EVOLUTION AND ANIMAL LIFE

PREFATORY NOTE

IN the present volume the writers have tried to give a lucid
elementary account, in limited space, of the processes of evo-
lution as they are so far understood. We have turned to ani-
mals for illustrative purposes, nearly to the exclusion of refer-
ences to plants, simply because both authors are zoologists
and bave made use of the facts most familiar to them.

The book is composed primarily of the substance of a univer-
sity course of elementary lectures delivered jointly by the
authors each year to students representing all lines of college
work. This fact, and the desirable limiting of the book to a
convenient size for the general reader and student, account for
the extremely laconic treatment of various important moot
points concerning the evolution mechanism, and for the omission
of certain discussions which otherwise might well have been
included. But on the whole the authors feel that the interested
general reader will find this small volume a fairly comprehensive
introduction to our present-day knowledge of the factors and
phenomena of organic evolution.

To the general reader we may perhaps with propriety ad-
dress the following words, used to the students in the opening
lecture of the course :

We cannot talk long without saying something others do
not believe. Others cannot talk long without saying something
we do not believe. We wish you to accept no view of ours
unless you reach it through your own investigation. What we
hope for is to have you think of these things and find out for

yourselves.

D. S. J.
V. L. K.

LKLAND STANFORD JUNIOR UNIVERSITY,
March 30, 1907.

TABLE OF CONTENTS

CHAPTER I.— EVOLUTION DEFINED.

Organic evolution and bionomics, 1; Meaning of evolution, 2;
Encasement theory, 2; Theory of epigenesis, 3; Evolution of the
species or transmutation, 3; Cosmic evolution, 4; Spencer's form-
ula of evolution, 5; Biologic evolution and oosmic^yj2lujtioja.not
the same, 6; Usefulness of the term bionomics, 7; The flux of na-
ture, 7; Comprehensiveness of the science of organic evolution, 8;
The immanence and permanence of law, 9; Evolution not neces-
sarily progress, 10; Theory of descent, 10.

CHAPTER II.— VARIETY AND UNITY IN LIFE.

Range of variety, 12; Meaning of species, 13; Number of species,
14; Extinct species, 16; Changes of species with time and place,
18; Variety in life a factor in the history of the globe, 21; Unity
in life, 22.

- CHAPTER III. — LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRES-
SION.

Live things and lifeless things, 25 ; The basic distinction between
life and non-life, 26; Protoplasm, 26; Chemical make-up of proto-
plasm, 27; Physical make-up of protoplasm, 28; The cell, 30; The
simplest animals, 32; Differentiation and animal types, 32; The
genealogical tree, 36; Primary conditions of life, 38; Origin of life,
41: Spontaneous generation, 42; Where did life begin on the
earth, 47.

CHAPTER IV. — FACTORS AND MECHANISM OF EVOLUTION.

The fact of descent, 48; Darwinism not synonymous with de-
scent, 49; Factors in descent, 49; Variation, 50: Selection, 51;
Prodigality of production, 52; Heredity, 53; Isolation, 53; Muta-
tion, 54; Orthogenesis, 55; Lamarckisrn and inheritance of ac-
quired characters, 55; Adaptation, 56.

vii

viii TABLE OF CONTENTS

CHAPTER V. — NATURAL SELECTION AND STRUGGLE FOR EXIST-
ENCE: SEXUAL SELECTION.

Natural selection the chief determining agent in adaptation, 57;
Adaptation to conditions of life, 58; The crowd of animals, 59;
Reproduction by multiplication, 59; Numbers of individuals al-
most stationary, 60; Struggle for existence, 60; Discriminate
death, 61; Natural selection, 62; Interdependence of species, 63;
Animal and plant invasions, 64; Doctrine of Malthus, 67; Limits
to the capacity of natural selection, 68; Survival of the existing,
69; Actual standing of Darwinism, 70; Secondary sexual dif-
ferences, 71; Classification of secondary sexual characters, 72;
Theory of sexual selection, 75; Criticisms of the theory, 77; The
sexual selection theory largely discredited, 78.

CHAPTER VI.— ARTIFICIAL SELECTION.

Natural selection and artificial selection, 80; Steps in the pro-
duction of new races, 81 ; Selected traits quantitative, 81 ; Race
traits qualitative, 84; Hybridization, 88; Plant amelioration, 90;
Work of Luther Burbank, 90; Panmixia, or cessation of selection,
104; Reversal of selection, 104; Transmission and heredity, 105;
Artificial selection and natural selection, analogous processes,
106; Race-forming by sports, 107.

CHAPTER VII. — VARIOUS THEORIES OF SPECIES-FORMING AND
DESCENT CONTROL.

Segregation of isolation, 108; Geographic and physiologic iso-
lation, 109; Romanes's championship of physiologic isolation, 109;
The Lamarckian theory of species-transformation, 111; Ortho-
genetic evolution, 112; Species-forming by mutation, 114; The un-
known factors of evolution, 115.

CHAPTER VIII. — GEOGRAPHIC ISOLATION AND SPECIES-FORMING.

Migration and faunal distribution, 117; Closely related species
not found in the same region, but in contiguous regions, 120; The
American warblers, 120; Barriers, 122; The Hawaiian Drepanidse,
124; Adaptive and non-adaptive characters, 127; The American
orioles, 128; Species traits not necessarily useful, 129; The persist-
ence of the sufficiently fitted, 130.

• CHAPTER IX.— VARIATION AND MUTATION.

Actuality and extent of individual variation, 131 ; Darwin's
laws of variation, 137; Quetelet's determination that fluctuating
variation follows the law of probabilities, 140; Discontinuous varia-

TABLE OF CONTENTS IX

tion, 141; Discontinuous and continuous variation, 141; Congeni-
tal and acquired variation, 142; Determinate variation, 150; The
causes of variation, 154, 156; Variation as related to amphimixis
and parthenogenesis as mutations of de Vries, 154.

^CHAPTER X.— HEREDITY.

Hereditary variancy defined, 163; Atavism or reversion, 166;
Telogony, 166; Prenatal influences, 167; Not all transmission is
heredity, 168; Determination of sex, 170; Homologies and analo-
gies, 172; Vestigial organs, 174; Significance of vestigial organs,
181; Heredity and its "laws," 181; Galton's law of ancestral in-
heritance, 184; Mendel's law of alternative inheritance, 187;
Modification of Mendelism, 188.

M^ CHAPTER XI. — INHERITANCE OF ACQUIRED CHARACTERS.

The Lamarckian principles of evolution, 196; Neo-Lamarckism
and Neo-Darwinism, 197; Acquired characters, 198; Effects of use
and disuse, 199; Environmental modifications not inherited, 200;
Examples of non-inheritance of acquired characters, 201 ; Heredity
unproved, 203; Convergence of characters and parallelism, 204;
Actual effects of environment, 205 ; Ontogenetic species, 206.

CHAPTER XII. — GENERATION, SEX, AND ONTOGENY.

Generation and ontogeny, 211 ; Spontaneous generation or abio-
genesis, 212; Simplest modes of generation, 213; Parthenogenesis,
215; Differentiation of reproductive cells, 217; Simplest many-
celled animals, 218; Effects of sex, 220; Sex dimorphism, 221;
The life cycle, 223; The egg, 224; Numbers of young, 225; Em-
bryonic and post-embryonic development, 227; Developmental
stages, 229; Continuity of development, 231; Metamorphosis or
apparent discontinuity, 234; Significance of facts -of develop-
ment, 234; Divergence of development, 234; The duration of
life, 240; Death, 241.

CHAPTER XIII. — FACTORS IN ONTOGENY, AND EXPERIMENTAL
DEVELOPMENT.

Processes in ontogeny, 244 ; Extrinsic and intrinsic factors, 245 ;
Mechanism versus vitalism, 246; Functions of protoplasm, 247;
Ultimate structure of protoplasm, 248; Theories of organic units,
250; Cell division, 251 ; Mitosis, or karyokinesis, 252; Somatic and
germ tissues, 257; Reproduction in protozoa, 260; Maturation,
264; Fertilization, 267; Cleavage, 269; Reduction of the chromo-
somes, 269; Preformation versus epigenesis, 276; Examples giving

X TABLE OF CONTENTS

evidence for each, 278; Mechanism versus vitalism, 281 ; Artificial
parthenogenesis, 283; Regeneration and regulation, 285.

i
CHAPTER XIV.— PALEONTOLOGY.

Fossils and theL' significance, 289; Fossil-bearing rocks and
their origin, 292; Geological epochs, 296; Conditions of extinct
life, 297; Divergent types and synthetic types, 299; Parallelism
between geologic and embryonic series, 300; Orthogenesis, 301;
Significance in evolution of the facts of paleontology, 301 ; Dur-
ation in time of species, 302; History of the vertebrates, 305;
Man, 307.

CHAPTER XV. — GEOGRAPHICAL DISTRIBUTION.

Zoogeography, 309; Relation of species to geography, 311;
Laws of distribution, 314 ; Species debarred by barriers, 315 ;
Species debarred by inability to maintain their ground, 315;
Species altered by adaptation to new conditions, 315; Effects of
barriers, 316; Faunas and faunal areas, 316; Remains of animal
life, 322; Subordinate remains of provinces, 323; Faunal areas of
sea, 323 ; Analogies between language and fauna, 325 ; Geographic
distribution and the theory of descent, 326.

CHAPTER XVI.— ADAPTATIONS.

The principle of fitness and general adaptations, 327^ Origin
of adaptations, 327; Types and classification of species adapta-
tions, 328; Adaptations for food-securing, 329; Adaptations for
self-defense, 330; Adaptations brought about by rivalry, 331;
Adaptations for defense of young, '338; Special adjustments to
surroundings, 343.

CHAPTER XVII. — PARASITISM AND DEGENERATION.

Parasitism defined, 347; Kinds of parasitism, 348; Simple
structure of parasites, 350; Gregarina, 351; Parasitic hemospor-
idia: the cause of malarial fevers, 351 ; Tapeworm and other flat
worms, 354 ; Trichina and other round worms, 355 ; Sacculina, 358 ;
Parasitic insects, 359; Parasitic vertebrates, 361 ; Parasitic plants,
362; Degeneration through quiescence, 363; Degeneration through
other causes, 363; Immediate causes of degeneration, 366; Ad-
vantages and disadvantages of parasitism and degeneration, 367.

CHAPTER XVIII. — MUTUAL AID AND COMMUNAL LIFE AMONG ANI-
MALS.

Man not the only special animal, 369; Animal societies, 369;
Commensalism, 370; Symbiosis, 373; Symbiosis between animals

TABLE OF CONTENTS xi

and plants, 376; Social life, gregariousness, 380; Solitary and com-
munal bees and wasps, 383; The honey-bee community, 387; Ants,
391; Termites, 3'.)4: Division of labor the basis of communal life,
395; Advantages of communal life, 397.

CHAPTER XIX.— COLOR AND PATTERN IN ANIMALS.

Color among animals, 398; Protection by color, 400; Protection
of color, 402; Significance of color and pattern, 404; Table of in-
sect colors, 405; General protective resemblance, 406; Variable
protective resemblance, 407; Special protective resemblance, 411;
Warning colors, 416; Terrifying appearances, 418; Directive col-
oration, 419; Recognition marks, 420; Mimicry, 421 ; Criticism and
general considerations of the theory of protective and mimicking
color pattern, 424.

CHAPTER XX.— REFLEXES, INSTINCT, AND REASON.

Irritability, 426; Nerve cells or fibers, 427; Brain or sensorium,
427; Mechanical reflexes, 428; The tropism theory, 429; The
theories of the method of trial and error, 429; Instincts, 430; In-
stincts of feeding, 432 ; Instincts of self-defense, 433 ; Instinct of
play, 435; Climatic instincts, 436; Environmental instincts, 438;
Instincts of courtship, 438; Instincts of reproduction, 439; In-
stincts concerned with the care of the young, 439; Variability of
instinct, 442; Reason, 443; Mind, 448.

CHAPTER XXL— MAN'S PLACE IN NATURE.

Post-Darwinian conception of humanity, 452; Man's place
among the other animals, 453; Classification of the primates, 455;
Evidences from comparative anatomy of man's relation to lower
animals, 456; Special physiological evidence, 457; Evidence from
embryology, 460; Evidence from paleontology, 461; Conclusions
from ethnology, 462; The earliest man, 464- The genealogy of
man, 466; Theology and Darwinism, 467.

EVOLUTION AND ANIMAL LIFE

CHAPTER I
EVOLUTION DEFINED

Grau, theurer Freund, 1st alle Theorie,
Und grim des Lebens gold'ner Baum.

— GOETHE.

Men of science repudiate the opinion that natural laws are rulers
and governors of nature, looking with suspicion on all " necessary " and
universal laws. — BROOKS.

THIS volume treats of the elements of the science of Organic
Evolution. To this science belongs the consideration of the
forces which govern the changes in organisms. It includes the
influences which control development in the individual and in
the species which is the succession of individuals, together with
the laws or observed sequences of events which development
exhibits. From another point of view, this is the science of
life — adaptation. The term Bionomics (j&'os, life, vo/xos, order
or custom), first suggested by Prof. Patrick Geddes, is essen-
tially equivalent to the older term Organic Evolution, the
science of the facts, processes, and laws involved in the mutation
of organisms. For many reasons, this new name, Bionomics,
with its technically exact meaning, should be preferred to the
phrase Organic Evolution, as, unlike the latter, it involves no
philosophic assumptions.

That organs and organisms do change from day to day, and
place to place, and from generation to generation is an observed
fact, which now admits of no doubt. The orderly arrangement
of our knowledge of this process constitutes a branch of science.
To use the word evolution in regard to this process is to use a
philosophic term in connection with a group of scientific facts.
For the word evolution means unrolling. It carries the thought

1

2 EVOLUTION AND ANIMAL LIFE

that something which was previously hidden is now brought to
light. This leads naturally to the philosophic suggestion that
whatever is evolved must be previously involved. This may
be true as a matter of words, but not necessarily so as a matter
of fact, unless we reduce these words to the simple meaning
that the actual now must have been the possible before; what-
ever actually takes place was a possibility before it happened.

The word evolution, then, belongs to philosophy rather than
to science. In the philosophy of nature the idea that present
conditions are brought about through unrolling or unveiling
has had a long existence. The word evolution has been fre-
quently applied to the process of growth and maturity of the
individual animal or plant, and again to the process of deriva-
tion of species from ancestral organisms, and again to the pro-
gressive changes in the forms of inorganic bodies, as planets
or mountains. Each one of these meanings is essentially dis-
tinct from the others, and each is distinct from the theory
of evolution which existed in the dawn of biological science.
When men first began to notice the changes in the animal
embryo, through which, from the formless egg, little by little,
the individual was built up, becoming at each stage of the
process larger, mora specialized, and more like the parent
from which it sprang ,~ it was natural to regard this process as
an unrolling. It wafe natural, too, to suppose that the egg
was not really formless, but that the beginnings of each part
of the final organism existed within it in fact, if we could
see them. Hence evolution took the form of a theory of
encasement. Men imagined that the egg of the chicken con-
tained a minute chicken, and that within this chicken were
the germs of the eggs the future hen would bear; and again,
that encased within each of these eggs was an endless series of
the eggs and chickens of all the future. In like fashion, -men
conceived that in the small human egg were the bodies and
gmbryos of countless future generations. In some theories,
this idea of encasement was applied not to the egg, but to the
male germ, the homunculus or minute man in whom the gener-
ations of the future were enfolded and from which they un-
rolled.

The perfection of the microscope as an instrument of pre-
cision did not verify these theories of encasement. The egg
still appeared essentially formless, a mass of undifferentiated

EVOLUTION DEFINED 3

protoplasm, or at least without traceable lineaments of the
future embryo. It was a single cell, apparently essentially like
any other cell, a single one of the units of structure of which
living organisms are made.

Thence arose the theory of upbuilding or epigenesis (eTri,
upon, yeVeo-is, birth) of organisms, by the addition of cell upon
cell, to the original germ or egg. Each egg cell by segmentation
divides into two daughter cells, and these, through the influence
of heredity, naturally arrange themselves so that a new organ-
ism is formed similar to the parent organism. It was recognized
that the form was predetermined by the ancestry, but no
longer that the embryo was literally released from encasement
within the structure of the egg. The evolution of the individual
is thus conceived as the realization of an hereditary tendency.

But "hereditary tendency" is again a metaphorical ex-
pression. In biology, we know no "influence" or "tendency"
which is not localized somewhere. Any act or modification of
an act is a function of some particular organ. To account for
the likeness involved in the facts of heredity, we must expect
to find some form of organic mechanism.

Such mechanism must exist within the germ cell itself, and
its existence as the "physical basis of heredity" is now well
established. In a later chapter we shall discuss the nature
of this physical basis, the structures within the nucleus of the
germ cell which control or preside over the development of the
individual. From our knowledge of the operation of the cell
in heredity we recognize the facts of epigenesis, and with these
a theory of individual evolution, much more subtle than the
old theory of encasement.

We may therefore still imagine the maturing of the individ-
ual organ as a process of evolution, or unrolling, of the
hereditary plan as hidden in the structure of its cells. We may
also speak of the same process as a development. To envelop
is to make snug. Development is its opposite. To develop
is to make free or independent.

From the evolution of the individual it is natural to extend
the use of the word evolution or the word development to the
changes which characterize the history of a species or other
group of animals or plants, a process which has also been called
transformism or transmutation. This word transmutation de-
scribes the process more literally than either evolution or

4 EVOLUTION AND ANIMAL LIFE

development. That species do change their structure with
time or with space is a matter of common scientific observation.
With the lapse of time, generation following generation, direct-
ive influences combine to modify the line of descent. Witli
the separation of individuals by barriers of land and water
and varying climate, differing lines of descent are brought into
existence. The fact of descent with modification large or small
is a matter of common knowledge in the biology of to-day, veri-
fied in the hundreds of thousands of species of organisms now
known and classified. To call this transmutation of species is
but to state the fact. To call it evolution is to suggest a theory
that all these changes are but the unrolling of the plan — a move-
ment toward some predetermined end. That this is true we have
no means of knowing, and the results as they appear to us seem
to be determined by proximate causes alone. Among these
proximate causes are differences in structure and in degrees of
adaptability among individuals, the operation of the rule of
the survival of the best adapted, the inheritance by individuals
of the traits of the immediate ancestry, and the effects of cli-
matic changes, and of migrations hampered and unhampered
by the presence of physical barriers. The effects of influ-
ences like these are considered by most writers as the es-
sential elements in "organic evolution." But a few writers
give external influences a secondary place, confining the term
evolution solely to the results of causes resident within the
individual.

Speaking broadly we find as a fact that transmutation of
species through the geologic ages has been accompanied by
increasing divergence of type, by the increased specialization
of certain forms, and by the closer and closer adaptation to
conditions of life on the part of the forms most highly special-
ized, the more perfect adaptation and the more elaborate
specialization being associated with the greatest variety or
variation in environment. Accepting for this process the name
of organic evolution, Herbert Spencer has deduced from it the
general law that as life endures generation after generation, its
character, as shown in structure and function, undergoes con-
stant differentiation and specialization. In this view, the
transmutation of species is not merely an observed process, but
a primitive necessity involved in the very organization of life
itself.

EVOLUTION DEFINED 5

A process of orderly mutation is observed not only in living
things but in inanimate objects as well. The features of the
surface of the earth pass through a slow process of unrolling—
from primitive chaos to the diversified earth of to-day. Mani-
festly we cannot imagine a homogeneous earth which could
forever retain its homogeneous condition. At least our universe
and our earth have not done so. A cooling earth must lose
its perfect rotundity, its surface must become diversified, its
relation to the sun must cause its equator to differ from its
poles. A single homogeneous form of life on this earth could
not remain uniform because it would be thrown under varying
conditions. It could not be the same under the tropical sun as
under the arctic cold, and the individuals adapted to either
would tend to reproduce individuals likewise adapted. There
must, then, exist in all things a " tendency " to become special-
ized and differentiated. In accordance with this tendency, it is
conceived that nebulous masses have been concentrated into
planets and the generalized creatures of geologic time have been
succeeded by variant and specialized forms, their lineal de-
scendants.

The universal formula of the process of evolution is com-
pactly stated by Herbert Spencer in these famous words:

"Evolution is a continuous change from indefinite incoherent
homogeneity to a definite coherent heterogeneity of structure and
function, through successive differentiations and integrations. In
its physical aspect evolution is further an integration of matter with
concomitant dissipation of motion."

This formula applies more or less to all forms of orderly
change, that is, change due to a persistent cause, a continuous
force. Thus solar systems are conceivably formed from nebulae.
Thus continents and mountain chains, islands and river basins
are shaped. Thus organisms are derived from parent organisms.
Thus all the variant chemical elements may have been (hypo-
thetically) derived through influences as yet not even imagined,
from the unknown and probably unknowable primitive element,
protyl. The general movement is from the simple to the
complex, from the homogeneous to the heterogeneous, from
the inexperienced to the experienced, from the undivided to
the divided, from the inchoate to the integrated. Whatever
2

6 EVOLUTION AND ANIMAL LIFE

happens in time or is encountered in space promotes evolution.
But the kind of evolution thus produced is very different in
different kinds of objects.

Biological evolution and cosmic evolution are not the same.
From the biological side a certain objection must be made to
this philosophical theory of universal or cosmic evolution. In
organic and inorganic evolution there is much in common so
far as conditions and results are concerned; but these likenesses
belong to the realm of analogy, not of homology. They are not
true identities because not arising from like causes. The evo-
lution of the face of the earth forces parallel changes in organic
life, but the causes of change in the two cases are in no respect
the same. The forces or processes by which mountains are
built or continents established have no homology with the
forces or processes which transformed the progeny of reptiles
into mammals or birds. Tendencies in organic development
are not mystic purposes, but actual functions of actual organs.
Tendencies in inorganic nature are due to the interrelations of
mass and force, whatever may be the final meanings attached to
these terms or to the terms matter and energy. It is not clear
that science has been really advanced through the conception of
the essential unity of organic evolution and cosmic evolution.
The relatively little the two groups of processes have in common
has been overemphasized as compared with their fundamental
differences. The laws which govern living matter are in a large
extent peculiar to the process of living. Processes which are
functions of organs cannot exist where there are no organs.
The traits of protoplasm are shown only in the presence of
protoplasm. For this reason we may well separate the evolu-
tion of astronomy, the evolution of dynamic geology and of
physical geography, as well as the purely hypothetical evolu-
tion of chemistry, from the observed phenomena of the evolution
of life. To regard cosmic evolution and organic evolution as
identical or as phases of one process is to obscure facts by
verbiage. There are essential elements in each not shared by
the other — or which are at least not identical when measured
in terms of human experience. It is not clear that any force
whatever or any sequence of events in the evolution of life is
homologous with any force or sequence in the evolution of
stars and planets. The unity of forces may be a philosophical
necessity. A philosophical necessity or corner in logic is un-

EVOLUTION DEFINED 7

known to science. We can recognize no logical necessity until
we are in possession of all the facts. No ultimate fact is yet
known to science.

For reasons indicated above the term evolution is not
wholly acceptable as the name of a branch of science. The
term bionomics is a better designation of the changing of
organisms influenced through unchanging laws. It is a name
broader and more definite than the term organic evolution, it
is more euphonious than any phrase meaning life adaptation, it
involves and suggests no theory as to the origin of the phenom-
ena it describes.

It is a matter of common observation that organisms change
from day to day, and that day by day some alteration in their
environment is produced. It is a conclusion from scientific
investigation that these changes are greater than they appear.
Not only do they affect the individual animal or plant, but they
affect all groups of living things, classes or races or species.
No character is permanent, no trait of life without change;
and as the living organism and groups of organisms are un-
dergoing alteration, so does change take place in the objects
of the physical world about them. "Nothing endures," says
Huxley, "save the flow of energy and the rational order that
pervades it." The structures and objects change their forms
and relations, and to forms and relations once abandoned they
never return; but the methods of change are, so far as we can
see, immutable. The laws of life, the laws of death, and the
laws of matter never change. If the invisible forces which
rule all visible things are themselves subject to modification
and evolution we have not detected it. If these vary, their
aberrations are so fine as to defy human observation and com-
putation. In the control of the universe we find no trace of
"variableness nor shadow of turning."

But the objects we know do not endure. Only the shortness
of human life allows us to speak of species or even of individuals
as permanent entities. The mountain chain is no more nearly
eternal than the drift of sand. It endures beyond the period of
human observation; it antedates and outlasts human history.
So may the species of animal or plant outlast and antedate the
lifetime of one man. Its changes are slight even in the lifetime
of the race. Thus the species, through the persistence of its
type among its changing individuals, has come to be regarded

8 EVOLUTION AND ANIMAL LIFE

as something which is beyond modification, unchanging so long
as it exists.

"I believe/7 said the rose to the lily in the parable, "that
our gardener is immortal. I have watched him from day to
day since I bloomed, and I see no change in him. The tulip
who died yesterday told me the same thing."

As a flash of lightning in the duration of the night, so is the
life of man in the duration of nature. When one looks out on
a storm at night he sees for an instant the landscape illumined
by the lightning flash. All seems at rest. The branches in the
wind, the flying clouds, the falling rain, are all motionless in
this instantaneous view. The record on the retina takes no
account of change, and to the eye the change does not exist.
Brief as the lightning flash in the storm is the life of man com-
pared with the great time record of life upon earth. To the
untrained man who has not learned to read these records,
species and types in life are enduring. From this illusion arose
the theory of special creation and permanence of type, a theory
which could not persist when the facts of change and the forces
causing it came to be studied in detail.

But when men came to investigate the facts of individual
variation and to think of their significance, the current of life
no longer seemed at rest. Like the flow of a mighty river, ever
sweeping steadily on, never returning, is the movement of all
life. The changes in human history are only typical of the
changes that take place in all living creatures. In fact, human
history is only a part of one great life current, the movement of
which is everywhere governed by the same laws, depends on the
same forces, and brings about like results.

Organic evolution, or bionomics, is one of the most com-
prehensive of all the sciences, including in its subject matter
not only all natural history, not only processes like cell division
and nutrition, not only the laws of heredity, variation, segre-
gation, natural selection, and mutual help, but all matters of
human history, and the most complicated relations of civics,
economics, and ethics. In this enormous science no fact can
be without a meaning, and no fact or its underlying forces can
be separated from the great forces whose interaction from
moment to moment writes the great story of life.

And as the basis to the science of bionomics, as to all other
science, must be taken the conception that nothing is due to

EVOLUTION DKFTXKI) 9

chance or whim. Whatever occurs comes as the resultant of
moving forces. Could we know and estimate these forces, we
should have, so far as our estimate is accurate and our logic
perfect, the gift of prophecy. Knowing the law, and knowing
the facts, we should foretell the results. To be able in some
degree to do this is the art of life. It is the ultimate end of
science, which finds its final purpose in human conduct.

"A law," according to Darwin, "is the ascertained sequence
of events." The actual sequence of events it is, in fact, but
man knows nothing of what is necessary, only of what has been
ascertained to occur. Because human observation and logic
can be only partial no law of life can be fully stated. Because
the processes of human mind are human, with organic limita-
tions, the study of the mind itself becomes a part of the science
of bionomics. For it is itself an instrument or a combination
of instruments by which we acquire such knowledge of the
world outside of ourselves as may be needed in the art of living,
in the degree in which we are able to practice that art.

The necessary sequence of events exists, whether we are able
to comprehend it or not. The fall of a leaf follows fixed laws
as surely as the motion of a planet. It falls by chance because
its short movement gives us no time for observation and calcu-
lation. It falls by chance because, its results being unim-
portant to us, we give no heed to the details of its motion. But
as the hairs of our head are all numbered, so are numbered all
the gyrations and undulations of every chance autumn leaf.
All processes in the universe are alike natural. The creation
of man or the growth of a state is as natural as the formation
of an apple or the growth of a snowbank. All are alike super-
natural, for they all rest on the huge unseen solidity of the
universe, the imperishability of matter, the conservation of
energy, and the immanence of law.

We sometimes classify sciences as exact and inexact, in
accordance with our ability exactly to weigh forces and results.
The exact sciences deal with simple data accessible and capable
of measurement. The results of their interactions can be
reduced to mathematics. Because of their essential simplicity,
the mathematical sciences have been carried to great com-
parative perfection. It is easier to weigh an invisible planet
than to measure the force of heredity in a grain of corn. The
sciences of life are inexact because the human mind can never

10 EVOLUTION AND ANIMAL LIFE

grasp all their data. The combined effort of all men, the flower
of the altruism of the ages, which we call science, has made
only a beginning in such study.

But, however incomplete our realization of the laws of life,
we may be sure that they are never broken. Each law is the
expression of the best possible way in which causes and results
can be linked. It is the necessary sequence of events, therefore
the best sequence, if we may imagine for a moment that the
human words "good" and "bad" are applicable to world
processes. The laws of nature are not executors of human
justice. Each has its own operation and no other. Each
represents its own tendency toward cosmic order. A law in
this sense cannot be " broken." "If God should wink at a
single act of injustice," says the Arab proverb, "the whole
universe would shrivel up like a cast-off snake skin."

The laws of nature have in themselves no necessary principle
of progress. Their functions, each and all, may be denned as
cosmic order. The law of gravitation brings order in rest or
motion. The laws of chemical affinity bring about molecular
stability. Heredity repeats strength or weakness, good or ill,
with like indifference. The past will not let go of us; we cannot
let go of the past. The law of mutual help brings the perpetua-
tion of weakness as well as the strength of cooperation. Even
the law of pity is pitiless, and the law of mercy merciless. The
nerves carry sensations of pleasure or pain, themselves as indif-
ferent as the telegraph wire, which is man's invention to serve
similar purposes. Some men who call themselves pessimists
because they cannot read good into the operations of nature
forget that they cannot read evil. In morals the law of compe-
tition no more justifies personal, official, or national selfishness
or brutality than the law of gravitation justifies the shooting
of a bird.

The science of bionomics centers about the theory of descent,
the belief that organs and species as we know them are derived
from other and often simpler forms by processes of divergence
and adaptation. According to this theory all forms of life
now existing, or that have existed on the earth, have risen from
other forms of life which have previously lived in turn. All
characters and attributes of species and groups have developed
with changing conditions of life. The homologies among
animals are the results of common descent. The differences

EVOLUTION DEFINED H

arc due to various influences, one of the leading forces among
these being competition in the struggle for existence between
individuals and between species, whereby those best adapted
to their surroundings live and reproduce their kind.

This theory is now the central axis of all biological investi-
gation in all its branches, from ethics to histology, from'anthro-
pology to bacteriology. In the light of this theory every
peculiarity of structure, every character or quality of individual
or species, has a meaning and a cause. It is the work of the
investigator to find this meaning as well as to record the fact.
"One of the noblest lessons left to the world by Darwin," says
Frank Cramer, "is this, which to him amounted to a profound,
almost religious conviction, that every fact in nature, no matter
how insignificant, every stripe of color, every tint of flowers,
the length of an orchid's nectary, unusual height in a plant, all
the infinite variety of apparently insignificant things, is full of
significance. For him it was an historical record, the revelation
of a cause, the lurking place of a principle." It is therefore a
fundamental principle of the science of bionomics that every
structure and every function of to-day finds its meaning in some
condition or in some event of the past.

CHAPTER II

VARIETY AND UNITY IN LIFE

"L'espece, c'est un etre qui dans ses generations successives
presente toujours les memes caracteres d 'organisation; il faut ajouter
dans les memes localites, et les memes circonstances exterieures."-
RAMBUR, 1842.

" THAT mystery of mysteries as it has been called by one of
our greatest philosophers " — this is Darwin's phrase regarding

the problem before us, the
origin of species — the origin/
or cause of variety in the
life of the globe.

That variety exists, that
there are many kinds and
types, grades and grada
tf^~M!M '"J tions in animal and vege-

11 Krai \ i table life is evident to a11-

/ jiMH$^n Birds and trees' beetles

f f fHIJISw 71 v and butterflies> fishes and

£ \ \JSlilrJ' / J ^ flowers, ferns and blades of

grass, all these are objects
of constant recognition.
The green cloak which
covers the brown earth is
the shield under which
myriads of organisms,
brown and green, carry on

I''IG. 1. — Long-horned boring beetle from
Central America (one-half natural size).1

their life work, and still
farther below the level of
our ordinary notice exists
a range of life scarcely less

irThis figure and the others in this chapter are introduced simply to
illustrate graphically the variety of animal form.

13

VARIETY AND UNITY IN LIFE

13

varied. Pasteur has defined fermentation as "life without air."^
A host of chemical changes in organic matter, fermentation,
putrefaction, infection of disease — all these are the work of
minute organisms none the less real because invisible and as
varied in form and structure as in the differing effects their
presence may produce.

Each kind of animal or plant, that is, each set of forms ^
which in the changes of the ages has diverged tangibly from its
neighbors, is called a species. There is no absolute definition
for the word species. The word kind represents it exactly in
common language, and is just as susceptible to exact definition.

Fi<;. 2. — Kangaroo rat from the California-Mojave desert (one-half natural size).

The scientific idea of species does not differ materially from the
popular notion. A kind of tree or bird or squirrel is a species.
Those individuals which agree very closely in structure and
function belong to the same species. There is no absolute test,
other than the common judgment of men competent to decide.
Naturalists recognize certain formal rules as assisting in such a
decision. A series of fully intergrading forms, however varied
at the extremes, is usually regarded as forming a single species.
There are certain recognized effects of climate, of climatic iso-
lation, and of the isolation of domestication. These do not
usually make it necessary to regard as distinct species the
extreme forms of a series concerned.

In the words of the entomologist Rambur, " A species is a
group of beings which in successive generations show the same
characters of organization, unchanged so long as the locality
and external conditions remain unchanged.''

14

EVOLUTION AND ANIMAL LIFE

The number of species actually existing is far beyond ordi-
nary conception. The earliest serious attempt to catalogue the
species of animals and plants was made by Linnaeus. In the
tenth edition of his " Systema Naturae " in 1758, in the 823 pages

FIG. 3. — Brittle or serpent stars — species undetermined. (Natural size.)

devoted to animals, he describes and names some four thousand
different kinds. Great as this number seemed, Linnaeus ven-
tured to suggest that probably his pages did not include half of
those kinds of animals actually existing.

To-day our records contain descriptions of more than one
hundred and fifty times as many kinds of animals as were known
to Linnaeus and all his predecessors and all his associates of a
century and a half ago. Each year, since 1864, there has been
published in London a volume called the "Zoological Record."
Each of the volumes — larger than the whole " Systema Naturae"
— contains the names of the animals new to science which have
been added to the system in the year of which it treats. In the

VAKIKTV AM) UNITY IX LIFE

15

record of each year we find about twelve thousand species, about
three times as many animals as in the whole " Systema Naturae."
Yet the field shows no signs of exhaustion. As the volumes of
the " Zoological Record " stand on the shelves, it is easy to see
that the later volumes are the thickest; and those of the new
century, with a general revival of interest in systematic zoology

FIG. 4. — California quail, Lophortyx californicus. (Two-thirds natural size.)

and the study of geographical distribution, are the thickest of
all. The depths of the sea, the jungles of the tropics, the crev-
ices of the coral reefs, the tundras of the north, the limbs of

16

EVOLUTION AND ANIMAL LIFE

trees, the hair of mammals, the feathers of birds, the body
tissues of mosquitoes, all places where animal life is found, are
being examined with an eagerness not less than that of the early
explorers, while the investigators of to-day are armed with
every appliance that science can devise. Yet now, as in Lin-
nseus's time, it is certain that not half of the number of species
of animal organisms is yet known. The 600,000, more or less,

FIG. 5. — Diamond rattlesnake, Crotalus adamanteus. (Photograph by W. K. Fisher.)

on our registers to-day are certainly far less than half of the
millions which actually exist.

In botany we find the same conditions. There are fewer
known species of plants than animals by half, and they are more
easily preserved and handled, while the work of collection and
investigation proceeds on a scale even more extensive, yet it
would be a bold statement to say that we know to-day half the
species of plants that exist.

All this refers to the forms now living, without reference to
the host which composes their long ancestry, extending back-
ward toward the dawn of creation. The species have come
down through the geological ages, changing in form and func-
tion to meet the varying needs of changing environment. This

VARIETY AND UNITY IN LIFE

17

enumeration takes no account of the still vaster myriads of forms
almost endlessly varied which have perished utterly in the
pressure of environment, leaving no trace in the line of descent.

Of these extinct forms of animals and plants we know a few,
one here and another there: here a bone, there a tooth, here a

18

EVOLUTION AND ANIMAL LIFE

mass of shells, there a piece of petrified wood, an insect in the
marl bed or a leaf preserved flat in the shale. Each of these
fossils is a record of past life, true beyond impeachment, but
the fragments are so few, so scattered, so broken, as to give
only hints of the history they represent.

Moreover, as we extend our studies of species we find that
they change with space as well as with time. These changes

FIG. 7. — Common lizard or swift, Sceloporus undulatus. (Photograph by W. K. Fisher.)

are in large degree a response to external conditions. As
conditions change, so do forms change to fit their surroundings.
A movement over the surface of the earth, any movement in
space, brings organisms in contact with barriers. A barrier
means a change in conditions of life. As distance in space
brings barriers, so does the passage of time bring events which
are barriers also. Time brings new events; events mean
changes in conditions, and change brings about divergence.
Neither time nor space flows evenly.

Variations in turn become greater with lapse of time and

VARIETY AND UNITY IN LIFE

19

space, for these again bring other events and disclose other
barriers. A closer observation will show us that the range of
variety is far greater than is indicated by the number of species.
There is not one blade of grass in the meadow exactly like any
other blade. There is not a squirrel in the forest like any
other squirrel, not a duck on the pond like any other duck in
every detail of its structure. If we compare two rose leaves
we shall find differences in size, in serration of the margin, in
the length of the stalk, in the hairs
on the surface, in the intensity of
the green, in the number of breath-
ing pores on the lower side. In
every structure and function where
difference is possible variation will
appear. The squirrels or the ducks
will differ in shade of color, in dis-
tinctness of marking, in length of
limb, in breadth of organ, in every
way in which there is play for in-
dividualism.

Nor are these differences limited
to matters of color or form. There
are like variations in function, in
tendency, in disposition, in endur-
ance. No two men ever bore the
same features, no two ever held the
same character, no two ever lived
the same life. The traits of the in-
dividual, however small, appear on
every hand. It is by little traits of
emphasis that we recognize our friends. The same individu-
alism is possessed by the lower animals and by plants, though
the differences in stress and emphasis in color and figure are
most marked in creatures of the most highly specialized organi-
zation. In all animals and all plants like differences obtain.
No two individuals of any species are ever quite the same. No
two germ cells of the same parent are ever quite alike. No two
cells in the body are ever exactly identical. Among plants of
the same kind in the field, some are cut down by frost while
others persist; some are destroyed by drought while others en-
dure; some are immune to attacks of rust while others are ex-

Fro. 8. — Sea cucumber, Cucu-
maria, sp. (Natural size.)

20 EVOLUTION AND ANIMAL LIFE

FIG. 9. — Blunt-nosed salamander, Amblystoma opacum. (Photograph by W. K. Fisher.)

FIG. 10. — White pelicans, Pelecanus erythrorhynchus, and whooping-crane, Grus ameri-
cana. (Photograph by W. K. Fisher.)

VARIETY AND UNITY IN LIFE

21

terminated by such parasites. Fill a bottle with flies. All in
time will die of suffocation, but a certain few will outlast the
many. Bring in a number of wolf cubs. Some will become
relatively tame— some will remain wolves, and between the
most fierce and the most docile we shall find all ranges of
variation. "What is one man's food is another man's poison.'2-
This proverb is a recognition of the principle of individuality
which accompanies everywhere the formation of species, and
being everywhere present, it must be an integral part of the

FIG. 11. — Silver fox, Vulpes pennsylvanicus argentatus. (Photograph
by W. K. Fisher.)

process. Such differences are not matters of structure alone.
Psychological differences, differences in instinct, in adapta-
bility, in rate of nerve processes are just as marked as differ-
ences in anatomy. They may separate one species from an-
other. They may be just as decided within the limits of the
species itself. Moreover, the beginning of variation is not with
the individual organisms. No two cells are absolutely alike,
and in the variance of the germ cells, from which individuals
spring, all the elements of their future variation are involved.
Without further discussion, it is evident that variety in life
is a factor in the history of our globe, that it may be expressed
in terms of number of species, but that the actual range of varia-
tion is far greater than the number of species, and that if causes
are to be judged by range of effects, we must find in the origin of
3

22

EVOLUTION AND ANIMAL LIFE

species the operation of world-wide forces, the cooperation of
great influences, far-reaching in time and space, as broad as the
surface of the globe and as enduring as its life. To consider
these causes so far as known is the purpose of this work. Our
problem is no longer the "mystery of mysteries/' for in a large
way by the work of Darwin and his successors the influences
promoting variety in life are already known. We know many
of the different factors which produce divergence in form and
adaptation to conditions. But the relative value of these
factors is less certain, and from time to time other and more

+J*

1 j

\

J

t

V

*J

•^t

t/51^

^*-

1?

