given
destination. The forces of cohesion and gravity
undoubtedly have some effect, but the great com-
plexity of results cannot be attributed to so small a
number of simple constant forces. What, then, is
the source of this energy of resistance? The energy
of the primary motion and the energy of resistance
cannot have the same source, for it is a fundamental
principle that masses set in motion by energy from
a common source tend to pursue a straight line, and
so, moving radially from the centre, they cannot in-
terfere with each other. Therefore, while we ascribe
the primary motion of the masses to the energy set
free by chemical action, we must ascribe the modi-

SOURCE OF ORGANIC ENERGY. 29

fications in the form and nature of that motion to
other sources; namely, the external forces acting
upon the body from various directions.

These two sets of forces, the internal and the ex-
ternal, are not the only factors which determine the
ultimate form of bodies,-—another and very impor-
tant factor is the nature of the substance of which
they are composed, whether it be rigid, or elastic,
solid, liquid, or viscous.

In order to understand the phenomena of living
matter as a display of physical forces, we must take
the most comprehensive view of the action of forces
upon matter in general,—the inanimate as well as
the animate. We must take note not only of those
effects which are made evident by experiment, but
also of those which are rendered probable by deduc-
tion, — remembering that in the apparent dissipation
of a force, even after it has escaped the observation
of our senses and the analysis of our instruments, it
still acts undestroyed—in a state where we can fol-
low it only with our imagination. We must dismiss .
from our minds all ideas of motionless, changeless
matter, and picture to ourselves a universe where
nothing is at rest, where every atom is quivering in
ceaseless vibration, where each minutest particle of
matter is held in its line of motion through space, by
the action of opposing forces, pulled now this way,

30 DEVELOPMENT AND HEREDITY.

then that way, by the resultant action of an almost
infinite number of forces, —forces of attraction,
repulsion, and gravitation, which extend toward us
from the extremest region of the stars and forever
change in form and in the endless complication of
their interaction.

A mass of matter appears to be at rest, because
the forces which act upon its component particles are
in a state of equilibrium. As the forces emanating
from each particle act upon all the other particles,
no particle can be taken away or added without alter-
ing the statical condition of the whole mass and
changing the co-ordination of its form-conserving
forces, so that a new state of equilibrium must
result, in which each particle stands in a new rela-
tion to every other particle. Further, every change
in the external forces which act upon a mass of
matter must cause a change in the equilibrium of the
mass, necessitating either the movement of the mass
as a whole or some change in the internal relations
of its particles. This change in the relation of the
particles may be permanent or it may be temporary.

These principles are fundamental, and they are so
important for our present inquiry that I quote here
a few illustrations of the subtle nature of the changes
produced upon inanimate matter by various forces,
and of the persistence of the effects. The first

SOURCE OF ORGANIC ENERGY. 31

is from Professor W. R. Grove’s essay on the
Correlation of Physical Forces, relating experiments
of M. Niepce de St. Victor : —

“An engraving which has been kept for some
days in the dark is half covered by an opaque
screen, and then exposed to the sun; it is then
removed from the light, the screen taken away,
and the engraving placed opposite, and at a short
distance from photographic paper; an inverted image
of that portion of the engraving which has been
exposed to the sun is reproduced on the photo-
graphic paper, while the part which has been cov-
ered by the screen is not produced. If the engrav-
ing, after exposure, is allowed to remain in contact
with white paper for some hours, and the white paper
is then placed upon photographic paper, a faint
image of the exposed portion of the engraving is re-
produced. Similiar results are produced by mottled
marble exposed to the sun; an invisible tracing on
paper by a fluorescent body, sulphate of quinine, is,
after insolation, reproduced on the photographic
paper. Insolated paper retains the power of pro-
ducing an impression for a long period, if it is
kept in an opaque tube hermetically closed.”

The following is from the same source :—

“As with heat, light, and electricity, the daily
accumulating observations tend to show that each

32 DEVELOPMENT AND HEREDITY.

change in the phenomena to which these names are
given is accompanied by a change, either temporary
or permanent, in the matter affected by them, so
many recent experiments on magnetisation have
connected magnetic phenomena with a molecular
change in the subject-matter. Thus M. Wertheim
has shown that the elasticity of iron and steel is
altered by magnetisation, the coefficient of elasticity
in iron being temporarily, in steel permanently,
diminished.

“ He has also examined the effect of torsion upon
magnetised iron, and concludes from his experiments,
that in a bar of iron, arrived at a state of magnetic
equilibrium, temporary torsion diminishes the mag-
netism, and that the untwisting, or return to its
primitive state, restores the original degree of mag-
netisation.

“M. Guillemin observed that a bar of iron, slightly
curved by its own weight, is straightened by being
magnetised. Mr. Page and Mr. Marrion discovered
that a sound is emitted when iron or steel is rapidly
magnetised or demagnetised, and Mr. Joule found
that a bar of iron is slightly elongated by magnet-
isation.

“Again, with regard to diamagnetic bodies, Mr.
Matteucci found that the mechanical compression of
glass altered the rotary power upon a ray of polarised
light which it transmitted.”

SOURCE OF ORGANIC ENERGY. 33

The following .quotation is from Professor P. G.
Tait’s book on the Properties of Matter. It re-
lates to an experiment in which a vertical wire, fas-
tened above and suspending a heavy weight below,
was first twisted and then allowed of itself to untwist
and return to its original position : —

“We have already seen that even brittle bodies
may be completely changed in form by small but
persistent forces. And there is no doubt that all
elastic recovery in solids is gradual; so that, for in-
stance, in the torsion vibrations which we have just
considered, even when there is no sensible viscous
resistance, the middle point of the range does not
coincide with the original untwisted position of the
wire. It is always shifted toward the side to which
torsion was applied, and to a greater extent the
longer the wire has been kept twisted before being
allowed to vibrate. With every vibration, however, it
creeps slowly back towards the original undisturbed
position, but usually comes to rest before reaching it.
But even after the oscillations have ceased, the wire
still continues to untwist, more and more slowly,
sometimes not even approximately reaching its un-
disturbed position until hours or even days have
passed.

“These phenomena are seen in a more striking
form when we dispense with oscillation. Thus, for

34 DEVELOPMENT AND HEREDITY.

example, suppose the wire to be kept twisted through
go° to the right for six hours, then for half an hour
go” to the left, and then so gradually let go that
there is no oscillation. When it is left to itself, it
turns slowly toward the right, gradually undoing part
of the effect of the more recent twist, then stops,
and twists still more slowly to the left, thus undoing
part of the quasi-permanent effect of the earlier twist.
Thus the behaviour of such a wire, strictly speaking,
is an excessively complex one, depending, as it were,
upon its whole previous history ; though, of course,
the trace left by each stage of its treatment is less
marked, as the date of that stage is more remote.”

Professor J. Clerk Maxwell has expressed this last
idea in almost the same words, “the stress at any
given instant depends not only on the strain at that
instant, but on the previous history of the body.”

From all of this we may learn, first, how slight a
force may change the structure of a body; second,
how complex may be its results; and third, how
subtle and persistent may be the effects of a force
upon a body even though its action be only tempo-
rary. The nature of the changes which various
forces cause in bodies, will be more fully understood
by a careful consideration of the following account
of the constitution of bodies, quoted from Professor
Maxwell’s article on that subject, in the Brztesh
Encyclopedia : —

SOURCE OF ORGANIC ENERGY. 35

“We know that the molecules of all bodies are
in motion. In gases and liquids the motion is
such that there is nothing to prevent any molecule
from passing from any part of the mass to any
other part; but in solids we must suppose that
some, at least, of the molecules merely oscillate
about a certain mean position, so that, if we con-
sider a certain group of molecules, its configuration
is never very different from a certain stable con-
figuration about which it oscillates.

“This will be the case even when the solid is
in a state of strain, provided the amplitude of the
oscillations does not exceed a certain limit; but if
it exceeds this limit the group does not tend to
return to its former configuration, but begins to
oscillate about a new configuration of stability, the
strain in which is either zero, or at least less than
in the original configuration.

“The condition of this breaking up of a configu-
ration must depend partly on the amplitude of the
oscillations, and partly on the amount of strain in
the original configuration; and we may suppose
that different groups of molecules, even in a homo-
geneous solid, are not in similar circumstances in
this respect.

“Thus we may suppose that in a certain number
of groups the ordinary agitation of the molecules

36 DEVELOPMENT AND HEREDITY.

is liable to accumulate so much that every now and
then the configuration of one of the groups breaks
up, and this, whether it is in a state of strain or
not. We may in this case suppose that in every
second a certain proportion of these groups break
up, and assume configurations corresponding to a
strain uniform in all directions.

“Tf all the groups were of this kind, the medium
would be a viscous fluid.

“But we may suppose that there are other groups,
the configuration of which is so stable that they will
not break up under the ordinary agitation of the
molecules, unless the average strain exceeds a cer-
tain limit, and this limit may be different for different
systems of these groups.

“Now if such groups of greater stability are dis-
seminated through the substance in such abundance
as to build up a solid frame work, the substance will
be a solid, which will not be permanently deformed
except by a stress greater than a certain given
stress.

“But if the solid also contains groups of smaller
stability and also groups of the first kind which
break up of themselves, then when a strain is ap-
plied the resistance to it will gradually diminish as
the groups of the first kind break up, and this will
go on till the stress is reduced to that due to the

SOURCE OF ORGANIC ENERGY. 37

more permanent groups. If the body is now left to
itself, it will not at once return to its original form,
but will only do so when the groups of the first kind
have broken up so often as to get back to their
original state of strain.

“This view of the constitution of a solid, as con-
sisting of groups of molecules, some of which are in
different circumstances from others, also helps to
explain the state of the solid after a permanent
deformation has been given to it. In this case,
some of the less stable groups have broken up and
assumed new configurations, but it is quite possible
that others, more stable, may still retain their orig-
inal configurations, so that the form of the body is
determined by the equilibrium between these two
sets of groups; but if, on account of rise of tem-
perature, increase of moisture, violent vibration, or
any other cause, the breaking up of the less stable
groups is facilitated, the more stable groups may
assert their sway, and tend to restore the body to
the shape it had before its deformation.”

According to the theory expressed in the preced-
ing paragraphs, it appears that the effect of a force
acting upon a body is greater in its molecular dis-
placement, and also more permanent, where all the
groups of molecules have a small degree of stability.
Thus the effects would be more permanent in a

38 DEVELOPMENT AND HEREDITY.

viscous body than in a rigid or elastic body. Fur-
thermore, it follows from the theory, that the less
homogeneous the body acted upon by an external
force, the more complicated will be the resulting
changes in the body.

We might, therefore, expect the action of the
various kinds of forces upon such a viscous body as
living matter to be very pronounced and permanent.
Also the results ensuing from the viscous nature of
living matter would be still further intensified by two
conditions. First, living matter is not homogeneous ;
it is formed of a mixture of different compounds rang-
ing in their physical states from solid particles to
liquids. This fact must add greatly to the complex-
ity of the molecular changes. The second condition
is also a source of further complexity, and makes
living matter peculiar among viscous substances; for
not only does living matter change in the configura-
tion of its groups of oscillating molecules, as do other
viscous bodies, but it is also undergoing a constant
chemical change in its molecules, thus increasing its
physical instability by adding thereto a chemical
instability.

Living matter is therefore a substance which is
eminently fitted to respond readily to the forces
acting upon it, and to retain the impression of those
forces. It is in a state of mobile equilibrium, which

SOURCE OF ORGANIC ENERGY. 39

may be compared with a hoop rolling down an in-
clined plane, —if the stroke of a stick disturb its
balance, the hoop wabbles and the curves followed
by every point in the hoop are changed throughout
the rest of its course. In living matter, so long as
the chemical substances combine and separate in
constant, regular flow, and so long as no change
occurs in the forces acting upon the molecules,
there exists a steady mobile equilibrium. But let
there be at some point a little less heat or light,
and the conditions of chemical combination are
changed; there may be a falling off in the assimi-
lation of some element, the composition of the
molecules is changed, and as a consequence they
assume a new configuration of oscillation, which
produces a new condition of strain affecting the mass,
and an almost endless series of effects ensue, the
chemical changes producing physical changes, and
vice versa.

It is evident that the combinations of forces
which maintain living matter in its state of mobile
equilibrium must be exceedingly complex. Every
particle of living matter has its own peculiar mo-
tion in the body of an organism, now carried
along in circulating fluids, then perhaps deposited
in some appendage where it performs a_ great
variety of movement, and eventually it is cast off

40 DEVELOPMENT AND HEREDITY.

and its place taken by other similar particles, To
account for all these movements of each particle,
we are compelled to suppose a certain combination
of forces acting in a variety of directions, and
impelling the particle through its devious course
so long as it remains a part of the living aggre-
gate. Every movement of each particle means
that a certain amount of energy has been imparted
to it, and every time the motion of the particle
deviates from the straight line, we know that an
additional amount of energy has acted upon it.
We know that part of this energy in the organism
has its source in the oxidation of the food; but
this alone, coming from a single source, could not
maintain a mobile equilibrium of forces. The only
other source of energy for the organism lies in
the existence and movements of matter outside of
its own body. The energy of such movements is
transmitted to the organism in the form of pres-
sure, impacts, strains, heat, light, electricity, atmos-
pheric vibrations, etc.

If we expect to explain organic phenomena as
mechanical processes we must allow due weight
to these forces of the environment as causes of
organic change.

CHAPTER III.

PROOF OF THE ACTION OF FORCES OF THE ENVIRON-
MENT UPON ORGANISMS. — EFFECT OF CHANGES
OF ENVIRONMENT.

THE action of the forces of the environment
upon the organism can not be accurately measured
with the present means at our command, but that
such forces do act, and profoundly affect the or-
ganism, may be sufficiently proven. We do not
understand the laws of this action well enough to
predict in all cases what the effects will be; but
when we see a change in the forces of the envi-
ronment followed by a change in the forces dis-
played by the organism, then we must believe
that the former change is the cause of the latter
change, for every change must have its cause.
We are concerned here only with those changes
appearing in the organism, which we call growth
and development: we can be satisfied when we

can trace the cause of these changes either to
41

42 DEVELOPMENT AND HEREDITY.

forces outside of the organism or to energy in the
living matter which composes the organism. A
very minute portion of energy and matter is suf-
ficient to compose a germ; all the rest of the
matter and energy which has to be added before
the germ becomes an adult organism must accord-
ingly be received from the environment. Thus we
see at once how dependent each individual organ-
ism is upon its environment. A few illustrations
will make clearer the relation of the organism to
the forces of its environment.

Among the higher animals all individuals begin
their existence in the same manner. Each one is
dependent on the energy of heat for its first devel-
opment, and each is dependent on the peculiar
combination of forces which make up its nutriment.
The peculiar nutriment, and also the other pecul-
iar environment of each individual, such as the
medium in which it exists, are absolutely essential
to its development. It is, perhaps, a general idea
that all this peculiar environment has merely a
passive value for the germ’s development; and
that when the germ is removed therefrom it dies,
not for want of the forces which have sustained
its life, but because of the fatally aggressive action
of a new environment. A moment’s reflection
must show how unwarranted such an idea is, The

ACTION OF FORCES ON ORGANISMS. 43

germ in its proper environment is caused to live
and develop by the action of the same forces that
have acted on the germs of its ancestors for an
immense number of generations, and caused the
development of each in turn. If the forces be
altered beyond certain narrow limits, or removed,
then malformation, arrested development, or death,
is the result.

It is impossible for us to observe, or to imagine
with any degree of accuracy, the action of the
forces which affect the organism while it is yet
within the membrane of the egg. We know, how-
ever, that these forces must act almost exactly
alike for all individuals of the same race. This is
shown by the similarity of the results. But when
the organism comes out of the egg-membrane, —
is hatched or born, — it is still lacking a large part
of its complete development. During this later
period of development we can trace to some extent
the particular effects of some of the forces which
cause development. Of the first importance in
this respect is the food of the organism. Without
this, of course, no growth nor development would
occur. The quantity and quality of food seem
also to have a decisive effect on the degree and
direction of development, beside influencing the
general strength and activity of the organism. The

44 DEVELOPMENT AND HEREDITY.

whole system of medicine rests upon the fact that
certain elements taken into the body have power
to influence the growth, or repair, or development,
of the different organs of the body. The effect
of a change of food is shown by Daubenton’s state-
ment that the intestine of the domesticated cat
is considerably longer than that of the wild cat.
One of the most striking facts of this connection
is that reported by Dr. Born, and also by Professor
E. Yung, in regard to the influence of food on the sex
of tadpoles. According to these experiments,! the
feeding of an unmixed diet to the larve until after
their metamorphosis, has a tendency to produce
females. Among 1443 tadpoles thus fed until after
metamorphosis, and then examined by Dr. Born,
ninety-five per cent were found to be females and
five per cent males. These were raised in a num-
ber of different aquaria, and in some of the aquaria
the proportion of females was one hundred per cent.
In one of Dr. Born’s aquaria, which received acci-
dentally a mixture of diet, the males were found
in the proportion of twenty-eight per cent. These
latter specimens were also larger, approaching, in

1G, Born, “ Experimentelle Untersuchungen ueber d. Entstehung
d. Geschlechtsunterschiede,” Breslauer Grtelicher Zeitschrift, 1881.
E. Yung, “ De l’influence des milieux physico-chemiques sur les étres

vivants,” Archives des Sciences physiques et naturelles, Mars, 1882,
troisiéme periode, tome vii.

ACTION OF FORCES ON ORGANISMS. 45

size and general development those which grew
in their natural surroundings. Dr. Born has shown
that in their natural conditions of development the
number of males among the young is equal to the
number of females.

Professor Yung’s experiments were carried on pri-
marily to determine the effect of different foods on
the general development of the tadpoles. He had six
glass jars arranged, so that all the conditions of light,
temperature, etc., should be as much the same as
possible, and so that the only differences in the six
jars should be the different food placed in each one.
At the beginning of the experiment, on the first. of
April, fifty specimens in the earliest tadpole stage
were placed in each jar. The mortality was very
great, and on the thirtieth of June, there remained of
the three hundred tadpoles, seventy-four which had
completed their metamorphosis into frogs. In two
of the jars, all died without metamorphosis, and in
the other four, the proportion of females ranged from
seventy to seventy-five per cent. Professor Yung’s
general results were as follows: “1. That tadpoles of
the frog, taken from the same batch of eggs, develop
in a very different manner, according to the kind of
food which they receive. 2. The different kinds
of food in question, taken as a single article of diet,
favour development in the following order, — flesh of

46. DEVELOPMENT AND HEREDITY.

beef; fish ; coagulated albumen of hen’s egg; yellow
of hen’s egg; albuminoid substance of frog’s egg
with liquid albumen of hen’s egg, substituted later
in the experiment ; vegetable substances (alge).
3. A purely vegetable diet is insufficient to transform
the tadpole into the frog.” These experiments afford
conclusive evidence that food is not merely the
source of supply of raw energy to a developing
organism; and also that the innate forces in the
organism do not wholly control its development.
Food is one of the stimulating forces which guide
and determine the developmental reactions of the
organism.

The effects of heat on life and development are so
marked and the universal necessity for its presence
so well known, as for example, in hatching eggs and
germinating seeds and sustaining all life, that this
part of the forces of the environment needs but a
bare mention. Heat and light are original sources of
almost all of the energy which displays itself in the
structure and movements of living things. The
peculiar effects of light upon animals, however, have

1 Professor Guyot has expressed a “ Law of Development and Life”
as follows: “Throughout the entire realm of nature, in the animal
world as well as in the vegetable, the development of life increases in
energy, and in the variety and perfection of the types, with the increas-

ing intensity of light and heat, from the poles to the equator.” —
Guyor’s Physical Geography.

ACTION OF FORCES ON ORGANISMS. 47

not been so generally recognised. Professor Yung in
his series of highly ingenious experiments has reported
some very interesting and important facts in this
connection. He conducted with great care an ex-
periment in which a number of tadpoles were allowed
to develop under lights of different colours. None of
those in coloured light developed so well as those
which were in the natural light of day. There were
certain marked differences in the development under
the different lights. Thus tadpoles under a violet
light increased faster in size, but were not so active
as those under white or yellow light. Under violet
they grew better, but did not develop as well as
under yellow light. Professor Yung’s observations
proved unmistakably that light may exert a potent
influence upon the developing organism.

The most apparent phenomenon resulting from
the action of light on the animal kingdom is the de-
velopment of pigment. Certain transparent animals
being excepted, it is an almost universal rule that
animals exposed to light develop some pigment in
the skin. This pigment increases in quantity or
brilliancy as we approach the equator, where the light
is most intense. There is thus a direct ratio be-
tween the amount of light and amount of pigment.
This ratio appears also in another manner which is
so universal that it must have great significance. I

48 DEVELOPMENT AND HEREDITY.

refer to the fact that the parts of the same animal
which receive less light develop less pigment.
Thus there is less colour on the under side of all quad-
rupeds, snakes, fishes, crawling insects, crustaceans,
worms, mollusks, echinoderms, and sea-anemones.
Among many of the flying insects and many birds
this condition is less marked or absent for the obvious
reason that while in the air they are exposed to a
nearly equal light on all sides. Further, it is a mat-
ter of general observation that animals which during
their lives are not exposed to light do not develop
pigment. Such animals, whether they live as par-
asites buried in the flesh or in the alimentary canal
of other animals, or whether they live in caves
where no light penetrates, remain colourless, often
semi-transparent ; and if they have eyes no pigment
develops in them. This has been explained as the
result of natural selection, which is supposed to act
in such a way as to prevent the waste of the energy
that would be necessary to produce pigment where
it is not necessary to the life of the organism. But
the facts just mentioned are so striking and uni-
versal, that it is difficult to imagine how natural
selection could have acted to produce the same
result in all cases under such varying conditions,
especially when we remember the prodigal way in
which energy is wasted in producing ornamental

ACTION OF FORCES ON ORGANISMS. 49

colours and useless corrugations and excrescences,
and in reproducing such a number of useless rudi-
mentary organs. The plain facts lead to the con-
clusion that pigment is caused by light acting upon
the tissues, and where there is no light there can
be no pigment.

In the case of the rooster described by Darwin,
which was reared in the dark, and never crowed,
but grew up with the instincts of a hen, we have
an instance of arrested and distorted development
which shows how extremely important the light is for
the proper development of an animal whose race has
always lived in the light. Professor Yung’s experi-
ments showed not only the importance of light, but
that different kinds of light have a distinctly differ-
ent effect upon the development of an organism.
Ordinary white light, as shown by Professor Yung,
is most favourable to the normal development.

Of the light that falls on an animal, a part is
absorbed or reflected by the pigment in the skin,
and a part is transmitted through the semilucent
tissues beneath. It is this latter portion of light
which penetrates the body, which we would natu-
rally expect to influence the organism’s vital func-
tions. Other things being equal, a light coloured
or white skin admits more light, and a dark or
black skin less light. In nature, therefore, the

50 DEVELOPMENT AND HEREDITY.

colour of the light thus admitted to act in the inte-
rior of the tissues of the body, is dependent on the
colour reflected by the environment of the animal,
and also on the colour absorbed or turned back by
the skin. If a brown or yellow colour should be
everywhere reflected on an animal by its environ-
ment, this colour would preponderate in the light
which entered the body, perhaps to the disadvan.
tage of the animal’s internal physiological func-
tions; but since in such cases the same colour is
reflected back by the pigments of the animal, there
would thus be a tendency to restore the normal
balance of white light, which is most favourable
to organic functions. Living matter seems to be
in a general way capable to a certain extent of
photographing colours when exposed for many gen-
erations. An example is found in the fishes, crabs,
shrimps, worms, polyzoa, and hydroids, which live
among the masses of yellow sea-weed found float-
ing in the Gulf Stream, and which a// partake of
the yellow colour of the weed. Another instance
is that of the animals of the great Sahara desert,
all of which, both those preying and those preyed
upon, share alike the tawny colour of the desert.
The fact that the colouration of animals increases
toward the equator leads easily to the supposition
that the lack of pigment or whiteness of animals

ACTION OF FORCES ON ORGANISMS. 51

in the extreme north may be primarily due to the
small amount of light in those regions.!

The arrangement of the pigments in organisms
seems to be governed by some peculiar law of
colour relations; a law which seems to be deter-
mined more by the nature of light and of living
matter than by the particular nature of the organ-
ism. The existence of such a law is suggested by
the repetition of the same colours and combinations
of colours in widely different plants and animals.
Thus we find the brilliantly coloured spot on the
peacock’s tail-feathers almost exactly reproduced
both in form and colour on the fins of certain fly-
ing-fish found among the Bahama Islands. Dr.
James McCosh has stated that patches of colours
which adjoin each other on plants and animals are
generally complementary colours, or else they are
separated only by a white or black streak. How
generally true this is may be observed in any good
collection of stuffed birds. But it is not univer-
sally true, as is shown by the brilliant Central
American macaw. This wonderful bird presents

1JIn all these cases the same colour prevails in all the classes of
animals, both in those which are offensive and those which are the
prey of others, and this universality of the colour seems not to be
wholly accounted for by the theory of protective colouration; never-

theless the view here advanced and the theory of protective coloura-

tion are in nowise mutually exclusive,

52 DEVELOPMENT AND HEREDITY.

all the colours of the spectrum in their proper
sequence, running from red on the head to violet
on the tail. This case is so remarkable that it
alone seems to prove that the colouration of ani-
mals is dependent on some peculiar reaction of the
physiological forces toward light, and not solely on
the protection or ornamentation of animals.

Every organism which is capable of motion is
influenced in its growth by the external resistance
which is offered to its bodily motions, thus chang-
ing the strain and pressure upon the various parts.
The ordinary exercise of its organs which a young
animal, stimulated by the forces of its environment,
performs during the period of its post-natal develop-
ment, we know to be of great importance for the
proper growth of the animal. By exercise of the
legs and wings the nervous co-ordinations are per-
fected which control the walking and flying. This
principle is recognised and acted upon by the train-
ers of racing colts. Not only are the nervous co-
ordinations perfected in this way, but the full
growth of bone and muscle, and of the other organs
of the body, are dependent upon exercise. The
effects of exercise in the training for athletic con-
tests and gymnastic feats are so well known that
it is hardly necessary to mention them. What I
wish to emphasise is the influence of the ordinary

ACTION OF FORCES ON ORGANISMS. 53

motions and exercise of young animals as one of
the causes of their development. In a young mam-
mal, if compelled to live without motion, there
would undoubtedly be a lack of muscular and ner-
vous development ; also there would be a most im-
perfect circulation of the blood, causing a host of
effects which would stunt the growth and eventu-
ally probably kill the animal. The cruelty of the
experiment has probably prevented its ever being
tried, but the evidence of this sort to be found
among the human race is only too convincing.
Every animal has a limited number of methods of
motion and exercise which are demanded by any
normal environment. This limited number of mo-
tions must influence the development of the ani-
mal in: certain definite directions, favouring some
points of growth more than others; just as many
forces acting in various directions on a body resolve
themselves into one single resultant direction of
force along which the body moves.

The records of pathology are full of evidence
showing the effects of abnormal forces upon the
tissues of the human body. Almost all diseased
growths of tissues may be traced as resulting from
some change in the normal forces acting on the
tissues, or to the introduction of a totally new
force or stimulus. The change of a living surface

54 DEVELOPMENT AND HEREDITY.

of mucous membrane into skin, when exposed per-
manently to the air, is a case of this kind, showing
the effect of a change of medium. If now we
admit the effect of a change of forces, or the intro-
duction of new forces, as a cause of disease, then
we must attribute an equal power to the unchanged
forces, for the normal life and growth of tissues
is as much a phenomenon of organic forces and
reactions as is the diseased growth.

A striking illustration of the effects of abnormal
action of forces on growth is shown in the change
in the shape of the human foot which results from
the pressure of the shoe. We see how the foot fits
itself into the shoe as though pressed into a mould,
and toward middle life, becomes thus permanently
distorted. This manner of growth under pressure
throws light on the problem of how the internal
organs of the body, and the muscles and bones
which press against each other, are moulded to fit
with a perfect contact of surfaces. So we must
conclude that mechanical pressures and strains play
a large part in modelling the shape of the body.

These illustrations, drawn from the more readily
observable phenomena of animal life, might be multi-
plied indefinitely, both in kind and number. While
we can observe these facts more readily among the
higher animals, we cannot suppose that the lower

ACTION OF FORCES ON ORGANISMS. 55

animals are exempt from a similar action of similar
forces.

Illustrations of the influence of various external
forces upon growth are found in great number among
plants. Thus, the sunlight may cause plants to
grow toward it, or away from it; and the amount
of light modifies the growth of plants in many other
ways. Gravity also has a powerful influence upon
the general direction of growth, and moisture deter-
mines the direction of growth of roots.

A number of experiments of very great interest
for our present enquiry have been very ably dis-
cussed by Professor Detmer ;! and his observations
seem so important, that I give here extensive quo-
tations from his paper —only taking the liberty to
translate the quotations into English.

“In the botanical realm a long series of facts
have been ascertained, which teach us that the
influence of external conditions upon the plant is
often very profound. Their operation modifies not
only the outward appearance of the parts of the
plant, but in a most striking manner changes even
the anatomical structure.

“If one prepare a cross-section of the bipennate
twig of Zhuja occidentalis, it will be found, upon

1 Detmer, “Zum Problem der Vererbung,” Pfluger’s Archiv f
Physiologie, Ba. 41, 1887.

56 DEVELOPMENT AND HEREDITY.

microscopical examination, that the mesophyllum, the
upper side of the twig, is differently constructed
from that of the lower side. The green cells on the
upper side are rich in chlorophyll bodies, and have
a pallisade-like form: the cells on the under side
are poor in chlorophyll, and have a nearly iso-diamet-
rical form. The dorsi-ventral structure of the Thuja
twig expresses itself in other ways, which, however,
we will not discuss here. Let the Thuja twigs be
taken in early spring, before the growth of the buds
has started, and without separating the twigs from the
plant, let them be tied in an inverted horizontal posi-
tion, so that the natural under side is turned toward
the zenith, —#it will then be found, as was first
shown by Frank, that the year’s additional growth
certainly assumes a dorsi-ventral nature, but is essen-
tially different in its anatomical structure from those
parts of the twig which were of previous growth.
The upper side of the newly grown parts of the
twig, although it is the continuation of the under
side of the older part, has an anatomical structure
like the upper side of the older part. Thus, the
dorsi-ventral structure of the Thuja twig, which
appears in the development of palisade-parenchym
on the upper side, and soft parenchym on the under
side, is the result of the working of an external
force; and in this case, according to all that we
know, we must regard the light as the cause.

ACTION OF FORCES ON ORGANISMS. 57

“Kohl has proved by experiment that the leaves
of Yropeolum plants display different properties
according as they are ‘grown in a moist or a dry
atmosphere, the other conditions of growth being
the same. The leaves grown in dry air are provided
with a thick cuticle, and under the epidermis is a
strong growth of collenchym tissue; while the leaves
raised in moist air show a thin cuticle and no
collenchym.

“ According to Stahl’s observations, the anatomical
structure of the leaves of some plants shows a great
difference as a result of having developed in strong
light or in shadow. The assimilative parenchym of
beech leaves which grow in sunlight, for example,
consist almost entirely of palisade cells, while the
palisade cell in the leaves grown in shadow are very
few; a circumstance which, though it cannot be
discussed here, is of great significance for the
function of the leaves. Hetureicher also gives
examples from which it appears that the condi-
tions of light have a decided influence upon the
anatomical structure of the leaves of different indi-
viduals of the same species of plant. The newer
botanical literature has acquainted us generally with
numerous facts concerning the relations between the
morphological and anatomical peculiarities of plants,
and the climatic influences or other conditions of life
to which the organisms are exposed.”

58 DEVELOPMENT AND HEREDITY.

From the facts in the previous paragraphs of this
chapter, it will be clearly seen that organisms do
not become what they are in their adult form, by a
simple unfolding of innate energy. Their develop-
ment, and even life itself, is dependent upon and
caused by the never-ceasing action of the forces
of their environment. As an eminent physiologist
has said, generation after generation are the same
because they live and develop under the same con-
ditions. If the forces of the environment change,
the growth of the organism must also change.
Hence the effects of domestication on animals
and plants, the degeneration of northern dogs, etc.,
in tropical regions, and the modifications of feral
races.

CHAPTER IV.

