EX LIBRIS.

Bertram C, <&. SlEtnfcle,

ffl., m.&c., 8,3.6., JF.K.S,

THE process of specialization which it would see
been necessitated by the immense growth of sc
during the past fifty years has gone far towards e
minating the older type of biologist who Was posset
not only of a fairly complete knowledge of what botany
and zoology had to teach, but also had a large acquaint-
ance with geology. Nowadays a man is a botanist or a
zoologist or a geologist, and perhaps has no very great
acquaintance with the intricacies of the sciences cognate
with his own. One welcomes, therefore, books like Pro-
fessor Dendy's Outlines of Evolutionary Biology (London J
Constable and Co. 1912) which give a general account of
the phenomena of life and of the theories associated with
them, with examples drawn from both kingdoms of
Nature.

We do not intend to enter into any detailed criticism
of the book, and will content ourselves for the most part
by saying that the task attempted in this book has been
Well carried out; that it presents a fair picture of the
state of scientific opinion'on most biological problems and
that it should find a place in the library of all our colleges
and of all persons working at bio-philosophical problems.
No book is worth much which does not reveal something
of the personality of its writer and, to our mind, Professor"
Dendy reveals himself as too much affected by Weisman-
nian views. No doubt " biophores " and " germinal
selection " might and would explain a good deal at
present unexplained. But then, because an explanation
explains, it is not necessarily, therefore, the true explana-
tion, and no one will claim that there is anything in the
way of ascertained fact which tends to prove the truth or
the theories alluded to.

Further we must take exception to the Statement that
the gap between man and ape is so small that " there is
little room for connecting links between them." Mr A. R<
Wallace, in his latest work says that " there is not, as often
assumed, one '* missing link ' to be discovered, but at
least a score such links, adequately to fill the gap between
man and apes " (The World of Life, 1911), and we fancy
that most persons who have devoted attention to mam-
malogy would agree with him. We can commend this
book to teachers.

B.C.A.W,

OUTLINES OF
EVOLUTIONARY BIOLOGY

OUTLINES OF
EVOLUTIONARY BIOLOGY

BY

ARTHUR BENDY, D.Sc., F.R.S.,

Professor of Zoology in the University of London (King's College)

Zoological Secretary of the Linnean Society of London ;

Honorary Member of the New Zealand Institute ; formerly

Professor of Biology in the Canterbury College

(University of New Zealand), and Professor of

Zoology in the South African College,

Cape Town.

LONDON

CONSTABLE & COMPANY LTD

10 ORANGE STREET LEICESTER SQUARE W.C.

1912

PREFACE

BIOLOGY, the fundamental science of living things in all their
manifold relations, is a study which, at the present time, is but
little encouraged by educational authorities in this country. It
has no place in the ordinary school curriculum and even in our
Universities it has been thrust into the background, partly
because University authorities devote so much of their attention
nowadays to subjects which are considered more likely to
bring in a direct pecuniary reward to the student, and partly
because of the immense elaboration of the various branches of
biological science, such as Zoology, Botany, Physiology, Com-
parative Anatomy and Embryology, that has taken place in
recent years, and the claims of these to more or less separate
recognition.

The student, if he studies Biology at all for its own sake,
which is seldom the case, usually confines himself almost
entirely to one or other of these branches, which he finds
treated more or less as an independent science, with an extensive
literature of its own, and he runs a grave risk of losing sight of
the general principles which underlie all and from which all
derive their chief educational value.

The medical student, it is true, usually takes a year's course
in what is called Biology, but his curriculum is, perhaps
unavoidably, dominated by the type-system and by what is
thought likely to be of direct service to him in his future
anatomical and physiological studies, so that in the brief time
which the medical authorities allow him to devote to the
scientific foundation of his professional work he has but little
opportunity for a philosophical treatment of the subject.

When we at length come to realize the meaning of Man's
position as, for the time being, the highest term of a great
evolutionary series which stretches far back into the dawn of

vi PEEFACE

the earth's history, and to appreciate the importance of the fact
that he derives his existence from the same ultimate sources and
is subject to the same natural laws as all the other living things
with which he shares the earth, we shall perhaps see the
necessity for making Biology, in the widest sense of the term,
one of the foundation stones of our educational system.

In the meantime those who wish to familiarize themselves
with the rapidly accumulating results of biological investigation
and the bearing of these results upon human problems ought
not to be debarred from so doing by want of the necessary
knowledge of fundamental facts and principles, and it is largely
with a view to the requirements of such students that the present
work is offered to the public.

That even the elementary study of biological theory should,
wherever possible, be preceded by a systematic course in Zoology
and Botany, based upon the type-system and including laboratory
work, is, no doubt, indisputable. Unfortunately, under existing
conditions, regular laboratory work is, for most people, im-
possible. We are apt to forget, however, that in reality we all
of us spend our lives in a biological laboratory, where we are
surrounded by living organisms which we can hardly avoid
studying. In this way we learn much of the nature of living
things and are to some extent prepared for the study of biological
principles.

The problems of life, however, cannot be satisfactorily solved
if we confine our attention to the higher and more familiar forms
of plants and animals. Man, in particular, is far too complex a
type to begin with in a philosophical treatment of the subject.
The logical method of study is, no doubt, to follow as closely as
possible the course which we believe to have been taken in the
actual evolution of living things, beginning with the simple and
ending with the complex. This method, of course, is attended
with certain practical difficulties, mainly due to the microscopic
size of the more primitive organisms, but these difficulties are
not insurmountable and need not be considered in relation to the
present work.

As I wish this book to be of use to those who have had no
special biological training, as well as to students who have taken
the ordinary first year's course, I have, in the earlier chapters,
dealt in a very elementary manner with the structure and
functions of both plants and animals. I have described Amoeba

PREFACE vii

and Hsematococcus in considerable detail and used these familiar
organisms as pegs on which to hang some elementary ideas with
regard to the nature of living things and the differences between
animals and plants. Otherwise I have as far as possible avoided
the type-system as being altogether unsuitable for a work of
this kind, though of course I have been obliged to refer to
numerous different organisms in illustration of special points.

It was only by rigidly excluding from the earlier part of the
book everything that was not considered essential to the under-
standing of general principles that it has been possible to find
space for even a brief presentation of the evidence upon which
the theory of organic evolution rests, and for a discussion of the
principal factors which appear to have co-operated in determining
the course of that evolution.

Although the entire work is intended to be of an elementary
character, it has been impossible, in connection with the theory
of heredity, to avoid, on the one hand, a considerable amount of
cy tological detail, and, on the other, some discussion of theoretical
speculations of a highly controversial nature. In dealing with
these vexed questions, which underlie the whole problem of
organic evolution, I have endeavoured to present the views of
opposing schools of thought as fairly as possible, but I must
confess that I have ventured to lay considerable stress upon
ideas which, though widely accepted elsewhere, have not as yet
met with much appreciation in this country, though that they
will do so in the future can hardly be doubted.

By way of introduction to the discussion of the factors of
organic evolution a chapter has been devoted to the views of
Buffon, Erasmus Darwin and Lamarck, and another to those of
Charles Darwin, Robert Chambers and Alfred Russel Wallace,
and I have endeavoured to present the opinions of these classical
authors as far as possible by means of quotations from their
own writings.

In a work such as the present the employment of numerous
technical terms is, of course, unavoidable, but it is hoped that
the meaning of these is sufficiently explained in the text, and the
use of the index should obviate any difficulties in this respect,
especially if tho book is read systematically from the first chapter
onwards.

It is a pleasure to record my thanks to many colleagues who
have ungrudgingly helped me in various ways. Amongst these

viii PKEFACE

I should like especially to mention Professor Poulton, Professor
Herbert Jackson, Dr. Stapf, Dr. Daydon Jackson, Dr. Sibly and
Professor G. E. Nicholls ; while to Mr. E. W. H. Eow, of the
Zoological Department at King's College, I am much indebted
for the trouble which he has taken in reading through the
proof-sheets and for many valuable suggestions.

I owe a sincere acknowledgment to my publishers, Messrs.
Constable & Co., for their generosity in the matter of illustrations.
Most of these are either entirely new or, in some cases, specially
re-drawn from original memoirs. I have made extensive use of
photomicrography for microscopic subjects, and in the prepara-
tion of the photographs at King's College have received much
assistance from Dr. Eosenheim, Mr. Eow, and my assistant
Mr. Charles Biddolph.

For the loan of the blocks from which the remainder of the
illustrations have been printed, or for permission to copy, I am
indebted to the generosity of the following :—

Messrs. George Allen and Son, •

" The American Journal of Science,"

Mr. Edward Arnold,

Messrs. George Bell and Sons,

Messrs. A. and C. Black,

The Cambridge University Press,

The Clarendon Press,

Messrs. Constable & Co.,

Messrs. Duckworth & Co.,

Herr W. Engelmann,

Herr Gustav Fischer,

" The Journal of Experimental Zoology,''

Messrs. Kegan Paul, Trench, Triibner & Co.,

The Council of the Linnean Society of London,

Messrs. Longmans, Green & Co.,

Mr. John Murray,

" The Quarterly Journal of Microscopical Science,"

The Council of the Eay Society,

Messrs. Smith, Elder & Co.,

The Trustees of the British Museum,

Messrs. T. Fisher Unwin,

Messrs. F. Warne & Co.,

Verlag des Bibliographischen Instituts, Leipzig und Wien.

PREFACE . ix

I must also express my gratitude to the numerous authors
whose work I have made use of, and whose names are mentioned
in the appropriate places.

I am indebted to the Council of the Royal Society of Arts for
permission to make use of the Aldred Lecture which I delivered
before the Society in 1909, and which is to a large extent
reprinted from their Journal in Chapter XIV.

ARTHUR DENDY.

KING'S COLLEGE, LONDON,
December, 1911.

COEEIGENDA.

P. 382, line 7— for " her " read " its."
,, ,, 9 — -for " she" read "life."
;> >? 10— for "herself " read "itself."

CONTENTS

PART I.— THE STRUCTURE AND FUNCTIONS OF
ORGANISMS— THE CELL THEORY

CHAPTER I

PAGE

Introductory : The nature of life — The living organism viewed as a
machine — The essential functions of the living body — The source
of energy in living things 1

CHAPTER II

Amoeba as a typical organism — The properties of protoplasm . . 12

CHAPTER III
Hsematococcus — The differences between animals and plants . . 27

CHAPTER IV

The cell theory — Unicellular organisms — Differentiation and division
of labour — Co-operation — The transition from the unicellular to
the multicellular condition — The early development of multicellular
animals and plants .......... 36

CHAPTER V

The cell theory as illustrated by the histological structure of the higher
animals and plants —Limitations of the cell theory — The cell as
the physiological unit 51

CHAPTEE VI
The multiplication of cells — Mitotic and amitdtic nuclear division . 69

PART II.— THE EVOLUTION OF SEX

CHAPTER VII

Limitation of the powers of cell-division — Rejuvenescence by conjuga-
tion of gametes — The origin of sex in the Protista . . .81

xii CONTEXTS

CHAPTER Till

PAGE

Sexual phenomena in multicellular plants — The distinction between
somatic cells and germ cells — Alternation of sexual and asexual
generations — Suppression of the gametophyte in flowering plants . 95

CHAPTEK IX

Sexual phenomena in multicellular animals — Structure and life history
of Hydra and Obelia — Alternation of generations — The ccelomate
type of structure — Secondary sexual characters — The evolution
of sex 113

CHAPTER X

Origin of the germ cells in multicellular animals — Maturation of the
germ cells — Reduction of the chromosomes — Sex determination in
insects — Different forms of gametes — Mutual attraction of the
gametes — Fertilization and parthenogenesis . . . . .129

PART III.— VARIATION AND HEREDITY
CHAPTER XI

Variation — Meristic and substantive variations — Fluctuations and
mutations — Somatogeuic and blastogenic variations — Origin of
blastogenic variations 14<s

CHAPTER XII

Heredity — General observations — Darwin's theory of pangenesis and
"Weismann's theory of the continuity of the germ plasm — The
nucleus as the bearer of inheritable characters . . . .161

CHAPTER XIII

The inheritance of acquired characters and the mnemic theory of

heredity 176

CHAPTER XIV

The Mendelian experiments in hybridization — The doctrine of unit
characters and the purity of the gametes — Galton's law of
inheritance 194

PART IV.— THE THEORY AND EVIDENCE OF
ORGANIC EVOLUTION: ADAPTATION

CHAPTER XV

Organic evolution versus special creation — Spontaneous generation and

biogenesis — The origin of living things . . . . . .211

CONTENTS xiii

•
CHAPTER XVI

PAGE

The continuity of life— The conception of species— The principles of

taxonomy — The taxonomic evidence of organic evolution . . 221

CHAPTER XVII

Connecting links — Homology and analogy —Convergent evolution —

Change of function — Vestigial structures — Reversion . . . 232

CHAPTER XVIII

Ontogeny — The recapitulation hypothesis — Interpretation of the onto-

genetic record — Palingenetic and csenogenetic characters . . 263

CHAPTER XIX

The stratified rocks — Geological periods — The age of the habitable earth
— The geological record — The succession of the great vertebrate
groups 282

CHAPTER XX
Fossil pedigrees — Ancestry of birds, horses, elephants and whales . 305

CHAPTER XXI

Geographical distribution — Areas of distribution — Barriers to migra-
tion— Means of dispersal — Changes in the physical conditions of
the earth's surface — The evidence afforded by the study of geogra-
phical distribution with regard to the theory of organic evolution . 319

CHAPTER XXII

Adaptation to environment in animals — Deep sea animals — The coloura-
tion of animals — Protective and aggressive resemblances — "Warning
colours — Mimicry — Epigamic ornamentation ..... 334

CHAPTER XXIII

Adaptation to the environment in plants —Alpine plants, desert plants
and lianes — The modification of flowers in relation to insect-
fertilization . . . . . . . . . . .350

PAET V.— FACTORS OF ORGANIC EVOLUTION

CHAPTER XXIV
Views of Buffon, Erasmus Darwin and Lamarck 365

CHAPTER XXV

Robert Chambers and the "Vestiges of Creation" — Natural selection

— The views of Charles Darwin and Alfred Russel Wallace . . 383

xiv CONTENTS

CHAPTER XXVI

PAOK

Selection not confined to the organic world — Illustrations of the action
of natural selection in the struggle for existence — Degeneration
— Flightless birds — Extermination of the Morioris — Sedentary
animals — Parasites — Co-operation of natural selection and the
so.-called Lamarckian factors of evolution — The influence of
internal secretions upon growth — Increase in size beyond the
limits of utility 395

CHAPTER XXVII

Artificial selection — Continuous and single selection — The mutation
theory of the origin of species — Mutual adaptation — Unit
characters — Isolation — Physiological selection — Non-adaptive
characters — The evolution of man 410

INDEX 4'29

OUTLINES OP EVOLUTIONAEY
BIOLOGY

PART I.— THE STRUCTURE AND FUNCTIONS
OF ORGANISMS— THE CELL THEORY

CHAPTER I

Introductory : The nature of life — The living organism viewed as a machine
— The essential functions of the living body — The source of energy in
living things.

THERE are many ways in which the extremely heterogeneous
materials of which the earth is composed may be classified.
Physicists often speak of them as solid, liquid or gaseous
respectively, but we know that one and the same substance may
exist in any of these conditions, changing from one to the other
in accordance with changing conditions of temperature and
pressure. Chemists, on the other hand, tell us that all sub-
stances are made up of certain definite chemical elements,
occurring free or in a state of combination with one another,
and that the almost infinite variety of gases, liquids and solids
with which we are familiar has arisen mainly from this power
of combination in very diverse but perfectly definite ways. The
tendency of modern research, however, is to show that even the
so-called chemical elements are not really so elementary in their
nature as has been supposed, and it is quite conceivable that
they may all have a common origin.

Our present purpose is to investigate a condition of matter
which altogether transcends the classifications of the chemist
and the physicist, a condition in which it exhibits those peculiar
phenomena which we regard as manifestations of Life, and the
study of which constitutes the science of Biology. It is not

B. B

2 OUTLINES OF EVOLUTIONARY BIOLOGY

that living matter differs fundamentally from not-living matter
in chemical or physical characters. On the contrary we recog-
nize in it exactly the same chemical elements — or rather some
of them — as we find in other constituents of the earth, and
these elements appear to obey precisely the same chemical and
physical " laws " in both cases, but in the living body they are
associated and combined with one another in such a manner as
to give rise to substances much more complex than any found
outside the animal and vegetable kingdoms, and possessed of
peculiar properties which raise them to an altogether higher
plane of existence. The complex substances in question con-
stitute protoplasm, which is the one essential constituent of
every living tiling, upon the peculiar properties of which the life
of the organism depends.

We may begin our investigations into the nature of this life
by examining the well-worn but none the less valuable analogy
of the flame of a candle. Chemists and physicists have taught
us that flame consists of incandescent matter, raised to a high
temperature by the process of combustion, or chemical union
with the oxygen of the atmosphere, and that the flame can exist
only so long as the combustion goes on and a sufficiently high
temperature is thereby maintained to render the burning matter
luminous, or in other words to produce those vibrations of the
invisible and intangible ether which we recognize as light. The
flame, then, is the outward and visible sign of certain chemical
and physical processes, of the action and reaction between the
material which is being burnt and the atmosphere which
surrounds it.

Similarly biologists have learnt to recognize life as the expres-
sion of the constant interaction which goes on between the living
organism and its environment, or, in the words of Herbert
Spencer, " the continuous adjustment of internal relations to
external relations." So far as we have yet been able to analyze
it, this interaction also consists of chemical and physical pro-
cesses, amongst which combustion plays a large part ; but the
whole business is vastly more complex than the processes
involved in the production of a flame, and so far many of its
details have defied analysis.

We may vary our analogy by looking upon the body of a living
organism, whether plant or animal, as an extremely elaborate
engine or machine, whose existence depends upon a perfect

ORGANISMS AND MACHINES 3

adaptation to its environment, and whose action consists in
continual self -adjustment to changes in that environment.
What we call the life of the organism consists of the sum total
of all the activities which it thus exhibits. The question at once
arises : How, then, does a living organism differ from a mere
man-made machine ? and this question is one which it is by no
means easy to answer. An organism, however, is not merely a
piece of apparatus which has the power of maintaining itself for
a longer or shorter period in a state of equilibrium with its
environment and thereby preserving itself from destruction, for
it also has the power of reproducing its kind by a process of self-
multiplication. In the case of an artificial machine, where there
is little or no automatic adjustment, the forces of the environ-
ment very soon get the upper hand ; the metal work becomes
corroded by oxidation, or worn away by friction, and presently
the whole affair comes to a standstill. Oxidation and friction,
and innumerable other chemical and physical agencies also tend
to destroy the machinery of the living body, but for a longer or
shorter period they are held in check by automatic processes of
repair and renewal, and when the inevitable end does come it is
usually not until the organism has produced at least enough off-
spring to take its place in the struggle for existence.

One of the most brilliant writers of the nineteenth century,
Samuel Butler, has indulged in the somewhat fantastic sugges-
tion that some day the construction of machines might be so
perfected that they also would be able to reproduce their kind, and
the little steam-engines would be seen playing about the door of
the engine shed. It certainly does not seem possible that
machines will ever multiply in this way, but should they do so,
and should they at the same time be able to feed and grow, it is
difficult to see why they should not be as much entitled to be
called living organisms as any of the plants and animals which
inhabit the earth to-day. They would, however, still be totally
different from plants and animals both in structure and
composition.

One of the most remarkable and characteristic features of the
living things which inhabit this earth is that they are all com-
posed of very similar materials, which are very different in their
nature from any which we should be likely to choose in the
construction of a machine. In making an engine we select those
substances which seem best calculated to resist the destructive

B 2

4 OUTLINES OF EVOLUTIONARY BIOLOGY

action of the environment ; hard and rigid metals which will
bear heavy strains, and as far as possible such as will be proof
against the chemical action of the atmosphere ; and we do our
best by means of oil and paint to protect even these from injurious
influences.

A living body may also have its hard protective structures, as
the shell of the oyster and the scales of the fish, or its rigid
levers, as the bones in our own limbs, but the really essential
part of the organism is built up of just those materials which are
most liable to destruction by chemical and physical agencies — of
that almost liquid and extremely unstable substance which we
have already referred to as protoplasm, and of its various
derivatives. The experience of every day teaches us how
rapidly the bodies of animals and plants decay when they are
left exposed to the atmosphere after life has become extinct ; and
this decay is simply the destruction caused by the disintegrating
forces of the environment. A disabled steamboat may lie
rusting on the shore for many years without undergoing much
change, but the dead body of a stranded jelly-fish thrown up
beside it will become disorganized and disappear in the course of
a few hours, and yet the jelly-fish when alive was undoubtedly
the more complex and perfect piece of apparatus.

The body of an organism, moreover, undergoes destruction
not only after death ; it is always undergoing destruction, and
its very life depends upon its destruction, just as the flame of a
candle depends upon the destruction of the candle. But as it is
destroyed it is constantly built up again; new protoplasm is
formed and new tissues take the place of those which are worn
out. The life of the organism is, in fact, the outcome of the
constant struggle between destructive and constructive forces,
and the keener the struggle the more vigorous will be the life —
just as the flame will be brighter or hotter in proportion to the
activity of the combustion to which it owes its existence.

Life, like the flame, is a manifestation of energy, and the
living body is, like the steam-engine, a machine for transforming
one kind of energy into another. Moreover, the ultimate source
of the energy is the same in both cases. That of the steam-
engine is derived from the combustion or oxidation of coal, which
contains stores of energy derived millions of years ago from the
light and heat of the sun by the green plants which flourished
in the vast forests of the Carboniferous epoch. The green plants

SOUKCE OF ENEEGY OF OKGANISMS 5

of to-day still derive supplies of energy from the same source
and lock up in their leaves and stems what they do not them-
selves expend, while the animals in turn derive their energy
from the green plants upon which they directly or indirectly
feed. Hence both plants and animals are ultimately dependent
upon the sun for their existence.

Even the most superficial examination is sufficient to demon-
strate the fact that the body of any of the more familiar animals
or plants is, as we have already indicated, an extremely complicated
thing. Whatever may be the degree of complexity, however, and
however much one organism may differ from another in details
of structure, whether it be a microscopic alga or an oak tree, an
Amoeba or a man, there are always certain things which have to
be done, certain actions or functions which have to be performed,
in order that its life may be maintained.

In the first place, the organism must safeguard itself as far as
possible from the destructive influences of its environment. It
must not only be able to protect itself from such physical agents
as heat and cold, mechanical impact and friction, but it must be
able to select a situation where life is possible, and to escape
from other organisms by which it is liable to be attacked. All
this involves the expenditure of energy in some form or another ;
it may be in the manufacture or secretion of protective envelopes
or shells, such as we find even in some of the simplest Protozoa,
or it may be in actively moving away from the source of danger.
Thus it appears that the very first thing necessary for the
maintenance of life is the expenditure of energy. This
energy, though ultimately derived from the sun, is, as we have
already seen, derived immediately from the combustion of
fuel, very much as in the case of a steam-engine, but with the
important difference that in the living organism the fuel is, at
any rate to a large extent, the actual substance of which the
body is composed. In this respect the comparison with a candle
is especially apt, for it is the combustion of the actual substance
of which the candle is composed that liberates the energy
manifested in the light and heat of the flame.

Now combustion is, of course, simply another name for what
chemists term oxidation, or combination with the element
oxygen, a process which is often accompanied by the liberation
of a considerable amount of energy in the form of heat and
light, though this is by no means always the case. In the

6 OUTLINES OF E VOLUTION AKY BIOLOGY

higher animals sufficient heat is evolved to maintain the
temperature of the body at a level considerably above that of the
surrounding atmosphere, and such animals are accordingly
termed " warm-blooded " ; in plants, on the other hand, and in
the " cold-blooded " lower animals, the amount of heat evolved
is not as a rule sufficient to raise the body temperature to any
great extent, if at all. Heat, however, is only one form in which
energy may be manifested, and in living organisms it is, as a
matter of fact, much more conspicuously manifested in the form
of motion, especially in animals, while in not a few cases even a
low temperature combustion may liberate energy in the form of
light, as in the glow-worm and numerous other luminous
animals and plants.

When a piece of charcoal is burnt in the air it enters into
combination with the elementary oxygen gas of the atmosphere,
and another invisible gas which we term carbon dioxide, or
carbonic acid, is produced, in accordance with the equation —

C + 02 C02

(Carbon) (Oxygen) (Carbon Dioxide).

In this process energy is set free in the form both of heat and
light. We must now inquire a little more carefully whence this
energy really comes, for although this is a question primarily
for the chemist and physicist it is also clearly one which the
biologist cannot afford to leave unanswered.

In accordance with the principles of the conservation of
energy and the indestructibility of matter we believe that the
quantities of energy and matter which exist in the universe
are fixed and constant. Neither energy nor matter can be
created and neither can be destroyed, though each may express
itself in a great variety of ways and change more or less readily
from one mode of expression to another. Thus, as we have
already seen, the energy of the sun's rays may be utilized in
building up the bodies of green plants, and locked up, as it were,
in the substances of which these are composed.

We also know that different chemical elements have a very
strong " affinity " for one another, their atoms tending to
unite and form compound molecules when they are brought
within the sphere of each other's attraction. Once united
they can only be separated again by the expenditure of energy,
and when they unite a corresponding amount of energy is

ENEEGY OF CHEMICAL AFFINITY 7

liberated. We may say, for example, that in the elements
carbon and oxygen, so long as they remain separate, a certain
amount of energy remains latent. We call this potential
energy. When the carbon and oxygen atoms are allowed to
come together and unite, this potential energy of chemical
affinity is liberated as kinetic energy, and manifested in the
form of light and heat.

It is from the potential energy of chemical affinity that the
energy of a living organism is immediately derived. Protoplasm,
the fundamental constituent of both plants and animals, contains
chemical compounds of extremely complex structure, composed
of many elements and containing a large amount of potential
energy locked up in them. Moreover, these proteids, as
they are termed, are extremely unstable bodies, readily breaking
up on oxidation into simpler and more stable combinations and
thus liberating energy. It is the presence of these unstable
proteids which confers upon protoplasm its peculiar fitness to
form what has been so aptly termed by Huxley " The physical
basis of life." They play the part of the gunpowder in a
cartridge, ready to produce a manifestation of energy as
soon as the proper stimulus is applied.

It is, then, the breaking up of proteids, or of some other
complex substances, usually by recombination of their con-
stituents with oxygen, which furnishes the constant supply of
energy which an organism requires. This process, however,
can only go on so long as the supply of combustible matter on
the one hand and of oxygen on the other is adequately main-
tained, and this brings us to the consideration of two of the
most important functions which every living organism must
perform, nutrition and respiration.

Under the head of nutrition we must include all those
processes which are concerned in building up the body, in
making good the waste of substance necessitated by the expendi-
ture of energy and thus providing new stores of fuel for the
use of the organism. The first step in nutrition is the taking
into the body of suitable food material. In the case of the
typical animal this material must contain in some form or
other all the necessary supply of potential energy, locked up in
more or less complex and unstable chemical compounds such
as it can obtain only from the bodies of other organisms. The
green plant, on the other hand, by virtue of the chlorophyll

8 OUTLINES OF EVOLUTIONARY BIOLOGY

which it contains, has the power of absorbing energy directly
from the sun's rays and using this to build up the complex
proteids from very simple constituents. The feeding of the
organism, whether plant or animal, is comparable to the stoking
of the engine, but with this difference, that the food material,
unlike the fuel in the engine furnace, has usually to undergo
complex chemical processes, which may actually result in the
formation of new protoplasm, before it is available as a source
of energy.

Supplies of energy are, of course, useless unless they can
be liberated when required. The fuel must be burnt, and
for this purpose, as we have already said, a supply of oxygen
gas is necessary, and the function which is concerned in pro-
viding this supply we call respiration. The term respiration,
however, is one in the employment of which we shall have to
exercise a certain amount of care. It is naturally associated
in our minds with the mechanical act of breathing which we
ourselves perform. We can see a man breathing, but we cannot
see an oak tree breathing ; nevertheless the oak tree performs
the function of respiration just as efficiently as the man. We
have to learn to dissociate the essential part of this function,
which is common to all living things, from the subsidiary com-
plications which have been introduced in the case of the higher
animals during the course of their evolution from lower forms.

Respiration, in the scientific acceptance of the term, is simply
the exchange by the organism of the carbon dioxide gas which
has been formed in the body in the process of combustion for
the oxygen gas which is required for that combustion. It is
therefore a double function — oxygen being taken in and carbon
dioxide got rid of by one and the same process. This process
is, in its essential features, an extremely simple one, depending
upon the physical principle of osmosis or diffusion, in accordance
with which two gases of different densities tend to change places,
until equilibrium is established, whenever they are placed in
the necessary relations with one another, as when they are
separated only by some membrane through which both can

Carbon dioxide, however, is not the only waste product
resulting from the breaking up of the complex proteids, for
these also contain hydrogen, nitrogen, sulphur and phosphorus,
and other products of decomposition are accordingly formed

METABOLISM 9

which contain amongst them all of these elements. These must
also be eliminated from the body, and this process of elimination
of waste products constitutes the function of excretion. This
function may be performed in a variety of ways and by a variety
of organs. In so far as the carbon dioxide is concerned it is, as
we have already seen, an essential part of the function of
respiration. Urea, on the other hand, a nitrogenous substance
which is perhaps the most characteristic waste product in the
higher animals, is eliminated by special excretory organs, such
as the kidneys. In the case of the higher plants the waste
products are for the most part stored up in the leaves and
got rid of when these are shed.

We have thus seen that the supply of energy to a living

Living Protoplasm

Food Material. / \Waste Products.

FIG. 1.— Diagram of Metabolism.

organism, and therefore also its life, depends upon a series
of complex chemical processes which take place within the body.
All these processes collectively are spoken of as metabolism, and
we may distinguish between two sets of metabolic changes :
those which are constructive and lead to the building up of
new living protoplasm out of food material, and those which
are destructive and lead to the decomposition of the body
substance, the liberation of energy and the formation of waste
products. The former are termed anabolic and the latter
katabolic.

We may roughly illustrate these elementary conceptions of
the chemical processes which take place in the living body by
the accompanying diagram, in which a mass of living protoplasm
is represented as balanced in a very unstable position on the
apex of a triangle. It is constantly undergoing destruction,

10 OUTLINES OF E VOLUTION AKY BIOLOGY

accompanied by the liberation of energy and resulting in the
formation of waste products, substances which, in falling down
one side of the triangle to a lower level, to a greater or
less extent lose their potential energy. On the other side of
the triangle, however, complex food material, containing fresh
supplies of potential energy, is supposed to be taken in, or, in
the case of green plants, actually built up from simple con-
stituents by the energy of the sun's rays, and used in repairing
the waste of the protoplasmic body.

If the constructive processes proceed more vigorously and
rapidly than the destructive, if the food supply is abundant and
the expenditure of energy comparatively low, the body may
grow, though, as we shall see presently, only within certain
limits. If the reverse is the case and the expenditure exceeds
the income, the body may dwindle away and finally die. If it
is to remain in a condition of healthy equilibrium a just balance
must be maintained between the two sides of the account.

Perhaps the most characteristic property of living things is,
as we have already suggested, the power of reproduction. This is
the last resort of the organism in the struggle for existence. The
individual, which owes its very life to the perishable nature of
its body, always succumbs to the destructive influences of the
environment sooner or later, but before yielding to the inevitable
it will, under normal conditions, have produced offspring which
will carry on the struggle for another generation.

The phenomenon of reproduction is intimately associated
with that of growth, and may be traced back to the division of
a simple ancestral mass of protoplasm into two parts whenever
its size increased to such an extent that the ratio of surface to
volume became too small for the necessary intercourse between
the organism and its environment. With this division of the
protoplasmic body the proper proportion is restored, and hence
reproduction by multiplication of protoplasmic units may be
looked upon as primarily the direct consequence of super-
abundant nutrition.

We have now learnt to look upon an animal or a plant as a
complex and extremely delicate piece of mechanism, constantly
employed in collecting energy directly or indirectly from the
sun's rays and in using that energy to maintain an incessant
struggle against the destructive forces of its environment. This
incessant getting and spending, winning and losing, constitutes

THE NATURE OF LIFE 11

what we call the life of the organism. In considering what is
the meaning of all this we must remember that, primarily at any
rate, every living thing exists for its own benefit, and that
living, like virtue, is its own reward. The organism also,
however, exists for the benefit of future generations, to which
it may hand on the lamp of life before its own flame is finally
extinguished. There is a race life as well as an individual life,
and we cannot realize too clearly that in the economy of nature
the former is of infinitely greater importance than the latter.

The ideas which we have just been considering are by no
means of modern origin. More than' three centuries ago the
philosopher Descartes endeavoured to explain the human body
as a machine, but as a machine under the control of the " soul,"
which he curiously located in that part of the brain known as
the pineal gland. His ideas of physiology, however, were,
naturally, of the crudest description, and immense strides have
been made in this direction since his time. Chemists and
physicists have helped us much towards a correct understanding
of the living mechanism, but when they have done their best it
may well be that the question " What is Life? " will still remain
unanswered, and that we may still have to take refuge in the
idea of an unknown " soul " to explain the difference between
living and not-living things. The " soul " of Descartes'
philosophy corresponds more or less closely with the " vital
force " of some more recent writers and the " entelechy " l of
others, but whatever term we employ it must be rather as a
cloak for our ignorance than as an expression of any definite
opinion as to what it is that really animates the living body.

1 Vide Driesch, " The Science and Philosophy of the Organism," (London,
A. & C. Black, 1908), Vol. 2, pp. 137, 138.

CHAPTER II

Amoeba as a typical organism — The properties of protoplasm.

IN illustration of the general principles dealt with in the fore-
going chapter we may now consider a definite concrete example
of a living organism. Probably none is better suited for this
purpose than the familiar Amoeba, which may be regarded as a
kind of pocket edition of a typical animal.

Amoebae may be found creeping about on the mud at the
bottom of ponds and ditches. Although of microscopic size and,
usually at any rate, invisible to the naked eye, they are by no
means the smallest or simplest of living things, but exhibit
within the narrow limits of their gelatinous bodies a considerable
amount of structural differentiation.

In general appearance (Fig. 2) an Amoeba resembles nothing
so much as an irregular speck of translucent jelly, but if we
watch it for a few minutes under the microscope we soon find
that it is something more than this. If in an active and
healthy condition it never maintains the same shape for long
together, but manifests an ever-changing irregularity as it
slowly creeps about from place to place, throwing out
irregular projections of its body first in one direction and
then in another and withdrawing old projections as new ones
are put forth.

The viscid substance of which the entire body is composed is
protoplasm, but this protoplasm is not homogeneous throughout ;
on the contrary, it exhibits a characteristic differentiation into
parts or organs, which can be more or less readily distinguished
from one another and each of which has its own duties or
functions to perform.

Inasmuch as it consists of but one protoplasmic unit, however,
we may speak of the body of an Amoeba as a single cell.1 As
in all other typical cells, the most fundamental differentiation

1 The origin and meaning of this term will be discussed more fully in a later
chapter.

AMCEBA

13

which it shows is into cell-body and nucleus. The cell-body
forms by far the greater part of the organism, and the

E.

D.

FIG. 2. — Amoeba.

A, B. The same individual in two phases of active movement, showing change of form.

C. Another specimen, of a different species. Note the numerous short projections at

the hinder end due to contraction, while at ect. the commencement of a new pseudo-
podium is indicated by a thickening of the ectoplasm.

D. A specimen with two nuclei.

E. Diagram of reproduction by simple fission.

c.v. Contractile vacuole ; ect. ectoplasm or ectosarc; end. endoplasm or endosarc ; f.p.
food particles; f.v. food vacuole; nu. nucleus; psd. pseudopodium. (The arrows
show the general direction in which the animal is moving.)

protoplasm of which it is composed is often distinguished as
cytoplasm.

14

The nucleus (Fig. 2, nu.), composed of a somewhat different
variety of protoplasm sometimes known as karyoplasm or
nucleoplasm, is a very definite body of more or less rounded
form, sometimes shaped like a bun, and easily distinguishable
from the cytoplasm even in the living animal by its more highly
refractive character. Its position is by no means constant, for
it floats about from place to place in the interior of the almost
liquid cell-body.

The cytoplasm is very imperfectly differentiated into inner and
outer portions. The former, in which the nucleus is lodged, is
often called the endosarc or endoplasm (Fig. 2, end.}, the latter
the ectosarc or ectoplasm (Fig. 2, ect.). The ectoplasm must be
regarded as a feebly developed protective layer ; it is the part which
comes into direct relation with the surrounding water and through
which all intercourse between the Amoeba and its environment
must take place. Though soft and gelatinous, it is a good deal
firmer and denser than the endoplasm, and it is also more trans-
parent, for the endoplasm contains imbedded in it numerous more
or less opaque particles of various kinds which give it a coarsely
granular character. Most of these particles are minute, but
others are generally present of comparatively large size and
enclosed in drops of clear liquid. These latter are food particles
(Fig. 2, f.p.) undergoing digestion, and they can frequently be
identified as the bodies of other organisms which the Amoeba
has taken in, microscopic plants or animals smaller than itself.
The drops of liquid in which they occur are termed food
vacuoles (Fig. 2, /.#.).

Another spherical drop of liquid (Fig. 2, c.v.) may be observed
somewhere near the surface of the cell-body. This is perfectly
clear and contains no solid particles ; moreover it undergoes a
rhythmical dilatation and contraction, gradually increasing to a
maximum size and then suddenly disappearing owing to the
discharge of its contents into the surrounding water. If the
spot where this " contractile vacuole " disappears be carefully
watched another drop of liquid is seen gradually to accumulate
there, and the process is repeated.

We are told that in the early days of chemistry, before the
highly specialized apparatus which is now used was thought of,
the originator of the atomic theory performed his experiments
with the ordinary domestic crockery. So also an Amoeba is able
to perform, with the extremely simple apparatus at its disposal,

LOCOMOTION IN AMCEBA 15

all those actions or functions which are really essential for the
maintenance of life.

In the higher animals the primary differentiation of the body
is into an outer protective and an inner digestive layer, each of
very complex structure. The Amoeba accomplishes the same
end in its own primitive manner by the differentiation into
ectoplasm and endoplasm. In most of the higher animals,
again, we find very well developed organs of locomotion in
the form of limbs. The Amoeba has no permanent organs of
locomotion at all but merely temporary projections of the body,
the so-called pseudopodia (Fig. 2, psd.), whicli are put forth when
required. In both cases, however, movement is effected in
essentially the same way, by contraction and expansion of the
living protoplasm. In the higher animals this power of con-
traction is localized in the muscles, which are highly specialized
for the purpose and have no other duties to perform, while in
the Amoeba any part of the body, or at any rate of the ectoplasm,
may contract or expand as occasion requires.

In the process of formation of a new pseudopodium we see
first a thickening and protrusion of the clear ectoplasm (Fig. 2,
C., ect.), accompanied by a streaming in of the endoplasm, and
the latter seems to bulge out the ectoplasm as it flows forwards.
The pseudopodium is withdrawn again by a reversal of the pro-
cess, the endoplasm streaming out from it into the central mass
of cytoplasm and the ectoplasm contracting after the retreating
endoplasm. Thus at the posterior end of an actively creeping
Amoeba one frequently sees numerous blunt projections which are
the last remnants of retracted pseudopodia (Fig. 2, (7.). The
shape of the pseudopodia when fully extended differs very much
in different kinds of Amoebae. In some species they are com-
paratively short, thick and blunt, as in our illustration, while in
others they are very long and slender, radiating outwards from
the body of the animal in all directions. They are used by their
possessor not only as organs of locomotion but also as tactile
organs and, as we shall see directly, for the capture of prey.

The protrusion and retraction of pseudopodia imply, of course,
the expenditure of energy, and this energy must be derived from
the combustion of the body of the Amoeba. In this way the
protoplasm is gradually used up, and, unless the animal is to die
of starvation, it must be replaced, which brings us to the
consideration of the manner in which an Amoeba performs the

16 OUTLINES OF E VOLUTION AKY BIOLOGY

important function of nutrition. As it crawls slowly about the
pseudopodia come into contact with all sorts of solid particles in
the surrounding water. Some of these will be inorganic, grains of
sand and so forth, others will be the dead or living bodies of other
organisms, sometimes much smaller than the Amoeba itself.
The Amoeba has the remarkable power of distinguishing amongst
these different kinds of particles those which are good for food
from those which are not. How it does so we do not know ; we
can only say that the presence of a particle which is good for
food stimulates the living protoplasm in a way quite different
from that in which it is stimulated by the presence of a mere
grain of sand. In the latter case the Amceba will simply pass to
one side and avoid the object ; in the former it will put forth
pseudopodia which will close around and envelop it. The food
particle is thus passed through the ectoplasm into the interior
of the body. There is no definite mouth, but food is taken in
wherever it happens to come into contact with the surface of the
body, and the aperture closes up after it. Thus a temporary
mouth is formed as occasion demands. Similarly there is no
permanent digestive cavity or stomach, but merely a temporary
food vacuole into which a digestive fluid is doubtless secreted
by the surrounding protoplasm.

Digestion, as in higher animals, is essentially a process of
solution, whereby those parts of the food which are digestible are
dissolved and rendered capable of diffusing from the digestive
cavity into the surrounding body. The higher animals make
use chiefly of three classes of food material,, proteids, carbo-
hydrates (e.g., starches and sugars) and fats. It is said that
Amoeba can only digest proteids, which of course it must obtain
from the protoplasmic bodies of other organisms. When diges-
tion is complete a certain amount of insoluble residues from the
food will remain over ; these constitute the faeces and have to be
got rid of. There is, however, no permanent vent or anus, and
the faeces are cast out through the ectoplasm at the hinder end
of the body as the animal crawls along.

Owing to the minute size of the whole organism there is no
need for a complex circulatory system, such as is found in the
higher animals, for the distribution of the digested food; it merely
soaks into the surrounding protoplasmic body from the food
vacuoles, and, by anabolic changes which are not fully under-
stood, is converted into new, living protoplasm.

RESPIRATION IN AMCEBA 17

It is not easy, if it be possible at all, actually to observe the
process of respiration in so small an animal as an Amoeba, but
we know perfectly well from the analogy of higher organisms
what must take place. Oxygen is required for the combustion of
the protoplasm from which the energy of the organism is
derived, and this oxygen occurs in a state of solution in all
ordinary water which is exposed to the air. At the same time
carbon dioxide, or carbonic acid gas, must be produced as one of
the products of the combustion, by oxidation of the carbon in the
protoplasm. This waste product (COa) will first of all be
dissolved in the water which forms the greater part of the bulk
of the living organism, while at the same time the water by
which the animal is surrounded may be regarded as a very
dilute solution of oxygen. The outermost layer of the ectoplasm
may be looked upon as a thin membrane separating the two
solutions.

We know from experiment that whenever two gases, or solutions
of gases, of different density, are separated from each other by a
thin organic membrane, they will pass through that membrane
in opposite directions until a state of equilibrium is established
between the two. This process of osmosis or diffusion is, as we
have already seen, the essential feature of respiration in all plants
and animals, although probably the purely physical process is
controlled to some extent by the living protoplasm.

In the case of the Amoeba then, the carbon dioxide diffuses
out through the ectoplasm into the surrounding water while
the oxygen from the surrounding water diffuses in, and the
necessary exchange of gases is brought about. No specialized
organs of respiration, such as we meet with in the higher
animals, are required. The whole surface is a respiratory sur-
face, and all parts of the interior are within reach by the simple
process of diffusion, aided doubtless by the circulation of the semi-
liquid protoplasm which is constantly going on inside the body.

Other waste products must be formed by the combustion of the
protoplasm in addition to carbon dioxide. What these are we
do not exactly know in the case of the Amoeba, but it is evident
that they must contain nitrogen, which is one of the essential
constituents of all proteids. These waste products must be got
rid of by some process of excretion, and it is usually supposed
that they are passed in a state of solution to the contractile
vacuole and thence periodically expelled to the exterior, so that

B. c

18 OUTLINES OF EVOLUTIONARY BIOLOGY

the contractile vacuole is probably to be regarded as the special
excretory organ of the animal.

In all but the lowest animals there is a more or less specialized
nervous system, whose function it is to place the different parts
of the body in communication with one another and, through
the mediation of the sense organs, with the external world or
environment. The action of this system depends primarily
upon one of the fundamental properties of living protoplasm,
the power of responding to stimuli by some definite change
in its own condition. The stimulus is, in the first instance,
supplied by some factor of the environment, such as light, heat,
electricity or mechanical impact. It may appear to originate in
the central nervous system itself, but this is probably secondary.
It has the effect of liberating stored energy in those parts of
the organism which are sensitive to that particular stimulus,
in somewhat the same way that the stimulus of heat may
have the effect of liberating the stored energy in a charge of
gunpowder. The living molecule has, indeed, actually been
described as explosive. In both cases potential energy is
converted into kinetic energy, and the effect which is produced
may be out of all proportion to the amount of energy represented
by the liberating stimulus itself.

In the higher animals, then, the stimulus received from the
external environment, whatever its nature may be, acts
primarily upon some special sense organ or receptor, whence it
is transmitted along highly specialized tracts of tissue, the
nerves, to some part of the central nervous system, where it
usually gives rise to what we call a sensation. The central
nervous system, again, not only has the power of receiving stimuli
through afferent or sensory nerves, but also of sending stimuli
through efferent nerves to the various organs of the body,
whereby their functions are controlled and regulated. The con-
traction of muscles and the secretion of glands are all con-
trolled in this manner, and the entire working of the body is
co-ordinated by the action of the nervous system.

In the Amoeba, however, we see no trace of a special nervous
system, nor of sense organs, but nevertheless the organism is
certainly capable of receiving and responding to stimuli ; in
other words it is irritable. Thus the protoplasmic body responds
by contraction to the stimuli of mechanical impact, heat, light,
"electricity and chsmical reagents, and, as we have already seen,

VITALISM 19

the presence of particles which are good for food causes the pro-
trusion of pseudopodia in a definite and purposive manner.
Probably the whole of at least the ectoplasm is to some extent
sensitive to stimuli of certain kinds, and it is also probable that
stimuli may be conducted from one part of the body to another
without the existence of special nervous tracts.

One of. the most difficult problems in connection with the
physiology of Amoeba — and indeed of any living organism — is
that of automatism. Does an Amoaba do anything really auto-
matically or spontaneously, or are all its actions the result,
direct or indirect, of the application of external stimuli to the
explosive molecules of living matter ? Is the organism merely a
machine run by the environment, or is it something more ? Here,
of course, at the very beginning of our investigations, we are face
to face with the old question, already referred to, of the existence
of an animating principle or " soul," which exercises some sort of
control over the physical and chemical processes upon which the life
of the organism depends. This is a question which perhaps falls
within the province of the philosopher rather than that of the
biologist. The theory of vitalism, by postulating the existence
of some such special vital force in all living things, undoubtedly
enables us to avoid many difficulties, but it is doubtful if it really
explains anything. As a matter of fact the more we study living
organisms by actual observation and experiment, the more
fully are we able to interpret their behaviour in terms of
chemistry and physics, but this is a very different thing from
saying that chemistry and physics will ultimately yield a
complete explanation of vital phenomena.

It is quite possible, for example, that the movements of the
Amoeba may all ultimately be interpreted in such terms, for
Biitschli has shown that they can be closely imitated by minute
artificially prepared drops of oil-foam surrounded by water. The
substance of which these droplets are composed is of course
totally different chemically from protoplasm and is in no sense
alive, but it seems highly probable, if not certain, that since
purely physical processes (amongst which surface tension seems
to play an important part) are capable of producing strikingly
amceboid movements in the oil-foam, they may also be largely,
if not solely, responsible for the similar phenomena of movement
in the living protoplasm of the Amoeba itself, which seems clossly
to resemble an oil-foam in its physical properties.

c 2

20 OUTLINES OF EVOLUTIONAEY BIOLOGY

Like other organisms, the Amceba sometimes undergoes a period
of rest, during which its various activities are more or less com-
pletely suspended. Under these circumstances the pseudopodia
are withdrawn, the body is rounded off and a protective envelope
or cyst is secreted by the protoplasm. This, however, is only a
temporary state, perhaps necessitated by unfavourable conditions
of the environment, and sooner or later the organism emerges
from its retirement and resumes its activity.

If in the course of its wanderings the Amoeba meets with an
abundant supply of food and takes in more than is actually
required to make good the waste of protoplasm ; if, in other
words, anabolism preponderates over katabolism, the organism
may increase in size by growth, by the addition of new particles
of protoplasm in excess of those which are used up. These new
particles are deposited, not on the surface, but throughout the
whole mass of protoplasm, between those which are already
formed. Thus growth takes place, not by accretion, as in a
crystal or a snowball, but by intussusception, and we have here
a characteristic though by no means absolute distinction between
the growth of not-living and that of living matter.

As in all organisms, however, there is a limit to the size which
the body may attain, and this limit varies with different species
of Amceba. It depends primarily, no doubt, upon the necessary
relation between surface and volume. As we have seen, all
interchange between the organism and its environment has to be
maintained through the surface, and a given area of surface
cannot supply the wants of more than a certain volume of
protoplasm. As the animal grows the volume must necessarily
increase in a much higher ratio than the surface, and the pro-
portion between the two is rapidly altered. This is probably
not the whole explanation of the limitation of growth in an
Amoaba, the problem being doubtless complicated by other factors,
but we may take it as quite certain that increase beyond a certain
size, if possible, would inevitably result in death. Such a
calamity is avoided by the simple expedient of dividing into two
parts whenever the limit of safety is reached. The nucleus
divides first and the two halves move away from one another,
then the protoplasm constricts into a bridge between the two
nuclei, the bridge narrows and finally ruptures, and instead of
one Amceba there are now two, each exactly resembling the
parent (Fig. 2, #.).

PHYSICAL PEOPEETIES OF PEOTOPLASM 21

This simple fission of a single protoplasmic unit, or cell, forms,
as we shall see later on, the essential feature of ordinary repro-
duction throughout the animal and vegetable kingdoms. It will
be observed that in this process generation succeeds generation
without the intervention of anything which we can speak of as
death. There is indeed no room for death in the history of
these simple organisms, unless it be death by accident, for every
time fission takes place the entire body is used up, and nothing is
left over to die. Nor is there any distinction to be drawn between
parent and offspring, for the two new individuals are in all
respects similar to one another, and neither can be said to precede
the other in point of time.

We have now become sufficiently well acquainted with the
nature of protoplasm to profit by a more detailed examination of
its physical and chemical properties. We have seen that, as it
occurs in the body of an Amoeba, it is a viscid, more or less
liquid, colourless substance, almost transparent but exhibiting,
under moderately high powers of the microscope, a granular
appearance due to the presence of numerous minute and more or
less opaque particles. These particles may be regarded as
impurities, and indeed protoplasm can never be obtained in a
perfectly pure state, for it is constantly undergoing chemical
change, both constructive and destructive, and the impurities
owe their origin partly to the food materials which are taken in
and partly to katabolic processes which give rise ultimately to
waste products.

Even apart from these impurities, however, the protoplasm
itself never exhibits a perfectly uniform structure. It is by no
means homogeneous but shows a more or less distinct differentia-
tion into different portions, as, for example, into nucleoplasm and
cytoplasm, ectoplasm and endoplasm, and so forth. In other
words it is an organized substance. Moreover, it has a character-
istic minute structure or texture which can to some extent be
recognized under high powers of the microscope and concerning
the interpretation of which different observers are as yet by no
means all agreed. According to Professor Biitschli it is a kind
of microscopic foam, consisting of exceedingly minute drops of
a more liquid substance separated by very thin layers of denser
material, and thus resembling physically such a foam as can
be prepared from a mixture of oil, salts of various kinds, and

22 OUTLINES OF EVOLUTIONARY BIOLOGY

water. If this really be a correct account of the minute structure
of living protoplasm it helps us, as we have already seen, to
explain its characteristic movements in terms of well known
physical phenomena.

Other competent observers, however, maintain that the appear-
ance of foam-structure is a delusion and that what Biitschli
interprets as thin sheets separating the droplets from one another
are in reality very delicate fibres arranged in a network. These
fibrillae are supposed to be contractile and thus to be responsible
for the movements of the protoplasm as a whole. But whence
comes the contraction of the fibrillae ?

Various considerations, again, and especially the phenomena
of heredity, oblige us to postulate for protoplasm an even more
minute fundamentaTstructure than the microscope is capable of
revealing to us. It is, in all probability, made up of ultra-
microscopic material units, each composed of a group of molecules,
which units, or "biophors," must themselves be regarded as living
bodies capable of nourishing themselves, growing and multiplying
by division.

It is difficult to form a satisfactory idea of the chemical com-
position of protoplasm because it is impossible to analyze it in the
living condition ; indeed, in the living condition it is constantly
undergoing chemical change, and the moment it dies it ceases to
be protoplasm. It is certain, however, that it is not a definite
chemical compound, but a mixture of several distinct sub-
stances : proteids, mineral salts and water. Moreover, different
samples of protoplasm, taken from different organisms or from
different parts of the same, organism, may differ widely from one
another in chemical composition. Thus the difference between
nucleoplasm and cytoplasm is largely a chemical one, depending
to some extent upon the relatively large amount of phosphorus
present in the former.

By far the most characteristic and important of the chemical
constituents of protoplasm are, of course, the proteids. These
form a remarkable class of substances which do not occur in
nature except in the bodies of plants and animals. They are
definite chemical compounds containing the elements carbon,
hydrogen, oxygen, nitrogen, sulphur and frequently phosphorus,
and they have an extremely complex and unstable constitution,
readily splitting up on oxidation into simpler and more stable
compounds and thereby liberating kinetic energy. Many

different kinds of proteids are known to us and have received
special names ; such are the albumen which occurs in the white
of an egg, the casein which is met with in cheese, the legumin
which is characteristic of peas and beans, the gliadin and
glutinin of flour, and so forth.

These proteids are, for the most part at any rate, colloid
substances, that is to say they are more or less gelatinous and
incapable of diffusing through organic membranes. This may
be accounted for if we assume that the highly complex molecules
of 'which they are composed are too large to pass through the
very minute pores which occur in such membranes and which
readily allow of the passage of the molecules of simpler,
crystalloid substances. The colloid nature of the proteid con-
stituents of protoplasm plays a very important part in determining
its properties and behaviour. Crystalloid mineral salts and other
diffusible substances in a state of solution can pass through a
cell-wall or membrane by osmosis, and thus the living proto-
plasm receives fresh supplies of nutriment, but the colloid
proteids are as a rule formed inside the cell and cannot usually
pass out again until they have undergone some chemical change
whereby they are rendered diffusible.

The mineral salts which we find in the protoplasm, usually in
a state of solution, are of very various kinds, compounds of
sodium, potassium, calcium and other elements with various
inorganic and organic acids, such as sulphuric, hydrochloric,
malic and citric acids.

Finally, water must always be present in living protoplasm and
usually forms a very large percentage of the whole mass.

Whatever view we may take with regard to the question of
vitalism, there can be no doubt that the most distinctive property
of living protoplasm is its power of controlling chemical and
physical processes so as to make them yield results different from
those which would be obtained if we were dealing with not-living/
matter. The various processes upon which depend the functions
of movement, nutrition, respiration and excretion all appear to be
controlled in this manner, but the general principle is perhaps
most beautifully illustrated in the case of many of the lower
animals and plants, in which the protoplasm secretes a protective
or supporting skeleton of some mineral substance, such as silica,
or carbonate of lime. Silica, for example, in the inorganic world,
occurs abundantly in a state of solution in water, from which it

24 OUTLINES OF E VOLUTION AEY BIOLOGY

may be deposited in different forms, in shapeless masses as in
the case of flints, or in symmetrical crystals as in some specimens

FIG. 3. — Different forms of Eadiolarian Skeletons. In the central figure the
protoplasmic psseudopodia are seen coming out from the openings in the
shell. (From Haeckel's " Kunstformen der Natur.")

of quartz, whilst the beautiful opal, chemically speaking, is
merely a hydrate of silica, or silicic acid.

SELECTIVE POWERS OF PROTOPLASM

25

Many simple unicellular organisms, such as the Radiolaria
(Fig. 3) amongst animals, and the diatoms amongst plants, have

FIG. 4. — Different forms of Foraminiferan Skeletons. (From Haeckel's
" Kunstformen der Natur.")

the power of taking up dissolved silica from the water in which
they live and using it for building skeletons. These skeletons,

26 OUTLINES OF EVOLUTIONARY BIOLOGY

however, which are really composed of opal, do not consist either
of shapeless masses or of geometrical crystals, but assume beauti-
fully symmetrical forms which vary with each particular kind of
organism and which are wholly different from any forms occurring
in the inorganic world. The same is true of those somewhat
more highly organized members of the animal kingdom, the
siliceous sponges, whose skeletons consist of spicules of opal
(Fig. 88), often of most beautiful and characteristic shape, and each
one as a rule formed by the activity of the living protoplasm
within the compass of a single cell.

Whilst many unicellular organisms and many sponges thus
manufacture for themselves skeletons of a siliceous character,
others, living perhaps in the same water, secrete skeletons which
are composed of carbonate of lime. Such, for example, are the
Foraminifera, the dead calcareous shells of which (Fig. 4)
accumulate to-day at the bottom of the deep sea in many places
in the form of ooze, while in the Cretaceous period of the earth's
history the^ played the principal part in building up the chalk
cliffs on the south coast of England. Each of these microscopic
shells, which are often of extreme beauty and frequently
ornamented with elaborate patterns, is formed as a protective
envelope by a simple protoplasmic organism closely resembling
an Amoeba.

It is evident, then, that we must attribute to living proto-
plasm a very remarkable power of selection or choice, for it is
able, as it were, to pick out certain materials from its environ-
ment for its own purposes and to reject others. We have already
seen this in the case of the selection of food materials by the
Amoeba ; we see it again in the selection of the materials for
skeleton formation in other simple organisms. The fact that
one organism will select silica while another selects carbonate of
lime from the same sample of sea water and for the same purpose,
must correspond to some deep-seated difference in the protoplasm
of which they are composed, and illustrates very well the diverse
potentialities of this remarkable substance.

CHAPTER III

Ilsematococcus — The differences between animals and plants.

IN striking contrast to Amoeba, which, though primitive,
is nevertheless a very typical example of an animal organism,
stands Haematococcus or Sphserella, which, by botanists at any
rate, is regarded as a very simply organized member of the
vegetable kingdom. Like Amceba, this organism is of microscopic
size, consisting of only a single cell or protoplasmic unit.

Hcematococcus pluvialis (also known as Sphcerella lacustris)
occurs in temporary pools of stagnant rain-water or, in the rest-
ing condition, in dried-up mud or dust. Though individually
invisible, or barely visible, to the naked eye, it may occur in such
dense associations as to give the water a bright red (or some-
times green) colour and form a red crust on the sides of the
vessel in which it is cultivated. A closely related, if not identical,
species (Hcematococcus nivalis) is the cause of the red patches
which are sometimes observed on the snow-fields in Arctic
regions. Cultivation is easy, and the same stock may be kept
going for many years and multiplied indefinitely. Twenty
years ago or more I had a sample given to me in Australia,
descendants of which are now flourishing in full vigour in
England. It can be dried up when not required and when
wanted again in the active condition needs only to be supplied
with fresh rain-water and placed in the sun.

In the resting condition each individual consists of a spherical
protoplasmic body (Fig. 5, A) of a bright red or green colour, or
sometimes partly green and partly red, with a more or less
centrally placed nucleus (mi.). It differs from an Amoeba in the
presence of a very distinct, firm cell-wall (c.iv.) on the outside, as
well as in its definite shape and characteristic colour. The cell-
wall is composed of cellulose, one of a group of chemical com-
pounds known as carbohydrates. These compounds are all
characterized by the fact that they contain only three elements,
carbon, hydrogen and oxygen, the hydrogen and oxygen occurring

28

OUTLINES OF EVOLUTIONARY BIOLOGY

in the same proportions as in water (H20). Thus the chemical
formula for cellulose is (C6Hi005)n, while that for glucose or grape
sugar, another carbohydrate, is CeHiaOe- The presence of very
definite cell-walls, composed of cellulose and formed as a secretion
by the living protoplasm, is very characteristic of vegetable as
contrasted with animal organisms.

The protoplasm which lies inside the cell-wall is, as we have
already said, either red, green or parti-coloured. The green

FIG. 5. — Structure and Life-history of Hcematococcua plttvialis.

A. Resting stage with thick cell-wall.

-B. Division into four motile zoospores within the old cell-wall.

C. Free-swimming zoospore.

D. Division of the resting cell into 32 microzooids or gametes.

E. Free-swimming gamete.

F — G. Conjugation of two gametes.
H. Zygote with four flagella, formed by conjugation.
J. Zygote with flagella withdrawn.

K. Resting cell formed from the zygote by secretion of a thick cell-wall.

c.w. cell-wall ; fl. flagellum ; mi. nucleus ; py. pyreiioid ; vac. vacuole.

(Figs. D — K adapted from Peebles.)

colour is due to the presence of that extremely characteristic
vegetable pigment known as chlorophyll, a remarkable pro-
duct of the activity of the living protoplasm with which we are
all familiar in the case of ordinary green plants. The red
pigment, known sis haematochrome, is but a slight chemical
modification of the green chlorophyll, and the one may readily
be converted into the other. If some nitrogenous substance,
such as a small quantity of manure water, be placed in a
vessel containing red Hsematococcus, the latter will in a short
time assume a bright green colour, whence we may conclude that
the red colouration is probably an effect of nitrogen starvation.

ILEMATOCOCCUS 29

When a dried -up sample of Haematococcus is supplied with
fresh rain-water and placed in the sunlight it undergoes a
remarkable change. The protoplasmic body within the cell-wall
undergoes division, first into two and then into four parts
(Fig. 5, B). This is effected by a process of simple fission exactly
comparable to that which we have already described in the case
of Amoeba. The four parts or daughter cells (sometimes called
zoospores) are for a short time kept together within the old cell-
wall, but presently this ruptures and they escape into the
surrounding water.

It will now be seen that these so-called zoospores differ greatly
in structure from the resting Haematococcus. Instead of being
spherical they are more or less oval or pear-shaped in outline
(Fig. 5, C). Each has secreted a new cellulose wall of its own
(c.w.), but this is separated from the main protoplasmic body by
a considerable space, or vacuole, filled with water (vac.), across
which stretch delicate threads of colourless protoplasm, which
keep the protoplasmic body in position. The main mass of
protoplasm is coloured red or green, as before, and contains the
nucleus (nu.). At one end it is drawn out into a kind of beak,
from which two very long and slender threads of colourless
protoplasm (ft.) pass outwards, through minute apertures in the
cellulose wall, into the surrounding water. Owing to their
whip-like form and characteristic lashing movements, these are
termed flagella.

It is by the very rapid movements of the flagella that the
locomotion of the active Haematococcus is effected. They are
carried in front, and the body of the organism is pulled through
the water by their action much as a boat is pulled by a pair of
sculls, at a rate which, though very slow when judged by our
own standards, appears very rapid when considered in relation to
the minute size of the organism. The movements of the flagella
are somewhat complex and of an undulatory kind. They appear
to be entirely automatic, but it seems probable that they must
be performed in response to stimuli which we are unable to
recognize. The flagella are much more definite and highly
specialized structures than the pseudopodia of an Amoeba. Like
the latter, however, they owe their value as organs of locomo-
tion to that inherent power of contraction which is one of the
most characteristic features of living protoplasm. It is probable
that each really consists of several very slender filaments, lying

30 OUTLINES OF EVOLUTIONARY BIOLOGY

side by side, and that the complex undulatory movements are
due to alternating contractions and relaxations of these.

The presence of a firm cell-wall makes the protrusion of
pseudopodia in the case of Haematococcus quite impossible, and
at the same time prevents the organism from taking in any
solid food, for there is no aperture through which such food
could pass. It must therefore depend entirely for its food supply
upon substances which are able to pass through the cell-wall
in a state of solution. These substances are all very simple
chemical compounds, consisting of certain mineral salts and
carbon dioxide gas, which, amongst them, contain all the elements
necessary for the formation of protoplasm. They are, however,
very stable bodies, with little or no affinity for oxygen gas and
little potential energy. They cannot, therefore, by themselves
supply the energy which the organism requires for its vital
activities. Energy has to be supplied from the environment
and the simple food materials thereby partially deoxidised and
combined together in more complex and less stable molecules
containing stores of potential energy which can be liberated by
oxidation as required.

The energy which Haematococcus uses for the building up of
its complex molecules is, as we have already observed for green
plants in general, the radiant energy of the sun, in the form of
light. The process is known as photosynthesis, and can only
take place in organisms which possess chlorophyll or some
functionally equivalent pigment, such as haematochronie. In
some way or other the pigment absorbs the energy of the light
rays and renders it available for the process of constructive
metabolism (which in plants is also spoken of as assimilation).

The first step in this complex process involves a chemical
decomposition, carbon dioxide, obtained in solution from the
surrounding water, being decomposed with evolution of free
oxygen gas.

We have already seen that in the combustion of charcoal
the reverse of this takes place, carbon and oxygen combining
to form carbon dioxide and the act of combination being accom-
panied by the liberation of energy. It is obvious that if energy is
liberated in the one process a corresponding amount must be
absorbed in the other.

When a glass jar of water containing Haematococcus, or any
green aquatic plant, is placed in bright sunlight the decomposition

PHOTOSYNTHESIS 31

of carbon dioxide in the plant takes place so rapidly that
minute bubbles of oxygen may often be seen rising to the surface
of the water. If, for example, we cut off a leafy branch of the
common Canadian water weed, known as Elodea canadensis, and
fix it under water in such a jar, it is possible to arrange the
experiment so that a regular stream of small oxygen bubbles will
be given off from the cut end, and it is further possible to adjust
the experiment so delicately that the interposition of a dark
screen between the jar and the sunlight will cause the immediate
cessation of the stream of bubbles, which will start again the
instant the screen is removed. This simple and beautiful
experiment clearly demonstrates the dependence of the process
of decomposition of carbon dioxide upon the presence of
sunlight.

The oxygen liberated in this way is not (with the exception of
a relatively small quantity used in respiration) required by the
organism, and is accordingly at once discharged into the sur-
rounding medium. The carbon, on the other hand, is needed for
the manufacture of new protoplasm. It is never actually set free
as carbon, but its molecules are probably recombined — under the
influence of the sunlight — with the molecules of water to form the
carbohydrate known as glucose or grape sugar. This process
may be represented by the equation—

6C02 + 6H20 = 602 + C6H1206

(Carbon Dioxide) (Water) (Oxygen) (Glucose).

It is probable that a simpler compound, possibly formaldehyde
(CH20), is formed as an intermediate product, while, on the other
hand, the glucose appears to be rapidly converted into starch,
which is the first visible product of the process of photosynthesis
in the plant cell.

Starch, like glucose and cellulose, is a carbohydrate, and,
though differing in many of its chemical and physical properties,
has the same general formula as the latter (C6Hi005)n. This
means simply that the elements carbon, hydrogen and oxygen
are present in the same proportions as in cellulose, but they must
be linked together differently in the molecule.

The first step in the actual construction of the proteid molecule

is then the combination of carbon with the elements hydrogen

and oxygen to form a carbohydrate. In the higher plants starch

\first appears in the chlorophyll-containing cells of the leaves in

32 OUTLINES OF EVOLUTIONARY BIOLOGY

the form of starch grains, which may afterwards be converted
into sugar again and then, in solution, transferred to other parts
of the plant, where it is redeposited and stored up, once more in the
form of starch grains, for future use, as in the potato and in starch-
containing seeds such as peas and beans.

Both starch and chlorophyll are, at any rate usually, formed in
the cell in connection with specialized portions of the protoplasm
known as plastids. These are regarded as living bodies which
are specially concerned in the formation of chlorophyll, starch and
other substances. When they contain chlorophyll they are termed
chloroplastids, and in the higher plants they take the form of
numerous minute " chlorophyll corpuscles " of definite shape,
which occur in abundance in the cells of all green parts, and in
connection with which the starch grains are formed (vide Fig. 26).
In Haematococcus practically the whole central mass of cytoplasm
is coloured by the chlorophyll (or haematochrome) and may
perhaps be regarded as a single large chloroplastid. The starch,
however, is collected around small, specialized, proteid bodies
imbedded in the general mass of cytoplasm. These are known
as pyrenoids (Fig. 5, C, py.).

We have thus seen how the green plant obtains the carbon,
hydrogen and oxygen which it requires for the manufacture of
protoplasm. Other elements, however, have to be combined with
the molecules of carbohydrate before proteids can be formed. These
are nitrogen, sulphur and, sometimes at any rate, phosphorus, all
of which are obtained by green plants by the decomposition of
mineral salts — nitrates, phosphates and sulphates — which exist in.
solution in the water or damp soil in which the plant grows. In
the higher plants these substances are taken up by the root-hairs
and transmitted to the leaves by a system of vessels and tracheids
analogous to the circulatory system of animals. In such a plant
as Haematococcus they simply diffuse into the protoplasmic body
from the surrounding water by the process of osmosis. Exactly
what happens when they meet with the carbohydrates we do not
know, but further chemical combinations must take place
under the influence of sunlight, which finally result in the
formation of new proteid molecules which are added to the
already existing protoplasm.

Eespiration in Haematococcus probably takes place exactly
as in Amoeba, but it is more easily studied in the case of the
higher plants. In daylight the process is greatly obscured by

CONJUGATION OF GAMETES 83

the absorption of carbon dioxide and the evolution of oxygen gas
which accompany photosynthesis. In darkness, however, the
gaseous interchange which forms the essential feature of respira-
tion can readily be detected, carbon dioxide, produced by oxidation
of the protoplasm, being given off and oxygen taken in.

No special organ of excretion has been observed in Hsemato-
coccus, though a contractile vacuole occurs in closely allied forms,
and waste products must simply diffuse into the surrounding
water through the permeable cell-wall.

We have already seen that Haematococcus multiplies itself by
simple fission within the old cell-wall. This process usually
results immediately in the production of four new individuals.
Under favourable circumstances it may be repeated very rapidly,
without the organism going through any true resting stage, so
that in a short space of time the number of active zoospores
may be very largely increased. The individuals thus produced
are usually all of the same form, and ultimately of the same size,
as the parent. Occasionally, however, a somewhat different
process of multiplication takes place. Instead of dividing into
four relatively large zoospores a resting individual may divide into
thirty-two or sixty-four much smaller "microzooids" (Fig. 5, D),
which differ from the ordinary active form in the absence of the
characteristic cell-wall with its underlying vacuole.

The microzooids (Fig. 5, E) swim actively about by means of
their flagella. Sooner or later, however, they come together in
pairs (Fig. 5, F, G), and the members of each pair fuse completely
with one another to form a single individual (Fig. 5, H) with
four flagella, which presently loses its flagella, secretes around
itself a thick cell-wall, and enters upon the resting state
(Fig. 5, J, K). From this resting individual new generations will
be produced by the ordinary method of division into zoospores.

We have here an excellent illustration of what is usually
termed sexual reproduction, the essential feature of which is the
union or conjugation of two sexual cells or gametes (in this case
the microzooids) to form a single cell, the zygote, which is the
starting point of a fresh series of cell generations. This important
process will be discussed more fully in a subsequent chapter.

We have spoken of Amoaba as an animal, and, as we have seen,
many people regard Hsematococcus as a plant. We must next
endeavour to find out what it is that really differentiates a plant
from an animal. Of course amongst the more highly organized

B. D

34 OUTLINES OF E VOLUTION AEY BIOLOGY

members of the animal and vegetable kingdoms we can point to
many obvious distinctions. The higher plants are fixed and
stationary, while the animals move about from place to place by
means of special organs of locomotion. The animals have
complex digestive, respiratory, excretory, nervous and sensory
organs, which are wanting in the plants. Lastly, the animals
have no chlorophyll and cannot therefore, like the green plants,
obtain their supplies of energy directly from the sun's rays by
photosynthesis, but must depend upon the potential energy con-
tained in the complex molecules of their food, which they obtain
ready made from the bodies of other organisms.

Amongst the lower organisms, however, we find that most of
these distinctions disappear. Thus many of the lower plants
move about actively while many of the lower animals, such as
the sponges, hydroids and corals, are fixed and stationary in the
adult condition, though still showing their animal nature in other
respects, such as their method of nutrition. This mixture of
what were once regarded as distinctively animal and vegetable
characters in such forms as the corals and hydroids gave rise to
the name " zoophytes," or animal-plants, by which these organisms
were known to the older naturalists.

When we descend to forms still lower in the scale of organiza-
tion, consisting each of a single cell, we find that every dis-
tinction may disappear except that of the presence or absence of
chlorophyll and the mode of nutrition immediately dependent
thereon : — plant-like or holophytic as in Haematococcus, animal-
like or holozoic as in Amoeba.1

It cannot be maintained, however, that even these characters
form an absolute distinction between plants and animals, for, in
the first place, many undoubted plants, such as the Fungi, have
lost their chlorophyll by degeneration, and, in the second place,
while botanists claim Haematococcus and the forms closely related
to it as plants, zoologists claim them as animals, chiefly because
they are so closely related in structure to other unicellular
flagellates which contain no chlorophyll that we cannot refuse to
include them in the same group.

1 The nutrition of any typical green plant is holophytic, that of any typical
animal holozoic. The latter term implies the taking in of solid food derived from
the bodies of other organisms, and is thus distinguished from the saprophytic type
of nutrition met with in many of the lower animals and plants (e.g. Fungi), which
consists in the absorption of liquid food derived from the decaying bodies of other
organisms.

PLANTS AND ANIMALS 35

The explanation of the difficulty really lies in the fact that
both plants and animals originally sprang from common unicel-
lular ancestors which were neither the one thing nor the other.
The first appearance of chlorophyll initiated the great cleavage
between the animal and vegetable kingdoms. Thenceforward
the two great groups developed each along lines of its own. By
virtue of their chlorophyll the green plants became the great
proteid manufacturers of the world, and the animals became
dependent upon them for their food supply. Animals are largely
dependent upon green plants in another respect also, for the
latter, as we have seen, split up the carbon dioxide, formed as a
waste product in the respiration of both groups, and thus set free
fresh supplies of the necessary oxygen.

Closely correlated with the differences in their mode of nutri-
tion are the great differences in the mode of life of the higher
plants and animals. Plants have no need to move from place to
place in search of food, which they obtain from the air and the
soil, and the supplies of which are constantly renewed by wind
and rain. Highly organized animals, on the other hand, would
soon exhaust their supplies if they remained always in the same
place, and it is doubtless the necessity for actively seeking out
and even contending one with another for fresh supplies that
has brought about the wonderful elaboration and perfection of
their organization, while the fact that in so many cases they
devour one another, instead of remaining directly dependent upon
vegetable organisms, must have greatly intensified the struggle
for existence and correspondingly increased the rate of progress
in their evolution.

D 2

CHAPTER IV

The cell theory — Unicellular organisms — Differentiation and division of
labour — Co-operation — The transition from the unicellular to the
multicellular condition — The early development of multicellular animals
and plants.

WE have seen that, although Haematococcus and Amoeba differ
widely from one another in various respects, they nevertheless
exhibit a fundamental agreement in structure, for each consists
essentially of a single nucleated mass of protoplasm, each is a
single cell.

The history of the term cell is a curious one, and affords a
good illustration of the manner in which our scientific conceptions

FIG. 6. — Thin Section of a Bottle Cork, showing cellular Structure, X 170.
(From a photograph.)

gradually become modified and improved as our knowledge
increases. Nothing was known of cells before the invention of
the microscope, but in the latter half of the seventeenth century
this invention opened up an entirely new field of research, and
enabled the earlier microscopists to lay the foundations of the
modern science of biology. To Eobert Hooke has been assigned the
credit of first observing the cellular structure of vegetable tissues,
and his observations, published in 1665, were soon afterwards

OKIGIN OF THE TEEM CELL

37

confirmed by Nehemiah Grew, who published his great work on
the Anatomy of Plants in 1672.

It is now a matter of common knowledge that many vegetable
tissues, such as cork and pith, when seen in section under the
microscope, exhibit a honeycomb-like appearance, being composed

CortH

V-

PHKS

Fia. 7. — Part of a cross Section of a Maize Root, showing cellular

Structure, X 84. (From a photograph.)

Cort., cortex; pi., pith; V., vessels.

of rectangular or polygonal, or it may be spherical chambers
separated from one other by firm walls. This structure is very
well shown in Fig. 6, which represents part of a thin section of an
ordinary bottle cork, and in Fig. 7, which represents part of a
transverse section of a root of maize. It was the resemblance to
a honeycomb that led to the application of the term cell to
these chambers. The earlier observers naturally attached most

38 OUTLINES OF EVOLUTIONARY BIOLOGY

importance to the cell-walls, which indeed are alone visible in
dead tissues such as dry cork and pith.

It was not until 1846 that von Mohl first gave the name
protoplasm to the slimy contents of the cells in living tissues.
It then gradually became evident that this protoplasm was the
really vital constituent of the cell, and that it was identical in
nature with the substance of which minute naked organisms
such as the Amoaba are composed, and which was already
known by the name sarcode, given to it by Dujardin in
1835.

The cell theory, first propounded in a very imperfect form by
Schleiden and Schwann, about the year 1838, rapidly developed,
during the course of the nineteenth century, into one of the most
fertile generalizations of natural science. At the present day the
term cell is extended to protoplasmic units which may have no
cell-walls at all, and to which therefore it is etymologically quite
inapplicable, and for a long time a cell has been defined simply
as a single nucleated mass of protoplasm.

In accordance with the cell theory such nucleated masses of
protoplasm are the organic units of which the bodies of all living
things are built up. The simpler organisms, such as Amoeba
and Haematococcus, consist each of a single unit only, and are
therefore said to be unicellular, while the more complex forms,
both of animals and plants, consist each of many such units united
together in a muiticellular body. Moreover, we now know that cells
never originate de novo but multiply by division, so that each one
is the immediate descendant of a pre-existing cell, a very
important fact which was emphasized by Virchow in his often
quoted phrase " Omnis cellula e cellula."

The zoologist includes under the name Protozoa all those
unicellular organisms which he claims as members of the animal
kingdom, whilst the unicellular plants are relegated to the domain
of the botanist under the name Protophyta, but, as we have
already seen, it is impossible to draw a rational line of demarcation
between these two groups and they are often included together
under Haeckel's term Protista.

Tbe outstanding feature in all these simple forms of life is that
the single cell is a complete and self-supporting organism. It
has to perform all the necessary vital functions for itself, by means
of such simple organs, temporary or permanent, as can be
produced by differentiation within the microscopic limits of its

PABAMCECIUM

39

protoplasmic body.1 It is surprising what a high degree of
organization, as indicated by complexity of structure, may be
attained in such a case.

Differentiation and division of labour aretheresultsof progressive
evolution, and at the same time the means by which further
progress is effected. Even in Amoeba and Hsematococcus we see
clearly enough the operation of these two great principles. Many
unicellular organisms, however, exhibit a far higher degree of
organization. The Protozoon, Paramceeium (Fig. 8), so common
in infusions of decaying vegetable matter, swims actively about by
means of innumerable short vibratile cilia which project all over
the surface of the body. It has a definite mouth, through which

PV

MY

tNECFV

FlG. 8. — Paramcecium anrelia, X 300. (From Marshall and Hurst's
"Practical Zoology.")

AV, anterior contractile vacuole (dilated) ; EC, ectoplasm with trichocysts ; EN, endo-
plasm ; EP, micronucleus ; FV, food vacuole ; M, mouth ; MY, contractile fibrillee ;
N, meganucleus ; OG, groove leading to mouth : PV, posterior contractile vacuole
(contracted) ; TR, discharged trichocyst threads ; X, cilia.

solid food particles are taken in, and a less definite anal spot at
which faecal matter is ejected. It has special weapons of offence or
defence (trichocysts) which can shoot out from the surface of the
body long threads when the animal is irritated. It has two
contractile vacuoles, each with a system of radiating canals
discharging into it, and it has two nuclei, large and small
(meganucleus and micronucleus), which appear to fulfil different
functions and each of which doubtless has a complex structure of
its own. How complex the structure of the nucleus may be we
shall be better able to judge when we come to speak of the
phenomena of nuclear division in a subsequent chapter.

1 The organs into which a single cell may be differentiated are sometimes spoken
of as organellae, but if we define an organ as any part of an organism which is
specialized for the fulfilment of some particular function, it is quite unnecessary to
distinguish the organs of a single cell by a special term.

40 OUTLINES OF EVOLUTIONARY BIOLOGY

Paramoecium and many other Protozoa, again, have specialized
contractile threads of protoplasm lying just beneath the surface
of the body, comparable to the muscle fibres of higher animals,
which enable them to perform movements of a different kind
from those effected by flagella or cilia. By means of such con-
tractile fibres, localized mainly in a long stalk, the bell-
animalcule, Vorticella (Fig. 9, 10), can instantaneously draw in
its ciliated disc and pull itself out of harm's way on the approach
of danger. Other examples of these highly organized ciliate
Protozoa (Infusoria) are shown in Fig. 9. These forms should
be compared with the Radiolaria and Foraminifera represented in
Figs. 3 and 4.

Another very important factor in the progress of organic
evolution is supplied by the principle of co-operation between
different organic units. This is illustrated to some extent in the
process known as colony-formation met with in certain
Protozoa and Protophyta. Carchesium, Epistylis and Zootham-
nium (Fig. 9, u— 15), for example, are colony-forming Infusoria
closely related to Vorticella. Vorticella itself multiplies rapidly
by simple longitudinal fission. The bell-shaped protoplasmic
body at the end of the stalk divides into two parts, one of which
s\vims away, attaches itself to some foreign object, and develops
a new stalk. If, instead of separating, the two daughter cells
remained together and went on dividing, and if this division also
extended each time to the upper part of the stalk, the organism
would presently arrive at a brancbing, tree-like condition. This
is what has happened in Carchesium, Epistylis and Zootham-
nium, and also in various other Protozoa.

This arborescent type of colony-formation, moreover, is by
no means the only one met with amongst the Protista. It is
characteristic of stalked forms. Other forms, which are not
stalked, may give rise to free-swimming or floating, solid or
hollow aggregates of plate-like or it may be spherical shape.
The development of such colonies is perhaps nowhere better
seen than in the small group of unicellular organisms to which
Haematococcus belongs, the members of which are known, on
account of their plant-like character and their possession of
flagella, as Phytoflagellata.

HaBmatococcus, or the closely related Chlaniydomonas, may be
taken as the starting point of the series. These two are always
solitary, the individuals separating completely from one another

CILIATE PROTOZOA

41

FIG. 9. — Various forms of Ciliate Protozoa (Infusoria), highly magnified.
(From Haeckel's " Kunstforrneii der Natur.")

1, Codonella ; 2, 3, Dictyocysta (shell only); 4, Tintimiopsis (shell only); 5, Cyttaro-
cyclis (shell only); 6, Petalotricha (shell only); 7, 8, Stetitor ; 9, Freia ; 10, Vor-
ticella ; 11, 12, Carchesium ; 13, Ejiistylis ; 14, 15, Zootliamirium.

42

OUTLINES OF EVOLUTIONARY BIOLOGY

after each cell-division. In Pandorina (Fig. 10), however, which
is another common fresh-water organism, the individuals pro-
duced by fission remain together and form a solid ball, composed
of from sixteen to sixty-four cells, enclosed in a gelatinous
envelope. These little colonies swim about actively by means of
their flagella, which project from the surface in pairs, one pair
belonging to each individual. Multiplication is usually effected
by the division of each individual cell into sixteen, so that as
many daughter colonies are formed as there were cells in the
parent colony, and these daughter colonies finally separate

from one another. Eudorina
(Fig. 40) forms slightly larger
colonies of a similar kind, but
with the component individuals
somewhat widely separated from
one another by the gelatinous
matrix. Finally, in Volvox
(Fig. 11), one of the most
familiar and beautiful of the
microscopic fresh -water organ-
isms, we find the individual
cells, each one still closely
resembling a HcTBmatococcus,
arranged side by side in the
gelatinous wall of a hollow
sphere, with their flagella pro-
jecting from the surface, whilo
daughter colonies are frequently seen swimming about freely in
the interior of the sphere.

In all these colonies the gelatinous matrix or ground substance
is formed as a secretion by the cells which it serves to hold
together. It is noteworthy that the connection between the
individual cells is much more intimate in Volvox than it is in the
lower types, for they are all united together by extensions of
their protoplasmic bodies which give a reticulate appearance to
the wall of the sphere. Volvox, moreover, attains a relatively
large size, and there may be as many as 22,000 cells in a single
colony, though about half that number appears to be more
usual.

We meet with another example of the formation of hollow
spherical colonies in the case of the beautiful Radiolarian,

FlG. 10. — Pandorina morum, X 400.
(From Vines' " Botany.")

A, free-swimming colony ; B, conjugation
of two gametes.

RESULTS OF COLONY FORMATION 43

Sphaerozoum (Fig. 12), which floats about in the surface waters of
the open ocean and has in each of its component cells a supporting
skeleton of branched siliceous spicules.

The co-operation of a larger or smaller number of cells to form
a colony at once opens up new possibilities with regard to

FIG. 11. — Volvox aureus. (From Weismann's "Evolution Theory," after
Klein and Schenck.)

A, mature colony containing daughter colonies (t) and ova (o) ; B, group of 32 developing
spermatozoa seen end on ; C, the same seen sideways ; D, mature spermatozoa,
x 824.

differentiation and division of labour. If a sufficiently good
understanding, so to speak, can be established between the
different members (or zooids) of the colony it will no longer be
necessary for each one to do everything for itself. At the expense
of becoming mutually dependent upon one another they will be
able to specialize in different directions, some identifying them-
selves with one necessary duty or function and some with

44 OUTLINES OF EVOLUTIONARY BIOLOGY

another. As a result of this specialization the various functions
may be much more efficiently and economically performed, but a
no less important result will be that the different individuals will
no longer be able to lead independent lives — if separated from
one another they will perish because unable to perform for
themselves individually all the functions which are necessary for
their existence.

In colonies of Protista we meet with little if any of this
differentiation and division of labour ; from the physiological
point of view the cell units remain almost if not quite inde-
pendent of one another, and that is why they are still classed

amongst the unicellular organ-
isms. There can be no doubt,
however, that it was this habit
of colony-formation that led to
the origin of true multicellular
animals and plants — Metazoa
and Metaphyta — from unicel-
lular ancestors. The compo-
nent cells of a colony gradually
became integrated to form an
individual of a higher order,
and this process was accom-
panied by that differentiation
and division of labour which
in course of time led to the
astonishing complexity of
structure which characterizes
the higher members of both the animal and vegetable kingdoms.
In contrast to this complexity of the organism as a whole we
shall find that the individual cells of one of the higher animals
or plants are usually much simpler in structure than the more
highly organized Protista. After what we have said about
differentiation and division of labour the reason for this should
be sufficiently obvious.

Perhaps no more convincing demonstration of the applicability
of the cell theory to the higher plants and animals could be
given than that which is afforded by the study of development,
for, however highly organized a plant or an animal may be in the
adult condition, it always commences its individual existence as
a single cell — the fertilized ovum or zygote — and attains the

FIG. 12. — A colony of Sphferozoum,
cut in half. (After Haeckel, in
" Challenger " Report.)

SEGMENTATION OF THE OVUM 45

adult state by means of a longer or shorter series of cell-divisions.
The earlier cell-divisions constitute what is termed the segmenta-
tion of the ovum, and result in the formation of a number of
daughter cells, which exhibit little or no differentiation amongst
themselves and are known as blastomeres. Sooner or later,
however, differentiation sets in and leads to the formation of
more or less highly specialized tissues or cell aggregates.

In the primitive, fish-like Amphioxus,1 for example, the ovum
(Fig. 13, I) is a spherical, nucleated cell about 215 o^h inch in
diameter. After fertilization by a male gamete or spermatozoon
it divides first into two equal and similar embryonic cells or
blastomeres (Fig. 13, II) by a vertical cleavage. Another vertical
cleavage, at right angles to the first, divides each blastomere
into two smaller ones (Fig. 13, III). This is followed by a hori-
zontal cleavage, which results in the formation of eight cells,
in two tiers of four each, the four upper ones being slightly
smaller than the four lower (Fig. 13, IV). The blastomeres go on
dividing and presently arrange themselves in the form of a
hollow sphere, whose wall is composed of a single layer of cells
(Fig. 13, VII). The embryo has now reached the blastula (or
blastosphere) stage of its development, a stage which is passed
through by all multicellular animals whose life-history follows
a typical course unmodified by secondary features.

If we allow for the fact that the cells all remain together
instead of separating from one another after each division, it is
obvious that the segmentation of the fertilized ovum into blasto-
meres is identical with the process of multiplication by fission in
such a protozoon as Amo3ba. The fact that each blastomere is
equivalent to a single protozoon and multiplies in a similar
manner has been experimentally demonstrated in an extremely
interesting way by Herbst, whose observations were made upon
the development of the common sea urchin (Echinus). He
found that, if the eggs are allowed to develop in sea water from
which every trace of calcium has been removed, the blastomeres
actually do separate after each division and give rise to 808
individual cells, which swim about separately like so many
flagellate Protozoa instead of remaining united together and
co-operating with one another to form the normal blastula. The
blastula stage itself, of course, corresponds very closely in general
features with such a protozoon colony as we meet with in the case

1 The external appearance of the adult Amphioxus is represented in Fig. 118.

46

OUTLINES OF EVOLUTIONARY BIOLOGY

FIG. 13. — Early Development of Amphioxus. (Adapted from Ziegler's
Models, after Hatschek.)

J, the fertilized egg ; II, two blastomeres formed by the first cleavage ; III, stage with
four blastomeres; IV, stage with eight blastomeres; V, stage with sixteen blasto-
meres; VI, stage with thirty-two blastomeres, cut in half vertically ; VII, blastula or
blastosphere stage, cut in half ; VIII, early stage of gastrulation, cut in half ;

BLASTULA AND GASTEULA 47

of Volvox or Sphserozoum (compare Figs. 11 and 12), a point to
which we shall have occasion to return in a later chapter.

In the blastula of Amphioxus the cells are still all very much
alike, except that those at one pole of the sphere are somewhat
larger than the others. Differentiation, however, now sets in in
a very marked manner, and the cells thereby become divided into
two distinct groups. That portion of the wall of the hollow
blastula which is formed by the larger cells becomes pushed
inwards or invaginated (Fig. 13, VIII), much as a tennis ball
may be pushed in by the pressure of the thumb, until it comes
into contact with the inner surface of the remainder of the wall.
In this way the original cavity (blastocoal) is obliterated and
the embryo takes on the form of a double cup (Fig. 13, IX, X).
The cavity of this cup is an entirely new formation. It is the
primitive digestive cavity of the animal and is known as the
enteron or gastral cavity. Its mouth gradually contracts to a
narrow aperture, the blastopore. The outer layer of cells forming
the wall of the cup is termed the epiblast and the inner the
hypoblast, and the two are continuous with one another all round
the blastopore. The stage now reached is spoken of as the
gastrula stage.

In nearly all Metazoa the blastula stage of development is
followed by one exhibiting the essential features of the gastrula,
or at any rate some indication thereof. The primary differentia-
tion of the component cells of the body into an outer epiblast
(which becomes the ectoderm of the adult) and an inner
hypoblast (which becomes the endoderm of the adult), the one
serving for protection and for the maintenance of all the
necessary relations with the external environment, and the other
surrounding a gastral cavity and concerned with the digestion of
the solid food which the animal captures, is closely correlated
with the characteristic animal or holozoic method of nutrition.

In later stages of development, in all animals higher than the

IX, young gastrula in longitudinal section ; X, older gastrula in longitudinal section ;
XI, XII, XIII, transverse sections of older embryos, showing the formation of the
coelomic pouches, notochord and neural tube; XIV, longitudinal section of embryo
of about the same age as XII ; XV, side view of embryo of same age as XIV with
the epiblast stripped off from one side to show the mesoblastic somites formed from
the ccelomic pouches.

bp. blastopore ; lie. blastoccel or segmentation cavity ; c.p. ccelomic pouch ; ent. enteron ;
ep. epiblast; liyp. hypoblast; m.s. mesoblastic somites; not. notochord; n.p. neuro-
pore or anterior opening of neural tube ; n.pl. neural plate; n,t. neural tube (central
nervous system, formed by folding of neural plate) ; o.c.p. openings of coelomic
pouches into enteron.

48 OUTLINES OF EVOLUTIONARY BIOLOGY

Coelenterata (a group which includes such forms as jelly-fish, sea
anemones and corals), a third layer of cells makes its appearance
between the first two and forms the mesoblast (giving rise to the
mesoderm of the adult). In Amphioxus this mesoblast arises as
a series of hollow, pouch-like outgrowths of the hypoblast (Fig. 13,
XI, XII, c.p.), and the cavities which these pouches contain give
rise to the body cavity or coelom, while their walls give rise to
ruesodermal tissues.

In accordance with what is commonly known as the germ-
layer theory, these three layers of cells, epiblast, mesoblast, and
hypoblast, can be recognized in the embryos of all the higher
animals, and from them all the various parts of the adult body
are derived. The epiblast gives rise to the outer skin or
epidermis, the nervous system and the essential parts of the
sense-organs ; the hypoblast gives rise to the lining epithelium of
the alimentary canal and its various outgrowths, including the
digestive glands, while from the mesoblast arise the various
connective and skeletal tissues, the blood-vessels and the essential
organs of reproduction. The cells of which each tissue is com-
posed acquire a characteristic structure of their own, and in
this way the histological differentiation of the body is gradually
completed.

With the early stages in the development of Amphioxus,
described above, we may compare the corresponding processes in
the life-history of a flowering plant. Here the ovum, instead of
being at once liberated from the parent, undergoes all the
earlier stages of its development within the so-called ovule,
which,. with the contained embryo, will ultimately be set free as
the ripe seed (compare Chapter VIII.).

As a definite example we may take the common weed known as
shepherd's purse, Capsdla bursa-pastoris (Fig. 14). The ovum
lies in a cavity in the ovule termed the embryo-sac. It is at first
a single nucleated mass of protoplasm without any cell-wall.
After fertilization it divides into two cells separated by a wall,
and the process is repeated by a series of divisions parallel to the
first one until we have a row of cells. The cell at one end of
this row, known as the basal cell (Fig. 14, A, B, fc.c.), is larger
than the others and is attached to the wall of the embryo-sac.
At the opposite end is a rounded cell, called the embryonic cell,
from which the body of the young plant will be chiefly formed.
The remainder of the row, including the basal cell, is known as

EARLY DEVELOPMENT OF CAPSELLA

49

the suspensor, and serves for the attachment and nutrition of the
embryo, which at first develops entirely at the expense of the
parent, having no means of feeding itself. All the cell-divisions

0 D

FIG. 14. — Four stages in the early Development of a Flowering Plant,
CapseJ/a bursa-pastoris, X 200. (From Scott's " Structural Botany,"
after Hanstein.)

b.c., basal cell of suspensor; ct., cotyledons growing out ; d, dermatogen ; e, embryonic
group of cells; g.p., growing point of stem; h, uppermost cell of suspensor;
pe, periblem ; pr,pl, cells of plerome; s^ s2 cells derived from h.

so far have taken place in planes parallel to one another, thus
giving rise to a single row of cells, but the embryonic cell now
divides into four parts by the formation of two cell-walls at right
angles to each other and to the preceding divisions (Fig. 14,
A, e), and each of these again divides into two by the formation
B. E

50 OUTLINES OF EVOLUTIONARY BIOLOGY

of a wall parallel to the earlier divisions (Fig. 14, B, e). The
embryo now consists of eight cells or " octants." The next divi-
sions are parallel to its surface and cut off a superficial covering
of cells, known as the dermatogen (Fig. 14, C, d), from which
the whole of the epidermis of the adult shoot will be derived.
The mass of rapidly dividing cells within this soon becomes
differentiated into two parts, the plerome (Fig. 14, D,pr,pl), lying
in the axis of the embryo, and the periblem (Fig. 14, D, pe), lying
between the plerome and the dermatogen. The plerome will
give rise to the vascular system of the plant and the tissues asso-
ciated therewith, while the periblem will give rise to the cortical
tissues of the stem and root and the mesophyll or middle layer of
the leaves. The periblem of the root, and the root-cap, are really
formed from the uppermost cell of the suspensor (Fig. 14, C, li).
By further cell-multiplication and differentiation in the three
primary layers — dermatogen, periblem and plerome — all the
various tissues of the adult plant are produced.

Thus we see that in the higher plants and animals alike the
development of the individual from the fertilized egg consists in
the first place of a process of cell-division, and in the second place
of differentiation between the cells thus produced, accompanied
by grouping of the differentiated cells to form the more or less
sharply defined tissues of the adult.

CHAPTEK V

The cell theory as illustrated by the histological structure of the higher
animals and plants — Limitations of the cell theory — The cell as the
physiological unit.

IN all the higher animals and plants the constituent cells of
the adult body are grouped in more or less well denned tissues,
which originate from the fertilized ovum in the manner indicated
in the last chapter, and the cells of each tissue co-operate
with one another in the fulfilment of some common function.
The study of the microscopic structure of tissues is termed
histology.

As examples of animal tissues we may take blood, epithelium,
fat, cartilage, muscular tissue and nervous tissue, as met with
in typical vertebrates.

Blood is exceptional in that it is a liquid tissue, a condition
which is of course necessary in order that it may circulate through
the blood-vessels and perform its functions as the distributor
throughout the body of food material and oxygen, and the carrier
of carbon dioxide and other waste products from all the various
parts of the body to the special organs of respiration and excre-
tion. Floating in the liquid portion, or plasma, are found two
kinds of cells, the white and red blood-corpuscles.

The white corpuscles, or leucocytes (Fig. 15, a.), closely
resemble Amoebae. They are colourless, nucleated cells, exhibiting
amoeboid movements, and they have the remarkable power of
creeping through the thin walls of the blood-capillaries into the
surrounding tissues. Like the Amoeba they feed, in part at any
rate, by taking in and digesting the bodies of other minute
organisms, and they grow and multiply by simple fission. They
exhibit a much greater degree of independence than most of the
cells of the body and can even live outside the body for a time if
kept in suitable culture media and at the proper temperature.
Their most important function appears to be to defend the body
from the attacks of bacteria and other harmful micro-organisms.

s 2

52 OUTLINES OF E VOLUTION AEY BIOLOGY

If these gain entrance into the tissues at any weak spot they are
set upon by the leucocytes and literally devoured. This process
is known as phagocytosis, and in this capacity the leucocytes are
often spoken of as phagocytes. It is obvious that the health of
the body must depend largely upon the activity of the phagocytes
and their efficiency in dealing with disease-producing " germs."

The white blood-corpuscles, then, differ in no essential parti-
cular as regards their structure and mode of life from so many
Protozoa." It is true they cannot live permanently outside the

a.

' FIG. 15. — Blood Corpuscles of the Frog, X 326. (From a photograph.)

a., white corpuscle or leucocyte; b., red corpuscles or hsematids. The nuclei appear
light-coloured in the photograph owing to their having been stained blue in the
preparation.

body, but that is also the case with many parasitic Protozoa
which live in the blood of other animals. That they are not
independent organisms, but form an integral part of the body
in which they occur, is, however, obvious from the fact that
they have a common origin with all the other tissues from the
developing ovum.

The red corpuscles, or haematids, are very different bodies.
They float passively in the blood-stream and serve as the carriers of
oxygen gas from the respiratory organs to the various tissues.
Unlike the leucocytes they have definite and constant outlines,
though, owing to the flexible nature of the thin cell-membrane by
which they are enclosed, they may undergo temporary distortion.

HISTOLOGY OF HIGHER ANIMALS

53

Individually of a pale yellow colour, and so small as to be quite
invisible to the naked eye, they occur in such vast numbers as to
give the blood its characteristic scarlet or purple colour. It is
estimated that in a cubic millimetre of human blood there are
about five millions of these red corpuscles.

In the frog the haematids are flattened oval cells about 0'02 mm.
in longer diameter, with a centrally placed nucleus (Fig. 15, b.).
In man they are a good deal smaller, and circular in out-
line, like biscuits, and, as in all the Mammalia, the nucleus

FIG. 16.— Epithelium from the Mesentery of a Frog, x 280. (From a

photograph.)

The underlying tissues are seen indistinctly through the transparent epithelial cells,
whose outlines only are visible.

has entirely disappeared. They owe their red colour, and
their power to act as carriers of oxygen, to the presence in them
of a peculiar pigment known as haemoglobin, with which the
oxygen appears to enter into a state of loose chemical combination
from which it is easily liberated again when required by the tissues.
They may indeed be regarded as mere bags of hemoglobin, formed
from highly specialized cells which have lost all power of indepen-
dent existence. They cannot even multiply by division, but, as
the old ones are worn out, they are replaced by the formation
of new ones from less specialized cells in various parts of the
body.