«*
%

V

y

/

**

*

v

M

t

V

FIG. 12. — Lizard walking. (After Marey.)

subtle factors are brought to light, or the great forces them-
selves are analyzed into finer component elements.

But with all that we may say of the universality of variation
and the prevalence of individualism, we are equally impressed
with the underlying unity. There are only a few types of
structure among animals, and in these few the beginnings in
development are the same. The plants show similarly a few
modes of development, arid all the range of families and forms
is based on the modification of a few simple types. Moreover
all living forms, plants and animals alike, agree in the funda-
mental elements. All are made of a framework of cells, each
cell a source of energy, containing in all cases a semifluid net-
work of protoplasm, which is found wherever the phenomena
of life appear. In all the cells is the mysterious nuclear sub-
stance which seems to direct the operations of heredity. The
same laws or methods of heredity, variability, and response to

VARIETY AND UNITY IN LIFE

23

outside stimulus hold in all the organic world. We call animals
and plants "organic" because they are made up of organs,
cells, and tissues so grouped that like structures perform like
functions. We could not use
a generic term like organic,
were it not for the structural
resemblances existing in each
individual in great groups of
organisms. All organisms
have the impulse to repro-
duction. All are forced to
make concession after con-
cession to their surroundings
and in such concessions prog-
ress in life consists. At
last each organism or each
alliance of organisms is dis-
solved by the process of
death.

The unity in life is then
not less a fact than the
diversity. However great
the emphasis we lay on in-
dividuality or diversity, the
essential unity of life must
not be forgotten. Whatever
solution we may find to the
problem of the origin of
species, must also explain
why species and individuals
may be so much alike in all
large details of structure.
To knowr the origin of species
we must also know why
species admit of natural
classification. Why is variety
in life based on essential
unity?

From the fundamental unity of the species of to-day, we
may infer the similar unity of species in past time. From the
knowledge of variety in unity comes the likening of species of

FIG. 13. — Starfish walking. (After
Marey.)

24 EVOLUTION AND ANIMAL LIFE

animals or plants to the separated twigs of a tree, of which
the trunk is more or less concealed. "We can only predicate
and define species at all/' says Dr. Elliott Cones, " from the mere
circumstance of missing links. Our species are twigs of a tree
separated from the parent stem. We name and arrange them
arbitrarily, in default of a means of reconstructing the whole
tree, in accordance with nature's ramifications." To continue

FIG. 14. — Heron flying. (After Marey.)

the figure, in our studies of the origin of the twigs of the tree,
the existence of the trunk must not be forgotten. In the life
of the earth variety in unity, unity in variety are nowhere
separated.

Another equally striking simile is this: A species is an
island, a genus, an archipelago, in a sea of death. The species
is clearly definable only as its ancestors and cousins have dis-
appeared, only in the degree that the stages in its development
are unrepresented in our records. The genus is a group of
species, an archipelago of islands, and there may be every
conceivable degree of width or breadth of channel which seems
to separate one island or group of islands from another.

CHAPTER III

LIFE, ITS PHYSICAL BASIS AND SIMPLEST
EXPRESSION

There can be little doubt that the further science advances the
more extensively and consistently will the phenomena of nature be
represented by mathematical formulae and symbols. But the man of
science who, forgetting the limits of philosophical inquiry, slides from
these formulae and symbols into what is commonly understood by
materialism, seems to me to place himself on a level with the mathe-
matician who should mistake the x's and y's with which he works his
problems for real entities, and with this further disadvantage as
compared with the mathematician, that the blunders of the latter
are of no practical consequence, while the errors of systematic materi-
alism may paralyze the energies and destroy the beauty of a life. —
HUXLEY.

IN practice the distinction between a live thing and a lifeless
one is usually of the simplest, but to define this distinction in
terms so precise that the definition may be used as an invariable
criterion is a problem of considerable difficulty. The sheep
grazing in the field and the soil under its feet; the grass and
flowers on the one hand, and the stones on the other hand, in
the same pasture; there are no difficulties in the distinction
here. Nor, indeed, even when we come to consider the simplest
kinds of organisms, the tiny one-celled plants and animals that
teem in stagnant waters of the wayside puddle. As we examine
a drop of this water under the microscope we know without
question what in it is alive and what in it is dead.

But let us attempt to put into words, into definite declaratory
phrases, the characteristics of organisms and we find ourselves
curiously impotent. When we come to study analytically
organic nature and inorganic nature, things animate and

25

26 EVOLUTION AND ANIMAL LIFE

things inanimate, we find structures and behavior among inor-
ganic things which cannot be readily distinguished in defining
words from structures and behavior that are usually taken as
characteristic of organisms. On the other hand we shall find in
organic nature the very same chemical elements, and for the
most part the same combinations of elements, that we find in
the great mass of inorganic world substance. So that some
biologists by a detailed and keen, if somewhat sophisticated,
analysis of the alleged differences between animate and inani-
mate matter show that these differences are not absolute, and
leave you with a stone in one hand and a grasshopper in the
other logically unable to define the fundamental difference
between the two, and yet morally certain of this absolute
difference.

As a matter of fact there is one distinction between living
matter and non-living matter which even the cleverest of the
modern physicochemical school of biologists has as yet been
unable to explain away. And that is the inevitable presence in
living matter and the inevitable absence in non-living matter
of certain highly complex chemical combinations of carbon,
hydrogen, oxygen, nitrogen, and sulphur, called proteids OT-
albuminous compounds.

The actual presence of these chemical substances in living
matter is made manifest to us by the physicochemical behavior
of these substances: that is, by our observation or recognition
of their peculiar attributes. This behavior or these peculiar
attributes or activities are those fascinating ones which we are
accustomed to call the essential life processes. What these
activities are we indicate in a not very precise way by the
words organization, assimilation, growth, reproduction, motion,
irritability, and adaptation. These essential life processes we
have come by constant experience to associate always and only
with a substance called protoplasm. Huxley long ago called
protoplasm, therefore, the physical basis of life.

But protoplasm we have found to be a complex of substances
or chemical compounds. Of these, a certain few are indispen-
sable and fundamental, while others may be absent or present
without affecting the particular capacities which make proto-
plasm the physical basis of life. This protoplasm too must be
organized in a particular way in order that life may persist in
the organism. It must appear in two conditions, and proto-

.LIFE, ITS PHYSICAL BASIS AND SLMPLKST EXPRESSION 27

plasmic stuff representing these two conditions must be disposed
in certain definite relations. Protoplasm must occur as a cell
or cells to be capable of performing the necessary activities of
life. Hence we must consider at the very beginning of any dis-
cussion of life the two things, protoplasm and the cell.

The elements that compose protoplasm are the familiar ones,
carbon, nitrogen, hydrogen, oxygen, sulphur, phosphorus,
potassium, sodium, etc.; but these elements, or some of them,
are found in protoplasmic cells in certain complex combinations
which arc not found elsewhere in nature, and which therefore
actually and absolutely distinguish chemically living proto-
plasm from all lifeless matter. These particular combinations
are certain albuminous compounds or proteids, composed of
C, H, O, N, and S, and their complexity is extreme: the atoms
in a single molecule often number more than a thousand. The
molecules also are very large, which is probably the reason of
their characteristic nondiffusibility through animal membranes
or artificial parchment.

In addition to these characteristic albuminous compounds
and various derivatives, of them, protoplasm usually contains
certain native albumins and certain other characteristic com-
pounds known as carbohydrates and fats (which differ essen-
tially from the albuminous substances in lacking nitrogen as a
composing element). There are also various salts and gases
and always water to be found in living substances. Water is
absolutely necessary to the physical condition of half fluidity
which gives to protoplasm its essential capacity for motion on
itself. The commoner salts found in living substances are
compounds of chlorine as well as the carbonates, sulphates, and
phosphates of the alkalies and alkali earths, especially common
salt (sodium chloride), potassium chloride, ammonium chloride,
and the carbonates, sulphides, and sulphates of sodium, potas-
sium, magnesium, ammonium, and calcium. The gases found
in living matter are oxygen and carbon dioxide. These, when
not in chemical combination, are almost always dissolved in
water, although rarely they may be in the form of gas bubbles.

To sum up the relation of living matter to chemistry we may
say that life is always associated with protoplasm, and that this
protoplasm is made up of a few familiar inorganic elements,
particularly those of lowest atomic weight; that it does not
include any special so-called vital or life element, that is, any

28 EVOLUTION AND ANIMAL LIFE

elementary substance other than occurs in the inorganic world.
These elements are combined in protoplasm into certain most
extremely complex compounds, which are always present where
life is, and never elsewhere, and hence the essential chemical
characteristic of living matter is the presence of these complex
as yet unanalyzed, albuminous compounds.

It is obvious that this chemical half-knowledge of proto-
plasm makes no satisfying revelation to us explanatory of the
qualities of this life stuff. How is it then with the physical
structure of protoplasm? We know that many simple chemical
substances put together in particular physical relationship to
each other will give a capacity of performance or function quite
different from and beyond that which they possess when simply
brought together without definite order or arrangement. Is
protoplasm a machine with a capacity for doing extraordi-
nary things, with its powers due primarily to its physical
make-up? Unfortunately we have no satisfying answer to this
question. While chemists are balked in their analysis of the
protoplasmic make-up by the complexity of the compounds
they meet, a complexity too much for their present technic to
resolve, physicists are similarly balked in their attempt to re-
solve and expose the ultimate physical structure of protoplasm.

This ultimate structure of protoplasm is ultramicroscopic,
and its study is checked by the limitations of microscopes.
When we examine protoplasm with the highest powers of the
microscope we see plainly that it is not as it appears under lower
powers, structureless and homogeneous. On the contrary it
reveals an apparent granular or fibrillar or alveolar or reticu-
lar structure. We find that protoplasm varies in its physical
make-up at different times or in different cells. We also find
that the difficulties of interpreting just what one sees when using
the highest microscopic powers make it impossible to be really
certain of understanding what is seen. But however various
our interpretations of the finer structure of protoplasm, they
agree that any bit of protoplasm is a viscous colloidal mass
composed of at least two substances of somewhat different phys-
ical make-up. One of these substances is evidently denser than
the other and is arranged in the form of grains, rods, threads,
or droplets scattered through a ground mass. Concerning this
dimorphic condition of protoplasm practically all biologists are
agreed. The names, hyaloplasm, paraplasm, or others of sim-

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 29

ilar significance are applied to the viscous hyaline ground
substance, while the denser parts are variously called micro-
somes, granules, fibrils, spongioplasm, etc.

The important part of all this is the fact that all the biologists
are not agreed on any certain kind of intimate structure of

FIG. 15. — Different types of cells composing the body of the squirrel or other highly
developed animal : A, liver cell; /, food materials; n, nucleus; B, complete cell;
C, nerve cell, with small part of its fiber; D, muscle fiber; E, cells lining the body
cavity; F, lining of the windpipe; G, section through the skin. (Highly magnified.)

protoplasm as revealed by the highest powers of the microscope,
but they all agree that there is a fine and real structural organ-
ization of what at first glance appears to be homogeneous
structureless life stuff. That is, as Delage expresses it, it is seen
that protoplasm is not simply an organic chemical compound,
but that it is an organized substance; that is, it possesses a
structure of a higher order than the automatic structure of those
chemical molecules which compose non-living so-called organic
substances. But at the same time we are deceived if we expect

30

UTION AND ANIKAL LIFE

to be able to find in this physical organization of protoplasm
any satisfactory explanation of its wonderful properties.

We have said that it should always be held dearly in mind
that the full life capacity of protoplasm is realized only when it
is in that differentiated and organized condition typical of the
structural unit or celL The essential thing about the cell is
not that it has a definite shape or sue or that it is truly cell- or
saclike, but that it is a tiny mass of protoplasm with

substances secreted by or held in it. The protoplasm itself is

differentiated into at least two parts, an inner, denser, smaller
part called the nucleus, and an outer surrounding, usually larger,
portion called the cytoplasm. Such a differentiated or organized
protoplasmic unit can perform all of the essential functions of life
and persist in this performance indefinitely unless destroyed by
extrinsic causes. The cell itself may not have any indefinite
existence as a unit, but it will be the progenitor of an indefi-
nitely prolonged series of cells. A single part of this cell, that is,
a bit of protoplasm either of the nucleus or the cytoplasm, or the
whole of either can perform for a while most of the activities of
life; but such a part always lacks the capacity for reproduction,
that is, for persistence as living matter. Thus it is obvious that

, ,, i , pTS PHYSH u. BASE \\i' BIMP] i ft i KP1 31

llr.l i-liint. (Aflrr V.

M

i . • IS.—

ioffu«ion. Thp.lnrkul.
upol n, lh, ,, ., ,r ii th« nucleus. ( M"< ' <

32

EVOLUTION AND ANIMAL LIFE

if such protoplasmic cells, composed of nucleus and cytoplasm,
exist singly they form living units. And we have actual ex-
emplifications of this condition in the structure and life of the
simplest organism.

The simplest organisms are independently living, single proto-
plasmic cells (Figs. 16-21). There are thousands of kinds of

FIG. 19. — Plasmodium of a slime mold on wood, Trichia favaginea: A, plasmodium X 2;
B, spores; C, spore with contents escaping; D, ciliated swarm spore, showing
flagellum, f, and nucleus, n; E, two amoeboid swarm spores; F, part of plasmodium
under glass slide; G, a part of F, more highly magnified. (After Campbell.)

these single-celled organisms recognizably different by charac-
teristics of shape and size, habit and habitat. We try to distin-
guish them as single-celled animals (Protozoa) and single-celled
plants (Protophy ta) , on the basis of alleged differences in their
habit of food-taking and general nutrition. This distinction is
often most arbitrarily made, and botanists and zoologists are
constantly claiming the same organisms as belonging to their
respective fields of study. Many naturalists, conspicuously
Haeckel, have repeatedly suggested the convenience and even
the necessity of grouping most of these unicellular organisms
into a phylum or kingdom to be called the Protista, the members

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 33

of which shall not be recognized as sufficiently specialized to be
called either plants or animals, but simply organisms. But
this suggestion seems to meet with little practical favor from
students of systematic biology.

For a basis, therefore, of any study of the evolution of life,
an acquaintanceship with the life and struc-
ture of the simplest organisms is a necessity.
As the authors have already tried in another
book ("Animal Life") to present a simple
account of this life together with an account
of certain less simple or slightly complex or-
ganisms (Figs. 22-26) whose physiology and
structure reveal successive stages in organic
complexity and specialization, and as the space
in this book is limited, the authors must
refer their present readers to chapters I, II,
and III of "Animal Life " for an account
of the life of the simplest and slightly com-
plex organisms.

The differentiation and growing com-
plexity of the body of those many-celled
animals which differ from and are, we may
say, beyond and higher than the simple many-
celled forms, are by no means always along
the same line (Figs. 27-37). It is familiar
knowledge that animals can be classified or
grouped into a number of great divisions
called branches or phyla. For example, the
starfishes, sea urchins, sea cucumbers, etc.,
constitute one phylum, the Echinodermata ;
the crustaceans, insects, spiders, etc., con-
stitute another phylum, the Arthropoda, and
all the animals with a backbone or with a
notochord constitute another, the Chordata.
of these phyla there is a fundamental or type structure (Fig.
27). All of the Echinodermata, for example, are built on the
radiate plan. They recall the starfish with its five or more
arms radiating from a central disk. The Arthropods are all
animals with a body composed fundamentally of a series of
successive segments, some or all of these segments bearing
pairs of jointed appendages; and so on. We need not pursue

FIG. 20. — Paramce-
cium aurelia. At
each end there is
a contractile vac-
uole, and in the
center is one of
the nuclei. (After
Verworn.)

Now for each

34

EVOLUTION AND ANIMAL LIFE

FIG. 21. — A group of stalked one-celled
animals, Carchesium, sp. (Adapted from
Davenport, from a photograph of the liv-
ing animals.)

grouped into two regions and
the appendages limited to
the anterior one of these
two. The Myriapods, which
are also Arthropods, have a
structure more in conformity
with what may be called the
racial or typical plan for the
whole phylum; that is, the
body is made up of a se'ries
of many successive similar
segments, each segment bear-
ing a pair of jointed ap-
pendages. In -that general
line of descent to which
man belongs, and which is
distinguished by the name
of the phylum Chordata,
there are of course various
subordinate lines which we
recognize under the names

further the general classifi-
cation of animals into phy-
la. Nor need we explain
in any detail the structural
types or fundamental struc-
tural plans which distin-
guish the various principal
lines of descent in the
animal kingdom.

Branching out from each
of the principal lines are
hosts of subordinate lines.
Some of the Arthropods, as
the insects, have their body
segments grouped into three
regions and their jointed
appendages confined to the
anterior two of these re-
gions. Others, as the spiders,
have the body segments

B

FIG. 22. — Gonium pectorale, a colonial pro-
tozoon: A, seen from above; B, seen from
the side. (After Stein.)

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 35

\

FIG. 23. — Pandorinn sp., a colonial
protozoon. (Highly magnified.)

of fishes, amphibians, reptiles,
birds, and mammals.

In all the subdivisions of the
main groups there are also to be
recognized differentiated and di-
vergent lesser lines of descent, and
within these still lesser ones.
While, as already noted, the main
divisions of the animal kingdom
are called phyla and the divisions
of the phyla, classes, the subdi-
visions of the classes are usually
called orders. The next subdi-
vision is that into families, each
in. turn being a cluster of genera.

The genera are composed of species and the species finally of
sub-species, varieties, and individuals. Each one of these

names refers pri-
marily to a special
line or mode of
differentiation and
at the same time
refers to the fact
that the members
of each of these
different! at ed
groups are genet-
ically related to
each other, that
is, related by
blood, by actual
ancestral descent.
All these differ-
entiated groups
indicate diverging
lines of evolution,
some of them
short, and but

FIG. 24.— A fresh-water polyp, Hydra vulgaris: A, in ex- slightly divergent

tended condition and in contracted condition; B, cross from the main
section of body, showing the two layers of cells which

make up the body wall. lme "OIIl Which

36

EVOLUTION AND ANIMAL LIFE

they arise; others, on the contrary, long, important, and widely
divergent.

The traditional tree which is drawn to explain animal
classification illustrates at the same time the two fundamental
facts upon which this classification is
based, namely, differentiation of struc-
ture, and corresponding divergence of
descent. All the branches of this gene-
alogical tree lead back, as they do in a
real tree, to its trunk, and the trunk
of this tree, springs from the simplest of
the many-celled animals, namely, from
those primitive forms which resemble in
essential characters animals like the

FIG. 25. — Longitudinal section through the body
of a sea anemone: oe., oesophagus; m.f., mesen-
terial filaments; r., reproductive organs.

FIG. 26. — One of the sim-
plest sponges, Calcolyn-
thus primigenius. A part
of the outer wall is cut
away to show the inside.
(After Haeckel.)

simpler polyps. Indeed it seems certain that this tree trunk
can be traced farther back; that it must spring in the begin-
ning from forms essentially like the lowest organisms that we
know to-day, namely, single, simple cells living independently.
From the Amoeba to Man; that is the history of descent, or
ascent if one prefers. The course has been a continuous one,
both in point of time and in point of gradual transformation.

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 37

/

FIG. 27. — Diagram showing fundamental structure of types of several animal phyla:
1, sea anemone; 2, starfish; 3, worm; 4, centipede; 5, clam; 6, honeybee; 7, sala-
mander. In each figure the central nervous system is indicated by the black lines.
(After Haeckel.)

But great periods of this time are shut away from us without
record of their duration, and long series of the gradually
changing forms are lost to us without hope of discovery. And
yet in its large outlines we know the history of all this time
and the character of all these graded series.

38

EVOLUTION AND ANIMAL LIFE

We should give at least brief attention to what may be called
the primary, or necessary, conditions of life. We know that
fishes cannot live very long out of water and that birds cannot
live in water. These, however, are conditions which depend on
the special ecological habits of these two particular kinds of
animals. The necessity of a constant and sufficient supply of
oxygen is a necessity common to both. It is one of the primary
conditions of their life. All animals must have air. Similarly
both fishes and birds and all other animals must have food.

This, then, is an-
other of the pri-
mary conditions of
animal life.

If water be held
not to be included
in the general con-
ception of food,
then special men-
tion must be made
of the necessity of
water as one of
the primary condi-
tions of life. Proto-
plasm, the basis of life, is a fluid, although thick and viscous.
To be fluid its components must be dissolved or suspended in
water. In fact, all of the really living substance in an animal's
body contains water. This water, so necessary for the animal,
may be derived from the general food, all of which contains
water in greater or less quantity, or it may be taken apart from
the other food by drinking or by absorption through the skin.
We know, too, that if the temperature is below a certain
minimum point or above a certain maximum, these points vary-
ing for different animals, death takes the place of life. It is
familiar knowledge that many animals can be frozen without
being killed. Insects and other small animals may lie frozen
through winter and resume active life again in the spring. An
experimenter kept certain fishes frozen in blocks of ice at a tem-
perature of — 15° C. for some time and then gradually thawed
them out unhurt. There is no doubt that every part of the
body, all of the living substance, of these fish was frozen, for
specimens at this temperature could be broken and pounded up

FIG. 28.— The fiddler crab, Gelasimus.
Miss Mary Rathbun.)

(Photograph by

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 39

into fine icy powder. But a temperature of —20° C. killed the
fish. According to L. J. Turner, the Alaska mud-fish (Dallia),
was fed frozen to Esquimaux dogs. One of these thawing in
the stomach of the animal made its escape alive. Frogs lived
after being kept at a temperature of —28° C., centipedes, at

FIG. 29. — The piddock, Zirphoca crispata, a rock-boring mollusk. (Natural size, from

life.)

a temperature of —50° C., and certain snails endured a tempera-
ture of — 120° C. without dying.

At the other extreme, instances are known of animals living
in water (hot springs or water gradually heated with the organ-
isms in it) of a temperature as high as 50° C. Experiments with
Amoebic show that these simplest animals contract and cease
active motion at 35° C., but are not killed until a temperature
of 40° to 50° C. is reached.

Variations in pressure of the atmosphere also constitute

40

EVOLUTION AND ANIMAL LIFE

conditions which may determine the existence of life. The
pressure or weight of the atmosphere on the surface of the
earth is nearly fifteen pounds on each square inch. This
pressure is exerted equally in all directions so that an object on
the earth's surface sustains a pressure on each square inch of

FIG. 30. — Cephalopods. Lower figure, the devil-fish or octopus, Octopus punctatus.
The upper figure represents the squid, Loligo pealii, swimming backward by
driving a stream of water through the small tube slightly beneath the eyes. (From
life, one-third natural size.)

its surface of fifteen pounds. That is, all animals living on the
earth's surface or near it live under this pressure and under
no other condition. The animals that live in water, however,
sustain a much greater pressure, this pressure increasing with
distance. Certain ocean fishes live habitually in great depths, at
from two to nearly five miles, where the pressure is equivalent to
that of many hundred atmospheres. If these fishes are brought
to the surface their eyes bulge out, their scales fall off because
of the great expanse of the skin, and the stomach is thrust
wrong side out. Indeed the body itself sometimes bursts. On

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 41

the other hand if an animal which lives normally on the surface
of the earth is taken up a very high mountain or is carried up in
a balloon to a great altitude where the pressure of the atmos-
phere is much less than at the earth's surface, serious conse-
quences may ensue, and if too high an altitude is reached, death
occurs.

Some animals require certain organic salts or compounds
of lime to form bones or shells, etc. These salts may be re-
garded as necessary articles of nutrition, though their function
is not that of ordinary food. These are peculiar demands of
special kinds of animals. There might also be included among
primary life conditions such necessities as the light and heat of
the sun, the action of gravitation, and other physical conditions

FIG. 31 .—Long-horned boring beetle, Ergates sp— larva, pupa and adult insect.

without which existence of life of any kind would be impossible
on this earth.

Finally we may refer briefly to the "grand problem" of the
origin of life itself. Any treatment of this question is bound to
be wholly theoretical. We do not know a single positive thing
about it. We have some negative evidence. That is, we have

42

EVOLUTION AND ANIMAL LIFE

no recorded instance — and men have searched diligently for
examples — of spontaneous generation. No protoplasm has
been seen, or otherwise proved, to come into existence except
through the agency of already existing protoplasm. All life
comes from life. All those former beliefs of spontaneous
appearance of bees from the carcasses of oxen, flies from

decaying flesh, hair worms
from horse tail hairs in
water troughs, and bacteria
and infusoria in infusions
of beef or hay have been
shown on scientific investi-
gation to be utterly with-
out basis of fact.

But if protoplasm and
life do not appear, are not
being generated spontane-
ously in this earth epoch,
may they not have been
in earlier ages? Geologists
and biologists attempt to
explain most of the things
that happened in earlier
geologic ages by what they
observe to be happening
now. They would answer,
on this basis, that what
evidence we now have
should lead us to believe

that the generation of life has never occurred. But there must
have been a beginning. Life has not always been. The ac-
cepted geological theory of the making of our earth precludes
the existence of life on it until the globe was cool enough for
organisms to exist. We know that there is a maximum of
temperature beyond which protoplasm inevitably coagulates.
When and where was this beginning of life? The biologist can-
not admit spontaneous generation in the face of the scientific
evidence he has. On the other hand he has difficulty in under-
standing how life could have originated in any other way than
through some sort of transformation from inorganic matter.
As a matter of curiosity we may glance at a few of the

FIG. 32. — Ascidian or sea squirt.

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRKSSloX 43

FIG. 33. — Blacksnake, Bascanion constrictor. (Photograph by W. K. Fisher.)

FIG. 34. — Hawkbill turtle, Eretmochelys imbricata.

44

EVOLUTION AND ANIMAL LIFE

speculations that biologists have allowed themselves concerning
the origin of living substance on the earth. A speculation that
is interesting only because it was suggested by a great scientific

FIG. 35. — Golden eagle, Aquila chrysaetus.

man — a physicist, however, not a biologist — is Lord Kelvin's
theory that living substance was brought to this earth from
celestial regions by meteorites. A more acceptable theory is
that at some earlier geologic age the conditions of earth, atmos-
phere, temperature, etc., were at one time of such a favorable

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 45

nature that just that fortunate coincidence of all necessary con-
ditions and elements occurred which allowed C, H, O, N, to unite
in those great, almost infinitely complex, molecules which com-
pose the albuminous compounds whose existence is the only real
chemical characteristic peculiar to living matter. But we have

FIG. 36.— African or two-toed ostrich, Struthio camelus.

Graham.)

(Photograph by William

46

EVOLUTION AND ANIMAL LIFE

already indicated that the production of such compounds would
not necessarily be the production of protoplasm. What of the
complex definitive physical organization of protoplasm on
which we predicate so much of its capacity?

The botanist Schaffhausen believes that water, air, and the
necessary mineral substances have been directly combined under
the influences of life and heat and have given birth to an

FIG. 37. — Opossum, Didelphys virginiana. (One-tenth natural size; photograph by

W. K. Fisher.)

uncolored protococcus which next became Protococcus viridis.
Delage asks: " If the thing is so simple why does not the author
produce one of these protococci in his laboratory? On lid ferait
grace de la chlorophylle ! " Nageli holds that when the albumi-
nous compounds had their birth in an aqueous liquid, as they
were not soluble in water, they were precipitated. This pre-
cipitate was formed of minute particles, a sort of crystal which
he calls micellae. These micellae are the materials from which
organisms were formed. An inorganic crystal deposited in a
saturated solution of the same nature determines a deposit
on its surface in the form of tiny crystals, by which means it

LIFE, ITS PHYSICAL BASIS AM) SIMPLEST EXPRESSION 47

increases in size. In the same way, when some of these albu-
minous micellae are formed anywhere, they facilitate further
precipitation within their sphere of influence in such a way that
the formation of other micellae, instead of going on uniformly
in the liquid mass, is localized at certain points. Thus are
found aggregates of an albuminous nature which constitute the
primitive protoplasm. This is Nageli's suggestion, and Nageli
is one of the most thoughtful biologists who has ever lived !

Granting that protoplasm must have had a natural, spon-
taneous beginning on this earth, being neither brought to it
from other worlds nor created extranaturally on this world,
biologists indulge in some speculations as to the probable
whereabouts of this first appearance of life, and as to whether
living substance was formed spontaneously but once only or
several times, and perhaps in several places. It is not necessary
here to follow up such speculations. The only one of them with
any scientific evidence at all for it is the theory that life began
at the poles or perhaps particularly at the north pole. The
evidence for this is based, first, on the fact that in accordance
with the cosmic theory of world evolution, the poles of the
earth must have been first in a condition under which life might
exist, and, second, on facts revealed by the study of the geo-
graphical distribution of living and fossil organisms. There
seems to be some slight scientific foundation for the claim that
the first organisms lived in polar regions.

CHAPTER IV
FACTORS AND MECHANISM OF EVOLUTION

Even in the latest and maturest formulations of scientific research,
the dramatic tone is never lost. The causes at work are conceived
in a highly impersonal way, but hitherto no science has been content
to do its work in terms of inert magnitude alone. Activity continues
to be imputed to the phenomena with which science deals, and activity
is, of course, not a fact of observation but is imputed to the phenomena
by the observer. Epistemologically speaking, activity is imputed to
phenomena for the purpose of organizing them into a dramatically
consistent system." — THORSTEIN VEBLEN.

THERE is to-day no doubt in our minds of the truth, the
actuality, of descent. It is not the theory of descent: it is the
fact, the law, of descent, of which we talk and write. Organ-
isms are blood-related: they are transformed, descended from
one another. This, which is the common knowledge of present-
day post-Darwinian science, was the belief of many naturalists
even before the days of Darwin. " From the Greeks to Darwin "
was not all darkness nor complete freedom from taint of the
"pernicious evolution doctrine." Goethe, Erasmus Darwin,
Lamarck, to mention only familiar names, were evolutionists:
they believed in the transmutation of species, believed in descent.
But it was Darwin who gave the waiting naturalists substantial
and satisfactory reasons for the beliefs that were in them; who
gave them strength to have the courage of their convictions.

While Darwinism, in our present-day use of the name, is
not synonymous with descent and evolution, but is the name
of a causomechanical explanation of it, or a group of causal
factors, yet it might justifiably be more broadly used, and held
still to mean, what it certainly did to the world generally for a
good many years after the "Origin of Species" appeared, the

48

FACTORS AND MECHANISM OF EVOLUTION 4<J

general theory of organic evolution and the particular doctrine
of the descent of man from the lower animals. For it was
Darwin who really proved these things to be realities. But in
biology to-day Darwinism is the name which refers to certain
particular causal factors or determining agents in the actual
production and control of the transmutation of species and the
progress and direction of the lines of descent. And the modern
scientific adverse criticism of Darwinism which is beginning to
find its echoes in popular literature must never be mistaken to be
disparagement or adverse criticism of the doctrine of descent,
the law of organic evolution. So we are not in this book dis-
cussing the probabilities of the truth or untruth of evolution
nor presenting evidences or argument to justify a belief in the
doctrine. As the days have long passed when the shape of the
earth, or the behavior of the members of the solar system,
was a fit subject for debate, so the days are now by when the
truth or falsity of the law of organic descent is a debatable
thesis. The earth is subspherical, the planets revolve about
the sun, and species of organisms descend from other species.

But in what particular way, or as the effect of what particular
causal factors, this descent or transformation of species, that is,
kinds of organisms, comes about, — here there is unlimited field
for debate and polemic, for hypothesis and investigation, for
deduction and determination. It is the factors of organic
evolution, the factors of each of the particular phases or aspects
of evolution phenomena, that are the subject of present-day
biological study and discussion. The mechanism and method
of evolution is a subject, with its score of moot questions, its
enormous opportunity and inspiration, for fact gathering, fact
arranging, and fact interpreting. So in biological science to-
day, no less but even more than in those first exciting days
after the "Origin of Species," the subject and problems of
evolution are the inspiriting and absorbing matters which chiefly
occupy the attention of biologists. What these factors are that
compose the chief subject of present-day evolution study and
discussion may be summarily set out in the present chapter.

In the first place it is obvious that there can be no transfor-
mation or change of species unless there is an ever-present actual
variation. By variation is simply meant, in the larger sense,
that no two individual organisms in the world, nor for that
matter any two that have ever been in the world, are exactly

50 EVOLUTION AND ANIMAL LIFE

alike! This refers not only to individuals of different species
of plants and animals, but to individuals of the same species
and even (and this in a way is most important of all) to indi-
viduals born of the same parents. It is indeed this last condi-
tion that is the actual basis and fundamental beginning for
species change. That this variation does exist is absolute fact,
and there is no discussion of it.

To what extent or degree, what parts of an organism are
chiefly affected, whether or no this variation shows a regularity
in its occurrence or a determinateness of tendency or direction,
whether or no this variation is based on inheritance and if so
in what degree of similarity or identity — all these and a dozen
other questions are the moot problems in connection with the
great factor variation. These are undecided things, which
means, on the whole, that variation, apart from the observed
and admitted actuality of the occurrence, is itself a great
evolution problem.

The variation alone, however, presumably does not make
new species nor maintain lines of descent. If this variation is,
as it seems to be, almost unlimited in its range of appearance,
then as species are of definite character and number and as lines
of descent are even more definite and more limited as to number,
there must be some factor which determines what kinds or lines
of variation may or shall persist and what shall be extinguished.
Is there something incident to the causes of variation that
determines what lines of descent shall be established by it or
based on it, or is there some added factor which, having no
control over the initial appearance of variation, has absolute
control over its persistence and headway? Darwin's factors of
selection, more particularly natural selection, is the explanation
of this control offered in the famous "Origin of Species.'7 And
natural selection has been in the minds of biologists until to-day,
at least, undoubtedly that factor in evolution which has been
believed to have the chief control in the forming of species and
the direction of descent lines.

But in reference to this particular factor three schools of
biologists have gradually grown up; namely, first the school
headed by Weismann, who has believed and contended that
natural selection is almost the only factor which, on a basis of
fortuitous, that is, uncontrolled, variation, has produced the
species and lines of descent as we know them; second, the

FACTORS AND MECHANISM OF EVOLUTION .51

•

school which holds that natural selection has practically nothing
to do with species-forming but only, and in a large general way,
with the control of descent; and, third, the compromise school,
which attributes to natural selection an important part in both
species-forming and control of general descent lines, but recog-
nizes the simultaneous existence and the considerable im-
portance of several other species-forming and descent-modifying
factors. In addition to these three schools one must note that
a number of active \vorking biologists repudiate the factor of
natural selection entirely, holding it to be a vagary and an
artifact of logic.

Associated with natural selection in the general theory of
selective action is Darwin's conception of sexual selection. This
factor was presumed by Darwin to play a part only in the forma-
tion and control of those often very obvious but never well-
understood characteristics of a secondary sexual character
which distinguish the sexes in many species of animals. Let
one recall these characters in the pea fowl, the bird of paradise,
the pheasant, some of the butterflies, the lamellicorn beetles,
many fishes, and so on. According to the theory of sexual
selection the females have chosen for their consorts those males
best endowed by variation with these ornamental character-
istics, so that by this selection there has come about a gradual
cumulation of the characteristics culminating in such bizarrerie
as we are familiar with in numerous living animals.

The word selection will certainly bring to the mind of the
reader also a third kind of selective process, namely, that called
artificial selection, and this kind of selection is, of course, a
factor, and an important one, and has been such for some eighty
centuries, in the modification of plant and animal forms. But
however widely differing and extraordinarily modified culti-
vated and domesticated kinds of animals and plants may be,
these different kinds are not looked on by biologists as having the
validity, that is, the stability and characteristics of origin, that
the different species of animals and plants found in nature have.

All the different kinds of pigeons, for example, are known to
be due primarily to the artificial modification of a single wild
kind, the rock dove of Europe, and all of these different artifi-
cially produced kinds agree in an important physiological charac-
teristic, namely, that of being able to mate freely with each other
and with their common ancestor. As this physiological char-

52 EVOLUTION AND ANIMAL LIFE

acteristic is precisely one of the criteria largely used in deter-
mining species limits in nature, naturalists call the artificially
produced kinds by another name than species; they call them
races or varieties, meaning by this to indicate obvious struc-
tural and functional differences. Thus artificial selection, while
a factor in determining the extent and character of the modifi-
cation of many kinds of animals and plants, is not considered a
factor in the determination of natural lines of descent. Its
value in this regard lies in the clew it gives to natural processes
of the same kind.

Selection by nature among the variations which appear is
made possible only by several other factors or actually existent
conditions. One is the "prodigality of production" or the
constant tendency to overpopulation due to reproduction by
multiplication or in a geometrically progressive ratio. Every
mature female or hermaphroditic plant or animal produces,
at least in the condition of eggs or germ cells, more than one
new individual like itself. (There are a very few exceptional
cases, compensated for, however, in other ways.) Most produce
many new individuals and some reproduce enormously. Cer-
tain fishes lay millions of eggs; so do certain oysters; many
insects produce thousands of young; many plants produce
myriads of seeds. But not all can grow up: there is neither
room nor food for all. There must inevitably be a selection by
active or passive, guided or fortuitous, means.

It is a necessary assumption, for the effectiveness of the
natural selection factor, that this selection is actually based on
the fitness or advantage of some of the variations as compared
with others. The trying out or determination of the advantage
of these variations comes about as an inevitable active or passive
competition for life among the overabundantly appearing new
individuals. This is the "struggle for existence/' and the
"survival of the fittest" is the expression of the assumed fact
of the success of the individuals advantageously (i. e., most
fitly) varying. The unfit and the less fit are assumed to com-
pose the thousands and hundreds of thousands who must die
where only tens or hundreds can live at one time.

But if natural selection, which is, so far, obviously one
of but individuals alone, is to produce new species and control
descent lines, it has to depend on a further factor, one named by
a familiar word, but not at all explained by it, namely, the factor

FACTORS AND MECHANISM OF EVOLUTION .",:;

heredity. Although we can rely in our theory building on the
fact that no two individuals are exactly alike, yet we can equally
certainly rely on the fact that the offspring of any individual
will be much more like other individuals of the species to which
the parent belongs than like individuals of other species, and
also, in the main, more like the parent than like other indi-
viduals of the same species. Heredity is the name we use for
expressing this fact of likeness of young to parent.

Some biologists seem to mean by heredity a force or dominat-
ing influence which brings about this likeness ; while others use
the word heredity to name rather the processes which are gone
through with by the young in becoming, in its total develop-
ment, like the parent. The essential connotation of the word
is, however, simply the fact that this likeness does exist and that
we may rely on its continuing to occur. So that when the
struggle for existence weeds out, if it does, those individuals
of a too abundant population which possess variations of
disadvantage or of no special advantage, leaving those to
survive and produce offspring which do possess specially
advantageous or fit variations, the fact of heredity permits us
to assume the almost certain perpetuation of these advan-
tageous variations by insuring their reappearance in the off-
spring of the "saved" individuals. Thus while we may liken
the causes that produce ever-appearing variations to a centrif-
ugal force making for difference and instability, heredity (if
used as the name for the causes that produce likeness) may
be conceived as a centripetal force, making for stability and
sameness.

But at least one other factor seems to be necessary in
species-forming and that is the factor of isolation, separation,
or segregation, as it is variously named. By this is meant that
those individuals showing similar variations must in some way
be segregated, made to live and breed together, in order that the
particular variations (which from the point of view of the
student of species-forming may be called also the particular
varietal differences that are to become in time so developed and
fixed as to be true species differences) may be maintained.

For it is obvious that if an individual possessing certain

particular variations mate with another of its species possessing

different variations, the offspring of this union will likely not

possess in pure form the variations of that particular parent

5

54 EVOLUTION AND ANIMAL LIFE

we are for the moment interested in. The offspring may show
a blend of the different characters of the parents, or a mosaic
of them, or may show the characters of either one alone, or,
indeed, characters of wholly new type. The important thing
is, however, that there is no certainty — indeed there is almost
certainty of the opposite — that any particular variation will be
fostered and fixed if miscellaneous interbreeding is allowed.
So that a segregation of individuals having certain common
variations or varietal characters is necessary for the perpetua-
tion of these characters.

Now the most usual way, probably, in which this segregation
or isolation is brought about is by topographic or geographic
barriers ; a group of individuals gets isolated from others of their
species by some physical barrier, and the variations that appear
among them, due often to some cause incident to the special
locality and hence common to all of them, are readily preserved
and fostered by the enforced breeding among themselves.
But such an isolation may conceivably be brought about in
several other ways, and observation has shown that probably in
some cases so-called biologic isolation occurs, that is, that n
restriction of miscellaneous interbreeding among individuals of
one species, and an enforced selective breeding among certain
ones possessing certain variations or differences in common,
does really obtain. Such isolation is also called physiologic, or
sexual, isolation.