ACTION OF THE NERVOUS SYSTEM INTERMEDIATE
BETWEEN THE FORCES OF THE ENVIRONMENT AND
THE ULTIMATE EFFECTS IN THE ORGANISM.— UNI-
VERSALITY OF NERVOUS ACTIVITY, OR THE NER-
VOUS PROPERTY, IN LIVING MATTER,

WE have seen in the previous chapter| that the
life and development of an organism are \sustained
and moulded by the forces acting upon the organism
from the environment. When these forces are
altered the growth and development are altered or
life may cease. The manner of the transmutation
of the force in the environment to the force dis-
played by the organism is very obscure. We can
assert, however, that the forces of the environment
must very readily affect that instable chemical
and molecular condition which we have seen to
be a characteristic of living matter. This alteration
in the chemical and molecular constitution must
produce the effects which we see as movements of

the mass and changes of its shape.
59

60 DEVELOPMENT AND HEREDITY.

When we look for the zimediate effects of the
external forces, we find that they are not the move-
ments and changes of shape and growth of the
organism. We find that there is an intermediate
link which physiology describes as nervous activity.
In the same way, when we look for the zmmediate
causes of movement and change of shape and
growth, we find that they are not the external
forces, but consist in this same nervous activity.
This idea, from the psychologist’s point of view,
has been expressed by Professor Ladd as follows :—
“The forces of external nature continually storm
the peripheral parts of the animal’s body. In
order that any of these forces may act as the stimuli
of sensations, they must be converted into molecular
motions within the tissues of the body. In order,
further, that the masses of the body may constantly
be readjusted to the external changes of which
the sensations are signs, the molecular motions
must, in turn, be converted into movements of
these masses. In other words a process of constant
interchange must take place between the animal
organism and external nature.” !

We know that those forces which act from with-
out upon living matter, have their immediate action
chiefly or wholly upon the nervous organisation.

lLadd’s Elements of Physiological Psychology, p. 220.

ACTION OF THE NERVOUS SYSTEM. 61

They cause chemical and molecular changes in the
nervous system. This is most easily observed where
there is a nervous system with end-organs capable
of being very sensibly affected by the slightest
variation in the forces of the environment, as, for
example, the eyes are affected by variations in the
intensity of light, and the skin by variations of
temperature. But even in cases where there is
no nervous system, all organisms are affected by
light and heat, and there is reason to believe that
could we test the other forces equally well, we
would find that they produce an equally great effect
upon living matter, whether a nervous system were
present or absent.

When the forces of the environment enter the
mass of living matter, it cannot be that they are de-
stroyed. If they be no longer recognisable in their
original form as light, heat, etc., they must be trans-
muted to some other of the various forms of energy.
The only other form of energy which can be discov-
ered as these forces vanish at the periphery of the
living mass, is that form of energy which appears as
nervous activity. Therefore, we may safely conclude
that the energy of the forces acting from without,
persists within the living matter as nervous activity
and change of nervous condition.

On the other hand, when we look for the imme-

62 DEVELOPMENT AND HEREDITY.

diate causes of the motions, changes of shape, and
functions of animals, we find in like manner that it
is the nervous system which is implicated. If we
examine all the changes that take place in an animal,
it appears that before most of them occur, there is
a peculiar activity of the nervous system. What-
ever may be the primary source of the forces causing
the change, we know from the science of physiology
that every display of physiological forces, the per-
formance of every function, is subject to the control
of the nervous system. The swallowing of food, the
secretion of the juices which digest it, its propulsion
through the alimentary canal, the absorption of nutri-
ment through the walls of stomach and intestine,
the distribution of the nutriment throughout the
body, respiration and circulation, glandular secretion,
assimilation in the various parts of the body, repro-
duction, locomotion, muscular action, both voluntary
and involuntary,—all of these are caused by the
activity of the nervous system. A nervous disturb-
ance which is at first purely mental may check or
alter the whole nutrition and growth of a body, —as
when a person loses appetite and grows thin in con-
sequence of a severe mental shock or disappoint-
ment. While appetite or hunger is a sensation
localised in the stomach, in reality it is caused by
the excessive consumption of the nutriment in the

ACTION OF THE NERVOUS SYSTEM. 63

blood by means of the assimilation taking place
throughout the body. Loss of appetite therefore
means a cessation of assimilation. Thus we see that
the nervous system controls all the functions of the
living body, as a unit. The result of the combined
functions of the body, is the sustenance and growth
of the body.

Since growth is controlled by the nervous system,
it follows that development also must be to a certain
degree controlled by the nervous system; for de-
velopment is either a change in the direction of
growth or it implies new growth, z.e. the addition of
new material to some part even though the material
be taken from an adjacent part. Growth cannot be
simply an excess of assimilation above waste. The
new material added by growth must form new tissues
and new parts. So long as these tissues and parts
which are added to an organ, remain of the same
nature as the rest of the organ, and cause no consid-
erable change of function, the organ is said to grow;
but when the addition of new parts makes the per-
formance of new functions possible, then develop-
ment is said to occur. A little reflection on the
matter will show how arbitrary is the distinction.
For no development can take place without growth,
and no growth can take place in any organ without
modifying in some degree, however slight, the func-

64 DEVELOPMENT AND HEREDITY.

tion of that organ. All researches into the history
of organic evolution, point to the conclusion that all
development has occurred gradually by slightly addi-
tional growth of various parts, in successive genera-
tions, —the different parts assuming new functions
as their changes of mass and form made the per-
formance of the new functions possible.

When development occurs, therefore, it means
that growth takes place at a certain definite point.
Now what is the cause which determines that new
material shall be added at this particular point, rather
than anywhere else?) We have seen, in the previous
chapter, that a great variety of forces in the environ-
ment constantly act upon the organism and produce
a variety of effects; but we cannot suppose that
these are the zmmediate causes of the complicated
processes of growth. Since the activity of the ner-
vous system regulates all the functions of the body
and controls its assimilation as a whole, we must
suppose that it also controls the growth or develop-
ment of each part. The nervous system is, of. all
parts of an organism, the most sensitive to change,
and the forces of the environment must act chiefly
upon the nervous system, and through it upon the
whole organism. In view of all we know of the
nervous system and its functions, and from analogy
of its known control over other organic processes, we

ACTION OF THE NERVOUS SYSTEM. 65

must conclude that it is-a nervous activity which
immediately controls the machinery of growth, and
thus determines the growth or development of any
and every particular part.

The facts concerning the nervous system, which
have been referred to above, have been attained
chiefly by the study of man and the higher verte-
brate animals, but few will doubt that they hold true
equally of all the lower animals possessing a nervous
system. For, in the first place, their tissues are
similar in composition and form of elements; and,
moreover, the investigations of the physiology of
the snail,! and other invertebrate animals, show that
they are affected similarly by various drugs, and
that in their physiological economy they are gen-
erally the same as the higher animals. The differ-
ent functions of their organs have also been shown
to be under the control of a central nervous system.
But there is a great difference in the extent of
control which the visible central nervous system
exercises over the bodies of different animals. As
a general rule, the lower the animal in the scale of
organisation the less centralised is the control exer-
cised by the nervous system over the component
organs. The organs may be able to perform their

1 Contribution & histoire phystologique de Pescargot (Helix poma-
tia), par Emile Yung. Bruxelles, 1887.

66 DEVELOPMENT AND. HEREDITY.

functions when all connection with the meagre
central nervous system is cut off. In many cases
no nerves can be traced to certain organs, and yet
they perform their functions perfectly. The lower
we descend in the scale of life the less noticeable
does the visible central nervous system become both
in its structure and in its function. The organs
seem to act independently of visible nervous struct-
ures, and yet they act harmoniously for the main-
tenance of the entire organism. In many of the
lowest animals no trace of a visible nervous structure
has been found; yet here also the organs perform
all their functions as a harmonious whole. All the
actions and functions necessary for the life of the
individual and of the race are performed with as
much regularity and perfection as among animals
possessing highly developed nervous structures. The
Junctions of a nervous system are performed, though
the nervous structures are not discernible. So great
is the similarity in motions and in the performance
of all the functions of life between these nerve-
less organisms and the higher animals, that we are
compelled to believe that, just as some of the low-
est animals can digest food in all parts of their
minute bodies, so also the nervous property may
reside in all parts. Since the same results of nutri-
tion, growth, etc., are accomplished among the low-

ACTION OF THE NERVOUS SYSTEM. 67

est as among the highest animals, we must conclude
that the controlling nervous activity is everywhere
of the same fundamental character. A _ distinct,
separate, and visible nervous structure is there-
fore not essential to the display of nervous activity.
In fact an elementary form of nervous activity is
recognisable in that sensitiveness or irritability,
which is common to all living matter. The move-
ments which result from this so-called irritability,
we must believe to be caused by internal molecular
changes of the mass; and these molecular changes
must in turn be caused by the action of some
external force.

As there are certain animals, of a low degree of
organisation, which possess no nervous structures, so
also there is a period in the life of each of the higher
animals when there is no nervous structure present.
At the beginning of their individual existence, all of
the higher animals resemble closely the full-grown
forms of the very lowest of animals. They consist
of a single cell, and even after many cells have de-
veloped, there is still no trace of nervous structures.
Yet during this time the functions of nutrition, as-
similation, growth, and development are performed
harmoniously, all the activities of the organism unit-
ing to produce a common result. Long before the
cells of the nerve tissues have attained their com-

68 DEVELOPMENT AND HEREDITY.

plete development, muscular action has begun in the
rhythmic pulsation of the heart. Evidently, the ner-
vous forces are active before the nervous structures
appear. There is no moment in the embryonic life
of an animal when we can say, “ow the nervous
system assumes control over the life processes.” The
nervous activity does not appear suddenly, nor sud-
denly concentrate itself in the nervous structures.
This nervous activity is something co-extensive with
the life of the animal. The nervous system is a
mechanism which has developed gradually — both in
the individual and in the species, — as the increasing
bulk and complexity of organisation made it neces-
sary for a more perfect action of the nervous force.
We must not allow ourselves to be blinded by a ter-
minology which is, unfortunately, inexact; but we
must always bear in mind that it is not the visible
material substance of the nervous system which con-
trols the organism, but it is the forces which act
through the material substance, building up the po-
tential energy of the nervous system, and controlling
and ‘determining its growth, as well as the growth of
the rest of the body.

When we turn from the animal to the vegetable
kingdom, we find that the general rule holds good,
and that plants display certain forms of nervous ac-
tivity. All plants, under certain circumstances, show

ACTION OF THE NERVOUS SYSTEM. 69

the fundamental property of irritability. As related
in character to this irritability, may be classed the
phenomena of some of the experiments of the pre-
vious chapter, in which the forces, — for example, the
sunlight, — may be supposed to act directly upon the
parts of the plant which are affected and changed.
But there is another class of phenomena, in which
the connection of the change of forces and the
change in manner of growth, while none the less
certain, is much more obscure and complicated, and
indicates a co-ordination of the internal forces, which
must partake of the nature of nervous co-ordination.
An illustration of this class is found in an experi-
ment referred to by Professor Detmer, in which the
tip end of a young pine tree is broken off. The re-
sult of the removal of the tip is that one of the side-
branches turns upward, and, losing its own function
and flat-spreading shape, it assumes the function and
symmetrical shape of the lost tip of the vertical pine-
stem. The side branch thus undergoes a complete
change in the manner of its growth. Another strik-
ing experiment of this class relates to a habit of
growing plant-stalks, according to which the stalk
bends sideways or downwards, while the tip end
swings slowly in a circle around the main stem, —
a process called zutation, The following description

70 DEVELOPMENT AND HEREDITY.

of the experiment is taken from a review! of Dr. Max
Scholtz’s “paper on the nutation of the flower-stalk
in poppies and of the terminal shoots in Virginian
creeper. In both cases, the nutation is dependent
on the action of gravity, but has nothing to do with
the weight of the bud. In the case of poppies, the
downward curvature of the stalk takes place, with
sufficient force to lift a weight equal to twice that
of the flower-bud. If, however, the flower-bud be
removed, there is no longer any nutation; the stalk
straightens itself. Véchting had already shown that
this is the case even if the amputated bud be tied
on again with thread. Dr. Max Scholtz further
states that, if a weight three times as heavy as the
bud is substituted for it, the stalk still straightens
itself and lifts up the weight. The state of the case,
then, is this: the upper part of the flower-stalk, dur-
ing a certain stage of its growth, is, ina high degree,
positively geotropic if it remains in connection with
a developing flower-bud, but not otherwise. The
author has further succeeded in determining the
exact part of the flower-bud which governs the geo-
tropism of the stalk. If the pistil is excised, nuta-
tion ceases, the stalk becoming negatively geotropic ;
but if all the other whorls of the flower are removed

1 “New Contributions to the Biology of Plants,” by D. H.S., Vat
ure, September 15, 1892. :

ACTION OF THE NERVOUS SYSTEM. 71

and the pistil left, then nutation goes on as usual.
But beyond this, if the ovules are extirpated, but
the wall of the ovary left standing, the nutation is
stopped. Hence we arrive at the striking conclu-
sion that the presence of developing ovules in the
young ovary determines the reaction of the flower-
stalk towards gravity.”

In these two experiments, we see that the destruc-
tion of a part of a plant changes that co-ordination
of forces which produces the growth and controls
the movement of the growing parts. When we
see the effects of a local injury made apparent in
a distant part of the plant, we must believe the
living part of the plant to be capable of some action
like nervous conductivity. And when we see that
the manner of growth and movement is in part
dependent upon some localised centre, we are
further led to the belief that the co-ordinations of
forces in the plant are similar to the nervous co-
ordinations which control the functions of animals.

Other experiments show more clearly the nervous
nature of the co-ordinations controlling the activities
of plants. They show not only how these co-
ordinations in the protoplasm of the plant react to
the external forces, but also how the reaction
becomes by repetition ingrained in the living matter
of the plant, so that for a limited time after the

72 DEVELOPMENT AND HEREDITY.

stimulus has been removed the reaction still con-
tinues. Such cases are particularly interesting, as
they show that a very simple organism without
any visible nervous structures, when accustomed
to performing certain actions as a result of external
forces, will still continue to perform those actions
from force of habit, after the external forces have
been withdrawn. This is illustrated in the experi-
ments described in the following passages from
Professor Detmer’s paper. (I begin the quotation
with his introductory remarks, because they seem
to me so very significant, and are so harmonious
with my own views.) “For many years I have
entertained the conviction — which forced itself upon
me first in 1876 on the occasion of my investigations
of the periodicity of the root-pressure upon the sap
—that the phenomena of after-effects or subsequent
reactions (NMachwirkungsphanomene) differ only in
degree and not in their essential nature from the
phenomena of inheritance,—a view which may
become the starting-point for investigations of
heredity. Of course the after-effects play their
part only in the individual life of an organism,
whereas heredity stretches out its grasp over the
individual life and onward to the next generation ;
but nevertheless the essential similarity of the two
classes of phenomena cannot escape the attentive

observer.

ACTION OF THE NERVOUS SYSTEM. 73

“T will remark beforehand that I pass over here
a line of phenomena of after-effects which may be
observed in the vegetable kingdom ; so, for example,
the after-effects displayed in bending upward from
the earth, and in the inclining toward the light.
When a shoot that has been placed in a horizontal
position for a while, still continues its bending
growth after it has been changed to a vertical
position, we see a simple case of after-effect; also
when leaves that have grown in darkness and then
have been illuminated for a short time, still continue
to grow toward the temporary source of light after
it has been removed, we see another simple case
of after-effect. Both of these cases may be ac-
counted for by saying that those movements of the
plants which are produced by external influences,
do not immediately cease at the instant when the
external influences are withdrawn, but continue for
a certain time afterwards. Other after-effects are
of a much more complicated nature. They may, of
course, be placed in relation to those mentioned, by
intermediate connecting links.

“Let the whole top be cut off of vigorous speci-
mens of Helianthus, Ricinus, Cucurbita, etc., which
have previously developed for a long time under
normal conditions; then let a glass tube be fast-
ened over the cut end of the stump, and the whole

74 DEVELOPMENT AND HEREDITY.

object under experiment be placed in darkness
with unchanging conditions of temperature and
humidity. In this way, as is known, it is possible
to prove that the flow of sap, which can now be
observed in the glass tube, shows a periodical
alternation. The root-pressure does not always
drive out, from the cut end of the stump, the same
amount of fluid in a given unit of time; but a
variation of the sap-flow makes itself apparent in
such a way that in general the maximum flow
occurs in the afternoon hours, and the minimum
flow occurs in the early morning hours. The sap-
flow may continue for days; and Baranetzky and
I each concluded from our investigations of the
root-pressure, that the cause of the daily periodicity
of root-pressure lies in the daily and nightly alter-
nation of light and darkness to which the plants
were exposed before the experiment. In support
of this view, for example, is the fact that when the
tops are cut off of etiolated plants which have been
raised in complete darkness, the sap under the
pressure of the roots continues to flow from
the stump, but no periodicity can be detected in
the flow.

“Tt is a well-known fact that darkness has a
quickening effect upon the growth of plants, while
illumination makes the growth slower. When,

ACTION OF THE NERVOUS SYSTEM. 15

therefore, suitable plants are exposed to the changes
of day and night,— the other conditions remaining
constant,— if the growth movements of the limbs
be measured, it will be found that the growth at
night is considerably greater than during the day.
Now it is remarkable as Baranetzky has found, that
these daily periods of growth do not disappear as
soon as the plant is placed in a constant darkness,
but on the contrary may be observed for a long
time as after-effects.

“By observing the position of the leaves of
mimosa pudica and acacia lophanta, it is found that
in the day-time they are spread out, but on the
approach of darkness they fold their upper surfaces
together. The alternating movements which come
here into consideration, have their cause in the
change in the conditions of illumination. If vigor-
ously developed specimens of szmosa or acacia be
brought into constant darkness, the alternating
movements continue for days (by my experiments
four or five days); the leaves are folded together
at night-time and spread out in the day-time.
Pfeffer shut off all light from specimens of mzmosa
in the day-time, but illuminated them artificially
all through the night. After a long time, the plants
under experiment were brought into constant dark-
ness; then they spread their leaves at evening,

76 DEVELOPMENT AND HEREDITY.

and folded them together during the day-time, —
proof enough that the rhythm of the after-effect
movements is totally dependent upon the manner
of the alternation of illumination to which the
plants have been exposed.

“Many of our trees and bushes lose their foli-
age in the autumn and form winter-buds which
do not unfold until spring, and this is evidently a
peculiarity of the plants which is originally due to
the climatic conditions of our latitude. If a twig
covered with winter-buds be cut off in the fall,
and the stem put in water and set in a hot-house,
the buds will not immediately open as one might
perhaps expect, but on the contrary months often
pass before the buds unfold. From this it follows
that the season of plants can only be considered
as a phenomenon which is directly dependent on
external conditions. These conditions once cer-
tainly determined the growing period of the plants,
but this period has gradually become through after-
effects and inheritance, more and more fixed in the
organism, until now it is not easily made to dis-
appear. This can, however, gradually occur under
the influence of changed climatic conditions. A
proof is furnished, for instance, by the circumstance
that our cherry has become an evergreen tree in
Ceylon.”

ACTION OF THE NERVOUS SYSTEM. 7

These latter experiments bring out a fact of
fundamental importance, namely, that in the con-
stitution of the plant there must arise an associa-
tion between the interval of time and the increase
of activity. This association can only be regarded
as of an elementary nervous or psychic nature.

The facts in the case lead to the conclusion that
the greatest part of the molecular change, which
is brought about in living matter by the action
of external forces, is that change which occurs in
the nervous organisation.

We have seen that the reactions of a piece of
metal, under a given strain, are governed by the
whole previous history of the metal; and the same
we see may be true of organisms. We see that
the changes in manner of growth, resulting from
changed conditions, have a tendency when long
continued, to become permanently ingrained in
the constitution of the organism. There are cases
where, the previous condition having been restored
after a long lapse of time, the changed manner
of growth still continued to show itself through
several generations: though finally, under the orig-
inal conditions of the race, in a few generations
the normal characteristics of the race reappeared.
It is well known that the same varieties of plants
—wheat, etc. — germinate and ripen in the short

78 DEVELOPMENT AND HEREDITY.

summer of a cold climate more rapidly than in
the longer summer of a more southerly locality.
It has been shown by experiment that if wheat
be taken from a southern locality to one much
farther north, that the wheat will in a few gener-
ations grow and ripen as quickly as the native
wheat; and further, if the seed of this same wheat
be returned again, and raised in the south, it
requires several generations to regain its normal
rate of growth. In this case of the wheat, the
difference observed in the different localities must
be primarily due to a difference in the “forcing ”
qualities of the season, z.¢. the more sudden change
of temperature and other seasonal conditions gives
a stronger and more effective stimulus, and a
gradual change gives a weaker stimulus. How-
ever, the chief interest lies in the persistence of
the effects. We have only to imagine the peculiar
conditions of an environment to have acted for
an indefinitely greater time, in order to account
for the production of permanent methods of growth
which produced the so-called “fixed” or hereditary
characteristics of species.

CHAPTER V.

NERVOUS ACTIVITY AND ALSO DEVELOPMENT DEPEND-
ENT ON ASSOCIATION AND REPETITION.

WHILE investigating the causes of evolution it
is always necessary to bear in mind the complex
character of living matter. We must remember
that in addition to those of its properties which
may be visibly observed in its natural state and
under experiment, and those which may be proved
by chemistry, and those which it possesses in com-
mon with certain inanimate bodies, it also has one
other essential property, namely, that which is
expressed in its capacity for development. Not
only has this capacity been the sustaining basis
of the whole evolution of the various forms of life
from the amceboid up to man, but it allows the
process of evolution to be constantly repeated —
more or less imperfectly, in each individual life.
A consideration of protoplasm from this point of
view, with a full appreciation of this fact, is neces-

sary to a well-rounded conception of the fundamental
79

80 DEVELOPMENT AND HEREDITY.

properties of the “physical basis of life.’ Some
have rather vaguely imagined that this capacity
for development was something acguzred by natural
selection, or that it had originated somehow in the
same manner as the generic or class characteristics
originate. This supposition, however, lies beyond
the limits of legitimate assumption. For if we
accept any theory of evolution, we must believe the
capacity for development is universal in living
matter and co-extensive with life itself. The facts
warrant this belief, for even the simple growth of
the Protamceba implies some change and adaptation
to the requirements of its increasing size,—a change
which is development. This capacity for develop-
ment,—for growth and change and constant repe-
tition, can be expressed in terms of other and
perhaps more familiar phenomena, which may give
us more definite ideas of its action, and of the part
it plays in determining the particular directions of
evolution. It may be regarded as a peculiar im-
pressionability of living matter under the action of
various forces.

We have already got some insight into the
nature of the action of forces, and their effects upon
matter in general: also we have observed the effects
of forces of the environment upon living matter.
We have seen that all the energy displayed by

DEPENDENCE ON ASSOCIATION. 81

organisms has its source in the food and environ-
ment,— except perhaps those primitive forces whose
interaction produces the simplest form of life, and
which may perhaps be considered as somehow
resident in living matter, or as properties of the
component elements. But no life can continue,
and no life does exist as we know it, without the
constant action of forces from the environment.

The energy or combination of forces residing in
the embryonic germ, and controlling and determining
its development, may seem to be an exception to the
above general statement. But the whole history
of evolution, so far as it has been interpreted, tells
us that this combination of forces, or this potential
energy, has been acquired gradually through long
ages. The particular form of potential energy which
exists in a chicken’s egg, and determines into what
it shall develop, did not exist in any living thing
during the palzozoic era. It must have been ac-
quired by the action of environment upon certain
organisms.

In the previous chapter we have seen that forces
act upon living matter chiefly through the nervous
system, or where such a structure is absent, the
forces affect what we recognise as the xervous
state of the organism. We have seen that where
the forces of the environment act upon the organs

82 DEVELOPMENT AND HEREDITY.

of locomotion and nutrition, it is only through the
mediation of the nervous system. The nervous
system is of all parts of the body the most mobile,
and most sensitive to the action force: it controls
the nutrition, assimilation, and growth; therefore we
conclude that it is through this method of physio-
logical activity which we know as nervous action,
that the forces of the environment act upon the
growth and development of the individual and of
the race.

We may suppose that when the kinetic energy
which acts from the environment has once passed
the periphery and entered the mass of living matter,
it no longer acts according to the laws of heat or
light, or whatever its external form may have been,
but, operating upon a mass of different nature, it
assumes a new form, and acts according to the
laws of nervous activity. Therefore in order to
understand the effects of the forces of the environ-
ment we must know the laws of nervous activity.
For this purpose we must resort to the methods of
investigation of modern psychology. It is among
the higher animals, possessing well-developed
nervous structures, that we must look for the most
perfect display of nervous activity. Where the
activity is displayed by certain definite organs, we
can separate and analyse its phases and investigate

its laws.

DEPENDENCE ON ASSOCIATION. 83

Psychological investigation teaches us that for
the causes of nervous or psychic change, we must
look to the forces acting from without upon the
organism. That all of the psychic changes are
accompanied by the display of energy in some form
of material change in the nervous structures, is the
most striking and far-reaching conclusion of modern
psychology. All the facts of physics and biology,
which have any bearing upon the subject, render
it most probable that in the nervous system, as
elsewhere, no energy is destroyed and none is
originated; no molecule moves without due appli-
cation of force from without, and the whole mechan-
ism thus moves perfectly subservient to the law of
the conservation of energy. Yet we cannot observe
the process in exactly the same manner as we ob-
serve and test other simpler physical processes.
Forces which act upon living matter elude us in
the wonderful intricacy of their results.

The general data of Biology go to show that no
physical change can take place ina living animal
without directly or indirectly affecting the psychical
condition of the animal. The psychical change
may follow immediately as a sensation, and may
remain as a new association in memory, or it may be
a subconscious nervous co-ordination ; or again the
psychical change may be only a gradual change of

84 DEVELOPMENT AND HEREDITY.

the state of feeling, increasing or decreasing the
vitality or general nervous activity of the animal.
This statement holds true in so many cases, and
in such variety of cases, that it may be regarded
as a general law. It means that whether or not
the physical change results in any immediate action
on the part of the animal, there remains recorded
in its nervous mechanism some change that will
affect future action, either as memory or as the
beneficial result of practising some nervous co-
ordination, thus making future action easier, or else
as a generally weakening or intensifying effect on
the nervous activity. We know this statement to
be true, in cases where the nervous system itself
has been affected, and if we believe that the nervous
system has been evolved from undifferentiated
living matter, then we must accept the statement
in its broader application to all living matter.

While we have no exact knowledge of the nature
of the molecular changes in the nervous system, nor
can we in any instance form a clear conception of
how they succeed each other in the sequence of
stimulation and reaction, yet still we may gain a
certain knowledge of the laws of their operation
by studying the psychical changes which accompany
the physical changes. We have in our own experi-
ence and daily observation a multitude of facts,

DEPENDENCE ON ASSOCIATION. 85

showing the relation and sequence of stimuli,
psychical effects, and reactions. Such facts are our
knowledge of how we and our fellow-men and
domestic animals are affected by all the varied
stimuli which constitute our and their environment,
causing conscious or unconscious cerebration which
may at some time exert a decisive influence upon
our movements. That this idea of the operation of
energy upon living organisms may be extended to
embrace even the material aspect of the higher
mental functions, seems to be the conclusion of
physiological psychology. Mr. Herbert Spencer
has attempted the statement of the case as follows:
“No thought, no feeling, is ever manifested save
as a result of a physical force. This principle will
before long be a scientific commonplace.” Professor
Wundt, without assuming a causal relation between
the two series of phenomena, has given us a better
conception, as follows :—

“This is what the analysis of the process of
sensation comes to, viz. that logical necessity and
mechanical necessity differ, not in their essence,
but simply according to our way of regarding
them. That which is given to us by _ psycho-
logical analysis as a continuity of logical opera-
tions (Schiisse), is given also by physiological
analysis as a continuity of mechanical effects (Kraft-
wirkungen). ... Logic and mechanism are identi-

86 DEVELOPMENT AND HEREDITY.

cal; they are both only the form of essentially the
same contents (gletchartigen Inhalt). Thus from
the occurrence of psychical changes we may infer
the occurrence of corresponding physical changes,
even though the latter by their subtlety may be
beyond detection by our present means of physical
observation.

It is the peculiar property of living matter which
we recognise as an elemental nervousness, and which
so wonderfully transmits and transforms the forces
of the environment, that I hope to show is the basis
of that capacity for development which is inherent
in all organisms. In order, therefore, to understand
the nature of development, we must analyse the fun-
damental properties of nervous action. In doing
this, we need not hope to establish any obscure or
hitherto unnoticed nervous property as a cause of
phenomena so universal as development, but shall
turn our attention rather to properties that are
equally universal and apparent.

If we turn to that development with which we are
most familiar, and which we can most easily analyse,
namely, our own later individual mental and bodily
development, we find that it is due to the effects
of repetition of actions and thoughts, and to asso-
ciation of the same. Says Mr. Sully :1 “The com-

1 « Mental Elaboration,” Jind, No, Ix., October, 1890,

DEPENDENCE ON ASSOCIATION. 87

mon maxims of education,— ‘Exercise strengthens
faculty,’ ‘Practice makes perfect,’ — illustrate this
fundamental fact of our psychic life, viz. that the
results of our several actions persist, rendering a
renewal of these actions easier, and also contributing
to the development of higher forms of activity.”
How true and universal is the maxim, ‘ Practice”’
(or repetition) “makes perfect,” all will admit ; but a
few illustrations will make clearer its bearing upon
the present subject. Thus we know that an action,
at first extremely difficult, such as fingering a musical
instrument, becomes, after long practice, so easy that
it can be performed while the attention of the per-
former is directed to other matters. The successive
actions follow each other without separate volitions,
and when such a series of actions becomes well prac-
tised, it requires considerable effort to omit any one
action in the series. Other kinds of actions may be
repeated so frequently as to become habitual and be
performed unconsciously, as brushing a fly from the
face, or smoothing the hair or beard. The repetition
of a mathematical solution, or an abstruse train of
philosophical speculation, enables the mind eventu-
ally to follow the chain of reasoning with perfect
ease. All of our actions and thoughts are perfected
by repetition, and what at first required a long time
for its performance, becomes eventually performed in

88 DEVELOPMENT AND HEREDITY.

a very brief time. In fact, were it not for the ability
we possess to make our mental and corporeal actions
habitual and more rapid of execution, life would be
too short for us to aspire to more than its bare
necessities.

We can observe this principle acting in the growth
of habits among animals, especially our domestic
animals, with which we may be familiarly acquainted.
A young bird’s first attempts at flight are very im-
perfect, but after repeating the action a number of
times, perfect power of flight is attained. We find
the same awkwardness and inability generally where
we see young animals first attempting to co-ordinate
their muscular efforts for a definite purpose. The
same thing is evident where older animals are placed
in new conditions of life, where they are required to
attempt entirely new movements. The structure of
the nervous system gives evidence that the same
state of affairs prevails wherever we find a distinct
nervous system ; and the fact that all protoplasm par-
takes somewhat of the nature of nervous tissue, in-
dicates that, throughout the whole realm of life, the
performance of an action tends to become more
easily and quickly executed by frequent repetition.

The difference in the effort with which an action
is performed for the first time, and again after much
practice, has its correlative in a difference of nervous

DEPENDENCE ON ASSOCIATION. 89

molecular conditions. The progress of the change
may be compared to the wearing away of the
roughness of the machinery and the reduction of
friction and resistance to a minimum. Also, new
arrangement and co-ordination of the nervous mech-
anisms have been accomplished, for where at first
each separate part of an action required a distinct
impulse to start it, after practice the whole of a
complicated series of actions may be produced by
one initial impulse; for example, a trill played on
the piano:—just as if at first five impulses had
to be sent from the brain to the five fingers, and
afterward only one impulse should be sent from the
brain to the arm, which should then send on the five
secondary impulses to the fingers. The change of
structure resulting from frequent repetition of the
same series of nervous molecular changes is in gen-
eral a permanent one; for when an art or habit is
once acquired the skill persists, and even after long
lapse of disuse is more readily revived, or acquired
more readily the second time than the first.
Besides rendering individual actions easier of exe-
cution, repetition has the effect of uniting successive
actions when frequently performed in the same
succession. A series of actions often repeated be-
come thus so associated that when the first action
of the series is performed the rest will generally

90 DEVELOPMENT AND HEREDITY.

follow without any additional stimulus. The fre-
quent repetition of a series of nervous molecular (or
material) changes, tends to make the changes in
the series always follow each other in the same
order. We know, when a train of ideas has often
passed through the mind in a certain order, that
thereafter those ideas, when the first one has been
suggested, will always come to mind in the same
order. This is also well illustrated in the case of
recalling or humming a tune, — which we may sup-
pose to be correlated with as many nervous mole-
cular changes as there are musical notes. Every one
knows how the different notes follow each other,
associated together in measures and rhythms. The
principle is the same in playing a tune on an
instrument where the movements of the fingers
follow in regular order. The different movements
become firmly associated together. The same is
true of a great many kinds of actions performed
by artisans and skilled labourers who may be con-
stantly repeating the same series of actions. The
arts of knitting and spinning are good examples, as
being each a series of actions that can go on
indefinitely, while the attention of the operator is
devoted to something else.

