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THE EVOLUTION OF
LIVING ORGANISMS

Are EVOLUTION OF
LIVING ORGANISMS

BY

EDWIN S. GOODRICH, F.R.S.

FELLOW OF MERTON COLLEGE, OXFORD

REVISED EDITION

Hondon and Edinburgh:
T.C. & E.C. JACK, LTD. | T. NELSON & SONS,LTD.

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PREFACE

Wiruin the limits of this little volume it would be
scarcely possible to deal with the vast subject of Organic
Evolution from all points of view. That evolution has
taken place is now universally admitted by students of
Biology. I have, therefore, discussed not so much the
evidence for the doctrine of transformation, as the
results of modern research on the nature and relative
importance of the factors which have contributed to the
production of the various forms of life we see around us.
No attempt has been made to give an historical account
of theories of evolution, and many important point:
have been only very briefly treated, owing to lack of
space. As far as possible the use of technical terms has
been avoided; but if the meaning is to be clear and
definite, it must sometimes be expressed in technica
language. Such terms are explained in the text, ano
the reader will easily find the explanation on turning
to the index at the end of the book.

EDWIN 8S. GOODRICH.

Merton Cotiece, Oxrorp,
March 18, 1912.

PREFACE TO THE SECOND
EDITION

Tuis second edition has been entirely revised. Although
this little book remains substantially the same, yet I
hope that the correction of some errors, the removal
of some obscurities, and the insertion of some additions,
will help to make it a more trustworthy and intelligible
introduction to the study of a vast subject of ever-increas-_
ing interest.

EDWIN 8. GOODRICH.
October 12, 1918.

CONTENTS

THE NATURE AND ORIGIN OF LIFE . . .

THE CELLULAR STRUCTURE OF ORGANISMS: RE-
PRODUCTION AND DEATH . ° . :
4

DARWINISM AND HEREDITY ; se s

VARIATION AND THE FACTORS OF INHERITANCE:
DETERMINATION OF SEX . . .

THE STRUGGLE FOR EXISTENCE AND NATURAL
SELECTION ° ° . . : F

ISOLATION AND SEXUAL SELECTION . .

. PHYLOGENY AND CLASSIFICATION : . .

THE GEOLOGICAL RECORD OF SUCCESS AND
FAILURE . . . . . ‘

PSYCHOLOGY AND THE EVOLUTION OF INTELLI-
GENCE . ‘ : : ° . .

A COURSE OF READING ON EVOLUTION . .
INDEX ° : . . . . . .

THE EVOLUTION OF
LIVING ORGANISMS:

4
CHAPTER I

THE NATURE AND ORIGIN OF LIFE

THE variety of living things is so great, the manifesta-
tions of life so diverse and wonderful, that it may seem
at first sight impossible to explain them all as the result
of the continuous and uniform action of certain funda-
mental “natural causes.” Yet this is the ambition of
the scientific student of nature. But, before attempting
to describe the process of evolution, and estimate the im-
portance of the several factors which have contributed to
bring about the development of life, let us see what
science can teach concerning the physical and chemical
properties of living organisms.

All living things have certain characteristics in com-
mon as regards their structure and composition, their
properties and activities. They all feed, grow, and re-
produce. But if we say that in the possession of these
characteristics living matter differs from non-living
matter, we do not mean to imply that the difference is
absolute, that a perfectly hard and fast line can be drawn
between them, that the gap separating the living from

10 EVOLUTION OF LIVING ORGANISMS.

the non-living can never have been bridged. On the
contrary, although far from being able to give a complete
scientific explanation of all the phenomena of life, we
have made so much progress towards that final goal of
the evolutionist that we seem fully justified in believing
that the transition from the non-living to the living has
indeed occurred, and even in hoping that some day the
very origin of life will be explained.

The organisms now living on this earth are known as
plants and animals. The study of plants is called
Botany, that of animals Zoology ; while Biology is the
name given in England to the science of living things in
general, including both animals and plants. Now one of
the first results of a deeper knowledge of living organ-
isms is to show us how much there is in common between
them, and how much they differ from their non-living
surroundings. There can be no doubt that even the
simplest plant or animal now living is the product of a
long series of evolutionary changes the initial steps of
which have long since disappeared, or are at all events
unknown to us.

In the first place, all living matter contains substances
of peculiar molecular structure and composition, far more
complex than any compounds found in inorganic nature.
Such complex compounds occur only in living matter
itself, or in its products. But the difference between
these organic and the inorganic substances is only one of
degree, and many of the most characteristic of them have
been artificially made in the chemical laboratory.

Built up of the ordinary elementary chemical sub-
stances of nature, chosen indeed as we might expect from
among the commonest elements on the surface of the
earth, these organic compounds may be grouped in three
classes: carbohydrates (such as sugars, starch, and
cellulose), fats, and proteins. Of these the proteins are
by far the most important. For while the molecule of
fat or carbohydrate consists entirely of various combiua-

NATURE AND ORIGIN OF LIFE. 11

tions of the three elements carbon, hydrogen, and oxygen,
the protein molecule always contains in addition nitrogen
and sulphur, and hence can attain a far higher degree
of complexity endowing it with many new properties.
Small quantities of other elements, such as phosphorus,
iron, chlorine, potassium, sodium, calcium, and mag-
nesium are also found combined with the five essential
elements mentioned above in the protein molecule. So
complex are the proteins that the exact chemical formula
of even the simplest varieties has not yet been made
out. We know, however, that the molecule is very
large, containing some hundreds or even thousands of
chemical atoms, with certain characteristic groupings
of C, H, O, and N. Further, the proteins display
several physical properties whieh play an important part
in the processes of life.

Correlated with this complexity of chemical composi-
tion is the most striking and important of all the physico-
chemical properties of living matter, namely a capacity
for perpetual change. It involves both an exchange of
material and a transformation of energy. This funda-
menial process, the basis of all vital activities, is called
metabolism. Briefly to explain the process it will be
best perhaps to take first the case of animals. What is
said of metabolism in animals applies in principle to
living organisms in general. An animal is perpetually
taking in food and oxygen, and perpetually giving off
carbon dioxide and waste products. Hence the neces-
sity for respiration (oxygen being derived from the air)
and nutrition. Now the food consists chiefly of fats,
carbohydrates, and proteins ; and since energy has been
consumed in the building up of these compounds, it will
again be freed if they are broken down or split up into
simpler compounds. During their whole life animals
perform work, and give off heat; and the energy so
spent is derived from the food. The stored-up potential
energy of the food is continually being converted into

12 EVOLUTION OF LIVING ORGANISMS.

kinetic energy (various forms of motion) and heat. The
freeing of the energy is brought about by the burning or
oxidation of the food materials, which are thus more or
less completely broken down from a highly complex
state into such comparatively simple compounds as
water, carbon dioxide, and urea. Hence the necessity
for excretion to remove these waste oxidised substances.
In ourselves the urea is passed out by the kidneys, and
the carbon dioxide by the lungs. An organism may
be compared to a heat engine, which derives its energy
from the fuel supplied to it. Throughout the process
neither matter nor energy is either gained or lost, but
merely changed from one form to another.

One of the most important and fundamental generali-
sations of biology is that the principles of the conserva-
tion of energy and of the conservation of matter hold
good in living things as they do in inorganic nature. All
the matter which enters an organism as food and oxygen
eventually leaves it as waste product, except in so far as
it is retained for purposes of growth. Similarly all the
energy brought in is balanced by work done and heat
given off. Neither new matter nor new energy is pro-
duced, although both are undergoing unceasing change.
This is the metabolic process in terms of physics and
chemistry ; it is quite characteristic of living things.
There is no life without metabolism, and no metabolism
without life.

In the living engine not only is the food consumed,
but the machinery itself is involved in the process of
change, so that the food material brought in does not
for the most part merely pass through as fuel; it serves
to build up that complex living substance, or machinery,
which is perpetually breaking down again into non-living
matter. Chemical instability, a tendency to unite with
other substances, or to break up into simpler groups, is
manifested by these highly complex protein molecules.
There is thus a double process continually going on in

NATURE AND ORIGIN OF LIFE. 13

metabolism. A building up or synthesis of substances
to form higher and higher compounds in the making of
which energy is absorbed—this is the anabolic process;
and a corresponding breaking down of these highest
compounds into simpler ones, and eventually into waste
products, during which energy is freed—called the kata-
bolic process. In living substance there are, then, two
series of compounds: one leading up to the highest com-
plexity, and the other leading down to waste products
from which energy is no longer drawn. This mixture of
a double stream of substances undergoing these physico-
chemical changes is the living matter itself, the very
physical basis of life, known as protoplasm.

In physical structure protoplasm is a viscid or semi-
liquid colourless substance seen under the highest
powers of the microscope it appears to be composed of
the minutest globules of a more liquid substance enclosed
in a meshwork of denser fluid. When dead protoplasm
is analysed it is found to be composed of a variety of
anabolic and katabolic proteins, associated with water
and certain mineral salts. Protoplasm is the essential
living substance present in all living organisms, the
seat of all their activities. There is no protoplasm apart
from life, and no life without protoplasm.

Animals have lost the power of building up new proto-
plasm from inorganic matter, indeed from any compounds
lower than the simplest nitrogen-containing proteins or
their immediate derivatives (albumoses, peptones, amino-
acids). They inevitably starve sooner or later on a diet
without protein, however much fat or carbohydrate it
may contain, since they are unable to replace the highest
nitrogen compounds which necessarily break down in
the metabolism of protoplasm. Speaking generally,
animals can build up the most elaborate proteins from
the simplest, and fats from proteins or simpler carbo-
hydrates ; but the carbohydrates themselves they are in-
capable of forming from inorganic matter. Thus although

14 EVOLUTION OF LIVING ORGANISMS.

they run the risk of starvation if no proteins are avail-
able, yet by feeding on other organisms they make
use of the best fuel, with the greatest amount of poten-
tial energy, and may have large quantities of surplus
energy to dispose of—for the amount of energy freed
by the breaking down of the food material much exceeds
that consumed in the anabolic processes in the ordinary
course of life.

All animals depend ultimately on plants for their food.
Carnivorous live on herbivorous animals, and these in
turn on plants, whose powers of synthesis are more
complete.

Even the ordinary green plants can only build up pro-
toplasm from inorganic material in the presence of sun-
light. This they accomplish with the help of the green
substance, chlorophyll, which decomposes the carbon
dioxide of the atmosphere, freeing the oxygen, and com-
bining the carbon with water to form starch, C,;H,)0,,
thus storing up energy from the sun. With the help
of inorganic nitrogen compounds derived from the soil,
protein can then be formed.

Many lower plants have the power of building up pro-
toplasm without chlorophyll and sunlight. Fungi can
synthesise protein from carbohydrates and inorganic
salts of nitrogen, but depend on organic compounds
for their supply of carbon. Among the multitude of
bacteria which abound in the soil there are many which
can build up proteins, and therefore protoplasm, from
inorganic compounds alone ; and some indeed which can
make use for this purpose of the free nitrogen of the
air.

All the phenomena of life are associated with the
physico-chemical processes of metabolism taking place
in protoplasm alone. The three most characteristic
properties of living matter: irritability, or the power
to respond to stimuli, growth, and reproduction, all
depend on metabolism. An excess of anabolic over

NATURE AND ORIGIN OF LIFE. 15

katabolic processes leads to increase of protoplasm, to
growth, and ultimately to reproduction. An excess of
katabolic over anabolic processes leads, on the contrary,
to reduction and to death.

Like all physico-chemical processes, metabolism is
limited by definite conditions. The essential elements
must be present, and a sufficiency of food to balance
the waste. Water is another essential, since all the
chemical reactions really take place in solutions. Free
oxygen is also necessary for the burning of the food
material, except in the rare case of certain parasitic
organisms and bacteria, which can obtain it from com-
pound substances. Moreover, metabolism can only take
place at all within a somewhat narrow range of tem-
perature, the exact limits of which vary with different
organisms. No metabolism is, of course, possible at
a temperature so high as to coagulate or destroy the
proteins, or so low as to stop chemical action.

Most, perhaps all, of the processes of metabolism take
place with the help of special proteins, known as ferments
or enzymes, which have the property of facilitating and
hastening chemical actions. Just as a small trace of
platinum black will cause an indefinitely large amount of
hydrogen peroxide (H,O,) to decompose into oxygen and
water, so a small quantity of ferment will cause an inde-
finitely large amount of carbohydrate, fat, or protein to
break up into simpler substances. Such ferments, which
are not themselves affected, which are not involved in the
end products of the actions they facilitate, are called
catalytic, and play a most important part in the mechan-
ism of life.

In the foregoing pages we have seen that living organ-
isms contain no special vital elements differing from those
of non-living matter, and are actuated by no special vital
force. From the physico-chemical point of view life is
@ process involving perpetual change in a complex of
elaborate compounds continually being built up and con-

———

16 EVOLUTION OF LIVING ORGANISMS.

tinually being broken down. A stream of non-living
matter with stored-up energy is built up into living
matter, and again passes out as dead matter, having
yielded up the energy necessary for the performance of
the various activities of the protoplasm. All the pheno-
mena of metabolism on which these activities are based
are strictly limited by external conditions and the pro-
perties of the material taking part in it.

We cannot, therefore, speak of a special living chemical
substance. The full attributes of life are only possessed
by a mixture of substances, some very complex, others
more simple, which make up protoplasm asa whole. The
living process forms a chain every link of which is
essential. Nosingle link by itself can be said to be living,
nor can we really draw a hard and fast line marking off
where the life process begins and where it ends. The
protoplasm itself contains many substances, such as yolk
of starch granules, which are merely materials for future
use in the building up of new protoplasm, or again
granules of waste products, or special products of decom-
position stored for future use (digestive ferments, secre-
tions) which may never again enter the life cycle.

It was mentioned above that irritability, or the power
of responding to a stimulus, is one of the most important
characteristics possessed by all living matter. A mani-
festation of metabolism, it depends on the fact that the

_ material of protoplasm is in a state of unstable equili-

brium capable of being disturbed by a stimulus. Stimuli

‘are those things or conditions in the environment which

can bring about disturbance or response. Naturally the
character and amount of the stimulus has no direct re-
lation to the character and amount of the response. Just
as the pressure of a button may ring a bell, explode a
mine, or start an engine, so a stimulus applied to living
protoplasm may cause a plant to grow, an animal te
move, or a man to embark on a course of action. The

extent and nature of the response depend on the structure
(2,081)

NATURE AND ORIGIN OF LIFE. 17

of the mechanism stimulated, and on the amount of
energy stored init. But the mechanism may be so dis-
posed as to vary in its response according to the intensity
and duration of the stimulus. An external stimulus fires
off, so to speak, an internal metabolic change ; this may
give rise to other changes, some of which may react on
the first. So the motiun of a mechanism may regulate
its own action ; as does, for instance, the governor of a
steam-engine. An internal change may become an in-
ternal stimulus starting new reactions. But there is no
real distinction between internal and external stimuli;
what is external to a part may be internal to the whole.

We must think, then, of living organisms as marvel-
lously complex mechanisms, with their parts so adjusted
as to set going, regulate, or restrict each other’s action.
A chain of such interactions forms the self-repairing,
self-regulating mechanism so essential for the continu-
ance of the process of life.

It is by means of this fundamental attribute of irrita:
bility that protoplasm comes into relation with its en-
vironment. It is the secret of adaptation. Of the
stimulating factors of the environment some, like tem-
perature and water, are of a general nature, ever present
and necessary ; others, like sound or light or some par-
ticular chemical compound, are more special, and not
always essential.

When first we approach the problems of evolution we
are apt to ask for definitions; to seek for distinctions
separating the living from the dead, the organic from the
inorganic : we try to discover hard and fast lines between
species and varieties, between plants and animals, be-
tween the conscious and the unconscious. But as we
study the question deeper, and extend our field of vision,
we come to recognise that the definitions are usually
misleading, the distinctions artificial, the sharp lines
arbitrary. The breaks in nature, if breaks there seem
to be, are gaps in our knowledge, and diminish in size and

18 EVOLUTION OF LIVING ORGANISMS.

number as science advances. If we analyse one by one
the distinguishing features of living matter, they can all
be paralleled in inorganic nature. No one universally
present in the first is universally absent in the second.
Complex chemical compounds, the properties of which
cannot be discovered in those of their component ele-
ments, cyclic changes of matter and energy, even self-
regulating mechanisms, occur in the non-living world.
As the untutored savage explains the movements of a
watch by attributing them to a spirit which has entered
into it, so many writers hold that the activities of living
matter are due to some special and mysterious vital
force. They attempt to bridge the gaps in our know-
ledge by merely giving thema name. This is no scientific
method ; science advances by explaining, that is de-
scribing, the unknown in terms of the known. If vital
force is merely a general term for those new properties
manifested in new mechanisms of increasing complexity,
there is no harm in it ; certainly living matter must dis-
play properties not found in simpler substances. But it
has no intelligible meaning if used to denote some force
added, so to speak, from without over and above the
ordinary properties, acting on the physico-chemical
mechanism but not of it. Like a vortex, the metabolic
process in living matter draws in inorganic substance
and force at one end, and parts with it at the other ; it is
inconceivable that these should, as it were, pass outside
the boundaries of the physico-chemical world, out of
range of the so-called physico-chemical laws, at one point
to re-enter them at another.

When we maintain that the physico-chemical processes
of metabolism, with which are correlated all the pheno-
mena of life, form a continuous series without break, we
do not in the least mean to assert that this is a complete
and satisfying explanation of life from all points of view.
It is only one aspect of the problem, and the psychical is
another. Doubtless there are still other aspects within

NATURE AND ORIGIN OF LIFE. 19

the wider scope of philosophy. But we hold that the
various aspects cannot be described in the same terms,
that they neither overlap nor break into each other’s
continuity. They are one-sided abstractions based on a
reality, with the ultimate nature of which it is not the
function of natural science to deal.

But, it will be asked, if life is thus correlated with a
physico-chemical process, why cannot living substance
be made in the laboratory? The answer is that the ex-
pectation is premature, perhaps never to be realised.
Since we are still ignorant of the intimate structure of
the proteins, we can hardly be expected to manufacture
them. Moreover, it would not be sufficient to make one
or even several such compotnds, but the whole chain of
growing complexity. If the machine is to work, the
mechanism must be complete. There is, also, a long
history behind even the simplest organism found at the
present day ; there must be a vast difference between
the very simplest of these and the initial stages in the
evolution of protoplasm. It would be as absurd to ex-
pect an experimenter to build up an organism in a labora-
tory as, for instance, to expect a refined civilisation to
arise In a day among the savages of Central Africa.
Great advances have already been made in the synthesis
of organic compounds of carbon and nitrogen; but if
any stage in the development of living substance were
artificially made, it would probably be so different from
the protoplasm of modern plants and animals that we
should scarcely recognise it as living at all, even if we
had it before us.

An attempt to reconstruct in imagination what we
believe may have been the history of the origin of living
matter may be made not altogether without profit.

Before the principle of the continuity of life, to which
we shall refer later, was established, it was thought that
living things arose spontaneously from organic com-
pounds. Moulds, the bacteria of putrefaction and fer-

20 EVOLUTION OF LIVING ORGANISMS.

mentation, infusoria, and suchlike forms, were supposed
to develop de novo in liquids of appropriate composition.
But Pasteur and others in the last century definitely
proved that such organisms really arise from germs or
spores originally present in the liquids; that neither
putrefaction nor fermentation will take place, nor organ-
isms of any kind appear, in substances which have been
thoroughly sterilised, or from which all living germs
have been rigidly excluded. So far as we know, living
organisms at the present day do not develop spontane-
ously, but are all derived from pre-existing organisms.

We must, however, suppose that at some period in the
earth’s history, when conditions were favourable and
perhaps very different from those of the present time,
living protoplasm made a first appearance. Possibly
these conditions will never be repeated, either in nature
or in the laboratory, and the first stages in the evolution
of life may never be discovered. The temperature,
moisture, pressure, and other conditions must have been
such as to allow of the formation of high compounds of
various kinds. Many of these would be quite unstable,
breaking down almost as soon as formed ; others would
be stable, and merely persist and accumulate. Still
others might, possibly with the help of some katalytic
substance, tend to reform as fast as they broke down.
Once started on this track such a self-repairing compound
or mixture would inevitably tend to perpetuate itself, and
might combine with, or “feed” on, other compounds less
complex than itself, as was long ago suggested by
Lankester. For any chemical action will continue so
long as the conditions are favourable; a trail of gun-
powder will inevitably explode from end to end provided
it be continuous, and will go on burning so long as
powder is supplied.

The principle of the survival of the fittest applies with
all its force to such initial steps in the evolution of life.
The more completely self-regulating mixtures would out-

STRUCTURE OF ORGANISMS. 21

last the others. And so we may imagine did the nicely
balanced mixture of anabolic and katabolic proteins
finally become elaborated into protoplasm. Innumer-
able compounds must, of course, have failed to establish
themselves in this way owing to too great fixity or too
great instability. For many reasons it seems probable
that life originated in the sea; protoplasm contains the
same elements as sea-water, and in much the same
proportions.

Speculating further, we may suppose that it became an
advantage for some of this vaguely defined metabolising
substance to become separated off into individual masses,
which came to acquire the structure of cells. From this
point onwards we can appeal to known evidence for our
history of the evolution of life.

CHAPTER II

THE CELLULAR STRUCTURE OF ORGANISMS :
REPRODUCTION AND DEATH

ALTHOUGH all living organisms necessarily contain some
protoplasm, yet they are far from being entirely made up
of it. All the parts of an organism are not truly alive,
but only that portion which is protoplasmic. However,
the substances of which it is composed are either about
to be assimilated into protoplasm, or are the products of
protoplasm. Indeed the great bulk of a plant or animal
may be formed of the accumulated products of its past
activity. Such, for instance, is the woody supporting
tissue of a tree, the skeleton of a coral, the shell of a
snail, the bony substance or hair in ourselves. While
the living element is continually undergoing change, the
dead deposit may continue unaltered during the life of
the organism, and even afterwards.