54

OUTLINES OF EVOLUTIONARY BIOLOGY

The term epithelium is applied to any layer of cells covering
a free surface. An epithelium is therefore primarily a protective
layer, but it frequently becomes modified for other purposes. It
may, for example, become glandular, certain of its cells taking on
the function of secretion, or it may become sensory, with cells
specially adapted for the reception of stimuli. It may consist of
a single layer of cells or of several layers one above the other.

A good example of the single-layered type is found in the
peritoneal epithelium which covers the surface of the mesentery
or membrane supporting the intestines in the coelom or body
cavity. Fig. 16 represents a portion of such an epithelium in

which the cell-outlines have
been rendered very distinct
by staining with silver
nitrate; the nuclei, how-
ever, are not shown by this
method. Each cell has the
form of a thin, flat, poly-
gonal plate, and they all
fit accurately together at
their edges. With this
Figure should be compared
Fig. 28, A, which represents
a single-layered epithelium
prepared in such a way as
to show both nuclei and cell-
outlines.

If we gently scrape the inside of the cheek with some clean,
blunt instrument, and examine the milky-looking product under
the microscope, we shall find that it contains a number of
flattened, scale-like bodies (Fig. 17), either entirely separated
from one another or still more or less, connected together by their
edges, and probably to some extent overlapping. These also are
epithelial cells, which have formed part of the special epithelium
known as the epidermis, which is derived from the epiblast or
external cell -layer of the embryo and covers the outer surface of
the body. If we examine our preparation more carefully we shall
find that the cells have an irregularly rounded contour and that
they measure about 0*08 mm. in diameter. There is a more or
less centrally placed nucleus (which appears dark in the figure
owing to the manner in which it has been stained) and the

FIG. 17. — Five isolated Epithelial Cells
from the inner Surface of the human
Cheek, x 4'JO. (From a photograph.)
The nuclei are stained darkly.

HISTOLOGY OF HIGHER ANIMALS

55

cytoplasm is granular. They are, however, dead, or at any rate
moribund cells. Owing to the constant friction to which the

epd.<

der.

FIG. 18. — Vertical Section of Stratified Epithelium from the Mouth of a
foetal Cat, X 280. (From a photograph.)

der., dermis; epd., epidermis; s.m., Stratum Malpighii, or layer of actively dividing cells.

surface of the body is exposed such cells are always being rubbed
off, and it is these which, by accumulating in places where the
friction is less severe, form the so-called scurf of the hair.

FIG. 19. — Section of Adipose Tissue from which the Fat has been dissolved
out, leaving the thin-walled Cells empty and shrivelled, X 175. (From
a photograph.)

As they are worn away their places are taken by other cells
which arise from a deeply situated layer, at the lower limit of the

56

OUTLINES OF E VOLUTION AEY BIOLOGY

epidermis, in which cell-division goes on actively throughout life.
In this way a many-layered or stratified epithelium is formed, as
shown in Fig. 18, which represents a small portion of a thin
vertical section through the epidermis (epd.) in the mouth of a
fostal cat. At the lower limit of the epithelium, resting imme-
diately upon the connective tissue of the dermis (der.), is seen
the layer of actively dividing cells (s.m.). The cells cut off from
this layer are gradually pushed outwards, becoming flattened and
scale-like as they approach the surface.

Fat, or adipose tissue, consists of an aggregation of more or

FIG. 20.— Section of Cartilage, showing the Cartilage Cells (c.c.) imbedded in
the transparent intercellular Matrix or Ground Substance (m.), X 390.
(From a photograph.)

less globular cells, swollen out by the accumulation within them
of drops of oil. If the oil is dissolved out by suitable reagents
the empty cells are left with their cell-walls or membranes in a
somewhat shrivelled condition, as shown in Fig. 19, and the
tissue now bears a curious resemblance, when seen in section, to
vegetable parenchyma, such as is seen in sections of pith
(compare Fig. 7).

Cartilage, or gristle, is one of the skeletal tissues, serving for
the support of the body and the protection of special organs.
In some of the lower vertebrates, such as the dog-fish, it forms
practically the whole of the internal skeleton, but in higher forms

HISTOLOGY OF HIGHER ANIMALS

57

it is to a greater or less extent supplemented or even replaced by
bone. It consists mainly of a tough, translucent matrix, or
intercellular substance, which is formed as a secretion by the
cartilage cells and in which the latter are imbedded at wide
intervals (Fig. 20). In this respect it differs greatly from the
epidermis, in which the cells lie close together and little or no
intercellular substance is developed. The cartilage grows by
repeated division of the cells which it contains and the secre-
tion of additional intercellular matrix between
them. The frequent arrangement of the
nucleated cells in pairs, as shown in the
illustration, is an indication of recent cell-
division. Bone is a more complex tissue than
cartilage and is further strengthened and
hardened by the deposition of calcareous
salts, chiefly phosphate of litne, in the matrix.

Muscular tissue is specialized in a totally
different direction from any of the foregoing.
Its function is to contract, and by so doing
to bring about the various movements of
which the higher animals are capable. There
are two very distinct kinds of muscular tissue,
the one comparatively simple and the other
much more complex in structure. The former,
which is known as unstriped muscle (Fig. 21),
consists of greatly elongated cells, the muscle-
fibres, associated in sheets or bundles. Each
has a centrally placed nucleus and its cellular
nature is at once obvious. The wall of the
alimentary canal, outside its lining epithelium,
is composed chiefly of muscle-fibres of this
kind. Their rhythmical and co-ordinated contraction causes
the characteristic peristaltic movement whereby the onward
passage of the food is secured. These and similar movements
effected in other organs by the action of unstriped muscular
tissue take place quite independently of the will — whence the
term " involuntary " is often applied to this type of muscle.

Striped or striated muscular tissue is usually under the control
of the will, and is hence often spoken of as " voluntary," but it is
found in the higher animals wherever very sharp, precise move-
ments are required, as for example in the walls of the heart, the

FIG. 21. — Un striped
Muscle Fibres
from theWall of
the Rabbit'sln-
testine, X 300.

nu., nuclei.

58

OUTLINES OF EVOLUTIONARY BIOLOGY

contraction of which serves to pump the blood through the blood-
vessels. It is more especially associated, however, with the
movements of the limbs, the bones of which form a system of
levers operated by the muscles which are attached to them. It
differs greatly in min ute structure from unstriped muscle, though
consisting essentially of greatly elongated, nucleated fibres
endowed with remarkable powers of contraction. These fibres,
and the fibrillge into which they are subdivided, are characterized
by a transverse striation of alternate light and dark bands.
Their structure is very complex and in the fully developed
muscle it is difficult if not impossible to recognize the limits

n.

FIG. 22. — Striped Muscle-Fibres (m.) from the Tail of a larval Axolotl,
showing their nuclei (n.), X 560. (From a photograph.)

between the constituent cells. Fig. 22 represents a number of
striated muscle-fibres from the tail of a larval axolotl, in which
each fibre is seen to be provided with several distinct nuclei.

The nervous system, as we have already pointed out, serves to
place the different parts of the body in communication with one
another and exercises a controlling and co-ordinating influence
over the whole, while through the mediation of the special organs
of sense it keeps the organism in close touch with its environ-
ment. The tissue of \\hich it is composed (Fig. 23) consists
of nerve-cells and nerve-fibres, but the fibres are merely out-
growths of the cells. A cell and fibre together form a neuron
— a single unit of the nervous system. The nerve-cells, or
rather their bodies, occur chiefly in the brain and spinal cord,
which constitute the central nervous system, but also in small

HISTOLOGY OF HIGHEE ANIMALS

59

local aggregations, or ganglia, in various parts of the body.
The nerve-fibres extend outwards from the central nervous
system in long, slender bundles, the nerves, which are distributed
to the various organs.

The body of a nerve-cell contains the nucleus and is usually
much branched into slender processes or dendrons (Fig. 24),
which are quite distinct from the nerve-fibre and are supposed
to afford the means of transmitting impulses between one nerve-
cell and another, with the dendrons of which they interlace.

FIG. 23. -Nervous Tissue, as seen in a thin Section of the Brain (Medulla
oblongata) of the Monk Fish. Two large nucleated Nerve Cells are
shown imbedded in a Mass of smaller Cells and Fibres, X 168. (From a
photograph.)

Like other higher specialized tissue-cells of the animal body the
neurons have lost the power of multiplication by division. More-
over there appears to be no provision, in some adult vertebrates,
for their renewal when worn out or injured. A certain number
are formed in the course of the development of the embryo and
these have to serve the animal for the whole of its life.

All the different kinds of cells met with in the body, a few of
which have been thus briefly described, are derived from the
apparently simple unicellular ovum by repeated subdivision and
gradual differentiation. The functions which in an Amceba are
all performed by a single protoplasmic unit are in one of the

60

OUTLINES OF EVOLUTIONARY BIOLOGY

higher animals distributed amongst thousands of millions of
such units, arranged, so to speak, in regiments and armies, each
group with its own duties to perform and all co-operating for the
common good under the supervision and control of the central
nervous system. The marvellous perfection of the whole
machinery is the result of that differentiation and division of
labour which was first rendered possible by the union and co-
operation of the individuals
of a protozoon family to form
a multicellular body.

Turning now to the higher
plants, we shall find that, in
accordance with their much
lower degree of functional
activity, their organization
is far less elaborate than in
the higher animals. In cor-
relation with their stationary
habit all those organs and
tissues which are specially
concerned with locomotion
are absent, and in further
correlation with this character
there are no nervous system
and no special organs of sense.
Of the functions concerned
with the life of the individual
— that is, other than repro-
ductive functions — that of
nutrition is alone highly

FIG. 24. — Diagram of a Neuron,
(From Hertwig.)

Nervenfortsatz, a nerve-fibre coining off
from the body of the cell.

developed. The entire plant
is little more than a piece of
apparatus for extracting carbon, water and mineral salts from the
air and soil, and converting these, with the aid of the sun's rays,
into organic substances. Although the higher plants often attain
a much larger size than any animals, this does not indicate a
higher degree of organization, for it is brought about simply by
the repetition of similar parts — such as roots, branches and
leaves — and the accumulation of dead cell-walls in the form of
wood and bark. As we have already seen, a green plant, instead
of spending the energy which it derives from the sun on its own

HISTOLOGY OF HIGHER PLANTS

61

activities, stores most of it up in the complex chemical compounds
which it manufactures.

Nevertheless, though the degree of histological differentiation
is not nearly so high as it is in the higher animals, we find in
the higher plants also a considerable variety of cells and tissues,
derived, as we have already pointed out, from the dermatogen,
periblem and plerome of the embryo.

We may illustrate this point by a study of some of the cells
and tissues which occur in the well-known spiderwort of our
gardens, Tradescantia virginica. If we examine the flowers of
this plant we shall find that the stamens are covered with long

FIG. 25.' — Structure of a Hair from a Stamen of Tradescantia virginica.

A. End of a hair as seen under a low power of the microscope. The hair is made up of

a single row of cells.

B. A single cell more highly magnified.

c.w. cell-wall; nu. nucleus; p.u. primordial utricle; vac. vacuole filled with coloured
or colourless cell-sap.

slender hairs. It will be convenient to make these hairs the
starting point of our inquiry. If we study them first under a low
magnifying power we shall see that each hair (Fig. 25, A) is
made up of a single row of cells, arranged like the beads in a
necklace ; most of the cells are elongated, but towards the apex
of the hair they become short and spherical.

If we now concentrate our attention on one of the larger cells
and study it carefully under a moderately high power of the
microscope, we shall find that it exhibits the appearance shown
in Fig. 25, B. It measures about 0*27 mm. in length by
0*08 mm. in breadth and consists of a thin-walled bag filled
with living protoplasm. The wall (c.w.) is transparent and
colourless and is composed, as in Haematococcus, of cellulose.

62 OUTLINES OF EVOLUTIONABY BIOLOGY

The granular, colourless protoplasm does not fill the interior
of the cell in a uniform manner, but is arranged partly as a
thin lining to the cell-wall, known as the primordial utricle
(p.n.), and partly in irregular strings which branch and anas-
tomose and stretch across the cavity of the cell in various
directions. These strings of protoplasm tend to converge towards
an irregular mass in which the nucleus is situated. The nucleus
itself (nu.) is a nearly spherical body of denser protoplasm, about
0'024 mm. in diameter. The extensive space which lies inside
the primordial utricle and between the strands of protoplasm
is filled with a more fluid liquid known as the cell-sap. It is
to this cell-sap that the flowers of Tradescantia owe their
colour; if the flowers are blue the cell-sap will be found to
be blue and if they are white it is because the cell-sap is
colourless.

The most striking feature of the cell which we are examining
still remains to be noticed. The protoplasm is in constant move-
ment. This is at once evident from the characteristic streaming
of the small granules which it contains. Both in the primordial
utricle and in the network of threads a constant circulation is
kept up, though not in a very definite manner, as the threads
themselves are constantly undergoing slow changes in their
arrangement. The arrows in Fig. 25, B indicate approximately
the course taken by the streaming protoplasm at the time when
the drawing was made.

The streaming of the protoplasm appears at first sight to be
an essentially vital phenomenon, but it is probably merely ther
mechanical result of chemical and physical processes going on
in the cell, such as the diffusion of various substances in solution
from one cell to another, which must take place in the process of
nutrition. If the cell is killed by the addition of alcohol the
physical and chemical conditions are at once altered and the
movement ceases ; the protoplasm is coagulated and, if we are
dealing with a cell containing coloured cell-sap, the nucleus
absorbs the colouring matter with great avidity and becomes
deeply stained, while the cytoplasm stains only very slightly or
not at all. We are thus able to make the cell stain itself
differentially, without the aid of any extraneous colouring matter,
the cell-sap acting as what is termed a nuclear stain.

It will be necessary to restrict our further observations on
Tradescantia to the structure of the leaf (Fig. 26). The leaves

HISTOLOGY OF HIGHER PLANTS

63

of this plant are long and narrow, somewhat resembling those of
grasses. There is a pronounced midrib and the so-called veins

J).

FIG. 26. — Histology of the Leaf of Tradescantia viryinica.

A. Part of a transverse section of the leaf.

B. Piece of the epidermis stripped from the lower surface of the leaf.

(7. Chlorophyll cells from the mesophyll, as seen in a longitudinal section of the leaf.

D. Portions of four vessels from a vascular bundle, with spiral and annular markings, as
seen in a longitudinal section of the bundle.

(All more or less highly magnified.)

a.c, air-cavity ; c.c. chlorophyll cell ; cp. chloroplastids ; ep.c. epidermic cell ; epd. epi-
dermis; g. c. guard-cell ; rues, mesophyll ; mi. nucleus ; par. colourless parenchyma ;
s/(.. sheath of vascular bundle, composed of thin-walled cells containing starch grains ;
sk. thick-walled skeletal tissue; st. stoma ; i.b. vascular bundle; ves. vessels.

run parallel with one another from base to apex, as in all typical
Monocotyledons.

If we cut a thin transverse section of a living leaf and
examine it under the microscope in a drop of water (Fig. 26, A) we

64 OUTLINES OF EVOLUTIONARY BIOLOGY

shall see at once that it is made up of three principal tissue-
systems, the epidermis, the mesophyll and the vascular bundles.
The epidermis (epd.) covers the upper and lower surfaces in the
form of two single layers of cells, the mesophyll (mes.) occupies the
space between these two layers, and the vascular bundles (r.fc.) —
corresponding to the veins of the leaf — are imbedded in the
mesophyll at fairly regular intervals (only one is shewn in the
figure). We must examine each of these tissue-systems sepa-
rately, and in order to gain a correct idea of the form and
arrangement of the cells of which they are composed it will be
necessary to study them from various points of view.

The epidermis may be stripped off bodily from the surface of
the leaf and then exhibits under the microscope the appearance
shown in Fig. 26, B. It is composed of elongated cells, placed
side by side in a single layer. The amount of protoplasm which
these cells contain is very small, but the nucleus (nu.) is
frequently conspicuous. Their external walls are specially
thickened in relation to their protective function, a feature
which can only be seen in sections (Fig. 26, A). At frequent
intervals little slit-like openings occur in the epidermis. These
are the stomata (st.) which lead into air-spaces in the mesophyll.
Each stoma is bounded by a pair of specially modified epidermic
cells known as the guard-cells (g.c.) — the only epidermic cells
containing chlorophyll — which have the power of opening and
closing the stoma like a pair of lips and thus regulating the
amount of aqueous vapour which passes through the stomata in
the process of transpiration. On the outer side of each guard-
cell lies another epidermic cell of much smaller size than the
ordinary kind, and these, together with the guard-cells, form a
kind of roof (or floor) to the air-cavity (Fig. 26, A, a.c.).

The mesophyll contains the chlorophyll-bearing cells, by
which the assimilation of carbon dioxide is effected and which
are at once recognized by their green colour. They appear more
or less round or oval in transverse sections (Fig. 26, A, ex.], but
are really considerably elongated, parallel to the length of the
leaf, as shown in Fig. 26, C. They come in contact with one
another by numerous short protuberances, between which lie the
spaces in which the air, containing carbon dioxide and aqueous
vapour, circulates. Each contains a nucleus (nu.) and numerous
small, biscuit-shaped green bodies, the chloroplastids or chloro-
phyll corpuscles (cp.), imbedded in the cytoplasm.

HISTOLOGY OF HIGHER PLANTS 65

In certain places the green mesophyll cells are interrupted by
groups of cells containing no chlorophyll. At intervals beneath
the upper epidermis we see masses of large, thin-walled, colourless
cells forming a parenchyma or ground-tissue (par.), while beneath
the vascular bundles we find bands of very thick-walled cells (sk.)
which play an important part in the mechanical support and
strengthening of the leaf.

The vascular bundles are surrounded each by a sheath of thin-
walled cells (sli.) containing numerous small starch grains.
Within this lie the bast or phloem and the wood or xylem, com-
posed of the elongated tubular elements through which the sap
circulates. The raw sap consists of water with mineral salts
in solution, and ascends through the xylem, while elaborated
sap, containing the proteids which have been manufactured in
the leaf under the influence of sunlight, descends through the
phloem. For our present purposes we may confine our attention
to certain of the xylem elements, known as the spiral and
annular vessels. These consist really of dead cell-walls, forming
long narrow tubes each composed originally of a row of cylindrical
cells placed end to end. In the course of their development the
transverse dividing walls between these cells are absorbed, the
protoplasm disappears, and nothing remains but a long hollow
tube whose walls are strengthened by spiral or annular thicken-
ings. Portions of these vessels are represented separately in
Fig. 26, D ; in A they are seen only in transverse section (res.).

Such are the principal kinds of tissue met with in a typical
flowering plant, and such is the way in which it carries out the
principles of co-operation, differentiation and division of labour
amongst its constituent cells.

To most people it will probably appear that the fundamental
truth and general applicability of the cell theory are sufficiently
firmly established by considerations such as those with which we
have been dealing. Ifc has, however, certain undoubted limita-
tions, and upon these limitations some biologists are inclined to
lay a good deal of stress.

Thus, from the point of view of the cell theory, we regard the
cell as the organic unit ; there are, however, units of a lower
order, of which the cell itself is composed. The chloroplastids of
one of the higher plants, for example, exhibit a good deal of
individuality, being capable of independent growth and multi-
plication, while, as we shall see later on, it is necessary for

B. F

66

theoretical reasons to postulate the existence of some such bodies
as Professor Weismann's biophors as the .primary units of which
living protoplasm is built up.

Then, again, although it may be questioned whether any
absolutely unnucleated organisms, such as Haeckel's Monera,
really exist, there can be no doubt that the most simply organized
living things known to us, the Bacteria, which probably stand a
long way below the point where the animal and vegetable king-
doms part company, and which are the most abundant of all
living organisms, do not show that sharp differentiation into
cell body and nucleus which is sa characteristic of typical cells,

FIG. 27. — Bacillus saccobranchi. Bacteria from the Blood of a I'i*h (Sacco-
branchus) stained so as to show the distribution of the chromatin
(nuclear) material, which is represented in black, and which may be
arranged in small scattered granules throughout the cell, or in an irregular
network, or in an irregular, more or less twisted rod, X 2000. (After
Dobell in the " Quatferly Journal of Microscopical Science.")

the nuclear constituents being more or less scattered throughout
the cytoplasm (Fig. 27).

A difficulty of another kind is met with in the fact that in a
good many cases the division of the nucleus is not followed — at
any rate not immediately — by corresponding division of the
cytoplasm. Some Amoebae constantly have two nuclei, and we some-
times get relatively large masses of protoplasm containing many
nuclei formed by repeated division. These are termed syncytia.
We meet, in fact, with all degrees of separation of the cytoplasm
into distinct cells, and a great many of the cells even of highly
developed plants and animals may remain connected together
throughout life by thin strands of protoplasm. We have already
noticed an example of this continuity of the protoplasm in the

CONTINUITY OF LIFE

67

case of Volvox ; another is shown in Fig. 28, B, which represents
a syncytial epithelium. .

On the other hand, in some of the lower organisms — the
Myxomycetes, slime-fungi or Mycetozoa, as they are variously
called — numerous originally separate Amoeba-like individuals
may fuse together to form plasmodia, which may continue to
feed and grow and undergo nuclear division until they form great
sheets of living protoplasm containing perhaps hundreds or
thousands of nuclei (Fig. 29). These and similar facts, how-
ever, interesting and
instructive as they un-
doubtedly are, cannot
be regarded as con-
stituting a serious
invalidation of the cell
theory.

There is no more
fundamental or more
stimulating conception
in the domain of bio-
logical science than
that of the continuity
of life as formulated by
this theory. We have
to imagine the whole
organic world as con-
sisting of a continuous
stream of living pro-
toplasm, which com-
menced to flow many
millions of years ago and has continued without interruption
ever since. At every cell-division the stream branches and
physical continuity is more or less completely interrupted, but
this in no way invalidates the conclusion that if all living things
did not actually have a common origin in a single primordial
protoplasmic unit, they probably at least originated from several
such units which themselves arose under unknown conditions
from inorganic matter.

The modern science of cytology, which is contrasted with
histology as the study of individual cells rather than that of
tissues or cell combinations, and which is yielding such important

F 2

FIG. 28.

A . A single-layered epithelium with very distinct cell-

outlines, from the brain of a reptile (Sphenodon
pimctatus}, x 750.

B. A syncytial epithelium, without cell-outlines, from

another part of the brain of the same animal,
x 750.

n u. nuclei.

68 OUTLINES OF EVOLUTIONAEY BIOLOGY

results both for pure biology and for medical science, owes its
origin and development entirely to the elaboration of the cell

FIG. 29. — Part of the Plasmodiura of a Mycetozoon (Budhamia utricularis),

X 50. (From a photograph.)
The small dark spots are the very numerous nuclei.

theory under the influence of improved methods of microscopical
investigation.

There is another point of view with regard to the living cell
which we may briefly refer to in this place. It is not only the
morphological or structural unit of the body but also the physio-
logical or functional unit. All the essential vital processes take
place within the bodies of cells, and all the different materials
required for and produced by these processes pass in and out of
them. Each cell may be looked upon as a microscopic laboratory
in which the complex chemical reactions comprised under the
term metabolism take place, and although, in the higher
organisms, the cells have become mutually dependent upon one
another, yet each retains a certain degree of physiological as
well as of morphological individuality.

CHAPTER VI

The multiplication of cells — Mitotic and amitotic nuclear division.

WE have already seen that the possibilities of structural
differentiation within the limits of the individual cell are by no
means exhausted by the distinction between cytoplasm and
nucleus, but it is only when we come to study in detail the pro-
cess of cell- division that we begin to gain any adequate concep-
tion of the fundamental complexity of the organic unit. We
have hitherto spoken of this process, as it occurs for example in
Amoeba, as though it were a simple matter, initiated by constric-
tion of the nucleus into two parts and concluded by a correspond-
ing division of the cytoplasm. The researches of the last forty
years, however, rendered possible by the improvements in
microscopical apparatus and micro-chemical technique, have
taught us that in the vast majority of cases the process of cell-
division is one of extreme complexity, accompanied by remark-
able phenomena which reveal a previously unsuspected degree of
structural differentiation within the nucleus itself. To these
phenomena Schleicher in 1878 gave the name karyokinesis, for
which Flemming, in 1882, proposed to substitute mitosis. Both
these terms are still in common use.

In a typical cell (Fig. 30) the cytoplasm (cyt.} is a semi-liquid
substance usually enclosed in a thin cell-membrane (animal cells)
or a thicker cell- wall (plant cells). It exhibits a microscopic
structure which is variously interpreted as reticular (fibrillar),
alveolar (foam-like), or simply granular, the probability being that
the real truth is expressed by a combination of these different
views. It may or may not contain plastids of various kinds (e.g.
chloroplastids in green plants).

The nucleus (nw.), consisting of the so-called nucleoplasm or
karyoplasm, is usually a spherical, more or less centrally situated
body enclosed in a definite nuclear membrane (n.m.). Within
this membrane the karyoplasm is differentiated into various con-
stituents. In the first place there is a network or reticulum of

70 OUTLINES OF EVOLUTIONARY BIOLOGY

delicate protoplasmic threads, known as the linin network, the
meshes of which are filled with a clear, probably liquid ground-
substance known as nuclear sap. These two together seem to
differ but little from the cytoplasm which lies outside the
nucleus.

The most characteristic constituent of the nucleus is another
substance, to which, on account of the readiness with which it
becomes coloured by certain dyes, the name chromatin1 has been
given. This substance usually occurs in the form of small
granules (ch.g.) scattered over the linin network, so that when
very close together they appear to form a chromatin network,

nucl.

FIG. 30. — Diagram of a typical Cell.

cli.g., chromatin granules; c.m., cell-membrane; c.s., centrosomes lying in centrosphere
cyt., cytoplasm ; n.m., nuclear membrane; nil., nucleus; nucl., nuoleolus.

while not infrequently a specially large aggregation of chromatin
substance forms a nucleolus or karyosome (nud.).

Owing to the presence of this chromatin the nucleus as a whole,
under low powers of the microscope, appears to be deeply coloured
by such stains as various preparations of logwood and carmine
and the basic aniline dyes. We have already had occasion to
observe this staining property in the case of the hair-cells of
Tradescantia, where, it will be remembered, the nucleus becomes
deeply stained by the coloured cell-sap as soon as the cells are
killed by the action of alcohol. Other stains, again, affect the
cytoplasm rather than the nucleus, and these various chemical
reactions enable us to differentiate fairly sharply between the
different constituents of which the cell is composed, though it is
a matter of some doubt exactly how far the structure of the

1 Greek xpwjua, a colour.

MITOTIC DIVISION OF CELLS 71

living protoplasm really corresponds to the appearances
exhibited in our preparations.

The differences in their staining reactions of course indicate
corresponding differences in chemical composition between the
chromatin and the cytoplasm, and analysis has shown that the
chromatin is characterized by the presence of relatively large
quantities of phosphorus. This is contained in the complex
nucleinic acid, with which various albuminous bodies may be
combined to form the chromatin substance.

When a typical animal cell is about to divide another structure
makes it appearance, usually just outside but occasionally inside
the nucleus. This is the centrosome (Fig. 30, c.s.), a very
minute body which has peculiar staining properties and which
is surrounded by a differentiated area of protoplasm known
as the centrosphere or attraction sphere (Fig. 31, A, csph).
It has been questioned whether a centrosome and attraction
sphere are always present or whether they make their
appearance only when the nucleus is about to undergo
mitosis. This process certainly seems to be initiated by the
centrosome, which may divide into two parts long before the
nucleus itself commences to do so, so that two centrosomes often
appear alongside the so-called resting nucleus (Figs. 30; 31, A).
Presently the two centrosomes move away from one another,
both still keeping close to the nucleus, and each is now seen to be
surrounded by its own attraction sphere (Fig. 31, B). Around
each attraction sphere delicate threads or nbrillre of the cyto-
plasm become radially arranged to form a star or aster, and
the rays of the asters which lie between the two centrospheres
combine to form a spindle (Fig. 31, C, A-sj>).

In the meantime remarkable changes have commenced in the
nucleus itself. The chromatin granules, together with the linin
by which they are apparently held together, have arranged them-
selves in the form of a long coiled thread, the spireme (Fig. 31, B),
and presently the nuclear membrane begins to disappear (Fig. 31,
C, km), being apparently dissolved in the general protoplasm.
In this way the distinction between cytoplasm and nucleoplasm
is obliterated.

The spireme thread breaks up into a number of short
lengths known as chromosomes (Fig. 31, C, chrs), the actual
number being, with certain exceptions, a constant character for
each species of plant or animal. The centrosomes at about this

72

OUTLINES OF EVOLUTIONARY BIOLOGY

time take up their positions at points corresponding to two
opposite poles of the original nucleus, with the spindle of fine

Fio. 31. — Diagram of the principal Stages in the milotic Division of the
Nucleus in a typical Animal Cell. (From Weismann's " Evolution
Theoiy," adapted from E. B. Wilson.)

aeq, equatorial plate; chr, chromatin ; chrs, chromosomes; cs, centrosome; csp/i, centro-
sphere, containing one or two centrosomes ; Jek, nucleolus ; km, nuclear mem-
brane; kn, nucleus; ksp, sp, nuclear spindle; p, aster; tie, daughter nucleus;
tz, daughter cell ; zk, cytoplasm forming the cell body.

protoplasmic fibres stretched between them (Fig. 81, D), and the
chromosomes " go on the spindle," arranging themselves in a
so-called equatorial plate across its widest part (Fig. 31, D, ai'q).

MITOTIC DIVISION OF CELLS

73

The shape of the chromosomes varies much in different cases ;
they may be more or less spherical, but they are frequently short,
rod-like bodies, often shaped like a V (Fig. 82, A). Each one is
composed, like the spireme thread from which they are derived,
of an aggregation of chromatin granules, held together by a
linin basis. The chromatin granules are sometimes arranged
like the beads on a necklace (Figs. 32, B ; 77), and are known as
chromomeres. The number of the chromosomes also varies
greatly, from as low as two in a variety of the horse-worm
(Ascaris) to as many as one hundred and sixty-eight in the shrimp
Artemia. In cases where the chromosomes are very small each one
may perhaps be equivalent to only a single chromomere.

Either before or after taking up its position in the equatorial

B

-Jd

FIG. 32. — Sperm-mother- cells of a Salamander, during Mitosis. In A the
chromosomes are shown ; in B the spireme thread is split lengthwise,
and also shows very clearly the chromomeres of which it is made
up. (Prom Weismann's " Evolution Theory," after Hermann and
Driiner.)

c, dividing centrosome ; clir, chromosomes ; Jd, chromomeres ; zk, cytoplasm.

plate, each chromosome splits longitudinally into two parts
(Fig. 81, D), in fact the splitting can sometimes be observed in
the spireme thread even before it breaks up transversely into
chromosomes (Fig. 32, B). The result of this splitting is that
the number of chromosomes is doubled ; but the daughter chromo-
somes very soon separate into two equal groups, one of which
moves towards each centrosome (Fig. 31, E). Each group contains
one of the two halves of each parent chromosome.

Having migrated to opposite poles of the spindle the two
groups of daughter chromosomes there form the foundations of two
new nuclei (Fig. 31, F). The chromosomes break up into granules
again ; a new nuclear membrane is formed, whereby a portion of
the general cytoplasm is separated off to form the linin network
and ground-substance of the nucleus ; the asters and nuclear

74 OUTLINES OF EVOLUTIONARY BIOLOGY

spindle more or less completely disappear — though the centre-
some may certainly persist in some cases if not in all — and the
newly constituted nucleus (Fig. 31, G, tic) enters upon a longer or
shorter period of inactivity accompanied by growth.

In the meantime the cytoplasm which constitutes the cell-body
has also divided into two parts in a plane which passes through
the middle of the nuclear spindle and at right angles to its
length. In animal cells this division is usually effected by a
constriction which starts from the outside (Fig. 31, F, G) and
in plant cells by the deposition of a cell-plate (Fig. 34, E, c.p.)
in the equator of the nuclear spindle. This cell-plate forms the
foundation of the double cell-wall which will separate the two
daughter cells ; it must not, of course, be confounded with the
equatorial plate formed temporarily by the chromosomes.

Various attempts have been made to explain the dynamics of
this remarkable process of mitosis or karyokinesis, the essential
features of which are always much the same though the details
vary considerably in different cases. The centrosomes, with
their centrospheres, asters and spindle, sometimes spoken of
collectively as the achromatic figure, are usually regarded as a
special mechanism for bringing about the equitable partition of
the chromatin substance between the two daughter nuclei. This
substance is evidently so important that no rough and ready
division will suffice. It is probable, as we shall see later on, that
the chromomeres of which each chromosome is composed have
different properties, and that it is necessary, in ordinary cell-
division, not only that the chromosome as a whole shall be
divided into two parts but that eacli daughter nucleus shall have
its share of each individual chromomere (compare Fig. 77). In
other words a qualitative as well as a quantitative division of
the chromatin material has to be effected, and this is secured by
the longitudinal splitting of the chromosomes. A transverse
division would only result in the separation of the chromomeres
into two groups, but the longitudinal division involves each one.

According to some observers the fibres of the nuclear spindle
are actively contractile and actually pull the two halves of each
split chromosome asunder. Others maintain that the centro-
somes attract the chromosomes in somewhat the same way as the
poles of a horse-shoe magnet attract iron filings sprinkled
between them.

Of late years the electro-magnetic explanation has been coming

MITOTIC DIVISION OF CELLS 75

more prominently to the front. Thus Gallardo has suggested
that the chromatin substance is charged with negative and the
cytoplasmic colloids with positive electricity, while the centro-
somes are capable of acquiring a positive potential higher than
that of the general cytoplasm. Increase of this potential causes
the centrosome to divide and the radiations which form the
asters and spindle indicate lines of force in the cytoplasm. The
two daughter centrosomes, inasmuch as they bear like charges of
electricity, repel one another. In a similar way the chromosomes
divide under the influence of their high negative charges and
the two halves of each repel one another and are at the same
time attracted by the positive centrosomes. The two new groups
of negatively charged chromosomes then attract the positive
cytoplasm in opposite directions and thus the division of the cell
body follows upon that of the nucleus.

Whatever may be the physical explanation of these complex
phenomena, we must think of them as lying at the root of all
normal processes of growth and multiplication in the higher
plants and animals. With comparatively rare exceptions, some
of which will be mentioned later on, every one of the innumer-
able series of cell-divisions initiated by the fertilized ovum,
and continued throughout life in the growth and repair of tissues,
is accompanied by complicated processes similar to those above
described. The process of cell-multiplication, however, is
frequently confined in adult organisms to certain regions.
Thus, as we have already seen, in the higher animals the growth
of the epidermis depends upon cell-divisions which go on only in
its deepest layer, the stratum Malpighii (Fig. 18, a.m.). Most of
the cells in the body sooner or later lose the power of division,
but they are then usually short-lived, as in the case of those
cells which form the outer layers of the epidermis and which
rapidly become converted into more or less horny scales to be
cast off on reaching the surface.

The majority of the tissues are thus renewed throughout
life by the mitotic activity of some unspecialized cell-group, a
high degree of specialization in the tissue cells of the higher
organisms being always, as we have already seen in the case of
red blood corpuscles and nerve cells, accompanied by the loss of
the power of multiplication.

The limitation of cell-multiplication to definite circumscribed
regions of the body is perhaps best seen in the case of the higher

76

OUTLINES OF EVOLUTIONARY BIOLOGY

plants, where the various meristematic or actively dividing
tissues remain in an undifferentiated embryonic condition and
give rise to those additions to the permanent tissues whereby
growth is effected. Such actively dividing meristem is found at
the growing points of stems and roots, where it serves to bring
about growth in length, and in the cambium, which serves, by
the addition of new elements to the wood and the bast, to bring
about growth in thickness.

The microscopic appearance of such a meristematic tissue,

FIG. 33. — Part of a longitudinal Section of the actively growing Eoot of a
Hyacinth (Galtonia candicans) showing the Nuclei of the Cells in various
stages of mitotic Division, A X 280 ; B X 640. (From photographs.)

when suitably stained and prepared for examination, is shown in
Fig. 33, taken from photographs of part of a longitudinal section
of the growing point of the root of a hyacinth (Galtonia candicans).
The cell-walls are as yet thin and inconspicuous and filled with
dense protoplasm, while the conspicuous nuclei exhibit all stages
of mitosis, the whole forming a striking contrast to the dead
tissues, such as cork and wood, of which the bulk of many plants
is made up, and which consists merely of cell-walls without any
protoplasmic contents (cf. Figs. 6 and 7).

Mitosis in the cells of the higher plants is usually, though by

MITOSIS IN PLANT CELLS

77

no means always, characterized by the absence of recognizable
centrosomes. The actual appearance of some of the principal
stages in the process is shown more highly magnified in Fig. 34,

A,

FIG. 34. — Six selected Stages in the mitotic Division of the Nucleus in the
growing Boot of Oaltonia candicans, X 1120. (From photographs.)

A. Resting nucleus with large nueleolus (Nuls.),

B. Spireme stage, with coiled chromatin thread.

C. The spireme thread has broken up into chromosomes which are forming the equa-

torial plate.

D. The chromosomes have split longitudinally and the two groups of daughter chromo-

somes thus formed are passing to opposite poles of the spindle.

E. Formation of the cell-plate (c.p.) across the equator of the nuclear spindle.

F. Completion of the cell-division, and disappearance of the individual chromosomes in

the daughter nuclei.

which represents a series of six selected stages arranged in proper
sequence, reproduced from photo-micrographs.

Fig. 34, A represents the so-called resting stage of the nucleus,
in which it will be noticed that there is, in addition to
the minute, scattered chromatin granules, a large spherical
chromatin nueleolus or karyosome (Nuls.). 13 shows the spireme

78

OUTLINES OF EVOLUTIONARY BIOLOGY

stage, with the chromatin granules collected together in a long
spirally coiled thread and the nucleolus still very conspicuous.
C shows the group of chromosomes formed by transverse breaking

UCCj*

as.---
A.

chr.

C.

FIG. 35. — Mitosis in the segmenting Egg of the Horse- Worm (Ascarfs m-fytilo-
cephahi), x 770. (From photographs.)

A. Lateral view of the egg during the first cleavage; showing the nuclear spindle (sp.),

the equatorial plate (aeq.), one of the two centrosonies (c.s.), the other being out of
focus, and the asters (as.) formed by fine threads of protoplasm radiating from
around the centrosonies. Polar bodies (p.b.) are also shown.

B. The same stage viewed from one pole, showing the four V-shaped chromosomes (chr.)

in the equatorial plate.

C. The first division is completed and the nuclei have again passed into the spireme stage.

A polar body (p.b.) is still visible.

D. Each of the first two blastomeres has again reached the stage represented in A

and B.

up of the spireme thread. The karyosome has now disappeared,
having apparently been used up in the formation of the chromo-
somes. D shows the two groups of daughter chromosomes
formed by longitudinal splitting of the parent chromosomes and
retreating towards the two ends of the spindle, which is only

MITOSIS IN ANIMAL CELLS 79

faintly visible. E shows the commencement of the cell-plate
(c.p.) across the middle of the spindle, and F the two young
daughter cells each with a new nucleus in which the chromo-
somes have again broken up into granules.

It is only by the examination of large numbers of examples
that all the minute details of the process can be elucidated, but
the main features as represented in the above figures can very
easily be made out.

For comparison with the process of mitosis as seen in typical
plants such as Galtonia, we may take the first division of the
fertilized egg in the horse-worm, Ascaris, a classical subject from
the study of wbich much of our knowledge of nuclear division in
animal cells has been derived. In this case there are only four
chromosomes, but they are large and conspicuous, and charac-
teristically V-shaped when forming the equatorial plate on the
spindle.

Fig. 35 is again taken from actual photographs. In this
figure, A represents a side view of the entire egg-cell during
the division of the nucleus, with spindle (sp.), asters (as.),
centrosomes (one only of which, cs., appears in the photograph,
the other being out of focus), and equatorial plate (acq.). 13
shows a similar stage viewed from one pole, so that the spindle
itself does not appear, but the four chromosomes forming the
equatorial plate are distinctly visible. C shows the two daughter
cells or blastomeres resulting from the first division of the egg,
each with the nucleus preparing for further division, and I)
represents a later stage in which the nucleus of each daughter
cell is again actually in process of division and shows the
separate chromosomes very distinctly.

It must not be supposed that the phenomena of mitosis are by
any means confined to the higher animals and plants ; they are
observable throughout the animal and vegetable kingdoms, in
unicellular as well as multicellular forms. The process has long
been known to take place, for example, in at any rate some
Amcebre, and it probably occurs wherever there is a clearly
differentiated nucleus. The Bacteria and their allies, in which
the chromatin granules are scattered throughout the cell body
and there is no proper differentiation into cytoplasm and nucleus,
apparently form exceptions to the general rule.

There are, however, even amongst the higher animals, some cases
of cell-division which do not exhibit mitotic phenomena, but in

80

which the nucleus appears simply to constrict into two or more
parts (Fig. 36) . This is known as direct or amitotic nuclear division.

It is frequently met with in
degenerating cells and patho-
logical tissues, but it is doubtful
if it ever occurs (in the higher
organisms at any rate) in cells
which are destined to undergo
long - continued multiplication.
We may therefore regard it as
a more or less abnormal process
with which we have no need to
concern ourselves any further. The phenomena of mitosis, on
the other hand, are thoroughly normal and practically universal,
and, as we shall see later on, they are of the deepest significance
from the point of view of the theories of heredity and variation.

FlG. 36. — Amitotic nuclear Division
as seen in Cells from the Cavity
of the Puraphysis in Sphenodon,
X 1000.

nu. nuclei.

PART II.— THE EVOLUTION OF SEX
CHAPTEE VII

Limitation of the powers of cell-division — Eejuvenescence by conjugation
of gametes — The origin of sex in the Protista.

BY the process of cell-division an unbroken continuity has
been established in the chain of living things from the earliest
appearance of unicellular organisms to the present day. Every
cell is the descendant of pre-existing cells and, in accordance
with the theory of evolution, all cells which exist to-day, distri-
buted amongst the bodies of countless millions of different
organisms, could, if our knowledge were sufficiently complete, be
traced back to a single ancestral cell.

It by no means follows from these considerations, however,
that there is, under natural conditions, no limit to the ordinary
process of cell-division. On the contrary it is well known that
in any cell family, whether belonging to a unicellular or a multi-
cellular organism, the power of multiplication tends to become
exhausted, and, if that particular cell family is to continue its
existence, has to be in some way renewed.

Take, for example, an ordinary ciliate or flagellate Protozoon,
which multiplies by simple fission. If a single individual be
isolated and placed in water containing suitable food material,
and kept under suitable conditions of temperature, light and so
forth, it will multiply very rapidly, until possibly hundreds of
generations of separate cells have been produced and the total
number increased perhaps to millions. But under ordinary
circumstances a time presently arrives when the individuals
begin to show signs of exhaustion, accompanied by physical
degeneration, and to slack off in their rate of multiplication.
They may be stimulated to renewed activity for a time by special
feeding or by constantly varying the culture medium,1 but in a

1 Mr. L. L. Woodruff has kept a culture of Paramcecium under observation for
nearly three and a half years, taking precautions to prevent the possibility of con-
jugation, but constantly varying the culture medium. During this time more than
two thousand generations of Paramcecium were produced by repeated fission — an

B. G

82

OUTLINES OF EVOLUTIONARY BIOLOGY

state of nature the chief if not the only means by which the
family can be kept from speedy extinction is conjugation, the

FIG. 37. — Life History of Copromonas. (From Bourne's " Comparative
Anatomy," after Dobell.)

cv., contractile vacuole; cpJi., cell pharynx; cst., cell mouth; fv., food vacuole;
N., nucleus; R., reservoir; tr., flagellum. (For further explanation see text.)

exhausted individuals approaching one another and finally uniting
in pairs. In this way they appear to become rejuvenated and

average of about one division every fifteen hours — and at the end of the period the
organisms were still in a perfectly normal condition.

83

their failing powers of multiplication by cell-division are com-
pletely restored.

For the purpose of studying this process of conjugation in its
primitive simplicity we can hardly do better than take the minute
flagellate form Copromonas, which is found in water frequented
by frogs, from the excrement of which it derives its nutriment.
The adult organism (Fig. 37, A) consists of a very minute ovoid
mass of protoplasm with a single flagellum (tr.) springing from
the narrow end. Alongside the base of the flagellum is a definite
cell mouth (cytostome, cst.) through which solid particles of food
are taken into the interior of the body. Close to this there is
a contractile vacuole (CT.), accompanied by a "reservoir" (K)
into which it discharges. The nucleus (N) is situated nearer to
the broadly rounded hinder end of the body, which may also
contain a number of food-vacuoles (/#.).

If the food supply be abundant the individual Copromonas will
grow and presently divide into two by simple longitudinal fission,
which commences at the narrow anterior end (Fig. 37, B — D).
The division of the nucleus is said to be amitotic. The two
daughter cells separate, feed, grow and repeat the process, and in
this way a whole swarm of monads is produced. In the course of a
few days, however, they appear to become exhausted and conjuga-
tion sets in, the individuals uniting in pairs (Fig. 37, 2 — 5). The
result of each such union is a single larger individual, which may
either undergo a period of rest within the protection of a special
envelope or cyst (Fig. 37, 7), or at once assume the ordinary
form and begin to multiply with renewed activity (Fig. 37, V;.
For the continued existence of the species it is probably necessary
that the encysted monads should at some time or another be
swallowed by frogs and passed out again in the faeces, in order
that they may be brought in touch with the necessary food
supply.

We have here, as in the case of Haematococcus described in
Chapter III., a perfectly typical example of conjugation1 occurring
at longer or shorter intervals in the life cycle of the organism. The
whole process consists in the union of two separate cells, known
in this connection as gametes, to form a single cell known as the
zygote, and it is of the utmost importance to observe that not only
is there a union between the cytoplasm of the two gametes but
the nuclei also unite to form a single zygote nucleus. Indeed,

1 Also known as syngamy or zygosis.

G 2

84 OUTLINES OF EVOLUTIONAEY BIOLOGY

as we shall see later on, it is the union of the nuclei which is
the really important part of the business, for in some cases (e.g.,
Paramo3cium) the union of the two cell bodies is a merely
temporary affair, a necessary preliminary to an exchange and
subsequent union of nuclei.

In such simple cases as that of Copromonas we see all the
essential features of the sexual process which occurs so constantly
throughout the animal and vegetable kingdoms. It is evident
that in itself conjugation is not a process of reproduction, for its
immediate result is to halve the total number of cells instead of
doubling it. It has in fact exactly the opposite effect to that of
cell-division. It is a process which appears to be necessary for
the rejuvenescence, at longer or shorter intervals, of exhausted
cells, whereby they are endowed with renewed powers of multi-
plication by ordinary cell-division. At the same time it forms
the starting point of all those remarkable structural modifications
of the organism, whether unicellular or multicellular, which
accompany the evolution of sex.

In Copromonas and in Haernatoeoceus, although there is a
true sexual process, there is apparently no sexual differentiation
at all ; there is no distinction between male and female gametes ;
the two conjugating cells are exactly alike, and the conjugation
is therefore said to be isogamous. In Copromonas, moreover, the
gametes or sexual cells are indistinguishable from the ordinary
individuals, every individual being at least a potential gamete.
Starting from such a case as this we find, even amongst the
unicellular plants and animals, every stage in the evolution of highly
specialized male and female gametes, differing widely from the
ordinary individuals and from each other. Conjugation will then
take place between two dissimilar gametes, and is said to be
anisogamous.

The first hint, so to speak, of sexual differentiation is to
be observed in the behaviour of the conjugating gametes; it is
a physiological rather than a structural or morphological
phenomenon, and consists in the fact that one gamete is active
while the other remains comparatively passive. We shall find
that this distinction lies at the root of all sexual differentiation
throughout the animal and vegetable kingdoms. The more active
gamete is spoken of as male and the more passive as female. The
passivity of the female is intimately associated with and probably
to a large extent dependent upon the fact that it contains more

SEXUAL DIFFEBENTIATION 85

cytoplasm and is therefore more heavily weighted than the male
gamete. This cytoplasm, moreover, is in many cases densely
charged with food material, which constitutes the capital with
which the zygote, formed by the union of the two gametes, has
to begin its new life cycle.

It is quite clear that the primary distinction between the sexes
is a simple case of division of labour accompanied by a
corresponding structural differentiation. Two ends have to be
secured by the gametes. They must come together in order that
they may conjugate, and therefore one or both must be capable
of active locomotion. They must also contain between them
sufficient material, either in the form of actual protoplasm or of
some substance that can easily be worked up into protoplasm, to
give the new individual which results from their union a fair
start in life.

A cell body heavily weighted with food material is, however,
clearly incompatible with great activity, so one of the two gametes
remains unencumbered and becomes specialized as the active
partner, charged with the duty of seeking out its mate
and bringing about their union, while the other, more or less
burdened with the necessary supplies, passively awaits the event.
The conjugation of such differentiated gametes leads to a more
satisfactory result than can be attained in cases of isogamy
like that of Copromonas, for the new individual will have a better
chance in life owing to the greater amount of capital with which it
commences. Such sexual differentiation of the gametes finds its
most complete expression in the formation of female ova and
male spermatozoa, which are especially characteristic of the
higher animals (compare Fig. 69), though they also occur in
many plants and even in some unicellular forms. The process
of conjugation in such a case is often spoken of as the fertiliza-
tion of the ovum by the spermatozoon.

These considerations enable us to understand at once the great
difference in size which usually distinguishes the male from the
female gamete, whence the general terms microgametes and mega-
gametes so often applied to them. The microgamete is as small
as possible in order that its activity may not be impaired ; the
megagamete is swollen out with nutrient material.

We may illustrate these general principles by a brief description
of a few more cases of conjugation amongst unicellular organisms.

Bodo, or Heteromita (Fig. 38), is a very minute flagellate monad

86 OUTLINES OF EVOLUTIONARY BIOLOGY

FIG. 38. — Life History of Bodo, or the Springing Monad, very highly
magnified. (From Dallinger and Drysdale.)

A. Ordinary individual.

B., C. Multiplication by longitudinal fission (the nucleus divides and the cell body and

both flagella split 'lengthwise).
D — F. Multiplication by transverse fission (the nucleus and cell body divide, the trailing

flagellum splits lengthwise and a new anterior flagellum is budded out at one end).
G. Two gametes about to conjugate.
H. Conjugation.

J. Zygote formed by conjugation, witli flagella still attached.
K. Fully formed zygote.

L. Escape of spores by rupture of the zygote wall.
M. Development of the spores,
a./., anterior flagellum; a'.f., new anterior flagellum sprouting out; mi., nucleus;

sp., spores; t.f., trailing flagellum ; zyg.nu., zygote nucleus.

LIFE HISTORY OF BODO 87

which occurs in long-standing infusions of cod's head. It differs
from Copromonas chiefly in the possession of two flagella and in
the absence of a cell mouth, all its food being taken in in a state of
solution by diffusion through the thin cell membrane. The two
flagella both spring from the beak-like anterior extremity. One
(A, a.f.) extends forwards and by its movements enables the
organism to swim actively about, the other (A, £/.) hangs down
and is trailed behind during active locomotion. The monad
anchors itself by the trailing flagellum and then, by coiling and
uncoiling the latter, executes characteristic springing movements.
Asexual reproduction (i.e., reproduction without any sexual
process) is effected by simple fission, which may be either
longitudinal (B, C) or transverse (D — F).

There are no structurally differentiated gametes or sexual
cells, but conjugation (G — J) is effected between two apparently
similar individuals which are indistinguishable from the ordinary
form. It is noteworthy, however, that one of the two gametes at the
time of union is anchored, while the other swims actively up to it,
and thus we get a slight indication of physiological differentiation
into active and passive, or male and female. The male gamete
also arises by a somewhat peculiar method of fission. Conjugation
of the two gametes produces a zygote which has somewhat the
shape of a triangular sac (K). The flagella disappear and in the
interior of the sac cell-division goes on with great rapidity, giving
rise to an immense number of very minute spores, which ultimately
escape from the corners of the sac in the form of very fine dust
(L, sp.). Each spore no doubt is a minute nucleated cell, but it is
so small that the nucleus cannot at first be made out. It grows
by absorbing liquid food from the infusion in which it lives, and
as it grows the nucleus becomes apparent, flagella are put forth,
and the adult form is gradually attained (M). We have here a
striking illustration of the fact that the most obvious result of
conjugation is an increase of the power of cell-division.

As a case of complete morphological as well as physiological
differentiation between male and female gametes in a unicellular
organism we may take that of Coccidium scltubergi, which occurs
as a parasite in the intestine of a centipede (Lithobius forficatus).
The life history of this remarkable protozoon is very com-
plicated and it is not necessary for our purposes to describe it
in detail. The adult organisms occur in the form of spherical
nucleated cells, each actually inside one of the epithelial cells

88 OUTLINES OF EVOLUTIONAEY BIOLOGY

which line the intestine of the host and upon which the parasites
feed (Fig. 39, A.). The latter increase in number very rapidly
by a kind of multiple fission, and successive generations of
parasites attack fresh epithelial cells of the host until the
epithelium is more or less completely destroyed. After many
generations have been produced asexually in this manner a
sexual process sets in. Megagametes and microgametes are pro-
duced ; the former by growth of an ordinary individual into a
large spherical ovum, or egg-cell (Fig. 39, B., ? gam.), the latter
by growth and division of an ordinary individual into a number

$ gram.

A. B.

FIG. 39. — Coccidium schulergi, highly magnified. (After Schaudinn.)

A. A full grown Coccidium lying within an epithelial cell of the host.

B. Male and female gametes about to conjugate.

coc., the Coccidium ; c.r., the cone of reception put out by the female gamete ; nil., nucleus
of female gamete ; nil. coc., nucleus of adult Coccidium ; nu.ep., nucleus of epithelial
cell of host ; £ gam., male gamete or spermatozoon ; % gam., female gamete or ovum.

of very much smaller spermatozoa or sperm cells (Fig. 39, B.,
cf gam.).

The ovum is densely filled with food granules and has no
power of locomotion. The spermatozoon closely resembles a
flagellate monad, being provided with a pair of flagella by means
of which it swims actively about ; the body, however, is long and
slender and consists almost entirely of nuclear (chromatin) material.
It seeks out the ovum, which exercises a peculiar attraction upon
it, and the two conjugate, the spermatozoon boring its way into
the ovum and their nuclei fusing to form the zygote nucleus.
The only trace of activity, apart from nuclear phenomena, which
the ovum exhibits is the protrusion of a small " cone of recep-
tion " (Fig. 39, B. c.r.) from the surface of the cell towards the
approaching spermatozoon, which seems to indicate that the

ORIGIN OF SEX IN PLANTS 89

attraction is mutual. The process of conjugation is followed as
usual by cell-division on the part of the zygote, which in this
case results in the formation of a small number of comparatively
large spores, enclosed in tough protective envelopes. From
these spores new individuals are produced which, under favour-
able circumstances, commence the life cycle afresh. It is
extremely interesting to observe that we have here, in a
unicellular Protozoon, as complete a sexual differentiation of
the gametes as we meet with in any of even the most highly
organized plants and animals.

We must now briefly notice the sexual phenomena exhibited
by those interesting Protista which we have already had occasion
to refer to in Chapter IV. under the name Phytoflagellata. It
will be remembered that in Haematococcus (Fig. 5) the process of
simple fission sometimes results in the production of a relatively
large number (32 — 64) of small individuals instead of the usual four
comparatively large ones. These small individuals are specialized
gametes, differing from the large ones not only as regards size
but also in the absence of the characteristic cell-wall of the latter.
There is, however, no differentiation into male and female.
Conjugation is of the isogamous type and produces a zygote
which grows into an ordinary resting cell which will presently
begin to multiply actively by ordinary fission.

We have also seen that this organism forms the starting point
of a series of forms, represented by the genera Haematococcus,
Pandorina, Eudorina and Volvox, which illustrate progressive
stages in the process of colony formation. The same series also
shows us very clearly the differentiation between male and female
gametes — the origin of sex in the vegetable kingdom. It will be
remembered that Pandorina forms colonies of sixteen or thirty-
two cells enclosed in a common envelope (Fig. 10, A). A sexual
multiplication is effected by each cell of the colony dividing into
2, 4, 8, and finally 16 or 32, which form a daughter colony within
the parent, to be liberated presently by softening of the parental
envelope. Occasionally, however, the individual cells of a colony
divide each into eight gametes. These are small cells, each with
a pair of flagella, which escape and swim about separately. They
exhibit no clear distinction into male and female, but some are
comparatively large, some small, and some intermediate in size.
They conjugate in pairs (Fig. 10, B), and the two members of a
conjugating pair are often, though apparently not always, of

90

OUTLINES OF EVOLUTIONARY BIOLOGY

different sizes. It is probable that we have here a kind of
foreshadowing of that sharp differentiation into large (female)

8 S

£-. 00

173 a

^~

3

^3

C

S,

„

|

ft

-2

7t

3

•M

a

—

X

O

0)

'£

T3

cS

_g

^£

^

O

.IH

~ x'

ce'_g

1

ri

O

w

oH

S

o

g

hi

a

.i

O a:
5.1

P< ""
«f 5 '
ai

•S

1:

1 "*

o

>>

'-H

o

O

be be .5
^ -2 —

1

0>

5S

"S""

3n3

<D

9

S"i

S J

>>S

^ s

—

bcj

Jf

8^ '

|

-- o>

'^ S

= 1

T

O c3

G "S

lg>

^?

a"^

"c.2

7-i

— Qi

^^ .2

r s

o

^ >

"o.i:

^^

£

11

2°

3* 2

S*

N

^

^"

megagametes and small (male) microgametes of which we have
already spoken.

In Eudorina, which forms colonies somewhat similar to those of
Pandorina, this differentiation is already fully expressed (Fig. 40).
The megagametes or ova1 differ scarcely at all from ordinary

1 Often termed by botanists oospheres.

SEXUAL DIFFERENTIATION IN PROTOPHYTA 91

individuals. They are shown imbedded in the gelatinous
matrix of the large colony on the left of the figure. The micro-
gametes or spermatozoa,1 on the other hand, are much smaller,
club-shaped bodies, having a characteristic yellowish colour and
with a pair of flagella at the narrow, pointed end (Fig. 40, M3).
They are produced in bundles of sixty-four by repeated divi-
sion of a mother cell (Fig. 40, II — VI). Thus the male and
female gametes, spermatozoa and ova, do not occur together
in the same colony, but the colonies, when they consist of
gametes and not of ordinary individuals, contain only one or the
other kind. Hence the sexual differentiation is in this case
extended from the gametes themselves to the colonies which
bear them, and we may recognize colonies of three kinds : (1)
Asexual, which produce no gametes and reproduce by ordinary
fission of all or any of the component cells, (2) male sexual, which
produce microgametes or spermatozoa, and (3) female sexual,
which produce megagametes or ova. The bundles or colonies of
microgametes (Fig. 40, MI, M2) swim about actively by means of
their flagella, apparently in search of the larger and much less
active female colonies (Fig. 40, I). Having found such a
colony the now separated microgametes (Fig. 40, M3) bore
their way in, ultimately conjugating with the megagametes to
form zygotes.

In any one colony of Pandorina or Eudorina the constituent
cells are all of one kind, all ordinary asexual cells or all male
gametes or all female gametes. In Volvox (Fig. 11) we meet with
a further advance. Even in asexual colonies which do not produce
gametes at all we find the cells differentiated in so far that some
only are capable of giving rise (asexually) to daughter colonies,
while in the female colonies only some of the cells form female
gametes or ova (Fig. 11, A, o). The male gametes (Fig. 11, B — D)
are very similar to those of Eudorina. They unite with the
female gametes to form zygotes, which, after a period of rest,
develop into new Volvox colonies.

In order to emphasize the fact that the process of conjugation
is essentially a nuclear phenomenon we may now turn to the
case of Paramoecium. The general appearance and structure of
this protozoon have been described in Chapter IV. (Fig. 8). It
multiplies by simple transverse fission, and under favourable
conditions continues to do so until exhaustion sets in, when its

1 Often termed by botanists spermatozoids.

92

OUTLINES OF EVOLUTIONARY BIOLOGY

failing powers are restored by conjugation. The conjugation in
this case, however, is not quite like that which takes place in
the other unicellular organisms which we have been studying,
and the term conjugant may be applied, in preference to the
term gamete, to the individuals concerned.

It may easily be observed that the union of the two con-
jugants (Fig. 41) is a merely temporary affair. They remain
attached together by their mouth-bearing surfaces for a short

I -meg.

mtg.-\ —

St.

FJG. 41. — Diagram of the Process of Conjugation in Faramoecium.

gam., gametic nuclei; M., mouth; meg., meganucleus; inic., microiiucleus ; mig.,
migratory nucleus ; p. b., products of the division of the microiiucleus which dis-
appear (polar bodies); St., stationary nucleus ; zyg., zygote nucleus. (For further
explanation see text.)

time and then separate again and continue their independent
lives. Before separating, however, they evidently undergo some
kind of rejuvenescence whereby their vigour and power of multi-
plication are completely restored. This is accounted for by the
fact that during the time of their union certain complex nuclear
processes take place, the net result of which is an exchange of
chromatin material between the two conjugants.

It will be remembered that Paramcecium differs from most
Protozoa in the possession of two nuclei, large and small, or
meganucleus and microiiucleus (Fig. 41, A, meg. and mic.). The

93

I latter is alone concerned in the process of conjugation, the
meganucleus in the meantime breaking up and being absorbed
into the cytoplasm, to be replaced in the manner described later
on. We may confine our attention, therefore, to the behaviour
af the micronucleus. In each conjugant this divides mitotically
into two daughter nuclei (Fig. 41, B, mic.' and p.b.) each of which
again divides, so that there are now four micronuclei (Fig. 41, C).
Of these four three (^.£>.) go to the bad, being apparently absorbed
into the cytoplasm, while the remaining one (mic.") divides once
more, so that each conjugant has now again two micronuclei
(Fig. 41, D). These two, though similar in appearance, differ
strikingly in their behaviour, one of them remaining quiescent
while the other passes over into the body of the other conjugant.
They are therefore known respectively as the stationary (st.) and
the migratory (miy.) micronuclei. In this way the migratory
micronuclei of the two conjugants change places with one another,
as indicated by the arrows, and the sole object of the temporary
union of the two conjugants appears to be to enable this inter-
change to take place. When it has been effected a true conjuga-
tion occurs between the two micronuclei in each cell (Fig. 41, E,
gam.), derived one from each conjugant. This is the real sexual
process. The migratory and stationary nuclei are gametic nuclei
and the result of their union is a zygote nucleus (Fig. 41, F, zyg.}.
Moreover, we have here again an evident distinction into
male and female gametic nuclei, characterized in the usual
way by the activity of the one and the passivity of the other.
The two conjugants themselves, however, cannot be distin-
guished as male and female, for each produces both male
and female gametic nuclei and may therefore be regarded as
hermaphrodite.

After the interchange of gametic nuclei has taken place the two
conjugants separate as ex-conjugants (Fig. 41, F). The zygote
nucleus in each divides repeatedly by mitosis and from the
daughter nuclei thus produced both micronuclei and meganuclei
are formed. Presently the ex-conjugants themselves begin
to divide once more by fission and the new micronuclei
and meganuclei are distributed amongst the new individuals
(Fig. 41, G).

The essential feature of this very complicated process is clearly
the same as in the simpler cases which we have examined, and
consists in the union of two nuclei belonging to different cells to

94 OUTLINES OF EVOLUTIONARY BIOLOGY

form a single zygote nucleus which has renewed powers of
multiplication by division. As already observed, the micronucleus
alone takes part in the process, the meganucleus being merely
concerned in the life of the individual and its asexual multiplication
by simple fission.

CHAPTER VIII

Sexual phenomena in multicellular plants — The distinction between somatic
cells and germ cells — Alternation of sexual and asexual generations —
Suppression of the gametophyte in flowering plants.

WHEN we consider the habit of colony formation which is so
common amongst the Protophyta, and which we have discussed
in the cases of Pandorina, Eudorina and
Volvox, we see at once that it is impossible to
draw any strictly logical distinction between
such primitive forms and the true multi-
cellular plants or Metaphyta. . The common
fresh water alga, Spirogyra, for example,
might be regarded either as a colony of single
cells or as a very simple multicellular plant
in which the constituent cells exhibit little
or no differentiation amongst themselves.
In any case it forms a very convenient start-
ing point for the consideration of the sexual
phenomena met with in Metaphyta, being in
this respect actually in a much more primitive
condition than either Eudorina or Volvox.

The fully developed Spirogyra plants con-
sist of long green filaments of hair-like
dimensions, which float in loose slimy
masses in clear fresh water. Each filament
consists of a single row of cylindrical cells
placed end to end, each cell being enclosed in
a thin, transparent wall of cellulose (Fig. 42,
c.w.), whereby its protoplasmic contents are
completely separated from those of adjacent
cells. The cytoplasm forms a thin primordial
utricle (p.u.), lining the cell-wall and enclosing
a large vacuole (vac.) filled with colourless, watery cell-sap, in
which a more or less central mass of cytoplasm, containing the

FlG. 42.— Part of a
filament of Spiro-
gyra, showing
one complete cell
and parts of two
others ; highly
magnified.

c.w., cell-wall; cr., chro-
ma tophore ; nu.,
nucleus ; p.u., pri-
mordial utricle;
pyr., pyrenoid ; -str.,
strands of proto-
plasm; vac., vacuole.

96

OUTLINES OF EVOLUTIONARY BIOLOGY

nucleus (nn.), is suspended by slender radiating protoplasmic
threads (str.). So far, both in structure and arrangement of its
component cells, the plant closely resembles a single hair of
Tradescantia (compare Fig. 25). It differs, however, in the

presence in each cell of one or more
conspicuous chloroplastids or chroma-
tophores (cr.), coloured bright green
by chlorophyll and wound spirally
round and round inside the cell-wall,
like pieces of ribbon. It is from
these characteristic structures that
the name Spirogyra is derived.

The filaments increase in length by
transverse fission of the component
cells. Every cell, however, or to speak
more accurately its protoplasmic con-
tents, must also be looked upon as
a potential gamete. Conjugation in
some species takes place between the
cells of two filaments which are lying
side by side, parallel with one another
(Figs. 43, 44). In others it lakes place
between adjacent cells of the same
filament, but we may confine our
attention to the former case. The
first indication of the process is seen
in the formation of a small, hollow pro-:
tuberance on the wall of one of a pair
of cells which happen to be more or
less opposite to each other (Fig. 44, a.).
This is shortly followed by the forma-
tion of a similar protuberance on
the wall of the other cell (/>.). The
two protuberances meet (c.) and fuse
together, and the cell-walls at the
point of union are dissolved away (d.). Thus a hollow canal is
formed placing the cavities of the two conjugating cells in free
communication with one another. In the meantime changes
are going on in the protoplasmic contents of the conjugating
cells, essentially similar in the two members of each pair but
with one cell still taking the lead and the other lagging somewhat

FIG. 43. — Conjugation in
Spirogyra, showing in one
Filament solitary Cells
(s.c.) which have failed
to mate, and in the other
Zygotes (zyg.), X 82.
(From a photograph.)

CONJUGATION IN SPIROGYRA

97

behind. The chloroplastid breaks up ; the primordial utricle

retreats from the cell-wall towards the middle, and the entire

protoplasmic contents round themselves off into a compact

nucleated mass — -the gamete (yam.). The time has now arrived

for the all-important event ; the gamete from one cell-chamber

creeps through the canal into the other chamber and conjugates

with the gamete which there awaits

it (Fig. 44, <?.). The result is a

zygote or zygospore (Figs. 43, 44,

ziffj.} which surrounds itself with a

thick protective envelope within

the cell-wall of the parent and

after a period of rest germinates,

giving rise by repeated transverse

division to a new filament.

In this case it will be observed
that the two conjugating gametes
are morphologically alike, and we
may therefore describe the process
of conjugation as isogamous. As in
the case of Bodo, however, there is
a physiological distinction between
the two as regards their activity ;
the gamete which takes the lead
throughout the whole process and
finally crawls through the canal of
communication is obviously to be
regarded as male, and its partner
as female. Moreover, one entire
filament may produce nothing but
male gametes and may therefore
be regarded as a male plant (Fig.
44, $ ), while the other may produce
only female gametes and therefore be regarded as a female plant
(Fig. 44, ? ). In the closely related form, Zygogonium, there is
no distinction of any kind between the two sexes, both gametes
exhibit the same degree of activity and they meet each other half-
way, in the connecting canal between their respective chambers,
to form the zygote.

In Spirogyra, as we have just seen, every cell of the adult plant
is a potential gamete or germ cell ; in the case of Volvox, it will

B. II

FIG. 44. — Diagram of two con-
jugating filaments of Spiro-

a. — e., pairs of cells in successive
stages of conjugation ; £ gam.,
male gametes; <£gam.y female
gametes ; S.C., solitary cell ;

98 OUTLINES OF EVOLUTIONARY BIOLOGY

be remembered, this is not so, certain cells only being concerned
in the sexual process. We have here a kind of fore-shadowing of
that distinction into somatic or body cells and sexual or germ
cells which is so characteristic of the higher organisms, both
plants and animals. We see a somewhat similar fore-shadowing

FIG. 45. — Fucus vesiculosus, about half nat. size. (From Vines' "Botany.")
b, air bladders ; /, fertile branch.

even in the unicellular Paramcecium, except that the distinction
is here confined to the nuclei, the meganucleus being concerned
with the life of the individual and the asexual process of ordinary
cell-division, while the micronucleus is alone concerned in the
sexual phenomena. We can, in this case, distinguish between
somatic nucleus and germ nucleus, though not between somatic
and germ cells.

99

The loss of the power to act as germ cells or gametes which
the vast majority of the constituent cells in a typical multicellular
organism usually suffer is undoubtedly one of the penalties
which they have to pay for their high degree of speciali-
zation. The germ cells themselves, on the other hand, always
remain in a more primitive, less specialized condition, and
may, in fact, be regarded as so many unicellular Protista
enclosed within the multicellular body. They resemble the
free-living Protozoa and Protophyta also in that they exhibit a
certain degree of independence and are in the majority of cases
actually set free from the parent body as unicellular individuals ;

FIG. 46. — Fticus vesiculosus ; Section through a female Conceptacle, X 50.
(From Vines' " Botany," after Thuret.)

short-lived, it is true, unless they happen to meet one another
and conjugate, but nevertheless enjoying that liberty which is
only possible to independent organisms.

All this is very beautifully illustrated in the case of the common
brown seaweed or bladder- wrack, Fucns vesicidosus (Fig. 45).
The entire plant consists of flattened branches, copiously
subdivided and attached by a root, usually to some rock between
tide-marks. Here and there the branches are swollen out to form
air-bladders (/>), which serve as organs of flotation. The
histological structure of the plant is comparatively simple and
need not detain us ; we are concerned only with its reproductive
processes.

Fucus vesiculosus is dioecious or unisexual, there being distinct

H 2

100 OUTLINES OF EVOLUTIONARY BIOLOGY

male and female plants. In both cases certain branches (/),
usually described as fertile, are characterized by the presence of
numerous minute spherical pits, opening on to the surface by narrow
mouths. These pits or conceptacles, one of which is -represented
in vertical section in Fig. 46, contain the sexual organs, male
antheridia or female oogonia, as the case may be, intermingled
with hair-like structures known as paraphyses. The antheridia
(Fig. 47, a) are attached to the branching paraphyses in the male

a

FIG. 47. — Fucus vesiculosus.

a, branching paraphysis from male conceptacle, bearing antheridia ; b, an oogoiiium
surrounded by unbranched paraphyses and with its contents divided into eight ova ;
c, a discharged ovum surrounded by spermatozoa, one of which will fertilize it ; d, a
developing embryo ; all x 160. (From Vines' " Botany," after Thuret.)

conceptacles in the form of small sacs in which the male gametes
(spermatozoa) are produced. These are minute, nucleated, pear-
shaped cells, each with two flagella ; they are produced in large
numbers in each antheridium and set free by rupture of the wall
of the latter to make their way out of the opening of the
conceptacle by their own activity.

The oogonia are oval sacs, a good deal larger than the antheridia,
and occur amongst the hair-like paraphyses in the female con-
ceptacles (Fig. 46). In each oogonium (Fig. 47, 1) eight female
gametes (egg cells, ova or oospheres) are formed by cell-division.
These are spherical, nucleated cells, very much larger than the

ALTERNATION OF GENERATIONS IN PLANTS 101

spermatozoa, owing to the great amount of cytoplasm which they
contain. They are liberated by rupture of the oogonium and
discharged through the opening of the conceptacle on to the
surface of the plant. There they are found by the spermatozoa,
which swarm around them in large numbers, endeavouring to
conjugate (Fig. 47, c). Finally a single spermatozoon succeeds
in boring its way into each large egg cell, and fertilization is
effected by the union of the male and female nuclei. The zygote,
well supplied with food material by the egg cell, begins to undergo
cell-division immediately, forming a multicellular embryo
(Fig. 47, d) which attaches itself by roots and grows into a plant
resembling the parents.

Here we have a perfectly typical case of differentiation of the
gametes or germ cells into large passive female ova and small
active male spermatozoa, and conjugation is anisogamous, the
ovum being " fertilized " by the spermatozoon.

In the ferns, mosses and other more highly organized plants
a new complication is introduced by the fact that two distinct
forms of the plant alternate with one another in the life cycle.
In one only of these forms, known accordingly as the gametophyte,
does a sexual process occur ; the other, known as the sporophyte,
reproduces by means of unicellular spores, which are produced
asexually and develop into new individuals without any process
of conjugation. The gametes or germ cells, borne on the gameto-
phyte, on the other hand, conjugate, and the zygote develops,
not into another gametophyte but into a sporophyte, while,
conversely, the spores produced by the sporophyte develop into
gametophytes.

This alternation of sexual and asexual generations is a
phenomenon of very wide-spread occurrence in the vegetable
kingdom, and, as we shall see in our next chapter, something of
the same kind occurs also in certain multicellular animals.

Take, for example, any ordinary fern. The conspicuous plant
(Fig. 48) is the sporophyte. It is very highly organized and shows
the typical differentiation into root, stem and leaf met with in all
the higher groups of the vegetable kingdom. Some or all of the
leaves sooner or later produce on their lower surfaces sporangia
(Fig. 48, A, C), little sac-shaped structures in which the spores
arise by division of mother cells into fours. These spores are
liberated, by rupture of the sporangia, in the form of fine brown
dust, which may be carried to considerable distances by the wind.

102 OUTLINES OF EVOLUTIONARY BIOLOGY

•FiG. 48. —The Sporophyte Generation of a Fern, Aspidium filix mas. (From

Strasburger.)

A, section through a sorus or group of sporangia, covered by the indusium ( x 20, after
Kny) ; B, lower surface of a pinna, showing the indusia covering the sori ; C, lower
surface of a pinna with the sori exposed.

LIFE HISTORY OF A FERN

103

FIG. 49. — Diagram of a young Prothallus
(pth.) formed by Germination of a
Fern Spore.

rh., rhizoid or root hair; sp.c., spore coat.

If one of them alights on a suitable spot, in a moist and shady
situation, it may germinate (Fig. 49). Its thick outer wall rup-
tures and a delicate tube is
put forth, containing the pro-
toplasm and nucleus. Cell-
division takes place and results
presently in the formation of
the gametophyte.

The gametophyte of the
fern (Fig. 50) is known as a
prothallus. It is an indepen-
dent, self-supporting plant,
but much less highly orga-
nized than the sporophyte, consisting usually of a green, heart-
shaped plate of cells, not more than perhaps a quarter of an
inch in diameter, and attached to the substratum by delicate
hair-like rhizoids. It develops no vascular system but never-
theless obtains its food
in the same way as the
sporophyte, absorbing
water containing dis-
solved mineral salts
from the soil by means
of its rhizoids, and
splitting up carbon
dioxide, obtained from
the air, by aid of its
chlorophyll. Such pro-
thalli are frequently
to be found attached to
the surfaces of flower-
pots and walls in damp
greenhouses and other
places where ferns are
grown.

The sexual organs,
male antheridia (Fig.
50, an) and female archegonia (Fig. 50, ar), are, like the rhizoids,
found on the lower surface of the prothallus, both usually occurring
on one and the same plant, which is therefore mono3cious or
hermaphrodite. The antheridia (Fig. 51, a) are essentially similar

FIG. 50. — The Gametophyte Generation or Pro-
thallus of a Fern, Aspidium filix mas, X 8.
(From Strasburger.)

A, lower surface of a sexually mature prothallus, showing

antheridia (an), archegonia (ar), and rhizoids (rJi).

B, an older prothallus with the young sporophyte genera-

tion or fern plant (b, w) attached to it.

104 OUTLINES OF EVOLUTIONARY BIOLOGY

to those of Fucus, being hollow sacs in which the male gametes
are developed. The latter (Fig. 51, s.) are active spermatozoa,

each having a spirally coiled body,
consisting chiefly of chromatin
material, and bearing a bunch of
cilia at one end, by the vibration of
which the gamete swims actively
about in any dew or other moisture
which may be deposited on the
prothallus.

The archegonia (Fig. 52) differ
considerably from the oogonia of
Fucus, having a characteristic
structure which is more or less
accurately repeated in the corre-
sponding organs of all the higher
plants. Each consists of a hollow
swollen venter, sunk in the tissue
of the prothallus, and a long neck
which projects from the surface,
and the wall of which is composed
of four rows of cells. The venter
contains a single relatively large ovum or oosphere (Figs. 52, A, o,
and 52, B), above which an axial row of canal cells (A"', A")

FIG. 51. — Antheridiumof a Fern,
discharging Spermatozoa
(Antherozoids) from its
opening, highly magnified.
(From Vines' " Botany.")
a, antheridium ; s, spermatozoon.

FIG. 52. — Archegonia of a Fern, Pdypodium vulgare, X 2J.O. (From

Strasburger.)

A, young, showing ovum (o) and canal ce\ls(K', J£"),and with the end of the neck closed.
S, mature, with the end of the neck open.

extends into the neck. When the ovum is ready for fertilization
the canal cells degenerate into mucilage and the cells at the end

CONJUGATION IN FERNS 105

of the neck separate so as to form an opening (Fig. 52, B). The
spermatozoa appear to be attracted to the opening by an acid
secretion discharged therefrom. One of them makes its way
down the neck to the ovum and fertilizes it by the usual process
of conjugation.

The zygote begins to develop, by cell-division, within the
venter of the archegonium,and forms a young sporophyte, which
for some time remains attached to the prothallus as shown in
Fig. 50, B, drawing nutriment therefrom by means of a special
temporary organ known as the foot. Presently, root, stem and
leaf are developed and the sporophyte becomes self-supporting.

One very remarkable fact in connection with the sexual
process in the fern remains to be noticed. The gametes, as we
have seen, are normally produced in special sexual organs and
are themselves perhaps as highly differentiated in relation to
the function of conjugation as gametes ever are. It has been
found, however, that if the normal sexual union between ova
and spermatozoa be prevented a conjugation may take place
between nuclei from adjacent vegetative cells of the prothallus,
resulting in the formation of an embryo sporophyte by so-called
apogamy. In these cases it is obvious that the sexual process
is not really suppressed, but simply transferred to ordinary
prothallial cells, which, though they do not conjugate under
normal circumstances, have retained the power of so doing when
occasion arises. It seems probable, however, that in some cases
true apogamy, or suppression of the sexual process, occurs, the
embryo sporophyte arising from the prothallus without any
conjugation of gametes.

The gametophyte or prothallus of an ordinary fern is, as we
have already seen, produced by the development of a unicellular
spore (Fig. 49), and in most cases is monoecious or hermaphro-
dite, bearing both male and female sexual organs and male and
female gametes. In the less commonly known and not very
fern-like " heterosporous " forms, however (Isoetes, Salvinia and
Marsilea), the gametophyte is dioecious or unisexual, there being
distinct male and female prothalli, and this sexual differentia-
tion affects not only the prothalli themselves but the spores from
which these are developed. Hence in these forms we find small
microspores which produce male prothalli, and large megaspores
which produce female prothalli. The spores themselves are set
free from the parent sporophyte, but the prothalli are very much

106 OUTLINES OF EVOLUTIONARY BIOLOGY

reduced in size and never become free from the spores ; they
nevertheless develop antheridia and archegonia respectively,
in which spermatozoa and ova are produced, and from the
conjugation of these arise zygotes or fertilized ova which
develop into new sporophytes.

We have briefly noticed these heterosporous ferns because,
as regards the sexual phenomena which they exhibit, they
constitute a very interesting connecting link between the ordinary
(homosporous) ferns, which produce only one kind of spore, and the
highest members of the vegetable series, the flowering plants.

In the flowering plants an alternation of sexual and asexual
generations can still be traced, but here the gametophyte is so
much reduced in size and has become so degenerate in structure
that it is quite inconspicuous, and can only be detected by micro-
scopical examination and recognized as constituting a distinct
generation in the light of our knowledge of lower forms.

The flowering plant itself is the sporophyte, and it is hetero-
sporous, producing microspores and megaspores. The pollen
grains are the microspores, while the megaspores are represented
by the embryo sacs enclosed within the ovules or unripe
seeds. The microspores, like the spores of ferns, are set free
from the parent sporophyte, the megaspores. however, are never
set free as such, and in neither case does the gametophyte become
free from the spore.

The terms pollen grain and embryo sac were applied to
the structures in question long before their true nature as
microspores and megaspores was recognized, and they have '.
become so firmly established that it is hardly possible to avoid
using them.

If we examine any typical, fully developed flower, such as is
represented diagrammatically in Fig. 53, we shall find that it
consists of four whorls or circlets of specially modified leaves.
Beginning at the outside we find first the calyx (Ke), composed
of a number of sepals, which usually, but by no means always,
retain the green colour characteristic of leaves and serve mainly for
the protection of the inner parts of the flower while in the bud ;
then the corolla (/£), composed of petals, which may be brightly
coloured and serve to attract insects ; then the andrcecium,
composed of stamens (a,/); and lastly, in the centre of the
flower, the gynoecium or pistil (n, g, F), composed of carpels.
The stamens and carpels are often spoken of as the essential

STRUCTURE OF A FLOWER

107

parts of the flower. They are really to be regarded as spore-
bearing leaves or sporophylls.

Each stamen consists usually of a long stalk or filament (/),
bearing an anther (a) at its extremity. The anther is a bilobed
structure, and each lobe contains two chambers, or pollen sacs,
in which the pollen grains (p) are formed by the division of
mother cells into fours, just as the spores of an ordinary fern
are developed within the spo-
rangia. The pollen sacs are, in
fact, nothing but sporangia—
and microsporangia, because they
contain microspores.

The pistil is formed of a vary-
ing number of carpels, which,
either singly or united, give rise
to a closed chamber below, the
so-called ovary1 (F), surmounted
by a longer or shorter style (g),
ending above in a rounded viscid
surface, the stigma (?i). In the
interior of the ovary, attached

to the carpels, are developed the FIG. _53.— Diagram of a typical
ovules (S), which are nothing
but sporangia (megasporangia)
enclosed each in a double enve-
lope (i). In each ovule a single
embryo sac or megaspore (em)
is produced. Only one ovule
is represented in the diagram, but there are usually a large
number in each ovary.

Having thus briefly described the parts of the sporophyte
with which we are immediately concerned, we must turn our
attention for a few moments to the gametophyte. The male
gametophyte, which never consists of more than a very small
number of cells, is developed from the pollen grain or microspore.
This latter is at first a perfectly typical unicellular spore,
with a single nucleus surrounded by cytoplasm, and the whole
enclosed in a thick protective cell-wall often ornamented with
microscopic sculpture of various patterns. The commencement

1 This is a very unfortunate name because the structure in question is an entirely
different kind of organ from the ovary of animals.

Flower in vertical Section.

(From Vines' " Botany.")
a, anther ; em, embryo sac ; E, ovum ;
/, filament of stamen ; F, wall of
ovary ; g, style ; i, integument of
ovule ; K, corolla ; Ke, calyx ; n,
stigma ; p, pollen grains ; ps, pollen
tube ; S, ovule.

108 OUTLINES OF EVOLUTIONARY BIOLOGY

It-

of germination of this spore in typical cases (Fig. 54) is
marked by the division of the nucleus into two. Around one of
these two cytoplasm collects to form a naked " antheridial cell "
(HI) ; the other, with the remainder of the cytoplasm, constitutes
the " vegetative cell " (k), which may or may not divide again.
The antheridial cell divides into two " generative cells." The
vegetative cell, or cells, represents the last vestige of the
body of the male prothallus ; the generative cells are male gametes.
The germination of the pollen grain and development of the

male prothallus are completed by
the putting forth of the pollen tube
(Fig. 53, ps, Fig. 54), which takes
place if the pollen grain is fortunate
enough to alight upon the stigma of
a flower of the right kind. The
pollen tube forces its way through
the loose tissue of the style to the
ovary and comes into intimate rela-
tions with one of the ovules contained
therein. *

The female gametophyte is repre-
sented by a few cells formed by
division of the rnegaspore (embryo
sac), or rather of its nucleus, and to
some extent of its cytoplasm, within
the ovule. The process is a some-
what complicated one, but, without
going into details, we may note that
at the time when the ovule is ready
for " fertilization " the embryo sac in a
typical flowering plant (Angiosperm) contains seven cells, one of
which is a female gamete (ovum or oosphere), while the others
may be taken to represent the female prothallus, including a
vestige of an archegonium. The arrangement of these cells is
shown in Fig. 55 (at, <>, k, s).

The embryo sac or megaspore (E) is surrounded by a cellular
layer known as the nucellus (Fig. 55, K), which represents the wall
of the sporangium, and this in turn by two other coats (ai and ii),
the outer and inner integuments of the ovule, which grow up
around the nucel-lus. The entire ovule is attached to the wall
of the ovary by a stalk or funiculus (/), upon which it is

FIG. 54. — Germination of the
Pollen Grain of Lilium
martagon, X 375. (From
Strasburger, after
Guignard.)

k, nucleus of the vegetative cell of
the pollen tube ; m, antheridial
cell ; g, male gametes formed
by division of the antheridial
cell.

CONJUGATION IN FLOWERING PLANTS

109

frequently bent sharply round as shown in Fig. 55. Opposite
to the spot where the funiculus is attached to the ovule an
aperture is left in the integuments known as the micropyle (w).
It is through this micropyle that the tip of the pollen tube
usually forces its way in search of the female gamete (<?), which
lies at the end of the embryo sac close to it. The wall of the
pollen tube and that of the embryo sac are absorbed where
they come in contact with one
another, and thus a passage is
opened for the male gamete,
which passes down the pollen
tube into the embryo sac and
there conjugates with the ovum.
The zygote thus formed develops
within the embryo sac into an
embryo sporophyte l ; the integu-
ments of the ovule become har-
dened to form the seed coat ;
reserve food material, such as
starch or oil, is stored up
either in the embryo itself or
in the endosperm2 around it, and
the ovule and its contents
separate from the parent sporo-
phyte as the ripe seed (Fig.
56, A).

When the seed ripens the
development of the contained
embryo is suspended for an
indefinite period, to be resumed
again if and when the seed finds
a suitable situation in which to germinate. The embryo may then
continue its development into the adult sporophyte (Fig. 56, B).

It is interesting to notice that no less than three distinct
generations take part in the formation of the seed. The seed
coat belongs to the parent sporophyte, while the contents of the

1 Vide pp. 48— oO (Fig. 14).

2 The second generative nucleus from the pollen tube conjugates with the nucleus
in the middle of the embryo sac (Fig. 55 A'), and the " endosperm " arises by repeated
division of the nucleus resulting from this conjugation. The whole process is
somewhat complex and it is not essential to our present purpose to describe it in
detail.

FIG. 55. — Diagram of the unfer-
tilized Ovule of a Flowering
Plant (Angiosperm) in longi-
tudinal Section. (From Vines'
" Botany.")

ai, outer integument ; at, antipodal cells ;
e, ovum ; E, embryo sac ; f, funiculus ;
ii, inner integument ; k, central or
definitive nucleus of embryo sac ; K,
nucellus ; m, micropyle ; s, synergidae.

110 OUTLINES OF EVOLUTIONARY BIOLOGY

test.

rcucL.

embryo sac at first represent the female gametophyte, which dis-
appears as the zygote develops into the embryo of another
sporophyte. In this way a very intimate relation is established
between each sporophyte generation and the one which precedes
it, and the gametophyte is crushed out of existence, as an inde-
pendent generation, between the two.

Accompanying this almost total suppression of the gametophyte
we find a delegation of certain responsibilities connected with the
sexual function to the asexual sporophyte, and the development
by the latter of what may be termed vicarious sexual characters.

These characters find
their expression in that
most remarkable feature
of all the flowering
plants, the flower itself.
Thus the use of the
terms male and female
may be extended in the
first instance to the sta-
mens and pistil, though
these are really merely
the spore-bearing leaves
of the asexual genera-
tion. Similarly the

transference of the pol-
jen graing frQm stamens

/./ i

to Stigma IS Otten SpOKen

Qf ag the fertilization of

the flower, though it is
obviously not the true process of fertilization but only a necessary
preliminary to the conjugation of the gametes, and is therefore
more accurately spoken of as pollination.

It will have been observed from the foregoing description that
not only is the male gametophyte of the flowering plant reduced
almost to the point of disappearance, but the male gamete itself
has apparently suffered great degeneration. It is no longer an
active spermatozoon, swimming about by means of flagella or
cilia, as in Eudorina or in the ferns, but a shapeless nucleated
mass of protoplasm which has at the most a sort of amoeboid
power of locomotion. By far the greater part of the travelling
which it has to accomplish in order to reach the ovum is

-Garden Pea (Pimm aativum).

A, ripe seed split in half ; B, seed germinating.
eo*., cotyledons or seed leaves ;pl., plumule or shoot of

embryo; rod., radicle or root of embryo; test.,

testa or seed coat.

POLLINATION 111

effected while it is still enclosed within the pollen grain and at
the expense of some external agency, for it is riot until the pollen
grain has alighted upon the stigma and the pollen tube is put
forth that the " generative cell " begins to exercise its own feeble
powers of locomotion.

The transference of the pollen to the stigma is effected usually
in one of two ways, either by the action of the wind or by the
agency of insects. Flowers which are pollinated in the first of
these two ways are said to be anemophilous, and they are usually
small and inconspicuous, as in the grasses and plantains, and many
forest trees. It is a very extravagant method of pollination,
involving the production of enormous quantities of pollen, most
of which is wasted, for only a very minute percentage of the
pollen grains will ever chance to alight upon stigmas. We
realize this when we see the enormous quantities of yellow
pollen dust which are blown off the pine trees in spring time.
The entomophilous or insect-pollinated flowers, on the other hand,
have hit upon a much more economical way of doing the
business. The development of nectar or honey as a bait for
their insect visitors, and of gaily coloured petals or sepals and
sweet scents as a means of attracting them, and in many cases
of elaborate mechanical contrivances to secure that the insect
shall not obtain its reward without doing the work of pollination,
all co-operate in bringing about the desired end, and the study
of these various adaptations forms one of the most interesting
chapters in biological science. We shall refer to it again when
we come to deal with adaptation and natural selection.

In many cases a complete sexual differentiation is manifested
by the entire flowers themselves, some having stamens without
carpels and others carpels without stamens, and being spoken of
as "male" or "female" flowers accordingly. We see this, for
example, in the case of the vegetable marrow, where both kinds
of flower are borne on the same plant, while in other cases, such
as the weeping willow and the Japanese Aucuba, the whole plant
may be either " male " or " female," producing flowers of the one
kind only. Thus the terminology which strictly speaking is
applicable only to the sexual gametophyte, has, as a matter of
convenience, been extended to the asexual sporophyte in order to
describe the secondary sexual characters which have been trans-
ferred to it in consequence of the suppression of the former.

The telescoping of successive generations one within the other

112 OUTLINES OF EVOLUTIONARY BIOLOGY

— the embryo sporophyte into the ovule of the preceding sporo-
phyte generation, with the gametophyte crushed in between —
is the most characteristic feature of the life cycle of the higher
plants.

From the homosporous ferns upwards throughout the vegetable
series it is obvious that the gametophyte is not nearly so well
adapted in its organization to the conditions under which the
higher plants have to live as is the sporophyte. There has
apparently been a kind of rivalry between the two alternating
generations, in which the gametophyte has had much the worst
of it. The sexual function itself, however, is far too important
to be altogether abandoned, and so the successful sporophyte has
finally taken over many of the responsibilities connected there-
with, while the poverty-stricken gametophyte has ultimately
become entirely parasitic upon its rival.

CHAPTER IX

Sexual phenomena in multicellular animals — Structure and life history of
Hydra and Obelia — Alternation of generations — The coulomate type of
structure — Secondary sexual characters — The evolution of sex.

IN multicellular animals or Metazoa, as in mulfcicellular plants,
a sharp distinction can usually be drawn between the somatic cells
wliich build up the various tissues and are concerned with the
life of the individual, and the germ cells or gametes which are
concerned with the propagation of the race and which alone (in
most cases) have the power of separating 'from the parent soma
or body and giving rise to new individuals.

In nearly all the Metazoa the gametes are sexually differen-
tiated into relatively large, passive ova and much more minute,
active spermatozoa which swim about by means of flagella.
The actual gametes arise by subdivision of undifferentiated
primordial germ cells. In the sponges, whose organization has
not advanced very much beyond that of complex colonies of
Protozoa, the primordial germ cells are merely wandering
amoeboid cells, resembling the white blood corpuscles of verte-
brates. Some of these round themselves off and give rise to
more or less spherical ova, others divide into spermatozoa, and
probably the entire sponge itself is in most cases either male or
female, producing one kind of gamete only. In the sponge the
.germ cells are not localized in definite organs but scattered
singly or in groups throughout the gelatinous ground-substance
of which the body is largely composed.

In the great majority of Metazoa, on the other hand, the
germ cells are segregated in well-defined organs termed gonads.
As a rule each gonad produces only ova, when it is known as an
ovary; or spermatozoa, when it is known as a spermary or testis;
only occasionally does it produce both, as in the case of the ovo-
testis of the snail. The gonads may accordingly be spoken of as
female, male, or hermaphrodite as the case may be, and the same
terms are also applied to the animals themselves, a male or a

B I

L14 OUTLINES OF EVOLUTIONARY BIOLOGY

female animal possessing either male or female gonads, while a
hermaphrodite animal may either possess a combined ovo-testis
or both ovaries and testes separately.

In illustration of these points we may briefly describe the
structure and life history of the common fresh water polype,

FIG. 57. — The fresh water Polype (Hydra) cut in half longitudinally and
greatly enlarged. (From Marshall and Hurst's " Practical Zoology.")

A, mouth ; B, hypostome ; C, digestive cavity ; D, ectoderm ; E, mesogloea ; F, endoderm ;
G, tentacle; H, testis ; I, ovum in ovary; K, bud ; L, foot.

Hydra (Fig. 57), so frequently found attached to aquatic plants
in ponds and ditches. Hydra is a member of the great group
Coelenterata, which includes the sea-firs, jelly-fish, sea -anemones
and corals, and which are distinguished by the fact that they
retain throughout life the fundamental features of the gastrula
(compare Fig. 13, IX, X). The body of a typical Coelenterate
animal consists essentially of a simple sac (Fig. 57) whose wall

HYDEA

115

is composed of two layers of cells. The cavity of the sac (C) is
the digestive or gastral cavity (enteron), and it has only a single
opening to the exterior, the mouth (A), usually surrounded by a
ring of tentacles (G). The wall of the sac is solid and there is
no body cavity or ccelom surrounding the digestive tube as in
higher animals (Ccelornata). The outer cell layer (D) of the body
wall is the ectoderm (epiblast of the embryo), the inner (F) is the

N,

FIG. 58. — A small portion of a thin longitudinal Section through the Body
Wall of Hydra viridi*, x 800. (From Marshall and Hurst's " Practical
Zoology.")

A, a large ectoderm cell ; B, its nucleus ; C, its muscle process ; D, an undischarged
thread cell ; E, its trigger process ; F, a thread cell with discharged thread ; G, inter-
stitial cells ; H, mesogloea ; I, endoderm cell ; K, vacuole ; L, nucleus of endoderm
cell ; M, green algal cells living in the endoderm cells ; N, flagellum of eudoderm cell.

endoderm (hypoblast of the embryo) and between the two is a
gelatinous, non-cellular supporting lamella, the mesoglcea (E).

Hydra itself is a very small form, but easily recognizable by
the naked eye. The body is long and slender or short and thick,
according to its state of contraction, and the same is true of the
tentacles, which may be visible as mere knobs around the mouth
at the unattached end of the animal, or as long slender threads
extended through the water like fishing lines and serving for the
capture of the minute organisms upon which the Hydra feeds.
The mouth is situated on the top of a conical projection,

i 2

116 OUTLINES OF EVOLUTIONARY BIOLOGY

the hypostome (Fig. 57, B), which lies within the circle of
tentacles.

The endoderm, which immediately lines the gastral cavity, is
made up of a single layer of relatively large cells (Fig. 58, I)
whose function is digestive. The ectoderm is made up of several
kinds of cells, some larger than others. The larger ones (A)
are much broader at their outer than at their inner ends
and the interstices thus left between the latter are filled up
by small interstitial cells (G). The endoderm cells and the
larger ectoderm cells both send out prolongations of their bodies
into the gelatinous mesogloea (H) which lies between them, and
these processes, having the form of elongated fibres (C), are the
seat of that power of contraction which Hydra possesses in such
a high degree — they are in fact muscular.

Hydra has two very distinct methods of reproduction, asexual
and sexual respectively. The former consists in a process of
budding, little hollow outgrowths of the body being formed,
which elongate, acquire mouth and tentacles, and for a time
remain attached to the parent (Fig. 57, K). In this way tem-
porary colonies of polypes may be produced, but sooner or later
the buds separate and begin to lead independent lives.

Sexual reproduction is effected by means of ova and sperma-
tozoa, which are essentially similar to those of higher animals.
They are produced in gonads — ovaries and testes — and as
both kinds of gonad usually occur in the same individual the
animal is hermaphrodite. The testes (Fig. 57, H) take the form
of little swellings situated at a short distance beneath the ring
of tentacles and formed each by an accumulation of interstitial
ectoderm cells. These are the primordial germ cells, by the
division of which the spermatozoa are formed. The spermatozoon
is perfectly typical, resembling a flagellate protozoon, with an
ovoid head consisting almost entirely of chromatin and a long
cytoplasmic tail or flagellum by means of which it swims
actively about when shed into the surrounding water by rupture
of the testis.

There is usually only a single ovary, appearing as a larger
projection from the body wall nearer to the attached end of
the animal ; it consists at first, like the testes, of a heap of
primordial germ cells formed by the multiplication of interstitial
cells. In each ovary, however, only a single cell develops into a
mature ovum. (Fig. 57, I), its sister cells, which may all be

HYDRA

117

regarded as potential ova, being sacrificed for the benefit of the
one ; in fact they are simply devoured by the voracious egg

FIG. 59. — Development of Hydra. (From Bourne's " Comparative Anatomy,"
partly after Brauer.)

A, the mature ovum, full of yolk granules and still attached to the body wall of the
parent ; B, section of blastula or blastosphere produced by segmentation of the ovum ;
C, the embryo becoming solid by migration into the blastocoel of cells cut off from
the wall of the blastula to form the hypoblast ; D, solid embryo composed of epiblast
and hypoblast and enclosed in a protective shell ; E, embryo flattening itself out
within the shell ; F, embryo emerging from the shell and with the gastral cavity
appearing in the endoderm (hypoblast) ; G, empty shell after the escape of the
embryo.

blc, blastocoel ; ec, ectoderm (epiblast) ; en, endoderm (hypoblast) ; i, solid mass of hypo-
blast cells ; mg, mesogloea ; sh, shell ; she, outer layer of shell ; sJii, inner layer of shell.

cell, which puts forth pseudopodia and feeds upon them like a
hungry Arnreba. In this way the ovum attains a relatively large
size and its cytoplasm becomes loaded with yolk corpuscles

which will serve later on
embryo.

Some of the surrounding cells of the

for the nutrition of the developing

M-
':,hyth.

A.

FlG. 60. — OMia geniculata.