By the statement that the series of actions takes
place without requiring attention, is meant that

DEPENDENCE ON ASSOCIATION. 91

each separate act in the series can occur without a
separate stimulus from beyond the local nervous
co-ordination which controls the whole series, —so
that, after the start, each separate act in the series
has a sufficient guiding stimulus in the action
immediately preceding. There are many perform-
ances where the stimulus of the preceding action
is necessary for all but the initial action of the
series. In a performance involving a series of
associated actions, the performer often finds it im-
possible to start in the middle of the series and go
on to the finish, but must start at the beginning
in order to have for each action the guiding stimulus
of the preceding action. This is true of players
of musical instruments in certain musical passages
which are peculiarly difficult of execution. Other
performances of manual skill may have the individ-
ual movements so firmly associated that they can
not be performed properly when separated. Such
strength of association seems to have’ its counter-
part in the nest-building instinct of birds. These,
when they find a nest already half-built, do not
undertake to finish it, but instead they start a new
nest on top of it; or else they tear it down and
then start a new nest from the beginning. In the
same way they frequently tear down their own
half-built work, or sometimes build a second nest

92 DEVELOPMENT AND HEREDITY.

on top of one that they have already completed.
In the case of the musical performer, we know
that each individual action in the series must have
had its own separate stimulus before there arose
any subordinate nervous co-ordination, and also each
stimulus must have been practically independent of
the preceding action. It was only by the frequent
repetition of this series of stimuli in regular order
that the subordinate nervous co-ordination was
effected, so that finally the initial stimulus in the
series of stimuli is sufficient to guide the whole
series of actions.

We know that if a certain train of ideas has
passed through our minds often enough to become
firmly associated, and if then a new stimulus shall
call up a new idea at the end of this train, then
there is a tendency for that new idea to be added on
when next the train of ideas shall repeat itself in
our minds. If the new stimulus should call up the
new idea at the end of the associated train of ideas
several times, then the new idea would remain in
that association a long time; but if the new stimulus
should never again occur the new idea would
gradually fade away. We see how in this way the
final parts of an associated series may gradually
disappear when their originating stimuli wholly
disappear. But it is important here to notice that

DEPENDENCE ON ASSOCIATION. 93

the strength of association alone will retain and
hold the idea or action in its place in the series
for a long time after the originating stimulus has
disappeared. Thus the final movement of a finger,
in playing a well-practised trill on a piano, cannot
be dropped immediately. It requires a well-directed
effort and some practice to get rid of it. Neither
can an action be dropped easily from the middle
of such a series as we have been considering.
The association with the preceding and the suc-
ceeding members of the series, is so strong that
the middle member can be dropped out only with
great difficulty. Another characteristic of such
associated series is that the first portion of the
series is more strongly associated and more deeply
impressed in the nervous structure. This is readily
seen when the series nas not been performed for
a long time and is weakened by disuse: the first
portion is more easily recalled, because, as the series
was learned, the first parts were more often repeated,
and the final parts were added on gradually one by
one. For instance, suppose we have a series of
four parts, and let number one be practised twelve
times before attempting the combination of one
and two; then let the combination of one and two
be practised twelve times, and so on with one, two,
and three: we see that number one will have been

94 DEVELOPMENT AND HEREDITY,

repeated thirty-six times before number four has
been attempted; and as the performance must
always begin with number one, it will always have
the advantage of number four by thirty-six repeti-
tions.

These characteristics of nervous action which have
been mentioned in the few preceding paragraphs,
have a very great importance in organic development.
The importance, however, must not be measured
by the space here devoted to them. Illustrations
of the principles might be multiplied without end,
and volumes of detailed facts might be collected
as proof; but I believe the principles are so gen-
erally known and accepted, that it is sufficient merely
to recall them to mind. Their importance consists
in the fact that they are not peculiarities of a certain
few processes of thought and action, but are uni-
versal characteristics of nervous action, so far as
we can trace that action ; and accordingly they are
dependent upon some fundamental property of living
matter. As we believe nervous action to be depend-
ent upon the nervous system, so we must believe
that the laws of nervous action must be similar
according as the different nervous systems are simi-
lar. In the gradation of instinct and intelligence
which we find as we ascend from the lower animals
to the higher, we have a large body of evidence lead-

DEPENDENCE ON ASSOCIATION. 95

ing us to the conclusion that the differences which
we find in the nervous action of animals, are differ-
ences of degree only and not of kind. The difference
lies in the complexity of organisation, and not in
the essential properties on which nervous activity is
based. We may infer, therefore, that these funda-
mental properties and laws of nervous activity are
the same for all animals which have a nervous
system. As the evolution of nervous systems from
undifferentiated protoplasm and undifferentiated tis-
sues, is a corollary of evolution— and we see it
occur in the development of each individual — there-
fore we may expect to find these properties existing
in some degree wherever we find living matter. We
have already had proof in a previous chapter of
the existence in plants of that fundamental nervous
property which displays so remarkably the effects of
repetition ; and we saw also how the effects of the
repetition remained for some time after the periodic
stimulus had been withdrawn. It may be objected
that I have assumed too much in stating that the
power of association and the effects of repetition are
so universal among organisms, but every co-ordination
of movements shows a power of associations; and
every animal whose actions are intelligible to us,
seems to be able to learn, —and plants show rudi-
ments of the same capacity; z.¢. an activity neces-

96 DEVELOPMENT AND HEREDITY.
sitated in an organism by external influence, is
performed with greater facility after frequent repe-
tition than before.

We have learned that each action of an organism
must have its sufficient stimulus— meaning by

“stimulus,” the action of force upon- the nervous
organisation. We are compelled to this belief by
the law of the conservation of energy. No molec-
ular, or atomic change or action can originate itself,
but must be part of the universal series of cause and
effect. The molecules of an organism must be sub-
ject to those laws of molecular mechanics, which, we
have reason to believe, apply universally. We must
therefore seek the cause of the actions of organisms
in all the forces which operate actively on the organ-
isms from the outside, or which may be stored as
potential energy within; and this latter potential
energy, we must remember, can have had ultimately
no other source than the world outside the organ-
ism. If we undertake to treat of the forces acting
upon organisms and the changes caused thereby, in
the terms of force, molecular change, etc., we are
quickly brought to a standstill. But if we treat of
them in the correlative terms of stdmulus, phystologt-
cal reaction, and nervous or psychical response, we may
proceed with some certainty, though not with mathe-
matical accuracy. As every molecular change de-

DEPENDENCE ON ASSOCIATION. 97

mands a sufficient cause in the form of force, so
every activity of an organism demands a sufficient
stimulus. If, now, we look for the stimuli which
produce these activities, we are obliged to include
every force that affects the organism; and such
forces we must recognise as the causes of the
activity, growth, and development of each individual
organism, and also of the changes occurring in indi-
viduals during the evolution of the species.

Among the countless forces affecting organisms,
we find a number which operate alike upon all.
These are gravity, light, heat, electrical conditions,
and the various combinations of energy comprised
under the head of nourishment. To these therefore
we may ascribe those characteristics common to all
organisms; and to the nourishment especially may
we ascribe that amount of energy which is expended
in the motion, heat, light, and electricity, produced
by organisms. But in so far as these forces are
universal, they give us no clue to the origin of the
diversity among organisms. The nourishment, it is
true, differs in different organisms, but this seems
a secondary rather than a primary cause of diversity.
Evidently, then, it is to the varying conditions and
quantities of the above-named forces acting as
stimuli, and also to the great number of those par-
ticular stimuli which are always peculiarly affecting

98 DEVELOPMENT AND HEREDITY.

each individual, that we must look for the causes of
the diversity of form and habit among organisms.
The action of these stimuli is cumulative in its
effect. After a stimulus has acted upon an organ-
ism, and the organism has returned to what is called
its “normal condition,’ we must not suppose that
the second normal condition is the same as the
condition of the organism before the action of the
stimulus; for the stimulus has caused a molecular
change, and this change persists until some other
force undoes it or intensifies it. The viscous living
matter retains its impression, and is more impression-
able than a solid body, of which Professor Maxwell
has said, “the stress at any given instant depends,
not only on the strain at that instant, but on the pre-
vious history of the body.” When a stimulus has
acted upon an organism, and called forth a respon-
sive change or action, then at the second time
that stimulus acts, and the third time, and always
afterward, the tendency is to produce the same
response. If we were now to take account of all
the stimuli acting on an individual during its life-
time, we would find that the vast majority of them
are constantly repeating themselves. By observing
the world around us, we see that in the lives of
animals — man included — each day is very much like
the day before. The stimuli and their responsive

DEPENDENCE ON ASSOCIATION. 99

actions form themselves into recurrent sequences, —
repeating themselves in groups with an established
regular order in each group. Those stimuli, arising
from chemical- changes in the process of nutrition,
are always present ; and though they have their varia-
tions, yet, in the healthy organism, they remain
generally the same, save that they are subject more
or less to recurrent periods of greater intensity.
The stimuli causing muscular motion, or protoplas-
mic contraction, are also frequently repeated. In
the animal kingdom, the muscular motions on which
the life of the individual depends, are repeated daily
and many hourly or even momentarily. Even the
stimuli causing the process of reproduction are, in
most animals, repeated many times during the life
of an individual.

We have already observed it to be a nervous
property of organisms, that a series of actions fre-
quently repeated tends to become more and more
readily performed. The repetition of the series
establishes a co-ordination in the mechanisms gov-
erning the actions, so that each action seems to
have some guiding stimulus in the action preced-
ing; and though some of the stimuli which were
originally necessary to the separate acts of the
series may be withdrawn, yet under the influence of
the remaining stimuli the whole series will be
executed with its usual regularity. Series of actions

100 DEVELOPMENT AND HEREDITY.

frequently repeated become fixed in their relation
to each other, and though the stimuli may vary
slightly from what they originally were, yet the
original series of actions will still result. The varia-
tion of the stimuli, however, if constantly repeated,
finally makes itself felt in a variation of the actions
of the series.

Professor Jos. Jastrow has recognised this law
of nervous action of which we have treated in this
chapter, and he has excellently described it as
what he calls the law of habit. “This law de-
clares that every reaction of an organism to a condi-
tion in its environment renders the repetition of
that reaction quicker, easier, more certain, more
uniform; and the existence of habits implies an
environment sufficiently constant to repeatedly pre-
sent to the organism the same or closely similar
conditions. Mere existence in a world so full of
regularities, of rhythm and law, of recurrence of
the same needs, results in the performance of defi-
nite actions in definite ways; and it equally results
that the earliest experiences will produce the
strongest impressions, and will gradually render more
difficult the learning of other modes of reaction,
even though these others, owing to a change of
conditions, would be more useful.” !

1“ Problems of Comparative Psychology,” Professor Jastrow, Popu-
lar Science Monthly, November, 1892.

DEPENDENCE ON ASSOCIATION. 101

If we could imagine an organism possessing the
fundamental nervous properties described in this
chapter, yet coming into existence with no inherited
potential energy in shape of co-ordinations or tenden-
cies, we could understand how, during an endless
existence under the influence of stimuli of environ-
ment, it would acquire great facility of reaction to
its every-day surroundings. It would acquire co-
ordinations which would affect its entire life-activity,
and thus influence its manner of growth and devel-
opment. A change in the environment entailing
new stimuli would effect after a while new co-ordi-
nations, and lead thus to new directions of growth.

CHAPTER VI.

THEORY OF DEVELOPMENT AND HEREDITY ILLUS-
TRATED AMONG PROTOZOA.

AFTER we have made full allowance for the
action of stimuli on an organism during the period
of its growth from inception to maturity, and have
allowed the great importance and necessity of this
action to the developing organism, there still remains
unaccounted for a sum of forces which are innate
in the organism, and which determine its essential
nature and specific character. The question arises,
what is the origin of those innate forces which
determine whether a developing organism will be
a horse or a dog? If we accept evolution, even
in its simplest form, we must believe that the
peculiar co-ordination of forces necessary to produce
a horse, could not have existed before the appear-
ance of vertebrates on the earth. If the conti-
nuity of life remained unbroken, the co-ordination
of forces which produce the horse, must have been

gradually acquired by living matter in the long
102

ILLUSTRATED AMONG PROTOZOA, 103

course of its unceasing development. Suppose
the vertebrates to be descended from worms, then
‘in like manner the co-ordination of forces necessary
to produce a worm must have been acquired, and
so on, back to the simplest form of living matter.
The raw energy, and matter in a crude state, we
may suppose to have existed anywhere at any time,
but the co-ordinations of forces necessary to pro-
duce a vorticella, a worm, a fish, or a horse, have
been acquired by the primitive form of living
matter by a long and slow process. It must have
been a series of causes and effects according to
laws, with no fortuitous variations, —a process that
acted in the living continuity of organic matter,
without regard to the destruction of the “unfit’”’
or the lopping off of side-branches of development.
This constant acquirement of new co-ordinations
of directive forces, indicates a universal capacity
of development, showing itself at every step in the
long course of evolution and co-extensive with life
itself. This capacity is that which makes possible
every new variation and every advance in develop-
ment. We are compelled to believe that it was
present in the archaic protoplasm, the first living
matter on the globe, just as much as in the living
forms of to-day. If we seek to know this capacity
in other terms, we can only ascribe it to the un-

104 DEVELOPMENT AND HEREDITY.

stable, impressionable constitution of living matter,
and to the effects of repetition known as retention
and association. An examination of the general
method of development shows that it is everywhere
the perfecting of a process by continued repetition
of the process.

In nature we find that no individual organism
acquires any of its parts or organs suddenly and
in their full perfection. Each organ or part is the
result of slow growth, and develops gradually
from the imperfect to the more perfect state.
Moreover, in tracing the course of evolution, we
find the same law to be true of the appearance
of organs in the species or class. The organs do
not appear suddenly as new and perfect parts in
any one generation, but they are the product
of the long process of graded development through
many generations. The proper development of
each organ, and the function of the organ, are
inseparably linked and interdependent on each
other. The function must have a controlling effect
on the development of the organ, and vzce versa the
structure of the organ must limit the function.
It follows, therefore, that both in the individual
and in the race, the organ and its function have
been gradually perfected. Take, for example, the
function of an organ like a leg or wing, which

ILLUSTRATED AMONG PROTOZOA. 105

requires the co-ordination of the movements of
several muscles. The nervous co-ordination neces-
sary to perfectly regulate such muscles so that
each shall act in its required time, and in proper
succession one after the other, can only have been
perfected by practice in the individual, and therefore
by practice in the race. One of the effects of
this practice has been an association of primarily
independent activities,— subconscious nervous asso-
ciation similar to mental association. This associa-
tion we must suppose to be a new arrangement of
the nervous elements, whereby actions formerly
independent should always occur most readily in
a certain definite relation to each other. In this
way only can we picture to ourselves the origin
of such complicated muscular movements as the
rhythmic beating of the heart, the origin of the
mechanism by which the equilibrium of a living
body is maintained, or the origin of any bodily
agility or manual dexterity. It must be regarded
as a universal property of living matter, that the
effects of its activity are perpetuated in itself as
the perfecting results of repetition, and as associa-
tion, —the activity itself being caused, as we have
seen, by energy absorbed from the environment.
The effects, also, of organic activity, of practice and
repetition, must, in some way, be transmitted from
generation to generation.

106 DEVELOPMENT AND HEREDITY.

Let us now attempt to apply these general princi-
ples, and try to explain the development of a hypo-
thetical first organism ; for, since the state of living
matter is dependent on all its past history, we must
naturally begin at the beginning in order to explain
the changes which living matter has undergone. It
is possible in this way to illustrate the theory by a
typical case, if we carefully keep within the limits of
the general principles of evolution, as it may be in-
ferred from the visible organic world and the traces
of its past history. Let us imagine an organism,
consisting of a simple, undifferentiated mass of pro-
toplasm, to have come into existence without any
inherited powers or tendencies. There are no ner-
vous co-ordinations or associations. All of its move-
ments and reactions are dependent merely on its
chemical and physical conditions and upon the forces
which affect it. In the first instant of its existence,
this highly sensitive and mobile organisation of mat-
ter is subject to the action of a variety of forces.
There. are the forces of pressure exerted by the
medium in which the organism lives, and by the sup-
porting body upon which it rests. There are also
the forces exerted by things brought into contact
with the organism by movements of the surrounding
medium. Some strike or press against the outer
surface of the organism. Others are forced into the

ILLUSTRATED AMONG PROTOZOA. 107

soft mass and dissolved ; and if they be of a certain
chemical composition, operate as nutriment, and lib-
erate new forces internally or add to the potential
energy of the living mass. The forces of light, heat,
electricity, and gravity, all act upon the organism. To
all of these forces the living mass reacts in a purely
mechanical fashion. The reality of the effects of
these forces upon inanimate matter is well known,
and living matter, as we have seen, is also affected
by them. The effects upon living matter are much
more complicated than upon inanimate matter. This
is due to the peculiar constitution of living matter,
it being a viscous, semi-fluid mass, composed of a
physical mixture of water and various chemical com-
pounds of varying degrees of density and viscosity.
Under the continued action of the forces of the
environment, the mass of living matter, like all
other bodies, tends to attain a state of rest. In
the nature of things, however, this equilibrium
toward which the organism and its environment are
tending cannot be attained, for the reason that the
environment does not remain the same, but is con-
tinually changing. There is an alternation of day
and night, light and darkness. The energy of the
sun causes changes of temperature and of density
in the medium in which the organism exists, be it
either air or water. These changes of temperature

108 DEVELOPMENT AND HEREDITY.

and density cause currents in the medium. Sur-
rounding objects are set in motion, and perhaps the
organism itself is moved about. Some of the forces
which act upon the organism are constant; such
are gravity and the pressure of the medium. Other
forces are regularly periodical, as the light and heat
of the sun. Still other forces are intermittent and
irregularly periodical, such as the contact and col-
lision with surrounding bodies, the impulses that
shove and transport the organism, and the contact
with particles of nutriment. Some of this last class
of forces may act very rarely, others at lesser in-
tervals, and still others will act very frequently.
Each of the forces which act upon the organism
—be it constant, regular, or irregular — will pro-
duce its own peculiar mechanical reaction in the
mass of living matter. Thus some of the reactions
will be repeated at regular intervals, others will be
repeated irregularly with greater frequency. In or-
der to understand the effects of this repetition, we
must recall to mind that peculiar property of living
matter which we have called elementary nervousness,
which makes possible and necessary the formation
of co-ordinations and associations as the result of
repetition of the necessitated reactions. As already
pointed out, this property seems to be coextensive
with organic life, and we must therefore give it

ILLUSTRATED AMONG PROTOZOA. 109

due weight in estimating the effects of environment
upon our hypothetical organism.

The forces of the environment have produced a
deeper effect than we could detect from a physical
point of view by a purely physical analysis. We
know as a general fact that each reaction of a ner-
vously constituted body produces an enduring effect
within the body. Each separate action of force, or
each stimulus, causes its peculiar mechanical reac-
tion. Each sequence of stimulus and reaction will
have its effect upon the nervous molecular condi-
tion of the organism; and the more often the
sequence is repeated, the more marked will be the
effect. With frequent repetition, the response comes
more easily and more quickly. Should any of the
stimuli form a regular series frequently recurring,
then the series of reactions, often repeated, become
associated together by co-ordination of the nervous
elements. Because of this association, all of the re-
actions in the recurrent series tend to occur together.
Each reaction of itself tends to produce the succeed-
ing reaction. Thus several reactions in a series
might ensue upon the action of a single stimulus.
For example, let us suppose a nutritious particle to
be borne by the current toward our first organism.
The molecules which separate from the nutritious
particle as gas will be the first to strike the organ-

110 DEVELOPMENT AND HEREDITY.

ism, and stimulate as smell; next, the particle itself
strikes against the organism, and there is a stimulus
of contact ; then the process of absorbing — caused
by contact, capillarity, etc. — brings the protoplasm
to flow toward and around the particle. Thus the
sequence of reaction is first smell, then contact, and
then movement in the direction whence the smell
and contact have come. If this simple sequence
were repeated often enough, an association of the
reactions would arise. This association might event-
ually become so strong —through repetition — that
the movement in the direction toward the gaseous
effluvia might follow after the smelling, even when
the stimulus of contact did not occur. We can
thus imagine how the co-ordinations arose which
control the search for food and locomotion.

Let us suppose, further, that owing to scarcity of
food or to increase of size, the organism is stimu-
lated to movement by hunger, or perhaps by asphyxi-
ation caused by remaining in water fouled by its own
carbon dioxide. The loss of food and oxygen would
cause a stoppage in the natural chemical change
going on in the mass; thus there would be a dis-
turbance in the inner equilibrium of forces, which
might readily cause slight movement in different
parts of the soft mass. The hunger and dearth of
oxygen would probably be conditions frequently

ILLUSTRATED AMONG PROTOZOA. 111

repeated. At any rate, there would be almost
constant variation in the chemical changes of the
protoplasm. Thus after each movement of the
protoplasm, the stimulus remains to start a new
movement. By constant repetition of the reaction,
nervous co-ordinations would be formed and per-
fected, so that movements at first slow would
become more and more rapid, until they finally
resemble a tetanus, or else the quick movements of
the cilia or flagella of the infusoria. Some of the
shell-amcebe make movements intermediate in form
and rapidity between the ordinary slow amoeboid
motion and the quick motion of the flagellum of
an infusorian. Any movements of the body would
thus constantly tend to become a means of loco-
motion.

The periodical and occasional stimuli acting on
an organism so primitive would necessarily be of
few kinds. The same stimuli and series of stimuli
would be frequently repeated. Those which were
repeated most frequently would of course have the
greatest effect, while those that occurred very rarely
would have little or practically no effect. In the
course of time, therefore, as this process of repeti-
tion goes on, the organism would acquire a facility
of reaction due to nervous co-ordinations. In these
nervous co-ordinations we see a development caused

112 DEVELOPMENT AND HEREDITY.

by the forces of the environment acting upon the
organism. This development is also an adaptation,
inasmuch as it enables the organism to perform
more readily those actions which the environment
makes necessary for its existence.

It may be objected that the stimuli are too few
in number and too constantly the same to produce
any such variety of form and habit as is found even
among many unicellular animals. But if we con-
sider the matter more closely, we shall see that
there is an inevitable gradual change in the influence
of the stimuli, and that this change induces new
effects, and opens the way for new stimuli to an
almost infinite degree. Let us follow our hypo-
thetical organism a step farther. By the assimila-
tion of new particles of matter the bulk is doubled,
and the mass is increased more rapidly than the
surface. The amount of oxygen necessary for the
life of the protoplasm increases as the mass in-
creases; but the amount of oxygen that can be
absorbed from the surrounding medium increases
only as the surface of the protoplasm increases.
Therefore in the process of growth the mass would
at some time become too great for the amount of
oxygen that can be taken in by the surface. The
result would be the asphyxiation of the interior
part of the mass of protoplasm. In the movements

ILLUSTRATED AMONG PROTOZOA. 113

caused by this constant and severe stimulus, we
may suppose the organism to have extended itself
to the utmost in different directions, — for the less
spherical its shape, the greater its relative sur-
face, — until finally the protoplasm parted and seg-
regated in two masses. This is the first step in
the multiplication of individuals. The two masses
of protoplasm are each of the same size, we will
suppose, as was the original mass at the beginning
of its existence. They are each, in the moment
when they come into existence, subjected to the
same permanent stimuli of the environment as acted
upon the parent at the beginning of its existence.
Also the occasional and periodic stimuli act upon
them in the same manner as they acted upon the
parent. Therefore, as a result of this similarity of
conditions, the two organisms naturally follow the
same course of development that was followed by
the parent organism. But there is this difference
between the parent and progeny: the two of the
latter generation are, at the first moment of their
existence, composed of protoplasm that has already
reacted many times to all the stimuli that affect it.
Once before it has been of the same size, and has
responded to the same constant and periodic stimuli
that affect it now. As those stimuli determined its
growth then, so they determine it now. Owing to

114 DEVELOPMENT AND HEREDITY.

that associative property of protoplasm which we
have discussed in the previous chapter, the reactions
to stimuli, z.¢. the growth or development, take place
the second time more easily and more rapidly than
the first time. Therefore, during the second cycle
of life, this unbroken continuity of protoplasm, which
now forms the second generation, would form its co-
ordinations more easily and more rapidly. The re-
actions and co-ordinations of the first organism have
left their impress upon the protoplasm, and this
impress affects the development of the second gen-
eration. In the third and fourth and every succeed-
ing generation we have a repetition of the entire
process of development. This repetition tends to
perfect the process of development,—to make it
occur more rapidly and more easily. It tends to
combine the reactions which we call growth and
development in an indissoluble chain of association ;
and it tends also to make those reactions always the
same.

The same permanent external forces acting upon
an organism affect it differently at different periods
in the development of the organism; for, though the
forces themselves remain the same, they will neces-
sarily produce different results in the organism as
the size of the latter increases and as the complexity
of its co-ordinations increases, z.¢. as the internal

ILLUSTRATED AMONG PROTOZOA. 115

conditions become more complex. We must bear in
mind that all changes in the constitution of the or-
ganism are the result of the action of forces which
are too numerous and intricate for us to follow. The
illustrations here given, however, will show the nature
of their action. Thus we see that, during the course
of life of an individual, both the constant and peri-
odical forces acting upon it produce a gradually
changing series of effects, as the state of the or-
ganism is gradually changed by its continued reac-
tions. These unchanged forces act as a gradually
changing series of. stimuli, and the life of the organ-
ism is thus a gradually changing series of reactions.
The reactions follow each other in a certain regular
order, as the organism changes in size and complex-
ity, and they are the same in each succeeding gener-
ation. When the same protoplasm, or continuity of
protoplasm, has repeated this series several thousand
times in as many generations, we can easily under-
stand how all the successive reactions in the series
become so firmly associated and co-ordinated that
they all tend to follow the first stimulus, even though
many, or even half, of the succeeding original stimuli
remain in abeyance.

Suppose all the forces of the environment to remain
the same for a great length of time. As each genera-
tion differs from the preceding generations in the

116 DEVELOPMENT AND HEREDITY.

number of times it has reacted to the series of
stimuli which make up an individual life, the con-
clusion follows, that in the latest generation the
associations will be strongest and the co-ordinations
most complex. This latest generation, owing to
its more complex constitution thus acquired, will be
affected by the same unchanged forces in a manner
slightly different from the manner in which each of
its predecessors was affected. The repetition neces-
sitates that there will be a new element —z.e. the
change in the internal resistance—added to the
stimuli affecting each successive generation. The
same unchanged forces of environment tend thus to
produce new effects and increase the complexity of
constitution in each generation.

As the difference between two successive genera-
tions depends on the fact that the later generation
has performed the series of reactions constituting
development one time more than the preceding
generation, it is evident that the difference between
the two will be most pronounced when the later
generation has almost finished its round of develop-
mental reactions. Therefore at this period in the
life of the second generation, —when the organ-
ism is approaching maturity,—the element in the
stimuli which is caused by the change in the rela-
tion between the unchanged external forces and the

ILLUSTRATED AMONG PROTOZOA. 117

newly added increase of internal complexity, will
be most effective in producing new features of
development. This explains the general fact in the
organic world that new variations of growth in
organisms appear, as a rule, about the time when
the individual completes its ontogenetic develop-
ment, — after the ancestral or generic characteristics
have developed. It would be difficult to determine
to what extent an unchanging environment can
produce changes of development in a long succes-
sion of generations, according to the principle under
discussion. Unless we disregard the most general
conclusions of Physics, Biology, and Psychology,
we must believe that some change, however small,
is produced in this manner in each generation —
just as we must believe that even the first revolu-
tion of a steel shaft contributes its share toward
causing the changes which, after many millions of
revolutions, become apparent in the crystallisation
and breaking of the shaft.

While we might suppose a certain amount of
development to take place in an unchanging environ-
ment, yet, asa matter of fact, all environments change.
Carrying analysis to the farthest limit, we find that
a changeless environment does not exist; the plane-
tary motions and the transmutations of solar energy
are factors of constant change, and from these and

118 DEVELOPMENT AND HEREDITY.

other causes arise a constant succession of changes
in the environment of organisms. Thus the combina-
tion of forces acting upon all organisms change
more or less slowly. Thus the relation between
the external forces of environment and the internal
forces of the organism is changed; and some new
adjustment of the structure and condition of the
organism must be the result. As a species of
organisms develops into more highly organised
and more complex beings, the later generations
will come under the influence of new stimuli occa-
sioned by their own wider range of activity. New
stimuli of this kind, too, can be effective in pro-
ducing new developments only after the individuals
of each generation have attained, in their onto-
genetic development, to that degree of growth
which brings them into contact with the new
stimuli, —just as a child cannot be developed by
practising the physical and mental pursuits which
are necessary for the development of a young
man. New stimuli, thus brought to bear on an
organism, are effective only toward the end of the
already acquired course of development.