A microscopic examination of living organisms teaches

22 EVOLUTION OF LIVING ORGANISMS.

us that the protoplasm is always present in the form of
cells. We now come to another of the great generalisa-
tions of biology: the so-called “cell theory,” founded by
Schleiden and Schwann more than seventy years ago, and
much modified and extended since then. Briefly it may
be summed up as follows: The cell is a small mass of
protoplasm consisting of a nucleus and surrounding cell-
body. All plants and animals are made up of such cells,
either singly or in aggregates. Growth is due to the
increase in size and multiplication of the cells. They
reproduce by division into two, and all the cells of the
body of an individual are thus derived from a single
original cell. Differentiation in multicellular organisms
is related to the progressive division of labour among
the cells, comparable to the division of labour among the
individuals of a civilised community. The body of a
multicellular organism is thus an aggregation, not of
separate units brought together, but of a multitude of
related cells remaining in association, and building up its
tissues. Asarule the tissue-cells remain in actual proto-
plasmic continuity; but in animals certain cells may
become free and lead a quasi-independent life, as for
instance the white corpuscles of the blood. The activity
of an organism is the sum of the activities of its com-
ponent cells. All living phenomena are ultimately ques-
tions of cell-life ; all organic products due to the action
of cells (Fig. 1).

While the cell-body consists of ordinary granular
protoplasm (cytoplasm), the nucleus is formed of special
protoplasm differing from it both in structure and in
chemical composition. In the nucleus may be distin-
guished by appropriate methods, besides a more fluid
nuclear sap, a meshwork of substance known as linin, in
which are suspended masses of another substance called
chromatin, from the fact that it colours deeply with
certain stains after having been killed and coagulated
with appropriate reagents.

STRUCTURE OF ORGANISMS. 23

For the life of a cell both body and nucleus are essen-
tial. An interchange of material takes place between
them; one cannot live without the other. If a cell is
divided into halves, only that which contains the nucleus
will continue to live, grow, and reproduce. The cell is
the smallest known unit of life.

In the Bacteria there is no well-defined nucleus, the
chromatin being scattered. They probably represent a

Fia. 1.—Enlarged view of stained microscopic preparations to show the
structure and multiplication of cells and their nuclei in plant tissue
A (section through the tip of a root), and in animal tissue B (epi-
thelium of the gill of a larval Salamander). a, b,c, d represent
successive stages in the divisions of cells.

primitive condition before the typical cell-structure was
completely differentiated.

Among the lowest plants, Protophyta, and the lowest
animals, Protozoa, the whole individual consists of a
single isolated cell, which consequently has to perform all
the functions of life. Such organisms may, nevertheless,
become highly differentiated, developing special cell-
organs—a mouth for taking in food, motile processes for
locomotion, contractile vacuoles for excretion, and so

24 EVOLUTION OF LIVING ORGANISMS.

forth. But in the multicellular plants, Metaphyta, and
animals, Metazoa, where the cells form tissues and organs,
starting from primitive cells capable of performing
all the essential processes of life (irritable, metabolic,
growing, and reproductive) the cells become as differ-
entiation advances adept at performing certain functions,
but correspondingly incapable of carrying out others.
The more they become adapted in special directions, the
more they are apt to lose their other powers. So the
cells of the alimentary, nervous, muscular, and other
tissues of animals can only continue to live in association
with each other, supplying each other’s wants. More-
over, the specialised cells of differentiated tissues can
usually only produce by division cells like unto them-
selves, and may lose the power of reproducing at all.

Multicellular organisms all start from a cell in a primi-
tive undifferentiated condition, rich in unspecialised
protoplasm, capable of becoming adapted to perform any
of the necessary functions of life. Of such are built up
the early embryonic stages, and the actively growing
tissues of adults. But soon they become burdened with
the accumulated products of their own activities, their
potentialities are narrowed down, their irritability re-
stricted so as to respond only to special stimuli, their
capacity for regeneration exhausted.

Speaking generally, the lower the organism the less
differentiated are its cells, and the greater are their
powers of growth and regeneration. A single cell of the
fresh-water alga Vaucheria or of the fungus Mucor will
grow into a new plant; the propagation of plants by
cuttings is familiar. While the regenerating powers in
a man are restricted to the simple growth of tissues, the
healing of wounds and so forth, a newt or a crab will re-
generate a whole limb ; a worm deprived of its head or
tail will replace the missing parts; a fresh-water polyp,
Hydra, may be cut into several pieces, each of which can
grow into a complete animal. Speaking generally, also,

STRUCTURE OF ORGANISMS. 25

the younger the organism the greater are its powers of re-
generation. Thus the embryo of a sea urchin, while still
composed of only a few cells, say two, four or eight, may
be divided into two, four or even eight separate cells, each
of which is capable of growing into a complete larva.

All that is necessary for complete regeneration to take
place is protoplasm retaining its potentiality of develop-
ment, and present in sufficient quantity for metabolism to
be carried out in full. So soon as the potentiality
becomes limited by specialisation the power of regen-
eration becomes restricted.

We must now return to the consideration of the repro-
duction of cells. A unicellular organism does not increase
indefinitely in bulk ; when it exceeds that size which is
normal for the adult of the species, it tends to divide into
two. The nucleus divides first, then the cell-body, form-
ing two daughter cells, each with its own nucleus. The
daughter cells then separate, growing into adults similar
to the original parent. It is the same with the cells of
the higher organisms. Each multicellular animal or
plant starts life as a single cell, which grows and divides
repeatedly. In every case the nucleus divides, each half
passing into one of the two daughter cells. But here,
instead of the cells separating to lead an independent life,
they remain more or less closely associated as parts of a
single complex individual (Fig. 1).

Thus not only is every cell derived from a pre-existing
cell, but every nucleus is formed from a pre-existing
nucleus, just as all protoplasm is derived from pre-existing
protoplasm. The continuity of protoplasm, of cells, and of
nuclei, is one of thé Most important facts established in
modern biology.

Rarely the nucleus divides “directly,” by simple con-
striction into two halves; usually the division is “indirect,”
by an elaborate process known as karyokinesis, and taking
place as follows. The chromatin gathers together into a
coiled thread, the linin network becomes disposed as a

26 EVOLUTION OF LIVING ORGANISMS.

system of fibres radiating through the cytoplasm from
two minute bodies, the centrosomes. Between these
centrosomes the fibres join across, forming a spindle.
The centrosomes can be seen to originate from the nucleus
or its neighbourhood, as a single body which divides, the
two halves moving to the opposite sides of the nucleus.
The chromatic thread now breaks up into a definite
number of separate pieces, the chromosomes, which
arrange themselves in a circle round the equator of the
spindle. Each chromosome now divides into two halves
which travel to the opposite ends of the spindle. There
they join together to form a thread; the thread breaks
up into granules, the system of fibres disappears, and
thus a new nucleus is reconstituted similar to the resting
nucleus of the original cell.- A division of the cell-body
then yields two nucleated cells (Fig. 1). Asa rule the
centrosome persists to give rise to that of the next
division. Now it is important to notice the continuity of
substance during this process of division. Cytoplasm,
linin, centrosome, and chromatin are all parcelled out to
the two daughter cells ; above all each daughter nucleus
receives the same number of chromosomes, and apparently
exactly the same amount of chromatin. The number of
chromosomes is approximately constant in each species,
but differs widely even among allied forms.

Although the process of karyokinesis may differ: in
detail in various forms, yet it is essentially the same in all
plants and animals.

Why, it may be asked, has the cell structure been uni-
versally adopted? To this question we can give no more
definite answer than this: that there seems to be some
proportion of mass to surface, and of nucleus to cytoplasm,
within which it is necessary for the protoplasm to keep if
metabolism is to be satisfactorily carried out. The size of
cells varies considerably ; much more than the size of
nuclei. It bears no direct relation to the size or com-
plexity of the organism to which the cell belongs. A cell

STRUCTURE OF ORGANISMS. 27

may be so small as scarcely to be visible with the high
powers of the microscope, or as large as the yolk of an
egg, for instance, the greater portion of which is, however,
made up of yolk granules.

If living organisms did not reproduce, they would
sooner or later be extinguished, if not by natural at all
events by accidental death. Therefore only those
creatures survive which can multiply, and indeed multi-
ply rapidly by the separation of some portion of their
substance capable of growing into a new individual.
This may give rise to a vegetative or asexual mode of
reproduction by fission, or by the formation of special
reproductive cells, or spores, or cellular bodies (buds).
Mention has already been made above of asexual re-
production in connection with growth and regeneration.
Propagation by shoots developed on runners or on tubers
is common among the higher plants, and gemmation or
bud-formation is a widely distributed mode of reproduc-
tion among animals. It is found not only among lowly
organised forms like the zoophytes, and polyps (Ccelen-
terata), and sponges (Porifera) ; but even in such highly
differentiated groups as the marine bristle-worms (Cheeto-
poda), sea-mats (Polyzoa), and sea-squirts (Tunicata),
But while in the Bacteria asexual reproduction by fission
and by spores is the only known method of propagation,
in all other groups of. plants or animals sexual reproduc-
tion occurs, though the asexual method may also be
retained.

The typical sexual reproduction of multicellular plants
and animals takes place by means of special cells of two
kinds set apart for the purpose, and called the germ-cells
or gametes. Each gamete of one kind by fusion with a
gamete of the other kind in the process of fertilisation
gives rise to a single cell, the zygote, which grows into a
new individual. The two kinds of cells have become
differentiated along divergent lines, being adapted to the
particular functions they have to perform. One, the

28 EVOLUTION OF LIVING ORGANISMS.

ovum, is quiescent, stored with food-material to provide
for the nutrition of the developing embryo, and is gener-
ally of large size. The other, the spermatozoon of
animals or spermatozoid of plants, is on the contrary
small, active, and usually furnished with a vibratile whip-
like “ tail,” with the help of which it bores its way into
the ovum. The individual bearing ova is of the female
sex; that bearing the spermatozoa, of the male sex.
But the names male and female are often conveniently
extended to the gametes themselves. Hermaphrodites
give rise to both kinds of germ-cells.

In fertilisation one male gamete fuses with one female ;
and not only do the cell-bodies unite, but the nuclei of the
cells also combine into one nucleus. Thus the nucleus of
the resulting zygote contains chromatin from two in-
dividuals, since the cells usually come from different
parents. For it is only exceptionally that hermaphrodites
are self-fertilising. Unsuccessful male gametes, which do
not reach an ovum, perish sooner or later ; and likewise,
unfertilised ova die, except in those rare cases where
parthenogenesis occurs. Instances of parthenogenetic re-
production are the fresh-water stonewort, Chara crinita,
of which only females occur in northern Europe, and the
plant lice (Aphidse) and certain other insects, which
propagate in this manner in the summer.

The differentiation of the germ-cells and of sex has had
a profound influence on the evolution of organisms. To
secure the nourishment, fertilisation, distribution, and
survival of the reproductive cells is their chief function ;
to this end have been developed that wonderful diversity
and elaboration of structure, both bodily and mental,
found in living nature.

Among the unicellular forms sexual reproduction may
be of a much simpler kind than that described above.
Often one individual may divide up into a number of
small “male” gametes, each capable of fertilising
another individual playing the part of a “female” ovum.

STRUCTURE OF ORGANISMS. 29

Or, as in many of the lower multicellular algz and in
Monocystis, a parasitic protozoon found in the earth-
worm, two individuals may each form a set of similar
gametes without visible sexual differences, which may
fertilise each other. Again, two similar whole individuals
may fuse, and subsequently give rise to new cells; a
common form of reproduction among fresh-water alge.
The infusorian Paramecium merely exchanges cytoplasmic
and nuclear material with another individual during a
temporary union or conjugation.

Although these various simpler modes of fertilisation
do not in all probability represent actual phylogenetic
stages in the evolution of sex, yet they give us some
notion of how sex may have become developed. For its
very first origin one must perhaps go back to that early
time in the history of protoplasm before definite forms
had become differentiated, and when it might have been
beneficial for two masses of protoplasmic material, differ-
ing slightly in composition, to mix and combine their
properties together.

But whatever may have been its origin, fertilisation
now gives a necessary stimulus to development, and
brings about some sort of rejuvenation. Even among
those plants and animals which propagate freely asexu-
ally recourse is had sooner or later to sexual reproduction.
It would seem, as Biitschli maintained, that prolonged
reproduction by fission in the protozoa in some way
exhausts their vitality. Certain it is that under con-
ditions which would lead to their death if unable to con-
jugate, they can be seen to start with renewed vigour on
a fresh career of growth and multiplication after conju-
gation has taken place.

It has already been mentioned that in multicellular
organisms active growth occurs among the less differen-
tiated cells which have retained a primitive richness in
protoplasm, an embryonic character. Now the germ-
cells, usually produced in enormous numbers, are derived

30 EVOLUTION OF LIVING ORGANISMS.

from such undifferentiated cells set apart for the purpose
of reproduction, sometimes from the very earliest stages
of embryonic development. They may be traced back,
and occasionally even be distinguished under the micro-
scope through an unbroken lineage of embryonic cells to
the fertilised ovum. 'Weismann’s famous theory of the
continuity of the germ-plasm is founded on these facts.
According to it, the germ-plasm, that special protoplasm
of the gametes which is handed on by them from genera-
tion to generation, and gives rise to new individuals, is in
a sense independent of the body or soma in which it
develops. Whereas the rest of the multicellular organism,
the soma, undergoes differentiation and dies, the germ-
cells continue for ever giving rise to new generations, the
germ-plasm passing from parent to offspring. This brings
us to the consideration of the origin and biological sig-
nificance of death.

Is death an essential inevitable attribute of life? If
by death we mean mere decay, mere katabolic changes,
the answer is yes. In this sense we begin to die as soon
as we are born. But this is not what is meant by death
in ordinary language. We do not speak of death unless
there is a corpse. And the question is, putting aside ac-
cidental death through violence or disease: does natural
death occur always, or does it even occur at all? Would
an organism shielded from all unfavourable conditions
continue to live for ever, or is it wound up, so to speak, to
live for only a limited time? It has already been men-
tioned that among the bacteria and unicellular organisms
propagation by a process of fission and spore formation
may go on indefinitely if the conditions are favourable.
True it is that the individual may be said to disappear in
the splitting ; but death can have no sting for an organ-
ism which divides into two living halves. Even when
sexual union is necessary no corpse need be produced.

Death, then, appears among living organisms when a
soma or body becomes differentiated from the germ-cells.

STRUCTURE OF ORGANISMS. 31

_ The soma dies; but the germ-cells live on, passing from
one mortal parent to another. As pointed out by Weis-
mann, germ-cells, like the unicellular organisms just
mentioned, are potentially immortal. Since the first
appearance of living substance on this earth death has
never interrupted the main streams of life. Death is,
so to speak, a by-product of multicellular organisms.

But even the soma might conceivably go on living for
ever, provided only it continue growing, or the wear and
tear of life be perpetually repaired. Weare familiar with
plants which can be propagated by cuttings for an indefi-
nitely long period, and we know of others, such as trees,
which, preserving vigorous embryonic cells at their grow-
ing surfaces, may prolong their natural life for thousands
of years. Even in the best regulated organism, however,
those essential proportions between surface and volume
can hardly be so well preserved ; that nice co-ordination
of parts, that power of repairing injuries and waste, can
hardly be so accurately adjusted as to continue working
smoothly forever. Especially is this the case in animals,
where both size and shape are almost always definite and
limited. If one may be allowed to use metaphorical
language—rather than go on resisting the wear and tear
of life, nature sacrifices the old and battered soma, and
trusts the germ-cells to start a new individual afresh.

So the length of life of an organism may become defi-
nitely adapted to its needs; as in the case of annual
plants and most animals, where the energies of the in-
dividual are exhausted in securing the success in life of
the next generation. Thus a large number of animals
in the colder regions live only for one season, leaving
behind them their eggs tosurvive the winter and develop
next year. Frequently the male sex dies as soon as fer-
tilisation has been accomplished. A definite relation
can be traced between the length of life of the individuals
of a species and the number of young produced, and the
care required to bring them up.

32 EVOLUTION OF LIVING ORGANISMS.

CHAPTER III
DARWINISM AND HEREDITY

ScaRcELY more than half a century ago Darwin, in his
immortal work on The Origin of Species, first gave a
satisfying truly scientific explanation of the evolution of
living organisms. Before his time many authors had
recognised that the various forms of life have been
evolved from one another by gradual transformation.
From the earliest times, indeed, philosophers had specu-
lated on the possible modification of organisms, laying
stress now on external environment, now on internal
factors. All such speculations failed to convince, being
either obviously inadequate, or calling in mysterious
evolutionary forces of which no scientific explanation
could be given. Darwin first clearly showed how over-
whelming is the evidence that evolution has actually
taken place; but his great merit is to have shown that
it can be accounted for by the action of “natural causes,”
which can be seen at work at the present time, can be
tested by observation and experiment, and leave no room
for any mysterious governing causes in addition ; that,
in fact, a complete scientific aspect of the process of
evolution can be described as an unbroken series of
“natural” events, a sequence of cause and effect, a series
of steps each one strictly determined by that which came
before, and determining that which follows after.
Darwin and Wallace simultaneously discovered the
great principle of natural selection, the keystone of the
Darwinian explanation of evolution. Like many great
truths, when stated it appears extraordinarily simple and
obvious. It can be described in the single phrase: the
survival of the fittest in the struggle for existence. If we
ask who are the fittest? the answer is—those which
survive. This is no mere argument in a vicious circle ;

DARWINISM AND HEREDITY. 33

it is the definition of the word fittest from the point of
view of the scientific observer. When one organism is
said to be more or less fit than another, no aspersion is
cast on the moral worth or esthetic beauty of either
from a purely human standpoint; but the fact is ex-
pressed that in the environment in which they live, one
will succeed and the other fail. So when we speak of
higher and lower forms in evolutionary series, we have
jno desire to made invidious distinctions, but take into
account the fact that some organisms are more complex
and elaborate in their structure, bodily and mental, than
others.

According to Darwin, the chief factors which contrib-
ute to the process of evolution are, variation, heredity,
and the struggle for existence’ He pointed out that man
has modified the characters (bodily and mental structure)
of domestic animals and plants by continually selecting
and breeding from those individuals which varied in
directions favourable to his purpose, and gave the name
natural selection to the similar process going on in nature.
While the doctrine of evolution has been universally
accepted, there is still much difference of opinion as to the
relative importance of the various factors concerned.

The impetus given by Darwin to the study of biology
was enormous, especially with regard to variation and
heredity, subjects which before his time had received
little serious attention. We must now examine more in
detail these factors of evolution.

However closely related organisms may be, they always
differ from each other: parents and offspring, brothers
and sisters are never quite alike. The differences between
them are called variations. Familiar to us among forms
which we know well and observe closely, as for instance
individuals of the human species, or our domestic animals
and plants, variation is no less universally present among
all sorts of living organisms. Variability occurs in vari-

ous degrees, in all possible directions, and extends to
(2,081)

34 EVOLUTION OF LIVING ORGANISMS.

every sort of character, structural or functional. Shape,
size, colour, scent, number and relative position of parts,
are all subject to variation. The most complex capaci-
ties, such as fertility, the power of resistance to disease,
and intellectual ability are notoriously variable. More-
over, variation occurs at every stage of life: the seed,
seedling, young and adult plant, the egg, embryo, young
and adult animal, all show variations.

These variations can often be accurately measured, and
the statistical study of variation begun by Quetelet and
Galton, and carried on by W. F. R. Weldon, K. Pearson,
and others, has yielded many important results. If we
took a number of sticks picked up at random, and
measured their length, we should find that while sticks
of medium length are the most numerous, the sticks
become rarer and rarer as they approach the extreme of
length and shortness. If the number of sticks be large
enough, the length occurring most frequently (the modal
value or mode) would be found at or near the mean
between the two extremes. In fact the length is, as we
say, determined by chance, and the results obey the laws
of probability, being due to a number of independent
causes acting at random on the individual sticks. It is
the same with variations, as shown by the examples on
the following page.

These results can be graphically represented in the form
of a curve, which will be found to agree with a normal
curve of error. The vertical passing through the apex of
the curve will represent the mode, and the distance from
it along the base will give the range of variation.*

Asymmetrical or skew curves occur when for some
reason the variation is more limited on one side than the
other.

These, then, are the materials from which natural
selection has to choose; but to understand their nature

* The reader is referred to Mr. Watson’s volume on Heredity, in this
series, for a fuller account of variation and its mathematical treatment.

DARWINISM AND HEREDITY. 35

and importance in evolution we must analyse them
further in the light of heredity.

When the characters of the parent reappear in the off-
spring we say in popular language that they are inherited.
There is a mechanistic aspect of heredity just as there is
of life. Why, we may ask, do the reproductive egg-cells
of a snail, a fiy, and a fish, all under the same conditions,
and bathed by the same water, reproduce the same
bodily structure, the same functional capacities, the same

Frequency of different lengths of beans measured by
De Vries.

bf

Lengthinmm. .| 8 | 9 | 10 | 11 | 12 | 13 | 4 | 15 | 16

Number of beans. | 1 2 23 | 108 | 167 | 106 | 33 7 1

Frequency of different types of beech leaves measured by
K. Pearson.

Number
of veins |10/11];12] 138; 14] 15| 16] 17} 18] 19} 20} 21
in leaf.

Number of
leaves of | 1] 71 341]110/ 318 | 479 | 595 | 516 | 307 | 181} 36] 15j 1

each type.

psychological powers, the same complex kind of individu-
ality as each of the parent forms? The answer is that
they are formed of the same protoplasm as the parent ;
they are chips of the old block. Both parent and off-
spring develop from the same starting-point ; a particu-
lar mixture of substances, having a peculiar intimate
architecture, or specific structure, and undergoing a
particular kind of metabolic change. Under approxi-
mately similar conditions they are bound to develop into
approximately similar organisms. This particular kind

36 EVOLUTION OF LIVING ORGANISMS.

of substance and particular association of qualities is
transmitted by means of the reproductive cells; hence
the importance of the principle of the continuity of living
protoplasm established above (p. 25). The direct con-
tinuity of living substance, obvious in the case of the
unicellular organisms which reproduce by fission, is no
less essential in all other modes of propagation. In this
way only can the physico-chemical and other properties
of one generation pass on to another. The process of
metabolic change has continued without interruption since
its first appearance from generation to generation.

The full importance of heredity is perhaps, even yet,
not always appreciated. This or that striking peculiarity
reappearing in parent and child is pointed at as having
been inherited, as if the countless resemblances in the
whole organisation were not also dependent on heredity.
Not only the similarities, but all the vast differences
which distinguish man from the lowest animal have
been built up through heredity. Without it no evolution
could take place. For we must think of the continuous
stream of protoplasm, the physical basis of evolution, as
the means of piling up, so to speak, hereditary differences
along diverging lines.