A, part of hydroid colony; B,
free - swimming medusa ;
both x 13. (From photo-
graphs.)

bst., blastostyle; gon., gonad;
gth., gonotheca ; hyc., hy-
drocaulus ; kyd., hydranth ;
hyth., hydrotheca; mn.,
mannbrium ; p.s., perisarc ;
r.c., radial canal ; ten., ten-
tacles.

ectoderm at first
form a covering
or envelope for the
growing ovum,
but this is pre-
sently ruptured
and the mature
egg is exposed on
the surface of the
body of the parent
Hydra. There it

is found by a spermatozoon, which is
attracted towards it and by its own
activity bores its wajr into the ovum.
This act of fertilization is concluded in
the usual manner by the fusion of the
nucleus of the spermatozoon (male pro-
nucleus) with that of the ovum (female
pronucleus) to form the zygote nucleus.
The fertilized ovum or zygote (Fig.
59, A) undergoes segmentation while
still remaining attached to the parent
Hydra. In this way a single-layered,
hollow blastula (B) is formed, which
becomes converted into a two-layered
embryo by migration of cells into the in-
terior to form the at first solid hypoblast
or endoderm (0, D, ?'). The epiblast
or ectoderm cells (D, ec) now secrete a
thick horny protective envelope (sh,
shi) around the embryo, which falls
off from the parent and undergoes a
period of rest at the bottom of the

pond. After a time the interrupted
development is resumed, the horny envelope is ruptured, and
the embryo escapes (F). The gastral cavity appears in the
midst of the endoderm cells, the mouth is formed by perforation
at one end, and the tentacles bud out. With the formation of
the mouth the gastrula stage is reached, but it will be noted that

OBELIA 119

this condition is arrived at by a somewhat different route from
that which leads to the corresponding stage in Amphioxus (com-
pared Fig. 13, I— X).

Closely related to Hydra are a large number of marine Coelen-
terates, which, from their obviously animal nature combined
with their plant-like mode of growth, were known to the older
naturalists as zoophytes. One of the most familiar examples of
these is Obelia (Fig. 60), which is frequently found attached to
rocks or seaweeds near low water mark.

Obelia differs from Hydra in several interesting particulars.
In the first place the asexual process of multiplication by means
of budding takes place in a very regular manner, and the buds,
instead of separating from the parent, remain connected together
to form permanent colonies (Fig. 60, A) in which the constituent
individuals or persons (sometimes called zooids) are arranged
in a perfectly definite way. The colony is comparable to an
arborescent colony of Protozoa such as Zoothamnium or Epi-
stylis (compare Fig. 9, n— is), but the individuals of which it
is composed are units of a higher order than single cells.

In the second place the colony develops a common skeleton,
secreted by the ectoderm, which takes the form of a slender
tube of horny perisarc (ps.) enclosing all the branches and
expanding at the end of each into a little cup or hydrotheca (Itytli.),
occupied by a single zooid. Lastly the colony is polymorphic,
the zooids exhibiting a certain amount of differentiation and
division of labour amongst themselves.

A network of root-like branches at the base of the colony
creeps over the substratum and serves for attachment. From
this network, which is not shown in the illustration, arises a
little forest of vertical stems, each of which has a characteristic
zig-zag outline (Fig. 60, A). From each angle of the stem a
short branch is given off which terminates in a single hydra-like
zooid known as a hydranth (hyd.), enclosed in one of the horny
cups or hydrothecae, from the mouth of which its tentacles are
extended into the water.

The structure of the hydranth is similar in all essential
respects to that of Hydra. ' In the middle of the ring of tentacles
is the mouth, situated on a projecting hypostome and leading
into the digestive cavity, and the different hydranths of the
colony are all placed in communication with one another by the
tubular hydrocaulus, or common stalk (hyc.), enclosed in the horny

120 OUTLINES OF EVOLUTIONARY BIOLOGY

perisarc. These liydranths are the nutritive individuals of the
colony, whose function it is to capture and digest the prey.

Very often we find, in the angle between a hydranth-bearing
branch and the main stem, a short branch bearing an individual
or zooid of a different kind, enclosed in a horny cup of totally
different shape. These individuals are somewhat club-shaped ;
they have no mouth and no tentacles, and they are termed
blastostyles (Fig. 60, A, bst.). The horny cup in which each is
enclosed is urn-shaped, with constricted mouth, and is distin-
guished by the name gonotheca (gth.). The function of the
blastostyle is exclusively reproductive and it is entirely dependent
for its nutrition upon digested food received from the hydranths
through the hydrocaulus. It reproduces by budding, and the
buds are often found attached to it in large numbers, within the
sheltering gonotheca, as shown in the figure.

The colony itself increases in size by the formation of new
buds in regular succession, alternately on the right and left sides,
beneath what is, for the time being, the topmost hydranth of each
main stem, each new bud giving rise to a hydranth-bearing
branch which overtops its immediate predecessor. The buds
formed on the blastostyle. on the other hand, do not develop into
hydranths at all, but into another kind of individual known as
a medusa, or medusoid person, which presently detaches itself
from the parent and escapes through the mouth of the gonotheca
as a free-swimming individual.

The medusa of Obelia (Fig. 60, B). though larger than the
hydranths, is still very small, not more than about i^th of an
inch in diameter. At first sight it looks very different from
a hydroid individual (hydranth), but it can easily be shown to
have the same fundamental plan of structure. It consists of a
circular disk surrounded by a fringe of tentacles (ten.). From the
middle of "one surface projects a handle-shaped structure, the
manubrium (win.), corresponding to the hypostome of the hydranth
and bearing the mouth at its extremity. This mouth leads into
a central digestive cavity from which four radial canals (r.c.) run
outwards through the gelatinous ground-substance to a circular
marginal canal. The medusa swims actively about by muscular
contractions of the disk, and in doing so it assumes a bell-like
shape with the manubrium projecting from the convex surface,
like the handle of the bell.

Most medusae— the larger of which are familiar to us as

ALTERNATION OF GENERATIONS 121

" jelly-fish " — are more definitely bell-shaped, but the manu-
brium lies in the hollow of the bell, like its clapper; in comparison
with these the Obelia medusa may be said to turn itself inside out
in the act of swimming. In accordance with their active life the
medusae have a fairly well developed nervous system and a number
of sense organs around the margin of the bell. The point with
which we are immediately concerned, however, is their mode of
reproduction.

Unlike the hydranths and blastostyles of which the Obelia
colony is composed the medusae are sexual individuals, bearing
sexual organs or gonads from which the gametes are set free.
The gonads (Fig. 60, B, gon.) are masses of germ cells lying one
beneath each radial canal, on the same side of the disk as the
manubrium. The sexes are distinct, the medusae being either
male or female, and the gonads accordingly producing either
spermatozoa or ova, which, when mature, are discharged into the
water by rupture of the gonad.

The ova are fertilized by the spermatozoa and undergo
segmentation much as in the case of Hydra, developing
ultimately into a hydroid individual (hydranth), which by
budding gives rise to a new Obelia colony.

It is obvious that we have here a case of alternation of sexual
and asexual generations (metagenesis), comparable to that which
occurs in the fern, except that the sexual generation arises from a
multicellular bud and not from a unicellular spore. The sexual
medusoid individual is produced by budding from the asexual
hydroid, and itself gives rise, through the conjugation of gametes
and the development of the zygote thus produced, to the asexual
hydroid again. The process is somewhat complicated by the fact
that the hydroid also produces other asexual individuals by bud-
ding, but this really amounts to little more than does the forma-
tion of numerous branches by the budding of one of the higher
plants, for in the latter case also each bud may be regarded as,
potentially at any rate, a perfect individual.

We may draw an even closer comparison between the alterna-
tion of hydroid and medusoid generations in the Ccelenterata and
that of sporophyte and gametophyte in the higher plants, for in the
former, as in the latter, we see in many instances a more or less
strongly developed tendency towards the suppression of the sexual
generation (sometimes called the gamobium in animals).

In Tubularia, for example,' the medusoid individual never

122 OUTLINES OF EVOLUTIONARY BIOLOGY

separates from the parent colony, but remains attached to it as a
bud, though still showing clear evidence, in its bell-like shape and
in the presence of the manubrium and of radial and circular
canals, of its medusoid nature. It is in fact merely a degenerate

ten.

FIG. 61. — A single Hydranth of Tubularia, with medusoid Individuals
(Gonophores) budded out between the two Circles of Tentacles ; highly
magnified. (After Alltnan.)

act., actinula larva enclosed in gonophore ; act. ', actinula larva escaping; ent., eiiteron
or digestive cavity ; gon., gonophore ; m., mouth ; per., perisarc ; ten., tentacles.

medusa or gonophore (Fig. 61, gon.). It still retains its sexual
function, producing either ova or spermatozoa, and in this case
the fertilized ova actually develop into young hydroids or
" actinula larvae " (Fig. 61, act., act.') before escaping from the
parent gonophore. We have here a telescoping of the successive
generations quite comparable to what we find in a flowering

SUPPRESSION OF MEDUSOID 123

plant, and again it is the sexual generation which has undergone
reduction.

In other cases the medusoid bud or gonophore more or less
completely loses its medusa-like structure and becomes a mere
sac containing the gonad, and it has been suggested that in
Hydra we may have the last stage in this progressive reduction
of the sexual generation, the medusoid having altogether dis-
appeared, after having transferred its sexual functions to the
hydroid.

It seems almost incredible that the germ cells themselves
which are originally produced by one generation should in this
manner be transferred to what is really the preceding generation,
but as a matter of fact we see all stages in this process in different
genera of Hydrozoa. Even in Obelia and certain other medusa
the germ cells do not originate in the gonads beneath the radial
canals but in the ectoderm of the manubrium, reaching their
final position by a process of migration. In Eudendrium, where
the medusoid is reduced to a mere vestige, the germ cells
no longer originate in the sexual individual at all, but in the main
stem of the hydroid colony, whence they migrate into the
medusoid bud. If we imagine them ceasing to migrate from their
place of origin in the hydroid, while the medusoid is no longer
formed, we reach the condition of Hydra, with a complete trans-
ference of the sexual function to what should be the asexual
generation.

In the coelenterates, however, it is not always the asexual
(hydroid) generation which predominates, for in some cases this
is suppressed more or less completely and the medusa or jelly-
fish reproduces its own kind directly without the intervention of
a hydroid phase.

In coelenterate animals such as Hydra and Obelia, as we
have already seen, the body consists of a single hollow tube
whose wall is made up of only two cell layers, the ectoderm
on the surface and the endoderm lining the digestive cavity, with
a supporting layer of gelatinous consistency — the mesogloaa
—between the two. In the case of the larger jelly-fish
this inesogloea attains a great thickness ; it is, however,
never a true cell layer like the ectoderm and endoderm, but
rather of the nature of intercellular substance secreted by the
cells on either side of it.

In Hydra and Obelia the germ cells arise from the ectoderm,

and the gonads (Fig. 62, A, yon.} are accordingly situated close
to the surface of the body and no special ducts are required for
the conveyance of ova or spermatozoa to the exterior.

In the great group Coelomata, which includes practically all
animals higher in the scale of organization than the coelenterates,
we find a different state of affairs. The mesoglosa has been
replaced by a true cellular mesoderm (Fig. 62, B, mes.), formed of
cells derived from ectoderm or endoderm, or from both, and in
the thickness of this layer a cavity is developed, the coelom or

eruL

gon.

A.

FIG. 62. — Comparison of Ccolenterate and Invertebrate Ccclornate Types of

Structure.

A. Diagram of a transverse section of a coelenterate animal.

B. Diagram of a transverse section of an invertebrate coelomate animal.

ccel. coelom or body cavity; ccel. ep. ccelomic epithelium lining body cavity; d.v. dorsal
blood-vessel ; ect. ectoderm ; end. endoderm ; ent. enteron or digestive cavity ;
g. a. genital aperture; g.d. genital duct (gonoduct) ; gon. gonad ; mes. mesoderm;
mesgl. mesoglcea ; n.c. double nerve cord ; som. somatopleure or body wall ; spl.
splanchnopleure or gut wall ; v.v. ventral blood-vessel.

body cavity (coeL), which more or less completely surrounds the
digestive tube or alimentary canal (compare the development of
Amphioxus, Fig. 13, XI — XIII) . The outer layer of the mesoderm
unites with the ectoderm to form the body wall or somatopleure
(som.), while the inner layer unites with the endoderm to form the
gut wall or splanchnopleure (spl.), and the body thus acquires the
form of a double tube. The body cavity is lined by a layer of
epithelial cells (ccel. ep.), and it is from this coelomic epithelium
that the germ cells arise. The gonads (Fig. 62, B, gon.) therefore
project into the body cavity and into this cavity the mature germ
cells are primarily discharged. In the great majority of cases

SECONDARY SEXUAL CHARACTERS 125

special genital ducts or gonoducts (g.d.) are developed which
pierce the body wall and serve for the passage of the germ cells
to the exterior. The sexes are usually distinct in the higher
forms (Vertebrata), but many of the invertebrate coelomates
(>v/., the earthworm) are hermaphrodite, the same individual
bearing both male and female gonads (testes and ovaries) with
the corresponding gonoducts (vasa deferentia and oviducts).

Fertilization of the ova by the spermatozoa, or in other words
conjugation of the gametes, may take place either within the
body of the parent, as in most terrestrial forms, or externally, as
in a very large proportion of aquatic animals. In the former case
special organs are developed for the transference of the sperma-
tozoa from one individual to another, and such transference
usually occurs even in hermaphrodite forms, which, as a rule, are
incapable of self-fertilization. Further modifications may arise
in connection with the nutrition of the embryo, which may
remain within the body of the parent — in an enlarged portion of
the oviduct known as the uterus — until it has reached an
advanced stage of development. This takes place more par-
ticularly in the females of the higher vertebrates.

In connection with the sexual differentiation, more
especially in the higher animals, numerous secondary sexual
characters may arise which are not directly connected with the
organs of reproduction. Such are the various ornamental out-
growths of hair, feathers and so forth, which distinguish the males
of many vertebrates and are supposed to appeal to the aesthetic
sense and thus to contribute towards the mutual attraction
between male and female, and the special weapons, such as
antlers and spurs, which male animals frequently develop and
which are used in combat for the possession of the females.

Although not directly connected with the gonads these
secondary sexual characters seem to depend for their development
in some curious way upon the presence of these organs. Thus
it is well known that if the testes be removed by castration the
secondary sexual characters will not, in most cases at any rate,
develop properly. We see an excellent illustration of this in the
case of the antlers of the stag, which are confined to the male and
do not develop at all if the animal be castrated in early youth,
while if the operation be performed after the antlers are fully
developed these are prematurely cast off and replaced by
imperfect ones.

126 OUTLINES OF EVOLUTIONARY BIOLOGY

How this intimate correlation between gonads and secondary
sexual characters is brought about is still uncertain, but there is
strong reason to suppose that it is due to the secretion by the
gonad of some specific substance (hormone), perhaps of the
nature of a ferment, which circulates throughout the body-
chiefly no doubt in the blood — and controls the development
of the characters in question. That internal secretions may act
in this way upon organs remote from their own place of origin is
well known and is strikingly exemplified in the case of the
corpora lutea of the mammalian ovary. These structures appear
on the surface of the ovary in the places whence ova have been
discharged, and are apparently of a glandular nature. As the
fertilized ova pass down the oviduct they begin to develop, and
on reaching the uterus fix themselves to the wall of the latter, in
which they become imbedded. The spot where fixation takes place
is far distant from the ovary, but it has been demonstrated that
if the corpora lutea on the surface of the latter be destroyed the
embryos will not become fixed at all. There is a close correlation,
then, between the presence of the corpus luteum and the fixation of
the embryo, and this is explained by supposing that the corpus
luteum secretes some substance which, circulating in the blood,
reaches the uterus and stimulates its epithelial lining to respond
to contact with the embryo. Moreover, the reaction, whatever its
cause, appears to be mutual, for if the discharged ovum does not
get fertilized the corpus luteum does not attain its full develop-
ment and soon disappears.

We have now very briefly traced the evolution of sexual
characters from their starting point in the male and female
gametes of the Protista to their culmination in the higher plants
and animals. We have seen how sexual differentiation, which
primarily concerns the gametes themselves, is gradually extended
to the colonies or to the multicellular individuals from which the
gametes arise, or even to a preceding, originally non-sexual
generation. The various structural modifications thus brought
about are all directed towards one end, the conjugation of the
gametes. The mutual attraction which undoubtedly exists
between the gametes themselves is not sufficient, at any rate in
the case of the more highly developed and complex organisms,
where the distances between their places of origin are relatively
very great, to secure their union.

In the flowering plants their own efforts are supplemented by

THE EVOLUTION OF SEX 127

all the elaborate devices for securing pollination, and by far the
most active part in the process is played by external agencies,
especially by those insects which have become the vicarious
fertilizers of the flowers.

In the -higher animals, on the other hand, the necessity for
bringing the gametes into close proximity with one another has
led to the development of all those secondary sexual characters, both
bodily and mental, which play so conspicuous a part in the drama
of life. Throughout the whole course of this remarkable process
of evolution, except in the case of certain obviously degenerate
forms, we observe that same fundamental distinction between the
sexes which we first noticed in the gametes of unicellular
organisms, and which in the higher animals is extended with
the sexual differentiation itself from the gametes to the complex
multicellular body which bears them. The female is the more
passive partner and is especially concerned with the nutrition
and rearing of the offspring, and her bodily organization is
especially adapted to her maternal functions. These functions
constitute an inevitable handicap in the struggle for existence,
and the females and young of the higher animals are in most
cases largely dependent upon the less burdened and consequently
more active and vigorous males for their protection.

The explanation of this progressive sexual differentiation is
undoubtedly to be found in the advantages to be derived from
division of labour and the accompanying possibilities of special-
ization. The origin of conjugation itself, upon which all sexual
phenomena are based, is another, and more fundamental, question.
At first, as we have seen, the conjugating gametes were apparently
exactly alike one another and exhibited no visible sexual differen-
tiation at all. The habit of conjugation probably arose from
the necessity of making good some disturbance of equilibrium in
the protoplasm of the cell. It has been supposed that, as the result
of repeated fission, some condition of inequality was gradually
set up amongst the daughter cells, whereby some of them came
to have too much of one constituent and too little of another,
while others were in the opposite condition. In this way the
successive unicellular generations gradually became more and
more enfeebled — as we saw in the case of Paramoscium — and, owing
perhaps to some sort of polarization, those which had become
modified in opposite directions came to exercise an attraction
upon one another which resulted in conjugation and restoration

128 OUTLINES OF EVOLUTIONARY BIOLOGY

of the proper equilibrium. It may be that from the very first
the inequalities of fission resulted in the accumulation of more
active protoplasm in some cells and a greater amount of reserve
material in others, and that this was the starting point of the
differentiation into male and female.

Although fusion of the nuclei (karyogamy) of the two
gametes appears now to be the most important feature of con-
jugation, we must suppose that it was preceded by plastogamy1 or
fusion of the cytoplasm, which obviously constitutes the natural
preliminary to karyogamy. In some of the lower Protozoa such
plastogamy has been sometimes observed unaccompanied by
karyogamy, and it is possible that in some cases plastogamy
alone is sufficient to bring about rejuvenescence and renewed
activity in cell-division.

1 Sometimes called plasmogamy.

CHAPTER X

Origin of the germ cells in multicellular animals — Maturation of the germ
cells — Reduction of the chromosomes — Sex determination in insects —
Different forms of gametes — Mutual attraction of the gametes —
Fertilization and parthenogenesis.

IN many multicellular animals the distinction between somatic
cells and germ cells becomes manifest at a very early stage in the
development of the individual. An extreme instance of this is
seen in the parasitic round-worm of the horse, Ascaris mcgalo-
cephala. Here the distinction in question precedes all other
histological differentiation. The two cells or blastomeres into
which the fertilized ovum first divides (Fig. 85, C, D) are originally
similar to one another, but as they prepare for the next mitotic
division of the nucleus a remarkable difference is, according to
the observations of Professor Boveri, established between them.
Both at first (in the case of the variety known as iinivalens)
exhibit two elongated chromosomes (the variety bivalens, which is
represented in Fig. 35, having four), but in one the thickened
ends of the two chromosomes are thrown off into the surrounding
cytoplasm, where they degenerate, while the more slender middle
portions break up into a number of short pieces. Thus two
differentiated cells are produced, one with two large chromosomes
and the other with numerous small ones. The latter gives rise
by its subsequent divisions to somatic cells only. The former is
a primordial germ cell ; for some five or six times it will divide
like its parent cell into a somatic cell and a primordial germ cell,
but after these early divisions the primordial germ cells will
give rise to their own kind only, until the time comes for the
production of the actual gametes. The somatic cells, on the
other hand, will gradually become differentiated into all the
various tissue cells of the adult.

Perhaps the most significant part of this remarkable process
as observed in Ascaris is the elimination of chromatin material
from the nuclei of the somatic cells when these are first

B. K

130 OUTLINES OF EVOLUTIONARY BIOLOGY

differentiated from the germ cells. The result of this is that the
germ cells alone retain the full complement of chromatin
derived from the parents, and their nuclei are accordingly
actually much larger than those of the somatic cells.

The differentiation into somatic cells and germ cells cannot
usually he traced so far back in the development of the individual
as in Ascaris, but in a great many animals the distinction can
be recognized at a very early stage. In certain insects, for
example, the primordial germ cells can be traced back to a large
" pole-cell " which lies at one end of the segmenting ovum, and

in the arrow-worm, Sagitta,
they can be identified at the
gastrula stage (Fig. 63, p.//.r.).
As we shall see later on, this
early segregation of the germ
cells is of very great interest
from the point of view of the
theory of heredity. It can
hardly be said, however, at
any rate in the present state
of our knowledge, to be a
phenomenon of universal or
even general occurrence, and
in the majority of creloinate
animals the germ cells are first
recognizable in the ccelomic
epithelium at a compara-
tively late stage of develop-
ment (Fig. 62, B). In plants also, in cases where there is
a well developed gametophyte this appears to attain its full
development before the germ cells are recognizable, and the
entire life of the sporophyte is passed without any distinction
between somatic and germ cells manifesting itself. Moreover,
the fact that the ordinary cells of a fern prothallus can, on
occasion, act as germ cells,1 prevents us from admitting any
absolute distinction between the two categories.

The primordial germ cells may undergo extensive multiplication
by ordinary mitotie division before giving rise to the actual
gametes. In niulticellular animals the process of gametogenesis
(formation of gametes), which is either oogenesis or sperrnato-

1 Vide, p. 105.

FIG. 63. — Section of the Gastrula of an
Arrow Worm (Sagitta) showing the
primordial Germ Cells. (After O.
Hertwig.)

bp., blastopore ; ent., enteron ; ep., epiblast ;
hyp., hypoblast; p.g.c., primordial germ
cells.

MALE AND FEMALE PRONUCLEI

131

genesis according to whether ova or spermatozoa are produced
thereby, is accompanied by nuclear phenomena of very great
interest, whereby the maturation of the germ cells is effected.
Before describing this process we must lay stress upon certain
preliminary considerations.

As we have already seen, each kind of animal or plant is
characterized by the appearance of a definite number of chromo-
somes in the nuclei of its cells at the time when these are under-
going division by mitosis. Although not absolutely constant in
all cases the number is usually the same for all the different
somatic cells of which the body
is composed. It is usually an
even number, and (with certain
exceptions) it remains the same
in successive generations of
individuals.

It will also be remembered
that the zygote or fertilized
egg from which the individual
develops is formed by the con-
jugation of two gametes, ovum
and spermatozoon, and that in
this process the nuclei of the
gametes, sometimes called the
male and female pronuclei,
unite, or at any rate co-operate
as a single nucleus. Fig. 64 is
taken from an actual photograph of an egg of Ascaris in process
of fertilization ; the spermatozoon has already entered the
ovum and the male and female pronuclei (pro.) are seen lying
side by side in the cytoplasm. Each pronucleus brings with it
its own set of chromosomes, and hence the zygote nucleus has
double the number of chromosomes possessed by either of the
gametes. Thus it appears at first sight that every conjugation
or sexual union of gametes (zygosis) must be accompanied
by a doubling of the number of chromosomes, and we might
therefore expect to find each successive generation with twice as
many chromosomes in its nuclei as the preceding one. That
this is not actually so depends (in the case of animals) upon the
fact that the nuclei of the gametes contain only half the number
of chromosomes characteristic of the somatic cells ; if the

K 2

pro

FIG. 04. — Ovum of the Horse Worm
(Ascaris megalocephala) during
the Process of Fertilization,
showing the inale and female
Pronuclei (pro), X 770. (From
a photograph.)

13-2 OUTLINES OF EVOLUTIONARY BIOLOGY

somatic cells have eight the mature ovum or spermatozoon will
have only four, and so on. This reduction of the number of
chromosomes (meiosis) is the essential part of the process of
maturation which the animal germ cells undergo, and it is

Spermalogenesis
1

Spermatogonia ^P

Oogenesis

Multiplication of Epithelial
Cells ( Spermatogonia and
Oogonia) in the Gonad
(Test is or Ovary) by ordinary
Somatic Mitosis

? Oogonia

Primary /J^

Spermatocytes >r

Secondary
Sperm

Spermatozoa

Synaps/s or Pairing xs!
of Che Chromosomes v/

Reducing Division
f Meiosis )

. Primary

Male Gametes

Polar
Bodies i

- Conjugation of '
'

0 ©

L Secondary Oocytes
4 / and Polar Bodies

l Mature Ova
Polar todies

Mature Ova

Female Gametes

C&J Zygote or Fertilised Ovum

TT

Segmentation of
the Zygote by
Ordinary Somatic
Mitosis

(The Figures indicate the numbers
of Chromosomes present)

FIG. 65. — Diagram of Oametogenesis.

effected by a modification of the mitotic nuclear division, in
which the chromosomes are separated into two groups, half the
total number of entire chromosomes going into one daughter
cell and half into the other.

In a typical case of spermatogenesis the first stage is the

GAMETOGENESIS 133

multiplication of cells (the so-called spermatogonia) derived from
the germinal epithelium, in the testis, by ordinary cell-division.
Let us suppose the number of chromosomes found in the nuclei
of the somatic cells to be eight ; it will, of course, remain the
same so long as the character of the mitosis undergoes no
change, each chromosome splitting into two at every nuclear
division. Presently, however, we find the chromosomes arranged
in pairs instead of all appearing separately in the mitotic figure,
and the cells, which have increased considerably in size by the
absorption of nutriment, may now be termed primary spermato-
cytes. This pairing of the chromosomes (synapsis) marks the
onset of the "reducing division " ; a nuclear spindle is formed, the
paired chromosomes arrange themselves upon it, and the two
members of each pair separate and travel towards opposite poles.
Thus two new nuclei are formed each with only four chromosomes.
The reduction is now complete and the new generation of cells,
with reduced nuclei, may be termed secondary spermatocytes.
One more mitotic division takes place, this time involving the
splitting of each chromosome, so that there is no further reduc-
tion in their number, and giving rise to the minute spermatids,
each of which develops a long, vibratile, cytoplasmic tail and
forms a spermatozoon. Hence we see that each primary
spermatocyte gives rise to four spermatozoa, with reduced
nuclei containing half the number of chromosomes found in the
somatic cells. The essential features of the whole process are
represented diagrammatically in Fig. 65.

The process of oogenesis takes place in essentially the same
manner ; the so-called oogonia,1 derived from the germinal
epithelium of the ovary, multiply and give rise to oocytes.
Synapsis and reduction in the number of the chromosomes take
place as they do in spermatogenesis, but, owing doubtless to the
fact that it takes a comparatively large amount of cytoplasm to
form the body of an egg, we find that only one perfect ovum
arises from each primary oocyte, the other three forming the
" polar bodies." Owing to their minute size as compared with
the ovum itself, two of the polar bodies (Fig. 35, A, p.b.) appear
to be cast out of the latter as it undergoes maturation, while
the third is formed by division of the first ; it will be clear,
however, from a careful study of Figs. 65 and 68 that the

1 This term is very unfortunately chosen and must not, of course, be confounded
with the same term as applied to the female organs of such plants as Fucus.

134 OUTLINES OF EVOLUTIONARY BIOLOGY

FIG. 66. — Some Stages in the Spermatogenesis of a Grasshopper (Stenolothrus
virididm}. (After Meek.)

1. A secondary spermatogonium in mitosis, with seventeen chromosomes (eight pairs and

one accessory). Polar view.

2. Resting or growth stage following the mitosis of the secondary spermatogonium. The

ordinary chromosomes have become resolved into a network but the accessory
chromosome retains its individuality.

3. Primary spermatocyte formed by growth of 2, preparing for mitosis.

4. Mitosis of the primary spermatocyte, showing the pairing (synapsis) of the sixteen

ordinary chromosomes on the nuclear spindle and the accessory chromosome
unpaired. Side view.

5. Later stage in the mitosis of the primary spermatocyte, showing separation of the

paired chromosomes. Side view.

6. Still later stage in the division of the primary spermatocyte into two secondary sper-

matocytes ; the chromosomes massing at the two poles of the spindle (one mass only
will contain the accessory chromosome). Side view.

7. A secondary spermatocyte in mitosis, showing the reduced number of ordinary chromo-

somes (8) and the accessory chromosome. (The sister-cell would of course contain no
accessory chromosome). Polar view.

8. Later stage in the mitosis of the secondary spermatocyte ; each chromosome (including

the accessory one, which lags behind the others) has split into two parts and the two
groups are separating. Side view.

9. A spermatid, formed by division of a secondary spermatocyte with an accessory

chromosome. It will form a single spermatozoon.

x The accessory chromosome.

SPERMATOGENESIS IN INSECTS 135

process is merely one of repeated cell-division as in the case of
spermatogenesis. The three polar bodies consist almost entirely
of chromatin and each of course contains the same reduced
number of chromosomes as the ovum itself; they undergo no
further development, however, and finally disappear. The truth
of the view, now generally held, that the polar bodies are
merely ova which have not sufficient cytoplasm to allow of their
development, is demonstrated by the fact that in one of the
turbellarian flat-worms, according to Francotte, the exceptionally
large first polar body may occasionally be fertilized and actually
develop as far as the gastrula stage.

The details of the process of gametogenesis vary very much
in different cases, but the above outline may be regarded as
generally applicable.

In a large number of insects it has been found that the male
animal possesses an odd number of chromosomes in the somatic
nuclei, due to the presence of a single "accessory" chromosome
or " monosome," while the female possesses an even number
(one more than the male owing to the presence of two
" accessory " chromosomes). This leads to a curious complica-
tion in the process of spermatogenesis. The accessory chromo-
some (Fig. 66, X) can often be distinguished by its appearance
from the others, and at the time of synapsis (Fig. 66, 4) it has
no mate. Hence in the reducing division the chromosomes are
separated into two unequal groups, one of which contains the
accessory chromosome while the other does not (Fig. 66, 5). Two
kinds of spermatozoa are accordingly produced in equal numbers,
one kind with an odd number of chromosomes and the other with
an even number.

The matured ova, on the other hand, all have the same number
of cbromosomes, because the accessory chromosome has a synaptic
mate. Fertilization of an ovum by a spermatozoon containing
an accessory chromosome results in the production of a female
animal with an even number of chromosomes in its somatic cells ;
fertilization by a spermatozoon which has no accessory chromo-
some results in the production of a male animal with an odd
number of chromosomes in its somatic cells, as shown in Fig. 67.

Thus it appears that the chromosomes, at any rate in some
cases, have a very important influence on the determination of
sex, and that the latter is not, as has often been supposed, merely
the result of nutritional and other environmental influences upon

136 OUTLINES OF EVOLUTIONARY BIOLOGY

the developing organism, but a character of far more deeply
seated origin.

It must not be forgotten that the chromosomes, as such, are
only recognizable during the process of mitosis ; in the resting
condition of the nucleus they appear to be broken up into larger
or smaller granules of chromatin scattered through the linin
reticulum. The observations on accessory chromosomes above
mentioned, and others to be referred to presently, point to the

6 (Unreduced Cells) $( Unreduced Cells)

FIG. 67. — Diagram illustrating the Correlation between the Number of
Chromosomes and the Sex in certain Insects. (The numbers of chromo-
somes given in this diagram are arbitrarily chosen and are obviously
different from those which occur in Stenobothius, as shown in Fig. 66.)

conclusion, however, that, in spite of this, the different chromo-
somes preserve some sort of individuality from one cell-genera-
tion to another. In other words, we have reason to believe that
the chromosomes which make their appearance at the onset of
each mitosis are, taken each as a whole, the same as those which
become disintegrated at the close of the preceding mitosis, though
it is very possible that the constituent parts of the old chromo-
somes (chromatin granules, chromomeres or ids), after absorbing
nutriment and increasing in size during the resting period, may
come together in new combinations to form the new chromosomes
each time division of the nucleus takes place.1

1 Vide Farmer, Croonian Lecture, Proc. Koyal Soc., Scr. B, Vol. 79, 1907.

PAIRING OF CHROMOSOMES 137

It is also highly probable, though by no means certain, that the
chromosomes which are derived from the male parent remain
throughout life distinct from those which are contributed by the
female parent. According to this view every ordinary somatic
cell has two sets of chromosomes, paternal and maternal respec-
tively, and this again is strongly supported by the observations
on the germ cells of insects above referred to, where all the
chromosomes appear to be duplicated, with the exception of the
accessory chromosome in the male animal.

There is reason for believing, therefore, that in ordinary cases
every paternal chromosome in an unreduced nucleus has an equiva-
lent or " homologous " mate derived from the female parent, and
that the phenomenon of synapsis l represents a pairing of these
homologous paternal and maternal mates. The reducing division
which follows on synapsis consists in the separation of the mates
once more, one of each pair going to each daughter cell, so that
the matured germ cells are left with a single instead of a double
set of chromosomes.

If we assume that, as seems highly probable, the chromo-
somes of each paternal or maternal set are not all identical
but differentiated amongst themselves — a differentiation which
in some cases is actually visible, as shown in Fig. 66 — and
that one of each kind is necessary to make up the full com-
plement of the nucleus of the gamete, the importance of
the pairing of homologous chromosomes which takes place in
synapsis becomes at once evident, for one of each pair goes to
each daughter nucleus, which will therefore be certain to receive
a chromosome of each kind instead of a chance assemblage.
The chromosome of each kind which it receives, however, may be
either the paternal or the maternal representative of that kind,
and as these, though essentially homologous, may differ from one
another to some extent in accordance with individual peculiarities
of the parents from which they were derived, it will be seen that
the matured gametes may differ widely amongst themselves in
their nuclear constitution. This, as we shall see presently, is a
very important matter from the point of view of the theories
of heredity and variation.

This somewhat complex subject will be rendered more readily

1 The pairing of the chromosomes, for which we have used the term "synapsis,"
is spoken of by some writers as "syndesis," and by others as "conjugation." The
use of the latter term seems likely to lead to confusion.

138 OUTLINES OF EVOLUTIONAKY BIOLOGY

intelligible by a careful study of Fig. 68, which represents in a
very diagrammatic manner the formation of the polar bodies and
the distribution of the maternal and paternal chromosomes in the
maturation of a typical animal ovum.

Periodic reduction of the number of chromosomes is clearly a
necessary consequence of the sexual process, for a doubling of

FIG. 68. — Diagram of the Maturation of a typical Animal Ovum, showing
the Behaviour of the Maternal and Paternal Chromosomes and the
Formation of Polar Bodies. (The somatic number of chromosomes is
supposed to be four ; the maternal chromosomes are shaded and the
paternal not, and the differences between the two of each set are
indicated by their shapes.)

A, ordinary somatic mitosis in an oogonium, each chromosome split ; B, daughter
oogonium ; C, synapsis in primary oocyte ; D, reducing division, formation of first
polar body ; E, commencement of formation of second (or third) polar body by ordinary
mitosis and of division of the first ; F, mature ovum with three polar bodies.

chr., chromosomes; c.s., centrosome ; n.m., nuclear membrane; p.b. 1 — 3, polar bodies.

the number at every zygosis or conjugation of gametes could not
go on indefinitely without some such compensation. In animals,
as we have seen, the reduction takes place during the maturation
of the germ cells, but it is by no means necessarily associated with
this process. In the ferns, where alternating sexual and asexual
generations are represented by independent and well developed
organisms, the reduction takes place in the process of spore-
formation by the sporophyte. Hence the gametophyte (pro-
thallus) to which the spore gives rise has in all its somatic cells

POLAB BODIES IN PBOTOZOA

189

Head

m.p.

nuls.

the reduced number of chromosomes, while the sporophyte (fern
plant proper), owing to the fact that it develops from a fertilized
ovum and receives a set of chromosomes from each parent, has the
full number, or, perhaps we should say more correctly, the double
number.

Even amongst the Protozoa the phenomenon of nuclear reduc-
tion has been observed, and it probably occurs wherever the sexual

process takes place. Thus,
in the case of Copromonas,
the life history of which we
have already dealt with,
the nucleus of each of the
similar gametes, previous
to uniting with its mate to
form the zygote nucleus,
gives off two "polar bodies"
which undergo degenera-
tion in the cytoplasm (Fig.
37, 3, 4), and something very
similar may be observed in
the maturation of the ovum
of Coccidium. In the con-
jugating ParamcBcium,
again, the products of divi-
sion of the micronucleus
(Fig. 41, B, C) which undergo
no further development may
be regarded as polar bodies.
As we have already seen,
the mature gametes through-
out both the animal and
vegetable kingdoms usually
sexual di.norphism, which
into active

Tail

FIG. 69. — Diagram of typical Sperma-
tozoon and Ovum, the former much
more highly magnified than the
latter.

ax., axial filament; e.g., chromatiii granules;
cs., ceiitrosome ; cyt., cytoplasm ; m.p.,
middle piece; n.m., nuclear membrane;
mi,., nucleus; mils., nucleolus ; v.m.,
vitelline membrane; y-g., yolk granules.

exhibit a very strongly marked

attains its fullest expression in the differentiation

spermatozoon and passive ovum (Fig. 69).

A typical spermatozoon, as we have also pointed out, closely
resembles a flagellate protozoon. It consists of a " head " and
a " tail," connected together by a " middle piece " (m.p.). The
head contains the nucleus (int.), which almost entirely fills it,
being covered with only a very thin envelope of cytoplasm. The
middle piece contains a ceiitrosome (cs.). The tail, or flagellum, is

140 OUTLINES OF EVOLUTIONARY BIOLOGY

ch.

the active locomotor organ, by the movements of which the
spermatozoon swims about ; it contains an axial filament (ax.)

and may or may not be
provided with a lateral
undulating membrane.
A typical ovum is a
relatively large, spheri-
cal cell with a con-
spicuous nucleus (nu.)
surrounded by a large
quantity of cytoplasm
(cyt.), and the whole
enclosed in a delicate
vitelline membrane
(v.m.). The actual size

sh.m,.

a.c.

yh.

NU

FIG. 70. — Diagram of the Structure of a
Bird's Egg.

a.c., air chamber; alb., albumen; ch., chalazse, of the OVUm depends

twisted cords of dense albumen which serve to Ql1T,^af 0nfii-aliT ^ i flir,

keep the " yolk " in position ; g.d., germinal al11 6lJ (

disk; nu., nucleus; sh., shell; sh.m., shell amount of food material

membrane; v.m. .vitelline membrane ',yk., "yolk." ,

(deutoplasm or yolk),

which is stored up in the cytoplasm in the form of granules (y.g.).
In Amphioxus (Fig. 13, I) the amount of food material is very
small, and the egg is only about ^io^1
inch in diameter.

An extreme contrast to the egg of
Amphioxus is seen in that of a bird
(Fig. 70), where the amount of yolk
is enormously large and the active
protoplasm is confined to a minute
" germinal disk " (c/.d.), containing the
nucleus (nu.), which lies within the
vitelline membrane (v.m.) on the top
of the "yolk" (yk.), while the ovum
proper is entirely enclosed in accessory
structures — the " white " or albumen
(<dl>.) and the shell (sh.), with its lining
membrane (sh.m.).

The mammalian ovum, on the other
hand, is, like that of Amphioxus, very
minute, in the rabbit (Fig. 71) again only about ^^th inch in
diameter. It is enclosed in an envelope known as the zona
radiata (Z), which lies outside the vitelline membrane, and it

FIG. 71. — Ovum of a Babbit,
X 200. (From Marshall's
" Vertebrate Embry-
ology," after BischofP.)

MO, spermatozoa which have
penetrated the zona radiata ;
N, nucleus; NU, nucleolus;
Z, zona radiata.

ATTRACTION OF GAMETES 141

contains very little deutoplasm. This is correlated with the fact
that it develops within the body of the parent at the expense of
food material derived from the blood of the latter. There is
reason to believe, however, as we shall see later on, that the
small size of the mammalian egg is a secondary feature.

The plant egg-cell may also be loaded up with food material,
so as to attain a large size, as in the green alga, Chara,
where the contrast between the minute flagellate spermatozoon
and the relatively gigantic ovum, filled in this case with starch
grains, is very striking. In the higher plants, however, where,
as in the case of the Mammalia, the developing embryo is not
dependent for its nutrition upon food supplies stored in the egg-
cell, the latter remains quite small, as, for example, in the fern
(Fig. 52) and the flowering plant (Fig. 55, e).

According to some authorities one of the most important
differences between ovum and spermatozoon in animals lies in the
fact that the centrosome of the former disappears finally during
the process of maturation, the centrosome of the zygote being
contributed by the spermatozoon alone. In view of the fact,
however, that a definite centrosome is not usually recognizable at
all in the higher plants we cannot attribute very great importance
to its supposed absence in the animal ovum, and we shall also see
presently that centrosomes appear in developing eggs which have
not been fertilized by spermatozoa.

We have already had occasion to refer to the existence of
some attractive force whereby the male and female gametes
are brought together in conjugation. Many observers main-
tain that this is simply a case of positive chemotaxis, or
the chemical stimulation of the protoplasm of one gamete by
a specific secretion of the other in such a way as to cause
them to respond by approaching one another (or by the male
gamete approaching the female).

It is a well known fact that certain spermatozoa are attracted
by specific chemical substances. Thus the free-swimming sperma-
tozoa of ferns and mosses are attracted by weak solutions of
malic acid and cane sugar respectively, and those of Coccidium
are attracted by nuclear matter discharged from the ovum in the
process of maturation.

There can be no doubt that, whether the attracting substance
be secreted by the germ cells themselves or by some other part
of the organism, chemotaxis sometimes plays a very important

142 OUTLINES OF EVOLUTIONARY BIOLOGY

part in bringing the gametes together. So also, of course, do
many other factors in various plants and animals, but we must
distinguish between factors which act directly upon the gametes,
such as chemotaxis, and those which act indirectly through the
soma or body of the organism, as for example through the visual
and olfactory senses of the higher animals.

It is probable, however, that chemotaxis itself is but a secondary
factor which serves to bring the gametes within the range of one
another's direct influence. Thus in Coccidiurn (Fig. 89) the
cheniotactic action seems to be exhausted after a certain number
of spermatozoa have been attracted to the neighbourhood of the
ovum and a fresh attraction appears to be exerted by the ovum
itself or by its nucleus.

The term cytotropism, or cytotaxis, has been applied to the
attraction which, according to some observers, is sometimes
set up between two adjacent cells, and something of this kind
probably comes into play in the mutual attraction of gametes.
It can probably act only at very short distances, and hence the
necessity for some preliminary means of attraction such as
chemotaxis. That chemotaxis alone is not a sufficient explana-
tion of the phenomenon in question is suggested by the case of
Spirogyra. The conjugation of the gametes in this plant has
already been described in Chapter VIII. It will be remembered
that the process may take place between the cells of two filaments
lying close together, parallel with one another (Figs. 48 and
44), and is then inaugurated by those of the male filament.
Each of these cells which happens to lie opposite to a cell of the
female filament puts forth a hollow protuberance of its wall, which
is presently met by a similar protuberance from the wall of the
female cell, the two projections uniting to form a canal through
which the protoplasmic body of the male gamete creeps inside the
cell- wall of the female gamete to conjugate with the latter. It
sometimes happens, however, that, owing to inequalities in the
sizes of the cells, there may be a cell in one filament which lies
between two cells of the opposite filament and for which there is
no mate, all the adjacent cells being coupled. In such cases the
solitary cell (Figs. 48 and 44, S.C.), if it exhibits any of those re-
markable activities which are shown by the conjugating cells on
either side of it, merely makes preliminary advances which are
prematurely checked, as though there were a competition for
partners in which it was unsuccessful. Here it is obvious that

PARTHENOGENESIS 143

in the case of two cells lying opposite to one another, though not
in contact, and though each is enclosed in a firm cell-wall, some
stimulus is transmitted from one to the other which calls forth
a definite response manifested in the formation of the connecting
canals and the conversion of the protoplasmic contents of the
cells into gametes. The insufficiency of the principle of chemo-
taxis to account for these phenomena appears to he indicated
by the fact that cells which have no mates do not form either
complete connecting canals or gametes, though exposed equally
with their more fortunate neighbours to the influence of any
chemical substances dissolved in the surrounding water. The
only explanation appears to be that the solitary cell cannot
attract, or at any rate retain, the attention of a mate to stimulate
it to complete the process of conjugation.1

The cytotropic attraction of the gametes, as we have already
observed, probably depends upon some difference of polarity
between the two. That it is mutual is demonstrated by such
cases as that of Zygogonium (see p. 97) and by the fact that even
when the ovum is too heavily laden with food material to take any
active part in the process of conjugation it yet in many cases puts
out a definite "cone of attraction" towards the advancing sper-
matozoon, as seen in Coccidiurn (Fig. 39). What the real nature
of this primary attraction between the gametes is we do not know ;
it may ultimately be explicable in terms of some force already
known to us, or it may be one of those cases where it will be
convenient to cloak our ignorance by the assumption of some
special vital force of which we know nothing.

Although as a general rule an egg does not develop unless
fertilized by a spermatozoon, this is by no means always the
case, and many instances are known of parthenogenesis or the
development of unfertilized eggs. This may either be a normal
occurrence in the life cycle or it may be artificially induced.

Natural parthenogenesis occurs chiefly in insects, especially
amongst the aphides or plant lice. In these animals males and
perfect females appear only in the autumn. Fertilized eggs are
then laid which hibernate through the winter and hatch in the
spring, producing imperfect viviparous females. In these imperfect
females eggs are formed which develop parthenogenetically within
the body of the parent and give rise to fresh generations of
viviparous forms. This reproduction by means of unfertilized eggs

1 See, however, the footnote on p. 189.

144 OUTLINES OF EVOLUTIONARY BIOLOGY

is repeated again and again throughout the summer months and
thus the aphides multiply with great rapidity. In the autumn,
however, males and perfect females are again produced. The
viviparous imperfect females, as well as the males, are generally
winged, the perfect females are wingless. We have here another
kind of alternation of generations, in which forms which repro-
duce parthenogenetically alternate with others which exhibit
the normal sexual process ; to this type of alternation the
term heterogeny is sometimes applied. A very large number
of parthenogenetic generations may intervene between two
sexual ones.

Another well-known case of parthenogenesis is that of the hive-
bee, where the eggs laid by the queen may either be fertilized or
not, in the former case giving rise to females (workers or queens)
and in the latter to males (drones). Other instances occur amongst
those parasitic flat-worms known as flukes (Trematoda) and in
some Crustacea (Cladocera, the so-called water fleas).

The most remarkable cases of natural parthenogenesis, how-
ever, are those to which the special term paedogenesis has been
applied, in which the imperfect females do not even wait to
attain maturity before producing their offspring, but actually do
so in the larval condition, as in Chironomus and some other
two-winged flies (Diptera).

In general we may say that parthenogenesis occurs in cases
where it is desirable to take advantage of a brief season of favour-
able conditions to multiply the race as rapidly as possible. It is
necessary to make hay while the sun shines. When adverse
conditions set in, such as the advent of winter in the case of the
aphides, or discharge from the body of the host in the case of
parasitic flukes, the vast majority of the race will perish, but
a sufficient number will be able to protect themselves in some
way (like the encysted cercariae of the fluke), or a sufficient
number of fertilized and protected eggs will be produced (as in
the aphides) to tide over the evil time and form the starting
points for fresh generations at the first favourable opportunity.

It seems possible, also, that in some cases parthenogenesis
may be continued indefinitely without fertilization ever occurring,
for in certain species of minute rotifers and crustaceans no males
have as yet been observed.

Recent researches have shown that parthenogenesis can be
artificially induced in cases where it does not occur naturally at

ARTIFICIAL PARTHENOGENESIS 145

all, and Professor Loeb1 goes so far as to maintain that the
problem of fertilization is really one of physical chemistry. He
holds that the development of the egg is to be regarded as a
chemical process which depends mainly on oxidation, and finds
that the unfertilized eggs of various animals (sea-urchins and
worms) will undergo development (at any rate up to a certain
point) after exposure to the action of certain chemical reagents.
The unfertilized eggs of a sea-urchin, for example, developed into
larvae after being placed for two hours in sea water, the osmotic
pressure of which had been raised about 60 % by the addition of
some kind of salt or sugar, but this " hypertonic '' solution must
contain a sufficient quantity of free oxygen. In another case the
unfertilized eggs of the worm Chaetopterus were stimulated to
develop into larvae by the mere addition of potash and acids,
without the osmotic pressure of the sea water being raised.

Exactly what takes place under these circumstances we do not
know, and any speculation on this point is perhaps somewhat
premature, but it is quite clear from the experiments of Loeb and
other workers in the same field that we can no longer regard
fertilization as an indispensable condition of development even in
the case of eggs which do not naturally exhibit the phenomenon
of parthenogenesis. These experiments may also throw some
light upon the process of normal fertilization, especially as
regards the nature of the actual stimulus which causes the
fertilized egg to begin segmenting.

The casting out of polar bodies during the maturation of the
ovum led many of the earlier observers of this phenomenon to
believe that the matured ovum is incapable of development
because it has an imperfect nucleus, the importance of the
nucleus as taking the lead in cell-division having been established
at a comparatively early date. The imperfect nucleus of the
matured ovum was termed the female pronucleus, and it was
supposed to be converted into a perfect segmentation nucleus by
union with the male pronucleus brought into the egg by the
spermatozoon (Fig. 64), and the power of cell-division was supposed
to result from this completion of the nucleus. The observations
upon which this belief was based were perfectly correct, but the
conclusions drawn from them have not been sustained by recent
investigations, for in cases of artificial parthenogenesis development

1 "Die chemische Entwicklungserregung des tierischeii Eies" (kiinstliche Par-
theuogeiiese). Berlin, 190'J.

B. L

146 OUTLINES OP EVOLUTIONARY BIOLOGY

takes place in spite of the fact that reduction of the nucleus of
the ovum has occurred in maturation and has nofc been made
good by union with the nucleus of a spermatozoon.

In cases of parthenogenesis it is clear that the developing
organism is provided only with maternal chromosomes, but we are
now also acquainted with cases which form the exact converse to
this, only paternal chromosomes being present. Such cases may
arise when an ovum is artificially enucleated, so that all the
maternal chromosomes are removed, and then fertilized by a
spermatozoon. It has actually been found possible in this way to
induce the development of enucleated eggs, and the phenomenon,
our knowledge of which we owe mainly to Delage, is known as
merogony. Although it is obvious that, as a result either of
artificial parthenogenesis or of merogony, the developing organism
may start life with only half the normal number of chromosomes,
it is possible that, as maintained by Delage, this number may be
subsequently doubled in some way.

It cannot therefore be the union of male and female pronuclei
that furnishes the stimulus to development ; this union, or
amphimixis as Weismann terms it, has another significance, and,
as we shall see later on, is most probably connected with the
transmission of inherited characters from parent to offspring.

Boveri, followed by other observers, has put forward the view
that the unfertilized egg cannot develop because the centrosome,
which is to be regarded as the dynamical centre of the cell, has
been eliminated during the process of maturation. It is the
spermatozoon that, in the act of fertilization, brings with it
the centrosome upon the activity of which the cell-divisions of the
fertilized egg depend. This is probably perfectly true in cases
of normal fertilization and development in animals, but the view
that the centrosome of the spermatozoon supplies the essential
stimulus to development seems to be hopelessly negatived by the
phenomena of parthenogenesis, in which new centrosomes
undoubtedly arise in unfertilized eggs.

It seems therefore as if we were for the present thrown back
upon Loeb's hypothesis of a chemical stimulus, which he main-
tains for normal fertilization as well as for artificial partheno-
genesis. In the former case the necessary chemical substances
are supposed to be brought in by the spermatozoon. Loeb
believes that there are two of these substances. One, which he
terms a lysin, is supposed to bring about the formation on the

STIMULUS TO DEVELOPMENT 147

surface of the egg of the " fertilization membrane," while the
other is supposed to prevent the -cy tolysis or disintegration of the
ovum, induced by the formation of the fertilization membrane,
from going too far. It remains to be seen whether this " lysin
theory " will stand the test of time.

What it is that stimulates the unfertilized ovum to develop in
normally occurring parthenogenesis we do not yet know. In
most cases of this kind the process of maturation seems to differ
more or less from that which takes place in eggs which are
destined to be fertilized by a spermatozoon, and it may be that
these differences have something to do with the power of the egg
to develop parthenogenetically, but the discussion of this very
difficult problem is altogether beyond the scope of the present
work.

L 2

PART III.— VARIATION AND HEREDITY
CHAPTER XI

Variation — Merislic iind substantive variations — Fluctuations and muta-
tions— Somatogenic and blastogenic variations — Origin of blastogenic
variations.

THE term variation is used in more than one sense ; it may
be denned in the first instance as the process whereby closely
related organisms come to differ amongst themselves. It is a
matter of everyday experience that neither animals nor plants
exhibit absolutely fixed and constant characters, which are
handed on without alteration from parent to offspring. This
is very well seen in the case of human families, in which
there is rarely any difficulty in distinguishing the different
members by more or less pronounced and characteristic
individual traits. One may be fair and another dark, one short
and another tall, one with brown eyes and another with blue,
one clever and another stupid, and so on. In this way they vary
amongst themselves and deviate from the common parents of the
family often to a very considerable extent.

We cannot, however, avoid extending the use of the term
variation from the process itself to the results of that process,
and speaking of organisms as exhibiting variations, but this usage
is not likely to cause any confusion.

Variations are of many kinds and may be classified in different
ways according to the point of view from which we regard
them.

Meristic variations, which are variations in the number of the
repeated parts of an organism, are sometimes contrasted1 with
substantive variations, which depend upon the structure (in-
cluding shape, size and colour) of the organism or its parts.

Small fluctuating or continuous variations, which fluctuate

1 Bateson, " Materials for the Study of Variation." (Macmillan <Sc Co., London,
1894.)

MERISTIC VARIATIONS 149

about a mean or average condition, are contrasted with larger
discontinuous variations, or mutations.

Somatogenic variations, which affect the body of the
organism and are acquired in the life-time of the individual, are
contrasted with blastogenic or germinal variations, which arise as
a consequence of some modification in the germ cells.

These three methods of classification obviously overlap one
another. Thus a meristic variation may be continuous or
discontinuous, somatogenic or blastogenic, and so on, but it will
be convenient to deal with each group separately.

Meristic or Numerical Variations. In a very great number of
organisms certain parts are repeated with a greater or less
degree of regularity and constancy, and it is upon this repetition
that the symmetry of the organism very largely depends. An
ordinary star-fish, for example, is radially symmetrical, with five
similar arms or rays radiating from a common centre ; meristic
variation, however, not infrequently gives rise to six-rayed
individuals. The common jelly-fish, Aureliu aurita, again, usually
has four principal radii, but specimens are occasionally found with
two, three, or six.

All vertebrate animals, on the other hand, and many inverte-
brates, are bilaterally symmetrical, and at the same time meta-
merically segmented, i.e. with repetition of similar parts in linear
series one behind the other. The vertebral column, for example,
is made up of a larger or smaller number of morphologically
equivalent units, the vertebrae, to which the ribs are attached in
linear series on each side, and the limbs are arranged in two pairs,
in each of which the same fundamental structure can be traced.
Many instances occur in different animals of variation in the
number of vertebrae, and in man the occasional occurrence of
thirteen ribs instead of the normal twelve on each side is well
known. Cases of polydactylism, or -the development of extra
digits on hand or foot, also come under the head of meristic
variation.

In the vegetable kingdom, where the repetition of similar
parts, such as the leaflets of leaves, is even more conspicuous
than amongst animals, meristic variation is again a common
phenomenon. A clover leaf, for example, normally consists of
three leaflets, but the occasional discovery of a specimen with four
has been a source of satisfaction to superstitious folk from time
immemorial. We meet with similar variations in the number

150 OUTLINES OF EVOLUTIONARY BIOLOGY

of petals in flowers and the number of flowers in inflorescences.
An example of this will be given later on in the case of the
grape hyacinth.

Substantive Variations. Examples of this type of variation are
seen in the shape, size and colour of the human body and of its
various parts. The total height of the individual, the shape of
such features as the nose, ears and fingers, the colour of eyes
and hair, are all subject to very great variation. The same is
true of the shape and size of leaves and the colour of many
flowers. Numerous physiological variations, which doubtless
depend upon unrecognized and perhaps unrecognizable differences
in structure, may also be classed under this head, as for example
the variations in the egg-laying powers of fowls and the milk-
giving powers of cows, and in the percentage of sugar present
in the roots of the sugar beet.

Fluctuating Variations. The variations which come under this
heading are also spoken of as normal, individual or continuous.
They may be either meristic or substantive. They are
characterized by the fact that they pass into one another by very
minute or even insensible gradations, which fluctuate on either
side of a mean or average condition, so that the extent to which
a given organism (or part of an organism) varies can be
graphically represented in the form of a curve. Fluctuating
meristic variations lend themselves most readily to this graphic
method of treatment, for all we have to do is to count the
numerically varying parts in a sufficiently large number of cases
and plot our curve accordingly.

The manner in which this is done is represented in the
accompanying diagram (Fig. 72), which expresses graphically the
results obtained by counting the number of flowers in 202 in-
florescences of the common grape hyacinth (Mtiscari sp.). The
number of flowers in an inflorescence ranged from 20 to 42, and
the number of inflorescences showing each of the different numbers
of flowers ranged from 1 to 29.

The base line of the diagram is divided into a number of equal
parts (abscissa?) corresponding to the number of flowers (from 20
to 42). At one side an ordinate is erected on the base line and
divided into a number of equal parts corresponding to the
number of the individual inflorescences (from 1 to 29) bearing the
different numbers of flowers. Lines are then ruled parallel to
the base line and to the ordinate respectively, so as to divide the

FLUCTUATING VARIATIONS

151

paper into a number of equal squares (or the proceeding may be
simplified by working with already squared paper). The diagram
is completed by shading in an appropriate square for each
inflorescence, the position of the square being determined by the
number of flowers which it contains, and by the number of

20 21 22 25 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
Number of flowers in each Inflorescence

FIG. 72. — Fluctuating Variation in Inflorescences of the
Grape Hyacinth.

similar inflorescences which have preceded it in the enumeration.
Thus an inflorescence with twenty-five flowers must be repre-
sented by a square added to the column above the number 25
on the base line, and so on. If now we join the middle points of
the tops of all the columns by straight lines, we get a zig-zag line
which represents the curve of variation as determined by our actual
observations. It is obviously a very imperfect result, but its

imperfection is undoubtedly due in the main to the fact that the
number, of observations upon which it is based has not been
sufficiently large to give a fair average.

The larger the number of individual cases examined and
recorded the more closely will our zig-zag line approximate to
a regular curve. Even as it is, with all its imperfections, it
shows certain very characteristic features. It rises steeply
in the middle and falls away from the highest point, at first
suddenly and then more gradually, on either side. This is a
graphic representation of the facts, (1) that the most frequently
occurring number of flowers in the inflorescence (28) lies not very
far from midway between the two extremes, and (2) that the

Stature in Inches.

FIG. 73. -^Fluctuating Variation in Human Stature. (From Lock.)

more the number of flowers deviates from this mean on either
side the smaller is the corresponding number of inflorescences ;
or, to put it more generally, the number of individuals exhibiting
any given degree of deviation from the mean condition of the
species is inversely proportional to the amount of that
deviation.

Fig. 73 is a diagram constructed on the same principles as
the above, but based upon totally different material and a very
much larger number of observations. It is taken from Mr. R. H.
Lock's work on " Variation, Heredity and Evolution," and shows
the variation in stature observed amongst 4,426 British members
of the University of Cambridge. The dots in this diagram
correspond to the middle points of the tops of the vertical columns
in the preceding one. The statures are reckoned in the nearest

NORMAL CURVE OF VARIATION 153

whole numbers of inches. It is clear that a zig-zag line formed
by joining the dots in the diagram would approximate fairly
closely to the curve actually drawn.

Such a normal curve of variation agrees very closely with what
mathematicians term the curve of frequency of error, which is
the graphic representation of the mode of occurrence of chance
deviations from a mean or average and may be derived from the
theory of probability.

Such a curve may be experimentally produced by drawing a
vertical line on a target and firing a large number of shots at it.
There will be a more or less strongly marked tendency for the
shots to hit the line, depending upon the skill or otherwise of the
marksman. Most of them will probably strike to the right or
left of the line and fairly near it, but a few will probably be
very wide of the mark on either side. If the distances of the
striking places from the vertical line be measured and tabulated
the result may be expressed in the form of a curve which, if the
number of shots be large enough, will probably closely resemble
the curves described above.

The fact that the normal curve of fluctuating variation for any
kind of organism is practically identical with the mathematical
curve of frequency of error suggests very forcibly that the
variation in question is due to chance or accident causing each
individual in the course of its development to depart more or
less from the mean or average condition of the species to which
it belongs. These deviations must depend upon numerous factors.
They are to some extent, no doubt, due to the direct influence of
the environment, such as the effect of nutrition upon the size of
the organism, but they may also depend largely upon the vary-
ing characters of the germ cells from which the organism
develops, and especially upon the permutations and combinations
of characters which happen to take place in the maturation of
the germ cells and in their sexual union or amphimixis.

In short, the general tendency is doubtless for each individual
to conform to the type of the species to which it belongs, but
many accidental circumstances combine to prevent the realization
of this tendency and deviations from the type (or mean) take
place in accordance with the laws of chance.

Mutations. The term mutation, or discontinuous variation, is
applied to the process whereby new and more or less conspicuous
characters appear suddenly and spontaneously, without any

154 OUTLINES OF E VOLUTION AEY BIOLOGY

obvious reason. In extreme cases the organisms exhibiting such
mutation are often spoken of as sports or monstrosities ; the
latter term, however, is also frequently applied in the case of
purely artificial modifications, which must not be included under
this heading. Such artificial modifications are not, at any rate
as a rule, inherited by future generations, while true mutations
usually, if not invariably, are.

Mutations differ from fluctuating variations not only in that they
usually deviate more widely from the type of the species, but
also in the much greater rarity of their occurrence and in the
fact that they do not fluctuate about the old mean or average
condition ; nevertheless it may be questioned whether we can
logically draw a hard and fast distinction between the two.

Mutations may be eitber meristic or substantive. Human
beings with six fingers or toes, in place of the normal five,
are not infrequently met with, and this meristic mutation (hexa-
dactylisrn) is well known to be transmitted by heredity.

The classical instance of the Ancon or otter sheep, on the
other hand, affords one of the best known examples of substantive
mutation. To quote the words of Huxley,1 " It appears that one
Seth Wright, the proprietor of a farm on the banks of the Charles
Eiver, in Massachusetts, possessed a flock of fifteen ewes and a ram
of the ordinary kind. In the year 1791, one of the ewes pre-
sented her owner with a male lamb, differing, for no assignable
reason, from its parents by a proportionally long body and
short bandy legs, whence it was unable to emulate its relatives
in those sportive leaps over the neighbours' fences, in which they
were in the habit of indulging, much to the good farmer's vexa-
tion." The inheritance of this peculiarity was so strong that
this single individual actually became the starting point of a new
breed.

Mutations, more or less pronounced in character, are also not
infrequently met with in the vegetable kingdom. Foxgloves, for
example, are sometimes found in which some or all of the petals
are converted into stamens, and it has been proved by experi-
ment that this peculiarity is handed down from parent to
offspring.

Professor Hugo de Vries has, as is well known, made a special
study of mutation amongst plants. In the year 1886 this dis-
guished botanist found a large number of specimens of the evening

1 " Darwininna," p. 35.

MUTATIONS

155

primrose, (Enotkera lamarckiana, growing in a field near Amster-
dam, whither they had made their way from a neighbouring
garden. The plants were in a state of intense variability and their
seeds gave rise to several quite distinct new forms, which, if they
had occurred in a state of nature, would have been considered
as separate species.1 Professor De Vries attributes great import-
ance to mutations as the starting points of new species, which he
believes to arise in this sudden manner rather than by fluctuating
variation.2 We shall discuss this point in a subsequent chapter.
The difference between fluctuating variation and mutation is
sometimes illustrated by means of the model known as Gallon's
polygon (Fig. 74). A thick slab of wood is cut into the form of
a polygon, with unequal sides and capable of resting in a position
of more or less stable equilibrium upon any of its edges, the

FIG. 74. — Model of a Polygon in two Positions, illustrating the Difference
between Fluctuating Variation and Mutation.

degree of stability depending upon the position of the centre of
gravity above the edge upon which it rests.

The polygon may be supposed to represent an organism, or
rather a number of successive generations of an organism,
whose stability (or adherence to type) tends to be more or less
disturbed by the unknown factors which cause variation. If the
model be pushed it may be made to rock backwards and forwards
on either side of a mean or average position, and if the oscillation
does not exceed certain limits it will return to rest in that position.
This oscillation may be compared to fluctuating variation. If the
disturbing force be sufficiently great, however, the model will topple
over into a new position of stability and come to rest on another

1 Considerable doubt has, however, been thrown upon the interpretation of these
observations,, the results being conceivably due to the splitting up of some unknown
hybrid form, such as is well known to take place in other cases (sec Chapter XIV.).

2 "The Mutation Theory." Trans, by Farmer and Darbishire. London, 1910.

156 OUTLINES OF EVOLUTIONARY BIOLOGY

edge. This may be compared to the process of mutation or discon-
tinuous variation, whereby an organism acquires a new type of
structure.

Concerning the forces which bring about mutation in organisms
we know little or nothing. It is possible that in some cases the
mutation may be gradually prepared within the germ cells long
before it manifests itself — the final upsetting of tbe equilibrium
only taking place on the addition of the last straw. If such be
the case there is no need to suppose that mutations differ
essentially in nature from small, fluctuating variations. Tower's
observations on the artificial production of mutations in certain
beetles, however, indicate very clearly that such modifications
may apparently arise quite suddenly as the result of some change
in the environment acting directly upon the germ cells of the
parent. These observations will be referred to again at the close
of the present chapter.

Somatogenic Variations. Characters are said to be somatogenic
or "acquired" when they arise in the life-time of the individual
exhibiting them and owe their origin to the direct influence of
the environment upon the sorna or body. They are, usually at
any rate, not transmitted by heredity to succeeding generations,
except perhaps to a very limited and inappreciable extent.1
Under this heading are included the effects of use and disuse
of organs, and numerous cases in which modifications of the
body are artificially produced, as well as those in which they are
due to natural causes.

Amongst the effects of use and disuse we may mention, on the
one hand, the enlargement or the atrophy of special organs con-
sequent upon the extent to which they are employed, and, on the
other, the effects of education. The muscles of an athlete may
be greatly increased in size by constant use, and similarly if one
of the two kidneys be removed the other, having more work
thrown upon it, becomes enormously enlarged ; but we should not
expect these modifications to be handed on to the next generation.
Children have been taught to speak ever since man first became
differentiated from his speechless ancestors, but every child has to
learn the art anew and if brought up amongst foreigners will
come to speak a language different from that of its parents.

The small feet of Chinese ladies and the slender waists of
many Europeans are artificially produced somatic modifications

l This may be, however, a very important exception.

ARTIFICIAL MONSTROSITIES 157

which are well known not to be inherited. They are merely
distortions, effected by easily recognizable mechanical agencies.

Much more remarkable and difficult to understand are those
cases in which the addition of specific chemical reagents to the.
water in which aquatic larvae are developing produces such
definite and extensive modifications of structure as to give rise to
veritable monstrosities. The "Lithium larvae" of sea-urchins and
frogs have long been known and more recently Stockard has
described1 the "Magnesium larva" of the fish Fundidus hctero-
clitus (Fig. 75). He found that, when the developing embryos
of this fish are subjected to the influence of magnesium salts
dissolved in sea water, a large percentage of them acquire a
" cyclopean " character, with a single median eye in place of the
ordinary pair. Such embryos may hatch and swim about in a
perfectly normal manner, but it is not known whether they can
be reared to the adult condition. These observations seem to
indicate that the cyclopean monsters which sometimes occur in
man and other mammals may also be somatogenic variations due
to some unknown environmental influence.

Blastogenic or Germinal Variations. In this category are included
those variations which are believed to owe their origin to some
modification in the germ cells from which the organism
exhibiting them has developed.

It is important to observe that the term congenital, sometimes
used in this connection, is not synonymous with blastogenic, for it
is obvious that animals which, like the mammalia, remain within
the womb of the mother during the early stages of development,
may come to develop purely somatogenic characters before birth,
due to environmental influences acting in utero (e.g. poisoning of
the foetus due to parental alcoholism).

It is very doubtful, as we have already said, whether we
can really draw any absolute distinction between blastogenic
and somatogenic characters, and it seems by no means impos-
sible that somatogenic modifications may sooner or later
make an impression upon the germ cells and thus ultimately
become blastogenic. This point, however, will be discussed
later on.

Blastogenic modifications are from their very nature as
attributes of the germ cells handed on by heredity from gene-
ration to generation. All true mutations must be regarded as

1 "Journal of Experimental Zoology," February, 1909.

158 OUTLINES OF EVOLUTIONARY BIOLOGY

B.

c.

E.
FIG. 75. — Free-swimming Larva? of Fundulus heteroclitus. (From Stockard.)

A. Normal larva, with anteriorly placed mouth (M).

B. Incompletely cyclopean larva, with the two eyes joined and occupying the position

usually taken by the mouth.

C. Completely cyclopean larva, with single antero-median eye. Dorsal aspect.

D. Lateral aspect of same, showing the ventral mouth (M).

E. Ventral aspect of same ; ys, yolk sac.

TOWER'S EXPERIMENTS ON BEETLES 159

blastogenic and many so-called fluctuating variations may also
perhaps belong to the same category.

The modifications of the germ cells by virtue of which the
offspring come to differ to a greater or less extent from their
parents are, as we have seen, often attributed in large
measure to permutations and combinations of different characters
which take place in the sexual process (amphimixis) and the
preceding nuclear reduction. It has long been suspected, how-
ever, that the germ cells themselves, apparently independently of
the body in which they are enclosed, may be influenced by the
environment to which an animal or plant is exposed, and the
observations of Tower1 upon beetles of the genus Lcptinotarsa
may be referred to in this connection.

This observer considers that all permanent variations in these
beetles, so far as can be discovered, arise in the germ cells and
are in no wise the results of inherited somatic modifications. He
attributes their appearance to the direct action of the environment
upon the germ plasm and supports his views by a series of very
interesting experiments. He subjected the parents to environ-
mental stimuli of various kinds during the growth and matura-
tion of their germ cells, and then, after the ova had been fertilized,
allowed the development of the young to take place under normal
conditions. The parents, having already reached their final state,
were not themselves visibly affected by the stimuli, but a large
percentage of the offspring showed surprising modifications
which were strictly inherited. These modifications appear to be
in no way adaptive. They seem to bear no relation to the nature
of the stimulus which calls them forth and to be of no value to
the organism in the struggle for existence.

We may cite one example of Tower's experiments. Four males
and four females of Leptinotarsa dccemlineata (the potato beetle)
were exposed during the earlier part of the laying period (the
eggs being matured and laid in successive batches) to extremely
hot, dry conditions accompanied by low atmospheric pressure.
The eggs were removed as soon as laid and reared under natural
conditions. From 506 larvao thus reared 96 adult beetles were
obtained, of which 82 were of a form known as pallida, 2 of a form
known as immaculothorax, and the remainder unmodified. During

1 " An Investigation of Evolution in Chrysomelid Beetles of the Genus Lcptino-
tarsa," by William Laurence Tower. (Publications of the Carnegie Institution,
Washington, 1906.)

the later part of the laying period the same parents were kept
under normal conditions and yielded 319 eggs from which 61
normal beetles were obtained and none of the other forms, and
these normal beetles continued to breed true for three generations,
after which they were killed.

The two specimens of the immacidoiliorax form obtained in
the earlier part of the experiment unfortunately died from
disease, as also did all but two of the pallida. The remaining
two, however, both being male, were crossed with normal
females, yielding hybrid offspring with the dccemlineata charac-
ters dominant, and these hybrids, breeding inter sc, gave off-
spring which separated out in a characteristic Mendelian fashion1
into pallida, decendincata and hybrids again. There can be no
question therefore that the pallida characters, first due to modi-
fication of the germ cells by the action of changed environment,
were strictly heritable.

It should be observed that the forms ^flWwfa and immaculothorax
also occur occasionally, but rarely, in a state of nature as sports
or mutations, a fact which suggests that sports or mutations in
general may owe their existence to the apparently direct action
of the environment upon the germ cells. It is, of course, possible,
or even probable, that the change in the environment merely acts
as a kind of liberating stimulus, which enables characters already
latent in the germ cells to express themselves in the developing
organism, which, under normal conditions, they are unable to do.

We shall have to return to the question of the origin of blasto-
genic variations in future chapters.

1 See Chapter XIV.

CHAPTER XII

Heredity — General observations — Darwin's theory of pangenesis and
Weismann's theory of the continuity of the germ plasm — The nucleus
as the bearer of inheritable characters.

WHEN we study the life histories of the unicellular Protista
we find ourselves face to face with the problem of heredity in
its simplest form. The Amoeba divides into two parts by simple
fission, the division of the cell body being preceded by that of
the nucleus. The two daughter cells exactly resemble one
another, and, except in point of size, also resemble the parent,
while the latter ceases to exist as an individual in the very act of
reproduction. Here we may suppose that we are dealing with a
division which is qualitative as well as quantitative, that every
organ possessed by the parent cell is divided into two similar
parts and the total inheritance thus fairly apportioned between
the offspring, which will therefore be exactly similar to one
another and will need only to feed and grow in order to become
exactly similar to the parent.

The young Amoebae may be supposed, to resemble the parent
because they arise by duplication of the parent and of its organs j1
moreover, there is a perfect continuity of the living substance, or
protoplasm, of which the body is composed from one generation
to the next, and the whole of the body of the parent is used up
in providing the bodies of the offspring. Thus, although the
individuality of the parent comes to an end, the Amoeba never
dies, for there is never anything left over to die, and, barring
accident, it goes on multiplying for ever. "We have here a
typical illustration of the so-called immortality of the Protista.

The case is very different amongst the higher plants and
animals. Here, as we have already seen, each individual starts

1 We cannot, however, say this of all Protista, for in many cases the division
takes place asymmetrically and entirely new organs have to be formed by one or both
of the daughter cells, as, for example, in the transverse fission of Bodo (Fig. 38, D — F).
In such cases it looks as if the nucleus might be the real seat of the inherited
tendencies, and as if it were able, by its influence, to mould the daughter cell into
the form of the parent,

B. M

1G2 OUTLINES OF EVOLUTIONARY BIOLOGY

its life as a single cell — the fertilized ovum or zygote — which is
strictly comparable to a unicellular protistan. Like the Amoeba
it divides (under favourable circumstances) repeatedly, but the
products of division, instead of separating from one another and
going each its own way as an independent unicellular organism,
all remain together and co-operate with one another to form a
multicellular body of greater or less complexity. The cells of
which this body is composed become differentiated and specialized
in various directions. In so doing the vast majority of them
lose their faculty for independent existence, and when their powers
of division have become exhausted the tissues into which they
are combined become worn out and ultimately die. Thus the
body or soma, as a whole, must suffer death sooner or later. The
only cells which are, even potentially, exempt from this fate are
the germ cells. These, instead of becoming highly specialized
constituents of the soma, remain in the condition of more or less
independent Protista, and have the power of separating sooner
or later from the parent body. Like most of the Protista
they have also the habit of conjugating in pairs and thereby
renewing their powers of cell-division, and thus arise the zygotes
or fertilized eggs from which the new individuals take their
origin.

The eggs from which the individuals of different kinds of
plants and animals arise are for the most part extraordinarily
similar to one another. The ovum of a rabbit, as we have
already seen, is a minute nucleated mass of protoplasm about
^J(jth inch in diameter, that of a human being is a similar but
somewhat larger cell, and if the ova of a hundred different kinds
of mammals were mixed together it would be an extremely
difficult, if not impossible task to sort them all out, even after
the most minute microscopical examination. Even where con-
spicuous differences exist, as between the eggs of a mammal and
those of a bird, these are due almost entirely to the development
of accessory features, such as protective envelopes and food-yolk,
which have little to do with the nucleated mass of protoplasm
which constitutes the really vital part of the egg. Yet each kind
of fertilized egg, if it develop, will give rise to an organism
resembling the parent from which it was itself derived. More-
over, the resemblance will not be merely a general one, it will be
specific, and probably even more than specific, for it may include
minute individual characters peculiar to one or other of the

PRE -FORMATION AND EPIGENESIS 163

parents. Everyone is familiar with eases of this kind. It may
be some abnormality of fingers or toes, or a lock of white hair in
some special situation in a dark-haired man, or even some
trifling nervous habit, that is thus indelibly impressed upon the
organism and handed on from one generation to another.

Inasmuch as the only possible connection between parent and
offspring is (in most cases) through the germ cells, it follows that
there must be something in these germ cells which, so to speak,
represents all the inheritable characters of the parents and is
capable of giving rise to a repetition of these characters in the
course of individual development.

Two sharply contrasted views as to what takes place in the
development of the egg were maintained by the older embryo -
logists, and, in a modified form, survive to the present day. The
so-called " evolutionists," or " pre-formationists," maintained
that the egg contains in itself a complete miniature of the
organism into which it develops, and that the process of
development consists simply in an unfolding (" evolution ") and
growth of this miniature. This idea, of course, carried to its
logical conclusion, involves the further supposition that every
egg contains in miniature the bodies of all future generations,
like nests of boxes one within the otber.

In opposition to this view the upholders of the theory of
" epigenesis " maintained that there is no pre-formation of organs
in the egg but that the different parts of the adult organism
become gradually differentiated from the simple undifferentiated
ovum during the course of development. This view, which is
said to have originated with Aristotle and was strongly supported
by the great pioneer embryologist C. F. Wolff about the middle
of the eighteenth century, almost entirely superseded the crude
ideas of the pre-formationists, but at the present day the latter
are being revived to some extent, but in a more refined form, as a
result of modern experiments in embryology. It cannot be
disputed that in some cases certain parts of the adult organism
can be traced back to corresponding portions of the egg, which
cannot therefore be entirely undifferentiated, and it is probable
that in the end the truth will be found, as in so many other
cases, to lie in a compromise between the two extreme views.

The great problem which has to be solved by any theory of
heredity is — How do the apparently simple germ cells of a multi-
cellular organism come to be representative of all the other

M 2

164 OUTLINES OF EVOLUTIONARY BIOLOGY

cells of the body, so that when they develop they will give
rise to all those different kinds of cells arranged in the same
way as in the parent ? We must now briefly consider some
of the numerous attempts which have been made to answer
this question.

Darwin, in 1868, put forward his theory of Pangenesis as a
provisional hypothesis to explain the facts of heredity, and this
theory, though it seems never to have met with any large
measure of acceptance, is of considerable historical interest.1
He supposed that all the constituent cells of which the body is
composed not only multiply by ordinary cell-division, so as to
build up the various tissues, but also, throughout life, give off
extremely minute " genmmles " which wander tbrough the body
and are collected in vast numbers in the germ cells. The
gemmules, although so small as to be invisible even with the
highest powers of the microscope, are supposed to be capable of
absorbing nutriment and multiplying by division, and each one
is supposed, in some mysterious and unexplained manner, to
represent the particular cell of the body from which it was
derived, and to be capable, at the proper time and in the proper
place, of impressing the character of its parent cell upon a
corresponding cell of the new organism which develops from the
germ cell.

In Darwin's own words : —

" The development of each being, including all the forms of
metamorphosis and metagenesis, depends on the presence of
gemmules thrown off at each period of life, and on their develop-
ment, at a corresponding period, in union with preceding cells.
Such cells may be said to be fertilized by the gemmules which
come next in due order of development. Thus the act of
ordinary impregnation and the development of each part in each
being are closely analogous processes. The child, strictly
speaking, does not grow into the man, but includes germs which
slowly and successively become developed and form the man. In
the child, as well as in the adult, each part generates the same
part. Inheritance must be looked upon as merely a form of
growth, like the self-division of a lowly organized unicellular
organism. Reversion depends on the transmission from the
forefather to his descendants of dormant gemmules, which
occasionally become developed under certain known or unknown
conditions. Each animal and plant may be compared with a bed

1 For a complete exposition of the theory see Darwin's "Animals and Plants
under Domestication" (Ed. 2, Vol. II., Chapter XXVII.).

PANGENESIS 165

of soil full of seeds, some of which soon germinate, some lie
dormant for a period, whilst others perish." :

Darwin did not recognize the modern distinction between
somatogenic characters, which are acquired hy the body or soma
during its individual life-time, and blastogenic or germinal
characters, which are supposed to originate in the germ cells; or
rather, in accordance with his theory of pangenesis, he believed
that somatogenic modifications might be transferred to the germ
cells and thus become blastogenic. In other words, he was a
firm believer in the inheritance of acquired characters, a doctrine
which, as we shall see presently, is now much discredited, and
he endeavoured to explain by means of his theory how such
characters may be transmitted from parent to offspring.

Suppose some part of the body in a particular multicellular
individual were to become modified by use or disuse, or by the
direct action of the environment. Then the gemmules given off
from the modified cells would also bo affected in a corresponding
manner and would carry information of the change to the germ
cells. It would be as if some constituency with many repre-
sentatives changed its political opinions and instead of sending
conservative members to the House of Commons took to sending
liberals. When the proper time came the new representatives
would vote according to their instructions ; but we must also
suppose that the old ones could never be turned out and that
there would be a struggle between the two. At first the old ones
would be the more numerous and would outvote the new ones;
presently, however, the new ones, being constantly reinforced,
would come to outnumber the old ones and perhaps be able to
give effect to the altered views of their constituency. As Darwin
says, " It is generally necessary that an organism should be
exposed during several generations to changed conditions or
habits, in order that any modification thus acquired should
appear in the offspring." It would probably be more in accord
with the facts if, instead of " several generations " we said
" a large number of generations." In this sense, we may
well believe that acquired characters can be inherited, without
expecting to be able to demonstrate such inheritance by cutting
off the tails of a few generations of mice.

The theory of pangenesis certainly explains a great deal, but
it involves so many improvable assumptions as to the nature

1 Loc. cit., pp. 398-9.

166 OUTLINES OF EVOLUTIONARY BIOLOGY

and behaviour of the " gemmules " that it cannot be accepted as
more than what Darwin himself termed it, a provisional
hypothesis or speculation.

It is interesting to observe that Darwin finds the cause of
variation in the direct influence of the environment, including
under that term the effects of use and disuse upon the organism.
In this respect he agrees with the views of Lamarck and differs
widely from those of Weismann and many other modern
biologists, who deny, either totally or in part, the possibility of
the inheritance of acquired or somatogenic characters. It
will have been noticed that the theory of pangenesis is of an
essentially pre-formationist character, for it assumes the existence,
within the fertilized egg, of an immense number of material
particles (gemmules) which in some way or other represent the
different inheritable characters of the body.

The celebrated theory of heredity which we owe to Professor
Weismann ' is based upon what he terms the " Continuity of
the Germ Plasm." The general idea of continuity is, of course,
by no means a new one ; indeed the protoplasmic continuity of
parent and offspring, through the germ cells, must form the
material basis for the transmission of characters on any theory
of heredity. Weismann, however, gives much greater precision
to the idea than any of his predecessors. He identifies the
chromatin of the nucleus as the actual hereditary substance, the
bearer of all inherited tendencies, and draws a very sharp dis-
tinction between the somatic cells which, with almost endless
diversity of form .and function, build up the body of one of the
higher plants or animals, and the germ cells, which play little if
any part in the life of the individual in which they are lodged
but are destined, under favourable circumstances, to give rise to
the next generation.

We have already seen that the germ cells are frequently
separated from the somatic cells at a very early stage in develop-
ment. It may be that the distinction between the two is actually
inaugurated by the very first division of the fertilized ovum, as
in the horse worm, Ascari? meyalocepliala (p. 129), or it may be
recognizable at the gastrula stage, as in the arrow worm, Sagitta

1 For full accounts of this theory see the English translations of Weismann's two
chief works on the subject, "The Germ Plasm " (Contemporary Science Series,
1893) and "The Evolution Theory" (Edward Arnold, 1!)04).

WEISMANN'S THEORY OF HEREDITY 167

(p. 130), but such cases appear to be very exceptional and the
segregation of the germ cells usually takes place much later and at
different stages of the development in different species of plants
and animals. The exact time at which the separation of the two
groups of cells takes place, however, does not seriously affect the
argument. In any case the ultimate distinction between germ
cells and somatic cells is supposed to lie in the fact that the
former retain each a complete sample of the ancestral germ
plasm, in which at any rate all the essential characters of the
organism are in some way or other represented, while the latter,
by a series of differential divisions, gradually undergo a further
segregation into the different tissue cells of the adult, each of
which contains (in an active condition) only a sample of that
part of the ancestral germ plasm which is appropriate to its own
particular requirements.

Hence the germ cells, complete in themselves like so many
Protista, alone retain the power of giving rise to new generations
of complete individuals. The somatic cells have sacrificed this
power to their need for specialization. Certain phenomena,
however, such as the regeneration of lost parts in many animals
and the propagation of plants by buds and cuttings, necessitate
the supposition that some at any rate of the somatic cells must
retain a more complete sample of the ancestral germ plasm than
is necessary for their own development. For the hereditary sub-
stance of any particular cell Weismann adopts Nageli's term
" idioplasm," and in order to account for the phenomena just
referred to he is obliged to postulate the existence, in the
somatic cells in question, of " accessory idioplasm," which is only
called into activity under exceptional conditions, as, for example,
when it becomes necessary for a crab to regenerate a lost limb
or for an earthworm to renew a portion of its body which has
been bitten off by a bird or chopped off by a spade.

Weismann's theory involves the assumption of great complexity
of structure in the germ plasm, which, as we have already
seen, he identifies with the chromatin substance of the nucleus
of the germ cells. He finds it necessary to assume the existence,
not only of " determinants," which correspond more or less closely
to Darwin's gemmules, and each of which is supposed to be
responsible for the development of some special inherited feature
of the organism, but also of structural units respectively of a
lower and a higher order. Thus each determinant is supposed

1G8 OUTLINES OF EVOLUTIONARY BIOLOGY

to be made up of "biophors," which are themselves the lowest
vital units, but each of which is in turn made up of molecules in
the chemical sense of the term ; while, on the other hand, the
determinants are supposed to be grouped in " ids," each of which
is a complete ancestral germ plasm, theoretically sufficient in
itself to determine all the different characters of an entire
individual. The ids may in some instances correspond to the
chromosomes, but these appear generally to be composite bodies
(" idants ") made up each of a large number of ids (chromomeres).
The determinants, and of course the biophors also, are far below
the limits of visibility even with the aid of the most powerful
microscope ; the ids, however, frequently appear during the
process of mitosis and may give the spireme thread or the chro-
mosomes into which it divides a characteristic beaded appearance
(Fig. 32, B). Biophors, determinants, ids and idants must all be
looked upon as living entities, growing by the absorption of
nutriment and multiplying by division.

In accordance with Weismann's theory the germ cells them-
selves may be regarded as so many unicellular organisms, which
multiply by fission and periodically, if they chance to meet with
mates, conjugate with one another. Their cytoplasm as well as
their chromatin is directly continuous from generation to genera-
tion just as it is in a dividing Amoaba, and theoretically there is
no reason why the constant succession of germ cells should ever
be interrupted by death. The soma, or body, however, stands in
a very different position. It may be regarded as a kind of
appendage thrown off from the chain of germ cells after each
conjugation, and resulting from the fact that most of the cells
arising from the segmentation of the zygote not only remain
together in intimate association with one another but become
specialized in various directions and co-operate with one another
to form a complex multicellular individual. Having exhausted
its powers of growth and renewal this individual body sooner or
later dies ; but the germ cells periodically renew their powers of
cell-division by conjugation and, under favourable conditions, go
on for ever.

According to Weismann, inherited characters are transmitted
not from soma to soma but from germ cell to germ cell, by virtue
of the continuity of the germ plasm. The soma has little if any
influence upon the germ cells which it contains beyond that which
is involved in supplying them with protection and nourishment.

WEISMANN'S THEORY OF HEREDITY 169

There is no other means of communication between the soma
and the germ cells, and hence somatogenic characters, which
are acquired in the life-time of the individual body as the direct
result of the action of the environment (including use and disuse
of organs), cannot be transmitted to the germ cells and therefore
cannot be inherited. The only characters which can be inherited
are blastogenic characters, which arise by modification of the
germ plasm in the germ cells themselves.

This denial of the transmission of so-called acquired charac-
ters constitutes the most important difference between the
theories of heredity propounded by Weismann and Darwin.
Both these theories postulate the existence of ultra-microscopical
material particles, determinants or gemmules, but Weismann's
theory allows of no transference of such particles from soma to
germ cell, only from germ cell to germ cell and from germ cell to
soma. There is supposed to be no mechanism for the trans-
mission of somatogenic characters to the next generation. Accord-
ing to the older view the germ cells give rise to the soma and the
soma to the germ cells alternately. According to the newer one
the germ cells give rise to the soma and at the same time to the
next generation of germ cells, while the soma gives rise to nothing
but itself and ultimately perishes.

The contrast between the two views is clearly expressed in the
accompanying diagram (Fig. 76), in which, for the sake of sim-
plicity, the complication introduced by the process of conjugation
of the germ cells has been ignored.

The inheritance of somatogenic characters being denied, Weis-
mann is obliged to seek the origin of variations from some
source other than the action of the environment and use and
disuse. We shall return to this point presently. In the meantime
we must point out that Weismann's theory harmonizes very well
with the phenomena of mitosis, and especially with the remarkable
modifications of those phenomena which accompany the matura-
tion of the germ cells.

The entire process of mitosis serves to emphasize the import-
ance of the chromatin substance of the nucleus. It is evidently
of the utmost consequence that this substance should be accur-
ately apportioned between the daughter cells. We accordingly
find the elaborate mechanism of centrosomes and nuclear spindle,
and a splitting of eacli individual chromosome, which takes place
longitudinally when the chromosomes themselves happen to be

170 OUTLINES OF EYOLUTIONAEY BIOLOGY

elongated in form. If^as Weismann maintains, and as can
actually be demonstrafetTfn many cases, each elongated chromo-
some is made up of a row of chrornomeres or ids, which may be
supposed to differ to some extent from one another as to the

_- _ i — ^

determinants which they contain, it is obvious that longitudinal
splitting of the chromosome is the only way in which a qualita-
tive as opposed to a mere quantitative division of the hereditary

FIG. 76. — Diagram to illustrate the contrast between Darwin's Theory of
Pangenesis and Weismann's Theory of the Continuity of the Gerin
Plasm.

The figures represent an imaginary organism with four processes given off from the soma
or body, which is supposed to contain only a single germ cell (dotted). Three genera-
tions are represented, and for the sake of simplicity the complication introduced
by the periodical conjugation of male and female genii cells is omitted. Figs. A,
B, C show how an acquired character — the elongation of one of the processes as a
result of its use for some special purpose — may be supposed to affect the germ ceils
through the migration of gemmules (indicated by the small arrows), and thus IM-
transmitted by heredity in accordance with the theory of Pangenesis. Figs. A', B',
C' show how such an acquired character is, in accordance with Weismann's theory,
unable to make any impression upon the germ cells and is therefore not transmitted
by heredity. In the first case the germ cells of each generation are supposed to arise
, from the soma; in the second case they are supposed to arise directly from the
preceding generation of germ cells, which also gives rise to the soma in which they
are enclosed, ae indicated by the large arrows.

substance can be brought about. This mode of division, then,
otherwise difficult to explain, is fully intelligible in accordance
i with Weismann's theory.

It is not necessary to suppose, however, that such division
always results in identical daughter chromosomes. We may
assume either that all the individual ids divide into similar
halves, containing similar determinants (as represented very
diagrammatically in Fig. 77, A), in which case the two daughter
chromosomes will be exactly alike, or that some or all of the ids

WEISMANN'S THEORY OF HEREDITY

171

divide each into two dissimilar halves containing different deter-
minants (Fig. 77, B), when the daughter chromosomes will be
unlike each other. In the former case the division is said to be
integral and in the latter differential. It is by differential division
that Weismann believes the histological differentiation of the
soma to be brought about.

When we consider the phenomena of maturation and fer-
tilization we find them in no less striking harmony with
Weismann's views. We have already pointed out, in Chapter X.,
that each particular species of plant or
animal is, as a general rule, characterized
by the appearance of a definite and con-
stant number of chromosomes in all the
cells of the body during the process of
mitosis. At some period in the life-cycle,
however, in typical plants during the
process of spore-formation and in animals
during the maturation of the ova and
spermatozoa, this number is reduced to
half by separation of the entire chromo-
somes into two groups, one of which
passes to each of two daughter cells.
Thus the mature germ cells have only
half the number of chromosomes charac-
teristic of the species (or, in the case of
typical plants, of the sporophyte genera-
tion). The full number is made up again
by the union of male and female gametes
to form the zygote or fertilized egg.

To the combination of the maternal and paternal chromosomes
in the nucleus of the zygote Weismann has given the name
amphimixis, and he sees in this mingling of ancestral germ
plasms the cause of that mixture of paternal and maternal
characters which we commonly find in animals and plants. If, for
the sake of simplicity, we imagine that each chromosome consists
(as appears to be sometimes the case) of only a single id or
chromomere, and that eight of these are present in the nucleus of
the mature germ cell, we may represent the effect of repeated
amphimixis upon the constitution of the nucleus by means
of the diagram (Fig. 78), which shows how the ids must become
more and more diversified in each successive generation. In this

77. — Diagram of
(A) integral and (B)
differential Division
of a Chromosome con-
sisting of five Ids or
Chromomeres.

172 OUTLINES OF EVOLUTIONARY BIOLOGY

diagram A represents an unreduced nucleus composed of sixteen
ids, eight paternal and eight maternal, all the paternal and all the
maternal ids respectively being supposed to he alike. After
reduction and union with another mature germ cell containing
also only one kind each of paternal and maternal ids, hut both
differing in some respect from those of its mate, the nucleus of
the next generation will contain four kinds of ids, two paternal

FIG. 78.— Diagram illustrating the Composition of the Germ Plasm
(Chromatm Substance of the Nucleus) out of Ancestral Ids, and the
Effect thereon of repeated Amphimixis. (From Weismann's " Evolution
Theory.")

A — 1) the unreduced nucleus, containing sixteen chromosomes (each consisting of a
single id), of four successive generations. In A the germ plasm consists of only two
kinds of ids ; in B of four ; in C of eight, and in f) of sixteen. mJ, p-T, maternal
and paternal ids.

and two maternal, as shown in B, while in the next generation
there may be eight kinds, as shown in C, and in the next sixteen,
as in P.

We must, then, in accordance with the views of Weismann,
look upon the germ plasm of any one of the higher organisms as
being made up of a larger or smaller number of ids, each one
representing the inheritance received from some more or less
remote ancestor on the paternal or maternal side, though of course

WEISMANN'S THEORY OF HEREDITY 173

it is quite possible that there may be a number of ids of the same
kind. When, during the process of maturation, half the total
number of chromosomes (each made up of one or more ids) are
eliminated from the germ cell, it appears to be largely a matter
of chance which shall go and which shall remain, and the nature
of the new combination of ids resulting from the process of
amphimixis must also be a matter of chance, depending upon
what luck the germ cells happen to have in their mating.

As has been well said, a new shuffling of the cards must take
place in each generation. The characters of the organism
developed from any zygote will depend upon the hand dealt out
to it in the processes of reduction and amphimixis, and as it can
rarely, if ever, happen that any two hands will be exactly alike, so
it will rarely, if ever, happen that any two organisms, however
closely related, will exactly resemble one another in all their
characters. Indeed, the only cases in which even an approxima-
tion to exact resemblance is known, at any rate amongst the
higher animals, are those of " identical " twins, and the explana-
tion of these is that they have arisen by an integral division of a
single fertilized ovum, followed by separation of the first two
daughter cells or blastomeres thus produced.

We therefore find in the processes of reduction and amphimixis,
in the permutation and combination of ancestral characters, an
abundant source of variation. This, however, is not supposed
to explain fully the origin of variations, and Weismann accord-
ingly invokes the aid of another hypothesis, his theory of
" Germinal Selection." In accordance with this theory the
determinants of which the ids are composed are supposed to be
differently situated with regard to their facilities for obtaining the
nutriment necessary for their growth and multiplication. There
is a kind of struggle for existence going on amongst them. Those
which are more successful in obtaining supplies, having once got
a start, will tend to supplant those which are less successful.
Some will become weaker and some stronger, and thus, as the
result of differences in nutrition, variation is set up amongst the
determinants themselves. If these vary it follows that their
determinates, or the parts which they control in the developing
organism, will vary also.1

1 There would seem, however, to be a serious objection to the theory of germinal
selection in the fact that the nucleus of any given germ cell contains many ids, and
that similar determinants must as a rule recur in each id. We can hardly suppose
that the corresponding determinants in each id are always subject to precisely the

174 OUTLINES OF EVOLUTIONARY BIOLOGY

In this way Weismann seeks to avoid the necessity of believing
in the transmission from parent to offspring of modifications
which result from the direct action of the environment upon the
body itself, without, however, altogether denying that external
influences, and especially nutrition, may act upon the germ plasm
through the body and thus cause modifications in the offspring.

We have seen that Weismann's theory of the continuity of the
germ plasm involves the acceptance of the chromatin of the
nucleus as the actual material basis of hereditary transmission.
There can be no doubt that the phenomena of ordinary mitosis
in the case of tissue cells, and those of maturation and fertiliza-
tion in the case of the germ cells, strongly support this view,
while the simple fact that the spermatozoon, consisting almost
entirely of chromatin substance and with a minimum of cyto-
plasm, contributes equally with the ovum to the characters of the
offspring in normal cases, seems almost conclusive as to the pre-
dominating importance of the chromatin in this respect. Some
observers, however, still maintain that the cytoplasm plays a very
important part in heredity.

It is probable that much light will be thrown upon this
question by the development of that extremely important branch
of biological science known as experimental embryology, which is
as yet in its infancy. We have already pointed out that in certain
cases eggs containing no nucleus may be fertilized by spermatozoa,
and may then develop up to a certain point, and that this process
is termed merogony. Some years ago Boveri claimed to have
fertilized enucleate fragments of the eggs of one genus of .sea-
urchins (Sphrerechinus) with the sperm of another genus
(Echinus), and obtained larvae with only paternal characters.
He concluded from this experiment that the nuclear substance is
alone responsible for the transmission of inherited characters.
Unfortunately it seems that his results are open to a different
interpretation, and they have been severely criticized. They
certainly cannot be regarded as by any means conclusive.

More recently Godlewski has succeeded in fertilizing eggs of
the common sea-urchin (Echinus) with sperm of the feather star
(Antedon), belonging not only to a distinct genus but to a widely

same advantages or disadvantages of position. It seems much more likely that
variations in this respect in different ids would tend to neutralize one another, the
kind of determinant which is unfavourably situated in one id being favourably
situated in another, so that each kind would, on an average, have the same chance
of nutrition.

EXPERIMENTS IN HEREDITY 175

different order of echinoderms. The larvae of these two types
are easily distinguished even in early stages of development. The
larvae produced hy fertilization of normal eggs of Echinus with
sperm of Antedon are said to he of the maternal type. This
result is in itself very remarkable, but Godlewski was also able to
fertilize enucleate egg fragments of the sea-urchin with sperm of
the feather star. Eight larvae produced in this way reached the
blastula stage, but only four developed as far as the gastrula.
These four, however, were again of the maternal type, and could
only l)e distinguished by their size from those of the pure Echinus
culture. From these experiments Godlewski concludes that
cytoplasm as well as chro matin must be concerned in the trans-
mission of hereditary characters, for no maternal chromatin was
present in the eggs from which the larvae developed. Whether
these results, which are in direct opposition to those of Boveri,
will be confirmed or refuted by further observations remains to
be seen. Walker, in his recent work on heredity,1 has accepted
Godlewski's conclusions and made use of them in support of the
theory that the chromosomes are the bearers of individual
characters only, while racial characters may be transmitted by
" the whole protoplasm of the cell."

1 " Hereditary Characters and their Modes of Transmission,'' by C. E. Walker
(London, Edward Arnold, 1910).

CHAPTER XIII

The inheritance of acquired characters aud the mnemic theory of heredity.