The course of development may be summed up
as follows: the primitive mass of protoplasm with
which we started acquires nervous co-ordinations
which influence its activity and growth; as it divides

ILLUSTRATED AMONG PROTOZOA. 119

and redivides, it adds continually new co-ordinations
to those already acquired; the process of growth
and development comes by repetition to have the
character of reflex action, — becomes more rapid.
As the same forces act on each generation, and form
a series of stimuli which are similar for each gener-
ation, so each generation repeats in its life the
course of development followed by all its ancestors ;
or, more concisely, the continuity of the protoplasm
repeats in each generation the same series of re-
actions to stimuli that it has repeated, with addi-
tions, in each previous generation since it began
its existence. We have here the explanation of
growth, development, and inheritance. The illus-
tration just given shows how fundamentally the
three are related in their nature, being in reality
only different phases of one process. :

CHAPTER VIL

APPLICATION OF THE THEORY TO MULTICELLULAR
ORGANISMS, AND THE RELATION OF THE GERM-
CELLS TO THE BODY,

Ir we admit the efficacy of the causes of which
we have been treating, and believe that they have
acted in the described manner so as to produce the
diversity noticed among the single-celled protozoa,
and also to account for the simple phenomena of
heredity among them, then we are immediately led
on to inquire how a theory, which may be plausible
in regard to organisms composed of single cells,
will appear when applied to those organisms which
are each composed of many cells. Evidently, if there
be a physiological difference between the many-celled
and the single-celled organisms, their organic phenom-
ena cannot be explained by the same theory. There
has been a general impression among biologists,
that the dividing line between the metazoa and the
protozoa is peculiarly unique among all the divisions

in the classification of organisms, and that it indi-
120

. APPLICATION TO METAZOA. “421

cates a profound difference of constitution between
the two groups. The importance of the division,
from a morphological point of view, in its bearings
on the cell theory, has, I think, led to the assumption
of a physiological importance which has no sure foun-
dation ; for the assumed difference seems to be based
wholly on morphological evidence, whereas what phys-
iological evidence may be obtained points to a physi-
ological similarity between the two groups. Thus,
as nearly as can be ascertained, they have each
nearly the same chemical elements for the basis of
their composition; the general phenomena of nutri-
tion are the same; they each alike react to anzs-
thetics and various physical and chemical stimuli ;
they each ‘have the power of separating off a small
portion of the substance of their body, and thus
reproducing their kind. The prevailing difference
which is generally agreed to exist between the two
groups is, that in one case the protoplasm composing
the body is intersected by the cell walls, running in
all directions, while in the other case there seems to
be no partition of the protoplasm; and this continu-
ous mass of protoplasm, which composes the entire
body of one of the protozoa, is considered as equiva-
lent to one of the cells of a metazodn. In the
simpler forms of the metazoa, each cell has a nu-
cleus ; in the more complex of the protozoa, a single-

122° DEVELOPMENT AND HEREDITY.

celled animal may have several nuclei. Thus we see
that the nucleus affords no basis for the assumption
of such a profound distinction between the two
groups. The presence of cell walls seems, there-
fore, to be the one distinguishing characteristic
separating the metazoa from the protozoa. This
characteristic, however, loses much of its impor-
tance in view of certain conditions obtaining among
the protozoa. If we examine a Radiolarian, we find
that its protoplasm is not perfectly homogeneous ;
it possesses a skeleton, which is partly internal, and
this makes certain divisions in the protoplasm. Ac-
tinosphera, which has no hard skeleton, has its
protoplasm divided by membranes, in the form of
concentric spheres, and intersecting radial planes,
thus forming a true cellular structure, except that
a nucleus is not present in every cellular space. I
have seen a green zodspore of CEdogonium being
digested in one of these cellular spaces of the Acti-
nospheera, and observed, after the chlorophyl bodies
were broken down, and when the green mass of the
zodspore seemed completely dissolved in the proto-
plasm of the Actinospheera, that the green colouring-
‘matter filled even into the corners of the cellular
space, but did not penetrate beyond the membrane
which limited it. We find here, among the Radiola-
rians, a plan of structure closely approaching that

APPLICATION TO METAZOA. 123

of the metazoa, and the reason for it seems to be not
far to seek; for an animal as large as Actinosphera,
composed as it is of protoplasm, which is externally
fluid and radiating, could not maintain its spherical
shape without the support given by the intersecting
membranes inside of it. This seems to be the pri-
mary function of cell walls, wherever we find them.
No animal could attain to any more than a micro-
scopic size, and possess any permanence of shape or
rigidity of body, without the aid of cell walls; and
we find in the simplest of metazoa, and in the sim-
plest tissues of all metazoa, that the cell walls form
an indispensable means of support. Wherever we
find them, they are the means of support, and, in
comparison of their supporting function, their func-
tion in limiting physiological activity sinks to a sec-
ondary importance. Among the protozoa, portions
of protoplasm, which are not separated from each
other by cell walls, perform separate functions, —as,
for instance, the digestive and contractile parts of
a vorticella, — thus showing that a physiological limit
and a cell wall are not identical. Neither must we
suppose that the many-celled animals are of a higher
grade of complexity than the single-celled animals,
for —leaving sponges aside —none can maintain
that the sluggish Hydra is a more highly developed
organism than the graceful Stentor or Paramoecium,

124 DEVELOPMENT AND HEREDITY.

with their differentiated cilia, cesophagus, pulsating
vacuoles, etc. We may therefore conclude that
there is no justifiable ground for the assumption of
any fundamental physiological difference between
the protozoa and the metazoa, other than that differ-
ence caused by the greater size of the latter, and its
consequent necessities of mechanical support. We
are accustomed to consider the universal phenom-
enon of an embryonic organism passing from a
unicellular to a multicellular stage without thereby
inferring a radical physiological change. In the
words of Professor Whitman, “the fact that phys-
iological unity is not broken by cell boundaries
is confirmed in so many ways that it must be
accepted as one of the fundamental truths of
biology.”

It may be conceded, therefore, that such an ani-
mal as Hydra possesses its cell walls chiefly for
support, that its protoplasmic continuity is unbroken,
and the effects of irritability may be transmitted
from any point to all parts of the animal; in short,
that its physiological unity is complete. If we turn
from the simpler forms of the metazoa to the more
complex forms, we find a different condition of
things. A mammal, for example, is composed of a
variety of organs which make up an individual.
These organs are separate from each other to a

APPLICATION TO METAZOA. 125

high degree of independence morphologically, and
are different both in tissue and in function. It is
difficult at first glance to conceive how the results
of irritability and sensibility can be transmitted
from one organ to another so perfectly as in the
case of Hydra or the single-celled organism. But
we find that all the organs of the mammal are
united by particular nerve-tracts to a central ner-
vous system, which is affected by nervous impulses
from every part of the body, and in its turn is
capable of affecting every part of the body. To
make good the comparison between the Hydra and
the mammal, we must try to conceive how the
nervous system of the latter has been evolved
from the generally unstable condition of the former,
and for this I think no other explanation can be
given than that usually set forth,—to the effect
that nervous molecular impulses passing through a
body would seek the channels of least resistance ;
by repeatedly passing through the same channels
they would cause a differentiation of the tissue,
thus forming definite nerve-tracts. Whether we
refer it to the principle of natural selection or the
principle of improvement by repetition of action, in
either case the facts warrant our conclusion that
the complex nerve-tracts perform their office more
perfectly than the undifferentiated body tissue. In

126 DEVELOPMENT AND HEREDITY,

such an organism as the Hydra the effects of irrita-
tion spread in circles from the point stimulated,
whereas in the mammal they are carried in definite
channels to the nervous centre, and from there
transmit their effects throughout the body, the
nervous centre acting like the banker's clearing-
house in adjusting the balance of credit. Thus we
see that the physiological union between the parts
and organs of a mammal is as complete as the
union between the parts of a unicellular organism.
There is no reason to suppose that a change can
be produced in one part of a multicellular organism
without affecting in some way the entire organism.

But there remains a difference between the uni-
cellular and multicellular organisms in regard to
reproduction and inheritance. In the first the
organism simply divides into a number of pieces
of protoplasm, and each piece possesses the char-
acter of the entire mass. In the second, particles
of protoplasm are separated off from the organism
and continue to live and grow into new individuals,
while the main body of the parent organism remains
unchanged for a period, and then dies. An under-
standing of this phenomena is best obtained by
following the generally accepted view that the
metazoa are descended from the protozoa. The
course of this evolution must have been something

APPLICATION TO METAZOA. 127

like the following hypothesis: The single-celled
organism, by means of frequent repetitions, in the
course of generations began to attain a larger size,
and then to differentiate its external surface into a
denser protective layer—the result of stimuli of
contact. When such an organism as this would be
divided into a number of pieces by the natural
process of reproduction, those parts of the proto-
plasm which had not undergone a grosser material
differentiation would be like the protoplasmic germs
of all its ancestors, and capable of responding to
the same stimuli, and therefore of developing in the
same manner. The only difference between these
and the ancestral germs would be the increased
complexity of their nervous co-ordinations, as was
pointed out in the previous chapter. But, on the
other hand, the part of the organism which has
been differentiated into the denser outer layer would
be in structure so different from the germs of the
species that it would be incapable of responding
to any of their accustomed stimuli, and therefore
incapable of repeating the development ; but if some
of the protoplasm remained within the denser pro-
tective outside layer, it might continue to live, thus
continuing directly the life of the old organism. If
all of the organism should be developed into highly
differentiated parts or organs, and no part of it be

128 DEVELOPMENT AND HEREDITY.

like the original germ-like form, then the process
of its development could not be repeated and its
regeneration would be impossible. But at every
step in the evolution a part of the protoplasm
retains its original qualities, only changing its ner-
vous condition to a condition of greater complexity
of co-ordinations. In this way the original proto-
plasm gradually adds to itself the co-ordinations for
developing in each generation, first cell walls, and
then differentiated organs. Thus are produced parts
which cannot subdivide in particles, and cannot, by
reacting to the external forces, repeat in each par-
ticle the development of the species. The differ-
entiated organs, for some essential reason that we
will consider later, have not the power of enduring
indefinitely, and therefore together form that part
of the organism which dies. At one extreme of
the scale we see the new-born protozoa bursting
and casting off the dead membrane which protected
the protoplasm during the period of “rest” or incu-
bation; at the other extreme we see the body of
an animal die long after it has ceased to produce
young.

Professor Weismann has recently advanced a theory
of heredity, in which he holds the view that the
changes which occur in the body (soma) of an indi-
vidual cannot affect the germ-cells. This view is

APPLICATION TO METAZOA. 129

so opposed to facts which show a most intimate vital
relation between the body and the germ-cells, that
I think it would never have obtained credit were
it not a part of a theory which was developed with
great ingenuity, and which was partly based on the
profound truth of the continuity of the co-ordina-
tions of living matter.

Professor Weismann has regarded the germ-cells
as parts upon which no molecular change could
be induced from without. All protoplasmic motion
is generally believed to be due to molecular changes,
and Professor Romanes has shown that in the undif-
ferentiated tissues of the jelly-fish, an irritation at
one part causes movement (molecular changes) to
be propagated -throughout the mass. We would
be unable to imagine the evolution of nerves, did
we not assume that all living matter possesses to
some degree the property of transmitting nervous
molecular changes; and wherever vital communi-
cation exists between two parts, it must be possible
for molecular changes in one part to induce molec-
ular changes in the other. The evidence on this
point is somewhat obscured by the general presence
of special nervous courses, yet the evolution of nerve
courses compels us to the conclusion that their
properties exist in some degree in the undifferen-
tiated tissues, as in the case of the jelly-fish, where

130 DEVELOPMENT AND HEREDITY.

the nerve courses had been removed. The sensitive-
plant is a notable instance of this transmission of
molecular change from one point to another with-
out the aid of nerves. There is, further, some
ground for believing that a molecular change may
be transmitted even where there is no vital con-
nection between the parts. This seems to be the
case in several observed instances, where the pro-
toplasm of the egg-cell becomes unusually active
at the approach of the spermatozoén, yet before
the latter has touched the outer surface of the egg.
Also, we have the well-known case of certain conju-
gating alge (eg. Spyrogyra), two cells of which,
though not touching each other, are seen to exert
a mutual influence, causing outgrowths of the cell
walls of each cell toward the other, and inducing at
the same time a peculiar molecular condition of the
protoplasm. Some psychological investigators claim
that molecular changes can be transmitted from one
organism to another through considerable distances
of space where there is no vital connection! Hence
it seems not only unnecessary to suppose that the
changes in the body (soma) of an animal cannot change
the molecular condition, or nervous co-ordinations, of
the germ-cells, but rather, it seems impossible that
the somatic changes should not affect the germ-cells.

1 See Reports of British Association for Psychical Research.

APPLICATION TO METAZOA. 131

The effect of changes in the body upon the germ-
cells is a subject at which we can only arrive by
indirect evidence; but when we come to examine
the converse of the proposition, the effects of the
-germ-cells upon the body, shown by their pres-
ence or absence, a mass of direct evidence lies
at hand. It is a matter of almost universal obser-
vation. The removal of the germ-cells seems to
affect profoundly the constitution of the animal.
If the operation be performed on a cow at a period
of full flow of milk, the secretion of milk is said,
in some cases, to become permanent. If performed
on a very young cow, the growth becomes more
rapid and the beef will be more marketable. It
might be supposed from this that the germ-cells
act in some way as secretory glands, diverting
nutriment from the body; but that this is not their
only nor their strongest action is shown by the
marked change of instinct caused by their degen-
eration or eradication. In his book, Azzmals and
Plants under Domestication, Darwin speaks as fol-
lows on this subject :—

“Tt is well known that a large number of female
birds, such as fowls, various pheasants, partridges,
peahens, ducks, etc., when old or diseased, or
when operated on, assume many or all of the sec-
ondary male characters of their species. In the

132 DEVELOPMENT AND HEREDITY.

case of the hen-pheasant this has been observed
to occur far more frequently during certain years
than during others. A duck ten years old has
been known to assume both the perfect winter
and summer plumage of the drake. Water-
ton gives a curious case of a hen who had
ceased laying, and had assumed the plumage, voice,
spurs, and warlike disposition of the cock. When
opposed to an enemy, she would erect the hackles
and show fight. ... The females of two kinds of
deer, when old, have been known to acquire horns;
and, as Hunter has remarked, we see something
of an analogous nature in the human species. On
the other hand, with male animals, it is notorious
that the secondary sexual characters are more or
less completely lost when they are subjected to
castration. Thus, if the operation be performed
on a young cock, he never, as Yarrell states, crows
again; the comb, wattles, and spurs do not grow
to their usual size, and the hackles assume an
intermediate character between true hackles and
the feathers of the hen. Cases are recorded of
confinement, which often affects the reproductive
system, causing analogous results. But characters
properly confined to the female are likewise acquired
by the male; the capon takes to sitting on eggs,
and will bring up chickens; and what is more

APPLICATION TO METAZOA. 133

curious, the utterly sterile male hybrids from the
pheasant and the fowl act in the same manner,
‘their delight being to watch when the hens leave
their nests, and to take on themselves the office of
a sitter. “*

When we consider these facts and remember how
large a part of all the instincts and activities of ani-
mals are dependent upon the influence of the germ-
cells, it seems highly probable that the growth and
activity of an animal should exert a retroactive in-
fluence on the germ-cells.

In most animals the period for the ripening or
maturation of the germ-cells is determined by the
changes of season — influences which can only affect
the germ-cells through the medium of the body.
Finally, since the germ-cells possess the potentiality
of producing all the nervous and mental traits of the
body from which they are derived, we cannot sup-
pose them utterly disconnected from the nervous
system and inaccessible to its influence. There
seems to be a most profound and intimate connec-
tion between the two.

We find, therefore, no essential difference between
the development and reproduction of the protozoa
and the same process among the metazoa. If in-
heritance of acquired characters be admitted for

1 Vol. IL., pp. 26, 27.

134 DEVELOPMENT AND HEREDITY.

unicellular organisms (and I can conceive no other
reasonable interpretation of the facts), then the same
must be admitted for multicellular organisms. For,
in neither case, is it a process dependent on the
malleability or plasticity of the body or soma of the
organism ; but it is a process of acquiring new ner-
vous co-ordinations, — just as it is a chiefly nervous
difference which we observe to exist between the
amoeba, with its slow pseudopoda, and the infusorian,

with its quivering cilia.

CHAPTER VIII.

THE INHERITED IMPULSE OF DEVELOPMENT, OR TEN-
DENCY OF GROWTH.

WE have seen that the process of development of
living things was somewhat as follows. The first
living matter was subjected to a variety of constant
and intermittent forces, and frequent action of com-
bination of forces. These caused mechanical reac-
tions. Repetition made these reactions more rapid
and easy, and association combined them into reflex
series of co-ordinated actions. For the most part,
these forces acted on the living things in monoto-
nous rounds of repetition, which would have admitted
of no change in the individual were it not for two
peculiarities of living matter. First, the process of
assimilation caused an increase in bulk; and second,
the constant repetition of reactions caused by asso-
ciation an increase in the complexity of structure
of the paths through which the forces expended
themselves in the organism. The repetition and

association developed the nervous elements into
135

136 DEVELOPMENT AND HEREDITY.

combinations and co-ordinations which were always
becoming more and more complex. In the inter-
action of the environment on the individual, the
environment always remained the same, but the in-
dividual was constantly changed by the action of the
environment as each new development in the indi-
vidual brought it into a new relation with the
environment. Thus the life of the individual would
consist of constant growth and development.

When now a small part of the living matter of
this individual separates from the main mass and
begins an independent existence, it cannot continue
the progress of development without a break; for,
owing to its small size, and perhaps its lack of cer-
tain organs, the forces of the environment do not
act upon it in the same way as upon its full-grown
parent, and so the reactions of the small germ can-
not be the same. But the environment must act
upon the small germ almost exactly as it acted upon
the parent at that time when the parent was a small
germ of similar size and conditions. Thus the
second individual would follow almost the identical
course of development that the parent followed, and
so on, generation after generation. The continued
sameness of environment is the cause of the simi-
larity of successive generations.

The unbroken continuity of living matter thus

THE INHERITED IMPULSE. 137

expands itself and goes through the same series of
activities in each generation, —activities which are
familiarly classed as the phenomena of birth, growth,
and development. This repetition has the effect of
making the processes easier and more rapid, and by
association they become interlocked as links in a
chain of events which make up the life of each
individual.

Reasoning from analogy and the general principle
of evolution, we may suppose the first living matter
to have made only the simplest mechanical responses
to stimuli; but as the effect of each repetition of the
same actions impresses itself on the living matter,
the latter becomes more complex with each repeti-
tion, and thus we can imagine how, with the increas-
ing complexity of living matter, there arose a corre-
spondingly wonderful complexity of responses to
stimuli.

As the round of activities which make up the life
of each generation is constantly repeated by each
generation, it follows that the continuity of living
matter must become more and more complex ; that
is, the associations of actions — both animal and
vegetable processes — become more strongly bound
together and fixed in relation to each other. As
each generation, therefore, possesses a greater com-
plexity of structure than the previous generation, so

138 DEVELOPMENT AND HEREDITY.

each generation will present new characteristics to
be acted upon by the forces of environment. Thus,
though the environment remain the same, the simple
continuance of a race is continually bringing that
race into new relations with the forces of nature.
This developing action of the environment is some-
what modified by death and the process of reproduc-
tion. The same relation of force and matter is
necessary each time to produce successively the
same result, and so if any generation is to resemble
the one preceding, it must pass through the same
course of growth. When the first living organism
had attained its maturity, and part of it had sepa-
rated off to form a new generation, the second
organism could only come to resemble the first by
possessing the capacity of responding in the same
way to the stimuli of environment. Thus it would
pass through the same process of growth, and when
it had reached a development equal to that of its
parent, it would nevertheless be different, for it
would be composed of living matter which had twice
passed through the same round of activities. The
same would be true for the third and each succeeding
generation. But this added complexity of co-ordina-
tions, or fixity of associations, would be apparent as
such only when each generation reached a point of
growth equal to that of its parents at their maturity.

THE INHERITED IMPULSE. 139

That is, when each repetition was completed, its ef-
fects would be equal to a new element in the result
produced by the action of the environment on the
organism. Thus all generations, from the first to the
last, must begin alike, and start alike on the same
course of growth, but each repetition adds something
to each generation not possessed by the preceding
generation, until the full-grown of the ten-thousandth
generation may differ vastly from the adult of the
first generation. We see here the explanation of the
greatest general law of embryology, — that each indi-
vidual, in its development from inception to maturity,
repeats the history of the evolution of its race.

This added complexity in each generation may
at first thought seem inadequate to account for
such great consequences. But we must remember
that this repetition is constantly strengthening and
co-ordinating those nervous elements which control
the entire growth and development, and which,
above all things, chiefly receive and transmit the
influence of the formative forces of the environ-
ment.

This principle which I have endeavoured to set
forth here explains a number of well-known facts.
Since each generation begins its life by responding
to the customary stimuli that caused the growth
of all its ancestors, and, when it has finished that

140 DEVELOPMENT AND HEREDITY.

round of responses, —that is, has attained matu-
rity, —it then stands in a new relation to its envi-
ronment through the changed state of its nervous
system, therefore we should expect any change
of growth, any variation new to the race, to follow
at or near the period of maturity. As is well
known, this is what really occurs in nature; the
newest variations in the race are the last to
appear in the growth of the individual, the
specific differences appear later than generic differ-
ences, and these latter later than class differences.
Remembering that growth, development, and repro-
duction are merely forms of organic action, and
under the control of nervous force, we must believe
that they are subject to the fundamental laws that
control nervous action in general, as we discern
those laws in the involuntary movements of our
bodies and in the working of our minds. Now
it is a familiar fact that repetition increases facil-
ity and rapidity of action, and thus we observe
that the frequent repetition of the processes of
growth and development have rendered it possible
for the brief life of an individual to encompass
the development which the race has been untold
ages in acquiring. We find the proofs abundant
in nature that new characteristics of animals tend
to appear earlier and earlier in each subsequent

THE INHERITED IMPULSE. 141

generation. Mr. Francis Galton has described
some wonderful cases where diseases which were
hereditary appeared earlier by a great many years
in the children, and also still earlier in the grand-
children.

This general principle has been summed up by
Professor Weismann as follows : —

“The line of direction of development is essen-
tially the following :—

“1, The development commences with a state
of simplicity and advances gradually to one of
complexity.

“2, New characteristics first make their appear-
ance in the last stage of the ontogeny.

“3. Such characters then become gradually carried
back to the earlier ontogenetic stages, thus displac-
ing the older characters until the latter disappear
completely.” ?

We come now to consider the important part
played by nervous association. Since growth and
all the customary responses to stimuli of the environ-
ment are carried on only through the mediation
of nervous forces, it follows that these responses
will be subject to the laws of nervous phenomena.
We have already seen that we must regard living
matter as something that is constantly repeating

1 Weismann, Studies in the Theory of Descent, Vol. I., p. 274.

142 DEVELOPMENT AND HEREDITY.

a certain round of activities, — birth, development,
growth, and reproduction. This round takes place
over and over again, the result of the same round
of stimuli acting on living matter in ‘each genera-
tion, with only slight variations, which, as already
explained, are generally added only at the end of
the round. Now as this round, which we call the
life of the individual from inception to reproduction,
repeats itself, there is acquired not only facility
of response to stimuli, but those responsive actions
become associated together by the fundamental
property of nervous association, in such a way that
the chain of associated activities becomes almost
inseparable. On the same principle that a thought
in the mind calls up an associated thought, or one
tone of music calls up another, or one action in
an oft-repeated series of: actions calls up the next
subsequent action or actions, so the initial stimulus
being given to an incipient organism, its responsive
activity each time tends to produce by association
the next activity in the oft-repeated series, and
so on through the successive steps of growth and
development. By association the tendency to
develop in a given direction is already present; but
in point of fact the same succession of stimuli
which has acted upon previous generations is also
continually present to compel the same routine

THE INHERITED IMPULSE, 143

of growth activity. Now in this succession of
stimuli, suppose one of them to be dropped out
through some change in the environment, as pointed
out in Chapter V: the mere power of association
will remain sufficient to retain the corresponding
response in the chain of responsive activities. The
gap becomes bridged over ; and, though the stimuli
change within certain limits, the series of responses
remains the same for a time, though the change
in stimuli must eventually make its impression on
the chain of activities. Thus we see in the incipi-
ent crganism an indwelling, potential energy which,
to some extent, independent of the environment,
impels the organism along that course of growth
which repetition has made customary to its race,
and thus determines the specific character of the
organism — whether it shall be dog or horse.
This, then, is that inherited impulse of growth
which, in combination with external forces, con-
stantly drives the organism forward on its course
of development, and, even while the environing
forces remain the same, is continually exposing
the developing individual to new stimuli, because
it is continually changing the individual. This
constant change in the stimuli which act upon an
organism is a point which cannot be too strongly
emphasised in our effort to imagine the immediate

144 DEVELOPMENT AND HEREDITY.

causes of growth and development. The forces
of the environment may remain the same, but the
instant they act upon an organism they produce a
change in the organism ; this change puts the organ-
ism in a newrelation to the forces of the envi-
ronment, which again alters the action of the
environment upon the organism; ze. makes it a
new stimulus, and a new change in the organism
ensues, and so on, to an apparently infinite extent.
I have used the word “stimulus” as indicating the
action of a force on an organism at any one instant.
The stimulus is dependent on the relation of the
forces of the organism to the forces of the environ-
ment; while the latter may remain unchanged,
the organism changes, and the relation between
the two being changed, the stimulus is changed,
z.e. the force effects the organism differently.

The inherited impulse, which is produced by the
association of an often-repeated series of activities,
must, from its nature, be strongest in the initial
stages of the life of an individual, for those stages
have been oftenest repeated. The impulse will be
weakest for those characteristics which were the
last to be developed in the species, for these have
been repeated the least number of times, or gener-
ations. We see thus the aptness of Darwin’s method
of statement, that “characteristics become fixed by

THE INHERITED IMPULSE. 145

inheritance.” As the latest stages of development
have been repeated the least, we see the explanation
of the fact that these newest characteristics are the
most likely to vary slightly in consecutive genera-
tions and in different individuals. Professor Haeckel
remarked that “heredity is memory” ; and in this
sense we may say that the latest developments have
not been thoroughly committed to memory, z.¢. have
not been repeated often enough to become indissol-
ubly associated in the inherited impulse.

CHAPTER IX.

ILLUSTRATION OF THE METHOD OF GROWTH AND DE-
VELOPMENT, AND OF THE ORIGIN OF VARIATIONS.

In the previous chapter it was said that, while the
forces of the environment remained the same, yet
the constant change in the relations of the organism
to the environment, might produce new changes in
the organism to an apparently infinite extent. (I
refer to the organism here as a species, something
which passes through a succession of rounds of life,
ze. generations.) Should any one be inclined to
doubt this statement, let him consider for a moment
that the very fact of life implies a constant change,
— metabolism and katabolism of the body substance.
In order that this may go on, food must be supplied.
There are alternate periods of hunger and satiety,
for, in the animal kingdom, food is obtained period-
ically. This brings periods of activity and rest, and
thus already we have the elements of nervous change
and development. After a number of such periods,

the animal may return to its previous condition in
146

ORIGIN OF VARIATIONS. 147

every respect except as regards its nervous system.
This, by necessity of its nature, bears the record of
the changes, and is changed by them. As the ner-
vous system is changed, so the rest of the organism,
which is dependent upon it, must eventually be
effected by that change. And such change is going
on constantly. The importance of the diversity of
influence upon the nervous element of living matter
is shown by the relatively low state of development
among plants, where all periodicity and alternation of
condition is reduced to the minimum of day and night,
summer and winter, and different soils and climates.

But while theoretically organic evolution may pro-
ceed indefinitely without a change of environment,
yet I am disposed to believe it would be like one
of those infinite curves which forever approaches a
straight line. There would come a time when the
evolution would be inappreciable. Practically, how-
ever, we know that all organisms, in the course of
their evolution, have been subjected to change in
their environment. There have been geologic and
climatic changes, and changes caused for each one
by other organisms, affecting mutually the conditions
of existence. All the facts of natural history tend
to prove that the most highly developed organisms
have, in their evolution, suffered the most numerous
and extensive changes of environment,

148 DEVELOPMENT AND HEREDITY.

On the other hand, animals whose environment
has changed little or not at all in the course of
geologic time, have changed very little. Thus take
the marine fauna, for example: the sharks of to-day
differ very little from the oldest fossil fishes, and the
echinoderms seem, in some. cases, scarcely to have
altered since archzean times. The echinoderms are
so well defended by their armour that they have but
few enemies. Their greatest struggle is with their
own kind; and this is not battle, but simply crowd-
ing each other out of existence. They have nearly
the same conditions on the sea-bottoms of to-day
that existed there in past geologic ages. The same
is true for the sharks. They were lords of the ear-
liest oceans, and have remained so until now. In
that earliest time when all fishes were nearly alike
in structure, great size would have been an advan-
tage, and it is apparently by their size that the
sharks have held their own. Fishes that were un-
able to cope with them must have been driven to
the estuaries and rivers, where, among the rocks and
débris of flooded currents, we may suppose they de-
veloped their ganoid armour. A few ganoids of great
size still inhabit the fresh water, whence their de-
scendants must have returned, as the osseous fishes
to the oceans. Farther on in the course of evolu-
tion, we must suppose that forms which could not

ORIGIN OF VARIATIONS. 149

maintain themselves against the competition in the
water were driven to living an amphibious life partly
on land, and so gave rise to reptiles and mammals.
A similar course of evolution seems to have occurred
from the worms through the crustaceans to the
insects. We see, therefore, how important has been
the effect of change of environment. It is sugges-
tive also to observe that, in primitive types, devel-
opment of size seems to precede development of
diversity in structure, thus, the sharks in salt
water, the ganoids in fresh water, the carboniferous
amphibians and reptiles, and, finally, the modern
whale and elephant, as primitive forms among marine
mammals and herbivores.

In order to clearly illustrate this idea of the
nature of variations of growth and development,
let us suppose that a species of carnivorous animals
should gradually accustom themselves to live more
and more around and in water, in order to eke
out the naturally insufficient food supply, by feed-
ing on fishes, and also to render themselves securer
from larger enemies. Where the life of the animal
had been previously passed on land and its motions
had been running and leaping, it would now pass
its life in water, and its chief motions would be
swimming and diving. A different set of muscles
would be called into play ; there would be a differ-

150 DEVELOPMENT AND HEREDITY.

ent stress on the bones; and a different pressure
of the surrounding medium would affect both circu-
lation and respiration. In short, the sum total of
the forces acting upon the organism would be
radically changed. Can we imagine, then, in the
interplay of the internal and external forces which
mould the structure of the animal, that no change
will appear in the structure as the result of the
changes in the moulding forces? Let us follow
the development of one of the young after the
species has made the change in its mode of life.
The germ begins existence with its inherited
impulse of development, and up to the moment
of its birth is subject to the same conditions and
the same stimuli that have been customary in its
race; therefore, through this period of its life it
grows and develops in the same way that its ances-
tors developed during the corresponding period
of their lives. But soon after it is born, and long
before it has attained its full growth and develop-
ment, it follows its mother into the water and
begins to swim. The change of condition chiefly
affects the legs. In the first place the extremities
of the legs, which are deepest in the water, are
subjected to the greatest pressure of the medium,
and therefore the blood circulation in those parts
meets with greatest resistance. Many of the

ORIGIN OF VARIATIONS. 151

muscles used in running are not called into play
at all, while those of the shoulder chiefly used in
swimming. are employed almost constantly. The
muscles of the lower part of the legs not being
used, and the blood supply there being hindered
by the pressure of the water, it follows that growth
in those parts will be hindered, and the legs tend
to become shorter. Again, the legs no longer
support the weight of the body, and a different
stress and tension is therefore brought to bear on
the young developing bones, thus introducing a
new element of change. Further, the buoyancy
of the legs will tend to float them upward and
outward, causing eventually a change of position.
Again, we must take account of the new co-ordi-
nations and new training of the nervous system
under this new system of stimuli. The land
motions of the animal — walking, running, leaping —
will never have been properly learned, perhaps
the two latter not learned at all, through want of
practice, while through practice it grows more
and more perfect in the art of swimming. A young
animal growing to maturity under such circum-
stances as we have pictured would, without a
doubt, show in its locomotive mechanism and ner-
vous co-ordinations a noticeable difference from
another of its species which might have lived the

152 DEVELOPMENT AND HEREDITY.

normal life of a land animal. The amount of differ-
ence is immaterial ; we may suppose it greater or less.

In the production of this difference there are two
kinds of influences at work. First, a negative influ-
ence, from the withdrawal of the heretofore normal
stimuli of growth, and second, a positive influence,
in the application of an entirely new system of stim-
uli; and, in either case, we must remember that
these stimuli form a system, z.e. by their sameness
and constancy they tend always to produce a certain
definite impression on the organism, —a certain defi-
nite direction of growth. A system of stimuli calls
forth a corresponding system of reactions, which
comprise the nutrition, growth, development, and
other exhibitions of energy by the animal. Through
frequent repetitions in previous generations, this sys-
tem of reactions has become welded together in all
its parts by the power of association; therefore,
when some of the stimuli are withdrawn, the zz-
hevited impulse is not immediately affected. So
that, after a number of generations had led an
aquatic life, if the ordinary stimuli of exercise on
land were restored, the young would develop after
the fashion of its land ancestors, as has generally
been observed to be the case in similar circum-
stances. While we do not understand, with any
exactitude, the laws of association and “forgetting,”

ORIGIN OF VARIATIONS. 153

yet we are certain of the fact that long disuse
weakens co-ordinations, and a cessation of stimuli
weakens associations and tends finally to obliterate
them. Therefore the withdrawal of those stimuli
sustaining that part of the inherited impulse which
causes the development of the peculiarities of a land
animal would, in the course of generations, destroy
that part of the inherited impulse. A gradual ret-
rogression must precede the addition of new parts.
Thus it seems probable that the legs of such car-
nivores as we have imagined would first go through
a period of shortening before they would assume the
breadth of flippers like those of a seal.

It is not so difficult to imagine how, under such
conditions as I have just mentioned, a limb might
gradually grow smaller, its growth being checked
earlier and earlier in each generation. Since, in its
early development, the brain surpasses all other or-
gans of the body, we may suppose that the part of
the nervous system which governs the growth and
action of a limb would be developed —in fact, zzst
be developed — before the limb shall have attained
any size or functional power. But if, through un-
toward circumstances, that part of the nervous sys-
tem be not stimulated to activity, then its co-ordina-
tions which govern the growth of limb in both indi-
vidual and species, will gradually be weakened, and

15+ DEVELOPMENT AND HEREDITY.

its efficiency be lost, thus causing a degeneration
of the limb in succeeding generations.