In ordinary sexual reproduction, the specific substance
containing the essential factors of inheritance must be
passed on from one generation to the next in the germ-
cells. It forms either the cell as a whole or only its
nucleus ; many authors indeed identify it with the chro-
matin. Moreover, it is carried in the germ-cells of both
sexes, since inheritance is equal from both parents. The
eharacters due to these factors usually reappear in the
same order in the offspring in which they developed in
the parent ; but some of them may remain for ever latent,
though capable of reappearing in later generations.

Now the older writers on evolution generally assumed
that the course of heredity and the progress of evolution-
ary change were greatly influenced by the direct action of

DARWINISM AND HEREDITY. 37

the external environment and of the characters of the
parents on those of their offspring. It was supposed that
the changes directly induced in the body of the parent by
such stimuli as temperature, moisture, nutrition, repeated
use, or exercise and disuse, are inherited as such, and
would reappear in the progeny. The theories of evolu-
tion propounded by Erasmus Darwin and by Lamarck
were founded on this suppésition ; and even in the time
of Darwin it was not yet questioned whether characters
thus acquired by the parent in the course of its lifetime
are directly inherited. It was not till Weismann criti-
cally examined the evidence for “the inheritance of ac-
quired characters” that the theory was definitely over-
thrown. He showed convincingly that mutilations (such
as the repeated cutting off of the tail in dogs), the effects
of use and disuse (such as callosities produced by friction,
the enlargement of muscles or other organs, the fruits of
education, &c.), or any direct modification due to the
action of any particular stimulus, have never in any
single instance been proved to be transmitted as such
from one generation to another, while the evidence that
they are not is overwhelming. These conclusions of
Weismann, which had been to some extent foreshadowed
by Pritchard and Galton, are the most important con-
tribution to the science of evolution since the publication
of Darwin’s Origin of Species. Let us now examine the
question more closely, and further analyse the “ varia-
tions” dealt with above.

Owing to that universal property of irritability or the
power of response to stimuli already described, all organ-
isms are the result of the interaction of two sets of
factors : the factors of inheritance, and the factors of the
environment. By factors of the environment we mean
all those conditions or stimuli which are capable of in-
fiuencing the differentiation, growth, behaviour, or in
other words the metabolism of the organism. By factors
of the inheritance, on the contrary, we mean that com-

38 EVOLUTION OF LIVING ORGANISMS.

plex association transmitted from the parents of sub-
stances with properties or capacities which make up the
specific inheritance characteristic of each organism, and
which may be called its assemblage of germinal factors,
or its germinal constitution. Not only are organisms as
we see them before us necessarily the results of the
combined action of these two sets of factors ; but so also
is every part of them, every structure, every activity,
every organ, every habit. Therefore the characters
observed and measured, dealt with in statistics and ex-
periments, are likewise their products. The inheritance
might be compared to a musical instrument, the stimuli
to the player, and the organism and its characters to the
music produced. What particular tune is produced will
depend on the player; but the range of possible sound
will be limited by the structure of the instrument, by the
factors which make up its capacity to respond to the
touch of the player.

In a sense, then, every character, every variation, is
partly acquired and partly inherited ; and no character
%3 More acquired or more inherited than another. Hence
the popular distinction between acquired and not ac-
quired characters, between those which have been de-_
veloped in the course of the individual’s lifetime and
those which are inborn, is misleading. The very terms
involve a fallacy. We cannot point to this or that bodily
or mental structure and say this is acquired and that is
not. What we see before us is the result of both sets of
factors in every case.

And yet there are characters which reappear in off-
spring and others which do not. The fact is that we
must revise the popular conception of inheritance just as
we have revised that of “acquired characters.”

Obviously if either of the sets of factors be altered the
resulting organism will be changed ; variation will occur.
What is called the normal mental or bodily structure is
that developed under the usual complex of environmental

DARWINISM AND HEREDITY. 39

stimuli. Any divergence from this normal structure may
be due either to some change in the germinal constitution
in a constant environment, or to an altered environment
acting on an unchanged germinal constitution. The two
results differ to some extent; they should not be called
respectively inherited and acquired, the real distinction
being that in the one case the new character is called
forth by a new stimulus, whereas in the other case the
stimulus remains unchanged, but brings forth a new
result because it acts on altered germinal factors. This
new deviation or change of character, the variation, will
reappear in succeeding generations, provided the stimulus
be present also.

That new character (varzation) which is due to ger-
minal change will always reappear in a constant environ-
ment, provided the germinal constitution continues the
same. The other new character (variation) will never
reappear unless the causal stimulus is also present. If
the new stimulus be removed the character necessarily
goes also. The real difference between characters which
always appear and those which do not is that the former
depend on stimuli which are always present, while the
latter depend on stimuli which may be absent.

It follows that there are two quite distinct kinds of
variation for which new names must be found. The
name “mutations” may be adopted for differences due
to the changes in the inheritance, to the alterations in
germinal constitution ; and we may call “modifications”
those induced by changes in the factors of the environ-
ment. The former are transmitted in the germ-plasm,
but not the latter. By mere inspection these two kinds
of variations cannot, of course, be recognised ; they can
only be distinguished by systematic observation and
experiment. A few instances will help to make this
clear.

At first sight nothing could appear more firmly fixed
by heredity than the greenness of plants; for countless

40 EVOLUTION OF LIVING ORGANISMS.

generations green plants have reproduced themselves on
earth. Yet take the seed of the greenest plant you like,
and grow it in a dark cellar—the new plant will come up
not green at all. The stimulus of light has been denied
it, so the green “character” does not appear; it is not
transmitted. But the capacity to become green is never-
theless transmitted, as can be shown at once by bringing
the pale seedling into the light, when it will soon turn
green. This capacity depends on the factors constituting
the inheritance. Just in the same way, if the seed be
sown in soil devoid of iron the young plant will not be
green, however much light there may be; the stimulus
of iron has been removed. Add a trace of iron to the
soil, and the plant will turn green. Again, Englishmen
in the tropics become sunburnt ; this “character,” dark
pigmentation, is due to the interaction between the en-
vironmental stimulus, sunlight, and the transmitted
germinal constitution. Their children will be likewise
sunburnt, provided they are exposed to the rays of the
tropical sun. But they will not be sunburnt if they are
sent home to England. Once the maximum effect of the
stimulus has been reached, once the maximum response
has taken place, they will not be increasingly burnt,
however many generations of their parents may have
lived in the tropics ; and the children, if removed from
the action of the sun, will not be sunburnt at all. The
negro, on the other hand, will continue to be black and
his children will be black, even when he lives in a tem-
perate climate among white fellow-men. For the black
character of the negro is called forth not by the external
stimulus of sunlight, but by some other external or in-
ternal stimuli which are not removed when he changes
his abode.

This example illustrates a most important principle in
the evolution of organisms. Obviously those characters
will be most constant which depend for their appearance
on ever present stimuli. And, except in so far as an

DARWINISM AND HEREDITY. 4]

organism can choose its environment, the so-called in-
ternal stimuli are those whose presence is most assured.
An organism, therefore, must have such a germinal
transmitted constitution that it can develop fully under
the ordinary stimuli of its environment, and it will
develop fully with the greater certainty the more it does
so with the help of internal stimuli. Hence, in the
struggle for existence, those may succeed best which are,
so to speak, freed from the dependence on merely ex-
ternal stimuli. This explains the importance of self-
regulating processes. A familiar instance is that of the
warm-blooded mammals. In them has been evolved an
automatic mechanism for keeping the temperature of the
body at the most favourable point for the carrying on of
the process of metabolism, and they have become thus
to a great extent independent of the changes in their
environment ; while the cold-blooded reptiles and in-
sects, for instance, are at the mercy of the surrounding
temperature, unable to live an active life except in warm
weather.

Two more very instructive examples may be mentioned
to illustrate our discussion on the factors of inheritance.
Many plants live both in the alpine heights and in the
low plains, and acquire a characteristic structure in these
two different habitats. So different in appearance may
the two forms become, that a botanist, not knowing of
their common origin, would certainly place them in
separate species. Now, the French botanist Bonnier
divided a common dandelion (Taraxacum vulgare), and
grew one half in the lowlands and the other half in the
mountains. While the former grew into a tall and slender
plant, the half raised in the alpine heights grew into a
plant of very different appearance, with longer roots,
much shorter stems, smaller and more hairy leaves,
larger and brighter flowers. Each variety will reproduce
its like in its own locality ; but seeds of the alpine plant
will produce only the lowland form if sown there, and

42 EVOLUTION OF LIVING ORGANISMS.

vice versa, the seeds of the lowland form will grow into
the alpine form in the mountains. Moreover, if either
form be transplanted into the other region, it will soon
grow into the variety characteristic of its new habitat.
This change is accomplished by the new-growing tissues,
for the already formed tissues are no longer capable of
altering. Once fully differentiated they are “fixed.” So
we see that organisms are moulded by their environ-
ment ; it is not the developed result which is transmitted,
it is not the modification which is inherited, but the
capacity for modification in certain directions, the
modifiability.

The other example is taken from some experiments on
Primula sinensis, a well-known garden plant of which
there are several constant races or varieties with different
coloured flowers. If the variety with red flowers (P. s.
rubra) be grown in a hot-house at a temperature of be-
tween 15° and 20° centigrade, it will yield white flowers.
Brought back to a normal temperature it will again
bring forth red flowers. Which modification appears
depends on the stimulus. Now, there is another variety
(P. s. alba) which has white flowers at any temperature
from the normal to 20°. At atemperature between 15°
and 20° these two varieties would both bear white
flowers, yet they would not be the same; they differ in
their hereditary factors, in their modifiability. In one
ease the factors are such that they remain unresponsive
to change of temperature, in the other case they are
responsive.

The argument may be briefly summarised as follows :
An organism is moulded as the result of two sets of
factors: the factors or stimuli which make up its en-
vironment, the conditions under which it grows up ; and
the factors of inheritance, the germinal constitution,
transmitted through its parent by means of the germ-
cells. No single part or character is completely “ac-
quired,” or due to inheritance alone. Every character

DARWINISM AND HEREDITY. 43

is the product of these two sets of factors, and can only
be reproduced when both are present. Only those char-
acters reappear regularly in successive generations which
depend for their development on stimuli always present
in the normal environment. Others, depending on a new
or occasional stimulus, do not reappear in the next
generation unless the stimulus is present. In popular
language the former are said to be inherited, and the
latter are said not to be inherited. But both are equally
due to factors of inheritance and to factors of environ-
ment; in this respect the popular distinction between
acquired and not acquired characters is illusory. In
every case it is the capacity to acquire, to become modified
or to respond, which is really transmitted ; the direction
and extent of the modification depends on the stimulus
encountered. The presence of a given hereditary factor
cannot be determined by mere inspection of the characters
of an organism ; the factor may be present, but the cor-
responding character fail to show itself owing to the
absence of the necessary stimulus. On the other hand,
dissimilar stimuli acting on different factors may give
apparently similar results. Heredity must be defined
afresh as the transmission of the factors of inheritance,
and not as the reappearance of characters in successive —
generations.

Modifications, then, are not transmitted as such. They
could only be “transmitted” if the new environment
produced such a change in the factors of inheritance
themselves, that when replaced in the old environment
they continued to respond as if the new stimuli were still
present. We will not say that such a thing is impossible ;
but it is in the highest degree improbable, and it is very
difficult to conceive how such a result could be reached.
At all events no case of the supposed transmission of
modifications has yet been brought forward which could
not be explained in other ways.

44 EVOLUTION OF LIVING ORGANISMS.

CHAPTER IV

VARIATION AND THE FACTORS OF INHERITANCE:
DETERMINATION OF SEX

Let us return again to the consideration of the variation
universally shown among all living organisms. Within
the range of modifiability of a given organism the modi-
fication may vary in amount according to the duration
or intensity of the stimulus. Could we obtain a large
number of creatures all endowed with the same inherit-
ance, and grow them under perfectly uniform conditions,
they would give rise to exactly similar individuals—
there would be no variation. Grown on the contrary
under variable conditions, the resulting diversity would
give us a measure of the modification. In the same way
with a number of individuals grown under exactly the
same conditions, the resulting variation would give us a
measure of the hereditary differences. Such simple con-
ditions are never quite realised in nature or in experi-
ments ; but we can get approximations.

Take, for instance, the case of the beans worked out by
Johannsen.

The flowers of the common bean, Phaseolus vulgaris,
fertilise themselves; all the offspring of one bean, then,
may have approximately the same hereditary factors.
Since the weight of the bean-seeds is affected by various
independent environmental factors, such as number and
position of the beans in the pod, richness of the soil, light
and shade, and so forth, and the particular combinations
of favourable and unfavourable stimuli fall haphazard on
each particular bean, the variation in weight of the beans
of each plant will be found to follow the normal curve of
probability {p. 34). The mean weight will be the most
frequent, and round it the modifications will fiuctuate
in decreasing number towards the two extremes. Now,

FACTORS OF INHERITANCE. 45

Johannsen took a number of beans from a large field, and.
found that the offspring of each differed to some extent
from that of the others in the position of the mean weight.
While the whole population of beans varied between
20 and 90 centigrams, the offspring of one bean might
vary between 20 and 65 centigrams with a mean of about
50, and the offspring of another between 40 and 90 with
a mean of about 60 centigrams. So the curve of varia-
tion of the whole population is made up of a number
of curves, each of which has its own mean, each of which
corresponds to a strain derived from one bean. Each
strain, with its own inheritance giving a characteristic
mean round which fluctuation may take place, is called
a “pure line.” Inheritance is uniform within each pure
line, so that light or heavy beans taken from the same
line will yield offspring varying about the mean charac-
teristic of the line. A bean weighing 55 centigrams
might belong to either of the pure lines mentioned above ;
to which of them it belongs would only be revealed by the
average weight of the beans derived from it.

_ If the various strains making up a population were
really pure, of uniform inheritance, then under uniform
conditions they should yield, not an even curve, but a
series of steps—there would be a sudden jump from one
strain to the other. It is, therefore, the modifications
which smooth down these steps to a graduated curve
corresponding to the variation of all the strains com-
bined.

Pure lines can rarely if ever persist in nature, except
perhaps in such forms as Bacteria, where sexual reproduc-
tion is not known to occur. Even here inheritance must
sooner or later split into new strains owing to some altera-
tion in its factors. But practically all organisms repro-
duce sexually ; there is a perpetual crossing and mixing
of strains, self-fertilisation being quite abnormal. With
these few exceptions, factors of inheritance are brought
into the zygote by the gametes of two parents. What

46 EVOLUTION OF LIVING ORGANISMS.

happens in such a case? For an answer we, must turn
to the facts and theories of heredity.

In his great work, Animals and Plants under Domesti-
cation, Darwin brought out the provisional hypothesis
of Pangenesis, the first complete corpuscular theory of
heredity. He supposed that each cell of an organism
gives off a living gemmule or pangene, that the pangenes
are collected together in the germ-cells, and that they
give rise in the next generation to cells similar to those
from which they have been derived. Darwin thus sug-
gested a means of transmission both for hereditary factors
and for modifications in accordance with the views then
prevalent. When Weismann showed that modifications
are not transmitted as such, the first part of the hypo- .
thesis of Pangenesis became unnecessary, and his elabo-
rate theory of inheritance by means of corpuscles, the
determinants, is an extension of Darwin’s theory of the
distribution of the pangenes from the germ-cells in
development. Each independently variable organ or cell
is supposed to be represented in the germ-plasm by a
separate determinant, itself compounded of several of
the hypothetical ultimate units of life, the biophors.
These, and the determinants formed of them, are sup-
posed to multiply, and to be transmitted along the con-
tinuous stream of germ-cells. Ingenious as is the theory,
it depends too much on unproved assumptions to carry
conviction ; moreover, when applied in detail it soon
lands us in a maze of difficulties from which there appears
to be no escape. As we pointed out in the first chapter,
life is not the attribute of any one special substance, and
the conception of unit biophors is not really tenable.

Modern theories of the mechanism of heredity, while
less comprehensive are more satisfying, because they are
based on direct observation and experiment, and depart
as little as possible from the conclusions immediately
deducible from the evidence. They were founded on the
work of Mendel, whose observations and conclusions

FACTORS OF INHERITANCE. 47

published in 1866 were not appreciated at that time, and
lay neglected and forgotten until again brought to light
in 1900 by the botanists Correns, Tschermak, and De
Vries. The illuminating researches of Mendel have since
been confirmed and extended by a vast number of ob-
servers, among whom one may mention Bateson, Punnet,
and Doncaster, in this country.

The laws of inheritance have been worked out by cross-
ing closely allied races of plants and animals differing
from each other by certain easily recognisable characters,
and observing the results in the subsequent generations.
For example, if two individuals of the snap-dragon
(Antirrhinum majus) are crossed, one belonging to a con-
stant race with crimson flowers and the other belonging
to a constant race with ivoryywhite flowers, the hybrid
offspring will all bear pink flowers like those of neither
of the parents. But if these hybrids are now inbred
(crossed among themselves), a second generation will be
obtained of mixed character. One quarter of this second
generation will bear crimson flowers like those of the ©
first parent, two quarters (half of the whole number)
will bear pink flowers like those of the first hybrid,
while the remaining quarter will produce white flowers
like those of the second parent. The crimson and the
white-flowered plants of this second generation will breed
true if crossed among themselves, like the original pa-
rental stocks; but the pink-flowered plants will never
breed true. On the contrary they will always split, like
the original hybrid, into three kinds and in the same
proportions (lIn—2n—1n). This example well illustrates
the fundamental principle of the segregation of the heredi-
tary factors in the gametes of the hybrid individual,
which is the foundation stone of Mendel’s theory of
heredity. The facts are accounted for as follows:
Individuals belonging to each of the constant parent
stocks give rise to germ-cells or gametes of similar in-
heritance ; all the gametes of the first parent will con-

48 HVOLUTION OF LIVING ORGANISMS.

tain the factor producing the crimson character, all the
gametes of the second parent the factor producing the
white character. On crossing the two factors will meet
in the zygotes giving rise to the hybrid offspring. If both
factors were now transmitted to each of the gametes of
the hybrid, it would of course breed true. The splitting
of the offspring of the hybrids is accounted for on the
simple supposition that the factors segregate ; that the
crimson-producing factor passes into half the total num-
ber of gametes, and the white-producing factor into the
other half in each hybrid parent. The gametes are then
“pure” in respect to the particular factor. It follows
that when the gametes of the hybrids fertilise each other
three kinds of zygotes will necessarily result; one half
of the total number containing both factors will give rise
to new hybrids ; while one quarter with white-producing,
and one quarter with crimson-producing factors only will
give rise to pure individuals like the parents. Another
ease in point is the “Blue” Andalusian fowl, which
never breeds true, being a hybrid between the Black
Andalusian and the splashed White, both constant races.
A cross between these two will always yield “ Blues”
only ; while the Blues if interbred always split into In
Black, 2n Blue, and 1n splashed White.

True-breeding individuals, containing only one kind of
gamete, are called homozygotes ; they have grown from
zygotes formed by the union of gametes containing the
same factors, and give rise to gametes of uniform value.
Hybrids derived from mixed zygotes are called hetero-
zygotes, and give rise to gametes of different factorial
value, each factor or set of factors passing into half the
total output of gametes. This conclusion is confirmed on
crossing the offspring of the hybrid with the homozygote,
when half the resulting generation will resemble the
homozygote, the original pure parent, and the others will
resemble the heterozygote.

Now the hybrids or heterozygotes may be more or

FACTORS OF INHERITANCE. 49

less intermediate in appearance between the two parent
forms, or like neither of them (as in the case of the Blue
Andalusian) ; or again they may closely resemble one of
the parents. Indeed the resemblance of the heterozy-
gotes to one of the homozygote parents may be so com-
plete as to deceive a keen observer. For instance, as
shown by Mendel himself, if two races of the common
pea (Pisum satwum) be crossed, one being tall and the
other dwarf, tne first heterozygote generation will all be
tall. These interbred yield one-quarter dwarf plants,
and three-quarters tall plants. Of these tall plants one
in every three breeds true to tallness, is a tall homozy-
gote, while the other two prove to be heterozygotes,
splitting again into the three kinds. The character tall-
ness, then, always shows itself in those individuals to whom
its factor has been transmitted, whether they be homo- or
heterozygotes. It dominates over the dwarf character
which is suppressed ; and therefore the one is called the
dominant character and the other the recessive. From
this, and indeed from the whole study of heredity, it
follows that we can never judge of the true inheritance
or gametic constitution of an individual by mere inspec-
tion of its “characters.”

Since the phenomena of inheritance are dealt with in a
special volume of this series, we will not attempt to give
a complete account of the Mendelian interpretations, but
will only summarise some of the main conclusions in so
far as they affect the doctrine of evolution in general.
It is held that in an organism a number of characters may
be distinguished capable of varying independently, and
of being isolated or followed separately in breeding ex-
periments, and each depending on a special transmissible
germinal factor or factors. These characters are known
as “unit characters,” and the factors which govern them
as “unit factors.” The whole inheritance would be
made up of the sum of its unit factors. No character

appears in an individual unless the corresponding factor
(2,031) 4

50 EVOLUTION OF LIVING ORGANISMS.

has been transmitted to it; yet it may inherit factors
which remain undeveloped, and the presence of which
may be revealed by their transmission to and effects on
later generations. Now when two races are crossed,
such as the tall and dwarf peas mentioned above, the two
opposed or allelomorphic characters, as Bateson calls
them, may be due not to the result of two different
factors, in the one case representing tallness and in the
other shortness, but merely to the presence in the first of
a factor for tallness which is absent in the second. Thus
a pure recessive would be without a factor present in the
pure dominant and in the hybrid.* Moreover an appar-
ently simple character may be due not to a single factor,
but to the co-operation of several, which must all be
present at the same time and in the same zygote for the
character to appear. Yet they are sufficiently independ-
ent to be capable of segregation, and to give rise each to
some different character. Thus the colour of an animal
or plant may be a complex character due toa group of
factors transmitted as a whole, and so far constant in the
species, but which can be analysed out into a number of
separate strains, each breeding true to its own new
colour. The grey colour of the wild mouse, for example,
has been shown to depend on the co-operation of at least
six different factors. Change any one of them and the
resulting colour will be changed also. A colour factor
may be unable to show itself in the resulting character,
unless accompanied in the zygote by an independent
colour-developing factor. Thus a white individual may
hold colour factors, but appear as an albino because it
lacks the essential colour-developing factor ; and there
may be as many different varieties of partial albinos as
there are colour factors in the species capable of segrega-
tion. If crossed they will always breed true to albinism,

* This “‘ Presence and Absence theory” is not universally accepted,
many authors believing that both allelomorphic characters are i dl
ssented in the germ-plasm by factors.