No biological question during the last fifty years has given rise
to more acute and vigorous controversy than that of the inherit-
ance or non-inheritance of " acquired " characters. The following
paragraph may be quoted to show the manner in which Weismann
stated the case and endeavoured to give precision to the
terminology employed : —

" By acquired characters I mean those which are not preformed
in the germ, but which arise only through special influences
affecting the body or individual parts of it. They are due to the
reaction of these parts to any external influences apart from the
necessary conditions for development. I have called them ' somato-
genic ' characters, because they are produced by the reaction of
the body or soma, and I contrast them with the 'blastogenic1
characters of an individual, or those which originate solely in
the primary constituents of the germ (' Keimesanlagen '). It is an
inevitable consequence of the theory of the germ-plasm, and of
its present elaboration and extension so as to include the doctrine
of determinants, that somatogeuic variations are not transmissible,
and that consequently every permanent variation proceeds from
the germ, in which it must be represented by a modification of
the primary constituents." l

As already pointed out, the view here expressed is directly
opposed to that of Lamarck and Darwin, who believed that
characters acquired in the life-time of the individual, either as a
result of the use or disuse of organs or of the direct action of the
environment, might be handed on by heredity from parent to
offspring, and Darwin's theory of pangenesis was essentially an
endeavour to imagine some mechanism by which such trans-
ference of acquired characters might be effected.

We may at once agree with Weismann that blastogenic
characters alone are transmitted from parent to offspring, but the
real question is — Can a somatogenic character be converted into,

1 " The Germ- Plasm : A Theory of Heredity," by August Weismann (Con-
temporary Science Series, 1893), p. 392.

INHERITANCE OF ACQUIRED CHARACTERS 177

or give rise to, a blastogenic one? In other words — Can a
modification of the body or soma, arising in the life- time of the
individual and itself in no way due to inheritance, affect the
germ cells in such a way that the offspring developed from
them will exhibit a corresponding modification of its soma ?

It is useless either to deny or to assert the possibility of the
inheritance of such characters on any purely a priori grounds.
The fact that no satisfactory mechanism for the transference of
such characters from parent to offspring has yet been demon-
strated does not justify us in denying the possibility of such
transference. Our decision must depend upon an unbiassed
examination of the evidence which can be brought forward on
each side.

It is, of course, obvious that inasmuch as any organism differs
to a greater or less extent from its ancestors, the differences
being as a general rule greater in proportion to the remoteness
of the particular ancestor with which it is compared, the
differentiating characters must have been acquired, in the
ordinary sense of the word, during the interval which separates
the two generations in question. For example, there can be no
reasonable doubt that birds are descended from ancestors which
were reptilian in character and had no feathers. Feathers have
unquestionably been acquired somehow or other during the pro-
gress of the bird's evolution. This, however, is not the sort of
acquisition the inheritance of which is in dispute. Weismann and
his followers would deny altogether that feathers originated as
somatogenic characters ; they would say that certain apparently
fortuitous modifications in the constitution of the germ cells
themselves were responsible for the first appearance of feathers —
probably in an extremely rudimentary form — and that this new
character proving to be of value in the struggle for existence was
preserved and fostered by natural selection until after a long
process of evolution the elaborate plumage of existing birds was
perfected.

In striking contrast to such a case as the above we have
innumerable cases of the more or less sudden appearance of
somatic characters during the life-time of an individual as the
obvious result of the action of some external or environmental
influence, or of the use or disuse of some organ by its possessor,
and it is to such cases that Weismann and his followers would,
rightly or wrongly, confine the discussion.

B. N

178 OUTLINES OF EVOLUTIONARY BIOLOGY

Artificially or accidentally produced mutilations afford a very
good example of such obviously somatogenic characters, and with
regard to the inheritance of these Weismann's position is clearly
summed up in the following passage : — " As far back as the
eighteenth century the great philosopher Kant, and in our own
day the anatomist Wilhelm His, gave their verdict decidedly
against such allegations, and absolutely denied any inheritance
of mutilations ; and now, after a decade or more of lively debate
over the pros and cons, combined with detailed anatomical
investigations, careful testing of individual cases, and experi-
ment, we are in a position to give a decided negative and say
there is no inheritance of mutilations." l It is, however, as
already pointed out, all a question of evidence, and we may here
quote a definite case in order to show the nature of the evidence
with which we have to deal :—

" A female (and very prolific) cat, when about half-grown, met
with an accident. ' Her fine, long tail was trodden on and had
a compound fracture, two vertebrae being so displaced that they
ever after formed a short offset between the near and far end of
the tail, leaving the two out of line. At first I noticed that out
of every litter of kittens some had a tail with a querl in it.' With
successive litters the deformity increased, until ' not a kitten of
the old cat had a straight tail, and it grew worse in her progeny
until now we have not a cat with a normal tail on the premises '
(in a cat-population of six or eight, exclusive of young kittens).
' The tails are now in part mere stumps, some have a semicircular
sweep sideways, and some have the original querl. Perhaps the
deformity was somewhat aggravated by in-and-in breeding and.
by artificial selection practised by my Chinaman, who, with the
perversity of his race, preferred the crooked tails, and thus
preserved them in preference to the normal kittens. There are
no other abnormally-tailed cats in the neighbourhood.' "

Professor Brewer quotes this remarkable case2 from "that
keen observer and eminent scientist, Professor Eugene W.
Hilgard of the University of California," along with others of a
like kind from various sources.

It is of course essential to the stability of Weismann's theory
as a whole that evidence such as this should be rebutted. I am
not aware that he has anywhere criticized this particular case,
but his general remarks in somewhat similar cases may be
quoted with advantage in this connection :—

1 "The Evolution Theory" (London, 1904), Vol. II., p. 65.
. 2 Vide Cope's " Primary Factors of Organic Evolution " (Chicago, 1896). pp. 432-3.

INHERITANCE OF ACQUIRED CHARACTERS 179

" In the first place, the assertion that congenital stump-tails
in dogs and cats depended on inherited mutilation proved to be
unfounded. In none of the cases of stump-tails brought forward
could it even be proved that the tail of the relevant parent had
been. torn or cut off, much less that the occurrence, in parents or
grandparents, of short tails from internal causes was excluded.
At the same time anatomical investigation of such stump-tails as
occur in cats in the Isle of Man, and in many Japanese cats, and
are frequently found in the most diverse breeds of dogs, showed
that these had, in their structure, nothing in common with the
remains of a tail that had been cut off, but were spontaneous
degenerations of the whole tail, and are thus deformed tails, not
shortened ones (Bonnet).

" Experiments on mice also showed that the cutting off of the
tail, even when performed on both parents, does not bring about
the slightest diminution in the length of tail in the descendants.
I have myself instituted experiments of this kind, and carried
them out through twenty-two successive generations, without any
positive result. Corroborative results of these experiments on
mice have been communicated by Ritzema Bos and, indepen-
dently, by Rosenthal, and a corresponding series of experiments
on rats, which these two investigators carried out, yielded the
same negative results.

" When we remember that all the cases which have been
brought forward in support of an inheritance of mutilations
refer to a single injury to one parent, while, in the experiments,
the same mutilation was inflicted on both parents through
numerous generations, we must regard these experiments as a
proof that all earlier statements were based either on a fallacy
or on fortuitous coincidence. This conclusion is confirmed by
all that we know otherwise of the effects of oft-repeated mutila-
tions, as for instance the well-known mutilations and distortions
which many peoples have practised for long, sometimes incon-
ceivably long, ages on their children, especially circumcision, the
breaking of the incisors, the boring of holes in lip, ear, or nose,
and so forth. No child of any of these races has ever been
brought into the world with one of these marks ; they have to be
re-impressed on every generation." *

It cannot be seriously questioned that in the majority of cases
mutilations are not visibly inherited, but the fact that no one has
as yet succeeded in producing experimentally an inheritable
mutilation does not prove that such never occur accidentally.

Weismann's arguments can hardly be regarded as conclusive
against such strong evidence as that afforded by Professor Hilgard's

1 Weismann, "The Evolution Theory" (London, 11)04), Vol. II., pp. 05-G.

N 2 '

180 OUTLINES OF EVOLUTIONARY BIOLOGY

cats. Moreover, there appears to be considerable difference of
opinion as to whether or not the effects of circumcision are ever
inherited. Darwin, in discussing this point,1 admits that it is
possible that all the recorded cases of apparent inheritance of
such effects may be accidental coincidences, but the fact that
there are such recorded cases prevents us from accepting the
sweeping generalization which Weismann makes in the last
sentence of the paragraph above quoted.

Darwin himself was convinced that the effects of operations
are sometimes inherited by the well known and often quoted case
of Dr. Brown-Sequard's guinea-pigs. In these animals not only
did epilepsy appear in the offspring of parents which had been
rendered epileptic by injury to the nervous system, but certain
structural modifications (e.g. loss of toes) appeared in the
offspring when corresponding structural modifications had been
produced in the parent (indirectly) by similar injury. Weismann,
however, endeavours very ingeniously to explain away these
observations, and the correctness of Brown-Sequard's results has
been seriously impugned.

It cannot be denied, however, that the prima facie evidence for
the occasional transmission of such characters as those produced
by mutilation is very strong. We may allow much for coinci-
dence, but well authenticated cases like that of the cats above
referred to, and like the following, are too numerous to be all
explained away in this manner : —

" A person, when a boy of ten years, cut the terminal
phalange of the little finger of his left hand with a sickle. The
joint was not injured, nor was the function of the finger seriously
impaired. There was, however, an obvious deformity. The
finger was ill-shaped and crooked, and the nail abnormal. He
married and had two children, the first a son, with normal
fingers, the second a daughter, who had the little finger of the
corresponding (the left) hand deformed from birth in the same
manner. The function of the finger was not seriously injured,
but the deformity was precisely the same in shape, even to the
malformation of the finger-nail. She died at thirty, without
children, consequently no observation on a succeeding generation
could be noted. None of his other kindred had malformed
fingers, nor had any ancestor of the child for at least three
generations, and there was no knowledge of any such in the
more remote ancestry.

1 •• Animals and Plants under Domestication " (2nd Ed., 1882), Vol. I., p. 467.

INHERITANCE OF ACQUIRED CHARACTERS 181

" (This case was related to me in full detail by the father with
the deformed finger, and with whom I was personally acquainted.
He was an eminent physician, the president of a large and
reputable medical college, and his name is well known to the
profession.) " l

It seems only reasonable, then, to admit that suddenly
acquired somatogenic characters are occasionally, though probably
very rarely, transmitted by heredity in a sufficiently pronounced
form to be recognizable, but even if this were not so we should
not be justified in concluding that all the characters which an
organism exhibits have their origin in the first instance in modi-
fications of the germ plasm. It must be remembered that a great
many somatogenic modifications are the result of what we may
call the normal action of the environment, and that such action
may extend over very long periods of time, in which many
thousands of generations may be produced. In such cases the
stimulus, whatever it may be, to which the organism responds by
modification in bodily structure, is repeated a vast number of
times, and therefore seems much more likely to produce an
inheritable effect than a single accidental or experimental injury.
A single drop of water falling on a stone makes no visible
impression, but if the dropping goes on for a long time the stone
will gradually be worn away, and it is by no means essential to a
belief in the heritability of acquired characters that such
characters should be immediately inherited in full perfection.
It may be merely a question of time.

It is a well known fact that the "habit" and even the
structure of a plant are largely determined by the conditions
under which its growth takes place.2 A plant growing in a hot-
house may acquire a very different habit from another of the
same species growing in the open air. Many alpine or sub-alpine
planls have become adapted to their peculiar environment by
various highly characteristic modifications, amongst which a
reduction in the size of the leaves is one of the most conspicuous,
and such plants may be induced to change their mode of growth
by simply removing them to sufficiently warm and sheltered
situations, in which their habit comes to approach that of their
lowland relatives. In such cases it is difficult to say how far

1 This is another of the striking cases collected by Professor Brewer and quoted
in Cope's " Primary Factors of Organic Evolution," pp. 433-4.

2 The reader should consult the quotations from Lamarck bearing upon this point
given iu Chapter XXIV., especially the case of Ranunculus ayuatilis.

182 OUTLINES OF EVOLUTIONARY BIOLOGY

the habit of the plant, whether alpine or lowland, is really
inherited, or how far it "may he produced afresh in each generation
as a direct response to environmental stimuli.

It appears, however, from the observations of Bordage,1 that
peach trees in the climate of Reunion gradually acquire an
almost evergreen habit, and that this character is hereditarily
transmitted, being exhibited by seedlings of the modified trees
when grown in situations where the peach is usually deciduous.
Dr. Bordage considers, and it appears to us that he has good
reason for so doing, that his observations and experiments defi-
nitely prove, in the case of plants, the hereditary transmission
of characters acquired under the influence of a change of climate.

To take another example, it has been shown that in rats and
mice certain modifications in bodily structure are produced as the
result of raising or lowering the temperature to which the young
animal is exposed during its growth, and such modifications appear
to be inherited. Sumner 2 found, as the result of a large number
of careful measurements, that mice reared in a warm room (about
21° C.) differed considerably from those reared in a cold room
(about 5° C.) as regards the mean length of the tail, foot and ear,
which were longer in the former than in the latter. The same
differences occurred to a recognizable extent in the offspring of
the warm room and cold room parents, although these offspring
were all reared together in a common room under identical
temperature conditions.

Observations such as these, which are rapidly accumulating,
lead us to hope that the question of the inheritance or non-
inheritance of somatogenic characters which have undoubtedly
arisen in response to the direct action of the environment
may, before long, be answered conclusively. They doubtless require
confirmation and extension, but they afford very strong evidence
in favour of the inheritance of such characters. It has been sug-
gested that in such cases the stimulus of changed conditions affects
the body and the germ cells simultaneously and in a parallel
manner, rather than that the body is modified first and then in-
fluences the germ cells, but such a suggestion seems like a last
attempt to avoid at all costs the necessity for believing in the

1 Edmond Bordage, "A propos do Theredite des caractt-res acquis " (Bulk-tin
Scicntitique dela France et de la Bulgique. Tome XLIV, Paris, 1910).

2 Francis B. Sunnier, ''An Experimental Study of Somalic Modifications and
their Reappearance in the Offspring," (Archiv fur Eutwicklungsmecbanik der
Organ ismcn, Bd. 30, 1910).

INHERITANCE OF ACQUIRED CHARACTERS 183

transmission of " acquired " characters, and is quite as difficult
to accept as the latter, while, as Sumner points out, it does not
affect the question of the importance of the environment in
determining the course of evolution.

Professor Henslow x believes that the direct action of the
environment, in its widest sense, coupled with the responsive
power of protoplasm, is the sole and efficient cause of adaptive
variations in plants, without any aid from natural selection.
This of course implies a firm belief in the inheritance of
somatogenic modifications. Although these may be acquired
slowly throughout the course of a long series of successive
generations, they may become gradually more and more fixed .
and permanent, until finally they attain a degree of stability
which entitles them to be considered as truly blastogenic.

Professor Eigenmann 2 has arrived at very similar conclusions
as a result of his careful study of animals which live in dark
caves. A very characteristic feature of such animals is the
bleaching which they undergo, due to the loss of pigment. Pro-
fessor Eigenmann regards this character as due in the first place
to the direct influence of the environment (i.e. the absence of
light) upon the individual. He tells us " The bleached condition
of animals living in the dark, an individual environmental adap-
tation, is transmissible, and finally becomes hereditarily fixed."
In Amblyopsis, one of the blind cave fishes, the fixation of this
character has become so complete that even when the young are
reared in the light they develop without pigment — the originally
somatogenic character has apparently become blastogenic. In
the well known European Proteus — a tailed amphibian which
lives in subterranean waters — on the other hand, it appears that
the bleached condition has not yet become hereditarily established,
for this animal becomes darker when exposed to the light. Pro-
fessor Eigenmann points out that " natural selection cannot
have affected the coloration of the cave forms, for it can be of no
consequence whether a cave species is white or black."

We do not know how many generations it may take to effect
the fixation of a character acquired slowly under the influence
of a constantly repeated or continuous environmental stimulus,
but the fact that human beings have not yet learned to speak

1 "Origin of Plant Structures " (International Scientific Series, Vol. LXX VII.,
1895).

2 " Cave Vertebrates of America. A Study in Degenerative Evolution " (Wash-
ington : Carnegie Institution, l'J09).

184 OUTLINES OF EVOLUTIONARY BIOLOGY

without being taught shows that the number may, in some cases
at any rate, be very large. Speech, however, is a comparatively
recent acquisition in the human race and many of the higher
animals perform actions without being taught which their ancestors
may very well have originally learnt to perform by observation
or experience. It would be extremely difficult to explain the
nest-building habits of birds, and other so-called " instincts,"
except as having been originally acquired in this way in the
life-time of the individual.

On the whole, then, the available evidence seems to indicate that
suddenly and exceptionally acquired characters, such as mutilations,
are occasionally but only rarely inherited to such an extent as to be
recognizable, while, on the other hand, characters which are due to
the continued action of some external stimulus, extending perhaps
over many generations, in the long run become so firmly impressed
upon the organism that they affect the germ cells as well as the
somatic cells and thus become truly blastogenic.

We must look upon the germ cells as highly conservative
bodies, possessed of great inertia, which can only be induced very
slowly in normal circumstances, or by some sudden revolution,
of the nature of which we know nothing, in abnormal circum-
stances, to alter their constitution. This conservatism is no
doubt of great value as a check upon the inheritance of accidental
and unfavourable modifications. The adaptation of the individual
to its particular environment is to a large extent provided for by
somatogenic modifications which arise in its own life-time. What
suits one individual, however, may not suit its successors, which
may have to live under more or less widely different conditions,
and for progressive evolution what is needed is an average
adaptation of the race to an average environment. Hence the
germ plasm in its evolution lags behind the soma and is only
very slowly impressed with those characters which experience
has shown to be of permanent value to the race. It is a kind of
second chamber which acts as a useful check upon too radical
tendencies.

That the germ plasm is to some extent capable of modification by
environmental stimuli, however, is clearly shown by the experiments
of Tower referred to in Chapter XL, but it is extremely difficult to
see how such stimuli as he mentions could affect it except by
acting primarily upon the soma in which the germ cells are
enclosed. Then the soma might be modified in response to the

INHERITANCE OF ACQUIRED CHARACTERS 185

stimulus in the first instance, and this modification, whatever its
nature (and it might of course he quite unrecognizable hy us),
might affect the germ cells — in which case the germ cells could not
he entirely shut off from the influence of the soma.

The chief obstacle in the way of our belief in the inheritance of
acquired characters lies, as we have already seen, in the difficulty
of imagining any mechanism adequate to bring about the con-
version of somatogenic into blastogenic modifications. If, as
Weismann insists, the germ cells are really incapable of being
influenced by the body, the difficulty does indeed seem insur-
mountable ; but there seems to be no valid reason why we should
follow Weismann in this respect. Nor does it seem necessary
that we should adopt any theory which postulates the migration
of material particles, such as the gemmules of Darwin's pan-
genesis, in order to get over the difficulty,

Herbert Spencer long ago indicated the direction in which the
solution of this problem must be sought. " It is," he says, "an
unquestionable deduction from the persistence of force, that in
every individual organism each new incident force must work its
equivalent of change ; and that where it is a constant or recurrent
force, the limit of the change it works must be an adaptation of
structure such as opposes to the new outer force an equal inner
force. The only thing open to question is, whether such re-adjust-
ment is inheritable ; and further consideration will, I think, show,
that to say it is not inheritable is indirectly to say that force does
not persist. If all parts of an organism have their functions
co-ordinated into a moving equilibrium, such that every part
perpetually influences all other parts, and cannot be changed
without initiating changes in all other parts — if the limit of
change is the establishment of a complete harmony among the
movements, molecular and other, of all parts ; then among other
parts that are modified, molecularly or otherwise, must be those
which cast off the germs of new organisms. The molecules of
their produced germs must tend ever to conform the motions of
their components, and therefore the arrangements of their com-
ponents, to the molecular forces of the organism as a whole ; and
if this aggregate of molecular forces is modified in its distribution
by a local change of structure, the molecules of the germs must
be gradually changed in the motions and arrangements of their
components, until they are re-adjusted to the aggregate of mole-
cular forces. For to hold that the moving equilibrium of an

186 OUTLINES OF EVOLUTIONARY BIOLOGY

organism may be altered without altering the movements going
on in a particular part of it, is to hold that these movements will
not be affected by the altered distribution of forces ; . and to hold
this is to deny the persistence of force." 1

In other words, the whole complex system of forces which
determines the constitution of the germ cells must be in a state
of equilibrium with the system of forces which determines the
constitution of the body, and any disturbance in the latter must
be met by a corresponding disturbance in the former. The same
idea may be expressed by saying that modifications of the soma
aet as stimuli, which make more or less permanent impressions or
engrams " upon the germ cells in much the same way that
stimuli of various kinds received through the sense organs make
impressions upon certain cells in the brain. In the case of the
brain cells the original impressions may become dormant and be
revived later on as memories, which are a kind of reproduction
of the phenomena to wnich the impressions were originally due.
In the case of the germ cells it is supposed that the engrams also
become dormant, but are aroused to activity again in the course
of the development of these cells into new organisms, exerting
their influence in such a manner as to bring about a kind of
reflection, in the body of the offspring, of the parental characters.
This is the central idea of the "Mnemic"a theory of heredity,
associated more especially with the names of Hering, Samuel
Butler a and Semon.4

According to some authorities it is through a continuity of
protoplasm from cell to cell, or especially through the nervous
system in the case of the higher animals, that stimuli are trans-
mitted from the soma to the germ cells, but the assumption of
any definite material paths for such transmission seems an
altogether superfluous encumbrance of the theory.

It has long been known that it is possible to send messages, or if
we like to call them so, stimuli, from one place to another without
any material means of communication. The very existence of
life on the earth depends upon stimuli received in the form of

1 " Principles of Biology " (London, 1881), Vol. II., p. 387.

2 From the Greek /II/TJ/XT/, memory.

8 See Butler's "Life and Habit" (2nd Edition, London, 1878) and "Un-
conscious Memory" (New Edition, London, 1910), the latter of which contains a
translation of Bering's lecture " On Memory as a Universal Function of Organized
Matter."

4 Semon, " Die Mneme als erhaltendes Prinzip im Wechsel des organischen
Geschehens," 3rd Ed., Leipzig, 1911.

INHERITANCE OF ACQUIRED CHARACTERS 187

light and heat from the sun -across an intervening space of almost
inconceivable extent which is supposed to contain nothing but
the hypothetical and intangible ether. To account for such
transmission we postulate the occurrence of vibrations or undula-
tions in this ether, and the truth of this theory may now be
regarded as conclusively demonstrated. The stimulation of the
sensory cells of the retina by certain of these vibrations gives rise,
when transmitted along the optic nerve to certain cells in the brain,
to the sensation which we recognize as light, while other
vibrations are perceived by us as heat.

The rapid development of wireless telegraphy in recent years
has familiarized us with another type of vibration in the ether,
by taking advantage of which we are able to send messages over
immense distances from one instrument to another, without the
aid of wires or any other material connections. The Rontgen
rays, again, can make impressions upon sensitive photographic
plates after passing through solid objects which are quite imper-
vious to the ordinary light vibrations.

In view of these facts it seems absurd to deny that the living
cells of the soma, in which doubtless complex vibrations, possibly
comparable to those which are responsible for the phenomena
of light and electricity, are constantly going on, may conceivably
influence the germ cells without our being able to demonstrate
the existence of material connections by which the necessary
stimuli might be transmitted.

The different means of communication between one cell and
another in the living body may be compared to the different
methods by which messages are transmitted between distant
members of a civilized human community. The circulation of
fluids (such as the blood) distributes throughout the body definite
substances secreted by certain cells, which may act as stimu-
lants upon other and far distant cells. We may compare this
to the transmission of messages by letter post. The nervous
system of the higher animals, and probably also the delicate threads
of protoplasm which so frequently connect one cell with another
both in animals and plants, provide a means of communication
which is closely comparable to ordinary telegraphy over conducting
wires. Why should we deny the possibility that a third means
of communication, analogous to wireless telegraphy, may also
exist ?

Darwin's hypothesis of pangenesis depends upon the

188 OUTLINES OF EVOLUTIONARY BIOLOGY

existence of the first of these means of communication. So also
does the modern theory of hormones, chemical suhstances which
are believed to exercise a most important controlling influence
over parts of the body that may be far remote from the
place where they themselves are secreted,1 and the aid of
which has also been invoked to account for the phenomena
of heredity. Others, again, as we have seen, have called in the
aid of the second means of communication to account for the
transmission of stimuli from somatic to germ cells, but their
view is hardly supported by what we know of the arrangement of
the nervous system throughout the animal kingdom, and the
difficulty of accepting it is still greater in the case of plants, in
which no definite nervous system is developed. The third method
of communication, however, seems open to no objection, and,
whether it may be supplemented by the others or not, seems
amply sufficient to account for the facts.

The faculty of receiving and responding to stimuli of various
kinds is one of the most characteristic features of living proto-
plasm. In the higher animals this function is more or less
concentrated in definite sense cells, each of which is adapted for the
reception of stimuli of one particular kind. The visual sense cells
are adapted for the reception of stimuli from the light-vibrations
of the ether, the auditory sense cells are stimulated by vibrations of
the air or water in which the animal lives, the olfactory sense cells
are stimulated by the chemical action of material particles, and
the tactile cells by the mechanical stimuli of contact and pressure.
In such an organism as the unicellular Amreba, on the other
hand, all parts of the superficial protoplasm are probably sensitive
to stimuli and no special organs of sense are developed. We may
well suppose, therefore, that the undifferentiated germ cells of the
multicellular animals and plants are sensitive to stimuli received
from the somatic cells, though it is impossible in the present state
of our knowledge to determine the nature of these.

It is not difficult to demonstrate that one cell may actually be
stimulated by another without the existence of any protoplasmic
connection between the two, as, for example, in the mutual
attraction of male and female gametes. Such stimulation, it is
true, is usually attributed, mainly at any rate, to the secretion
of some specific chemical substance which diffuses into the sur-
rounding water, and classed accordingly under the head of

i Vide p. 126.

INHERITANCE OF ACQUIRED CHARACTERS 189

ehemotaxis. In the case of Spirogyra, however, discussed in
Chapter X,1 it certainly seems as if ehemotaxis were not sufficient
to account for the phenomena.

Belief in the possihility of the transmission of acquired
characters from somatic to germ cells by no means obliges us to
throw over Weismaun's theory of determinants. Indeed it seems
necessary to postulate the existence in the germ plasm of some
such material primordia as the responsible agents in the trans-
mission of heritable characters in order to account for the
Mendelian phenomena of hybridism, and especially the existence
of interchangeable unit characters, which will be dealt with in
the following chapter. Moreover, the theory of determinants, as
we have already seen, harmonizes very well with what we know
of the microscopic structure of the gerrn plasm and its behaviour
in cell-division and conjugation.

It may be that it is the determinants themselves that are
influenced by stimuli received from the somatic cells. It is even
possible that each different kind of determinant ib " tuned "
to respond to vibrations of a particular wave-length, emanating
from corresponding determinants in the nuclei of somatic cells of
a particular kind. Modifications of these somatic cells may then,
by altering the character of the vibrations in the determinants of
their own nuclei, affect the corresponding determinants in the
distant germ cells in a similar manner. By means of some such
hypothesis as this, which would be strictly in accordance with
modern physical ideas on the subject of the transmission of
electrical energy from one group of electrons to another at a dis-
tance, any a priori difficulty in accepting the possibility of the
inheritance of acquired characters might be removed.

The case of neuter insects has frequently been adduced as a
serious stumbling block in the way of the theory of the origin of
permanent, heritable characters from somatogenic modifications,
for these neuters possess features of their own which are not
shared by the males and perfect females and which, as the
neuters do not themselves produce offspring, they cannot trans-
mit directly to future generations. According to Weismann's
theory these characters are supposed to result from fortuitous
favourable variations of the germ plasm of the parents, which are
accumulated and intensified under the influence of natural

1 Pp. 142-ii. I have perhaps laid too much stress upon this case ; chemotaxis
may have more to do with it than appears at first sight.

190 OUTLINES OF EVOLUTIONARY BIOLOGY

selection, the welfare of the insect community as a whole, rather
than that of its constituent individuals, being in this case the
determining condition through which natural selection operates.
The case of such communities, however, is exactly^ analogous to
that of individuals, ejtcept that the single cells are replaced by
complete and separateTfTuTtTcellular units. The whole community
may be looked upon as one individual of a higher order, and the
problem of the transference of stimuli from the bodies of the
neuters to the germ cells of the perfect insects differs in no
essential respect from that of the transference of similar stimuli
from somatic cells to germ cells in an ordinary individual. The
close association in which members of such communities live
may be supposed to facilitate the transfer of such stimuli (arising
from modification of the somatic cells of the neuters in response to
the environment) to the germ cells of the males and perfect
females, without the aid of material conductors.

In the present state of our knowledge, of course, any suggestions
which may be put forward on this subject must be regarded as mere
hypotheses, incapable of demonstration, and as such they will
doubtless appear to many people to be unwarrantable. If, however,
they serve to show that there is no a priori reason for denying the
possibility of the transmission of somatogenic characters to the
germ cells, and thence to future generations, they will serve a
useful purpose.

We must, however, again point out that, under normal cir-
cumstances, it probably takes many generations before any
impression produced by modification of the soma upon the germ
cells becomes so deeply ingrained as to find full expression in the
offspring produced by their development. The impression, once
made, appears to be equally difficult to remove, and hence has
probably arisen that conservatism or inertia of the germ cells to
which we have already alluded. Occasionally, however, it appears
that a suddenly acquired somatogenic character at once makes a
deep impression upon the germ cells. Why this should be so we do
not know, but it is perhaps no more remarkable than the well
known fact that the germ cells themselves may occasionally throw
aside their conservatism and give rise to sports or mutations.1

It is well known that, apart altogether from the highly
specialized brain cells, living protoplasm frequently has the power

1 It is possible, however, that the suddenness of a mutation is apparent rather
than real, at any rate in some cases ; compare p. 156.

MNEMIC THEOKY OF HEREDITY 191

not only of receiving and responding immediately to stimuli of
various kinds, but also of storing up impressions or engrams for
future use. This is demonstrated quite clearly by the phenomena
known as " after-effects " in various plants and animals. One of
the best known examples of such after-effects is seen in the daily
periodicity of plant growth. The light of the sun acts as a
restraining or inhibiting stimulus upon the rate of growth of
ordinary plants. In consequence of this the plant grows most
rapidly in the early hours of the morning after a prolonged
exposure to darkness, and most slowly in the afternoon, after
long exposure to daylight. It has been shown that this daily
periodicity, or variation in rate of growth in correspondence with
the periodic variation in environment, is continued when the
plant is kept in perpetual darkness, and the direct stimulation of
changing environment thereby rendered impossible. In other
words, the plant, though entirely devoid of any nervous system
in the ordinary sense of the term, establishes a habit, which must
depend upon something analogous to memory on its part.

The mnemic theory of heredity is based, as we have seen, upon a
comparison of the phenomena of inheritance with those of memory.
The latter are reasonably explained by supposing that impressions
received by certain cells of the brain may be stored up as
" engrams " in these cells for use on future occasions, when,
under appropriate stimulation, they give rise to mental condi-
tions corresponding to those produced by the original stimuli.
The stimulus which evokes a memory is commonly the
repetition of some stimulus which was originally associated
with the thing remembered. Thus the gight of a person whom
we have not seen for a long time, or even of his portrait, will
evoke a whole train of memories connected with that person.

The degree of accuracy with which we remember any occurrence
depends partly upon the nature of the occurrence itself and
partly upon the frequency with which it has been brought under
our notice. The first time we take a walk we may have to pay
attention to every turning and every signpost in order to find our
way at all, but if we take the same walk many times we at length
become so familiar with it that we may be thinking of other
things the whole time and not consciously notice a single land-
mark from start to finish. As Samuel Butler has most forcibly
pointed out, the more perfect memory becomes the less is it
accompanied by consciousness of the things remembered, as when

192 OUTLINES OF EVOLUTIONARY BIOLOGY

a skilled musician, playing a composition for perhaps the
hundredth time, is quite unconscious of the individual actions
which he performs by memory in striking the notes, while a
beginner, who has only played the same piece once or twice
before, may be acutely conscious of every note which he strikes.
The development of an individual organism from the egg is,
according to the mnemic theory, merely an unconscious repetition,
by memory, of acts which have been performed many thousands
of times by its ancestors, and, as in the performance of a piece of
music, each successive act constitutes the stimulus which calls
forth the next.

The fertilized ovum maybe looked upon as being charged with
the latent memories of past generations. The unconscious memory
of what it did last time it was a fertilized ovum prompts it to divide
into two cells, this re-arrangement of its constituents supplies
another stimulus which prompts it to a further division, and so on
through all the stages of ontogenetic development. In the words
of Professor Francis Darwin, "the rhythm of ontogeny is actually
and literally a habit." l

Progressive evolution takes place owing to the fact that each
successive generation may add a little bit to the record on its own
account, this addition being the result of some new experience
due to some difference in its environment as compared with the
environment of its predecessors.

There are, then, two distinct sets of factors which determine
the individual development of any organism, first the inherited
tendencies, or " engrams," and second the stimuli provided by
the environment during its own life-time. An egg placed under
very unusual conditions, as, for example, too low or too high a
temperature, may be unable to develop at all, but if the
change of conditions be but slight the organism may adapt
itself, educate itself and furnish itself with a new store of
experiences, which in course of time may be impressed upon the
germ plasm and thus added to the record and handed on to
future generations.

We have already had occasion to notice, in the case of
artificially produced cyclopean fish lame, how an alteration of the
environment not sufficient altogether to prevent development
may so far overcome the inherited tendencies of the organism
as to give rise to the production of monstrosities (p. 157).

1 Presidential Address to the British Association, Dublin, 1908.

ACTION OF THE ENVIRONMENT 193

Such cases clearly prove the immense influence of the environ-
ment in determining the character of the organism. If we admit
that inherited tendencies also owe their origin in the first instance
to the action of the environment on previous generations, it is
difficult to avoid the conclusion that every organism is ultimately
indebted for all its characters to the action of external stimuli
upon responsive protoplasm.

B.

CHAPTER XIV

The Mendelian experiments in hybridization — The doctrine of unit
characters and the purity of the gametes — Galton's law of inheritance.

WHATEVER view we may adopt as to the mechanism of heredity
there can be no doubt as to the existence of a material foundation
for the transmission of characters from parent to offspring. We
know that the protoplasm which forms the material basis of all
living things is continuous, by means of the germ cells or gametes,
in an unbroken stream from one generation to another. At each
conjugation of gametes, however, two such streams are mingled,
and the offspring receives part of its initial stock of protoplasm
from one parent and part from the other, each part bringing with
it all the potentialities which may have been derived from a long
line of ancestors. Bearing these facts in mind, we must now
direct our attention to some results which have been obtained
by the "Mendelian" method of attacking the problem of
heredity.

In the early part of the nineteenth century much attention was
paid by horticulturists to experiments on the hybridization of
plants. It was found that, within certain limits, it was possible
to fertilize the flower of one variety of plant with the pollen
of another variety, and in this manner to alter the character
of the offspring. Many crosses or hybrids between different
varieties were thus obtained, differing in various ways from
the parent plants. It was also found that by repeatedly fertilizing
the flowers of one variety with the pollen of another the
descendants of the first could be entirely changed into the second.
This clearly proved that all the distinctive characters of the male
parent were in some way represented in the pollen grains, and
could be transmitted by these to future generations, but it also
proved something more, it proved that it was possible to
eliminate the special characters of the female parent, or at least
to prevent them from manifesting themselves in the offspring.

As a rule it is only possible to bring such experiments to a

MENDEL'S WORK ON HYBRIDIZATION 195

successful issue when working with closely related species or
varieties. The flower of one kind of orchid, for example, may
perhaps be fertilized by the pollen of a different orchid, but the
pollen of such a plant as a lily would probably not have the
slightest effect upon it.

The first observer to throw a clear light upon the meaning of
the remarkable results obtained by hybridization was Gregor
Johann Mendel, a native of Austrian Silesia, born in 1822, whose
work has recently attracted so much attention. As Abbot of /
Briinn, he was the happy possessor of a garden, and presumably
also of that peace and quiet which are so essential to intellectual \
work. He was not content with merely casual experiments ; he
had time to think about what he was doing and he attacked the
problem in the true scientific spirit. Unfortunately for science,
however, his seclusion was a little too complete. He was content
to publish his results in 1865 and 1869 in the proceedings of a local
natural history society, where they remained buried and almost
unnoticed for more than thirty years. Thus, by the irony of fate,
our own illustrious countryman, Charles Darwin, although a con-
temporary of Mendel, probably never heard of those remarkable
discoveries which bid fair to solve some of the problems in which
he himself was so deeply interested.

In order to gain an insight into the nature of Mendel's work
we cannot do better than turn to his original memoir, entitled
" Experiments in Plant Hybridization," of which an excellent
translation has been published by Professor Bateson,1 one of the
leading exponents of what is now termed Mendelism. Even fifty
years ago, experiments in hybridization were, as we have already
seen, no new thing. Mendel had many predecessors in this line
of research, but it was reserved for him to introduce exact
statistical methods into the work, and it is to these methods that
he owed his success. It was already known that hybridization,
or the crossing of distinct species and varieties, might result
in the production of several distinct types of offspring from
the same hybrids. Mendel was not content with this know-
ledge : he proceeded by numerous and often repeated experi-
ments, extending over several years, to classify the offspring
produced by hybridization, to follow out their descendants from
generation to generation, and above all to find out the exact
numerical proportions in which the different types appeared in

1 Vide Bateson, " Mendel's Principles of Heredity " (Cambridge, 1909).

o 2

196 OUTLINES OF E VOLUTION AKY BIOLOGY

successive generations. It was the discovery of these proportions
that furnished the clue to the mystery.

Mendel found the material which he required for his experi-
ments -chiefly in the numerous garden varieties of the common
edible pea. How these varieties first originated we do not know.
Possibly they arose as sudden and apparently spontaneous
variations, or mutations. No less than twenty-two such varieties
were chosen for experiment.

The next thing was to select certain differentiating characters
upon which to concentrate attention, and this is a very important
point. Of course, all the varieties agreed with one another in
most of their characters ; in other words, they were all edible
peas exhibiting the specific characters * of the plant known to
botanists as Pis um sativum, but they differed in numerous minor
features. Of these differentiating characters Mendel selected
seven for the purposes of his experiments, amongst which I need
mention only two : (1) the form of the ripe seed, which, roughly
speaking, may be either rpund and smooth or angular and
wrinkled ; and (2) the difference in colour of the seed-contents or
cotyledons, which may be either yellow or green and which usually
determine the colour of the seed as a whole.

The two differentiating characters of each pair were artificially
united by cross-fertilization, the flowers of a round-seeded pea
being fertilized by the pollen of a wrinkled-seeded pea, those of a
green -seeded pea by the pollen of a yellow- seeded pea, and so on.

In this way a number of hybrid plants, represented in the first
instance by seeds, were obtained.1 These hybrids constituted
what modern writers term the Fl or "first filial" generation,
and the curious fact was observed that every hybrid closely
resembled one of the two parents, instead of being, as is- frequently
the case in other hybrids, intermediate in character between
them. Further experiments with the offspring of the hybrids,
however, showed that, although only one of the two contrasted
characters manifested itself in the hybrid, the other was there
also in a latent or dormant condition, and appeared again in

1 It must be remembered that the pea-seed consists chiefly of the young plant
in a dormant condition, and that its characters are therefore, with the exception
of those appertaining to the seed-coat, which are not considered in this
chapter, those of the sporophyte generation which follows the. conjugation of the
gametes. The fact that the complete life-cycle of the plant really includes two
generations, a well developed sporophyte and a vestigial gametophyte, does not
affect the Mendelian conclusions.

HYBRIDIZATION IN PEAS 197

subsequent generations. The character which appears in the
hybrid is said to be dominant, while that which is suppressed is
said to be recessive. In the case of these peas, then, one character
is always dominant over the other in the hybrid, and, moreover,
it is always the same character, and it does not matter whether
it is derived from the male or from the female parent. Thus the
round form of seed is dominant over the wrinkled, the yellow
colour of the seed-contents over the green, and so on.

Having obtained the hybrids, the next step was to follow the
history of the offspring throughout successive generations. For
this purpose the flowers of the plants raised from the hybrid
seeds were allowed to fertilize themselves with their own pollen,
no further crossing being permitted. The seeds thus obtained
(constituting the F2 or " second filial " generation) were now
again apparently of two kinds, resembling the two original parent
forms from which the hybrid was produced. Moreover, these two
kinds occurred in definite proportions, three of the dominant to
one of the recessive — three round seeds to one wrinkled, three
yellow to one green, and so on. Of course, these proportions are
only averages, and, in order to eliminate errors of chance, large
numbers of observations must be made — the greater the number
the more reliable the result. To take an actual example from
Mendel's work, 7,324 seeds were obtained in the second trial year
from the hybrids between round and wrinkled. Of this number
5,474 were found to be round, and 1,850 wrinkled — showing the
ratio of 2'96 to 1. Again, out of 8,023 seeds produced by the
hybrids between green and yellow, 6,022 were yellow and 2,001
green, the ratio being 3*01 to 1. These experiments have often
been repeated during recent years, especially by Mr. A. D.
Darbishire,1 and it is found that the average ratio of three to one
is always maintained.

The significance of this proportion is not at first sight obvious.
It is necessary to continue the experiment for at least another
generation in order to gain further insight into the matter. We
have as the result of the self-fertilization of our hybrids
apparently only two kinds of seeds or plants, which we may
call D, showing the dominant character, and R, showing the
recessive character, in the proportion of 3 D to 1 R. If these

1 Some beautiful illustrations of Mendelian results in both plants and animals
are given in Mr. Darbishi re's recent work "Breeding and the Mendelian Discovery "
(Cassell & Co., Ld., 1911).

198 OUTLINES OF EVOLUTIONABY BIOLOGY

plants are again cultivated and allowed to fertilize themselves we
find that every one of the B's breeds true, and no matter how
many generations we raise they will always remain of the
recessive type. These are now called "extracted recessives."
When, however, we cultivate the plants showing the dominant
character in the same way we soon find that, though similar
to one another externally, they are not in reality all alike,
for one-third of them will yield nothing but D's, while the
remaining two-thirds will yield D's and B's in the same
proportion of 3 to 1 as the original hybrid. Moreover, the
D's obtained from the one-third will continue to breed true
from generation to generation, and do what we will we can
never get an B out of them again. They are called " extracted
dominants."

It is obvious then, from these experiments, that the apparent
D's are not all true D's, but that two-thirds of them contain the
recessive character in a latent or dormant condition, and are,
therefore, still hybrid in composition. If we indicate those which
we can thus prove by breeding to contain the recessive character
by the letters D (B), we can sum the whole story up in the following
simple diagram :—

D x R (Original Parents)

Generation

D D D D D D® D® R D D(R) D® R R R R R - - - F

Extracted dominants

FIG. 79. — Monohvbridism.

Thus we see that from generation to generation of the
offspring of the hybrid there goes on a constant sorting out into
three categories, the two original parental types and the hybrid
form being always produced in definite proportions. If we
assume that each kind produces on an average an equal number
of offspring, we shall see from the diagram that in the course of a
few generations of undisturbed reproduction by far the greater
number of the plants will have finally reverted, in equal propor-
tions, to one or other of the original types, but a certain number

EXPLANATION OF MENDELIAN EESULTS 199

will always remain in the hybrid condition and will continue
to split up as before in the characteristic Mendelian proportions.
We see, then, from Mendel's experiments, that in the case of
peas, where one of the two contrasted characters is dominant
over the other, the offspring of the first hybrid appear, in the
proportion of three apparent dominants to one recessive, but that
further analysis shows that the real proportion is

1 D : 2 D (R) : 1 R.1

This proportion has since been observed not only in peas but
in a large number of other cases, including animals as well as
plants. It is evidently a phenomenon of very common
occurrence, though we cannot as yet say that it occurs
universally whenever two forms with contrasted characters are
crossed. Moreover, as we have already noticed, the phenomenon
of dominance is not always shown, and the hybrid may exhibit
a character intermediate between those of the two parents or
different from either. The occurrence of Mendelian propor-
tions, however, is sufficiently frequent to demand explanation,
and this explanation we must now seek.

Suppose we take a large number of black and an equal number
of white counters, all alike in shape, size and weight, and after
shaking them up thoroughly in a bag, draw them out two at a
time, and one on top of the other, without looking. Each pair
that we draw out may consist of white over white, black over white,
white over black, or black over black. If we draw out a sufficiently
large number of pairs entirely at random in this way and then count
them, we shall find that they occur in the proportion : — 1 W W :
1 B W : 1 W B : 1 B B, or, since B W and W B may be looked
upon as the same, 1 W W : 2 W B : 1 B B, or one all white, two
white and black, and one all black ; this being, of course, only
what is to be expected in accordance with the mathematical law
of probability. This is also the same as the simple Mendelian
proportion, and at once suggests that the latter may, perhaps, be
explained in a similar way as the result of random union of
characters in accordance with the laws of chance.

We know that each individual plant or animal produced by
sexual reproduction is formed by the union of two germ cells or
gametes. Now these gametes are formed in very large numbers

1 This is sometimes written more fully 1 D 1) : 2 D (R) : 1 R R. The reason
for so doing will be obvious from what follows.

200 OUTLINES OF EVOLUTIONARY BIOLOGY

in the parent organisms, and if we suppose that each one contains
only one of the two contrasted characters with which we are
experimenting, and if we further suppose that the two kinds are
produced in equal proportions and that they unite at random,
then, in accordance with the law of chance, we should expect
to get in the offspring the proportion of one with the one
character, two with both characters and one with the other
character, exactly as Mendel found in his experiments.1 There
seems to be no other possible explanation of the Mendelian
phenomena.

The conclusions to be drawn from these results are of
fundamental importance. In the first place, we learn that small
individual characters may be separately represented in the
germ cells and separately transmitted from parent to offspring.
This indicates that the entire organism may be built up of a
number of " unit characters," and if we can once establish
the general occurrence of such unit characters we shall have
taken a long stride towards the understanding of the laws of
heredity.

In the second place, we may conclude from these experiments
that, as regards the unit characters with which we are dealing,
we have a complete segregation amongst the germ cells or
gametes, each of which carries only one of each pair of contrasted
characters (technically termed allelomorphs), and in this respect
it makes no difference whether the germ cell be male or female.
Thus we arrive at what is sometimes termed the doctrine of the
" purity of the gametes " — purity, that is to say, writh regard to
each character of any such contrasted pair, and without reference
to the other innumerable characters which must be represented
in the germ cells in order that the whole complex structure of the
organism may be developed from them.

We may now examine an example of a class of practical
problems which may be solved by the application of -such simple
Mendelian principles.

Fowl-fanciers are well acquainted with a particular breed of
fowl known as the blue Andalusian. It has long been known
that this kind of fowl cannot be made to breed true. Some of

1 A little consideration will show that if the contrasted characters are repre-
sented in equal proportions of both male and female gametes, the facts that a
male must always unite with a female, and that the actual number of sperm cells
produced is usually very much larger than the number of ova, will not affect the
result.

HYBRIDIZATION IN FOWLS 201

the offspring will be white and others white with black splashes,
the remainder being like the parents. It has been shown,
moreover, that the three kinds of chicken occur, on an average, in
definite proportions, a quarter being black, a half blue, and a
quarter white splashed with black. Here we have the familiar
Mendelian proportion 1:2:1, suggesting that the blue Andalusian
is really a hybrid, and that the so-called " wasters " are the parent
forms. It is easy to prove that this is the case, for if the two
kinds of waster are mated we invariably get the blue Andalusian
again. The old idea of the so-called practical breeder would have
been to go on destroying the wasters and carefully selecting the
blues in order to maintain the "purity of the breed." We now
know, however, that there is no such thing as a pure blue Andalu-
sian breed ; the blue is really a hybrid, and you can get more blues
by mating the wasters than by breeding from the blues
themselves. This case is also interesting as affording an example
of a hybrid which differs in character from either of the parent
forms.

If only a single pair of alternative characters or allelomorphs is
dealt with in the experiment the case is termed one of mono-
hybridism, and such cases yield the proportion 1:2:1 (or
apparently 1 : 3 in cases of dominance) amongst the offspring of
the hybrid, i.e. in the F2 generation. It will frequently happen,
however, that the two varieties which are united in the formation
of the hybrid will differ from one another as regards more than
one pair of contrasted characters. We may illustrate this by a
case of dihybridism, in which two pairs of contrasted characters
are involved. The case selected is one which shows us how,
under certain circumstances, we can obtain a hybrid exhibiting
an entirely new combination of characters, which in spite of its
hybrid nature will continue to breed true.

Suppose a gardener had only two kinds of peas, one with round
green seeds and the other with wrinkled yellow seeds, and that he
wished to obtain peas with wrinkled green seeds. Mendel has
shown us how he may get what he desires, both surely and
speedily, from the material already in his possession.

The necessary procedure will be evident from the following
scheme : —

Let R = round, W = wrinkled, Y = yellow, and G = green.
We know from previous experiments that R is dominant to W,
and Y to G.

202 OUTLINES OF E VOLUTION AKY BIOLOGY

E G crossed with W Y gives the hybrid, E (G) (W) Y, which, owing
to the dominance of E and Y, will appear in the form of round
yellow seeds (the brackets around the G and W indicating the
recessive nature of green and wrinkled). That is not what our
gardener wants, and if he knew no better he would naturally be

FIG. 80.— An Example of Dihybridism in Peas.

A, wrinkled yellow peas ; B, round green peas ; C, hybrid peas (Fi generation, round
and yellow in appearance) produced by crossing A and B ; D, plant of the FI
generation, with seeds of the F'2 generation, produced by self-fertilization and of the
four different kinds, round yellow, round green, wrinkled yellow, and wrinkled green,
lying in the pods. (Photographed from a preparation by Mr. Darbishire, who has
reproduced the same subject in colours in his work on " Breeding and the Mendelian
Discovery ").

much disappointed with the result. If, however, the hybrids be
allowed to fertilize themselves and reproduce, then their off-
spring will appear in the apparent proportions : —

9EY : 3 EG : 3 WY : 1 W G,

no account being taken of the non-apparent recessive characters.
The meaning of the proportion 9:3:3:1 exhibited in this

DIHYBEIDISM

203

case will be readily understood from the accompanying table
(Fig. 81) :-

Germ
c-ells

R Y

KG

W Y

W G

I

R Y

RG
VV Y
WG

R Y
BY

R(G)
R Y

(W)Y

R Y

(W) (G)
R Y

R Y
B(G)

R G
RG

(W)Y
R(G)

(VV) G
R G

R Y
(W)Y

R(G)
(W)Y

W Y
W Y

W (G)
W Y

R Y

(W) (G)

RG

(W) G

W Y

W(G)

WG
YV G

FIG. 81. — Dihybridism.

The constitution of the hybrid (in so far as the characters in
question are concerned) is E (G) (W) Y. Each of its germ cells,
however, contains one character, and one only, from each con-
trasted pair. Each may, therefore, contain E Y, R G, W Y or
W G. These different kinds of germ cells will occur on the
average in equal numbers, and on self-fertilization of the hybrid
flowers they will unite in pairs at random. The possible
ways in which such union may take place are shown in the table,
and if allowance be made for the phenomenon of dominance, in
accordance with which G disappears from view whenever it meets
Y, and W whenever it meets E (as indicated by the brackets in
the table), we get the apparent proportion of 9 round yellow
seeds, 3 round green seeds, 3 wrinkled yellow seeds, and 1 wrinkled
green seed. The number of wrinkled green seeds will be small at
first, but they will breed true, containing as they do only the
extracted recessive characters, for if a dominant character were
present it would necessarily show itself.

Thus we see that one way of producing new forms of plants
and animals is by the artificial combination of characters
which already exist in different varieties. These elementary or
unit characters can be brought together by the process of hy-
bridization, and new organisms produced in somewhat the same

204 OUTLINES OF E VOLUTION AEY BIOLOGY

way as that in which the chemist is able to produce new compounds
by analysis and synthesis from other substances.

Probably no man has made more successful use of the process
of hybridization in the production of new and valuable forms of
plant-life than Luther Burbank, at his celebrated California!!
nurseries. Mr. Burbank himself is unfortunately not a writer,
and for a scientific account of his work we are indebted to
Professor Hugo de Vries, who in his book on " Plant Breeding"1
describes what he himself saw and heard during his visits to
Mr. Burbank's farms. Some idea of the commercial value of
Mr. Burbank's work may be formed from the fact that special
companies have been formed for the propagation and sale of
some of these wonderful hybrids. White blackberries, stoneless
prunes, " plumcots " and thornless cacti are only some amongst
the many novelties which he has produced by crossing different
varieties by hybridization and thus combining two or more
desirable qualities in one plant.

Take, for instance, the case of the stoneless prune. It had
somehow or other come to the knowledge of Mr. Burbank that
about 200 years ago there existed in France a plum known as the
" Prune sans noyau." He succeeded in obtaining specimens of
this fruit, but it proved to- be of little or no commercial value
owing to its poor quality. Its one valuable feature was its stone-
lessness, and Burbank set to work to transfer this character, by
hybridization, to a variety of good quality. He succeeded, and
there appears to be no reason why the stoneless character should
not be similarly implanted upon all the different varieties of
plums now in cultivation.

Similarly, the "plumcots" are hybrids between plums and
apricots, which Professor de Vries speaks of as " most delicious
and beautiful fruits."

It must be admitted that Bui-bank's work is practical rather
than scientific. He apparently pays no attention to the law of
Mendel in his operations, and just goes on hybridizing until he
gets what he wants. In this process enormous numbers of plants
which do not fulfil his requirements have to be destroyed for
every one that is worth preserving.

He is not necessarily concerned with the production of pure
breeds ; of forms, that is to say, which will breed true from seed,

1 Hu2;o de Vries, " Plant Breeding " (London, Kegan Paul. Trench, Triibner & Co.,
Ld., 1907).

PEACTICAL APPLICATIONS 205

or, in other words, of forms which will hand on their desirable
qualities to future generations by heredity. This does not matter
in the case of fruit trees and other plants which can be propa-
gated by buds independently of sexual reproduction. As a
general rule, a fruit tree cannot be relied upon to come true
from seed, the characters which it has received from different
ancestors not being permanently combined, but separating out
and undergoing fresh combinations in the sexual process, pro-
bably in accordance with Mendelian principles. The seedlings
will therefore be " degenerate," as horticulturists say, and will no
longer exhibit those valuable qualities which depend upon the
confluence in one individual of particular lines of ancestry.

The majority of Burbank's productions could not survive in a
state of nature at all ; they are essentially artificial and have to
be artificially propagated by means of buds or cuttings. In this
respect they are quite different from the pure races which it is
possible to produce by hybridization carried out in accordance
with Mendelian principles, in which new and permanent com-
binations of unit characters, capable of being transmitted by
heredity, are effected.

Professor Bateson, Professor Biffen, Mr. Hurst and others have
lately done much to demonstrate the possibilities of progress in
this direction, and the value which the application of the
Mendelian principles of heredity must have from the economic
point of view. We know now that such cereals as wheat and barley
obey the Mendelian laws of hybridization as regards various
important characters. So also do horses as regards the colour of
their coats, and human beings as regards the colour of their eyes,
and in some other respects, especially as regards the inheritance
of certain diseases. In short, it appears certain that Mendelian
principles have a very general application, and the economic
value of the knowledge of such principles must be enormous.
Professor Biffen has shown, for example, that susceptibility and
insusceptibility to that destructive disease in wheat known as
" rust," behave as Mendelian characters, and that it is possible
by a very simple process of hybridization to confer immunity
from this disease upon naturally susceptible varieties.

From the theoretical point of view the great value of the
Mendelian experiments lies in the possibilities which they present,
at any rate in certain cases, of analyzing the constitution of the
hereditary substance (germ plasm), and thereby gaining some

206 OUTLINES OF E VOLUTION AEY BIOLOGY

insight into the real nature of heredity. We may regard the
existence of " unit characters " as being conclusively demonstrated
by these experiments, and the fact that we are able to interchange
these characters and to add and subtract particular characters to
and from the organism seems to indicate very clearly that the
germ plasm must contain material primordia or " factors," as they
are often called, which are responsible for the development of
these characters.

In so far as they demonstrate the existence of such factors —
which evidently correspond very closely with Weismann's hypo-
thetical determinants — the Mendelian experiments may be taken
as affording confirmation of Weismann's theory of the constitu-
tion of the germ plasm. Their results also harmonize very well
with those of recent cytological investigations. We have seen
that there is strong evidence for regarding the chromatin
substance of the nucleus as the material basis of heredity. We
have further seen that this chromatin is very complex in
structure and that the chromosomes, or units of the highest order
of which it is composed, can in many cases be optically resolved
into chromomeres or units of the next lower order. According
to Weismann's theory these chromomeres or ids are in their
turn made up of the determinants. Moreover the associa-
tions and redistributions of chromosomes which take place
in the process of " reduction " and in the conjugation of the
germ cells seem to afford ample opportunity for those permuta-
tions and combinations of factors the occurrence of which is demon-
strated by the Mendelian experiments. As Professor Farmer
observes, " The facts of meiosis "J are seen to fall completely into
line with conclusions drawn from experiments on breeding as far
as the numerical distribution of characters is concerned." '

The doctrine of the " purity of the gametes " teaches us that
any given mature germ cell contains only one member of any
given pair of alternative factors or primordia (allelomorphs).
This may be accounted for by the pairing of the chromosomes
and the subsequent halving of their number which take place at
some period or other prior to the formation of the gametes.
There is good reason to believe that each member of such a pair
of chromosomes is homologous with or morphologically equivalent

1 I.e. the phenomena accompanying the reduction in the number of chromosomes
which takes place periodically in all typical organisms (vide. Chapter X.).

2 Croouiau Lecture. Proceedings of the Royal Society, B, Vol. 79, 1907.

HOMOZYGOTE AND HETEROZYGOTE 207

to the other, the difference between them lying in the fact that
one is paternal and the other maternal in origin.1 Reduction (or
halving of the total number of chromosomes) is accomplished by
the distribution of the two members of each pair to different
daughter cells. Hence if the paternal and maternal chromosomes
contain different alternative factors (or determinants), derived
from different parents, so also will the gametes when these are
formed.

Suppose we represent the two members of any pair of factors
by the letters a and b, then any one gamete may contain either
a or b, but not both. When the gametes unite in conjugation the
zygote will again have both maternal and paternal chromosomes
and there will be three possibilities as to its constitution in
regard to the characters in question. If a gamete containing a
happens to unite with another containing a the zygote will
contain the pair a a, and will be termed in Mendelian phraseology
a " homozygote." 2 If a gamete containing b happens to unite
with another containing b the zygote will contain the pair b b and
will again be a homozygote. If, on the other hand, a gamete
containing a unites with another containing b the zygote will
contain the pair a b, and such a zygote is termed a " heterozygote "
or hybrid.

In accordance with what is termed the " presence or absence
hypothesis " one of the two " factors" in an allelomorphic pair may
be merely negative in character ; i.e. it may have no real existence
as a material primordium or determinant. In the words of
Professor Bateson, "All observations point to a conclusion of great
importance, namely that a dominant character is the condition
due to the presence of a definite factor, while the corresponding
recessive owes its condition to the absence of the same factor. This
generalization, which so far as we yet see, is applicable throughout
the whole range of Mendelian phenomena, renders invaluable
assistance in the interpretation of the phenomena of Heredity.
The green pea, for instance, owes its recessive greenness to the
absence of the factor which, if present, would turn the colouring
matter yellow, and so forth." 3

It is obvious that if, in our general formula, we take a to
represent the dominant factor and b the recessive, we might

1 Compare Chapter X.

2 This term has been extended to the fully developed organism which arises from
the zygote.

3 Bateson, " Mendel's Principles of Heredity" (Cambridge, 190'J), pp. 53 — 54.

208 OUTLINES OF EVOLUTIONARY BIOLOGY

substitute not a for b without altering the result, and the differences
between the paternal and maternal chromosomes of any given pair
may depend merely upon the absence from one or the other of
one or more factors which are present in its mate.

Mendelian phenomena are often greatly complicated and
rendered very difficult of interpretation by the fact that the
activity of a given factor, or its power to influence the development
of the zygote, may be dependent upon the presence of another
factor belonging to a different " allelomorphic pair." In other
words, a given character may depend not merely upon the
presence of a single factor in the zygote but upon the co-operation
of two or more factors, and if these factors happen to be separated
in the process of reduction and do not happen to come
together again in the union of the gametes to form the zygote,
then the character in question l will not appear in the organism
developed from the zygote. In this way it may happen that a
character which was present in the ancestors of a particular
organism may disappear for many generations and then suddenly
re-appear as the result of some cross or hybridization in which
the necessary factors happen to be brought together again in the
zygote. In this way are explained those cases of " reversion " to
ancestral types which Darwin found to occur so frequently as the
result of cross-breeding.

One of the most thoroughly investigated cases of this kind 2 is
that of certain white sweet peas belonging to the variety known
to horticulturists as " Emily Henderson." Plants of this variety
are not really all alike but differ from one another in the shape of
their pollen grains, and when the two kinds, distinguishable only
in this way, are crossed, the offspring are frequently found to
possess purple flowers resembling those of the wild Sicilian pea
from which our cultivated varieties have been derived. It is not
necessary to go into the somewhat complicated analysis of this
case in any detail, but it has been demonstrated by the researches
of Professor Bateson and Professor Punnett that the re-appearance
of the lost colour in the hybrid is due to the re-union in the zygote
of certain factors which had become separated at some period or
other in the ancestral history of the white-flowered parents.

The existence of interactions of this kind between the different
factors or determinants in the germ plasm, giving rise often to

1 Professor Bateson terms such characters " compound characters."

2 Bateson, op. cit., pp. 89 et setj.

GALTON'S LAW OF INHERITANCE 209

very complex results as regards the characters of the offspring, is
a sufficient explanation of the fact that many cases of hybridism
are at present incapable of interpretation in terms of the
Mendelian theory, and such cases cannot be legitimately used as
arguments against the validity of that theory. It is obviously
impossible to explain the results of any particular cross until we
have some knowledge, not only of the extremely complex con-
stitution of the parental germ plasm on both sides, but also of
the way in which the factors which meet in the zygote may be
expected to influence one another.

Mendelian phenomena, of course, can occur only as a result
of crossing or hybridization at some stage or other of the
ancestral history, and it does not seem likely that such crossing
has had any very great influence upon the evolution of plants and
animals in a state of nature. In cases of monohybridism, as we
have seen above, the hybrid form in the course of a few generations
would probably be quite swamped by the numerical preponderance
of the pure extracted forms which have reverted to one or other
of the original parental types, the hybrid itself never being
permanently fixed. New and permanent combinations may, of
course, sometimes arise naturally through dihybridism, but they
would at first be produced in very small numbers and could only
be expected to survive in the struggle for existence if they
happened to possess some material advantage over the parent
forms. Thus it appears that in a state of nature the results of
hybridization tend to be automatically eliminated.

Attention has frequently been called to the alleged discrepancy
between the results of the Mendelian experiments and those
obtained by the late Sir Francis Galton and others by statistical
methods based upon the quantitative estimation of characters in
a number of successive generations. As a result of his investiga-
tions Galton was able to formulate the following " Law " of
inheritance : — " The two parents contribute between them on the
average one-half, or (0'5) of the total heritage of the offspring ;
the four grand-parents, one-quarter, or (0'5)2 ; the eight great-
grandparents, one-eighth, or (0'5)3, and so on. Thus the sum
of the ancestral contributions is expressed by the series { (0'5) -f-
(0'5)2 + (0'5)3, &c.}, which, being equal to 1, accounts for the
whole heritage."1

This series has been modified by Professor Karl Pearson for

1 Proceedings of the Royal Society of London, Vol. LXL. 1897. p. 402.
B. P

210 OUTLINES OF EVOLUTIONARY BIOLOGY

certain reasons into which we cannot enter in this place, but it
may be taken as a substantially correct expression of the results
obtained by biometries! inquiry. If we remember that Galton's
Law merely expresses the average results which may be antici-
pated from the interbreeding of a large population, in which
hybridization probably plays a very small part, it does not appear
in any way antagonistic to the Mendelian theory. It is of course
not applicable to individual cases of hybridization, where we are
concerned, not with small variations in the same characters, but
with the permutations and combinations of alternative character
units.

Intimately bound up with Galton's Law of Inheritance is
another important generalization known as the Law of Filial
Regression, which we owe to the same distinguished philosopher.
The law of ancestral inheritance teaches us that on an average
the individual derives half its characteristics from its immediate
parents and the remaining half from its more remote ancestry.
If the immediate parents, or one of them, happen to depart from
the average condition of the race in respect of any character
there will be a tendency on the part of the offspring to inherit
the deviation in question ; but not to the same extent, for the
influence of the more remote ancestry will tend to counteract
that of the parents and cause a partial return — or regression —
towards mediocrity. As a result of his statistical investiga-
tions Galton estimated that the offspring of parents exhibiting
a marked deviation from the average would tend to inherit that
deviation to the extent of only one-third of its magnitude in the
parents (the mean of the two parental deviations being taken as
the standard of comparison). In the words of Professor Karl
Pearson, " It is the heavy weight of this mediocre ancestry which
causes the son of an exceptional father to regress towards the
general population mean ; it is the balance of this sturdy common-
placeness which enables the son of a degenerate father to escape
the whole burden of the parental ill. Among mankind we trust
largely for our exceptional men to extreme variations occurring
among the commonplace, but ... if we could remove the
drag of the mediocre element in ancestry, were it only for a few
generations, we should sensibly eliminate regression or create a
stock of exceptional men." l

1 Grammar of Science, 1900, pp. 456 — 457.

PART IV.— THE THEORY AND EVIDENCE OF
ORGANIC EVOLUTION: ADAPTATION

CHAPTER XV

Organic evolution versus special creation — Spontaneous generation and
biogenesis — The origin of living things.

AT the present day we see the surface of the earth teeming with
hosts of living things, incalculable in number and of endless
diversity in form and structure. Every situation where life is
possible is occupied by plants or animals of some kind or other,
all specially adapted in bodily organization to the conditions
under which they have to maintain their existence. From the
bleak and inhospitable summits of high mountain ranges to ocean
depths which can be measured in miles ; from the perpetually
frozen circumpolar regions to the torrid zone on either side of
the equator, living things abound. Seas, rivers, lakes, dry land
and air have all alike been taken possession of by representatives
of the animal and vegetable kingdoms. A single drop of water
may contain thousands of organisms, and every chalk-cliff, coal-
seam or peat-bog testifies to the countless myriads which have
lived in past times and whose remains have contributed in no
small measure to the formation of the earth's crust.

In the animal kingdom alone it is probable that at least a
million different kinds or species are living on the earth at the
present day. Some half million or so have already been dis-
covered, named and more or less imperfectly described, while
every exploring expedition brings back many which have never
before been seen.

Nevertheless there must have been a time when no living
things whatever existed on the earth. According to the
nebular hypothesis our planet is still gradually cooling from a
molten state, preceded probably by an incandescent gaseous
condition, which must have rendered the existence of any proto-
plasmic organism a physical impossibility. Protoplasm becomes

p 2

212 OUTLINES OF EVOLUTIONARY BIOLOGY

coagulated and thereby destroyed as living substance at com-
paratively low temperatures, and cannot have been present on the
earth until the surface of the latter had not only solidified but
also cooled down to something approaching its present degree
of heat.

Living things must therefore have first put in an appearance
at some definite period well advanced in the earth's history-
According to the biblical account given in the Book of Genesis
their advent was due to a series of special acts of creation per-
formed by the Creator at the appropriate time, and this account
forms the basis of the doctrine of Special Creation which still
survives amongst uneducated people. Upholders of this doctrine
maintain that each kind of plant or animal was not only created
as a separate kind and in a state of full perfection, but was
specially designed to suit the conditions under which it was
placed, and in this way the marvellous adaptation of plants and
animals to their environment was supposed to be accounted for.
The fish was created to swim in the sea, the horse to run on
dry land, the monkey to climb in the tree and the bird to fly in
the air, and each was constructed in accordance with its foreseen
requirements. Moreover, each species was supposed to be-imniut-
able, propagating its own kind by reproduction but never changing
into a different kind.1

Such a view of the origin of living things could only have arisen
in a state of almost complete ignorance of the phenomena which
have to be accounted for, and at the present day it has been
entirely superseded, as a scientific theory, by the doctrine of
Organic Evolution. This doctrine teaches us that species or kinds
are not immutable but are subject to changes whereby one may
give rise to another, and that all existing species are the more or
less modified descendants of pre-existing ones. Modern biologists,
moreover, maintain that the further back we trace the ancestral
history of living things the less diversity of structure do they
show, until finally, if we could trace them back to their origin,
we should find all the different lines of descent converging towards
a common starting point far back in geological time. We shall
see presently that the adaptation to environment which forms
such a characteristic feature of all living organisms can be
explained just as well in accordance with the theory of organic

1 Compare the quotation from Linnaeus on p. 222 (1st footnote) and that from
Buffon on p. 369.

SPECIAL CREATION AND EVOLUTION

213

evolution, or transmutation of species, as in accordance with that
of special creation.

The fundamental difference between these two opposing and
irreconcilable doctrines can be expressed by means of the
accompanying simple diagrams (Fig. 82) : —

• • --' b c de abcde

?????/' ? * t ' ? J * 5 I Z 3* 5 I 3 J45678 I Z34567 I 23* 1254512345

B' C' D'

Special Creation

Evolulion

FIG. 82.

The letters A — E are supposed to represent five closely related
existing species in each case. According to the doctrine of
special creation each of these species has persisted without altera-
tion from the time when it was first created, so that the
ancestral species may be represented by the letters A' B' C' D'
and E', and the lines of descent of existing species run parallel
with one another. The idea of evolution, on the other hand,
is expressed by showing these lines of descent diverging from
a remote ancestral species, x, which may have been quite different
in character from any of the existing species.

Any existing species at the present day is made up of a
larger or smaller number of individual organisms, and we know
as a matter of fact, proved by daily observation, that these

214 OUTLINES OF EVOLUTIONAKY BIOLOGY

individuals arise by some process of reproduction from pre-exist-
ing pareut individuals. This is represented in the upper parts
of both diagrams, where a 1 — 8, b 1 — 7, c 1 — 4, d 1 — 5 and
e 1 — 5, on each side, are supposed to represent small groups of
individuals of the species A B C D and E respectively, each
group being descended from some common ancestor (or pair of
ancestors) which itself belonged to the species.

The study of this diagram is alone sufficient to afford strong
presumptive evidence in favour of the view of the evolutionist as
against that of the upholder of special creation, for the evolu-
tionist in his imagination extends backwards into the past the
processes which he sees taking place constantly at the present
day, and endeavours to account for the origin of species in
accordance with what he knows to be true of the origin of
individuals. In other words, the diagram expressing the idea of
organic evolution is consistent throughout, whereas that which
represents the idea of special creation is made up of two incon-
gruous portions, and is therefore less likely to be correct. The
great weakness of the doctrine of special creation, however, lies
in its failure to explain countless facts of comparative anatomy,
embryology, geographical distribution and palaeontology, all of
which are readily explicable in terms of evolution. We shall
deal with some of these facts in subsequent chapters.

It is obvious, then, that in order to be logically consistent the
evolutionist need not postulate more than a single starting point
for the evolution of the whole organic world, though he is not
obliged to limit himself in this way should evidence be forth-
coming that there has been more than one starting point. This
brings us to the consideration of the extremely difficult and at
present quite insoluble problem of the origin of the first living
organisms.

Here again we may first inquire whether our experience of what
takes place at the present day throws any light upon the problem.
Many people in the past have held, and some few still maintain,
that living things may, even now, arise from not-living matter.
This is known as the doctrine of " Spontaneous Generation " or
abiogenesis. One of the earliest expressions of this idea is met
with in classical mythology, in the story of the birth of Aphrodite
from the sea-foam ; and Vergil, in the Georgics, accepts the prin-
ciple involved therein apparently in perfectly good faith.

Vergil's account of the manner in which a swarm of bees may

SPONTANEOUS GENERATION 215

be produced from the decomposing body of a dead ox affords a
very striking illustration of that want of scientific training in
observation and reasoning which has led to so many erroneous
beliefs. He tells us, with much elaboration of detail, that if the
body of an ox is well beaten and enclosed in a suitable chamber a
swarm of bees will arise from it within a certain number of days.
We now know perfectly well what may really happen under these
circumstances. In the first place the supposed bees are not bees
at all, but drone-flies, which superficially resemble bees, though
easily recognizable as belonging to a totally different order of
insects by the fact that they possess only two wings instead of
four, and consequently any husbandman who followed Vergil's
instructions must have been grievously disappointed in his
expectations of honey. In the second place the drone-flies are not
spontaneously generated from the body of the ox, or from any-
thing else, but are hatched out, first in the form of maggots, from
eggs which were laid by pre-existing flies. These maggots,
.having fed abundantly on the decaying carcase, presently undergo
their metamorphosis and emerge as flies.

Many similar instances of alleged spontaneous generation, all
resting upon gross ignorance of the real facts of the case, might
be collected from the writings of ancient and mediaeval authors.
The cruder stories, which could be easily disproved by simple
observation, were soon cast aside as fables under the influence of
modern scientific methods. The invention of the microscope,
however, and the consequent revelation of a new world of
hitherto invisible organisms, led to a revival of the doctrine of
abiogenesis. It was noticed that organic infusions, even after
boiling, presently became densely filled with various kinds of
micro-organisms, especially Bacteria, and as the boiling was
supposed to have killed any organisms that might have been in
them at first it was argued that living things arose in them
de novo, by spontaneous generation. This was, however, merely
a revival of Vergil's story of the swarm of bees in a more refined
form. Had Vergil's ox been protected from flies none of the
alleged bees would have been produced, and the careful experi-
ments of such observers as Tyndall and Pasteur have conclusively
demonstrated that if the organic infusions are adequately pro-
tected from the countless microscopical germs which float in the
air they will remain free from living organisms, and consequently
from putrefaction, for an indefinite period, provided always that

216 OUTLINES OF EVOLUTIONARY BIOLOGY

they are themselves first properly sterilized. This sterilization,
however, cannot always be effected by a single boiling, for the
germs of some organisms are extremely resistant to heat and
special precautions have to be taken accordingly.

Every story of alleged spontaneous generation has, so far,
failed to stand the test of scientific criticism and experiment, and
accordingly biologists are now almost unanimous in maintaining
the contrary doctrine of biogenesis, which teaches that, at the
present day at any rate, living organisms arise only by repro-
duction from pre-existing living organisms. Nevertheless there
must, as we have seen, have been a time when there were no
living things on the earth, and therefore living things must have
either reached the earth from some outside source or have arisen
on the earth itself from matter which was previously not-living,
or in other words, by spontaneous generation. It seems as if we
had got to take our choice between these two alternatives, neither
of which is at all easy to accept.

It has been maintained by some that organic life is eternal and
is transferred from one world to another — possibly on meteorites —
in the form of minute germs or "Cosrnozoa," but no one has ever
seen such Cosmozoa, and it is difficult to imagine any which
would be capable of surviving such a journey. It is perhaps less
difficult to believe that under certain conditions, of which we at
present know nothing and which perhaps no longer exist on the
earth's surface, living protoplasm may have arisen from in-
organic matter as the result of chemical and physical processes.
In this connection we must remember that protoplasm — the
physical basis of life — contains no chemical elements that are
not also found in inorganic matter and is, as a matter of fact,
constantly being built up from inorganic constituents in the
bodies of living organisms, though apparently only by the action
of pre-existing protoplasm.

Whether physical and chemical forces, such as we are
acquainted with, alone sufficed to bring about the transformation
of not-living, inorganic material into living protoplasm, or
whether, as some people suppose, some unknown " vital force "
was (and still is) involved in the process, must remain, for the
present at any rate, an open question.

The time has evidently not yet arrived for solving the great
problem of the origin of life, but we may, at any rate, legitimately
speculate upon the nature of the living organisms which first

THE FIRST LIVING THINGS

217

appeared on the earth. In the first place we may safely assume
that they were extremely simple in structure, for it is generally
agreed that the evolution of the higher forms of life has been
accompanied by a gradually increasing complexity of organization.
It is also certain that they cannot have been animals, for animals,
as we have seen in a previous chapter, are dependent for
their food supply upon other living things, being themselves
unable to build up the proteid molecule from inorganic constitu-
ents. Green plants are the great proteid manufacturers at the
present day, but only by virtue of the fact that they contain
chlorophyll, which enables them to utilize the energy of the sun's

FIG. 83. — Different Forms of Disease-producing Bacteria, X about 1500.
(From Strasburger, after Fischer.)

a, pus cocci ; b, erysipelas cocci ; c, gonorrhoea cocci ; d, bacilli of splenic fever; e, bacilli
of tetanus ; /, bacilli of diphtheria ; g, tubercle bacilli ; h, typhoid bacilli ; i, colon
bacilli ; k, cholera bacilli.

rays in the process of photosynthesis. Now chlorophyll itself is a
very complex substance, and we cannot suppose that the first living
things already possessed it. If, then, they had no other organisms
to feed upon, and if they possessed no chlorophyll, how did they
obtain the energy necessary to enable them to maintain life
at all?

The simplest and at the same time the smallest organisms known
to us at the present day are the Bacteria (Fig. 83), many of which
are so minute as to be hardly visible even under the highest
powers of the microscope. These Bacteria are neither plants
nor animals, but occupy a position lower than either. They have
not even attained to the dignity of perfect cells, for they exhibit
no proper differentiation into cell body and nucleus, the chromatin
substance, which in a typical cell is aggregated in the nucleus,

218 OUTLINES OF EVOLUTIONARY BIOLOGY

being scattered in minute granules throughout the cytoplasm
(ride Pig. 27). They contain no chlorophyll and the colourless
protoplasm is enclosed in a very thin and delicate cell wall. In
form they vary greatly, being sometimes spherical, sometimes
rod-shaped and sometimes corkscrew-shaped, and frequently
they occur united together in chains. Some of them swim about
by means of cilia or flagella, others are motionless, and they
multiply by simple fission and also by the formation of spores,
which have extraordinary powers of resistance and thus serve to
secure the wide dispersal of these organisms by air and water.

A great number of different kinds of these Bacteria are already
known to us, and doubtless a vast number still remain to be dis-
covered. They occur practically everywhere ; earth, air and water
are full of them, and they exercise a most
profound influence — sometimes injurious
and sometimes beneficial — upon the lives
of other living things. Some of them are
the active agents in the putrefaction and
fermentation which are rapidly set up in
dead organisms, and others are responsible
for many of the most grievous ills which
living flesh is heir to. Thus most of them

FIG. 84. - Nitrosomonas live either as saprophytes (upon dead
enropa-a, x 600. organic matter) or as parasites (upon

(From Percival's living organisms) and obtain their supply
< ' Agricultural Bacte- * . /

riology.") o* energy from already formed proteids

and other complex chemical compounds.

Some, however, are said to live free in the soil and to be able
to derive their food supply, and with it their energy, from purely
inorganic substances. These are called nitrifying Bacteria. One
form, Nitrosomonas (Fig. 84), converts salts of ammonia into
nitrites, and another, Nitrobacter, converts nitrites into nitrates.
In this way they perform an important function in preparing the
food material in the soil for the use of green plants, but their chief
interest for us lies in the fact that they are able, in some manner
which we do not understand, to build up their own proteid mole-
cules directly from inorganic constituents without the aid of chloro-
phyll. They show us that such a thing is at any rate possible,
and that therefore the first living things may have been able to
maintain their existence without either possessing chlorophyll
or having other organisms to feed upon.

SPONTANEOUS GENEKATION 219

It seeuis likely that of all the organisms known to us these
Bacteria come nearest to the first living things, and yet they
probably stand very far from them and represent a much later
stage in the process of evolution. Minute and apparently
simple as the Bacteria are, it seems more than probable that the
first living things were very much smaller and simpler, so that
even if we had them under the highest powers of our micro-
scopes we should be unable to recognize them. Weismann has
suggested that they may have been single biophors, i.e. vital
units of the first order, such as he believes to constitute the
ultimate living particles of protoplasm, and he has actually
proposed the name " Biophoridse " for these hypothetical free-
living primordial organisms.

These considerations throw a new light upon the question
of spontaneous generation, for if living matter is first formed
in such ultra-microscopic particles and can only be recog-
nized as living matter after it has reached a comparatively
high stage of evolution, it is obvious that we are not entitled to
say that it is never formed from not-living matter at the present
day. We cannot see it being formed, and we probably never
shall see it being formed, but it is possible that it is still
being " spontaneously generated " all the same. We are not
logically obliged, as we said before, to content ourselves with a
single starting point for organic evolution, and it would be quite
impossible to prove that all the different kinds of Bacteria, the
simplest organisms known to us, have descended from a single
ancestor. They may equally well have been derived from a
number of ancestral protoplasmic units which originated inde-
pendently from inorganic, not-living matter. If such an evenl
can have taken place once it may have taken place many times,
and may still be taking place around us, though the imperfect
means of observation at our disposal will not allow us to demon-
strate the fact. We may safely affirm, however, that no living
organism which we are at present capable of recognizing as such
has arisen, or ever will arise, by spontaneous generation, but that
all organisms known to us have been derived from pre-existing
organisms by some process of reproduction ; that, in the course
of long ages, they have undergone slow changes, whereby they
have become more and more diversified and usually more com-
plex in structure, and that in this way the evolution of the
animal and vegetable kingdoms has been brought about.

220 OUTLINES OF EVOLUTIONARY BIOLOGY

In concluding this chapter it may be well to remind the reader
that the idea of organic evolution is no novelty, but can be
traced back to the philosophy of ancient Greece. It will be more
convenient, however, to postpone what we have to say about the
history of the evolution theory to a later chapter and in the
meantime to familiarize ourselves with the theory itself and the
evidence upon which it is based.

CHAPTER XVI

The continuity of life — The conception of species — The principles of
taxonomy — The taxonomic evidence of organic evolution.

IN accordance with the theory of evolution we may picture
to ourselves the entire animal and vegetable population of the
earth, both past and present, as forming one vast, tree-like
organism, all parts of which, if we knew enough about their
history, could be traced into actual, although of course not simul-
taneous, protoplasmic connection with all other parts. This
tree commenced its growth far back in geological time, and its
branches became ever more and more ramified in succeeding
ages, and more and more diversified in character as they
diverged from one another. Only the youngest twigs of the
tree, however, are still actually alive, being represented by the
animal and vegetable population of the earth for the time being.
All the older parts have died away, most of them without leaving
any traces of their former existence, and thus has arisen the
actual discontinuity which is found to occur between the sur-
viving groups of organisms at any given period.

The tendency to structural variation which all organisms
exhibit, whatever may be its cause, is responsible for the
progressive diversity which is gradually set up between thfe
different branches of the organic tree. The combined action of
the forces of heredity and variation bring about " descent with
modification." How it is that such modification always leads,
in the long run, to a more or less perfect and often very
specialized adaptation of both plants and animals to the con-
ditions under which they live will be discussed subsequently.
At the moment we have to deal with things as we actually find
them.

In any scheme of zoological or botanical classification the
lowest unit must, of course, be the individual. The untrained
observer, acquainted only with the more familiar plants and
animals, sees no difficulty in arranging these in apparently

222 OUTLINES OF E VOLUTION AEY BIOLOGY

sharply circumscribed groups. A child collecting flowers by
the wayside will tell you without difficulty how many kinds he
has found, and these kinds — speaking generally — correspond
to the different species recognized by the botanist.

Even amongst the higher and better known plants and
animals, however, the so-called species are not always sharply
distinguishable from one another, for we not infrequently meet
with intermediate forms, or connecting links, between what are
usually recognized as distinct species. Linnaeus himself, who
pronounced most explicitly in favour of the doctrine of special
creation,1 was forced by his profound knowledge of the animal
and vegetable kingdoms to recognize this fact in the case of
certain groups of plants 2 ; and when we come to study some of
the lower members of the animal kingdom, as, for example, the
sponges, the difficulty of distinguishing species sharply from one
another frequently becomes so great that the definition of any
particular one is little more than a matter of individual
opinion. The more we study the animal and vegetable king-
doms, in short, the more clearly is the fact impressed upon us that
if wre could have before us all past and present individuals we
should find it impossible, except in an arbitrary manner, to
arrange them in species at all, for each kind would be found
to be connected with others by a series of small gradations.

According to Darwin, the differences which separate existing
species from one another have, at any rate in most cases, arisen
by the gradual accumulation of small successive variations. If
this be the case the fact that we are able logically to distinguish
such species from one another at all can only be due to that dis-
appearance of the older portions of the organic tree whereby
discontinuity has arisen between the surviving branches. In
any case it is quite certain that such destruction of former
connecting links has played a very large part in the separation of
those groups of individuals to which the term species is usually
applied. This process is clearly illustrated in the accompanying
diagram (Fig. 85), in which each enclosed area is supposed

1 " Species tot sunt, quot diversas formas ab initio produxit Infinitum Ens ; quae
delude fornise secundum generationis inditas leges produxere plures, at sibi semper
similes, ut Species mine nobis non sint plures, quara qua? fuere ab initio."

Linnaeus. Gen. Plant. (1737).

2 " Species Rosarum difficillime limitibus circumscribuntur et forte natura vix
cos posuit." LiniiiEiis. Spec. Plant., Ed. 2, p. 705 (1762). 1 am indebted to my
friend and colleague at the Linnean Society, Dr. B. Daydon Jackson, for showing
me these quotations.

DEFINITION OF SPECIES

223

to represent the limits of an existing species. Within each one
the individuals differ but slightly from one another, not more,
perhaps, than might be expected amongst the offspring of the
same parents. The three species represented have, however,
become more or less widely separated from one another by the
dying away of the branches from which they arose, and which
are represented in the diagram by dotted lines.

Darwin has told us that he looked " at the term species as
one arbitrarily given, for the sake of convenience, to a set of
individuals closely resembling each other, and that it does not
essentially differ from the
term variety, wrhich is
given to less distinct and
more fluctuating forms.
The term variety, again,
in comparison with mere
individual differences, is
also applied arbitrarily, for
convenience' sake." l

This, of course, is no defi-
nition of the term species
and was not intended as
such. A definition in ac-
cordance with the above
views might, however, be
given as follows : — " A
species is a group of
individuals that closely

FIG. So. — Diagram to illustrate the Separa-
tion of Species by the Dying out of
connecting Links.

resemble one another
owing to their descent from common ancestors, which has
become more or less sharply separated from all other co-existing
species by the disappearance of intermediate forms." Such a
definition would be in accord with the practice of many systematic
naturalists, who are in the habit of uniting species previously
considered as distinct whenever intermediate forms are found,
the former species being reduced to the rank of varieties of one
and the same species.

It is obvious that the element of time must be taken into
consideration in forming such a conception of species. At any

1 " Origin of Species," Ed. 6, p. 42. The reader should compare the very similar
views of Lamarck on the species question, given in Chapter XXIV.

224 OUTLINES OF EVOLUTIONARY BIOLOGY

given time the species living on the earth would form naturally
limited groups, which we should be able to define more or less
sharply if we had sufficient knowledge of the then existing
fauna and flora to enable us to trace their boundaries. If,
however, we were to include extinct forms in our survey, it is
obvious that the number of existing species which we should
recognize would be inversely proportional to the extent of our
palaeontological information, for the gaps between the survivors
would gradually be filled up by the discovery of intermediate
fossil forms.

Naturalists have long recognized the fact that there is another
way in which more or less sharply defined groups of individuals
may arise, and that is by the occasional and sudden appearance of
" sports," or " mutations " as they are now generally called,1 which
" breed true," handing on their special peculiarities from genera-
tion to generation. Darwin was of opinion that such sports differ
only in degree from the ordinary variations to the accumulation of
which he attributed the chief importance in the evolution of species.
At the present day, as we have already had occasion to point out,
some observers, and especially Professor Hugo de Vries,2 consider
them to be totally different in kind from fluctuating or small
successive variations, and regard them as affording the sole means
by which new species originate.

According to the mutation theory any species may, from time
to time, throw off such sports, either singly or in groups, and they
are from the first distinguishable by well defined, though it may,
be minute, characters from the parent form. These mutations
are regarded by de Vries as constituting " elementary species,"
which are already fully characterized and will not change again
unless by giving rise to fresh mutations. De Vries maintains
that many of the species of flowering plants as defined by
Linnaeus are really " aggregate species," each made up of a larger
or smaller number of such elementary species. The common
weed known as Draba verna, for example, regarded by Linnaeus
as constituting a single species, occurs under about 200 more or
less distinct forms. All of these so-called elementary species are
believed to come true from seed and to have arisen as sudden
mutations.

1 Compare Chapter XI.

a Vide " The Mutation Theory," by Hugo de Vries. English translation by
Farmer and Darbishire (Kegan Paul, Trench, Triibner & Co. Ld., London, 1910).

CLASSIFICATION 225

According to this view a newly arisen species, at its first origin,
is already sharply distinguished from the parent species and the
extinction of intermediate forms is not required to separate the two.

The elementary species, however, though apparently constant
in their characters, are often so similar to one another, and
they are moreover so numerous, that for practical purposes of
classification it would be undesirable to regard them as distinct
species in the ordinary sense of the word. It is far more con-
venient to maintain the older view that they are merely varieties
or subspecies, and their occurrence in no way invalidates our
conception of the slow, tree-like evolution of the organic world.
If de Vries be right as regards the importance he attributes to
them as the sole means by which species have originated, which
is extremely doubtful, it merely means that evolution has taken
place in a rather more jerky fashion than we previously supposed.
It will be necessary to return to this question when we come to
discuss the factors of organic evolution.1

It is not sufficient for the purposes of classification to arrange the
individual plants or animals with which we are acquainted in their
respective species. These species in turn must be arranged in
groups of higher order, which are termed genera. Each genus
stands to the species included within it in much the same relation
that the species themselves stand in with regard to individuals,
and related genera, though often sharply definable, are only so by
virtue of the disappearance of connecting links. The characters
by which genera can be distinguished from one another are of
a more deep-seated and fundamental nature than those which
distinguish species. Individual opinion, however, differs greatly
as to the exact limits which should be assigned to each, and it is a
common thing for an old established genus to be subdivided into
several as the result of the discovery of new species and the more
exhaustive study of old ones.2 It is all a matter of convenience,
and one may justifiably make a separate genus for any group
of closely related species which can be distinguished by some
common character from all other co-existing groups of species.
Many naturalists, however, consider that the mutual relation-
ships of species are more accurately expressed by making use of
subgenera as intermediate groups between genera and species.

1 Vide Chapter XXVII.

2 A striking illustration of this is afforded by the recent history of the old genera
Peripatus and Amphioxus, each of which is now subdivided into several.

B. Q

226 OUTLINES OF EVOLUTIONARY BIOLOGY

Genera in turn are grouped in families, separated from one
another by still more fundamental characters, and with sub-
families as intermediate groups where necessary. Families are
grouped in suborders and orders, orders in subclasses and classes,
classes in phyla (or subkingdoms), and phyla in kingdoms. Of
kingdoms only two are recognized, animals and plants, and even
these cannot be sharply separated from one another, because
amongst the lowest forms of life (Protista) the distinction between
animals and plants, as we have already seen, ceases to exist.

The arrangement of individual organisms in species, and the
grouping of these to form genera, families, orders, classes, phyla
and kingdoms, constitutes the work of the systematist and is
usually spoken of as classification, while the study of the
principles in accordance with which classification should be
carried out forms a special branch of biological science sometimes
known as taxonomy.

The first and greatest of the modern systematists was Karl von
Linne, now usually known as Linnaeus, a Swedish naturalist who
was born in 1707 and died in 1778. In his great work, the
" Systema Naturae," which is now recognized as the starting point
of modern systematic zoology and botany, he described all the
species of plants and animals then known, and endeavoured to
arrange them in a " natural system " in accordance with their
mutual resemblances and differences. As an aid to the
accomplishment of this formidable task he invented the binomial
system of nomenclature, in accordance with which each kind of
plant or animal is referred to by the name of the genus as well as
by that of the species to which it belongs. The necessity for the
constant repetition of longer or shorter descriptions as a means
of identification every time there is occasion to refer to a
particular species is thus avoided, while at the same time the use
of the generic name makes it possible to employ the same
specific name in a number of different genera without risk of
confusion, a very important point in consideration of the
enormous number of species which have to be distinguished from
one another.

The binomial system of nomenclature has proved so well
adapted to its purpose that it has survived practically unchanged
to the present day, although the number of known species has
increased enormously in the interval and the Linnsean system
of classification has undergone profound modification at the hands

NOMENCLATURE 227

of later investigators. Very many of the more familiar plants
and animals are, however, still known by the names which
Linnaeus gave to them.

It still frequently happens that newly discovered species have
to be named and new genera defined, and various attempts have
been made to formulate a code of rules for the guidance of those
engaged in this work. This is necessary in order to secure, so
far as possible, uniformity of method, and to avoid confusion
such as would arise by giving the same name to different genera
or different species within a genus, or different names to the same
genus or species. The chief points so far agreed upon are that
generic and specific names (as well as those of larger groups)
should be given in either Latin or Greek and that strict attention
should be paid to the law of priority, the name first given
taking precedence over any which may be proposed subsequently,
as is often the case owing to ignorance of the work of previous
writers.

The greatest latitude has, however, been allowed in the past
with regard to the coining of generic and specific names. Even
the resources of the Latin and Greek languages have proved hardly
sufficient to meet all requirements, and many generic names have
been given which, though euphonious enough, have no meaning
whatever. It is said that one eminent naturalist (Dr. Gray)
used to make up such names by drawing letters out of a hat, and
it is certain that Dr. Leach derived the generic names of a whole
series of parasitic Crustacea (Cirolana, Conilera, Nerocila, &c.) from
anagrams on his wife's name, Caroline. The most satisfactory
names, on the other hand, are those which convey some definite
information as to the characters of the genus or species concerned
or as to the place where it is found.

If we have to classify a number of inorganic bodies we may
set about our work in a variety of ways. We may arrange them
according to colour, or size, or weight, or shape, or texture, or
chemical composition, or according to the places they come from,
the precise method adopted depending upon what it is that we
wish to express by means of our classification.

It is exactly the same with plants and animals, the way in
which we classify them will depend upon our point of view. A
fisherman classifies fishes largely according to their food-value, and
various other animals according to whether or not they are good for
bait, and such classification serves its purpose quite satisfactorily.

Q 2

228 OUTLINES OF EVOLUTIONARY BIOLOGY

The aim which the zoologist or botanist sets before himself in
classification is the expression of the " natural affinities " of the
plants or animals investigated. The existence of such natural
affinities was clearly recognized long before the explanation of
them was known, and even by upholders of the doctrine of special

creation. Thus it was
Individual
Brown Bears

Individuals

Species

Genera

Families

Orders

Classes

Species Arctos

\

Genus Ursus

Family Ursidae:

Order Carnivore

Class Mammalia

Phylum Vertebrata

well known to Linnaeus
that individuals fall
naturally into species,
that species may be
grouped in genera,
genera in families, and
so on in ever widening
circles. It also soon
came to be recognized
that natural affinities
could be best determined
by taking into account
as many characters as
possible instead of rely-
ing merely on one or
two. A system of classi-
fication based on a small
number of characters
only is always more
or less artificial, and
though it may serve its
purpose for a time it
will have to be amended
by future workers who
are able to bring a more
complete knowledge to
bear upon the problem.
Thus it comes about
that our views on the
classification of the animal and vegetable kingdoms are always
undergoing change, old systems are constantly being discarded
as too artificial and more natural ones proposed in their stead.

As our knowledge progresses it becomes more and more evident
that a natural classification assumes a tree-like form. The
division of the whole organic world (with the exception of the

Kingdom Animalia

The Organic World

FIG. 86. — Diagram to show the Tree-like Form
assumed by a natural Classification.

THE TAXONOMIC TEEE 229

lower Protista) into plants and animals is represented by the first
branching of the tree, the division of each of these great king-
doms into phyla by the next branching, and the subdivision of
phyla into classes, orders, families, genera, species and individuals
by the further ramifications, as shown in the accompanying
diagram (Fig. 86), which represents a mere fragment of the whole
tree' selected to show the systematic position of a single species of
animal, the common brown bear, Ursus arctos.

This great truth affords one of the most striking pieces of
evidence in favour of the theory of organic evolution, 'for the
tree-like form assumed by a natural classification bears an
unmistakable resemblance to the tree-like development of
the whole organic world which evolutionists believe to have
taken place. The two results represent, indeed, but slightly
different aspects of the same truth ; the resemblance between
them is no mere coincidence, but the fact that we are able
to classify organisms in a tree-like manner indicates very clearly
that these organisms have been produced by tree-like evolution.

The systematist is now able to replace the vague groping after
" natural affinities " by a much clearer conception of the aims of
taxonomy. What he has to strive after is the elucidation of the
actual pedigrees of existing organisms, the unravelling of what is
termed their phylogeny or ancestral history. This means neither
more nor less than the ultimate reconstruction of the whole vast
tree of life. Piece by piece this is gradually being done, and it is
the great German biologist, Ernst Haeckel, who has led the way
in this particular department of biological investigation. Some
of the main branches of the tree have already been reconstructed
in a satisfactory manner, but it is a work of immense difficulty
and practically unlimited extent, and at every step opinions
differ as to the value of conflicting evidence and the interpreta-
tion of obscure facts. The greater part of the tree is dead and
gone beyond recall, and such fragmentary information concerning
extinct forms as may be supplied by the discovery of fossils can
only be supplemented by indirect evidence derived from the study
of existing organisms.

We must now inquire a little more fully into the relation
between the tree-like form assumed by a natural system of
classification, or the taxonomic tree, as we may term it, and the
phylogenetic tree, which represents the actual pedigrees of
organisms. In the former (Fig. 86) the main trunk of the tree,

230 OUTLINES OF EVOLUTIONARY BIOLOGY

Existing Groups

as we have seen, represents the entire organic world ; in the
latter it represents the remote Protistan ancestors from which
both plants and animals have sprung. The first great branching
represents in the one case the systematist's main division of
organisms into vegetable and animal kingdoms, and in the other
the gradual differentiation of the ancestral Protista (which at one

time constituted the

, .

entire organic world)
into the primitive
unicellular plants or
Protophyta on the
one hand and the
primitive unicellular
animals or Protozoa
on the other ; and so
on with subsequent
ramifications.

The parallelism
between the two is
sufficiently striking
to justify us in believ-
ing that it would be
complete if only our
knowledge of classi-
fication and phylo-
geny were so ; we
should then doubtless
see at once that the
taxonomic tree and
the phylogenetic tree
are, after all, one and
the same thing, for we should arrange all organisms strictly in
accordance with the course of their evolution.

One point remains to be noticed in connection with the phylo-
genetic tree. The branching has been monopodial rather than
dichotomous or polychotomous, each branch, in addition to giving
off lateral branches, being itself continued on, so to speak, at
each forking. The descendants of any given ancestral group
did not all undergo modification to the same extent, but some
adhered more or less closely to the ancestral condition and have
continued to do so to the present day. Even the primitive

Ancestral Cocltniernes

Ancestral Protista

FIG. 87. — Diagram illustrating the Relation
between Classification and Phytogeny .

CLASSIFICATION AND PHYLOGENY 231

ancestral Protista are still represented by descendants which
probably differ but little from their remote progenitors and which
have not yet reached the level of either plants or animals, and
there are still living unicellular forms in abundance which, though
they may perhaps be distinguished as plants or animals, have
never been able to learn the secret of forming multicellular
bodies. So it is with most of the great ancestral groups ; they
are represented still by descendants which have undergone com-
paratively little change in structure since remote geological
periods. The algse are still algte, the coelenterates are still
coelenterates and the fishes are still fishes, though each of these
great groups has in past time given rise to descendants which
have gradually become modified into higher types.

Many subordinate groups, such as the trilobites and ammonites
and the winged reptiles of the secondary period, have of course
become extinct during past geological ages, but the fact remains
that even at the present day there still exist on the earth organic
types which represent, in a more or less unmodified form, all the
principal stages of organic evolution, and thus it is that, even if
we had only living plants and animals to consider, our classifica-
tion of the organic world would still assume a tree-like form,
with the simplest organisms at the bottom and the most complex
at the top of the tree. It would be difficult indeed to explain
this fact in accordance with the theory of special creation and
immutability of species.

The monopodial branching of the organic tree and the relation
which the natural classification of existing animals bears to the
phylogenetic or ancestral series is diagrammatically represented
in Fig. 87. On the right hand side are shown some of the
principal stages in the evolution of vertebrates from ancestral
Protista, and at the top of the tree are shown the existing groups
by which these stages are actually represented at the present
day. Only a few stages are represented and the great majority
of the lateral branches of the tree are supposed to have been
lopped off.