Many biologists, and the number of them has increased
rapidly in the last few years, due primarily to the activity and
leadership of the botanist de Vries (Amsterdam), believe that
species-forming is achieved without the aid of the selection
factor; that the actual production of species is a function of
variation ("mutation" the special kind of variation efficient
in species-making is called), and that the influence of selection
is only of a more remote and generally restraining, and thus
directive, nature. Such biologists may be said to believe in
species-forming by heterogenesis or saltation, as contrasted
with species-making by slow, gradual transmutation. And
de Vries and his followers have adduced a few apparently
undeniable examples of species-forming by heterogenesis. At
least this influence seems to have produced forms to all in-
tents and purposes apparently similar to natural species. So
the particular kind of variation* called mutation, which is the

FACTORS AND MECHANISM OF EVOLUTION 55

*

basis of this sort of species-making, must be added to our list
of evolution factors.

•Some other biologists, of whom the botanist Nageli, the
eoologist Eimer, and the paleontologist Cope are rcpresenta-
£ives (all three of these men, however, having evolution theories
and beliefs distinct and peculiar to each), believe in what may
be called orthogenetic evolution. That is, that the lines of
descent are determined by the appearance of certain special
determinate lines or tendencies of variation or change, this non-
fortuitous and determinate variation being itself determined
by certain causes either (in Nageli 's belief) inherent in life, or
(in Eimer's belief) extrinsic to life but imposed upon it, as for
example the influence of climate, etc. So that orthogenesis or
determinate variation should also find a place in any list of
assumed evolution factors.

While it is apparent that variation is ever present and also
apparent that heredity or the fact of likeness is always ever to
be relied on, the exact relationship or correlation of these two
evolution factors is not so apparent. That heredity often
preserves or perpetuates variations after they have occurred is
well proved, but it is also proved that some variations appearing
in the parent are not handed on to the parent's offspring, nor
indeed to any future generations of the line. And the general
answer to the natural query raised by this condition is that
variations which are congenital or blastogenic, that is, are
determined at birth for it (although they appear of course only
after development), are heritable (that is, will be passed on
from parent to offspring); but that variations or modifications
acquired during the lifetime of the individual, that is, those which
are impressed on it by extrinsic influences during its "growing
up " or development, will not be heritable. Thus such modifica-
tions in body parts as may be produced by use or disuse, or by
other functional stimulation or lack of it, changes caused by
mutilation or disease, etc., are believed by most biologists to be
non-heritable. Hence it is that only the congenital variations
are looked on by these biologists as of importance in the matter
of species-forming. Yet the whole pre-Darwinian evolution
theory of Lamarck was founded on the assumption that the
modifications in individuals due to use, disuse, and other func-
tional stimulation, in a word that all body change and adapta-
tion, all characters acquired during the lifetime of an individual,

56 EVOLUTION AND ANIMAL LIFE

can be, in some degree at least, handed on by inheritance to the
offspring. And there are to-day many Lamarckian evolu-
tionists. So that in our list of possible evolution factors the
so-called Lamarckian factor should not be omitted. And in
connection with it may be considered, by and large, the imme-
diate influence or non-influence on individuals and on species
of all environmental conditions; and particularly the results
of such influence during development. In fact the study of
development has come largely to be a study of the actual in-
fluences or factors that determine and guide growth, instead of
one purely descriptive and comparative as in the older days
of embryological study. Some of these factors are apparently
strictly inherent in the protoplasmic germ cells and in the
embryo substance: others are as obviously extrinsic or epigenetic.
And the determination of the relative influence and power of
these two sets of developmental factors and of the various
members of each set is one of the most eagerly worked-at
problems of modern biological study.

Finally, the general term adaptation should be mentioned
in any list of evolution factors; although it is more usually
looked on, not as a factor, but as an evolution problem and
indeed one of the greatest of the problems. Adaptation is
precisely one of the things evolutionists are trying to find the
causes or causal factors of. But nevertheless the adaptability
of life stuff, its plasticity and capacity of advantageous reac-
tion, is, to many biologists, a fundamental fact in organic
nature, like gravitation or chemical affinity in inorganic nature:
a thing basic and inexplicable, and in itself a factor whose con-
sequences are to be determined but not further to be ques-
tioned as to their cause.

CHAPTER V

NATURAL SELECTION AND THE STRUGGLE
FOR EXISTENCE; SEXUAL SELECTION

The tendency to regard natural selection as more or less unnecessary
or superfluous which is so characteristic of our day, seems to grow out
of reverence for the all-sufficiency of the philosophy of evolution, and
pious belief that the history of living things flows out of this philosophy
as a necessary truth or axiom. — BROOKS.

La selection naturelle est un principe admirable et parfaitement
juste. Tout le monde est d 'accord aujourd'hui sur ce point. Mais
ou Ton n'est pas d'accord, c'est sur la lirnite de sa puissance et sur la
question de savoir si elle pent engendrer des formes specifiques
nouvelles. II semble bien demontre aujourd'hui qu'elle ne le peut.

— DELAGE.

OF all the various factors of organic evolution the one
which has been most relied on as the great determining agent /
is that called Natural Selection, the survival of the individuals
best fitted for the conditions of life, with the inheritance of
those species-forming adaptations in which fitness lies. The
primal initiative is not in natural selection, but in variation,
germinal and individual. This may be slight variation (fluc-
tuation) or large deviation (saltation), but in any case all
difference in species or race must first be individual. The
impulse to change, once arisen, is continued through heredity.
From natural selection arises the choice among different lines
of descent, the adaptive tending to exclude the non-adaptive,
while traits which are neither helpful nor hurtful, but simply
indifferent, may be borne along by the current of adaptive
characters. Finally separation or isolation tends to preserve a
special line of heredity from being merged in the mass which
constitutes the parent stock or species.

Without individual variation, no change could take place;
all organisms would be identical in structure. Without heredity,

57

58 EVOLUTION AND ANIMAL LIFE

if we could conceive such a condition, no change would persist.
Without selection, there would be no premium placed on
adaptive characters, and organisms would persist in every
degree of variance with their surroundings. Without some
degree of isolation, every change would be lost by cross-breeding
with the mass. In a world of varying conditions with varying
organisms, it is not conceivable that species should, through
all their generations, undergo no change. Nor in the changes
of any species is it possible that any one of the factors or con-
ditions named above should be wholly absent. But the effects
of each one may show themselves in many different ways, and
each may be modified by other facts or conditions. We have
compared the history of species to the flow of a river. A single
rock may change the course of a stream. In like manner
incidental circumstances may determine the evolution of a
species. Or using a different metaphor we may compare the
course of a species with that of a glacier. The movement of a
glacier depends on the law of gravitation "resident'7 within
its molecules. Its course is determined by th^ topography of
its bed. To this bed it is perfectly fitted, but the condition of
its surface depends on circumstances related neither, to the law
of gravitation nor to the form of its bed. A species of animal or
plant is well fitted to its conditions in life. This natural
selection rigidly enforces ; but its surface characters, wThich are
not essential to its life, are determined by other influences, and
in this both selection and environment play but a minor part.

All animals feed upon living organisms or upon that which
has been living. Hence each animal throughout its life is busy
with the destruction of the other organisms or with their
removal after death. If these creatures, animals, or plants on
which animals feed, are to hold their own, there must be an
excess of birth and development to make good the drain upon
their numbers. If the plants did not restore their losses the
animals that feed on them would perish. In like fashion flesh-
eating animals are dependent on those which feed on plants.

But throughout nature there is a vast excess in the process
of reproduction. More plants sprout than could find standing
room were all to grow. More seeds are developed than can find
place to sprout. More animals are born than can possibly
survive. The process of increase among animals is rightly
called multiplication. Each species tends to increase in geo-

NATURAL SELECTION; SEXUAL SELECTION 59

metric ratio, but as it multiplies it finds the world already
crowded with other multiplying species. A single pair of any
species whatsoever, if not checked by adverse conditions, would
soon fill the whole earth with its progeny.

An annual plant producing two seeds only would have
1,048,576 descendants in twenty-one years, if each seed sprouted
and matured. But most plants produce hundreds or thousands
of seeds. The ratio of increase is a matter of minor importance.
It is the ratio of increase above loss which determines the fate
of species. Those species increase in numbers in which the
gain exceeds the rate of destruction through the influence of
other species or the adverse conditions of life. Where few
enemies exist the ratio of increase need not be large. One of
the most abundant of birds is the fulmar petrel of the mid-
Pacific. It lays but one egg yearly, but it has few enemies and
the low rate of increase suffices to cover the sea with fulmars
within the region it inhabits.

It is not easy to realize the inordinate numbers any species
would attain were it not for the checks produced by the presence
of the activity of other organisms. Certain protozoa, at their
normal rate of increase — if none were devoured or destroyed —
might fill the entire ocean within a very short time. It is said
that the conger eel lays 15,000,000 eggs yearly. If each hatched
and the conger grew to maturity, in a few years there would be
no room for any other kind of fish in the sea. The codfish has
been known to produce 9,100,000 eggs each year. If each egg
were to develop, in ten years the sea would be solidly full of
codfish.

The female quinnat salmon of the Columbia, Oncorhynchus
tchawytscha, ascends the river at the age of about four years,
and lays 4,000 eggs, after which she dies. Half these eggs
develop into males. If each female egg came to maturity, we
should have at the end of fifty years 8,000,000,000,000,000,-
000,000,000,000,000,000,000,000,000 female salmon and as
many males as the offspring of a single pair. It takes about
one hundred of these salmon to weigh a ton. Could all these
fishes develop, in a very short time there would be no room
for them in all the rivers of the North, nor in all the waters of
(hr s<>a.

If each egg of the con in ion house fly should develop and each
of the larva- should find the food and temperature it needed,

60 EVOLUTION AND ANIMAL LIFE

with no loss and no destruction, the people of the city in which
it happened would suffocate under the plague of flies. When-
ever any species of insect develops a large percentage of the eggs
laid, it becomes at once a plague. Thus originate plagues of
locusts, grasshoppers, and caterpillars. But the crowd of life
renders these plagues rare. Scavenger-beetles and bacteria
destroy the decaying flesh where the fly would lay its eggs.
Minute creatures, bacteria, protozoa, other insects, are parasitic
within the larva itself. Millions of flies starve to death. Mil-
lions more are eaten by birds and predaceous insects. The
final result is that from year to year the number of flies does
not increase. Linnaeus once said that " three flies will devour
a dead horse as quickly as a lion." Quite as soon would three
bacteria with their descendants reach the same result. "Even
slow-breeding man," says Darwin, "has doubled in twenty-five
years. At this rate in less than a thousand years there literally
would not be standing room for his progeny. The elephant is
reckoned the slowest breeder of all animals. It begins breeding
when thirty years old and goes on breeding until ninety years
old, bringing forth six young in the interval and surviving to be
a hundred years old. If this be so, after about 800 years there
should be 19,000,000 elephants alive descended from the first
pair." A few years of still further multiplication without check,
and every foot of the earth would be covered by elephants.

Similar calculations may be made in regard to any species of
animal or plant whatsoever. Each one increases at a rate which
without checks would make it soon cover the earth. Yet the
number of individuals in a state of nature in any species re-
mains about stationary. With the interference of man, in
many species the numbers slowly diminish; very few increase.
There are about as many squirrels in the forest one year as
another, as many butterflies in the field, as many frogs in the
pond. Wolves, bears, deer, ducks, singing birds, fishes, all suf-
fer from man's attacks or man's neglect and grow fewer year
by year. It is manifest that the tendency to reproduce by
geometric ratio meets everywhere with a corresponding check.
This check is known as the Struggle for Existence.

The struggle for existence is threefold: (a) Among individuals
of one species, as wolf against wolf or sparrow against sparrow;
(6) between individuals of different species, as rabbit with wolf
or blue-bird with sparrow; (c) with the conditions in life — as

NATURAL SKLECTION; SEXUAL SELECTION 61

the necessity of the robin to find water in summer or to keep
warm in winter. All three forms of the struggle for existence,
intraspecific, interspecific, and environmental, are constantly
operative and with every species. In some regions or under
some conditions the one phase may be more destructive, in
others another. Any one of these may be in various ways
modified or ameliorated. When the conditions of life are most
easy, as with most species in the tropics, there the conflict of
individuals and the conflict of species is most severe. It is not
possible to say that any one of these three forms of struggle and
selection is more potent than the others. In fact, the first and
the second are in a sense forms of the third. All struggle is,
strictly speaking, with the conditions of life. Those individuals

FIG. 38. — Praying mantis, eating a grasshopper. (Adapted from photograph
from life by Slingerland.)

which endure this struggle survive to reproduce themselves.
The rest die and leave no progeny.

Because of the destruction resulting from the struggle for
existence, more individuals in each species are born than can
mature. The majority fail to reach maturity because for one
reason or another they cannot do so. All live that can. Each
animal tries to feed itself : many try to take care of their young.
But in self protection and in propagation of the species very few
individuals succeed in comparison with the vast number which
the process of reproduction calls into being.

The destruction in nature is not indiscriminate. In the
long run and for the most part, those creatures least fitted to
resist are the first to perish. It is the slowest animal which is
soonest overtaken by the pursuers. It is the weakest which is

62

EVOLUTION AND ANIMAL LIFE

crowded aside or trampled on by its associates. It is the least
adaptable which suffers most from extremes of heat and cold.
By the process of Artificial Selection the breeder improves his
stock, destroying his weakest or least comely calves, reserving
the strong and fit for parentage. In like fashion, on an in-
conceivably large scale, the forces of nature are at work modify-
ing and fitting to the demands of their surroundings the different
species of animals. Because the processes and results of the
struggle for existence seem parallel with those of artificial
selection, Darwin suggested the name of Natural Selection
for the sifting process as seen in nature. To the general re-
sult of natural selection, Herbert Spencer has applied the term

FIG. 39. — The Australian ladybird, Vedalia cardinalis, feeding on cottony cushion scale,
Icerya purchasi. (From life.)

Survival of the Fittest. By fitness in this sense is meant only
adaptation to surrounding conditions, for the process of natural
selection has no necessary moral element, nor does it necessarily
work toward progress among organisms. With changing con-
ditions species undergo change. Some individuals, by the
possession of slight advantageous variations of structure or of
instinct, meet these new demands better than others. These
survive, the others die. The survivors produce young sharing
in part, at least, their own advantages, and with renewed selec-
tion the degree of adaptation increases with successive genera-
tions.

To the process of natural selection we must, in most cases,
probably ascribe the adjustment of species to surroundings.

NATURAL SELECTION; SEXUAL SELECTION 63

Natural selection does not create species, it enforces adaptation.
If a species or a group of individuals cannot fit itself to its
environment, it will be crowded out by others which can do so.
It will then either disappear entirely from the earth, or it will be
limited to that region or to those conditions to which it is
adapted. A partial adjustment tends to become more perfect,
for the individuals least fitted are first destroyed in the struggle
for existence. Very small variations may sometimes, therefore,
lead to great changes. A side issue apparently unimportant
may perhaps determine the fate of a species. Any advantage
however small may possibly turn the scale of life. " Battle
within battjes must be continually recurring, with varying suc-
cess, yet in the long run the forces are so nicely balanced that
the face of nature remains for a long time uniform, though
assuredly the merest trifle would give the victory to one organic
being over another."
Darwin says:

" I have found that the visits of bees are necessary for the fertili-
zation of some kinds of clover; for instance, twenty heads of white
clover (Trifolium repens) yielded two thousand two hundred and ninety
seeds, but twenty other heads protected from the bees produced not
one. Again, one hundred heads of red clover (Trifolium pratense)
produced two thousand seven hundred seeds, but the same number of
protected heads produced not a single seed. Humble-bees alone visit
red clover, as other bees cannot reach the nectar. . . . Hence we may
infer as highly probable that, if the whole genus of humble-bees became
extinct or very rare in England, the heartsease and red clover would
become very rare or wholly disappear. The number of humble-bees
in any district depends in a great measure on the number of field mice,
which destroy their combs and nests; and Colonel Newman, who has
long attended to the habits of humble-bees, believes that more than
two-thirds of them are thus destroyed all over England. Now the
number of mice is largely dependent, as everyone knows, on the num-
ber of cats; and Colonel Newman says: 'Near villages and small towns
I have found the nests of humble- bees more numerous than else-
where, which I attribute to the number of cats that destroy the mice.'
Hence it is quite credible that the presence of feline animals in large
numbers in a district might determine, through the intervention
first of mice and then of bees, the frequency of certain flowers in
that district."

64 EVOLUTION AND ANIMAL LIFE

Huxiey carries this calculation still further by showing that
the number of cats depends on the number of unmarried women.
On the other hand, clover produces beef, and beef strength.
Thus in a degree the prowess of England is related to the number
of spinsters in its rural districts! This statement would be true
in all seriousness were it not that so many other elements come
into the calculation. But whether true or not, it illustrates the
way in which causes and effects in biology become intertangled.

There was introduced into California from Australia, on
young lemon trees, twenty-five years ago, an insect pest called
the cottony cushion scale (Icerya purchasi). This pest in-
creased in numbers with extraordinary rapidity, and in ten years
threatened to destroy completely the great orange orchards of
California. Artificial remedies were of little avail. Finally, an
entomologist was sent to Australia to find out if this scale insect
had not some special natural enemy in its native country. It was
found that in Australia a certain species of ladybird beetle
attacked and fed on the cottony cushion scales and kept them
in check (Fig. 39). Some of these ladybirds (Vedalia cardi-
nalis) were brought to California and released in a scale-infested
orchard. The ladybirds, having plenty of food, thrived and
produced many young. Soon they were in such numbers that
many of them could be distributed to other orchards. In
two or three years the Vedalias had become so numerous
and widely distributed that the cottony cushion scales began
to diminish perceptibly, and soon the pest was nearly
wiped out. But with the disappearance of the scales came
also a disappearance of the ladybirds, and it was then dis-
covered that the Vedalias fed only on cottony cushion scales
and could not live where the scales were not. So now, in order
to have a stock of Vedalias on hand in California, it is necessary
to keep protected some colonies of the cottony cushion scale
to serve as food. Of course, with the disappearance of the pre-
daceous ladybirds the scale began to increase again in various
parts of the State, but with the sending of Vedalias to these
localities the scale was again crushed. How close is the inter-
dependence of these two species!

There is little foundation for the current belief that each
species of animal has originated in the area it now occupies, for
in many cases our knowledge of palaeontology shows the reverse
of this to be true. Even more incorrect is the belief that each

NATURAL SELECTION; SEXUAL SELECTION 65

species occupies the district or the surroundings best fitted
for its habitation. This is manifested in the fact of the extraor-
dinary fertility and persistence shown by many kinds of animals
and plants in taking possession of new lands which have become,
through the voluntary or involuntary interference of man, open
to their invasion. Facts of this sort are the "enormous in-
crease of rabbits and pigs in Australia and New Zealand, of
horses and cattle in South America, and of the sparrows of
North America, though in none of these cases are the animals
natives of the countries in which they thrive so well " (Wal-
lace). The persistent spreading of European weeds to the
exclusion of our native plants is a fact too well known to every
farmer in America. The constant moving westward of the
white weed and the Canada thistle marks the steady deteriora-
tion of our grass fields. The cockroaches in American kitch-
ens represent invading species from Europe. The American
cockroaches live in the woods. Perhaps a majority of the
worst insect pests of the United States are of European or
Asiatic origin. Especially noteworthy are cases of this type
in Australia and New Zealand. In New Zealand the weeds
of Europe, toughened by centuries of selection, have won an
easy victory over the native plants.

Dr. Hooker states that, in New Zealand " the cow grass has
taken possession of the roadsides; dock and watercress choke
the rivers; the sow thistle is spread all over the country, growing
luxuriantly up to 6,000 feet; white clover in the mountain dis-
tricts displaces the native grasses." The native Maori saying
is: "As the white man's rat has driven away the native rat, as
the European fly drives away our own, and the clover kills our
fern, so will the Maoris disappear before the white man himself."

Prof. Sidney Dickinson gives the following notes on the
rabbit and other plagues of Australia:

"The average annual cost to Australasia of the rabbit plague is
£700,000, or nearly $3,500,000. The work which these enormous figures
represent has a marked effect in reducing the number of rabbits in the
better districts, although there is little to suppose that their extermina-
tion will ever be more than partial. Most of the larger runs show very
few at present, and rabbit-proof fencing, which has been set around
thousands of square miles, has done much to check further inroads.
Until this invention began to be utilized it was not uncommon to find

66 EVOLUTION AND ANIMAL LIFE

as many as a hundred rabbiters employed on a single property whose
working average was from three hundred to four hundred rabbits per
day. As they received five shillings a hundred from the station owner,
and were also able to sell the skins at eight shillings a hundred, their
profession was most lucrative. Seventy-five dollars a week was not
an uncommon wage, and many an unfortunate squatter looked with
envy upon the rabbiters, who were heaping up modest fortunes, while
he himself was slowly being eaten out of house and home.

"The fecundity of the rabbit is amazing, and his invasion of remote
districts swift and mysterious. Careful estimates show that, under
favorable conditions, a pair of Australian rabbits will produce six
litters a year, averaging five individuals each. As the offspring them-
selves begin breeding at the age of six months, it is shown that, at this
rate, the original pair might be responsible in five years for a progeny
of over twenty millions. That the original score that were brought to
the country have propagated after some such ratio, no one can doubt
who has seen the enormous hordes that now devastate the land in
certain districts. In all but the remoter sections, the rabbits are now
fairly under control; one rabbiter with a pack of dogs supervises
stations where one hundred were employed ten years ago, and with
ordinary vigilance the squatters have little to fear. Millions of the
animals have been killed by fencing in the water holes and dams during
a dry season, whereby they died of thirst, and lay in enormous
piles against the obstructions they had frantically and vainly striven
to climb, and poisoned grain and fruit have killed myriads more. A
fortune of £25,000 offered by the New South Wales Government still
awaits the man who can invent some means of general destruction,
and the knowledge of this fact has brought to the notice of the various
colonial governments some very original devices.

"Another great pest to the squatters is developing in the foxes,
two of which were imported from Cumberland some years ago by a
wealthy station owner, who thought that they might breed, and
give himself and friends an occasional day with the hounds. His
modest desires were soon met in the development of a race of foxes
far surpassing the English variety in strength and aggressiveness,
which not only devour many sheep, but out of pure depravity worry
and kill ten times as many as they can eat. When to these plagues is
added the ruin of thousands of acres from the spread of the thistle,
which a canny Scot brought from the Highlands to keep alive in his
breast the memories of Wallace and Bruce; the well-nigh resistless
inroads of furze; and, in New Zealand, the blocking up of rivers by

NATURAL SELECTION; SEXUAL SELECTION 67

the English watercress, which in its new home grows a dozen feet in
length, and has to be dredged out to keep navigation open, it may be
understood the colonials look with jaundiced eye upon suggestions
of any further interference with Australian nature.

"Not to be outdone by foreign importations, the country itself
has shown in the humble locust a nuisance quite as potent as rabbit,
fox, or thistle. This bane of all men who pasture sheep on grass has
not been much in evidence until within the last few years, when
the great destruction of indigenous birds by the gun and by poisoned
grain strewn for rabbits has facilitated its increase. The devastation
caused by these insects last year was enormous, and befell a district a
thousand miles long and two thousand wide. For days they passed in
clouds that darkened the earth with the gloomy hue of an eclipse,
while the ground was covered with crawling millions, devouring every
green thing and giving to the country the appearance of being carpeted
with scales. It has been discovered, however, that before they attain
their winged state they can easily be destroyed, and energetic measures
will be taken against them throughout all the inhabited districts of
Australia whenever they make another appearance."

The conditions of the struggle for existence are not neces-
sarily felt as an individual stress to the individuals which sur-
vive. The life they lead is the one for which they are fitted.
The struggle is painful or destructive only to those imperfectly
adapted. Men in general are fitted to the struggle endured by
their ancestors as they are adapted to the pressure of the air.
They do not recognize the pressure itself but only its fluctua-
tions. Hence many writers have supposed that the struggle
for existence belongs to animals and plants and that man is
or should be exempt from it. Competition has been identified
with injustice, fraud, or trickery, and it has been supposed that
it could be abolished by acts of benevolent legislation. But
competition is inseparable from life. The struggle for existence
may be hidden in social conventions or its effects more evenly
distributed through processes of mutual aid, but its necessity is
always present. Competition is the source of all progress.

The first suggestion of the doctrine of natural selection
came to Darwin through the law of population as stated by
Thomas Malthus. The law of Malthus is in substance as fol-
lows: Man tends to increase by geometrical ratio — that is, by
multiplication. The increase of food supply is by arithmetical

68 EVOLUTION AND ANIMAL LIFE

ratio — that is, by addition; therefore, whatever may be the ratio
of increase, a geometrical progression will sooner or later outrun
an arithmetical one. Hence sooner or later the world must be
overstocked, did not vice, misery, or prudence come in as checks,
reducing the ratio of multiplication. This law has been criti-
cised as a partial truth, so far as man is concerned. This means
simply that there are factors also in evolution other than those
recognized by Malthus. Nevertheless, Malthus's law is a sound
statement of one great factor. And this law is simply the ex-
pression of the struggle for existence as it appears among men.

The doctrine of organic evolution was first placed on a firm
basis by Darwin, because Darwin was the first who clearly
defined the force of natural selection. Darwin, however, rec-
ognized other factors, known or hypothetical, and was inter-
ested more in showing the fact of descent and one cause of
modification than in insisting on the all-sufficiency of the cause
especially defined by himself.

In later times, Weismann and his followers have laid more
exclusive stress on natural selection and its Allmacht or ex-
clusive power in bringing about organic evolution. This view
is known as Neo-Darwinism and the school of workers who
profess it as Neo-Darwinians. Few investigators question
the far-reaching influence of natural selection, but there are
many phases in organic evolution which cannot be ascribed
to it. Hence the search for other factors has been assiduously
prosecuted, and doubts of Darwinism have been widely ex-
pressed; but this doubting has been thrown not so much on
the Darwinism of Darwin, nor, as a rule, on the law of natural
selection, but rather on the Allmacht claimed for it by Weis-
mann and his associates.

Without attempting any elaborate discussion of questions
still far from settled we may venture these suggestions :

1. Given the facts of individual variation, of inheritance,
and some check to freedom of migration, natural selection would
accomplish some form of organic evolution; species would be
formed by the survival of the adapted, adaptations would be
perpetuated, and minor differences would develop in time into
deep-seated differences.

2. With natural selection alone, however, the actual facts in
organic evolution as we know them would apparently not be
achieved.

NATURAL SELECTION; SEXUAL SELECTION 69

3. In other words, while natural selection furnishes the
motive force of change, other influences, extrinsic and intrinsic,
help to direct the channels in which life runs. It is necessary to
consider other causes for the great body of indifferent characters
or traits not produced by adaptation, and apparently not yield-
ing either advantage or disadvantage in the struggle for life.

4. The formation of species of animals and plants through
natural selection finds an analogy in the formation of rivers
through gravitation. Gravitation is the motive power carrying
the waters from the uplands to the sea. The courses of streams
are determined by a number of minor influences acting in con-
currence with gravitation, the final result far more complex than
the single cause would produce.

5. In like fashion, while natural selection is the motive
element in descent or evolution, the total result is due to a
concurrence of causes, and is too complex to be explained by
natural selection, by the principle of utility, or the survival of
the fittest alone, and the varying effects must be ascribed to a
variety of causes.

Certain minor traits, as color patterns, relative proportions
of parts, survive — apparently without special utility, but because
these traits were borne by some ancestors or group of ancestors.
This has been called the Survival of the Existing. In making
up the fauna or flora of any region those organisms actually
present when the region is first stocked must leave their qual-
ities as an inheritance. If they cannot maintain themselves
their breed disappears. If they maintain themselves in iso-
lation their characters remain as those of a new species. In
hosts of cases, the survival of characters rests not on any
special usefulness or fitness, but on the fact that individuals
possessing these characters have inhabited or invaded a certain
area. The principle of utility explains survivals among com-
peting structures. It rarely accounts for qualities associated
with geographic distribution. The nature of the animals which
first colonize a district must determine what the future fauna
shall be. From their actual specific characters, largely traits
neither useful nor harmful, will be derived for the most part
the specific characters of their successors.

It is not essential to the meadow lark that he should have
a black blotch on the breast or the outer tail feathers white.
Yet all meadow larks have these marks, as all shore larks possess
G

70 EVOLUTION AND ANIMAL LIFE

the tiny plume behind the ear. Any character of the parent
stock, which may prove harmful under new relations, will be
eliminated by natural selection. Those especially helpful will
be intensified and modified. But the great body of characters,
the marks by which we know the species, will be neither helpful
nor hurtful. These will be meaningless streaks and spots,
variations in size of parts, peculiar relations of scales or hair or
feathers, little matters which can neither help nor hurt, but
which have all the persistence heredity can give.

In regard to natural selection our knowledge seems positive.
In regard to most other factors of organic evolution we have
to deal so far not with clearly demonstrated facts but with
"probabilities of a higher or lower order/' their value to be
ultimately shown by experiment.

In this connection the following words of Dr. Edwin Grant
Conklin are very pertinent :

"On the whole, then, I believe the facts which are at present at our
disposal justify a return to the position of Darwin. Neither Weis-
mannism nor Lamarckism alone can explain the causes of evolution.
But Darwinism can explain those causes. Darwin endeavored to show
that variations, perhaps even adaptations, were the result of extrinsic
factors acting upon the organism, and that these variations or adap-
tations were increased and improved by natural selection. This is, I
believe, the only ground which is at present tenable, and it is but
another testimony to the greatness of that man of men that, after
exploring for a score of years all the ins and outs of pure selection and
pure adaptation, men are now coming back to the position outlined
and unswervingly maintained by him."

Finally we ought not to suppose that we have already
reached a satisfactory solution of the evolution problem, or
are, indeed, near such a solution.

"We must not conceal from ourselves the fact," says Roux, "that
the causal investigation of organisms is one of the most difficult, if
not the most difficult, problems which the human intellect has at-
tempted to solve, and that this investigation, like every causal
science, can never reach completeness, since every new cause ascer-
tained only gives rise to fresh questions concerning the cause of this
cause."

NATURAL SELECTION; SEXUAL SELECTION

71

In order to explain certain important phenomena outside
the apparent range of natural selection, a theory of another sort
of selective activity is recognized by many biologists. This is
the theory of Sexual Selection first propounded by Darwin.

FIG. 40. — Male and female humming bird; showing sex dimorphism. (After Gould.)

Differences between male and female individuals of the same
species are the rule rather than the exception (Fig. 40). Many
of these differences are what might be called the necessary ones
due to the particular functions assumed by each individual in this
differentiation of sex. Of this nature are, besides those funda-
mental ones of the primary reproductive ones, such others as

72 EVOLUTION AND ANIMAL LIFE

those specially connected with the care and rearing of the
young; as the mamma? of female mammals, the brood pouches
of the female kangaroos and opossums, etc. But a moment's
reflection calls to mind the existence of a host of other differ-
ences between males and females of the same species which
plainly have no such immediate relation to the distinct functions
or duties assumed by each in the business of production and
care of young. For example, the long plume feathers of the
male bird of paradise, the curious chitinous horns of the male
leaf-chafer beetles (Fig. 41), the brilliant plumage of many male
birds as contrasted with the sober dress of the females, and a
host of other distinguishing characteristics of the sexes in many
animal species. Now these differences are all conveniently
named by the phrase " secondary sexual differences," and the
explanation of their origin has come to be one of the most

FIG. 41. — Male and female Scarabeid beetles, Phaneus mexicanus, showing sex dimor-
phism; the male with prominent dorsal horn on head. (From specimens.)

puzzling of biological problems. The most familiar and, for
many years, a widely accepted solution of this problem, is that
embraced in the theory of sexual selection proposed and fought
for by Darwin and Wallace, but later discarded by the latter
of these great naturalists.

Before taking up the sexual selection explanation of dis-
tinguishing sex characters, it is well to pay a little further
attention to the characters themselves. And for this purpose a
rough grouping or classification may be attempted.

The characters may be of special use to the possessor (male
or female) or for the benefit of the young, such as weapons of
offense and defense (antlers of male deer, stings of female bee
and wasp, tusks of male swine, etc.), or special organs for mat-
ing (seizing and holding organs of certain male crabs, suckerlike
holding pads on the feet of male water beetles (Fig. 42), or special
locomotory organs (presence of wings in the male and their

NATURAL SELECTION; SEXUAL SELECTION

73

absence in the female in numer-
ous insect species), or special
sense organs "(the much more
expanded antennae of male cecro-
pia, promethea, polyphemus, and
other bombycine moths, as com-
pared with those of the female),
or special structures for the care
of the young (milk glands of
female mammals, brood pouches
of female marsupials, pits on the
back of the male of the frog Pipa
(Fig. 43), for carrying the eggs,
etc.), or recognition marks (the
eye spots, collars, wing bands,
tail blotches, and such other con-
spicuous color spots and mark-
ings possessed by the males and
wanting in the females of various
bird species), or, finally, char-
acters connected with special
habits of one sex differing from
those of the other (the pollen
baskets and wax plates of the

worker female honey bees, the winglessness of certain female
parasitic insects, the males being nonparasitic and winged, etc.).

The special characters may be
apparently for the purpose of attract-
ing or exciting the other sex, as the
brilliant colors, markings, and other
ornamentation of many male birds,
some mammals, and some reptiles
and very many fishes, and the cries
and songs, special odors, and curious
antics or dancing of the males of
various animals (mammals, birds,
spiders, insects, etc.). In many of
these cases the special secondary
sexual characters appear only during
-A male frog Pipa ih breedmg season; in others they

americana, carrying eggs in pits

on its back. (After Darwin.) are persistent.

FIG. 42. — Fore leg of male water beetle,
Dyticus, showing special suckerlike
expansion of the leg. (After Miall.)

74 EVOLUTION AND ANIMAL LIFE

The characters may also be of the type called reciprocal,
that is, organs which exist in functional condition in one sex, but
in the other appear in rudimentary and often nonfunctional
forms, as the reduced horns of female antelopes and goats, the
undeveloped stridulating organs of female crickets and katydids,
small spurs on the female pheasant, reduced mammae of male
mammals, undeveloped mimicry of male butterflies, etc.

FIG. 44. — Male (A) and female (B) of the fly, Calotarsa insignia Aid., showing secondary
sexual characteristics on the feet of the male. (After Aldrich.)

Finally the characters may be indifferent, that is, without
any apparent utility; as the reduced wings of numerous female
insects, the rudimentary alimentary canal of the male Rota-
toria, absence of antlers of female deer, loss of wings in insect
females, small differences in size and markings between males
and females, slight differences in wing form in hummingbirds,
dragon flies, and butterflies, differences in number of tarsal
and antennal segments in insects, etc.

The explanation of these various differences between males
and females plainly cannot be a single one. The extreme vari-
ety of the secondary sexual differences of itself makes it neces-
sary to find more than one explanation for their existence. To
take the most obvious case, it is apparent that the useful
characters, such as the fighting antlers of the male deer, can be
explained probably by natural selection. At least these char-
acters fall readily into line with precisely that type of useful
•specialization for whose explanation we rely on natural selec-
tion. So practically all those secondary sexual characters of
our first category, namely, those obviously useful to the pos-
sessor or to its young, such as organs of offense and defense,
brood pouches, food-producing or gathering organs, special

NATURAL SELECTION; SEXUAL SELECTION

75

means of locomotion, etc., may be considered to offer no special
problem. Although indeed the reason why these useful char-
acteristics should be possessed by but one sex is by no means
always, or perhaps even often, plain to us.

But the real problem presented by secondary sexual char-
acters is that thrust on us by the nonuseful and even appar-
ently disadvantageous differences. Why the male bird of para-
dise should be decked out in a plumage certain to make it
a conspicuous object to every enemy it has, and of a weight and
difficulty of manipulation that must mean a constant demand
on the strength and attention of the bird, is a question that
demands a special answer. In the same case with the bird
of paradise are the peacock, the gorgeous male pheasant (Fig. 45) ,

FIG. 45. — Male and female argus pheasant; the male is shown in characteristic "courting
attitude." (From Tegetmeier's " Pheasants.")

many hummingbirds (Fig. 40), etc. Now to explain these ex-
traordinary secondary sexual differences the theory of sexual
selection has been devised.

This theory, in few words, is that there is practically a
competition or struggle for mating, and that those males are

76 EVOLUTION AND ANIMAL LIFE

successful in this struggle which are the strongest and best
armed or equipped for battle among themselves, or which are
most acceptable by reason of ornament or other attractiveness
to the females. In the former case mating with a certain
female depends upon overcoming in fight the other suitors, the
female being the passive reward of the victor; in the second case
the female is presumed to exercise a choice, this choice depend-
ing upon the attractiveness of the male (due to color, pattern,
plumes, processes, odor, song, etc.). The actual fighting among
males, and the winning of the females by the victor is an ob-
served fact in the life of numerous animal species. But a spe-
cial sexual selection theory is hardly necessary to explain the
development of the fighting equipment, antlers, spurs, claws,
tusks, etc. This fighting array of the male is simply a special
phase of the already recognized intraspecific struggle; it is not a
fight for room or food, but for the chance to mate. But this
chance often depends on the issue of a life and death struggle.
Natural selection would thus account for the development of
the weapons for this purpose.

For the development, however, of such secondary equal char-
acters as ornament, whether of special plumage, color, .pattern, or
processes, and song, and special odors, and "love dancing," the
natural selection theory can in no way account; the theory of
sexual selection was the logical and necessary auxiliary theory,
and when first proposed it met with quick and wide acceptance.
Wallace in particular took up the theory and applied it to ex-
plain many cases of remarkable plumage and pattern develop-
ment among birds. Later, as he analyzed more carefully his
cases, and those proposed by others, he became doubtful, and
finally wholly skeptical as to the theory.

The theory as proposed by Darwin was based on the follow-
ing general assumptions, for the proof of each of which various
illustrations were adduced. First, many secondary sexual
characters are not explicable by natural selection; they are not
useful in the struggle for life. Second, the males seek the
females for the sake of pairing. Third, the males are more
abundant than the females. Fourth, in many cases there is a
struggle among the males for the possession of the females.
Fifth, in many other cases the females choose, in general, those
males specially distinguished by more brilliant colors, more
conspicuous ornaments, or other attractive characters. Sixth,

NATURAL SELECTION; SEXUAL SELECTION 77

many males sing, or dance, or otherwise draw to themselves
the attention bf the females. Seventh, the secondary sexual
characters are especially variable. Darwin believed that he
had observed certain other conditions to exist which helped
make the sexual selection theory probable, but the conditions
noted are sufficient if they are real.

Exposed to careful scrutiny and criticism, the theory of
sexual selection has been relieved of all necessity of explaining
any but two categories of secondary sexual characters; namely,
the special weapons borne by males, and special ornaments and
excitatory organs of the males and females. For examination
has disclosed the fact that males are not alone in the possession
of special characters of attraction or excitation. Regarding
these two categories Plate in his able recent defense of Darwinism,
says "the first part of this theory, the origin of the special
defensive and offensive weapons of males through sexual selec-
tion, is nearly universally accepted. The second part of the
theory, the origin of exciting organs, has given rise to much
controversy. Undoubtedly the presumption that the females
compare the males and then choose only those which have the
most attractive colors, the finest song, or the most agreeable
odor, presents great difficulties, but it is doubtful if it is possible
to replace this explanation by a better." Some of these diffi-
culties may be briefly enumerated.

The theory can be applied only to species in which the
males are markedly more numerous than the females, or in which
the males are polygamous. In other cases there will be a female
for each male whether he be ornamented or not; and the unor-
namented males can leave as many progeny as the ornamented
ones, which would prevent any accumulation of ornamental
variations by selection. As a matter of fact, in a majority of
animal species, especially of the higher vertebrates, males and
females exist in approximately equal numbers.

Observation shows that in most species the female is wholly
passive in the matter of pairing, accepting the first male that
offers. Note the cock and hens in the barnyard, or the fur seal
in the rookeries.

Ornamental colors are as often a characteristic of males of
kinds of animals in which there is no real pairing, as among
those which pair. How explain by sexual selection the remark-
able colors in the breeding season of many fishes, in which the

78 EVOLUTION AND ANIMAL LIFE

female never, perhaps, sees the male which fertilizes her dropped
eggs? In many fishes the spring ornamentation of the males is
just as marked and just as brilliant as in the birds or other
animals of much higher intelligence and corresponding power of
choice. Witness the horned dace, chubs, and stone rollers in
any brook in spring.