FACTORS OF INHERITANCE. 51

since they are devoid of the necessary factor for the pro-
duction of their colour. In certain cases whiteness seems
to be due not to the absence of a factor, but to the
presence of an inhibiting factor. Such a white race can
behave as a dominant. A cross between a dominant
white and a recessive white fowl will yield some coloured
offspring in the second generation. So the factors may
influence each other’s results when meeting in a zygote,
may interact in such a way as to produce characters
differing more or less completely from those produced by
any of them separately.

We have seen that when two varieties are crossed
differing in the presence or absence of one hereditary
factor, as with the tall and skort peas, a dominant and a
recessive form result in the second generation, in the pro-
portion of three to one. If, now, the varieties differed by
two factors, as for instance tall purple-flowered and
dwarf white-flowered peas, the hybrid would show both
the dominant characters, would be tall and purple, and
would give rise to four kinds of gametes in equal propor-
tions. One might contain both factors, one the factor
for tallness, one the factor for purple, and one neither
factor ; the gametes would appear in equal proportions,
since the factors segregate independently by “ chance.”
The product of random fertilisation among these gametes
will yield zygotes in the proportion of nine with both
factors, three with the factor for tallness only, three with
that for purple only, and one with neither, the pure
“extracted recessive.” The number of possivle combi-
nations increases rapidly with the number of factors.

The most complete analysis of the factors of inheritance
yet made is that of the fruit fly (Drosophila ampelophila),
recently worked out by Prof. Morgan and his pupils.
By breeding experiments about 125 different factors
have been distinguished, the eye-colour alone being
influenced by some 30. Moreover, it has been shown
by ingenious experiments that these factors are carried

52 EVOLUTION OF LIVING ORGANISMS.

in sets by individual chromosomes in the gametes, and
further it is claimed that the actual position of the
factors in the chromosomes can be determined.

The conclusion is that the total inheritance transmitted
in a gamete may be interpreted as made up of definite
independent unit factors contributing to the develop-
ment of corresponding characters. While the factors
are either entirely absent or present, the characters due
to them may be more or less developed according to inter-
action with other factors of inheritance and with factors
of the environment. The origin of at all events the
majority of domestic races of animals and plants is ac-
counted for on the theory that the complex of factors of
inheritance possessed by the primitive wild form has been
split up among a number of races, each distinguished by
the loss of some factor or set of factors belonging to the
original stock. The various domestic races of pigeons,
fowls, sweet-peas, and so forth, would differ from each
other in holding a different selection from out of the
whole number of available original factors. If this were
true we might expect to be able to bring together again
the separated fragments of inheritance and reconstruct
the complete set by crossing various races with the neces-
sary elements. And this can indeed be done, giving rise
to “reversion,” the reappearance of an ancestral com-
bination of characters. Thus the original wild form of
sweet-pea found in Sicily can be reconstructed by cross-
ing the “Bush” and the “Cupid” domestic varieties.
The “ Agouti” colour of the wild rabbit reappears on
crossing a yellow with a Himalayan variety ; and the
plumage of the wild “blue rock pigeon” (Columba livia)
can be reproduced on crossing certain very unlike domestic
races.

But do all factors of inheritance “mendelise”? In
answer to this question it may be said that the law of
segregation, the law of Mendel, appears to hold good with
all sorts of characters and in all sorts of plants and

FACTORS OF INHERITANCE. 53

animals. No ‘thoroughly established case has yet been
brought to light in which the factors have been proved
not to segregate. Many of the supposed instances of
blended inheritance, with the formation of series of inter-
mediates, have been shown to be due not to failure of
segregation, but to incomplete dominance, multiplicity
of factors, direct effect of environment, and other com-
plications which blur the result. Yet there are still some
cases known, such as the colours of human races, which
have so far defied analysis into segregating factors ; and
it seems possible, in the light of recent work, that the
rule is not universal, and that segregation may sometimes
be incomplete or even not occur at all.

Now most of these experiments have been made on
domestic varieties, differing,from one another by only
a few well-marked unit characters ; differences due prob-
ably to the dropping out of certain hereditary factors, to
“retrogressive mutations.” We may now inquire what
happens if two natural “species” are crossed. This is a
most interesting question, and one directly affecting the
general problem of evolution, since the experiment might
possibly reveal some hidden but genuine distinction
between a “species” and a “variety.”. The variety
might mendelise but not the species; as, indeed, was
suggested by De Vries. Such, however, does not appear
to be really the case ; but the problem is much compli-
cated owing to the large number of factors of varying
importance which may distinguish two even closely
allied natural “species.” In so far as some of the unit
characters are conspicuous and dominant, it may be
possible or even easy to trace out the segregation in later
generations; but the task may become too difficult if
the distinguishing characters are small and very numer-
ous. In such a case thousands of “intermediate”
heterozygotes may be produced, each with its particular
inheritance, and a bewildering number of intermediate
forms result. This is after all perhaps not a matter of

54 EVOLUTION OF LIVING ORGANISMS.

great general importance, since hybridisation, or the
crossing of different “species,” can play but a small part
in evolution, and need hardly be considered as a factor
at all.

The term “unit” character is apt to be misleading.
For practical purposes it is convenient to fix the attention
on some conspicuous result, such as the colour of a flower
or the height of a plant, but it must always be remem-
bered that the influence of the factor may—possibly
always does—extend over the whole organism.

In Chapter ITI. it was shown that two kinds of varia-
tion can be distinguished : modifications and mutations.
The latter alone are due to changes in the inheritance,
and may be further classified into retrogressive mutations,
due to the loss of some factor, and progressive mutations,
due to the gain of some factor.*

Of the origin of progressive mutations we know practi-
cally nothing. So far as experiment has shown, the multi-
tudes of domestic varieties of our plants and animals are
all, or almost all, of a retrogressive kind ; that is to say,
are due to the separation and rearrangement of the factors
of inheritance already present, not to the addition of new
factors. New characters may thus come out, but the
unit factors remain the same though apparently reduced
in number.

Similar retrogressive mutations are known to occur in
nature. Common instances are albino animals and white-
flowered plants. The botanist De Vries, who has made
a special study of these variations, has collected and
studied a large number of cases. But it cannot be that
all mutations are of this negative nature. All the differ-
ences that distinguish man from the primitive stock
whence arose the whole animal kingdom can hardly be

* Mutations, whether progressive or retrogressive, are not necessarily
related, as might be supposed, to the complicated changes undergone by
the chromatin in the maturation of the germ-cells, since they occur in the
Bacteria, and among other forms which reproduce asexually.

FACTORS OF INHERITANCE. 55

due to man having lost certain factors which this early
ancestor possessed, or to the factors being shuffled. At
some period in evolution new factors must have been in-
troduced into the inheritance, and the process is presum-
ably still going on.

Speculating as to the possible origin of mutations we
are inevitably led to the conclusion that any transmis-
sible differences arising between two originally identical
germinal constitutions formed of the same factors of in-
heritance must come from the environment; for any
closed system must reach a state of equilibrium, and con-
tinue unchanged unless influenced from without. Muta-
tions, then, are produced by the addition to, subtraction
from, or alteration in, the faptors of inheritance already
present.

This whole system of factors may appear to the reader
somewhat artificial; yet it is the legitimate deduction
from facts for which no other explanation can at present
be given. What the factors are we can only surmise.
They are not distinct corpuscles of living matter like the
hypothetical gemmules of Darwin or biophors of Weis-
mann, representing separate cells or parts of the organ-
ism. Rather should they be conceived as definite
chemical substances forming part of, or entering into,
the chain of metabolising compounds and thereby in-
fluencing the course of metabolism in such a way as to
bring about a certain definite result.

If such a substance is destroyed, or fails to pass into
the gamete, a retrogressive mutation arises. If the
physico-chemical structure of a factor is altered by the
shifting and rearrangement of the chemical atoms, for
instance, a mutation may result. A truly progressive
mutation would be produced when new atoms or com-
pounds become permanently involved in the metabolic
cycle of the germ-plasm.

One must be careful not to assume that because a
character appears to vary independently it is necessarily

56 EVOLUTION OF LIVING ORGANISMS.

represented by a separate substance, present from the be-
ginning in the germ. Such a supposition would soon
lead us into that tangle of difficulties which have proved
fatal to the corpuscular theories of heredity. It is the old
controversy between the supporters of “epigenesis” and
“evolution” in development (evolution being used here
in the restricted sense of an unfolding during ontogeny
of parts already present on a small scale in the germ).
While the latter held that the parts of the adult are de-
veloped by the mere unfolding of corresponding elements
preformed in the germ, the “epigenetists” held that
they are formed de novo in every embryo from an un-
differentiated substance. In modern language we should
say that the structure of the adult is gradually developed
by a series of epigenetic changes, each one of which is
strictly determined by the stage preceding it, and strictly
determines that which follows it. So every feature of
a landscape, every detail of a melting view, is doubtless
strictly predetermined by the nature of the geological
strata and the surrounding conditions, but cannot be said
to have been preformed in previous geological epochs.
If every separately variable character had to be repre-
sented by a separate unit the number of units in every
gamete would become fantastically large. Moreover,
characters are variable independently at different stages
of growth—not the butterfly only, but the chrysalis,
the caterpillar, and the egg may vary ; must there be a
unit for every stage of every character? Again, it is
difficult to account for the marshalling and development
of all these units in proper order without calling in the
aid of some mysterious governing force. These are some
of the difficulties which have led to the downfall of the
corpuscular theories of heredity.

Great as has been the advance in our knowledge of the
inheritance of characters through breeding experiments
conducted on mendelian lines, there is some danger of
the factorial theory becoming a mere formal explanation

FACTORS OF INHERITANCE. 57

of results. So far we are presented with a picture of
independent particles without any clue as to how they
co-operate or succeed in producing the finished result.
Students of genetics are apt to overrate the importance
of an explanation of the mere mechanism of transmission.
Not until the factors have been brought into relation
with the general metabolism, with growth and repro-
duction, will the theory of heredity approach com-
pletion.

It is not possible as yet to describe scientifically the
process of embryonic development, but some sort of
epigenesis there must be. All we need assume is that
there are factors enough in the inheritance to produce a
certain definite result. When, under the same conditions,
the results come to differ, when a mutation arises, no
doubt some corresponding change must have taken place
in the inheritance, some alteration in the composition and
properties of what we vaguely call the factors (p. 52),
A consideration of the changes undergone by complex
chemical compounds may give us some notion of what
the factorial changes may be. The student of Organic
Chemistry is familiar with long series of closely allied
compounds, formed of molecules often containing vast
numbers of atoms. The properties of these compounds
all differ from one another, owing to the elements not
being the same, or to their not being present in exactly
the same proportions, or again to their being differently
grouped within the molecule. The properties may be
altered at will by removing this or that atom, by intro-
ducing a new element, by substituting one group for
another, or by merely shifting the grouping within the
molecule. The resulting changes will be definite, will
appear large or small, important or not, according as they
strike our senses. In some such fashion do we imagine
the changes occur in the factors of inheritance.

Mutations are generally said to arise spontaneously ;
this, of course, is only another way of saying that we do

58 EVOLUTION OF LIVING ORGANISMS.

not yet know what causes them to appear. Darwin be-
lieved that great and sudden changes in the environment
increased variability, a conclusion which is probably well
founded, but is difficult to prove, since we know so little
about. the range of variability in nature under usual and
unusual conditions. Some recent experiments seem to
have clearly established the most interesting fact that
mutations can be brought about by altering certain
stimuli in the environment. But it must not be for-
gotten that the experimental conditions may only have
favoured the survival of pre-existing mutations, and not
actually caused their first appearance. The American
investigator Tower, experimenting on the Colorado
beetle (Leptinotarsa), found that on exposing some in-
dividuals at a certain stage and for a certain time to
extremes of heat and dryness there appeared in their
offspring beetles differing remarkably from the parents
in colour and pattern. That these new forms were real
mutants, and not modifications, was proved by their
breeding true under normal conditions, and when crossed
with the parent form giving the proper mendelian pro-
portion of parent and new types in the second generation.
Similar results have been obtained by Morgan with the
fly Drosophila, and by others with plants. For factorial
ehanges of this kind to produce lasting results, constant
inheritable mutations, they must of course be persistent.
It is a mistake to confuse, as is often done, such changes
brought about by the direct effect of external or internal
stimuli on the germ-plasm with alleged cases of the so-
called inheritance of acquired characters.

These artificial mutations are all of the retrogressive
kind, and the mutants, the new forms, are all recessives.
So that the result of the experiment is the loss or
suppression of one or more of the factors of inheritance.
But we may look forward to the possibility of being able
some day to produce progressive mutations in any desired
direction ; a discovery of such immense practical value

FACTORS OF INHERITANCE. 59

that it would eclipse even such triumphs as the practical
application of electricity, or the use of steam power.

A word may here be said about the determination of
sex, although space will not allow us adequately to discuss
this most interesting and important problem in evolution.
_ What determines that a given individual shall be male,
female, or hermaphrodite? Since, in the vast majority ~
of cases, when the sexes are separate, they appear in
equal numbers, it might seem that here is an excellent
instance of mendelian segregation. We may suppose, as
Geoffrey Smith suggested, that one sex is a homozygote
and the other a heterozygote for sex-determining factors.
The heterozygote parent would then yield male-producing
and female-producing gametes in equal numbers, and the
sex of the offspring would bé determined at fertilisation.
Working on quite different lines, M‘Lung, Wilson, and
others have shown that in certain insects there are two
kinds of spermatozoa, one with the normal number of
chromosomes, and the other with one of these reduced in
size or absent. These observations have been extended
to other forms, and it is now established that in, at all
events, a large number of organisms, the egg develops
into a male or a female according as it is fertilised by a
male-producing or female-producing gamete. Sex, then,
would in these cases be determined from the first by the
presence of the necessary germinal factors. Yet there
are undoubted instances where the sex of an animal is
influenced by the environment. In the case of the spider
crab, IJnachus, attacked by the parasitie crustacean
Saceulina, not only does the male undergo “parasitic
castration,” and acquire the external characters of the
female, as shown by Giard, but, as Geoffrey Smith proved,
on recovery from the attack the male crab develops ova
as well as spermatozoa. This seems to show that one
sex may really carry the factors for both, a conclusion
borne out by the well-known fact that female birds often
in old age acquire the secondary sexual characters of the

60 EVOLUTION OF LIVING ORGANISMS.

male. Further, Balzer has recently given evidence that
the larve of the strange worm-like animal Bonellia become
either female or male according as they live an inde-
pendent life or become parasitically attached to their
mother. The difficult problem of the determination of
sex cannot yet be considered as solved. The evidence
seems to show that the sex of an individual is fixed in
different ways and at different times. Some animals lay
male-producing and female-producing eggs; among the
Hymenopterous insects, such as the bee, the egg produces
a female or a male according as it is or is not fertilised ;
while in other cases, as mentioned above, sex is deter-
mined by the kind of gamete which fertilises the ovum.
Moreover, although in the vast majority of cases the sex
of an individual appears to be irrevocably fixed at or
before fertilisation, yet there is evidence that in certain
cases it is influenced by environmental conditions.

CHAPTER V
THE STRUGGLE FOR EXISTENCE AND NATURAL SELECTION

Or all the primary factors of evolution the struggle for
existence is that which lends itself least to controversy.
When once stated and understood it must be recognised.
Unfortunately, it is not always understood. The phrase
is a metaphor to express the undeniable fact that more
organisms are born into the world than can survive in it.
As Darwin says, unless “the truth of the universal
struggle for life be constantly borne in mind the whole
economy of nature, with every fact on distribution,
rarity, abundance, extinction, and variation, will be dimly
seen or quite misunderstood.” The enormous rate at
which even the most slowly reproducing creatures are
capable of increasing is not always fully realised. Organ-

é

THE STRUGGLE FOR EXISTENCE. 61

isms have a tendency to increase in geometrical ratio;
consequently their powers of multiplication are prodi-
gious. To take an instance quoted by Wallace, a single
pair of flies (Musca carnaria) produce 20,000 carrion-
eating larve, which will hatch out into flies ready to re-
produce in about a fortnight, giving rise in turn to some
200 million hungry larve. Linnzus did not exaggerate
when he said that a dead horse would be devoured by
flies as quickly as by a lion. The fertility of parasites
is proverbial: the eggs of tapeworms and other such
internal parasitic worms may be counted by the million.
Among the vertebrates the fish are the most prolific—a
single cod can lay over nine million eggs, though of course
they may not all be fertilised? Equally remarkable is the
power of reproduction in plants. It has been ealculated |
that a single cholera bacillus would give rise to sixteen
hundred trillions of bacilli in a day if propagating freely,
forming a solid mass weighing 100 tons. Doubtless,
such unrestrained reproduction rarely if ever occurs;
but every organism is, so to speak, ready to seize the
opportunity of spreading to an indefinite extent and is
trying, as it were, to extend its range, to colonise fresh
regions. Every available spot, from the top of the high-
est mountain to the lowest depth of the sea, is invaded,
every possible mode of life is adopted, as far as adapta-
bility will allow. The various organisms are packed as
tight as possible, exerting mutual pressure. If any one
increases, it is at the expense of some other. So the
fauna and flora of an old established district are made up
of a multitude of interdigitating species, availing them-
selves of all the resources of the region, fitting close to-
gether like the stones of a mosaic, and each occupying
that place for which it is best suited. Under ordinary
conditions a balance is soon struck, a state of unstable
equilibrium established, in which the surviving forms
have reached an average number of individuals about
which they fluctuate within comparatively narrow limits.

62 EVOLUTION OF LIVING ORGANISMS.

The severity of the struggle may vary according to age
and the surrounding conditions, but it is never quite
absent. Besides the competition for food, for light, for
water, for space generally, and the unceasing struggle
against unfavourable conditions of climate, there is the
never-ending war against enemies, parasites, and dis-
eases. All these factors contribute towards the death-
rate of the species.

How great are the possibilities of increase and how
much they are kept in check under ordinary conditions,
is seen when the balance of factors restraining it is in
any way disturbed, as for instance owing to some change
in the climate or the intervention of man. The occasional
immense swarms of locusts, of butterflies or other insects,
of lemmings, the great epidemics of diseases, are ex-
amples of the temporary removal of barriers to repro-
duction. Similar expansion of some one species at the
expense of others is continually taking place on a small
scale even in the most stable fauna and flora. The ex-
traordinarily rapid increase of horses and cattle intro-
duced by the Spaniards in South America, of rabbits in
Australia, of terrible insect pests like the Phylloxera
which attacked the vines in Europe, or the European
gipsy moth (Lymantria dispar) which destroys the forest
trees in North America, the spread of the watercress
(Nasturtium officinale) blocking the rivers of New
Zealand, or the American water-weed (Hlodea canadensis)
filling the rivers of western Europe, these are only a few
of hundreds of similar cases of the possible result of in-
troducing a species into a new country where it does not
meet with the ordinary checks occurring in its native
habitat.

Individuals of similar habits stand most in each other’s
way ; therefore, competition is often more severe between
closely allied than distantly related forms. It is most
severe within the limits of the species itself, between
individuals, families, tribes, and social aggregations.

THE STRUGGLE FOR EXISTENCE. 63

The intensity of the struggle may be judged by the
death-rate. In a region where the population has
reached a stationary state, the number of each species
remains approximately the same. On the average, then,
only one out of the whole progeny can survive to repre-
sent each parent individual of the previous generation.
All except these fortunate individuals are destroyed
sooner or later before they have succeeded in leaving
offspring to perpetuate their kind, or in securing the
success in life of any of their progeny. The greatest
amount of destruction takes place when the organisms
are still quite young.

The struggle for existence, natural selection, and adap-
tation, are so intimately connected, that it is scarcely
possible to treat of one of these without at the same time
dealing with the other two. While the struggle may be
considered as the primary factor, to the action of which
the selection is due, it is also the very means whereby
adaptation is brought about. The transmission of heredi-
tary factors is, of course, necessary for the selection to
be effective. Obviously natural selection acts merely as
a sieve, separating individuals so formed as to survive
in the struggle for existence, from others which are not.
It may be represented equally well as a survival of the
fit, or as an elimination of the unfit. This point is worth
insisting on, because, strangely enough, it has been often
brought forward as a serious criticism that natural selec-
tion is, after all, merely a process of elimination.

In the long-run those organisms less well adapted to
survive will have less chance of leaving successful off-
spring behind them, will be crushed out in the struggle,
will be eliminated. The only difference between the
well adapted and the badly adapted in a given environ-
ment is that in the first case the total inheritance, the
hereditary mechanism or factors, react to stimuli in such
a way as to lead to success, while in the second case they
do not. Selection, therefore, is between that kind of

64 EVOLUTION OF LIVING ORGANISMS.

organisation which responds in the right way, and that
which does not. If one may be allowed to speak meta-
phorically, the problem before the organism is to acquire
such an organisation that under given conditions it will
react in the right way. Therefore, those individuals
which vary in their hereditary organisation in the right
direction necessarily have a better chance of succeeding.
Success in the struggle is the only criterion of “right”
and “wrong” in variation. The action of selection, it
must be remembered, on the hereditary factors is in-
direct, since it only acts on the combined product of
these and the environment (see p. 38)—on the characters
of the organism as presented to it. It is these characters
and their variations which pass through the sieve. The
scope of the selection is limited to the variations pre-
sented ; its direction is determined by the environment.
This direction will always necessarily be towards better
adaptation ; adaptation is the keynote to evolution. If
the right kind of variation does not occur the organism
runs the risk of death, and will inevitably be exterminated
if the struggle is severe enough.

When the history of Biological Science in the last fifty
years comes to be written, we fancy that the impartial
historian will not dwell with much pride on the account
of the criticisms of Darwinian doctrines, The violent
attacks of theologians, based for the most part on ignor-
ance and prejudice, may be forgiven and forgotten ; but
many of the criticisms from the pens of the biologists
themselves are scarcely better founded. It is often said
that of late years Darwinism has lost ground, and that
natural selection can no longer be regarded as a satisfyin
explanation of, or even as an important factor in, the
process of evolution. Doubtless there is some truth in
the saying, at all events in so far as it appears that the
doctrine is not what some misguided enthusiasts may
have represented it to be, that it does not explain every-
thing, that many problems remain unsolved. Yet the

THE STRUGGLE FOR EXISTENCE. 65

Darwinian theory still stands unassailable as the one
and only rational scientific explanation of evolution by
“natural” forces, whose action can be observed, tested,
and measured. Nevertheless, the critics are quite right
in demanding convincing evidence for every step in the
argument. .