CHAPTEK XVII

Connecting links — Homology and analogy — Convergent evolution —
Change of function — Vestigial structures — Eeversion.

IN presenting the evidence of organic evolution it is convenient
to separate that derived from the study of comparative anatomy
from that afforded by taxonomy, but the. separation is not
a logical one, for classification is necessarily based upon com-
parative anatomy, and much of the evidence might be equally well
dealt with under either heading.

The mere fact that we are able to arrange existing organisms
in progressive series, of which the extremes are connected by
intermediate forms, in itself suggests that one form has been
derived from another. This gradation is equally obvious whether
we study entire organisms or confine our attention to their com-
ponent parts or organs. It is illustrated very clearly by the
accompanying diagram of the modifications exhibited by the
siliceous spicules of certain sponges (Fig. 88). These micro-
scopic organs of the sponge, each of which arises within a single
cell, exhibit an astonishing diversity and beauty of form. The
extreme types are as different from one another as we can well
imagine, but in the particular group of sponges from which our
illustration is taken (the Tetraxonida) all may be derived from the
same four-rayed architype (number 1 in the centre of the
figure) and numerous intermediate forms mark out the lines
along which evolution has taken place. An analogous diagram,
with a triaxonid architype, could easily be constructed for those
remarkable deep-sea sponges known as the Triaxonida or
Hexactinellida, and another for the calcareous sponges.

Amongst the higher animals also innumerable illustrations of
the same general principle of gradation in structure, even in co-
existing types, are met with, and are to be explained in the
way indicated in the last chapter, as expressions of phylogenetic
relationship. Take, for example, those now widely contrasted
groups of vertebrate animals the Reptilia and the Mammalia. The

CONNECTING LINKS

233

reptiles as a group are distinguished from the mammals in
many ways. Amongst other things they usually have a scaly
epidermic covering, but they never have hair ; they lay large,

J9.

FIG. 88. — Series of tetraxon Sponge Spicules, showing how all may be
derived from the same fundamental type, highly magnified. (From
Dendy, Article " Sponges " in Encyclopaedia Britannica, Ed. XI.)

heavily-yolked eggs and do not suckle their young ; they
have a cloaca into which the urinary and genital ducts,
as well as the alimentary canal, discharge ; they have a very
complex shoulder girdle (Fig. 89), consisting of a consider-
able number of more or less separate bones or cartilages,

234 OUTLINES OF EVOLUTIONARY BIOLOGY

ci.

'EpC.

including suprascapula, scapula, coracoid, epicoracoid, clavicle
and interclavicle.

The mammals, on the other hand, are characterized by the
possession of an epidermic covering of hairs ; they generally
have very minute eggs almost destitute of yolk, and they always
suckle their young ; they usually have no cloaca but separate
openings for the alimentary canal and urino-genital ducts, and
the shoulder girdle (Fig. 90) is very greatly reduced, being often
represented by only a single bone, the scapula, with a small

remnant or vestige of
the coracoid completely
fused with it, though a
slender clavicle is some-
times present as well.

There exists in Aus-
tralia and some of the
adjacent islands, how-
ever, a small but very
interesting group of
animals known as the
Monotremata, repre-
sented by the duck-billed
Platypus (Ornithorbyn-
chus, Fig. 91) and the
spiny anteater (Echidna,
Fig. 92). The fact that
these animals possess
hair and suckle their
young justifies us in
classifying them amongst the Mammalia, but at the same time
they exhibit certain characters in respect of which they differ
from all typical mammals and agree with the reptiles. Thus they
lay large, heavily-yolked eggs, the alimentary canal and urino-
genital ducts discharge into a common cloaca, and the shoulder
girdle (Fig. 93) is made up of a considerable number of separate
bones almost exactly as in reptiles, most remarkable amongst
which is the interclavicle, which is found in no other mammals.

Here then we have animals still existing which undoubtedly
occupy an intermediate position between the reptiles and the
typical mammals, and the three groups exhibit a progressive series
of structural peculiarities. Such definite connecting links as the

fnt

FIG. 89. — Shoulder Girdle and Sternum of a
Lizard ( Varanus) seen from below.

Cl. Clavicle ; Cor. Coracoid ; Ep. C. Epicoracoid ;
Gl. Glenoid cavity ; Int. Interclavicle ; £. Ribs ;
Sc. Scapula; St. Sternum.

LIMBS OF VERTEBRATES

235

Monotremata, however, are more usually met with amongst the
extinct animals of past geological periods. They indicate the
paths along which the more highly organized groups have pro-
gressed during their evolution from more lowly organized
ancestral groups, which latter may or may not still be represented
by surviving forms at the present day.

It would be difficult to explain the occurrence of such graduated
series of organs and organisms by the theory of special creation,
and it would be no less difficult to explain in this way those
remarkable facts of comparative anatomy which are grouped

Fia. 90. — Shoulder Girdle and part of Sternum of a Rabbit, seen from below.

Cl. Clavicle; Cor. Coracoid process of Scapula (= Vestige of Coracoid) ; Gl. Glenoid
cavity; R. Ribs; Sc. Scapula; St. Sternum.

together under the terms homology and analogy (or homoplasy)
to which we shall next refer.

We have already had occasion to point out that all living
organisms are more or less perfectly adapted to the conditions under
which they exist, and are accordingly provided with organs suitable
for their various requirements. Thus all typical vertebrates have
organs of locomotion formed from two pairs of limbs, but these
differ very greatly in form and structure in accordance with their
adaptation to very diverse conditions of life. In terrestrial
vertebrates — amphibians, reptiles and mammals — both pairs of
limbs usually take the form of ambulatory legs, adapted for
moving the body on dry land. In birds the hind limbs are
generally used for the same purpose and similarly constructed, but

236 OUTLINES OP E VOLUTION AEY BIOLOGY

the fore limbs are modified to form wings for flying in the air ;
and the same is true of the fore limbs of bats and of the extinct

FIG. 91. — The Duck-billed Platypus (Ornithorliynclms anatinus). (From
British Museum Guide.)

flying reptiles known as Pterosauria (pterodactyls). The Cetacea
(whales and porpoises), the Sirenia (dugongs and manatees) and
the seals, amongst mammals ; and the turtles and extinct

FIG. 92. — The Echidna or Spiny Anteater (Echidna aculeata). (From British

Museum Guide.)

Ichthyosauria and Plesiosauria amongst reptiles, on the other
hand, have limbs which take the form of paddles and are
specially adapted for swimming in water.

We find, then, three chief types of locomotor organs amongst

LIMBS OF VERTEBRATES

237

the air-breathing vertebrates : ambulatory legs, wings and
paddles, employed in totally different methods of locomotion and
differing widely from one another in general form and appear-
ance. Yet when we come to examine' these organs closely we very
soon discover the remarkable fact that all are constructed on
essentially the same plan ; all belong to what is known as the
pentadactyl or five-fingered type of appendage.

We ourselves retain this type of limb-structure in a compara-
tively primitive condition (Fig. 94), although our fore limbs have
taken on new functions as organs of prehension. The fore limb

FlG. 93. — Shoulder Girdle and part of Sternum of Ornithorhynchus, seen

from below.

Cl. Clavicle ; Cor. Coraeoid ; Ep. C. Epicoracoid ; Gl. Glenoid cavity ; Int. Interclavicle ;
B. Ribs ; Sc. Scapula ; St. Sternum.

consists of arm, forearm, wrist and hand (manus) with five
fingers ; the hind limb of thigh, leg, ankle and foot (pes) with five
toes. Each segment of the limb is supported by an internal
skeleton of bone which is clothed with muscle and skin. The bones
are articulated with one another at the joints and moved like so
many levers by the contraction of the muscles attached to them.
The bone of the arm is the humerus and the corresponding bone of
the thigh is the femur. In the forearm we find the radius and
ulna and these are represented in the leg by the tibia and fibula.
The wrist is made up of a number of small carpal bones and the
ankle of a number of more or less similar tarsals. These small
bones are arranged mainly in two rows, and the distal or far row

238 OUTLINES OF E VOLUTION AKY BIOLOGY

affords support to the five metacarpal bones in the palm of the
hand or the five corresponding metatarsal bones in the foot. Each
metacarpal or metatarsal bone in turn forms the support of a
series of phalanges which constitute the skeleton of the fingers or
toes. The thumb and great toe have each two phalanges and
each of the other digits has three.

When the human arm is stretched out at right angles to the
long axis of the body, with the thumb towards the head, it is in
the primitive position of the pentadactyl limb. The thumb and
the radius then lie on what is called the preaxial or anterior border,

FIG. 94. — Skeleton of the fore Limb (A) and hind Limb (B) of Man, showing
the pentadactyl Structure. (From photographs.)

c. carpals; fe. femur; fi. fibula: hum. humerus; m.c. metacarpals; mt. metatarsals ;
ph. phalanges; r. radius; t. tarsals ; ti. tibia; ul. ulna; I — V, digits.

the little finger and the ulna on the postaxial or posterior border
of the limb. For convenience of comparison with other types
the digits are numbered from in front backwards, the thumb
being No. I and the little finger No. V. Similarly in the hind
limb, which in man has become permanently twisted out of its
primitive position, it is easy to show that the great toe is really
the preaxial digit (No. I) and the little toe the postaxial (No. V).
In Fig. 91 both limbs are represented in the primitive position.

Starting from the primitive pentadactyl type let us next inquire
more closely how different groups of air-breathing vertebrates have
solved the problems of locomotion on land, in air and in water.

In considering the modifications of the limbs adapted for

LIMBS OF VERTEBRATES 239

locomotion on dry land we may conveniently confine our attention
to the hoofed mammals or Ungulata. In the elephant the limbs
remain, as in man, in a primitive condition, with five well
developed digits in each. The entire limb is stout and massive
and the foot1 remains very short and affords a broad support for

FIG. 95. — Skeletons of Man and Horse, photographed from a group in the
Natural History Museum. (From Lankester's " Extinct Animals.")

E, Elbow bone (olecranon process of ulna) ; H, Heel bone in ankle (the hock of the
horse); K, Knee joint (the stifle of the horse); P, Hip bone; Sh, Shoulder bone
(scapula) ; T, Vertebrae of tail (coccyx in man) ; W, Wrist or carpus (the so-called
knee in the horse's fore leg).

the heavy body. This type of limb is somewhat clumsy and not
adapted for very rapid locomotion.

In the more typical ungulates we always find a reduction in the
number of digits, accompanying an uplifting of the hinder part
of the foot from the ground, until finally the animal comes to

1 The term foot in the case of quadrupeds is commonly used to include both
manus and pes.

240 OUTLINES OF EVOLUTIONARY BIOLOGY

walk and rest on the extreme tips of one or two toes only. The
assumption of this " unguligrade " condition is accompanied by
elongation of the limb and especially of the remaining
metapodial bones (metacarpals and metatarsals), so that the wrist
and ankle in the horse, for example, are uplifted high above the
ground and form the so-called knee and hock respectively (Fig. 95).

C.

I.

-/Ph.

FIG. 96.— Skeleton of (A) the right fore Foot and (B) the right hind Foot
of a Tapir, X g. (From photographs.)

c. carpals ; m.c. metacarpals ; m.t. metatarsals ; ph. phalanges ; t, tarsals ; II — V, digits.

It is also accompanied by a reduction of the ulna in the fore limb
and of the fibula in the hind limb to mere vestiges. Thus the
entire limb becomes long and slender and adapted for rapid
locomotion by running on hard open ground.

The disappearance of digits from the manus and pes in the
Mammalia follows a very simple law. The first to disappear is
always the preaxial digit (No. I), which is the shortest of the
series (having only two phalanges while the remainder have

241

-N

three each) and consequently the first to leave the ground as
the hinder part of the foot is uplifted. The next to go is the
postaxial digit (No. V) from the opposite side, the next No. II
and the next No. IV.

In some ungulates the long axis of the foot passes through
the middle digit (No. Ill) and in others between the third and
fourth digits. The true ungulates are
accordingly divided into two series, odd-
toed or perissodactyl and even-toed or
artiodactyl.

To the former belong the tapir
(Fig. 96), with four digits in the fore foot
and three in the hind ; the rhinoceros,
with four or three digits in the fore foot
and three in the hind, and the horse
(Fig. 97), with only the middle (third)
digit remaining in both manus and pes,
but with vestiges of Nos. II and IV in
the functionless " splint-bones " which
lie alongside the greatly elongated meta-
podial (=. metatarsal or metacarpal) of
No. III.

In the artiodactyl series (Fig. 98) we
find the hippopotamus, with four well-
developed toes on each foot ; the pig,
with four toes on each foot but the two
outer ones greatly reduced; the deer,
with the two outer digits still further
reduced ; the sheep and oxen, with the
second and fifth digits reduced to small
nodules of bone, and the camels and
llamas, in which all traces of digits other
than the third and fourth have completely disappeared. The
gradual reduction of the number of digits in the artiodactyl series is
accompanied by fusion of the two remaining metapodials to form
a single " cannon bone " (welt shown in Fig. 98, deer and camel),
whereby the now greatly elongated and uplifted foot acquires
much greater strength and rigidity.

It is very easy to see that all the different varieties of
perissodactyl and artiodactyl limbs are modifications of one
and the same primitive pentadactyl type. We must next

B. T>

M

FIG. 97. — Fore and hind
Feet of a Horse, show-
ing reduction to a single
functional Digit (No.
Ill) with Vestiges of
two others (II and IV)
in the form of splint
bones, x £. (From
Lull, after Marsh.)

242 OUTLINES OF EVOLUTIONARY BIOLOGY

consider the way in which this type has been adapted to form
organs of flight in different vertebrate groups.

The wing of the extinct pterodactyls (Fig. 99) was formed

by an expansion of the skin of the body stretched between the
fore and hind limbs, and the phalanges of the little finger were
enormously elongated so as to aid in the support of its anterior

LIMBS OF VERTEBEATES

243

margin. The surface of the flying membrane was probably
covered with scales. In the bats (Fig. 99) four of the digits

c«

P

of the hand, Nos. II — V, are elongated and take part in the
support of the flying membrane, the surface of which is either
naked or covered with fine hair. In the birds (Fig. 99) the

B 2

flying membrane is formed chiefly of feathers, and the distal
part of the wing is supported mainly by the second digit ;
the first and third digits are greatly reduced, and the
other digits are absent. In all these different types of wing
the skeleton is again clearly of the pentadactyl type, modified
by enlargement, diminution or total suppression of certain of its
constituent bones.

In the paddles of the modified aquatic mammals, such as the
sirenians, seals (Fig. 100) and whales (Fig. 101), the whole limb

c.

m.c.

Ill

FIG. 100.— Skeleton of right fore Foot (A) and right hind Foot (B) of a
Seal (Otaria hookeri), X 5. (From photographs.)

c. carpals ; m.c. metacarpals ; m.t. metatarsals; ph. phalanges; I — V, digits.

is very much shortened and flattened and the digits are enclosed in
a common integument so as to offer a greater resistance to the
water, but the skeleton still reveals the essential pentadactyl
features, and the same is true of those aquatic reptiles, the
turtles, plesiosaurians (Fig. 141) and ichthyosaurians (Fig. 142).
In the whales, however, a curious increase in the number of
phalanges (hyperphalangy) may often be observed, and in the
ichthyosaurians supplementary small bones are developed in
such positions that the entire skeleton of the paddle assumes
the form of a mosaic pavement.

245

246 OUTLINES OF EVOLUTIONARY BIOLOGY

We can only explain the occurrence of the same type of
skeleton — and that a very complex type — in all these different
kinds of organs of locomotion on the assumption that it has been
inherited from common ancestors. We cannot believe that one
and the same type of skeleton was necessarily the most suitable
for all these different cases, including ambulatory legs, wings and
paddles, and was therefore specially and independently created for
each. We must conclude rather that each organism receives a
certain kind of material by inheritance from its ancestors and
has to adapt it to its own requirements as best it may ; has, in
short, to cut its coat according to its cloth, and, whatever the

'

S.I.

FIG. 102. — A swimming Crab (Porttmm depurator), showing jointed ambu-
latory appendages and also swimming appendages (s.l.). (From a
photograph.)

shape of the coat may ultimately develop into, the cloth will
retain, more or less evidently, traces of the original pattern.

This conclusion is greatly strengthened when we turn to other
groups of animals and see how they have solved the same problems
with the aid of different materials. The vertebrates are not the
only animals which have organs of locomotion in the form of
jointed appendages. Many members of the great group
Arthropod a — insects, crustaceans and spiders— have ambulatory
limbs which externally bear considerable resemblance to those
of vertebrates. If we examine these limbs, however, we find that
they are constructed upon a wholly different plan. In the first
place the skeleton is external instead of internal, and is composed

HOMOLOGY AND ANALOGY 247

of the hard cuticle secreted by the epidermis, there being no true
bone. The muscles therefore lie inside the skeleton instead of
outside. The number and arrangement of the limb-segments,
again, is totally different, and there is, of course, no pentadactyl
structure (compare Fig. 102).

Thus, with different material at their disposal, the arthropods
have solved the problem of constructing an ambulatory appendage
in a very efficient manner, but quite differently from the
vertebrates. Many of them have also solved the problem of
locomotion in water by modification of certain of the limbs to
form paddles, as in the case of the swimming crab (Fig. 102).
Many insects, on the other hand, have acquired the power of
flight by means of wings, which, though bearing some external
resemblance to those of vertebrates, are totally different in
structure and origin not only from the latter but also from the
other appendages of the arthropods themselves.

We are now in a position to define the meaning of the terms
homology and analogy as used by biologists. Homologous
organs are such as have the same essential structure,
which they owe to inheritance from common ancestors,
though they may be very differently modified in adaptation to
different functions. The pentadactyl limbs of air-breathing
vertebrates, however much they may differ amongst themselves,
are all homologous organs in so far as their essential pentadactyl
structure is concerned.

Analogous or homoplastic organs, on the other hand, bear only
a superficial resemblance to one another, which they owe not to
common ancestry but to adaptation of fundamentally different
structures along similar lines for similar functions. The
ambulatory appendages of arthropods and vertebrates are
analogous but not homologous organs, so also are the wings of
birds and insects.

The evolutionary process by which analogous but not
homologous structures have come to resemble one another is
sometimes spoken of as convergence, and the result may be looked
upon as an illustration of the general principle that similar causes
tend to produce similar effects. The necessity for the adaptation
of different organs and organisms to the same environment and
the same mode of life results in a superficial resemblance between
the organs and organisms thus adapted.

One of the most familiar examples of convergent evolution

248 OUTLINES OF E VOLUTION AKY BIOLOGY

is afforded by the whales, dolphins and porpoises (Figs. 101
and 161) with their extraordinary external resemblance to
fishes. The fore limbs, as we have already seen, are modified
to form paddles, but they retain the pentadactyl structure and
are thus totally different from the fins of fishes, which have
never reached the pentadactyl stage. The hind limbs have
completely disappeared, but vestiges of the limb bones and their
supporting girdle, which in some cases are found buried deeply
beneath the skin in the pelvic region (Fig. 101, f,p), still bear
witness to their former presence. The powerful tail fin has
obviously no real relationship to the tail fin of a fish, for it

lies horizontally instead of
vertically. There are, of
course, no gills, as in fishes,
but the animal comes to
the surface to breathe air
by means of lungs.

Not only is the whale
not a fish, but it belongs
to the group of vertebrates
most remote from fishes.
It is a warm - blooded
mammal, suckling its
young and exhibiting other
characteristically mamma-
lian features. It has seven
cervical vertebrae, a number
which is curiously constant
throughout the mammalian series, but owing to the extreme
shortness of the neck these vertebras are all crushed together to
form practically a single bone (Fig. 103). In the giraffe we also
find seven cervical vertebrae, but they are all greatly elongated
in accordance with the enormous length of the neck (Fig. 104).
On the hypothesis of special creation we should certainly have
expected the whale to have fewer vertebras in its neck than the
giraffe, and we can only suppose that the number seven has
been inherited from some common mammalian ancestor. The
resemblance of the whale to the fish, in short, is simply due to the
fact that both have acquired the external form best adapted for an
active aquatic life.

The winga of pterodactyls, bats and birds are equally good

FIG. 103. — The seven cervical Vertebrae
of a Whale, fused together in one
Mass. (From Reynolds' "Vertebrate
Skeleton.")

CONVERGENT EVOLUTION

249

examples of convergent evolution within much narrower limits.
They can only be regarded as homologous to the extent of all being

FIG. 104. — Skeleton of the Giraffe, showing the seven separate and greatly
elongated cervical Vertebrae. (From Brehm's "Thierleben.")

pentadactyl. They do not owe their special characters as wings
to descent from a common ancestral winged form, but have been

250 OUTLINES OF EVOLUTIONARY BIOLOGY

evolved independently from more primitive pentadactyl types of
limb.

We find analogous cases in other vertebrate groups. Several
of the lizards, such as the so-called slow worm or blind worm
(Fig. 105) and the " glass snakes " (Ophisaurus), have, by loss
of their limbs, come so closely to resemble snakes as to be

FIG. 105. — The Slow Worm, Anyuis frayilis, X J. (From a photograph.)

indistinguishable by most people. The Coeciliidae (Fig. 106)
amongst amphibians, the Amphisbfenidae amongst lizards and
the Typhlopidae amongst snakes have all adapted themselves to a
burrowing underground life, like the earthworms. Their limbs
have completely disappeared, the body has become cylindrical and
worm-like and they have lost the use of their eyes, but though they

FIG. 106. — A worm-like, limbless Amphibian, Urm.typhlus africanus. (From
British Museum Guide.)

have all come to resemble one another closely as the result of
convergent evolution they are not in reality at all nearly related.
The loss of limbs, the assumption of the worm-like form, &c.,
have taken place quite independently in each group.

The mammalian fauna of Australia consists mainly of members
of the single order Marsupialia or pouched mammals, but
different representatives of this order have come, by convergence,
to resemble closely members of widely different orders of

CONVERGENT EVOLUTION

251

mammals found in other parts of the world. The so-called
marsupial wolf of Tasmania (Thylacinus) closely resembles a
typical carnivore in its habits and general structure. Its teeth
are adapted for a predaceous life, and the entire skull (Fig. 107, B),
with its dentition, bears an extraordinary general resemblance to
that of a dog (Fig. 107, A), being distinguishable only by details
of structure which would hardly be noticed except by an

FIG. 107.— A. Skull of Dog, side view ; B. Skull of Thylacine, side view.
(From photographs.)

anatomist. These details, however, are quite sufficient to show
that there is in reality no close relationship between the two.
Thus the dog (Fig. 108, A, AI) has in each upper jaw three
incisor teeth (i.1 — i.3), one canine (c.), four premolars (p.m.1 — p.m.4)
and two molars (m.1 — m.2) ; and the dentition of the lower jaw is
the same except for the presence of a third molar (m.3), which is,
however, in a vestigial condition. The thylacine, on the other
hand (Fig. 108, B, BI), has four incisors (i.1 — i.4) in the upper and

252 OUTLINES OF EVOLUTIONAKY BIOLOGY

FIG. 108.— A, A!, Skull and Mandible of Dog ; B, BI, Skull and Mandible
of Thylacine, to show the arrangement of the teeth, &c. (From
photographs.)

i. J. — i.4, incisors of upper jaw;i.i — i.s, incisors of lower jaw; c., canine; p.m.1 — p.m. 4,
premolars of upper jaw; p.m.j — p.m.4, premolars of lower jaw; m.1 — m.4, molars
of upper jaw ; m.i — m.4, molars of lower jaw.

CONVERGENT EVOLUTION 253

three (i.i — i.3) in the lower jaw ; and one canine (c.), three
premolars (p.m.i — ») and four molars (m.i — 4) in each jaw. The
thylacine skull is further distinguished from that of the dog by
the presence of large vacuities in the hinder part of the bony
palate and by the strongly marked inflection of the angles of the
mandibles. These two characters, both of which are shown in
the illustrations, as well as the arrangement of the cheek teeth
and other minor features which it is unnecessary to specify, are
fundamental peculiarities of the great group Marsupialia and at
once indicate the true affinities of Thylacinus.

The case of the Australian marsupial mole, Notoryctes, is
equally striking. The powerfully built digging limbs, the soft,
close fur, the absence of external ears and the loss of sight, as
well as the general shape of the body, all in adaptation to its
burrowing habits, cause it to assume a wonderfully mole-like
aspect, while in reality it comes nowhere near the true moles as
regards genetic relationship.

Perhaps the most remarkable of all known cases of convergent
evolution, however, is met with in the Ungulata. We have
already seen that amongst the typical ungulate mammals of the
present day we can distinguish two series, odd-toed or perisso-
dactyl, and even-toed or artiodactyl. In the odd-toed series the
reduction of the digits culminates in the horse, with a single
perfect digit in each foot and vestiges of two others in the form
of splint-bones. This extreme modification of the unguligrade
type is so peculiar that it is difficult to believe that precisely the
same line of evolution has been followed independently by two
different groups of animals. Yet such appears to be actually
the case. There is an extinct group of ungulates known as the
Litopterna, whose remains have been described by Ameghino
from Tertiary beds of Patagonia. The small size of the
brain-cavity, the characters of the dentition, cervical vertebrae,
carpus and tarsus, indicate that they were more primitive forms
than the true Perissodactyla, and, as Dr. Smith Woodward
observes, they reached their maximum of specialization at an
earlier period than the latter. We find amongst them forms
(Theosodon, Fig. 109, A, B) with three well-developed toes as in
the rhinoceros, forms (Proterotherium, Fig. 109, C) with one well-
developed toe and two small ones as in some of the extinct
ancestors of the horse, and forms (Thoatherium, Fig. 109, D, E)
with a single toe as in existing horses ; and in all cases the axis

254 OUTLINES OF E VOLUTION AEY BIOLOGY

IV

FIG. 109.— Feet of Ungulates
belonging to the extinct
group Litopterna, from
Lower Tertiary deposits of
Patagonia. (From Smith
Woodward's "Vertebrate
Palaeontology," after
Ameghino.)

m

A, B, Theosodon lydekkeri ; right fore and hind feet, x J.

C, Proterotherium intermixtum ; right fore foot, x |.

D, E, Thoatherium crepidatum ; left fore and hind feet, x

CONVERGENCE AND CHANGE OF FUNCTION 255

of the foot passes down the middle of the third digit. If we
compare the foot of Thoatherium (Fig. 109, D, E) with that of a
horse (Fig. 97) it is hard indeed to imagine that the animals
possessing such closely similar and highly specialized limbs are
not nearly related to one another. Expert palaeontologists,
however, tell us that they are not, and we must helieve that the
resemblance is due entirely to adaptation to a similar mode of
life in the two cases. We shall have something to say as to how
this adaptation was brought about in the case of the horse when
we come to discuss the ancestral history of that animal in
Chapter XIX.

It will be readily understood from the examples which we have
been considering that the phenomena of convergence provide
many pitfalls for the unwary biologist, and have led to many
other mistakes in classification besides the popular error of
placing the whales amongst the fishes. We have already noticed
how the limbs of arthropods have come to bear a superficial
resemblance to those of vertebrates, though so absolutely different
in their essential structure that no anatomist would dream of
regarding them as homologous organs. Many aquatic arthropods,
belonging to the class Crustacea, like the lobsters, cray-fishes
and crabs, breathe by means of gills which bear a superficial
resemblance to those of fishes, but are again by no means
homologous structures, and there are other resemblances between
arthropods and vertebrates, due probably to convergence, which
have led more than one observer to conclude that vertebrates are
descended from arthropod ancestors, a conclusion which is by no
means justified by the facts.

We may now further consider the process known as " change
of function," in which an organ primarily adapted and used for
one purpose takes on a new and altogether different duty and
becomes modified accordingly. The lungs of air-breathing
vertebrates, for example, are generally believed to be homologous
with the swim-bladder of fishes, for both arise in the same way as
outgrowths of the front part of the alimentary canal, in the
region of the throat. The swim-bladder of a fish is a hydrostatic
organ ; it is filled with gas, the amount of which can be regulated
by suitable means, and assists the animal in maintaining its
proper position in the water. In the dipnoan fishes, such as
the Australian mud-fish (Neoceratodus, Fig. 110), the South
American Lepidosiren and the African Protopterus, the walls of

256 OUTLINES OF EVOLUTIONARY BIOLOGY

the swim-bladder have become highly vascular and it serves as
an organ of respiration. Air is taken into it through the mouth,
so that the blood circulating in its wall is oxygenated, and the
arrangement of the heart and blood-vessels has become modified
accordingly. Functional gills, however, are still present.

In the amphibians the swim-bladder has become completely
converted into a pair of lungs, and in this way was rendered
possible that great step in the evolution of the vertebrates, the
migration from water to land. Thus in the air-breathing verte-
brates the gills have been entirely supplanted and replaced
functionally by the lungs, which are still to be regarded as
homologous with, or morphologically equivalent to, the air-
bladder of fishes.

The hand of man affords another beautiful example of change

FIG. 110.- The Australian Mild-Fish, Neoceratodus forsleri, greatly reduced.
(From British Museum Guide.)

of function — from locomotion to prehension — but it has under-
gone surprisingly little structural modification in the process.
The proboscis of the elephant is also a very efficient organ o£
prehension, but it is formed by enormous elongation of the nose,
which is primarily an organ for conveying air to and from the
olfactory organs and lungs. Still more remarkable is the con-
version of muscle fibres in the torpedo and the electric eel
(Gymnotus) into powerful electric batteries, capable of giving
severe shocks and therefore valuable as weapons of offence and
defence. In this case the change of function is accompanied by
very profound modifications in structure.

Wherever we turn we find that novel requirements are met, not
by the sudden creation of new organs, but by the gradual modifi-
cation of old ones. As we have already said, the organism has
to do its best with the material which it has inherited from its
ancestors ; and yet the power of living protoplasm to meet every
new requirement by a suitable modification of bodily structure

VESTIGIAL ORGANS 257

appears to be almost unlimited. We must remember, however,
that such modifications, in a state of nature at any rate, only
take place very slowly and gradually.

Traces or vestiges of organs which have ceased to be of any
use to their possessors persist with astonishing pertinacity in
many organisms. Evidently their complete removal must be
an extremely slow process. We have already noticed the per-
sistence of vestiges of the pelvic girdle and leg bones in the
whale (Fig. 101), of the reduced metapodials or splint bones in

FIG. 111. — A New Zealand Kiwi, X &. (From a photograph of a stuffed

specimen.)

the feet of the horse (Fig. 97), and of the coracoid in the typical
mammalian shoulder girdle (Fig. 90). Unless we are to believe that
such structures have been put where they are on purpose to
mislead us we cannot possibly explain their occurrence on the
theory of special creation. They are, at any rate in many
cases, perfectly useless to their possessors, and the only rational
way of accounting for their presence is by supposing them
to be inherited from remote ancestors in which they were
functional.

We may now briefly notice a few other instances of such
B, s

258 OUTLINES OF EVOLUTIONARY BIOLOGY

vestigial structures. A characteristic feature of oceanic islands
is the presence on them of birds which have more or less lost the
power of flight. Such were the dodo of Mauritius, the solitaire
of Eodriguez, and the gigantic moas of New Zealand. All these
forms are now extinct, but we still find in New Zealand several
flightless birds surviving. One of the most interesting is the
kiwi or Apteryx (Fig. Ill), a moderate-sized bird of nocturnal

habits, now rapidly be-
coming exterminated.
The whole body is covered
with coarse, hair - like
plumage and externally
shows no trace of wings.
There is, however, a
minute remnant of a
wing present on each
side, completely con-
cealed in the plumage
and entirely functionless
as an organ of flight.
Yet it still possesses the
pentadactyl skeleton,
though in a greatly
reduced condition (Fig.
112). It is said that
when the kiwi goes to
sleep it still endeavours
to tuck its long beak
under its wing after the

FIG. 1 1 2. — Skeleton of a Kiwi, showing the manner of other birds,
vestigial Wing Bones, behind which a j th extinct moas to
piece of Black Paper has been placed, .

x i- (From a photograph.) which thejkiwis are per-

haps related, even the

last vestiges of the wings seem to have disappeared, for amongst
the enormous quantities of the remains of these gigantic birds
which occur in New Zealand no wing bones have ever been found.
There is very strong reason to believe that the remote ancestors
of existing vertebrates possessed an additional pair of eyes on
the top of the head, behind the still existing lateral eyes.
Traces of this second pair— the pineal or parietal eyes — are yet
to be found in lampreys and lizards, and especially in that

VESTIGIAL ORGANS

259

remarkable New Zealand reptile the tuatara (Sphenodonpunctatas).
This animal (Fig. 113) is usually regarded as the oldest surviving

FIG. 113.— The New Zealand Tuitara, X £. (Drawn by Miss V. E. Dondy
from a model and photographs.)

type of terrestrial vertebrate, the order to which it belongs (the
Rhynchocephalia) dating back to the close of the Palaeozoic period
of the earth's history (Permian epoch).

In the adult tuatara there is still a fairly well developed pineal

FIG. 114.— Section through the Pineal Eye of the Tuatara, X 68 (From

photograph.)

le., lens ; n., nerve ; ret., retina.

eye (Fig. 114), with retina, lens and nerve, but it is only about
inch in diameter and lies deeply buried beneath the

s 2

260 OUTLINES OF EVOLUTIONARY BIOLOGY

skin in the parietal foramen in the middle of the roof of the
skull. It may to some extent still be functional, though probably
of far less importance than in the extinct amphibia and reptiles
of the Palaeozoic and Mesozoic periods, in which the presence of
a comparatively large parietal foramen indicates that the eye was
better developed. In the case of the tuatara it has been demon-
strated by anatomical and embryological investigation that the
remaining pineal eye is the left-hand member of the original
pair. In the lampreys it appears to be the right-hand member.

Turning now to the frog, we sometimes find in this animal a
minute light-coloured spot on top of the head (Fig. 115), between
the paired eyes. This marks the position of a little sac-shaped

vesicle which lies
beneath the skin and
is known as Stieda's
organ, consisting merely
of a number of undiffer-
entiated cells surround-
ing a central cavity and
attached to the end of a
kind of stalk (Fig. 116).
The vesicle represents
the functionless vestige
of a pineal eye and the
stalk probably repre-
sents its disappearing
nerve.

In the birds and mammals all trace of eye-like structure has
disappeared from this region, but a vestige of one or both of the
pineal eyes is probably to be recognized in the so-called pineal
gland lying on the roof of the brain, which attained such celebrity
in the seventeenth century owing to its identification by the
philosopher Descartes as the seat of the soul.

In studying the mammalian dentition we again meet with
plenty of illustrations of vestigial structures. The whale-bone
whales (Balsenidae) have no teeth in the adult at all, but in the
foetus vestiges of teeth can be found imbedded in the gums. The
young Australian duck-billed Platypus (Ornithorhynchus) has
vestigial teeth which are entirely replaced by horny pads in the
adult. The third molar in the lower jaw of the dog (Fig. 108,
AI, m.3) is, as we have already seen, evidently in a vestigial

FIG. 115. — Head of Frog (Rand temporaria),
showing Stieda's Organ between the lateral
Eyes. (From Studnicka, after Stieda.)

VESTIGIAL ORGANS

condition and can be of very little if any use to its possessor,
and there is reason to fear that in man himself the teeth are
disappearing a good deal more rapidly than we could wish,
though in this case the disappearance seems to be chiefly due
to disease.

A better example of a vestigial structure in man is the
coccyx at the hinder end of the vertebral column (Fig. 95), which
represents the last remnant of the ancestral tail, and is occasion-
ally accompanied by vestiges of the muscles by which an ordinary
tail is moved. The hair on the chest, again, is a vestige of the

r.s.

FIG. 116. — Section through the vestigial Pineal Eye of the Frog (Tadpole),

X 168. (From a photograph.)
epd., epidermis ; p.e., pineal eye ; r.s., roof of skull ; v.n., vestigial stalk and nerve.

hairy coat which once clothed the entire human body, as it still
does that of the apes.

Examples of vestigial organs might be multiplied to an
indefinite extent, but enough has perhaps been said to show that
such structures are of very common occurrence and to indicate
their value as evidences of organic evolution. They occur, of
course, not only in the adult condition but also, as in the case of
the embryonic gill-slits of air-breathing vertebrates, at earlier
stages of development, but such cases will be more conveniently
dealt with in the next chapter.

Closely akin to the occurrence of vestigial structures is another
phenomenon known as atavism or reversion, by which we mean

262 OUTLINES OF EVOLUTIONARY BIOLOGY

the sudden and sporadic reappearance of some ancestral structure
which had either been completely lost or very greatly reduced.
The elder Pliny has placed it on record that Caesar the Dictator
had a horse whose fore feet were like those of a man. This
statement evidently refers to a more or less complete return to
the ancestral pentadactyl condition. Marsh has given a figure
of the fore foot of a horse in which the second digit is fairly
well developed, with three perfect phalanges, though not so large
as the third, while the first is represented by a splint bone, and a
very similar case is exhibited in the Natural History Museum at
South Kensington.

In man the occasional excessive development of the canine
teeth is regarded as due to reversion to an ancestral condition
similar to that of the anthropoid apes, in which the canine teeth
are very large and used as weapons. The occasional occurrence
in man of vestigial tail muscles, and even of a short tail, may
also be regarded as due to reversion. Whether or not all such
cases are capable of being explained on Mendelian principles, as
suggested in Chapter XIV, it is impossible to say, but this does
not affect their importance as evidence of the truth of the theory
of organic evolution.

Although, in the present chapter, we have so far drawn our
illustrations entirely from the animal kingdom, it must not be
supposed that the same general principles cannot be equally well
demonstrated in the case of plants. Indeed the whole chapter
might be re-written entirely from the botanical point of view.
Change of function is very well shown, for example, in the
"pitchers" of Nepenthes and Sarracenia, formed from modified
leaves and serving as traps for catching insects. Convergent evolu-
tion is illustrated by the strong superficial resemblance which exists
amongst the various Alpine cushion plants belonging to widely
different orders, all of which have assumed the form best suited
for withstanding the peculiar hardships of their environment.
The leaves of the parasitic dodder, reduced to tiny scales (Fig.
184), and the staminodes or functionless stamens of many flowers,
again, might well serve as examples of vestigial structures.
Considerations of space, however, forbid us to pursue this fertile
line of inquiry any further.

CHAPTER XVIII

Ontogeny — The recapitulation hypothesis — Interpretation of the onto-
genetic record — Palingenetic and caenogenetic characters.

IN dealing with the cell theory, in Chapter IV, we have
already laid stress upon the fact that every multicellular animal
or plant commences its individual life as a unicellular egg or
ovum, and gradually passes by slow stages of cell-division and
differentiation into the adult condition. There are, of course,
apparent exceptions to this rule in the case of animals and
plants which reproduce by budding or by some analogous process,
where the bud arises from a group of cells belonging to the
parent, but even in these cases reproduction by means of
germ cells is resorted to at more or less frequent intervals and
the budding must be regarded merely as an additional method of
multiplication interpolated in the life-cycle.

The individual organism, then, does not come into existence
fully formed in all its perfection, but passes through a longer or
shorter process of development to the adult condition. In fact
it undergoes a process of individual evolution which constitutes
its individual life-history or ontogeny, and the length and
complexity of this process are proportional to the complexity of
organization of the adult. Thus the complete life-cycle of many
of the lower multicellular animals and plants is passed through
in the course of a few months, while a man requires twenty
years or more to attain his full development.

Although, for the sake of convenience, the ontogeny of any
given organism may be divided up into a number of stages, yet
these stages cannot in reality be sharply distinguished from
one another. Even the division or segmentation of the ovum
(Figs. 13 and 119), whereby the organism passes from the uni-
cellular to the multicellular condition, does not take place
suddenly but by means of the slow and complicated process of
mitosis or karyokinesis (Fig. 31) ; and in cases where there is an
apparently abrupt change, or metamorphosis, from one condition

264 OUTLINES OF EVOLUTIONARY BIOLOGY

to another, as, for example, at a later stage, from the chrysalis to
the butterfly, the development really progresses quite slowly
and gradually internally, although the external appearance may
for a long time remain unaltered and give the impression that
it is completely at a standstill.

The fact that the young organism cannot commence its life at
the stage reached by its parents, but has to make a fresh start
from the beginning and go through a whole series of stages

FIG. 117. — A. Chick Embryo of about 5| days, X 5. B. Babbit Embryo of
about 13 days, X 5. In both cases the foetal membranes and yolk-sac
have been removed. (From photographs.)

before reaching the adult condition, is very significant from the
point of view of the evolutionist. Still more significant is the
fact that different organisms all commence at the same stage —
as unicellular eggs — and come to diverge further and further
from one another in structure as their development progresses.
If we examine a number of series of embryos belonging to
different vertebrate types, no matter how widely the adult
animals may differ from one another, we shall find that as we
trace their life-histories backwards they gradually converge until,
while still at a relatively advanced stage of development, the
different embryos come to resemble one another so closely that

265

in many cases it is doubtful if even an experienced embryologist
could distinguish between them. This great generalization
appears to have been first reached by the embryologist von Baer,
who in 1827 discovered the mammalian ovum. It may be
illustrated by the chick and rabbit embryos represented in
Fig. 117.

Von Baer's generalization contained the germ of what is now
known as the Eecapitulation Hypothesis, or, as Haeckel has
termed it, the Biogenetic Law, which states that every organism
in its individual life-history recapitulates the various stages
through which its ancestors have passed in the course of their
evolution. In other words, ontogeny, or the life-history of the
individual, is a repetition of phylogeny, or the ancestral history
of the race to which the individual belongs.

We have already referred, in Chapter IV (Fig. 13), to some of
the earlier stages in the development of that primitive fish-
like animal Amphioxus (Fig. 118). Let us next inquire how
these stages may be interpreted in accordance with the recapitu-
lation hypothesis. The unicellular ovum (Fig. 13, I) obviously
represents the remote protozoon ancestors which were common
to the whole animal kingdom, and which are also represented
at the present day by independent unicellular organisms such
as the Amo3ba. The segmentation of the ovum into primitive
embryonic cells or blastomeres (Fig. 13, II — VI) represents the
transition from the condition of the simple protozoon to that of
the protozoon colony, in which the individual cells, instead of
separating, as in the dividing Amoeba, remain together, but still
without undergoing any marked differentiation and division of
labour. The arrangement of the blastomeres in the form of a
hollow sphere, the blastula or blastosphere (Fig. 13, VII), with a
single layer of cells surrounding a central cavity, represents the
formation of such a protozoon colony as we see in the existing
Volvox (Fig. 11) or Sphaerozoum (Fig. 12). The process of
gastrulation, whereby the single-layered blastula is converted
into a two-layered gastrula (Fig. 13, VIII — X), with primitive
digestive cavity (enteron) and primitive mouth (blastopore), repre-
sents the transition from the protozoon colony to the coelenterate
stage of evolution, the latter being still represented at the present
day by such forms as Hydra (Fig. 57), Obelia (Fig. 60), the jelly-
fish, the corals and all their numerous relations, which retain in
their organization all the essential features of the gastrula,

though generally complicated by the development of tentacles,
skeleton, &c. The development of coelomic pouches (Fig. 13,
XI — XIII, c.p.) as outgrowths of the primitive digestive cavity,
and the conversion of these into mesoblastic somites (Fig. 13,
XV, ni. 8.), arranged serially or metamerically down each side of
the body, mark the transition from the unsegmented and
ccelenterate condition to the metamerically segmented l and
coalomate condition. The cavities of the coalomic pouches form
the coalom or body cavity, which is at first transversely sub-
divided into compartments, as it still is in the adult earthworm,
while their walls form the third germ-layer or mesoblast, lying
between the epiblast which covers the surface of the body and

FIG. 118. — The Lancelet (A?)<pMcxusla,r,ceolatus), X 2. (From a photograph.)

the hypoblast which lines the digestive cavity. Along the mid-
dorsal line of the body a strip of epiblast sinks down (Fig. 13,
XI, n.pL) and becomes folded into the form of a tube (Fig. 13,
XII — XIV, 7i.t.), the rudiment of the central nervous system
(brain and spinal cord), and beneath this tube a long strip of
hypoblast becomes nipped off from the roof of the gut, forming
the notochord (Fig. 13, XI — XIV, not.) or axial skeletal rod —
the foundation around which in higher types the vertebral
column is built up. A little later the front part of the gut
becomes pierced by gill- slits for purposes of respiration and
the primitive chordate condition is thus fully attained.

Amphioxus (Fig. 118) does not progress much beyond this
stage in its development. It never acquires any limbs but swims
about by lateral undulations of the body caused by the contraction

1 Metameric segmentation has, of course, nothing to do with the segmentation of
the ovum, but is the term applied to the transverse division of the body in many
animals into a series of joints or segments arranged longitudinally, each of which
typically repeats the essential features of all the others.

THE RECAPITULATION HYPOTHESIS 267

of the lateral sheets of muscle derived from the mesoblast. No
skull and no vertebral column are formed, the notochord
remaining throughout life as almost the sole representative of the
internal skeleton. The brain does not become distinctly
differentiated from the spinal cord and no paired organs of sense
are developed, nor does that division of the body into head and
trunk, which is so characteristic of typical vertebrates, take
place. Amphioxus is a chordate animal but it is not a true
vertebrate. It probably, however, represents fairly closely a
stage of evolution through which the ancestors of the vertebrates
have passed, though it becomes somewhat modified by secondary
features in the later stages of its development.

It is easy enough to interpret the earlier development of Amphi-
oxus in terms of the recapitulation hypothesis and to recognize
in the ontogeny certain stages of the phylogenetic history. The
ontogenetic record, however, is by no means always so readily
decipherable. It is generally more or less obscured by secondary
features superposed upon it, much as an ancient picture may
be concealed by another painted over it. These secondary
features are characters which are developed in relation to the
requirements of the embryo itself at its various stages, for it is
not only the adult organism which becomes modified in structure
in the course of evolution but all the stages of its life-history
are likewise subject to adaptation.

In the first place nourishment has to be provided for the
developing embryo, and this necessity is present from the very
first division of the fertilized ovum. The blastomeres into
which it divides cannot go about and seek their own food supplies
like so many Amoebae, they have sacrificed the power of leading
independent lives for the sake of remaining together and building
up a more or less complex multicellular body ; but none the less
they require feeding. Hence we find that all eggs contain a
larger or smaller quantity of nutrient material stored up in
the cytoplasm. This usually takes the form of definite granules
or particles of food-yolk (deutoplasm), the amount of which
varies immensely in different cases. If there is very little the
egg is said to be microlecithal as in Amphioxus ; if there is
much it is said to be megalecithal, as in the frog and still more
so in the bird, and the size of the egg, as we have already had
occasion to point out in Chapter X. depends almost entirely upon
the amount of food-yolk which it contains.

268 OUTLINES OF EVOLUTIONARY BIOLOGY

The possession of food-yolk is, of course, of immense import-
ance to the embryo, for the stage which it is able to reach in its
development before it has to begin to provide for itself will
depend upon the amount of nutrient material which it has at its
disposal. The presence of this food-yolk, however, must also
act as a mechanical hindrance to the development, just as the
supply of provisions which he carries with him must impede the
progress of a traveller, though enabling him ultimately to
accomplish a longer journey.

In Amphioxus the egg contains hardly any food-yolk and the
process of segmentation and the formation of the blastula and
gastrula take place in an almost perfectly typical manner.
There is here practically no hindrance to development and no
obliteration of the ontogenetic record. There is, however, just
sufficient nutrient material to cause some of the cells — those
which will invaginate to form the hypoblast of the gastrula — to
be slightly larger than the remainder, which are destined to form
the epiblast (Fig. 18, VII, VIII).

Let us compare with this the corresponding stages in the
development of the frog (Fig. 119). Here the egg is, as we have
already indicated, much larger, owing to the greater quantity of
food-yolk which it contains. This is chiefly collected in the
lower half of the ovum, while the upper half is deeply pigmented
and thus readily distinguished. Segmentation begins as in
Amphioxus by the formation of two vertical clefts, or cell-divisions,
one after the other and at right angles to one another, whereby
the four-celled stage is reached (Fig. 119, I — III). Each cleft
begins at the pigmented pole and throughout the whole of the
segmentation the yolk-laden half of the egg lags behind the
pigmented half because of the mechanical hindrance which the
food-yolk opposes to the process of cell-division. The first
horizontal cleft (Fig. 119, IV) divides each of the four blasto-
meres into two daughter cells of very unequal size, an upper
pigmented cell with little food-yolk and a much larger, lower,
yolk-laden cell with little pigment. After this, segmentation
continues to progress more rapidly in the upper than in the
lower half of the egg (Fig. 119, V), until we reach the blastula
stage (Fig. 119, VI), where we find the large yolk-laden cells
forming the lower half of the wall of the hollow sphere and pro-
jecting into and partly obliterating its cavity, while the small
pigmented cells form the upper half.

EARLY DEVELOPMENT OF THE FROG

269

The process of gastrulation is profoundly modified by the
presence of the food-yolk, and instead of a simple invagination
of the lower half of the blastula wall to form the hypoblast, we
find the small (epiblast) cells, now arranged in several layers,
spreading themselves over the yolk-containing cells until they
completely enclose the latter except for a small circular area

yk.c.

yk.c. u.l.bp.

V*

, FIG. 119. — Early Stages in the Development of the Frog (partly from
Ziegler's models and partly adapted from Marshall.)

I, the fertilized ovum ; II — V, segmentation of the ovum ; VI, blastula ; VI I, modified
blastula, with wall composed of more than one layer of cells ; VIII, commencement
of gastrulation; IX, the modified gastrula stage (VI — IX in vertical section).

blc., blastocoel or segmentation cavity; bp., blastopore ; ent., enteron; ep., epiblast;
hyp., hypoblast ; u.l.bp., upper lip of blastopore ; yk.c., yolk-containing cells.

which corresponds to the blastopore or aperture of invagination
in Amphioxus blocked up by yolk cells (Fig. 119, VIII, IX).
Indeed, practically the whole of the primitive digestive cavity or
enteron in the frog's gastrula may be regarded as being filled
with yolk cells, from which hypoblast will be derived, and it
only gradually opens out later on in development as the yolk is
used up. Hence the coelenterate stage of the ancestral history

270 OUTLINES OF EVOLUTIONARY BIOLOGY

(Fig. 119, IX) is very much disguised in the ontogeny, simply
owing to the presence of a large quantity of nutrient material
in the egg.

In the development of the bird's egg (and also in that of reptiles,
which is closely similar) it is still more difficult to recognize the
process of gastrulation. The quantity of yolk (Fig. 70) is so
enormous that it entirely prevents cell-division from taking
place in the greater part of the egg, and segmentation is con-
sequently confined to a small yolk-free area at one pole of the
spherical mass, where it gives rise to a layer of cells known as the
blastoderm. This blastoderm gradually spreads over the yolk and
from it the embryo is formed by a kind of pinching-off process,
the yolk remaining enclosed in a bag — the yolk-sac — attached to
the lower surface of the embryo, until it is all absorbed by means
of a special set of blood-vessels developed in the yolk-sac, and
used up in the nourishment of the growing embryo. The
embryo itself meanwhile becomes provided with special organs
for its protection and respiration, the so-called foetal membranes.
Of these the amnion, formed by outgrowth of the embryonic
body-wall, forms a sac filled with fluid, which surrounds the
embryo at a distance and leaves it free room for growth, while
the allantois, formed by outgrowth of the embryonic gut-wall,
forms a highly vascular organ which comes into relation with
the extremely porous shell and thus provides for the necessary
gaseous interchange.

The yolk-sac, the amnion and the allantois are all organs
which have been developed in accordance with the special
requirements of the embryo, and which must therefore be left
out of account in reconstructing the direct ontogenetic record.

In the dogfish also (Fig. 120) we find an excellent illustration
of the formation of a special yolk-sac to contain the enormous
supply of food material for the developing embryo, but in this
case neither amnion nor allantois is developed.

The Mammalia, on the other hand, have adopted an improved
method of nourishing their young. The developing embryos are
retained within the body of the parent and obtain their food
supply from the blood of the latter by means of a special organ
known as the placenta, which is developed partly from the
allantois of the embryo or " foetus," and by which the latter is
attached to the wall of the uterus. This method seems to have
proved much more satisfactory than the provisioning of the egg

INFLUENCE OF FOOD YOLK 271

with food-yolk, and we accordingly find that the mammalian
ovum (Fig. 71) has become reduced in size again until it is
no larger than that of Amphioxus, i.e. about 2sotn incn m
diameter. The early stages of segmentation have also gone
back to the primitive type, but, strange to say, a yolk-sac is still
formed although there is no yolk for it to contain — affording an
admirable example of a vestigial embryonic organ. Here one
record has been superposed upon another in the ontogeny and
we have a recapitulation of an embryonic stage of the reptilian
ancestors of the mammals. After birth, of course, the young

FIG. 120.— Embryo of a DogBsh, with Yolk-Sac (Y.S.) attached, X 2. (From

a photograph.)

mammal is nourished by the secretion of milk, and is thereby
enabled to reach a very advanced stage of development before it
has to begin to find its own food. The chick, owing to the very
large amount of food-yolk with which the egg is supplied, is also
able to reach a highly developed condition before it hatches and
begins to look after itself.

With animals whose eggs contain insufficient food-yolk, and
which are not provided with any other special means of
nourishing the developing young, the case is very different. They
are obliged to hatch and begin life on their own account while
still a long way from the adult condition — when only a small
part of their ontogenetic journey has been accomplished.
Amphioxus, for example, hatches as a free-swimming ciliated
embryo shortly after the gastrula stage has been passed through,
and completes its development at the expense of food supplies
which it obtains for itself.

272 OUTLINES OF E VOLUTION AEY BIOLOGY

Embryos which, having hatched at a comparatively early stage
of their development, lead independent, self-supporting lives, are
commonly spoken of as larvae, and amongst such larvae we again
find abundant instances of special larval organs developed in
relation to their special requirements.

The tadpole of the frog (Fig. 121, 4—11) affords an excellent
example of a free-living larva. It has certain special organs,
such as its horny jaws and the suckers on the under surface of

FIG. 121. — Stages in the Life History of the Frog. (From Brehm's "Thier-

leben.")

1, freshly laid eggs; 2, eggs slightly magnified, adhering together by their gelatinous
envelopes to form " spawn " ; 3, young tadpole still enclosed in the jelly ; 4, newly
hatched tadpoles adhering by their suckers to a blade of grass ; 5, 6, tadpoles with
external gills ; 7, 8, tadpoles with the gills concealed by the operculum ; 9, 10, tad-
poles with conspicuous hind legs but with the front legs still concealed beneath the
operculum ; 11, tadpole during metamorphosis, with front legs protruded ; 12, young
frog with tail not completely absorbed.

(For microscopic details of the early stages see Fig. 119.)

the head, which are developed in relation to its larval life, but
there can be no question about the phylogenetic significance
of the tadpole stage as a whole. In every essential feature of its
organization the tadpole, before its legs develop, is a fish. It
swims like a fish by means of its well developed tail, it breathes
like a fish by means of its gills and gill-slits, and its internal
structure is that of a fish. There can be no doubt that it
represents a fish-like stage in the evolution of the frog.

In the tadpole, owing to its free-living, aquatic mode of life,

ABBREVIATION OF ONTOGENY

273

au.

— m.s.

the characteristic piscine organs are fully functional, but even in
the embryos of higher air-breathing vertebrates — reptiles, birds
and mammals — while enclosed within the egg-shell or within the
womb of the parent, we can still recognize clear traces of a
similar fish-like stage in their evolution, for gill-slits are present
in all (Fig. 122, g.s.) and the
arrangement of the principal veins
and arteries is unmistakably fish-
like.

Difficulties in the interpretation of
the ontogenetic record arise not only
from the addition of new features
in the form of embryonic or larval
organs but also from the abbreviation
of the different stages through which
the organism passes. The develop-
ment of any of the higher organisms
must be looked upon as a kind of
compromise, due to the necessity of
attaining as perfect a condition as
possible in as short a time as pos-
sible, for the sooner the adult condi-
tion is reached the sooner will the
organism be able to reproduce its
kind, and hence any abbreviation of
the earlier stages of development will
be advantageous to the species and
will be favoured by natural selection.
For this reason the fish-like stage
in the development of reptiles, birds
and mammals is passed through very
rapidly and the fish-like structure is
no longer fully developed. Although gill-slits are present the
gills themselves are not, and other piscine characters are only
represented by transient vestiges.

Turning now to some of the lower groups of the animal king-
dom, we may briefly notice a beautiful illustration of the law of
recapitulation afforded by the life-history of the common feather-
star, Antedon (Fig. 123). This animal, found abundantly in
comparatively shallow water off the British coast, belongs to the
great phylum Echinodermata, which also include? the star-fish,

B. T

FIG. 122.— Chick Embryo of
about 3 Days, with the
Fretal Membranes and
Yolk-Sac removed. (Pho-
tographed from one of
Ziegler's models.)

au., auditory organ ; e., eye ; g.s., gill
slits; h., heart; m.s., meso-
blastic somites ; n., nasal organ.

274 OUTLINES OF EVOLUTIONARY BIOLOGY

sea-urchins and sea-cucumbers. The subdivision to which it
belongs — the Crinoidea — is characteristically a deep-water group.
It is also a group of great antiquity, the remains of crinoids —
such as the well-known " Encrinites " of the Carboniferous period
—being abundant in certain Palaeozoic formations.

Both the Palaeozoic crinoids and the surviving deep-sea mem-
bers of the group, such as Pentacrinus (Fig. 125), are stalked
forms, the " calyx," with its radiating arms, being attached to

FIG. 123. FIG. 124.

FIG. 123. — The Feather Star (Antedon li/ida), nat. size.
FIG. 124. — Pentacrinoid Stage in the development of Antedon, X 14.
dr. cirri ; st. stalk.

the end of a long, jointed, calcareous stem, the lower extremity of
which is permanently fixed to the sea-bottom. Antedon, and the
other shallow-water crinoids of the present day, on the other
hand (Fig. 123), have no stalks but in place thereof a number of
slender " cirri " whereby they temporarily attach themselves to
seaweed or other objects.

Now in the course of its development from the egg the feather-
star passes first through a free-swimming larval stage and then
through a fixed stalked stage (Fig. 124), known, from its resem-
blance to the deep-sea genus Pentacrinus, as the pentacrinoid

LARVAL ORGANS

275

FIG. 125. — A Deep-Sea Crinoid, Penta-
crinus (Isocrinus) decorus, about nat.
size.

stage. Later on the young animal develops cirri and falls off
the end of
the stalk,
assuming
once more
an active
modeoflife.
Here we
recognize
very clearly
in the pen-
tacrinoid
stage of the
ontogeny
a repeti-
tion of the
stalked
condition

through which the ancestors of the feather-star
must have passed.

We also find amongst invertebrate animals
abundant examples of special larval organs.
The body of a caterpillar, for instance, is built
up very largely of such structures, and it has
to undergo an almost complete reconstruction
within the protective envelope of the chrysalis
before it reaches the adult condition and emerges
as a perfect insect. It is therefore extremely
difficult to interpret the caterpillar stage in
terms of the ancestral history.

The ophiuroids or brittle stars (Fig. 126), so
commonly met with between tide-marks, have
remarkable, free-swimming " Pluteus " larvae
(Fig. 127), totally different in appearance from
the adult. These larvae occur in immense
numbers in the surface waters of the ocean.
They are provided with long slender arms
supported on calcareous rods, which probably
serve, in part at any rate, to protect the very
delicate body from the attacks of other small animals. Before
attaining the adult condition the larva undergoes a complex

T 2

276 OUTLINES OF EVOLUTIONARY BIOLOGY

metamorphosis, during which the long arms are completely lost
and the form of the body entirely changed. The echinoids or
sea-urchins pass through a very similar larval stage.

The free-swimming " Zosea " larva of the crab (Fig. 128) is also
provided with conspicuous defensive spines which are lost in the

FIG. 126.

Fia. 127.

FIG. 126. — A Brittle Star (Ophiura ciliaris), X f.
FIG. 127.— The Pluteus Larva of a Brittle Star, x 62.

adult condition (Fig. 102) and which have doubtless been developed
in relation to the special conditions of the larval life.

In many cases the development of larval organs causes the
young animal to differ so widely from the adult that the relation-
ship of the two for a long time remained unsuspected, and hence
the special names which many larvse have received. In other
cases, however, it is the adult animal which has become so pro-
foundly modified in relation to special conditions of life as to

INTERPRETATION OF ONTOGENY

277

obscure its genetic affinities, which may still be clearly indicated
by embryonic or larval stages.

The common Ascidian, for example (Fig. 129), is a sac-shaped
organism, permanently attached to rock or seaweed in the adult
condition, which by the uninitiated would hardly be taken for an
animal at all. Before their life-history was known the Ascidians
were placed by zoologists amongst the " Molluscoida," a group of
invertebrates supposed to resemble the mollusca or shell-fish. It
was only the discovery that
the typical Ascidians pass
through an active tadpole
stage in their development
(Fig. 130), with a muscular
tail, a notochord and a central
nervous system all formed as
in vertebrates, that demon-
strated their true position
as degenerate members of
the great phylum Chordata
(= Vertebrata in the widest
sense of the term), the tad-
pole stage being, of course,
a recapitulation of the ances-
tral, fish-like, chordate con-
dition.

It will be readily under-
stood from the foregoing
illustrations that in endea-
vouring to reconstruct the
ancestral history of an organ-
ism from the stages which it
passes through in its indivi-
dual development, we have to distinguish very carefully between
two sets of characters. In the first place there are characters which
are due to inheritance from the adult condition of very remote
ancestors and which really indicate stages in what we may call
the direct line of evolution. These are termed palingenetic or
ancient characters. Such, for example, are the essential features
of the gastrula stage, which occurs so generally and which indi-
cates a remote ccelenterate ancestor (the Gastrsea of Professor
Haeckel) for at any rate the great majority of multicellular animals.

FIG. 128. — The Zoaoa Larva of a
Crab (Portunus), X 30. (From a
photograph.)

278 OUTLINES OF EVOLUTIONARY BIOLOGY

Such also are the essential characters of the tadpole stage of the
frog and of the ascidians, and the vestigial gill-slits of higher
vertebrates, which indicate descent from some gill-breathing, fish-
like form.

In the second place there are characters which have been
acquired secondarily by the embryo or larva in relation to its

tn.a-p.

ex.ap.

FIG. 129. — A Simple Ascidian (dona
intestinalis], X £.

a.p., processes for attachment ; in.ap.,
inhalant aperture ; ex.ap., exhalant
aperture.

FIG. 130. — Tadpole Larva of a
Compound Ascidian (Diplosoma
crystallinum), x 35. (From a
photograph.)

p., papillae for attachment ; t., tail.

special embryonic or larval requirements. These are termed
csenogenetic or modern characters, and it is these which tend
more or less to obscure the ontogenetic record. Such are the
presence of food-yolk, the foetal membranes of air-breathing
vertebrates and the various special larval organs to which we
have above referred. It is difficult, however, to draw a hard and
fast line between palingenetic and caenogenetic characters, and
we must always remember that embryonic stages, as well as the

PHYLOGENY AND ONTOGENY

279

St3ges

adult, undergo modification during evolution, and that some
caenogenetic characters are very much more ancient than others.
The relation which exists between the phylogenetic and onto-
genetic stages in the evolution of an organism are very diagram-
matically expressed in Fig. 131, which is drawn with special
reference to the familiar case of the frog. The vertical line is
supposed to represent the ancestral history, some of the principal
stages of which are indicated.
The horizontal line at the top
represents the individual life-
history, with the stages indi-
cated which represent those
selected in the ancestral his-
tory. The ovum represents
the protozoon ancestor ; the
blastula the hollow, spherical
protozoon colony; the gastrula
the coelenterate ; the meta-
merically segmented embryo
the segmented worm, and the
tadpole the fish-like stage.
The oblique lines represent
the lines of evolution of the
different ontogenetic stages
themselves, in the course of
which caenogenetic characters,
such as the acquisition of
food-yolk by the ovum and of
suckers and horny jaws by

Protozoan Colony
'Hollow Sphere)

Protozoon

FIG. 131. — Diagram illustrating the
Eelation between Ontogeny and
Phylogeny.

the tadpole, have arisen. The
shorter horizontal lines repre-
sent the ontogeny at the different ancestral stages, and their
increase in length from below upwards is obviously due to the
successive addition of new stages to the ontogenetic record, while
the shortness of the horizontal lines as compared with the corre-
sponding portions of the vertical line may be taken to indicate,
though very imperfectly, the abbreviation of the ontogeny.

In the vegetable kingdom we find illustrations of the law of
recapitulation and of the complication of the ontogenetic record
by the development of caenogenetic characters, quite as striking
as any which we find amongst animals. Take, for example, the

280 OUTLINES OF EVOLUTIONARY BIOLOGY

life-history of some of the Australian acacias, or wattles. These
are leguminous plants and many of them have the pinnately
subdivided leaves so characteristic of the order. Others, however,
have the pinnate leaves in the adult entirely replaced by narrow
" phyllodes," which closely resemble simple undivided leaves
like those of some of the eucalypts, but are really only flattened
leaf-stalks or petioles. The development of such phyllodes is no
doubt to be regarded as an adaptation to the heat arid drought of

the Australian climate,
which delicately sub-
divided leaves of the
ordinary leguminous
type are ill-suited to
withstand, and the re-
semblance of the phyl-
lodes to true foliage
leaves of the undivided
type — and especially to
those of the eucalypts
— affords a very good
illustration of the
phenomenon of conver-
gence. If, now, we
examine the seedlings
of a phyllode-bearing
acacia (Fig. 132) we
shall find that imme-
diately after the coty-
ledons or seed - leaves
they bear typical pin-
nate leaves, and then
leaves of intermediate form, thus clearly recapitulating the
ancestral condition from which the phyllode-bearing forms were
derived. A very similar state of things occurs in the common
European gorse or furze, another leguminous plant, in which
the leaves of the adult have become modified to form spines,
while those of the seedling still retain the ancestral ternate
condition, somewhat resembling those of the clover.

The cotyledons or true seed-leaves of flowering plants, on
the other hand, afford excellent examples of caenogenetic characters
developed in relation to the special requirements of the embryo.

FlG. 132. — Seedling of a phyllode-bearing
Acacia (Acacia pycnantha). (From Stras-
burger.) (The cotyledons have already
been shed.)

RECAPITULATION AND HEREDITY

281

They are very commonly modified to form receptacles for a
store of food material for the young plant in the form of
starch, as in the peas and beans (compare Fig. 56), and may
never even leave the seed-coat or emerge above the ground.
In such cases they develop no chlorophyll and differ absolutely
from ordinary foliage leaves, without, of course, indicating any
ancestral stage through which the latter have passed. Even
when they emerge above the ground, as in the beech (Fig. 133),
and become green, their form is usually widely different from
that of the foliage leaves and
appears to have been developed
largely in adaptation to the neces-
sity for close package within the
narrow compass of the seed-coat,
the shape of the cotyledons being
governed, as Lord Avebury has
pointed out, by that of the seed
within which, in a dormant condi-
tion, they pass most of their lives.
In short, the embryological
investigation of both animal and
vegetable organisms leaves no
doubt as to the general truth
of the recapitulation hypothesis,
and must convince any unbiassed
observer that, however much modified it may be by abbrevia-
tion and by the superposition of secondary features, the life-history
of the individual is essentially a condensed epitome of the
ancestral history of the race. The law of recapitulation, indeed,
may be regarded simply as a logical extension of the law of
heredity, for every organism tends to inherit the characters not
only of its immediate progenitors but of all its ancestors, and
these characters appear in the individual life-history in the same
order as that in which they first appeared in the ancestral
history — in other words, ontogeny is a repetition of phylogeny
and can only be explained in terms of organic evolution.

FIG. 133. — Seedling of Beech,
showing the Cotyledons.
(From Lubbock.)

CHAPTER XIX

The stratified rocks — Geological periods — The age of the habitable earth —
The geological record — The succession of the great vertebrate groups.

THE materials of which the solid crnst of the earth is made up
are, as it is perhaps hardly necessary to point out, extremely
varied, but whatever may be their physical or chemical characters
they are all spoken of by geologists as rocks. In accordance
with their mode of formation these rocks may be divided into two
chief categories, first, those which, like granite and basalt, have
been formed directly by cooling of molten matter, of which the
earth was perhaps at one time entirely composed, and, second,
those which, like sandstone, shale, limestone and coal, have been
formed by accumulation of detritus derived from pre-existing
rocks or from the remains of dead organisms. The first are
commonly termed igneous and the second sedimentary, the
latter deriving their name from the fact that they have been
deposited under water. Owing to the manner in which they
have been laid down, usually in the form of gravel, sand, mud or
ooze, which have subsequently become hardened and consolidated
by the pressure of more recent deposits, the sedimentary rocks
form a series of layers or strata of varying thickness superposed
one upon another. The stratification must at first have been
horizontal, but, owing to the irregular shrinkage of the earth's
crust and to the action of volcanic upheavals, it has usually been
more or less disturbed, so that the planes of bedding have been
tilted at various angles and frequently exhibit a high degree of
curvature or even contortion. Moreover, rocks which were
originally laid down on the bed of the sea have in many cases
come to form part of the dry land and even to be uplifted far above
sea-level in the form of mountain ranges.

The formation of sedimentary rocks must have been going on
ever since the earth became cool enough to admit of the con-
densation of aqueous vapour into water, and it is still going on
in the same way at the present day. Every river is continually

THE STEATIFIED ROCKS 283

carrying down to the sea immense quantities of sand and mud,
which will ultimately be deposited in the form of new strata ;
every cliff by the sea-shore is slowly or rapidly weathering away
under the influence of atmospheric agencies — rain, frost, tides
and wind ; and far out, beyond the reach of the detritus derived
from the land, the sea-bottom is being covered by the slow
accumulation of ooze or mud derived almost entirely from the
remains of marine organisms, while the activities of corals and
other reef -building animals in their turn make no small contribu-
tion to the grand total.

The sedimentary rocks naturally contain the remains of vast
numbers of organisms which flourished on the earth at more or
less remote periods of its history, and the study of these fossils
may justly be expected to yield results of the greatest importance
from the point of view of the theory of organic evolution. We
have here, in fact, the only really direct evidence of the course
which the evolution of the animal and vegetable kingdoms has
actually taken, and this evidence constitutes what is commonly
known as the geological record.

The relative ages of the different sedimentary rocks and of
the different geological formations which they build up are of
course determined primarily by their position with regard to one
another, each layer or bed being necessarily of more recent date
than the one below it.1 The series of strata met with in any
particular locality may, however, be very different from that
which occurs in other localities, for while mud may be accumu-
lating in one place, sandstone may be forming in another and
limestone in a third, and over other areas, which lie above
sea-level, rock formation will be replaced by destruction or
denudation. Nevertheless, geologists have found it possible to
compare the series of stratified rocks found in different parts of
the world one with another, and, by the aid of the fossils which
they contain, to identify a number of more or less well defined
epochs in the geological history of the earth.

At the bottom of the stratified series lies an immense thickness
of rocks which seem to have been originally sedimentary,
but many of which have undergone profound changes due to
pressure and heat since they were laid down. These rocks
indicate a very long period of the earth's history, the

1 Except in a few cases where contortion has been so extensive as to reverse the
relative positions.

284 OUTLINES OF EVOLUTIONARY BIOLOGY

GEOLOGICAL TIME-SCALE, AS INDICATED BY THE STRATIFIED EOCKS.

1

w

Epochs, with relative duration
indicated by thickness of
stratified rocks.

Some representative
British Formations.

Range in Time (if
Animal Groups.

rf

« £

•~ a T.

J ^ £ £ |

| | f 1| = g

S S | cSa JB S

Cainozoie or Tertiary.

Kccriit and Pleistocene 4000 feet.

Alluvium, Peat, boulder Clay, Jkc.

Pliocene 13000 feet.

Norwich and Red Crag, Coralline

: : : : 9

Miocene 14000 feet.

Oligocene 12000 feet.

Hamstead, Bembridge, Osborne
and Headon Beds.

Eocene 20000 feet.

Barton and Bagshot Beds, London
Clay, Thanet Sands, &c.

Mesozoic or Secondary.

Cretaceous 44000 feet.

Chalk, Greensand, Gault, Wealden,
&c.

Jurassic 8000 feet.

Oolites, Lias.

Triassic 17000 feet.

White Lias (Rhictic) Elgin Sand-

stone, &c.

Palseozoic or Primary.

Permian 12000 feet.

Magnesian Limestone, Red
Marls, &c.

Carboniferous 29000 feet.

Coal Measures, Millstone Grit,
Mountain Limestone, &c.

Devonian 22000 feet.

Old Red Sandstone.

Silurian 15000 feet.

Ludlow and Wenlock Shales and
Limestones, &c.

Ordovician 17000 feet.

Slates, Flags, Sandstones, &c., of
Bala, Caradoc, Llandeilo, Skid-
daw, &c.

Cambrian 26000 feet.

Lingula Flags, Harlech Series, &c.

Pre-Cambrian (extent unknown).
1

Archaean Schists, Gneiss and
other Crystalline Rocks; largely
metamorpliif.

9

THE AGE OF THE HABITABLE EARTH 285

pre-Cambrian epoch, during which organic evolution must have
been taking place, though only somewhat doubtful indications
of organic remains have been met with. Probably the pre-
Cambrian rocks originally contained abundant fossils, which have
been destroyed by the treatment to which they have been sub-
jected. All the subsequent periods of the earth's history,
however, are represented by rocks containing organic remains in
greater or less abundance. These deposits indicate a division of
the earth's history since pre-Cambrian times into three main
eras — primary or palaeozoic, secondary or mesozoic and tertiary
or cainozoic. Each of these three is subdivided into a series of
epochs, as shown in the accompanying table, in which are also
mentioned some of the principal rock formations by which the
different epochs are represented in the British Islands.

The thickness of the strata deposited during each period differs
greatly in different parts of the world, and in North America the
various formations appear to be much better developed than in
North Western Europe. It is of the utmost importance that our
estimates of this thickness should be as accurate as possible, for
they constitute important data from which to calculate the age
of the habitable earth and the duration of the different geological
periods.

Sir Archibald Geikie,1 in 1892, very cautiously estimated the
total thickness of the stratified rocks, where most fully developed,
from the bottom of the Cambrian onwards, as not less than
100,000 feet. He also estimated that it would take from 730 to
6,800 years to add a foot to this thickness by accumulation of
material derived from the denudation of the land. Of course the
actual time must vary very greatly according to the nature of the
material deposited and the agencies at work. There is, however,
no reason to suppose that denudation and sedimentation took place,
on an average, more rapidly in past geological time than at the
present day. If, then, we take the average between the most
rapid and the slowest rate of denudation and sedimentation, and
the thickness of the stratified rocks as calculated by Professor
Geikie, we arrive at 376,500,000 years as the age of the habitable
earth exclusive of the pre-Cambrian period.

Sir Archibald Geikie appears, however, to have assumed for
the purposes of his calculation that material derived from the
wearing away of the land has been spread out over an equal area

1 Presidential Address, British Association, Edinburgh , 1892.

286 OUTLINES OF EVOLUTIONARY BIOLOGY

of sea-bottom, so that every foot in depth worn off the land is
taken to indicate a foot in thickness piled up in new deposits.
This hardly seems to be a justifiable assumption, for in many
localities sediment derived from a large area must be accu-
mulated over a small one, and the increase in thickness of the
newly forming rock must therefore take place at a much greater
rate than the decrease in thickness of that which is being worn
away.

Professor Sollas, in his presidential address to the Geological
Society of London in 1909, gives the total maximum thickness of
the sedimentary rocks from the bottom of the Cambrian to the top
of the most recent formations as being no less than 253,000 feet.
I have accepted this estimate in drawing up the geological time-
scale on page 284, in which the supposed thickness of the
deposits belonging to each of the main subdivisions of the three
great eras is indicated both numerically and by the relative depth
of the space allotted to each.

Professor Sollas, however, estimates a much more rapid rate of
accumulation of these deposits — as high, indeed, as one foot per
century — which, even with his own increased estimate of thickness,
would give only 25,800,000 years as the total lapse of time since
the commencement of the Cambrian epoch. But he also points
out that there is good reason for believing that the pre-
Cambrian epoch must have been as long as all the succeeding
epochs put together, and the total lapse of time since the formation
of the stratified rocks (including the pre-Cambrian) commenced
might accordingly be estimated at 50,600,000 years.

We must also, of course, make abundant allowance for the fact
that denudation has been going on at all times since the deposition
of the stratified rocks commenced, and that the total thickness of
these rocks has been enormously reduced thereby, the same
material having been used over and over again in rock formation.

Where opinions differ so much, and where there is so much
uncertainty as to the value of the data themselves, it is obvious
/that any estimate of geological time derived from the thickness
'of the sedimentary rocks can have only a doubtful value, and that
it is highly desirable that such an estimate should be checked
by making use of other sources of information.

Several other methods of estimating the age of the earth have
been employed. Perhaps the most satisfactory is that which is
especially associated with the name of Professor Joly, and which

THE AGE OF THE HABITABLE EARTH 287

depends upon the amount of sodium present in solution in sea-
water. If we knew the total amount of sodium present in the
ocean, and if we also knew the rate at which it has been accu-
mulating since the ocean was first formed, it would be a simple
matter to calculate the age of the ocean and therefore that of
the stratified rocks, whose existence must have commenced with
that of the ocean itself. The greater part at any rate of the
salt contained in the sea must have been derived from the
disintegration of the land and carried down by rivers. Estimates
have been made, based upon careful analysis, of the amount of
sodium contained in the sea and in the water of various rivers,
as well as of the amount of water which flows into the sea
annually, and Professor Sollas has come to the conclusion that
" on a review of all the facts, the most probable estimate of the
age of the ocean would appear to lie between 80 and 150 millions
of years," which is considerably in excess of the estimate which
he arrives at from the consideration of the thickness of the
sedimentary rocks.

We cannot assume, however, that the earth has been habitable
during the whole of this period, for when the ocean was first
formed both it and the land must have been at far too high a
temperature to admit of the existence of protoplasmic organisms.
Possibly if we assume 100,000,000 years as the total age of the
habitable earth, and therefore as the time which has been available
for the evolution of the organic world, we shall be as near as we
can get to the truth in the present state of our knowledge.

We - must now turn to the consideration of the geological
record itself, and in the first place we may ask, what is to be
legitimately expected from such a record ? Have we any right
to expect anything like a complete representation, by fossil
remains, of the past history of the organic world ? Obviously
the answer to this question must be a decided negative. The
imperfection of the geological record is due to many causes. It
must have been imperfect when first laid down, for the great
majority of organisms usually disintegrate and pass away
without leaving any recognizable remains behind them at all.
In most cases it is only hard skeletal structures that have any
chance of being preserved. Bodies composed entirely of soft
tissues, such as jelly-fish and many worms, are rarely repre-
sented by fossils. Even hard structures, such as animal
skeletons, will only be fossilized if they happen by some

288 OUTLINES OF EVOLUTIONARY BIOLOGY

fortunate chance to find their way to some place where rock
formation of a suitable kind is going on. The remains of land
animals may be carried by rivers to some sea or lake and buried
in a suitable accumulation of mud or sand, but it is more likely
that they will not. Marine animals, provided they have hard
skeletons, have, of course, many more opportunities of attaining
a lasting monument, and we often find great thicknesses of rock,
such as chalk and limestone, composed almost entirely of their
remains.

Even when the record has been successfully established,
however, it is liable to destruction by various agencies. The

A. B. C.

FIG. 134. — Three Species of Trilobites. (From British Museum Guide.)
A., Agnostus princeps ; B., Olenus cataractes ; C., Staurocephalus murcliisoni.

rocks containing it may be uplifted above sea-level and planed
right away by sub-aerial denudation, or they may be sunk so
far beneath later accumulations as to be profoundly altered by
the action of heat and pressure, by which means any fossils
which they contain may be completely destroyed. Then, again,
we must remember that only a relatively small proportion of
the earth's crust is accessible for investigation. Deep-lying
rocks may be brought to the surface by upheaval, and denudation
of overlying strata, or they may be reached by deep mining, but
we know practically nothing of the strata which now lie beneath
the bed of the sea, and even the dry land has only been tentatively
scratched by inquisitive man in a few places.

Even were the geological record far less perfect than it
actually is, therefore, its imperfection could not legitimately be

THE GEOLOGICAL RECORD 289

used as an argument against the theory of organic evolution.
From the nature of the case it must be imperfect and the
surprising thing about it is that it should have yielded so much
evidence as it has done. The earlier portions of the record, how-
ever, have apparently been completely destroyed. The fact that
alread}^ in the Cambrian epoch highly organized invertebrate
animals, such as trilobites (Fig. 134) and brachiopods, were in
existence, shows us that evolution must then have been going on
for an immensely long period. This period, as we have already
indicated, is represented by the pre-Cambrian rocks, in which
the organic remains have been so far destroyed as to be only
recognizable in a few doubtful cases.

The investigation of what remains of the geological record
has, however, yielded some very remarkable and conclusive
results. In the first place we have learnt that life on the earth
has been continuous without a break from the commencement
of the Cambrian epoch to the present day, and in the second
place we have learnt that the higher groups of animals and
plants have appeared on the earth in exactly the order which we
should expect on the assumption that each has arisen from some
preceding and more lowly organized ancestral group.

In discussing this point we may confine our attention to the
Vertebrata, the evolution of which seems to have taken place
entirely during post-Cambrian times. The most lowly organized
group of true vertebrates existing at the present day is that of
the Cyclostomata, which includes the lampreys and hag-fishes.
These are distinguished from true fishes by the absence of jaws,
the mouth being suctorial, and by other primitive features. The
skull and vertebral column remain cartilaginous throughout life
and there are no paired limbs. Neither is any dermal armour
of any kind developed. The only structures which they possess
which seem at all likely to be preserved as fossils are the horny
teeth. Certain bodies known as conodonts, which occur as
fossils in strata ranging from the Lower Silurian to the
Carboniferous, may perhaps represent such teeth. If so they
are perhaps the earliest vertebrate remains as yet known.1

The remarkable group of fishes known as Ostracodermi,
however, makes its appearance in the Upper Silurian and
continues on to the Upper Devonian. These animals agree

1 Palseospondylus, from the Lower Old Red Sandstone of Scotland, is probably
also a cyclcstome.

B. U

290 OUTLINES OF EVOLUTIONARY BIOLOGY

with the cyclostomes in the absence of true jaws and (in most
known forms) of paired fins, and in the extremely primitive
character of the vertebral column. They had also, like the
cyclostomes, median fins, but they differed from the latter con-
spicuously in the presence of a strongly developed dermal armour,

FIG. 135. — Restoration of Cephalaspis murcMsoni , from the Lower Old Red
Sandstone, X ^. (From British Museum Guide, after Smith Wood-
ward.)

to the existence of which their fossil remains owe their preser-
vation. As representative genera of this group we may men-
tion Cyathaspis (Upper Silurian), Pteraspis (Lower Devonian),

B-

FIG. 136. — Pterichthys milleri, from the Lower Old Red Sandstone; restored
by R. H. Traquair. A. Dorsal ; B. Ventral ; C. Lateral views, x \.
(From Smith Woodward's " Vertebrate Palaeontology.")

Cephalaspis (Upper Silurian and Lower Devonian, Fig. 135) and
Pterichthys (Lower Devonian, Fig. 136).

The origin of the Ostracodermi is unknown, but their relation-
ship to the cyclostomes has led to their inclusion with the latter in
one class, the Agnatha (jawless vertebrates), which stands at the

THE GEOLOGICAL RECORD

291

very bottom of the vertebrate series and was probably derived from
some primitive chordate ancestor, not unlike Amphioxus, which
had no hard structures to be handed down to us in a fossil
condition. It is true that the ostracoderms bear a curious
resemblance to certain extinct contemporaneous arthropods, the
Eurypteridae (Fig. 137),
but this resemblance is
probably merely super-
ficial and due to conver-
gence owing to similar
conditions of life; it
cannot be accepted as
evidence of the origin of
vertebrates from arthro-
podan ancestors, which,
on other grounds, is ex-
tremely improbable.

The most primitive
group of true jaw-bearing
fishes is that of the Elas-
mobranchii, including the
existing sharks, dog-fishes,
skates and rays. These
have well developed paired
fins and a dermal armour
of scales or denticles, but
their primitive character
is shown by the fact
that the internal skeleton
remains cartilaginous

FIG. 137. — Pteryyotus osiliensis, an Upper
Silurian Eurypterid, dorsal view, re-
duced. (From Wood's ' ' Palaeontology, "
after Schmidt.)

throughout life. Frag-
mentary indications of
elasmobranchs occur
already in the Upper
Silurian, but the first satisfactory specimens have so far been met
with in rocks of Lower Devonian age (Old Red Sandstone). A
carboniferous form, Acanthodes, is represented in Fig. 138.

The remarkable Dipnoi, feebly represented by the mud-
fishes or lung-fishes of the present day — the Australian Neo-
ceratodus (Fig. 110), the African Protopterus and the South
American Lepidosiren — seem to have arisen during the

u 2

292 OUTLINES OF EVOLUTIONARY BIOLOGY

Devonian period and to have attained their maximum of
development and specialization during that epoch. This group
is of especial interest, as we have already had occasion to notice,
as indicating how the transition from an aquatic to a terrestrial
mode of life was first rendered possible to vertebrate animals by
the conversion of the swim-bladder into a pair of lungs. The
dipnoids, however, have remained aquatic in habit, and retain
their gills for aquatic respiration as well as being able to breathe
air directly by means of their lungs.

The ordinary bony fishes (Teleostei), to which the vast
majority of existing species belong, do not attain any prominence
as a group before the secondary era, but they had fore-runners
in some of the early ganoids which date back to lower
Devonian times. The teleosts are the most specialized of all

FIG. 138. — Kestoration of Acantliodes ivardi, from the Coal Measures of
Staffordshire, X ^. (From British Museum Guide.)

fishes and it is not from such a group that we should expect the
next great advance in organization to take its origin.

There are the strongest anatomical and embryological grounds
for believing that the Amphibia, a group which at the present
day includes the frogs, toads, newts and salamanders, are
the descendants of fish-like ancestors which became adapted to
an air-breathing mode of life by the development of lungs. The
dipnoid fishes, which, as we have already seen, attained their
maximum of development in the Devonian epoch, show us
clearly enough how such a transition probably took place, and
it was doubtless either from some dipnoid form, or from some
other primitive type of fish which had also succeeded in con-
verting its swim-bladder into lungs, that the Amphibia arose.
The fact that amphibian remains first occur in Lower Carboni-
ferous rocks is in complete harmony with this hypothesis as to
the origin of the group.

THE GEOLOGICAL RECORD

293

The earliest known amphibians are the Stegocephalia, so-
called on account of the strongly developed bony armour
which covered the head and formed the roof of the skull, as it
did already in many of the earlier fishes. These Stegocephalia
included both small, salamander-like forms such as Branchio-
saurus (Fig. 139) and large crocodile-like creatures — the
labyrinthodonts. Some of them survived into the j-Triassic

FIG. 139. — Branchiosaurus amblystomus ; restoration of Skeleton (A) and
ventral Armour (B) by II. Credner, nat. size. Lower Permian. (From
Smith Woodward's " Vertebrate Palaeontology.")

epoch, after which they appear to have become entirely extinct,
and although a fair number of Amphibia still exist at the present
day, the group must be regarded as having attained its maximum
of development in late Palaeozoic and early Mesozoic times. The
existing frogs and toads are very highly specialized forms,
completely adapted to a terrestrial life in the adult condition but
for the most part resorting to water to deposit their eggs, which
hatch out in the form of aquatic fish-like larvae or tadpoles,

294 OUTLINES OF EVOLUTIONARY BIOLOGY

breathing by means of gills (Fig. 121). The newts, on the other
hand, though developing lungs, retain the fish-like form and the
aquatic habit throughout life.

In one respect the imperfection of the geological record as
regards the origin of the Amphibia from fishes is especially to
be regretted. One of the chief distinctions between the two
groups lies in the fact that whereas the Amphibia always
have pentadactyl limbs (except when these have been lost
by degeneration), the fishes have only fins, which are not
pentadactyl. The conversion of the fin into a pentadactyl limb
was no doubt an adaptation to the terrestrial mode of life, but
neither comparative anatomy nor embryology nor yet the evidence
of the geological record have so far enabled us to discover how the
transformation of the one type of limb into the other actually took
place. We can only hope that light may yet be thrown upon this
difficult problem by the discovery of the fossil remains of inter-
mediate forms.

The reptiles and the amphibians are closely related to one
another, but existing reptiles are readily distinguished from
existing amphibians in a variety of ways. For example, they
produce much larger eggs, within which the young are nourished
up to a very advanced stage of development by means of the yolk,
while the embryo is provided with the special protective and
respiratory total membranes known as amnion and allantois,
which are not found in amphibians. A reptile, again, never
develops gills at any stage of its existence, while an amphibian
not only has gills in the larval but sometimes also in the adult
state.

Fossil remains naturally afford no clue as to when the develop-
ment of the total membranes and the suppression of the larval
gills took place, and we are thrown back entirely upon skeletal
peculiarities as a means of distinguishing between extinct
members of the two groups. From one point of view this is
unsatisfactory, but the very difficulty which we experience in
drawing a hard and fast line between the fossil reptiles and
amphibians only serves to emphasize the fact that the one group
has been derived from the other.

One of the chief osteological peculiarities which distinguish
existing reptiles from amphibians is, as Dr. Smith Woodward
observes, " the degeneration of the parasphenoid bone and its
functional replacement in the basi-cranial axis by the pterygoids.

THE GEOLOGICAL RECORD 295

. . . If this feature in the palate has always been distinctive,
Palfeohatteria, from the Lower Permian of Saxony, is the
earliest member of the class Reptilia hitherto discovered ; and it
is certain that during the Upper Permian age there were numerous
reptiles both in Europe and America, probably also in South
Africa."1 So far as the evidence goes, therefore, the appearance
of the Reptilia succeeds that of the Amphibia in exactly the way
demanded by the theory of organic evolution.

The reptiles as a group attained their maximum of develop-
ment in Mesozoic times, and of the nine great orders into
which the class has been subdivided only four persist at the
present day, and one of these four is represented by only a single

FIG. 140. — Skeleton of Pariasatirus baini, X BV (From Smith^Woodward,

after Seeley.)

species. This species, Sphenodon (=Hattena) punctatus (Fig. 113),
now confined to certain small islands off the coast of New
Zealand, is the sole surviving representative of the order
Rhynchocephalia, and not only constitutes the oldest existing type
of reptile but also makes a very close approach to the oldest type
known from fossil remains — the extinct Palfeohatteria of the Lower
Permian. The other existing orders of reptiles are the Chelonia
(turtles and tortoises), the Squamata (lizards and snakes) and the
Crocodilia (crocodiles and alligators).

Many of the extinct Secondary reptiles were far more highly
specialized and remarkable than any existing forms. Different
members of the class were adapted to the most diverse modes of
life, the group having taken full possession of sea, land and air.

1 A. Smith Woodward, " Outlines of Vertebrate Palaeontology for Students of
Zoology " (Cambridge, 1898).

296 OUTLINES OF EVOLUTIONARY BIOLOGY

The earliest and at the same time the least specialized of the
extinct orders were the Anomodontia or Theromorpha, which Dr.
Smith Woodward tells us were "directly intermediate in skeletal

be
W

OJ

g

•si

II

M
f.

characters between the highest Labyrinthodonts (Mastodonsaurus
and its allies) and the lowest Mammals (Monotremata)." These
animals, whose remains have been found only in Permian and
Triassic deposits, were terrestrial creatures with limbs modified
for walking on dry land, so that they often bore a close general

THE GEOLOGICAL RECORD 297

resemblance to mammals. One of the best known is the celebrated
Pariasaurus (Fig. 140), described by the late Professor Seeley
from the Karoo formation of Cape Colony. This animal attained
a length of some ten or eleven feet.

The Sauropterygia or Plesiosauria (Fig. 141) were long-
necked lizard-like forms, sometimes of large dimensions, which
had become re-adapted to a marine life, and whose limbs, while
retaining the typical pentadactyl structure, gradually became
modified to form paddles. The group seems to have persisted

FIG. 143. — Skeleton of Tyuanodon lernissarfensis, from the Wealden of
Belgium, X g\j- (From British Museum Guide.)

throughout the whole of the Secondary period and shows a gradual
evolution from the Trias onwards.

The Ichthyopterygia or Ichthyosauria (Fig. 142), which also
range throughout the whole of the Secondary period, were more
completely adapted to life in the ocean and acquired a remarkably
fish-like form, like the whales, dolphins and porpoises amongst
existing mammals.

The Dinosauria or Ornithoscelida formed an enormous goup
of land reptiles which seems to have arisen in the Triassic period
and reached its climax in Jurassic and Cretaceous times.
Some of them were carnivorous, others herbivorous; some walked
on all fours and others on their hind legs, and they often attained
gigantic dimensions. The well known Iguanodon (Fig. 143), from the

298 OUTLINES OF EVOLUTIONARY BIOLOGY

THE GEOLOGICAL RECORD 299

Wealden deposits of Europe, was a huge herbivore, which seems to
have supported its massive body, chiefly at any rate, on its hind
limbs, which were much more strongly developed than the front
pair. The American Brontosaurus (Fig. 144) and Diplodocus
walked on all fours, and the latter attained a length of 80 feet,
but a large proportion of this was taken up by the enormously
long neck and tail. In both these forms the head was of
astonishingly small dimensions in proportion to the rest of the
body. Other Dinosaurs, such as Stegosaurus (Fig. 145) and
Triceratops (Fig. 146), developed an extraordinary dermal
armature of bony plates or spines.

Perhaps the most remarkable of all the extinct reptiles of
the Secondary period, however, were the Ornithosauria (Ptero-
sauria or pterodactyls), which at that time occupied the place now

FIG. 147. — Skeleton and Outline of Pteranodon occidentals, from the Upper
Cretaceous of Kansas, U.S.A. ; X 5%- (From British Museum Guide.)

filled by the birds and bats. These animals were very perfectly
adapted for flight, the wing being formed, as already described in
Chapter XVII, by an extension of the skin supported by the arms,
legs and tail, and especially by the enormously elongated fifth digit
(Fig. 99). Some of these flying reptiles attained a very large
size, the total expanse of the wings in Pteranodon (Fig. 147)
being about eighteen feet.

The origin of birds from reptilian ancestors has been quite
conclusively demonstrated on anatomical and embryological

FIG. 144. — Skeleton of Brontosaurus excelsus, from the Upper Jurassic of

Wyoming, U.S.A.; x T^. (From British Museum Guide, after O. C.

Marsh.)
FIG. 145. — Skeleton of Stegosaurus urtyulatus, from Jurassic of Wyoming ;

X aV (From Smith Woodward's "Vertebrate Palaeontology," after

O. C. Marsh.)
FIG. 146. — Skeleton of Triceratops prorsus, from Cretaceous of Wyoming;

X SV (From British Museum Guide, after 0. C. Marsh.)

300 OUTLINES OF EVOLUTIONARY BIOLOGY

grounds, and Professor Huxley included both birds and reptiles
in one large group, the Sauropsida. The structure of their
wings shows, however, that birds cannot have arisen from
pterodactyls but must have sprung from some less specialized
form, mainly through the development of a dermal exoskeleton
in the form of feathers, which constitutes the chief distinguishing
feature of the group. It is, therefore, quite in accordance with
expectation that the earliest known bird, Archaeopteryx (Fig. 151),
should have been found in the lithographic slates of Solenhofen
in Bavaria, which are of Upper Jurassic age, and should exhibit
characters intermediate between those of existing birds and
reptiles. We shall have occasion to refer to this remarkable
connecting link more in detail in our next chapter.

The Mammalia, so far as existing evidence indicates, appeared
at an earlier date than the birds, but the earliest known mammals
were far less highly specialized forms than birds, and although
what appear to be mammalian remains have been found as low
down as the Trias, the group did not attain much importance
before the Tertiary epoch, when it replaced the reptiles as the
dominant group of vertebrates. Opinions have been a good deal
divided as to whether the mammals arose directly from amphibian
ancestors or from some primitive group of reptiles. On the
whole the evidence seems to be decidedly in favour of the latter
view, which fits in very well with the first appearance of the
mammals just when the reptiles were beginning to become
dominant, and with the fact that, as we have already seen, ,
some of the earlier reptiles were extraordinarily mammalian
in aspect.

The earliest mammals were small forms of doubtful affinities.
Dr. Smith Woodward remarks in this connection : — " It is as yet
impossible to determine at what particular stage in the evolution
of the vertebrate skeleton the lung-breathers first acquired the
characteristic mammalian circulatory system, the milk -producing
glands, and a dermal covering of hair," these being the principal
features, apart from the skeleton, which distinguish mammals
from reptiles at the present day. The difficulty is increased by
the fact that some of the anomodont reptiles had acquired a
dentition very similar to that of primitive mammals, while the
earliest known mammals are represented by little more than a
few fossil jaws and teeth.

The Mammalia, both living and extinct, may be divided into

THE GEOLOGICAL RECORD 301

three main subclasses. (1) Prototlieria, with the single surviving
order Monotremata, represented by the Australian spiny ant-
eater (Echidna, Fig. 92) and duck-billed Platypus (Ornithorhyn-
chus, Fig. 91), which, as we have already seen, are intermediate
both in anatomical structure and in their method of reproduction
between the reptiles and the more typical mammals. (2) Meta-
theria, with the sole order Marsupialia, represented to-day by
the Australian kangaroos, wombats, phalangers, thylacines and
numerous other pouched mammals, and by the American opossums
(Didelphyidae) and Caenolestes. These are in some respects primi-
tive forms, in which the young are born at a very early stage of
their development (there being at the most only a very feebly
developed placenta) and usually carried about, attached to the
teat of the mother, in a marsupium or pouch. They are readily
distinguished by osteological features, especially the structure of the
jaws and teeth (Fig. 108), from the higher mammals. (3) Entlieria,
the dominant mammals of the present day, with a large number
of orders — edentates, whales, sirenians, ungulates, rodents,
carnivores, insectivores, bats and primates ; with well developed
placenta and retaining the young in the womb until a very
advanced stage of development. There can be no doubt that
the Eutheria are on the whole more highly organized forms
than the Metatheria and the latter than the Prototlieria. This
is especially indicated by the gradual acquisition and perfection
of the characteristically mammalian method of nourishing the
young by means of the placenta and mammary glands.

The order of appearance in time of the principal subdivisions
of the class Mammalia, as indicated by the geological record,
entirely supports the evolutionary hypothesis. The earliest
known fossils which may possibly be referable to the class are of
Triassic age. Tritylodon is represented by an imperfect skull
from the Karoo formation of South Africa. This has generally
been regarded as mammalian, but Dr. Smith Woodward, notwith-
standing the fact that it has tuberculated grinding teeth with
deeply cleft roots, considers that it probably belonged to an
anomodont or theromorph reptile. Microlestes is represented by
double-rooted, multituberculate teeth obtained from European
Rhaetic formations, and may also possibly be reptilian.

In deposits of Lower and Upper Jurassic age we find more
convincing evidence of the existence of true mammals. The
celebrated Stonesfield slate of Oxfordshire (Lower Jurassic) has

302 OUTLINES OF EVOLUTIONARY BIOLOGY

yielded a fragment of a jaw, containing three small multituber-
culate grinding teeth, to which the name Stereognathus has been
given, and one of the Middle Purbeck beds of Swanage (Upper
Jurassic) has contributed the well known jaws of Plagiaulax
(Fig. 148). Remains of closely related forms, supposed to have
belonged to the same family (Plagiaulacidae), have been discovered
in Upper Jurassic and Upper Cretaceous formations of North
America, and the family appears to have survived, both in Europe
and North America, into early Tertiary times.

The Plagiaulacidae and related forms have been grouped together
in the extinct order Multituberculata, which Dr. Smith Wood-
ward places provisionally amongst the Prototheria, side by side
with the surviving order Monotremata, no remains of which are
p known to occur before Tertiary

times. It is possible, however,
that the Multituberculata may be
metatherian rather than proto-
therian.

The remains of undoubted Meta-

theria (Marsupialia) first occur, so
FIG. 148.— Mandible of Plagiaulax

minor, x 4. (From Smith far as we yet know, in the same
Woodward's " Vertebrate beds as the earliest Multituber-
Paleeontology," after Fal- cukta (leayi Qut of account

COUGl • )

the enigmatical Tritylodon and

Microlestes). Mandibles of Phascolotherium (Fig. 149) and
Amphitherium have been found in the Stonesfield slate, while
Triconodon and Spalacotherium are similarly represented in the
mammal bed at Durdlestone Bay near Swanage.