Choice on a basis of ornament and attractiveness implies a
high degree of aesthetic development on the part of the females
of animals of whose development in this line we have no other
proof. Indeed, this choice demands aesthetic recognition among
animals to which we distinctly deny such a development, as
the butterflies and other insects in which secondary sexual
characters of color, etc., are abundant and conspicuous. Sim-
ilarly with practically all invertebrate animals. Further, in
those groups of higher animals where aesthetic choice may be
presumed possible, we have repeated evidence that preferences
vary with individuals. Certainly they do with men, the animal
species in which such preferences certainly and most conspicu-
ously exist.

In some human races hair on the face is thought beautiful;
in others, ugly. Besides even if we may attribute fairly a cer-
tain amount of aesthetic feeling to such animals as mammals and
birds, is this feeling so keen as to lead the female to have
preference among only slightly differing patterns or songs?
Yet this assumption is necessary if the development of ornament
and other attracting and exciting organs is to be explained by
the selection and gradual accumulation through generations of
slight fortuitously appearing fluctuating variations in the males.

There are actually very few recorded cases in which the ob-
server believes that he has noted an actual choice by a female.
Darwin records eight cases among birds. Since Darwin, not
more than half a dozen other cases, all doubtful, have been
noted. Also a few instances, all more illustrative of sexual
excitation of females resulting from the perception of odor or
actions, than any degree of choice on their part, have been
listed.

In numerous cases the so-called attractive characters of the
males, described usually from preserved (museum) specimens,
have been found, in actual life, to be of such a character that
they cannot be noted by the female. For example, the brilliant
colors and curious horns of the males of the dung beetles are, in

NATURAL SELECTION; SEXUAL SELECTION 79

life, always so obscured by dirt and filth that there can be no
question of display to the female eye about them. The dancing
swarms of many kinds of insects are found to be composed of
males alone with no females near enough to see; it is no case of
an excitatory flitting and whirling of many males before the eyes
of the impressionable females. Of many male katydids singing
in the shrubbery will not for any female that particular song be
loudest and most convincing that proceeds from the nearest
male, not the most expert or the strongest stridulator? Simi-
larly with the flitting male fireflies ; will not the strongest gleam
be, for any female, that from the male which happens to fly
nearest her, and riot from the distant male with ever so much
better, stronger light? Even in the human species, propin-
quity is recognized as the strongest factor in the choice of mates.

Several other serious objections can also be urged against
the sexual selection theory, but the most important one of them
all is that all the evidence (though it is little in quantity as
yet, although of good quality) based on actual experiment, is
strongly opposed to the validity of the assumption that the
females make a choice among the males based on the presence
in the males of ornament or attractive colors, pattern, or special
structures. Such experiments have been undertaken by Diiri-
gen and Douglas with lizards, and by Mayer with moths.

It must be said, however, in closing this brief discussion
of the sexual selection theory, that no replacing or substitute
theory of anything like the same plausibility has yet been
offered to take its place.

There is no question that, in many cases, brilliancy of
breeding colors, development of processes, and the like, is
often correlated with superior vigor. This is especially true
among fishes and birds. This reason could, however, not at
all account for such structures as the highly specialized stridu-
lating organs of certain insects. The problem of the secondary
sexual characters, especially of those which seem to stand in
opposition to the natural selection theory, is one of the most
pressing in present-day biology.

CHAPTER VI
ARTIFICIAL SELECTION

We can command Nature only by obeying her laws. This prin-
ciple is true even in regard to the astonishing changes which are super-
induced in the qualities of certain animals and plants in domestication
and in gardens. — LYELL.

VARIETIES are the product of fixed laws, never of chance. With a
knowledge of these laws we can improve the products of nature, by
employing nature's forces in ameliorating old or producing new species
and varieties better adapted to our necessities and tastes. Breeding
to a fixed line will produce fixed results. There is no evidence of
any limit in the production of variation through artificial selection;
especially if preceded by crossing. — LUTHER BURBANK.

THE name Selection has been long used for the process by
which breeds or races of domestic animals or plants have been
formed in the past, and for the process by which the skill-
ful breeder can develop new forms at will. This latter proc-
ess, called by Youatt "the magician's wand/7 by which the
breeder can summon up any form of animal which may meet
his needs or please his fancy, has been especially designated as
Artificial Selection. By it we have derived all of our famil-
iar hosts of varieties of domesticated animals and plants. The
similar process in nature was accordingly designated by Darwin,
Natural Selection. It refers to the development or increase
of traits adaptive or advantageous in the life of a species,
through the survival for reproduction of a greater proportion of
individuals possessing the characters in question than of those
which do not. In any race, it is the individual which succeeds
in reaching maturity which determines the future of the race.
The qualities of the multitude which die prematurely are
naturally not repeated in heredity. In general, the forms pro-

80

ARTIFICIAL SELECTION

81

duced in artificial selection are not those which could arise
or even exist in nature. In nature, hardiness or power of
resistance in competition or the struggle for existence is all
important. In artificial selection stress is laid chiefly on char-
acters useful or attractive to man. From the standpoint of
self dependence, the improvements due to artificial selection
constitute a sort of retrogression.

In general, the production of a new race of animals or plants
in domestication is the
outcome of the work of
a number of factors, in
which human or artificial
selection plays a leading
part, a part which in-
creases in importance
with the degree of intel-
ligent choice concerned
in it.

In the formation of
a new race of animals or
plants, we may have the
following stages or fact-
ors:

1. Unconscious se-
lection with more or less
complete isolation.

2. Conscious selec-
tion of the most desira-
ble individuals.

3. Conscious selec-
tion directed toward
definite or special ends.

4. Crossing with other races or with other species (known
as hybridizing), in order to increase the range of variation, or
to add or combine certain specific desirable qualities or to elimi-
nate those undesirable, this accompanied by conscious selection
directed toward definite ends. On this series of processes
breeding as a fine art must depend.

Taking as an illustration some of the breeds of medium
wool sheep found in Southern England: we have (1) the domes-
tication of sheep in each of the different counties or natural

FIG. 46.— White-crested black Polish cock.
(After photograph.)

82

EVOLUTION AND ANIMAL LIFE

areas. In the beginning men are satisfied with sheep as sheep.
Little attention is paid to the distinction among individuals.
Those which are feeble, ill nourished, untamable, scant-fleeced,
or otherwise unfit will be eliminated, a process which will tend
to improve the stock, without giving the race distinctive quali-
ties, except as compared with the wild original. To form dis-
tinct races, the factor of isolation must enter. Those in one
county, for example, will be, at the beginning, somewhat

different from those in an-
other. Each herd will show
its own traits in time, these
due primarily to differences
in the original stock,
secondarily to the pre-
dominance of one form of
variation over others. Ex-
changes of sheep will, by
cross-breeding, tend to
unify the type of sheep in
some one county, or on
some side of a barrier across
which sheep are not driven.
With this, there will be also
variations in the character
of the unconscious selec-
tion. One type of sheep
will flourish in a meadow
county, another on a moor,
and still another on the
rocky hills. At any rate,
as the environment varies,
so will the character of the

selection. Thus as a final result, in Southern England, the
Southdown sheep of Sussex have tawny faces and legs; the
sheep of Hampshire have black faces, ears, and legs, with a
black spot under the tail ; this black spot is lacking in the sheep
of Devon. In the Cheviot sheep the face and ears are white, the
head free from wool, while the ears, unlike those of most of the
others, stand erect. In the dun-faced Shropshire sheep, the
faces are more or less covered by wool. All these are hornless,
while the more primitive Dorset sheep with white face and ears

FIG. 47. — Silver-laced Wyandotte cockerel.
(After photograph.)

ARTIFICIAL SELECTION

83

have almost always small curved horns which are white, not
black, as in the still more primitive Irish breed. Most of these
distinctive traits offer neither advantages nor disadvantages
either to the sheep or its owner. They are nonadaptive or

FIG. 48.— Typical Dorset ewe, horned. (After Shaw.)

FIG. 49.— Polled Welsh sheep, a primitive type, lean and scant wooled. (After Youatt.)

indifferent characters. These characters are therefore asso-
ciated with the hereditary traits of the original stock. They
are preserved through segregation and they are lost when herds
from different counties freely intermingle. Free interbreeding
would give a new and relatively uniform race of sheep over the
whole area occupied by these separate breeds.

84 EVOLUTION AND ANIMAL LIFE

At this point we may conceive that (2) conscious selection of
the more desirable individuals appears. Through its agency,
Hampshire, Shropshire, Cheviot, and Southdown sheep alike,
and the others in their degree, tend toward larger size, more
wool, plumper bodies, earlier maturity, greater docility, greater
fertility, or whatever virtues the average shepherd may prize in
a sheep. While in race traits, the breeds (uncrossed) tend to
diverge from one another, in these adaptive qualities, their
tendency is to run parallel — or even to converge toward greater
resemblance.

With conscious selection (3), there is first a tendency to
emphasize the qualities of desirable breeds. If, for example,

FIG. 50. — Typical Southdown ewe. (After Shaw.)

the Hampshire is a favorite breed, the individuals showing most
distinctly black ears, legs, and face will be preferred by breeders
to those having these parts pale. Again, new points of special
excellence will appear in the breed and these will be deliberately
emphasized, and perhaps by continuous selection a new breed
will be formed having one or more of these as a distinctive trait.
According to Somerville, one may chalk out on a wall any
form or type of sheep he may like, and then in time reproduce
it through selective breeding.

In Nova Scotia, Mr. A. Graham Bell has developed a new
breed of sheep by selection, its distinctive character being in the
increased milk flow, with an increased number of teats.

- ARTIFICIAL SELECTION 85

At Chillenham, in England, is still preserved a herd of the
original wild white English cattle, from which most or all of the
British breeds are said to be descended. It is stated that Lord
Cawdor has offered to reproduce this herd, by selection alone, in
three or four generations, using the relatively primitive Welsh
cattle as his base of operations.

In general, those characters which are usually affected by
selection, whether natural or artificial, are characters of degree.
They are matters of more or less, a greater or less degree of
strength, swiftness, size, endurance, fertility, capacity to lay

FIG. 51. — Typical American merino ewe, a highly specialized breed with fine close-set
wool. (After Shaw.)

on fat, docility, intelligence, or of whatever it may be. Lender
ordinary conditions these characters selected are not traits of
quality. They do not represent a new thing, a new acquisition,
but a different degree of development of an old one, or, at most,
a change in their relative arrangement, an alteration of bio-
logical perspective.

The characters which distinguish true breeds as well as true
species are not of this order. They are in their essence quali-
tative and not quantitative. They are not, as a rule, adaptive.
One set of species or race traits is as good as another, if the good
qualities or adaptive qualities are represented in an equally

FIG. 52. — Heads of various British breeds of domestic cattle, showing variations in
shape of head and condition of horns. (After Romanes.)

FIG. 53. — Various races of pigeons, all probably descended from the European rock
dove, Columba livia. (After Haeckel.)

88 EVOLUTION AND ANIMAL LIFE

high degree. The Southdown sheep are valued — not for their
Southdown traits, but for the excellence of their mutton, a
trait with which middle length of wool, tawny legs, naked faces,
drooping ears, and absence of horns have nothing necessarily to
do. We value these race traits only for the other qualities

FIG. 54. — Skulls (in longitudinal section) of two breeds of domestic fowl, showing the
large modification in the cranium: upper figure, Polish cock; lower figure, Cochin
cock. (After Darwin.)

which have been in a high degree associated with them in the
heredity of the race.

Under crossing and selection, much bolder attempts are
possible. When parents widely divergent are crossed, many
very different results are attained. In general the progeny, at
least after the first generation, diverge very widely from one
another. Some will have the good traits of both parent stocks ;
some will have the undesirable ones; some will show a mosaic of
parental characters; some a more or less perfect blend of char-
acters, this blend being definable as a finer type of mosaic.
Some will diverge widely from either stock, often showing traits
either remotely ancestral or wholly new. From desirable vari-
ations of this sort new races may be developed, each succeeding
generation tending to give greater fixity.

In general, wide crosses or hybrids are more successful with
plants than with animals, because the mutual adjustment

ARTIFICIAL SELECTION

89

traits become more important in the more highly specialized
organisms. Among animals, related species often cannot be
crossed at all; the germ cells refuse to intermingle. Sometimes
there is a very imperfect mingling and the resultant animal is
divided within itself and does not live long. An example of this
is seen in Dr. Moenkhaus's cross of the silverside (Menidia)
with the killifish (Fundulus). The unmixed chromosomes of the
germ-cell nucleus are seen unblended, through several segmen-
tations of the egg.

In the case of the mule, the cross of the horse with the ass,
the hybridization is readily effected, but the resultant offspring
is sterile. Presumably the hereditary difference in the repro-

Fir,. 55. — Wild boar contrasted with modern domestic pig. (After Romanes.)

ductive organs in the two parental strains is too great to allow
the normal development of generative organs in the progeny.

In general, crosses between closely related species are fertile,
the degree of fertility being less as the parent species are more
widely differentiated. Among animals, any great difference

90 EVOLUTION AND ANIMAL LIFE

between the parent stocks renders hybridization impossible.
But among plants, when hybrids are actually formed, fertility
rather than sterility may be taken as the rule. This is the case
with Mr. Luther Burbank's Primus berry, a cross between the
Siberian raspberry (Rubus cratcegifolius) and the Calif ornian dew-
berry or blackberry (Rubus ursinus). In this form the fruit
excels in size and abundance either parent, and the hybrid
breeds true from the seed, and ripens before either parent begins
to bloom. It was fixed in the first generation, being in this re-
gard a rare exception to the general rule of the aberration of
hybrids. In this and in other respects the Primus, known to
be an intentional cross of two species, behaves as though it
were a distinct species. In like fashion, the Logan berry, the
product of an accidental cross at Santa Cruz, in California, of
the European raspberry with the native dewberry, behaves also
like a distinct species, and is also much superior in productive-
ness to either parent.

The fine art of the horticulturist is seen in the selection and
fixing of the variations produced by crossing and hybridization.
While most of the forms thus obtained are worthless, a few
will show decided advances. Often as m.uch progress may be
made in a single successful cross or hybridization as in a dozen
or even a hundred generations of pure selection.

By selection alone, however, important results may be
obtained, with time and patience. Given a variation in a de-
sired direction there is perhaps no actual limit bounding the
possibilities of selection unless arising through external or me-
chanical conditions. Thus selection for speed of horses is limited
by the strength of the material of which a horse's leg is com-
posed. The increase in the number of petals may be limited
by the space on which petals can stand, and the number of
leaflets in a leaf by the length of the rhachis. Still there are
known cases in which a positive limit has been reached in at-
tempting to modify organisms by selection alone.

Accidental crossing within a species may form a useful basis
for selection. Thus from the seeds in a single potato ball of the
Early Rose variety, crossed by insects with an unknown parent,
Mr. Luther Burbank reared potatoes of many different sorts:
red potatoes, white potatoes, elongate potatoes, potatoes rela-
tively smooth and potatoes all eyes and "eyebrows." Among
all these, one form, long, white, smooth, and mealy, seemed far

ARTIFICIAL SELECTION

91

superior to the others. From the subdivision of the tubers of
this seedling arose the Burbank potato, the most valuable
variety in its economic relations now cultivated in America.
But with the choice of this form for preservation, selection
ceased, as all plants of the Burbank potato in cultivation are

FIG. 56. — Heads of timothy, showing improvement by selection.
(After Hays.)

subdivisions of a single original plant. New forms would come
from further selection of the Burbank potato seed.

As illustrations of the more complex art of hybridization and
selection, we give in the following paragraphs a brief account of
the work of Luther Burbank, the most ingenious and successful
of all recent experimenters in plant breeding.

92

EVOLUTION AND ANIMAL LIFE

Burbank has originated and introduced a remarkable series
of plums and prunes. No less than twenty varieties are included
in his list of offerings, and some of them, notably the Gold,

FIG. 57. — Four types of plumcot: colors, red and yellow of various shades. (Photo-
graph by Burbank; about one-half diameter.)

Wickson, Apple, October Purple, Chalco, American, and Climax
plums and the Splendor and Sugar prunes, are among the best
known and most successful kinds now grown. In addition, he
is now perfecting a stoneless plum, and has created the inter-

\

^

FIG. 58. — Seedlings from one hybrid plum. (After photograph by Burbank.)

esting plumcot by hybridizing the Japanese plum and the
apricot. The plumcot, however, has not yet become a fixed
variety and may never be, as it tends to revert to the plum.

ARTIFICIAL SELECTION 93

The stoneless and seedless plum is being produced by
selection from the crossing of the descendants of a single fruit
in a small wild plum with only part of a stone with the French
prune; the percentage of stoneless fruits is gradually increasing
with succeeding generations. The sugar prune, which promises
to supplant the French prune in California, is a selected product
of a second or third generation variety of the Petit d'Agen, a
very variable French prune. The Bartlett plum, cross of the
bitter Chinese simoni and the Delaware, a Burbank hybrid, has
a fragrance and flavor extraordinarily like that of the Bartlett
pear. The Climax is a cross of the simoni and the Japanese
Iriflora. The Chinese simoni
produces almost no pollen,
only a few grains of it ever
having been obtained, but
these few grains have en-
abled Burbank to revolu-
tionize the whole plum
shipping industry. Most
of Burbarik's plums and
prunes are the result of

multiple crossings, in which HIHIHHHH _J

the Japanese Satsuma has Fl(! 59 _The larger plum is the dirert ,eefl_

played an important part. ling of the smaller, produced by crossing

Hundreds of thousands of the trif°liaia (JaPan> n'um and the little

„. . , maritima (Atlantic Coast) plum. (After

seedlings have been grown photograph by Burbank.)
and carefully worked over

in the twenty years' experimenting with plums, and single
trees have been made to carry as many as 600 varying seed-
ling grafts.

Burbank has originated and introduced the Van Deman,
Santa Rosa, Alpha, Pineapple "No. 80," the flowering Dazzle,
and other quinces; the Opulent peach, cross bred from the Muir
and Wager; the Winterstein apple, a seedling variety of the
Gravenstein; and has made interesting, although not profitable,
crosses of the peach and nectarine, peach and almond, and plum
and almond.

Next in extent, probably, to his work with plums is his long
and successful experimentation with berries. This work has
extended through twenty-five years of constant attention, has
involved the use of forty different species of Rubus, and has

94

EVOLUTION AND ANIMAL LIFE

resulted in the origination arid introduction of a score of new
commercial varieties, mostly obtained through various hybridi-
zations of dewberries, blackberries, and raspberries.

PIG. 6(). — Seedlings of one kind of hybrid plum: colors almost black, deep crimson,
light crimson, scarlet, deep yellow, and shades of orange and yellow, green striped,
spotted and speckled; long and short stems; sweet, sour, bitter, good, bad, and
indifferent, firm and soft; flesh, yellow, white, pink, red, crimson, striped, and
shaded; stones of various shapes and sizes, large, small, oval, round, of different
colors, some clingstones, some freestones; foliage varying as much as the rest, and
growth from short arid stalky and dwarf to rampant exuberance. ^Photograph by
Burbank; about one-quarter diameter.)

ARTIFICIAL SELECTION

95

Among these may especially be mentioned besides the
Primus already spoken of, the Iceberg, a cross-bred white
blackberry derived from a hybridization of the Crystal White
(pistillate parent) with the Lawton (staminate parent), with

FIG. 61 .—Seedlings of the Japanese quince, Pyrus Japonica: colors, orange yellow, or
almost white, with crimson dots and splashes. (From photograph by Burbank.)

96 EVOLUTION AND ANIMAL LIFE

beautiful snowy-white berries so nearly transparent that the
small seeds may be seen in them; the Japanese Golden May-
berry, a cross of the Japanese R. palmatus (with small, tasteless,
dingy yellow, worthless berries) and the Cuthbert, the hybrid
growing into treelike bushes, six to eight feet high, and bearing
great, sweet, golden, semitranslucent berries which ripen before
strawberries; the Paradox, an oval, light-red berry, obtained in
the fourth generation from a cross of Crystal White Blackberry
and Shaffer's Colossal Raspberry. While most of the plants
from this cross are partly or wholly barren, this particular out-
come is an unusually prolific fruit producer.

An interesting feature of Mr. Burbank's brief account, in his

r

FIG. 62. — Three walnuts: at left Japanese walnut, at right English walnut,
and in middle a hybrid of these two. (From photograph by Burbank.)

"New Creations" catalogue of 1894, of the berry experimenta-
tion is a reproduction of a photograph showing "a sample pile
of brush 12 feet wide, 14 feet high, and 22 feet long, containing
65,000 two- and three-year old seedling berry bushes (40,000
Blackberry X Raspberry hybrids and 25,000 Shaffer X Gregg
hybrids), all dug up with their crop of ripening berries." The
photograph is introduced to give the reader some idea of the
work necessary to produce a satisfactory new race of berries.
"Of the 40,000 Blackberry-Raspberry hybrids of this kind
'Paradox' is the only one now in existence. From the other
25,000 hybrids two dozen bushes were reserved for further
trial."

ARTIFICIAL SELECTION

97

Leaving Burbank's other fruit and berry creations un-
noticed, we may refer to his curious cross-bred walnut results
(Fig. 63), the most astonishing of which is a hybrid between

FIG. 63. — At left, leaf of English walnut, Juglans regia; at right California black
walnut, Juglans californica; and in the middle a leaf of the hybrid Paradox, first
generation. (From photograph by Burbank.)

Juglans Californica (staminate parent) and J. niyra (pistillate
parent), which grows with an amazing vigor and rapidity, the
trees increasing in size at least twice as fast as the combined
growth of both parents, and the clean-cut, glossy, bright green

98

EVOLUTION AND ANIMAL LIFE

leaves, from two to three feet long, having a sweet odor like that
of apples. This hybrid produces no nuts, but curiously enough
the result of the reverse hybridization (i. e., pollen from nigra on

FIG. 64. — Hybrid seedling cactuses, Opuntia, after six months growth, showing num-
erous varieties. (From photograph by Burbank.)

pistils of Californica) produces in abundance large nuts of a
quality superior to that possessed by either parent.

Of new vegetables Burbank has introduced besides the Bur-
bank and several other new potatoes, new tomatoes, squashes,
asparagus, etc. Perhaps the most interesting of his experiments
in this field is his attempt, apparently destined to be successful,
to produce a spineless and spiculeless and unusually nutritious
cactus (the spicules are the minute spines, much more danger-
ous and harder to get rid of than the conspicuous long thornlike
spines) edible for stock, and indeed for man. This work is
chiefly one of pure selection, for the cross-bred forms seem to
tend strongly to revert to the ancestral spiny condition.

Among the many new flower varieties originated by Bur-
bank may be mentioned the Peachblow, Burbank, Coquito, and
Santa Rosa roses, the Splendor, Fragrance (a fragrant form),
and Dwarf Snowflake callas, the enormous Shasta and Alaska

ARTIFICIAL SELECTION

99

daisies, the Ostrich plume, Waverly, Snowdrift, and Double
clematises, the Hybrid Wax Myrtle, the extraordinary Nico-
tunia, a hybrid between a large, flowering Nicotiana and a
Petunia, several hybrid Nicotianas, a dozen new gladioli and
ampelopses, several amaryllids, various dahlias, the Fire poppy
(Fig. 65), (a brilliant, flame- colored variety obtained from a
cross of two white forms), striped and carnelian poppies, and a
blue Shirley (obtained by selection from the Crimson field poppy
of Europe), the Silver Line poppy (obtained by selection from
an individual of Papaver umbrosum, showing a streak of silver

FIG. 05. — At left, leaf and flower of the pale yellow poppy, Papaver pilosum; at right
leaf and flower of the snow white poppy, Papaver somniferum; and in the middle,
leaf and fire-crimson flower of the first generation hybrid of these two. (From
photograph by Burbank.)

inside) with silver interior and crimson exterior, and a Crimson
California poppy (Eschscholtzia) , obtained by selection from the
familiar golden form.

Perhaps his most extensive experimenting with flowers has

100

EVOLUTION AND ANIMAL LIFE

been done in the hybridizing of lilies, a field in which many plant
breeders have found great difficulties. Using over half a hun-
dred varieties as basis of his work Burbank has produced a mar-
velous variety of new forms (Fig. 66). "Can my thoughts be
imagined/' he says, in his " New Creations " of 1893, "after so
many years of patient care and labor [he had been working over
sixteen years], as, walking among them [his new lilies] on a
dewy morning, I look upon these new forms of beauty, on which

FIG. 66. — An improved seedling lily with two petals. (From photograph
by Burbank.)

other eyes have never gazed? Here a plant six feet high with
yellow flowers, beside it one only six inches high with dark
red flowers, and further on one of pale straw, or snowy white,
or with curious dots and shadings: some deliciously fragrant,

ARTIFICIAL SELECTION 101

others faintly so; some with upright, others with nodding
flowers; some with dark green, woolly leaves in whorls, or with
polished light green, lancelike, scattered leaves."

FIG. 67. — An extraordinary apple, one-half being bright red and sour, and the other
half greenish yellow and sweet; note in photograph the sharp line of demarkation
between the different halves. (From photograph by Burbank.)

So far no special reference has been made to the more
strictly scientific aspects of Burbank's work. Burbank has
been primarily intent on the production of new and improved
fruits, flowers, vegetables, and trees for the immediate benefit
of mankind. But where biological experimentation is being
carried on so extensively it is obvious that there must be a
large accumulation of data of much scientific value in its rela-
tion to the great problems of heredity, variation, and species-
forming. Burbank's experimental gardens may be looked on,
from the point of view of the biologist and evolutionist, as a
great laboratory in which, at present, masses of valuable data
are, for lack of time and means, being let go unrecorded.

Of Burbank's own particular scientific beliefs touching the
"grand problems" of heredity we have space to record but
two: first, he is a thorough believer in the inheritance of ac-
quired characters, thus differing strongly from the Weismann
school of evolutionists; second, he believes in the constant
8

102

EVOLUTION AND ANIMAL LIFE

mutability of species, and the strong individuality of each plant
organism, holding that the apparent fixity of characteristics is a
phenomenon wholly dependent for its degree of reality on the

FIG. 08. — Seedlings of the Williams early apple, showing all the colors ever found in
apples. (From photograph by Burbank.)

length of time this characteristic has been ontogenetically re-
peated in the phylogeny of the race.

In like fashion to this working with plants, breeds of
animals have been established by crossing and selection with a

ARTIFICIAL SELECTION

103

view to the preservation of the best traits of both. In estab-
lishing the stock farm at Palo Alto, Leland Stanford had the

FIG. 69. — Improvement in geranium: at left, the original wild form, and at
right the latest improved form. (From photograph by Burbank.)

conception of strengthening the trotting horse by a cross with
the larger running horse or thoroughbred. The result was the
formation of a peculiar type of horse, large, strong, supple,

FIG. 70. — Sports found among crossed amaryllids, the size and form markedly changed;
the flowers are three inches in diameter. (From photograph by Burbank.)

and intelligent, very clean of limb and sleek of coat. This
group of horses held for some years the world's records for

104 EVOLUTION AND ANIMAL LIFE

speed in their various classes and ages, and the experiment
was in the highest degree successful. In one sense such at-
tempts are not experiments. The skillful breeder knows that
out of the many combinations possible in crossing, some few
will fall in line with his plans. He has only to preserve these,
and to clinch them by in-and-in or segregated breeding to
bring about a result he may have deemed possible or desir-
able. It is possible, by intentional selection, to turn a non-
essential or race character into a selective or adaptive one.
The Hampshire sheep have black ears, but by persistent se-
lection the ears could probably be made white. Probably also
the horns of the Dorsets could be bred on Hampshires by
making use of possible occasional reversions to the horned
stock. This result could be attained very rapidly by a cross-
ing with Dorset stock, but this triumph of the breeder's art
has rarely any homologue in the wild state or in the condition
of unconscious selection.

When selection ceases, the adaptive characters are likely to
decline or disappear. Under cessation of selection, called by
Weismann panmixia, no premium is placed on traits of excel-
lence, from the human standpoint, such as long wool, plump-
ness or symmetry of form; and only the purely vegetative ad-
vantages of the individual count. But while the traits of
excellence disappear, the race traits or nonadaptive characters
persist unchanged. A herd of neglected Hampshire sheep is
still a herd of Hampshires. The black face, ears, and legs
remain black, with no tendency to fade.

When the worst individuals are selected for breeding, we
have the reversal of selection. A flock of Hampshire culls,
feeble, loose-jointed, scant-wooled, unsymmetrical, could be
used in breeding, and the adaptive characters usually sought
for could be bred out of them. But they would still be Hamp-
shires, for the hereditary characters which had persisted with-
out the aid of selection would persist after selection ceases or
even if it is reversed. When these same characters are made the
object of selection, they are subject to the same laws as ordinary
adaptive characters.

What is true of a breed of sheep — a product of geographical
isolation with segregative breeding — is true in a general way of
any wild species of animals or plants. Its adaptive characters
are due to natural selection. These change more rapidly than

ARTIFICIAL SELECTION 105

the nonadaptive characters, and respond more readily to the
conditions of panmixia or of reversal of selection.

In matters of breeding we must distinguish between animals
actually best and those potentially best. An animal is at its
actual best when in prime condition, at the prime of its life.
Another of far finer heredity, of far stronger ancestry, may be
at any given time actually the inferior of the first. It may be
too old, too young, in too poor condition to represent its own
best status.

It is generally recognized that, for all breeding purposes, the
animal potentially best is superior to one which, otherwise
inferior, may be actually best at the time. The tendency of
heredity is to repeat the traits of the ideal individuals, which
the parents ought to have been. More exactly, the tendency of
heredity is to produce individuals which, under like conditions
of food and environment, would develop as the parents have
developed.

But it is also recognized that the actual physical condition of
the parent affects the offspring. A sick mother is likely to bear
an enfeebled child. Immature or declining sires do not beget
offspring as strong as those begotten by them when they are in
perfect strength and health. In this matter, apparently, we have
to deal with two different elements, as Weismann and others have
pointed out. The first is true heredity, the quality of the germ cell,
which is not affected by the condition of the parent. Weak or
strong, the offspring is of the same kind or type as the parentage.

The second element has been called Transmission. Its
relations are with vegetative development. The embryo is ill
nourished by the sick mother, and it enters on life with lowered
vigor. The momentum, if we may use such a figure of speech,
is reduced from the first, and the lost vitality may never be
regained. The defects of the male parent are perhaps of less
moment, but whatever their nature their results would be of
the same kind. They would not enter into the heredity of the
offspring, but they might play a large part in retarding its
development. In the category of transmission, not of heredity,
would belong the theme of Ibsen's " Ghosts" (Gjengdngere) , the
development of softening of the brain in the son of a debauchee,
the alleged cause being that the father's nervous system was
vermoulu (worm-eaten), if we are to accept the ghastly drama
as an exposition of possible facts.

106 EVOLUTION AND ANIMAL LIFE

. The role played by the phenomena of transmission as distin-
guished from that of heredity has never been clearly ascertained.
Many eminent writers ascribe to it a large importance. It is
a central element in Mr. Casper RedfiekTs theory of heredity,
and he brings together a considerable array of facts and statis-
tics to justify his conclusions. But the value of statistics in
such matters is easily exaggerated, because of the difficulty in
ascertaining the real causes behind the phenomena we try to
record. It is fair to say as a broad proposition that, as a sound
mind requires a sound body, soundness both of mind and body
are factors in giving to offspring the best possible start in life.
The heredity unchanged, there is still a great value in vigor of
early development.

The relation of these matters to the theory of organic evo-
lution is mainly here: artificial selection as a process is of the
same general character as natural selection; both represent a
form of isolation or segregation, which prevents indiscriminate
mating, and which holds certain groups of individuals as the
agents of reproduction of the species within a given time or in
a special area.

Artificial selection intensifies useful or adaptive characters,
using these words in a broad sense. At the same time, it per-
petuates a series of characters, in no wise useful, and in no
fashion adaptive. The,3e characters remain unchanged for long
periods, and hence have more value in race distinction or in
classification than the strictly adaptive characters have. A
Southdown sheep is plump and fat, on the whole perhaps more
so than any other type of sheep. Nevertheless, it is not by its
plumpness that we know a Southdown. It is rather by the
character of its wool, the color of its face and feet, the form of
its head. So it is with breeds and races generally. They are
formed primarily by isolation in breeding, the separation of a
few from the many by geographical or similar causes, by the
perpetuation of the traits of these few (the "survival of the
existing"), all this being modified by the new range of natural
and artificial selection and the new reactions under the varying
conditions of a new environment.

It interests us to know that a similar process takes place in
nature. Geographical and topographical barriers are crossed
in migration. These isolate a portion of a species under new
conditions, with new reactions to the environment, and a

ARTIFICIAL SELECTION 107

new range of natural selection. Adaptive characters change
rapidly, and in ways more or less parallel, with similar altera-
tions in related species. Characters nonadaptive, often slight
in appearance and bearing no relation to the life of the animal,
become slowly but surely fixed as characters of the species.
As two closely allied breeds of animals are never found in the
same region unless purposely restrained from free interbreeding,
so two closely related species never develop in the same breeding
area. As the nearest relative of some given breed of domestic
animals is found in a given region nearly related geographically,
so is the nearest relative to any given wild species found, in
most cases, not far away. It is to be looked for on the other
side of some geographic, topographic, or climatic barrier. In
other words, the interrelation of variation, heredity, geographic
isolation and environmental features generally seems to be the
same in the formation of domestic races as in that of the
formation of natural species. The principal new element intro-
duced in the art of selective breeding is that of purposeful
crossing, the removal of the barriers which separate well-
differentiated forms, for the purpose of beginning a new series
to be selected toward a predetermined end.

It has been recently repeatedly stated that most races of
domesticated animals or plants find their origin in a mutation
or saltation of some sort/ In our judgment, there is not suffi-
cient evidence to prove this view. There are few cases of
either races or species known to have originated in this way.
That such is in fact the general law of race or species origin,
we see little reason to believe. One of the few well-known
illustrations of race-forming through saltation is that of the
Ancon sheep. In 1791, in Massachusetts, a ram was born with
unusually short legs. As this character was useful, preventing
the sheep from leaping over stone walls, the owner of this sheep
used the ram for breeding purposes, and succeeded in isolating
a short-legged strain of sheep known as the Ancon sheep. So
far as known to us, this type of sheep differed in this character
alone from the common sheep of Connecticut. With the later
advent of the more heavy- wooled, and therefore more profitable,
Merino, the Ancon sheep disappeared. A recent similar case of
race origin from a prepotent sport is that of the polled Here-
fords arising in Kansas from a hornless Hereford bull.

CHAPTER VII

VARIOUS THEORIES OF SPECIES-FORMING
AND DESCENT CONTROL

The four factors named, variation, inheritance, selection, and sepa-
ration, must work together in order to form different species. It is
impossible to think that one of these should work by itself or that
one could be left aside. — ORTMANN.

As mentioned in the introductory chapter on the factors of
evolution (Chapter IV) , and as referred to several times in the
chapter on natural selection, the factor of the segregation or
isolation of groups of individuals must be taken into account in
any discussion of species-forming causes. This factor has long
been recognized by biologists, that phase of it, and undoubtedly
the most important of its several phases, called geographic
or topographic isolation or segregation being very clearly
stated and its importance emphasized by Moritz Wagner in
1868. Alfred Russel Wallace gave much attention, in his years
of active investigation, to the general subject of geographical
distribution, and was a pioneer in calling the attention of natu-
ralists to the great significance, in the light of the evolution
theory, of the facts of the geographical distribution of both
animals and plants. To-day, especially among American
biologists, the factor of topographic segregation is recognized
as one of the most important of species-molding influences.
Indeed it seems self-evident to many naturalists that natural
selection is impotent as an actual cause of species-forming with-
out some effective sort of isolation factor to assist it. Because
of the importance in the eyes of present-day naturalists of the
geographic isolation factor we have given (Chapter VIII), a
brief special discussion of this factor. In addition, in Chapter
XIV, will be found a discussion of the more general subject
of geographical distribution.

108

VARIOUS THEORIES OF SPECIES-FORMING 109

But it is conceivable that isolation may be effected in other
ways than by actual segregation or geographic separation of
individuals. Anything that could lead to exclusive or dis-
criminate breeding among certain individuals of a species would
result in the isolation of these individuals from the rest of the
species as effectively as their actual separation from others by
a geographic or topographic barrier. Now there are various
influences or conditions that might conceivably bring about
such a state of affairs, and some of these have been actually
observed to exist. It is of interest to note that this kind of
isolation differs, in a rather important way, from purely geo-
graphic isolation in that the latter is almost sure to be wholly
indiscriminate as regards the individuals comprised in an isolated
group, while the former, which has been called physiological
isolation, will be discriminate. That is, there will be a struc-
tural or physiological peculiarity common to all the " isolated "
individuals, it being by virtue of this common peculiarity
(something not common to other individuals of the same
species) that the isolation actually exists.

Romanes has been the chief champion of the physiological
isolation factor. And we may advantageously refer directly to
his writings for a specific statement of different forms or phases
of this kind of isolation. In "Darwin and After Darwin/' III,
p. 7 et seq., he writes:

"Now the forms of discriminate isolation, or homogamy, are very
numerous. When, for example, any section of a species adopts
somewhat different habits of life, or occupies a somewhat different
station in the economy of nature, homogamy arises within that section.
There are forms of homogamy on which Darwin has laid great stress,
as we shall presently find. Again, when for these or any other reasons
a section of a species becomes in any small degree modified as to form
or color, if the species happens to be one whe^e any psychological pref-
erence in pairing can be exercised — as is very generally the case among
the higher animals — exclusive breeding is apt to ensue as a result
of such preference ; for there is abundant evidence to show that, both
in birds and mammals, sexual selection is usually opposed to the
intercrossing of dissimilar varieties. Once more, in the case of plants,
intercrossing of dissimilar varieties may be prevented by any slight
difference in their seasons of flowering, of topographical stations, or
even, in the case of flowers which depend on insects for their ferti-

110 EVOLUTION AND ANIMAL LIFE

lization, by differences in the instincts and preferences of their
visitors.

"But, without at present going into detail with regard to these
different forms of discriminate isolation, there are still two others,
both of which are of much greater importance than any that I have
hitherto named. Indeed, these two forms are of such immeasurable
importance that were it not for their virtually ubiquitous operation,
the process of organic evolution could never have begun, nor, having
begun, continued.

"The first of these two forms is sexual incompatibility — either
partial or absolute — between different taxonomic groups. If all hares
and rabbits, for example, were as fertile with one another as they are
within their own respective species, there can be no doubt that sooner or
later, and on common areas, the two types would fuse into one. And
similarly, if the bar of sterility could be thrown down as between all
the species of a genus, or all the genera of a family, not otherwise pre-
vented from intercrossing, in time all such species, or all such genera,
would become blended into a single type. As a matter of fact, com-
plete fertility, both of first crosses and of their resulting hybrids, is rare,
even as between species of the same genus; while as between genera of
the same family complete fertility does not appear ever to occur, and,
of course, the same applies to all the higher taxonomic divisions. On
the other hand, some degree of infertility is not unusual as between
different varieties of the same species; and, wherever this is the case,
it must clearly aid the further differentiation of those varieties. It
will be my endeavor to show that in thifc latter connection sexual
incompatibility must be held to have taken an immensely important
part in the differentiation of varieties into species. But meanwhile we
have only to observe that wherever such incompatibility is concerned,
it is to be regarded as an isolating agency of the very first importance.
And as it is of a character purely physiological, I have assigned to it
the name Physiological Isolation; while for the particular case where
this general principle is concerned in the origination of specific types,
I have reserved the name Physiological Selection."

If the factors of variation, heredity, natural selection, and
isolation are, in the minds of most naturalists, the chief factors
in species-forming and descent control, and a combination of
these factors is, in the belief of these same naturalists — the so-
called selectionists or Neo-Darwinians — a sufficient causal ex-
planation of organic evolution, there are many other natural-

VARIOUS THEORIES OF SPECIES-FORMING 111

ists who have no such high esteem of the value of natural
selection. These believe, variously, that (a) to the selection
factors other auxiliary or helping ones are to be added, or (6)
that various other factors are equally potent in species-forming,
or (c) that these other factors are the more important ones, or
finally (d) that the selection factors are of no importance at all,
that is, have no reality. Before Darwin, the French naturalist
Lamarck had clearly enunciated an explaining theory of species
transformation, and there are to-day many naturalists who
believe that the Lamarckian explanation, or its fundamental
assumption, is true, or, at least, that it is based on the more
important and effective factors in evolution. These natural-
ists have been called Neo-Lamarckians. Some of these have
formulated theories of their own based on Lamarckian funda-
mentals, but developed in directions more or less obviously away
from characteristic Lamarckism.