The modern developments of the study of heredity and
variation on mendelian lines, far from weakening the
case for natural selection, seem to have definitely disposed
of the only rival theory, the doctrine of Lamarck founded
on the supposed “inheritance of acquired characters.”
Fortuitous changes in the inherited organisation are left
as the only elements of primary importance, the only
stones of which the edifice is built.

Misunderstanding lies at the root of many of the ob-
jections urged against Darwinism. For instance, almost
every detractor of natural selection has brought forward
with wearisome reiteration the criticism that the theory
does not account for the origin of variations. Of course
it does not; it was never meant to do so. No one has
tried to drive this point home more persistently than
Darwin himself: “Some have even imagined that natu-
ral selection induces variability, whereas it implies anty
the preservation of such variations as arise ;” . . . “un.
less such occur natural selection can do nothing” (Orzgin
of Species). What selection alone can do is to preserve
these variations, and to pile one individual difference on
the top of another in the direction of adaptation, leading
to ever increasing changes along divergent lines.

Another “criticism” is that natural selection is nov
the means whereby adaptation is brought about because
organisms must have the desired structure before they
are chosen, must be preadapted. But if by adaptation
we merely mean the appearance of a favourable variation,
the statement amounts to an obvious truism. Further,
if by adaptation we merely mean that an organism has

such a structure that it will live, all living things are
(2,031)

66 EVOLUTION OF LIVING ORGANISMS.

necessarily adapted—viability is the first test they have
to pass. Any that fail to satisfy it are eliminated at once.
But organisms in competition are further tested in par-
ticular directions, and when we can see both the advan-
tage gained and the means whereby it is obtained, the
special structure evolved, we speak of it in teleological
language as adapted for this purpose. Thus a sense
organ becomes specially adapted to receive only certain
stimuli; a flower is adapted to be fertilised by insects.
Now each step in the formation of such a structure, each
fortuitous favourable variation selected to build it up,
may, in a sense, be said to have been preadapted. But
the word adaptation also implies just that process of
gradual building, that continued selection of variations
in a particular direction from among variations in all
other possible directions, which is represented in the
elaborated organ, yet cannot in any intelligible sense be
said to be present in each individual step.

To prove that natural selection is a real working
principle, it must be shown that the death-rate is selective.
Unfavourable conditions, diseases, enemies of all kinds,
dog the path of every individual from birth to death ;
the death-rate is consequently enormous, as was shown
above. But is it really selective? Are, for instance, the
two flies which alone survive out of every 20,000 (p. 61)
different on the average from those 19,998 which perish ?
That is the important point now to be considered.

Doubtless in many cases the death of organisms is due
to pure “accident,” causes which exterminate without
selective action, as for instance in the wholesale destruc-
tion brought about by some natural cataclysm; but such
eases must be comparatively rare, and may be neglected.
Death of this kind simply fails to select; it does not
prevent: it merely delays the work of selection. The
survival of the fittest, the elimination of the unfit, is easy
enough to prove in the case of well-defined races, species,
and larger groups, and has been eloquently described by

THE STRUGGLE FOR EXISTENCE. 67

Darwin, Wallace, and others. Every observant naturalist
now realises how intense is the struggle, how rapidly
one species can be replaced by another, how completely
the geographical distribution, the abundance or rarity,
or the extermination of an organism is dependent on the
success of its competitors. It is the selective action of
this competition, combined with the selective action of
the inorganic factors of the environment, which regulates
the distribution of living things and the diversity in the
fauna and flora found in different regions. If this selec-
tive action could be stopped, the whole fauna and flora
would soon become almost uniform throughout the
world.

The well-known instance of cats and clover, quoted by
Darwin, well illustrates the extraordinarily complex
interrelationship between different organisms of most
diverse structure and most distant affinity. Red clover
(Trifolium pratense) is almost exclusively fertilised by
humble-bees, and the abundance of bees in a district is
much influenced by the number of mice, which destroy
their nests and eggs. Now since the number of mice
depends to a great extent on the abundance of cats, these
animals will affect the distribution of the red clover in the
neighbourhood. So the goats introduced into St. Helena,
by eating up the seedling trees, not only destroyed the
forests, and greatly altered the general flora and fauna of
the island, but have actually changed the climate itself.
The notorious tse-tse fly (Glossina morsitans), carrier of
the deadly disease germ (7rypanosoma Brucet), renders
certain districts of South Africa uninhabitable for im-
ported dogs, horses, and cattle; while the distribution
of man himself has been greatly intluenced by the mos-
quito Anopheles, and the malaria parasite which it trans-
mits. But the continuous steady struggle which goes on
between more closely allied forms is perhaps best seen in
the case of plants. It can be watched in the forest and
on the mountain slope, in the desert and in the tropical

68 EVOLUTION OF LIVING ORGANISMS.

swamp. The survival of some forms, the extinction of
others less well adapted, the alternate spread of one
species and retreat of another as the climatic conditions
oscillate from dryness to dampness, from heat to cold,
these are familiar to every botanist.

We need not.dwell longer on this aspect of the question.
These and similar facts are not ignored by the most
sceptical of the opponents of Darwinism; nor is their
significance denied. But they will say—“Granted that
the great gaps between the widely divergent forms have
been produced by the extinction of the intermediate ones,
granted that the struggle between differing species leads
to the selection of the best adapted, these well-marked
‘species’ were already present. Show us,” they will
ask, “selection between varieties differing but little from
each other, and between the individual variations them-
selves.” Instances of the success of mere “varieties”
are not unknown. Of late years it has been observed that
dark varieties of various moths have tended to increase
in northern England, aud may even have superseded the
original pale forms, as in the case of the peppered moth
(Amphidasys betularia). Familiar to us all, though per-
haps rarely understood, are the constant struggles be-
tween the various races of mankind, which have played
and still play so large a part in the history of the world ;
here there is ample material for the study of the selective
value of all sorts of racial characters. More difficult is it
to demonstrate the action of selection among individuals,
and to express the result in statistical form. Only on
rare eecasions can we directly compare the eliminated
individuals with the survivers. One good example we
owe to the American zoologist Bumpus. After a severe
storm he ¢ollected 136 injured specimens of the common
sparrow (Passer domesticus); and out of this number
72 revived, while 64 did not recover. On measuring all
the birds, and comparing the dead with the survivors,
it was found that the former on the average were

THE STRUGGLE FOR EXISTENCE. 69

heavier and larger than the latter; but significant also
was the discovery that the range of variability was dis-
tinctly smaller among the survivors than the eliminated,
thus proving that those individuals which departed least
from the “ideal type” had on the whole the best chance
of surviving. Weldon has also shown, in the case of a
terrestrial mollusc, Clausdia laminata, that extreme vari-
ations tend to be eliminated. Selection of this kind pre-
serves the mean character of the race. Tower obtained
the same results on comparing Chrysomelid beetles
(Leptinotarsa decemlineata) before and after hibernation ;
the extreme variants died off, and the survivors ap-
proached nearer to the mean.

The fact that many commonly occurring mutations,
such as albinism or variegatéd leaves among plants, fail
to establish themselves is also clear evidence that con-
stant elimination of these variations must take place.

Of all the agents of elimination among human beings
disease is the most potent in modern times, as Reid and
others have well shown. Some notion of the death-rate
and its causes in the human species may be gathered from
the official government reports. Im the year 1909 there
died in England and Wales 518,000 individuals (in round
numbers), of whom 100,000 were infants under one year
of age. Some 18,000 persons were killed by violence or
accident, while the remaining 500,000 died of disease.
Of these tuberculosis carried off 55,000, pneumonia
36,000, cancer 32,000, measles 12,000, diarrhcea 10,000,
influenza 9000, whooping-cough 7000, diphtheria 5000,
scarlet fever 3000, enteric fever 2000, and other diseases
smaller numbers. This death-rate is selective in so far
as it affects the fertility and prevents the reproduction
of susceptible individuals.

The germs of most of these diseases are so widespread
that infection cannot be avoided; therefore a constant
elimination takes place of individuals unable to resist
their attacks. If the resistance to a particular dangerous

70 EVOLUTION OF LIVING ORGANISMS.

disease is “natural,” that is to say depends on characters
developed in the ordinary environment, then natural
innate immunity is soon established by selection, and the
disease is eventually stamped out, only immune indivi-
duals surviving. But if resistance depends partly or
entirely on “acquired immunity,” developed only under
the stimulus of the disease itself, then the disease will
persist, but become less dangerous as the capacity to
resist it is increased by selection of the individuals who
recover most easily. How effectually selection acts is
realised on observing the rapid and fatal spread of in-
fectious diseases imported by one race into a region in-
habited by another race where they were not previously
prevalent. Unless the invaded race happens by chance
to be preadapted to resist the new disease, it falls an easy
victim. Naturally each race is only adapted by selection
to resist the diseases of its own habitat; town life is as
fatal to the prairie Indian as winter frost to the tropical
plant.

Disease has played a most important part in the evolu-
tion of the human races. It is not so much by acts of
violence that the Spanish and other invaders of America
or the European colonists of Australasia have conquered
and almost exterminated the native inhabitants of those
countries, as by the introduction of diseases the natives
were unable to withstand. Small-pox, measles, and
tuberculosis have almost cleared the fine indigenous
races from off the North American continent, severely
handicapping them in the struggle against their Euro-
pean rivals. This is merely another instance of the con-
stant struggle between species or varieties which we have
discussed above.

There can be no doubt, then, that the death-rate is
selective. The next point we have to deal with is the
part played by selection in the process of evolution. What —
will be the effect of selection on succeeding generations ;
or, in other words, what will be the combined effect of

THE STRUGGLE FOR EXISTENCE. 71

elimination and inheritance? It has been far too easily
assumed by Wallace, and other writers, that the result
of the continued selection of any character must neces-
sarily lead to its gradual increase. For instance it was
supposed that if, out of a number of birds varying in
' wing-length from 5 to 7 inches, with a mean of 6 inches,
individuals with a wing-length of 7 inches were chosen
for breeding, the mean length of the wing of the offspring
would be raised. And that if for a number of generations
the parents were always selected from among the birds
with greatest wing-length, the average wing-length of
the progeny would be gradually raised from 6 to 7 inches,
from 7 to 8 inches, and so on indefinitely, so long as the
selection continued. But Ahis is not necessarily the
case. * .

In the first place, if the variations selected are modi-
fications induced by the environment in individuals
endowed with the same hereditary factors, no cumulation
at all will take place, however much the selection may be
prolonged. Thus the selection of individuals which have
become immune to a disease in the course of their life-
time will not increase that immunity, nor free the pro-
geny from the necessity of becoming immune, unless the
capacity to acquire immunity itself varies and is selected.
To take a simpler case—that of the beans mentioned on
p- 45. Selection of the heaviest bean-seeds from any one
strain or pure line of uniform hereditary capacity will
not alter the mean weight of seeds within that strain ;
the offspring of such a selected bean is no more likely to
be heavier than is that of any other bean from the same
strain. But if heavier beans are selected from the whole
group of strains, differing in their hereditary capacities,

* Whether the direction of variation is influenced by selection, whether
the continued selection of a character, and indirectly of its germinal
factors, encourages their further development, is indeed a most funda-
mental question. At present, however, it cannot fully be answered, since

the evidence is quite uncertain. On the whole, current opinion is agai
the view.

©
=

= {LIBRARY

72 EVOLUTION OF LIVING ORGANISMS.

then the mean weight of the offspring will be raised, and
raised rapidly to the highest possible level—that of the
strain with the highest mean weight. For since there
are more heavy bean-seeds among those belonging to the
strains with the higher mean, they will have a better
chance of being selected. The offspring of lines with
lower mean weight will be gradually eliminated, and at
last only those of the strain with the greatest mean
weight will remain. Obviously the same result would be
reached at once if this strain could be distinguished at
the beginning of the experiment, and alone chosen for pro-
pagation. Moreover, the effect of selection will come to
a stop as soon as this highest point is reached ; unless in
the meantime new changes have arisen in the hereditary
factors, leading to further possible increase in weight.
This limitation is a necessary consequence of the sieve-
like action of selection—it can only select what is already
there. It must also be understood that the modifica-
tions due to the environment do not materially alter
the results of selection, which acts only by the indirect
choice of particular hereditary constitutions. Cumula-
tion of results can only take place in so far as new
mutations occur in the required direction.

The same conclusions have been reached on statistical
grounds, and may be expressed as follows. Comparing
the characters of offspring with those of parents, grand-
parents, and great-grandparents, it is found that the
resemblance decresses rapidly; so that the “correla-
tion” with parent: is about } or ‘5, with grandparents ‘3,
with great-grandparents ‘2, and so on. The “correla-
tion” is the ratio expressing the deviation of each genera-
tion from the mean of the species. This is the “law of
ancestral inheritance” worked out by Galton and modi-
fied by Pearson. Pearson has defined it as a rule for
predicting the average value of a character in the off-
spring from the value of that character in.the ancestors.
From this point of view, therefore, the contributions to

THE STRUGGLE FOR EXISTENCE. 73

inheritance from distant ancestors are negligible, and
selection through very few generations is sufficient to
yield a practically pure and constant race.

As was long ago pointed out by Darwin, the method
and results of natural selection are quite comparable to
the methods used and the results obtained by man in
artificial selection. The wonderful diversity of domestic
races of plants and animals, showing all sorts of new
developments in size, shape, and colour, in function,
habits, and mental qualities, is due to the selection of
individual variations in this or that direction. One
mutation after another is isolated and bred from, and
so almost any desired form is obtained.

_ Now it is often maintained that these domestic varie-
ties are not of the same value as natural 1aces; that if
allowed to roam wild, or if selection be relaxed, they re-
turn to their original condition, degenerate, or regress.
This conclusion is, however, mostly based on misconcep-
tions. There is no good reason to believe that domestic
races are fundamentally different from or inferior to
natural races, either in mode of formation or in con-
stancy when formed. If they are generally unable to
compete with wild races when let loose, it is because they
have not been selected with this end in view. If they
appear to degenerate when removed from the care of
man, it is because they no longer enjoy the exceptionally
favourable conditions of abundant nutrition, protection
from enemies, and so forth. Those domestic races which
fail in the struggle for existence among wild competitors
are like natural species which, when introduced into a
new country, fail to establish themselves because the
conditions are unsuitable.

But what, it may be asked, is the effect on domestic
races of mere relaxation or cessation of selection? The
race will probably tend to regress, that is to say, to
‘return to the original form from which it was selected.
There is true regression and false regression. The latter

74 EVOLUTION OF LIVING ORGANISMS.

is brought about by the crossing of the new variety with
the original species with which it may now come into
contact. This kind of regression by hybridisation, as De
Vries has shown, is very difficult to prevent with plants
fertilised by wind or by insects. It leads to the forma-
tion of intermediate forms ; but the new hereditary con-
stitution can never be quite suppressed. True regression
is due to the tendency of the offspring of parents which
deviate from the mean to return to that mean of the
race ; it is the inevitable consequence of the interbreeding
of individuals endowed with unequal inheritance. If the
race is quite uniform, composed of homozygotes with the
same hereditary factors, there can naturally be no re-
gression ; this has been well established by numerous
and prolonged experiments. But if there is inequality of
inheritance ever so slight, regression will take place on
the cessation of selection. It is a universal phenomenon
common to all impure races, whether artificial or natural.

In the foregoing chapter it has been shown that
natural selection acts by eliminating the unfit and so
leaving the fit to continue the race ; that this selection is
effective not only between widely divergent forms, but
also between individuals differing from one another
by ordinary variations; and lastly, that the process of
natural selection is strictly analogous to that of artificial
selection practised by man. But some important points
still remain to be discussed.

Are variations continuous or discontinuous is a ques-
tion which has given rise to much controversy. Con-
tinuity here means gradation of variation from one ex-
treme to another, so that, with regard to the measurement
of a particular character, the individuals of a race could
be arranged in a series leading gradually from those
having the character developed to its greatest extent to
those in which it is least developed. The more gradual
the transition, the more perfect the continuity, the more
even would be the curve formed by the ascending series.

THE STRUGGLE FOR EXISTENCE. = 75

We have already seen (p. 34) that when dealing with
large numbers of individuals most characters conform to
this rule and vary continuously. On the other hand,
discontinuous variation would give rise not toa graduated
curve but to a series of steps or jumps, from one stage to
the next above. A tendency towards discontinuity is
shown in “meristic” variation. Thus, a cell will either
remain single or divide into two, the antenna of an insect
may have four, five, or six joints, the number of dorsal
vertebree in a mammal may vary from twelve to thirteen,
or from thirteen to fourteen, and so on. But even
meristic variations need not be discontinuous, and seldom
is gradation more perfectly shown than in the variation
of segmental nerves in the plexus supplying the limb of
a vertebrate (pp. 93, 107).

The whole subject has been greatly confused by the
failure to distinguish between continuity in the variation
of characters and continuity in the changes of hereditary
factors. Modifications vary continuously, because the
incidence and quality of the factors of the environment
are due to chance. Mutations may be discontinuous if
the changes in the factors giving rise to them are sudden
and large enough. But the variation of characters due
to mutation may also be more or less continuous, owing
for instance to their reactions being modified by other
factors, as in incomplete dominance (p. 49). Again an
apparently simple character may become more and more
pronounced when it is really a complex character, de-
pending on the co-operation of a number of separate
factors reinforcing each other—as with the red colour of
wheat studied by Nilsson-Ehle, which develops only to
ts full extent when three factors are all present. The
gradation in the mean weight in the different strains of
the beans already so often mentioned (pp. 45, 53, 71) is
perhaps also due to the co-operation of several factors.
Moreover, it is possible that the factors themselves may
show grades of strength and development; certainly if

76 EVOLUTION OF LIVING ORGANISMS.

the presence and absence theory is correct, the character
of the heterozygote in the case of the ‘blue Andalusian
fowl (p. 48) may be explained on the supposition that
' the single dose of a factor in the heterozygote does not
produce the same result as the double dose in the homo-
zygote parent or offspring.

Here may be mentioned the recent researches of
Jennings on the Protozoon Difflugia, of Castle on rats,
and of Morgan on Drosophila, all of which yield incon-
trovertible evidence of the occurrence of “continuity” in
mutation—that is to say, of the possibility of isolating
strains differing from each other by quite small characters
or in the degree of development of some one character.
But while Morgan would interpret the gradation as due
to the cumulative effect of numerons modifying factors
brought to bear successively on the unit factor of the
character, Castle maintains that this unit factor itself
undergoes gradual change.

Variations, then, may be continuous or discontinuous,
and there is no hard and fast distinction between the two
kinds (pp. 57, 93). An interesting question we have now
to consider is whether large or small variations are the
more important in evolution. Darwin was uncertain on
this point, but finally came to the conclusion that natural
selection dealt chiefly with the smaller. Certainly they
are the more numerous, and considering how severe may
be the struggle for existence, it can hardly be doubted
that they greatly influence the death-rate. The attempts
which are sometimes made to fix an arbitrary limit to the
“selection value” of a character are futile ; it all depends
on the intensity of the struggle at any particular time.
A character, useless during the greater part of the life of
an organism, may prove of vital importance on a par-
ticular occasion. The slightest difference in weight
between two seeds carried by the wind may decide that
the one will reach a favourable spot and not the other ;
the smallest inferiority in powers of resistance may cause

THE STRUGGLE FOR EXISTENCE. 77

one man to perish of a disease while another recovers.
Nevertheless, selection is of course ready to avail itself of
large variations or sports, if they are in the right direction.
However, it has been shown above (p. 68) that extreme
variations seldom succeed, doubtless because they tend
to upset that nice adjustment of parts, that harmony of
function so essential in competition. Indeed, we can
hardly imagine that the complex adaptations so com-
monly found, the marvellous cases of protective resem-
blance between organisms and their surroundings, and
similar developments, could have arisen otherwise than
by the accumulation of small differences step by step.

For it must never be forgotten that variation is not
itself adaptive. Obviously iff it were adaptive, a species
would never become extinct, since the right kind of varia-
tion would then always be presented to meet the demand.
Variation, on the contrary, is blind, fortuitous, but takes
place in many directions. Great as may be the number
of possible directions, it is not infinite. In a sense,
variation is limited; for all changes in the hereditary
constitution are, in the end, due to additions to, subtrac-
tions from, or rearrangements in factors already present.
Therefore, new developments are greatly influenced by
the general hereditary constitution of the race, and, we
may add, some sorts of mutation are liable to occur
more often than others, and repeatedly in the same or
even in different groups (albinism, &c.).

The next point to consider concerns the usefulness
of characters. It is often urged against the theory
of natural selection that many characters are useless.
Now, we have just maintained that variations are not
adaptive—they may be useful, harmless, or harmful.
But this does not invalidate the Darwinian doctrine; on
the contrary, if variations were always useful selection
could not take place. But if it could be shown that a
cumulation of harmful or even useless variations had
occurred, we should have a serious difficulty to deal with.

78 EVOLUTION OF LIVING ORGANISMS.

Darwin used to say that a single instance of such useless
evolution would be fatal to his whole theory: it need
hardly be added that none has ever been found.

Natural selection can never be prophetic ; organs can-
not be developed before they are needed. Certainly
organisms may on rare occasions by chance, so to speak,
find themselves ready adapted to meet new conditions ;
so protoplasm may possess properties which have never
yet been made use of in the struggle for existence—as, for
instance, the power of responding to galvanic stimuli.
But every step in the process of evolution by selection
must of necessity be useful, must lead to survival.

Apparent exceptions to this rule may be due to corre-
lation. Factors of inheritance are able to affect not one
but several characters, or even the whole organism, so
that alteration of a character by selection may lead to
correlated alterations in other parts. Thus changes may
be brought about not for their own sake, and characters
useless or even harmful may be developed, so long as the
advantage gained is not counterbalanced. Correlation
plays an important part in evolution. It. has been
elaborately studied mathematically by Pearson and
others, and has been proved to occur extensively, and
often in most unexpected directions.

At the same time, the tendency shown by the detrac-
tors of Darwinism to assert that this or that character is
useless, because they cannot find a use for it, is strongly
. be deprecated. Every day naturalists are discovering
the functions of the most insignificant-looking organs.
Little importance can be attached to the statement often
made that the characters which distinguish nearly allied
species are of no value to them; in fact no character
should be accepted as useless until it has been definitely
proved that it exerts no influence on the death-rate.
Some few years ago it might have been held—indeed it
was held—that such organs in man as the thyroid gland,
the pituitary gland, the suprarenal glands, and others,

THE STRUGGLE FOR EXISTENCE. 79

are useless structures, functionless vestigial remnants.
They are now known to be of the greatest importance,
altering the composition of the blood or secreting sub-
stances essential for the regulation of the processes cf
metabolism. He would be a rash man indeed who
would now assert that any part of the human body is
useless.