The question, how does growth begin in a new
direction, and how do new parts originate, involves
the whole problem of the physiology of growth. The
human body may need four or five years to attain
the last twenty pounds of its weight, and yet if
twenty pounds’ weight be lost by sickness, it may be
recovered in as many weeks. Why should this be so
if it be not that the nervous elements controlling
assimilation are more perfectly developed and more
efficient because of their continued exercise? Again,
why does a muscle grow larger and stronger by exer-
cise? The contraction of the muscle actually weak-
ens it, and also consumes its substance, so that each
contraction causes loss of both strength and material.
Therefore, after each contraction, there must be a
restoration of strength and substance; and, as the
muscle grows stronger and larger by repetition of
the exercise, it follows that each restoration must
give the muscle more than the previous exercise
consumed, z.e. the restorative process becomes more
and more perfect by repetition. Thus we see that
the trophic nerves, like all others, gain facility by
practice. And we must remember also that the
trophic nerves, like all others, are only active as
the result of stimulation.

ORIGIN OF VARIATIONS. 155

Another case, parallel to the preceding, is found
in the thick growth of skin on the palms of the
hands and soles of the feet, and sometimes the
elbows, of children who play a great deal on the
floor. The wear and tear of the skin on these
places stimulates the trophic nerves to activity, and
by practice, the restoration soon surpasses the de-
struction. From these parts which are exposed to
blows and friction, we must distinguish those parts,
like the toes in a shoe, which are subject to a con-
stant pressure. As is well known, the natural growth
of such parts may be greatly hindered and changed,
even though the pressure be never strong enough to
cause pain.

Let us look again at the supposed case of a trans-
formation of a carnivore from a terrestrial to an
aquatic animal. While the absence of the terrestrial
stimuli have been reducing the length of the limb
and the former vigour of development, its constant
use in swimming will tend to produce some new
changes. As the foot is moved backwards and for-
wards in the water, the parts corresponding to the
palm and back of the hand will receive alternately a
greater and lesser pressure. This alternation of
pressure, however, will make the greatest impression
on the edges of the paw. The middle part of the
paw is supported by bones, but the softer, pliable

156 DEVELOPMENT AND HEREDITY.

edges will be bent first one way and then the other
by the resisting water as the paw is moved forward
and backward. No matter how small a part of the
edge of the paw be thus affected, there will be in
that part of the paw a greater wear and waste of
tissue, and a correspondingly increasing restoration,
which would cause a growth on the edges of the
paw, and thus increase the area of its flat surface.
It will perhaps be objected to this that the edges of
the paws of land animals are also continually sub-
jected to the alternations of pressure, and pulling
and pushing in all directions, according to the rough-
ness of the ground on which they tread, and yet,
nevertheless, the paw does not increase beyond a
certain small surface. But notice the difference
between the great variety of stresses brought to
bear on a land animal in walking, running, jumping,
and in striking against various obstacles, and then
contrast the simple alternation caused by swimming.
We must believe that the shape of the paw of the
land animal is the result of all the forces acting upon
its development, and in the same way the different
shape of the aquatic animal’s paw is the result of
the different forces acting upon it; and one cause
of difference we may recognise, I think, in the con-
ditions I have here mentioned. Any plastic viscous
body may be moulded by separate repeated blows.

ORIGIN OF VARIATIONS. 157

On such bodies it is not necessary that the pressure
or blows on different sides of it be simultaneous.
But every blow will have its effect in the end upon
the ultimate shape. Now if the various blows and
pressure upon a plastic body, like a limb, be limited
in their direction and force, though unlimited in
number, they must of necessity repeat themselves,
and thus tend to force the limb into a certain defi-
nite shape, — just as when we roll a piece of soft clay
between the palms of the hands, the equal pressure
applied successively to all parts of the lump forms it
into a sphere, though the pressure is never applied
at more than two points at once. When this pres-
" sure is no longer applied the ball may at once begin
to lose its shape.

It would be almost impossible to analyse fully the
action of new forces on a mechanism so complex as
the leg and paw of an animal, but they must always
be subject to certain fundamental mechanical laws.
Thus, the larger the flat surface of the paw becomes,
the more powerful must be the muscle, or the slower
must be the swimming stroke. Such laws as this
must limit and determine development in a variety
of ways. For instance, the slower the swimming
stroke became, the less would be the alternation of
pressure on the edges of the paw, and thus the
resulting stimulus of growth would be lessened. It is

158 DEVELOPMENT AND HEREDITY.

interesting to note, in this connection, that different
groups of fishes, also whales, sea-cows, seals, and
porpoises, all preserve the same general proportion
between the size of the body, and the size of the fin
or flipper.

If we admit, as I think we must, that one of the
most conspicuous properties of living matter is its
power to restore losses caused by waste and wear,
and to increase the restoration beyond the loss so
that the result is growth, then we have one of the
keys to unlock the mysteries of development. We
have already seen one application of this principle
in the previous paragraphs. We have next to con-
sider its application to the bones, the hard parts
which determine the shape of the body. According
to Professor Martin, “the experience of physicians
shows that any continued pressure, such as that of
a tumor, will cause the absorption and disappear-
ance of bone almost quicker than that of any other
tissue; and the same is true of any other continued
pressure.”! The bones, though apparently so un-
yielding, are among the most easily modified parts
of the body. Remembering that, as heretofore, we
have in mind a succession of generations acted upon
by the environment as though it were a single indi-
vidual, I think we can explain the shape of the bones

1 Martin’s Human Body.

ORIGIN OF VARIATIONS. 159

as resulting from various applications of pressure,
strain, and shock. The characteristic structure of the
joints is chiefly due to pressure. As is well known,
constant pressure prevents growth, as is shown in
the artificially compressed heads and feet of some
races of people. Let us suppose the development
of a typical ball and socket joint from the apposi-
tion of the undifferentiated surfaces of two bones.
We would have a surface of a highly movable bone
opposed to a large surface of a relatively immovable
bone. The permanent tension of the muscles keeps
up a constant pressure of one bone upon the other.
The movement of one bone, which we will suppose
to be a long bone, brings different parts of its end
surface in opposition to the larger surface of the
other bone; but whichever way the long bone points,
its end presses against the same spot on the other
bone. That part of the immovable bone would
therefore be checked in its growth as compared
with the surrounding part, or its waste would not
be repaired, and a depression would result at that
point, forming a socket. Suppose the movable bone
has at first opposed a sharp point to the immovable
bone, — in that case, that point of the movable bone
would be under constant pressure, and by the stop-
page of growth would tend to become blunt and
rounded. Wherever, through roughness in the first

160 DEVELOPMENT AND HEREDITY.

form of this rude ball, a point is subjected to greater
pressure than the surrounding parts, that point will
be reabsorbed or hindered in its growth until the
ball finally presses evenly on all parts of the socket.
We know that the toes of civilised people become
reduced in size and changed in form by the wearing
of shoes. Also, it is stated by medical authorities
that the wearing of corsets changes the shape of
the ribs. In these two cases it is not necessary
that the pressure be great enough to be uncomfort-
able, and during the periods of sleep the pressure
is entirely removed. This shows that, in order to
change the shape of bones, the pressure need not
be constant, if it be only long continued each time,
and active the greater part of the time. The pres-
sure of the tendons, nerves, arteries, muscles, etc.,
upon the bones must also have its effect upon the
shape of the latter. The strain, or pulling of the
muscles, ligaments, etc., upon the bones must pro-
duce a great effect. This is shown by the ridges
caused by the insertion of the muscles, and by the
long projections of bones where great tendons are
inserted. As pressure on a bone, at any spot, pre-
vents growth at that spot, so strain or pull seems to
induce growth; though, where there is a constant
changing in the direction of the pull, the varying
effects would tend to be neutralised, and, therefore,

ORIGIN OF VARIATIONS. 161

in most cases we would find only a slightly raised
ridge. An illustration of the effects of pressure
and pull on the bones may be found in the flattened
shape of the ribs, caused by the pressure of the in-
ternal organs and the external muscle and skin, and
by the pulling of the intercostal muscles. At the
back, where the spinal muscles arise from the ribs,
their shape is nearly round in cross-section. The
spines themselves may be regarded as chiefly due
to the steady pull of the symmetrical dorsal muscles.
The effects of a pulling upon growth are more
easily seen on the softer parts of the body, as the
elongation of the lobes of the ears and of the lips
caused by the strain of the ornaments of savages.
The mere absence of a pressure in some cases causes
growth. Thus the removal of one of the molar
teeth causes the growth of a protuberance of flesh
from the inner wall of the cheek into the empty
space. We cannot, of course, trace out with accu-
racy the causes of the various effects which we see
‘in the shaping of the skeleton and flesh, but we know
that these forces which I have mentioned do act,
and that their action is extremely complicated.
How powerful their action is, may be readily seen
by instituting on a growing limb, or even a full-
grown one, some abnormal conditions of pressure
and pull. It is well known that there would be a

162 DEVELOPMENT AND HEREDITY.

pronounced result. At first glance, one might say
that such an experiment was merely interference
with the natural growth, and no true test of the
forces causing the shape of organs. But a moment’s
reflection shows that if we can prove that a certain
condition of stress produces one arrangement of the
component particles, then every other arrangement
of particles must be due to some other condition of
stress, and a normal limb must have its condition of
stress as well as an abnormal one.

The effect of shock or sudden jarring upon bones
may be compared to the effect of exercise upon
muscle—z.e, it causes the repair to exceed the
damage or waste. Shock differs from pressure in
that pressure does not relax often enough to
admit. of the repair of its ravages, whereas shock
is only momentary, and not only admits, but seems
to stimulate, repair. Pressure is like the over
exercise of muscle, causing degeneration; shock is
like exercise, causing growth. If a bone be struck
on one end, or if a row of articulated bones be
struck, the shock is passed from one end of the
bone, or the row of bones, to the other. Accord-
ing to the discoveries in the science of physics,
the continued repetition of such shocks tends
fo cause a new arrangement in the particles of
the object struck. A bar of metal takes on a new

ORIGIN OF VARIATIONS. 163

molecular arrangement, and may become crystallised
in structure. A bone presents this difference:
its particles are undergoing a gradual change all
the time, and this mobility of its particles must, from
its nature, take away from the elasticity of the bone.
When a bone is compressed by a blow, its imperfect
elasticity would prevent it from returning to its
original shape. We must suppose, therefore, that
those vital restorative processes in the bone
which prevent it from crystallising under the
many shocks it receives, at the same time cause
a repair in such directions that the bone resumes
its former shape. It is well known that in many
cases such shocks, repeated blows, or friction
cause the repairs to exceed the waste, and excres-
cences of bone are the result, as on the hands or
feet. The bony excrescences on the heads of
horned animals have generally been explained
as having arisen in this way under the intensifying
effects of inheritance and selection. The action
of shocks would naturally cause the restoration
and increase to take place in the direction of the
compression caused by the blow; nor must it be>
supposed that the effect of the shock and its conse-
quent increase is merely on the surface of the
bone, but it takes place wherever the shock causes
a molecular displacement. The structure of the

164 DEVELOPMENT AND HEREDITY.

spongy part of bones shows that the bone septa
between the cavities are arranged along the lines
of greatest stress. To such causes must we ascribe
the length of the bones in the legs of quadrupeds
which run and do not use their legs for climbing,
digging, or swimming. The shock of the feet
striking the ground furnishes the motive for the
lengthening of the bones. In recognising this as
an efficient cause, we must not forget that other
forces, which cannot be defined, are at work limiting
and modifying the effects.

The giraffe seems to present the most remarkable
illustration of the lengthening of the bones as the
result of the frequent repetition of such shocks. As
is well known, this animal feeds on the foliage of
trees. From the earliest youth of the species,
and the earliest youth of each individual, it must
have been stretching upwards for food, and, as
is the custom of such quadrupeds, it must have
constantly raised itself off its fore feet, and as it
dropped must have received a shock that made
itself felt from the hoofs through the legs and
vertical neck to the head. In the hind legs the
shock would not be felt. It is impossible to imagine
that an animal which, during the greater part
of every day of its life (both its individual and
racial life), performed motions so uniform and

ORIGIN OF VARIATIONS. 165

constant, would not be peculiarly specialised as
a result. The forces acting upon such an animal
are widely different from the forces acting upon
an animal which eats the grass at its feet like
an ox, or one which must run and climb like a goat
or a deer, and the resultant modifications of growth
in the several cases must also be different. The
principle of increased growth in the direction of
the shock resulting from superabundant repair
of the momentary compression, explains how the
giraffe acquired the phenomenal length of the
bones of its fore legs and neck; and the absence
of the shock in the hind quarters shows why they
remained undeveloped and absurdly disproportionate
to the rest of the body. We may thus recognise
in this principle not only the cause of the peculiar
difference between the giraffe and other animals,
but even of the difference in size between its fore
and hind quarters.

The disturbance which permanently changes
the complex combination of forces acting upon a
living organ, as has been pointed out, must change’
the mobile equilibrium existing in the organic
processes of growth, waste, and repair. A change
in one set of organs entails a change in their rela-
tions to adjacent organs, and the forces acting
on the latter are accordingly modified. Thus the

166 DEVELOPMENT AND HEREDITY.

change in the fore quarters of the giraffe produce
a peculiar relation to the hind quarters, whereby
the gallop of the animal is rendered unlike that of
all other hoofed animals, as Mr. Herbert Spencer
has pointed out.1 The unity of action, however,
is still preserved, and the parts, though changed
by their mutual interaction, still work harmoniously
together. The changes by which the hinder parts
have adjusted themselves to the gradual changes
of the fore quarters, we can only attribute to the
change in the forces which the two sets of organs
bring to bear on each other. These secondary
changes, due to the mutual action of parts on each
other, embrace a large class. of phenomena, and
probably the effects thus produced cannot always
be distinguished from those effects directly due
to primary external causes. We see from this
example that a comparatively slight change in the
habit of an animal may, in the course of generations,
produce a profound change in its growth, affecting
every organ.

From what has been said of the nature of the
hereditary impulse and of the action of the forces
which produce the growth of organisms, it follows
that any alteration of the forces, in order to produce
the greatest change in the organism, must act upon

1 The Factors of Evolution, 1885.

ORIGIN OF VARIATIONS. 167

the organism during the period in which the devel-
opment of the parts, and the greatest amount of
growth takes place. In the growing and develop-
ing animal there is a constant and rapid physical
change taking place, a process of building up which
may be easily modified. In the adult the physical
change is not so great; the ordinary restoration
does not exceed the waste and wear, and any pro-
nounced change in the forces acting on the organism
is likely to interfere and decrease the restoration
without causing growth in any new direction. This
seems probably due to the fact that in the adult
the nervous associations are stronger, and co-ordi-
nations once broken are not easily replaced by
new ones.

The phenomena of acclimatisation illustrate this
principle. One cannot read the passage relating
to this subject in Darwin’s Plants and Animals
under Domestication without being convinced that
it is the action of the changed environment upon
the young and developing organisms which produces
the changes of constitution which are denoted by
acclimatisation. The fact that acclimatisation does
not take place in plants propagated by buds and
grafting seems to show that, in order that the
changed forces may produce their greatest effect,
they must act upon the organism during the period

168 DEVELOPMENT AND HEREDITY.

of development of parts, as distinguished from the
period of simple growth in size.

When the development is modified by a change
in the forces of the environment, the change of
forces will produce its effect first upon those parts
which are the last to perfect their development ;
z.e. the parts which, in their developing state, are
subjected longest to the action of the new arrange-
ment of forces will be the first to succumb to
their modifying influence. We find a striking illus-
tration of this in the observations of Professor Eimer
on the lizard’s tail. The tail of the lizard is the
last organ to lose its embryonic character. At
a time when the other organs are all well developed,
the tail still retains at its tip end the neurenteric
canal, which joins the central canal of the spinal
cord with the cavity of the rudimentary caudal
intestine. At the same time the tip of the verte-
bral column shows only the embryonic notochord.}
This method of growth, viewed in the light of the
present theory, explains the following quotation
from Professor Eimer: “The appearance of new
characters always takes place at definite parts of
the body, usually the posterior end, and during
development — with age—passes forwards, while

1“ Contribution to the Embryology of the Lizard,” Henry Orr,
Journal of Morphology, 1887.

ORIGIN OF VARIATIONS. 169

still newer characters follow after from behind.
Thus during life, eg. in lizards, a series of mark-
ings pass in succession over the body from behind
forwards, just as one wave follows another, and
the anterior ones vanish while new ones appear
behind. (Law of wavelike evolution, or law of
undulation.)” 1 Even more striking is the follow-
ing quotation in the same connection: ‘“ Wurten-
berger finds that in ammonites all structural
changes show themselves first on the last (the
outer) whorl of the shell,—as in living animals,
e.g. in my lizards, at the tail,—and that then such
a change in the following generations is pushed
farther and farther towards the beginning, —as,
é.g. in my lizards, towards the head,—until it
prevails in the greater number of the whorls... .
Then still newer characters may arise again on
the most external whorl— just as in lizards at the
tail—and drive back the former, and so on. The
ammonites also only at an advanced age, only
when they have as exactly as possible gone through
the course of development inherited from their
parents, acquire the power to vary in a new direc-
tion; but this power can be inherited in such a
way that in following generations it always ap-
pears a little earlier, until it itself characterises the

1 Organic Evolution, Eimer. Trans. by Cunningham (Macmillan & Co.).

170. DEVELOPMENT AND HEREDITY.

greatest part of the period of growth. This ‘Law
of precocious inheritance,’ as Wurtenberger calls
it, also holds —for it obviously coincides with the
law of abbreviated development —for living ani-
mals.” 1 This passage is one of a very large
number in Professor Eimer’s work on Organic
Evolution in which not only the facts recorded,
but also the very phraseology, lends itself un-
changed for the illustration of the present theory.

These examples given in this chapter are not in-
tended to set forth exactly how the parts attained
their present form and function, but to indicate the
class of forces which has produced the given result.
We know that these forces do act, and we know also
that the action of any force must produce its effect.
When we admit the existence of the hereditary im-
pulse as I have described it, with its capacity for
modification by the action of these forces through
long series of generations, then we have the expla-
nation of development. And, understanding the
action of the forces and the nature of the hered-
itary impulse, we have no necessity to suppose any
other occult factor or agent. We see the result
before us, we know that the forces act, and the
theory of the hereditary impulse makes the process
intelligible and the result logical.

} Organic Evolution, p. 31, Eimer, Trans. by Cunningham (Mac-
millan & Co.).

CHAPTER X.
CORRELATION OF GROWTH.

In considering that elementary form of reproduc-
tion, in which the small mass of protoplasm which
constituted the parent organism simply divides and
gives rise to two similar but smaller organisms, we
saw that the plan of development, or rather the
co-ordinations which determine future development,
are not destroyed. The same is true of the germ-
cells, or embryos, of many higher animals. They
may be divided into several parts, and each produce
a complete animal. The embryo of the small marine
animal, Pyrosoma, divides regularly into four parts,
each of which produces an individual. And in the
group of the Tunicata, to which Pyrosoma belongs,
there are many forms in which, as soon as the em-
bryo is differentiated into the three cell layers, —
hypoblast, mesoblast, and epiblast, —it divides itself
into several similar embryos, which may develop into
complete individuals, or may, perhaps, first divide

again before attaining maturity. We see this same
71

172 DEVELOPMENT AND HEREDITY.

thing illustrated in other animals. Many worms may
be divided into a number of parts, each of which will
grow again into a complete individual. The Hydra
may be chopped to small bits, and the pieces will
develop into an indefinite number of Hydras. There
are some conclusions to be drawn from these phe-
nomena; namely, the co-ordination of forces, which
determines development, is not to be regarded as a
definite, localised mechanism, wound up and ready to
go when touched. If such were the case, we ought
to find one such mechanism allotted to a certain defi-
nite number of cells; but, instead, we find that each
piece, regardless of the number of cells, or whether
it be the half or the twentieth of the Hydra, is capa-
ble of producing only one new individual. The
power of living matter to produce new individuals
is dependent, therefore, not on the quantity of that
matter nor, apparently, upon the unfolding of any
limited mechanical arrangement of latent energy.
The quality upon which development depends seems
to reside in a small piece as well as in a large piece,
and, moreover, equally in all parts. How this may
be, it is not easy to conceive and much less easy to
express. I think we can best compare the inherence
of the plan and potentiality of development in the
clump of protoplasm to inherence of ideas and po-
tentialities of volition in the brain substance, not as

CORRELATION OF GROWTH. 173

though each idea and potentiality were located there
in its own minute definite limit of space, and at-
tached to a definite mechanism of matter, for, says
Professor Ladd: “Everything which psychology
teaches as to the character of the mental phenom-
ena, and everything which physiology teaches as to
the nature of cerebral functions, discourages the
puerile attempt to connect separate mental images
or ideas with isolated nerve-cells as their product.”
But, rather, we should think of development and
mental potentialities as dependent upon certain states
of living matter, which states are the results of the
entire past history of that living matter, and which
thus determine the method of response to external
stimuli, and the direction which shall be taken by
the new energy constantly entering from the outside.
However, I do not venture here to attempt an elu-
cidation of that point.

What it concerns us to observe is, that a simple
piece of living matter, torn from an individual, may
grow into a single individual, or, the piece being
further divided, it may give rise to several individuals.
But all the individuals are alike. There was but one
plan of development in the piece, no matter whether
it was producing one individual or a dozen. In
other words, the piece of living matter could react

1 Ladd, Elements of Physiological Psychology, p. 555.

174 DEVELOPMENT AND HEREDITY.

to the forces of the environment only in one man-
ner. If it were torn into two pieces or into a dozen,
each could still react in only one given manner, and in
all cases with the same result. While there is but a
single plan of development possible to the larger
mass, yet each separate part of the mass has the
power of carrying out that plan. The potentiality
of development is divisible, and yet in each part
the potentiality is complete. The single plan of
development might, of course, be modified in some
of the separate pieces by exposing them to peculiar
conditions of environment.

These inherited co-ordinations of growth, or the
inherited impulse of growth, as I have called it,
tend to guide the development of each separate
piece of a living mass in the same direction. The
living matter in each particular environment can
respond to the combination of forces acting upon
it in only one way. It repeats this response over
and over again, and thus has /earned in its past
history only one plan, and it must always follow that
as nearly as the circumstances of environment will
allow. To this singleness of plan must we attribute
all such phenomena as the appearance of rudimen-
tary mammez in the male animal. These differ from
many other rudimentary organs in the fact that in
one sex they are necessary for the life of the species,

CORRELATION OF GROWTH. 175

and therefore cannot be regarded as in a process of
gradual disappearance owing to the loss of function
and stimuli of growth.

We find the same singleness of plan influencing
growth where the growing parts are not completely
separated but remain dependent upon one another.
Thus the different segments of a worm, which grow
by budding from one another, are similar both in
external appearance and in internal organs. In as-
cending the scale of segmented animals, we find
more and more that the different influences of a
more complex existence have modified the particular
growth of the various segments, preserving only a
general fundamental similarity of structure. How-
ever, the change in the development of one segment
of an animal seems to influence a change in the
development of the other segments — as though the
living matter would not learn an entirely new plan
for only one segment, but applied it as far as possi-
ble to all, Thus Professor Weismann mentions cer-
tain eye-like markings which appeared first on one of
the anterior segments of a caterpillar, and then
developed on the other succeeding segments.! Pro-
fessor Weismann explained satisfactorily the first
appearance of the marking on one segment as an
instance of protective colouring, whereby the eye-

1 Weismann, Studies in the Theory of Descent (Macmillan & Co.).

176 DEVELOPMENT AND HEREDITY.

like spots gave the caterpillar a reptilian aspect
calculated to terrify its feathered enemies. The
spots on the other segments, however, are useless
and inexplicable, I think, except in the way I have
suggested. In the same way among other arthro-
pods, where we find the appendages of one segment
developed in a peculiar manner for a particular pur-
pose, the appendages of adjacent segments tend to
show a similar modification of growth, though pur-
poseless and without particular function. For ex-
ample, the appendages of the crawfish behind the
pair of great claws are similarly modified in shape,
but are too small and weak to perform anything like
the same function. The large claws were undoubt-
edly developed by certain stimuli of growth, which
modified the co-ordinate forces controlling their
development ; but those co-ordinate forces con-
trolled not only the development of the large claws,
but also the development of the other appendages,
so that consequently the adjacent appendages became
secondarily modified in the same manner, but in a
lesser degree. This singleness of plan in living
matter, or the inability of one mass to contain more
than slight variations of one arrangement of devel-
opmental forces, is illustrated in a striking manner
in the similarity of the fore and hind limbs of
vertebrates, even when in man the uses of the two
pairs of limbs are so different.

CORRELATION OF GROWTH. 177

When we consider how every activity of living
matter has been gradually acquired through constant
repetition, each new method of activity or growth
being added to and conditioned by that method of
growth which already existed, we can understand
how this singleness of plan necessarily arose. Every
new part and function could only be a modification
of that which already existed. Bearing in mind this
singleness of plan, and remembering the power of
the hereditary impulse in preserving shape, we can
attain to an understanding of the phenomena of
nearly similar parts performing different functions,
in cases where, upon the old theory of use and
environment, we might expect as great a difference
between the parts as betweeri the functions. For
example, if we expected use and function alone to
determine the shape and structure of different parts,
we could never explain the similarity between the
human toes and fingers. For the toes and fingers
are very similar in form, in spite of the fact that
their functions are widely different. This similarity
of form is due originally to the singleness of plan,
and secondarily to the strength of the hereditary
impulse which has not yet fully succumbed to the
influence of the change of function.

But apparently, the leg and arm being developed
from a single co-ordination of forces, the one limb

178 DEVELOPMENT AND HEREDITY.

cannot be greatly changed without changing the
fundamental co-ordination of forces, and thus chang-
ing the other limb. Thus the development of the
leg is not the result of forces and stimuli acting
upon it alone, but it is also modified by forces act-
ing upon the arm, and wice versa. To this same
principle must we also attribute the exact (sym-
metrical) similarity between right and left limbs.
The slightest deviation from the type of growth in
the right limb tends to appear equally in the left ;
if one hand is large or small, the other is the same,
and the feet correspond in relative size to the hands.
Little things only skin-deep, as moles, may be found
exactly corresponding on opposite sides, and even
the ridges of the skin on the finger-tips, which are
said to differ in every individual, resemble each
other in the right and left hands. That the devel-
opment of one arm influences the development of
the other is most strikingly shown in the nervous
and muscular co-ordinations and changes acquired
by the practice of writing. Generally, the right arm
alone has been trained to write. But if we take a
pencil in each hand, we can write with each hand
the same letters, except that the left hand, accord-
ing to its symmetrical relation to the right, produces
the letters in a symmetical reverse order. If a name
written thus with the left hand be held before a

CORRELATION OF GROWTH. 179

mirror, it will appear in the mirror often with all
the peculiarities of the writer’s signature. The
significance of this fact is appreciated when we
remember the long and difficult practice which was
necessary before the right hand attained its present
power of writing, and how profoundly this practice
has, unknown to us, affected the left hand.

There is still another kind of correlation of parts
which is, not like what we have discussed, depend-
ent upon the similarity of form, but rather is de-
pendent upon a similarity in the origin of the tissues
of the correlated parts. In every species of multi-
cellular animals, the egg, or fertilised germ-cell,
divides into a number of parts, each forming a cell;
this division is repeated many times, forming thus
a cluster of cells which gradually arrange themselves
around a central cavity in two or three layers, one
inside the other, generally in a spherical or oval
shape. The outer layer is called the “epiblast,” the
inner is called the “hypoblast’”’; the middle layer,
called the “ mesoblast,” is secondary, being an out-
growth of the hypoblast. In all embryos the epi-
blast produces the skin and nervous system, while
the hypoblast and its product, the mesoblast, pro-
duce respectively the digestive and muscular sys-
tems. In the group of Ascidians it is very common,
as we have seen, for embryos which have reached

180 DEVELOPMENT AND HEREDITY.

the stage where they consist of epiblast and meso-
blast, to divide by budding or constriction into a
number of separate embryos. Further, there is, in
this group, a truly wonderful variety of methods by
which the young animals grow out as buds from
the older ones. Now, a careful examination of all
these methods of budding shows that invariably
there enter into the composition of the bud, or new
individual, parts of both the original epiblast and
hypoblast, also of the mesoblast, if it be already
developed. Not only is this true of the budding
of the ascidians, but also of large groups of the
hydroids, and, I believe, wherever this method of
reproduction obtains. Apparently, each layer has
the power to develop, under favourable conditions,
into certain organs, and into these definite organs
only. Not as though one layer should produce an
arm and another a leg, but one layer produces the
muscles of the body, including muscles of both arm
and leg; and another layer produces the skin of the
entire body, including arm and leg. Thus in each
germ-layer there is a singleness of plan which per-
mits each layer to produce only certain kinds of
tissue.

We see, therefore, that, at a very early stage in
the development of the embryo, the parts which are
to develop into the various systems of organs become

CORRELATION OF GROWTH. 181

distinct from each other. Consequently, we might
expect that whatever would retard or accelerate the
development of one single layer would affect in a
similar manner all the parts derived from that layer,
z.e. there would be a correlation between all the
parts derived from the same germ-layer. This kind
of correlation we find between the hair, teeth, and
claws of animals, all of which are derived secondarily
from the skin and primarily from the epiblast. The
single origin of all parts of the muscular system
explains why relatively strong muscles in one part
of the body are generally accompanied by strong
muscles in the other parts. Since we have observed
a similar origin for the skin and nervous system, it
is interesting to note in man the curious correlation
between the colour of the skin, or “ complexion,” and
the mental or nervous temperament.

The analysis of the correlation of parts helps us
to apprehend, if not the method, at least the degree
in which the conditions and interacting forces of a
developing organism are complicated. :

The specialisation of growth of the tissues goes
farther than the primary division into the three
germ-layers. Each of these three layers is further
divided into a number of secondary tissues, each
layer developing only its own peculiar tissues. Any
one of these tissues, if partly destroyed, has the

182 DEVELOPMENT AND HEREDITY.

power of reproducing itself and developing new
tissue of the same kind, but only of the same
kind, and no other. Muscle tissue will not produce
epidermis, nor will glandular tissue produce muscle.
We see thus that there is in each tissue but a single
plan of development, —a single set of co-ordinations
which guide and limit the activity of the tissue.

There is still another example of the singleness of
plan found in the reproduction of lost parts among
the lower animals, such as the reproduction of the
lost tail of a lizard, or the lost leg of a newt. There
is not, in these cases, a simple addition of new tissue
layer by layer on the wounded stump; but the part
to be reproduced is formed first in embryonic size
and condition, and passes thus through nearly the
same phases of growth that were followed by the
original part, repeating the original single plan of
growth.

CHAPTER XI.
DIMORPHISM AND POLYMORPHISM IN SPECIES.

WE have seen that the growing time of seeds,
and the time for putting forth leaves, is determined
by the annual changes of the seasons and by the
approach of the season of rapid increase of temper-
ature. We find an almost exactly similar series of
phenomena among animals, and especially is it the
case among insects. Insects, like plants, can flourish
only in warm weather, their lives being dependent,
in a large degree, both on the warmth and on the
plants. Each species of a temperate climate, there-
fore, is subjected by the regular return of the sea-
sons to a regularly recurring alternation of its
environment. Among many kinds of insects, only
the adult forms remain over from season to season,
passing the winter ina state of torpor. The young
aré developed only in summer time, and, accordingly,
all generations are developed under the same condi-
tions. Consequently, all these generations, as we

would expect, are alike. On the other hand, there
183

184 DEVELOPMENT AND HEREDITY.

are certain insects, like the butterflies, which develop
to maturity perhaps two generations in the summer,
the latter giving rise to a third generation, which
passes the winter in some larval form, and does not
reach its full development until the following spring.
This winter generation thus takes a much longer
time for its development, and also develops under
very different circumstances from the summer gen-
erations. It would seem, therefore, that the winter
generation ought to be different from the summer
generations, as a result of the different combination
of external forces to which it was subjected during
the time of its development. Such is found to be
the case in many species which have been most
carefully examined. According to Professor Eimer,
“the majority of the species of our (German) white
butterflies (Pieridze) show very strikingly a winter
and a summer form.” Several species of butterflies,
showing this alternation of form, have been subjected
to numerous experiments, especially the species de-
scribed in the following passage from Professor
Eimer : —

“Since the fourth decade of this century, it has
been known that the two butterflies, Vanessa Le-
vana and Vanessa Prorsa formerly considered as
different species, are really one and the same. And,
indeed, in these two forms of the butterfly we have

POLYMORPHISM IN SPECIES. 185

two generations developed at different seasons of the
year. V. Levana is its winter form; V” Prorsa the
summer form. The chrysalis of Levana remains
dormant during the winter. The butterfly emerges
in the spring, breeds immediately, and its progeny
go through their whole development in the summer ;
from their chrysalids emerge the 2 Prorsa whose
progeny then pass the winter as chrysalids, and in
the spring produce the Levana. These two forms
of butterflies are differently coloured and marked, and
it is in complete agreement with many other exam-
ples of the effect of warmth on the formation of
pigment in the integument that the summer form
(exposed to warmth), Prorvsa, is much more deeply
coloured than the winter form (exposed to cold),
Levana. The former is deep black, the latter
brown-yellow, in its ground colour.”