Still other fundamental causal factors than the Darwinian
ones of selection and the Lamarckian ones of accumulated effect
of use, disuse, and functional stimulation are assumed in certain
other theories of species change and general evolution. Nageli,
a botanist and natural philosopher, believed in a special inherent
vitalistic principle or force in living matter which tends to pro-
duce progressive differentiation and evolution. Von Kolliker,
Korschinsky, and de Vries believe that species-forming occurs
by definite sudden small (or larger) fixed changes or mutations,
so that for them a mutational or discontinuous variation is the
fundamental causal factor in species transformation. Numerous
paleontologists believe that variation follows determinate lines
in its occurrence, so that evolution is orthogenetic, with its lines
primarily fixed by determinate variation.

We may then examine briefly some of the more important
special theories or groups of theories put forward by biologists
either as auxiliary and subordinate to the more generally
known Darwinian theory, or as alternative with or substitutes
for this theory.

First to be mentioned should be the transmutation theory
of Lamarck. In its simplest expression it is, that every individ-
ual organism is, throughout its lifetime, reacting to environ-
mental stimuli and conditions in such ways as to change its
structure and its habits in greater or less degree from the
structure and habits of its parents and ancestors, this change

112 EVOLUTION AND ANIMAL LIFE

coming about specifically from the varying effects of use or
disuse of parts, and the functional stimulation of other parts
in response to such extrinsic conditions as light, contact, tem-
perature, pressure, color, etc., etc. The changes effected will,
in the nature of things, be essentially adaptive. Now, these
adaptive changes, these variations, or new characters acquired
during the lifetime of the individual will be, in Lamarck's belief,
inherited, if not in full, at least in partial degree, by the offspring.
These in turn submitted to similar or to different environ-
mental influences will continue the changes either cumulatively
or diversely. By this steady direct change and adaptation to
environment the species is ever modifying and transforming.
Evolution marches, and marches adaptively and advanta-
geously.

But modern naturalists find a most unfortunate impediment
to this simple, direct, and sufficient explanation of species-
forming and evolution in the apparent untruth of the assumption
that the characters acquired by an individual in its lifetime are
transmitted by inheritance to its young. This question, fun-
damental to the Lamarckian theory, of the inheritance or non-
inheritance of acquired characters has long been one of the most
hotly debated points in evolution biology. As we have devoted
a number of pages to its particular discussion in our later chap-
ter on heredity (Chapter X), we need not anticipate that
discussion here. It is sufficient to say that as far as scientific
proof, that is, evidence from actual observation and experiment,
goes, those naturalists led by Weismann, who deny this inheri-
tance, have at present distinctly the better of the argument.

The orthogenetic evolution theories of various authors,
based upon the assumed occurrence of variations in determinate
lines or directions (a restricted and determinate variation as
compared with the nearly infinite, fortuitous, and indeterminate
variation assumed in the selection theory) , are of several types.
The mention of twro will reveal pretty well the more important
characters of all. Not a few biologists have always believed in
the existence of a sort of mystic, special vitalistic force or prin-
ciple by virtue of which determination and general progress of
evolution is chiefly fixed. Such a capacity, inherent in living
matter, seems to include at once possibility of specific adapta-
tion and the possibility of progressive or truly evolutionary
change. Not all evolution is in a single direct line, to be

VARIOUS THEORIES OF SPECIES-FORMING 113

sure ; ascent is not up a single ladder or along a single genealogi-
cal branch, but these branches are few (as indeed we actually
know them to be, however the restriction may be brought about)
and the evolution is always progressive, that is, toward what we,
from an anthropocentric point of view, are constrained to call
higher or more ideal life stages and conditions.

Other naturalists also seeming to see this course of determin-
ate or orthogenetic evolution, but not inclined to surrender their
disbelief in vitalism, in forces over and beyond the familiar
ones of the physicochemical world, have tried to adduce a
definite causomechanical explanation of orthogenesis. The
best and most comprehensible types of this explanation are
those essentially Lamarckian in principle, in which the direct in-
fluence on living matter of environmental conditions, the direct
reactions of the life stuff to stimuli and influences from the
world outside, are the causal factors in such an explanation.
But while every naturalist will grant that such factors do change
and control in considerable degree the life of the individual,
most see no mechanism or means of extending this control
directly to the species.

The stumbling block of heredity, the means and mode of
inheritance, as we so far know them, are directly in the way of
any general acceptance of such a theory of evolution under the
direct control of such "primary factors of life." Ontogenetic
species, that is, conditions of structure and habit common to
many individuals of one kind, the conditions due to sameness
of intrinsic and extrinsic factors in development, constitute a
category of organisms which at any given time and place seem
very real, and are for the moment truly real. But their
environment is remaining fairly constant. We speak easily of
the flux of Nature: her everchangingness. And in the large
we are speaking only of the truth. But during our brief
period of observation of the few generations of this or that kind
of animal or plant that come under our eyes and microscopes,
the nature environing these generations may be nearly uniform.
What are the changes in the desert in a score or a hundred or a
thousand of years? What changes in life conditions on the
barren storm-swept peaks of the mountain ranges? What in
the waters of that brackish bay or sweet-water lake apart from
the paths of man? Ontogenetic species have a seeming of
reality, but so far as our present knowledge goes it is only a

114 EVOLUTION AND ANIMAL LIFE

seeming: reality vanishes with the death of the individual:
their young can perpetuate their specific peculiarities only if
the environmental conditions of their development are identical
with those which attended the growing up of their parents.
Variations in this environment will determine variations in
them, and their father's kind will exist no more.

The authors of this book believe that more characters of
species than are commonly thought are of this shifty, ephemeral
character; that not a few so-called true species are only onto-
genetic species held for a number of generations true to a type
simply because the environment, the extrinsic factors in the
development of all the individuals in these successive genera-
tions, are the same. But how these individual characteristics
and changes can be put into the heredity of the race we do not
understand. " There is no fixity in species other than that
due to the long-repeated ontogenetic reiteration of this or that
characteristic," says Luther Burbank. And he speaks from the
conviction forced on him through thirty years of the closest sort
of observation and personal experience of the life of plants.
And yet, however strongly our own minds respond to a desire
to believe this — it would be so clarifying— the obstinate "no
mechanism" objection stands boldly up^to check our sympa-
thetic reasoning.

Finally we should refer to the theories of heterogenesis or
species-forming by mutations or saltations, which have been
proposed at various times as a substitute for the theory of
species-forming by the gradual transformation through selec-
tion. During the discussion in the first few years after the
appearance of Darwin's "Origin of Species," the German zoolo-
gist von Kolliker expressed the belief that the change from
species to species would probably be found to be more sudden
and more distinctive than Darwin's theory permitted one to
assume. Later, the Russian botanist Korschinsky, on a basis
of general observation and some not very extensive personal
experimentation, definitely formulated a theory of species-
forming by heterogenesis which he placed strongly in contrast
with Darwin's theory of gradual transformation by selection,
which later theory he claimed should be wholly given up. But
not until the publication of de Vries's work, Die Mutations-
theorie, in which are recorded the results of close personal
observation and experimentation for twenty years on race and

VARIOUS THEORIES OF SPECIES-FORMING 115

species-forming in plants has the theory of species-forming by
mutations, or sudden fixed changes (lesser or greater) had any
considerable adoption or even general attention.

At the present moment, probably because of a strong re-
action against the too blind acceptance and general over-
emphasis of the selection doctrines, and because, too, of the
unusually extensive character of de Vries's experimentation and
observation, and his trenchant criticism of the weak places
in the other theories, with the generally weighty character of
his work and reputation, because of all this the theory of species-
forming by mutations has at the present moment a fairly large
body of adherents among reputable biologists. And yet the
actual evidence of tested observation on which the theory rests
is curiously meager. One hastens to admit, however, that
similar evidence for the theory of direct species-forming by
selection is also meager. While apparently no one has ever
seen a case of species-making by the natural selection of slight
fluctuating variations, de Vries seems to be almost the only
one who has observed actual cases of species-making by hetero-
genesis, and he has seen very few. And in the nature of things,
the opportunities for this kind of evidence, that is, that of actual
observation, ought to be much larger in the case of hetero-
genesis than in that of general transformation by the selection
of slight variations. An account of the exact character of
the de Vriesian mutations is included in our later chapter on
variation and mutation. Our readers should realize, that
however much they may see of this theory in present-day
popular scientific literature, and however strongly the case
may be put in favor of the mutation theory of species origin,
this theory is not accepted by the great body of biologists as
entitled to replace the Darwinian theory.

We may close this chapter with a reference to a pregnant
sentence of the American paleontologist, Osborn, in a lecture
entitled "The Unknown Factors of Evolution": "The general
conclusion we reach from a survey of the whole field is that for
Buffon's and Lamarck's factors we have no theory of heredity,
while the original Darwinian factor, or Neo-Darwinism, offers
an inadequate explanation of evolution. If acquired varia-
tions are transmitted, there must be, therefore, some unknown
principle in heredity; if they are not transmitted, there must be
some unknown factor in evolution." Our present plight seems

116 EVOLUTION AND ANIMAL LIFE

to be exactly this: we cannot explain to any general satisfac-
tion species-forming and evolution without the help of some
Lamarckian or Eimerian factor; and on the other hand, we
cannot assume the actuality of any such factor in the light of
our present knowledge of heredity. The discovery of the
"unknown factors of evolution" should be the chief goal of all
present-day biologic investigation.

CHAPTER VIII

GEOGRAPHIC ISOLATION AND SPECIES-
FORMING

"For me, it is the chorology of organisms, the study of all the
important phenomena embraced in the geography of animals and
plants, which is the surest guide to the knowledge of the real phases in
the process of the formation of species." — MORITZ WAGNER.

A FLOOD of light may be thrown on the general problem
of the origin of species by the study of certain evidence as to
the 1 actual origin of species with which we may be familiar, or of
which the actual history or the actual ramifications may in
some degree be traced.

In such cases, one of the first questions naturally asked is
this: Where did the species come from? Migration forms a
large part of -the history of any species or group of forms. The
fauna of any given region is made up of the various species of
animals living naturally within its borders. The flora of a
region is made up of the plants which grow naturally within
its limits. Of all these, animals and plants, the inhabitants
of most regions are apparently largely migrants from some
other region. Some have entered the region in question
before acquiring their present specific characters; others come
after having done so. Which of these conditions apply to any
given case can sometimes be ascertained by the comparison
of the individuals along the supposed route of migration.

Thus, Dr. C. Hart Merriam has undertaken to show the
actual origin of nine species of Californian chipmunks (Eutamias)
by an elaborate study of their distribution, adaptations, and
1 nmsformations. He finds them closely related to one another,

1 A paper published in "Science," 1906, by the senior author, under
the title "The Actual Origin of Species," has been freely quoted from in
this chapter.

9 117

118

EVOLUTION AND ANIMAL LIFE

71. — Some chipmunks of California, showing distinct species produced through
isolation. (From nature, by William Sackston Atkinson.)

GEOGRAPHIC ISOLATION AND SPECIES-FORMING 119

but not derived from one another by direct lines of descent. A
closer study indicates that some of them " came from closely
related forms in remote geographic areas, others from antece-
dent forms now extinct, and not more than three or four from
species still inhabiting the region."

The nature of any fauna bears an immediate relation to
the barriers, geographic, climatic, topographic, or bionomic,
which may form its boundaries. By bionomic barriers we
mean any condition of any sort which may check free inter-
breeding, or which may tend to cause divergence within a
species. A thickly peopled level area may be in this sense a
barrier, because it prevents the animals on the one side of the
area from interbreeding with those on the opposite side. If
the two extremes have diverged to become different species,
the individuals in the middle area, whose presence in a sense
constitutes the bionomic barrier, are usually variously inter-
mediate in the characters and habits which they possess.

Whenever the individuals of a species move evenly over an
area, its members freely interbreeding, the character of the
species remains substantially uniform. Whenever freedom of
movement and consequent freedom of interbreeding is checked,
the character of the species is rapidly altered. It is changed
even though external conditions seem to be absolutely identical
on both sides of the barrier, and if there is no visible distinction
in the original stock on the two sides. Presumably, there are
subtle differences in the environment, producing changes in
the process of selection and adaptation. Doubtless, there are
differences equally subtle produced by the processes of varia-
tion and their repetition by inheritance.

The pregnant phrase of Dr. Cones applies in these cases:
"Migration holds species true: localization lets them slip."
In other words, free interbreeding swamps incipient lines of
variation, and this in almost every case. On the other hand, a
barrier or check of any sort brings a certain group of individuals
together. These are subjected to a selection different from
that which obtains with the species at large, and under these
conditions new forms are developed. This takes place rapidly
when the conditions of life are greatly changed, so that a new
set of demands is made on the species, and those individuals not
meeting it are at once destroyed. The process is a slow one,
for the most part, when the barrier in question interrupts the

120 EVOLUTION AND ANIMAL LIFE

flow of life without materially changing its conditions. But
this is practically a universal rule: A barrier which prevents the
intermingling of members of a species will with time alter the
relative characters of the groups of individuals thus separated.
These groups of individuals are incipient species, and each
may become in time an entirely distinct species if the barrier
is really insurmountable. In the great water basin of the
Mississippi, many families of fish occur and very many spe-
cies are diffused throughout almost the whole area, occurring
in all suitable waters. Once admitted to the water basin, each
one ranges widely and each tributary brook has many species.
In the streams of California, mostly small and isolated, the
number of genera or families is much smaller. Each species,
unless running to the sea, has a narrow range, and closely re-
lated species are not found in the same river. The fact last
mentioned has a very broad application and may be raised to
the dignity of a general law of distribution.

Given any species in any region, the nearest related species
j is not likely to be found in the same region nor in a remote
I region, but in a neighboring district separated from the first
1 by a barrier of some sort, or at least by a belt of country, the
^ breadth of which gives the effect of a barrier.

Always the species nearest alike in structure are not
found together nor yet far apart, and always a check to
interbreeding lies between. Where two closely allied forms
are not found to intergrade, they are called distinct species.
If we find actual intergradation, the occurrence of specimens
intermediate in structure, the term subspecies is commonly
used for each of the recognizable groups thus connected.

Widely distributed across the United States and from
southern Canada to Arizona, we have the yellow warbler,
Dendroica wstiva. This bird is chiefly yellow, olive on the
back with chestnut streaks on the sides, the tail feathers colored
like the body, and without the white spot on the outer feathers
shown in most of the other wood warblers composing the genus
Dendroica.

The yellow warbler throughout its range is very uniform
in size and color. Its nearest relative differs in having a
shade less olive on the back and the brown streaks on the sides
narrower. This form is found in the Sonoran region, and, as
along the Rio Grande it intergrades with the first, it is called

GEOGRAPHIC ISOLATION AND SPECIES-FORMING 121

a subspecies, Dendroica (estiva sonorana. Further south, in
central Mexico, this form runs larger in size and is recorded as
Dendroica ccstiva dugesi. Northward, through to Alaska, we
have an ally of the parent bird, but smaller and still more
greenish. This is Dendroica cestiva rubiginosa.

In the West Indies, the golden warblers migrate not from
north to south, but from the shore to the mountains, and,
possibly in consequence of the less demand of flight, 'the wing
is shorter and more rounded, while the tail is longer. As these
forms do not clearly intergrade with those of the mainland,
and, for the most part, not with each other, they are held to
represent a number of distinct species, although doubtless
derived from the parent stock of Dendroica (estiva. Some of
these West Indian forms are relatively large, the wing more
than five inches long, and the longest known of these, the type
of the species for this reason, found in Jamaica, is called Dendro-
ica petechia. On the island of Grand Cayman is a similar
bird, a little smaller, Dendroica auricapilla. Of a deeper
yellow than petechia, and equally large, is the golden warbler
of the Lesser Antilles ranging from island to island, from Porto
Rico to Antigua. This form, first known from St. Bartholomew,
is Dendroica petechia bartholemica. A smaller bird, a little
different in color, takes its place in the Bahamas. This is
Dendroica petechia flaviceps.

In Cuba, the golden warbler is darker and more olive, with
other minor differences from the form called bartholemica, of
which it may be the parent. This is Dendroica petechia gund-
lachi. A similar bird, but with the crown distinctly chestnut,
is Dendroica petechia aureola, the golden warbler of the Gala-
pagos and Cocos Islands, off the coast of Ecuador and Peru.
Scattered over other islands are smaller golden warblers with
the wing less than five inches long, and with the crown tawny
red, as in aureola. These are known collectively as Dendroica
ruficapilla, the type being from Guadeloupe and Dominica.
More heavily streaked, with the crown darker in color, is the
golden warbler of Cozumel, Dendroica ruficapilla flavivertex,
and with very similar but with darker crown is Dendroica
ruficapilla flavida, of the island of St. Andrews. Always, the
nearest form lies across the barrier, and among these island
forms the chief barrier is the sea. With a darker chestnut
crown is Dendroica ruficapilla rufopileata, of the island of

122 EVOLUTION AND ANIMAL LIFE

Curasao, and still darker bay is the crown of Dendroica ruficapilla
capitalis, the golden warbler of the Barbadoes.

Still other golden warblers exist, with the chin and throat
chestnut as well as the crown. One of these, olive green on the
back, is Dendroica rufigula, of Martinique. The others are
more yellow. One of these, with the sides heavily streaked,
inhabits the isthmus region, Dendroica ei^ythacoides, called a
distinct species, because no intergradations have been made
out. Another, more faintly streaked, replaces it on the Atlantic
coast from Yucatan to Costa Rica, Dendroica bryanti, while
the Pacific coast, from Sinaloa to Costa Rica, has another form,
with still fainter markings, Dendroica bryanti castaniceps. An
extreme of this form with the throat and breast tawny, but not
the crown, is found in Jamaica again and is known as Dendioica
eoa. In this case, which is one typical of most groups of small
birds, the relation of the species to the barriers of geography
is so plain as to admit of no doubt or question.

Given the facts of individual fluctuation and of heredity,
it is manifest that while natural selection may produce and
enforce adaptation to conditions of life, the traits which dis-
tinguish these species bear little relation to utility. The
individuals which, separated from the n;ain flock, people an
island, give their actual traits to their actual descendants, and
the traits enforced by natural selection differ from island to
island. If external conditions were alike in all the islands the
progress of evolution would perhaps run parallel in all of them,
and the only differences which would persist would be derived
from differences in the parent stock. As some difference in
environment exists, there is a corresponding difference in the
species as a result of adaptation. If great differences in con-
ditions exist, the change in the species may be greater, more
rapidly accomplished, and the characters observed will bear
a closer relation to the principle of utility.

Doubtless, wide fluctuations or mutations in every species
are more common than we suppose. With free access to the
mass of the species, these are lost through interbreeding.
Isolate them, as in a garden, or an enclosure or on an island, and
these may be continued and intensified to form new species or
races. Any breeder or any horticulturist will illustrate this.

It is not claimed that species are occasionally associated
with physical barriers, which determine their range, and which

GEOGRAPHIC ISOLATION AND SPECIES-FORMING 123

have been factors in their formation. We claim that such
conditions are virtually universal among species as they exist
in nature. When the geographical relations of the origin of a
species cannot be shown it is usually because the species has not
IHVII critically studied, from absence of material or from absence
of interest on the part of naturalists. In a few cases, a species
ranges widely over the earth, showing little change in varying
conditions and little susceptibility to the effects of isolation.
In other cases, there is some possibility that saltations, or
suddenly appearing characters, may give rise to a new species
within the territory already occupied by the parent form.
But these cases are so rare that in ornithology, mammalogy,
herpetology, conchology, and entomology, they are treated as
negligible quantities.

One of the most successful workers in this field is Rev. John
T. Oulick, whose studies of the localization of species and sub-
species of land snails in Oahu Island (Hawaii) have become
classical. According to Mr. Gulick, the land snails of the
wooded portion of Oahu have become split up into about
175 species of land shells represented by 700 or SCO varieties.
He frequently finds a genus represented in several successive
valleys by allied species, sometimes feeding on the same and
similar plants. In every case, the valleys that are nearest to
each other furnish the most nearly allied forms, and a full set
of the varieties of each species presents~a~lninute gradation
between the more divergent types found in the more widely
separated localities. Similar conditions are recorded among
the land snails in Cuba and in other regions. In fact, on a
smaller scale, the development of species of land and river
mollusks has everywhere progressed on similar lines with that
of birds and fishes. To recognize isolation as practically a
necessary condition in the subdivision of species need not
necessarily eliminate or belittle any other factor. Isolation is
a condition, not a force. Of itself it can do nothing. Species
clinnge or diverge with space and with time: with space, be-
cause geographical extension divides the stock and brings
new conditions to part of it; with time, because time brings
always new events and changes in all environment. The
beginning of each species must rest with its variability of
individuals.

One of the most remarkable cases of group evolution is

124 EVOLUTION AND ANIMAL LIFE

that of the song birds of Hawaii which constitute the family of
Drepanidse. In this family are about forty species of birds,
all much alike as to general structure, but diverging amazingly
from each other in the form of the bill, with, also, striking
differences in the color of the plumage. In almost all other
families of birds the form of the bill is very uniform within
the group. It is correlated with the feeding habits of the bird,
and these in most groups of wide range become nearly uniform
within the limits of the family. With a great range of com-
petition, each type of bird is forced to adapt itself to the special
line of life for which it is best fitted. But with many diverging
possibilities and no competition, except among themselves, the
conditions are changed, and we find Drepanidse in Hawaii
fitted to almost every kind of life for which a song bird in the
tropics may possibly become adapted. (Plate II.)

In spite of the large differences to be noted there can be
little doubt, as Dr. Hans Gadow, Mr. Henry W. Henshaw and
others have shown, of the common origin of the Drepanidse.
A strong peculiar goatlike odor exhaled in life by all of them
affords one piece of evidence pointing in this direction. There
is, moreover, not much doubt that the whole group is descended
from some stock belonging to the family of honey creepers,
Coerebidse, of the forests of Central America. Each of the
Hawaiian Islands has its species of Drepanine birds, some olive
green in color, some yellow, some black, some scarlet, and some
variegated with black, white, and golden. The females in
most cases, like the young, are olive green. On each island,
most of the species are confined to a small district, to a single
kind of thicket or a single species of tree, each species being
especially fitted to these localized surroundings. With the
destruction of the forests some of these species are already rare
or extinct. With high specialization of the bill they lose their
power of adaptation. In each of the several recognized genera
there are numerous species, mostly thus specialized and local-
ized, relatively few species being widely distributed throughout
the islands.

Most primitive of all, least specialized and most like the
honey-creeper ancestry, is the olive green Oreomystis bairdi of
the most ancient island, Kauai. This bird has a small straight
bill, not unlike that of the slender-billed sparrows. It is said
to be the most energetic and ubiquitous of the group, feeding

PLATE II. — 1, Chloridops kona Wilson, Hawaii; 2, Pseudonestor xanthophrys
Rothschild 3, H e.micjnathus procerus Cabanis, Kauai. (From specimens.)

if )1

GEOGRAPHIC ISOLATION AM) SPECIES-VoKMING

on insects on the trunks of trees. It we assume that Oreo-
mystis, or some other of the genera with short and slender bills,
represents the original type of Drepanidse, we have two lines of
divergence, both of them in directions of adaptation to peculiar
methods of feeding.

Next to Oreomystis. on the one hand, we have Loxops and
Himatione, with the bill pointed, a little longer than in Oreo-
mystis, and slightly curved downward. The species, red or
golden, of these two genera are distributed over the islands,
each on its own mountain or in its own particular forest.
\'(*tiaria, another genus, remarkable for its beautiful scarlet
plumage, has the bill very much longer and strongly curved
downward. Vestiaria coccinea, the iiwi of the islands, lives
among the crimson flowers of the ohia tree (Metrosideros) and
the giant lobelia, where it feeds chiefly on honey, which is
said to drop from its bill when shot. According to Mr. S. B.
Wilson, the scarlet sickle-shaped flowers of a tall climbing
plant (Strongylodon lucidus) found in these forests " mimic in
a most perfect manner both in color and in shape the bill of
the iiwi " so that the plant is called nukuiiwi (bill of the iwii).

The next genus, Drepanis, has the sickle bill still further
prolonged, forming a segment of a circle, and covering nearly
fifty degrees. Drepanis pacifica, one of the species, has the bill
forming about one fourth of the total length. The species of
this genus, black and golden in color, were very limited in
range, and are now nearly or quite extinct. Still another
group with sickle bills, Hemignathus, diverges from Vestiaria
in having only the upper mandible very long and decurved,
the lower one being straight and stiff. The numerous species
are mostly golden yellow in color. The group contains long-
billed forms like Hemignathus procerus of Kauai, and short-
billed forms like H eterorhynchus olivaceus of Hawaii. In the
short-billed forms the two mandibles are quite unlike: the upper
very slender, much curved and about one fourth the length of
the rest of the body, the lower mandible half as long and thick
and stiff. These birds feed chiefly on insects in the dead limbs
of the koa trees in the mountain forests. Some or all of them
use the lower mandible for tapping the trees, after the fashion
of woodpeckers, while with the long and flexible upper one they
reach into cavities for insects or insect larvae or suck the honey
of flowers.

126 EVOLUTION AND ANIMAL LIFE

Mr. S. B. Wilson remarks: "Nature has shown great sym-
metry in regard to the species of this genus (Hemignathus
including H eterorhynchus} to be found in the Sandwich Archi-
pelago, three of the main islands having each a long-billed
and a short-billed form." This, of course, is most natural.
Both long-billed forms (Hemignathus) and short-billed forms
(H eterorhynchus) have spread from the island where they were
originally developed to "the other islands, each changing as it
is isolated from the main body of the species and subjected
to natural selection under new conditions. With the genus
H eterorhynchus, the forms with slender bills reach their culmina-
tion.

Going back to the original stock, to which Oreomystis
bairdi is perhaps the nearest living ally, we note first a divergence
in another direction. In Rhodacanthis , the bill is stout like
that of the large finch, not longer than the rest of the head,
and curved downward a little at the tip. The species of this
genus feed largely on the bean of the acacia and other similar
trees, varying this with caterpillars and other insects. The
stout bill serves to crush the seeds. In Chloridops, the bill is
still heavier, very much like that of the grosbeak. 'Chloridops
kona is, according to Mr. Robert Perkins, a dull, sluggish,
solitary bird and very silent; its whole existence may be summed
up in the words "to eat." Its food consists of the fruit of the
aaka (bastard sandal tree), and as this is very minute, its whole
time seems to be taken up in cracking the extremely hard
shells of the fruit, for which its extraordinarily powerful bill
and heavy head are well adapted.

"The incessant cracking of the fruits, when one of these birds is
feeding, the noise of which can be heard for a considerable distance,
renders the bird much easier to get than it otherwise would be. Its
beak is always very dirty with a brown substance adhering to it which
must be derived from the sandal nuts."

In Psittacirostra and Pseudonestor the bill suggests that of
a parrot rather than that of a grosbeak. The mandibles are
still very heavy, but the lower one, as in H eterorhynchus, is
short and straight, while the much longer upper one is hooked
over it. Pseudonestor feeds on the larvae of wood-boring beetles
(Clytanus) found in the koa trees (Acacia falcata), while the

PLATE III. — 1, Or corny stis bairdii Stejneger, Kauai; 2, H eterorhynchus oliva-
ceus La Fresnaye, Hawaii, 3, Drepanis funerea Newton, Molokai. (From
specimens.)

GEOGRAPHIC ISOLATION AND SPECIES-FORMING 127

closely related Psittacirostra eats only fruits, that of the ieie
(Freycinetia arbor ea) and the red mulberry (Morns sapyrifera)
being especially chosen. In all these genera, there is prac-
tically one species to each island, except that in some cases
the species has not spread from the mountain or island in which
we may suppose it to have been originally developed.

There are a few other song birds in the Hawaiian Islands,
not related to the Drepanida3. These are derived from the
islands of Polynesia and have deviated from the original types
in a degree corresponding to their isolation. In the case of the
Drepanidae, it seems necessary to conclude that natural selection
is responsible for the physiological adaptations characteristic
of the different genera. Such changes may be relatively rapid,
and for the same reason they count for little from the stand-
point of phylogeny. On the other hand, the nonuseful traits,
the petty traits of form and coloration which distinguish a
species in Oahu from its homologue in Kauai or Hawaii, are
results of isolation. These results may be analyzed as in part
differences in selection with different competition, different
food and different conditions, and in part to hereditary differ-
ence due to the personal eccentricities in the parent stock from
which the newer species was derived.

In these as in all similar cases we may confidently affirm:
the adaptive characters a species may present are due to
natural selection or are developed in connection with the
demands of competition. The characters nonadaptive which
chiefly distinguish species do not result from natural selection,
but are connected with some form of geographical isolation
and the segregation of individuals resulting from it.

The origin of races and breeds of domestic animals is in
general of much the same nature. In traveling over Eng-
hiiul one is struck by the fact that each county has its own
breed of sheep, each of these having its type of excellence in
mutton, wool, hardiness, or fertility, but the breeds distin-
guished by characters having no utility either to sheep or to
man. The breeds are formed primarily by isolation. The
traits of the first individuals in each region are intensified by
the inbreeding resulting from segregation. Natural selection
preserves the hardiest, the most docile, and the most fertile:
artificial selection those which yield the most wool, the best
mutton and the like. The breed once established, artificial

128 EVOLUTION AND ANIMAL LIFE

selection also tends to intensify and to preserve its nonadaptive
characteristic marks. The more pride the breeders take in
their stock, the more certain is the preservation of the breed's
useless peculiarities. Very few of the characters which usually
distinguish a breed of domestic animals have the slightest phys-
iological value to the species. Each of them would disappear
in a few generations of crossing, and in each race prized by the
breeder the actual virtues exist wholly independently of these
race marks.

Analogous to the race peculiarities of domestic animals
are the minor traits among the men of different regions. Cer-
tain gradual changes in speech are due to adaptation, the fitness
of the word for its purpose, analogous to natural selection.
The nonadaptive matters of dialect find their origin in the
exigencies of isolation, while languages in general are ex-
plainable by the combined facts of migration, isolation, and
the adaptation of words for the direct uses of speech.

In the animal kingdom generally we may say therefore:
Whenever a barrier is to some extent traversable, the forms
separated by it are likely to cross from one side to the other,
thus producing intergradations, or forms more or less inter-
mediate between the one and the other. For every subspecies,
where the nature of the variation has been carefully studied,
there is always a geographical basis. This basis is defined
by the presence of some sort of physical barrier. It is ex-
tremely rare to find two subspecies inhabiting or breeding in
exactly the same region. When such appears to be the case,
there is really some difference in habit or in habitat: the one
form lives on the hills, the other in the valleys; the one feeds
on one plant, the other on another; the one lives in deep water,
the other along the shore. There can be no possible doubt that
subspecies are nascent species, and that the accident of inter-
gradation in the one case and not in the other implies no real
difference in origins.

For a final example, we may compare the species of Ameri-
can orioles constituting the genus Icterus. We may omit from
consideration the various subspecies, set off by the mountain
chains, and the usual assemblage of insular forms, one in
each of the West Indies, and confine our attention to the
leading species as represented in the United States. (See
frontispiece.)

GEOGRAPHIC ISOLATION AND SPECIES-FORMING 129

The orchard oriole, Icterus spurius, has the head, back, and
tail all black, the lower parts chestnut, and the body relatively
small, as shown by the average measurements of different
parts. In the hooded oriole, Icterus cucullatus, the head is all
golden orange except the throat, which is black, the tail is
black, and the wings are black and white. This species, with
its subspecies, ranges through southern California and Arizona,
and over much of Mexico. Our other orioles have the tail black
and orange. In the common Baltimore oriole, Icterus galbida}
of the east, the head is all black and the under parts orange.
In the equally common Bullock oriole, Icterus bullocki, of the
California region, the head is yellow on each side, the belly
rather yellow than orange. The females of all the species are
plain olivaceous, the color and proportions of parts varying
with the different species, while in the males of each of the
many species black, white, yellow, orange, and chestnut are
variously and tastefully arranged. Each species again has a
song of its own, and each its own way of weaving its hanging
nest.

That which interests us now is that not one of these varied
traits is clearly related to any principle of utility. Adaptation
is evident enough, but each species is as well fitted for its life
as any other, and no transposition or change of the distinctive
specific characters or any set of them would in any conceivable
degree reduce this adaptation. No one can say that any one
of the actual distinctive characters or any combination of
them enables their possessors to survive in larger numbers
than would otherwise be the case. One or two of these traits,
as objects of sexual selection or as recognition marks, have a
hypothetical value, but their utility in these regards is slight
or uncertain. Naturalists now look with doubt on sexual
selection as a factor in the evolution of ornamental structures,
arid the psychological reality of recognition marks is yet un-
proved, though not impossible. It may be noted in passing,
that the prevalent dull yellowish and olivaceous hues of the
female orioles of all species seem to be clearly of the nature
of protective coloration.

It has been shown statistically that certain specific charac-
ters among insects have no relation to the process of selection.
Among honey bees the variation in venation of the wings
and in the number and character of the wing hooks is just as

130 EVOLUTION AND ANIMAL LIFE

great among the bees which first come from their cells as in a
series of individuals long exposed to the struggle for existence.

Among ladybird beetles of a certain species (Hippodamia
convergens), eighty-four different easily describable "aberra-
tions" or variations in the number and arrangement of the
black spots on the wing covers have been traced. These
variations are again just as numerous in individuals exposed
to the struggle for life as in those just escaped from the pupal
state. In these characters, there is, therefore, no rigorous
choice due to natural selection. Such specific characters,
without individual utility, may be classed as indifferent, so
far as natural selection is concerned, and the great mass of
specific characters actually used in systematic classification are
thus indifferent.

And what is true in the case of the orioles and the lady-
birds is true as a broad proposition of the related species which
constitute any one of the genera of animals or plants. All
that survive are sufficiently fitted to live, each individual, and
therefore, each species, matched to its surroundings as the dough
is to the pan, or the river to its bed, but all adaptation lying ap-
parently within a range of the greatest variety in nohessentials.
Adaptation is presumably the work of natural selection; the
division of forms into species is the result of existence under
new and diverse conditions.

CHAPTER IX

VARIATION AND MUTATION

It becomes imperative that we should carry out the most exact
research possible by means of experiment and also wean ourselves of
the convenient, but, as it seems to me, highly pernicious habit of theo-
retical explanations from general propositions. Otherwise there is
great danger that the bright expectation which Darwin has opened out
to us by his theory may be baffled — the prospect of gradually bringing
even organic Being within reach of that method of inquiry which
seeks to discern mechanical efficient causes. — SEMPER.

THUS far in our discussion of evolution factors and theories
we have taken for granted the actuality of the two fundamental
factors, variation and heredity. No one disputes their reality;
nor does anyone deny their fundamental and indispensable
character in relation to the origin of species and the evolution
of organisms. All the theories to explain evolution build on
these two basic factors or vital conditions. The subjects of
doubt or denial are such postulated factors as selection, muta-
tion, orthogenetic progress, etc.; variation and heredity never.

But the character, the influence, the regularity or irregu-
larity of variations, their behavior in heredity, whether trans-
missible or not, whether acquired or congenital, whether deter-
minate or indeterminate, etc. — these are the problems that the
factor variation or variability presents to biologists. Heredity,
too, has its problems. These we shall take up in another
chapter.

That variations exist is too obvious to everyone to need any
discussion. Any litter of kittens or puppies, of mice or pigs,
shows us the differences in pattern, shape, and physiology of in-
dividuals born at one time and of the same parents. In wild
nature the variations among brothers and sisters are no less real
than among these domesticated animals.

131

132

EVOLUTION AND ANIMAL LIFE

Collect a few thousand individuals, at one time in one place,
of a single species of insect, as a spotted ladybird beetle; then
go over these carefully, looking for variation in some single
characteristic, as the color pattern. What do you find? Let us

FIG. 72. — Diagram showing variation in elytral pattern of the convergent ladybird,
Hippodamia convergens : 1, Mode; 2-9, variations in size of spots; 10-17, variations
by coalescence of spots; 18-40, variations by reduction in number of spots. (After
Kellogg and Bell.)

VARIATION AND MUTATION

133

answer by calling attention to Figs. 72, 73, and 74 and what
these variations signify. Note also Fig. 75, showing the

PIG. 73. — Diagram showing variations in elytral pattern of convergent ladybird,
Hippodamia convergens: 1-5, Variations by different reduction in number of spots
in the two elytra; 6-9, variations by conditions of spots. (After Kellogg and Bell.)

variation in elytral blotching to be found in a series of individ-
uals of the California flower beetle, Diabrotica soror; see also
Fig. 76, showing the vari-
ations in the black and
yellow color pattern of the
abdomen of the common
yellow jacket (Vespa sp.);
and Fig. 77 showing the
variation in the pattern of
the prothorax in a series of
178 individuals of a common
Calif ornian flower bug, all
these individuals collected
at one time by sweeping a
net over a few rods of alfalfa
and Baccharis on the campus FlG 74._Diagram showing Variati0ns in

Of Stanford University. prothoracic pattern of the convergent

These are all Color and ladybird, Hippodamia convergens. (After

Kellogg and Bell.)

pattern variations; but in-
sects show variations in structural parts as well. Fig. 78 shows
a common red-legged locust and one of its hind tibiae enlarged
10

o

134

EVOLUTION AND ANIMAL LIFE

FIG. 75. — Diagram showing variations in elytral pattern of the California
flower beetle, Diabrotica soror. (After Kellogg and Bell.)

to show the spines. In eighty-nine individuals of this species
of locust collected at Ithaca, N. Y., the number of spines in the

outer row of the right tibiae
varies from nine -to fifteen,
in the inner row from eleven
to sixteen. One not given to
the systematic study of insects
may think spines on the hind
legs very trivial structures in-
deed; but the entomologist,
using exactly such character-
istics as the number of these
structures as a means in help-
ing him to distinguish and
define his species, knows how
considerable this variation
really is.

The dog-days cicada (Fig.
79) also has spines on its hind
tibiae, but only a few, usually,
indeed, two. But in any series

of individuals of this insect some individuals will be found with
but a single spine, some with three, and a few with four even,
although the very great majority will have two. For example,

FIG. 76. — Diagram showing variation in
pattern in the yellow jacket, Vespa ger-
manica. (After Kellogg and Bell.)

VARIATION AND MUTATION

135

in a series of 98 male individuals collected at Indianapolis, In-
diana, at one time, 12 individuals had one spine in the outer row
of the right tibiae, 83 had two spines, 2 had three spines, and one
had four spines. In the outer row of the left tibiae of the same
individuals, there were three spines in 6 individuals, two in 75,
and one in 17. In the inner rows of tibial spines in these same

FIG. 77. — Diagram showing variation in pattern of the prothorax of a flower bugt
(After Kellogg and Bell.)

individuals there were in the right tibiae, five spines in 5, four
spines in 40, three spines in 43, two spines in 9, and one spine in
1 individual: in the left tibiae, five spines in 2 individuals, four
spines in 48, three spines in 39, and two spines in 8.

In the paper from which we have taken these illustrations
of the actuality of variation, studied and statistically tabulated,
are given the data showing the actual extent and frequency of
variations in various characters, such as color patterns of head,
thorax, and abdomen, character of antennal segments, number
of tibial spines, character of elytral striation, character of vena-

136

EVOLUTION AND ANIMAL LIFE

tion, number of wing hooks, etc., in two dozen different insect
species. Long ago Dr. J. A. Allen, of the American Museum of
Natural History, gave similar data of the actual variation in

FIG. 78. — Red-legged locust, Melanoplus FIG. 79. — The seventeen-year locust, Cicada

femur-rubrum, and hind tibia, showing septendecim, and its hind tibia, showing
inner and outer rows of spines. (After
Kellogg and Bell.)

inner and outer spines.
and Bell.)

(After Kellogg

various familiar American bird species, his data referring
chiefly to variations in dimensions; as length of whole body,
length of tail, of wing, of bill, of tarsus and claw, etc.

CARDINALIS VIRGIMANUS 58 specimens, Florida.

7a/V.

: :*:

• • ••••• •••••

•• ••••• »•••••••••••

Length ofBird

•V.

Winy.

..v.

v..v. .:.

•.

•V.

» ••• •

FIG. 80. — Diagram showing variation in length of tail, body, and wing in fifty-eight
specimens of the cardinal, Cardinalis (formerly called virginiarius) , from Florida.
(After Allen.)

And anyone with means of collecting considerable series of
individuals of single species can, if he but give the time and
study to it, reveal similar variations in almost any part or
characteristic of any species or kind of plant or animal. " What

VARIATION AND MUTATION

137

parts vary?" some one asks. All parts vary, but some more
than others.

Darwin, in Chapter V of his "Origin of Species," postulated
certain so-called laws of variability, which attempt to answer
this question, "What parts vary?" These so-called "laws"
which to-day would hardly
be dignified with the name
of law, are summed up by
Darwin at the end of this
chapter as follows:

VARIATION OF

ICTERUS BALTIMORE.20J

Tail. • *

•• • •

•»
••

Tarsus.