The same may be said of the coloration of organisms,
and the rash statements so frequently made as to the
uselessness of the differences in colour which so often
distinguish species from each other. Doubtless colour
variations are not adaptive, and the colour differences
between species are not necessarily adaptive either.
Yet many possible uses of coloration have been brought
to light by the labours of Bites, Fritz Miiller, Wallace,
Poulton, and others, showing that it may often be of the
greatest importance in the struggle for existence ; as in
the case of the protective resemblance to surroundings
enabling an organism to escape from its enemies or to
approach its prey unnoticed ; of warning colours exhibited
by animals well able to defend themselves with poisonous
weapons ; of mimicry where a species gains advantage by
acquiring a resemblance to some distasteful or dangerous
form ; of recognition marks or sexual ornaments which
serve to bring the sexes together. An interesting ex-
periment on the selection value of colour differences was
performed by Di Cesnola on the praying mantis (Mantis
religiosa). This insect occurs in Italy in two varieties,
a green and a brown, adapted for concealment on green
or brown surfaces ; and it was found that green speci-
mens placed in brown surroundings and brown specimens
placed in green were invariably eaten by their enemies,
while individuals on a background which they matched
frequently escaped destruction.

There was a difficulty much felt by the early advocates
of natural selection which has been removed now that
the process of inheritance is better understood. It was

80 EVOLUTION OF LIVING ORGANISMS.

thought that a variation which appeared in one or even
in several individuals of a species would have little chance
of establishing itself, since it might be reduced and
finally swamped by constant interbreeding with the more
numerous individuals of the species not possessing it.
Such a swamping by intercrossing, however, does not
occur. We have already seen (p. 52) that factors of in-
heritance are as a rule transmitted complete ; even when
the appearance of the character to which it gives rise is
subject to various inhibiting influences, it is liable at any
time to reappear in full force, as is seen in: reversion.
Characters due to factors of inheritance will persist. un-
less eliminated by variation and selection, as can be
shown by experiments and by mathematical reasoning.
The relative scarcity of the mutation at the start does
not prevent that a number of individuals interbreeding
at random, some with and others without a certain
factor, will give rise to a population of impure hetero-
zygotes and pure homozygotes in which the proportion
of the three classes will be in equilibrium so soon as the
square of the number oi heterozygotes equals the number
of pure “dominants” multiplied by the number of pure
“recessives.” If this proportion is not already present at
the beginning it will soon become established, and will
continue, provided there is no selection to disturb the
equilibrium. In fact a species of interbreeding indi-
viduals of unequal hereditary constitution soon reaches
a state of stability.

CHAPTER VI
ISOLATION AND SEXUAL SELECTION

FR&E intercrossing can only mix hereditary strains, hence
the necessity for isolation if divergence along various

ISOLATION AND SEXUAL SELECTION. 81

lines of adaptation is to take place. .The importance of
isolation has been variously estimated by different
authors; but it must not be forgotten that it plays a
subsidiary part in evolution, and can do nothing with-
out selection and variation. Selection without isolation
may give rise to evolution in a straight line ; combined
with isolation it may lead to divergence into as many
lines as there are groups of individuals isolated. There
are several kinds of isolation: geographical, “ physio-
logical,” and isolation due to the adoption of different
habits and modes of life.

Means of dispersal are various. Some organisms are
borne passively by the wind, as seeds and microscopic
plants and animals, others by water currents, and still
others by their own activity“move from place to place.
In one way or another species are always trying, so to
speak, to spread over a wider area. Land organisms
become geographically isolated by the formation of
barriers such as deserts, or mountain ranges, or by the
separation of parts of a continent as islands. Marine
organisms may be divided by the uprising of dry land,
and the inhabitants of fresh water by the separation of
river basins. But whatever may be the barrier which
cuts off more or less completely a number of individuals
from the main stock, the result is the same—they tend
to diverge from the parent species. This divergence is
due to differences in the environment directly or in-
directly modifying the individuals, to the new fauna and
flora with which the isolated specimens come into contact
altering the course of selection, and lastly to the appear-
ance of new mutations. So many local races or species
become differentiated ; and when a number of closely
allied forms occupy neighbouring regions, they are more
unlike, generally speaking, the further they have strayed
from the original centre of distribution. The more com-
plete and the older the barrier the greater will be the

divergence. Thus the marine and littoral fauna on the
(2,031)

82 EVOLUTION OF LIVING ORGANISMS.

opposite coasts of the isthmus of Panama differ con-
siderably, but differ far less than does the fauna of the
Mediterranean from that of the Red Sea, these seas
having been separated for a much longer time. In the
Sandwich Islands there are some 300 species of the genus
Achatinella, almost every valley having its own peculiar
form of this mollusc. A similar variety of structure has
been described by Sarasin among the land molluscs of
the island of Celebes, where species become subdivided
into an astonishing number of local races still united by
transitional forms. Islands afford excellent illustrations
of divergence through isolation. While their fauna and
flora bear a general resemblance to those of the nearest
mainland, each island or archipelago usually acquires a
remarkable number of peculiar forms. For instance.
almost everyone of the West Indian islands has its repre-
sentative species of the golden oriole. The Galapagos
archipelago has its own reptiles, insects, and land mol-
luscs; out of some thirty species of land birds about
twenty-eight are peculiar: while each one of these
islands has developed its own race of the gigantic land
tortoise.

Another kind of isolation is that brought about by
parasitism. Since every parasite tends to restrict itself
to one particular kind of host, being often actually trans-
mitted from one individual to another, a parasitic species
becomes split up into as many divergent races as there
are varieties of host, until finally each host may acquire
its own peculiar species of parasite. A somewhat similar
subdivision and specialisation takes place in plants fer-
tilised by insects. The flowers tend to become special-
ised in structure and colour to attract particular kinds
of insects, while the insects undergo a corresponding
specialisation in order to derive nourishment from the
flowers.

Physiological isolation may result from incompati-
bility of habits or temperament, or from sterility.

ISOLATION AND SEXUAL SELECTION. 83.

Closely allied rival forms often meet or overlap in some
region, yet interbreed little, or not at all, although quite
capable of forming a fertile union and of producing
fertile offspring. A familiar instance is that of the
negroes and whites in America. Sterility in one form or
another is the most important physiological barrier, and
may be due to many different causes. For instance,
sterility may result from variation in the structure of
the copulatory organs, as in numberless kinds of insects ;
or merely in their size or shape. Intercrossing may also
be prevented if sexual maturity is reached at different
times of the year ; and self-fertilisation is made impossible
for most hermaphrodites by the spermatozoa developing
either before or after the ova in the same individual.
Lastly, isolation may result*from some variation in the
germ-cells themselves, causing fertilisation to be imper-
fect or sterile even if it take place; this may be called
true sterility. Further, the zygotes when formed may
fail to develop normally, or even if viable the hybrid
offspring may themselves be sterile. This form of ste-
rility is common among plants and animals; the mule-
produced from a cross between the horse and the donkey
is the most familiar instance. In these various ways,
then, divergences inevitably arise among groups of
organisms originally alike, and evolution in its course
moves along ever branching paths.

If a higher organism is to succeed in the struggle for
existence it must reproduce itself sexually ; hence the
importance in evolution of the various adaptations for
securing the fertile union of the sexes. The differences.
between the two sexes, other than those of the repro-
ductive organs themselves, are known as secondary sexual
characters. Many are of such a kind as to enable them
to find and recognise each other, and to accomplish the
act of copulation. To this class belong the diverse
organs developed in all sorts of animals for grasping the
female, the call notes of many insects, birds, and mammals,

84 EVOLUTION OF LIVING ORGANISMS.

and the strong scents given off by female hawk-moths,
male musk-deer and stags, and other animals at the time
of maturity. Conspicuous sexual differences are often
produced by the development im one sex of special organs
to receive stimuli from the other, as for instance in the
Crustacea and Insecta, where the olfactory organs, the
antenne, may be greatly enlarged and modified in the
males.

But there are many other secondary sexual characters
the use of which is by no means so obvious—such as the
beautiful patterns and colours developed in male butter-
flies and other insects, the marvellous wealth of orna-
mental colours and plumage in birds, and offensive
weapons like the antlers of deer—and it is one of Dar-
win’s greatest triumphs to have g*’ea us in the theory
of sexual selection a rational expianation of the evolu-
tion of these apparently useless characters. No hard
and fast line can be drawn between natural and sexual
selection. Sexual selection may be considered as sub-
ordinate to natural selection, as a special kind of natural
selection taking place within the limits of an interbreed-
ing set of animals or “species.” It is due to the com-
petition between individuals of one sex for the possession
of the other. Almost always it is the males which com-
pete for the females, either because they are more numer-
ous or because they are polygamous. This kind of selec-
tion takes place only among highly organised animals,
and is almost entirely restricted to the vertebrates and
arthropods (insects and spiders) It has been aptly
called the struggle for wife as opposed to the struggle for
life ; but failure means, in the end, extinction in the one
case as in the other.

Darwin has convincingly shown how severe is the com-
petition between the rival males, how universal is the
law of battle, leaving the most agile the strongest and
the best-armed in possession of the field. The general
superiority of the male sex in strength, pugnacity, and

ISOLATION AND SEXUAL SELECTION. 85

fighting weapons, such as the large canine teeth of male
mammals and the antlers of stags, is accounted for by
the survival of variations leading to victory in the fierce
struggles which take place between the males at the
breeding season. The correctness of this view can hardly
be doubted; but there is great divergence of opinion
when the principle of sexual selection is applied to the
more purely ornamental secondary sexual characters.
These characters are very commonly developed among
the higher animals, and one may mention as examples
the mane of the lion and bison, the ornamental patches
of colour and hair in monkeys, the beard in man; the
brilliant wattles and plumage in numberless birds, such
as the gorgeous feathers of pheasants, peacocks, birds of
paradise, and humming birds; the ornamental colours
in many fish, butterflies and spiders, the horn-like pro-
cesses of beetles, the attractive scents of butterflies, the
vocal sounds emitted by insects, frogs, and mammals,
and the beautiful song of birds. Darwin pointed out
that these brilliant and striking characters appeal to
the senses of the female, and are deliberately displayed
to her at the breeding season. Courtship with these
animals is often a lengthy and elaborate business, during
which the male may perform a regular dance and strike
attitudes to display himself to the best advantage. The
more attractive male will thus secure his mate more
rapidly and certainly than the less happily endowed, or,
in the case of polygamous species, will be followed by a
greater number of females. Even if the less successful
males eventually manage to pair, they will not have
such good or so many chances of leaving offspring.
A small percentage of. advantage in this respect is
sufficient to bring about a selective result. Now it
is characteristic of such structures and colours that
they develop only in one sex and generally only at ma-
turity ; frequently, as in birds, they are periodically
renewed at each breeding season. Moreover, they

86 EVOLUTION OF LIVING ORGANISMS.

‘appear only on those parts which are displayed’ in
courtship.

Much scepticism has been shown concerning the efii-
cacy of sexual selection as a factor in evolution, chiefly
on the ground that the theory seems to presume an
esthetic taste and power of choice in the female. But
this criticism is due, at all events to a great extent, toa
misunderstanding of the metaphorical language in which
it is convenient, if not necessary, to describe such facts.
Strictly speaking, “choice” and “taste” are but words
to express the fact that the female is more stimulated by
one kind of form, colour, scent or sound than by another.
In physiological language it is all a matter of stimulus
and response ; those males will succeed best which most
effectually stimulate the females. Doubtless in many
cases, here as elsewhere, the facts are but incompletely
known ; but in others the evidence is convincing enough.
We will here mention only one instance, that of the
spiders of the family Attide, so thoroughly studied by
Mr, and Mrs. Peckham. These very competent observers
show clearly, that when the sexes are differentiated the
males are the more brilliantly coloured ; that the young
male resembles the adult female, and that it is the male
that has departed from the ordinary ancestral colouring ;
that the male colours are visible and displayed during
courtship ; that the females pay attention to them and
exercise a “choice”; and lastly, that the more brilliant
males may be selected again and again. Moreover, it
must be remembered that sexual selection is known to
occur in the pairing of the human species, and that its
selective value has been statistically estimated.

Wallace has pointed out that natural selection must
often compel the female to keep to a more modest and
protective type of coloration than the male; for, until
she has laid her eggs or reared her young, a female may
be more essential for the propagation of the race than a
particular male; and there can be no doubt that pro-

PHYLOGENY AND CLASSIFICATION. 87

tective coloration or structure, mimiery, polymorphism,
and other such factors, greatly complicate the problem
of the evolution of secondary sexual characters.

CHAPTER VII
PHYLOGENY AND CLASSIFICATION

Ir is the task of the anatomist and systematist to study
and compare the structure of adult organisms (Compara-
tive Anatomy or Morphology), and also their develop-
ment (Embryology), in order to make out their affinities,
to discover the lines of descent which connect the various
diverging branches of the genealogical tree. These
branches are called Phyla, and Phylogeny is the name
given to the study of the pedigrees of organisms, the
tracing out of their blood-relationships. A true natural
classification is based on phylogeny. Much valuable
evidence for this science can be derived from Palon-
tology, the study of fossil extinct animals and plants,
which may reveal the actual ancestral forms or their
near relatives ; but a great deal can be gathered also from
a knowledge of the structure and development of the
living.

Since forms which differ widely in the adult condition
often resemble each other much more closely in the
young or embryonic stages, by observing their develop-
ment affinities can sometimes be discovered which would
not otherwise be suspected, or at all events would be very
doubtful. Familiar instances are those of the Tunicates
and Cirripedes. The former are the Sea Squirts, seden-
tary animals for the most part, which have undergone
degeneration owing to their peculiar mode of life. The
adult lives fixed to the sea bottom, and is of very simple
structure, showing little resemblance to an ordinary

88 EVOLUTION OF LIVING ORGANISMS.

vertebrate, yet the young free-swimming larva has all
the characteristics of the Vertebrate phylum, with /its
dorsal central nervous system, gill-slits, axial skeletal
rod or notochord, and tail. The cirripedes or barnacles
likewise have taken to a fixed habit in adult life, and have
lost almost all resemblance to the Crustacea from which
they have been derived, as is shown by the larval stages
typically crustacean. Other common instances of de-
generation are afforded by parasites. Often they be-
come simplified beyond recognition, but betray their true
affinities in their development. Many parasitic crustacea
of the order Copepoda lose practically all trace in the
adult condition of the characteristic appendages used in
the normal free-living forms for locomotion or for seizing
and cutting up their food. Indeed, parasitic animals,
able as they are to absorb the nourishment directly from
their host, generally tend to lose not only organs of loco-
motion and of special sense, but even the alimentary
canal, and spend all their energies in producing enormous
numbers of young to infect new hosts. The loss of the
organs of flight in flightless birds and insects, and of the
eyes in animals living in caves or the dark depths of the
ocean is also due to degeneration.

Degeneration, in fact, is a widespread phenomenon
among animals and plants, and leads to the loss of
any special structures mental or bodily which the organ-
ism no longer needs in the particular environment for
which it has become adapted. It is a return from a com-
plex to a simpler organisation ; but not to a truly primi-
tive or ancestral condition, for the path of retrogression
is generally very different from that followed by the
original progressive evolution. Now it is one of the great
merits of the doctrine of evolution by natural selection,
that it accounts for this simplification as easily as for the
development of complexity. For both progressive and
retrogressive mutations occur. Variation takes place
both in the + and inthe — direction, and selection of the

PHYLOGENY AND CLASSIFICATION. 89

one may be as advantageous as selection of the other.
Which will be chosen depends on the needs of the organism
at the time. On the other hand, it is very difficult to see
how any theory of evolution based on some supposed
internal perfecting force could possibly be reconciled
with these facts.

Strewn along the path of evolutionary change are thus
left derelict organs once of vital importance, but now no
longer of use, or at all events of less consequence in the
struggle for existence, owing to some change in habit
and environment. Such organs are known as vestigial ;
and unless they are turned to some new purpose, that is
to say, unless they vary in such a way as to become
adapted to fulfil some new function, they are apt to dis-
appear. The exact process“of disappearance is difficult
to describe, is in fact not thoroughly understood. If an
organ thrown out of work is an actual burden on the
organism, it will tend to become eliminated by selection
of retrogressive variations, and will also be apt to develop
incompletely in the individual. owing to disuse. But
if merely useless, it may remain indefinitely as a vestigial
structure. Such, for instance, are the vestigial hind
limbs in the dugong and in whales, and the teeth in em-
bryo baleen whales, the much reduced wing in flightless
birds like the emu or the apteryx, or the extinct moas of
New Zealand. It would be rash, however, to take it for
granted that even in these cases the vestiges are alto-
gether without function (p. 79).

But it is far more common for the apparent disappear-
ance of an organ to be due to its alteration and adapta-
tion to some new function. And this brings us to another
objection often urged against Darwinian doctrines. If
natural selection cannot be prophetic (p. 78), if organs
cannot develop before they are called into use, how can
one account for the initial stages in their development ?
Of what use can a complex organ be before it is com-
pleted? But this objection loses its force when it is

90 EVOLUTION OF LIVING ORGANISMS.

remembered that organs rarely if ever can be said to
“begin.” Entirely new functions and entirely new
organs are not suddenly developed. All are evolved by
the gradual transformation of, addition to, or subtrac-
tion from something already there. The wing of a bird,
unique as it is, has had nosudden beginning—it has been
gradually transformed from the fore-limb of the reptile.
The extinct Archeopteryx from the Jurassic strata has a
modified limb in a beautifully intermediate state. The
one-toed hoof of the horse is not a new organ; it is derived
from an ordinary five-toed foot by the gradual loss of the
lateral digits. Almost every stage of its history can be
traced in the fossils. What more complex or useful
organ than the human eye? Yet it is merely an instance
of the extreme specialisation of the property of response
to light generally distributed over the surface of the body
in the lowest forms. In the evolution of an organ by
selection every stage must be useful, and it is often diffi-
cult to picture the intermediate conditions; but we
must not jump to the conclusion that they could not have
existed. The heart of the amphibian has one ventricle,
in which the venous and arterial blood become more or
less mixed ; that of the bird has the ventricle completely
subdivided into two chambers, so that the venous is kept
quite separate from the pure arterial blood. Now if
reptiles were unknown, we could well imagine an op-
ponent of the theory of natural selection stating dog-
matically that the intermediate steps in the formation
of the dividing septum could not have been useful, and
therefore could not have been selected. To be effective
at all, he would say, the septum must be complete from
the beginning ; if the venous is to be separated from the
arterial blood an intermediate stage would be of no use.
Such arguments are constantly heard. Fortunately,
in this case, we can point to the reptilia, where these
very intermediate steps occur, and the incomplete
septum can be shown actually at work.

—

PHYLOGENY AND CLASSIFICATION. 91.

As a rule, evolution leads to specialisation and differ-
entiation along the ever diverging and forking branches
of the phylogenetic tree. Usually the more diverse
organisms become the more successful they are in the
struggle for existence, since they interfere less with each
other owing to their adoption of different modes of life
and different feeding habits. On a given plot of ground
more individual plants can live if they belong to several

Fie. 2.—Convergent evolution. Salamandra (1), 2 normal Urodele Am-
phibian ; and Siphonops (2), a legless Amphibian ; Agama (4), a nor-
mal Lacertilian; and Amphisbena (5), a legless Lacertilian. (2)
and (5) being adapted to a burrowing life have come to resemble
the Earth-worm (3) and each other.

species adapted in different ways, than if they belong to
one species only. But occasionally evolution leads to
convergence in function and structure. Organisms and
their parts may then come to resemble each other, an-
alogies are developed. Thus a burrowing snake like
Typhlops, a burrowing lizard like Amphisbzena, or
amphibian like Siphonops, acquire a resemblance to an
earthworm (Fig. 2); or again, among the Mammalia, the
“fiying ” squirrel, Pteromys, and the “flying” phalanger,

92 EVOLUTION OF LIVING ORGANISMS.

Petaurus, have become remarkably alike. Very striking
also may be the analogous resemblance between two
organs of different origin, but fulfilling the same definite
function, as the eyes of Polychsete worms, of Molluses,
and of Vertebrates. Mimicry and protective resemblance
also are special examples of convergence. Yet however
close the resemblance may be, it is generally essentially
superficial, and the separate origin of two structures is
usually betrayed not only by fundamental differences,
but also by innumerable details. Just as the expert can
often detect a forged antiquity by careful inspection, so
the comparative anatomist distinguishes analogies from
true homologies. It is worth insisting upon this subject,
because the French philosopher Bergson has recently, we
think, greatly exaggerated the extent and somewhat mis-
represented the significance of convergence.

Coming now to classification, it is the sorting out of
organisms into groups according to their natural affini-
ties. Individuals are grouped into species, species into
genera, these again into families, orders, classes, and
phyla, divisions of increasing size and importance.
Before the doctrine of evolution was accepted, it was
thought that these groups had definite limits based on
some separately created and fixed unit. At one time it
was supposed that the comparatively large genus was the
unit of creation, the species and varieties within it be-
ing merely fluctuations caused by external influences.
Linnzus maintained that the smaller group, the species,
is the originally created unit, and succeeded in establish-
ing his views so firmly that they grew into a dogma from
which the systematist even of the present day has not
entirely freed himself. But the consistent evolutionist
recognises that so-called “species” are merely closely
allied individuals descended from a common ancestor,
which normally interbreed, and are sufficiently alike to
be conveniently called by the same name. All sorts
of vain attempts have been made to draw up stricter

PHYLOGENY AND CLASSIFICATION. 93

definitions of a true species. Mere likeness counts for
little: males and females may often differ very consider-
ably from each other, and in polymorphic forms the off-
spring of the same parents may be of several different
types. Sterility has often been held to form a definite
barrier distinguishing true species from mere varieties
(p. 83); but there are all degrees of fertility, and crosses
between forms which no systematist would hesitate to
call species often yield offspring. Sterility is a character
variable like any other ; it occurs even between members
of undoubtedly the same species, as we know in the case
of our own.