The caterpillars, pupa, and eggs are perfectly
alike in both summer and winter forms. Experi-
ments have shown that the chrysalids, which would
naturally produce the summer form, can be made to
produce the winter form if they be kept at an unu-
sually low temperature. The application of warmth
to these, however, ensures the production of the
summer form. On the other hand, every second or
third generation—according to the species —as-
sumes the winter form, no matter how much heat

186 DEVELOPMENT AND HEREDITY.

be applied to it. Among a large number of speci-
mens of this winter generation which were devel-
oped under high temperature, there were a few —
generally imperfectly developed — individuals of the
summer form, showing that heat is not altogether
without effect upon this generation.

When we seek for an explanation of this phenom-
enon of a periodic change of form in insects which
spring successively from the same lineal stock, we
can compare it to the periodic change in the root-
pressure of plants, which we have discussed in
Chapter IV. In both cases, when undisturbed,
the periods of change are coincident with periods
of profound changes in the environment, —in the
one case the periods of day and night, and in the
other, periods of winter and summer. We have seen
that light caused the increase of root-pressure, and
we have seen that cold caused the development of
the winter form of the butterfly. We may therefore
safely conclude that as day and night cause the
periodic changes of root-pressure, so summer and
winter cause the periodic change of form of the
butterfly. In the one case the daily alternation of
stimuli have effected co-ordinations in the living
matter of the plant, which cause a periodicity in its
activity ; in the other case, the annual alternation of
stimuli have effected co-ordinations in the living

POLYMORPHISM IN SPECIES. 187

matter, which, possessing a continuous life itself,
develops periodically into the different forms of
successive generations of butterflies. We have seen
that the periodicity of the root-pressure does not
cease immediately upon the removal of the stimuli
which originally produced it; in the same way we
observe that the periodic development of the winter
form of the butterfly persists in spite of the applica-
tion of heat. The fact, however, that the winter
form, as we have seen, may be somewhat affected
by heat, strengthens the belief that the periodic
difference of development is due to co-ordinations
of a trophic-nervous nature, which have become in-
grained in the living continuity of matter, by the
long exposure to the successive changes of summer
and winter. Whether the periodicity could be
changed by a long exposure to a different succes-
sion of stimuli, as was done with the root-pressure,
is an interesting though difficult problem; but,
reasoning from analogy, we might look for a result
similar to that of the experiment on the root-
pressure.

In the case of the dimorphic butterflies which we
have been considering, if the chrysalids which are
about to produce the summer form be subjected to
different degrees of cold, there will be developed
several forms intermediate in appearance between

188 DEVELOPMENT AND HEREDITY.

the extreme summer and winter forms. Each form,
therefore, must be regarded as the result of the in-
fluence of a particular environment during develop-
ment. Of a number of such possible forms, we may
suppose the average alternations of seasons to pro-
duce alternately only the two extreme forms; or,
on the other hand, a change in the climate toward
greater heat might cause the omission of one of the
extreme forms, and instead, the production alter-
nately of one of the intermediate forms instead of
the extreme cold form. The following illustration
of this refers to Lycena agestis: “This butterfly
occurs in three forms: A and B alternate in Ger-
many as winter and summer form; B and C are the
winter and summer form in Italy. Thus the form
B occurs in both climates, but appears in Germany
as the summer, in Italy as the winter, form. The
German winter form A is completely wanting in
Italy, while the Italian summer form (var. Ad/ous)
does not occur in Germany.”

It is evident, from the facts and principles which
we have discussed in previous chapters, that a per-
manent new form is not the result of the brief
action of a new set of forces upon the individual
during the short period of a single lifetime. A new
form is produced by the long-continued action of
the. changed environment, which causes the gradual

POLYMORPHISM IN SPECIES. 189

acquisition of new methods of growth responsive to
new stimuli. Since each organism in its develop-
ment passes through the various stages of growth
which have been attained successively by its ances-
tors, it follows that when any organism fails to
attain its full development, it must remain at some
point of development which, while incomplete for
the present generation, was complete for some past
generation. A careful study of the dimorphic but-
terflies has led the authorities on those points to
the conviction that some of the forms — the winter
forms—are the incomplete or ancestral stages of
development. So that while the species in the course
of its existence has gone on developing new char-
acteristics, it has been forced by the alternation of
seasons to reproduce an ancestral form every second
or third generation. Thus we see that neither the
extreme summer nor winter forms are forms which
are newly and suddenly produced ; and the same is
true of the intermediate forms, which represent an
intermediate stage of development.

The main point to be deduced from all this is that
as a result of peculiar conditions of environment,
ancestral or imperfectly developed forms may exist
alternately or simultaneously in the same species
with more advanced and more highly developed
forms. This fundamental principle explains, I think,

190 DEVELOPMENT AND HEREDITY.

the wonderful polymorphism of bees and ants, and
other insects living in organised societies. The
application of the explanation will be made more
intelligible by the following interesting account of
the humblebee, which I take from Professor Eimer,
on Organic Evolution: “‘In spring, when the all-
vivifying sun has warmed the ground to a certain
depth, a female humblebee creeps forth from a hole
dug by herself in the ground, — usually in a position
exposed to the sun, —or from a rotten tree-bark, or
from a clump of moss, or from some other retreat in
which she has passed her winter sleep.’

“Thus Professor Eduard Hoffer commences his
description of the humblebees of Styria,! and he
goes on to describe the formation of the humblebee
family as follows :—

“ At first the female humblebee flies from flower
to flower, sipping honey; then she seeks a place in
which to build her nest. When she has found a
place, — any suitable hole, — she carries into it moss,
grass, leaves, hairs of animals, and fine needles of
the fir or pine, and builds a nest closed on all sides,
and provided with only a single opening directed
towards the rising sun, and usually concealed.
Then she collects honey and pollen, makes a cell

1 E. Hoffer, Die Hummeln Stiermarks, Lebensgeschichte und Be-
schreibung derselben. Graz. 1852.

POLYMORPHISM IN SPECIES. 191

of wax, fills it with pollen soaked with honey, and
lays a pair of eggs in it. From these, larva soon
emerge, which grow rapidly, and therefore require
much food. The mother now toils energetically day
and night for the welfare of her children, by day
collecting, and feeding the larve, by night biting
up and arranging the materials of the nest, coating
it with wax-like material, and warming the young.
She allows herself little rest, except when the
weather is bad. At last, in the beginning of May,
or, in some forms, several weeks later, the first young
humblebees creep forth. These are the workers,
much smaller than the mother or queen —are, in fact,
stunted queens. They fly forth at once to collect
honey and pollen, which they bring into the nest.
As long as there are but few of these workers the
mother continues to fly out also to the fields and
collect industriously, but afterwards she goes out
less; she now remains much at home, laying eggs
and tending them. At last she ceases entirely from
going out, her wings, as a rule, becoming useless.
Some of the workers tend their younger sisters who
are still in the cells, and feed them, work at the
construction of the nest, keep it clean, and lick and
warm the young bees when they creep forth. The
workers raise a great humming if the nest is dis-
turbed, and defend it by stinging the intruder.

192 DEVELOPMENT AND HEREDITY.

Thus the queen lives with her handmaids several
weeks, — about three months, —continually adding
to their number. Asarule, about July, young of a
much larger size emerge, likewise resembling the
queen, the so-called ‘small females,’ or large workers,
—1.e. females whose reproductive organs are devel-
oped, but who generally only produce drone eggs,
though under certain circumstances they can lay
eggs which hatch into females or workers. These
large workers and the small workers and the old
female, all three kinds, now lay drone eggs in large
numbers, from which males are hatched. Not till
the end of the summer does the mother again lay
queen eggs. There exists now, therefore, in the
family —(1) the old queen, who is perfectly incapa-
ble of flight and destitute of hairs; (2) numerous
young queens ; (3) the ordinary or small workers ; (4)
the large workers or small females ; (5) the drones or
males. All the workers throughout the summer fly
forth to collect, about a quarter of an hour before
sunrise, awakened by a peculiar humming, the voice
of the trumpeter. The males also go forth to the
fields, but only from ten in the morning to four in
the afternoon, and only for themselves. They do
no work in the nest—although Hoffer saw them
occasionally assist, but only when the roof was taken
away from the nest. On fine sunny days in July,

POLYMORPHISM IN SPECIES. 193

August, September, and the first half of October,
the young queens fly away from the nest, settle on
flower stems, broad leaves, fences or walls in the
sun, and are there sought by the drones of their own
or other nests, and fertilised during flight. When
the young queens are all thus in a condition which
enables them to found new colonies in the following
year, the whole family gradually disperses.

“The influence of nutrition is here directly before
our eyes. The first brood of workers, laid at a
time when the queen has to do everything her-
self, or with the aid of a few workers, can, of
course, not be so well fed as the second brood,
produced when the whole of the first brood render
assistance. This better nourished brood, according
to Hoffer, have developed sexual organs; their
more highly nourished condition shows itself also
in their greater size, and they sometimes lay eggs
from which females are hatched, which must, there-
fore, be fertilised. Lastly follow the young queens,
whose larve, with the assistance of both broods
of workers, can be fed best of all, and the eggs
of which have likewise received by fertilisation an
important addition of nutritive material.”

In this description we see a succession of three
different conditions of environment which act
upon the developing young bees, so that each

194 DEVELOPMENT AND HEREDITY.

brood develops under circumstances which are
different from those of the other broods. The dif-
ference lies chiefly in the amount of nourishment
supplied to each brood. These varying conditions
occur every year, and always in each year in the
same order, thus forming regular cycles in the
life of each species. We must recognise the cause
of the three unlike broods of females in the three
conditions of food-supply. While the species has
been advancing in its development to the produc-
tion of the new and more perfect forms of the large
queens, the regularly recurring period of the meagre
food-supply has caused each year the reproduc-
tion of the imperfect ancestral form —the stunted
worker.

The honey-bee is a species which has developed
further along the same lines. Its forms are more
specialised than those of the humblebee, and seem
to be more differentiated from one another. But
the difference is not so great that intermediate
forms may not occasionally occur.

In the case of the butterflies we have seen that
the change of the seasons was the cause of the
existence of two or more forms in a species, but
we have previously seen that heat and cold are
not the only conditions which advance or retard
development. Light is almost as important, and

POLYMORPHISM IN SPECIES. 195

the quality and quantity of food are more important
in the variety and extent of the effects dependent
upon them. The number of different forms which
might thus arise in one species would depend on
the number of different regularly recurring condi-
tions of development to which the young would
be exposed in the long course of generations.
Suppose two forms already existing, then we can
imagine a new condition to recur periodically in
the environment, which, without affecting the con-
ditions which produced the first two forms, would
by its continued action eventually produce a third
form. This new form would produce additional
complexity in the social organisation, which might
of itself tend to produce a fourth form. It would
be impossible to follow out in detail the condi-
tions which gave rise, for instance, to the eight
forms which exist in a species of white ants
(termites). The causes may lie untraceably hidden
in the geologically ancient history of the species.
Suppose a case where, in some such manner as
among the butterflies, two forms of ants have devel-
oped—one much advanced in structure beyond
the other, but both being fertile. Suppose now,
while both are produced simultaneously in the
species, that through some change in environment
the advanced form should become sterile, though

196 DEVELOPMENT AND HEREDITY.

still produced in equal numbers. Further changes
in environment might entail some periodic change
in the food-supply, or some periodic struggle with
enemies. In the former case we might have the
original form gradually producing an alternate new
form to meet the periodic demand for the new
method of getting food. In the latter case the
tendency would be to produce an alternate new
form to carry on the regularly recurring wars.
Through later changes of environment these new
forms might also lose their fertility, the old original
form being the only form remaining fertile. At
the same time, instead of being alternate or peri-
odic generations, all four forms might become
simultaneous, the difference of development in each
form depending on the treatment received by the
embryo. Forms of this kind once produced would
persist unless some hostile change of environment
and natural selection should eventually obliterate
them.

We must not forget to make allowance for the
effect of instinct in modifying the growth of these
social animals. While many are to be regarded as
imperfectly developed, some particular instinct seems
to be perfectly developed, or, perhaps, over-devel-
oped, thus influencing them to a particular line of
activity, which must have its effect upon their

POLYMORPHISM IN SPECIES. 197

growth. Thus we see, in the workers which have
been deprived of their full growth by lack of nour-
ishment, the strongly developed female instinct of
‘caring for the young and building the house. This
instinct is probably perfected in each individual
largely by its constant exercise. It is probable also
that the growth of individuals in a social body, espe-
cially the psychic growth, would be affected in some
way by the mysterious sympathetic nervous influ-
ence which shows itself among animals in the stam-
peding of cattle, the orderly wheeling and circling
of flocks of birds, in the movements of droves of
caterpillars and swarms of insects, and by the phe-
nomena of suggestion and hypnotism in man. These
instincts might be cultivated by some psychic influ-
ence, and, in turn, the instincts may influence the
bodily growth. The final conclusion of modern sci-
ence is, that no cause is too slight to produce an
effect.

CHAPTER XII.
DEGENERATION AND LAWS OF VARIATION.

TuE growth of every organism is the result of a
certain combination of forces. If any change, there-
fore, takes place in this combination of forces, then
a change, as the result of their action, must follow.
We have seen that a change in the daily illumination
of plants can change their periodic sap-flow. Also
we have seen that various coloured lights influence
differently the development of tadpoles, and that a
special diet changes the proportion of the sexes.
Exactly how particular forces cause particular ef-
fects, we do not understand; but that they do. act,
we are compelled to believe, not only by the general
principles of physical science, but by the special
experiments upon organisms. In the course of gen-
erations, the change of forces produces a change in
the hereditary impulse of the organism, and thus
a new specific character appears. The hereditary
impulse, which is the result of the long previous

history of the organism, is not immediately changed
198

DEGENERATION AND LAWS OF VARIATION. 199

by a change in the environing forces. There is a
certain inertia in the nervous co-ordinations. Any
single nervous reaction in a connected and oft-re-
peated series does not disappear the instant its
original stimulus is withdrawn. It will last for
some time afterward under the influence of asso-
ciation with the rest of the series. Eventually,
however, it must disappear, — sooner or later, accord-
ing to the firmness of the association. Thus when
the forces, acting as stimuli, which have directed the
growth of any part, are withdrawn, the power of
association will still tend to cause the growth of
the part, though less perfectly. If an organ be no
longer stimulated to perform its function, it is left
without one of the chief directive forces of its
growth. The power of association, which at first
maintains its normal growth, gradually diminishes ;
the zzherited impulse weakens, and the organ grad-
ually degenerates. Those organs will degenerate
most slowly which have existed longest in the race.
As examples of this may be mentioned the pineal
and thyroid glands among vertebrates; and these
degenerate organs probably owe their persistence
partly to the fact that they are associated in growth
with such important and unchanging organs as the
brain and alimentary canal.

The blindness of animals which dwell in caves

200 DEVELOPMENT AND HEREDITY.

is an interesting example of degeneration resulting
from a withdrawal of a single one of the growth.
stimulating forces. These animals, including fishes,
amphibians, crustaceans, and insects, are descended
from animals which lived above the surface of the
earth and possessed eyes. In the blind descendants
the eyes are present, but imperfectly developed.
There can be, I think, only one explanation of this
blindness, namely, that the eyes of many successive
generations have never been stimulated to their
full growth by light. If we admit the influence
of any force of the environment upon the develop-
ment of individual and race, then we must believe
that the action of light as a directing force was
of prime importance to the development of the eye.
It is a force which has always acted upon organisms
wherever eyes have been developed, and the eyes
have always stood in a direct relation to this force.
To suppose that eyes would develop and grow
equally well in darkness, is to accept a doctrine
of effects without causes, and causes without effects.
The theory has been advanced that the degeneration
of the eyes of cave-dwelling animals is due to natural
selection. It is supposed that the energy which
would be used in perfecting the eyes (which would
be useless in darkness) is saved in these blind
animals for other purposes. Therefore those with

DEGENERATION AND LAWS OF VARIATION. 201

the least developed eyes would save the most
energy, and so have an advantage over others in
the struggle for existence. This advantage, accord-
ing to that theory, would lead to the establishment
of a blind race, and the extinction of races with eyes.
When, however, we examine into the supposed
relation between the abundance of energy at the
disposal of an animal and the perfection of its
development, we find that the two conditions by
no means coincide. No animals have such an
abundance of energy at their disposal as those
parasitic animals which live in the bodies of their
hosts, continually bathed by nutritive juices, and
yet those animals have, above all others, degenerated
most in the growth and development of their parts.
They have abundance of energy at their command
for the performance of growth, development, and
all their functions, and yet from the time a race
begins a parasitic life, the evidences of its history
show that it constantly deteriorates. In some of
the crustaceans the organs of sense and locomotion
disappear, and the animal becomes a sac, incapable
of every function save nourishment and reproduc-
tion. Evidently, the abundance of readily converti-
ble energy is not the cause of a complete and perfect
development of an animal. And the fact that an
animal which is supplied only with an abundance

202 DEVELOPMENT AND HEREDITY.

of convertible energy cannot maintain the complete
development which characterised its ancestors,
proves that the formative forces causing development
do not all reside in the organism. Some of them
must come from the outside. A cursory view of
the animal kingdom shows that those races which
have been subjected to the greatest variety of action
of external influences have attained the greatest
complexity of development.

Wherever any force is withdrawn that has always
played a part in the development of any organ, that
organ will be changed in its development and will be
less perfect. Wherever many or all of the forces of
the environment are weakened, we may expect an
arrest of development or a case of atavism. When-
ever there is a general change in the forces of the
environment, we may expect a general change to
gradually ensue in the race of organisms. This
must, however, be limited in two ways. First, the
change cannot exceed certain limits without destroy-
ing the species. These limits of change are nar-
rower according as the species is more accurately
adapted to its environment, ze. the more its life
depends upon minute details. This fact precludes
the survival of species under extreme changes, and
the changes under which a species can survive are
generally so slight that the effect of the change can-

DEGENERATION AND LAWS OF VARIATION. 203

not be immediately detected. It is only after succes-
sive generations have been subjected to the change,
that its effect becomes apparent. In the second
place, the change is limited by the past history of
the species, by its constitution, the strength of its
hereditary impulse, and also by the amount and
rapidity of the changes which have been induced
during the time immediately preceding. Organisms
under changed conditions can only produce slight
variations of such structures as are already devel-
oped in the species. The changeability of any part
seems to be partly dependent upon the length of
time it has existed in the species. Darwin expressed
this fact and conclusion, as it appeared to him, by
saying that “characters become fixed by inheri-
tance,” which means, that characters which have
appeared through many generations are less apt
to vary than perfectly new characters. When a
character appears and continues through many gener-
ations, its growth is due, not to a temporary, but to
a permanent, cause acting upon each individual. As
the growth of the peculiar character is repeated gen-
eration after generation, it becomes firmly associated
with the nervous co-ordinations controlling the gen-
eral growth of the organism, and forms part of the
inherited impulse. Naturally, therefore, characters
will vary less the longer they have been subjected

204 DEVELOPMENT AND HEREDITY.

to the same unchanging environment. Darwin
speaks of the changes of growth, in a species re-
sulting from a general change in environment, as
the “breaking of the constitution of the species.”
After the constitution is broken, the species pro-
duces many variations of growth. It is to be sup-
posed that, if the new conditions were to endure for
a great length of time, the constitution of the spe-
cies, with its newly induced characters, would again
become “ fixed.” Our domesticated plants and ani-
mals are species which, being subjected to a change
of conditions from their wild state, have had their
“constitutions broken,” and ever since, being sub-
jected to constantly changing conditions of locality,
soil, climate, cultivation, and breeding, have contin-
ued in a highly variable state.

Any change in the stimuli acting upon a species,
in order to produce a change in the hereditary im-
pulse, must continue its action long enough to affect
the nervous co-ordinations. Asa rule, the hereditary
impulse is not affected by a change which acts on
only one generation. The so-called “individually
acquired characters,” which are produced by the
temporary action of peculiar forces upon a single
individual, are not inherited, because the nervous co-
ordinations are not so greatly disturbed that they can-
not, in the next generation, under the normal stimuli
of the species, react in the normal method of devel-

DEGENERATION AND LAWS OF VARIATION. 205

opment. It is essential that the inertia of the ner-
vous co-ordinations be overcome, and that there be
made a permanent change in associations, otherwise
any enforced change of growth will not pass over to
succeeding generations. A slight disturbance in the
manner of growth may be produced artificially in
many successive generations, without producing a
permanent change in the hereditary impulse. Thus,
for many successive generations, the tails of certain
breeds of game dogs have been cut off; and yet, the
tails of the latest generation continue to grow if not
disturbed. The explanation of this, and of all phe-
nomena of this class, seems to be, that the artificial
change of growth, acting late in the period of devel-
opment, and upon a single unimportant part, has not
produced so profound an impression upon the ner-
vous organisation as to overcome the powerful asso-
ciations which unite the co-ordinations governing
the growth of the tail with the co-ordinations gov-
erning the growth of the rest of the body. While
the tail is not important to the life of the dog, yet
the co-ordinations governing its growth have been
associated, by repetition, with the co-ordinations
governing the growth of the trunk since the earliest
appearance of the vertebrates, the tail being phylo-
genetically a much older organ than the legs. There-
fore, when there is a reaction of the co-ordinations of
growth of the trunk, there will also be, through

206 DEVELOPMENT AND HEREDITY.

association, a reaction of the co-ordinations causing
development of the tail. To eventually destroy this
association might require a more profound disturb-
ance of the nervous organisation than the mere
cutting off of the tail after it has developed.

A more profound disturbance is caused when
the nervous system itself is directly affected. The
operation of such a disturbance may in one or two
generations produce an effect in changing the
hereditary impulse. Thus general intemperance
and drunkenness in one or two, or a few genera-
tions, may produce insanity or other mental weak-
ness, which is handed on and reappears again and
again in future generations. This same principle
is illustrated in the celebrated experiments upon
guinea-pigs by Professor Brown-Séquard. By operat-
ing directly upon the nervous system, he was able
to cause certain well-marked changes of growth
to appear in the succeeding generation.

The strongly hereditary nature of insanity and
other mental diseases, —they being of all diseases
the most certain of inheritance, — and also the
frequency of their appearance in combination with
other deformities of growth, both of these facts
emphasise the relation of the nervous organisa-
tion to the phenomena of development and
heredity.

CHAPTER XIII.

EMBRYONIC ORGANS.— METAMORPHOSIS AND ALTER-
NATION OF GENERATIONS,

In our analysis of the development of an indi-
vidual organism from the germ to maturity we
saw that two kinds of causes were at work in the
process. One set of causes comprises all those
conditions in the living matter of the germ which
are the result of the entire past experience of the
continuity of living matter of which the germ
formed a part. We have seen also how these condi-
tions have arisen as the result of long continued
repetition of the reactions of living matter to the
forces of the environment, —the effects gradually
accumulating by the power of nervous association
through the long course of ages and generations,
and forming what we have called the hereditary
impulse. The other set of causes controlling
individual development are those combinations of
external forces which act constantly and _periodi-
cally upon the developing organism, supplying to

207

208 DEVELOPMENT AND HEREDITY.

it the matter and energy requisite to its growth,
and stimulating its activity. These two sets of
causes produce the development of every individual,
and no change can occur in the development of
new individuals except through some change in the
causes. These two sets of causes, however, are
not independent of each other. The whole chain
of associations comprising the hereditary impulse
may be present in the germ, but there will be no
development unless certain definite external forces,
in their proper sequence, supply energy and matter
to the organism as it progresses in its growth.

As the development of an organism is a process
of constant change, there must arise at every
moment a new relation of the forces which cause
the change. At each moment the action of the
various forces causes the developing organism to
change. This change results in a new arrange-
ment of the parts of the organism, ze a new
combination of its internal forces. Now, whether
the external forces remain the same, or whether
they change, we have in either case a new combi-
nation of the whole complex arrangement of forces
which act to produce development. This new
combination we must regard as the cause of the
immediately ensuing step in development. Thus
every step in development is the result of a partic-

METAMORPHOSIS. 209

ular combination of forces acting at the proper
moment. These combinations of forces follow each
other in fixed and regular sequence, and blend
one with the other. As the sequence is generally
the same for all organisms of a species, so the
organisms resemble each other. Where the se-
quence for any individual differs from the sequence
for the rest of the species, that individual will
differ in growth from the rest of the species, pro-
vided always that the difference be not so great
as to destroy the life of the individual.

The changes of these combinations of forces
are not equal in extent or activity at all points of
the sequence. Thus in the earliest development
of an organism, the changes of development are
more marked and more rapid — the earliest stages of
development having been repeated most often. In
the latter stages, as the organism approaches
maturity, the changes become slower, less marked
in degree, and less certain—the very last stages
that appear in some cases being wholly omitted
in others. Apparently, these very last reactions
of the organic matter to the forces at work upon it
have not been repeated often enough to become
firmly associated with the older and stronger part
of the hereditary impulse. The power of the hered-
itary impulse, in the latter stages, loses strength

210 DEVELOPMENT AND HEREDITY.

as its associations become weaker and as its
reactions are less practised. Growth, therefore,
becomes more sluggish. The internal arrangement
of forces is not so quickly and easily changed; the
external forces remain unchanged, and thus the
successive combinations of the two sets of forces
change less and less, until finally growth and
development practically cease.

If, at any intermediate stage in this course
of development, the particular combinations of forces
causing growth of that stage should be changed,
it follows that the growth itself at that stage would
be changed. We have reason to believe that the
manner of growth for some particular period of
development may be secondarily changed without
radically affecting either the preceding or suc-
ceeding growth. As an example of this, may be
mentioned the embryonic organs and embryonic
modifications which adapt the embryo to undergo
a partial development in the body of the parent,
and allow it to receive nutriment from the parent,
eg. the placenta. According to the generally
accepted theory these modifications have gradually
developed upon a form of embryo that was originally
adapted to perform its whole development outside
the body of the parent. Such modifications can-
not have arisen by the backward transference of

METAMORPHOSIS. 211

acquired characters, but must have developed solely
for the period of embryonic life for which they are
useful, and they must, further, be the result of
the peculiar conditions which came gradually to
surround that period of the life of the animal. The
placenta, etc, have probably been caused by the
new and peculiar stimuli which began to act on
the organism when the egg remained so long in the
body of the parent that part of its development was
performed under these novel conditions. This be-
lief is strengthened by viewing the similarity of
embryonic conditions, and the consequent similarity
of modifications which have arisen in such widely
different groups as the mammals, the selachian
fishes, the lophobranchiate fishes, and Doliolum
among the tunicates. We see therefore that this
associated chain of nervous reactions which form
the hereditary impulse resembles in its nature
the ordinary chain of associated reactions which
are established in an individual by practising any
regular series of movements. For in such a series
of reactions the middle part of the chain can be
gradually modified and changed without destroying
or weakening or changing the other associations in
the first and last parts. In such a series of well-
practised movements as the playing of a passage
of music upon an instrument, the first effort to

212 DEVELOPMENT AND HEREDITY.

change the movements in the middle of the passage
will be resisted by the strength of the associations,
but by a continued change of the stimulus this
resistance will be overcome, the movements will
be changed, and new associations will be established.
The modified series differs from the original series
by only a few links in one part of the chain. The
method and nature of the associations in this illustra-
tion and in the hereditary impulse, I believe to be
essentially the same.

In the same manner may be explained all the
temporary embryonic organs and conditions of
growth which exist in the embryo, but which can-
not be regarded as ever having existed in any adult
ancestral form. Thus the gill-clefts of the mammals
can be regarded as inherited from some adult fish-
like ancestors, but not the placenta; and the first
stage of the cranial flexure we cannot imagine to
have ever existed in any adult animals. The pla-
centa is undoubtedly the result of its immediate
surroundings, while the cranial flexure seems to be
due to its peculiar method of rapid increase of bulk,
and to be the manner of closing the neural canal.

Another class of phenomena suggests itself here,
as being somewhat akin to the phenomena of em-
bryonic growth; namely, the development of larval
forms and metamorphosis. An examination of the

METAMORPHOSIS. 213

embryology of the different groups of the animal
kingdom reveals the fact that in no case does the
development proceed with equal rapidity at all points
of its course from inception to maturity. As before
remarked, toward the end of its course, development
becomes slower and less marked in its changes. As
pointed out, this is due to lack of strength in the
association of the hereditary impulses, and conse-
quent lack of progressive change in the sum and
relation of the internal and external forces. This
progressive change in the relation between the in-
ternal and external forces is the cause of develop-
ment. The progressive change is most constant
and invariable for long successions of generations,
in the earliest stages of development before the
animal leaves the egg, or the body of the parent,
and therefore at this period the development is most
rapid and uniform. When the animal leaves this
earliest protection, and goes forth a very imperfectly
developed creature to shift for itself, it then meets
with conditions that are slightly different for each
generation and each individual. At this period of
its life, therefore, because of its variable relations
to variable conditions, the associated reactions of the
hereditary impulse will not be so strong, and devel-
opment will accordingly be slower and less uniform.
For it is a general fact, that those associations are

214 DEVELOPMENT AND HEREDITY.

the strongest where the separate acts of practice,
ze. the successive repetitions, are exactly the same;
any slight changes in the successive repetitions tend
to lessen their power to form nervous associations.
This larval period, when the imperfect animal is
shifting for itself, living a free life in a variable
environment, might be a period of considerable
diversity of individual conditions, and be followed
later by a period in which the conditions would be
the same for all individuals. Take, for example, the
development of the butterfly. First, comes the
embryonic period, where the conditions are the same
for all individuals of the species; the hereditary im-
pulse is accordingly strong, and the development
rapid and alike in all individuals. Then comes the
larval period, when the organism must seek its own
food, and the individuals must meet with diverse
experiences of life. In this period the development
is slower, and individuals of the same brood some-
times develop slight differences, ¢.g. in coloration.
Third, comes the chrysalis stage, when the condi-
tions are again uniform for all individuals. In this
stage the hereditary impulse is again very strong,
and the greatest changes of development take place,
and follow with great certainty and uniformity for
all individuals and generations. The rapidity of de-
velopment, however, at this stage, is modified by and

METAMORPHOSIS. 215

largely dependent upon, external conditions of tem-
perature, season, and climate.