Throughout the whole of the Secondary period the mammals
remained of insignificant size, and in a more or less primitive
condition, such as is represented at the present day by the
surviving monotremes and marsupials. The typical placental
mammals (Eutberia) are not known to us from formations of
earlier date than the Eocene. Then, all at once, they seem to have
branched out in every direction and taken possession of land, sea
and air just as the reptiles had done before them ; whales replacing
the Ichthyosauria and Plesiosauria, various groups of land
mammals replacing the Theromorpha and Dinosauria, and bats
sharing with the birds the kingdom of the air which had formerly
belonged to the pterodactyls.

As in the case of their amphibian and reptilian predecessors,

THE GEOLOGICAL RECORD

303

many of the mammalian groups in Tertiary times have run to
great size ; most of the larger forms, such as the primitive
ungulate, Tinoceras (Fig. 150), of the American Eocene, and the
giant ground sloth, Megatherium, of the American Pleistocene,
are already extinct, but it must not be forgotten that the existing
whales are amongst the largest animals that have ever lived, and
in bulk at any rate will bear comparison with the largest of
the great extinct reptiles.

The last term in the evolutionary series of the Mammalia is
man, whose advent, so far as we at present know, dates back only

FIG. 149. — Mandible of Phascolatherium lui-klandi ; x 3. (Prom Smith
Woodward's "Vertebrate Palaeontology," after Goodrich.)

to about the commencement of the Pleistocene or end of the
Pliocene epoch.1

There is one more point that is well worth emphasizing about the
evolution of the Vertebrata, as indicated not only by the geological
record but also by the facts of comparative anatomy. Each
successive great group appears to have arisen, not from the
most highly specialized members of some preceding great
group, but from comparatively undifferentiated forms. Thus the
Amphibia arose, not from bony fishes, but from primitive dipnoids
or ganoids ; the reptiles arose, not from frogs or toads, but from
primitive stegocephalian amphibia ; the birds arose, not from
pterodactyls, but from comparatively unspecialized reptiles ; the
mammals also arose from the more primitive reptilian forms, and
man himself, whose advent undoubtedly marks the commencement
of a fresh line of evolution, belongs to the order Primates, which
in respect of bodily organization, as seen, for example, in the

1 Vide Chapter XXVII.

304 OUTLINES OF EVOLUTIONARY BIOLOGY

typical pentadactyl limbs, is far more primitive than many other
mammalian groups.

Each great group seems to have begun in a small way, then
developed rapidly, branching out in many directions and becoming
the dominant group for the time being, only to dwindle away
again and give place to some new and vigorous off-shoot. The
dominance of any particular group has often been accompanied
by the attainment of enormous size by its individual members,1

FIG. 150. — Skeleton of Tinoceras ingens, from the Middle Eocene of Wyoming,
X BV (From British Museum Guide, after O. C. Marsh.)

and it is not impossible that this may have had something to do
with its subsequent decline or complete extinction.

So far as the animal kingdom is concerned, we may perhaps
say without exaggeration that the succession in time of the
different groups, as indicated by the geological record, amounts
to positive demonstration of the truth of the theory of organic
evolution. In the case of plants the record is perhaps not quite
so clear, but here also there can be no reasonable doubt that the
great groups succeeded one another in a manner consistent with
the theory, commencing with the algae and ending with the
flowering plants of the present day.

1 A possible explanation of this fact is suggested in Chapter XXVI.

CHAPTER XX

Fossil pedigrees — Ancestry of birds, horses, elephants and whales.

IN our last chapter we gave a brief outline of the general course
of evolution amongst vertebrate animals as indicated by the
geological record. We may now study in somewhat greater
detail certain branches of the great phylogenetic tree which are
especially well represented by fossil remains and therefore
particularly instructive from the point of view of the evolution
theory.

One of the most highly specialized groups of vertebrates that
have ever existed is that of the birds. We have already pointed
out that, on anatomical grounds, birds are classed together with
reptiles as Sauropsida. They agree with reptiles in their method
of reproduction by means of large, heavily yolked eggs, and in
the presence, in the embryo, of the characteristic foetal membranes,
amnion and allantois, as well as in certain morphological charac-
ters of the adult. They differ from reptiles, however, in many
striking features. Thus they possess feathers, which almost (but
not quite) completely replace the reptilian scales as a protective
exo-skeleton. The anterior limbs are modified to form wings,
constructed, as we have already seen in Chapter XVII, on an
entirely different plan from those of flying reptiles. The digits of
the hand are very greatly reduced ; only one of them, and that in
a vestigial condition, projects freely from the anterior border of the
wing, forming the so-called " ala spuria " (Fig. 99). The true tail
is greatly abbreviated and the caudal vertebrae reduced in number
and to a large extent fused together to form the " ploughshare
bone " which supports the tail feathers. Lastly, all existing birds
have completely lost their teeth, which are functionally replaced
by the horny beak.

The remains of the earliest known bird, Archaeopteryx (Fig. 151),
have been found in the celebrated lithographic stone of Solenhofen
in Bavaria, of Upper Jurassic age, which, owing to its extremely
fine grain, is peculiarly well suited for the preservation of even

B. X

306 OUTLINES OF EVOLUTIONAEY BIOLOGY

such delicate structures as feathers. This animal was about the
size of a rook and the presence of well developed feathers and
wings of the avian type is alone sufficient to show that we are
dealing with a true bird. It still exhibits, however, a number of
features which are usually met with in reptiles but have dis-
appeared in modern birds. The digits of the anterior limb are
not nearly so much reduced as in the latter, for three claw-bearing

FIG. 151. — Fossil Remains of Arcliceopteryx siemensi, showing the three
fingers in each wing, the long tail, feathers, &c. (From Lankester's
" Extinct Animals.")

fingers project from the anterior margin of the wing ; the tail is
elongated like that of a lizard and supported by about twenty
separate vertebrae each carrying a pair of feathers ; and numerous
teeth are present in the beak.

It is obvious that Archaeopteryx represents a stage in the
derivation of birds from reptilian ancestors, and this is exactly
what we should expect of the earliest birds in accordance with
the theory of evolution. Unfortunately, with the exception of a
few other toothed birds of Cretaceous date, Archaeopteryx is

EVOLUTION OF THE HORSE

307

almost the only link in the pedigree of birds which has so far
been discovered, and it teaches us nothing as to the origin of
those characteristic avian structures, the feathers, which it
possesses already in a fully developed condition.

It will be observed that Archaeopteryx occupies a position
between reptiles and typical birds exactly comparable with that
of the Monotremata between reptiles and typical mammals (see
Chapter XVII), the only difference being that the Monotremata
still survive side by side with mammals of the most highly
advanced type, while Archseopteryx has long since become extinct.

One of the most complete fossil pedigrees as yet known to us

FIG. 152. — Skeleton of Phenacodus, a five-toed Eocene Ungulate. (From
Lankester's "Extinct Animals.")

is that of the Equidae or horse family. As we have already seen
in Chapter XVII, the study of comparative anatomy indicates very
clearly that the highly specialized single-toed limbs of the horse
(Fig. 97) must have arisen from some primitive pentadactyl
type by gradual suppression of all the digits except the middle
one. Amongst the fossil remains of horse-like animals which
abound in various tertiary formations of Europe and America we
find a very complete series of stages in the evolution of the
modern horse, which entirely confirms this conclusion. Our
knowledge of this extremely interesting phylogenetic series is due
largely to the late Professor 0. C. Marsh and has been admirably
summarized by Mr. E. S. Lull in the American Journal of Science.1

1 Series IV, Vol. XXIII, 1907.

X 2

308 OUTLINES OF EVOLUTIONARY BIOLOGY

FIG. 153. — Outlines of Horses of different Geological Periods, showing their
Eelative Sizes. (From Lull.)

a, Protorohippus (Eocene) ; b, Orohippus (Eocene) ; c, Mesohippus (Oligocene) ; d, Meryc-
hippus (Miocene) ; e, Pliohippus (Pliocene) ; /, Equus (Recent).

EVOLUTION OF THE HORSE 309

It is generally admitted that the Equidse originated from the
Condylarthra, a group of primitive, five-toed, ungulate mammals
which made its appearance in early Eocene times, and the best-
known representative of which is Phenacodus (Fig. 152). The
evolution of the horses appears to have taken place chiefly in
America, though occasionally representatives of the group seem to
have migrated to or from Europe, doubtless by a former land
connection in the neighbourhood of Behring Strait. In com-
paratively recent times, however, the family became confined to
the old world and was only re-introduced to America by human
agency.

The course of their evolution has evidently been determined by
the development of extensive, dry, grass-covered, open plains
on the American continent. In adaptation to life on such areas
structural modification has proceeded chiefly in two directions.
The limbs have become greatly elongated and the foot uplifted
from the ground, and thus adapted for rapid flight from pursuing
enemies, while the middle digit has become more and more
important and the others, together with the ulna and the fibula,
have gradually disappeared or become reduced to mere vestiges.
At the same time the grazing mechanism has been gradually
perfected. The neck and head have become elongated so
that the animal is able to reach the ground without bending its
legs, and the cheek teeth have acquired complex grinding surfaces
and have greatly increased in length to compensate for the
increased rate of wear. As in so many other groups, the evolution
of these special characters has been accompanied by gradual
increase in size (Fig. 153). Thus Eohippus, of lower Eocene
times, appears to have been not more than 11 inches high at the
shoulder, while existing horses measure about 64 inches, and the
numerous intermediate genera for the most part show a regular
progress in this respect.

All these changes have taken place very gradually, and a
beautiful series of intermediate forms indicating the different
stages from Eohippus to the modern horse (Equus) have been
discovered. The sequence of these stages in geological time
exactly fits in with the theory that each one has been derived
from the one next below it by more perfect adaptation to the
conditions of life. Numerous genera have been described, but it
is not necessary to mention more than a few.

The first indisputably horse-like animal appears to have been

310 OUTLINES OF EVOLUTIONARY BIOLOGY

PQ-"

Hyracotherium, remains of which have been found in the
London Clay (Lower Eocene). Another Lower Eocene genus
was Eohippus, which seems to have arisen in Western Europe,
possibly from a hyracotherian ancestry, and migrated, by way
of Northern Asia, to America, where its remains occur in rocks
of the same age. In this animal (Fig. 154) the fore foot had
four well developed digits and the thumb was represented by a
splint bone ; in the hind foot the great toe had entirely dis-
appeared and the fifth digit was represented only by a splint

bone. In both fore and hind feet
the third or middle digit was already
conspicuously larger than any of the
others.

Eohippus was succeeded by Pro-
torohippus (Fig. 153, a), which was
some 3 inches higher and had lost
the vestigial thumb. Then came
Orohippus (Fig. 153, b), again a little
larger and with closely similar feet
(Fig. 155), but with a considerable
advance in the evolution of the
grinding teeth. The last of the
Eocene horses was Epihippus, still
with four toes in front and three
behind, but with the lateral toes
further reduced in size and another
distinct advance in tooth structure.
In Oligocene times there occurred
in North America Mesohippus and
Miohippus, and in Europe Anchi-
therium. Mesohippus (Fig. 153, c) was 18 inches or more in
height, with three digits and a vestige of the fifth in the fore
foot and three digits only in the hind foot (Fig. 156). Miohippus
attained a height of 24 inches and closely resembled Mesohippus
in the structure of its feet. Anchitherium is supposed to be
a European derivative of Miohippus.

In the Miocene period the horses appear to have attained their
maximum of development as a group, and a number of extinct
American genera are distinguishable. Merychippus (Fig. 153, </),
Protohippus and Neohippar ion were still three-toed horses, though
the lateral digits were now greatly reduced (Fig. 157). Pliohippus

FIG. 154. — a, Fore Foot and
l>, Hind Foot of Eohippus
pernix, X i- (From Lull,
after Marsh.)

EVOLUTION OF THE HORSE

311

(Fig. 153, 6-), which continued on into Pliocene times and attained
a height of 48 inches, had the second and fourth digits of each
foot represented by mere splint bones as in modern horses,
and had therefore already attained to the single-toed condition
(Fig. 158).

In Pliocene times, however, we still find a three- toed horse—
Hipparion — surviving in Europe, but the modern one-toed genus
Equus (Fig. 153, /) also makes
its appearance both in the old and
new worlds, becoming extinct in
the new world in Post-Pleistocene
times until re-introduced from
Europe by the agency of man.

The time occupied in the evolu-
tion of the genus Equus from
its remote ancestor Eohippus is
estimated by Professor Sollas at
five or six millions of years. This
period is sufficient to allow of a
very slow and gradual change
from one condition to the other.
Allowing five years for each
generation, Sollas arrives at the
conclusion that somewhere about
a million generations intervene
between the two extremes. The
total increase in height during
this time has been 53 inches, and
if this increase were spread fairly
uniformly over the whole period
it would only mean about 0*00005 inch for each successive
generation — an amount which would be quite imperceptible to
human observers.

In reconstructing such a pedigree as that of the horse from
palaeontological evidence it is of course necessary to bear in
mind that the great majority of extinct forms which come to
light will almost certainly not be actually in the direct line of
descent. Collateral branches will have been given off from the
phylogenetic tree in various directions, and it is much more likely
that any particular form discovered will belong to one of these
branches than that it will belong to the main stem. This fact,

IV

FIG. 155.— -a, Fore Foot and b, Hind
Foot of Orohippm agifis, X ^.
(From Lull, after Marsh.)

312 OUTLINES OF EVOLUTIONAEY BIOLOGY

R

however, by no means vitiates the general argument, for it is usually
possible to pick out pretty accurately those which come into or
near to the direct line, and even the collaterals afford valuable
evidence as to the general course of evolution. We may safely
say that the palaeontological evidence amounts to a clear

demonstration of the evolution
of the horse from a five-toed
ancestor along the lines indi-
cated above.

The ancestry of the elephants
is less well known than that
of the horses, but recent dis-
coveries in the Egyptian Ter-
tiary formations, which we owe
especially to the investigations
of Dr. Andrews, have done
much to elucidate the history
of this remarkable group of
mammals, and there can now
be no doubt as to the main
line of evolution which has led
up to the existing Proboscidea.
In some respects the elephants
have remained in a some-
what primitive condition, as
is indicated very clearly by the
fact that all the digits remain
well developed in both fore and
hind feet. It is in the struc-
ture of the head that they

IV

FIG. 156. — a, Fore Foot and b, Hind
Foot of Mesohippvs celer, x ^.
(From Lull, after Marsh.)

exhibit a high degree of special-
ization, marked particularly by
the elongation of the snout to
form a long prehensile trunk, by the enormous development of
the occipital region of the skull, by the enlargement of the
incisor teeth to form great tusks, by the shortening of the jaws
and by the increase in size and complexity and the reduction in
number of the cheek teeth. These changes have been accom-
panied by a huge increase in the size of the entire body, so that
most of the elephants are amongst the largest of known land
mammals, whether fossil or recent.

EVOLUTION OF ELEPHANTS

813

CL.

Like the horses, the elephants prohably originated from that
primitive ungulate group, the Condylarthra. The earliest known
form exhibiting proboscidean characters is Moeritherium, a
tapir-like animal whose remains have been found in the Middle
and Upper Eocene deposits of
the Egyptian Fayum. This was
a comparatively small creature,
about as large as a Newfoundland
dog. It probably differed but little
from other primitive ungulates,
but the skull (Fig. 159, 1) already
shows marked proboscidean ten-
dencies. The position of the nasal
bones, away back from the tip of
the snout, indicates that there
was in all likelihood a short pro-
boscis. The occipital region of
the skull is beginning to grow up
and air cells are beginning to
develop in the bones. The second
pair of incisor teeth in each jaw
are enlarged to form small tusks
and the hinder cheek teeth are
beginning to show an increase in
complexity of structure. The total
number of teeth however (86) is
only eight short of the full typical
mammalian dentition.

The next stage is represented
by Palaeomastodon (Fig. 159, 2)
from the Upper Eocene of the same
region, some species of which were
little larger than Mreritherium
while others attained almost

elephantine proportions. In this genus we notice a strong
accentuation ef the proboscidean characters. The occiput is higher,
the nasal opening in the skull further back, the upper tusks
better developed, the cheek teeth more complex ; while the canines
and all the incisors except the tusks in both jaws have dis-
appeared. It will be observed that as yet there is no shortening
of the jaws, but, on the contrary, the lower jaw has become

FIG. 151.— a, Fore
Hind Foot of
whitneyi, X %.

Foot and b,
Neoli ipparion
(From Lull.)

314 OUTLINES OF EVOLUTIONAEY BIOLOGY

considerably elongated, apparently serving as a support for the
lengthening proboscis.

In Tetrabelodon angustidens, from European Miocene formations,
this elongation of the mandible is much more marked, so that
the lower jaw is much longer than the upper one and the short
lower tusk comes to project almost as
far forward as the long upper one (Fig.
159, 3). From this time onwards, how-
ever, the chin shortens, thereby allowing
greater flexibility to the proboscis, so that
in the lower Pliocene we find Tetrabelodon
longirostris (Fig. 159, 4) with the lower
jaw only a little longer than the upper,
leading the way to the mastodons and
true elephants (Elephas), which also
appeared in Pliocene times and in which
the tusks have entirely disappeared from
the greatly abbreviated mandibles while
the cheek teeth have become enormously
enlarged and complicated (Fig. 159, 5).

We have here a wonderfully perfect
series of connecting links between the
most primitive known ungulate mammals
and the elephants. Only forms which
appear to lie in or near the direct line of
descent have been mentioned in the above
brief account. Other modifications of
the proboscidean type arose as lateral
offshoots from this main stem. One of
the most remarkable of these is Dino-
therium, with its great, downwardly
directed lower tusks (Fig. 160), which
appeared in Europe in the Pliocene period.

In the case of the Cetacea, a group which includes the whales,
porpoises and dolphins, we have as yet only a much more frag-
mentary pedigree, but still quite sufficient to justify, on the
palaeontological side, the conclusion, already arrived at on ana-
tomical grounds, that these extremely aberrant forms are the
descendants of typical terrestrial mammals which have become
re-adapted to an aquatic life and in accordance therewith have
re-acquired a superficial resemblance to their much more remote

FIG. 158.— a, Fore Foot
and &, Hind Foot of
Pliohipjnispernix, X \.
(From Lull.)

EVOLUTION OF ELEPHANTS

315

Recent
Pleistocene
Ufifier Pliocene

ELEPHAS
(short chin)

Lower Pliocene TETRABELODON

[LONGIROSTRIS STAGE]
Ufiner Miocene (shortening chin)

Middle Miocene TETRABELODON

[ANGUSTIDENS STAGE]
Lower Miocene (long chin]

~,- Mi oration from Africa

I/finer Oh gocene . f _ A J

' ' into Lurone -Asia

Lower Oligocene?\
UnnerEocene {

Middle Eocene

Lower Eocene

PAL AEO MASTODON
chin)

MOERITHERIUM
(short chin)

FIG. 159. — Some Stages in the Evolution of the Skull in the Proboscidea
(From British Museum Guide.)

316 OUTLINES OF EVOLUTIONARY BIOLOGY

fish-like ancestors.1 The fore limbs have become converted into
paddles while the hind limbs have entirely disappeared externally

FIG. 160. — Skull of Dinotherium giganteum, Lower Pliocene, X T^. (From
Smith Woodward's " Vertebrate Palaeontology," after Kaup.)

(Fig. 161). The tail has become flattened out into a horizontal
fin and there is frequently a well developed dorsal fin. The skull

FIG. 161. — The Dolphin, Delpliinus delphis, X

Guide.)

(From British Museum

has undergone very curious changes. The brain case is rounded
and strongly arched (Fig. 162) and the nasal apertures or blow-
holes lie far back, at or near the highest point of the head

1 Compare Chapter XVII.

EVOLUTION OF WHALES

317

(Fig. 101, //), while the jaws have become greatly elongated. The
teeth have in some cases completely disappeared, as in the whale-

FIG. 162.— Skull of the Dolphin, x J. (From British Museum Guide.)

bone whales (except for foetal vestiges), while in others they are
present in large numbers but have lost the typical mammalian

FIG. 163. — Dorsal and lateral views of the Skull of a primitive Whale,
Prozeuglodon atrox, X rV (From British Museum Guide.)

differentiation into incisors, canines, premolars and molars, being
represented by a continuous and uniform series, all of which are
conical in shape and single-rooted. Such teeth occur in both

318 OUTLINES OF EVOLUTIONARY BIOLOGY

jaws of the porpoises and dolphins (Fig. 162) and in the lower
jaw only of the sperm whale.

In the extinct shark-toothed Dolphins (Squalodontidae), whose
remains have been found in Miocene formations of Europe and
America, the teeth are still differentiated into incisors, canines,
prernolars and molars, and the molars have double roots and
compressed crowns with serrated edges.

Further back, in Eocene times, there existed, widely distributed
over the northern hemisphere, a group of still more primitive
whale-like animals known as Zeuglodontidse. In these the seven
vertebrae of the neck, which in existing whales are more or
less fused together into a solid mass (Fig. 103), are all separate,
and the typical dental formula is identical with that of primitive

,..31 4 3—2

carnivorous land mammals, viz. i 0, c. - r, p.m. -., m. — » — .

o 1 4 o

The genus Prozeuglodon, from the Egyptian Eocene, ap-
proaches so closely in the characters of the skull and teeth
(Fig. 163) to the primitive carnivores (Creodontia)of about the same
period as to leave no reasonable doubt about the derivation of the
Cetacea from that group, although it is quite possible that none
of the extinct forms so far discovered are actually in the direct
line of descent of any of the modern whales.

CHAPTEK XXI

Geographical distribution1 — Areas of distribution — Barriers to migration
— Means of dispersal — Changes in the physical conditions of the earth's
surface — The evidence afforded by the study of geographical distribution
with regard to the theory of organic evolution.

IT is hardly necessary to remind the reader that each species
of plant or animal, in a state of nature, is more or less sharply
restricted to a certain portion of the earth's surface, the entire
region over which it may be found, whether sea or land, being
termed its area of distribution. Such areas of specific distribu-
tion are nearly always continuous, without any considerable gaps
or intervals from which the species is entirely absent. This does
not, of course, mean that the species necessarily occurs in all
parts of its area of distribution at once, but that it is free to
range over the whole of it and may accordingly be found in any
suitable part of it at any time. It is necessary to introduce some
such qualifying word as " suitable " in this connection, because
each species is not only restricted in its range to a more or less
well-defined geographical area, but can only live continuously in
certain portions of that area, to the special conditions of which it
is structurally and physiologically adapted and which constitute
its habitat. Thus, for example, a fresh-water snail may perhaps
range over an entire continent, but it would be useless to look for
it except in fresh water. Individuals of a species may pass with
more or less freedom, according to the nature of the case, from
one habitat to another within the area of distribution, but it is
only on rare and exceptional occasions that they are able to
transgress the boundaries of the area itself.

True discontinuity in areas of specific distribution, as distin-
guished from mere discontinuity of habitats, is extremely rare.
We have a good example of it, however, in the case of the marsh

1 The reader is referred to Dr. Wallace's classical volumes on " Island Life " and
the " Geographical Distribution of Animals," and to Professor Heilprin's work on
the " Distribution of Animals" in the International Scientific Scries (Vol. LVIII,
1887), for further information on this subject.

320 OUTLINES OF EVOLUTIONARY BIOLOGY

tit (Parus palustris), which has two areas of distribution separated
from one another by an interval of four thousand miles — in
Europe and Asia Minor on the one hand and in Northern China
on the other.

The size of the area over which a species may range varies
immensely, in some cases comprising an entire continent, or even
more, and in others only a few square miles. Thus the leopard
ranges over the whole of Africa and most of Southern Asia, while
the Tuatara (Fig. 113) is confined to certain small islands off the
coast of New Zealand, and certain species of humming birds
are said to occur only on the volcanic peak of Chimborazo in the
equatorial Andes.

An area of generic distribution is the sum of the areas of
distribution of all the species which are comprised within the
genus, and thus genera have usually a much wider geographical
range than species. Families, again, have a wider range than
genera and orders than families, and so on with groups of still
higher value. In short, the more comprehensive the group the
larger will be its area of distribution, until we find that the sub-
kingdoms or phyla are cosmopolitan, ranging more or less over
the entire world, wherever a suitable habitat is to be found.

The reason why species are rarely, if ever, cosmopolitan in
their distribution is that they are confined within their own
limited areas by the existence of physical conditions which con-
stitute what are called barriers to migration. Such barriers
either form absolutely insuperable obstacles to the passage of the
species in question or they may be surmounted only at rare
intervals and by some happy chance.

The nature of the barriers varies, of course, with tbe species
concerned, and wbat is a barrier to one may be a high road
to another. In the case of marine animals the principal barriers
are continents and temperature conditions, while the deep sea
itself acts as a barrier to the distribution of shore-dwelling or
littoral forms. For terrestrial animals the chief barriers are
seas, rivers, mountain ranges, deserts and climate generally ; for
fresh-water animals land and sea. In the case of plants the
barriers to migration are very much the same.

It may be laid down as a general law that every organism,
whether animal or vegetable, at some period or other of its
existence is specially adapted so as to secure dispersal either by
its own exertions or by the action of some external agency. By

DISPERSAL OF PLANTS 321

some means or another it is able, not only to spread itself over
its own area of distribution, but also., when occasion offers, to
extend that area by surmounting its barriers.

The lower terrestrial plants, such as fungi, mosses and ferns,
are dispersed by means of spores, which, protected by special
envelopes, may be widely distributed by the wind. In the
higher plants the spores, as agents of dispersal, are replaced by
seeds, wbich, usually still within the fruit, maybe carried on the
wind, floated on rivers or ocean currents, carried about entangled
in the hair or feathers of animals, or actually eaten and passed
out uninjured in the freces. A great many seeds and fruits are
specially modified in structure to secure their distribution in one
or other of these ways, and the study of such adaptations consti-
tutes one of the most interesting chapters in botanical science.
We need only refer here to such fruits as the blackberry, whose
succulence tempts the birds to eat them and carry the seeds,
safely enclosed in their hard protective envelopes, to long
distances ; the various kinds of burs with their hooks for entangle-
ment in fur and feathers ; the winged fruits of the maple, elm
and ash, and the thistledown of the tbistles, adapted for floating
on the wind.

The dispersal of plants is in all cases passive and dependent
on external agencies, though sometimes aided by some purely
mechanical device in the plant itself ; in the case of animals it
may take place either passively or actively, by the exertions of
the animals themselves.

Beginning with the marine fauna, we find that the larger
forms — whales, porpoises, dolphins and fishes — owe their dis-
persal mainly to their own active powers of locomotion, while
the smaller animals, especially the invertebrates, are • largely
dependent in this respect upon oceanic currents.

Even animals which, like the sponges and corals, are firmly
fixed to the sea-bottom in the adult condition, have free-swim-
ming larval forms (Fig. 164) whose own limited powers of
locomotion may, under favourable circumstances, be enormously
supplemented by the action of currents. Such larval forms,
from the point of view of dispersal, play the part of the spores and
seeds of plants, and they occur not only in cases where the adult
is strictly sedentary in habit but also where its powers of loco-
motion are limited, as in many worms, snails, crabs (Fig. 128),
star-fishes, brittle stars (Fig. 127) and sea-urchins. The

B. Y

322 OUTLINES OF EVOLUTIONARY BIOLOGY

dispersal even of large and active fish, like the mackerel, is largely
assisted by the action of currents upon the floating eggs, and
this factor must be of still greater importance in the case of
the comparatively sluggish bottom dwellers, such as the turbot
and sole.

The more superficial waters of the open ocean are densely
populated with pelagic animals and plants and with pelagic eggs
and larvae in various stages of development, all drifting more or
less helplessly wherever the ocean currents may carry them, for
their own powers of locomotion are usually quite insufficient to

enable them to pursue an inde-
pendent course. This floating
population is technically spoken
of as " plankton " and its in-
vestigation, which is of great
importance for the solution of
practical fishery problems, has
lately attracted a great deal of
attention.

We must therefore regard all
the great ocean currents, such
as the Gulf Stream, as highways
thronged with life of many kinds,
including representatives of all
the more important groups of
marine animals, any one of
which may be on its way to
found a new colony and establish its own particular species in some
region far distant from its original home. Some of the wanderers
are only immature forms, belonging partly to shore-dwelling
species, others are adult. Almost all exhibit some special adap-
tation to their pelagic mode of life. The fish-eggs are floated
by oil-globules, and the larvae of the crabs, brittle stars and
sea-urchins are provided with defensive spines (Figs. 127, 128) ;
but the most general and characteristic feature of all the pelagic
host is transparency, whereby they are rendered inconspicuous
and less likely to become the victims of the numerous enemies
which feed upon the plankton. Adult jelly-fish, worms, molluscs
and crustaceans, and innumerable larval forms, all exhibit this
same peculiarity.

The effect of ocean currents upon the distribution of marine

FIG. 164. — Free-swimming Larva of
a Sponge, Orantia compressa ;
highly magnified.

(The larva swims by means of the rapid
undulations of the numerous flagella
with which it is provided.)

DISPERSAL OF ANIMALS 323

animals is shown in a very interesting manner in the case of the
Mediterranean and the Red Sea. In each of these there is a
surface current constantly flowing in from the open ocean and
bringing in vast numbers of individuals, both larval and adult,
which never find their way out again. Hence these almost
enclosed seas form a kind of trap for marine animals and we
accordingly find them to be inhabited by an exceptionally rich
and varied fauna.

The range of marine species, though sometimes very wide, is
usually more or less strictly limited, so that the shores of every
continent have their own characteristic fauna and flora. This is
no doubt partly accounted for by differences in climatic conditions,
food supply and so forth, but it is mainly due to the fact that,
in spite of the facilities for travel afforded by ocean currents,
the dangers incidental to a long voyage from one continent to
another are rarely surmounted, at any rate by shore-dwelling
organisms. We know that many such forms flourish quite as well
in some other part of the world as in their original home if they
can once overcome the initial difficulty of migration. Thus
the artificial introduction of the American oyster into British
seas has accidentally brought with it the introduction of the
remarkable limpet-like Crepidula, which attaches itself to the
oyster shells and runs riot over the oyster beds on the Essex
coast. That such occurrences may occasionally take place in a
state of nature, and a species thereby be enabled to extend its
area of distribution, there can be no reasonable doubt, for even
American turtles have occasionally been carried by the Gulf
Stream to the shores of Great Britain.

Amongst the higher forms of non-aquatic animals, and
especially birds and mammals, their own powers of locomotion
constitute the most important means of dispersal. Even these,
however, are frequently transported for long distances by those
external agencies which are chiefly responsible for the dispersal
of less highly organized forms.

Leaving out of account, for the moment, the action of man,
which has brought about immense changes in the geographical
distribution of the existing fauna, the chief agents to be noticed
in this connection are wind and water.

It is to the action of the wind that large numbers of winged
animals — insects, birds and bats — owe the wide distribution which
they enjoy. All actively flying land animals are liable to be

Y 2

324 OUTLINES OF E VOLUTION AEY BIOLOGY

carried out to sea in storms, and although the great majority of
these will inevitably perish a few will occasionally manage to
reach some distant haven where they may succeed in establish-
ing a colony and thus extending the range of the species.

During westerly winds American birds not infrequently make
their appearance on various parts of the coast of Europe, while
north of the 58th parallel of latitude the polar winds trend in
the opposite direction and with them we find a transference of
European birds, by way of Iceland and Greenland, to the
American continent.1 During storms, again, European birds are
cast upon the Azores, about 1,000 miles from the nearest
continental coast, and there is strong reason for believing that
the little wax-eye (Zosterops lateralis) has been transported in
this way from Australia to New Zealand, where it has succeeded
in establishing itself.

Water currents may play an important part in the dispersal
of two groups of terrestrial animals — those which occasionally
swim and those which are liable to be carried away on icebergs
or on floating masses of vegetation. Most quadrupeds swim well
and even if not habitual swimmers may be forced to take to the
water in times of flood. In this way they may cross large
rivers and even get carried out to sea and perhaps to some
neighbouring island, but they cannot cross large stretches
of open ocean, and are accordingly never found, except
when introduced by man, on islands far remote from any
continent.

In polar regions the floating ice affords a means of dispersal
to such animals as wolves and polar bears, while within the
tropics floating islands or rafts formed of matted vegetation play
the same part. Such islands have been observed floating out to
sea from the mouths of large rivers like the Ganges, the Amazon,
the Congo and the Orinoco. They serve as a means of transport
to many different kinds of terrestrial reptiles, birds and mammals,
and countless molluscs, worms and insects, to say nothing of
plants.

"If," says Sir Charles Lyell, "the surface of the deep be
calm, and the rafts are carried along by a current, or wafted by
some slight breath of air fanning the foliage of the green trees,
it may arrive, after a passage of several weeks, at the bay of an
island into which its plants and animals may be poured out as

1 Heilprin, op. cit., p. 47.

DISPERSAL OF ANIMALS 325

from an ark, and thus a colony of several hundred new species
may at once be naturalized." *

As a definite example of this kind of dispersal may be men-
tioned the fact that in 1827 a large boa constrictor, twisted round
the trunk of a tree, was carried by ocean currents from South
America to the Island of St. Vincent, where it was destroyed
after killing a few sheep.

A current flows from the North Island of New Zealand
southwards to Chatham Island, four hundred miles distant
from the nearest point on the New Zealand coast. This current
carries considerable quantities of New Zealand timber to the
island and its existence probably accounts for the fact that the
planarian worm, Gcoplana exulans, has been found both in the
North Island of New Zealand and on Chatham Island, but, as
yet, nowhere else. The land planarians habitually creep into
the crevices of decayed timber, and their eggs are enclosed in
tough, horny cocoons which may probably occasionally be
transported even over wide stretches of sea.

Small terrestrial animals are, of course, often accidentally
dispersed by human agency. Rats, mice and cockroaches have
been carried nearly all over the world by ships, and snails,
worms and other small creatures may be carried about with
timber and earth, especially around the roots of plants. When
I was in New Zealand I had some plants sent to me from
England in a tightly closed tin box. When they arrived, after
a voyage of some five or six weeks, I found an earthworm still
alive in the tin. Many invertebrates have doubtless been
unknowingly dispersed in this manner and great care has to be
taken to make due allowance for such possibilities in studying
problems of distribution. In exactly the same sort of way the
seeds of many plants are accidentally dispersed over the world
in ships' ballast, so that the same common European weeds
occur in the neighbourhood of the ports along all the great
routes of commerce.

The restrictions placed upon the dispersal of fresh water
animals are more severe than in the case of either marine or
terrestrial forms. One river system or one lake is separated
from another by intervening land or sea which fresh water
animals cannot as a rule cross by their own exertions. There
are of course exceptions, as in the case of the lampreys and eels,

1 Lyell's " Principles of Geology," Ed. 5, Vol. III., p. 44.

326 OUTLINES OF EVOLUTIONARY BIOLOGY

and other fish which go down to the sea periodically, but for the
most part the inhabitants of fresh water are largely dependent
upon external agencies for their dispersal. Accordingly we find
two groups of such animals, widely contrasted with one another
as regards their distribution. Those which do not go down to
the sea and which are not likely to be carried about by external
agencies, such as most of the fishes, have usually restricted areas
of specific distribution, and individual mountain lakes sometimes
contain peculiar species of fish which are found nowhere else in

FIG. 165.

FIG. 166.

FIG. 166. — Gemmule or Statoblast of a Fresh Water Polyzoon, Cristateila

mucedo, X 40. (From Sollas.)

FIG. 166.— Two Gemmules of a Fresh Water Sponge, Ephydatia (Spongilla)-
fluviatilis, X 60. (From a photograph. )

gem., gemmules ; sp., spicules of the parent sponge.

the world. Galaxias nigothoruk, for example, is a small fish
which occurs abundantly in lake Nigothoruk in Victoria
(Australia). This lake is in a very isolated position in a
mountainous region and the only outlet is by percolation under-
ground. There appears to be no natural means by which the
fish could be transferred to any other locality at the present
time, and it is not known to occur elsewhere.

On the other hand many fresh water invertebrates, such as
the Polyzoa, hydras and sponges, and above all the microscopic
Protozoa, are remarkable for their wide distribution. Identical
genera if not identical species of tliese groups occur almost all
over the world, and the reason for this is not far to seek, for all

CLIMATIC CHANGES 327

of them have some special character which enables them to be
easily dispersed by external agencies. The fresh water Polyzoa
and sponges produce minute buds (statoblasts or gemmules)
enclosed in hard protective envelopes (Figs. 165, 166), which are
likely to be carried about in the mud on the feet of wading birds
and mammals. The embryo of Hydra secretes its own protective
envelope (Fig. 59, D — G) within which it passes through a period
of rest embedded in the mud ; while many of the Protista (e.g.
Haematococcus) are capable of being dried up at some period or
other of their life-history and carried about by the wind in the
form of dust. Thus a sample of mud, taken from a pond and
dried up, may, after an interval of many months, if again placed
in water, give rise to an abundant fauna, amongst which even
such highly organized forms as Crustacea (e.g. Apus and
Branchipus, which lay specially protected eggs) will frequently
appear.

We must remember that the present distribution of animals
and plants is the outcome not only of the existing physical
conditions of the earth's surface but also of conditions which
obtained in past geological periods. From time to time these
conditions undergo great changes, which may concern not only
the climate of particular regions or of the entire world, but also
the relative distribution of land and sea.

The earth has been subject, at various periods of its history,
to climatic changes of two chief kinds, (1) cold or even glacial
epochs in temperate regions, and (2) mild or warm epochs in
arctic or antarctic regions. Probably alternations of these two
extremes have been not infrequent, but the case of which we
have most complete knowledge occurred in the Pleistocene period
and is usually known as the " Glacial Epoch " par excellence.

There is clear evidence that during a portion of the Pleistocene
period a very large part of the northern hemisphere, which now
enjoys a temperate climate, was covered with perpetual ice and
snow and reduced to a condition resembling that of Greenland
at the present time. Scandinavia and the whole of Northern
Europe were buried beneath the ice-sheet, and the same is truo
of the northern part of North America.

The glacial epoch in the north must have driven the greater
number of the northern plants and animals southwards, causing
a keen struggle for existence in which many species were
exterminated. Its influence was possibly intensified by the

328 OUTLINES OF EVOLUTIONARY BIOLOGY

fact that the glaciation was not continuous but alternated with a
succession of warm periods. The southern hemisphere also
experienced a glacial epoch during which warm and cold periods
alternated, and astronomers hold that the warm periods in one
hemisphere coincided with cold ones in the other. It has been
calculated that each warm or cold period lasted for about
21,000 years.

Other important changes in climate occurred long before the
great glacial epoch. Thus the fossil remains of a luxuriant
vegetation in Greenland and other northern localities indicate
the occurrence of a mild arctic climate in Miocene times. Such
a climate must have favoured migration between the old and
new worlds by way of what is now Behring Strait, which may
very well have been dry land at the time.

Owing to the gradual loss of the earth's heat by radiation and
the consequent shrinkage and crumpling of the solid crust,
variations in the level of the land are constantly taking place.
Areas which are at present separated by sea may have been
connected in former times and vice versa, and there can be no
doubt that the distribution of plants and animals has been
profoundly influenced in this way. Many cases of discontinuity
in distribution may be explained by the former existence of land
connections which no longer remain. It is necessary, however,
to be extremely careful how we invoke the aid of this principle,
which, as an easy way out of difficulties, is apt to lead us into
all sorts of unjustifiable speculations.

Those remarkable animals the lemurs, as a group, exhibit a
very curious discontinuity in their distribution, occurring in
Africa (and especially Madagascar) on the one hand and in
Southern Asia on the other. To explain this distribution it has
been suggested that in former times a continent — Lemuria —
existed in the Indian Ocean. Similarly, but with perhaps
greater justification, it is believed by many people that the
antarctic continent at one time extended much further north
than at the present day, so as to afford, possibly with the aid of
a chain of islands and with the co-operation of a mild antarctic
climate, a route along which migration might take place between
South America and Australasia. In this way may be explained
certain remarkable points of agreement between the fauna and
flora of Australia and New Zealand and those of South America.
The genus Fuchsia, for example, is typically South American,

CHANGES IN LAND AND SEA 329

but one or two species occur in New Zealand ; and the same is
true of Calceolaria. The evergreen beech forests of New Zealand
must be extraordinarily like those which Darwin described in
Patagonia. Even the same curious genus of fungus (Cyttaria)
is found on the beech trees in South America, New Zealand and
Tasmania. A fresh water lamprey, Geotria, also occurs both in
New Zealand and South America and similar cases could be
quoted from the invertebrate fauna.

It is very doubtful, however, whether such an extensive change
in the configuration of the earth's surface as the submergence of
an entire continent has ever taken place. According to
Dr. Wallace, who is recognized as the greatest authority on
tbe subject of geographical distribution, the existing continents
and oceans as a whole are permanent features, although their
outlines may be greatly affected by oscillations of the earth's
crust.

Perhaps tbe strongest argument against the former existence
of continents where we now have oceans lies in the fact that the
average depth of the sea is many times greater than the average
height of the land, no less than twelve thousand feet as com-
pared with one thousand, for the great depths of the ocean
extend over vast areas while the greatest heights of the land
are narrow mountain ranges. Hence, although large areas of
land might be submerged by a comparatively slight change of
level, it would take an enormous movement to bring any extensive
tract of the ocean bed to the surface.

In short, we are only justified in postulating the former
existence of land in places where the ocean is comparatively
shallow, but even this limitation leaves abundant opportunity
for changes in the relative distribution of land and sea which
would profoundly affect the distribution of plants and animals.
The actual occurrence of such changes is abundantly proved by
geological evidence and they are known to be going on at the
present day in many parts of the world.

There is good reason to believe that the principal groups of
terrestrial animals originated in the great northern land masses
and that the southern peninsular areas of Africa, Australasia
(now, of course, represented by detached islands) and South
America have been peopled mainly by successive waves of
migration from the north. We find in all these southern areas
primitive, ancient forms of life. Marsupials at the present day

330 OUTLINES OF EVOLUTIONARY BIOLOGY

are found only in Australasia and America, but the fossil remains
of such animals are widely distributed over the northern hemi-
sphere. The Onychophora, again, a small group of extremely
primitive arthropods, which until recently were all included by
zoologists in the single genus Peripatus (Fig. 167), are almost
confined to Australasia, South Africa and South America, in all
of which regions they are fairly abundant. It is more reasonable
to imagine that the ancestors of the Onychophora migrated
from the north, where the group has now become extinct,

than to invent imaginary continents
across which they may have wandered,
or even to suppose that they have
been so widely distributed as we now
find them by some external agency
such as floating timber.

The geographical distribution of
plants and animals would be quite
inexplicable on the supposition that
they had all been independently
created and deposited where they
now live. It is, however, easy enough
to explain it on the theory that the
earth has been peopled by the des-
cendants of common ancestors which
migrated from place to place as
occasion permitted and at the same
time underwent modification in many
different directions. We may now
briefly summarize the principal facts of distribution which justify
us in holding this view.

(1) The extent of the area of distribution of any group of
animals is directly proportional to its means of dispersal. Thus
flying animals are much more widely distributed than quadrupeds.
Birds occur abundantly on oceanic islands, but the only mammals
which occur there in a state of nature are bats and small forms
like rats and mice which may be carried on floating timber.
Nevertheless we know that when the larger mammals are trans-
ported by man to such localities they flourish exceedingly.
Many Protozoa, again, which are readily blown about in the form
of dust, are almost cosmopolitan even as regards their species.

(2) The degree of peculiarity of the fauna and flora of any

FIG. 167. — Peripatusmpensia,
from Cape Colony, X f .
(From a photograph.)

GEOGEAPHICAL ISOLATION 331

area is proportional to the length of time for which and the
extent to which that area has been isolated from other areas.
Thus Australia, which has probably been separated from the
Asiatic continent ever since the Cretaceous period, has a most
peculiar fauna and flora. We have already referred to the
numerous different kinds of marsupials — kangaroos, wombats,
phalangers, native bears, native cats and so forth — which have
not as yet been supplanted by the more recently developed
groups of mammals found in other parts of the world. Australia
is also still the home of those most primitive and reptile-like
of all the mammals, the Monotremata (Figs. 91, 92). The
Australasian forests, again, are composed principally of eucalypts
of many different species, which are found nowhere else in the
world. In New Zealand, which is even more isolated than
Australia, we find no less peculiar inhabitants, including the
wonderful tuatara (Fig. 113), the oldest surviving type of terrestrial
vertebrate, together with the kiwi (Fig. Ill) and other remarkable
flightless birds.

The reasons why the degree of peculiarity of the fauna and
flora of any region is proportional to the degree of geographical
isolation are not difficult to find. On the one hand ancient types,
such as the tuatara, the monotremes and the marsupials, may be
preserved from competition with more modern forms long after
they have been exterminated elsewhere. On the other hand, indivi-
duals accidentally introduced from distant areas at rare intervals
will have few opportunities of breeding with others of the same
species, and thus whatever variation occurs amongst them will
be less liable to be swamped by intercrossing with the parent
form. New races and ultimately new species will thus become
established more readily in such areas than elsewhere. This
principle of geographical isolation as a factor in the production
of new species is of great importance and we shall have to refer
to it again in a subsequent chapter.

The zoological or botanical affinities of the inhabitants of any
given area, not only with one another but also with those of
adjacent areas, are exactly what we should expect in accordance
with the views which we are advocating. It is impossible to
believe that the existing marsupials were (with the exception of
the few American species) all specially created in Australasia
when we know perfectly well that marsupials used to exist in
Europe in past geological times and can still exist in Europe

332 OUTLINES OF EVOLUTIONARY BIOLOGY

when transported there by human agency, and it is equally
impossible to believe that such animals as sheep and rabbits, to
which the Australian climate appears to be pre-eminently suited,
were specially created in Europe and Asia but never in Australia.
The existing condition of the Australian fauna is, however, easily
explained on the supposition that it was originally derived from
Asia at the time when marsupials and monotremes flourished in
the north, and that the island continent became separated from
the mainland before the more recent mammalian types, such as
sheep and rabbits, had arisen on the latter. Divergent evolution
within the limits of this isolated area is then quite sufficient to
account for the immense variety of marsupials occurring there at
the present day.

It is very instructive in this connection to contrast the con-
dition of the fauna of a comparatively recently separated
continental island, such as Great Britain, which is not far
removed from its parent continent, with that of the fauna of a
typical oceanic island which has never formed part of a continent
at all and is very widely separated from any other laud. The
native or indigenous population of continental islands always
exhibits a close relationship with that of the adjacent mainland,
from which it was originally derived and with wbich it is still
able to keep up a certain amount of intercourse. Such an island
will contain indigenous quadrupeds, and the great majority of the
species of plants and animals found in it will be identical with
those of the mainland. True oceanic islands, on the other hand,
such as St. Helena and the Sandwich Islands, are peopled
entirely by waifs and strays which have gained access to them at
rare intervals in one or other of the ways discussed in the earlier
part of this chapter. They never contain large quadrupeds and,
owing to their more or less complete isolation, the animals which
do occur almost always belong to peculiar species found nowhere
else in the world.

(8) Palaeontological investigations have demonstrated that the
present animal population of any tolerably isolated area is closely
related to the population of the same area in comparatively
recent geological periods. Thus in Australia we not only find
that at the present day marsupials are by far the most charac-
teristic features of the fauna, but also that the remains of extinct
marsupials, many of which belong to genera and species different
from any now living, are very abundant in the tertiary deposits

DISCONTINUOUS DISTEIBUTION 333

of the same region. Similarly in South America at the present
time the edentates (sloths, armadillos and ant-eaters) form the
most characteristic mammalian group, and the tertiary deposits
of that country have yielded the remains of a great number of
extinct forms belonging to the same order. It would be very
difficult to explain these facts on any theory of special creation,
but we can easily understand how a group of animals, having
once gained a footing in any area and finding itself secure and
more or less cut off from communication with other parts of the
world, would increase and vary, producing new species and
ultimately becoming the dominant group in that particular
region.

(4) Cases of discontinuous distribution are readily explicable
on the theory of evolution and migration. Either individuals
of the species in question have occasionally transgressed the
barriers to their dispersal and established new and distant
colonies, or possibly a large area of distribution has become
broken up into a number of smaller ones by geographical or
climatic changes rendering portions of it uninhabitable.

" Thus, for instance," says Romanes, " it is easy to understand
that during the last cold epoch the mountain hare would have
had a continuous range ; but that as the arctic climate gradually
receded to polar regions, the species would be able to survive in
southern latitudes only on mountain ranges, and thus would
become broken up into many discontinuous patches, correspond-
ing with these ranges. In the same way we can explain the
occurrence of arctic vegetation on the Alps and Pyrenees —
namely, as left behind by the retreat of the arctic climate at the
close of the glacial period." 1

1 " Darwin and after Darwin," Vol. I., p. 209.

CHAPTER XXII

Adaptation to environment in animals — Deep sea animals — The colouration
of animals — Protective and aggressive resemblances — Warning colours
— Mimicry — Epiganiic ornamentation .

IN the last few chapters we have discussed a number of facts
selected from that great and ever increasing mass of evidence
which leads us to the inevitable conclusion that the present con-
dition of the fauna and flora of the earth, with their almost
endless diversity of plants and animals, is the outcome of a long
process of organic evolution. It is desirable at this stage of our
inquiry to emphasize the fact that this evolution, in the main,
has been of a progressive character, and of such a character,
moreover, as to maintain a more or less perfect harmony between
the organism and its environment.

Adaptation in bodily organization and in corresponding
function, whereby each kind of plant or animal is enabled to
meet the constant demands made upon it and maintain its
existence in the endless warfare of life, is the great outstanding
feature of living things. So universal is this adaptation that
we are apt to take it for granted, and any want of it is at once
recognized as an exception and an anomaly. Anyone, for
example, who watches the slow and clumsy movements of a
tortoise cannot fail to be struck with the fact that the limbs of
this animal are but ill-suited for purposes of locomotion, but
even in this case there is compensation in that the tortoise
carries its place of refuge about with it and has therefore little
need to hurry itself.

We have seen in an earlier chapter how completely the
pentadactyl limbs of air-breathing vertebrates may become
modified from their primitive condition in correspondence with
changes in the mode of life. The fore limbs, adapted in the
first instance for locomotion on land, have become changed in
the whales, seals and dugongs into paddles ; in the pterodactyls,
birds and bats into wings, and in man into organs of prehension.

335

Indeed, given time enough, the power which an organism
possesses of altering its bodily structure in accordance with new
demands on the part of the environment seems, as we have
already pointed out, to be almost without limits.

This plasticity is illustrated in the most striking manner in
cases where the organism has been removed from what may be
regarded as the normal environment of the group" to which it
belongs, and to which the great majority of the group are adapted,
and come to live under new
and very different conditions.
Thus it is with the aquatic
and aerial mammals, which,
in encroaching upon the
domains of the fishes and
birds, have, by convergent
evolution, come to resemble
these in bodily form.

Wherever we turn we find
fresh illustrations of the same
principle. At great depths
of the ocean the conditions of
life are very different from
those which obtain in shal-
low water, and we find the
animals which inhabit these
abysses modified accordingly.
Fig. 168 represents two deep
sea sponges obtained by the
" Challenger " expedition ;
Cladorldza longipinna from a
depth of 3000 fatboms in the North Pacific and Axonidenna miralile
from a depth of 2250 fathoms in the South Pacific. It will be seen
at once that the form assumed by these sponges is very unusual
and quite unlike that exhibited by their shallow water relatives.
The great majority of the members of the group of sponges (the
Tetraxonida) to which they belong are indeed by no means
remarkable for symmetry of shape, but these two are beautifully
symmetrical, their form at once suggesting that of a parachute,
with a small conical body fringed by long radiating processes
surrounding a central root-like projection. This " Crinorhiza
form," as it is termed, is obviously an adaptation which serves

FIG. 168. — Two Deep Sea Sponges,
exhibiting the Crinorhiza Form.
A. Cladorhiza longipinna; B, Axoni-
derma mirabile ; nat. size. (After
Ridley and Dendy in " Challenger "
Reports.)

336 OUTLINES OF EVOLUTIONARY BIOLOGY

to prevent the sponge from sinking into and being smothered by
the soft mud or ooze which covers the bottom of the ocean at
very great depths, and it is interesting to observe that species of
several distinct though related genera have adopted the same
device, thus affording a beautiful example of the phenomenon of
convergence. Other sessile deep sea animals have found different
means of overcoming the same difficulty, especially in many
cases by the development of long stalks.

The absence of light at great ocean depths has led to the
acquisition on the part of many of the deep sea fishes of brilliant
phosphorescent organs, arranged like little lamps on various
parts of the body. In some cases at any rate these serve to
attract other animals upon which these fishes prey. Some of
them, again, develop long and delicate feelers by aid of which
they grope their way about in the dark.

In the brilliantly illuminated surface waters of the ocean con-
ditions are very different, and here we find that the most favourite
device for preserving life amidst a host of enemies is transparency,
but we have already alluded to this in the preceding chapter and
need not dwell upon it further. It is a phenomenon which falls
under the head of protective colouration, of which we shall find
better instances elsewhere.

The significance of the colouration of animals as a means of
adaptation to environment is a subject which has in recent years
developed into a special branch of biological science, and which
already has a copious literature of its own. Professor Poulton,
in his well known work on the Colours of Animals,1 has suggested
an elaborate scheme of colour classification from this point of
view. He distinguishes, in the first place, between apatetic
(deceitful) colours, sematic (warning and signalling) colours, and
epigamic colours (displayed in courtship), all of which afford
marvellous instances of more or less highly specialized adaptation.
We have not space to follow out the details of this classification
but we shall presently refer to examples of all the more important
types of colouration included therein.

Every observer of nature must have been struck with the
general harmony of colouration which exists between animals
and their surroundings. So complete is this harmony that our
sense of hearing is frequently a better guide to the whereabouts
of an insect, bird or mammal than our sense of sight. I

1 International Scientific Series, Vol. LXV1II.

PEOTECTIVE AND AGGRESSIVE RESEMBLANCE 337

remember standing with my gun in the midst of a dense patch
of scrub in Australia and hearing the pademelons1 hopping about
all around me. For a long time, however, I could see nothing
but the trees. My native guide pointed out where I was to aim,
but I only fired at a log from the side of which a pademelon
hopped away. Again he pointed, and this time at a small white
spot which I could just distinguish amongst the trees. I fired
once more, aiming at the white spot, and sure enough a pademelon
rolled over. Ft appeared that I had aimed at the white fur which
occurs on the breast of the animal and which to the experienced
eye of the native told all that he needed to
know. It is often supposed that conspicuous
patches of this kind serve as recognition marks
between individuals of the same species, but it
may be questioned how far the advantage of
being recognized by a friend compensates for a
disturbance of the colour harmony which reveals
an animal to its enemies.

The type of colouration which aids in the
concealment of an animal is termed, by Pro-
fessor Poulton, cryptic. It belongs, of course,
to the apatetic group. Concealment may be
desirable either as a means of escape from
enemies or for the purpose of ambuscading
prey, or possibly for both. In the former case
we may speak of it as protective resemblance
(procryptic colouration), in the latter as aggres-
sive resemblance (anticryptic colouration).

Protective resemblance is often of a very highly
specialized character, and may be due as much to adaptation
in actual form as to adaptation in colour ; frequently these two
factors unite in producing the result, and a third may be added,
viz., adaptation in habit or instinct. In the common stick
caterpillars of the geometer moths we see all three factors
co-operating. In colour and shape these caterpillars precisely
resemble small twigs. They move about with a characteristic
looping action amongst the leaves or branches of the bushes
which they frequent, but when at rest they stiffen themselves up
and stand out from the branch at the exact angle of a twig, and

FIG. 169. — Larva
of the Brim-
stone Moth
(Rumia crutae-
yata) resting
upon a Haw-
thorn twig ;
nat.size. (From
Poulton.)

A small species of kangaroo.

B.

338 OUTLINES OF EVOLUTIONAKY BIOLOGY

in this condition it is extremely difficult to detect them. Professor
Poulton remarks : —

"These caterpillars are extremely common, and between two and
three hundred species are found in this country ; but the great
majority are rarely seen because of their perfect resemblance to
the twigs of the plants upon which they feed."

As will be seen from the illustration (Fig. 169), which represents
the larva of the brimstone moth upon its food plant, the hawthorn,

the caterpillar is enabled to main-
tain its position for a long period
by attaching its head to a twig by
means of a silken thread.

Numerous moths so closely re-
semble in the colour and pattern
of the upper surface of their wings
the objects upon which they rest
in the daytime, such as the bark
of trees, that they are almost
invisible, but perhaps the most
perfect examples of protective
resemblance are met with in the
wonderful leaf insects. Fig. 170
represents an orthopterous insect,
PuLchriphyUium crurifolium, from
Ceylon. The whole insect is of a
bright leaf-green colour, and not
only are the wings shaped and
veined so as to resemble leaves,
but even the body and legs exhibit
leaf-like outgrowths.

In the well known Indian leaf

FIG. 170.— A Green Leaf Insect
( Pulchriphyllium crurifolium,
J ), from Ceylon ; xi. (From
a photograph.)

butterfly, Kallima (Fig. 171), the resemblance to a leaf is only
seen when the insect comes to rest with its wings folded together
above the body so as to expose their under surfaces. It is a dry,
dead leaf which is imitated this time, and stalk and midrib, veins
and colour markings, even down to such minutiae as rust spots,
are perfectly represented.

The Mantidse or praying insects feed upon flies, &c., which
they capture with marvellous dexterity with their serrated claws.
In some species the uniform green colouration doubtless serves,
not only to protect them from their own enemies, but also to

PROTECTIVE AND AGGRESSIVE RESEMBLANCE 339

prevent them from being seen by their victims before they have
come within range. Other species exhibit even more wonderful
adaptations both in form and colour. Thus the South African
Harpax tricolor sits amongst the pink and white flowers of the
heath, which are imitated by similarly coloured outgrowths of
the insect, and there awaits the approach of its unsuspecting
victims ; while in Mozambique the terrible Idolum didbolicum

I

FIG. 171. — An Indian Leaf Butterfly (Kallima inachis}; A., with wings
expanded ; B., with wings folded ; x if. (From a photograph.)

simulates, both in form and colour, a large flower, and thereby
deceives and attracts other insects in search of honey.

It is no doubt amongst the almost innumerable species of the
great group Insecta that cases of highly specialized adaptation
for purposes of concealment or deception are most frequently
met with. They also occur, however, and by no means
uncommonly, in other groups of the animal kingdom. A
familiar instance is afforded by the common British spider crab,
now known as Macropodia rostrata,1 of which excellent illus-
trations (under the name Cancer Phalangium) were given by

1 I am indebted to my friend, the Rev. T. 11. R. Stebbing. F.R.S., for information
as to the correct nomenclature, &c., of this species.

z 2

340 OUTLINES OF EVOLUTIONARY BIOLOGY

Dr. Macculloch, in the Transactions of the Linnean Society, as
far back as 1801. I am enabled by the courtesy of the Council
of the Society to reproduce here, on a reduced scale, Dr. Maccul-
loch's original plate (Fig. 172). The crab actually breaks off
fronds of seaweed and attaches them to the long hairs of its body,
thus disguising itself so effectually as to be quite unrecognizable
except by careful examination. Dr. Macculloch was of opinion

FIG. 172. — Eeduced Facsimile of Dr. Macculloch:s Plate of Macropodia
rostrata, in the Transactions of the Linnean Society. On the left is
shown a plant of the seaweed in which the crab dresses itself up ; on
the right the crab without the seaweed, and at the bottom the crab
dressed up.

that this dressing up of the crab in seaweed was an artifice
which assisted it in capturing its food (anticryptic), but it is
much more likely that it is protective (procryptic). The late
Professor Bell has told us how the slow and sluggish habits of
the crab render it an easy prey to fishes, and the stomach of a
thornback ray has been found entirely filled with them, so that
there appears to be ample reason for them to seek concealment.

In the case of Macropodia the adaptation for concealment
shows itself as an inherited habit or instinct more than in any
modification of bodily structure, but such an instinct is probably

PROTECTIVE AND AGGRESSIVE RESEMBLANCE 341

itself the effect of some structural modification, however impossible
to detect, in nervous tissue. In the Australian Phyllopteryx
eques, a fish which is closely related to the curious sea-horse
(Hippocampus) of our own coasts, we get precisely the same
idea, so to speak, carried out in a different manner. Both
Hippocampus and Phyllopteryx live amongst seaweed, to which
they attach themselves by means of their curious prehensile
tails. Hippocampus (Fig. 173) exhibits no special resemblance

FIG. 174.

FIG. 173.

FIG. 173. — A Sea-horse (Hippocampus antiquorum), x §. (From a

photograph.)

FIG. 174. — Plnjllopteryx eques, attached to seaweed. (From Giinther's
" Study of Fishes.")

to its surroundings, but in Phyllopteryx (Fig. 174) the body is
covered with cutaneous outgrowths which float out in the water
like fronds of seaweed and doubtless effect a most satisfactory
disguise. This is certainly a less troublesome plan than that of
dressing up in clothing borrowed from the outside world.

The well known colour changes of the chameleon and of
various flat fishes, not to mention numerous other instances
which might be cited from different groups of the animal king-
dom, are due to a complex apparatus, controlled by the nervous

342 OUTLINES OF EVOLUTIONARY BIOLOGY

system, whose function it is to bring about a varying adaptation
for concealment under varying conditions of the environment.
How perfect the adaptation may be will be realized by all who
have ever observed with what marvellous accuracy the colour
markings of a turbot in an aquarium are made to match the
sand or gravel upon which it is lying.

In striking contrast with the cryptic colouration by which an
animal seeks, as it were, to avoid observation, are those numerous
cases in which self-advertisement appears to be the main object
in view. The British army, which only in recent years has
learnt the advantages of khaki clothing when in the field, still
exhibits some of the most startling instances of conspicuous
colouration met with anywhere in the animal kingdom, though
whether these examples should be classed under the head of
warning colours, or regarded as belonging to the epigamic
category, is perhaps an open question. We must, however, con-
fine our attention in this place to a few examples of warning
colours met with amongst the lower animals.

We have seen that both warning and signalling colours, or
recognition marks, are spoken of as sematic. The former are
further distinguished as aposematic and the latter as episematic.
Aposematic colours are exhibited by many animals which possess
some special means of defence and find it advantageous to
advertize the fact. Wasps and hornets, with their conspicuous
orange- and black-banded bodies, are excellent examples. Such
animals do not seek to conceal themselves but rely upon their
warning colours to remind their enemies that they had better
leave them alone. It is not enough that they should possess the
power of making themselves disagreeable; the fact must be
clearly recognized, otherwise they would be constantly exposed
to experimental attack, and suffer injuries for which any damage
which they might inflict upon their pursuers would be but a
poor consolation. Orange, red and black, owing to their great
conspicuousness, especially when associated with one another,
are the colours most frequently met with in this connection, and
we find these colours, not only in noxious insects, but in various
vertebrate animals, such as poisonous reptiles, toads and sala-
manders. The Gila monster (Heloderma suspectum), of Mexico
and Arizona, is the only known poisonous lizard, and is con-
spicuously coloured in tints of blackish brown, yellow and
orange, while other members of the same group are usually

MIMICRY

343

coloured so as to harmonize more or less perfectly with their
surroundings.

If it is advantageous for a noxious species to advertize its true
character, it is no less so for an innocuous one to advertize a
false character, and gain credit for some power of making itself
objectionable which it does not really possess. The practice of
bluffing is by no means an exclusively human institution. Thus
we find many insects, which in themselves are quite inoffensive,
taking on the characteristic warning colouration of dangerous
species. The drone-fly mimics the bee, and though they belong
to widely different orders of insects,
the one having only two wings and the
other four, the resemblance is so close
as to have given rise, as we saw in an
earlier chapter, to the ancient belief
in the spontaneous generation of bees
from the carcases of oxen (on which, of
course, drone-flies had deposited their
eggs).

Most moths, as is well known, have
opaque wings, covered with microscopic
scales, but in the clear-winged moths
(Fig. 175, A) the wings have partially
lost their scales and become transparent,
and this anomalous feature, combined
with the colouration of the body, enables
these perfectly harmless insects to mimic
the dangerous hornets (Fig. 175, B).
Even a harmless snake may mimic
the warning colouration of a venomous
species, and thus secure for itself the respect which is properly
due only to the latter.

It is not necessary that an animal should be capable of
inflicting serious injury upon its enemies when attacked for it to
secure immunity from pursuit as soon as recognized. Many
butterflies and other insects, which are probably merely distasteful
or nauseous (or perhaps actually unwholesome) to birds, exhibit
aposematic or warning colouration. Amongst these we find
curious associations known as synaposematic groups, the
members of which, belonging to distinct species and often by
no means closely related to one another, seem to have combined

B.

FIG. 175. — A., a clear-
winged Moth (Sesia cra-
bronifurmis] mimicking
B. , a Hornet ( Ves/ia
crabra] ; both x f. (From
a photograph.)

314 OUTLINES OF EVOLUTIONARY BIOLOGY

together to share the expenses of a common advertisement and
thereby reduce the cost to each. Young birds have to learn
by experience which insects are good to eat and which are not.
In making their experiments no doubt they themselves suffer,
but the subjects of the experiment are probably actually killed.
Obviously, then, if one experiment can be made to serve for a
number of different species of insects there will be a corre-
sponding reduction in the death-rate, and hence it is that we

FIG. 176. — A Synaposematic Group of South American Lepidoptera, all X \-
(From a photograph.)

A, Tithorea harmonia ; B, Heliconius etliilla ; C, Perrhybris (Mylotliris) malenka,
g ; D, Dismorphia praxinoe, J ,• E, Pericopis angulosa.

find these groups of species all adopting the same type of
warning colour, and thus coming to resemble one another very
closely, although perhaps belonging to totally distinct families.

We may illustrate this somewhat complex phenomenon by
reference to certain South American Lepidoptera which take
part in the formation of such a synaposematic group. In
Fig. 176 A, B and D represent butterflies belonging to three
distinct families, while E is a moth, as may be seen at once by
its thick body and the absence of terminal knobs on the antennae.

All of these, in common with numerous other species which
inhabit the same area, have adopted the same characteristic
scheme of warning colouration, wherein the prevailing tints are
black and orange.

In such a synaposematic group, or mimicry ring, it is usually
possible to distinguish between certain species which seem to
have led the way in the development of the warning colouration,
and others which seem to have followed their example. In
the particular case under notice the original " models " belong
to the group Ithomiinae, of which Titkorea harmonia (Fig. 176, A)
is a representative. These are probably the most distasteful
members of the combination to birds. They have been imitated
by Helieoninae, such as Helicomus ethilla (Fig. 176, B), Pierinse
("whites"), such as Dismorphia praxinoe (Fig. 176, D), and
Hypsidae(a family of moths), as exemplified by Pericopisanyidosa
(Fig. 176, E), all of which may be regarded as mimics of the
Ithomiinae.

The case of the pierine mimics is particularly instructive,
and shows very clearly that these forms really imitate other
species, for the female is commonly a far more perfect mimic
than the male, which often departs little, if at all, from the typical
colouration of the group to which it belongs. Fig. 176, C repre-
sents a male pierine, Perrltybris (Mylothris) malenka, which is
at once recognizable from its colouration as a " white," although
even here, curiously enough, there is a faint trace of the warning
colouration on the under surface of the hind wings.1 The
female of the same species has the warning colouration well
developed, as it is in both male and female of Dismorphia
praxintie.

So different are the males and females of some of these
mimicking species that it would be difficult to believe, were it not
for breeding experiments, that they are really specifically
identical. The explanation of the difference is doubtless to be
found in the fact that it is much more important, from the point
of view of the species, that the females, heavily laden with the
eggs upon which the existence of future generations depends,
should be able to warn off the birds, than that the males should
do so, for the latter, having once accomplished the fertilization
of the eggs, is of no further value to the race.

1 Doubtless inherited incompletely from female ancestors, as in the case of the
vestigial nipples of man.

346 OUTLINES OF EVOLUTIONARY BIOLOGY

When an unquestionably harmless species mimics the warning
colours of an undoubtedly noxious one, the case is sometimes
spoken of as one of " Batesian " mimicry, after the distinguished
naturalist, H. W. Bates, who added so much to our knowledge of
the subject. In the case of a synaposematic group, or mimicry
ring, however, it is often impossible to say whether any particular
species is edible or not, and it may very well be that in some
cases all are more or less inedible, though undoubtedly some,
which are presumably the less objectionable forms, mimic others,
which are presumably the more objectionable. This kind of
mimicry, resulting in the development of a warning colour
common to a number of inedible species, is sometimes distin-
guished as " Miillerian " mimicry, after the naturalist Fritz
Miiller, who first suggested the correct interpretation of the
phenomenon.

Perhaps the most remarkable case of mimicry known
amongst butterflies is that of certain species of Papilio found in
Africa and Madagascar, which have formed the subject of
exhaustive study by Trimen, Poulton and others. In Madagascar
occurs Papilio meriones, a non-mimetic species in which the
male and female (Fig. 177, A) closely resemble one another and
both possess the " tail " on the hind wing which is such a charac-
teristic feature of the genus. We may take it, then, that this is a
primitive form. On the continent of Africa is found the wide-
spread Papilio dardamts, with several subspecies. In these sub-
species the male (Fig. 177, BI) retains the ancestral form, but in
most of them the female is mimetic; it has lost the Papilio tail
and closely mimics, both in shape and colour markings, some
one or another of various species of butterflies belonging to
different families which occur in the same region. Nor is this
all, for the female is likewise polymorphic, and different
individuals of the same subspecies resemble widely different
models. Thus the subspecies merope is known to have three
forms of female, a hippocoon form (Fig. 177, B2) which mimics
the danaine butterfly Amanris niarius (Fig. \ll,G),o.troplioinus
form (Fig. 177, B3) which mimics another danaine, Limnas
chrysippus (Fig. 177, D), and a planemoides form which mimics
the acrseine, Planema pogyei. Our illustrations, which are
reproduced from Mr. Trimen's original memoir, give a good
idea of the form and pattern of some of these insects, but they
lack the beautiful colouring of the original figures, which is

MIMICRY IN BUTTERFLIES

347

A. Papilio merion.es

Bi. Papilio dardanus (jnerope) o*

C . Amauris niavius

D. Limnas chrysippus.

Bz. Papilio dardanus (me rope)

{hippocoon form)

Ba. Papilio dardanus (merope)
(frrophon/us form)

FIG. 177. — Mimicry in Butterflies. (After Trimen, from coloured plates in
the Transactions of the Liunean Society, First Series, Vol. XXVI.)

348 OUTLINES OF EVOLUTIONARY BIOLOGY

necessary in order to give a true idea of the close resemblance
between mimics and models.

Breeding experiments carried out by Mr. G. F. Leigh with the
subspecies cenea of Papilio dardanus have shown that the eggs
laid by one and the same butterfly may give rise to at least

FIG. 178. — Male and Female of an Australian Lyre-Bird (Menura superba).
(Photographed from specimens in the British Museum, Natural History.)

three different forms of mimetic female, as well as, of course, the
male.

It is obvious that we have here the very opposite of a
synaposematic group, for, instead of concentrating upon a single
warning pattern, different individuals, even of the same species,
adopt totally different patterns in imitation of totally different
models. This case still requires a great deal of explanation, but
concerning the facts there can be 110 doubt.

EPIGAMIC ORNAMENTATION 349

Frequenters of any good museum of natural history will be
familiar with plenty of examples of epigamic ornamentation.
The highly elaborate and gorgeous plumnge of many male birds,
such as the peacock, the Argus pheasant, the Australian lyre-
bird (Fig. 178) and numerous species of humming birds, to say
nothing of less conspicuous examples, affords the best illustration
of this phenomenon. In all such cases the function of the
ornamentation appears to be to gratify the aesthetic sense of the
female during the period of courtship, and render her amenable
to the attentions of her mate. There is no doubt that in many
cases the cock bird deliberately displays himself to the best
advantage before the admiring eyes of the hen, who is credited
with a no less deliberate choice of the partner who most nearly
approaches her ideal standard of beauty. In all these cases the
plumage of the female is comparatively sombre and uninteresting.
An elaborate and gaudy tail would be a disadvantage during
the lengthy period of incubation, when it is desirable, both
for her own sake and that of her offspring, that the female
should be as inconspicuous as possible. Thus it is the male bird
that has had to adapt his clothing to the requirements of the
female, and she herself is unable to follow the fashions which she
imposes upon her mate.

It is obvious that no theory of evolution can be regarded as
satisfactory which does not offer some explanation of the origin of
such highly specialized and precise adaptations as those which we
have been considering in this chapter, and it was in order to
emphasize the need for such explanation that we have laid so
much stress upon them. We shall see in our next chapter that
no less remarkable and precise adaptations for special purposes
are also met with in the vegetable kingdom, and shall then pass
on to seek the necessary explanation.

CHAPTER XXIII

Adaptation to the environment in plants — Alpine plants, desert plants and
lianes — The modification of flowers in relation to insect-fertilization.

IN plants no less than in animals we find adaptation to the
conditions under which they have to live to be the most striking
feature of their organization. We have already noticed, in
dealing with the phenomenon of convergent evolution, the
manner in which Alpine plants of many kinds tend to assume the
compact cushion-like form which seems best suited to withstand
the rigorous climate to which they are exposed. Wherever a
plant may be found growing in a state of nature the character of
the environment is sure to be reflected more or less obviously in
its structure and habit. We see this equally clearly in the
water-storing plants of desert regions — the cacti of America
or the aloes of Africa— with their succulent stems or leaves and
other structural modifications which enable them to withstand
the effect of long-continued drought, and in the climbing lianes
of tropical forests, whose one object in life appears to be to reach
some elevation where they can expose their foliage to the light
and air. Just as we find those air-breathing mammals which
have taken to an aquatic life adopting the form and habits of
fishes as being best suited to their changed conditions, so in the
tropical forests of Queensland we find palms, members of a group
which elsewhere are types of self-supporting independence,
adopting the form and habit of climbing plants as the only means
of coping with the exigencies of the situation.

If the adaptation amongst plants usually appears less remark-
able than is often the case in animals, it is because the
relations of a plant to its environment are usually less complex
than those of an animal. The greater activity of animals is
associated with the development of highly specialized organs of
locomotion, sense-organs and nervous system, all of which are
alike subject to adaptation. Plants afford much more restricted
opportunities for the effects of the environment to show them-
selves, and it is in their relations with animals, or to speak more

ADVANTAGE OF CROSS-FERTILIZATION 351

accurately, in those organs which are immediately concerned
with these relations, that adaptation becomes most complex and
precise.

Even amongst animals, however, it would be difficult to find
better illustrations of accurate adaptation to highly specialized
conditions of the environment than those which we see in the
wonderful structural modifications whereby the majority of
flowers are adapted for pollination by insect agency. This
process of pollination is commonly referred to as the " fertiliza-
tion " of the flower, although, as we have seen in a previous
chapter, the real act of fertilization takes place inside the
so-called ovule and consists in the conjugation of the male and
female gametes, the sperm-cells and egg-cells.

The great majority of flowers produce both pollen and ovules,
containing respectively the male and female gametes ; in other
words they are hermaphrodite. It is obvious that in such cases
we have two possibilities with regard to fertilization, for the
flower may either be fertilized by its own pollen or by that of
some other flower. It may be either " self-fertilized " or " cross-
fertilized," and the cross-fertilization may be effected either by
pollen from another flower of the same plant or by pollen from a
different plant of the same kind, the latter being the more
advantageous.

It seems at first sight a strange thing that when a flower is
capable of self-fertilization such a roundabout and apparently
unnecessary proceeding as cross-fertilization should ever take
place at all. It has been shown, however, that cross-fertilization,
if not absolutely necessary, is at any rate of very great
advantage to the plant, or rather to the species, in which it
occurs.

Charles Darwin experimented for eleven years on this subject,
and proved conclusively that cross-fertilization yields better
results than self-fertilization, both as regards the number of seeds
produced and also as regards the quality of the offspring. It
would be impossible -to give an adequate account of his work in
this place, but we may briefly notice one series of experiments
and refer for the remainder to his classical volume on the " Cross
and Self Fertilization of Plants." He started with the " Con-
volvulus major" (Ipomcea purpurea), the flowers of which are
hermaphrodite and may be either cross- or self-fertilized in a
state of nature. It is an easy matter, by artificially conveying

352 OUTLINES OF EVOLUTIONARY BIOLOGY

the pollen on the tip of a feather, to bring about the fertilization
in any way which may be desired. Ten flowers were thus self-
fertilized with their own pollen, while ten others were cross-
fertilized with pollen from a distinct plant. The crossed and
self-fertilized seeds thus obtained were carefully cultivated under
exactly the same conditions, and it was found that the plants
raised from cross-fertilized seeds were taller than those raised
from self-fertilized seeds in the proportion of 100 to 76.

The same experiment was repeated with ten successive
generations of the same plants, and always with the same result
in favour of the cross-fertilized individuals. Moreover, it was
proved at the same time that the fertility of the plants produced
by cross-fertilization was greatly superior to that of the self-
fertilized plants, a much larger quantity of seed being produced.

These results, taken in conjunction with many others of the
same kind, clearly proved the advantages of cross-fertilization.
To quote Darwin's own words :—

" It has been shown that the offspring from the

union of two distinct individuals, especially if their progenitors
have been subjected to very different conditions, have an
immense advantage in height, weight, constitutional vigour, and
fertility over the self-fertilized offspring from one of the same
parents."

The theoretical explanation of this advantage is not an easy
matter, but is probably to be sought in the admixture of two
distinct series of hereditary tendencies in the offspring of a
cross.

In a state of nature it is a very rare thing for plants to be
exclusively self -fertilized, for, although self-fertilization may take
place in some cases to a very large extent, and although some
flowers are so constructed that self-fertilization alone is possible,
yet there is nearly always at least an occasional cross by the
introduction of pollen from a different plant.

Considering the advantages which arise from cross-fertilization,
we need not be surprised to find that a very large number of
flowers are provided with special adaptations whereby these
advantages may be secured to them. We may notice first certain
contrivances by means of which self-fertilization is more or less
effectually prevented and the injurious effects of perpetual close
inbreeding thereby avoided. These contrivances may be classed
under three principal heads.

FERTILIZATION OF FLOWERS. 353

(1) Separation of the Sexes. — In many flowers we find that only
stamens or pistil are developed, never both, so that we get
distinct male and female flowers, which renders self-fertilization
absolutely impossible, though it does not necessarily prevent
fertilization by pollen from other flowers on the same plant.

(2) Physiological Self-sterility. — In some flowers, to quote the
words of Darwin, " the ovules absolutely refuse to be fertilized
by pollen from the same plant, but can be fertilized by pollen
from any other individual of the same species." Again, in a
large number of plants, although self-fertilization may take place,
yet, if the pollen from another individual be brought to the
stigma, it takes precedence, so to speak, of the flower's own
pollen and renders the latter ineffectual ; it is said to be
prepotent.

(3) Dichogamy. — In a large number of flowers, in which both
male and female organs are present, the stamens and pistil
become mature at different times, so that self-fertilization
cannot possibly take place and, physiologically speaking, the
flowers are unisexual. Usually in these cases the pollen ripens
and is all shed before the stigma is ready to receive it. The
flower is then termed protandrous, as for example in the
common pink (Dianthus). More rarely the stigma ripens and
withers before the pollen is ripe, and such species are termed
protogynous, as in the fig-wort (Scroplmlaria nodosa).

Of course it would be worse than useless to prevent self-
fertilization unless some means were provided at the same time
for securing cross-fertilization. We saw in Chapter VIII that
the principal agents in conveying pollen from one flower to
another are wind and insects, and that flowers are accordingly dis-
tinguished as anemophilous or wind-fertilized and entomophilous
or insect-fertilized. The fact that the former are usually very
small and inconspicuous, as for example in the grasses, while the
latter are large and brightly coloured, affords strong presumptive
evidence that entomophilous flowers have become modified in
relation to the insects which visit them in search of food or
shelter. A great many different kinds of insects have this habit,
bees, flies, butterflies, moths and even beetles ; while in South
America some of the humming birds in like manner play the
part of pollen carriers.

We are in the habit of regarding bees as the most important
insects concerned in the cross-fertilization of flowers, and in

B. A A

354 OUTLINES OF EVOLUTIONARY BIOLOGY

.ati

at

countries where they are plentiful this is perhaps the case.
They visit the flowers in order to collect both honey and pollen,
to be used as food for themselves and their offspring. The honey
is usually found at the bottom of a long narrow tube formed by

the lower part of the corolla,
so that in order to reach it
the bee requires a corre-
spondingly long and narrow
instrument. This is pro-
vided in the shape of a very
complicated proboscis (Fig.
179), formed by modifica-
tion, especially elongation,
of certain of tha appendages
surrounding the mouth,
which in more primitive
insects, like the cockroach,
remain short and simple.
When not in use the pro-
boscis is neatly folded away
beneath the head, but when
a flower is visited it is un-
folded and inserted into the
tube containing the honey,
which is then drawn up into
the bee's stomach by means
of a special sucking ap-
paratus. In butterflies and
moths also a somewhat
similar proboscis is used for
the same purpose, but it
differs so much from that of
the bee in details of struc-
ture as to indicate that it has
been independently evolved
from the primitive mouth parts of some remote insect ancestor.
In both cases the mouth appendages have become specially adapted
for the very special purpose of sucking honey, and the necessity
for the fulfilment of the same function has led, as usual, to a
superficial resemblance between the two types of proboscis. We
have here, of course, another illustration of convergent evolution.

FIG. 179. — Head of a Bee, showing the com-
plex Proboscis formed from modified
Month Parts. (From Weismann's
" Evolution Theory.")

at, antennae ; Au, large compound eye; au,
ocellus ; la., labrum or upper lip ; le, outer
division of second maxilla (paraglossa) ; li,
ligule or tongue, formed by fusion of inner
divisions of second maxil lee ; md, mandible ;
mx*t mx^, first and second maxillse ; pi,
labial palp; p.m., maxillary palp.

FERTILIZATION OF FLOWERS. 355

For collecting pollen the bees make use of their legs, on which
special collecting brushes and baskets are formed by the stiff
hairs. The pollen is first moistened with honey to make it stick
together, and then picked up on the collecting brushes, placed
in the baskets on the hind legs and carried to the hive or nest.

Insects commonly visit a large number of flowers of the same
kind in rapid succession, and, whether they intentionally collect
pollen for their own purposes or not, it is evident that, after each
visit to a flower containing ripe pollen grains, they will uninten-
tionally carry away some of these, accidentally attached to various
parts of the body. The pollen which is thus unconsciously
carried away is equally unconsciously deposited on the stigma of
the next flower visited, and thus cross-fertilization is effected.
It may, of course, happen sometimes that the pollen is deposited
on the wrong kind of flower, but that is of no consequence, for
pollen is, with rare exceptions, quite incapable of fertilizing
flowers of any but its own particular species.

Under these circumstances it will obviously be advantageous
to a plant to make its flowers as attractive to insects as possible,
and, just as it is desirable for a nauseous insect to inform the
birds by means of warning colours of the fact of its unpalatability,
so also is it desirable for flowers which have honey to offer in
exchange for the service of pollination to make that fact known
to their insect visitors by means of brilliant colours and strong
scents. There can be no reasonable doubt that the colours serve
to attract the insects and to enable them to recognize the flowers
which they prefer, and that the same is true of the scents is very
clearly indicated by the fact that certain flowers, such as various
species of Arum and related genera, which are fertilized by
carrion-loving flies, have managed to perfume themselves in
accordance with the tastes of their visitors.

It is also desirable to ensure, by means of mechanical arrange-
ments, that the insects shall not get their honey without paying
for it, that is to say, without effecting fertilization, and this end
is usually secured by concealing the honey at the bottom of a
long tube, in such a manner that the insect must brush against
the stamens and stigma before it can reach it. " Sometimes, how-
ever, the insects become a little too clever for the flower and steal
the honey by biting a hole through the bottom of the tube with-
out ever touching the stamens or stigma at all. In this way the
red clover flowers are often robbed of their honey by the humble

A A 2

356 OUTLINES OF EVOLUTIONARY BIOLOGY

bee, Bombus terrestris, thus affording an example of imperfect
adaptation.

We may now consider a few definite examples of the manner
in which flowers may be specially adapted in structure so as to
secure the advantages of cross-fertilization by insect agency.

If a number of plants of the common primrose, the oxlip, the cow-
slip or the polyanthus (species of the genus Primula) be examined
carefully, it will be seen that in each case two quite different
forms of flower occur. In other words the flower is dimorphic.

The two forms are known to
gardeners as " pin-eyed " and
" thrum-eyed " respectively. In the
pin-eyed flowers (Fig. 180, A) the
style (g) is comparatively long and
the stigma (n) appears as a round
knob, like the head of a pin, in the
centre of the flower, at the entrance
to the long tube formed by the
lower part of the corolla. The
anthers (a) lie much lowyer down in
the tube, so that they are invisible
until the flower is cut open. In the
thrum-eyed flowers(B)the positions
of the stigma and anthers are
reversed ; the style being much
shorter the stigma lies only half
way up the tube, while the anthers
appear in the centre of the flower,

in the mouth of the tube. The two kinds of flower are
always found on separate plants, and the long-styled and short-
styled plants are said to occur in about equal numbers under
natural conditions. The pollen grains also differ in the two
kinds of flower, those of the thrum-eyed being larger than those
of the pin-eyed and of somewhat different shape. This particular
kind of dimorphism, which is sometimes known as heterostylism,
is extremely characteristic of the genus Primula, and it has
recently been shown that the distinguishing features of the two
forms are inherited in a Mendelian fashion.

The flowers of the primulas are fertilized by the agency of
insects such as humble bees, and Darwin found that if insects
were carefully excluded by covering the flowers with a net, little

B

FlG. 180. — Heterostyled Flowers
of the Oxlip (Primula datior]
in longitudinal section.
(From Vines' " Botany.")

A, long-styled ; B, short-styled flower,
a, anthers ; c, corolla ; f , ovary ;
g, style ; k, calyx ; n, stigma.

FERTILIZATION OF FLOWERS. 357

or no seed was produced. In order to reach the honey, which is
secreted at the bottom of the long tube of the corolla, the bee
thrusts its proboscis down the tube and in so doing brushes past
the stigma and the anthers. If a pin-eyed flower be visited the
pollen from the anthers is deposited comparatively low down on
the proboscis. If a thrum-eyed flower be visited the pollen is
deposited much higher up, in accordance with the more elevated
position of the anthers. Thus the bees carry about two different
kinds of pollen on two different parts of the proboscis, a fact which
has been established by microscopic examination of the proboscis
itself with the attached pollen.

If it be remembered that in the two different kinds of flower
the relative positions of the anthers and stigma are reversed, it
will be obvious that when a bee sucks honey from a long-styled
flower the stigma will be touched and pollinated by that part of
the proboscis which touches the anthers of a short-styled flower,
and vice versa. Hence it follows that pin-eyed flowers will be
fertilized by pollen from thrum-eyed, and thrum-eyed flowers by
pollen from pin-eyed.

Here, then, we have a very precise adaptation for a special kind
of cross-fertilization, and Darwin further proved by his experiments
that this is the only kind of fertilization which results in complete
fertility. Cross-fertilization in these plants is, however, possible
in no less than four distinct ways : — (1) A pin-eyed flower may
be fertilized by pollen from another pin-eyed ; (2) a thrum-eyed
flower may be fertilized by pollen from another thrum- eyed ;
(3) a pin-eyed flower may be fertilized by pollen from a thrum-
eyed ; (4) a thrum-eyed flower may be fertilized by pollen from
a pin-eyed. The first two methods have been termed by Darwin
"illegitimate unions" and they result in incomplete fertility; the
last two have been termed " legitimate " and result in complete
fertility.

The benefit derived from the existence of the two kinds of
flower lies in the intercrossing, not merely of two distinct
flowers, but of two distinct plants, for it will be remembered that
each plant bears only one kind of flower. Self-fertilization is
not absolutely prevented in this case, for the anthers and stigma
are mature at the same time in the same flower, but it is not
likely to take place, and if it does it results in incomplete fertility
and weakly offspring. But even if pollen should accidentally find
its way to a stigma of its own plant it need not necessarily

358 OUTLINES OF EVOLUTIONARY BIOLOGY

prevent cross-fertilization, for foreign pollen will be prepotent
even if deposited on the stigma some time afterwards.

" To test this belief," Darwin observes, " I placed on several
stigmas of a long-styled cowslip plenty of pollen from the same
plant, and after twenty-four hours added some from a short-
styled dark-red polyanthus, which is a variety of the cowslip.
From the flowers thus treated, thirty seedlings were raised, and
all these, without exception, bore reddish flowers, so that the
effect of pollen from the same form, though placed on the stigmas

twenty-four hours pre-
viously, was quite des-
troyed by that of pollen
from a plant belonging to
the other form."

The flowers of the
common sage, and of
other species of the genus
Sal via (Fig. 181), afford a
no less striking example
of profound structural
modification in adapta-
tion to the visits of insects.
These flowers are pro-
tandrous, the stamens
maturing and shedding
their pollen before the
stigma is ready to receive
it, so that self-fertiliza-
tion is absolutely prevented. Moreover, the stigma in young
flowers (Fig. 181, c/r') lies in such a position that it will not
be touched by visiting insects, which crawl right under it in
order to reach the honey at the bottom of the corolla-tube. In
older flowers the style elongates and curves downwards, so that
the stigma (Fig. 181, cjr") comes to lie in the mouth of the
corolla-tube and must be brushed against by insects in search of
honey.

The most remarkable feature of the flower, however, is a
special mechanical contrivance for placing the pollen on the back
of any insect which attempts to suck honey from it in its young
or male condition. There are only two fully developed stamens,
and these are most curiously modified in structure. Each anther

U

FIG. 181.- -Flower of ftalvia^ratensis. (From
Weismanu's " Evolution Theory," after
H. Miiller.)

gr' immature stigma; gr", mature stigma; st',
anther-lobe concealed in the " helmet " ; st",
anther-lobes lowered ; U, lower lip of corolla.

FERTILIZATION OF FLOWERS. 359 -

consists as usual of two anther-lobes united together by a con-
nective ; but the connective is very greatly elongated, so that the
anther- lobes are widely separated from one another instead of
lying close together at the top of the filament. The anther-lobe
at one end of each connective is imperfect and produces little or
no pollen ; the other is fully developed. The connective is
attached to the top of the very short filament by a movable joint,
so that it can be swung freely up and down in a vertical plane.
It has a long limb which curves upwards inside the " helmet " of
the corolla and terminates in the perfect anther-lobe, and a
short one which bears the imperfect lobe, as shown in the figure.
The two stamens lie side by side in the flower and the imperfect
anther-lobes are held downwards in the mouth of the corolla-
tube, exactly in the path of a visiting insect. The perfect anther-
lobes, on the other hand, are, at first, held up well above the
level of the insect's back (Fig. 181, st').

When a large insect, such as a bee, visits the flower, it alights
upon the large lower lip ( [/), which, as in so many other flowers,
affords a convenient platform and has doubtless been specially
adapted to that end. The head of the insect is then thrust into the
corolla-tube and pushes against the two imperfect anther-lobes.
The connectives turn on their pivots like a see-saw and the two
ripe anther-lobes (st") are clapped down on the back of the insect
and dust it with pollen. This happens when the insect visits a
young flower. When an older flower, in the female condition, is
visited, the pollen from the back of the insect is deposited on the
mature stigma (gr"), which now hangs directly in front of the
entrance to the corolla-tube.

It is, however, in the highly specialized order Orchidaceae that
we find perhaps the most remarkable contrivances for ensuring
cross-fertilization that are to be met with in the vegetable
kingdom. Many of these have been described and illustrated by
Darwin in his well-known work on the " Fertilization of Orchids."

In the common early orchis of Europe (Orcliis mascula,
Fig. 182) there are three sepals and three petals, and both sepals
and petals are brightly coloured. The lower petal is very large and
in front forms a tongue-like projection termed the labellum (u\
which serves as a landing platform, while behind it is produced
backwards into a hollow spur or nectary (sp, ri) in which honey
is secreted. The reproductive organs lie just above and partly
in front of the entrance to the nectary, so that an insect, in

360 OUTLINES OF EVOLUTIONARY BIOLOGY

poking its proboscis down to get at the honey, cannot fail to touch
them. Both male and female organs are very much modified in
structure, and united together to form the column (C). There
are three stigmas. Of these, however, only two (B, no) are
functional, and they lie at the back of the entrance to the
nectary. The third is specially modified to form a remarkable
organ known as the rostellum (B, C, »•), which projects above the

FIG. 182. — Adaptation in the Flower of Orchis mascuJa in relation to Insect-
Fertilization. (From Weismann's " Evolution Theory.")

A, side view of flower ; B, front view ; C, vertical section through the column ; J>, pol-
linia removed on the point of a pencil and still standing erect ; E, the same later on,
bent forwards.

ei, entrance to nectary ; n, nectary ; na, stigmatic surface ; p, pollinia ; r, rostellum ;
Sm, honey guide ; sp, spur; st. ovary (twisted) ; U, lower lip of flower (labellum).

other two in front of the mouth of the nectary. This rostellum,
or beak, consists of an exceedingly sticky body covered over by a
thin, membranous cap. The cap is so delicately adjusted that
the slightest touch is sufficient to push it down and expose the
sticky mass beneath, and it is so elastic that when the pressure
is removed it springs back into its original position and again
covers up the sticky substance.

There is only one perfect anther, consisting of two sacs which
stand up above and behind the rostellum. Each sac contains a
single coherent mass of pollen grains instead of the usual loose,

FERTILIZATION OF FLOWERS. 361

powdery pollen. Each mass of pollen grains, or pollinium
(It, C, p), is a pear-shaped body provided \vith a short stalk or
caudicle. The stalk is continued downwards into the rostellum
as shown in C, where it ends in a membranous disc attached
to the sticky substance. When the pollen is ripe the pollinia are
exposed by rupture of the sacs in which they lie, so that in the
mature flower they are only attached by means of their stalks or
caudicles, loosely fixed by the sticky substance of the rostellum.

If we take any slender object, such as a pencil point, and poke
it gently into the mouth of the nectary, we shall find on with-
drawing it again that it will bring with it either one or both of
the pollinia, attached to it by sticky cement. The pencil has, of
course, touched the rostellum and pushed down the little cap
which covers it. The sticky substance of the rostellum, thus
exposed, has cemented on to the pencil the disc at the lower end
of the caudicle, and thus the pollinium itself, or both of them, is
pulled bodily out with the pencil. The cement rapidly hardens
on exposure to the air.

When first attached to the pencil the pollinium is in a nearly
upright position (D), so that if the pencil were inserted into
another flower it would simply go back into its old place, without
touching the stigma and of course, therefore, without effecting
fertilization. After being exposed to the air for a short time,
however, the disc by which the pollinium is attached to the
pencil begins to contract, and in such a manner that the polli-
nium is pulled down until it comes to project straight forwards
(E). If now the pencil be inserted into a flower it will be found
that the pollinium exactly strikes against one of the stigmas and
dusts it with pollen.

In the economy of nature the proboscis of some insect takes the
place of our experimental pencil. Bees and moths have fre-
quently been observed with the pollinia of various species of
Orchis attached to them ; indeed Darwin figures one instance in
which no less than fourteen pollinia are attached to the proboscis
of a moth. Every time such an insect visits a flower in search
of honey it will effect cross-fertilization by pushing the pollinia
which it brings with it against the stigmas, and may perhaps
carry off another pollinium into the bargain.

In this case one can hardly fail to be astonished at the number
and complexity of the adaptations which have arisen in the
flower for the purpose of ensuring cross-fertilization. There is

362 OUTLINES OF EVOLUTIONARY BIOLOGY

the conspicuous colour of the sepals and petals, which serves as
an advertisement ; the formation of the long spur or nectary, with
its secreted honey ; the great development of the labelluni to
serve as a landing place, marked with the "honey-guide " (Sni)
to point out the way to the visitor ; the secretion of sticky
cement by the rostellum ; the formation of an elastic cap to keep
the cement from being dried up before it is wanted ; the agglu-
tination of the pollen into solid masses, which serve to fertilize
a large number of flowers in succession, losing a few grains at
each contact ; the remarkable form of these pollinia, with their
adhesive discs and their peculiar relation to the sticky substance
on the rostellum ; and, lastly, the wonderful mechanism by means
of which the pollinia become bent downwards after their removal,
so as exactly to adjust them for contact with the stigma of
another flower !

In some species of plants certain parts of the flower are highly
sensitive and respond to the slightest touch by a sudden and
vigorous movement, going off, so to speak, like a spring trap or
a hair trigger. This is well exemplified in a New Zealand orchid,
Pterostylis trulli folia, as described by Mr. Cheeseman. In this
flower the lower petal, or labelluni, is highly sensitive, and when
an insect alights upon it springs up and imprisons the visitor
in a cage. The parts of the flower are so arranged that the
insect can only escape by crawling through a narrow passage,
and in such a manner that it must carry away the pollinia and
also leave on the stigma some of any pollen which it may chance
to bring with it.

A still more remarkable orchid, Catasetum, actually throws the
pollinia at its insect visitors as soon as they touch the flower,
and the tiny projectiles attach themselves to the intruder's head.
Darwin has described how, on one occasion, when he experi-
mentally irritated this flower, the pollinium was thrown for a
distance of nearly three feet, when it stuck on to a window
pane.

The presence of irritable structures which aid in cross-
fertilization by insect agenc}T is, however, by no means confined
to orchids. Candollea (Stylidium) graminifolia is a common
Australian wild flower belonging to a totally different order, the
Candolleacese. It is found growing on open heaths and sends
up tall stems from the midst of a tuft of long, grass-like leaves,
each stem bearing a large number of rather small pink flowers

FERTILIZATION OF FLOWERS. 363

along its length. There are five petals, but only four are fully
developed, and these spread themselves out in the form of a
cross around the mouth of the tube in which the honey is
secreted. The anthers and stigma are borne on the end of a
long slender column which springs from the middle of the flower.
Close to its base the column is bent downwards at a sharp angle
so that it hangs out at one side of the flower, between two of
the fully developed petals and resting upon the aborted petal as
on a cushion. Near its apex the column is again bent at a sharp
angle, this time upwards. The flowers are protandrous, the
anthers becoming mature and shedding their pollen before the
stigma is fully developed.

An insect, visiting the flower and poking its proboscis down
the tube of the corolla in search of honey, must touch the column
at the first bend. This is the irritable spot, and no sooner is it
touched than the column springs over to the other side of the
flower and brings the anthers or stigma, as the case may be,
down on the back of its visitor. In tbis way pollen is deposited
upon or. removed from the insect's back and cross-fertilization is
effected. One of the most curious things about this case is that
the column loses its irritability at nightfall and refuses to jump,
however much you may tickle it.

The few examples of entomophilous flowers which we have now
studied must suffice to give some idea of how wonderfully minute
and perfect may be the adaptation of plants to highly specialized
environmental conditions. The dominating factor of the envi-
ronment here is, of course, the presence of insects, which can be
pressed into service as pollen carriers, and it is obviously in
relation to the requirements and tastes of these insects that the
various adaptations which we have been describing have arisen.
There can be little doubt that the insects select for their visits
those flowers which please them best and which are most readily
recognizable as the bearers of the coveted honey.

Although, as we have previously pointed out, some carrion-
feeding flies prefer scents which are repulsive to ourselves, yet
the vast majority of insect-fertilized flowers have scents and
colours which we appreciate perhaps no less than the insects for
whose gratification they primarily exist. As we shall see later on,
these scents and colours, though now in many cases improved
by human selection, doubtless arose in the first instance in
response to insect selection. If we pursue this line of thought a

364 OUTLINES OF E VOLUTION AEY BIOLOGY

little further, there seems good reason to suspect that man, who
appeared upon the scene at a very much later date and whose
ideas of what is beautiful are doubtless largely derived from the
contemplation of flowers, may owe much of his aesthetic develop-
ment to the fact that he has been educated by flower-loving
insects.

PAKT V.— FACTORS OF ORGANIC EVOLUTION
CHAPTER XXIV

Views of Buffon, Erasmus Darwin and Lamarck.

WE shall have occasion to point out in a subsequent chapter
that many organisms exhibit characters which it is extremely
difficult, if not impossible, to bring into any direct relation-
ship with the environment, but this fact does not invalidate
the generalization that the most striking feature of all living
things is adaptation to the conditions under which they have to
carry on their existence. In seeking for an explanation of the
means whereby organic evolution has been effected, this fact
must constantly be borne in mind, and, as we have already said,
no theory can be considered adequate which does not take
fully into account the phenomena of adaptation, and offer some
reasonable explanation of the wonderful harmony which exists
between living things and their surroundings.

We have said before that the theory of organic evolution is no
new thing, but can be traced back even to the ancient Greek
philosophers. In the middle ages such ideas were thrust into
the background, along with other fruits of Greek and Roman
intellectual activity, and to a large extent supplanted by the
teachings of dogmatic theology. With the revival of scientific
inquiry, however, the theory of organic evolution, as opposed to
the doctrine of special creation and the immutability of species,
again began to occupy a prominent place in the minds of
thinking men, and a brief consideration of the views of some of
the chief philosophical biologists of the last two centuries will
perhaps form the most fitting introduction to this part of our
subject.