It is true that De Vries has recently tried to define
“elementary species” as forms produced with a new
progressive mutation, due to the acquisition of some new
factor. Such forms, if they really occur suddenly, could
certainly be considered as definite discontinuous steps in
evolution. Especially among plants there are many
widely distributed “species” containing a large number
of local races or subspecies, each breeding true and ap-
parently differing from the others by one or perhaps a
few “unit characters.” Of the whitlow-grass (Draba
verna) some two hundred constant races have been dis-
tinguished by Jordan; and similar subdivisions of the
species of Viola, Helianthemum, &c., are known. But the
evidence as to the sudden origin of the mutations is
weak ; and, at all events, it is by no means yet established
that new factors may not appear gradually in increasing
intensity, so that discontinuity becomes negligible, and
the new characters develop in steps small enough to
satisfy the most exacting selectionist (p. 76). The diver-
sity of variation and the smallness of the steps in muta-
tion are well illustrated in the case of the hawkweed
(Hieractum) and the dandelion (Taraxacum), which re-
produce parthenogenetically ; as in the Bacteria, there
may be differentiated as many “pure lines” as there are
parents, and a bewildering number of races results,

94 EVOLUTION OF LIVING GRGANISMS.

each distinguished by some small constant variation in
character. Indeed, it is the universal experience of
naturalists engaged in the classification of quite modern
closely allied “species,” that the great difficulty of the
work is due to the fact that it is usually scarcely possible
to find any character at all sufficiently conspicuous and
constant to distinguish them from each other. Sudden

Species Species

oN Ve Species

Genus

Pe

Order
Fic. 3.—Diagram to illustrate the principles of classification.

mutations and sharp distinctions would be welcomed by
all systematists.

The only “fixed points” in a phylogenetic system of
classification are the points of bifurcation, where one
branch diverges from another (Fig. 3). It is here that
our divisions should be made; and our generic, family,
and ordinal distinctions naturally come at each fork
farther and farther down the stem of the phylogenetic
tree. The phylogeny of organisms, however, is but
incompletely known ; and often the actual point of diver-

THE GEOLOGICAL RECORD. 95

gence or origin of a group may be but vaguely inferred,
so we have to compromise and adopt divisions which only
indicate progressive grades of structure.

There is one more consideration to be borne in mind
when dealing with species. The living species around us
represent the extreme tips of the branches of the phylo-
genetic tree, which have succeeded in the struggle for
existence. If their ancestors are extinct these living
twigs become isolated from each other, and so real limits
become established, which of course would disappear if
the complete series of extinct forms were discovered. It
is the inevitable result of evolution by the elimination
of the less well-adapted that ever widening gaps come
to separate the diverging living representatives of the
various branches derived from a common stem.

CHAPTER VIII
THE GEOLOGICAL RECORD OF SUCCESS AND FAILURE

EVOLUTION is too often represented as a history of success
and progress; it is also one of extinction and failure.
Authors, seeming to forget that for one line of develop-
ment that succeeds there are a hundred that fail, are fond
of invoking some mysterious guiding principle, some in-
ternal perfecting agency—an élan vital, or what not—to
account for evolution; but there would appear to be
little scope for such mysterious forces in a world where
the majority of individuals are crushed out, where most
lines of development fail hopelessly to establish them-
selves. What guiding principle there may be behind the
whole of creation is a subject outside the scope of Natural
Science, and on which it can express no opinion. It
cannot even prophesy whether man or the bacillus will
eventually triumph in the struggle for existence ; indeed,

96 EVOLUTION OF LIVING ORGANISMS.

both would seem to be doomed to destruction in the end
by an unfavourable environment when this earth becomes
too hot or too cold to support life. In the meantime, it
is the great merit of the Darwinian principles of evolu-
tion that they account for the failures as well as for
the successes. The elimination of the unfit, leaving the
better adapted in possession, is a necessary part of the
process.

The living organisms of to-day show us the types
which have succeeded ; for the failures we must appeal
to the record of the past as revealed by a study of fossil
remains. This record, in spite of its incompleteness,
has much to teach. Every theory of evolution must be
tested by the results of paleontology ; no conclusion can
be accepted which is inconsistent with them.

In the first place, the conviction derived from a study
of living forms is confirmed, that evolution does not pro-
ceed along continuous straight lines, but, on the contrary,
along a multitude of diverging branches. Just as indi-
viduals are found to vary in all directions compatible
with their structure and composition, so groups become
differentiated in various directions, each adapted to a
particular mode of life. Having reached a certain favour-
able combination of characters, they start on this new
plane of structure to diverge, according to the principle
of adaptive radiation, as it has been called by Osborn,
many instances of which are found in the history of the
land vertebrates.

Derived from some fish-like aquatic ancestor in
Devonian or pre-Devonian times, the land vertebrates
appear in Carboniferous strata as clumsily-built Am-
phibia with four walking limbs. Like their modern
representatives, they spent their early life in water,
breathing by means of gills, and made use of lungs for
respiration in adult life on land. These primitive
amphibia soon diverged in various directions. Some
acquired a large size and formidable dentition, like the

THE GEOLOGICAL RECORD. 97

Labyrinthodonts ; others remained small, and were prob-
ably harmless herbivores; some became elongated, lost
their limbs, and were adapted to an eel- or snake-like
mode of progression (Aistopoda); while others (Bran-
chiosauria), losing the original scaly covering of the fish-
like ancestor, gave rise to the modern groups (the frogs
and toads, or Anura, and the salamanders and newts,
or Urodela). These latter are all specialised forms, the
existing Urodela being only the degenerate remnants of
a once flourishing class which have become more or less
completely readapted to an aquatic life. In fact, the
Anura are the only order which has succeeded and ex-
panded in recent times.

The Amphibian was the dominant type in Carbonif-
erous times; it now occupies a very subordinate place.
But from some unspecialised branch of it arose the more
thoroughly terrestrial Reptilia towards the end of the
Carboniferous or beginning of the Permian epoch. The
class Reptilia reached a higher grade of structure, and
soon almost completely superseded the Amphibia on
dry land. So successful were the reptiles that already in
Permian and Triassic times they had spread over the
whole earth, becoming adapted in various directions to
all sorts of life (Fig. 4). The earliest reptiles known so
closely resemble the primitive Amphibia that it is difficult
to say where one class begins and the other ends; but
these undifferentiated reptiles soon gave way to more
specialised successors. The Theromorpha gave rise to
remarkable forms: some with large fiat grinding teeth
(Placodontia) ; others active, vigorous creatures with a
formidable carnivorous dentition (Therocephalia and
Therodontia) ; while the highly-specialised Dicynodontia
retained only two huge tusks. None of these specialised
reptiles survived beyond the Trias. Other lines of dif-
ferentiation lead toward a return to aquatic life.
Plesiosaurs and Ichthyosaurs quite independently took

to marine life, and their limbs became transformed into
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THE GEOLOGICAL RECORD. 99

swimming-paddles. Neither group persisted beyond the
Cretaceous epoch. Among the most interesting extinct
reptiles are the Dinosauria. First appearing in the Tri-
assic, they flourished in the Jurassic and Cretaceous, but
became extinct before the beginning of the Eocene epoch,
Often of gigantic size—as for instance Cetiosaurus and
Diplodocus—these remarkable animals were the lords of
the earth in later Mesozoic times. Some were adapted
to a herbivorous vegetable diet, like the huge Iguano-
don; while others were aggressive carnivores, like
Ceratosaurus and Megalosaurus. But in spite of every
effort, so to speak, to succeed in all possible directions,
in spite of elaborate adaptations, terrible weapons,
formidable defensive bony plates, horns, and spines,
these splendid Dinosaurs all filed in the struggle for
existence by the end of the Cretaceous epoch (Figs. 4
and 5).

Some se rtadttatives of the large order Crocodilia still
persist in the tropics; but the Rhynchocephalia survive
at the present day only in the single species Sphenodon
punctatum, preserved by special legislation on certain
small islands off the coast of New Zealand. The Ptero-
sauria, admirably specialised for flight as they appear to
have been, had but a short success in later Mesozoic
times. On the other hand, the Chelonia (tortoises and
turtles), modestly taking refuge in their bony shell, are
fairly flourishing in warm countries even at the present
day. But of the vast array of reptilian forms the
Lacertilia (lizards) and Ophidia (snakes) are the only
two orders which have really increased and spread in
recent times. A few other orders linger on in reduced
numbers ; the majority have failed altogether (Figs. 4
and 5).

Bat although the reptilian type, once so successful and
widespread, has failed so signally in the struggle for
existence, it has given rise to other types which have
replaced it. The birds are doubtless descended from

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some primitive reptile allied to the Crocodilia and Dino-
sauria. The avian branch has undergone comparatively
little pruning. Quickly supplanting the Pterosauria
after the Jurassic epoch, they radiated along all sorts of
adaptive lines, most of which survive to the present day.
The beautifully adapted organisation of birds, with warm
blood, efficient lungs, sharp senses, quick movements,
and light feathers, has secured them a supremacy in the
air which has hardly been challenged even by the
mammals.

The Mammalia, that class of vertebrates to which we
ourselves belong, arose earlier than the birds, probably
from some primitive reptilian stock in Permian times.
Indeed, the Theromorph reptiles of the Trias so nearly
approach the mammalian typé of structure in the char-
acter of the skull, palate, lower jaw, and other important
points, that they are now generally held to have been,
if not the ancestors themselves, at all events closely
allied to them. Quite independently of birds, and on
different lines of specialisation, the Mammalia have
acquired a four-chambered heart, completely separating
the arterial blood from the venous, and a self-regulating
mechanism, keeping the blood at a constant high tem-
perature, independent of that of the surrounding environ-
ment (p. 41). Of very adaptive build, the mammals
soon diverged from the primitive ancestral egg-laying
type now almost extinct, but still preserved in the
archaic Monotremes living in Australia, the famous
Ornithorhynchus, and Echidna. Adopting the advan-
tageous method of nourishing their young during early
_ life in the mother’s womb, the placental mammals spread
rapidly over the earth, ousting the lower reptilian type
of organisation, and diverging in various adaptive direc-
tions, they became the dominant group in Eocene times
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in the struggle. Large groups have vanished altogether,
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THE GEOLOGICAL RECORD. 108

supials, once widely distributed, remain only in Aus-
tralia, where they have escaped from competition with
the more advanced Placentalia, and as scattered genera
in America. The Dugong and Manatee are now the
only representatives of the order Sirenia; while the
Edentata, including the gigantic ground Sloths (Mega-
sherium) and Glyptodonts, once all-powerful in South
America, are reduced nowadays to a few highly-special-
ised tree-sloths and armadillos (Fig. 6).

Most instructive is the history of the large order
Ungulata, which includes all the hoofed herbivorous
mammals. Starting in Eocene times from primitive
forms about the size of a fox, with complete unspecialised
dentition and five-toed feet, known as the Condylarthra,
and long ago extinct, the Ungulates branched out into
a number of sub-orders (Fig. 7). The Amblypoda de-
veloped into huge creatures, like Dinoceras, with large
tusks and four horns on the skull, but did not survive
beyond the Eocene age. A somewhat similar but quite
distinct group of massively-built herbivores, the Titano-
theria, lasted only into the Miocene, while the bighly-
specialised and aberrant sub-order Ancylopoda occurs
up to the Pliocene epoch. Two South American groups,
the formidable rhinoceros-like Toxodontia and the more
horse-like Litopterna, have left no descendants. The
Hyracoidea survive at the present day only in the little
coney, Hyrax, and a closely-allied genus; and of the
Proboscidea, including mastodons, mammoths, and
elephants, a large group once widely distributed over
both the Old and the New World, there persists but one
species in Asia and one in Africa. Even the large sub.
order Perissodactyla (the odd-toed Ungulates), although
still represented by a few rhinoceroses, tapirs, horses,
and asses, is by no means as widespread or successful as
it once was. The even-toed Artiodactyla are at present
the most flourishing group, with a large number of genera
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THE GEOLOGICAL RECORD. 105

the pig-like forms, or Suina, have not been very suc-
cessful; the hippopotamus is nearing extinction, the
Tylopoda (camels) are but the isolated relics of a not
very flourishing division, and many allied families have
failed altogether. Thus we see that the existing families
of Ungulates are but the scattered remnants of a far
larger number of groups which flourished more or less
successfully in the past (Fig. 7). And the same may be
said of the history of all organisms, whether vertebrates
or invertebrates, whether animals or plants. Many are
the forms developed, few are those which survive.
Those who believe in a guiding force directing the course
of evolution must admit that it has been singularly blind
and inefficient, leading more often to destruction than to
success, A

Still, it is sometimes argued, organisms seem to get
into a groove of specialisation, to pursue a road along
which they can no longer stop, to become overspecialised
by virtue of some sort of momentum driving them over
the limits of usefulness to inevitable destruction. Thus
over and over again we see, in the record left by fossils,
animals acquiring a larger and larger size, and then sud-
denly dying out. The large Amphibia of the Carboni-
ferous, the monster Dinosaurs of the Jurassic and
Cretaceous, the gigantic Moas (flightless birds of New
Zealand), and among the mammals the huge Amblypods
and Titanotheres, the giant sloths, and others, are all
extinct. Again, some animals develop certain organs to
an excessive extent, as, for instance, the canine teeth in
the extinct sabre-toothed tiger (Machairodus), or the
monstrous antlers of the extinct Irish elk. Now it is
quite probable that these animals died out owing to over-
specialisation, a narrow adaptation to a particular en-
vironment accompanied by a corresponding loss of power
of accommodation to changed circumstances ; but it is a
mistake to assume without clear proof that the course of
their evolution can have been useless. Variation (p. 77)

106 EVOLUTION OF LIVING ORGANISMS.

may be useless or harmful, and doubtiess these unsuc-
cessful forms may have got into grooves of variation ;
but variation is not evolution. And seeing that natural
selection looks not to a distant future but to the immediate
advantage, there is nothing in the history of these animals
which cannot be explained as due to the ordinary action
of selective elimination. We have every reason to sup-
pose that every step in increase of size gave some advan-
tage to the giant forms over their competitors. To assume
the contrary would be as wise as to argue that the huge
modern Dreadnoughts are useless ships of war because
they may possibly be driven off the seas by the relatively
small submarines and flying machines, or to deny that
the progressive stages in the development of these men-
of-war must each have surpassed in usefulness those
which went before.

This brings us to another interesting subject on which
palzeontology can throw some light—namely, the rate of
evolutionary change. Some organisms have changed
_very little through long geological periods. The mol-
luscan genus Nucula, and the genus Patella, which in-
cludes our common limpet, date back to the Silurian
epoch ; the Silurian scorpion (Palzeophonas) differs little
from modern forms; the Brachiopod Lingula has
changed very little since the Ordovician; and many
Protozoan skeletons are found in Cambrian rocks which
differ but little from those of the present day. There
can be no doubt, then, that the rate at which the various
branches of the phylogenetic tree have grown varies
very much. Presumably the persistent types have been
sufficiently well adapted to keep their own place against
competitors, though perhaps only where the struggle
has not been very severe. The fact that they have
changed little does not in the least prove that they have
not varied, but merely that the divergent variations have
not been selected. If variations are eliminated as fast
as they occur, an organism may continue unchanged for

THE GEOLOGICAL RECORD. 107

an indefinitely long time. Change is no exact measure
of variation.

It is sometimes objected that phylogenies are insecure
speculations, that they are made up of series put to-
gether according to preconceived theory, that the con-
clusions have often proved erroneous. As the gaps
become more completely filled up, certain forms are
found to have been placed in the wrong order or to
belong to diverging branches rather than to the same
stem. But there can scarcely be any doubt that in
the main these phylogenies represent the real course
of evolution, and such mistakes in details do not
shake our faith in the correctness of the conclusions
as a whole. : .

Very important also is the evidence. of palzontology,
concerning the gradual character of the transition from
-one form to the other. We need not describe in detail
the case of the horse, which is familiar to all, but shall
only mention that it can be traced from an unspecialised
comparatively small Eocene ancestral mammal, with five
digits on each foot, a normal short skull, and the full
complement of short rooted teeth. The complex pattern
on the grinding surface of the teeth can be seen to evolve
by almost insensible steps from the origina! six-cusped
form, just as the lateral digits gradually become reduced
on the feet. Exactly how gradual these transitions have
been we cannot often yet say, but the more complete
the evidence the smaller appear to have been the steps.
Wonderfully gradual are the transitions described by
Beecher in Trilobites and Brachiopods, by Hyatt and
others in Ammonites, and in numberless other cases,
Neumayer has traced the gradual evolution of Paludina
neumayri, found in the lowest Pliocene deposits of
Slavonia, into P. Hoernesi of the uppermost layers.
Hoernes has shown that the Pliocene Cancellaria cancel-
lata is intermediate between the Miocene and the recent
forms of the same species, while Hilgendorf has followed

i08 EVOLUTION OF LIVING ORGANISMS.

the transformation of Planorbis through the successive
geological strata of Steinheim.

In this record of the past we read the work of natural
selection, the drastic action of elimination, and see on a
large scale what is happening to-day not only among the
competing groups of organisms, but among the struggling
individuals. From the record we also learn that evolu-
tion does not proceed along an even course such as we
might expect to see pursued owing to the pressure of
some internal or external directive force. On the con-
trary, it is the rule that groups quickly expand, radiat-
ing in various directions of adaptation. This specialisa-
tion leads to a certain rigidity, a loss of adaptability in
other directions, and sooner or later to a failure to meet
new conditions, while some obscure side branch com-
mitted as yet to no special line of adaptation acquires
some advantageous combination of characters, enabling
it to compete successfully with the dominant race.
Evolution does not move along a straight line from one
dominant specialised form to another, but by the con-
stant uprising of new forms which supplant the old ones.
The evidence of Paleontology is all against the theory
of orthogenesis (the transformation of one group into

nother along a straight line in ladder-like fashion).
Adaptive radiation, a perpetual tendency to branch off
in various directions, and founded on individual varia-
tions of indeterminate character, is seen in the history of
all groups of organisms. The number of possible lines
of development is indefinitely great; the external en-
vironment decides which, if any, shall succeed.

So it is not from the specialised Amphibia that the
reptiles have been developed, but from some early un-
differentiated form ; neither is it from specialised Rep-
tilia that the Mammalia have been derived. And among
the mammals themselves, the Carnivora, Ungulata,
_Cheiroptera (bats), the Cetacea (whales), have not de-
scended the one from the other; but all have diverged

EVOLUTION OF INTELLIGENCE. 109

from some primitive adaptable ancestor. Man himself
preserves many archaic anatomical characters, and the
order Primates, of which he is the highest member,
although its history is still imperfectly known, can be
traced back to primitive Eocene forms. Extreme
specialisation may secure temporary triumph, and in
very uniform conditions even lasting success, but adapta-
bility is the most precious possession, and it is the crea-
tures most ready to meet new and changeable conditions
which have the future before them.

CHAPTER IX
rd
PSYCHOLOGY AND THE EVOLUTION OF INTELLIGENCE

WE have just seen that adaptability is one of the most
useful attributes an organism can possess. In plants it
may be highly developed, but along comparatively simple
lines of direct response. Here adaptability is shown in
the power to respond in different ways to a variety of
stimuli, The case of alpine and lowland forms has
already been mentioned (p. 41), and many others are
familiar to botanists. For instance, the plant Ranun-
culus acquires a very different shape and internal struc-
ture according as it grows on dry land or in water, and
the terrestrial or aquatic form can be developed in the
same individual, by parts above and below the surface
of the water. Also plants which in a normal climate
develop ordinary stems and leaves may in dry or desert
regions assume a very different form ; the stems become
succulent for storing water, and the leaves become re-
duced, spines being ‘often developed in their stead, thus
diminishing the surface whereby water is lost in tran-
spiration. Now these and similar changes are useful
responses enabling the plant to accommodate itself to
varying conditions. This manifold adaptability is not

110 EVOLUTION OF LIVING ORGANISMS.

due, as is so often assumed, to some mysterious power
of automatically responding favourably to stimulus, but
doubtless to the survival of those organisms which re-
spond in the right way. To some extent this power of
adaptation is possessed by all organisms, and by varia-
tion and selection may lead to the establishment of
definite series of responses in several possible directions.
Potentially dimorphic and polymorphic species are thus
produced both in the animal and in the vegetable king-
dom, but while the accommodation of plants is of this
simple and direct character that of animals is usually far
more complex.

On the dangers attending specialisation we have al-
ready dwelt (p. 108). Advantageous as it may be to
acquire a special structure adapted to fulfilling a certain
end with rapidity, sureness and precision, yet animals
have, so to speak, found it more profitable still to avoid
overspecialisation in one or even two or three directions ;
to develop a readiness to respond to all sorts of stimuli,
and a power of storing up the impression of past responses
so as to benefit by “experience”—in animals has been
built up a system of “behaviour” of such a kind that
the character of the final response to the initial stimulus
depends on the number and quality of the responses
previously called forth. This power has been acquired
with the development of elaborate discriminating sense
organs, and a complex conducting and co-ordinating
nervous system.

Now this consideration brings us to the threshold of
the vast domain of Physiological Psychology, which we
cannot fully explore in this little volume. Yet something
must be said about Instinct and Intelligence from the
point of view of a scientific explanation of Evolution, while
taking care not to trespass into the region of Philosophy.*

* The Mechanistic explanation here adopted should not be confused

with Materialism, a discredited system of Philosophy which denies the
existence of anything bui the material or physical.

EVOLUTION OF INTELLIGENCE. ill

It was maintained above (p. 18) that the physico-
chemical processes of life form an unbroken series of
changes ; to these correspond the chain of mental pro-
cesses. And we believe that to every mental process,
whether of the “highest” kind in the mind of man or of
the “lowest” in that of the most primitive organism,
there corresponds some physico-chemical change. How
intimately connected are the two sets of processes is
matter of common knowledge. The slightest disturbance
or interruption in the metabolism of the brain, due to an
injury, an anzesthetic, a poison, will have its echo in a
disturbance of the mental processes. The gradual] elabora-
tion of the sense organs and nervous system found in the
evolutionary series from the lowest forms up to man we
judge to be accompanied “by a corresponding develop-
ment of mental powers. So far as we know, neither
the mental nor the metabolic processes can take place
without the other. Yet, indissolubly bound together
as they are, the one is certainly not the product of the
other, nor can it interfere with the continuity of the
other (p. 18). The mental and the physical series can-
not break into each other’s continuity because they are
not independent of each other ; what exactly isthe nature
of the connection between them Philosophy may attempt
to define, but Science is not called upon to describe. We
may point out, however, that they appear to be two
aspects of one and the same series of events; one seen
from within and the other from without. It is the
artificial separation of these two abstractions, body and
mind, from the reality that has led to endless contro-
versies about the possible action of one on the other. So
in attempting to give a scientific explanation of evolution
we can neither speak of mental processes as produced by
or guided by physico-chemical processes, nor of meta-
bolism as directed along its course by mind. Several
theories of evolution seem ito fall into this error, notably
the “Mnemic” theory recently supported by Semon.