• • ••

Middle Toe.

•••• f •••

Hind, Toe.

Bill.

Length.

::*!*:••:.

"Our ignorance of the laws
of variation is profound. Not
in one case out of a hundred
can we pretend to assign any
reason why this or that part
has varied. But whenever we
have the means of instituting
a comparison, the same laws
appear to have acted in pro-
ducing the lesser differences
between varieties of the same
species, and the greater differ-
ences between species of the
same genus. Changed condi-
tions generally induce mere
fluctuating variability, but
sometimes they cause direct
and definite effects; and these
may become strongly marked
in the course of time, though
we have not sufficient evidence
on this head. Habit in pro-
ducing constitutional peculiarities, and use in strengthening, and
disuse in weakening and diminishing organs, appear in many cases
to have been potent in their effects. Homologous parts tend to
vary in the same manneV, and homologous parts tend to cohere.
Modifications in hard parts and in external parts sometimes affect
softer and internal parts. When one part is largely developed, perhaps
it tends to draw nourishment from the adjoining parts; and every part
of the structure which can be saved without detriment will be saved.

Width.

Bill, Widl

:::.::':

Fro. 81. — Diagram showing variation in di-
mensions in twenty male specimens of the
Baltimore oriole, Icterus galbula (formerly
called baltimare). (After Allen.)

138 EVOLUTION AND ANIMAL LIFE

Changes of structure at an early age may affect parts subsequently
developed ; and many cases of correlated variation, the nature of which
we are unable to understand, undoubtedly occur. Multiple parts are
variable in number and in structure, perhaps arising from such parts
not having been closely specialized for any particular function, so that
their modifications have not been closely checked by natural selection.
It follows, probably from this same cause, that organic beings low in
the scale are more variable than those standing higher in the scale, and
which have their whole organization more specialized. Rudimentary
organs, from being useless, are not regulated by natural selection, and
hence are variable. Specific characters — that is, the characters which
have come to differ since the several species of the same genus branched
off from a common parent — are more variable than generic characters,
or those which have long been inherited, and have not differed within
this same period. In these remarks we have referred to special parts
or organs being still variable, because they have recently varied and
thus come to differ ; but we have also seen . . . that the same prin-
ciple applies to the whole individual; for in a district where many
species of a genus are found — that is, where there has been much former
variation and differentiation, or where the manufactory of new spe-
cific forms has been actively at work — in that district and among
these species we now find, on an average, most varieties. Secondary
sexual characters are highly variable, and such characters differ much
in the species of the same group. Variability in the same parts of the
organization has generally been taken advantage of in giving secondary
sexual differences to the two sexes of the same species, and specific
differences to the several species of the same genus. Any part or organ
developed to an extraordinary size or in an extraordinary manner, in
comparison with the same part or organ in the allied species, must have
gone through an extraordinary amount of modification since the genus
arose ; and thus we can understand why it should often still be variable
in a much higher degree than other parts; for variation is a long-con-
tinued and slow process, and natural selection will in such cases not as
yet have had time to overcome the tendency to further variability and
to reversion to a less modified state. But when a species with an ex-
traordinarily developed organ has become the parent of many modified
descendants — which in our view must be a very slow process, requiring
a long lapse of time — in this case, natural selection has succeeded in
giving a fixed character to the organ, in however extraordinary a
jnanner it may have been developed. Species inheriting nearly the
same constitution from a common parent, and exposed to similar

VARIATION AND MUTATION 139

J^ectt
..Body

Lacerta ocellata

-JfindLegs

Neck

Lacerta viridis

N Hind Legs

.Tail

M Neck

I— N Body

Lacerta agilis M Hind Legs

Tail

— • Neck

*•* JBody

Lacerta muralis ^^^ Hind Legs

Neck

Lacerta velox ^^ Hind Legs

Tail

m- Neck

•« £o</y

Lacerta deserti M Hind Legs

FIG. 82. — Diagram showing variations in dimensions of lizards. (After Wallace.)

influences, naturally tend to present analogous variations, or these
same species may occasionally revert to some of the characters of their
ancient progenitors. Although new and important modifications may
not arise from reversion and analogous variation, such modifications
will add to the beautiful and harmonious diversity of nature.

140

EVOLUTION AND ANIMAL LIFE

Num. of

Vacates.

500

450

" Whatever the cause may be of each slight difference between the
offspring and their parents — and a cause for each must exist — we have
reason to believe that it is the steady accumulation of beneficial differ-
ences which has given rise to all the more important modifications of
structure in relation to the habits of each species."

Modern investigation of variation, which includes at least two
phases of study that have been developed since Darwin's time,
namely the statistical and quantitative, and the experimental

study of variation, has been
able to add much information
about the mode and the
character of variations, and
has effected a sort of classifi-
cation of them which helps at
once to express and to clarify
and organize our knowledge
of variability. But it has
added as yet no great funda-
mental generalizations really
worthy to be called laws.

One generalization there is,
perhaps, of application and
value far reaching enough to
be called law (although it ap-
plies to only a single category
of variation, but a large one),
and that is the law formulated

Classes...O

1 8 9 10

FIG. 83. — Diagram showing curves of
distribution of frequency of variation in
glands of swine. (After Davenport.)

in 1846 (ten years before Dar-
win's "Origin of Species"),
by the Belgian anthropologist,
Quetelet, on the basis of the
examination of the height and

chest measurements of soldiers. As it applies only to what
are variously called fluctuating, individual, continuous, or Dar-
winian variations, we may note before stating the law the cur-
rent mode of classifying the variations which occur in ' plants
and animals.

Variations may be either congenital or acquired: that is,
may be such as are apparently determined in the organism at
conception, or such as are imposed on it during its development

VARIATION AND MUTATION 141

by the influence of extrinsic factors. Or variations may be di-
vided into determinate and indeterminate; that is, those (if there
really are such) which are apparently controlled by some, to us
unknown, influences and are by these influences confined to cer-
tain definite lines or directions of change; and, on the other hand,
those which are apparently wholly accidental, or rather which
may represent any conceivably possible line or kind of change.
Finally, variations may be distinguished as to their general
character as discontinuous and continuous ; that is, variations oc-
curring irregularly, mostly large and comparatively rarely, and
small, abundant variations occurring in graded series. Among
the former are to be ranked the occasional sports and monsters
familiar to all breeders ; while in the latter, Darwin believed him-
self to have at hand the necessary ever-present materials to
serve natural selection as a basis for species transformation.
Hence the slight but abundant and ever-present fluctuating
continuous variations are often called "Darwinian variations."

Now the law of Quetelet applies solely to the Darwinian
variations. The law is, that these variations occur according
to the law of probabilities (or law of error) : that is, that the
slightest variations away from the modal or average type will
be the most abundant, and that the number of varying individ-
uals will be progressively less the farther away from the modal
type the variations of these individuals are. That is, if the vari-
ations in some characteristic of a species be determined for, say,
10,000 individuals of the species, and tabulated, and a curve
erected to express graphically the facts of this variability, this
curve will practically coincide with that one which would simi-
larly express the variation, if the variation actually occurred
according to the mathematical law of the frequency of error;
this theoretical curve being obtained by the formula deduced
originally by Gauss at the beginning of the last century. Fig.
83 shows graphically how certain studied cases of continuous
variation reveal the condition expressed by Quetelet 's law.

As compared with discontinuous and sport variation, con-
tinuous variation is by great odds the more common. Bateson,
an English student of variations, has attempted to show that
discontinuous variations are more common than is generally
believed, and has filled a large volume with accounts and illus-
trations of such alleged variations. But it has been proved
that many of these are cases of teratogenic regeneration, or ab-

142 EVOLUTION AND ANIMAL LIFE

normal restoration of injured parts. Others, too, are of a char-
acter which, to many people, will not seem to be discontinuous
at all, but continuous. For example, differences in number of
antennal or tarsal segments in insects are called by Bateson
cases of discontinuous variation if the differences are only by
one segment. But as the differences cannot well be less than a
whole segment, variations in number of segments, if represented
by all the successive numbers between the lowest and highest
number of segments observed, may fairly be called continuous:
that is, strictly gradatory.

It may be of interest to note, for the purposes of explaining
by concrete examples the various phases or categories of varia-
tion already named, some specific examples exemplifying each
category. The following are taken from a paper on variation in
insects, which records a number of statistical studies of varia-
bility made by the junior author of this book and Mrs. Bell-
Smith.

To distinguish absolutely between acquired variation and
congenital (or blastogenic) variation is a matter which can be
done in but comparatively few cases. Whether a variation be
congenital or whether it be acquired during the development or
life-time of the individual showing it, this variation cannot be
recognized until after a considerable part of the development
has been undergone; if it is a variation in an adult structure or
function, all of the development must have been completed.
The variation is apparent only after it is unfolded: only after
the part it appears in has reached its definite stage of completed
growth and development.

Now, who is to say whether this variation was or was not
imposed on the individual showing it, during this long develop-
ment and immature life as a result of some external influence
brought to bear on the varying part during the development?
We know that such extrinsic influences do modify parts and
functions during individual development, and so we must be
very careful when we claim that this or that variation is con-
genital and not acquired. Yet, how all-important it is to make
the distinction is apparent when we recall the fact that most
biologists are agreed that acquired characters (variations) can-
not be inherited, so that new species can be built up only on
the basis of congenital characteristics.

In the case of insect variations, a criterion for distinguishing

VARIATION AND MUTATION 143

between the congenital and acquired condition is at hand,
thanks to the unusual character of the development of certain
specialized insects, namely, all those that undergo a complex
metamorphosis.

"Without by any means exhausting the subject of the postembry-
onic development of insects, entomologists have become sufficiently
well acquainted with the phenomena attending this development to be
able to confirm absolutely (in essential characters) Weismann's dis-
coveries in the larva of the ' imaginal discs ' as the independent embry-
onic centers from which develop the wings, legs, antennae, and some
other parts of the winged adults (imagines) of insects with complete
metamorphosis. That is to say, in all the insects which hatch from the
egg in a larval condition markedly different from the definitive condi-
tion of the species in its fully developed, mature stage, many of the
adult organs, as the external parts of the head, and the legs and wings,
are produced not by a gradual development, growth, and transforma-
tion of the corresponding larval parts, but by a special development in
late larval life and during the pupal stage, the final structures being
formed from small groups of previously undifferentiated subembryonic
cells. These cells are derived in the case of the external parts just
named chiefly from i-nvaginations of the larval cellular skin layer. In
the larva (maggot) of a house fly, for example, there are no functional
legs or wings: there are no external signs (buds, pads) of these organs at
any time in the larval stage.

"In the larval life there can be no possible molding influence on
these future adult organs of the nature of a direct response or reaction
to the immediate environment. We might assume such an influence
possible if the wings and legs were slowly transforming external struc-
tures subject to attempts at or actual functional use in flight or crawling
during the larval life. At pupation, the wings and legs suddenly
appear as external parts, but still equally functionless, and now wholly
concealed and protected by the opaque chitinized wall of the puparium.
With the final issue of the adult, the wings and legs appear for the first
time in functional condition, and with the simple need of unfolding,
expanding, and drying the outer wall, an operation requiring but few
moments, they appear at this time in their definitive fully developed
condition. The wings have the arrangement of veins and number of
spines and fringing hairs; the legs have the armature of spines and
spurs and the number of segments which they retain unchanged through
the short or longer adult life. The wings and legs of the adult of all

144 EVOLUTION AND ANIMAL LIFE

insects with complete metamorphosis — and the insects of this category
include all the beetles (Coleoptera), two-winged flies (Diptera), moths
and butterflies (Lepidoptera), ants, bees, wasps, gall flies and ich-
neumons (Hymenoptera), and some other orders — are exposed during
their development to just one type of extrinsic influences, namely,
those of nutrition, temperature, humidity, etc. These influences
affect the whole body and metabolism of the body-developing insect.
But they have no direct relation to specific parts.

"An important special environing condition of life, and one that
certainly works direct and obvious influence on the body wall of certain
animals, is what may be called the chromatic condition of the environ-
ment. Color and pattern adapted to the needs of protection or ag-
gression are phenomena familiar throughout the animal series. Most
of such color and pattern conditions, catalogued under the head of
protective resemblance, mimicry, warning colors, etc., are fixed con-
ditions as far as the individual is concerned, presumably brought
about by the age-long action of natural selection.

"Not a few animals display the capabilities of achieving marked
adaptive changes, i. e., acquired variations, during their immature life
(postembryonic development). But it is obvious that insects of com-
plete metamorphosis, which possess in adult stage a color scheme and
pattern wholly different from that of the larva or pupa and one which
is not apparent until it appears in fixed definitive condition on the
emergence (and drying) of the imago from the pupal cuticle, cannot be
conceived to show, in their color pattern, variations due to individual
adaptive changes. That is, variations in this color pattern among the
individuals of a species are not acquired, but are strictly congenital,
except in so far as they are produced by the general influences of
nutrition, temperature, etc., working without reference to the external
chromatic conditions of the environment.

"Even such all-pervading influences as nutrition, temperature,
humidity, and light may be, and in many cases obviously are, so nearly
practically identical for all the members of one brood, or even for all
the individuals of the species, that they can have little or no influence
in causing variations. For conspicuous example, the case of the honey
bee may be noted. Here, all the larvae live side by side under identical
conditions (those of the hive) of temperature, humidity, and light, and
the distribution of exactly similar food to them in similar quantity is
probably as nearly exactly uniform as could be guaranteed under our
most careful artificial experimental conditions. The pupae are, more-
over, under identical conditions of temperature, moisture, and light, so

VARIATION AND MUTATION 145

that when the adults issue, the variations to be found in any of their
parts may with complete confidence be ascribed to prenatal influences,
to intrinsic causes. They are purely blastogenic. Similarly, the con-
ditions of life of the developing individuals of all the other social
insects, the termites, ants, and social wasps, are practically identical.

"The variations, therefore, in the color pattern of Diabrotica (Fig.
75), Hippodamia (Figs. 72, 73 and 74), and Vespa (Fig. 76) (insects
of complete metamorphosis with all adult external structures never
exposed to outside conditions until in definite unchangeable condition),
are congenital variations. Of the same nature are also the structural
variations in the character of the venation and the number of wing
hooks in the honey bee (see Fig. 94). But the variations in the pat-
tern of the prothorax of the flower bug (Fig. 77), and in the number
of spines on the tibiae of the red-legged locust (Fig. 78), and the cicada
(Fig. 79), may be in part acquired. In these latter cases the insects,
not having a complete metamorphosis, have during their immature
life these color and structural characters in formative condition, and
to some extent in use. They are therefore exposed to the continuous
influence of their environment."

It might be thought that we could determine whether varia-
tions are congenital or acquired in cases in which we are thor-
oughly acquainted with the character of the environment or ex-
trinsic influences which have surrounded the individuals during
their development. In experimental cases we can control this
environment and make it identical for all of a given lot of
individuals, or measurably varying for different lots. Then
by comparison we can determine what characteristics still vary
among those individuals exposed to identical environment—
these variations should be congenital — and what new kinds of
variations appear in those individuals exposed to different en-
vironments— these should be acquired variations. This has
been done experimentally for silkworm moths. By varying
the food supply, etc., marked variations have been produced in
the size of larvsc and moths, weight of silken cocoons, duration
of larval stages (instars). These variations are manifestly
acquired, and wherever in nature simple variations in dimen-
sions are found among individuals of a species, this is due un-
doubtedly, to greater or lesser extent, to differing conditions of
nutrition. But we know well that a practically identical food
supply given to domesticated animals or human beings can

146

EVOLUTION AND ANIMAL LIFE

never make all the individuals of a single brood or family of the
same size. Part of the dimensional variation is due, therefore,
to congenital causes. In a beehive, the condition of temper-
ature, humidity, and food supply are practically identical for
all the developing bees, and yet bees born of eggs laid by a
single queen, reared at the same time in
the same hive, vary largely in such easily
determinable and important matters as
venation of the wings, number of hooks
used in holding the two wings of one side
together, color pattern, etc. Undoubtedly,
these variations are strictly congenital, hence
inheritable, and therefore of a character to
serve as a basis for species change.

With regard to examples of continuous
and discontinuous variations, we take the
following from the paper on "Variation in
Insects'':

FIG. 84.— Head of deer
with one prong of
horns markedly dif-
ferent from the other.
(After Bateson.)

"By continuous variations we refer to those
variations mentioned above, variously called

fluctuating, individual, etc., which are present in any series of indi-
viduals of a species, and which cluster about the modal or most
abundantly represented forms of the species, as would be expected
from the law of error (law of probabilities) dis-
cussed above.

"Morgan, in 'Evolution and Adaptation/ ob-
jects to the use of ' continuous ' as a descriptive
name for these variations, on the ground that the
word suggests persistence or continuity through
successive generations. It seems to us, however,
that the name is an apt one, if ' continuous ' be
taken to mean that the occurring variations in
any (sufficiently large) set of individuals form a
continuous series, the extremes being connected
or immediately merging into each other by a
series of small gradatory steps. By ' discontinuous ' variation, we
would mean, in contrast to continuous, such considerable and radical
changes as have been variously called single variations, sports, saltations,
mutations, etc. ; that is, variations which are not members of graded
series and do not group themselves in orderly manner about the modal

PIG. 85. — Turtle with
two heads. (After
Bateson.)

VARIATION AND MUTATION

147

FIG. 86.— Child with six
toes on each foot. (After
Bateson.)

species form according to the law of error. Although often not large,
they are yet rarely so minute as those differences which distinguish the
adjacent members in any series of individuals arranged on a basis of
continuous or fluctuating variation. Mutations, according to the usage
of de Vries, discontinuous variations may or may not be. Thus, all
mutations might be called discontinuous
variations, although not all discontinuous
variations are necessarily de Vriesian muta-
tions, that is, certain to breed true under
varying conditions of environment.

" As a matter of fact, not all continuous
variation follows the law of error : the curve
or polygon of frequency is not infrequently
an unsymmetrical one : ' skewness ' prevails ;
that is, the highest part of the curve may be

nearer one end, or the curve may even be bimodal. But neverthe-
less the 'continuity' of the variations is unmistakable. In a suffi-
ciently large series the extremes of the range are perfectly connected
with the mode or modes and hence with each other by gradatory steps
very small in size. Whatever the largeness of the difference between
the extremes, any two adjacent members of the series are hardly distin-
guishable. This gradual
kind of variation, in-
sensible, but yet effective
(as regards widely sepa-
rated members of the
series), is most typically
illustrated in cases of
what Bateson calls 'sub-
stantive' variation, that
is, where the varying
characteristic is one of
pattern, of length, width,
or bulk, of the curving
of a vein or leg or spine.
Excellent examples of this

continuous substantive variation are presented by the abdominal and
face patterns of Vespa (see Fig. 76), and the elytral pattern of Dia-
brotica (see Fig. 75).

" According to Bateson, variations in number of antennal and
tarsal segments, number of spines, hairs, or other processes, and other

FIG. 87. — Eyestalks of a decapod dissected out:
on the right an antenna has regenerated out in
place of an amputated eye; opt., optic nerve.
(After Herbst.)

148

EVOLUTION AND ANIMAL LIFE

such numerical or, as called by him, meristic variations, must be looked
on as different in kind from the substantive variations — those capable
of perfect merging from one condition to another — in other words,
practically incapable of quantitative measurements. These meristic
variations are called discontinuous by Bateson. Typical examples are
the variation in the number of the costal wing hooks in bees and ants,
the number of tibial spines in the locust and cicada, the number of
metathoracic tactile hairs in biting bird lice, etc. But when one stops
to consider the fact that in all these cases variation could hardly occur

FIG. 88. — Variations in pattern of wings of Peronea cristana. (After Clark.)

by any steps less than those of one hook or one spine or one hair, that
a half hook or half antennal segment is inconceivable, serious doubts as
to the validity of Bateson's classification of variations as continuous
and discontinuous will certainly result. The doubt is strengthened
by the difficulty of a clean classification presented by such cases as that
of Hippodamia convergens (Figs. 72, 73 and 74). Here we have a
substantive variation in pattern, appearing, however, in such a way as
to demand numerical, i. e., meristic, expression. One specimen has
nine elytral spots, another ten, another eleven, and so on; the whole
range is indeed from naught to eighteen, with every number between
represented, each by various combinations of spots.

" But it is conceivable, and indeed is really the case among our
specimens, that these spots might be either of normal size, or of any
lesser size down to the limits of visibility. Some of the spots are of
the diameter of pin points, some of the pin shaft, and some of pin
heads. There is perfect gradation and continuity in this variation.

VARIATION AND MUTATION

149

And even in such cases as variations in spines and hairs, this gradation
might exist : and indeed it does. Although in our consideration of the
variation in the number of the tibial spines of the locust and cicada
and in the number of the tactile hairs of the bird lice, we have referred
to these variations only numerically, i. e., meristically, as a matter of
fact there are obvious differences in the length, i. e., size, of the spines
and hairs, so that it would be wholly fair to break down the unit differ-
ences and speak of differences by one quarter, one third, and two thirds
of a spine. For the tibial spines of the locust, we have actually re-
corded the conditions in the form of frac-
tions. But in the case of a hook or an
antennal or a tarsal segment it is a unit or
nothing.

^^To our mind, the distinction between
substantive and meristic variation is not
at all equivalent to a distinction between
continuous and discontinuous variation.
It is a distinction between two categories
of variation only in that one category in-
cludes such conditions as permit more
readily of extremely slight, nearly insensi-
ble, practically unmeasurable differences,
as those of pattern or shape or extent,
while the other category includes partic-
ularly conditions in which any variation
must of necessity be fairly obvious, and
usually capable of numerical expression.

" But we believe, nevertheless, that variations really discontinuous
occur among insects. For example, the occurrence of interpolated,
wholly new, and complete cells (determined by the presence of new
cross veins or branches of longitudinal veins) in the fore and hind
wings of drone honey bees (Figs. 93 to 96) and the occurrence of
curious malformations of venation among drone bees must be looked
on as sports or truly discontinuous variations. The regular occurrence
of a four-segmented foot, perfectly complete, functional in those numer-
ous specimens of cockroach (Fig. 89), in which natural regeneration
has taken place, may be looked on as an example of discontinuous
variation. Although no difference in tarsal segments less than that of
one is conceivable, it is quite conceivable that the foot with one fewer
than the normal number might be in such condition that it would be
obviously a five-segmented foot with one segment dropped out: in
11

FIG. 89. — Cockroach, showing
varying number of tarsal seg-
ments in legs. (After Kellogg
and Bell.)

150 EVOLUTION AND ANIMAL LIFE

other words, that when compared with a normal five-segmented foot it
would appear to be a modification of such a foot with some one segment
wanting. But that condition is not at all what appears after the cock-
roach regenerates a foot. The new foot is only very little, if any,
shorter than the normal five-segmented foot (see Fig. 89) : one cannot
say that it is precisely this or that segment which is lost. It is a new
kind of foot, apparently just as capable, as 'fit/ as useful as the five-
segmented kind. We have regularly occurring, in these cases of re-
generation, the development of an entirely changed organ, similar as
a whole to the old one, but different from it in all its parts; this differ-
ence not being one of incompleteness, or serial addition or subtraction,
but the difference of newness. It is the regenerative mutation of an
organ!"

In five years of experimental rearing of the silkworms for
the sake of studying phenomena of heredity and variation, the
junior author has been able to record numerous cases of discon-
tinuous or sport variation such as the absence of the usually
well-developed caudal horn of the larva, melanism in larvae,
double cocooning or absence of cocoon in the pupal condition,
congenital monstrous loss of a whole wing in the adult, striking
aberration of the wing pattern in the adult, etc. But the great
mass of variation ever present and readily observable among
the scores of thousands of silkworm individuals reared and
carefully scrutinized has been of the continuous (fluctuating or
Darwinian) type.

A special type or kind of discontinuous variation, that
exemplified by the so-called de Vriesian mutations, is discussed
at the end of this chapter.

The matter of determinate variation is discussed as follows
in the same paper :

"The theory of determinate variation is based on the hypothesis
that fluctuating variations are not in all cases, nor necessarily in any
case, purely fortuitous and scattering, but that because of some in-
trinsic or extrinsic influence they tend to occur along definite or
determinate lines. The need for the theory rests on the claimed inad-
equacy of slight fortuitous variation in offering selection a sufficient
' handle ' for action. The greatest logical difficulty with the theory is
that none of the influences which are known is adequate to cause such
an effect as that of producing persistent determinate variations. In
the case of any developing individual, determinate variation can be

VARIATION AND MUTATION 151

attained by controlling the environment (kind and quantity of food,
degree of temperature, humidity, and light, etc.), but if such variations
(modifications) acquired during development are not inherited, there
will be no advance, generation after generation, along any line. There
will be no cumulative effect of such determinate variation. The con-
stant repetition of a certain environment on generation after genera-
tion of a certain species would of course produce a constant repetition
of certain individual modifications (orthoplasy), but we do not know as
yet of any actual effect on the species of such persistent ontogenic
variations.

"The need, however, for some such factor in species-forming as de-
terminate variation is obvious and strongly felt. There are certainly
few selectionists left who honestly believe that the minute fluctuating
variations in pattern, in size, in curve of a vein, in length of a hair, etc.,
have that life-and-death value which is the sole sort of value that an
4 advantageous variation ' must have to be a serviceable handle for the
action of natural selection. As a matter of fact, no systematist will
have escaped having had it distinctly impressed on him that he recog-
nizes differences in the pattern of ladybird beetles, in the number of
fin rays in fishes, in the branching of a vein in flies' wings, that no
enemy, no agent of natural selection, can recognize, at least to the
extent of pronouncing sentence of death (or not pronouncing it) on
its basis. And further, no biologist really satisfies himself with the
worn statement: ' We must not presume to judge the value of these triv-
ial, these microscopic differences, for we do not know all the complex
interrelation and interaction of the organism and its environment.'
We do not ; but we do know for many cases that such differences are
not actually of life-and-death selective value, and reason compels us to
believe to a moral certainty that in other cases these fortuitous trivi-
alities have similar lack of life-and-death importance.

" Directly touching this point are our data of the variation of series
of honey bees collected from free-flying individuals after exposure
as adults to the rigors of outdoor life, as compared with the variation
in the series of bees, adults, but collected just when issuing from the
cells before being exposed as adults in any way to the external dangers
of living. Series of both drones and workers representing both exposed
and uncxposcd individuals were studied. The results of this examina-
tion are, that the variation among the exposed individuals is no less
than that among the unexposed individuals. This means that these
various, mostly slight, blastogenic, variations (although in such im-
portant organs as the wings), which occur among bees at the time of

152

EVOLUTION AND ANIMAL LIFE

their issuance as active, winged creatures, are not of sufficient advan-
tage or disadvantage to the individuals to lead to a weeding out by
death or saving of such varying individuals by immediate selective
action. Whatever the rigor and danger of the outdoor bee life, these
variations seem to be insufficient to cut any figure in the persistence or
nonpersistence of any individual in the face of this rigor.

"A case which really
seems to illustrate deter-
minate variation is that of
the variation of the flower
beetle, Diabrotica soror (Fig.
75). Among a thousand in-
dividuals collected on the
University campus in 1895, a
certain condition of variation
in the elytral pattern exists,
as represented graphically by
Fig. 90. In 1901 and 1902,

. . . . , other thousands collected

i : : : : : i : : : i : : : : ::::::::::::::: from the same place and
Q — ii::::::::::::::::::: examined to determine the

condition of the variation in
this pattern, show a dis-
tinctly different status, as il-
lustrated in Figs. 91 and 92.
(To be sure that a series of
a thousand individuals really
reveals the conditions of this
pattern variation, repeated
series of 1,000 individuals
each were examined and
found practically identical.)
The difference in the varia-

Classes'
"

55

•-in

64

an

203

MISCEL

FIG. 90. — Frequency polygon of variation of
elytral pattern in 905 specimens of the Cali-
fornia flower beetle, Diabrotica soror, collected
at Stanford University, 1895. (After Kellogg
and Bell.)

tion status between the 1895 lot and the 1901-2 lots consists in the
dominance in 1901-2 of one of the two modal conditions found to
exist in the species, which in 1895 was not the dominant one. There
has been a marked change in seven years, not in the pattern itself
but in the prevalence or dominance of one type of pattern. Has the
change been brought about by natural selection? Or is it the result
of a determinate variation caused by we know not what intrinsic or
extrinsic factors ?

VARIATION AND MUTATION

153

"When one straightens up after a careful microscopic examina-
tion of the pattern of Diabrotica to determine its variation, one
is sure that no other enemy of these flower beetles can be conceived
to use such discrimination as ours. Does the fly catcher swoop-
ing from its station on fence post or tree branch determine which
of two heavily flying Dia-
broticas shall be its prey on
the basis of 'two middle
spots on left elytron partial-
ly fused' in one and 'these
two spots not touching' in
the other? To our minds
the change in variation
status, the dominance of one
mode to-day which was the
subordinate mode in 1895,
is not due to the action of
selection. We do not indeed
hesitate to believe in those
'unknown factors of evolu-
tion' which may produce,
among other results, that
condition of affairs best
najned ' determinate varia-
tion.' This variation is not
necessarily to be conceived
of as purposeful or even
advantageous ; if by its
cumulation it becomes a
disadvantage of life - and -
death value, natural selec-

FIG. 91. — Frequency polygon of variation of
elytral pattern in 905 specimens of the Cali-
fornia flower beetle, Diabrotica soror, col-
lected at Stanford University, October, 1901.
(After Kellogg and Bell.)

tion, which is after all a
logical necessity and un-
doubtedly an actual actively regulative factor in species control, will
take care of it."

In the light of the foregoing discussion of the categories and
characters of variations, it is obvious that a well-grounded
knowledge of variability and variations, a knowledge based on
careful extensive statistical and experimental studies, is essen-
tial as a basis for any effective investigation of the factors and

154

EVOLUTION AND ANIMAL LIFE

processes involved in species-forming, that is, evolution. The
methods and phenomena of evolution are intimately linked
with — indeed throughout are based upon — the methods and
phenomena of variation. What causes variation is a contrib-
utory cause in evolution,
and one of the funda-
mental and all-important
causes.

Concerning the causes
of variation, at least of
those of congenital varia-
tion, we are almost wholly
in the dark. Only such
influences as can affect
the actual germ cells are
presumably potent to ef-
fect congenital variation.
Such influences are not
proved to the satisfac-
tion of many biologists
to be numerous. In the

1 1 1 1 1 1 1 1 1 1 I I 1 1 I 1 1 I 1 1 1 1 I I 1 1 1 1 1 1 ITT1 fusion of the germ cells
iT~tt | i M of £wo individuals, the

phenomenon called by
him amphimixis, Weis-
mann finds the most ef-

CUsses :
Variates 31 40

55

388 109 total 9Q5

FIG. 92. — Frequency polygon of variation of
elytral pattern in 905 specimens of the Cali-
fornia flower beetle, Diabrotica soror, col-
lected at Stanford University, October, 1902.
(After Kellogg and Bell.)

fective cause of variation.
Now the wider apart the
two parents are in struc-
tural and functional char-
acteristics, the greater is
the variation in their off-
spring likely to be. Hence hybridization, or the mating of
unlike parents, even to the degree of race and species un-
likeness, is a great resource of the breeder who would have in
his hands large variation. But if the parents are too unlike,
their mating, even if possible, proves sterile. Usually parents
must be of the same species, although experiment has shown
that considerable extraspecific hybridization is possible. Among
cultivated plants and animals the artificially selected races
differ very much, but these races are mostly easily hybridiz-

VARIATION AND MUTATION 155

able. The nature and results of fertilization and amphimixis
are treated in Chapter XIII.

But parthenogenetically produced individuals (that is,
young born from unfertilized eggs — as the honey-bee drones,
certain whole generations of various gall flies, saw flies, aphids,
etc., etc., regularly are) also vary.
In the case of male bees, male
ants, female aphids, etc., etc., the
individuals differ quite as much as
do individuals of the same species
of bisexual parentage. Comparing
the variation in drone bees (par-
thenogenetically produced) as COm- FIG. 93.— Fore ancThmd wings
pared With that Of the Workers of honeybee (drone), showing

(from fertilized eggs), we find that
this is true. The organs examined
for variation in these series of bees were the wings, organs
used by both drones and workers, and having no immedi-
ate relation either structurally or physiologically to the differ-
entiation of those two castes or kinds of individuals of the
honey-bee species. The workers are " incomplete " only in that
most of them are infertile: in no other structural or physiological
feature of their makeup are they less "complete" than the
drones. They are indeed distinctly the more specialized of

FIG. 94. — Part of costal margin of hind wing of honeybee, much magnified to show
hooks. (After Kellogg and Bell.)

the two, and according to one of the early Darwinian canons of
variation might be expected to differ more than the drones.
But the drones are males and, according to another commonly
accepted belief, this is the explanation for a larger variation on
their part, if such larger variation occurs. As a matter of fact,
it does. The drones, in all the many series studied, show mark-
edly more variation in the venation of the wings than do the
workers, while they show quite as much variation as the work-
ers in the number of the hooks which hold the two wings together
in flight. (See Figs. 93 to 96.) Both these characters, i. e.,
wing venation and wing hooks, are not so-called "male char-

156

EVOLUTION AND ANIMAL LIFE

acters": they are not to be compared with those secondary
sexual characters such as ornamental or aggressive spines,
horns, patterns, etc., which are the characteristics that give
males their special reputation for ultravariation.

FIG. 95. — Fore wings of honeybee (drone), showing variations in venation.
(After Kellogg and Bell.)

Finally, with regard to the causal influence in variation-pro-
ducing of the "primary factors of evolution," such as temper-
ature, light, humidity, pressure, and extrinsic physicochemical
conditions generally, summed up commonly in the phrase
climate and environment, we have one all-important considera-
tion to keep constantly
in mind. However po-
tent and obvious the ef-
fects of these influences
are on the individual,
we have no proof as
yet of a nature to com-
pel the general accept-
ance of biologists, that
such effects can be car-
ried directly over to the
race or species.

Only ten years after
Darwin published the

"Origin of Species," von Kolliker, the great German zoologist,
in criticising the assumptions on which species-forming by
natural selection was based in the Darwinian theory, proposed
an alternative theory of heterogenesis or species-forming by
leaps (saltations or mutations). These saltations need not of
necessity to be large, but must be changes definite and fixed.
Later, Korschinsky, a Russian botanist, outlined in some de-

FIG. 96. — Hind wings of honeybee (drone), show-
ing variations in venation. Note the interpola-
tion of the cells. (After Kellogg and Bell.)

VARIATION AND MUTATION 157

tail and with greater emphasis such a theory of species-form-
ing by mutations; and finally in 1901 Hugo de Vries, the
famous botanist of Amsterdam, published in extenso the details
of many years of observation and experiment on the subject of
mutations, and reformulated definitively a theory of species-
forming by mutational or saltational variation, the now famil-
iar mutation theory.

The following paragraphs from Morgan ("Evolution and
Adaptation," pp. 294-297, 1903) give a concise statement of
the actual details of the mutations in the evening primrose ob-
served by de Vries :

"We may now proceed to examine the evidence from which de
Vries has been led to the general conclusions given in the preceding
pages. De Vries, found at Hilversam, near Amsterdam, a locality
where a number of plants of the evening primrose, (Enothera lamarck-
iana, grow in large numbers. This plant is an American form that
has been imported into Europe. It often escapes from cultivation, as
is the case at Hilversam, where for ten years it had been growing
wild. Its rapid increase in numbers in the course of a few years may
be one of the causes that have led to the appearance of a mutation
period. The escaped plants showed fluctuating variations in nearly
all of their organs. They also had produced a number of abnormal
forms. Some of the plants came to maturity in one year, others in
two, or in rare cases in three, years.

"A year after the first finding of these plants de Vries observed
two well-characterized forms, which he at once recognized as new
elementary species. One of these was 0. brevistylis, which occurred
only as female plants. The other new species was a smooth-leafed
form with a more beautiful foliage than 0. lamarckiana. This is 0.
Icevifolia. It was found that both of these new forms bred true from
self-fertilized seeds. At first only a few specimens were found, each
form in a particular part of the field, which looks as though each might
have come from the seeds of a single plant.

"These two new forms, as well as the common 0. lamarckiana,
were collected, and from these plants there have arisen the three
groups or families of elementary species that de Vries has studied.
In his garden other new forms also arose from those that had been
brought under cultivation. The largest group, and the most impor-
tant one, is that from the original 0. lamarckiana form. The accom-
panying table shows the mutations that arose between 1887 and 1899

158

EVOLUTION AND ANIMAL LIFE

from these plants. The seeds were selected in each case from self-
fertilized plants of the lamarckiana form, so that the new plants ap-
pearing in each horizontal line are the descendants in each generation
of lamarckiana parents. It will be observed that the species, 0. ob-
longata, appeared again and again in considerable numbers, and the
same is true for several of the other forms also. Only the two species,
0. gigas and 0. scintillans, appeared very rarely.

"CENOTHERA LAMARCKIANA

Elementary Species

GENERATION.

n,io-n« \lhirla vuiufi- Rubri- Lamarck- Nan- T , Scin-
Gigas. Albida. gata nervis. iana. nella. Lata. ti])ans

8 Gener )

VIII.

1899 5 1

0 1,700 21 1

annual ;

V

7 Gener )

VII.

1898 9

0 3,000 11

annual )

V

6 Gener )

VI.

1897 11 29

3 1,800 9 5

1

annual )

V

5 Gener )

V.

1896 25 135

20 8,000 49 142

6

annual )

V

4 Gener )

IV.

1895 - 1 15 176

8 14,000 60 73

1

annual )

V

3 Gener )

III.

1890-91 -

1 10,000 3 3

biennial )

V

2 Gener )

II.

1888-89 [

15,000 5 5

biennial )

V

1 Gener )

I.

1886-87 [•

9

biennial )

"Thus de Vries had, in his seven generations, about fifty thousand
plants, and about eight hundred of these were mutations. When the
flowers of the new forms were artificially fertilized with pollen from

VARIATION AND MUTATION

159

the Howers on the same plant, or of the same kind of plant, they gave
rise to forms like themselves, thus showing that they are true elemen-
tary species. ' It is also a point of some interest to observe that all
these forms differed from each other in a large number of particulars.
"Only one form, 0. scintillans, that appeared eight times, is not
constant as are the other species. When self-fertilized, its seeds pro-

FIG. 97. — At left, section of chestnut, Costarica vesca, showing unusual variations; at
right, a branch of Mercurialis annua, which presents several variations. (After
de Vries.)

duce always three other forms, 0. scintillans, 0. oblongata, and 0.
lamarckiana. It differs in this respect from all the other elementary
species, which mutate not more than once in ten thousand individuals.
From the seeds of one of the new forms, 0. Icevi/olia, collected in
the field, plants were reared, some of which were 0. lamarckiana, and
others 0. Icevi folia. They were allowed to grow together, and their
descendants gave rise to the same forms found in the lamarckiana

1 0. lata is always female, and cannot, therefore, be self-fertilized.
When crossed with 0. lamarckiana there is produced fifteen to twenty per
cent of pure lata individuals.

160

EVOLUTION AND ANIMAL LIFE

family, described above, namely, 0. lata, elliptica, nannella, rubri-
nervis, and also two new species, 0. spatulata and leptocarpa.

"In the lata family, only
female flowers are produced, and,
therefore, in order to obtain seeds
they were fertilized with pollen
from other species. Here also ap-
peared some of the new species,
already mentioned, namely, al-
Mda, nannella, lata, oblongata, ru-
brinervis, and also two new species,
elliptica and subovata.

"De Vries also watched the
field from which the original forms
were obtained, and found there
many of the new species that ap-

Fio. 98. — Stamens of a hybrid willow,
Salix auritax purpurea, showing dif-
ferent degrees of varying.

peared under cultivation. These
were found, however, only as
weak young plants that rarely
flowered. Five of the new forms

were seen either in the Hilversam field, or else raised from seeds that
had been collected there. These facts show that the new species are
not due to cultivation, and that they arise year after year from the
seeds of the parent form, 0. lamarckiana."

FIG. 99. — A branch of a Japanese tree, Cryptomeria japonica, showing an atavistic
variation. (After de Vries.)

As to this we may observe: It has long been known that
individual variations of an extreme degree sometimes occur,

VARIATION AND MUTATION 161

and that these may be to a degree persistent in heredity. Of
such nature was the Ancon sheep, the Mauchamp sheep, the
iceberg blackberry, and numerous other races or forms known in
the domestication of animals, or the cultivation of plants. The
generally normal structure of such individuals distinguishes

P'IG. 100. — A branch of the green Georgine, in which the inflorescence leaves and some
of one branch (the right-hand one) are green like the rest of the plant, while the
other varieties are red and in normal condition. (After de Vries.)

them from monstrosities, which are usually freaks of develop-
ment rather than of heredity.