The celebrated French naturalist, Buffon (1707 — 1788), who
held the post of Superintendent of the Jardin des Plantes and,
in conjunction with his colleagues, published a large number of
volumes of Natural History, was one of those who had at any

366 OUTLINES OF E VOLUTION AEY BIOLOGY

rate strong leanings towards the theory of organic evolution,
although, unfortunately, he seems to have found it very difficult
to give a frank expression to his views. The following trans-
lated extracts will serve to illustrate his position.

Speaking of the arbitrary character of our systems of classifi-
cation he observes : —

" But Nature proceeds by unrecognized gradations and con-
sequently she cannot lend herself completely to these divisions,
since she passes from one species to another species, and often
from one genus to another genus, by imperceptible shades : so
that we find a great number of intermediate species and objects
which we do not know where to place, and which necessarily
upset the plan of the general system." l

Buffon certainly appears in this passage as no believer in the
immutability of species, and amongst the causes which bring
about their modification he attributes great importance to the
action of climate: —

"If we again consider each species in different climates, we
shall find obvious varieties both as regards size and form ; all
are influenced more or less strongly by the climate. These
changes only take place slowly and imperceptibly ; the great
workman of Nature is Time : he walks always with even strides,
uniform and regular, he does nothing by leaps ; but by degrees,
by gradations, by succession, he does everything; and these
changes, at first imperceptible, little by little become evident,
and express themselves at length in results about which we
cannot be mistaken." 2

In dealing with the animals of the old and the new worlds, and
after speaking of the extinction of the mammoth, he says : —

" This species was certainly the foremost, the largest and the
strongest of all the quadrupeds : inasmuch as it has disappeared,
how many other smaller ones, weaker and less remarkable, have
had to succumb also, without having left us either witness or
evidence of their past existence ? How many other species, having
become modified in their nature, that is to say, perfected or
degraded by the great vicissitudes of land and sea, by the neglect
or the culture of Nature, by the long influence of a climate
become contrary or favourable, are no longer the same that they
formerly were ? And, moreover, the quadrupeds are, next to
man, the beings whose nature is the most fixed and whose form
is the most constant: that of the birds and of the fishes varies

1 Buffon, " Histoire Naturelle," Tom. I, p. 13.

2 Op. tit., Tom. VI, pp. 59, 60.

VIEWS OF BUFFON. 367

more ; that of the insects still more, and if we descend to the
plants, which we must not exclude from animated nature, we
shall be surprised at the promptitude with which the species
vary, and at the facility with which they change their nature
while taking on new forms.

" It would not be impossible then, that, even without reversing
the order of Nature, all these animals of the new world may have
been originally the same as those of the old, from which they
may have been formerly derived ; one might say that having
been separated subsequently by immense seas or impassable
lands, they would, in the course of time, have received all the
impressions, suffered all the effects, of a climate itself altered in
character by the same causes which brought about the separa-
tion ; that in consequence they would in time have become
dwarfed, changed their nature, &c. But that should not prevent
us from regarding them to-day as animals of different species :
whatever may be the cause of this difference, whether it has been
produced by time, climate and country, or whether it be of the
same date as the creation, it is none the less real. Nature, I
admit, is in a continual state of flux ; but it is enough for man
to seize her as she is in his own time, and to glance backwards
and forwards in endeavouring to gain some glimpse of what she
may have been in former times and of what she may become in
the future." l

This passage also seems to be a sufficiently clear declaration
in favour of the theory of organic evolution and the action of
the environment in modifying species. Buffon, however, by no
means confined himself to the consideration of climate as a
factor in the production of such modifications. The following
quotation shows that he also realized the importance of the
principles of use and disuse and the inheritance of acquired
characters, which were destined to take such a prominent place
in the subsequent writings of Lamarck : —

" The llama, which, like the camel, passes its life in bearing
burdens and only rests by lying down upon its breast, has similar
callosities which are perpetuated in the same way by generation.
The baboons and the apes, whose most usual attitude, whether
awake or asleep, is sitting, have also callosities beneath the region
of the buttocks, and this callous skin has even become adherent
to the bones against which it is continually pressed by the weight
of the body ; but these callosities of the baboons and apes are dry
and healthy, because they do not arise from the constraint of
trammels or of the weight of a foreign burden, and because they

1 Op. clt., Tom. IX, pp. 126, 127.

368 OUTLINES OF EVOLUTIONARY BIOLOGY

are merely the effects of the natural habits of the animal, which
remains seated more willingly and for a longer time than in any
other posture : it is the same with these callosities of the apes as
with the double sole of skin which we carry beneath our feet ;
this sole is a natural callosity which our constant habit of walking
or resting upright renders more or less thick, or more or less
hard, according to the amount of friction to which the soles of
our feet are exposed." l

It has been maintained that Buffon not only anticipated
Lamarck's views as to the influence of the environment and the
principle of use and disuse, but also those of Malthus and
Charles Darwin with regard to the importance of the struggle for
existence and the process of natural selection. That some such
ideas were present in his mind seems sufficiently clear from the
following passages. After speaking of the invasions of the Huns
and Goths and other peoples he continues : —

" These great events, these conspicuous epochs in the history of
the human race, are, however, only trifling vicissitudes in the
ordinary course of living nature ; it is in general always constant,
always the same ; its movement, always regular, turns on two
fixed pivots, the one the unlimited fecundity given to all species,
the other the innumerable obstacles which reduce the product of
this fecundity to a fixed quantity and at all times leave only
approximately the same number of individuals in each species."

" The causes of destruction, of annihilation and sterility follow
immediately upon those of excessive multiplication ; and, inde-
pendently of contagion, the necessary consequence of too great
an accumulation of any living matter in one place, there are in
each species special causes of death and destruction, which we
shall indicate in the sequel and which alone suffice to compensate
for the excess of former generations." 3

" The least perfect species, the most delicate, the most heavily
burdened, the least active, the least well armed, &c., have already
disappeared or will disappear." 4

Even if, on the strength of this last passage, however, we can
claim that Buffon had conceived the idea of the survival of the

1 Op. tit., Tom. XIV, pp. 325, 326.

2 Op. cit., Tom. VI, p. 248.

3 Ibid., p. 251.

4 I tianslate this passage, which I have not found in the original, from a quotation
given by Osborn in his interesting work " From the Greeks to Darwin " (Columbia
University Biological Series). The student should refer to this work for the history
of the theory of evolution.

VIEWS OF BUFFON 369

fittest, the mere question of priority is a matter of small moment.
As a matter of fact it is now well known that this idea is very
much older than Buffon, and can be traced back, as Osborn
remarks, " to Empedocles, six centuries before Christ." l
Osborn also points out that : —

" Buffon 's ideas regarding the physical basis of heredity are
very similar to those of Democritus, and certainly contain the
basis of the conception of the Pangenesis theory of Darwin, for
he supposes that the elements of the germ-cells were gathered
from all parts of the body." 2

So far we have presented the views of Buffon as those
of a thoroughgoing evolutionist. He had, however, apparently
another side to his mind, which is extremely difficult to under-
stand in the author of the foregoing passages. In the fourth
volume of his Natural History, after discussing the possible
modification of species, he continues : —

" But no, it is certain, by revelation, that all animals have
participated equally in the grace of creation, that the two first
of each species and of all the species came forth complete from
the hands of the Creator ; and we must believe that they were
then much the same as they are now represented to us by their
descendants."3

The doctrine of special creation could hardly be more clearly
expressed.

Of course it is possible, as Samuel Butler suggests in his
interesting discussion of Buffon in " Evolution Old and New,"
that such passages as this may be ironical, but it seems more
likely that Buffon vacillated between what was then regarded as
religious orthodoxy and the more rational views which he knew
so well how to express. Indeed, Butler himself remarks, apropos
of another passage : —

" This is Buffon's way. Whenever he has shown us clearly
what we ought to think, he stops short suddenly on religious
grounds." 4

1 Op. cit., p. 117.

2 Ibid., p. 135.

3 " Histoire Naturelle," Tom. IV, p. 383.

4 " Evolution Old and New " (New Issue, London, A. & C. Fifield), p. 115. This
work contains numerous passages translated from Buffon and Lamarck, and I
•have found it of great use as a guide to the more salient passages in the voluminous
writings of the former. I have, however, in all cases made fresh translations from
the French.

B. B B

370 OUTLINES OF EVOLUTIONARY BIOLOGY

Buffon's inability to reconcile the logical consequences of his
own inductions with his religious convictions is perhaps nowhere
better illustrated than in the attitude which he adopted with
regard to the position of man in the animal kingdom. Such an
experienced observer as he was could not fail to realize the close
agreement in structure between man and the higher apes : —

" We shall see, in the history of the orang utan that, were we to
pay attention only to the form, we might equally well look upon
this animal either as the highest of the apes or as the lowest
of mankind ; because, with the exception of the soul, he lacks
nothing of all that we possess, and because he differs less from
man in bodily structure than he differs from other animals to
which the name of ape has been given." *

" I admit that, if one should judge only by form, the ape species
might be taken for a variety of the human species : the Creator
did not think fit to make for the human body a model absolutely
different from that of the animal ; He has included his form, like
that of all the animals, in one general plan ; but at the same
time that He bestowed upon him this ape-like material form, He
penetrated this animal body with His divine breath."2

The ape, on the other hand, is a mere animal, and

" in spite of his resemblance to man, far from being the second
in our species, he is not the first in the order of animals, since
he is not the most intelligent." 3

There can be no doubt that the views of Erasmus Darwin
(1731 — 1802) were largely influenced by the writings of Buffon, to
which he repeatedly refers. Darwin's "Zoonomia" was published
in 1794,4 but, perhaps because it was mainly a medical work, it
received but little attention from professional naturalists. The
section dealing with " Generation" contains his views on evolu-
tion. A few quotations will suffice to illustrate these :—

" Owing to the imperfection of language the offspring is termed
a new animal, but is in truth a branch or elongation of the
parent ; since a part of the embryon-animal is, or was, a part of
the parent ; and therefore in strict language it cannot be said to

1 " Histoire Naturelle," Tom. XIV, p. 30. The name "orang utan " was applied
by BuflFon to the chimpanzee.

2 Ibid., p. 32.
8 Ibid., p. 37.

4 My quotations are taken from an edition published by B. Dugdale at Dublin
iu 1800.

VIEWS OF EKASMUS DARWIN 371

be entirely new at the time of its production ; and therefore it
may retain some of the habits of the parent-system."

Here we have clearly expressed the idea of continuity which
plays such an important part in modern theories of heredity.
Erasmus Darwin was, however, a " Spermatist," that is to say
he believed

" that the embryon is produced solely by the male, and that the
female supplies it with a proper nidus, with sustenance, and
with oxygenation; and that the idea of the semen of the male
constituting only a stimulus to the egg of the female, exciting it
into life, (as held by some philosophers) has no support from
experiment or analogy."

It must be remembered that he wrote in the days before the
cell theory had shed its illuminating rays over the science of
embryology : —

"I conceive," says he, "the primordium, or rudiment of the
ernbryon, as secreted from the blood of the parent, to consist of a
simple filament as a muscular fibre,"

but this filament was not necessarily thread-like in form, for
he adds : —

" I suppose this living filament, of whatever form it may be,
whether sphere, cube, or cylinder, to be endued with the capability
of being excited into action by certain kinds of stimulus."

It thus absorbs nutriment and becomes organized by "accre-
tion of parts," and this leads to the development of new kinds
of " irritability." According to this view the appearance of a
new organ precedes its use : —

" the lungs must be previously formed before their exertions to
obtain fresh air can exist."

" From hence I conclude, that with the acquisition of new
parts, new sensations, and new desires, as well as new powers, are
produced ; and this by accretion to the old ones, and not by
distention of them."

But the exercise of these new powers in turn gives rise to the
development of more new parts, which

" are formed by the irritations and sensations, and consequent
exertions of the parts previously existing, and to which the new
parts are to be attached."

" From this account of reproduction it appears, that all animals
have a similar origin, viz. from a single living filament ; and

B B 2

372 OUTLINES OF EVOLUTIONARY BIOLOGY

that the difference of their forms and qualities has arisen only
from the different irritabilities and sensibilities, or voluntarities,
or associabilities, of this original living filament ; and perhaps in
some degree from the different forms of the particles of the fluids,
by which it has been at first stimulated into activity."

"From their first rudiment, or priniordium, to the termina-
tion of their lives, all animals undergo perpetual transformations;
which are in part produced by their own exertions inconsequence
of their desires and aversions, of their pleasures and their pains,
or of irritations, or of associations ; and many of these acquired
forms or propensities are transmitted to their posterity."

Dr. Darwin thus passes from the discussion of what is now
termed the ontogeny or development of the individual to that of
the phylogeny or development of the race. From consideration
of the former he endeavoured to gain some insight into the latter,
and it may be fairly claimed that he thus anticipated what is
known in modern biology as the Recapitulation Hypothesis :—

" From thus meditating on the great similarity of the struc-
ture of the warm-blooded animals, and at the same time of the
great changes they undergo both before and after their nativity ;
and by considering in how minute a portion of time many of
the changes of animals above described have been produced ;
would it be too bold to imagine, that in the great length of time,
since the earth began to exist, perhaps millions of ages before
the commencement of the history of mankind, would it be too
bold to imagine, that all warm-blooded animals have arisen from
one living filament, which the great First Cause endued with ani-
mality, with the power of acquiring new parts, attended with
new propensities, directed by irritations, sensations, volitions
and associations ; and thus possessing the faculty of continuing
to improve by its own inherent activity, and of delivering down
those improvements by generation to its posterity, world without
end ! "

Erasmus Darwin was perfectly familiar with the idea of
adaptation, as manifested, for example, in the colours of
animals :—

" The colours of many animals seem adapted to their purposes
of concealing themselves either to avoid danger, or to spring upon
their prey. Thus the snake and wild cat, and leopard, are so
coloured as to resemble dark leaves and their lighter interstices ;
birds resemble the colour of the brown ground, or the green

VIEWS OF EKASMUS DAEWIN 373

hedges, which they frequent; and moths and butterflies are
coloured like the flowers which they rob of their honey."

" A proboscis of admirable structure has been acquired by the
bee, the moth, and the humming bird, for the purpose of
plundering the nectaries of flowers. All which seem to have
been formed by the original living filament, excited into action
by the necessities of the creatures, which possess them, and on
which their existence depends."

In the following sentences we have at any rate a very close
approach to the idea of natural selection which forms the key-
note of the evolutionary theory as advocated by the illustrious
grandson of the writer :—

" A great want of one part of the animal world has consisted
in the desire of the exclusive possession of the females ; and these
have acquired weapons to combat each other for this purpose. . . .
So the horns of the stag are sharp to offend his adversary, but
are branched for the purpose of parrying or receiving the thrusts
of horns similar to his own, and have therefore been formed for
the purpose of combating other stags for the exclusive posses-
sion of the females ; who are observed, like the ladies in the
times of chivalry, to attend the car of the victor."

" The final cause of this contest amongst the males seems to be,
that the strongest and most active animal should propagate the
species, which should thence become improved."

Here the principle of selection seems to be clearly enough
recognized, at any rate that form thereof which Charles Darwin
afterwards distinguished as sexual selection.

The great French philosophical biologist,1 Jean Baptiste
Pierre Antoine de Monet, Chevalier de Lamarck, was born in
1744 and died in 1829. His most celebrated work, the
" Philosophie Zoologique," which contains the fullest expression
of his mature views on the theory of organic evolution, was
published in 1809, but these views appear to have been first
announced to the world in the opening lecture of the Course of
Zoology given at the Natural History Museum in Paris in 1800,

1 Lamarck was the originator of the term Biology for the Science of Living
Things.

and published in the following year in the form of a preface to
the " Systeme des Animaux sans Vertebres."1

It seems impossible to doubt that Lamarck, like Erasmus
Darwin, was largely influenced in his views by the writings of
his great compatriot Buffon, with whom he was on terms of
personal friendship, and to whom he refers in his published
work. Whether or not he was acquainted with the works of
Erasmus Darwin will probably never be known, but it is evident
that both drew part at any rate of their inspiration from the
same source. Owing largely to his official position as Professor
of Invertebrate Zoology in the National Museum, Lamarck was
able to bring forward a very imposing array of facts in support
of his opinions, and though these opinions were but the natural
development of those enunciated by his predecessors, he was able
to place the theory of organic evolution on a much firmer and
broader basis than it had previously enjoyed. He also doubtless
derived great advantage from the fact that he was an experienced
botanist as well as a zoologist, having published many important
botanical works before he turned his attention more particularly
to the zoological aspect of biology.

During the earlier part of his life Lamarck appears to have
accepted the still prevalent doctrine as to the immutability of
species, and it is perhaps significant that his conversion to
evolutionary views seems to have followed very rapidly upon the
extension of his investigations from the vegetable to the animal
kingdom.

In the " Philosophie Zoologique " he maintains that the first
living things arose by a process of spontaneous generation, which
may still take place, and that from the starting points thus pro-
vided the entire animal and vegetable kingdoms as they now exist
have arisen as the result of orderly and progressive evolution.

He devotes a large amount of space to the question of classifi-
cation and the conception of species, and arrives at the following
conclusions, which are, on the whole, in singularly close agreement
with those of modern biologists : —

" But these classifications, of which several have been so happily
imagined by naturalists, and the divisions and sub-divisions

1 Vide Packard's " Lamarck, the Founder of Evolution, his Life and Work "
(Longmans, Green & Co., 1901). This most interesting work contains translations of
portions of Lamarck's writings, and has for the first time made the work of the great
French philosopher available to the general reader in England and America. I have,
however, thought it desirable to give fresh translations from the original French.

VIEWS OF LAMARCK 375

which they present, are entirely artificial contrivances. Nothing
of all that, I repeat, occurs in nature, in spite of the foundation
which appears to be given to them by certain portions of the
natural series which are known to us, and which have the
appearance of being isolated. We may be certain that amongst
her productions, nature has really formed neither classes, nor
orders, nor families, nor genera, nor constant species, but only
individuals which succeed one another and which resemble
those which produced them. Now these individuals belong to
infinitely diversified races, which shade off under all forms and
in all degrees of organization, and each of which maintains
itself without change so long as no cause of change acts
upon it." l

" The name species has been given to every collection of
similar individuals which have been produced by other individuals
like themselves.

"This definition is exact; for every individual that enjoys
life always resembles very closely that or those from which it
sprang. But to this definition has been added the supposition
that the individuals which make up a species never vary in
their specific character, and that consequently the species has
an absolute constancy in nature.

" It is exactly this supposition that I propose to combat,
because the clear evidence obtained by observation shews that
it is unfounded.

" The supposition, almost universally admitted, that living
bodies form 82)e,cies constantly distinguished by invariable
characters, and that the existence of these species is as old as
that of nature herself, was established at a time when obser-
vations were insufficient, and when the natural sciences were
still almost non-existent. It is always contradicted in the eyes
of those who have seen much, who have for a long time followed
nature, and who have profitably consulted the great and rich
collections of our Museum."2

After speaking of the doctrine of special creation Lamarck
continues : —

"Without doubt, nothing exists except by the will of the
sublime Author of all things. But can we assign to Him laws
in the execution of His will, and fix the method which He has
followed in this respect ? Has not His infinite power been able
to create an order of things which should give existence

1 " Philosophic Zoologique," Tom. I, pp. 21, 22.

2 Op. cit., Tom. I, pp. 54, 55.

376 OUTLINES OF E VOLUTION AKY BIOLOGY

successively to everything that we see, as to everything which
exists and which is unknown to us ?

" Assuredly, whatever may have been His will, the immensity
of His power is always the same ; and in whatever manner this
supreme will may have been executed, nothing can diminish
its grandeur."1

Lamarck seems to have been the first to insist upon the
branching character of evolutionary series ; after speaking
of such series he goes on : —

" I do not wish to say thereby that existing animals form a
very simple series, everywhere equally graduated ; but I say
that they form a branching series, irregularly graduated, and
which has no discontinuity in its parts, or which, at least, has
not always had, if it be true that, in consequence of some species
having been lost, such discontinuity occurs anywhere. It
results from this that the species which terminate each branch
of the general series are connected, at least on one side, with
other neighbouring species which shade into them."2

Like Buffon, he lays great stress upon the action of the
environment in modifying organisms : —

" Many facts teach us that in proportion as the individuals of
one of our species change their situation, their climate, their
manner of living or their habits, they thereby receive influences
which little by little change the consistency and the proportions
of their parts, their form, their faculties, even their organization ;
so that everything in them participates, in the course of time, in
the transformations which they experience.

" In the same climate, very different situations and exposures
at first cause the individuals exposed thereto simply to vary ;
but, in the course of time, the continual difference in situation of
the individuals of which I am speaking, which live and reproduce
themselves successively in the same circumstances, causes in
them differences which become, in some way, essential to their
existence ; so that, after many generations have succeeded one
another, these individuals, which belonged originally to another
species, find themselves in the end transformed into a new species,
distinct from the other.

" For example, if the seeds of a grass, or of any other plant
natural to a damp meadow, be transported by any chance, first
to the slope of a neighbouring hill, where the soil, although more
elevated, is still sufficiently cool to permit of the plant main-
taining itself, and if afterwards, after having lived there and

1.Op. cit., Tom. I, pp. 56, 57.
2 Ibid., p. 59.

VIEWS OF LAMARCK 377

reproduced itself many times, it reaches, by slow degrees, the
dry and almost arid soil of a mountainous region ; if the plant
succeeds in living there, and perpetuates itself for a succession
of generations, it will then become so changed that the botanists
who come across it will make of it a distinct species." l

As Lamarck's views have so frequently been misrepresented it
is incumbent upon us to make ourselves thoroughly acquainted
with what he really meant by the action of the environment, and
on this point, fortunately, he is very precise :—

" Here it becomes necessary for me to explain the meaning
which I attach to these expressions : The environment (les
circonstanccs) influences the form and organization of animals, or
in other words, as it becomes very different it changes, in course
of time, the form and organization themselves by proportional
modifications.

" Certainly, if people took these expressions literally they
would attribute to me an error ; for, whatever the environment
may be, it does not directly effect any modification whatever in
the form and organization of animals.

" But great changes in the environment lead, in the case of
animals, to great changes in their requirements (besoins), and
such changes in their requirements lead necessarily to actions.
Now, if the new requirements become constant or very lasting,
the animals then adopt new habits, which are as lasting as the
requirements which have called them forth. . . .

" Now, if a novel environment, having become permanent for
a race of animals, has given to these animals new habits, or in
other words has led them to new actions which have become
habitual, there will have resulted therefrom the use of some
particular part [of the body] in preference to some other, and in
certain cases the complete disuse of some part which has become
useless.

"None of these statements should be considered as hypo-
thetical or as the expression of individual opinion ; they are, on
the contrary, truths which need only attention and the observa-
tion of facts to render them evident.

" We shall see immediately, by the citation of known facts
which attest it, on the one hand that new requirements having
made some part necessary, have really, by a series of efforts,
caused this part to arise, and that afterwards its continued
employment has little by little strengthened it, developed it, and
in the end considerably enlarged it ; on the other hand, we shall
see that, in certain cases, the novel environment and new
requirements having rendered some part quite useless, the

1 Op. cit., Tom. I, pp. 62, 63.

378 OUTLINES OF EVOLUTIONARY BIOLOGY

complete want of employment of this part has caused it to
gradually cease from developing like the other parts of the
animal ; that it has become reduced and attenuated little by
little, and that at length, when this want of employment has
been complete during a long period, the part in question has in
the end disappeared. All this is positive ; I propose to give the
most convincing proofs of it.

" In plants, where there are no actions, and consequently no
habits properly so-called, great changes in the environment have
none the less led to great differences in the development of their
parts ; in such a way that these differences have caused certain
of them to appear and develop, while they have caused many
others to dwindle away and disappear. But here everything is
effected by changes which take place in the nutrition of the
plant, in its absorptions and transpirations, in the amount of
heat, light, air and moisture, which it then habitually receives ;
finally, in the superiority which certain of the various vital
movements may acquire over others."1

Like his predecessors, and like those who followed him,
Lamarck adduces in support of these views the remarkable
modifications which have taken place in animals and plants
under the influence of domestication : —

" That which nature does with the aid of much time, we do
every day by suddenly changing, in relation to some living plant,
the conditions under which it and all the individuals of its species,
have existed.

" All botanists know that the plants which they transport from
their native place in order to cultivate them in gardens, undergo,
little by little, changes which, in the end, make them unrecogniz-
able. . . .

"Is not the cultivated wheat (Triticum sativwn) a plant
brought by man to the condition in which we now actually see
it? Who will tell me in what country such a plant occurs
naturally, that is to say except as the result of its cultivation in
some neighbouring place ?

" Where do we find, in a state of nature, our cabbages, our
lettuces, &c., as we now possess them in our kitchen gardens ?
Is it not the same with respect to the many animals which
domestication has changed or considerably modified?"

We may next consider two examples of the kind of evidence
which Lamarck brings forward in proof of the effects of the

1 Op. cit., Tom. I, pp. 221—223.

2 Ibid., pp. 226—227.

VIEWS OF LAMAECK 379

environment and of the use and disuse of organs upon plants
and animals living in a state of nature : —

" The following fact proves, with regard to plants, how much
the alteration of any important factor in the environment
modifies the parts of these living bodies.

" So long as Ranunculus aquatilis is sunk beneath the surface
of the water, its leaves are all finely divided and have their
divisions hair-like ; but when the stems of this plant reach the
surface of the water, the leaves which develop in the air
become broadened, rounded and simply lobed. If some runners
of the same plant succeed in pushing their way into a soil which
is merely damp, without being inundated, their stems are then
short, and none of their leaves are divided into hair-like
segments ; thus arises Ranunculus hcderaceus, which botanists
regard as a species when they come across it." 1

"With regard to habits, it is curious to observe the result
thereof in the remarkable form and in the stature of the giraffe
(Camclo-pardalis) : we know that this animal, the tallest of the
mammals, inhabits the interior of Africa, and that it lives in
places where the earth, almost always arid and without herbage,
obliges it to browse on the foliage of trees and to make con-
tinual efforts to reach it. As a result of this habit, maintained
for a long time in all the individuals of its race, the fore limbs
have become much longer than the hind ones, and the neck has
become so much elongated that the giraffe, without standing
up on its hind legs, raises its head and reaches a height of
six metres (nearly twenty feet)." 2

The following passage must suffice to give some idea of
Lamarck's views on the inheritance of " acquired " characters,
which his theory necessarily implies, though not in the
exaggerated sense of some modern writers : —

" These familiar facts are surely well suited to prove what is
the result of the habitual use by animals of some particular
organ or part ; and if, when we observe, in an animal, an organ
specially developed, strong and powerful, anyone pretends that
its habitual exercise has caused it to gain nothing, that its
continued disuse would cause it to lose nothing, and that, in
short, this organ has always been as it is since the creation of
the species to which the animal belongs, I would ask why our
domestic ducks can no longer fly like wild ducks ; in a word, I
would cite a multitude of examples relative to ourselves, which
bear witness to the differences which result in our own bodies

1 Op. tit., Tom. I, p. 230.

2 Ibid., pp. 256—257.

380 OUTLINES OF E VOLUTION AEY BIOLOGY

from the exercise or the want of exercise of any of our organs,
although these differences are not maintained in the individuals
of the next generation, for then their results would be much
more considerable.

" I shall show in the second part that when the will determines
an animal to some action, the organs which have to execute this
action are forthwith stimulated by the affluence of subtle fluids
(the nervous fluid) which become the determining cause of the
movements which the action in question requires. A multitude
of observations establishes this fact, which should no longer be
called in question.

" It results therefrom that numerous repetitions of these acts
of organization strengthen, extend, develop and even create
the organs which are needed. It is only necessary to observe
attentively what is happening everywhere in this respect to
convince oneself of the actuality of this cause of organic
development and modification.

" Now, every change acquired in an organ by a habit of use
sufficient to have produced it, is maintained afterwards by
generation, if it is common to the individuals which have united
in the act of fecundation for the reproduction of their species.
In short, this modification is propagated, and thus passes to all
the individuals which follow and which are subjected to the
same environment, without their being obliged to acquire it by
the means which really created it.

" The mingling in reproductive unions, however, between
individuals which have different qualities or forms, is necessarily
opposed to the constant propagation of those qualities and forms.
This is the reason why in man, who is subjected to so many
different modifying circumstances, the accidental qualities or
defects which he has chanced to acquire are not preserved and
propagated by generation. If, when any peculiarities of form or
any defects have been acquired, two individuals in this condition
should always unite, they would reproduce the same peculiarities,
and successive generations confining themselves to similar
unions, a special and distinct race would then be formed. But
the perpetual mingling between individuals which have not the
same peculiarities of form causes all peculiarities acquired as the
result of peculiar circumstances of the environment to disappear.
Whence one may be certain that if human beings were not
separated by the distances of their habitations, the mixed breed-
ing would cause the general characters which distinguish the
different nations to disappear." l

In the light of modern knowledge these views on the subject
of heredity are, of course, crude and inaccurate enough, but there

1 Op. cit., Tom. I, pp. 259—262.

VIEWS OF LAMARCK 381

is nothing absurd in them, and at the time when Lamarck wrote
it would scarcely have been possible to formulate anything
better.

It is, however, this assumption of the inheritance of somato-
genic characters that has probably done more than anything else
to prevent many modern biologists from accepting the so-called
Lamarckian factors of evolution. Those who hold with Weis-
mann that there is no possible mechanism by which a somato-
genic character can be converted into a blastogenic one are
forced to reject Lamarck's teaching, but the Weismannian
assumption that there is no such possibility of inheritance of
somatogenic characters rests upon no better foundation than the
Lamarckian assumption that there is. We have dealt with this
question at some length in an earlier chapter and need only now
remember that, if there* are many modern biologists who reject
the Lamarckian factors because they cannot reconcile them with
Weismannism, there are probably quite as many others who
accept them because they appeal irresistibly to their common
sense, and because they refuse to believe that Weismann's
difficulties are really insurmountable.

It is, of course, easy to ridicule Lamarck's views, and to say,
for example, that he maintained that an animal could develop
an organ by simply wishing for it, and ridicule of this kind has
undoubtedly done much to hinder the due appreciation of his
work. Such statements, however, are only made by those who
have never paid adequate attention to the writings of the great
French biologist.

It is evident from the passage last quoted that Lamarck also
did not fail to perceive the importance of isolation as a factor in
organic evolution, necessary to prevent the swampin effects of
intercrossing upon newly arisen species or varieties.

In another place he discusses the influence of the struggle for
existence in counteracting the effects of excessive multiplication,
and in so doing just misses the idea of Natural Selection :

" Animals eat one another, except those which live only upon
plants ; but the latter are liable to be devoured by carnivores.

" We know that it is the stronger and the better armed which
eat the weaker, and that the large species devour the smaller
ones. Nevertheless the individuals of one and the same race
rarely eat each other ; they make war on other races." l

1 Op. cit., Tom. I, p. 99.

382 OUTLINES OF EVOLUTIONARY BIOLOGY

Lamarck summed up his views as to the factors which have
co-operated in organic evolution, at any rate so far as the animal
kingdom is concerned, in a later work, the " Histoire Naturelle
des Animaux sans Vertebres," published in 1815, in the form of
four " Laws," which may be taken as replacing the two " Laws "
of the " Philosophie Zoologique." They are as follows :—

" First Law : Life, by her own forces, tends continually to
increase the volume of every body which possesses it, and to
extend the dimensions of its parts, up to a limit which she
herself imposes."1

" Second Law : The production of a new organ in an animal
body results from a new requirement which continues to make
itself felt, and from a new movement which this requirement
begets and maintains."2

" Third Law : The development and efficiency of organs are
constantly in proportion to the use of these organs,"3

" Fourth Law : All that has been acquired, traced out or
altered in the organization of individuals during the course of
their life, is preserved by generation, and transmitted to the
new individuals which originate from those which have
experienced these modifications."4

With regard to the position of man in the animal kingdom
Lamarck, unfortunately, does not seem to have been able, any
more than Buffon, to divest himself of the fetters of religious
orthodoxy. After pointing out at length the numerous ties by
which man, in his bodily organization, is united to the lower
animals, and especially his close relationship to the apes, he
saves himself, so to speak, in the following paragraph :—

" Such would be the reflections which one might make if man,
considered here as the pre-eminent race in question, were only
distinguished from the animals by the characters of his
organization, and if his origin were not different from theirs."5

1 " Histoire Naturelle des Animaux sans Vertebres," Tom. I, 1815, p. 182.

2 Ibid., p. 185.

3 Ibid., p. 189.

4 Jbid., p. 199.

5 " Philosophic Zoologique," Tom. I, p. 357.

CHAPTER XXV

Eobert Chambers and the " Vestiges of Creation " — Natural Selection —
The Views of Charles Darwin and Alfred Eussel Wallace.

FOE half a century after the appearance of the " Philosophie
Zoologique " the theory of organic evolution made but little
progress. The gap, however, was to some extent filled by the
publication, at first anonymously, of Robert Chambers' celebrated
book, "Vestiges of the Natural History of Creation." This work
first appeared in the year 1844 and rapidly passed through a large
number of editions. Though the views of its author can hardly
be said to mark any advance, but on the whole perhaps rather a
retrogression, the work, which gave rise to much controversy,
undoubtedly played a very important part in preparing the way
for the reception of Charles Darwin's " Origin of Species."
Chambers presented the evidence of organic evolution in a very
convincing manner, laying great stress upon that afforded by
the geological record and the facts of comparative anatomy and
embryology, and he included mankind in his general scheme of
evolution.

His views as to the modus opcrandi of organic evolution are
probably expressed as clearly as such views could be in the
following paragraphs :—

" The proposition determined on after much consideration is,
that the several series of animated beings, from the simplest and
oldest up to the highest and most recent, are, under the providence
of God, the results, first, of an impulse which has been imparted
to the forms of life, advancing them, in definite times, by genera-
tion, through grades of organization terminating in the highest
dicotyledons and vertebrata, these grades being few in number,
and generally marked by intervals of organic character which
we find to be a practical difficulty in ascertaining affinities ;
second, of another impulse connected with the vital forces,
tending, in the course of generations, to modify organic structures
in accordance with external circumstances, as food, the nature
of the habitat and the meteoric agencies, these being the

384 OUTLINES OF EVOLUTIONARY BIOLOGY

" adaptations " of the natural theologian. We may contemplate
these phenomena as ordained to take place in every situation,
and at every time, where and when the requisite materials and
conditions are presented — in other orbs as well as in this — in
any geographical area of this globe which may at any time
arise— observing only the variations due to difference of
materials and of conditions."1

" For the history, then, of organic nature, I embrace, not as a
proved fact, but as a rational interpretation of things as far as
science has revealed them, the idea of Progressive Development.
We contemplate the simplest and most primitive types of being,
as giving, under a law to which that of like-production is sub-
ordinate, birth to a type superior to it in compositeness of
organization and endowment of faculties; this again producing
the next higher, and so on to the highest. We contemplate, in
short, a universal gestation of nature, analogous to that of the
individual being ; and attended as little by circumstances of
a startling or miraculous kind, as the silent advance of an
ordinary mother from one week to another of her pregnancy.
We see but the chronicle of one or two great areas, within which
the development has reached the highest forms. In some others,
as Australia and the islands of the Pacific, development appears
to have not yet passed through the wrhole of its stages, because,
owing to the comparatively late uprise of the land, the terrestrial
portion of the development was there commenced more recently.
It would commence and proceed in any new appropriate area, on
this or any other sphere, exactly as it commenced upon our area
in the time of the earliest fossiliferous rocks, whichever these
are. Nay, it perhaps starts every hour with common infusions,
and in similar humble theatres, and might there proceed through
all the subsequent stages, granting suitable space and conditions.
Thus simple — after ages of marvelling — appears Organic
Creation, while yet the whole phenomena are, in another point
of view, wonders of the highest kind, being the undoubted results
of ordinances arguing the highest attributes of foresight, skill,
and goodness on the part of their Divine Author." 2

Progressive evolution is here clearly attributed to some inherent
tendency implanted in the first living things, and apparently the
writer imagines that the same or closely similar results may
have been arrived at along many different lines of evolution, each
commencing at a distinct starting point and at a different time and

1 " Vestiges of the Natural History of Creation." 12th ed., 1884, pp. 201, 202.
a Hid., pp.230, 231.

NATURAL SELECTION 385

place from all the others. The author of this hypothesis evidently
considers it a distinct improvement upon the views of Lamarck,
which he very briefly discusses. He thinks that Lamarck attributes
too much importance to the principle of use and disuse, which he
regards as "obviously insufficient to account for the great grades
of organization," though he admits that external conditions may
have been " a means of producing the exterior characters."
There can be little doubt, however, that, in relying upon a system
of more or less definite and continuously operating natural
causes as factors of organic evolution, Lamarck took up a far
more scientific position than Robert Chambers.

In insisting upon the great importance of Natural Selection as
a factor in organic evolution, Charles Darwin and Alfred Russel
Wallace made a great advance upon the position of any of their
predecessors.

We have seen in the last chapter that this principle had been
hinted at by more than one writer about the close of the eighteenth
and the commencement of the nineteenth centuries, and that it
can even be traced back to the philosophy of ancient Greece. In
the historical sketch which prefaces the later editions of the
" Origin of Species," Charles Darwin himself quotes a translation
from Aristotle which shows sufficiently clearly that the idea was
familiar to the great Greek biologist. In the same sketch he
also quotes a passage from Dr. W. C. Wells, from a paper read
before the Royal Society in 1813, in which the same principle is
recognized in the most explicit and unmistakable manner.

It was not, however, until the year 1858 that the part played
by natural selection in organic evolution began to be gene-
rally understood. In that year Sir Charles Lyell and Dr. J. D.
Hooker communicated to the Linnean Society certain papers,1
written by Darwin and Wallace, which at once called prominent
attention to the importance of this factor and contained the
essential parts of the theory of natural selection as subsequently
developed by both these writers.

Darwin's paper consisted of an extract from his as yet
unpublished work, together with an abstract of a letter to

1 These papers have been reprinted by the Linnean Society in the volume pub-
lished in connection with the Darwin-Wallace Celebration held on July 1st, 1908,
and it is from this volume that the quotations which follow are taken.

B. CO

386 OUTLINES OF EVOLUTIONARY BIOLOGY

Professor Asa Gray. A few quotations will suffice to indicate
his views : —

" De Candolle, in an eloquent passage, has declared that all
nature is at war, one organism with another, or with external
nature. ... It is the doctrine of Malthus applied in most
cases with tenfold force. . . . Even slow-breeding mankind
has doubled in twenty-five years ; and if he could increase his
food with greater ease, he would double in less time. But for
animals without artificial means, the amount of food for each
species must, on an average, be constant, whereas the increase of
all organisms tends to be geometrical, and in a vast majority of
cases at an enormous ratio. Suppose in a certain spot there
are eight pairs of birds, and that only four pairs of them
annually (including double hatches) rear only four young, and
that these go on rearing their young at the same rate, then at
the end of seven years (a short life, excluding violent deaths, for
any bird) there will be 2048 birds, instead of the original sixteen.
As this increase is quite impossible, we must conclude either
that birds do not rear nearly half their young, or that the
average life of a bird is, from accident, not nearly seven years.
Both checks probably concur. The same kind of calculation
applied to all plants and animals affords results more or less
striking, but in very few instances more striking than in man."

" Lighten any check in the least degree, and the geometrical
powers of increase in every organism will almost instantly
increase the average number of the favoured species. . . .
Finally, let it be borne in mind that this average number of
individuals (the external conditions remaining the same) in each
country is kept up by recurrent struggles against other species
or against external nature (as on the borders of the Arctic
regions, where the cold checks life), and that ordinarily each
individual of every species holds its place, either by its own
struggle and capacity of acquiring nourishment in some period
of its life, from the egg upwards ; or by the struggle of its
parents (in short-lived organisms, when the main check occurs
at longer intervals) with other individuals of the same or different
species.

" But let the external conditions of a country alter. If in a
small degree, the relative proportions of the inhabitants will in
most cases simply be slightly changed ; but let the number of
inhabitants be small, as on an island, and free access to it from
other countries be circumscribed, and let the change of conditions
continue progressing (forming new stations), in such a case the
original inhabitants must cease to be as perfectly adapted to the

VIEWS OF CHARLES DARWIN 387

changed conditions as they were originally. It has been shown
in a former part of this work, that such changes of external
conditions would, from their acting on the reproductive system,
probably cause the organization of those beings which were
most affected to become, as under domestication, plastic. Now,
can it be doubted, from the struggle each individual has to obtain
subsistence, that any minute variation in structure, habits, or
instincts, adapting that individual better to the new conditions,
would tell upon its vigour and health ? In the struggle it would
have a better chance of surviving ; and those of its offspring
which inherited the variation, be it ever so slight, would also
have a better chance. Yearly more are bred than can survive ;
the smallest grain in the balance, in the long run, must tell on
which death shall fall, and which shall survive. Let this work
of selection on the one hand, and death on the other, go on for
a thousand generations, who will pretend to affirm that it would
produce no effect, when we remember what, in a few years,
Bakewell effected in cattle, and Western in sheep, by this identical
principle of selection ? "

" In nature we have some slight variation occasionally in all
parts ; and I think it can be shewn that changed conditions of
existence is the main cause of the child not exactly resembling
its parents; and in nature geology shews us what changes have
taken place, and are taking place. We have almost unlimited
time ; "

" Another principle, which may be called the principle of diver-
gence, plays, I believe, an important part in the origin of species.
The same spot will support more life if occupied by very diverse
forms. We see this in the many generic forms in a square yard
of turf, and in the plants or insects on any little uniform islet,
belonging almost invariably to as many genera and families
as species. . . . Now, every organic being, by propagating so
rapidly, may be said to be striving its utmost to increase in
numbers. So it will be with the offspring of any species after it
has become diversified into varieties, or subspecies, or true
species. And it follows, I think, from the foregoing facts, that
the varying offspring of each species will try (only few will
succeed) to seize on as many and as diverse places in the
economy of nature as possible. Each new variety or species,
when formed, will generally take the place of, and thus exter-
minate its less well-fitted parent. This I believe to be the origin
of the classification and affinities of organic beings at all times ;
for organic beings always seem to branch and sub-branch like
the limbs of a tree from a common trunk, the flourishing and

c c 2

388 OUTLINES OF EVOLUTIONARY BIOLOGY

diverging twigs destroying the less vigorous— the dead and lost
branches rudely representing extinct genera and families."

The same paper also contains a sketch of the supplementary
theory of sexual selection, which will be seen to agree very closely
with the paragraph from Erasmus Darwin's " Zoonomia " quoted
in the last chapter : —

" Besides this natural means of selection, by which those
individuals are preserved, whether in their egg, or larval, or
mature state, which are best adapted to the place they fill in
nature, there is a second agency at work in most unisexual
animals, tending to produce the same effect, namely, the struggle
of the males for the females. These struggles are generally
decided by the law of battle, but in the case of birds, apparently,
by the charms of their song, by their beauty or their power of
courtship, as in the dancing rock-thrush of Guiana. The most
vigorous and healthy males, implying perfect adaptation, must
generally gain the victory in their contests. This kind of selec-
tion, however, is less rigorous than the other ; it does not require
the death of the less successful, but gives to them fewer
descendants."

The publication from which these quotations are taken itself
consists of extracts from Charles Darwin's manuscripts, selected
with a view to explaining, as clearly as possible, his theory of
natural selection, and is therefore especially suitable for citation.
In the following year (1859) the author's classical work, " The
Origin of Species by means of Natural Selection," made its
appearance, the first of that notable series of volumes on
philosophical biology which have made his name so famous.
In these works both the general theory of Evolution and the sub-
sidiary theory of Natural Selection are elaborated and supported
by an immense body of evidence drawn from published records
and personal observations, and so successfully was this done that
in a comparatively few years these theories met with general
acceptance on the part, not only of scientific men, but also
of the educated public. It must not be forgotten, either, that
Charles Darwin applied the doctrine of organic evolution
in a fearless and uncompromising manner to the origin of the
human race.

Dr. Wallace's contribution to the Linnean Society symposium
of 1858 was entitled " On the Tendency of Varieties to depart

VIEWS OF A. K WALLACE 389

indefinitely from the Original Type." The following quotations
will serve to show that his views on Natural Selection were closely
similar to those of Charles Darwin.

" The life of wild animals is a struggle for existence. The
full exertion of all their faculties and all their energies is
required to preserve their own existence and provide for that of
their infant offspring. The possibility of procuring food during
the least favourable seasons, and of escaping the attacks of their
most dangerous enemies, are the primary conditions which
determine the existence both of individuals and of entire
species."

" Even the least prolific of animals would increase rapidly if
unchecked, whereas it is evident that the animal population of
the globe must be stationary, or perhaps, through the influence
of man, decreasing. Fluctuations there may be ; but permanent
increase, except in restricted localities, is almost impossible.
For example, our own observation must convince us that birds
do not go on increasing every year in a geometrical ratio, as
they would do, were there not some powerful check to their
natural increase. Very few birds produce less than two young
ones each year, while many have six, eight, or ten ; four will
certainly be below the average ; and if we suppose that each pair
produce young only four times in their life, that will also be below
the average, supposing them not to die either by violence or want
of food. Yet at this rate how tremendous would be the increase
in a few years from a single pair ! A simple calculation will
show that in fifteen years each pair of birds would have increased
to nearly ten millions ! Whereas we have no reason to believe
that the number of the birds of any country increases at all in
fifteen or in one hundred and fifty years. With such powers of
increase the population must have reached its limits, and have
become stationary, in a very few years after the origin of each
species. It is evident, therefore, that each year an immense
number of birds must perish — as many in fact as are born. . . .
It is, as we commenced by remarking, ' a struggle for existence,'
in which the weakest and least perfectly organized must always
succumb.

" Now it is clear that what takes place among the individuals
of a species must also occur among the several allied species of
a group, — viz., that those which are best adapted to obtain a
regular supply of food, and to defend themselves against the
attacks of their enemies and the vicissitudes of the seasons, must
necessarily obtain and preserve a superiority in population ;
while those species which from some defect of power or organization
are the least capable of counteracting the vicissitudes of food

890 OUTLINES OF EVOLUTIONARY BIOLOGY

supply, &c., must diminish in numbers, and, in extreme cases,
become altogether extinct."

" Most or perhaps all the variations from the typical form of
a species must have some definite effect, however slight, on the
habits or capacities of the individuals. Even a change of colour
might, by rendering them more or less distinguishable, affect
their safety ; a greater or less development of hair might modify
their habits. . . . An antelope with shorter or weaker legs must
necessarily suffer more from the attacks of the feline carnivora.
... If, on the other hand, any species should produce a variety
having slightly increased powers of preserving existence, that
variety must inevitably in time acquire a superiority in numbers.
. . . All varieties will therefore fall into two classes — those
which under the same conditions would never reach the popula-
tion of the parent species, and those which would in time obtain
and keep a numerical superiority. Now, let some alteration
of physical conditions occur in the district — a long period of
drought, a destruction of vegetation by locusts, the irruption of
some new carnivorous animal seeking ' pastures new ' — any
change in fact tending to render existence more difficult to the
species in question, and tasking its utmost powers to avoid com-
plete extermination ; it is evident that, of all the individuals
composing the species, those forming the least numerous and
most feebly organized variety would suffer first, and, were the
pressure severe, must soon become extinct. The same causes
continuing in action, the parent species would next suffer,
would gradually diminish in numbers, and with a recurrence
of similar unfavourable conditions might also become
extinct. The superior variety would then alone remain, and
on a return to favourable conditions would rapidly increase
in numbers and occupy the place of the extinct species and
variety.

" The variety would now have replaced the species, of which it
would be a more perfectly developed and more highly organized
form. It would be in all respects better adapted to secure its
safety, and to prolong its individual existence and that of the
race. Such a variety could not return to the original form ; for
that form is an inferior one, and could never compete with it
for existence. . . . But this new, improved, and populous race
might itself, in course of time, give rise to new varieties,
exhibiting several diverging modifications of form, any of which,
tending to increase the facilities for preserving existence, must,
by the same general law, in their turn become predominant.
Here, then, we have progression and continued divergence deduced
from the general laws which regulate the existence of animals in

CHARLES DAE WIN AND LAMARCK 391

a state of nature, and from the undisputed fact that varieties do
frequently occur."

It will be evident from the above sketch of the theory of
Natural Selection, which I have thought it desirable to give in
the actual words of its chief exponents, that adaptation is
explained as the logical consequence of certain facts which can
at any time be verified by direct observation. (1) All
organisms tend to increase in a high geometrical ratio ; (2) there
is, partly as a direct result of such increase, a keen struggle for
existence, to which all organisms are more or less exposed and in
which vast numbers perish without leaving offspring; (3) all
organisms tend to vary in many directions ; (4) variations,
whether favourable or otherwise, tend to be transmitted by
heredity from generation to generation; though, as we have
already seen, there is at the present time much dispute as to
whether variations of a certain kind ought not to be excluded
from this generalization.

It follows inevitably from these premisses that in every genera-
tion there will be a more or less strongly pronounced tendency
towards the elimination of those individuals which are least well
adapted to their environment and a corresponding preservation
and encouragement of those which are best adapted, or, in
Herbert Spencer's celebrated phrase, a " survival of the fittest."
This process, continued from generation to generation for count-
less ages, has resulted in that marvellous perfection of adaptation
which we have seen to be such a striking feature of both plants
and animals.

Charles Darwin himself, however, was not satisfied with natural
selection as the sole factor concerned in bringing about pro-
gressive evolution and adaptation. Although, in the historical
sketch which he added to the later editions of the " Origin of
Species," he remarks : —

"It is curious how largely my grandfather, Dr. Erasmus
Darwin, anticipated the views and erroneous grounds of opinion
of Lamarck in his ' Zoonomia,' "

and although he himself at first appears to have attached very
little importance to Lamarck's opinions, yet we find in the last
chapter of the sixth edition of the " Origin of Species " abundant
evidence that he was obliged to admit the efficacy of the chief
" Lamarckian " factor, the principle of use and disuse, in

392 OUTLINES OF EVOLUTIONARY BIOLOGY

modifying species, and also, to some extent, that of the direct
action of the environment : —

" Disuse, aided sometimes by natural selection, will often have
reduced organs when rendered useless under changed habits or
conditions of life ; and we can understand on this view the
meaning of rudimentary organs.1 But disuse and selection will
generally act on each creature, when it has come to maturity
and has to play its full part in the struggle for existence, and
will thus have little power on an organ daring early life ; hence
the organ will not be reduced or rendered rudimentary at this
early age. The calf, for instance, has inherited teeth, which
never cut through the gums of the upper jaw, from an early pro-
genitor having well-developed teeth ; and we may believe, that
the teeth in the mature animal were formerly reduced by disuse,
owing to the tongue and palate, or lips, having become
excellently fitted through natural selection to browse without their
aid ; whereas in the calf, the teeth have been left unaffected, and
on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day." 2

" I have now recapitulated the facts and considerations which
have thoroughly convinced me that species have been modified,
during a long course of descent. This has been effected chiefly
through the natural selection of numerous successive, slight,
favourable variations ; aided in an important manner by the
inherited effects of the use and disuse of parts ; and in an
unimportant manner, that is in relation to adaptive structures,
whether past or present, by the direct action of external condi-
tions, and by variations which seem to us in our ignorance to
arise spontaneously. It appears that I formerly underrated the
frequency and value of these latter forms of variation, as leading
to permanent modifications of structure independently of natural
selection. But as my conclusions have lately been much mis-
represented, and it has been stated that I attribute the modifica-
tion of species exclusively to natural selection, I may be permitted
to remark that in the first edition of this work, and subsequently,
I placed in a most conspicuous position — namely, at the close of
the Introduction — the following words : ' I am convinced that
natural selection has been the main but not the exclusive means
of modification.' This has been of no avail. Great is the
power of steady misrepresentation ; but the history of science
shows that fortunately this power does not long endure." 3

1 Often now called " vestigial organs."
a "Origin of Species," Ed. vi. p. 420.
8 Ibid., p. 421.

A. R. WALLACE AND LAMARCK 393

It is of the greatest interest to recognize the fact that Darwin
himself saw nothing incompatible between the so-called
Lamarckian factors of use and disuse and the direct action of
the environment, and the principle of natural selection, but, on
the other hand, that the one set of factors might supplement the
other.

On the occasion of the unveiling of the statue of Charles
Darwin in the Natural History Museum at South Kensington,
Professor Huxley found occasion to observe tbat " science
commits suicide when it adopts a creed."1 This warning, it is
to be feared, has not been heeded by all of Darwin's followers.
Many of these have departed very far from the moderate and
rational position of their leader and, while attributing to natural
selection almost every advance which has been made in the
evolution of the organic world, are, as we have already seen,
obliged to justify their neglect of the " Lamarckian " factors by
denying altogether the possibility of the inheritance of acquired
characters, which Darwin, of course, freely admitted. Natural
selection, in the hands of these enthusiasts, and in spite of
Charles Darwin's efforts to maintain a just balance between this
and other factors, lias indeed become a creed.

Dr. Wallace from the first adopted an uncompromising
attitude towards the opinions of Lamarck. In the Linnean
Society paper from which we have already quoted he says : —

" The hypothesis of Lamarck — that progressive changes in
species have been produced by the attempts of animals to increase
the development of their own organs, and thus modify their
structure and habits — has been repeatedly and easily refuted by all
writers on the subject of varieties and species, and it seems to have
been considered that when this was done the whole question has
been finally settled ; but the view here developed renders such
an hypothesis quite unnecessary, by shewing that similar results
must be produced by the action of principles constantly at work
in nature. The powerful retractile talons of the falcon- and the
cat-tribes have not been produced or increased by the volition
of those animals ; 2 but among the different varieties which
occurred in the earlier and less highly organized forms of these
groups, those always survived longest which had the greatest facilities
for seizing their prey."

1 Vide Herbert Spencer's " Factors of Organic Evolution," p. 75.

2 Who ever said they had, except in the sense that an animal voluntarily uses its
claws on appropriate occasions and that constantly repeated use causes them to

Wallace's views l also differ from those of Darwin in that, at
any rate in later years, they have become strongly anthropo-
centric, and he now regards the whole of the organic world as
having been designed by the Creator for the ultimate reception
and benefit of mankind. He does not, it is true, go back to the
old idea that species have been separately and specially created
as we now find them, but he holds that the entire scheme of
evolution was planned out in the mind of the Creator, and even
suggests that the working out of this scheme may have been
delegated by the Supreme Being to a body of "organizing
spirits " : —

" At successive stages of development of the life-world, more
and perhaps higher intelligences might be required to direct the
main lines of variation in definite directions in accordance with
the general design to be worked out, and to guard against a
break in the particular line which alone could lead ultimately to
the production of the human form." 2

Such speculations as this would render natural selection and
all other natural factors of organic evolution superfluous, but we
cannot profitably discuss them in a work like the present. We
may point out, however, that they are in essential agreement with
the views of the author of the " Vestiges of Creation," to which
we have referred in the early part of this chapter, excepting that
Eobert Chambers did not venture to call in the assistance of
subordinate " organizing spirits " to carry out the plans of the
Creator.

increase in size and efficiency ? Lamarck did not suppose that an animal simply
willed organs to sprout out of its body !

1 For a full exposition of these views the reader should refer to Dr. Wallace's
" Darwinism " (London : Macmillan & Co., 1889).

2 " The World of Life, a Manifestation of Creative Power, Directive Mind and
Ultimate Purpose," by Alfred Russcl Wallace (London : Chapman and Hall, Ltd.,
1910), p. 395.

CHAPTER XXVI

Selection not confined to the organic world — Illustrations of the action of
natural selection in the struggle for existence — Degeneration Flight-
less birds — Extermination of the Morioris — Sedentary animals — Para-
sites— Co-operation of natural selection and the so-called Lamarckian
factors of evolution — The influence of internal secretions upon growth
— Increase in size beyond the limits of utility.

THE principle of selection is, of course, by no means confined
to living things. The various bodies which make up the
inorganic world owe their actual form and arrangement largely
to processes of selection which are constantly going on amongst
them. The outline of the sea coast is the result of the selective
action of atmospheric and tidal agencies upon the different kinds
of rock of which it is composed. The softer parts are destroyed
first, leaving the more resistant portions to stand out in the form
of bluffs or promontories, and to illustrate in the inanimate
world the principle of the survival of the fittest. We might even
say that the prominent headlands exhibit adaptation, for if they
were not adapted by their peculiar hardness to resist the dis-
integrating influences of the environment they would not be
there, but would have perished with those portions of the land
which formerly occupied the bays and inlets.

All things, in short, must be subject to the selective action of
their environment, and we need not hesitate to attribute to
natural selection a very large share in the modelling of the
features of the organic world as we now see it. We know what
we ourselves, by our so-called artificial selection, are able to do
in this way. The chief difference between artificial and natural
selection is that man selects for his own purposes and modifies
organisms to suit his own ends, while Nature selects to the benefit
of the species operated upon, which becomes thereby modified to
its own advantage and preservation in the struggle for existence.
But we cannot really draw a distinction between the two kinds
of selection, for even in a state of nature organisms are often
selected and modified to the advantage of other organisms.

396 OUTLINES OF EVOLUTIONARY BIOLOGY

As we saw in a previous chapter, the forms, colours and scents
of many flowers are probably the result of unconscious selection
by insects, extending over countless generations. It may be said
that the advantage gained in this case is mutual ; the insect gets
the honey and the flower gets fertilized. This of course is true,
but exactly the same is true of human selection. The sheep gets
the pasture and man gets the wool.

It seems impossible to explain on any other hypothesis than
that of the natural selection and gradual accumulation of chance,
favourable variations, those marvellous adaptations of animals
which lead to protective resemblance and mimicry, for although
we may admit that an organ which is actively employed may be
modified by the efforts of an animal to maintain itself by the
use of that organ, we can hardly extend the same principle to
such passive features as colour and ornamentation, or the out-
growth of leaf-like dermal appendages and so forth. It may be
questioned if, even with the aid of natural selection, we can fully
account for all the wonderful phenomena of mimicry, for why, if
it be an advantage to some species to adopt a common warning
colour and band themselves together in synaposematic groups,
should it be desirable for others to do just the reverse and split
up into a number of differently coloured forms, each of which
mimics some particular model ? We can only say that we do
not know all the factors of the environment, and that until we do
our inability to solve the problem cannot be justly considered as
an argument against the efficacy of natural selection.

It is, of course, extremely difficult, if not impossible, to obtain
direct evidence of the action of natural selection in modifying
species in a state of nature. Human life is all too brief to
admit of our making very satisfactory observations concerning
processes which extend perhaps over millions of years. Man
has, however, in a comparatively short space of time, so changed
the conditions of life for many of the lower animals as to lead,
albeit unintentionally, to the more or less complete extermina-
tion of many species, and by studying these cases we may hope
to arrive at sound conclusions as to what takes place in a state of
nature. After all, mankind is a part of nature and we have no
just reason for excluding his influence in our consideration of
the factors which have brought about the present condition of
the organic world.

It is well known that many of the birds of various remote

FLIGHTLESS BIRDS 397

islands have lost the power of flight. Such are the kiwi, the
kakapo, the weka, the notornis and the already extinct gigantic
moas of New Zealand ; the dodo of Mauritius, and the solitaire
of Rodriguez. Although belonging to several very distinct
families of birds, including ratites, parrots, rails and pigeons, all
the forms enumerated have undergone the same curious modifi-
cation, resulting in the most extreme cases (the moas) in the
complete loss of the wings, and in others in the reduction of those
organs to a more or less vestigial condition.1

This convergence is clearly due to the similarity of the con-
ditions under which these birds have had to live. One of the
most characteristic features of oceanic islands is the absence
from them of predaceous mammals, the natural enemies of birds,
which have never been able to cross the great stretches of open
ocean which separate such islands from the continental areas on
which the Mammalia have been evolved. Birds, however, and
even land birds, by virtue of their powers of flight, have been
able to reach these islands at more or less frequent intervals
and to establish themselves there. Finding abundance of food,
which they could obtain near the ground, and finding themselves
no longer under the necessity of constantly using their wings in
order to escape from their enemies, some of these birds, though
by no means all, gradually gave up flying and their wings under-
went a slow process of degeneration in accordance with Lamarck's
principle of disuse. No doubt such disuse, if continued only
through a single lifetime, could scarcely produce a visible effect
upon the next generation, but continued under the same con-
ditions throughout thousands of generations it has brought about
a permanent deterioration which can no longer be retrieved.

It is to be noted that this degeneration is the result of the
removal of the organism, to a certain extent, from the struggle
for existence. Natural selection can only act through the struggle
for existence and upon those organs which are of value in the
struggle. When the struggle ceases, natural selection ceases and
degeneration sets in, for there is no longer any reason why a
high standard of perfection should be maintained. All degrees of
imperfection now have equal opportunities of propagating them-
selves. The inferior individuals are no longer weeded out, and
the average condition of the species consequently deteriorates.

But observe what happens when a degenerate organism is

1 Compare Chapter XVII, Figs. Ill, 112.

398 OUTLINES OF EVOLUTIONARY BIOLOGY

once more exposed, by some unfortunate change in its environ-
ment, to the old struggle from which it had escaped. This has
actually taken place in the case of the flightless birds of New
Zealand and other remote islands. With the advent of Euro-
peans, predaceous mammals of many species — dogs, cats, rats,
weasels, stoats and ferrets — have been let loose upon their
helpless victims. These are once more exposed to a keen
struggle for existence, while at the same time they have lost
those very organs which are necessary to enable them to maintain
themselves in that struggle, and natural selection, having
regained her power, is rapidly exterminating them. It is not
too much. to say that in a few years' time there will be no
flightless birds left in New Zealand except in special reserves
where they are being protected by man.

It is highly instructive in -this connection to contrast the con-
dition of such a bird as the flightless parrot, or kakapo, with that
of its relative the kea. The kakapo is a large, heavy bird of
nocturnal habits and with practically no means of defence ; it
haunts the dense forest and is rarely seen except when hunted
out by dogs. The kea, on the contrary, is one of the strongest
fliers of the parrot tribe. It frequents high and more or less
inaccessible mountain regions and since the advent of Europeans
has learnt to make use of the sheep which they have introduced
as an additional food supply. It is doubtful whether the utmost
efforts of the sheep farmers, who annually expend large sums of
money for the purpose, will ever enable them to exterminate the
kea, and it is equally doubtful whether the efforts of the New
Zealand Government to preserve the unique flightless birds will
suffice to" prevent the complete extermination of the kakapo
within the next few years.

The aboriginal human population of remote islands has of
course suffered not less than the lower animals from the in-
vasion of their retreats by Europeans, although not always
exclusively at the hands of the Europeans themselves. There
are, perhaps, few more striking examples of the extermination of
a primitive native race than that afforded by the rapid disap-
pearance of the Moriori inhabitants of the Chatham Islands,
some four hundred miles to the east of New Zealand, during the
nineteenth century.1 At the time of my visit to these islands, in

1 Compare Dencly, " The Chatham Islands : A Study in Biology " (Memoirs and Pro-
ceedings of the Manchester Literary and Philosophical Society, Vol. XLVI., 19U2).

EXTERMINATION OF MORIORIS 399

January, 1901, there were only about a dozen pure-blooded indi-
viduals left ; some of these were of great age, while the youngest
was a lad of about 16, and they had all, I think, more or less
completely adopted European manners and customs. Under
these circumstances we are fortunate in possessing any reliable
record of this interesting people, and that we do so is largely
due to the energy and enthusiasm of Mr. Alexander Bhand, who
for more than thirty years lived amongst the Morioris and
made a special study both of that race and of their Maori
conquerors.

It appears from their language, customs and traditions, as
well as from their physical characteristics, that the Morioris are
closely related to the New Zealand Maoris. Their ignorance of
the art of tattooing, and their very inferior artistic faculties in
general, however, point to a very remote separation of the two
races.

Like the Maoris they trace their origin to an unknown father-
land called Hawaiki, from which they must have emigrated to
Chatham Island in canoes. In their new home they appear to
have found the conditions of life remarkably easy, indeed, as the
sequel shows, fatally so. With an abundant natural food supply
of fruit, shell-fish, &c., and with no enemies to contend with,
they multiplied until the islands were thickly populated, while
at the same time they doubtless became lazy and effeminate.

The discovery of the islands by the brig " Chatham," in 1790,
may be said to have sealed the fate of the unfortunate Moriori,
though it is doubtful whether any serious injury ensued until
the advent of the whaling and sealing vessels in 1828. These
vessels brought with them many undesirable visitors, and prob-
ably were the means of introducing a disease which soon played
havoc with the native race. On board some of the ships, moreover,
were Maoris from New Zealand, who, on their return, painted
such a glowing picture of the land of plenty, that a large number
of their fellow-countrymen determined to emigrate to the islands
en masse.

In order to effect this purpose they took possession of the brig
" Rodney " at Port Nicholson, in New Zealand, about the
beginning of November, 1835. They are said to have seized the
crew and compelled the captain to transport them, about 900 in
number, to their destination. At the time of the invasion the
Morioris are supposed to have numbered about 2000, and had they

400 OUTLINES OF EVOLUTIONARY BIOLOGY

attacked the new-comers on their first arrival, they might have
exterminated them with little trouble and prolonged for an
indefinite period the life of their own race. Unfortunately for
themselves, however, they had lost the art of self-defence. Owing
to the absence of competition they had, in this respect at any
rate, undergone degeneration. Killing was actually forbidden
by their laws, and peace had reigned too long and too securely to
give place at once to war when the emergency arose.

Just as the flightless birds of New Zealand have more or less
completely disappeared since the advent of carnivorous mammals,
so the Morioris, their happy isolation once broken, fell an easy
prey to the more virile Maoris. The latter proceeded to parcel
out the conquered country amongst themselves, claiming not
only the land but also the inhabitants thereof, many of whom
were massacred under circumstances of unutterable atrocity,
while the remnant were speedily reduced to the condition of
slaves. Under the changed conditions which had suddenly arisen
in their environment the Morioris were no longer fit to survive
in the struggle for existence, they had become degenerate in a
vital respect, and natural selection, as soon as opportunity arose,
stepped in and eliminated them.

It would be easy to multiply illustrations of the great generaliza-
tion that when removed from the struggle for existence all
organisms tend to become degenerate, the organs or faculties
which they no longer require atrophying and gradually dis-
appearing for want of employment. We see this very clearly in
the case of sedentary animals such as the ascidians (Figs. 129,
130). The young ascidian is a highly organized creature which
swims actively about by means of a muscular tail, in the same
way as the tadpole of a frog. Like the latter it has nervous
system, notochord and sense organs — though the sense organs
are of a type peculiar to itself — and is an undoubted chordate.
It never, however, progresses further in organization, so as to
attain the true vertebrate condition. On the contrary, it gives up
its active life and withdraws as far as possible from the struggle
for existence by fixing itself to some rock or seaweed and envelop-
ing its entire body in a thick protective envelope, within which
it undergoes extensive degeneration. The tail and notochord
completely disappear, so do the sense organs, none of these
being any longer required under the new conditions of life. The

DEGENERATION IN PARASITES

401

nervous system dwindles away to a mere ganglion, from which a
few nerves come off, and the entire animal is reduced to the
condition of a bag, with two openings through which the
remaining organs obtain their food supply and communicate
with the outside world by means of a stream of water maintained
by ciliary action.

Still more conspicuous is the degeneration undergone by
the great majority of parasites, whether animals or plants.
Sacculina, for example, in the earlier stages of its existence, is
an active crustacean which swims vigorously about by means of
well developed appen-
dages. It belongs to a
group, the barnacles or
cirripedes, which are
notorious for sedentary
habits and consequent
degeneration in the adult
condition. Sacculina,
however, not content with
a sedentary life, goes
further down hill and
becomes parasitic. It
attacks crabs, and in the
adult state is reduced to
the condition of a large,
irregularly shaped bag
(Fiff. 183) fixed to the
under surface of the crab s
abdomen by root -like pro-
cesses which penetrate the body of the host and extract nutri-
ment therefrom. With the exception of these root-like processes,
which are a special, csenogenetic development, adapted for nutri-
tion under new conditions of life, the only organs which have
not undergone degeneration are those of reproduction, for upon
these depends the perpetuation of the race and upon these,
therefore, natural selection is still able to retain her hold.

It is, indeed, a general rule amongst parasitic animals that the
reproductive organs are largely developed and very complicated,
for the conditions which have become necessary for the existence
of these animals are so complex and highly specialized, while
the chances of mating between different individuals for purposes,

FIG. 183.— Lower surface of a Swimming
Crab (Portunus depurator] with a Sac-
culina (Sac.) attached to it. (From a
photograph.)

B.

D D

402 OUTLINES OF EVOLUTIONARY BIOLOGY

of sexual reproduction are so remote, that the ova and spermatozoa
have to be produced in vast numbers to compensate for the
immense mortality which must take place amongst them and
amongst the young animals to which they may give rise. Thus

FIG. 184. — The Dodder, Cuscuta europcea. Part of a Plant parasitic on a
Branch of Willow, with germinating Seedlings on the right and
Section of Host and Parasite on the left. (From Strasburger. )

b, vestigial leaves ; Bl, flowers ; Cus, stem of parasite in section ; H, haustoria of parasite
in section; W, stem of host in section, with vascular bundles (v, c) ; t, seedlings.

one of the most frequent results, or at any rate concomitants, of
parasitism, and one which is well exemplified in the case of
Sacculina, is hermaphroditism, which affords many more chances
for the fertilization of the eggs than the unisexual condition.

We meet with precisely analogous phenomena in the case of
many parasitic plants. In the dodder (Fig. 184) the leaves and

INSUFFICIENCY OF NATURAL SELECTION 403

roots have disappeared almost completely and no chlorophyll is pro-
duced, but special nutritive organs, the sucker-like haustoria, are
developed on the slender, twining stems, and serve to extract the
necessary food from the host plant. The flowers, however, upon
which the perpetuation of the race depends, still remain in a
well developed condition.

Both Sacculina and the dodder have lost the power of inde-
pendent existence, and if, for any reason, they were to find
themselves suddenly confined to an environment where there were
no suitable hosts, their races would inevitably become extinct.
Nature would treat them just as she is treating the wingless
birds, and make them pay the penalty for the degeneration which
they have undergone.

It has sometimes been pointed out as an objection to the theory
of natural selection that it cannot account for the first origin of
favourable variations. The theory takes variations for granted
and assumes that some will be favourable and some not, that the
former will be fostered and accumulated from generation to
generation and the latter ruthlessly eliminated. It is further
alleged that variations are usually so slight at their first appear-
ance that they can have no selective value, and that something
is wanted to account for the increase of such variations along
apparently definite lines of utility. The theory also takes the
inheritance of variations for granted, and many people, as -we
have seen, consider nowadays that this is not altogether a
justifiable proceeding, that while some variations undoubtedly
are inherited, others, and amongst them many which would be
likely to be of the greatest value to the organism, are not.

We have, then, to go much deeper than the idea of natural
selection before we can reach a satisfactory working hypothesis
as to the manner in which organic evolution has taken place.
The problems of variation and heredity have already been dealt
with in earlier chapters, and it will be unnecessary to discuss the
matter now at great length, but there are certain points which
we must recapitulate in this connection.

We have seen that somatogenic or bodily variations in the
individual are undoubtedly brought about by the direct action of
the environment and by the use and disuse of organs. We have
also seen that blastogenic variations, which originate in the germ
plasm, may likewise be brought about by the action of the

D D 2

404 OUTLINES OF EVOLUTIONARY BIOLOGY

environment (as in the case of the potato beetle as demonstrated
by Tower), but that they probably also arise from the mingling
of different streams of ancestral tendencies in the process of
amphimixis or conjugation of gametes, and possibly in yet other
ways with which we are not acquainted.

Of course, natural selection can only influence a species
through variations which are capable of being inherited, and it
is, as everyone knows, urged by many modern writers that
somatogenic variations, due either to the direct action of the
environment or to the use and disuse of parts, cannot be
inherited and therefore have no significance in evolution, and
that natural selection must content herself with such fortuitous
and non-adaptive variations as may happen to arise in the germ
plasm. This indeed seems an extreme view, and it is just
here that the split between the extreme selectionists, who have
gone far beyond Charles Darwin in this matter, and the followers
of Lamarck arises. The curious thing about the controversy is
that there is no inherent incompatibility between the views of
the two schools. The theory of the inheritance, to a limited
extent, of acquired characters, indeed, appears to be just what is
necessary to supply the deficiencies of that of natural selection.

To say that acquired characters cannot be inherited because
we cannot see them being inherited in our own brief lifetimes1 is
like saying that a glacier does not move because we do not see it
or feel it moving as we walk over it. I have endeavoured to
show in an earlier chapter that it is not difficult to imagine a
mechanism by which somatogenic characters may gradually be
converted into blastogenic ones, and if this is in any way
possible there is no reason why we should deny the possibility
of their inheritance. No one, however, would be rash enough
to suppose that all that an animal or plant acquires in its
individual lifetime is transmitted to its heirs. Nature imposes
a heavy death duty and takes away by far the greater part of the
capital which has been accumulated by each individual. We
may suppose, however, that a fraction remains, however
unrecognizable by our limited powers, and that these
fractions, accumulating under the same influences throughout
thousands of generations, ultimately confer upon the organism
as a birthright that adaptation which is essential to its existence.

1 As a matter of fact, it appears from recent experiments that in some cases we
can see them being inherited (vide p. 182).

CO-OPERATION OF FACTORS 405

Even the individual can do much in its own lifetime to adapt
itself to its environment, and when the residua of all the
individual adaptations are summed up by inheritance the result
is such that we may well wonder how it can have been produced.
Throughout the whole process, of course, natural selection must
help by constantly weeding out inferiority, but it is probably
the direct influence of the environment, including the use and
disuse of organs in response to that influence, that is in most
cases the determining factor in bringing about adaptation.

It may well be, however, that there are also cases in which
natural selection alone, acting through the occurrence of purely
fortuitous variations, has, in the struggle for existence, been
sufficient to produce marvellous adaptations. This may have
been the case with protective resemblance and mimicry in form
and colour, and with the adaptation of flowers for fertilization by
insects, in all of which it is difficult to see how the direct action
of the environment or the use and disuse of organs could bring
about adaptive modifications. But the fact that natural selection
alone appears to have been sufficient in some cases must not
prevent us from admitting the action of other factors in other
cases. The fact that some carriages are pulled by motors affords
no justification for asserting that other carriages may not be
pulled by horses, or that the same carriage may not at one
time be pulled by a motor and at another by a horse, or even
by both together. Many factors must have co-operated to
bring about such a marvellously complex result as the present
condition of the organic world, and no sufficient reason has
yet been shown for denying ourselves the assistance of
" Lamarckian " factors in our endeavours to discover the processes
through which this result has come about.1

We may now turn our attention to a group of cases which
certainly appear to support the view that the factors at work in
determining any particular line of evolution are more complex
than might at first sight be supposed. It is a fact well known
to palaeontologists that many widely separated groups of the
animal kingdom have, during the course of their evolution, and
especially towards the end of that course, shown a strongly
marked tendency to enormous increase in size. We see this
in the extinct eurypterids (Fig. 137), giants amongst the

1 " To insist on ascribing complex results to single causes is the well-known vice
of narrow and untrained minds " (Morley's " Life of Gladstone." Vol. II., p. 68).

406 OUTLINES OF EVOLUTIONAEY BIOLOGY

•

arthropods; in the huge labyrinthodont amphibians; in many
groups of reptiles of the Secondary period, some of which
attained a length of 80 feet or more, and amongst mammals in
the extinct Tinoceras (Fig. 150) and the still surviving elephants
and whales. Comparative anatomists are familiar with similar
phenomena exhibited by individual organs, such as the extra-
ordinary development of horns and spines in many of the
extinct reptiles referred to (Fig. 145), the immense tusks of

FIG. 185. — Head of Babirusa alfnrus. (From Flower and Lyddeker's
" Mammals Living and Extinct.")

the babirusa (Fig. 185), and the gigantic and grotesque beak
and " helmet " of the hornbill (Fig. 186).

The exuberant development of some organs of this kind may
possibly be attributed to the action of sexual selection, and indeed
our daily experience of our own species seems to warrant us in
believing that there is no limit to the grotesque results which
may ensue from the unrestricted exercise of the jiesthetic faculties
of either sex, but it seems hardly reasonable to attempt to
explain all such bizarre and monstrous productions in this
manner.

In all the cases cited, and in many others which could be
adduced, either the entire body or some particular organ appears
to have acquired some sort of momentum, by virtue of which it
continues to grow far beyond the original limits of utility, although
perhaps in some cases a new use may be found which will assist
the species in maintaining itself in the struggle for existence.

EXTINCTION OF GIANT RACES

407

An enormous increase of mere bodily size, however, seems in the
long run to be always fatal to the race, whose place will be taken
by smaller and presumably more active forms. The gigantic
amphibians are all extinct, so are the really gigantic reptiles,
and of the gigantic mammals only a couple of species of elephants

FIG. 186. — A Hornbill, Biiceros rhinoceros, from North.- West Borneo. (Drawn
from a specimen in the British Museum, Natural History.)

and a few whales survive, all of which are being rapidly exter-
minated in competition with man.

There is perhaps some justification in recent developments of
physiological science for the belief that a race of animals may
acquire a momentum of the kind referred to ; that some brake
is normally applied to the growth of organisms and organs and
that sometimes this brake is removed, leaving the organism to
rush onwards to destruction like a car running away down hill.

Many modern physiologists hold the view that the growth of

408 OUTLINES OF E VOLUTION AEY BIOLOGY

the different parts of the animal body is controlled by internal
secretions, or hormones, the products of various glands. Thus
we know that disease of the pituitary body in man may lead to
acromegaly, one of the symptoms of which is great enlargement
of certain parts. The most dreadful of all the diseases to which
human beings are subject, cancer, is essentially due to an unre-
strained multiplication of cells, and consequent abnormal growth
of tissue, which may very possibly be correlated with the extent
to which some specific controlling secretion is produced in the
body. In short, we are justified in supposing that in the individual
growth may be normally inhibited or checked by specific secretions,
and that in the absence of these it may continue far beyond the
ordinary limits.

It is difficult to see any good reason why we should not apply
this principle to the race as well as to the individual, and,
paradoxical as it may appear, it even seems possible to explain
both the growth of the organism as a whole and that of its
various organs, beyond the limits of utility, as an indirect result
of natural selection.

When a useful organ, such as the tusk of a wild boar, is first
beginning to develop, or to take on some new function for the
execution of which an increase in size will be advantageous,
natural selection will favour those individuals in which it grows
most rapidly and attains the largest size in the individual lifetime.
If growth is normally checked and controlled by some specific
secretion, or hormone, natural selection will favour those
individuals in which the glands which produce this secretion are
least developed, or at any rate least active. The process being
repeated from generation to generation these glands (whatever
may be their nature, and we use the term "gland "for any cell
or group of cells which produces a specific secretion, whether
recognizable as a distinct organ or not) may ultimately be
eliminated, or at any rate cease altogether to produce the par-
ticular hormone in question. Moreover, this elimination may
take place long before the organ whose growth is being favoured
by natural selection has reached the optimum size. When it
has reached this optimum it is certainly desirable that it should
grow no larger, but is there now any means by which further
growth can be checked ? The inhibiting hormone is no longer
produced ; the brake has been removed, and further growth may
be supposed to take place irrespective of utility, until, when

EXCESSIVE GROWTH 409

the size of the organ gets too great to be any longer compatible
with the well-being of the race, natural selection again steps in
and eliminates the race. The same argument of course applies
'to the size of the body as a whole as well as to that of its con-
stituent parts.

It may be thought that many of the bizarre and almost
monstrous characters under discussion, such, for example, as
some of the excrescences of the dermal armature in extinct
reptiles (Fig. 145), can never have had any value as adaptations,
and that therefore natural selection could never have encouraged
them to increase so much in size as to get beyond her control.
Here, however, the principle of correlation comes in. Just as
many totally different organs are affected by disease of the
pituitary body, so the removal of the gland which controlled the
development of some undoubtedly useful organ, such as a frontal
horn, might at the same time permit the growth of all sorts of
excrescences which have no adaptive significance.

Thus it appears not impossible that, the normal checks to
growth being removed along certain lines by the action of natural
selection, a definite direction might be given to the course of
evolution, which the organism would continue to follow irrespec-
tive both of natural selection and of the principle of use and
disuse.1

In the present state of our knowledge, however, the above
suggestions can only be regarded as tentative. They are doubtless
open to much criticism, and it is unfortunately impossible to
subject them to the crucial test of experiment.

1 I have discussed this question at somewhat greater length in a paper read
before the British Association for the Advancement of Science (ride Report of the
Portsmouth Meeting, 1911).

CHAPTER XXVII

Artificial selection — Continuous and single selection — The mutation theory
of the origin of species — Mutual adaptation — Unit characters —
Isolation — Physiological selection — Non-adaptive characters — The
evolution of man.

IT has long been recognized that much light may be thrown
upon the problem of the origin of species by the careful
study of the methods which mankind has adopted for the
improvement of the various races of cultivated plants and
domesticated animals. Many such races have been so greatly
modified that, did they occur in what is commonly called a state
of nature, we should be obliged to regard them as distinct species.
The history of some of these is lost in antiquity and we have no
positive knowledge of the methods by which the improvement of
wild species was first effected. We may assume, however, with
some degree of confidence, that the earliest breeders and
cultivators would select for cultivation and propagation those
individuals which offered them the most valuable qualities, and
that they would reject such as exhibited marked signs of
inferiority. This process, repeated from generation to generation
through thousands of years, and aided in each generation by the
direct effects of cultivation, could not fail to bring about con-
spicuous results. For an account of what has been effected
in this manner the student should consult Charles Darwin's
classical work on the " Variation of Animals and Plants under
Domestication."

The almost unconscious efforts of our ancestors have given
place in modern times to deliberate and systematic attempts to
discover the principles upon which the improvement of cultivated
races, both of plants and animals, should be based.

Perhaps no species of plants have been more improved by
man than the various cereals upon which he relies so largely for
his food supply. Professor de Vries, in his interesting book on
" Plant-Breeding," l describes how such improvement has been

1 Kegan Paul, Trench, Triibner & Co., London, 1907.

CONTINUOUS SELECTION 411

effected in recent times. In the first place much care and
thought have been devoted to carrying out experiments in
accordance with the principle of continuous selection : —

" The general custom [in Germany] was to start such experi-
ments from the best local or improved varieties by an initial choice
of a certain number of typical heads. Such a group of selected
plants was called the elite, and this elite had to be ameliorated
according to the prevailing demands or even simply in accordance
with some ideal model. Year after year, the best ears of the
elite group were chosen for the continuance of the strain or
family, and slowly, but gradually, its qualities were seen to
improve in the desired direction. After some years, such a
family might become decidedly better than the variety from
which it had been derived. Then its yearly harvest would be
divided into two parts, after having been sufficiently purified by
the rejection of accidental .ears of minor worth. The best ears
were carefully sought out and laid aside for the continuance of
the elite strain, but the remainder were sown on a distant field
in order to be multiplied as fast as possible. By this means, after
a multiplication during two or three generations, its product
could be used as seed grain for the farm or sold to others for
the same purpose. Each year the elite would, of course, give a
new and better harvest which could be multiplied and sold in the
same manner." 1

By this method improvement may undoubtedly be effected,
but the selection has to be constantly repeated, otherwise the
improved strain rapidly deteriorates again. Indeed it may be
questioned whether it is possible in this way to effect any per-
manent improvement, at any rate in the case of cereals. One
reason for this appears to be that we are dealing all the time,
not with a single pure race, but with a mixture of distinct races.
We must also remember that many of the characters which it is
desired to perpetuate and increase may be the direct result of the
cultural methods employed, and, as we have already seen, we
cannot expect such causes to produce visibly heritable effects in
the course of a few generations, whatever they might do in the
long run.

We have had occasion to point out in an earlier chapter that,
according to Professor de Vries, new species arise, in a state of
nature, not by the accumulation in particular directions of small,
fluctuating variations, but by the sudden appearance of those

1 De Vries, op. clt.. p. 58.

412 OUTLINES OF EVOLUTIONARY BIOLOGY

more conspicuous variations known as mutations. De Vries
points out that many of the so-called Linnean species,
such as Draba verna, are in reality made up of a large
number of "elementary species" which have arisen in this
manner, and certain results which have been obtained in experi-
ments upon the improvement of cereals appear at first sight to
afford considerable support to these views.

It has long been known that an ordinary field of wheat con-
tains a larger or smaller number of " types," " mutations," or
" elementary species," which can be recognized by the experienced
eye, and it has been shown that if a single plant of one of these
types be isolated it will produce offspring like itself and continue
to breed true for an indefinite number of generations. Of course
it is necessary that there should be no crossing with other
types, but this is easily avoided, for, although accidental
crosses may occur, the cereals, with the exception of rye, are
usually self-fertilizing. Upon this knowledge is founded the
method of single as opposed to continuous selection, a single
selection of a suitable type being enough to establish the desired
strain.

One of the first to make use of the method of single selections
was Patrick Shirreff :—

" His first discovery was made in the year 1819. He observed
a plant of wheat which surpassed its neighbors by its high
degree of branching. It yielded 63 ears with about 2500
kernels. He saved the seeds, sowed them on a separate field
and at considerable distances apart so as to induce in all the
plants the same rich branching. He contrived to multiply it so
rapidly that it took only two generations to get seed enough to
bring it advantageously into the trade. He gave it the name of
Mungoswell's wheat, and it soon became one of the most profit-
able varieties of Scotland. It has found its way into England
and into France, where it is still considered one of the best sorts
of wheat." 1

The same method has been subjected to severe tests and
placed upon a thoroughly scientific footing at the Swedish
Agricultural Station of Svalof.

We have seen, in Chapter XIV, that hybridization may
occasionally give rise to permanent races or strains exhibiting
new combinations of characters, and that this takes place in

1 De Vries, op. fit., pp. 34, 35.

HYBRIDIZATION AND MUTATION 413

accordance with Mendelian principles. It cannot be doubted that
hybridization occurs occasionally in cultivated cereals, and
Professor de Vries is of opinion that the occurrence of the
different types or mutations is often the result of hybridization
at various periods in the history of the race : —

"Experience, however, shows that in ordinary fields almost
all possible combinations may be met with, and it is to be pre-
sumed that at least the greater number of them are due to
crosses in previous and, perhaps, in long-forgotten years." x •

Some of these combinations, as might be expected, are not
stable but split up into a number of varieties in the next
generation, but also : —

" We may conclude that some, and perhaps many, of the types
which may be selected and isolated in the fields and which prove
to be constant races must be of hybrid origin." 2

De Vries maintains that in the case of the cereals so many of
these " types " now lie ready to our hand that all we have to do
is to pick out those which we require and cultivate them in
isolation from each other and from the remainder.3 Professor
Biffen has shown, however, as we have already pointed out in
Chapter XIV, that it is possible by intelligent artificial hybridi-
zation to produce yet otber stable combinations or hybrids
which may surpass in value any which have accidentally arisen
in the past.

It appears, then, that many at any rate of the so-called
mutations or types amongst cereals are due to hybridization.
How far this applies to mutations in general it is quite impossible
to decide. That it is not always so, however, appears to be proved
by the occurrence of such mutations or sports as hexadactylism,
which are known to be inherited and which cannot have
arisen in this way.

De Vries tells us in another work that :—

" According to the theory of mutation species have not arisen
gradually as the result of selection operating for hundreds, or
thousands, of years but discontinuously by sudden, however
small, changes. In contradistinction to fluctuating variations

1 De Vries, op. cit., p. 80.

2 Ibid., loc. cit.

8 Ibid., p. 50. It seems strange, considering that de Vries admits that many
at any rate of the "types" have probably arisen by hybridization in the first
instance, that he should attribute so little value to artificial hybridization as a
means of improvement.

414 OUTLINES OF EVOLUTIONARY BIOLOGY

which are merely of a pins or minus character the changes which
we call mutations are given off in almost every manner of new
direction. They only appear from time to time, their periodicity
being probably due to perfectly definite but hitherto undiscovered
causes.

" The theory of the inheritance of acquired characters comes
under the heading of fluctuations. Acquired characters have
nothing to do with the origin of species. Nor can the theory of
descent be applied to the solution of social problems." l

There is here no suggestion of a hybrid origin for the
mutations in question. If, however, as seems probable, a large
proportion of so-called mutations are really the result of hybridi-
zation, and if, as we showed in Chapter XIV, hybrids tend to be
automatically eliminated in a state of nature — though of course
there is nothing to prevent a constant hybrid from being pre-
served if it happens to possess characters peculiarly favourable
to its own existence — it does not seem likely that such mutations
can have played any very great part in organic evolution. In
any case there is no need to suppose that the theories of muta-
tion and natural selection are mutually exclusive, for, however
new characters may arise, they must be subject to the action of
natural selection in the struggle for existence.

Professor de Vries' objection to small, fluctuating variations as
the material upon which natural selection operates in the
modification of species appears to be based upon the view that
such characters are acquired in the lifetime of the individual
and cannot be inherited. If, however, we admit that a sorna-
togenic character may, in the course of many generations
and under the continued influence of the same conditions which
originally called it forth, become converted into a blastogenic
character, this difficulty entirely disappears.

It is extremely hard to believe that mutations, which, apart
from the occurrence of hybridization, seem to occur very rarely
and at long intervals, can have afforded sufficient opportunity
for the production of all the marvellous adaptations which exist
in nature. Take, for example, the mutual adaptations which we
see between the length of the nectary in certain flowers and the
length of the proboscis in the insects which fertilize them. We
cannot suppose that either the elongated nectary or the elongated
proboscis arose by sudden mutations, for unless these mutations

1 " The Mutation Theory." English Trans., Vol. I., p. -'13.

MUTATION AND ADAPTATION 415

took place simultaneously in flower and insect, and in the same
locality, and in a sufficient number of each, a supposition which,
to say the least of it, is wildly improbable, the delicate correla-
tion between the two would be thrown out of gear and the
individuals exhibiting the mutations would be eliminated by
natural selection because they were no longer sufficiently well
adapted to the very special conditions of their environment.

We can only believe that the increase in length of nectary and
proboscis took place so slowly that their reciprocal adaptation
was never upset. A slight increase in the length of the nectary
obliged the insect to poke further into the flower for the honey
and thus increased the chances of fertilization. A slight increase
in the length of the proboscis enabled the insect to get more
honey and thus gave it a better chance of existence. After
perhaps many thousands of generations, under the influence of
natural selection, combined in the case of the insect with the
effects of use and disuse, the present enormous lengths of
proboscis and nectary have been attained, and no one doubts .the
fact that they have become blastogenic characters.

The same argument applies to all accurate adaptations to
special conditions of the environment. How can we explain the
facts of protective resemblance and mimicry except as due to the
accumulation under the influence of natural selection of what
Charles Darwin called slow successive variations ? How, again,
can the theory of mutation be applied to such cases as that of
the flightless birds on oceanic islands ? Who can doubt that
the reduction of the wings and the loss of the power of flight has
been brought about slowly and gradually as a result of disuse ?
and at the same time who would venture to argue that the
flightless birds are not specifically distinct from their actively
flying ancestors ? If it be urged that mutations may be so small
as to be almost imperceptible, then we must ask how do they
differ from fluctuating variations ? and if we are told that they
occur very rarely and do not fluctuate, that very answer is
sufficient to show that they can hardly have given rise to adaptive
modifications.

De Vries' theory of the origin of species -by mutation is
supposed to harmonize with the Mendelian principle of unit
characters, but we have to ask ourselves, how do new unit
characters arise in the first instance ? It seems at least as
probable that they arise by the gradual accumulation of slight

416 OUTLINES OF E VOLUTION ABY BIOLOGY

fluctuating variations under the control of natural selection as
that they originate in any other way that can be suggested in
the present state of our knowledge ; and even if these variations
are at first purely somatogenic, we may suppose that in the
course of many generations they gradually exert a cumulative
influence upon the germ plasm until the latter, so to speak,
topples over into some new position of equilibrium and a new
unit character arises. We cannot, however, now add to what we
have already said on this subject when dealing with the theory
of heredity.

One of the most important factors in bringing about divergent
evolution is undoubtedly isolation. However a new character
may have arisen it is liable to be swamped by the crossing of
the individuals which possess it with others which do not possess
it unless by some means or other the two groups are prevented
from interbreeding. We cannot fail to see the importance of this
principle when we study the fauna and flora of remote islands,
and their relationships to those of the nearest continental areas
or of other islands.

Take, for example, the case of the Chatham Islands, which, as we
have already seen, lie some 400 miles to the east of New Zealand.
There can be very little doubt that these islands were formerly
connected with the New Zealand mainland, and this connection
probably continued into Pleistocene times, when a great
depression took place which caused the two to be separated by
a wide tract of ocean. All who have studied the question are
agreed that the fauna and flora of the Chatham Islands are
simply isolated detachments of those of New Zealand. Many
species, especially of the plants, are identical with New Zealand
species, but many others, though closely related to those of New
Zealand, are considered by systematists to be specifically distinct,
and they occur nowhere else in the world.

We have here an excellent illustration of the effects of
geographical isolation, which we shall be able to appreciate
better, perhaps, if we confine our attention to a few typical cases.
The common New Zealand wood pigeon, Carpopliaya (Hcmiphagd)
novce-zealandife, is represented on the Chathams by a species
known as Carpophaga (Hemiphaga) chatliamensis, differing but
slightly from its new genus congener, and the New Zealand

GEOGRAPHICAL ISOLATION 417

lizard, Lyyosoma moco, is represented on Pitt Island (one of the
Chathams) by a very similar form described by Mr. Boulenger
under the name Lygosoma dendyi. The remarkable New Zealand
lance-woods (Psendopanax crassifolium and P. ferox) are repre-
sented on Chatham Island by the closely related Psendopanax
chathamicwm, and the Chatham Island ribbon-wood also differs
slightly from the common New Zealand species (Plagiantlius
betulimis).

The explanation of these differences is that the two portions
into which each of the species mentioned became divided when
the Chatham Islands were separated from New Zealand have
diverged from one another and followed somewhat different lines
of evolution. Owing, perhaps, to slightly different conditions of
the environment, or to other causes which it is impossible to
specify, one or both has become modified to a greater or less
extent in its own particular direction and, owing to the
geographical isolation, there has been no interbreeding between
the two sections to keep them both in the same average condition.

The principle of isolation fully explains why the fauna and flora
of oceanic islands in general are made up almost entirely of
peculiar species found nowhere else in the world. The ancestors
of these species were originally derived from some very distant,
probably continental area, and their descendants have had few
if any opportunities of interbreeding with the parent species,
from which they have gradually diverged further and further
under their new conditions of life.

Certain writers, such as Mr. Gulick and Dr. Romanes, have
maintained that the mere separation of a species into two or
more sections which are prevented from interbreeding would
suffice to bring about divergent evolution, irrespective of whether
or not the separate sections were exposed to different environ-
mental conditions. It would probably be impossible to divide
a species into two sections whose average qualities are identical,
and : —

" No matter how infinitesimally small the difference may be
between the average qualities of an isolated section of a species
compared with the average qualities of the rest o f that species, if
the isolation continues sufficiently long, differentiation of specific
type is necessarily bound to ensue." l

1 Romanes, " Darwin and after Darwin," Vol. Ill, " Isolation and Physiological
Selection," p. 13.

B. BE

418 OUTLINES OF EVOLUTIONARY BIOLOGY

If isolation is necessary for the establishment of new species
by divergent evolution, why, it may be asked, do we find closely
related species, which we must suppose to be descended from
common ancestors at no very distant period, actually inhabiting
the same areas at the present time ? The answer is that there
are other means by which groups of organisms can be prevented
from interbreeding besides geographical isolation.

It used to be supposed that one of the best tests of the specific
distinctness of any two forms was their incapacity for breeding
together and producing fertile offspring.1 The mule, it is true,
is the offspring of parents belonging to two distinct species, the
ass and the horse, but the mule is almost always sterile, and most
well characterized species2 are incapable of breeding together at
all. This fact has been regarded by many people as a most serious
difficulty in the way of comparing the origin of species by natural
selection with the results produced amongst domesticated plants
and animals by artificial selection, for the products of artificial
selection, however much they may differ from one another, if
they have been derived from the same parent species will
remain capable of breeding together with perfect fertility.

The solution of this difficulty is to be found in the theory of
" Physiological Selection," which we owe to Mr. Gulick, Dr.
Romanes and others. These writers point out that amongst
the endless variations to which plants and animals are subject
will be variations in the reproductive system, by which certain
individuals will be rendered infertile when crossed with others
of the same species, while remaining fertile with individuals
which have varied in the same manner as themselves. In this way
a species may be as effectively divided into two sections as by any
geographical barrier, and under these circumstances divergent
evolution may be expected to take place. According to this view
the mutual sterility which, to a greater or less extent, undoubtedly
does characterize distinct species in a state of nature, is the
cause and not the result of their distinctness, and cannot be
regarded as a reason for supposing that there is any essential
difference between the processes of artificial and natural
selection. In artificial selection it is merely another kind of

1 Although Lamarck pointed out a century ago that this is really no criterion of
specific distinction. The idea that it is so is clearly expressed by Buffon (" Histoire
Naturelle," Tom. VI, 1756, p. Ifi).

2 At any rate amongst the higher animals.

NON-ADAPTIVE CHARACTERS 419

isolation that has been employed to prevent the swamping
effects of intercrossing ; but it is an isolation that may be
broken through at any moment, and if all the different varieties
of some domestic plant or animal were turned loose to struggle
for existence and interbreed with one another and with the
parent species in a state of nature, they would probably in most
cases very soon cease to have any separate existence.

The theory of natural selection, combined with that of the
gradual inheritance of the effects of use and disuse and of other
modifications brought about by the long-continued influence of
the environment, affords a satisfactory explanation of the evolu-
tion of adaptive characters. Many if not all organisms, how-
ever, exhibit characters to which we can assign no adaptive value,
which do not seem to be of any particular use to the organism in
the struggle for existence, or which apparently might, so far as
utility is concerned, be equally well replaced by any one of a
number of alternative characters.

Amongst the microscopic Protozoa species are frequently
distinguished from one another by minute differences in the form
or ornamentation of the skeleton (compare Figs. 3 and 4).
Different species of the genus Lagena (Fig. 4),1 amongst the
Foraminifera, for example, exhibit different sculptured patterns
upon their flask-shaped calcareous shells. Are we to suppose
that it is of any consequence to the gelatinous, Amoeba-like
inhabitant of the flask whether its shell be ornamented in one
way rather than in another ? Does one pattern help a uni-
cellular foraminiferan or radiolarian more Lhan another in the
struggle for existence ? The same argument applies to the
extremely minute siliceous flesh-spicules or microscleres
which occur scattered without order through the ground sub-
stance of many sponges, and the form of which is regarded by
those who have studied the question as by far the most reliable
guide, not only in the recognition of species, but also in the
grouping of these species in genera and families. Take, for
example, the wonderful chelae (Fig. 187), characteristic of the
family Desmacidonidae. Different genera and species are dis-
tinguished by differences in the size, shape and number of

1 The two figures below the centre figure and two in the bottom right hand
corner represent four species of Lagena.

E E 2

420 OUTLINES OF EVOLUTIONARY BIOLOGY

the teeth of these microscopic and apparently useless organs —
useless at any rate so far as their generic and specific characters
are concerned, for what can it matter to the sponge whether the
number of the teeth be three or more or less, or whether the teeth
at the two ends of the spicule be equal or unequal? Yet, in the
course of evolution, such characters as these have become more

ElG. 187. — Siliceous Spicules (Chelae) of Sponges. (After Eidley and
Dendy, in " Challenger " Eeport.)