112 EVOLUTION OF LIVING ORGANISMS.

Now, if we only knew the whole series of physico-
chemical changes which correspond to the mental pro-
cesses, the whole story of evolution could be told from
beginning to end from the point of view of the scientific
abstraction as an unbroken chain of metabolic changes.
But unfortunately we are still very far from having
obtained this complete knowledge, except perhaps in
the case of plants; and so, in describing the behaviour
of animals, we constantly pass over from the physico-
chemical to the psychological side, and describe be-
haviour in terms of mental processes we know only in
ourselves, but infer from external evidence to take place
in animals also. The gaps in the physico-chemical
description are thus filled in from the other series. The
physiologist is, however, daily extending our knowledge
of the metabolic processes, and as fast as he advances
he discards the psychical “explanation.” But in doing
so he often ignores the passage from one abstraction to
the other, and erroneously concludes that mental pro-
cesses are insignificant, superfluous, or non-existent.
He imagines he has reduced them to mere physics and
chemistry ; an error into which we must be careful not
to fall. If the whole chain of physico-chemical meta-
bolic processes going on in even the human body and
brain be some day discovered, the mental processes will
still remain untouched and incapable of being described
in the same terms.

The current method of dealing with mental processes,
even morals and emotions, in a scientific description of
evolution is then a makeshift, a provisional but con-
venient method ; so the words feeling, memory, choice,
and will, constantly creep into our descriptions of animal
behaviour—no harm is done so long as their foreign
origin is recognised. Since the two sets of processes
correspond, we need not be astonished at finding that
the “laws” of variation, inheritance and natural selection
hold good in mental as in material evolution.

EVOLUTION OF INTELLIGENCE. 113

Having thus cleared the ground it may at once be
pointed out that instincts correspond to simple adapta-
tions fixed in definite directions ; they are formed of a
series of responses which must succeed each other when
a particular stimulus is applied and reach a predeter-
mined end, as surely as the piece of chocolate is yielded
up by the penny-in-the-slot machine when the right
coin is dropped in. We may pass from the simplest
“reflex action, such as the contraction of a muscle on
the application of a stimulus to the skin, to the most
elaborate instinct found in bees and other social insects,
along a series of such direct chains of reactions of gradually
increasing complexity.

The scientific analysis of the behaviour of animals has
only begun, and as a rule we“can trace but very incom-
pletely the actual physico-chemical processes which
underlie it. But important advances have lately been
made in this direction, chiefly by J. Loeb, who has sug-
gested a physico-chemical explanation of the tropisms.
Tropism is the name given to those simple instincts, the
material out of which the behaviour of the lower animals
is to a great extent composed, which appear as direct
responses to external stimuli. Thus when a moth flies
towards a flame, or a fly to a lighted window, it is said to
be positively heliotropic (or phototropic); when a worm
moves away from the light it is said to be negatively
heliotropic. Similarly Chemotropism leads animals to
move either away from or towards certain substances of
particular chemical composition, as for instance food and
water ; and Geotropism leads them to move either with
or against the force of gravity. Stereotropism is the name
given to the peculiar instinct shown by certain animals
to place as large a part as possible of the surface of their
body in contact with foreign objects, and so induces them
to creep into narrow chinks and crannies. Now although
these tropisms may be built up and specialised by selec-

tion into elaborate instincts, adaptations for special
(2,031)

114 EVOLUTION OF LIVING ORGANISMS.

purposes, they are really not primarily dependent on
specialised sense organs and nervous systems, but are
fundamental properties of protoplasm common to all
animals and plants. They depend on the universal
irritability of protoplasm, based on the instability of the
complex compounds of which it is composed, and their
capacity for being broken down, modified, or raised up
by the action of external stimuli; or, in physiological
language, they depend on changes in the metabolism
(p. 11). A protoplasmic connection between the various
parts of the mechanism, irritability and conductivity
(which is only a special form of irritability), are the only
essentials in the process.

Tropisms have long been known to occur in plants.
Thus shoots being negatively geotropic grow upwards,
while the positively geotropic roots grow vertically down-
wards, A negatively heliotropic plant grows away from
the light, and a positively heliotropic plant towards it,
and so on. These results are brought about by differ-
ences in the metabolism due to the unequal action of
the stimulus on the two sides of the plant structure.
In the case of heliotropism, for instance, the side away
from the source of light grows quicker than that towards
it ; thus curvature is caused, which ceases when the grow-
ing apex points directly to the source of light, and both
sides are equally affected.

Exactly the same thing happens with fixed anima:
like zoophytes. Now Loeb, with great ingenuity, has
applied the same explanation to the behaviour of free-
moving animals, which turn their head towards or away
from the source of a stimulus. For instance, in bilater-
ally symmetrical heliotropic animals the symmetrical
points on the right and left side are equally sensitive to
light ; and when light falls more on one side than on the
other there will be greater chemical activity on one side
than on the other, and the muscles on one side will be
more stimulated so as to move the head round towards or

EVOLUTION OF INTELLIGENCE. 115

away from the source of light. This inequality of muscular
action will last until the animal is so situated as to point
directly to or from the source of the stimulus. Thus when
a moth tends to fly straight into the flame of a candle it is
driven by its mechanism, not by an interfering emotion or
will. If one eye of a moth is darkened, the insect will be
unable to fly straight, but will continue to move in a circle.
Animals are more or less heliotropic according as they
possess more or less chemical substance capable of being
decomposed by light, and their susceptibility will vary
greatly according to temperature and other conditions
which hasten or retard the action. A weak acid added
to the water containing small Crustacea, like Cyclops,
can increase their heliotropigm, or even change the
heliotropism from negative to positive.

Another important element in the behaviour of
organisms is known as “differential sensibility,” or the
reaction to sudden and marked changes in the strength
of the stimulus. A shadow passing across a worm will
cause it to contract; other instances easily suggest
themselves.

In the lower animals, as in plants, behaviour can
be almost completely analysed into manifestations of
tropisms and differential sensibility. Their general —
motions when seeking food; their habit of gathering
together in dark, warm, or damp places; their sexual
instincts; their instincts connected with reproduction
(the laying of the eggs on appropriate substances for the
young to feed on, and so forth), can all be interpreted as
corresponding to metabolic changes, though all the steps
in the physico-chemical processes are not yet known to
us. Interesting is the “instinct” shown by the plant
Limaria cymbalaria. At first positively heliotropic, it
becomes negatively heliotropic after pollination, and so
pushes its fruits into dark crevices, lodging them in
places favourable for the subsequent germination of the
seeds. |

116 EVOLUTION OF LIVING ORGANISMS.

Instincts, or mental adaptations, are “hereditary ”
in the popular sense (see p. 42); that is to say, will
reappear in succeeding generations under normal con-
ditions, since they correspond to metabolic processes
which depend on the interaction of certain constant.
factors of inheritance with certain factors present in
the environment. They vary, and in all probability are
built up and preserved by natural selection according to
their usefulness in the struggle forexistence. But simple
reflexes and tropisms need not be useful; on the con-
trary, as in the case of variation generally (p. 77), they
may be useless 6r even injurious. Moreover, a tropism
which is useful in the normal environment may be fatal
under other circumstances—e.g. the moth and the candle.
In so far as they have been built up by selection, in-
stincts. will be found to be advantageous. Apparent
exceptions may be due to their being relics of past
adaptations fallen into disuse, or to some change in the
environment. What we have said in previous chapters
about. the development of the structural characters of
organisms applies equally to the evolution and specialisa-
tion of the metabolic processes to which the mental
characters correspond.

Sequences of interlocking and co-ordinated reflexes,
each one of which sets off the next, give rise to the most
complicated instinctive behaviour, as we see in the
actions of our own internal digestive organs; but in
the more elaborately differentiated. mstincts of animals
yet another element can be found, known as “ associative
memory.” For example, the homing instinct of insects,
enabling them to find their way back to the nest from
a distance, has been shown to be due to the retention of
effects produced by previous responses. to visual stimuli.
This lasting and cumulative effect. of responses to stim-
ulus enters very largely into the behaviour of the higher
animals and of man, but we know very little indeed of
the corresponding physico-chemical processes in such cases.

EVOLUTION OF INTELLIGENCE. 117

This cumulation of the effects of previous responses,
this formation of internal stimuli reacting on the be-
haviour is, of course, no new factor in the development
of organisms (p. 56). It is the epigenetic factor, the
means of differentiation in the ontogeny of the individuai.
Thus an organism will respond differently according as it
is starved or well-fed. The storage of nutriment in the
tissues of a plant provides an internal stimulus affecting
its response ; so a hungry animal will behave differently
from one that is satiated. In fact, the whole behaviour
of an organism is determined by the condition of the
mechanism at the time the stimulus is received, and this
again depends on the responses previously called forth.
It should never be forgotten, that an organism reacts as
a whole, each response beitfe closely or remotely related
to every other and to the entire organisation of the
organism,

Intelligent behaviour, from the physiological point of
view, is prompted by indirect responses. Influences are
brought to bear on the course of metabolic changes
diverting them in this or that direction, in a manner
which cannot be foreseen by an outside observer who is
not acquainted with the entire past history of the animal.
The final response may then appear to be spontaneous,
and even unrelated to the initial stimulus. The influences
above mentioned are, besides the external stimuli or
factors of the environment, either due to the transmitted
factors of the inheritance or the internal stimuli caused
by the lasting effects of previous responses.

Lastly, we are still in the dark as regards the evolution
of consciousness, the highest stage in the development of
the mental processes, for our knowledge of the ana-
tomical structure and physico-chemical processes which
accompany it is still too incomplete to enable us to de-
termine when it first made its appearance in the animal
series. We can tell approximately at what stage in the
ontogeny of individuals of our own species consciousness

118 EVOLUTION OF LIVING ORGANISMS.

begins to manifest itself as the tissues become differen-
tiated, but as to its phylogenetic origin, we can only say
that it appeared when the cerebral hemispheres reached
a high state of development.

Too often the Darwinian doctrines are represented as
teaching that success in the struggle for existence is
obtained only by tooth and nail, by blood and iron.
This is a very mistaken view. Brutality, fraud, greed
may secure temporary success; but the triumph of the
human race over the lower organisms, and again of the
lower civilisations over the higher, has been brought
about, on the contrary, through mutual help, co-operation,
self-sacrifice. These are the very bonds which hold
societies together. Religion, art, and science all play an
important part in evolution ; and morality appears not as
an external force working against a ruthless and unmoral
Cosmic Process, but as a prcduct of that very process,
and an all-important factor in its development. In the
iong run, it is those civilisations which are founded on
justice and liberty, on law and order, which will succeed
best and last longest.

Man has conquered in the struggle for existence not so
much because his body is more powerful, his movements
quicker, or his senses sharper than those of other animals,
but because of his great capacity for retaining the im-
pressions of past responses, and for bringing them to
bear on the response to new stimulations. To this he
owes his marvellous powers of adaptation to new and
varying conditions. And this great development of
associative memory has been accompanied by a corre-
sponding enlargement of the brain, especially of the
cerebral hemispheres. There can be little doubt that
the giant mammals of past ages failed, in spite of their
formidable offensive and defensive weapons, partly be-
cause their nervous system was not sufficiently de-
veloped. Their brain was absurdly small as compared
with that of ordinary mammals of to-day; indeed, it

EVOLUTION OF INTELLIGENCE. 119

was often scarcely larger than the brain of the gigantic
reptiles which preceded them. It need hardly be
pointed out that man owes his mastery of the world to
brain-power, not brute force. Most of the peculiarities
in the structure of the human body are closely related to
the immense development of the brain. The size and
shape of the brain-case has greatly influenced the de-
velopment of the skull; and even the erect attitude and
consequent modifications of the hind limbs, vertebral
column, and cther parts are probably an adaptation for
supporting the heavy weight of the brain. In other
respects the structure of man is not very specialised, and
differs but little from that of his nearest allies, the
anthropoid apes. ;

Evolution from the scientific point of view, as it ap-
pears to an outside observer, may be represented as a
vast and continuous series of changes in a continuous
stream of living matter. Each stage in the process is
determined by that which precedes it, and determines
that which follows. The scientific generalisations based
on the observation of non-living matter, the “laws” of
physics and chemistry, hold good when applied to the
changes in living matter. There is no special living
element, no mysterious life-force ; but life, scientifically
described, is a physico-chemical process taking place in
the complex compounds of the ordinary elements which
make up the protoplasm of living organisms. Meta-
bolism, as this process is called, is due to the instability
of these compounds, to the fact that external conditions
or stimuli applied to them may alter their structure and
composition. All the manifestations of life thus corre-
spond to metabolic processes. From the very first
origin of living protoplasm there has been an unbroken
continuity of living processes and substance. The living
organisms of the present day are the descendants of those
of the past, and all the various forms of life are but the
divergent streams from the original source of metabolism.

120 EVOLUTION OF LIVING ORGANISMS.

Variation is due to change in the metabolism; all
organisms are the products of the interaction of trans-
mitted factors of metabolism (the factors of the in-
heritance) and factors of the environment. The ever
changing and continuous stream of living matter is
diverted into this or that channel of differentiation and
specialisation by the environment through natural selec-
tion. The environment moulds the organism, and the
organism reacts on its environment, until in the case of
man he seems to have become master of it and to shape
his own destiny.

A COURSE OF READING ON EVOLUTION

BzsipEs the standard works of Darwin, The Origin of
Species, The Descent of Man, and Animals and Plants
under Domestication, the reader may be recommended to
consult the following books from among the vast number
written on the subject :

For the treatment of the evolution of organisms in
general: A. R. Wallace, Darwinism, 1889 ; A. Weismann,
The Evolution Theory, 2 vols. 1904. For metabolism and
general physiology: M. Verworn, General Physiology,
1899; and for the cell-theory, E. B. Wilson, The Cell,
1900, in addition. For a short general study of variation :
H. M. Vernon, Variation in Animals and Plants, 1903;
and for the statistical treatment of variation and hered-
ity, F. Galton, Watural Inheritance, 1889; and K.
Pearson, The Grammar of Science, 1900. For mendelism :
W. Bateson, Mendel’s Principles of Heredity, 1909 ; and
R. C. Punnett, Mendelism, 1911. For animal tropisms,
instincts, and intelligence: J. Loeb, Comparative Physi-
ology of the Brain, 1905; and C. Lloyd Morgan, Animal
Behaviour, 1900. Further, the student should read P.
Geddes and J. A. Thomson, The Evolution of Sex, 1889 ;
T. H. Morgan, Experimental Zoology, 1907 ; H. de Vries,
The Mutation Theory, 2 vols. 1910-11; A. Reid, The
Principles of Heredity, 1906. E. B. Poulton, Essays on
Evolution, 1908 ; and Sir E. Ray Lankester, The Advance-
ment of Science, 1890, and The Kingdom of Man, 1906,
which contain discussions of many important problems

in Evolution.
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INDEX

ACHATINELLA, 82.

“‘acquired characters,” 37.
adaptation, 64, 77, 110.
adaptive radiation, 96, 108.
Agama, 91.

albinism, 50.

Allelomorphic characters, 50.
Amphibia, history of orders, 96.
Amphidasys betularia, 68.
Amphisbzna, 91.

anabolic processes, 13.
ancestral inheritance, 72.
Andalusian fowl, 48, 76.
Anopheles, 67.

Antirrhinum majus, 47.
Archzopteryx, 90.
associative memory, 116.

Bacteria, 23, 45.

9.
Bateson, W., 46, 50, 121.
Beecher, 107.
behaviour, 112, and sequel.
Bergson, 92.
Biology, 10.
Bonnelia, 60.
Bonnier, 41.
Botany, 10.
Brachiopods, 106-107.
Bumpus, 68.
Biitschli, 29.

CARBOHYDRATES, 10.

Castile, 76.

Celebes, 82.

cell, the, and the cell-theory, 22-27.
—— division, 25.

Cesnola, Di, 79.

characters, eMac o

—— acqui 7.

— allelomorphic, 50.

chlorophyll, 14.

chromosome, 26.
chromatin, 22, 25.
Cirripedes, 87.

Clausilia laminata, 69.
classification, 92, and sequel.
Comparative Anatomy, 87.

#| consciousness, 117,

conservation. of energy and of
matter, 12.

continuity of protoplasm, etc., 25.

convergence, 91.

correlation, 72, 78.

Correns, 47.

Cyclops, 115.

Darwin, Ch., 32, and sequel, 46, 55,
60, 65, 67, 73, 76, 84, 121.
Darwin, Erasmus, 37.
Darwinism, 32, 37, 63.
—— criticism of, 64, and sequel, 89.
— 80: selective death-rate, 66,
0.

degeneration, 87.

‘Dinosauria, 99.

disease, 69, and sequel.
domestic races, 52, 73.
dominant 1a WAR 49,
Doncaster, 46.

Draba verna, 93.
Drosophila, 58.

ELIMINATION, 63, 95, 108.
—— selective, 63, 68.
Elodea canadensis, 62.
Embryology, 87.

energy of life, 11.
epigenesis, 56.

evolution as unfolding, 56.

Factors of environment, 37, 41.
—— of inheritance, 37, 42, 55.
fats, 10.

ferments, 15.

fertilisation, 28.

123

124

GALAPAGOS, 82.

Galton, F., "34, 37, 72, 121.

gametes (germ- cells), 27 :

Geddes, P., and Thomson, J. A.,
21

gemmation, 27.
germ-cells, 27, 29.
germ-plasm, 30.
Giard, 59.

Glossina morsitans, 67.
green-plants, 14.

HEREDITY and inheritance, 35, and
uel.

—— theories of, 46, and sequel.

hermaphrodite, 28,

heterozygote, 48.

Hieracium, 93.

Hilgendorf, 107.

homozygote, 48.

Hyatt, 107.

Hydra, 24,

Tamuoniry, 70.

Inachus, 59.

instincts, 113, 115.
intelligence, 117.
intercrossing, eyeupoe by, $0.
irritability, 14-17,

islands, fauna of, "32,

isolation, geographical, 81.
—— physiological, 82.

JENNINGS, 76.
Johannsen, 44.
Jordan, 93.

KARYOKINESIS, 25.
katabolic progesses, 13.

LAMARCK, 37, 6.
Lankester, E. R., 20, 121.
Leptinotarsa, 58, "69.

life and living matter, 9, and sequel.
— conditions, 15.

—— nature of, 15.

——- artificial, 19.

—— origin of, 19.

—— length of, 31.
Linnzus, 61, 92.

Loeb, J., 113, 124, 121.
Lymaniria dispar, 62.

MAMMALTA, 101.
—— origin of and orders, 101.
man, 109, 118.

INDEX.

Mantis religiosa, 79.

Mendel, 46, 49.

mendelism, 46, andjsequel.

mental processes and mind, 110,
and sequel.

metabolism, 11, and sequel.

Metaphyta, 24

Metazoa, 24.

mimicry, 91.

modification, 39.

—— not transmitted, 30, and sequel.

Monocystis, 29.

Morgan, C. Lloyd, 121.

Morgan, T. H., 58, 76, 121.

morphology, 87.

Mucor, 24.

Miller, Fritz, 79.

Musca carnaria, 61.

mutation, 39, and sequel.

—— retrogressive, 53.

—— progressive, 54.

—— origin of, 55.

Nasturtium officinale, 62.
natural selection, 63, and sequel.
Neumayer, 107.
Nilsson-Ehle, 75.
nucleus, 22.

Nucula, 106.
OSBORN, 96.

> 20s

PALZONTOLOGY, 87, 95, and ‘sequel.
Paludina, 107.
Pangenesis, 46.
Paramecium, 29.
parasitism, 82, 88.
parthenogenesis, 28,
Passer domesticus, 68.
Pasteur, 20.

Patella, 106.

Pearson, K., 34, 35, 72, 78, 121.
Peckham, 86.

Petaurus, 91.

Phaseolus vulgaris, 44,
Phyla and Phylogeny, 87.
Phylloxera, 62.

Pisum sativum, 49.
Planorbis, 108.

Poulton, E. B., 79, 121.
Primula sinensis, ‘42.
Pritchard, 37.

proteins, 10.

Protophyta, 24.
protoplasm, 13.

—— formation of, 13-16.

ee “oan eS

INDEX, | 125

Protozoa, 24.
psychology, 110.
Pteromys, 91.

Punnett, R. C., 46, 121,
pure lines, 45, 71, 93.

QueTeLer, 34.

Ranuncuuus, 109.

recessive character, 49.

reflex action, 113.
regeneration, 24.

regression, 73.

Reid, A., 69, 121.
reproduction, 27, and sequel.
—— asexual and sexual, 27.
—— of cells, 25.

Reptilia, higtony of orders, 97.

SAccuLINA, 59,

Salamandra, 91.

Sarasin, 82.

segregation of factors, 48.
selection, 70, 73.

—— value,

Semon, 111.

Sex, 2 27, and sequel.

ae determination of, 59.
sexual characters; secondary, 83.
—— selection, 84, and sequel.
Siphonops, 91.

soma, 30.

specialisation, 24, 105, 110.
species, 92,

—— ee 93.

Smith, G. W.,

spontaneous Petiin 19.
sterility, 83, 93.

stimuli, or conditions of environ-
ment,, 16, 37:

struggle for existence, 60, and
sequel.

TARAXACUM, 415, 93:
pti’ 58, 69.

rifolium pratense, 67.
Trilobites, 107.
tropisms, 113.
Trypanosoma Brucei, 6%
Tschermak, 47.
Tunicates, 87.
Typhlops, 91.

UNGULATA, 163,
unit characters, 49, 54.
—— factors, 49, 54.

VARIATION and variability, 33, and
sequel.
—— continuous: and: discontinuous,

» 93.
Vaucheria,. 24:

Verworn, M., 121%,

vestigial organs, 83.
Vitalism,

Vries, De, 35, 47, 54, 93, 127.

Wauvacr, A. B.,, 32, 61, 67, 71, 79,
86, 121.

Watson, J., 34:

Weismann, 30, 37, 46, 55, 121,
Weldon, W., F. R., 34, 69.
Wilson, EB. B., 59, 121.

ZooLoey, 10.
Zygote, 27.

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