In order to explain the origin of this complicated
method of development, we must look to the lower
and ancestral forms of insect life. In the gradual
evolution of the hereditary impulse, as I have de-
scribed it, we cannot suppose the ancestral line of
the butterfly to have come by a simple and direct
manner to its present complicated method of devel-
opment. The simple manner of development of the
hereditary impulse is by constant and similar repe-
tition in each cycle of life with the resulting associa-
tions, the adding of new slight characteristics at the
end of each cycle, the more rapid repetition of that
earlier part of the cycle which has been repeated
most often, and thus the backward transference of
the newly acquired characters. The result of such
a simple manner of evolution of the hereditary
impulse, would be that the earliest part of the devel-
opment of an individual would be most rapid and
invariable, and the later part would become slower
and slower as it progresses toward the end of its
course, and would be also less certain. There could
be no period of slow and variable development
succeeded by a period of decisive and invariable
development. The lower orders of insects, we find,
follow the simpler method of development, — the

216 DEVELOPMENT AND HEREDITY.

changes from egg to maturity occurring with grad-
ually decreasing rapidity throughout the whole period
of development, and no sudden and striking meta-
morphosis takes place in the last stage. This
method we must suppose also to have been followed
at some time in the past by the ancestors of the
butterfly. What causes may have effected a change
in this simple manner of development and produced
the present method, we cannot definitely determine.
But still, from our knowledge of the nature of the
hereditary impulse on which this change has been
wrought, and from our knowledge of the present
manner of growth, we may gain some idea of the
nature of these causes. Imagine some ancestral
form of the butterfly in which the development was
simple and gradual from beginning to end, without
any sudden metamorphosis, in fact, such a method of
development as that of the grasshopper. Suppose
such a change in the environment of this organism
that on reaching the stage of development equivalent
to the caterpillar stage, the organism should meet
with conditions of existence much easier than hith-
erto. Let its food become very abundant, and let
its external conditions remain for a long time un-
changed. This of itself, we have seen in discussing
parasitism, would be enough to hinder its develop-
ment, through the withdrawal of its customary

METAMORPHOSIS. 217

stimuli of development. Suppose, further, that the
new conditions of existence while remaining for a
long time the same for each individual, should dif-
fer slightly for successive generations. The effect
would be to hinder still further the action of the
hereditary impulse. The total effect, therefore, of
these new conditions would be to prolong the cater-
pillar state, —there would be a tendency to remain
in this state. But the hereditary impulse is not
destroyed, and the mature development of the insect
eventually occurs, the only change being the extreme
prolongation of the caterpillar stage. Suppose now
some change of climatic conditions, whereby as each
generation is about to abandon its protracted cater-
pillar stage, it encounters cold weather. Each cater-
pillar protects itself with a wrapping of leaves or a
web of its own spinning ; and because of the strong
associations of hereditary impulse, development pro-
ceeds —a supply of energy from the previous abun-
dance of nutriment being on hand. In this quiet,
motionless, and protected state, the conditions of
development are the same for each individual and
each successive generation, therefore the hereditary
impulse for this period thus becomes invariable in its
results, more rapid, and its various stages become
compressed. The last changes of development, as is
universal in the organic world, tend to occur earlier

218 DEVELOPMENT AND HEREDITY.

and earlier in successive generations, z.¢. tend to be
transferred from the end of development back toward
the beginning. But in the butterfly the backward
transference is limited by the slow and variable de-
velopment of the caterpillar stage. The changes of
development are therefore compressed and heaped
up, in the period during which each generation lies
quietly protected in the shelter it has made for itself.
Thus the greater part of its development may take
place in the chrysalis stage.

By following this line of supposition, we can see
how forms might arise in which, the larval life being
greatly prolonged, and the chrysalis life being abbre-
viated, the stored nutrition or potential energy of
the larva might suffice not only for the chrysalis
stage, but also for the adult stage, so that the mature
forms might reproduce before they had ever partaken
of food. The result would be that, in the course of
generations, the habit of taking food would be lost
by the mature form, especially if the supply of food
for the mature forms failed. When the habit of
eating were lost, the corresponding organs would
degenerate and disappear by reason of disuse. But
since it is necessary for an organism to attain matu-
rity in order to reproduce, the mature state, however
brief, must be its final stage of development so long
as the species exists, — even though it. be in the

METAMORPHOSIS. 219

larval stage that the organism. acquires the potential
energy that keeps the species alive.

The above explanation seems to me to account
not only for insect metamorphosis, but also for the
curious cases of extremely long larval life and very
brief maturity, such as the Ephemera and seventeen-
year locust (Cicada septemdecim). The great length
of the larval life of the latter is probably due to the
unchanging conditions of its existence underground,
especially the sameness of temperature and the dark-
ness. In this connection, another curious fact sug-
gests itself, namely, that the wings of certain ants
drop off after the hymeneal flight, a few hours after
the completion of development, which is ended by
the emergence of the young ants from their cells.
Apparently, the wings are developed through the
strength of the hereditary impulse.. Long-continued
habit has made it necessary for the ants to fly up in
the air for the act of reproduction. For this, wings
are necessary, and, by virtue of their powerful associa-
tion with the reproductive act, they are still retained
among the associations of the hereditary impulse,
though they are otherwise degenerate organs, and
not developed by the worker ants. That they should
drop off immediately after the hymeneal flight, seems
to be a curious case of the transference of a degen-
eration backward as far as possible without interfer-
ing with the reproduction of the species.

220 DEVELOPMENT AND HEREDITY.

The principle which I have illustrated by the case
of the butterfly seems to me to be applicable to all
cases of animal metamorphosis. Wherever animals
with an established method of development are in
active intercourse with their environment, and strug-
gling for their existence through a long period of
immaturity, it must follow that when the conditions
of their larval life change, the course of their larval
development must be affected. In any change of
environment it is hardly possible that all the stages
of larval life could be equally affected. The change
would probably be more favourable to some stage or
stages of life, and less favourable to others. The
result would be a prolongation of the favourable
stages of life, and an abbreviation and compression
at other stages. Thus may be explained the origin
of periods of sudden changes, or metamorphosis.

In the same category with metamorphosis should
be placed another class of phenomena, which is
usually described as “alternation of generations.”
But we must except here dimorphic species, already
discussed, and that kind of alternation of genera-
tions which consists in the alternation of sexual and
parthenogenetic generations, as in Aphis and other
arthropoda. This latter is a phenomenon that truly
deserves its name. The cases, however, of many of
the Hydroids and Tunicates are of another char-

METAMORPHOSIS. 221

acter. Let us consider Salpa as an example. In
this genus the fertilised egg develops into a small
transparent barrel-shaped creature, which swims free
and near the surface of the ocean. When the ani-
mal has attained its growth, there appears on the
ventral side of it a very small stem or rod-like
prolongation of the internal tissues, which is des-
tined to develop into-a colony of individuals. This
stem, or sto/oz, as it is called, is made up of tubes
developed from the entodermal and mesodermal
tissues, covered externally by ectodermal tissue.
As the stolon becomes longer, its distal portion
enlarges and divides — without complete separation
—jinto a series of transverse segments. Each seg-
ment receives thus a part of each of the three
fundamental tissues. As the segments are increased
in size, they are squeezed alternately right and left,
though all remain connected by a common circula-
tion. Each segment develops into modified barrel-
shaped individuals with male and female productive
tissues. At length the oldest part of the stolon,
having developed thus into a colony, breaks loose
from the rest of the stolon, which, in its turn, fol-
lows the same course of development. The indi-
viduals of the now free-swimming colony proceed
to reproduce sexually the eggs from which the first
described single form of Salpa develop. One thing

222 DEVELOPMENT AND HEREDITY.

remarkable about the development of the colony lies
in the fact that the eggs develop in the tissue of the
stolon before the stolon segments itself to form the
individuals.

For explanation of this method of growth, we
naturally turn to the ancestral forms of the Tuni-
cates. We find among single Ascidians growing
fast to rocks, etc., some forms which mature and
reproduce sexually, and at the same time produce
by simple budding one, or perhaps three or four
individuals, which remain fastened to the first near
its base. All of these individuals are essentially
alike. A grade higher in the scale, we find the
group of Botryllus, also stationary, where the
young one developed from the egg does not mature,
but buds into four individuals, and is said to dis-
appear by absorption into the four. These in turn,
before maturing, give off other buds, all of which
together form a colony. In these later buds, long
before development is complete, the egg-cells ap-
pear well differentiated, showing that in this respect
Salpa is not unique. This early development of
eggs, and the rapid reproduction by budding are
probably both the result of a great abundance
of food. In another stationary group, Perophera,
we find a manner of growth more nearly according
with that of Salpa. From the egg of Perophera

METAMORPHOSIS. 223

arises an individual which, long before it has com-
pleted its development, sends out a comparatively
large bud or stolon composed like all such buds
of entoderm, mesoderm, and ectoderm, and com-
parable to the first bud of the single Ascidian.
When the stolon of the young Perophera has at-
tained a considerable length, there appear at inter-
vals along it, several small swellings, comparable
to the subdivision of the young bud of Botryllus.
Each of these little swellings of the stolon de-
velop into sexual individuals, which remain united
in one colony, and never have an independent
existence. In external appearance all the individ-
uals of the Perophera colony are alike. Let us
compare now the development of the Perophera
colony with what seemed like the two generations
of Salpa. In each case there is a first individual
which produces a stolon. From the different por-
tions of the stolon are produced the other individ-
uals which make up the colony. Suppose a fixed
“Perophera-like colony to change its habit of life
and become free-swimming. Evidently a weak
immature individual dragging a long stolon would
be at a great disadvantage. The probable result
would be that the single individual would attain
its full growth before the appearance of the stolon,
and the stolon would be much diminished in size.

224 DEVELOPMENT AND HEREDITY.

This is apparently what has happened in the phy-
logenetic development of Salpa. Here the stolon
is not only much diminished in size, but to prevent
its retarding the progress of the animal through
the water, it is coiled up and enveloped in the
wall of the body. When a portion of the stolon
has developed into nearly perfect individuals, it
becomes pushed out and hangs free in the water,
and is finally broken off. The first individual of
Salpa is therefore homologous with the first indi-
vidual of Perophera. The free-swimming life of
Salpa is most favourable to a single untrammelled
individual, or to a perfect colony ; accordingly these
two stages in the growth of the colony become
more pronounced, and the intermediate stages be-
come abbreviated and compressed. Instead of two
distinct generations of Salpa, we have only two
peculiarly modified forms of the same colony, which
eventually separate by force of circumstance.

An extremely interesting example of meta-
morphosis is presented to us by the Medusz. The
study of morphology has shown us that these free-
swimming animals were originally derived from
stationary Hydroids. As was explained in the case
of the butterfly, this phylogenetic development
must have been gradual, and individual develop-
ment must have been originally without sudden
jumps or metamorphoses. The individual Medusa

METAMORPHOSIS. 225

developing simply, after the gradual manner of
development, would have repeated regularly all of
the stages of development through which its ances-
tors passed. But apparently secondary conditions
of environment have arisen which favour the hy-
droid stage and the perfect medusa stage, and at the
same time tend to eliminate the intermediate stages.
We see the result in the luxuriant development
of the hydroid state, with its fixed colonies of
variously differentiated individuals, and then the
sudden metamorphosis into the widely different
form of the Medusa with its new addition of nerves
and sense organs, indicating the total suppression
of a long series of intermediate stages of ontological
development. On the other hand, the secondary
changes of environment were in some cases unfavour-
able, not alone to the intermediate stages of develop-
ment, but also to the hydroid stage itself, and
resulted in the suppression of the latter also. Thus
we find Medusze in which the hydroid stage of
development is completely absent. This seems
to me the only way in which we can explain the
great similarity in the various adult Medusz, and
the great dissimilarity in their methods of individual
development. It would be impossible to explain
the details of these changes; the theory indicates
the manner in which their investigation should be
approached.

CHAPTER XIV.
ORIGIN AND SIGNIFICANCE OF SEX.

In order to understand the nature of sex, let us
return to the consideration of the processes of life
among unicellular organisms. In the first place,
we see that those constructive changes of the pro-
toplasm which are caused by the environment are
not destroyed when the protoplasm divides into
two parts. The elementary nervous organisation
remains effective in each of the two parts. The
results of the original stimuli acting upon the single
mass are shown in the development of each part,
as the magnetising of a steel bar will cause each
piece of the bar, when broken, to form a magnet.
What the nature of these changes in the protoplasm
may be, from a physical point of view, we cannot
conjecture. We have seen that they are not
destroyed by a division of the protoplasm, and we
have further to notice that they are not destroyed
when two separate bits of protoplasm unite and

fuse together in one mass. Here the illustration
226

ORIGIN AND SIGNIFICANCE OF SEX. 227

of the magnet again holds good. The two masses
blend in their union the effects of their past experi-
ences as individual organisms. From the union
of the two organisms there may arise one or many
organisms, each displaying the results of the
influence of environment upon the two parent
organisms. This is the process of conjugation
which prevails largely among unicellular organisms.
If we may judge from the phenomena of heredity
in higher organisms, we conclude that each parent
organism transmits the effects of its already induced
changes to the progeny, thus producing new and
generally intermediate forms. We know, from the
observation of the higher animals, that the poten-
tialities of development of both parents are present
in the initial stages of the embryo, and that the
development proceeds this way or that way, accord-
ing to the stimuli brought to bear upon the embryo.
‘Thus the young of frogs have the power of develop-
ing like the mother or the father, and, as shown by
Professor Yung, their manner of development is de-
pendent upon the quantity and quality of their food.
The honey-bees furnish another illustration, for
among them the food and treatment which the
embryo receives, determines whether it shall grow
into a queen or a worker. Also there have been
described a large number of cases showing that

228 DEVELOPMENT AND HEREDITY.

under peculiar and little known conditions male
characteristics may develop in the female, and female
characteristics may develop in the male. The dif-
ferent characteristics of the parents seem capable
of almost infinite variations of combinations, and
even the most distinguishing characteristics of the
two sexes may be combined in one individual.

It is within the bounds of legitimate inference to
believe that inheritance among unicellular organisms
is fundamentally the same process as in the higher
organisms. Therefore we may assume that we are
right in our conclusion that the conjugation of two
individuals combines two potentialities of develop-
ment, — or combines two hereditary impulses into
one. But, as has been pointed out heretofore, the
hereditary impulse does not alone determine the
development, for the development is dependent on
the action of external forces or stimuli, which sus-
tain and guide its activity. The environment modi-
fies the hereditary impulse, and determines within
limits the intensity and direction of its development.
We may suppose the hereditary impulse to be made
up of two sets of potentialities of growth — one set
derived from each parent. Each of the two bits of
protoplasm which mingle to form an individual has
its associated co-ordinations of reactions; which of
these reactions shall take effect depends upon the

ORIGIN AND SIGNIFICANCE OF SEX. 229

stimuli of the environment, for each developmental
reaction is caused only by certain definite stimuli
and cannot occur without them. The facts of in-
heritance show that the stimuli of an environment
may produce developmental reactions, some of which
are inherited from one parent, and some from the
other parent. If the stimuli acting on a developing
individual resemble more closely the stimuli that
have acted for a long time upon the protoplasm de-
rived from one parent, and differ slightly from the
stimuli that have acted upon the other parent, then
the individual will develop more after the fashion
of the parent whose environment was like its own
present environment. Mr. Herbert Spencer, after
discussing some of the phenomena of breeding
domestic animals, seems also to arrive at this same
conclusion.! When the conditions of stimuli have
long been the same for both parents, or both
parental races, then we might expect the union to
intensify the hereditary impulse, and, under the same
conditions of stimuli, to produce a more perfect
development along the same line —as in our most
approved cases of artificial breeding. On the other
hand, when the stimuli acting on the developing
organism are a selected combination of two different

1“ Inadequacy of Natural Selection,” by Herbert Spencer, Con-
temporary Review, March, 1893.

230 DEVELOPMENT AND HEREDITY.

series of stimuli that have acted separately on the
two ancestral lines, then we might expect the indi-
vidual development to combine characteristics of the
two parents, — the mixture of stimuli calling forth a
corresponding mixture of developmental reactions.
Experiments in crossing breeds, varieties, .and
species of plants and animals, show that the heredi-
tary impulses of the two organisms united in the
cross must be very similar if a perfect development
is to result. When the two organisms differ enough
to be classed as separate species, the offspring of the
union is rarely perfect. Two allied species have
the greater part of their development alike, —it is
only as their development nears its completion that
the lines diverge. In the same way, they have
had a common ancestor in comparatively recent
times, and before their divergence from that ances-
tor, they have had a common ancestral history.
From what we have learned of the origin of the
hereditary impulse, we may conclude, therefore, that
two allied species have their hereditary impulses
identical up to a certain stage of growth — the stage
represented in their racial history by the common
ancestor ; from this point, through the latter part of
their development, the hereditary impulses proceed
in opposite directions. The offspring of a cross of
two such species might therefore continue its devel-

ORIGIN AND SIGNIFICANCE OF SEX. 231

opment so long as the two inherited impulses were
alike, but when the impulses begin to impel growth
in opposite directions, the development must cease.
This explains why the imperfectly developed off-
spring of a crossed species resembles an ancestral
form. For, when the method of development of two
animals proceeds for a time on the same lines, and
then diverges, the result of the cross will develop
only as far as the point of divergence. Thus the
fancy breeds of pigeons, when crossed, produce the
ancestral form of the rock-pigeon. Also the stripes
on mules show a partial reversion to a zebra-like
ancestor of the horse and the ass. Since reproduc-
tion is a superabundant growth, these hybrid organ-
isms which do not attain a perfect development, nor
a full growth, are generally sterile.

Conjugation, or union of two cells to produce the
one or more individuals of the succeeding genera-
tions, is generally admitted to be the first step in
the evolution of the sexual method of reproduction ;
and in this connection I wish to direct attention to
some points which have not generally received the
consideration which their importance seems to de-
mand. From its almost universal existence among
the more complex organisms, the sexual method of
reproduction, whereby each individual born may
inherit from two parental races, appears to play a

232 DEVELOPMENT AND HEREDITY,

very important part in organic evolution. It seems
as though it might have originated among single-
celled animals, by the effort of simple protoplasmic
masses to absorb each other as food. Such a mutual
effort between two individuals would, in all proba-
bility, end in a drawn battle by the commingling of
the two masses of protoplasm. The sudden super-
lative growth —the doubling in size — would of itself
induce reproduction, z.e. subdivision, — the new gen-
eration would be formed of protoplasm derived from
the mixture of both parents, and would inherit devel-
opmental potentialities from each.

But, beside this double inheritance, there is
another very important feature of sexual reproduc-
tion; for although each of two organisms may be
capable singly of reproducing, yet the process of
conjugation gives a peculiar stimulus to the devel-
oping germ, which entails a more complex or a more
complete development. That this mixture of pro-
toplasm derived from two organisms is largely of
the nature of mere stimulus to growth activity, is
shown by the fact that among many organisms
accustomed to reproduce asexually, there may be
produced, without fusion of protoplasm, a germ
which will attain only a partial development soon
arrested, or perhaps develop into a weak individual.
A germ thus produced, and a germ produced by

ORIGIN AND SIGNIFICANCE OF SEX. 233

the fusion of two protoplasmic cells, have in other
respects the same surroundings, so that we cannot
attribute the lack of development of the former to
a lack of nutriment, but must regard it rather as
owing to the absence of that stimulus derived from
the fusion and mutual influence of two cells of
different potentialities, that is, different ancestries.
That this latter is important, is shown by the fact
that many plants cannot be fertilised by their own
pollen, when the same pollen is efficient with other
plants. This self-sterility of plants seems most
plausibly explained as the result of too close inter-
breeding, whereby the sexual elements of the
same growth have lost that peculiar differentiation
which is necessary to produce the mutual stimulus
for development of the germ. Says Darwin, “ Both
with plants and animals, there is the clearest
evidence that a cross between individuals of the
same species, which differ to a certain extent, gives
vigour and fertility to the offspring ; and that close
interbreeding, continued during several generations
between the nearest relations, if these be kept
under the same conditions of life, almost always
leads to decreased size, weakness, or sterility” ;
and again, “slight crosses, that is crosses between
the males and females of the same species, which
have been subjected to slightly different conditions,

234 DEVELOPMENT AND HEREDITY.

or which have slightly varied, give vigour and fer-
tility to the offspring.” Mr. Havelock Ellis, after
investigating in detail the ancestry of men of gen-
ius, comes to the following conclusion: “ Wherever
we find a land where two unlike races, each of fine
quality, have become intermingled and are in pro-
cess of fusion, there we find a breed of men who
have left their mark on the world, and have given
birth to great poets and artists.”

We have therefore to regard the consequences of
the sexual method of reproduction as of two kinds —
first as a profound stimulus to-the general develop-
ment of the germ; and second as a combining of
two sets of potentialities of development. The full
bearings of this latter consequence seem to have
been hardly recognised. The most certain and most
striking conclusions deduced from experiments in
breeding are —first, that uniting dissimilar forms
tends to produce an intermediate form ; and second,
that uniting similar forms tends to intensify the
peculiar characteristics of the parents. If new vari-
ations in form were really so scattered in time and
space, and affected only exceptional individuals here
and there, as has been sometimes supposed, it would
be difficult to imagine why “natural selection”
should maintain a method of reproduction which
would be constantly tending to obliterate all varia-

ORIGIN AND SIGNIFICANCE OF SEX. 235

tions ; for if the organisms were interbred closely
enough to preserve the variation, degeneration and
sterility would probably result. On the other hand,
crossing would obliterate the new variation. It is
equally difficult to imagine how new characteristics
can arise by means of the sexual method of reproduc-
tion, when the weight of evidence goes to show that
its strongest tendency is either to obliterate pecu-
liar and rare characteristics, or to intensify those
already existing in both parents. It seems, there-
fore, that the sexual method of reproduction must
have some other reason for existence than the pro-
duction of new forms by the mixture of different
germ-plasms. We must attribute all new changes
in the construction of organisms to changes in the
forces that act upon the organism while in process
of construction, —and, as I have pointed out, in
each generation forces are indirectly acting on the
construction of all generations to come after. Every
generation helps to form the hereditary impulse which
determines the developmental reaction of each suc-
ceeding individual to its environment. Where a
species continues for a great length of time in un-
changed conditions, the constant sameness of succes-
sive cycles of life makes the hereditary impulse fixed
and invariable, and the species becomes highly spe-
cialised to suit its surroundings. Eventually the lack

236 DEVELOPMENT AND HEREDITY.

of some change of stimulus seems to cause a lack of
development and growth. We can imagine a num-
ber of individuals reproducing asexually, forming a
variety so highly specialised and completely adapted
to certain limited local conditions that a compara-
tively slight change of the conditions might cause
the total destruction of the variety. The ill effects
of too great a degree of specialisation are well
known. On the other hand, a sexual mixture with
individuals living under slightly different conditions
— that is, a commingling of different developmental
potentialities —seems to give a greater power of
adapting to changes of environment. Each individ-
ual, instead of inheriting, so to speak, the experience
of a single straight line of ancestors, inherits from a
geometrical progression of ancestors almost the whole
experience of his race, and comes into the world
equipped with an hereditary impulse capable of re-
sponding to all the conditions under which any one
of his race can survive. Sexual reproduction thus
appears in a new light, as a means of widening the
range of responsive power by combining a great
number of inherited potentialities, and so enabling
the developing organism to cope with any of the
various conditions which have been met by its

numerous ancestors.

CHAPTER XV.

CONDITIONS OF DEVELOPMENT. — DEATH. —
CONCLUSION.

THE different methods of development of various
organisms cannot be accounted for by difference
in the chemical composition of the germs, nor by
difference in their cellular structure. We must recog-
nise some general property of living matter, by virtue
of which the individual and racial development of
organisms takes place. We can only express it
as that which is the basis of nervous co-ordination
and which makes all nervous activity possible. It
is something which is not destroyed by the division
of an organism, nor by the fusion of two organisms
into one. The latent potentialities of the two
organisms, which are based upon this property, may
be mixed in a great variety of combinations, —
combinations which are, however, dependent upon
the stimuli to which the organism is subjected during
its development. In no case can we recognise this

property of development as acting independently
237

238 DEVELOPMENT AND HEREDITY.

of the stimuli applied to the organism. Remember-
ing that by stimuli we mean all of the forces acting
on the organism, we must believe that every change
in the stimuli necessitates a change in the organ-
ism, —or else we are left to suppose that the pro-
cesses of development transcend the law of the
conservation of energy. We have seen how unnec-
essary and erroneous it is to suppose that the
germ is a complex mechanism of molecules so
charged with potential energy that it will unfold
and evolve itself into a predetermined adult form,
without regard to the stimuli of its surroundings.
All such suppositions fall short of explaining the
real nature of developmental change. They do
not touch the fundamental nature of the property
which makes development possible, nor explain how
the complexity of development has originated in
the course of evolution. From the manner in which
this property of development is exhibited in or-
ganisms, from its retention of latent potentialities,
from its ability to survive division and commingling
of the protoplasm, and from its power of reacting
to stimuli, we must conclude that it is a fundamental
property of living protoplasm. When we consider
the protoplasm’s responsiveness to stimuli, and to
the effects of repetition or practice, with the intricate
co-ordinations that may thereby be effected, also

CONDITIONS OF DEVELOPMENT. 239

the impressions made by stimuli which remain long
fixed as “memory,” we are led directly to suppose
that the property which is the basis of bodily develop-
ment in organisms is the same property which we
recognise as the basis of psychic activity and psychic
development. A broader view of living organisms
will only strengthen the supposition; for when we
know that certain purposeful material changes begin
in a germ and continue in an unbroken causal
series, until this finally becomes the series of brain
changes concomitant with the thinking processes
of a Shakespeare or a Newton, we can readily sup-
pose that the whole series rests upon the same
property of organic matter—the property which
is at once the basis of development and of psychic
activity. It would be difficult in the face of the
facts to imagine that organic development differs in
its nature from psychic activity, or rather —to speak
more accurately —from the material accompaniment
of psychic activity. For instance, the nest memory
of birds is transmitted by the sexual germs; now,
can we imagine that all the psychic qualities con-
gregate in one portion of a certain blastoderm cell
which shall later produce the epidermis, and does
that one portion of the blastoderm cell produce
those particular epidermis cells which later form
the brain? We have no evidence for such a view,

240 DEVELOPMENT AND HEREDITY.

but on the contrary, the facts of embryology and
morphology point to a negative conclusion. The
epidermis cells from which the brain develops seem
to be like all the other epidermis cells, and among
worms and other lower animals parts of the nervous
system when cut out may be replaced by a new
growth from other cells around them. Since, there-
fore, the brain cells possess the power of purposeful
response to stimuli, can the purposeful response
of the other cells, sprung from the same blastcderm,
be attributed to a totally different property ?

An instinct or habit of action is not essentially
different from a habit of growth. We have seen
how the habits of growth and development were
formed by long continued repetition of mechanical
reactions, and how they perpetuate themselves in
the hereditary impulse. In the same way, the
more complex reactions of an animal, such as feed-
ing, reproducing, nourishing, and protecting the
young, being repeated in each generation, tend by
repetition to become as much a part of the heredi-
tary impulse as the growth or development itself.

It is interesting to note the fact, that when any
action is repeated a great number of times, although
at first it is only with great difficulty that the action
can be performed twice exactly alike, yet finally,
after a great many repetitions, the successive per-

CONDITIONS OF DEVELOPMENT. 241

formances of the action assume almost an exact
similarity. This principle of nervous action ex-
plains why the instinctive actions of all individuals
of a species resemble each other so closely.

There is, however, generally a slight difference
between the instinctive actions of different individ-
uals. This is due to a difference in surroundings.
The delicate organisation with which the instinct
is connected responds to very slight differences.
This peculiar response to new conditions is what
is called “intelligence.” It is, however, only the
necessary response of a complex organisation to
new and unusual conditions. Instinct is the neces-
sary response to usual conditions; intelligence is
the necessary response to unusual conditions. Thus
by instinct a bird’s nest is planned; by intelligence
it is adapted to the peculiar requirements of the
location and materials at hand. The difference
between instinct and intelligence is therefore
neither fundamental nor very wide. The great im-
pelling motives of mankind may be classed as
instincts. Such are the instincts of self-preserva-
tion, procreation, loving and caring for one’s chil-
dren and friends, with the consequent desire for
power, knowledge, and wealth. The responsive
adaptation of each individual to those conditions
of environment which are new and unique in his

242 DEVELOPMENT AND HEREDITY.

own life, is the exercise of “intelligence,” and
varies with the fineness and perfection of the
organisation. To realise the instinctive character
of the great mass of human action, we have only
to look at a large collection of mankind as a
wholé, and notice the monotonous sameness in
their actions from year to year, as shown by statis-
tics of birth, marriage, pauperism, crime, illegiti-
macy, commerce, and manufacture.

The intelligent human action and the simplest
process of growth are thus alike the necessary
result of the stimuli which are momentarily acting,
and those which have acted upon the organism
through its racial and individual existence. This
is the logical outcome from the doctrine of the
conservation of energy. The evidence is such that
we cannot escape the conclusion. We must accept
it in spite of the serious problems which it raises
in regard to free will and moral responsibility ; the
solution of the paradox thus raised seems to lie
in the proper conception of Being, and the relation
of its parts.

The view that mind has developed according to
the principles of the theory here explained, will,
I think, simplify and elucidate some of the prob-
lems of psychology. At a glance one can see how
far Locke was correct in this theory of the mind

CONDITIONS OF DEVELOPMENT. 243

beginning its existence as a “tabula rasa.” The
mind truly began its existence as a blank sheet, but
that was in the very first of the whole series of
living organisms, and by the inherited results of the
experience of every succeeding generation it has
attained its present development. It is not within
the scope of the present work to explain in how far
mental phenomena are due to properties of some
mode of Being, and how far they are the result of
processes of development. It is enough to say that
all material form and activity (as of brain and nerves)
come under the latter category. Professor Héffding!
has pointed out in an admirable manner the paral-
lelism existing between the material and mental —
the body and mind.

The history of philosophy and of nations shows
how the development of knowledge and institutions
has followed the general principles of the theory
of development here set forth. For while the
material world has remained unchanged, each suc-
cessive generation of mankind has been altered
by the slowly accumulating mass of human experi-
ence, and so has reacted to the demands of envi-
ronment in a slightly different manner. The great
minds who have added to human knowledge have
but reacted in a natural way to the influence of

1 Outlines of Psychology. Macmillan & Co,

244 DEVELOPMENT AND HEREDITY.

human experience and the ideas and conditions
of the age and environment in which they lived.
An illustration of this is the fact that so momen-
tous a theory as that of organic evolution by natural
selection was discovered simultaneously and inde-
pendently by two great naturalists. The knowl-
edge already existing determines what shall be
the.next advance. Thus Aristotle, though as gifted
as Newton, could not discover the law of gravity,
for the idea was too remote from the knowledge of
his age. Newton could only succeed Copernicus,
Galileo, and Kepler. We are accustomed to say
that the development of human knowledge and
institutions has been logical, which is the same
as saying that their development follows the laws
of mental activity.

The similarity or identity of the general laws
of mental activity with those of development, points
to a fundamental identity in the nature of the two
classes of phenomena. The laws of mental activity
are more easily studied and more generally known
in their details than the laws of development. If
the identity, therefore, be real, we shall get a more
accurate understanding of development by explaining
it according to the laws of mental action. This
method will be found to yield at least very sug-
gestive results,

CONDITIONS OF DEVELOPMENT. 245

It is a matter of common observation that the
human mind will not develop to full power without a
certain amount of stimulus from its surroundings, —
without something to urge it to activity. All sys-
tems of primary education are based upon this fact ;
and even after the schools have done their work,
unless some change of conditions of life, infusion
of new ideas, or adversity and misfortune, occur
to compel the mind to great activity, it will not
attain its greatest development. It will not only
lack the power of rapid calculation and compre-
hensive judgment, but will be narrowed in the range
of its feelings, and wanting altogether in certain
sympathies. A general proof of this is furnished
by comparing the average mental development of
those who, having no high intellectual interests,
live always without change in quiet villages and
on farms, with the development of those who dwell
in cities and also travel. We see also that the
mind and the brain—its physical mechanism —
not only need an active stimulus from the environ-
ment to attain their normal development, but need
constantly new stimuli to retain those powers in
perfection. This principle has a very wide and
important application, as can be seen by reviewing
the conditions necessary to the development of
animals. They all require the action of stimuli

246 DEVELOPMENT AND HEREDITY.

in order to develop; and the degree of development
which they attain is greater as the number and
variety of the stimuli increase.

In the lives of human beings, at periods when
the intensity of the general stimuli to life and
action is greatly increased, there is a corresponding
increase in the emotional or bodily activity result-
ing in a change of bodily conditions or a change
of character. The history of nations shows certain
periods when the imagination has been strongly
stimulated by the infusion of new ideas from abroad,
or by ideas evolved at home, or by great discoveries,
so that for a few generations the material and
intellectual activity of the people has displayed
wonderful vigour,—the unusually strong stimulus
thus making an epoch of greatness in the life of
the nation, far above the average level of the national
existence. As examples of ideas or events which
have exerted a profound stimulus upon the life of
nations, may be mentioned Mohammedanism among
the Saracens, the crusades preached by Peter the
Hermit, the introduction in Europe of the art of
printing, the revival of classic learning, and the
discovery of the new world. Geographical position,
the opening of new routes of trade, civil and foreign
wars, have also had their effect in promoting
suddenly the intellectual and moral development

CONDITIONS OF DEVELOPMENT. 247

of nations. In regard to individuals also, we notice
that the great stress of social and political crises pro-
duces great men; and lives filled with many events
and strong and varied emotions produce the great-
est characters. The greater the strength and variety
of the stimuli have been, the greater has been the
resulting progress of development, — provided the
stimuli did not pass the limit beyond which they
become destructive.