The name "saltation," or in recent years "mutation,"1 has
been applied to extreme fluctuation, the immediate cause of
which is unknown. The experiments of de Vries on the salta-
tions of the descendants of the evening primrose (called (Eno-
thera lamarckiana) have drawn general attention again to the
possibility that saltation has had a large part in the process of

1 The word mutation was first used not for saltations but for the slow
fluctuation in successive geological periods.

162 EVOLUTION AND ANIMAL LIFE

formation of species. As to this it may be said that the possible
variation within each species is much greater than the range
of the individuals which actually survive. The condition of
domestication favors the development of extreme variation, be-
cause such individuals may be preserved from interbreeding
with the mass, and they may survive even if their characters
are unfavorable to competition in the struggle for existence.
Among plants it is noticed that new soil and new conditions
seem to favor large variation in the progeny, although the traits
thus produced are not usually hereditary. Cases more or less
analogous to those noted by Dr. de Vries are not rare in horti-
culture. The cross breeding of variant forms favors the ap-
pearance of new forms. Among actual species in a state of
nature, there are very few which seem likely to have arisen by a
sudden leap or mutation. The past and the future of de Vries'
evening primroses are yet to be shown. The species called by
de Vries (Enothera lamarckiana is not at present known in its
wild state anywhere in North America, the parent region of all
the species of evening primroses or (Enothera; so that we have
as yet no reason to assume that the various mutants of the
evening primrose are really comparable to the wild species of
the same group now existing in America.

While saltation remains as one of the probable sources of
specific difference, the actual role of this process in nature is
yet to be proved.

CHAPTER X
HEREDITY

"Vom Vater hab' ich die Statur,
Des Lebens ernstes Fiihren ;
Vom Mutterchen die Frohnatur
Und Lust zu fabuliren.

Urahnherr war der Schonsten hold,

Das spukt so hin und wieder.
Urahnfrau liebte Schmuck und Gold,

Das zuckt wohl durch die Glieder.

Sind nun die Elemente nicht

An dem Complex zu trennen;
Was ist denn an dem ganzen Wicht

Original zu nennen?"

— GOETHE, * " Zahme Xenien," vi.

HEREDITY is the rule of persistence among organisms. The
existence of such a law, or "ascertained sequence of events/' is
a matter of common observation. "Like produces like,"

1 "Stature from father and the mood

Stern views of life compelling;
From mother, I take the joyous heart
And the love of story-telling.

" Great -grandsire's passion was the fair.

What if I still reveal it?
Great-grandam's, pomp and gold and show,
And in my bones I feel it.

"Of all the various elements

That make up this complexity,
What is there left when all is done,
To call originality? "

BAYARD TAYLOR'S translation in part.
163

164 EVOLUTION AND ANIMAL LIFE

"Blood will tell," "Blood is thicker than water," these proverbs
in all languages indicate the general fact that each organism is
likely to resemble its parents, and that the basis of fundamental
resemblance among organisms is found in kinship by blood. It
is equally a matter of common observation that the law of hered-
ity is inseparable from a law of variation. No one organism is
quite an exact copy of another. The prevention of such a con-
dition is one of the effects of the process of double parentage.
Except in certain exceptional forms in which parthenogenesis
or hermaphroditism appear, each complex organism springs
from two organisms of the same species : the one male, the other
female. The resultant organism partakes of the qualities of
each of these in some degree, and through these to a degree also
it partakes of qualities of the parents or ancestors of each.

The phrase, "Kinship by blood," used in connection with all
studies of heredity, is a survival of an ancient theory that the
physical basis of heredity is found in the actual blood. " Blood
is quite a peculiar juice," as was observed by Mephistopheles,
but its peculiarities are not concerned with heredity. The func-
tion of blood is concerned with the nourishment of tissues and
the removal of their waste. The actual vehicle of' transfer of
hereditary qualities, the physical basis of heredity, is found in
structures within the protoplasm of the germ cell.

The germ cells, male or female, are alike in all characters
essential to this discussion. On the average, the potency of the
male and the female cell is exactly the same, there being nowhere
constant advantage of one sex over the other. Each cell, male
or female, is one of the vital units, or body cells, set apart for
the special purpose of reproduction. It is not essentially differ-
ent from other cells in structure or in origin, but in its poten-
tialities. Its function is that of repeating the original organism,
" with the precision of a work of art."

Heredity is shown in the persistence of type, in the existence
of broad homologies among living forms, in the possibility of
natural systems of classification in any group, in the retention
of vestigial organs, in the early development and subsequent
obliteration of outworn structures once useful to individuals of
the race or type.

In a general way, the individual inherits from both parents
the common structure of organisms of the species to which it
belongs. The special peculiarities of the individual organism

HEREDITY 165

are also inherited, but in much less certainty of degree. These
traits belonging to a member of a single generation have a
smaller " inheritance fund " on which to draw. In each gener-
ation some of these individual qualities are latent or "reces-
sive/' others are potent or "dominant." The recessive or an-
cestral characters reappear with a certain regularity. They
may form a sort of mosaic, by mixing with other dominant
traits, or they may make a more or less perfect blend. Resem-
blance to some remote ancestor occurs at times, being known as
atavism. Each ancestor has some claim in the formation of the
new individual, and behind the grandfather and grandmother
dead hands from older graves call in their direction. The past
will never let go, though with each generation there is a deeper
crust over it. These old claims grow less with time, because
with each new generation there are twice as many of these com-
petitors. Moreover past generations can affect the heredity
of the individual only through the agency of his immediate
parents. Out of these elements Mr. Galtori frames the idea of a
" mid-parent/' a sort of center of gravity of heredity, though, as
Dr. Brooks has observed, it is doubtful if this mid-parent is
more than a logical abstraction. The bluer the blood in any
species, that is, the more closely alike the ancestors are, the
more certain will be the personal resemblance among the de-
scendants.

But characters actually latent are very real in heredity.
Dr. Brooks says:

" When a son of a beardless boy grows up and acquires a beard, we
may say that he has inherited his grandfather's beard, but this is only
a figure of speech, and he actually inherits the beard his father might
have acquired, had he lived, nor would the case of a child descended
from a series of ten or a hundred beardless boys be different."

It is, moreover, certainly true that a beard can be as well
inherited from the mother — who has none — as from the father.
The inheritance is that of the beard the mother might have
developed had she been a man. And, in general, in matters of
heredity, the child is not derived from the parents as they
actually are, but from the parents as they might have been.
The traits transmitted in heredity are chosen from the whole line
of parental possibilities. And with the process of conception,
12

166 EVOLUTION AND ANIMAL LIFE

the union of the two parental germ cells, "the gate of gifts is
closed." No trait or quality can ever be acquired of which at
least the elements are not involved in the original inheritance.

"What is transmitted to the infant/' observes Dr. Archdall
Reid, "is not the modification [of the parent], but only the
power of acquiring it under similar circumstances. The power
to acquire fit modifications in response to appropriate stimula-
tion is that which especially differentiates high animal organ-
isms from low animal organisms."

Atavism or reversion is the process of "throwing back," by
which in some degree an individual resembles a distant ancestor.
Under the name of "atavism," according to Yves Delage, are

Zluded three very different things :
(a) The transmission in one family of individual characters,
ich, latent for several generations, suddenly reappear. This
is family atavism, and its nature is readily recognized.

(6) The reappearance, more or less regularly in a race, of
characters of an allied race, from which the first race may have
been derived. This is race atavism. Of this nature are the
zebra stripes sometimes seen in mules.

(c) The appearance of characters abnormal for -the race in
which they appear, but which are normal in other races sup-
posed to be ancestral. This is atavism of teratology. An illus-
tration is the occasional appearance in the modern horse of rudi-
ments of additional toes, with partly developed hoofs.

"Everything is possible in heredity," observes Delage.
"One may always find examples of election, of blending (of
mosaic), of combination, of resemblance direct, and of resem-
blance reversed. To give to these groupings the name of laws
would be an abuse of language, since not one of these rules is
exclusively true. In reality there is no law of resemblance be-
tween a child and its parents. All is possible, from a difference
so great that there is not a trait in common, to an almost perfect
identity with one or the other parent, with every intermediate
degree of blending of characters and combination of resem-
blances."

The name " telegony " is given to the supposed influence of
the first male on the future offspring of the female. This theory
of telegony rests mainly on a case of a mare which was first im-
pregnated by a quagga, and whose subsequent colts from males
of her own species had quagga-like markings. The supposed

HEREDITY 167

facts on which the theory is based are inadequate or unproved,
and it is probable that the phenomena called telegony have no
real existence.

Equally uncertain are the phenomena known as "prenatal
influences.'' In the process of evolution, the development of the
female has brought her to be more and more the protector and
helper of the young. She gives to her progeny not only her
share of its heredity, but she becomes more and more a factor in
its development. In the mammalia the little egg is retained
long in the body and fed, not with food yolk, but with the
mother's blood. The parent thus becomes an immediate and
most important part of the environment of the young. In man,
by the growth of the family the parental environment becomes
a lifelong influence. The father as well as the mother becomes a
part of it.

It has long been a matter of common belief that among
mammals a special additional formative influence is exerted by
the mother in the period between conception and birth. The
patriarch Jacob is recorded as having made a thrifty use of this
influence in relation to the herds of his father-in-law, Laban.
This belief is part of the folklore of almost every race of intelli-
gent men. In the translations of Carmen Silva, that gentle
woman whom kind nature made a poet and cruel fortune a
queen, we find these words of a Roumanian peasant woman:

"My little child is lying in the grass,
His face is covered with the blades of grass.
While I did bear the child, I ever watched
The reaper work, that it might love the harvests;
And when the boy was born, the meadow said,
' This is my child.'"

In the current literature of hysterical ethics we find all sorts
of exhortations to mothers to do this and not to do that, to
cherish this and avoid that on account of its supposed effect
on the coming progeny. Long lists of cases have been reported
illustrating the law of prenatal influence. Most of these records
serve only to induce scepticism. Many of these are mere co-
incidences, some are unverifiable, others grossly impossible.
There is an evident desire to make a case rather than to tell the
truth. The whole matter is much in need of serious study, and

168 EVOLUTION AND ANIMAL LIFE

the entire record of alleged facts must be set aside to make a
fair beginning.

There are also many phenomena of transmitted qualities
that cannot be charged to heredity. Just as a sound mind de-
mands a sound body, so does a sound child demand a sound
mother. Bad nutrition before as well as after birth may neu-
tralize the most vigorous inheritance within the germ cell. A
child well conceived may yet be stunted in development. Even
the father may transmit weakness in development as a handicap
to hereditary strength. The many physical vicissitudes between
conception and birth may determine the rate of earlyegowth
or the impetus of early development. In a sense, mjf im-
pulse of life comes from such sources outside the ^B cell
and outside heredity. All powers may be affected by it. Per-
fect development demands the highest nutrition, an ideal
never reached. In such fashion the child may bear the in-
cubus of Ibsen's "Ghosts/7 for which it had no personal re-
sponsibility. " Spent passions and vanished sins " may impair
germ cells, male or female, as they injure the organs that
produce them.

In a thoughtful article on problems of heredity (The Horse-
man, April 17, 1906), Mr. C. B. Whitford maintains that better
results in the trotting horse come from breeding from untrained
horses of good blood than from horses which have been elabo-
rately trained to the highest speed on the racecourse.

"Trotting horses that are overbred show the effects of their inten-
sified breeding in a variety of ways. But the usual difficulty is ex-
treme nervousness and want of ability to stand training. Sometimes
a horse of this kind will show great promise when he is first hitched to
a sulky. He will show great flashes of speed and will have a smooth,
easy action and the trotting instinct well pronounced."

But he is overnervous, lacks constitutional strength and will
not do well.

"The trouble with a horse of this kind is that he has not inherited
the necessary fuel with which to create energy. He is 'burnt out'
by heredity. That which he needed to train on was so largely used
up by his ancestry in their process of development that they had not
enough to transmit to their progeny."

HEREDITY

169

Fio. 101. — Diagram showing arrangement of bones in the hand or foot
of various animals : 1, man ; 2, gorilla ; 3, orang ; 4, dog ; 5, sea lion ; 6,
dolphin; 7, bat; 8, mole; 9, Ornithorhynchus. (After Haeckel.)

170

EVOLUTION AND ANIMAL LIFE

If this is true, it would appear that nervous overstrain of
the parent is unfavorable to normal nerve development of the
offspring. This would be apparently a case of transmission of
parental conditions, as above indicated, and not one of true
heredity.

It may be conceived that, at the moment of impregnation,
the resultant germ cell is sexless. It begins its development at
once, and, in the higher animals, turns very soon toward the
formation of those structures which distinguish the one sex or
the other. Each individual ultimately becomes either male or
female. Relatively few animals, and those among the lower

4

7

FIG. 102. — Limb skeletons of extinct and living animals, showing the homologous
bones: 1, salamander; 2, frog; 3, turtle; 4, Aetosaurus; 5, Plesiosaurus; 6, Ichthyo-
saurus; 7, Mososaurus; 8, duck.

forms, are ever really hermaphrodite, or representative of both
sexes at once.

Among the invertebrate animals the numerical relations of
the sexes are subject to great variation. Among vertebrates,
in general, the sexes are practically equal in number, as is shown
by count of large series of individuals. This is true whether the
species be monogamous, polygamous, or promiscuous in its sex
relations. It is therefore apparent that the sex tendencies in

HEREDITY

171

the germ are held on a very fine balance. A very slight impulse
the one way or the other determines the sex direction the em-
bryo shall take. Although much investigation and very much
speculation have been devoted to this problem, it is still } un-
solved. We are not able, in the vertebrate animals, nor in fact
in animals generally, to determine the nature of the stimulus, or
of any of the various impulses, if more than one exists, which

to..

FIG. 103. — Limb skeletons of various animals, showing homologous bones: 9, Orni-
thorhynchus; 10, kangaroo; 11, Megatherium; 12, armadillo; 13, mole; 14, sea
lion; 15, gorilla; 16, man.

leads the individual germ cell to develop as male or female. _
It is also possible that each germ cell is really bisexual from the
beginning. One sex or the other becomes dominant and the
other recessive as the embryo develops. But in this event we
are still in doubt as to the nature of the determining factor or

1 The latest studies of the problem are chiefly concerned with an attempt
to determine whether or not there exists a chromosome sex determinant,
and whether sex determination may not be brought under Mendel's law
of heredity (see later paragraphs in this chapter) in a modified form.

172 EVOLUTION AND ANIMAL LIFE

stimulus. Among ants and the social bees and wasps the males
develop parthenogenetically from unfertilized cells, the fertilized
cells yielding either females or workers which are sterile females.
But this specialized mode of development is peculiar to particu-
lar groups. For a few lower species it has been ascertained
that variation in nutrition may be a factor in sex determination.
Favorable nutrition seems to increase the number of females.
Most higher plants are hermaphrodite, the central leaves (car-
pels) in the bud which becomes the flower, yielding ovules or fe-

FIG. 104. — Limb skeletons of various animals, showing homologies of the bones;
at left, mole; next, giraffe; next, bat; next, porpoise.

male germ cells. The next whorl (stamens) yields male germ
cells or pollen. The outer whorls (corolla, calyx) serve as pro-
tective organs only, and are without sex.

The. bonds of union among organisms which stand at the
basis of all classification are known as "homologies " (Figs. 101-
104). A homology is a real likeness, as distinguished from one,
merely superficial or apparent. To superficial likeness we give
the name of analogy. Homology means fundamental iden-
tity of structure, as distinguished from incidental similarity of
form or function. Thus, the arm of a man is homologous with
the foreleg of a dog, because in either we can trace deep-seated
resemblance or homologies with the other. In each detail of
each bone, muscle, vein, or nerve of the one we can trace the

HEREDITY

173

corresponding details of the other. But in comparing the arm
of man with the "limb" of a tree, the arm of a starfish, or the
foreleg of a grasshopper, we find no correspondence in details.
In a natural classification, or one founded on fact, organisms
showing the closest homologies are placed together. An arti-
ficial classification is one based on analogies. Such a classifica-
tion might place together a cricket, a frog, and a kangaroo,
because they all jump, or a bird, a bat, and a butterfly, because
they all fly, even though
the wings are very dif-
ferently made (Fig. 105)
in each case.

The very existence
of such terms as animals
and plants, insects and
mollusks imply relation-
ships, and relationships
in different degrees.
Classification is the
process of reducing our
knowledge of these
grades of likeness and
unlikeness to a system.
By bringing together
those which are funda-
mentally alike, and
separating those which
are unlike, we find that

these traits are the outcome of long-continued influences.
Classification is defined as "the rational lawful disposition of
observed facts." It rests on the results of the operations of
natural laws, or forces which bring about inevitable results.

For it is a matter of common observation that the closest
homologies are shown by those animals which have sprung from
a common stock. The fact of blood relationship shows itself
always in homology. So far as we know, homology is never
produced in any other way, therefore the actual presence of
homologies among animals or plants implies, as we shall see in a
later chapter, their common descent from stock possessing these
same characters. In our primitive use of the trunk of the tree
to imply unity in life, we can see that this trunk represents

FIG. 105. — Diagram of wings, showing homol-
ogy and analogy: a, wing of fly; 6, wing of
bird; c, wing of bat.

174

EVOLUTION AND ANIMAL LIFE

homology, and that it is the representation of the current of
heredity. The resemblances arise from common origin, the

FIG. 106. — Ears of various anthropoid apes and of man, showing human vestigial
characters: 1, hairy human ear; 2, Barbary ape; 3, chimpanzee; 4 and 5, human
ears; 6, ear of human fretus; 7, orang-outang.

variations from the demand of differing external conditions. It
may be said that the inside of an animal tells what it is, the out-
side where it has been. In the internal structure, ancestral
traits are perpetuated with little change
through geologic ages. The external
characters affected by every feature of
the surroundings may be rapidly altered
through response to demands of environ-
ment and through the destruction of in-
dividuals whose life fails of adjustment.

It is in the persistence of heredity
that we find the explanation of vestigial
organs. An organ well developed in one
group of animals or plants may in some
other be reduced to an imperfect organ
or rudiment so incomplete as to serve
no purpose whatever. Such rudimentary
or functionless structures may be found
in the body of any of the higher animals and in most or all
of the higher plants. As a rule such structures are more fully

FIG. 107.— Head of a five-
months human embryo
showing embryonic hair-
covering. (After Ecker.)

HEREDITY

175

Russian dog man, showing extraor-
dinary covering of hair on the face.
(After Wiedersheim.)

developed in the embryo than in
the adult, becoming atrophied
with age. Familiar examples
are the appendix vermiformis
and the unused muscles of the
ears in man, the atrophied lung,
pelvis, and limbs of the snake,
the air bladder of the fish, the
"thumb" (or rather index fin-
ger), of the bird, the splint bone
of the horse, and the like.

The anatomist Wiedersheim
has recorded 180 vestigial or-
gans in man. These structures

OCCUr in all the Systems of Fl«- 108. — Andrian Jeftichjew, the

organs, integument, skeleton,
muscles, nervous system, sense
organs, digestive, respiratory,

circulatory, and urino-genital systems. Most of these rem-
nants of structures are to be found completely developed in
other vertebrate groups. Eleven of them are characteristic
as functional organs of fishes only, four of amphibians and
reptiles. The fact that structures are vestigial is shown often

by cases of atavistic de-
velopment.

Within the brain of
man, near the optic lobes,
is a little spheroid structure
scarcely larger than a pea,
known as the "pineal
gland" or conarium. It
has no evident function,
and Descartes once sug-
gested that it might be the
seat of the soul. It is
larger in the embryo and
still larger in the brains of
some of the lower verte-
brates. Recent investiga-
tions have shown that it

FIG. 109. — Pineal eye of lizard, Hatteria. . . -,-, -, , j

(After spencer.) is especially developed in

176

EVOLUTION AND ANIMAL LIFE

certain lizards, notably in a very primitive New Zealand lizard
of the genus Sphenodon (Hatteria) (Fig. 109), and that, in these
lizards, the pineal body ends in a more or less perfect eye-like
structure placed between the true eyes in the center of the
forehead. A trace of this eye is shown in the limbless lizard
called slow worm (Anguis), of Europe, and in several American
species. In the horned toad (Phrynosoma) (Fig. 110) its place

FIG. 110. — Head of lizard or horned toad, Phrynosom blainvillei, showing translucent
pearly skin covering the pineal eye. (From specimen.)

is covered by a translucent pearly scale. These lizards have
in fact three eyes, and the pineal body is the nervous gang-
lion from which the third eye arises. The natural conclusion
from this that all vertebrates originally had three eyes, is prob-
ably a too-hasty one. Perhaps the pineal body was an organ
of sense, which developed into an eye in the lizards and their
ancestors only, not in any of the Amphibians or fishes, and not
in any mammals or birds, although these are descended from
reptilian stock. Whatever the origin or primitive function of
the pineal ganglion, its existence in man as a vestigial organ
is due to the persistence of heredity.

HEREDITY

177

In the living species of horse, Equus, there is but a single toe,
with its basal bones. On each side of the base bone of this toe is
a small bone known as a splint bone. The splint bones are
apparently useless to the horse, but in extinct species of horse
these bones are developed as digits, bearing small hoofs. Occa-
sionally even now colts are born in which these splint bones bear
rudimentary hoofs. In the museum of Stanford University is
the leg of a high-bred colt
from Milpitas, California, bear-
ing a small hoof on each of
the two splint bones.

The remains (Fig. Ill) of
over thirty different ancient
horse-like animals' have been
found in the rocks of the
Tertiary era. The Eohippus,
the earliest of these horselike
animals, found in the oldest
Tertiary rocks, was little larger
than a fox, and its forefeet
had four hoofed toes, with the
rudiment of a fifth, while the
hind feet had three hoofed
toes. In the later rocks is
found the Orohippus, also
small, but with the rudi-
mentary fifth toe of the fore-
foot gone. Still later appeared
the Mesohippus and Miohip-
pus, horses about the size of

sheep, with three hoofed toes only, on both forefeet and hind
feet, but with the rudiment of the fourth toe in the forefeet,
of the same size in Mesohippus, smaller in Miohippus. Also,
the middle toe and hoof of the three toes in each foot was
distinctly larger than the others in both Mesohippus and Mio-
hippus. Next came the Protohippus, a horse about the size of
a donkey, with three toes, but with the two side toes on each
foot reduced in size, and probably no longer of use in walking.
The middle toe and hoof carried all the weight. Still later in
the Tertiary era lived the Pliohippus, an "almost complete
horse. " The side toes of Pliohippus are reduced to mere rudi-

FIG. 111. — Foot changes in evolution of
the horse: a, Equus, Quaternary (re-
cent) ; 6, Pliohippus, Pliocene; c, Pro-
tohippus, Lower Pliocene; d, Miohip-
pus, Miocene; e, Mesohippus, Lower
Miocene; /, Orohippus, Eocene. (After
FIG. 254 of "Animal Studies.")

178 EVOLUTION AND ANIMAL LIFE

ments or splints. This animal differs from the present horse
somewhat in skull, shape of hoof, length of teeth, and other
minor details. Lastly came the present horse, Equus, with the
splint bones or concealed rudiments of the side toes very small
and the hoof of the middle toe rounder. In spite of the great
difference between the one-toed foot of the living horse and
the dog's five-toed foot there was once a kind of horse which
had a five-toed foot, and there is after all a close relationship
between the foot of the horse and the foot of the dog.

FIG. 112. — Homology of digits of four odd-toed mammals, showing gradual reduction
in number and consolidation of bones above. (After Romanes.)

In man there is developed at the proximal end of the caecum
or blind sac of the large intestine a small structure as shown in
Fig. 113. This appendage has no function, and it is subject to
inflammation or suppuration, known as appendicitis. In the
embryo the appendix vermiformis is notably larger than in
the adult man; and in the lower animals, as in the dog or the
kangaroo (see Fig. 113), it may be recognizable as a prolon-
gation of the caecum, scarcely less in diameter than the intestine
itself. The appendix vermiformis is therefore a vestige of a
long caecum which had its part in the process of digestion.

In the embryo of all chordate animals, without exception,
respiratory or gill slits are developed, homologous with those
seen in the embryo of the fish. The presence of these slits or
their vestiges is one of the most important secondary distinctive
characters of the great group of Chordata, which includes the
vertebrates. The human embryo is, in this regard, at certain

HEREDITY 179

stages essentially similar to the embryo of the fish. But in the
course of development the gill slits in man and the higher ver-
tebrates disappear. Their position is, however, indicated by
the course of certain blood vessels. These follow the lines
blocked out in the embryo when they led to the gill slits, al-
though no other trace of these slits persists in the adult, and this
direction is not one which we could conceive as likely to have
arisen except for the results of inheritance from the lower ver-
tebrates.

In the veins of the higher animals valves are present, so
arranged as to prevent the flow of blood backward and espe-
cially downward from the heart. In the lower animals, these
valves are adjusted to the position on all fours. Their adjust-
ment is the same in man, notwithstanding his erect posture.
Apparently the adjustment of the valves was completed before
the position on all fours gave way to the erect posture.

FIG. 113. — At left, appendix vermiformis of kangaroo; at right appendix vermiformis
of human embryo. (After Wiedersheim.)

In the embryo of man there exists a regular tail, supported
by eight distinct bones, like the tail of any other mammal. In
the process of development, these bones are reduced in number
and are joined, forming the coccyx or rudimentary tail.

In various species of fishes, lizards, salamanders, crayfishes,
and other animals living in caves or buried in the ground, the
eyes are atrophied. Numerous cases (Fig. 114) of this sort have
been studied by Dr. Carl H. Eigenmann. He finds in general
that the young cave fish have normally developed eyes, but that

180

EVOLUTION AND ANIMAL LIFE

with growth atrophy sets in affecting different species differ-
ently, in some cases the muscles, in others the lenses, but in all
cases reducing the size of the organ to a functionless structure
more or less covered by the skin. In all cases, the ancestry of
these blind species can be traced to forms with well-developed
eyes inhabiting the same region. Among the species examined
are the blind fish of Mammoth Cave (Amblyopsis spelceus), the
cave blind fish of Kentucky and Indiana (Typlichthys subterra-

neus] , descended from
the Dismal Swamp fish
(Chologaster cornutus) ,
the Missouri blind fish
(Troglichthys rosce), the
blind fishes of the caves
of Cuba (Lucifuga sub-
terranea, and Stygicola
dentata) and the blind
goby of Point Loma
(Typhlogobius calif orni-
ensis) .

In Dr. Eigenmann's
opinion, the retention
of eyes in these species
is due to the influence
of heredity, the vesti-
gial structures being
each and all necessary

to life in the light. Their degeneration he ascribes to the
inheritance of the individual effects of disease, a matter we
discuss in another chapter.

Hundreds of cases of vestigial organs in plants have been
recorded, among which we may mention the barren stamen in
Pentstemon which completes the number of five usual in the
group of Scrophulariacese to which Pentstemon belongs. Other
illustrations are the rudimentary leaves, with rudimentary
stomata, found on the joints of species of cactus (Opuntia), etc.;
the cilia found on the spermatozoa of cycads, which would en-
able these structures to move freely in the water, although they
are not deposited in the water, and these cilia are never actually
used.

By the theory of special creation it was supposed that these

=—

FIG. 114, — Fishes showing stages in loss of eyes
and color: A, Dismal Swamp fish, Chologaster
cornutus, ancestor of the blind fish; B, Agassiz's
cave fish, Chologaster agassizi; C, cave blind fish,
Typhlichthys subterraneus.

HEREDITY 181

rudiments were created in accordance with the tendency
in creative processes to adhere to an ideal type. But it can-
not be too clearly understood that tendencies in biology exist
only as functions of particular organs. The tendency to
adhere to a type is a part of heredity, the function of the germ
cell.

In the light of our knowledge of organic evolution it is
clear that the presence of vestigial organs is simply a fact of
heredity. They are organs once useful, but which through
changed conditions of life have become needless.

It is a recognized fact that useless organs tend to dwindle
away, but the cause of this phenomenon is not so clear. It may
be due in part to (a) panmixia or cessation of selection, the
organ being no longer held to a high grade of efficiency, to (b)
reversal of selection, the advantage lying with those individuals
in which the organ is no longer functional or (c) the inheritance
of the results of functional disuse. The latter offers an explana-
tion which at first sight appears adequate, and its reality has
been stoutly maintained by various writers of the Neo-Lamarck-
ian school. In their views, changes in the individuals unques-
tionably due to individual or ontogenetic disuse are carried over
to the species as phylogenetic disuse. Against this view is
opposed its inconsistence with current theories of heredity, and
also the positive fact that there is as yet no proof of the in-
heritance of acquired characters.

When we say that, through heredity, the offspring inherits
the characters of the parent, we are speaking only a large and
general truth. The details of this inheritance reveal in what
regards this general statement must be modified. We have
already noted the inevitable occurrence of at least small varia-
tions in all body parts in all individuals. In addition to this ex-
ception to identical inheritance, certain characters of the parent
may not, as just mentioned, appear at all in the offspring. And
this may be due to any one of several causes.

First, certain parental characters are apparently really not
heritable, namely, those new characters which have been ac-
quired by the parent during its lifetime as the result of mutila-
tion, disease, special use or disuse of parts, any change of parts
due to direct reaction to a functional stimulus or to an environ-
mental stimulus or cause, such as a bleaching due to lack
of light, a thickening of the skin in certain places due to con-
13

182 EVOLUTION AND ANIMAL LIFE

tact, etc. At least, there is not recorded any satisfactory
proof of the inheritance of these acquired characters, and there
is definite proof that many of them are not inherited. And
most biologists, as helpful in many ways to a clearing up of
the problem of adaptation and species-forming as the actu-
ality of such inheritance would be, believe themselves un-
able to accept this fact, in the light of our present knowl-
edge. (This matter of the inheritance of acquired characters
is discussed in Chapter XI. The assumption of this inheritance
is a fundamental part of the Lamarckian explanation of evo-
lution.)

Second, certain characters, peculiar to sex are inherited only
according to sex and not by all the young. These characters
include not only the differing reproductive organs themselves,
but those many, various, and often most remarkably developed
so-called secondary sexual characters, such as the tufts and
plumes and brilliant plumage of male birds, the antlers of male
deer, the specialized antennae, skeletal processes, and color pat-
terns of many male insects, and the reduced wings of many female
insects, etc., etc. Even in cases of parthenogenetic reproduc-
tion (L e., reproduction in which the male takes no part), sex and
the sex characters of the offspring have no direct relation to the
sex and sex characters of the mother. The queen honey bee
produces, in fact, exclusively drones (male bees) when she lays
unfertilized eggs, while on the contrary the parthenogenetic
offspring of the Aphids (plant lice) are all females for several
generations, and then in a single generation both males and
females.

Finally, certain parental characters, even though blastogenic,
may not appear in the offspring, but be inherited by them in
latent condition, to appear in their young or perhaps even in a
later generation. It is obvious, too, that where a certain char-
acter in the mother is represented in the father by one of oppo-
site condition, as where the mother is very short, the father very
tall, the mother a brunette, the father light-haired, a given child
can inherit the character in only one condition. That is, in all
cases of biparental reproduction, and they compose the majority
of cases in both animal and plant kingdoms, the inherited char-
acters cannot be all those possessed by both parents, but must
be either those of one or the other, or a mosaic of them, or a
blend or fusion of them. And this introduces us to that phase

HEREDITY

183

of the study of the results of heredity which to-day is being most
investigated, the determination of the "laws" of inheritance
of characteristics.

The similarity or dissimilarity of the two mating parents is a
matter of much importance in regard to the results of inherit-
ance. To produce a fertile mating the two parents have at
least to be nearly allied. We are accustomed to take this for

FIG. 115. — Romulus, the striped colt of a horse mother and zebra father.
(After Ewart.)

granted, but the actual degree of phyletic relationship necessary
in fertile mating is a point of much biologic interest. In most
cases both parents must belong to the same species or kind, but
among animals and plants there have been noted exceptions
to this rule, these exceptions constituting the facts of hybridi-
zation.

Hybridism is practically limited to mating of different
species of the same genera. Only in a few recorded cases have
organisms of different genera mated in nature with the produc-
tion of offspring. In zoological gardens and menageries the

184 EVOLUTION AND ANIMAL LIFE

race feeling of the confined animals seems to break down, and
unusual cases of hybridism are occasionally noted. Also men-
tion must be made of the artificial induction of the fertilization
of sea-urchin eggs by the sperm cells of starfishes (animals not
only of different genera but of different families), and a few other
similar exceptional cases accomplished by Loeb and other ex-
perimenters. In many examples of hybridism the immediate
offspring are unable to produce young and so no continuous
series of generations results. In other fewer cases the off-
spring of hybridization are fertile, and thus constitute the
beginnings of a new race or variety of animal or plant. Many
of our domesticated animal races and cultivated plant varie-
ties have originated by hybridism often artificially induced
by man.

For the most part, however, both parents of any brood of
young belong to the same species, and hence they are at least as
like each other as the other members of the same species
have to be. But this may still permit great superficial dissimi-
larity: many attributes, such as size, color, texture, outline,
etc., of the body parts, especially the external ones, may be
quite different. For within any species there may be several
subspecies or varieties, the individuals of all of which are
capable of fertile mating with each other. And even where
there is no distinctly recognizable subspecific distinctions there
may yet be much superficial dissimilarity among the individ-
uals composing a single species. So in all studies of the results
of heredity, of the actual inheritance of parental characters, the
degree of likeness or unlikeness of the parents must be taken
into account.

So that the " laws " of heredity, as formulated on a study not
of its mechanism but of its results, refer to the character of the
parental union, whether pure or crossed, and if crossed whether
the parents are of different varieties of one species or of actually
different species. As a matter of fact the crossing of parents
with a few to many dissimilar characters has been the actual
means of getting at some of the most important evidence as to
the behavior of heredity that we have. For the very dissimilar-
ity of the parental attributes makes it possible to trace in the
progeny of succeeding generations the workings or results of
heredity with reference to these particular characters.

Galton's Law of Ancestral Inheritance may be stated in few

HEREDITY 185

words, although for an understanding of the character of the
evidence on which it is based, and for an appreciation of its whole
significance some full account of it, preferably Galton's own
statement and discussion of it in his memoir entitled "The
Average Contribution of Each Several Ancestor to the Total
Heritage of the Offspring/' published in 1897, should be read.
From a study of the carefully kept pedigree book of the kennels
of the Basset Hounds Club, with records extending through
twenty-two years, and a study of inheritance in the British
Peerage made possible by the complete genealogic records
kept for these families, together with a consideration of va-
rious other less detailed but at least helpful records of inher-
itance, Galton formulated the statement that any organism of
bisexual parentage derives one half its inherited qualities from
its parents (one fourth from each parent), one fourth from its
grandparents, one eighth from its great-grandparents, and so
on. These successive fractions, whose numerators are one and
whose denominators are the successive powers of two, added
together equal one or the total inheritance of the organism: thus

The English mathematician and natural philosopher, Karl
Pearson, has made computations showing that Galton's law
thus simply expressed is only a close approximation to the
actual inheritance relations, and that the fraction indicating the
contribution of any given ancestor must be slightly modified
by introducing into it another factor. In general, though,
the Galtonian formula received a very general acceptance
among biologists. And only recently, in the light of the discov-
ery of Mendel's investigations and conclusions and their confir-
mation in essential principle by the recent researches of various
botanists and zoologists, has Galton's law been looked on as
altogether too simple and incomplete a formulation of the facts
of inheritance. It is not yet quite certain whether Galton's
formula is consonant with the Mendelian formula or not. But
at best Galton's law only expresses a part of what may now with
confidence be said to be known of the regular course of inher-
itance.

Before taking up the actual Mendelian results and conclu-
sions, however, it is important for us to note the different modes
or kinds of behavior of inheritance which characteristics may
show in their transmission. Cuenot has made a rough but

186 EVOLUTION AND ANIMAL LIFE

suggestive classification of these inheritance categories as
follows :

In cross matings — and by " cross mating/' students of hered-
ity do not necessarily mean mating between distinct species or
even varieties, but mating between parents which disagree in
the condition of one or more specifically referred to characteris-
tics— in cross mating between the parents A and B, if we con-
sider a single pair of corresponding characters a and . b which
differ in the two parents, the young produced by the crossing
may (1) all present the same parental character a without any
trace of the character 6, the character a being then termed
dominant or prepotent or prevalent, the other recessive or
latent ; or, (2) the young may all agree in presenting a new char-
acter differing from the parental characters a and 6, this new
character apparently being a simple physical mixture or a real
chemical combination or blending of a and 6; or (3), the young
may differ from one another in regard to the parental characters
a and 6, some showing the character a, some showing the char-
acter b ; or (4) the young may differ among themselves in regard
to the characters a and 6, some showing the character a, some
the character 6, and some various characters intermediate be-
tween a and b; or (5) the young may show the characters a and
6 side by side in each individual in small separated parts, even
in neighboring but distinct cells. These differences undoubt-
edly depend partly on the nature of the characteristics them-
selves, partly on the kind of organism, and partly on extrinsic
influences. It is obvious also that for certain characteristics by
no means all five of these ways are open. Many characters are
so wholly antagonistic that no blend nor any mosaic of them can
occur in a single individual, leaving only ways (1) and (3), viz.,
exclusive or alternative inheritance open to them.

To these five general categories of the actual transmission
of certain obvious parental characters may here be added for
consideration those cases of the appearance in the young of a
character or characters having no obvious relation to either a
or 6, but sometimes explicable as reversions or reappearances
of characters possessed by ancestors more or less remote and
other times as obviously wholly new and heretofore never ex-
istent characters which, if pronounced, are called "sports" or
sudden or discontinuous variations. Also must be taken into
account the possible appearance among the young, of a few to

HEREDITY 187

many individuals showing many simultaneous, usually slight
but real differences from the parents in various parts and func-
tions. These are the differences called mutations by de Vries
and his followers, and are the basis of the at present consid-
erably accepted theory of species-forming by heterogenesis or
sudden complete fixed modifications of organic types. In the
light of the observations and experiments of de Vries, these mu-
tations are of special importance in any consideration of hered-
ity and variation. (See p. 157, Chapter IX, for a brief account
of these mutations.)

The Mendelian "laws" apply only, probably, to certain par-
ticular categories of inheritance, or rather categories of char-
acters. That is, so far as worked out, the Mendelian principles
seem to have definite application only to cases of inheritance in
which the characteristics under observation are mutually ex-
clusive or alternative in character; categories (1) and (3) in
our list in a preceding paragraph are the only ones under the
rule of the Mendelian principles, and there are even some ex-
ceptions in these categories. The various other kinds of inher-
itance, called blended or combined (where the two characteristics
fuse or blend to form a new condition), and mosaic or par-
ticulate (where both parental characteristics exist side by side
in each individual among the young), apparently require for
their explanation something besides the Mendelian principle.

At some time between 1855 and 1865 Gregor Johann Mendel,
an Augustinian monk in the small Austrian village of Brimn,
carried on in the gardens of his cloister pedigree cultures of peas
and some other plants from which he derived data which he read,
together with his interpretation of their significance, before
meetings of the Natural History Society of Briinn, and which
in the same year of their reading (1865) were published under the
title "Experiments in Plant-hybridization," in the Abhand-
lungen (vol. iv), of the society. Mendel was the son of a peas-
ant, and had been educated in Augustinian foundations and
ordained a priest. For two or three years he studied physics
and natural science in Vienna, and refers to himself in one of his
papers as a student of Kollar. He became abbot of his cloister,
and was for a time president of the Briinn Natural History
Society. Such are the essential details of the education and
work of the man whose name will undoubtedly live forever in
the annals of biological science.

188 EVOLUTION AND ANIMAL LIFE

Mendel's principal data were derived from the crossing of
varieties of peas (Pisum sativum) in which he found several pairs
of well-marked contrasting characters. Bateson gives a clear
and concise summary account of Mendel's methods and results
which we quote in the following paragraphs. For the purposes
of his experiments Mendel selected seven pairs of characters as
follows :

1. Shape of ripe seed, whether round; or angular and
wrinkled.

2. Color of "endosperm" (cotyledons), whether some shade
of yellow; or a more or less intense green.

3. Color of the seed skin, whether various shades of gray and
gray-brown; or white.

4. Shape of seed pod, whether simply inflated; or deeply
constricted between the seeds.

5. Color of unripe pod, whether a shade of green; or a bright
yellow.

6. Nature of inflorescence, whether the flowers are arranged
along the axis of the plant; or are terminal and form a kind of
umbel.

7. Length of stem, whether about six or seven feet long,
or about three fourths to one and one half feet.

"Large numbers of crosses were made between peas differing in
respect of one of each of these pairs of characters. It was found that
in each case the offspring of the cross exhibited the character of one
of the parents in almost undiminished intensity, and intermediates
which could not be at once referred to one or other of the parental
forms were not found.