A. A', front and side views of chela of Esperella lapidiformis, x 360.

B. B', front and side views of chela of JEsperiopsis pulchella, x 284.

C. C', front and side views of chela of Cladorhiza (?) tridentata, x 360.

or less fixed and constant and they are evidently handed down
from generation to generation by the ordinary process of heredity.1
We pointed out in an earlier chapter that the external form
of the entire sponge is, in some cases at any rate, explicable
as an adaptation to peculiar conditions of the environment.
We saw this is the case of the curious " Crinorhiza " form
(Fig. 168) which prevents the sponge from sinking into the soft
mud or ooze on which it rests. Let us glance for a moment,
however, at another deep sea sponge, Esperiopsis challenger^
dredged up by the " Challenger " Expedition from a depth of

1 The reader should refer back to Fig. 88 for other forms of sponge spicules.

NON-ADAPTIVE CHARACTERS

421

825 fathoms in the Malay Archipelago. In some respects this is
the most remarkable sponge that has ever been discovered. Its
form (Fig. 188) is absolutely unique and resembles rather that of
some graceful plant than those of other sponges. It belongs to a
group of sponges whose members occur
mostly in much shallower water and are by
no means distinguished by beauty or sym-
metry of shape. In spiculation, moreover,
and other minute anatomical features, it
exhibits no striking peculiarities ; indeed, so
closely does it agree with more ordinary
species of Esperiopsis that it has not as
yet been considered necessary to separate it
generically. How then can we account for
this wonderful form ? Can we say that it
is an adaptation to any special conditions of
the environment ? It hardly seems likely
that this is the case, for we know that the
relatives of this sponge get on well enough
with all sorts of other forms, for the most
part more or less irregular and ill-defined,
and we know of nothing in the conditions
under which it lives to make such a unique and
beautiful form especially advantageous. The
" Challenger" obtained no less than thirteen
specimens of this sponge at the same place,
and the form appears to be quite constant.
No doubt the undisturbed condition of the
at great depths favours symmetry of

FIG. 188. — Esperiofisia
challenyeri, X £.
(After Ridley and
Dendy, in " Chal-
lenger'' Eeport.)

sea

growth in sessile organisms like sponges,
but why this particular and absolutely unique
shape, so different from anything else that
has ever been met with ?

These are questions which we cannot
answer, and it must suffice to point out that such cases can hardly
be explained by the theory of natural selection. Many factors must
combine in determining the course of evolution of any particular
organism, and some of the characters which result from their
interaction may perhaps have no more direct relation to the
necessities of the organism in the struggle for existence than has
the colour of a pebble to its continued existence on the sea-shore.

422 OUTLINES OF EVOLUTIONARY BIOLOGY

It may even be questioned whether any large proportion of
specific characters can have arisen through the action of natural
selection. The characters by which we are accustomed to sepa-
rate one species from another — such as minute differences in
size, shape and colour — are usually so slight that we are hardly
justified in attributing to them any adaptive value. They may,
however, form the starting points from which, under the influ-
ence of natural selection and use and disuse, adaptive characters
may subsequently arise.

The application of the principles of organic evolution to the
problem of the origin and progress of the human race cannot be
adequately dealt witli in the present volume, while at the same
time we cannot altogether ignore it, for not only is it in man
himself that we find the most remarkable illustration of what
has been accomplished by evolution, but the future progress of
mankind must depend in large measure upon the correct
understanding of the principles in question.

Buffon, more than a century ago, pointed out the close resem-
blance in anatomical structure between man and the higher
apes, and it is clear that both he and Lamarck were only pre-
vented by religious scruples from definitely maintaining the
origin of the human species from ape-like ancestors, a view
which at the present time is universally accepted amongst
scientific men. This reluctance to admit the obviously close
relationship of the human species with the apes was one of the
evil results of the intellectual dishonesty and obscurantism of
the middle ages. If we go back to the days of Carthage, we
find that the explorer Han no did not hesitate to speak of the
"gorillas "l which he met with in Africa as hairy men and
women of the woods, and although this was doubtless going too
far in the opposite direction, it shows not only that he recognized
the relationship but also that he approached the question with a
mind entirely free from prejudice.

Man is one of the latest products of organic evolution, and his
appearance upon the scene possibly does not date further back than
Pliocene times. It is said that flint flakes of human workman-
ship have been discovered in early Pliocene deposits of Burmah,

1 Probably really chimpanzees.

ANTIQUITY OF MAN 423

but perhaps the earliest actually human fossil so far known is
a lower jaw of very massive form which was found in a deposit
of late Pliocene or early Pleistocene age near Heidelberg, and
described by Schoatensack in 1908 under the name Homo
Heidelberg ensis. This name implies that in the opinion of its
author the fossil man of Heidelberg was generically, but not
specifically, identical with the human beings which now exist.
The question of the distinction of species in the genus Homo,
however, is, as in most other genera, a very difficult one, and
opinions are divided as to whether only one or several species
should be recognized amongst the existing races of mankind.

So close is the anatomical agreement between the genus
Homo and the higher apes that there is little room for con-
necting links between them, the difficulty being rather to find
any definite characters by which they can be separated than to
discover reasons for bringing them together. Nevertheless, the
gap, small as it is, has been filled by the discovery, by Dr.
DuboTs in I»y4,~~bf the remains of a semi-human, ape-like
creature, to wliich the name Pithecanthropus erectus has been
given. These remains were found in Pliocene strata in the
island of Java and consist of a portion of a cranium, a thigh
bone and two molar teeth.1

In deposits of Pleistocene age undoubtedly human remains
become fairly abundant, and are found associated with the bones
of other mammals, of which many, such as the mammoth, the
cave bear and the woolly .rhinoceros, are now extinct.

The chief factors which contributed towards the gradual
transformation of ape-like into human creatures were doubtless
the same as those which have operated in the evolution of other
branches of the animal kingdom, namely, the efforts which the
ancestral forms were obliged to make in order to maintain them-
selves in the struggle for existence and the natural selection of
favourable variations. In no group of the animal kingdom do we
see better illustrated the importance of Lamarck's principle of
the effect of changed habits upon bodily organization.2

The anthropoid ancestors of man were undoubtedly, like

1 Some authorities regard these remains as truly human. The difference of
opinion is itself very instructive.

2 Darwin, notwithstanding what he says in the sixth edition of the " Origin of
Species " about the effects of use and disuse (quoted on p. 392), denies, in the
" Descent of Man " (Ed. 2, p. G19), that man has risen through his own exertions.
I can see no reason for such a pessimistic view.

424 OUTLINES OF E VOLUTION AEY BIOLOGY

existing Simiidae, arboreal in habit. Their limbs, whose primi-
tive pentadactyl structure indicates their origin from some
little-specialized mammalian type, had ceased to be used exclu-
sively as organs of locomotion on the ground and become adapted
for climbing trees. Hence the opposable toe and thumb, which
enabled their possessor to obtain a firm grasp of the branches.
In the existing apes hand and foot are very similar, and both,
as regards function, partake as much of the nature of hand as of
foot, whence the name " Quadrumana " applied to the group by
the older naturalists. In many of the lower apes or monkeys a
long prehensile tail, which can be twisted round the branches, is
of great assistance to the animals in their arboreal habits, but
in the higher forms, such as the chimpanzee, the gorilla and
the orang utan, the tail has already disappeared.

These tailless forms are still mainly arboreal, but when they
have occasion to come to the ground they assume a semi-erect
posture in locomotion. In walking they usually put their hands
to the ground, but generally resting upon the backs of the fingers,
instead of upon the palms as in the lower apes.1 Hence in these
large old-world apes the hands and feet are more completely
differentiated from one another.

The next step in the evolution of man was probably the
gradual abandoning of arboreal habits and the assumption of a
more completely erect attitude during locomotion. This appears
to have been the real starting point of his human career. It
was this change of habit which led to those structural modifica-
tions required to balance the body properly in its new position,
and to the further differentiation between hand and foot, the
latter being used to the complete exclusion of the former as an
organ of locomotion and at the same time losing its prehensile
character, so that one of the chief distinguishing features between
man and the higher apes is that in man the great toe has ceased
to be opposable.

In this way the hand was completely set at liberty for the
purposes of a prehensile organ. It was, however, no longer used,
as in the apes, chiefly for laying hold of branches in climbing
and for conveying food to the mouth, but more for grasping
loose objects and converting them into weapons or tools for
different purposes. Hanno observed that the anthropoid apes
which he met with in Africa defended themselves with stones,

1 Vide Beddard, " Mammalia " (Cambridge Natural History, Vol. X), p. 570.

EVOLUTION OF MAN 425

so that we can hardly say that the use of tools or weapons is an
exclusively human attribute ; but the hand of man is undoubtedly'
one of his most characteristic features, and by its aid man has
been able to make for himself an unlimited number of what are
really additional organs, derived not from his own body but from
his environment.

To the experience gained by the exercise of the hands in so
many different ways must also be attributed in large measure
the extraordinary mental and moral development which, more than
anything else, separates mankind from the apes. The constant
exploitation of the environment stimulated and exercised the brain,
which in turn suggested methods of employing the hands and
the tools which they had constructed to ever greater advantage ;
and thus both hand and brain progressed until they attained their
present wonderful state of efficiency.-

The development of the brain has, however, long since taken
the lead in human evolution and, considering the immense
difference in intellectual capacity, it is surprising that there
should be so little structural difference between the brain of
man and that of the higher apes. As Huxley has pointed
out : —

" It is only in minor characters, such as the greater excavation
of the anterior lobes, the constant presence of fissures usually
absent in man, and the different disposition and proportions of
some convolutions, that the Chimpanzee's or the Orang's brain
can be structurally distinguished from Man's."1

Intellectual capacity appears, however, to depend mainly upon
the size of the cerebral hemispheres, which is doubtless correlated
with the number of nerve cells present, and in this respect the
human brain is far in advance of that of any ape.

One of the most important characters which differentiate man-
kind from the apes is the faculty of articulate speech, but even this
undoubtedly had its beginnings in the inarticulate sounds made
by ape-like ancestors, either as spontaneous expressions of their
emotions or as a means of communicating more or less definite
ideas to one another. The development of speech provided a
new means for the transmission of experiences from one genera-
tion to the next, and as a consequence knowledge began to
accumulate in the minds of the human race. When oral tradition

1 Huxley. "Man's Place in Nature," p. 140 (Collected Essays, Vol. VII).

426 OUTLINES OF EVOLUTIONARY BIOLOGY

gave place to the establishment of written records, and methods
were invented for the indefinite multiplication of these, the
accumulation of knowledge took place at a much more rapid rate
and it became possible for every -human being, at any rate in
civilized communities, to benefit from the experience of all his
fellow men. The acceleration of intellectual and moral progress
which has been brought about in this way has led to results
which may well have deluded man into the belief that he is the
centre of the universe and that between himself and the lower
animals there is a great gulf fixed.

Man has indeed acquired a degree of control over his environ-
ment and over his own destiny which distinguishes him from any
of the lower animals, but at the same time the conditions of his
life have become far more complex and the young, at any rate in
civilized communities, have to go through a long course of
education before they are fit to enter upon the struggle for
existence on their own account. Amongst the lower animals all,
or almost all, the faculties necessary for existence are directly
inherited from the parents, incorporated in the organism itself,
but man inherits in this way only a relatively small proportion
of the powers which he requires to carry on his life. The greater
part of human experience is of too recent origin to have become
heritable ; it has to be acquired afresh by education in every
generation, and in this respect is strikingly contrasted with the
instincts of the lower animals.

The immense advance which civilized man has made since he
parted company with his ape-like progenitors is shown, not only
by the fact that he has already to a large extent subjugated the
remainder of the organic world and directed the forces of inanimate
nature into new channels to serve his own purposes, but also by
the intelligent forethought which he exercises for the future
welfare of his own race. At the present time this forethought
is being exercised in new directions, and a determined effort is
being made to apply our knowledge of the principles of organic
evolution to the furtherance of human progress. In spite
of many differences of opinion, such as that which still
prevails with regard to the relative importance of breeding
and education, we have undoubtedly arrived already at many
results which are of vital significance in this connection,
and the intelligent application of scientific principles must
here, as always, lead to further progress. We cannot, however,

HUMAN PROGRESS 427

control the future of the human race until we have familiarized
ourselves with the past, and learned to recognize the part
played by the numerous different factors which have been con-
cerned in the evolution, not only of mankind, but of the whole
organic world.

It must always be remembered that the problem before us is
one of extreme complexity, and that we cannot afford to neglect
any of the factors involved. Above all, we must avoid dogma-
tizing on an insufficient basis of fact. If, for example, the very
modern doctrine of the non-inheritance of acquired characters is
allowed to influence the actions of men and women, and if, after
all, this doctrine should prove to be erroneous, as • seems highly
probable at the present time, the attempt to apply biological
principles to the welfare of humanity may well end in disaster.
The penalty which each generation has to pay, in regard to
bodily and mental organization, for the mistakes and misfortunes
of its ancestors, may, in most cases, be a very small one ; but if
there is any penalty at all, and if the mistakes are continued
from generation to generation, it will surely be a cumulative one.

In dealing with problems of this kind a rational conservatism,
with a mind always open to conviction, seems the only safe
attitude to adopt.

INDEX

A.

ABBREVIATION of ontogeny, 273.
abiogenesis, 214 — 216.
Acacia pycnantha, seedling, Fig. 132.
acacias, life-history, 280.
Acanthodes, 291, Fig. 138.
accessory chromosomes, 135.
accessory idioplasm, 167.
accretion, 20.
achromatic figure, 74.
acquired characters and human
progress, 427.

,, ,, Bordageon, 182.

,, ,, Brewer on, 178,

180.
,, ,, Brown - Sequard

on, 180.
„ ,, Buff on on, 367,

368.
,, ,, Charles Darwin

on, 180, 393.
,, ,, de Vries on,

414.
,, ,, Eigenmann on,

183.

,, ,, Erasmus Dar-

win on, 372.

,, ,, Henslowon,183

,, ,, Herbert Spencer

on, 185.

,, ,, Ililgardon, 178.

,, ,, inheritance of,

165, 169, 176—
193, 404.

,, ,, Lamarck on,

379, 380, 382.

,, ,, meaning of

term, 156, 176,
177.

,, ,, Sumner on, 182.

,, ,, Weismann on,

176, 179.

actinula larva, 122.
activity of male, 84, 85.
adaptation, 235, 365.

,, and design, 212.
,, ,, fluctuating varia-

tion, 414, 415.

adaptation and mutation, 41 J, 415.

,, ,, natural selection,

396, 404, 405.

,, average, 184.

,, co-operation of factors in
bringing about, 405.

,, embryonic, 267

,, Erasmus Darwin on,
372, 373.

,, in animals, 334 — 349.

,, individual, 184.

,, in inorganic world, 395.

,, in leaves of acacias, 280.

,, in plants, 350 — 363.

,, mutual, 414, 415.

,, origin of, 183.

,, resulting from survival
of fittest, 391.

,, Eobert Chambers on,

383, 384.

adipose tissue, 56, Fig. 19.
aesthetic development of man, in-

flueuced by insects, 364.
aesthetic sense, in courtship, 349.
affinities, natural, 228, 229.
after-effects, 191.
age of habitable earth, 285 — 287.
age of ocean, 287.
aggregate species, 224.
aggressive resemblance, 337.
Agnatha, 290.
Agnosias princeps, Fig. 134.
air-bladders of Fucus, 99.
air-breathing vertebrates, origin,

256.

ala spuria, 305.
albumen, 23.
alcoholism, 157.
alimentary canal, 124.
allantois, 270.
allelomorphs, 200, 206.
aloes, 350.

Alpine plants, 181, 262, 350.
Alps, Arctic vegetation on, 333.
alternation of generations, 101.

in aphides, 144.

in ccelenterates, 121.

in ferns, 101—106.

in flowering plants, 106 — 112.

430

INDEX

Amauris niavius, 346, Fig. 177.
Amblyopsis, 183.

ambulatory appendages of arthro-
pods, 246, 247.

ambulatory legs of vertebrates, 235.
Ameghino, 253.
amitotic nuclear division, 80, Fig. 36.

in Copromonas, 83.
amnion, 270.
Amoeba, 12—21, Fig. 2.
,, mitosis in, 79.
,, sensitive to stimuli, 188.
Amphibia, geological range, 284.
,, limbs, 235.

,, origin, 292.

amphimixis, 146, 171, Fig. 78.
,, and variation, 173.
Amphioxus, adult, 266, 267, Fig. 118.
,, early development, 45 —

48, 265, Fig. 13.
,, free swimming embryo,

271.

,, ovum, 140.

Amphisbsenidse, 250.
Amphitherium, 302.
anabolism, 9, Fig. 1.
analogy, 235, 217.
anatomy, evidence afforded by, 232

—262.

ancestral history, 229.
Anchitherium, 310.
Ancon sheep, 154.
Andalusian fowls, blue, 200, 201.
Andrews, C. W., 312.
androecium, 106.
anemophilous flowers, 111, 353.
Anguisfrdyilis, 250, Fig. 105.
animals, compared with plants, 34,

35.
,, dependent on green plants,

35.

anisogamous conjugation, 84.
anisogamy in Fucus, 101.
Anomodontia, 296.
Antarctic continent, 328.
anteaters, 333.

Antedon, hybridization in, 174, 175.
,, recapitulation in develop-
ment, 273, 275, Figs. 123,
124.
antheridia, of Fera, 103, 104, Fig. 51.

„ ,, Fucus, 100, Fig. 47..

antheridial cell of pollen grains, 108,

Fig. 54.
anthers, 107.

anticryptic coloration, 337.
antiquity of man, 422, 423.

antlers, 125.
apatetic colours, 336.
apes, Buffon on, 367, 368.
,, and man, Buffon on, 370.
,, ,, ,, Lamarck on, 382.
,, ,, ,, relationship of, 422 —

425.

aphides, parthenogenesis in, 143.
Aphrodite, story of, 214.
apogamy in ferns, 10j.
aposematic coloration, 342.
Apteryx, 258, Figs. Ill, 112.
Apus, dispersal of, 327.
arborescent colonies, 40.
Archseopteryx, 300, 305—307, Fig.

151.
archegonium, of Fern, 103, 104,

Fig. 52.
,, ,, flowering plant,

108.

architype, 232.
Arctic climate, retreat of, 333.

,, vegetation on Alps and Pyre-
nees, 333.

areas of distribution, 319, 320, 330.
Argus pheasant, 349.
Aristotle, 163, 385.
arm, of man, 237, 238, Fig. 94.
armadillos, 333.
arrow worm, origin of germ cells,

130.

Artemia, chromosomes, 73.
Arthropoda, limbs, 246, 247.
arthropods and vertebrates, 255.
artificially produced characters, 156,

157.
artificial parthenogenesis, 145.

selection, 395, 396, 410—

413.
,, ,, and fertility, 418,

419.

artiodactyl limbs, 241, Fig. 98.
Arum, fertilization, 355.
Ascaris, chromosomes in, 73.

,, distinction between somatic

and germ cells, 166.
,, fertilization of egg, 131,

Fig. 64.

,, mitosis, 79.
,, origin of germ cells, 129.
ascidian, adult, 277, Fig. 129.
,, degeneration in, 400.
„ larva, 277, Fig. 130.
asexual reproduction, in Eudorina,

91.

,, ,, in Hydra,

116.

INDEX

431

asexual reproduction, in Obelia,

119.
„ ,, inPandorina,

89.

Aspidium, Figs. 48, 50.
asters, in mitosis, 71.
atavism, 261, 262.
attraction of gametes, 143.

„ ,, ,, in Coccidium,

141, 142.

,, ,, ,, in ferns and
mosses, 141,
142.
in Spirogyra,

142.

„ „ sexes, 125—127.
,, sphere, 71.
Aucuba, male and female plants,

111.

Aurelia, meristic variation in, 149.
Australia, fauna and flora of, 250,

328, 329, 331.
Australian climate, adaptation to,

280.

automatism, 19.
Avebury, Lord, 281.
average adaptation, 184.
axial filament, of spermatozoon, 140.
Axoniderma mirabile, Fig. 168.

B.

BABIRTJSA, tusks, 406, Fig. 185.
baboons, Buffon on, 367, 368.
Bacillus saccobranchi, 66, Fig. 27.
Bacteria, 217, 218, Fig. 83.

,, alleged spontaneous genera-
tion, 215.

,, origin, 219.

,, structure, 66, Fig. 27.
Badhamia, Fig. 29.
Bakewell, 387.

liahvna mysticetus, 245, Fig. 101.
Balsenidse, teeth, 260.
barnacles, degeneration, 401.
barriers to migration, 320.
basal cell, 48.
bast, 65.

Bates, II. W., 346.
Batesian mimicry, 346.
Bateson, W., 195, 205, 207, 208.
bats, 301.
,, dispersal, 323.
,, wings, 236, 243, Fig. 99.
battle, Charles Darwin on law of, 388.
,, Erasmus Darwin on law of, 373.
beech, evergreen, 329.

beech, seedling, 281, Fig. 133.
bees, alleged spontaneous genera-
tion, 214, 215, 343.
,, as pollen carriers, 354, 355, &c.
,, parthenogenesis, 144.
,, proboscis, 354, Fig. 179.
beetles, mutation in, 159.
Behring Strait, former land connec-
tion, 309.
,, ,, former mild climate

328.

Bell, 340.

Biff en, E. H., 205.
binomial nomenclature, 226.
biogenesis, 216.
biogenetic law, 265.
Biology, origin of name, 373.

,, science of, 1.
Biophoridse, 219.
biophors, 22, 66, 168, 219.
birds, compared with reptiles, 305.
dispersal, 323, 324.
egg, 140, Fig. 70.
evolution, 306, 307.
geological range, 284.
limbs, 235.
origin, 299, 300.
wing, 243, 244, Fig. 99.
blackberries, white, 204.
blastoccel, 47.
blastoderm, 270.
blastogenic characters, 176.

,, ,, inheritance,

169.
,, ,, origin, 159,

160.

,, variations, 149, 157— 160.

blastomeres, 45, 79.
blastopore, 47, 265, 269.
blastosphere, 45, 265.
blastostyles, 120.
blastula, 45, 265.

,, interpretation, 279.

„ of Amphioxus, 45, 47,

Fig. 13.

of frog, 268, Fig. 119.
of Hydra, 118, Fig. 59.
blind worm, 250, Fig. 105.
blood, 51, Fig. 15.
blue Andalnsian fowls, 200, 201.
boa constrictor, dispersal, 325.
Bodo, life-history, 85-87, Fig. 38.
body cavity, 48, 124.
body wall, 124.
Bombus terrestris, robbing clover,

355, 356.
bone, 57.

432

INDEX

bony fishes, 292.

Bordage, E., 182.

Boveri, T., 146, 174.

brachiopods, 289.

brain, in evolution of man, 425.

,, of man and apes, 425.
branching classification, Charles

Darwin on, 387.
,, evolutionary series, La-

marck on, 376.

Branchiosaurus, 293, Pig. 139.
Branchipus, dispersal, 327.
breathing, 8.
breeding experiments with Papilio,

348.

Brewer, 178.

brimstone moth, larva, Fig. 169.
brittle star, 275, Fig. 126.
Brontosaurus, 299, Fig. 144.
Brown-Sequard, 180.
Buceros rhinoceros, Fig. 407.
budding, 263.

in Hydra, 116.
in Obelia, 119, 120.
Buffon, on sterility of species when

crossed, 418.
views of, 365—370.
Burbank, L., 204.
Butler, Samuel, 3, 186, 191, 369.
Butscbli, O., 19, 21, 22.
butterflies, warning colours, 343 —

347.

butterfly, leaf, 338, Fig. 171.
,, metamorphosis, 264.
„ proboscis, 354.

C.

CACTI, 350.

,, thornless, 204.
csenogenetic characters, 278 — 281.
Csenolestes, 301.
Csesar's horse, 262.
Cainozoic Era, 284, 285.
calcareous skeletons, 26.
Calceolaria, distribution of, 328.
calcium, influence upon segmenta-
tion, 45.

callosities, Buffon on, 367, 368.
calyx, 106.
cambium, 76.
Cambrian Epoch, 284.
camel, Buffon on, 367.

„ feet, 241, Fig. 98.
Camelo-pardalis, Lamarck 011, 379.
cancer, nature of, 408.
Cancer pJialangium, 339.

candle, analogy of, 2, 5.

Candollea graminifolia, fertilization,

362.
cane sugar, attracting spermatozoa,

141.

canine teeth, reversion in, 262.
cannon bone, 241.
Capsella bursa-postoris, development,

48, Fig. 14.
carbohydrates, 27.
carbon, 6.

carbon dioxide, 6, 8.
Carboniferous Epoch, 28-1.
Carchesium, 40.
carnivores, 301.
carpals, 237.
carpels, 106.
Carpophaga chathamensis, 416.

„ nova: zealandia:, 416.

carrion flies, 355.
cartilage, 56, Fig. 20.
casein, 23.

castration, effect, 125.
Catasetuni, fertilization, 362.
caterpillar, larval organs, 275.

„ stick, 337, 338.
cats, inheritance of mutilation, 178.

,, stump-tailed, 179.
caudicle, 361.
cave-animals, bleaching inherited,

183.
cell, 12.

body, 13.
definition of, 38.
division, 69—80.

,, limits of, 81.
history of term, 36, 37.
membrane, 69.
plate, 74.
sap, 62
theory, 38.

,, limitations of, 65.
typical, 69, 70, Fig. 30.
wall, 27, 28, 69.
cellular structure, 36.
cellulose, 27, 28.
centrosorne, 71.

, , absence i n higher plants ,

141.

,, ,, ,, ovum, 141.

,, as stimulus to develop-

ment, 146.
,, in spermatozoon, 139,

Fig. 69.

,, in unfertilized egg, 146.

,, origin of, in zygote, 141.

centrosphere, 71.

INDEX

433

Cephalaspi*, 290, Fig. 135.
Ceratodus (see Neoceratodus).
cercariae, 144.
cereals, fertilization, 412.

,, improvement, 410— 413.

,, Mendelian inheritance in,

205.
cervical vertebrae, of giraffe, 248,

Fig. 104.

„ „ „ whale, 248,

318, Fig. 103.
Cetacea, evolution of, 314 — 318.

„ limbs, 236.

Chsetopterus, artificial partheno-
genesis, 145.
chalk, 26.

chamaeleon, colour change in, 341.
Chambers, Robert, views of, 383—

385.

change of function, 255, 256, 262.
Chara, gametes, 141.
characters, coinpound, 208.
"Chatham," H.M.S., 399.
Chatham Islands, 325, 398—400, 416,

417.

chelso, of sponges, 420, Fig. 187.
Chelonia, 295.
chemical affinity, 6, 7.
,, processes, 9.
,, stimulus to development,

146.

chemically modified larvae, 157.
chemotaxis, 141, 143, 189.
chick, embryo, 265, Figs. 117, 122.
chimpanzee, brain, 425.

„ Buffon on, 370.
Chinese women, small feet, 156.
Chironomus, psedogenesis, 144.
Chlamydomonas, 40.
chlorophyll, 7, 8, 28.
,, cells, 64.

,, corpuscles, 32, 64.

chloroplaslids, 32, 64.

,, in Spirogyra, 96.

Chordata, 277.
chordate condition, 266.
chromatiu, 70, 71.

,, in heredity, 166, 169, 174,

206.

chromatophores, in Spirogyra, 96.
chromomeres, 73, 168, 206.
chromosomes, 71, 73, 168.

and sex, 135, Fig. 67.
,, differential division,

171, Fig. 77.

,, in gametophyte, 138.

,, „ sporophyte, 139.

B.

chromosomes, integral division, 171,

Fig. 77.
,, maternaland paternal,

137, 207, Fig. 68.
„ pairing, 133, 137, 206,

Fig. 68.

„ reduction, 132, 133,

137, 138, 206, 207,
Fig. 68.
chrysalis, 264.
cilia, 39.

dona intestinaHs, Fig. 129.
circumcision, 179, 180.
Cirolana, 227.

cirripedes, degeneration, 401.
Cladocera, parthenogenesis, 144
Cladorhiza longipinna, Fig. 168.

,, (?) tridentata, spicules,

Fig. 187.
classes, 226.
classification, 225—228.

,, and phylogeny, 230,

Fig. 87.
artificial, 228.
,, Buffon on, 366.

,, Charles Darwin on,

387.

,, Lamarck on, 374, 375.

natural, 228, Fig. 86.
clavicle, 234, Figs. 89, 90, 93.
clear-winged moths, mimicry, 343,

Fig. 175.

cleavage of ovum, 75.
climate, adaptation to, 280.
,, changes in, 327.
,, influence of, Buffon on, 366.
,, ,, ,, Lamarck on,

376.

,, Miocene, 328.
clover leaf, meristic variation, 149.
coal, energy in, 4.

Coccidium, gametes, 87, 88, Fig. 39.
,, maturation of ovum, 139.
coccyx, in man, 261.
cockroaches, dispersal of, 325.
Codonella, Fig. 9.
Cceciliidse, 250, Fig. 106.
Coolenterata, 114.
coelenterate and gastrula, 265, 269,

270, 277.

ccelom, 48, 124, 266.
ccelomate and coelenterate types,

124, Fig. 62.
coalomic epithelium, 124.

,, pouches, 266.
cold-blooded animals, 6.
colloids, 23.

F F

434

INDEX

colony formation in Hydra, 116.
,, Obelia, 119.

,, „ „ Protozoa,40 — 44.

colour changes in animals, 341, 342.
colours of animals, 336, 337.

,, ,, ,, Erasmus Darwin

on, 372.

,, ,, flowers, 355.
column, of flower, 360, 363.
combustion, energy liberated by, 4, 6.

,, nature of, 2, 5, 6.

communication between cells, 187.
compound characters, 208.
conceptacles of Fucus, 100.
Condylarthra, 309, 313.
cone of attraction (or reception) in

Coccidium, 88, 143.
congenital variations ( characters), 157.
Conilera, 227.
conjugants, in Paramcecium, 92,

Fig. 41.
conjugation of chromosomes, 137,

Fig. 68.

„ gametes, 33, 82—85,
131, 141,
206, 207,
Fig. 65.

,, ,, iu Ascaris,

131, Fig.
64.
,, ,, in Bodo, 87,

Fig. 38.

,, ,, in Cocci-

dium, 88,
Fig. 39.

,, ,, in Ccelo-

mata, 125.

,, ,, in Copromo-

nas, 83,
Fig. 37.

„ ,, inEudorina,

91, Fig. 40.

,, ,, in ferns, 105.

,, ,, in flowering

plants,
109.

,, „ in Fucus,

101, Fig.
47.

,, ,, in Haema-

tococcua,

33, Fig. 5.

., ,, in heredity,

171.

,, ,, in Pando-

rina, 89,
Fig. 10.

conjugation of gametes in Para-
moDcium,
92, 93,
Fig. 41.

,, ,, in Spiro-

gyra, 9(3,
97, 142,
Figs. 43,
44.

„ in Zygogo-

iiiuni, 97.

,, of nuclei in zygote,

131, Fig. 64.
,, origin of, 127.
,, results of, 84, 87.
connecting links, 232—235, 305—

318.
,, ,, destruction of,

222.

conodoiits, 289.
conservatism of germ cells, 184,

190.

continental islands, fauna, 332.
continents, permanence, 329.
continuity of germ plasm, 166, 168,

Fig. 76.
„ life, 67.

,, ,, parent and offspring,

Erasmus Darwin on,
370.

,, ,, protoplasm, 66.

continuous selection, 411.

,, variations, 148, 150.

contractile tissue (see muscle fibres).

,, vacuole, 14, 18.

convergence, 247 — 255.

,, in Acacia, 280.

,, „ flightless birds, 397.

„ plants, 2fi2.
,, ,, proboscis of bees and

Lepidoptera, 354.
" Convolvulus major," fertilization,

351, 352.
co-operation, 40.

,, of factors in evolution,

421.

Copromonas, life-history, 83, Fig. 37.
,, nuclear reduction, 139.

coracoid, 234, Figs. 89, 93.

„ vestigial, 257, Fig. 90.
corals, 114, 2H5.

cork, cellular structure, 37, Fig. 6.
corolla, 106.
corpora lutea, 126.
corpuscles, blood, 51, 52, Fig. 15.
correlation, 409, 415. .
cortical tissues, 50.

INDEX

435

cosmopolitan distribution, 320.

Cosmozoa, 216.

cotyledons, caenogenetic characters,

280, 281, Figs. 56, 133.
cowslip, fertilization, 356.
crab, larva, 276, Fig. 128.

„ swimming, 247, Fig. 102.
creation, Lamarck on, 375, 376.

special, 212—214, Fig. 82.

Buffon on, 369.
,, ,, Linnaeus on, 222.

Creodontia, 318.
Crepidula, 323.
Cretaceous Epoch, 284.
Crinoidea, 274.

Crinorhiza form, 335, 420, Fig. 168.
Cristatella mucedo, statoblast, Fig.

165.

Crocodilia, 295.
cross -fertilization of flowers, 351 —

363.

crossing (see hybridization).
Crustacea, fresh water, dispersal,

327.

,, gilH 255.

cryptic coloration, 337.
crystalloids, 23.

currents, dispersal by, 321 — 325.
curve of frequency of error, 153.

,, ,, variation, 150, 153, Figs. 72,

73.

Cuscuta europcva, 402, 403, Fig. 184.
cushion plants, 262, 350.
Cyathaspis, 290.
cyclopean larvae, 157.
Cyclostomata, 289.
cyst, 20, 83.
cytology, 67.
cytolysis of ovum, 147.
cytoplasm, 13, 69.

,, in heredity, 174, 175.
cytostome, 83. "
cytotaxis, 142.
cytotropism, 142.

,, of gametes, 143.

Cyttaria, 329.
Cyttarocyclis, Fig. 9.

D.

DARBISHIRE, A. D., 197.

Darwin, Charles, views of, 164 — 167,
176, 180, 185, 195, 208, 222—
224, 329, 351,— 353, 356, 357,
359, 385—388, 391—393, 410,
415, 423.

Darwin, Erasmus, views of, 370 — 373.

,, Francis, 192.
death, 21, 162, 168.
De Candolle, 386.
decay, nature of, 4.
deep sea animals, adaptation in, 335,

336.

deer, feet, -241, Fig. 98.
degeneration, 397 — 403.

,, in ascidians, 400.

,, ,, flightless birds, 397.

,, ,, Morioris, 400.

results of, 398.
Delage, Y., 146.

Delphinus deljihis, Figs. 161, 162.
Democritus, 369.
dendrons, 59.
dentition of dog and thylacine, 251 —

253, Figs. 107, 108.
denudation, 285, 288.
dermatogen, 50.
Descartes, 11, 260.
descent with modification, 221.
design, doctrine of, 212.
Desmacidonidae, spicules, 419, 420,

Fig. 187.
determinants, 167, 206.

,, effect of stimuli upon,

189.

,, vibrations in, 189.

deutoplasm, 140, 267.
development, 263 — 281.

,, and unconscious memory,

192.
,, Erasmus Darwin on,

371, 372.

,, factors in, 192.
,, of birds and reptiles, 270.
,, of flowering plants, 48 — -

50, 109, Fig. 14.
of Frog, 268—270, 272,

Figs. 119, 121.

,, of Hydra, 118, Fig. 59.
Devonian Epoch, 284.
De Vries, Hugo, 154, 204, 224, 410—

414.

Dianthus, 353.
diatoms, 25.
dichogamy, 353.
Dictyocysta, Fig. 9.
Didelphyidae, 301.
differences between plants and

animals, 34, 35.
differential division of chromosomes,

177, Fig. 77.

differentiation, 39, 43, 44, 60, 119.
„ sexual, 85.

F F 2

436

INDEX

diffusion of gases, 8.
digestion, in Amoeba, 16.
digestive cavity, of Hydra, 115.
„ Medusa, 120.
„ Obelia, 119.
digits, reduction, 239 — 241.
dihybridism, 201—203, Figs. 80, 81.
dimorphic flowers, 356.
dimorphism of gametes, 139.
Dinosauria, 297.
Dinotherium, 314, Fig. 160.
dioecious, 99, 105.
Diplodocus, 299.
Diplosoma crystallinum, larva, Fig.

130.

Dipnoi, 255, 291.
Diptera, ppedogenesis, 199.
direct nuclear division, 80, Fig. 36.
discontinuity between species, 222,

Fig. 85.
,, in distribution, 319, 320,

333.
,, in evolutionary series,

Lamarck on, 376.
,, in organic world, 221.

discontinuous variation, 149, 153 —

156.

DismorpJn'a praxiiwe, 345, Fig. 176.
dispersal of organisms, 320 — 327.
distribution, geographical, 319 — 333.
disuse (sre use and disuse).

of wings, effects of, 397, 398.
divergence in evolution, 213.

A. E. Wallace on, 390.
,, Charles Darwin on, 387.

division of labour, 39, 43, 44, 60, 85,

119.
,, ,, between male and

female, 127.
dodder, 402, 403, Fig. 184.

,, vestigial leaves, 262, Fig. 184.
dodo, 258, 397.
dog, skull and dentition, 251 — 253,

Figs. 107, 108.
,, vestigial teeth, 260.
dogfish, embryo, 270, Fig. 120.
dolphin, Figs. 161, 162.
dolphins, convergence in, 248.
„ shark-toothed, 318.
domestication, Charles Darwin on,

410.

,, Lamarck on, 378.

dominant characters, 197, 207.
Draba verna, elementary species in,

224, 412.

drone flies, mimicking bees, 343.
mistaken for bees, 215.

Dubois, E., 423.

Dujardin, 38.

dynamics of cell-division, 74.

E.

ECHIDNA, 234, 301, Fig. 92.
echinoids, larval stage, 276.
Echinus, blastorneres, 45.

., hybridization in, 174.
ectoderm, 47.

,, of coelomates, 124.

„ Hydra, 115, 116, 118.
edentates, 301, 333.
education, 426.
eels, dispersal of, 325, 326.
egg cell (see ovum).
,, of Ascaris, mitosis, 79, Fig. 35.
,, ,, bird, 140, Fig. 70.
,, „ frog, 268.
eggs, dispersal of fish, 322.

,, similarity in different or-
ganisms, 162.
,, size, 267.
Eigenmaun, 183.
Elasmobranchii, 291.
electrical energy, transmission, 189.
electric eel, 256.

,, organs, 256.
electro-magnetic theory of mitosis.

74, 75.

elementary species, 224, 412.
elephants, ancestry, 312 — 314, Fig.

159.

„ limbs, 239.
Elephas, 314.
elite, 411.

Elodea, evolution of oxygen, 31.
embryo, fixation, 126.

of chick, 265, Figs. 117, 122.
,, ,, Fucus, 101, Fig. 47.
,, ,, mammals, 270.

,, rabbit, 265, Fig. 117.
embrjology, Erasmus Darwin on,

371, 372.
,, evidence afforded by,

261—281.
embryonic cell, 48.
embryo-sac, 48, 106, 107, 108.
embryos of birds and reptiles, 270.
Emily Henderson, sweet pea, 208.
Empedocles, 369.
Encrinites, 274.
endoderm, 47.

,, of coelomates, 124.

of Hydra, 115, 116, 118.
endoplasm, 14, 22.
endosarc, 14.

INDEX

437

endosperm, 109.
energy, conservation of, 6.
,, manifested in life, 4, 5.
,, of chemical affinity, 7.
,, source of, 4, 6, 7, 9.
,, ,, ,, in green plants, 30.

engrams, 186.
entelechy, 11.
enteron, 47, 265.

,, in Hydra, 115.
entomophilous flowers, 111, 353.
enucleate eggs, fertilization (see

merogony).
environment, control of, 426.

,, influence of, 5, 6, 166,

182, 183.

A.E.Wallace on, 390.

Buffon on, 366, 367.

Charles Darwin on,

387, 392.

Lamarck on, 377 —
379.

upon development,
193.

,, germ plasm,
159.

Eocene Epoch, 284.
Eohippus, 309, 310, Fig. 154.
Ephydatiafluviatilis, gemmules, Fig.

166.
epiblast, 47, 48, 266.

of Hydra, 115, 118.
epicoracoid, 234, Figs. 89, 93.
epidermis, 54, Fig. 17.

of plants, 50, 64, Fig. 26.
epigamic ornamentation, 336, 349.
epigenesis, 163.
Epihippus, 310.
epilepsy, in guinea pigs, 180.
episematic coloration, 342.
Epistylis, 40.

epithelium, 54, Figs. 16—18, 28.
epochs of earth s history, 284, 285.
equatorial plate, 72.
Equidse, pedigree of, 307 — 312.
Equus, 309, 311, Fig. 153.
eras of earth's history, 284, 285.
Esperella lapidiformi's, spicules, Fig.

187.
Esperiopsis challenger!, 420, 421, Fig.

188.
,, pulchella, spicules, Fig.

187.
eucalypts, 331.

leaves, 280.

Eudendrium, migration of germ
cells, 123.

Eudorina, 42, 90, Fig. 40.
Eurypteridse, 291, Fig. 137.

,, size of, 405.

Eutheria, 301.

,, first appearance of, 302.
evolution, factors of, 365 — 427.
in development, 163.
individual, 263.
of sex, 81—147.
progressive, 192, 334.
theory and evidence of,

211—364.
,, versus special creation,

212—214, Fig. 82.

ex-conjugants, in Paramceciuin, 93.
excretion, 9.

,, in Amoeba, 17.
experiments in heredity, 174, 175,

194—209.

explosive character of living mole-
cule, 18.
extinction of groups, 231.

,, ,, species, Buff on on, 366.
extracted dominants, 198.
,, recessives, 198.
eye-colour, Mendelian inheritance of,

205.
eyes, pineal, 258—260.

F.

FACTORS, co-operation of, 208.
,, in development, 192.

,, germ plasm, 206, 207.
,, of organic evolution, 365 —

427.

,, permutations and combina-
tions of, 206.
faeces, 16.

families, in classification, 226.
Farmer, J. B., 206.
fat, 56, Fig. 19.

feather star, hybridization, 174, 175.
,, ,, recapitulation in life-
history, 273—275,
Figs. 123, 124.
feathers, acquisition of, 177.
female animal, 114.

,, characters, 84, 85, 127.
,, dependence on male, 127.
femur, 237.
fermentation, 218.
fern, life-history, 101—105, Figs. 48—

52.

ferns, dispersal, 321.
fertility and cross-fertilization, 352,

357.
,, test of specific identity, 418

438

INDEX

fertilization, adaptation of flowers for,

351—363.
,, chemical stimulus in,

146.
,, development without,

145.

„ in heredity, 171.
,, membrane, 147.

of flowers, 110, 351—

363.

,, ,, ovum, 85, Fig. 65.

,, ,, ,, in Ascaris, 131,

Fig. 64.
,, ,, „ ,, Ccelomata,

125.

,, „ ,, ,, ferns, 105.

„ „ „ „ Fucus, 101.

,, „ ,, „ Hydra, 118.

,, ,, ,, ,, medus?e,12].

fibrillar structure of protoplasm,

22.

fibula, 237.
fig-wort, 353.
filament of stamen, 107.
finger, inheritance of mutilation,

180.
First Cause, Erasmus Darwin on,

372.

fish eggs, dispersal, 322.
fishes, bony, 292.
„ deep sea, 336.
,, geological range, 284.
fish-like stage in ontogeny and evo-
lution, 272, 273, 277, 278, 279.
fission, in Amoeba, 21.
„ „ Bodo, 87.
,, „ Copromonas, 83.
,, ,, Hsematococcus, 29.
fixation of embryo in uterus, 126.
flagella, in Bodo, 87.
,, ,, Eudorina, 91.
,, ,, Hsematococcus, 29.
flame, nature of, 2.
Flemming, 69.
flightless birds, 258.

,, ,, and fluctuating varia-

tion, 415.
,, ,, ,, natural selection,

397, 398.
flint, 24.

floating islands, dispersal by, 324.
flower, structure of, 106, lot, Fig. 53.
flowering plants, life-history, 106 —

112.
flowers, adaptation for fertilization,

351—363.
,, sexual characters, 110, 111.

fluctuating variations, 148, 150, 155,
Figs. 72, 73.

,, ,, and adapta-

tion, 414,
415.

,, ,, and natural

selection,
414.
,, ,, De Vries on,

413, 414.

flukes, parthenogenesis in, 144.
foam structure of protoplasm, 21.
foetal membranes, 270, 278.
foetus, in Mammalia, 270.
food materials of animals, 16.
,, ,, ,, green plants, 30 —

32.

,, nature of, 7.

,, vacuoles, 14.

food-yolk, 267, 278.

„ influence of, 268, 271.
foot, artiodactyl, 241.
,, of apes and monkeys, 424.
„ „ camel, 241, Fig. 98.
,, „ deer, 241, Fig. 98.
,, „ elephant, 239.
,, „ hippopotamus, 241, Fig. 98.
,, „ horse, 241, Fig. 97.
,, ,, ,, atavism in, 262.
,, ,, ,, evolution of, 310 — 312,

Figs. 154—158.
„ ,, Litopterna, 253 — 255, Fig.

109.

„ „ llama, 241.
,, „ man, 238, Fig. 94.
,, ,, oxen, 241.
,, „ pig, 241, Fig. 98.
,, ,, rhinoceros, 241.
„ ,, seal, 244, Fig. 100.
,, ,, sheep, 241.
,, „ tapir, 241, Fig. 96.
„ ,, ungulates, 239—241, 253—

255.

,, pentadactyl, 238.
,, perissodactyl, 241.
Foraminifera, 26, Fig. 4.
fossilization, 287, 288.
fowls, Mendelian inheritance in, 200,

201.

foxgloves, mutation in, 154.
Francotte, 135.
Freia, Fig. 9.

frequency of error, curve, 153.
fresh water animals, dispersal, 325,

326.

frog, early development, 268—270,
Fig. 119.

INDEX

439

frog, life-history, 272, Fig. 121.

,, ,, interpretation of,

279, Fig. 131.

, , pineal eye in, 260, Figs. 115,116.
frogs and toads, 293.
fruit trees, propagation, 205.
fruits, dispersal, 321.
Fuchsia, distribution, 328.
Fucus, 99—101, Figs. 45—47.
Functions of organisms, 5.
Fundulus, cyclopean larvae, 157,

Fig. 75.
Fungi, 34, 321.
funiculus, of ovule, 108.
furze, recapitulation in seedlings, 280

G.

Giilaxiaa niyothoruk, dispersal of,

326.

Gallardo, 75.
Galton, Francis, 209.
Gal ton's law of inheritance, 209.
Galton's polygon, 155.
Galtonia, mitosis in, 76, Figs. 33, 34.
gametes, 33, 83 (see also germ cells).

attraction of, 141, 142, 143.
,, conjugation of, 207.
,, evolution of male and female,

84, 85.
of Bodo, 87.
„ fern, 104.
,, „ flowering plant, 108, 109,

110.

„ Fucus, 100.
,, ,, Pandorina, 89.

„ Spirogyra, 96, 97, 142,

143.

„ Volvox, 91.
„ purity of, 200, 206.
,, sexual dimorphism of, 113,

139.

gametic nuclei in Paramoecium, 93.
gametogenesis, 132, Fig. 65.
gametophyte, 101.

,, chromosomes of, 138

of fern, 103, Fig. 50.
,, ,, flowering plant, 107,

108.
suppression of,

112.

gamobium, 121.
ganglion, 59.
ganoids, 292.
gastrsea, 277.
gastral cavity, 47.

,, ,, in Hydra, 115, 118.
,, ,, in Obelia, 119.

gastrula, 47, 265, 277.

,, interpretation of, 265, 279.
„ of Hydra, 118.

,, Sagitta, 130, Fig. 63.
gastrulation, in birds and reptiles,

270.

„ frog, 269, Fig. 119.
Geikie, Sir A., 285.
gemmules, dispersal of, 327.
,, in pangenesis, 164.
,, of fresh water sponge,

327, Fig. 166.
genera, 225.
generative cells, 108.
Genesis, Book of, 212.
genital ducts, 125.
geographical distribution, 319 — 333.
,, ,, summary, 330.

,, isolation, 416, 417.

geological formations, 284.

,, history of the earth, 284,

285.

,, periods, 284.
„ range of animal groups,

284.

record, 283, 287—304.
geometer moths, caterpillars, 337,

338, Fig. 169.

Oeoj)lana exulans, dispersal, 325.
Geotria, distribution, 329.
germ cells (see also gametes).

,, and somatic cells, 97, 98,
99, 113, 129, 166, 167,
168.

,, conservatism, 184, 190.
,, immortality, 162, 168.
,, independence, 99.
,, maturation, 138.
„ migration in Hydrozoa,

123.

,, origin in Ascaris, 129.
,, ,, Ccelouiates, 124,

130.

,, „ Hydrozoa, 123.

,, ,, plants, 130.

,, ,, Sagitta, 130.

,, potentialities, 163.
,, sensitive to stimuli, 188.
germinal disk, 140.

,, selection, 173.
,, variations, 149, 157 — 160.
germination of fern spore, 103, Fig.

49.
,, pollen grain, 108,

Fig. 54.

„ seed, 109, Fig. 56.

germ layer theory, 48.

440

INDEX

germ plasm, complexity, 167.

,, composition, 172, Fig. 78.

,, constitution, 205, 206.

„ continuity, 166, 168,

Fig. 76.

,, influenced by environ-

ment, 159.
gestation of nature, R. Chambers on,

384.

pigantic animals, 303, 304, 405—409.
Gila monster, 342.
gill slits, in Amphioxus, 266.

„ embryos, 261, 273, Fig.

122.

gills of crustaceans and fishes, 255.
giraffe, 2-18, Fig. 104.

,, Lamarck on, 379.
glacial periods, 327, 328.
glass snakes, 250.
gliadin, 23.
glucose, 28, 31.
glutinin, 23.
Godlewski, 174.
gonads, 113.

,, in Coelomates, 124.
,, ,, Hydrozoa, 124.
,, ,, medusae, 121.
gonoducts, 125.
gonophores, 122.
gonotheca, 120.
"gorillas," Hanno on, 42'2.
gorse, recapitulation in seedlings, 280.
gradation in nature, Buffon on, 366.
,, „ structure, 232.
,, of animals, Lamarck on,

376.
Orantia compressa, larva, 322, Fig.

164.
grape hyacinth, curve of variation,

150, Fig. 72.
,, sugar, 28, 31.
grasshopper, spermatogenesis, 134,

Fig. 66.
Gray, Asa, 386.
Gray, J. E., 227.
grazing mechanism, 309.
Great Britain, a continental island,

332.

Greenland, former mild climate, 328.
Grew, Nehemiah, 37.
gristle, 56.
growth, 10, 20.

control of, 408.

in Amoeba, 20.

,, animal tissues, 75.

,, plants, 76.

,, ,, periodicity of, 191.

guard cells, 64, Fig. 26.
guinea pigs, Brown-Sequard's experi-
ments, 180.

Gulf Stream, dispersal by, 322, 323.
Gulick, 417, 418.
gut wall, 124.
Gymnotus, 256.
gynoecium, 106.

H.

HABIT, of plants modified by en-
vironment, 181.
habitat, 319,
habits, adaptation in, 337.

,, influence of, Lamarck on,

377, 378, 379, 380.
,, in plants, 191.
Haeckel, Ernst, 66, 229, 265, 277.
hsematids, 52, Fig. 15.
hsematochrome, 28.
Hsematococcus, 27 — 34, Fig. 5.
,, conjugation, 89.

,, dispersal, 327.

haemoglobin, 53.
hair, vestigial, 261.
hairs of Tradescantia, 61, Fig. 25.
hand of man, 256, 424, 425.

,, ,, monkeys and apes, 424.
Hanno, 422, 424.
harmony, in coloration, 336.
Harpax tricolor, 339.
Hatteria (see Sphenodon).
Hawaiki, 399.

heat, in living organisms, 6.
Heidelberg, fossil man, 423.
Heilprin, A., 319.
Heliconime, 345.
Heliconius ethi/la, 345, Fig. 176.
Heloderma suspectum, 342.
Hemiphaya chathamensis, 416.

,, novae zealandia>, 416.

Henslow, G., 183.
Herbst, 45.

heredity, 161 — 210 (see also inherit-
ance).

„ Buffon on, 369.
,, Charles Darwin's theory,

164—166.
,, fertilization experiments,

174, 175.

Galton on, 209, 210.
,, in neuter insects, 189, 190.

„ Protista, 161.
„ Lamarck on, 380, 381.
„ Mendelian experiments,
194—209.

INDEX

441

heredity, mnemic theory, 186, 191,
192.

,, nature of problem, 163.

,, Pearson on, 210.

,, Weistaann's theory, 166 —

174.

Heriug, E., 186.

hermaphrodite, 103, 105, 113, 114.
hermaphroditism, 93, 116, 125, 402.
heterogamy, 144.
Heteromita, life-history, 85 — 87,

Fig. 38.

heterosporous ferns, 105.
heterostyled flowers, 356, Fig. 180.
heterozygote, 207.
hexadactylism, 154, 413.
Hilgard, E. W., 178.
Hipparion, 311.
Hippocampus antiquorum, 341, Fig.

173.

Hippopotamus, feet, 241, Fig. 98.
histological differentiation, 48.
histology, 51.
His, Wilhelm, 178.
hock, of horse, 240, Fig. 95.
holophytic nutrition, 34.
holozoic nutrition, 34.
Homo heidelhergensis, 423.
Homo, species of, 423.
homologous chromosomes, 137, 206,

207.

homology, 235, 247.
homoplasy, 235, 247.
homosporous ferns, 106.
homozygote, 207.
honey guide, 362.
honey sucking apparatus, 354.
Hooke, Robert, 36.
Hooker, J. D., 385.
hormones, 126, 188, 408.
hornbill, 406, Fig. 186.
hornets, colours of, 342, 343.
horns, excessive development, 406.
horse, evolution of, 307 — 312, Figs.

153—158.
horse's feet (see foot of horse).

,, skeleton, Fig. 95.
horses, Mendelian inheritance in, 205.
horse-worm (see Ascaris).
humble bee, robbing clover, 355,

356.

humerus, 237.

humming birds, as pollen carriers,
353.

„ „ distribution, 320.

,, ,, epigamic orna-

mentation, 349.

hyacinth, mitosis in, 76, Figs. 33, 34.
hybridization, 194—209.

,, and evolution, 209.

,, and mutation, 413,

414.

,, in cereals, 413.

Hydra, 114—118, 265, Figs. 57—59.
,, dispersal, 327.
,, suppression of medusoid, 123.
hydranth, 119.
hydrocaulus, 119.
hydrotheca, 119.
Hydrozoa, 123.
hyperphalangy, 244.
hypertonic solution, producing par-
thenogenesis, 145.
hypoblast, 47, 48.

,, of Amphioxus, 47, 266.
„ Frog, 269.
„ Hydra, 115, 118.
hypostome, 116, 119.
Hypsidse, 345.
Hyracotherium, 310.

I.

ICEBERGS, dispersal by, 324.
Ichthyopterygia (Ichthyosauria),

297, Fig. 142.
paddles, 236, 244.
Ichthyosaurus communis, Fig. 142.
idants, 168.
identical twins, 173.
idioplasm, 167.
Idvlnm diabolicum, 339.
ids, 168, 206.
igneous rocks, 282.
Iguanodon, 297, Fig. 143.
illegitimate unions, in Primula, 357.
immortality of germ cells, 162, 168.

,, „ Protista, 161.

immutability of species,

Buffon on, 369.

Lamarck on, 374.

Linnaeus on, 222.
individual adaptation, 184.

,, characters, transmission
of, 175.

,, variations, 150.
individuality of cells, 68.
inertia of germ cells, 190.
inflorescences, variation in, 150.
Infusoria, 40, Fig. 41.
inheritance (see heredity).

,, of acquired characters,

165, 176—193.
,, ,, mutilations, 178, 179.

442

INDEX

insect communities, as individuals,

190.

insectivores, 301.

insects, as pollen carriers, 353 — 355.
,, dispersal of, 323.
,, primordial germ cells, 130.
,, selection by, 363.
„ sex determination, 135, 136,

Fig. 67.

,, spermatogenesis, 135.
,, wings, 247.
instincts, adaptation in, 337.

,, origin of, 184.
integral division of chromosomes,

171, Fig. 77.
integration, 44.
integuments of ovule, 108.
intercellular substance, 57, 123.
interclavicle, 234, Figs. 89, 93.
intercrossing, swamping effects, 416.
interstitial cells, 116.
intussusception, 20.
invagination, 47.
invertebrates, dispersal, 325.

,, geological range, 284.

Ipomoea purpurea, fertilization, 351,

352.

irritability, 18.
irritable structures in flowers, 362,

363.

Isoetes, 105.
isogamy, 84, 89, 97.
isolation, 331, 416-419.

„ Lamarck on, 380, 381.
Ithomiinse, 345.

J.

JELLY-FISH, 114, 121, 265.

Joly, 286.

Jurassic Epoch, 284.

K.

KAKAPO, 397.

Kallima inachis, 338, Fig. 171.

kangaroos, 301.

Kant, 178.

karyogamy, 128.

karyokinesis, 69 — 79, Figs. 31 — 33.

karyoplasm, 14, 69.

karyosome, 70, 77.

katabolism, 9, Fig. 1.

kea, 398.

khaki clothing, 342.

kidney, effect of removal, 156.

kinetic energy, 7.

kingdoms, 226.

kiwi, 258, 331, 397, Figs. Ill, 112.
knee, of horse, 240, Fig. 95.

L.

LABELLUM, 359.
labyrinthodonts, 243, 406.
Lagena, non-adaptive characters,419.
Lamarck, views of, 176, 373 — 382,

418.

Lamarck's four laws, 382.
Lamarckian factors, A. E. Wallace

on, 393.

,, ,, Charles Darwin

on, 166, 392,
393.

,, ,, neglect of, 393.

,, ,, Robert Cham-

bers on, 385.

lampreys, dispersal, 325, 326.
„ distribution, 329.
„ pineal eye, 258, 260.
lance-woods, 417.
land connections, former, 328, 329.

,, planarians, dispersal, 325.
larva of Antedon, 274.

Ascidian, 277, Fig. 130.
crabs, 276, Fig. 128.
echinoids, 276.
frog, 272, Fig. 121.
Grautia, 322, Fig. 164.
ophiuroids, 275, Fig. 127.
larvae, 272.
larval forms, dispersal, 321, 322.

„ organs, 272, 275, 278.
Leach, 227.
leaf insects, 338, 339, Figs. 170, 171.

„ structure, 63-65, Fig. 26.
leg, of man, 237, 238, Fig. 94.
legitimate unions, in Primula, 357.
legumin, 23.
Leigh, G. F., 348.
Lemuria, 328.
lemurs, distribution, 328.
leopard, distribution, 320.
Lepidosiren, 255, 291.
Leptinotarsa, mutation in, 159, 160.
leucocytes, 51, Fig. 15.
level, changes of in land, 328.
lianes, 350.

life-history, 263 (see also ontogeny),
life, nature of, 2, 3, 4, 11.
Lilium, germination of pollen grain,

Fig. 54.
limbs, of arthropods, 246, 247.

,, vertebrates, 235 — 246.
Limnas chrysippus, 346, Fig. 177.
linin network, 70.

INDEX

443

Linnaean species, 412.
Linnaeus, 222, 226, 228.
Liimean Society, 340, 385.
lithium larvae, 157.
Lithobius, host of Coccidium, 87.
Litopterua, 253, Fig. 109.
lizards, pineal eye, 258.

„ shoulder girdle, 234, Fig. 89.
llama, Buffon on, 367.

„ feet, 241.
Lock, B. H., 152.
locomotion in Amoeba, 15.
Loeb, J., 145, 146.
Lubbock, Sir John, 281.
Lull, E. S., 307.
luminous organisms, 6.
lung-fishes (see Dipnoi),
luugs and swim-bladder, 255, 256.
Lyell, Sir Charles, 324, 385.
Lyyosoma dendyi, 417.

,, moco, 417.
lyre bird, 349, Fig. 178.
lysin theory of fertilization, 146, 147.

M.

MACCULLOCH, 340.
machine, analogy of, 2, 3, 10.
Macropodia rosirafa,339,340,Fig.l72.
magnesium larvae, 157, Fig. 75.
maize root, section, 37, Fig. 7.
male animal, 1 14.

,, characters, 84, 85, 127.
malic acid, attracting spermatozoa,

141.

Malthus, 386.
Mammalia, compared with Eeptilia,

232-235.
dispersal, 323.
geological history, 284,

301—304.
gigantic, 406.
limbs, 235.

nutrition of young, 270.
origin, 300.
ovum, 140, Fig. 71.
mammoth, Buffon on extinction of,

366.

man, aesthetic development in-
fluenced by insects, 364.
,, and apes, relations, 422, 423,
424, 425.
,, ,, ,, Buffon on,

370.

,, ,, ,, Lamarck

on, 382.
„ antiquity, 284, 303, 422, 423.

man, control of environment, 426.
,, evolution, 422 — 427.
,, ,, Charles Darwin on,

423.
,, influence on other organisms,

396.

,, limbs, 237, Fig. 94.
,, Mendelian inheritance in, 205.
,, progress of, 422 — 427.
,, races of, 423.
,, reversion in, 262.
,, vestigial hair, 261.
„ „ tail, 261.

Mantidae, 338, 339.
manubrium, 120.
Maoris and Morioris, 398 — 400.
marginal canal, 120.
marine animals, dispersal, 321.

„ fauna and flora, 323.
Marsh, O. C., 262, 307.
marsh tit, distribution, 319, 320.
Marsilea, 105.
marsupial mole, 253.
„ wolf, 251.
Marsupialia, 250, 301, 331.

,, distribution, 329, 330,

331, 332.

,, extinct, 302, 332.

Mastodon, 314.
Mastodonsaurus, 296.
maternal chromosomes, 137, Fig. 68.
„ functions, a handicap, 127.
matter, indestructibility of, 6.
maturation of germ cells, 132, 137,

138, Figs. 65, 68.
in heredity, 171.
Mauritius, 258.
Mediterranean fauna, 323.
medusae, 120, 121, 123, Fig. 60.
megagametes, 85, 88, 90.
megalecithal, 267.
meganucleus, 39, 92.
megasporangia, 107.
megaspores, 105, 106, 107, 108.
Megatherium, 303.
meiosis, 132, 206.
memory, 186, 191, 192.
Mendel, G. J., 195.
Mendelian experiments, 194 — 209.
,, inheritance in Leptino-

tarsa, 160.
in Primula, 356.
„ principles, application of,

205.

,, proportions, 199.

Mendelism, 195.

,, and mutations, 415.

444

INDEX

Meinira snperba, 349, Fig. 178.

meristem, 76.

meristic mutations, 154.

,, variations, 148, 149.
rnerogoiiy, 146, 174.
Merychippus, 310, Fig. 153.
mesentery, 54.
mesoblast, 48, 266.
mesoblastic somites, 266.
mesoderm, 48, 124.
mesogtea, 115, 116.
Mesohippus, 310, Figs. 153, 156.
mesophyll, 50, 64, Fig. 26.
Mesozoic Era, 284, 285.
metabolism, 9, Fig. 1.
inetacarpals, 238.
metagenesis, 121.
metameric segmentation, 149, 266,

279.

metamorphic rocks, 283.
metamorphosis, 263.
Metaphyta, 44, 95.
metapodials, 240, 241.
metatarsals, 238.
Metatheria, 301, 302.
Metazoa, 44.
mice, dispersal, 325.

,, experiments in heredity, 179,

182.

microgametes, 85, 88, 91.
microlecithal, 267.
Microlestes, 301, 302.
micronucleus, 39, 92.
micropyle, 109.
microsporangia, 107.
microspores, 105, 106, 107.
microzooids, 33.
migration from north, 329.
milk, 271.
mimicry, 343—348.

,, and fluctuating variation,
415.

,, and natural selection, 396,
405.

,, rings, 345.
umieral salts, 23.
Miocene Epoch, 284.
Miohippus, 310.
mitosis, 69—79, Figs. 31—35.

,, in heredity, 169.
mnemic theory of heredity, 186, 191,

192.

moas, 258, 397.
models and mimics, 345, 348.
Moeritherium, 313, Fig. 159.
moles, marsupial, 253.
Molluscoida, 277.

momentum in evolution, 406 — 409.
monads, 83.
Monera, 66.
monoecious, 103, 105.
mpnohybridism, 201, Fig. 79.
monopodial branching of phyloge-

netic tree, 230, 231, Fig. 87.
monosome, 135.

Monotremata, 234,296, 301, 302, 331.
monstrosities, 154, 157.
Morioris, extermination, 398 — 400.
mosses, dispersal, 321.
moths, clear-winged, 343, Fig. 175.
,, proboscis, 354.
,, protective resemblance, 338.
motion, in living organisms, 6.
mountain hare, distribution, 333.
mud, dispersal in, 327.
mud-fishes, 255, 291.
Miiller, Fritz, 346.
Miillerian mimicry, 346.
multicellular, 38.

,, organisms, origin of,

44.

Multituberculata, 302.
multiplication of cells, 69 — 80.
Mungoswell's wheat, 412.
Muscari, variation in, 150.
muscle fibres, 57, 58, 116, Figs. 21,

22, 58.

music and memory, 192.
mutation and adaptation, 414, 415.
,, ,, evolution, 414.

,, ,, hybridization, 413

414.

,, ,, Mendelism, 415.

,, ,, natural selection, 414.

,, theory, 224.

mutations, 149, 153—156, 224.
,, meristic, 154.

origin, 159, 160.
,, ,, of species from,

411, 412, 413.
,, substantive, 154.
mutilations, inheritance of, 178, 179.
mutual adaptation, 414, 415.
Mycetozoa (Myxomycetes), 67, Fig.
29.

N.

NAGELI, 167.

natural affinities, 228, 229.

,, selection, 395 — 409.

,, ,, and mutation, 414.

,, ,, A. E. Wallace on,

389—391.

,, ,, Buff on on, 368.

INDEX

445

natural selection, Charles Darwin on,

385—388, 392.
,, ,, Erasmus Darwin

on, 373.
,, ,, insufficiency of,

404.

,, ,, insufficiency of,

Charles Darwin
on, 392.
,, ,, summary of theory,

391.

,, system, 226.
nectaries, 359, 360, 414, 415.
Neoceratodus, 255, 291, Fig. 110.
Neohipparion, 310, Fig. 157.
Nepenthes, 262.
Nerocila, 227.
nerve-cells, 58, Figs. 23, 24.

,, fibres, 58.
nerves, 59.

nervous system, 18, 121, 266.
neurons, 58, 59.

neuter insects and heredity, 189, 190.
newts, 294.
New Zealand, fauna and flora, 258,

328, 329, 331.
nipples, vestigial, 345.
nitrifying bacteria, 218.
Nitrobacter, 218.
Nitrosomonas, 218, Fig. 84.
nomenclature, 226, 227.
non-adaptive characters, 419 — 422.
normal curve of variation, 153.

,, variations, 150.
notochord, 266.
Notornis, 397.
Notoryctes, 253.
nucellus, 108.
nuclear membrane, 69.
nucleinic acid, 71.
nucleolus, 70, 77.
nucleoplasm, 14, 22, 69.
nucleus, 13, 69.

,, division (see mitosis and
amitotic nuclear division).
,, in heredity, 161, 166, 174.
,, zygote, 83.
numerical variations, 149.
nutrition, 7 (see also food materials).
„ of embryo in Coolomata,
125.

O.

OBELIA, 119, 265, Fig. 60.

ocean, age of, 287.

oceanic islands, fauna, 330, 332, 397.

oceans, permanence, 329.

CEnothera, mutation in, 155.

oil-foam, 19, 21.

Olenus cataractes, Fig. 134.

Oligocene Epoch, 284.

ontogenetic record, obscuring of,

267, 278.
ontogeny, 263.

„ abbreviation of, 273.

„ a habit, 192.

„ and phylogeny, 265, 279,

Fig. 131.
,, Erasmus Darwin on, 371,

372.

,, interpretation of, 277.
Onychophora, distribution, 330.
oocytes, 133.
oogenesis, 133, Fig. 65.
oogonia, of Fucus, 100.

„ in oogenesis, 133.
oospheres, 90, 100, 104, 108.
ooze, 26, 283.
opal, 24, 26.
Ophisaurus, 250.
Ophiura dliaris, Fig. 126.
ophiuroids, 275, 276.
opossums, 301.

opposable great toe and thumb, 424.
orang, brain, 425.
" orang utan," Buffon on, 370.
Orchidace<e, fertilization, 359 — 362.
Orchis mascula, fertilization, 359 —

362, Fig. 182.
orders, 226.

Ordovician Epoch, 284.
organellse, 39.
organic evolution, doctrine of, 212

(see also evolution).
,, units, 38, 65.
organism, 3.

organisms, first appearance of, 212.
,, nature of first, 217, 219.
„ number and variety of,

211.

origin of, 214—216.
organizing spirits, A. E. Wallace on,

394.

organs, 12, 38.

origin of great groups, 303, 304.
„ ,, living things, 214—216.
„ ,, sex, 84, 85, 89, 126—128.
Ornithorhynchus, 234, 260, 301,

Fig. 91.

Ornithosauria, 299.
Ornithoscelida, 297.
Orohippus, 310, Figs. 153, 155.
Osborn, 369.
osmosis, 8, 17, 23.

446

osmotic pressure, effect on egg, 145.
Ostracodermi, 289, 290, Figs. 135,

136.

Otaria Jiookeri, paddles, Fig. 100.
otter sheep, 154.
ovary, of animals, 113, 116.

,, ,, flower, 107.
oviducts, 125.
ovotestis, 113.
ovule, 48, 106, 107, Fig. 55.
ovum, 85.

,, interpretation, 279.

,, maturation, 133, Figs. 65, 68.

,, of Auiphioxus, 45, Fig. 13.

,, Ascaris, 131, Fig. 64.
„ „ bird, 140, Fig. 70.
„ „ Chara, 141.
,, „ Coccidium, 88, Fig. 39.
,, ,, Eudorina, 91, Fig. 40.
„ fern, 104, Fig. 52.
,, flowering plant, 108, 109,

Fig. 55.

„ Fucus, 100, Fig. 47.
,, ,, Hydra, 116, Figs. 57, 59.
,, ,, mammals, 71, 271, Fig.

140.

,, ,, meduste, 121.
,, ,, plants, 141.
,, ,, rabbit, Fig. 71.
,, ,, sponges, 113.
„ „ Volvox, 91, Fig. 11.
„ segmentation of, 263, 265.
,, similarity in different or-
ganisms, 162.
size of, 267.
typical, 140, Fig. 69.
oxen, feet, 241.
oxidation, 5.
oxlip, fertilization, 356.
oxygen, 5, 6, 8, 31.
oyster, American, introduced, 323.

P.

PADDLES, 236, 244, 247.
pademelons, coloration, 337.
psedogenesis, 144.
pairing of chromosomes, 133, 137,

206, Figs. 65, 66, 68.
Palseohatteria, 295.
Palseomastodon, 313, Fig. 159.
palaeontology, evidence afforded by,

287—304.

Palseospondylus, 289.
Palaeozoic Era, 284, 285.
palingenetic characters, 277.
palms, climbing, 350.

Pandorina, 42, 89, Fig. 10.
pangenesis, 164—166, 369,' Fig. 76.
Papilio, mimicry in, 346 — 348,

Fig. 177.
parallel modification of body and

germ cells, 182.

Paramoecium, 39, 98, 139, Fig. 8.
paraphyses, of Fucus, 100.
parasites, peculiarities of, 401 — 403.
parasitism, of gametophyte, 112.
parenchyma, 65.
Pariasaurus, 297, Fig. 140.
parietal eyes, 258—260, Figs. 114—

116.

,, foramen, 260.
parthenogenesis, 143, 144.

artificial, 144—147.
Parmpalustris, distribution, 319,320.
passivity of female, 84, 85.
Pasteur, 215.

Patagonia, beech forests, 329.
paternal chromosomes, 137, Fig. 68.
peach trees, inheritance of acquired

characters, 182.

peacock, epigamic ornamentation,349.
Pearson, Karl, 210.
pea, Sicilian, 208.
,, structure and germination of

seed, Fig. 56.
peas, Mendelian experiments on,

196—199.

pedigrees, 229, 311.
Peebles, Florence, 28.
pelagic organisms, 322, 336.
pentacrinoid stage of Antedon, 274,

275, Fig. 124.

Pentacrinus, 274, Fig. 125.
pentadactyl limbs, 237, 294 (see also

limbs and feet),
periblem, 50.

Pericopis angulosa, 345, Fig. 176.
Peripatus, Fig. 167.

,, distribution, 330.
perisarc, 119.
perissodactyl, 241, 253.
peristaltic movement, 57.
peritoneal epithelium, 54, Fig. 16.
Permian Epoch, 284.
Perrliybris malenka, 345, Fig. 176.
persons in colony, 119.
Petalotricha, Fig. 9.
petals, 106.

phagocytes, phagocytosis, 52.
phalangers, 301.
phalanges, 238.

Phascolotherium, 302, Fig. 149.
Phenacodus, 309, Fig. 152.

INDEX

447

phloem, 65.

phosphorescent organs, 336.

phosphorus, 22, 71.

photosynthesis, 30, 31.

phyla, 226.

phyllodes, 2SO.

PhyUopteryx eques, 341, Fig. 174.

phylogenetic tree, 229.

phylogeny, 229.

,, and classification, 230,
231, Fig. 87.

„ ,, ontogeny, 265, 279,

Fig. 131.

,, Erasmus Darwin on, 372.
physical basis of life, 7.

,, conditions of earth, changes

in, 327.
physiological selection, 418.

„ unit, the cell, 6$.

Phytoflagellata, colony formation, 40,

42.
,, evolution of sex, 89

—91.

Pierinse, 345.
pig, feet, 241, Fig. 98.
pineal eyes, 258— 260, Figs. 114-116.

„ gland, 11, 260.
pin-eyed flowers, 356.
pistil, 106.

Pisum sativum, varieties, 196.
pitcher plants, 262.
Pithecanthropus erectus, 423.
pituitary body, 408.
placenta, 270.
placental mammals, appearance of,

302.

Playianthus betaJinits, 417.
Plagiaulacidse, 302.
Plagiaulax, 302, Fig. 148.
Planema pogyei, 346.
plankton, 322.
plants, compared with animals, 34, 35.

,, dispersal of, 321.
plasma, 51.
plasmodia, 67.
plasmogamy, 128.
plasticity of organisms, 335.
plastids, 32.
plastogamy, 128.
Platypus (see Ornithorhynchus).
Pleistocene Epoch, 284.
plerome, 50.

Plesiosauria, 236, 244, 297.
Plesiosaurns macrocephalus, Fig. 141.
Pliny, 262.
Pliocene Epoch, 284.
Pliohippus, 310, 311, Figs. 153, 158.

ploughshare bone, 305.

plumage of birds, 349.

plumcots, 204.

pluteus lame, 275, 276, Fig. 127.

polar bodies, 133, 135, 139, 145, Figs.

35, 65, 68.

pole-cell, in insect egg, 130.
pollen, collection of, 355.
grains, 106, 107.
,, sacs, 107.
tube, 108.
pollination, 110, 111.

,, adaptations for, 351 — 363.
pollinia, 361.

polyanthus, pollination, 356.
polydactylism, 149.
polymorphic colony, 119.
polymorphism in Papilio, 346, Fig.

177.

polype, fresh water (see Hydra).
Polypodiuin, archegonium, Fig. 52.
Polyzoa, dispersal, 327.
population, Buff on on, 368.

,, Charles Darwin on, 386.
,, Lamarck on, 381.
Wallace on, 389.
porpoises, convergence in, 248.
Portunus, 247, Fig. 102.

larva, Fig. 128.
postaxial, 238.
potential energy, 7.
Poulton, E. B., 336, 337, 346.
preaxial, 238.

Pre-Cambrian Epoch, 284, 285.
pre-formation in development, 163.
prehension, organs of, 256.
prepotency, 353, 358.
presence or absence hypothesis, 207.
Primary Era, 284, 285.
Primates, 301, 303.
primordia, in germ plasm, 206.
primordial germ cells, 113 (see also

germ cells, origin),
utricle, 62, 95.

Primula, fertilization, 356—358.
priority in nomenclature, 227.
Proboscidea, evolution, 312 — 314,

Fig. 159.
proboscis, of bee, 354, Fig. 179.

,, butterflies and moths,

354.

,. elephants, 256.

, , insects, Erasmus Darwin

on, 373.
,, mutual adaptation,

414, 415.
procryptic coloration, 337 — 342.

448

INDEX

progress, human, 426, 427.
progressive development, B. Cham-
bers on, 384.
,, evolution, 334.

,, cause of, 192.

,, Wallace on, 390.

series, 232, 234.

pronuclei, 131, 145, 146, Fig. 64.
protandrous, 353, 363.
protective resemblance, 337 — 342.
,, and fluctuating varia-

tion, 415.
,, ,, natural selection,

396, 405.
proteids, 7, 22.

Proterotherium, 253, Fig. 109.
Proteus, 183.
prothallus, of fern, 103.

,, „ flowering plant, 108.

Protista, 38.

,, dispersal of, 327.
,, immortality of, 161.
protogynous, 353.
Protohippus, 310.
Protophyta, 38, 95.

, , sexual differentiation in,

91.
protoplasm, 2, 38.

,, chemical composition,

22, 23.

controlling power, 23.
energy in, 7.
physical properties, 21.
selective action, 26.
sensitive to stimuli, 188.
sti'eaming, 62.
Protopterus, 255, 291.
Protorohippus, 310, Fig. 153.
Prototheria, 301, 302.
Protozoa, 38, &c., Figs. 3, 4, 9, &c.
,, non-adaptive characters,

419.

,, nuclear reduction, 139.
„ powers of multiplication,

81.

,, sexual differentiation, 89.
Prozeuglodon, 318, Fig. 163.
prunes, stoneless, 204.
Pseudopanax chathamicum, 417.
,, crassifolium, 417.

,, ferox, 417.

pseudopodia, 15.
Pteranodon, 299, Fig. 147.
Pteraspis, 290.
Pterichthys, 290, Fig. 136.
Pterosauria (pterodactyls), 236, 242,
299, Figs. 99, 147.

Pterostylis trullifolia, fertilization,

362.

Ptert/goius osiUensis, Fig. 137.
Pukhriphyllium crurifolinm, 338,

Fig. 170.

Punnett, E. C., 208.
purity of gametes, 200, 206.
putrefaction, 218.

Pyrenees, arctic vegetation on, 333.
pyrenoids, 32, Figs. 5, 42.

a

QUADRTJMANA, 424.
quartz, 24.

R.

BABBIT, embryo, 265, Fig. 117.

,, shoulder girdle, 234, Fig. 90.
racial characters, transmission, 175.
radial canals, 120.
Badiolaria, 25, Fig. 3.
radius, 237.

rafts, dispersal by, 324.
range in time of animal groups, 284.
Ranunculus aqnatilis and Ranunculus

hederaceus, Lamarck on, 379.
rats, 179, 182, 325.
recapitulation hypothesis, 265 — 281.
„ ,, anticipated

by Erasmus Darwin, 372.
receptor, 18.

recessive characters, 197, 207.
recognition marks, 337.
red blood corpuscles, 52, 53, Fig. 15.
,, clover, robbed by humble bee,

355, 356.

Bed Sea, fauna, 323.
red snow, 27.

reduction of chromosomes (reducing
division), 132—139, 206, 207, Figs.
65—68.

regeneration, 167.
regression, filial, 210.
rejuvenescence, 92.
reproduction, 10.
Beptilia, compared with Amphibia,

294.
,, compared with Mammalia,

232, 235.
fossil, 295—299.
geological range, 284.
giant, 406.
limbs, 235.
origin, 294, 295.

INDEX

449

respiration, 8.

,, in Amoeba, 17.

,, ,, embryos, 270.

,, ,, plants, 32, 33.

response to stimuli, 18.
Reunion, peach tree3 in, 182.
reversion, 261, 262.

,, Mendelian explanation,

208.

rhinoceros, feet, 241.
Bhynchocephalia, 259, 295 (see also

Sphenodon).
ribbon- wood, 417.
ribs, rneristic variation, 149.
right whale, 245, Fig. 101.
Bitzema Bos, 179.
rock-formation, 282.
rodents, 301.
"Rodney," brig, 399.
Rodriguez, 258.
Romanes, G. J., 333, 417, 418.
Rontgen rays, 187.
root-cap, 50.

root, growing point, 76, Figs. 33, 34.
Rosenthal, 179.
rostellum, 360.

rotifers, parthenogenesis, 144.
rudimentary organs (see vestigial
organs).

,, ,, Charles Darwin

on, 392.

Rumia crat«'<jata, larva, Fig. 169.
rust, in wheat, 205.
rye, fertilization, 412.

S.

SACCULIXA, degeneration, 401, Fig.

183.

sage (see. Sal via).
Sagitta, origin of germ cells, 130,

Fig. 63.

salt, in sea, 287.
Salvia, structure of flower and

fertilization, 358, 359, Fig. 181.
Salvinia, 105.

Sandwich Islands, oceanic, 332.
sap, 65.

,, nuclear, 70.
saprophytic nutrition, 34.
sarcode, 38.
Sarracenia, 262.
Sanropsida, 300.
Sauropterygia, 297, Fig. 141.
scapula, 234, Figs. 89, 90, 93.
scents, of flowers, 355.

B.

Schleicher, 69.
Schleiden, 38.
Schcetensack, 423.
Schwann, 38.
Scrophularia nodosa, 353.
scurf, 55.

sea anemones, 114.
,, coast, selection in, 395.
,, firs, 114.

,, urchins, artificial partheno-
genesis, 145.

,, ,, blastomeres, 45.

,, ,, hybridization, 174,

175.

,, ,, larva1, 276.

seal, paddles, 244, Fig. 100.
Secondary Era, 284, 285.

,, sexual characters in

Ccolomata, 125.
secretions, controlling growth, 408.

,, internal, 126.

sedimentary rocks, 282, 286.
sedimentation, rate of, 285, 286.
seed, 109, Fig. 56.
,, coat, 109.
,, leaves (ste cotyledons),
seedling of Acacia, 280, Fig. 132.

,, ,, pea, Fig. 56.
seeds, dispersal ,321.
Seeley, H. G., 297.
segmentation, metameric, 149, 266.
segmentation of ovum, 45, 263.
in Amphioxus, 45, Fig. 13.
,, birds and reptiles, 270.
,, frog, 268, Fig. 119.
„ Hydra, 118.
,, mammals, 271.
interpretation of, 265.
segregation of germ cells (see germ

cells, origin),
selection, artificial, 395, 396.

,, ,, Charles Darwin

on, 387.

,, by insects, 396.
,, continuous, 411.
,, germinal, 173.
,, natural (see natural selec-
tion).
,, not confined to living

things, 395.

„ sexual (see sexual selec-
tion).

,, single, 412.
self- fertilization, 351, 352, 353.
sematic colours, 336, 342.
Seinon, Richard, 186.
sensation, 16.

G G

450

INDEX

sense cells, stimulation of, 188.

,, organs, 18, 121.
sepals, 106.

series, progressive, 232, 234.
tiesia rrabroniformis, Fig. 175.
sex determination in insects, 135,

Fig. 67.
sexes, fundamental distinction

between, 127.

sexual characters transferred to
asexual generation, 110,
111.

,, differentiation, origin of, 128.
sexual phenomena, 84, 85.
evolution of, 126, 127.
in Bodo, 87, Fig. 38.
,, Coccidium, 88, Fig. 39.
,, Ccelomata, 125.
,, Copromonas, 83, Fig. 37.
,, Eudoriua, 91, Fig. 40.
,, ferns, 104, 105, Figs. 51, 52
,, flowering plants, 108, 109,

Figs. 54, 55.

,, Fucus, 100, 101, Fig. 47.
,, Ha>inatococcus, 33, Fig. 5.
„ Hydra, 116.
,, Obelia, 121.
,, Pandorina, 89, Fig. 10.
,, Paramoecium, 93, Fig. 41.
,, Spirogyra, 96, 97, Figs. 43, 44.
,, Volvox, 91, Fig. 11.
„ Zygogonium, 97.
sexual reproduction, 33.

,, selection, Charles Darwin on,

388.
,, „ Erasmus Darwin

on, 373.

Shand, Alexander, 399.
sharks, fossil, 291.
sheep, Ancon or otter, 154.

„ feet, 241.

shepherd's purse, 48, Fig. 14.
Shirreff, Patrick, 412.
shoulder girdle, of mammals, 234,

• Fig. 90.
,, ,, ,, Ornithorhynchus,

234, Fig. 93.
,, reptiles, 233, 234,

Fig. 89.

Sicilian pea, 208.
silica, 23, 24.
siliceous skeleton, 25.
Silurian Epoch, 284.
single selection, 412.
Sirenia, 301.

paddles, 236, 244.
size, increase of, 309, 405 — 409.

skeleton (see feet, limbs, paddles

and wings),
of extinct animals, Figs.

139—152.
-iraffe, Fig. 104.
horse and man, Fig. 95.
kiwi, Fig. 112.
Obelia, 119.
Protista, 23—26, Figs.

3, 4.

,, whale, Fig. 101.
skulls of dog and thylacine, 251,

Figs. 107, 108.
slime fungi (see Mycetozoa).
sloths, 333.

slow worm (see Anguisfragilit).
snails, dispersal, 325.
snakes, mimicry in, 343.
social problems, de Vries on, 414.
sodium, in sea water, 287.
sole of foot, Buff on on , 368.
solitaire, 258, 397.
Sollas, W. J., 286, 287, 311.
soma and germ cells contrasted (see

germ cells and somatic cells),
somatic and germ nuclei in Para-
moecium, 98.

somatogenic characters, supposed
non-inheritance, 169, 176 (see also
acquired characters),
somatogenic variations, 149, 156

157.

somatopleure, 124.
somites, 266.
soul, 11, 19.
South America, fauna and flora, 32S,

329, 333.

Spalacotherium, 302.
special creation, 212—214, 222, Fig.

82.

Buff on on, 369.
,, Lamarck on, 375,

376.
species, aggregate, 224.

,, Charles Darwin on, 223.

,, definition of, 223.

,, elementary, 224.

,, Lamarck on, 375.

,, modification of, Buffon on,

366, 367.

,, nature of, 222—224.
,, number of living, 211.
,, origin from mutations, 225.
„ supposed immutability of (see

immutability of species).
,, transformation of, Lamarck
on, 376, 377.

INDEX

451

specific characters, non -adaptive,

419—422.
speech, an acquired character, 156,

184.

,, evolution of, 425.
Spencer, Herbert, 2, 185, 391.
spermary, 113.
spermatids, 133, Figs. 65, 66.
spermatist (Erasmus Darwin), 371.
spermatocytes, 133, Figs. 65, 66.
spennatogenesis, 132, 133, 134, 135,

Figs. 65, 66.

spennatogonia, 133, Figs. 65, 66.
spermatozoa, 85, 139, Fig. 69.

,, chemo taxis in, 141.

,, development (see sper-

matogenesis).
,, of Chara, 141.

,, Coccidium, 88, 141,

Fig. 39.

,, Eudorina,91,Fig. 40.
,, fern, 104, 141, Fig.

51.

Fucus, 100, Fig. 47.
Hydra, 116.
medusa?, 121.
mosses, 141.
sponges, 113.
Volvox, 91, Fig. 11.
spermatozoids 91.

sphoerechinus, experiments in hybri-
dization, 174.

Sphserella (see Heematococcus).
Sphferozoum, 43, 265, Fig. 12.
tipltenodon punctatus, 259, 260, 295,

320, 331, Figs. 113, 114.
spicules (see sponges, spicules).
spider crab, 339, 340, Fig. 172.
spindle, nuclear, 71.
spines, excessive development, 406.
spiny anteater (see Echidna),
spireme, 71.

Spirogyra, 95 — 97, 142, Figs. 42 — 44.
splanchnopleure, 124.
splint bones, 241, 257.
sponges, deep sea, 335.

,, dispersal of fresh water,

327.
,, gemmules of fresh water,

327, Fig. 166.
,, germ cells, 113.
,, non - adaptive characters,

419—421.
spicules, 26, 232, 419, 420,

Figs. 88, 187.

Xj>on</iHa ftnriatilis, gemmules. Fig.
166.

spontaneous generation, 214 — 21(5,

219.

Lamarck on, 374.
sporangia of fern, 101, Fig. 48.

,, ,, flowering plant, 107,

Fig. 53.
spores, 101.

,, dispersal, 321.
,, formation, reduction of chro-
mosomes, 138.

,, germination, in fern, 103.
,, of Bodo, 87.
,, ,, Coccidium, 89.
sporophylLs, 107.
sporophyte, 101.

,, * of fern, 105, Fig. 48.
,, ,, flowering plants, 106.

,, number of chromosomes,

139.

sports, 154, 224 (see also mutations).
Squalodontidro, 318.
Squamata, 295.

stability of organisms, 155, Fig. 74.
stag, antlers, 125.
staining reactions, (52, 70.
stamens, 106.
staminodes, 262.
starch, 31.

star-fish, meristic variation, 149.
statoblasts, 327, Fig. 165.
stature, variation in human, 152.
Staurocephalus murchisoni, Fig. 134.
Stebbing, T. E. E., 339.
Stegocephalia, 293.
Stegosaurus, 299, Fig. 145.
Stenoliothrits uiridiilus, spennato-
genesis, Fig. 66.
Stentor, Fig. 9.
Stereognathus, 302.
sterility of species when crossed,

418.

St. Helena, an oceanic island, 332.
stick caterpillars, 337, 338, Fig.

169.

Stieda's organ, 2(50, Figs. 115, 116.
stigma, of flower, 107.
stimulation of protoplasm and cells,

188.
stimuli, 18.

,, to development, 145, 146.
stimulus, transmission between cells,

143.
,, ,, to germ cells,

186—190.
Stockard, 157.
stomata, 64, Fig. 26.
storms, dispersal by, 324.

INDEX

stratified epithelium, 56, Fig. 18.

rocks, 282—286.
striped muscle, 57, Fig. 22.
struggle for existence,

A. E. Wallace on, 389.
Buffon on, 368.
Charles Dai-win on, 386.
importance of, 397, 398, 400.
Lamarck on, 381.
stump tails, 179.
style, of flower, 107.
Stylidium graminifolia, fertilization,

362.

subclasses, 226.
subfamilies, 226.
subgenera, 225.
subkingdoms, 226.
suborders, 226.
substantive mutations, 154.

,, variations, 148, 150.

succulent plants, 350.
Sumner, F. B., 182.
sun, dependence of life upon, 5.
suprascapula, 234.
surface, relation to volume, 20.

,, tension, 19.
survival of fittest, 391.

,, ,, Buffon on, 368.

,, ,, Charles Darwin on,

387.
,, ,, in inorganic world,

395.

suspensor, 49.
Svalof, 412.

sweet peas, reversion in, 208.
swim bladder and lungs, 255, 256.
synaposematic groups, 343 — 346,

Fig. 176.

synapsis, 133, 137, 206, Figs. 65, 66,
"68.

synaptic mates, 135.
syncytia, 66.

syncytial epithelium, 67, Fig. 28.
syndesis, 137 (see also synapsis).
syngainy, 83 (see also conjugation).
Bystematist, work of, 226.

T.

TADPOLE, ascidian, 277, Fig. 130.

frog, 272, Fig. 121.

stage, 278, 279.
tail, in man, 261, 262, 424.
tails, mutilation of, 178, 179.
tapir, feet, 241, Fig. 96.
tarsals, 237.
taxonomic tree, 228, 229, Fig. 86,

taxonomy, 226.

aims of, 229.

teeth, of Cetacea, 317, 318, Figs. 162,
163.

,, ,, dog and thylacine, 251 —
253, Figs. 107, 108.

,, ,, elephants, 314.

,, „ horses, 309.

,, vestigial, 260.
Teleostei, 292.
telescoping of generations, 111, 112,

122.

temperature of body, 6.
tentacles, 115, 118.
Tertiary Era, 284, 285.
testis, 113, 116.
Tetrabelodon angiistideus, 314, Fig.

159.
,, lovgirostris, 314, Fig.

159.
Tetraxonida, spicules, 232, Figs. 88,

187.

Theosodon, 253, Fig. 109.
Theromorpha, 296.
Thoatherium, 253, Fig. 109.
thrum-eyed flowers, 356.
thylacines, 301.

Thylacinus, 251, Figs. 107, 108.
tibia, 237.
time, geological, 285 — 287.

, , importance in evolution, Buffon
on, 366.

,, in evolution of horse, 311.

,, scale, geological, 284.
Tinoceras, 303, 406, Fig. 150.
Tintinnopsis, Fig. 9.
tissue-formation, 45, 50, 51, 75 — 79.
tissues, 37, 51 — 65.
Tithorea harmonia, 345, Fig. 176.
tools, use of, 424, 425.
torpedo, 256.
tortoise, 33 i.

Tower, W. L., 156, 159, 184.
Tradescantia, histology of, 61 — 65,

Figs. 25, 26.
transformation of species (see

species),
transparency, of pelagic animals.

322, 336.

tree-like classification, 228, 229, Fig
86.

,, evolution, 221.
Trematoda, parthenogenesis, 14-1,
Triassic Epoch, 284.
Triceratops, 299, Fig. 146.
trichocysts, 39.
Triconodon, 302.

INDEX

trilobites, 289, Fig. 134.

Trimen, B., 346.

Triticum sativum, Lamarck on, 378.

Tritylodon, 301, 302.

tuatara (set Sjihenodon punctatus).

Tubularia, 121, Fig. til.

turbellarian, fertilization of polar

body, 135.

turbot, colour changes, 342.
turtles, dispersal, 323

limbs, 236, 244.
twins, identical, 173.
types, in wheat, 412.
Tyndall, 215.
Typhlopidrc, 250.

U

ULNA, 237.

unconscious memory, 191, 192.

ungulates, 301.

limbs of, 239—241.
unguligrade, 240.
unicellular, 38.
unisexual, 99, 105.
unit characters, 200, 206.

,, ,, origin of, 415, 416.

unstriped muscle, 57, Fig. 21.
Ureeotyphlus africanus, Fig. 106.
urea, 9.
lrrsus arctos, systematic position, 229,

Fig. 86.
use and disuse, effects of, 156, 166.

Buff on on, 367 — 368.

Charles Darwin on, 391, 392.

Lamarck on, 377, 378, 379, 380,
382.

E. Chambers on, 385.
uterus, 125.

V

VACCOLES, contractile, 39, 83.
Varanus, shoulder girdle, 234, Fig.

89.

variation, 148 — 160.
,, cause of, 166.
,, curve of, 150, 153, Figs. 72,

73.
., in grape hyacinth, 150,

Fig. 72.
,, ,, human stature, 152,

Fig. 73.
variations, blastogenic, 149, 157 —

160.

,, congenital, 157.

,, continuous, 148, 150.

variations, discontinuous, 149, 153

—156.
fluctuating, 148, 150,

155, Figs. 72, 73.
germinal, 149, 157—160.
,, individual, 150.

,, inheritance, 403, 404.

meristic, 148, 149.
,, normal, 150.

,, origin, 153, 173, 403,

404.

,, selective value, 403, 404.

,, somatogenic, 149, 156,

157.

,, substantive, 148, 150.

vasa deferentia, 125.
vascular bundles, 65.

,, system in plants, 50.
vegetable marrow, flowers, 111.
vegetative cell in pollen grain, 108.
Vergil, 214,

vertebrae, meristic variation, 149.
vertebral column, development, 266.
Vertebrata, geological history, 289

—304.

,, origin, 291.

Vesper crabro, Fig. 175.
vessels, of plants, 65.
vestigial organs, 257 — 262, 271, 345,

397.
,, ,, Charles Darwin on,

392.

vibrations, in determinants, 189.
Virchow, 38.
vital force, 11, 143, 216.
vitalism, 19.

vitelline membrane, 140.
volume, increase of, Lamarck on,

382.

,, relation to surface, 20.
Volvox, 42, 91, 97, 98, 265, Fig. 11.
Von Baer, 265.
Von Mohl, 38.
Vorticella, 40, Fig. 9.

w.

WAISTS, slender, 156.

Walker, C. E., 175.

Wallace, A. E., views of, 319, 329

389-391, 393, 394.
warm-blooded animals, 6.
warning colours, 342 — 348.

,, ,, and natural selec

tion, 396.

wasps, colours, 342.
waste products, 8, 9.

454

INDEX

wattles, life-history, 280.
weapons, use of, 424, 425.
weeds, dispersal, 325.
Weismann, views of, 146, 166—174,
176, 177, 178, 179, 180, 185, 189,
206, 219.
weka, 397, 398.
Wells, W. C., 385.
Western, 387.
whales, ancestry, 314—318.

,, cervical vertebra?, 248. Fig.
103.

,, convergence in, 248.
paddles, 244, Fig. 101.
size, 303.

,, systematic position, 301.

,, vestigial organs in, 257, 260,

Fig. 101.
wheat, improvement of, 411 — 413.

,, Lamarck on, 378.
wind, dispersal by, 323.
winged animals, dispersal of, 323.
wings, 236.

., convergence in, 248 — 250.

,, of bat, 243, Fig. 99.

,, „ bird, 243, 244, Fig. 99.

,, ,, insects, 247.

,, ,, pterodactyls, 242, Fig. 99.
wireless telegraphy, analogy of, 187.
Wolff, 0. F., 163.
wombats, 301.
wood, 65.

Woodward, A. S., 253, 294, 296, 300,

301, 302.
worms, dispersal, 325.

XYLEM, 65.

X.

Y.

YOLK, 140, 267.

,, corpuscles, 117.

,, influence on development, 208

—270.
,, sac, 270, 271, Fig. 120.

ZETTGLODONTHXE, 318.

zosea larva, 276, Fig. 128.

zona radiata, 140.

zooids, 43, 119.

zoophytes, 34, 119.

zoospores, 29.

Zoothamniuin, 40.

Zostero/is lateralis, dispersal, 324.

Zygogonium, conjugation, 97, 143.

zygosis, 83, 131.

zygospore, 97.

zygote, 33. 83, 87, 88, 91, 97, 101,

105, 109, 118.

,, conjugation of nuclei in,
131, Fig. 64.

PRADflURY, AONEW, & CO. LD., PRINTERS, LONDON AND TONBHIDGE,

FROM CONSTABLE'S LIST

Life Histories of Northern Animals,

AN ACCOUNT OF THE MAMMALS OF MANITOBA.

By Ernest Thompson Seton, Naturalist to the Government
of Manitoba. In two volumes. Large 8vo. Over 600
pages each. With 70 Maps and 600 Drawings by the
Author. Price 735. 6d. the set. Prospectus on application.

An important book of popular natural history on a strictly scientific
basis.

The Origin and Distribution of Life in
America,

By Robert Francis Scharff, Ph.D., B.Sc., F.Z.S., M.R.I.A.,

Keeper of the Natural History Collection at the National
Museum of Ireland, Author of " European Animals." Illus-
trated with Maps. Demy 8vo. 105. 6d. net.

European Animals : Their Geological History and
their Geographical Distribution.

By Robert Francis Scharff, Ph.D., B.Sc., F.Z.S.. M.R.I.A.

Numerous Illustrations. Large Crown 8vo. 75. 6d. net.

Extinct Animals,

By Sir E. Ray Lankester, F.R.S. With a Portrait of the
Author and 218 other Illustrations. Demy 8vo. New
Popular Edition. 35. 6d. net.

NATURE : — "We give the book a hearty welcome, feeling sure that
its perusal will draw many young recruits to the army of naturalists,
and many readers to its pages."

A Guide to the Study of Fishes.

By David Starr Jordan, President of Leland Stanford Junior
University. With Coloured Frontispieces and 427 Illustra-
tions. In two volumes. Folio. 505. net.

CONSTABLE & COMPANY LIMITED
10 ORANGE STREET W C

The Birds of the World.

A popular account by Frank H. Knowlton, M.S., Ph.D.,

Member American Ornithologists' Union, President
Biological Society of Washington, etc., etc. With Chapter
on Anatomy of Birds by Frederick A. Lucas, Chief Curator
of Brooklyn Museum of Arts and Sciences, and edited by
Robert Ridgway, Curator of Birds, U.S. National Museum.
4to. 305 net.

North American Trees :

Being Descriptions and Illustrations of the Trees growing
independently of cultivation in North America, North of
Mexico, and the West Indies.

By Nathaniel Lord Britton, Ph.D., Sc.D., Director-in-Chief
of the New York Botanical Garden. With the assistance of
John Adolph Shafer, Pharm.D., Custodian of the Museums
of the New York Botanical Garden. Illustrated. 8vo.
305. net.

A Manual of the Trees of North America.

By Charles S. Sargent. With 644 Illustrations by C. E.
Faxon. Demy 8vo. 255. net.

Indian Trees.

AN ACCOUNT OF TREES, SHRUBS, WOODY CLIMBERS,
BAMBOOS AND PALMS, INDIGENOUS OR COMMONLY
CULTIVATED IN THE BRITISH INDIAN EMPIRE.

By Sir Dietrich Brandis, K.C.I. E., Ph.D. (Bonn), LL.D.
(Edin.), F.R.S., F.L.S., F.R.Q.S., and Hon. Member of
the Royal Scottish Arboricultural Society, of the Society of
American Foresters, and of the Pharmaceutical Society of
Great Britain. Assisted by Indian Foresters. With many
Illustrations. Demy 8vo. 165. net. Third Impression.

Plant Physiology and Ecology.

By Frederic Edward Clements, Ph.D., Professor of Botuny
in the University of Minnesota. With 125 Illustrations.
Demy 8vo. 8s. 6d. net.

CONSTABLE & COMPANY LIMITED
7 10 ORANGE STREET W C

- I Q - 2/5 -

t

M

.

Outlines of evolutionary
biology.