When we extend the application of this last con-
clusion to the physical development of organisms,
it is found to explain a peculiar phenomenon. It
has been observed that the human race, under the
strenuous conditions of life existing in newly settled
countries, shows a higher birth rate of males than
of females, while in the easier conditions of older
countries the opposite is the case. The male or-
ganism is — from a morphological point of view —
a more advanced or specialised development than
the female, and the conclusion is easily drawn, that
the stronger stimuli of the life in a new country
cause greater activity of the vital forces in the
parents, which expresses itself in the production of
the more specialised sex. So the production of
females may be regarded as a form of atavism in-
duced in easy circumstances by the relaxation of
the stimuli of life. In the same way, among many

248 DEVELOPMENT AND HEREDITY.

of the lower animals which reproduce parthenoge-
netically, when food is plenty and other conditions
favourable, only females are produced, but when the
conditions of life reach a certain degree of severity,
males are produced. In Professor Yung’s experi-
ments with tadpoles, we saw that reducing the
variety of stimuli by limiting the food to only one
kind, had the effect of greatly increasing the per-
centage of females above the normal ratio.

To the inquiry, what are the most advantageous
conditions for life and development, not only for the
lower animals, but for man also, it may be answered
—great variety and strength of stimuli. As the
stimuli cannot all be of the same kind or cannot all
be pleasant and agreeable without decreasing the
variety, so adversity has its uses; and the strength
of the stimuli of adversity counts equally with the
stimuli of prosperity ; and perhaps more, for adversity
has more varieties than prosperity. What counts
chiefly is not the available supply of simple energy,
but the complexity of the forces acting. The parasite
buried in the body of its host, has a comparatively
boundless supply of energy at its command, but
nevertheless degenerates, for it is removed from the
operation of the forces which cause development.
Great wealth, unless accompanied by corresponding
anxieties, duties, and responsibilities, removes its

DEATH. 249

possessors from the operation of many of the stimuli
of life, and a corresponding degeneration ensues.
The last phenomenon we have to consider is death.
We have already given a hint of its origin in dis-
cussing the reproduction of the metazoa. In the
reproduction of the metazoa there is a division of
the protoplasm of the organism — certain parts di-
vided off repeat in each case the cycle of life from
the beginning. Another part, however, which has
become too highly specialised by the forces acting
upon it, cannot return to the original condition, —
cannot repeat the cycle of life, — but must continue,
without a break, its reactions to its environment.
There is an approach to a condition of the continu-
ous action of the same forces upon a body. A force
acting continuously on any inanimate body composed
of a complex molecular or mechanical system, shows
two phases of action. First, there is a time when
the force is used in producing changes within the
body, overcoming resistance and friction, and chang-
ing molecular conditions. This time may be com-
pared with the period of development of living
bodies. It is followed by a time when the force
acting reaches an equilibrium with the resistance, —
no longer produces internal changes, and in its con-
tinued action none of it becomes latent in the body,
but escapes in the same quantity as it entered. This

250 DEVELOPMENT AND HEREDITY.

seems to be the state which the living body is con-
stantly approaching after it has finished its develop-
ment. The conditions of life, however, are too com-
plex to offer more than a general simile, but it is
enough to give an idea of the nature of life and
death. Development and life consist in internal
changes; death is a cessation of change. In the
living body changes are most frequent and pro-
nounced in the earliest stages and gradually decrease
as life approaches its end. By the long continuation
of the same stimuli the reaction to them gradually
becomes less. This phenomenon is familiarly known
as the ability to grow accustomed to things. We
may become unmindful or even unconscious of im-
pressions which at first affected us either pleasantly
or painfully. When growth has ceased, new co-or-
dinations and new adaptations to external changes
are not easily formed. Also, as the energy of inter-
nal changes is lessened, the restorative and trophic
processes become weaker, and all the phenomena of
senescence follow, the earliest and strongest associa-
tions remaining to the last, —second childhood in
man. Finally the weakest wheel in the machine
breaks, and dissolution takes place.

The causes of death, then, may be summed up as,
—first, the inability of the specialised body (soma)
of the protozoa to form new adaptive co-ordinations,

CONCLUSION. 251

and this is perhaps included in the second and main
cause ; namely, the decrease in the internal changes
of the body, produced by the continued sameness of
the stimuli acting upon it. If the continued same-
ness of environment tends to weaken the reactions
and processes of life and bring on senescence and
death, then we would expect that changes of envi-
ronment would strengthen the reactions and exert a
favourable influence upon the processes of life. The
following passages from Darwin’s Origin of Species
gives the reply to this supposition: “It is an old
and almost universal belief, founded on a consider-
able body of evidence, which I have elsewhere given,
that slight changes in the conditions of life are bene-
ficial to all living beings. We see this acted on by
farmers and gardeners in their frequent exchanges of
seed, tubers, etc., from one soil or climate to another,
and back again. During the convalescence of ani-
mals, great benefit is derived from almost any change
in their habits of life... the principle of life, ac-
cording to Mr. Herbert Spencer, being that all life
depends on, or consists in, the incessant action and
reaction of various forces, which, as throughout
nature, are always tending towards an equilibrium;
and when this tendency is slightly aisturbed by any
change, the vital forces gain in power.”

In concluding this brief exposition of the theory

252 DEVELOPMENT AND HEREDITY.

of development and heredity, it may be well to
point out its relation to other scientific theories. In
the first place, it is an effort to extend the applica-
tion of the law of the conservation of energy to the
phenomena of living matter, and to resolve the pre-
mises given us by the science of physics to their
logical conclusion in the realm of Biology. On the
other hand, it is the extension to all living matter
of certain fundamental properties of life which psy-
chology has either proved or tacitly assumed to
exist in the higher animals, the possible existence
of which in the lower animals has aroused so little
interest that it has perhaps never been discussed.
I mean here those properties expressed by the law
of repetition and association. I think none who
will review the facts of animal existence will deny
the universality of those properties.

By pursuing the method of applying these wide
generalisations to the phenomena of life, the lat-
ter appear in a new light before us, and a har-
mony heretofore hidden becomes apparent. Growth,
development, and heredity are all due to one com-
mon principle. The complex relation of organism
to environment may be better understood; the
broad facts of embryology and morphology are
explained, and fall harmoniously into their place in
the system. Much, of course, is lacking in detail,

CONCLUSION. 253

and it still remains to test the theory in all its
applications. It opens up a new method of explor-
ation. Its bearing on psychological problems seems
especially important, as having to do with the
development of mind, and there seems to be a
new field opened in the direction of comparative
psychology. I have not attempted to include this
investigation in the present work, and what little
I have indicated in this line is not meant to be
applied in a materialistic sense to explain or sug-
gest the nature of mind.

It may be objected that by using psychic pro-
cesses and properties of living matter as means of
explanation of heredity, I have departed from the
modern purpose of natural science, which is to give
a mechanical explanation of the universe. But I
have in this only anticipated the conclusion which
all biological facts render most probable, that psychic
processes have always their physical counterpart.
When physiological psychology can discover and
express the exact inter-relation of the physical and
the psychical, then general biology can make use of
the expression ; but meantime it were foolish to neg-
lect the facts of psychology. Even though arrived at
indirectly, we can accept no other conclusion than
that all evolution, psychic and organic, is a definite
causal series. The theory here given seems to em-

254 DEVELOPMENT AND HEREDITY.

phasise this conclusion, and to exclude utterly chance
as an agent of development. If we apply the the-
ory to our own development, we must recognise
ourselves and our actions as the result of a def-
inite, accurate activity of creative force; and if we
believe there is what we call “purpose” or “ intelli-
gence” in our actions, which are the latter end
of the causal series, then we must believe that pur-
pose or intelligence was displayed in all other parts
of the causal series, for they appear to be all of
the same nature. There is here a suggestion as to
the nature and relations of the creative power and
the creation.

As before observed, this theory of development
and life, as a series of necessary causes and effects,
makes our ideas of our free will and moral respon-
sibilities seem paradoxical. This is not the placé
to attempt the solution of this problem, though I
believe it capable of satisfactory solution. There
need be no haste to reach a final conclusion in
the matter, for we are all convinced that we have
a certain degree of freedom of will. According
to this theory of heredity, there is a great re-
sponsibility resting upon each generation, since
its actions are helping to mould the character of
its posterity. There is a hopeful thought also
in the idea that heredity is not all powerful,

CONCLUSION. 255

and that environment may tend to correct a heri-
tage of evil character. A heritage of good character
also may be warped by environment, so that excel-
lence of parts in successive generations can be main-
tained only by constant vigilance and effort in every
succeeding generation. The moral development of
our race may be viewed as the “forgetting’’ of our
heritage from the brute and the savage, and the
“learning”? of the higher modes of thought and
emotion which are possible to us.

ESSAYS UPON HEREDITY

AND KINDRED BIOLOGICAL PROBLEMS.

By DR. AUGUST WEISMANN,

Professor in the University of Freiburg, in Breisgau.

Vol. I., Second Edition, $2.00. Vol. Il., $1.30.

_, ‘ Weismann’s attempt to solve an old problem has excited great interest in scien-
tific circles, and furnishes a fine example of legitimate scientific speculation.” — Prof.
G. MactosxiE, in The Presbyterian and Reformed Review.

“For some years students of biology have been aware that Dr. August Weis-
mann, professor in Freiburg University, has materially contributed to the solution
of important biological problems, not so much, indeed, by the discovery of new data
as by a more searching analysis and more trustworthy interpretation of facts already
known. Now, however, for the first time the outcome of his inquiries is made acces-
sible in English to the general reader by an authorized translation issued from the
Clarendon Press. That the version correctly reproduces the German original, we
have the assurance of Dr. Weismann himself, and that it is made in admirable Eng-
lish the reader will at once acknowledge. The essays collected in the volume before
us were published at divers times during the years 1881-1888. They all deal either
exegetically or controversially with two fundamental problems; namely, the origin
and significance of the phenomenon which we call death, and the limitations of the
phenomenon which is termed heredity. We should also premise that the discussion
of the latter subject leads him to dissent from one of Darwin’s conclusions by deny-
ing the transmissibility of acquired characters.” — The New York Sun.

“ American students of the more profound biological problems are fortunate in
having these important essays in so good an English form. Romanes has recentl.
said of Weismann’s works, ‘A remarkable series of papers, the effects of whic
have been to create a new literature of such large and rapidly increasing proportions
that, with the single exception of Mr. Darwin’s own works, it does not appear that
any publications in modern times have given so great a stimulus to speculative sci-
ence or succeeded in gaining so influential a following.’ The work before us is a
series of essays presented at various times and in various forms, but all bearing upon
one central thought, — that acgurred character cannot be transmitted by heredity.
The idea is a startling one, and is wide-reaching in its consequences. If demon-
strated, it demolishes Lamarckism at one blow. With it, it destroys the whole theory
—a favorite one with American workers — that a sfeczes may be directly modified by
its environment. It does away with the theory of the disappearance of parts from
disuse. It establishes the idea that nothing can be transmitted to posterity but what
is congenital in the ancestor. In other words, it reduces the working force in devel-
opment or evolution of species to natural selection operating upon variations in the
germ cell, The importance of these essays is thus clear. The essays are eight in
number, — the duration of Life; on Heredity; Life and Death; on the continuity of
the Germ Plasm as a foundation of Heredity ; Significance of Sexual Reproduction
in the theory of Natural Selection; the number of Polar Bodies and their Signifi-
cance in Heredity; on the Supposed Botanical Proofs of Transmission of Acquired
Characters ; the Supposed Transmission of Mutilations, — are devoted to stating the
theory and meeting in detail objections that have been urged against it. Some of the
essays are too technical for the general reader; but the first three and the last two
are simple, and clearly place the theory, its bearing, and the two factors that must
appear in the discussion, before the reader.” — Old and New Testament Student.

MACMILLAN & CO.,
66 FIFTH AVENUE, NEW YORK.

I

THE HISTORY OF HUMAN MARRIAGE.

By EDWARD WESTERMARCK,
Lecturer on Sociology at the University of Finland, Helsingfors.

8vo. Cloth. $4.00.

“Even those whose views are here opposed will, I think, acknowledge that Mr.
Westermarck is a careful investigator and an acute reasoner, and that his arguments,
as well as his conclusions, are worthy of the most careful consideration.... Every
reader of the work will admire its clearness of style, and the wonderful command of
what is to the author a foreign language.’? — ALFRED RussEL WALLACE.

“The author . . . is known throughout Europe as a scholar of unusual attain-
ments and ability. His work is introduced to the reading public by Professor Alfred
R. Wallace, who says of it: ‘I have seldom met a more thorough or a more philo-
sophic discussion of some of the most difficult, and at the same time interesting,
problems of the day. The origin and development of human marriages have been
discussed by such eminent writers as Darwin, Spencer, Morgan, Tylor, Lubbock,
and many others. On some points, Mr. Westermarck has arrived at different, and
sometimes at diametrically opposite, conclusions; and he has done so after a most
complete and painstaking investigation of all the available facts. With such an
array of authority on the one side, and a hitherto unknown student on the other,
it will certainly be thought that all the probabilities are against the latter. Vet I
venture to anticipate that the verdict of independent thinkers will, on most of these
disputed points, be in favor of the newcomer who has so boldly challenged the con-
clusions of some of our most esteemed writers.’ Professor Westermarck could hardly
have had a better presentation to scientific readers than these few words of Professor
Wallace....

“The exhaustive researches of the author (the bare list of titles of authorities
examined and quoted covers twenty-nine closely printed pages) culminate in the
following paragraph: ‘ Marriage, generally speaking, has become more durable in
proportion as the human race has advanced. Marriage has thus been subject to
evolution, in various ways, though the course of evolution has not always been the
same. The dominant tendency of this process, at its later stages, has been the exten-
sion of the wife’s rights. A wife is no longer the husband’s property; and according
to modern ideas, marriage is, or should be, a contract on the footing of perfect equal-
ity between the sexes. The history of human marriage is the history of a relation
in which women have been gradually triumphing over the passions, the prejudices,
and the selfish interests of men.’ The work is one of remarkable interest, and will
undoubtedly arouse lively debate in scientific circles.”? — Boston Transcript.

‘This is the most scientific treatise yet produced in this special field of investi-
gation, and we note three prime characteristics of a scientific method. The facts
upon which its theory is based have been collected in enormous quantity from the
literature and the observation of all ages, for, as the author says, the first condition
of success is that ‘ there should be a rich material; what is wanting in quality should
be made up in quantity.’ Again, in the interpretations of existing phenomena the
writer has sought to guard against the double danger of ‘ putting into them a foreign

2

meaning,’ and of ‘inferring, without sufficient reasons, from the prevalence of a
custom or institution among savage peoples, that this custom, this institution, is a
relic of a stage of development that the whole human race once went through.’
Finally, Mr. Westermarck is never satisfied that a custom or institution is explained
until he has endeavored to trace it to something fundamental in the physical or
psychical nature of the race.... We lay the volume down with the conviction that
it is a masterly contribution to a growing science, and that its author’s argument will
stand, even although one may not accept with him the anthropoid ape as a man and
a brother.” —Joun J. Hausey, in The Dial.

“Tt is scholarly, thoughtful, and comprehensive. There is nothing in it to stay
the souls of persons having ‘advanced views’ of the marriage relations; the author
finds that nature, like sentiment, reason, and religion, has decreed that one man shall
be the mate of one woman, and that any deviations from this rule bring enduring
penalties with them. The book contains much curious information about the mar-
riage customs of various countries, savage and civilized, but is devoid of anything
that can stimulate erotic imaginations. A portion of Professor Westermarck’s work
which might be read to advantage by almost any one is the chapter showing the
changes which have been made from time to time in woman’s degree of dependence
upon man and independence of him. Since the earliest period of written history,
woman, in most civilized countries, has been regarded as the inferior partner in the
family relation, and often has sunk to the position of a mere dependent. One of the
boasted glories of modern civilization is the elevation — too often nominally only —
of the wife to equality with the husband. Yet traditions of the oldest races, and the
practice of some modern peoples who are scarcely civilized, show that woman has
been at times the head of the family and absolutely her own ruler. ... The world
would not willingly go back to all primitive customs, ignorance, and discomfort, that
woman might fully reassert herself; but it is one of Civilization’s satires upon herself
that woman was never so independent of man as when she supported herself.”’ — The
New York Herald.

‘This is one of the most elaborate works on the history of social institutions that
we have met with. The author is lecturer on sociology in the University of Finland at
Helsingfors, yet his book was written by himself in English, which is to him a foreign
language. He modestly tells us in his preface that, as originally written, the book
contained some un-English expressions, which were corrected by his English friends;
but the ease and clearness of his style show that he is a master of the art of expres-
sion, and make his work far more interesting than works on such subjects are apt to
be. ... Asa descriptive history of marriage in the many forms it has assumed, the
work could hardly, in the present state of our knowledge, be surpassed... . He
shows a very wide as well as intimate knowledge of the facts, so far as they have
been discovered, and both his facts and his arguments will have to be considered by
all who may write on the subject hereafter. His opinions on certain fundamental
points are at variance with those of most previous writers, and hence his work is
likely to give rise to some controversy. He rejects the hypothesis that promiscuous
intercourse was once everywhere prevalent, and his arguments on this point deserve
careful attention. ... Whatever may be thought of some of Mr. Westermarck’s
theories, his work will be indispensable to all students of the early history of
marriage.” — Science. -

MACMILLAN & CO.,
66 FIFTH AVENUE, NEW YORE.

3

DARWINISM:

Being a Systematic Exposition of the Theory of Natural
Selection, with Some of its Applications.

By ALFRED RUSSEL WALLACE, LL.D., F.L.S.

Pp. Xiv., 494. $1.75.

“In all the mass of books that has appeared since The Orzgzu of Spectes, no one
volume is more valuable as a statement of theory, or more fascinating to the reader,
than that by Alfred Russel Wallace, on Darwinism.” — The Plain Dealer.

‘The book is written with all of Wallace’s usual clearness and force... . It is
full of suggestion, and will repay careful reading better than any scientific work of
recent years,”’ — The Chronicle.

“« The book is hardly less important than those of Darwin himself. . . . In popu-
lar as well as scientific interest, this may justly be called a fascinating book. Its
highest significance is that it still allows to man a spiritual meaning.” — Czucinnat?

Commercial Gazette.

“A most stimulating and delightful work. Certainly no scientific writer has set
forth the principal points of Darwinism with more brilliant and convincing clearness,
or has thrown more light, for the ordinary understanding, on phases of the question
which are obscure and difficult to grasp. It is only proper to call attention to the
style of the author, which is simple, bright, and vivid, a model for the scientific ex-
positor. Mr. Wallace has that most desirable of all gifts for the scientist, a powerful
and well-ordered imagination, which shows itself not only in its higher uses, but in
the charm which it lends to presentation.” — Eclectic Magazine.

“Dr. Wallace’s book is necessary to a thorough understanding of the present
status of conservative thought on the doctrine of evolution. To the student of sci-
ence, to the theologian, and to the cultivated layman, it alike appeals. Those who
master it will owe the distinguished author a debt of gratitude.” — The Beacon.

‘The number of writers on questions connected with Darwinism is so great, and
their views so varied in detail, that it would puzzle most people to tell exactly what
the Darwinism of Darwin really i is. A book, therefore, which defines Darwin’s theory
simply and definitely, restates the old and ’adds the new evidence that has been col-
lected to uphold it, indicates what has been contributed since Darwin’s day to the
strengthening of his theory, carefully criticised in order to dispose of what has been
brought against it, so that within itself it contains all that is necessary to a complete
understanding of ‘the evolutionary theory as it was held originally, and as it stands
to-day, is a book which is in the nature of a boon to a bewildered public. Such a
book is this one by Mr. Russel Wallace. This author is beyond question the best
man to undertake the task of elucidation and exposition. He was, be it remembered,
co-originator with Darwin of the theory of the origin of species. He has always been
in close agreement with Darwin’s views, and worked with him for the establishment
of the theory as a friend, not as a rival. By intimate knowledge of Darwin’s ideas,
and of the multitudinous “facts in natural history that go to support them, Mr. Wal-
lace is pre-eminently fitted for the task he undertook. Moreover, he has the lit-
erary ability to make both easily intelligible and interesting. His unpretending style
is very attractive in its clearness and simplicity, and for orderliness and compactness

his arrang t of agr and example could hardly be excelled. Finally, his
tone is admurable, calmly judicial, and without the least touch of polemical aggressive-
ness. ... The book is, of course, a treasury of curious knowledge in all branches

of natural ben, and, read on that account only, would prove abundantly interest-
ing; but in addition it is valuable to the reader, as an authoritative and skilfully pre-
pared. résumé, and to scientific men, by reason of the fresh material and argument it
contains.” — Boston Advocate.

MACMILLAN & CO.,
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4

NATURAL SELECTION AND TROPICAL
NATURE :

Essays on Descriptive and Theoretical Biology.

By ALFRED RUSSEL WALLACE, LL.D., F.L.S.

8vo. pp. 492. $1.75.

_ ‘This is a new, corrected, and enlarged edition of two volumes of essays printed
in 1870, 1871, and 1878. They are now re-worked into the present one-volume
edition, with some additions and some omissions. Mr. Wallace’s scientific position
remains in them unchanged. The somewhat free and miscellaneous arrangement of
the contents relieves to some extent the strain on the reader, and adds much to the
pleasure of reading. We note with interest his strong re-assertion of the fact that
there are limits in Nature to the operation of natural selection, and that it cannot
account for the development of man.” — The /udependent.

“* Among interesting subjects treated here are the tendency of varieties to depart
indefinitely from the original type, mimicry, and other protective resemblances among
animals. he remarks on the general phenomena of color in the organic world gives
very great enjoyment to the reader, who finds, in Mr. Wallace’s work, reverent studies
of the beauties and harmonies of creatures, and a conviction that the most insignifi-
cant parts of tiny organisms are worthy of earnest study, and capable of exciting
intelligent admiration.” — Pudlic Ledger.

“« Biologists of the present, though perhaps unanimous in acceptance of some form
of evolution, are divided into two camps as to what factor has been most potent in
the progress, — the direct modificative influence of the environment, or natural selec-
tion. Wallace stands for the latter. In 1870 he presented to the world a little vol-
ume of essays on Natural Selection. In 1878 appeared a similar volume on
Tropical Nature. Both were delightful, even charming, reading, They have
been out of print for some time, Just at this time, their reprinting, as in the
volume before us, is certainly desirable.... To differ with a few of his statements
and some of his conclusions, is not to lack appreciation of their value and beauty
of style.” — Christian Union.

“Somewhat more than one-half of the book is occupied with the subject just
named in this title, — Natural Selection. By this is meant, of course, that process
in nature by which organic life, in its ‘ struggle for existence,’ parts with those forms
which are feebler and less equal to such a ‘struggle,’ while the more vigorous ones
become permanent.... It is, in other words, the basal principle in that doctrine
of ‘evolution’ of which we hear so much. This doctrine Mr. Wallace adopts,
although under limitations which the extreme advocates of the evolution school in
science do not recognize. He believes in a creation and a Creator; but it is ‘creation
by law,’ that is to say, the establishment, by the Creator, of a law and method in the
physical universe, in accordance with which all organic life develops and multiplies.
.++ He is, then, a Darwinian within limitations. His treatment of the principles
which characterize this school in modern science is very lucid, while his array of
facts shows fruits of a lifetime devoted to studies of this nature. On one side, his
book may be read with a sensation of relief; it is a relief to follow in such studies a
man who, while thoroughly at home in his subject, a profound observer and a skilful
reasoner, is at the same time zo¢ an atheistical fanatic. ‘Ihe other essays in the
book, descriptive of organic life in tropical countries, will very much delight those
whom researches of this kind interest.” — The Standard.

MACMILLAN & CO.,
66 FIFTH AVENUE, NEW YORK.
5

INQUIRIES INTO HUMAN FACULTY, AND
ITS DEVELOPMENT.

By FRANCIS GALTON, F.R.S., etc.
8vo. Cloth. pp. xii., 387. $3.00.

“ The larger part of this book has already appeared in various periodicals; but
the matter is so interesting, and the treatment of the subject so unique, that it is well
worthy of preservation in this permanent form,” — The Literary World.

Scientific students of all kinds, especially psychologists, have anxiously awaited
this suggestive volume. . . . The volume contains colored plates and other illus-
trations, among them interesting specimens of composite portraiture. Although the
author intends it to be suggestive, and ‘renounces all claim to be encyclopedic,’ he
has given the world another important chapter of the new branch of science which he
mapped out in his remarkable book on hereditary genius.” — Phzladelphia Item.

‘*Mr. Galton has worked a complete revolution in the study of man.... The
book is of universal and profound interest to every human being. It is the beginning
of a new study of an old subject, which, if we are wise, will one day far in the future
make the human race a race of aristocrats. But the aristocracy will not be based on
money, nor on mere names, but on the proper breeding of men, and the proper laws
for human development, physical and mental.” — Vew Vork World.

“Mr. Galton’s exceedingly interesting Zuguzrives 7nto Human Faculty are so
novel and suggestive that they open the way for many extended lines of discussion
and argument concerning the future evolution of man. The book is made up largely
of magazine articles, but these are held together by the thread of a common purpose,
and form a connected outline. . . . His book affords a great amount of instruction,
and cannot fail to stimulate inquiry into the fascinating themes of which it treats.” —
Boston Traveller.

HEREDITARY GENIUS:

An Inquiry into its Laws and Consequences.

By FRANCIS GALTON, F.R.S., etc.
8v0. pp. 379. $2.50.

“ Hereditary Gentus is the title of a volume written in the sixties by Francis
Galton, F.R.S. To say that it created a sensation is to put it mildly, for a sensation
seldom lasts very long; but better would it be to say that by its subtle reasoning and
arrangement of facts, it made a deep impression on the minds of inquiring people.
The title hardly explains the contents of the book, as it is rather a study of vital laws
which governed the past and will govern the future. It strives to show the reader
the means by which the condition of the race may be ameliorated, and the standard
of mentality raised by natural methods. Many old precepts which have firm hold
on the mind are dissipated by an indisputable array of evidence, as by one fell blow;
and, while we are startled and surprised, we are pleased with the carnage and
advance of science. The book will open to one a field in which he can study with
profit, and make one the better for knowing a few vital principles which govern life,
and upon which the future happiness and usefulness of posterity largely depends.”
— Brooklyn Iliustrated Church History.

MACMILLAN & CO.,
66 FIFTH AVENUE, NEW YORK.
6

NATURAL INHERITANCE.

By FRANCIS GALTON, F.R.S., etc.

8vo. Cloth. $2.50.

‘Mr. Galton is a specialist in this department, and he has collected a vast amount
of material. His conclusions do not favor the extreme ideas of heredity held by many
writers. His argument seems to establish the fact that ‘ the chain of natural endow-
ments remains essentially the same through all ages, and of equal size throughout.’
There isa tendency, in spite of peculiarities, to return to about the average size,
height, intellect, etc. It is an interesting and valuable work.” — The Sunday School
Journal,

“Mr. Francis Galton’s studies of the science of heredity have an authoritative
value in sociology, since his arguments are based upon thorough investigations, and
accompanied by careful comparisons. Natural Inheritance, his latest study upon
the subject, is a valuable illustration of his manner in arguing from collected data... .
Tables and appendix add to the scientific interest of the work, which may be regarded
as a most suggestive and valuable contribution to the study of modern society.” —
The Boston Fournal.

“ The general feeling with which one leaves the book is an admiration of the pow-
ers of method to give meaning to facts; an appreciation of the complex system of
inferences that make each individual what he is; and a hope that posterity will be
able to utilize the fruits of such workers as Mr. Galton for the amelioration of man-
kind.” — The Christian Union.

“Mr. Galton is the highest authority on this subject; a new book by him is an
event in the scientific world. . . . Scientific men will enjoy particularly the study of
Mr. Galton’s processes, the usefulness of which is by no means limited to the investi-
gation of heredity.” — Boston Advocate.

“Mr. Francis Galton, whose studies in heredity have been life-long, whose meth-
ods are so patient, is the last man in the world to ventilate startling theories, or to
assert ill-digested things as facts. ... For perfecting philosophical inquiry, for pru-
dence and good judgment, Mr. Galton is to be considered as presenting the highest
examples of modern research.”’— The New York Times.

‘« This is Mr. Galton’s latest treatise on a subject and in a field of investigation
which he has made almost exclusively his own. Whatever one may think of his con-
clusions, no one who has any adequate conception of ‘Science’ can find fault with
either the spirit of the author or the manner in which his investigations are carried
on. It would be impossible here to do anything like justice to the details of his
method, which is in general observation, experiment, and generalization; but it is
simple justice to say that no effort to reach general laws by an induction from phe-
nomena was ever made with more scrupulous method. The problem he has under-
taken to solve is one of extreme difficulty, and he does not protss to have done more
than furnish a contribution toward its solution; but, by publishing what this work
contains, he has made it possible to secure the intelligent co-operation of hosts of co-
workers wherever his book is read, and in this way he has done a far greater service
to Science than perhaps he thought of when he wrote it. Whether Science will ever
be able to find out to what extent the nature of the child is determined by conditions
antecedent to birth is still very doubtful; but if the doubt on the subject is hereafter
set at rest, much of the credit of getting rid of it must always be accorded to Mr.
Galton for the labor, the patience, and the ingenuity which he has brought to bear
on the question of heredity.” — Toronto Week.

MACMILLAN & CO.,
66 FIFTH AVENUE, NEW YORE.
7

ORGANIC EVOLUTION AS THE RESULT OF THE INHERITANCE
OF ACQUIRED CHARACTERS ACCORDING TO THE
LAWS OF ORGANIC GROWTH.

By Dr. G. H. THEODORE EIMER.
Translated by J. T. Cunnincuam, F.R.S.E,
8v0o. $3.25.

“ Professor Eimer believes that he has demonstrated, contrary to certain portions
of Professor Weissman’s argument, that external conditions do modify organisms,
and that characters so acquired are veritably inherited. If he is right, then Darwin
ism receives a powerful support, for Darwin did not work out this part of his theory
as wellas others. But he does not maintain (and this he dwells on) that adaptation
to surroundings will explain all phenomena— that it has exclusive dominion. ‘ If
this exclusive dominion did belong to it, if everything were adapted, all evolution of
the living world would be also excluded; there would be stagnation.’ He brings
combinations of characters into play. Supposing that when a given animal became
adapted in a certain way other less necessary changes occurred in its frame, as, when
horns appeared, other parts of the body changed in sympathy. He is fond of an
analogy with crystals, which do not only form at the point of impact, but when once
started branch far and wide in certain lines of direction, changing the whole mass.
It would be impossible to follow Professor Eimer through the organic growth of the
living world, the influence of adaptation on the formation of species, and the sections
or acquired characters, mental faculties acquired and inherited, function as the cause
of evolution without and within, and the Law of Organic Form, Readers may be
warned that the earlier sections are the difficult ones, while those that follow contain a
very interesting and attractive store of information, anecdote, and running argument
applied to animal life. It may merely be said broadly that Eimer represents more
nearly the kind of scientific mind that Darwin had than do men like Kélliker, Von
Nageli, and Weissman, who call themselves physiologists, and base their arguments
on studies with the microscope rather than on wide generalizations on visible facts
concerning animals. Professor Eimer is a zodlogist rather than a physiologist. To
laymen the distinction is fine; but when we discover that one scientist assumes that
another must err because that other is a zodlogist, for example, we begin to perceive
that the training a man has undergone may plausibly enter into account.” — Vew
York Times.

MACMILLAN & CO.’5 NEW SCIENTIFIC BOOKS.

HERTWIG (0.). A Text-Book of Embryology: Man and Mammals. Translated
from the third German edition by Dr. Epwarp L. Mark, Hersey Professor of
Anatomy in Harvard University. Illustrated. 8vo. $5.25 xed.

HATSCHEK (B.). The Amphioxus and its Development. Translated by JAMES
Tuckey, M.A. $1.50 ed.

MIGULA (W.). An Introduction to Practical Bacteriology. Translated and edited
by M. and H. J. Camprety. Illustrated. $1.60 et.

SHIPLEY (A. G.). Zodlogy of the Invertebrata. 8vo. $6.25 ned.

IN PREPARATION.

GAMGEE (A.). A Text-Book of the Physiological Chemistry of the Animal Body.
With illustrations. Vol. II.

KORSCHELT and HEIDER. Text-Book of the Developmental History of the
Invertebrates. Translated under the supervision of Dr. E. L. Marx, Harvard
University, Mass. Fully illustrated. 8vo.

MACMILLAN & CO.,
66 FIFTH AVENUE, NEW YORK.
8

SO) SEP 1.58

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