" In the case of each pair of characters there is thus one which in
the first cross prevails to the exclusion of the other. This prevailing
character Mendel calls the dominant character, the other being the
recessive character.1

"That the existence of such ' dominant' and 'recessive' charac-
ters is a frequent phenomenon in cross breeding, is well known to all
who have attended to these subjects.

" By letting the cross-breds fertilize themselves Mendel next raised
another generation. In this generation were individuals which showed

1 "Note that by these novel terms the complications involved by the
use of the expression /prepotent' are avoided."

HEREDITY 189

the dominant character, but also individuals which presented the
recessive character. Such a fact also was known in a good many
instances. But Mendel discovered that in this generation the numer-
ical proportion of dominants to recessives is on an average of cases
approximately constant, being in fact as three to one. With very con-
siderable regularity these numbers were approached in the case of
each of his pairs of characters.

"There are thus in the first generation raised from the cross-
breds seventy-five per cent dominants and twenty-five per cent re-
cessives.

" These plants were again self-fertilized, and the offspring of each
plant separately sown. It next appeared that the offspring of the
recessive remained pure recessive, and in subsequent generations never
produced the dominant again.

" But when the seeds obtained by self-fertilizing the dominants were
examined and sown it was found that the dominants were not all alike,
but consisted of two classes: (1) those which gave rise to pure dom-
inants, and (2) others which gave a mixed offspring, composed partly
of recessives, partly of dominants. Here also it was found that the
average numerical proportions were constant, those with pure domi-
nant offspring being to those with mixed offspring as one to two.
Here it is seen that the seventy-five-per-cent dominants are not really
of similar constitution, but consist of twenty-five which are pure
dominants and fifty which are really cross-breds, though, like the
cross-breds raised by crossing the two original varieties, they only
exhibit the dominant character.

" To resume, then, it was found that by self-fertilizing the original
cross-breds the same proportion was always approached, namely:
25 dominants, 50 cross-breds, 25 recessives,
or ID : 2DR : 1R.

"Like the pure recessives, the pure dominants are thenceforth
pure, and only give rise to dominants in all succeeding generations
studied.

"On the contrary the fifty cross-breds, as stated above, have
mixed offspring. But these offspring, again, in their numerical pro-
portions, follow the same law, namely, that there are three dominants
to one recessive. The recessives are pure like those of the last genera-
tion, but the dominants can, by further self-fertilization, and exam-
ination or cultivation of the seeds produced, be again shown to be
made up of pure dominants and cross-breds in the same proportion
of one dominant to two cross-breds.

190 EVOLUTION AND ANIMAL LIFE

"The process of breaking up into the parent forms is thus con-
tinued in each successive generation, the same numerical law being
followed so far as has yet been observed.

"Mendel made further experiments with Pisum sativum, crossing
pairs of varieties which differed from each other in two characters,
and the results, though necessarily much more complex, showed that
the law exhibited in the simpler case of pairs differing in respect of one
character operated here also.

"In the case of the union of varieties AB and ab differing in two
distinct pairs of characters, A and a, B and b, of which A and B are
dominant, a and b recessive, Mendel found that in the first cross-bred
generation there was only one class of offspring, really AaBb.

"But by reason of the dominance of one character of each pair
these first crosses were hardly if at all distinguishable from AB.

"By letting the AaBb's fertilize themselves, only four classes of
offspring seemed to be produced, namely:
" AB showing both dominant characters.
"Ab showing dominant A and recessive b.
" aB showing recessive a and dominant B.
" ab showing both recessive characters a and b.
"The numerical ratio in which these classes appeared was also
regular and approached the ratio

QAB : 3Ab : 3aB : lab.

"But on cultivating these plants and allowing them to fertilize
themselves, it was found that the members of the
Ratios

1 ab class produce only ab's.

„ ( 1 aB class may produce either all aB's,

(2 or both aB's and ab's.
o j 1 Ab class may produce either all Ab's
(2 or both Ab's and ab's.

1 AB class may produce either all AB's

2 or both AB's and Ab's,
2 or both AB's and aB's,

4 or all four possible classes again,

namely, AB's, Ab's, aB's, and ab's,

and the average number of members of each class will approach the
ratio 1 : 3 : 3 : 9 as indicated above.

"The details of these experiments and of others like them made
with three pairs of differentiating characters are all set out in Mendel's
memoir."

HEREDITY 191

Perhaps the most striking thing about Mendel's work is the
singularly suggestive and luminous interpretation which he gave
of just why the pea characteristics were transmitted exactly
as they were; why, in general, the peculiar numerical ratio be-
tween dominant and recessive should be, and why it should
persist so uniformly. This interpretation or explanation is now
well known in biology as the theory of the " purity of the germ
cells," or, as Cuenot has called it, the theory of "gametes dis-
joints," or "la disjonction des characteres dans les gametes des
hybrides" (the separation of characters in the germ cell of
hybrids), the Spaltungsgesetz of de Vries.

This interpretation is simply that in the young of the first
generation after a cross-mating, although because of dominance
but one of the contrasting pair of parental characters will show
itself in the body make-up, yet when these young form their
germ cells the two parental characteristics will be represented,
but only one in any one germ cell; that is, in the case of
Mendel's peas that the pollen cells and ovule cells produced by
the cross-bred young would carry each one of the alternative or
mutually exclusive parental varietal characters. If this were
the case and if, on an average, the pollen cells and ovule cells
were evenly divided as to the two characteristics, then by
miscellaneous or random mating (mating according to the law
of probabilities) between these cells we should get in the de-
veloped young just such conditions with regard to the con-
trasting characteristics as Mendel actually did get in his peas.
For twenty-five per cent of the pollen grains representing the
dominant character would unite with twenty-five per cent of the
ovule cells representing the dominant character, twenty-five per
cent of the recessive pollen grains with twenty-five per cent
of the recessive ovule cells, and the remaining fifty per cent of
each kind with each other; that is, of every four pollen grains and
every four egg cells we should get by random pollination 1 pollen
dominant X 1 ovule dominant; 1 pollen recessive X 1 ovule re-
cessive; 1 pollen dominant X 1 ovule recessive; 1 pollen reces-
sive X 1 ovule dominant. This condition would bring it about
that the fully developed young would show the contrasting
characteristics (remembering the dominance of one of the char-
acteristics in those cases in which dominant and recessive are
united), in this condition: 3D, 1R. Which is exactly what
occurred in Mendel's peas, and has since been noted to occur

192 EVOLUTION AND ANIMAL LIFE

in many other cases recorded by post-Mendelian observers and
experimenters. These records are of both plants and animals,
and are fast multiplying.

Thus the so-called Mendelian laws of heredity refer to two
phases of the problem of inheritance — viz.: (1) how inherited
characters are actually distributed, and (2) the fundamental
cause, lying in the germ plasm, for this particular kind of dis-
tribution. Like Galton's formula, Mendel's law expresses the
regularity of heredity based on actual recorded statistics of
inheritance ; but it also gives a satisfying fundamental reason for
this regularity. Biologists, with few exceptions, see in the
establishment of the Mendelian principles of heredity in biologic
science the greatest advance toward a rational explanation of
inheritance that has been made since the beginning of the
scientific study of the problem.

The extraordinary fact that Mendel's work lay practically
unnoted for thirty-five years (actually the only reference to it in
'scientific "literature " in all that time seems to have been one by
Focke in 1881 in Die Pflanzenmischlinge, p. 109), has been
partly explained by Bateson as due to the driving interest felt
through all that time by biologists generally in other phases of
investigation; but it remains a curious commentary on the
•possibilities of the temporary obscurity that may be in store for
even the best scientific work. The "discovery" of Mendel's
work seems to have been made in 1900 by three investigators
almost simultaneously, who also discovered independently the
same important facts of the transmission behavior in inheritance
of exclusive or alternative characteristics. These men are de
Vries, Tschermak, and Correns, and their published papers not
only verify Mendel's particular work on the peas, but confirm
his principles or laws on the basis of much added experimenta-
tion and observation on other plants. In the last five years
zoologists, notably von Gnaita working with mice, Cuenot,
Darbishire, Davenport, Bateson and Castle with mice, rabbits,
guinea-pigs and chickens, McCracken with certain beetles, and
Toyama, Mrs. Bell and Kellogg with silkworms, have shown
that Mendelian principles obtain in animal as well as in plant
inheritance. For the results of all of these investigations in
large measure confirm our confidence in the Mendelian prin-
ciples of dominance and recessivity and of the purity of germ
cells. But also in nearly all of these studies the investigators

HEREDITY 193

have found some inconsistencies and have caught glimpses of
other principles which, when finally grasped, will undoubtedly
considerably limit the application of Mendel's laws, but will,
almost certainly, not detract from their importance, nor lessen
in any degree the high place in science that belongs to the
patient, persistent, clear-minded Augustinian monk of the
cloister gardens of Brunn.

One of the modifications of the Mendelian behavior of
hybrids which has been shown to exist in certain cases, is that
the young of the cross-mated parents may not all exhibit in
the same degree the dominant characteristic, although in the
subsequent generations the regular Mendelian three-to-one
splitting up into dominant and recessive appearance may
occur. The young of the first generation may include a very
few individuals showing the recessive character, as de Vries found
in mating two varieties of Papaver somniferum (ninety-seven
per cent showed the dominant character, three per cent the re-
cessive). Or the first generation may show a sort of pseudo-
blend condition, approaching but not duplicating exactly the
dominant characteristic, as occurs when Hyoscyamus pallidus
is crossed with H. niger (de Vries, "Die Mutationstheorie,"
Bd. II., p. 162.)

When silkworm moths of the race Shanghai, with white
cocoons, are crossed with moths of the race Yellow Var. with
rose-yellow cocoons, the hybrid offspring make straw-yellow
cocoons of a tint just between the two parent tints. The color-
ing matter of the grapevine Aramon has the chemical formula
C4eH36O20j and the coloring matter of the race Teinturier has
the formula C44H4oO2o" the hybrid- offspring called Petit-
Bouschet, of a crossing of these two, has coloring matter of the
formula C45H38O2o, exactly intermediate. Mendel himself got
as the result of a crossing between two pea races, one one foot
in height and the other six feet, hybrids measuring from six
to seven and one half feet high. These are specific cases of
blended inheritance and there are many others known.

Also when the plant Mirabilis jalapa 9 , with red flowers,
is crossed with a male variety with white flowers the hybrid
offspring exhibit red flowers (maternal type), white flowers
(paternal type), and flowers streaked with the two colors. So
when corn with blue kernels is crossed with corn with white
kernels, a hybrid is obtained exhibiting on a single ear blue

194

EVOLUTION AND ANIMAL LIFE

kernels, white kernels, kernels of intermediate bluish-white tint
and kernels streaked with blue and white. The streaked flowers
and kernels of these two cases are due to mosaic inheritance.

Or the apparent
dominance of the
contrasting charac-
teristics may be
proved to have
something of real
dominance about it,
as Miss McCracken
has so clearly shown
in her studies of
the inheritance of
dichromatism in the
little beetles Lina
lapponica (Fig. 117)
and Gastroidca dis-
similis. Here the

FIG. 116. — At left an ear of field corn; at right an ear of fij-cf two Ol* three

sweet corn; and in the middle a hybrid of these two, .

showing alternation of kernels resembling those of generations behave

each different parent. (After de Vries.) in true Mendcliail

manner, but with

successive generations the dominant character is plainly seen to
be gradually extinguishing the recessive character in the cross-
bred groups, so that in the seventh generation after the original
cross-mating the Mende-
lian ratio of 2 to 1 in
the cross-bred group is
changed to 28 to 1.

There may occur also
a breaking up or decom-
position of the parental
varietal characters, which
may mean that the domi-

nant and recessive Char-
acterS are not Simple

ones but are complex—

i. e., really the resultant of several combined characteristics; or
it may mean that there exists a real instability in the parent
type and that the stimulus or influence of the cross-mating is all

FlG. u7.—Lina lapponica, showing its two forms,
°ne black and one spotted. (After McCracken.)

HEREDITY 195

that is needed to break down this weak apparent stability of the
type and allow its component characters (the elementary units
of de Vries) to recombine into various new and differing types.
This condition seems to be that which results in the extraordi-
nary variation so commonly observed by plant and animal
breeders as brought out by hybridization, and which is con-
stantly made use of by these breeders. Luther Burbank de-
pends very largely on this initial abundant and eccentric varia-
tion induced by wide hybridizations for " starters " for his work
of producing "new creations."

So in accepting Mendel's laws of heredity we must bear
clearly in mind that they by no means apply to all, or, at any
rate, that our present knowledge of them does not include their
application to all, cases and categories of inheritance.

CHAPTER XI
INHERITANCE OF ACQUIRED CHARACTERS

Tout ce qui a ete acquis, trace ou change dans 1 'organisation des
individus, pendant le cours de leur vie, est conserve par la generation
et transmis aux nouveaux individus qui proviennent de ceux qui out
eprouve ces changements. — LAMARCK.

Heaven forfend me from Lamarck nonsense of a tendency to pro-
gression, adaptation from the slow willing of animals, etc. ; but the
conclusions I am led to are not wholly different from his, though the
means of change are wholly so. — DARWIN to HOOKER, 1848.

THE "fourth Law of Evolution," as expressed by Lamarck
in his "Zoological Philosophy," reads as follows: "All that has
been acquired, begun, or changed in the structure of individuals
in their lifetime, is preserved in reproduction and transmitted to
the new individuals which spring from those which have in-
herited the change."

This principle was used by Lamarck as one of the funda-
mental elements in his theory of the transmutation of species.
For nearly a hundred years it attracted little attention, being
accepted as a part of the law of heredity by most persons, even
by those most opposed to the essential part of Lamarck's theory,
the derivation or transmutation of species. Among others,
Darwin accepted it as one of the factors in evolution of forms.
With Herbert Spencer it became one of the fundamental prin-
ciples of the philosophy of Evolution. Mr. Spencer states the
proposition in this way: "Change of function produces change
of structure: it is a tenable hypothesis that changes of structure
so produced are inherited." For the supposed inheritance of
characters produced by the impact of environment or by
resultant activities of the individual the term progressive
heredity has been devised. The fact of the existence of pro-

196

INHERITANCE OF ACQUIRED CHARACTERS 197

gressive heredity, more or less taken for granted by writers of
the last century, was flatly denied by Dr. August Weismann,
who insisted that it was necessary that the theory of the
inheritance of characters acquired in the lifetime of the indi-
vidual should no longer be accepted without definite proof.

In the theory of heredity through the development of the
germ cell controlled by influences exerted by structures within
the nucleus, Weismann found no room for the inheritance of
characters not preestablished within this germ. External in-
fluences in general cannot reach the germ cells, and throughout
nature the germ cells are elaborately protected from the direct
influence of external conditions.

This attack upon an ancient theory roused its supporters
to defend their faith and to search for evidence to support it.
A temporary division of naturalists into two schools arose as a
result of this discussion. Those who held with Lamarck and
Spencer that characters gained in the life time of the individual,
and not received from ancestors possessing them, became hered-
itary, were known as Neo-Lamarckians. Those who, with
YVcismann, denied the existence of this factor and from a neces-
sity, real or fancied, laid special stress on the Darwinian principle
of natural selection, assumed the title of Neo-Darwinians. In
their hands the Darwinian principle became the all-powerful
factor in evolution, a theory of Allmacht which was soon
questioned from other quarters and by those not considered as
Neo-Lamarckians. Prominent among the leaders of the Neo-
Larnarckians were Herbert Spencer, Haeckel, Nageli, Cope,
Eimer, Hyatt, Gadow, Dall, Packard, and others. Among
the recognized Neo-Darwinians were Weismann, Wallace, Hux-
ley, Gray, Brooks, Lankester, and others.

After some years of controversy, mostly theoretical, the dis-
cussion has been tacitly dropped by biologists generally. It is
recognized that the sole crucial test is that of experiment, that
experiment is not easy, inasmuch as it is very difficult to show
that any given trait in heredity really belongs to the category
of acquired characters, and that in no case has it been indu-
bitably shown that any character not inborn has been inherited.
Moreover the studies of the germ cell and the physical basis of
heredity tend to show that the structures of the germ cell are
more complex and that the processes of heredity are in a sense
more mechanical than could have been supposed in the time of
14

198 EVOLUTION AND ANIMAL LIFE

Lamarck or even that of Darwin or Spencer. The characters
shown by any adult individual are all in a sense acquired char-
acters, their development dependent largely on nutrition and on
the influences of environment. The facts of heredity show that
it is not the actual traits of the parents, but rather their poten-
tialities which are inherited. Moreover, acquired characters
are simply matters of degree of development. They represent
in no case anything qualitatively new. Taking the modern
theories of heredity, it is perhaps not conceivable that "all that
is acquired, begun, or changed" in the physical or mental life
of the individual should produce a corresponding change in the
germ cells, or in the cells from wyhich these are thrown off.

On the other hand, Dr. Weismann has admitted the possi-
bility that one-celled animals and animals of simple structure
in which the germ cell shares in the general relation of the body
cells to the environment may be effected by developmental con-
ditions. In other words, the inheritance of acquired characters
may be a reality in the development of Protozoa, the simpler
Metazoa, and the lower types of plants, but this condition does
not obtain among the higher forms.

In much of the discussion on this subject the term "ac-
quired characters " is used with an uncertain or double meaning.
The term should be limited to traits of the individual which
were not inborn or blastogenic, and would not be exhibited in
the natural or usual development of the individual. In general,
such traits would arise either from the operation of use or disuse
of parts, or other functional stimulation derived from the en-
vironment.

An illustration of an acquired character resulting from use
and disuse would be the increased size of the arm in the black-
smith, or the decreased leg muscles of the tailor. The training
of a musician or of a mathematician would give increased power
along the lines of the training. The neglect of the musical or of
mathematical ability would lead to the relative mediocrity of
this form of ability. Education in a general way increases mental
capacity: neglect of education allows it to become relatively
less. The supposed inheritance of results of civilization forms
an important part of the philosophy of Herbert Spencer. Is
civilization the inheritance of the power gained by past suc-
cesses, or is it simply the acquisition of the machinery which
past successes have produced? As to this Herbert Spencer

INHERITANCE OF ACQUIRED CHARACTERS 199

remarks: "Considering the width and depth of the effects
which the acceptance of one or the other of these hypotheses
must have on our views of life, the question, Which of them is
true? demands beyond all other questions whatever the atten-
tion of scientific men."

Other illustrations of the supposed effect of use and disuse
are thus discussed by Dr. Edwin Grant Conklin:

"In the first place, this whole line of argument starts with the
assumption that the individual habits of an animal are inherited, and
that these habits ultimately determine the structure, an assumption
which really begs the whole question ; for, after all, the substratum of
any habit must be some physical structure, and if modified habits are
inherited it must be because some modified structure is inherited. I
take an example which will serve as an illustration of a whole class:
Jackson says that the elongated siphon of Mya, the long-necked clam,
is due to its habit of burrowing in the mud, or to quote his words:
' It seems very evident that the long siphon of this genus was brought
about by the effort to reach the surface, induced by the habit of deep
burial.' It certainly would be pertinent to inquire where it got this
habit, and how it happened to be transmitted. It is surely as diffi-
cult to explain the acquisition and inheritance of habits, the basis of
which we do not know, as it is to explain the acquisition and inheri-
tance of structures which are tangible and visible. Such a method
of procedure, in addition to begging the whole question, commits the
further sin of reasoning from the relatively unknown to the relatively
known.

"This case is but a fair sample of a whole class, among which may
be mentioned the following: The derivation of the long hind legs of
jumping animals, the long forelegs of climbing animals, and the elon-
gation of all the legs of running animals through the influence of an
inherited habit. All such cases are open to the very serious objection
mentioned above.

" Another whole class of arguments may be reduced to this propo-
sition: Because necessary mechanical conditions are never violated
by organisms, therefore modifications due to such conditions show
t lie inheritance of acquired characters. Plainly, the alternative propo-
sition is this: if acquired characters are not inherited, organisms ought
to do impossible things.

"Many of the arguments advanced to prove the inheritance of
characters acquired through use or disuse seem to me to prove entirely

200 EVOLUTION AND ANIMAL LIFE

too much. For example, Professor Cope argues very ably that bones
are lengthened by both stretch and impact, and that modifications
thus produced are inherited. Even granting that this is true, how
would it be possible for this process of lengthening to cease, since in
active animals the stretch and impact must be continual? Professor
Cope answers that the growth ceases when 'equilibrium' is reached.
I confess that I cannot understand this explanation, since the assumed
stimulus to growth must be continual. But, granting again that
growth may stop when an animal's legs become long enough to 'sat-
isfy its needs/ how on this principle are we to account for the shorten-
ing of legs, as, for example, in the turnspit dog and the ancon sheep
and numberless cases occurring in nature? If any one species was
able, by taking thought of mechanical stresses and strains, to add one
cubit unto its stature, how could the same stresses and strains be
invoked to decrease its stature?

"These evidences are, I know, not the strongest ones which can be
adduced in support of the Lamarckian factors. There are at present
a relatively small number of such arguments which seem to be valid
and the great force of which I fully admit. But the cases which I have
cited are, I believe, fair samples of the majority of the evidences so
far presented, and in the face of such 'evidence' it is not surprising
that one who is himself a profound student of the subject and a con-
vinced Lamarckian prays that the Lamarckian theory may be deliv-
ered from its friends." *

As to the inheritance of the effects of extrinsic forces on
the individual, we find little in the way of direct evidence.

In all the members of the large family of flounders and soles,
the adult fish rests flat on the bottom arid swims on its side, the
cranium being twisted so that both eyes appear on the upper
side. As a rule color cells are developed on the upper side only,
the lower cells remaining largely uncolored or white. In the
young of all species the head is symmetrical, both eyes being
normally situated, and the fish swims vertically in the water.
Little by little, as development goes on, the fish turns over to
one side, and the eye of the lower side passes around or through
the forehead to join its fellow on the upper side. On the upper
side pigment cells develop, while on the lower side they remain

1R. F. Osborn, "Evolution and Heredity," Wood's Holl Biological
Lectures, 1890.

INHERITANCE OF ACQUIRED CHARACTERS 201

imperfect. However, in a flounder reared under conditions in
which the light falls on the lower side, pigment cells are de-
veloped also on that side.

It has been claimed by certain writers, as Cunningham, that
the twisting of the head in the flounder is due to the inheritance
of an acquired character. A flat fish without air bladder, rest-
ing on the sea bottom, naturally falls on one side. The eye
thrust into the sand is naturally twisted around to the upper
side, and this tendency begun in very young individuals becomes
hereditary, while the lack of pigment on the under side is also
transmitted by inheritance. But it is just as easy to claim
that the first trait of adaptation is due to natural selection, and
that the whiteness of the blind side is ontogenetic, due to the
absence of light in the growth of the individual Jrr^fiy case, no
specific theory of the origin of the twist of the flounder's head
can be regarded as proved.

It is well known, as Dr. Conklin observes, that certain water
snails "if reared in small vessels are smaller than when grown
in large ones," and this case has been cited as showing the
influence of environment in modifying species. There is good
evidence, however, that this modification does not affect the
germinal protoplasm, for these same gasteropods will grow
larger if placed in larger vessels. It seems very probable that
the diminished size of these animals is due to deficient food
supply, but this has so little modified the somatic protoplasm
that, although they may be fully developed as shown by sexual
maturity, they at once increase in size as soon as more abundant
food is provided, and this takes place by the active growth and
division of all the cells of the body. In higher animals, once
maturity has been reached, there is little chance for growth,
apparently because many of the cells are so highly differen-
tiated that they can no longer divide; consequently the growth
is limited, and hence the size of the adult may depend in part
upon the amount of nutriment furnished to the embryo. This
limitation of growth is due to the high degree of differentiation
of the somatic cells. But as the germ cells are not highly dif-
ferentiated and are capable of division, it follows that they
would not be permanently modified by starving. It may be,
as Professor Brewer argues, that long-continued starving and
consequent dwarfing of animals may leave its mark on the
germinal plasm; but, as he also remarks, this influence must be

202 EVOLUTION AND ANIMAL LIFE

very slight as compared with the cumulative effects of selection
in breeding, and it is safe to assert that there is no such whole-
sale and immediate modification of the germinal plasm due to
nutrition as some people seem to suppose."

As a matter of fact experiment has shown that the results of
dwarfing due to lack of food are shown for three generations in
silkworms (these subsequent broods, of larvae being full fed
but producing dwarfed moths). But with succeeding genera-
tions the moths became larger and resumed their normal ap-
pearance.

Mutilations of any sort are not inherited. The tails of
sheep have been cut off for countless generations. Yet each
lamb is born with a tail. This law holds good for docked tails,
docked ears, pierced ears, and the many mutilations to which
domestic animals and men have been subject since the begin-
ning of civilization.

Influences of climate, of heat, of cold are not inherited so
far as experiment shows, nor has it been made clear that any
extrinsic influence exerted on the individual really modifies
the forces of heredity. Even Lamarck admits this. He ob-
serves: "Circumstances change the forms of animals. But I
must not be taken literally, for environment can effect no direct
changes whatever upon the organization of animals."

In Spencer's view, the phenomena of instinct are to be ex-
plained as the inheritance of habits of the individual. The
Neo-Darwmians see in the adaptations of instinct only the re-
sults of natural selection acting upon the endless variations to
which individual instincts are subject. In most cases the latter
view seems the most probable. In some cases it hardly offers
a plausible explanation.

The young mocking bird shows an inborn dread of owls and
cats, while it is relatively indifferent to the presence of dogs or
chickens. It seems hardly reasonable to suppose that all
mocking birds without this instinct of dread for these particular
animals have been destroyed, while the others have survived.
Still more deep seated is the dread of snakes possessed by all the
monkey species known to us, as well as by their human allies.
Most men and most monkeys have a different feeling in regard
to snakes from that exhibited toward any other sort of animals.
This feeling is inborn. It may be suppressed, but not often
wholly conquered. To call it an inherited experience is easy,

INHERITANCE OF ACQUIRED CHARACTERS 203

but in default of other evidence for the inheritance of experi-
ences, the explanation is not satisfactory. But it is not easy
to believe that in early times those without this instinct fell
victims to venomous snakes through their own fearlessness. It
is perhaps not necessary to take sides on this question. Any
view we may adopt rests for confirmation mainly on the im-
probability of what we conceive to be the opposite alternative.
Conklin further observes that the

" so-called facts [of progressive heredity] are merely probabilities of a
higher or lower order, and to one man they seem more important than
to another. No conviction based even upon a high degree of proba-
bility can ever be reached in this way. There is here a deadlock of
opinion, each challenging the other to produce indubitable proof.
This can never be furnished by observation alone. Possibly even
experiment may fail in it, but at least it is the only hope."

We shall not assist science, says Osborn,

"with any evolution factor grounded upon logic rather than upon
inductive demonstration. A retrograde chapter in the history of sci-
ence would open if we do so and should accept as established laws
those which rest so largely upon negative reasoning."

Meanwhile we may regard the theory of the inheritance of
acquired characters as a piece of useful scaffolding which has
served its purpose in the development of the facts of the deriva-
tion of species. At present most of it — perhaps all of it — must
be taken down, but it may be that from the same base will arise
a better constructed theory which will again serve a purpose
in the study of organic evolution.

Similar conditions in life tend to develop or encourage
analogous adaptations in groups of animals not homologous in
structure nor closely related by lines of descent. In many cases
these adaptations are so very similar and are so subtle in their
parallelism as to deceive even the trained naturalist. In other
cases, the convergence and its consequent analogies are less
perfect, and the separate influence of like selection under like
environment is easily traceable.

Examples of this sort are seen in the density of the fur of all
Arctic animals, whatever the group to which they belong. An-
other illustration is found in the white winter dress of weasels,

204 EVOLUTION AND ANIMAL LIFE

rabbits, owls, ptarmigans, and other birds and mammals, this
color aiding alike in defense or attack as against the back-
ground of snow. Similar convergence of characters is seen in
the gray hues of almost all desert animals, in the thorny stems
and scant thick foliage of almost all desert plants. In swift
streams, fishes of various types (sculpins, darters, gobies, cat-
fish, and minnows) protect themselves from the current by the
reduction of the air bladder, by the instinct to lie flat on the
bottom, and the instinct to make short quick darts from place to
place in the swift waters. To this end also, certain fins are in
each case especially increased in size and force.

Convergence of characters is shown in the black colors, soft
bodies, and luminous spots, characteristic of different groups of
deep-sea fishes. It also appears in the development of eellike
forms in groups of fishes which have no affinity with eels, and
of snakelike forms among .lizards and salamanders, which have
no real affinity with snakes. Like conditions of life bring about
like structures. We may instance the occurrence of blind fishes
of various groups in the different cave areas, these species being
derived in all cases from fishes of neighboring regions having
well-developed eyes. Thus the blind cave fish of Missouri (Trog-
lichthys rosce), and those of Indiana and Kentucky (Amblyopsis
spelceus, Typhlichthys subterraneus) , are separately derived from
the once widespread type of the Dismal Swamp fish (Cholo-
gaster cornutus) . The blind fishes of Cuba (Stygicola, Lucijuga)
are derived from ancestors of a marine cusk (Brotula) now found
in the Cuban seas. The blind catfish of Pennsylvania (Gronias
nigrilabris) is modified from an existing species (Ameiurus
pullus) found in the same region. The blind salamanders of
Austria and Texas are derived from former inhabitants of the
same regions which possessed well-developed eyes.

Parallelisms of this sort are found in every group of animals
and plants. It is generally easy to distinguish analogous
variations or results of convergence of characters, by the study
of comparative anatomy. Resemblances induced by like selec-
tion or by like conditions are usually superficial and do not
affect those structures which do not come into direct contact
with external conditions. But sometimes even deep-seated
characters have been reached and affected by environmental
influences. In this case a finer test is found in the study of
embryonic development. In general, creatures actually closely

INHERITANCE OF ACQUIRED CHARACTERS 205

related in descent have inherited common methods of develop-
ment. Thus to embryology we have looked for the final test
as to the real affinities of any given form. But even this test is
sometimes delusive, for selection and environmental influences
may affect embryonic development, as they may affect every
other character or instinct.

Certain writers carry this thought further, and find no real
basis for discrimination between homologies and analogies.
They would hold that the progressive inheritance of effects of
similar environment might in time produce forms not imme-
diately related into a condition of practical identity each with
the other.

Professor Hans Gadow observes :

" When Gegenbaur had become the founder of modern comparative
anatomy by putting it on the basis of evolution, it became gradually
an axiom that homologies determined the degrees of affinity, and now
in turn the position of an animal in the system is appealed to for de-
termining whether a given organ is homologous or only analogous.
It is a vicious circle. 'Only analogous' is the usual expression. In
reality these cases of analogy, hornoplasy, convergence have become
of supreme interest in our science. Their solution implies the greatest
of problems, and it is only the thoughtless orthodox fanatic who be-
lieves that similarity in structure must mean relationship. To him
two and two make four, no matter what the twos are composed of."

But most naturalists believe that homology, which involves
common descent, and analogy, which rests on similar experi-
ences, are quite distinct elements, and that they can always
be distinguished by recognized biological tests.

No one can question the vast influence of extrinsic or en-
vironmental influences on the history of a species of animal or
plant. In the analysis of such influences we find a wide variety
of opinions. According to some writers, these forces are dy-
namic, shaping the development of the individual, and by
heredity determining through the individual the future of the
species. Dall uses these striking words:

"The environment stands in relation to the individual as the ham-
mer and anvil to the. blacksmith's hot iron. The organism suffers
during its entire existence a continuous series of mechanical impacts,
none the less real because invisible."

206 EVOLUTION AND ANIMAL LIFE

Others, not questioning the reality of the direct effects of
environmental forces on the individual, find no evidence that
these impacts are perpetuated in heredity.

Besides direct effects of these outside influences, we have to
consider an infinite variety of reactions, which these forces or
impacts may set up in the organism. These again have been
supposed to be hereditary, for the species changes under them
in what seems to be very much the same fashion that the indi-
vidual does. Use and disuse in the species bring about parallel
results to those shown by use and disuse in the individual, and
are by some therefore referred to the same cause.

But again there is grave reason to question the fact of the
inheritance of such reactions, and to doubt whether the effects
of use and disuse in the species rest on the same set of causes as
the results of use and disuse in the individual.

There remains the supposition, adopted at least tentatively
by a large proportion, probably the majority, of the naturalists
of to-day, that the direct effects of environment, as well as the
reactions or indirect effects on the individual, are not repeated
in heredity, and that the selective influence of environmental
causes is the measure of their influence in the transformation of
species.

The question of the nature of dynamic forces in evolution is
one of the most recent and most interesting phases of the long-
continued discussion of the inheritance of acquired characters.

A vast range of variations are ontogenetic, or dependent on
influences affecting directly the life of the individual. These
ontogenetic variations are, strictly speaking, individual, ap-
pearing as collective only when many individuals have been
subjected to the same conditions. They may be divided into
environmental variations and functional variations, two cate-
gories which cannot always be clearly separated, as variations
due to food conditions partake of the nature of both.

More than thirty years ago, Dr. J. A. Allen demonstrated
that climatic influences affect the averages in measurements and
in color among birds. For example, in several species of birds,
the total length is greater in specimens from the north, while
the bills and toes are actually longer in southern specimens.
That this condition is due to the influence of climate on develop-
ment is apparently shown by the fact that numerous species are
affected in the same way. It is noticed also that specimens

INHERITANCE OF ACQUIRED CHARACTERS

207

B

from the northeast and the northwest of the United States are
darker in color than those from the interior, and again that red
shades are more common in the arid southwest. Similar effects
have been recently shown by a study of species of wasps. Modi-
fications of this type may be produced at will by subjecting
the larvse and pupae of certain in-
sects to artificial heat and cold.
The butterflies of the glacial regions
and those developed in the ice chest
have a pale coloration, and a warm
environment deepens the pigment.
The woodpeckers and other birds of
the rainy forests, northwest and
northeast, have always darker and
more sooty plumage than those birds
of the same type found in more
sunny regions.

A typical case is found in the
various species of sticklebacks (Fig.
119) constituting the genera Gaster-
osteus and Pygosteus of the Northern
Temperate Zone. In both genera,
the marine species are armed for the
whole length of the body by a series
of about twenty to thirty vertically
oblong enameled bony plates.

In brackish waters in Europe,
America, and Asia alike, the stickle-
backs in all the various species are
only partially mailed, having vari-
ously from three to fifteen bony
plates, these smaller than in the
marine forms and covering only the

anterior part of the body. In these fishes also, the spines of
the fins are less developed than in the marine forms. In
strictly fresh waters, sticklebacks of various types are found
entirely destitute of bony plates. These unarmed fishes have
been regarded as distinct species and as distinct subspecies.
At present they are usually simply regarded as variant " forms,"
to which distinctive scientific names need not be applied. It
has not been proved, but it is probably a fact, that the differ-

FIG. 118. — Specimen of Cera-
tium, collected (.4) out of the
Guinea Coast stream and (B)
out of the South Equatorial
stream; note the marked dif-
ference in development of the
spines. (After Weismann and
Chun.)

208

EVOLUTION AND ANIMAL LIFE

FIG. 119. — Specimens of the stickleback, Gasterostus cataphractus, collected in different
kinds of water, and showing marked variations in the number of lateral bony
plates and in the size of the dorsal fin rays; at the top, specimens collected in the
salt water (note many lateral plates and large dorsal fin rays); next figure below,
specimens collected in brackish water; next below, specimens collected in a river
mouth; at the bottom, specimens collected in a river, with no lateral plates and
small dorsal fin rays.

INHERITANCE OF ACQUIRED CHARACTERS 209

ence is one due to the environment of the individual. Those
in the sea find adequate salts from which to develop their
coats of mail. Those in fresh water do not find this, while
those in river mouths and other brackish situations develop
armature in intermediate degrees. In the genus Eucalia, a
stickleback confined to fresh waters of the Middle Western
States, plates are never developed.

The Loch Leven trout, Salmo levenensis, is distinguished
from the brook trout of England, Salmo eriox (fario), in its
native waters by certain obvious characters. These disappear
when the eggs are planted in brooks in England or in California,
and the species develops as the common English brook trout.
But it is conceivable that the obvious or ontogenetic traits of
the Loch Leven trout are not the real or phylogenetic distinc-
tions, and that the latter, more subtle, engendered through in-
dividual variation, inheritance, selection, and isolation, really
exist, although they have escaped the attention of ichthy-
ologists.

After the Loch Leven trout was planted in the Yosemite
Park in 1896, it remained for nine years unnoticed. In 1905
individuals sent to Stanford University were, so far as could
be seen, exactly like English brook trout. But it is conceivable
that differences in food and water have caused slight ontogenetic
distinctions. It is certain that in isolation from all parent
stocks they will in time develop larger differences which, after
many thousand generations, will be specific or subspecific. At
present, these trout are quite unlike the native rainbow trout
(Salmo irideus gilberti) of the Yosemite. The ontogenetic char-
acters will perhaps approach those of the latter, but the phylo-
genetic movement may be in quite, another direction.

Another ontogenetic species is the little char or trout (Sal-
velinus hides Cope) from Unalaska. In Captain's Harbor, Una-
hi.ska, the Dolly Varden trout, Salvelinus malma, swarms in
myriads, in fresh and salt water alike, reaching in the sea a
weight of from six to twelve pounds. A little open brook, which
drops into the harbor by an impassable waterfall, contains also
an abundance of Dolly Varden trout, mature at six inches and
weighing but a few ounces. This is Salvelinus tudes. In the
harbor the trout are gray with lighter gray spots, and fins scarcely
rosy. In the brook, the trout are steel blue, with crimson spots
and orange fins, striped with white and black. In all visible

210 EVOLUTION AND ANIMAL LIFE

phylogenetic characters, the two forms of trout are one species.
We have reason to believe that fry from the bay would grow up
as dwarfs in the brook, and that the fry from the brook would be
gray giants if developed in the sea.

But it is also supposable that in the complete isolation of the
brook fishes, with free interbreeding, there would be some sort
of phylogenetic bond. There may be a genuine subspecies,
tudes, characterized not by small size, slender form, and bright
colors, but by other traits, which no one has found because no
one has looked deeply enough.

In no group of vertebrates are the life characters more plastic
than among the trout. The birds have traits far more definitely
fixed. Yet differences in external conditions must produce cer-
tain results. We should not venture to suggest that the dusky
woodpeckers or chickadees of the rainy forests of the northeast
and northwest are purely ontogenetic species or that they should
be erased from the systematic lists. But it will be a great ad-
vance in ornithology when we know what they really are arid
when we understand the real nature of the small-bodied, large-
billed, southern races of other species of birds. .It would be
worth while to know if these are really ontogenetic purely, or
if they are phylogenetic through "progressive heredity/7 the
inheritance of acquired characters, such as are produced by
the direct effects of climate or as the reaction from climatic
influences. Or again may there be a real phylogenetic bond
through geographical segregation, its evidences obscured by
the more conspicuous traits induced by like experiences? Or
are there other influences still more subtle involved in the
formation of isohumic or isothermic subspecies?

To sum up, there is no convincing evidence that the direct
influence of environment is a factor in the separation of species,
except as its results may be acted upon by natural selection.
We have no proof to show that the environment of one genera-
tion determines the heredity of the next — and yet perhaps
most naturalists feel that the effects of extrinsic influences work
their way into the species, although a mechanism by which
this might be accomplished is as yet unknown to us.

CHAPTER XII
GENERATION, SEX AND ONTOGENY

"Unter jedem Grab liegt eine Weltgeschichte " (German proverb).

EACH animal, each plant, must have its individual beginning,
its "creation," and its individual development from this begin-
ning to full grown, completely developed condition. For no
organism is born fully developed. Even the simplest organ-
isms, the one-celled kinds, whose "creation" is accomplished
simply by the splitting in two of a previously existing individual
of their kind, are not produced full-fledged. They have at
least to increase from half size to full size, that is, to grow, and
there are very few if any of them that do not have to effect
changes in their body structure during this period of growth;
that is, they have to undergo some development. The begin-
ning, then, is always from a previously existing organism — but
how could it always have been? — and between this beginning
and the normal mature or full-fledged creature there has
always to be some development. The beginning is called
generation', the development, ontogeny.

We are all so familiar with the fact that a kitten comes into
the world only through being born as the offspring of parents
of its kind, that we shall likely not appreciate at first the full
significance of the statement that all life comes from life; that
all organisms are produced by other organisms. Nor shall we
at first appreciate the importance of the statement. This is a
generalization of modern times. It has always been easy to see
that cats and horses and chickens and the other animals we
familiarly know give birth to young or new animals of their own
kind ; or, put conversely, that young or new cats and horses and
chickens come into existence only as the offspring of parents
of their kind. And in these latter days of microscopes and

211

212 EVOLUTION AND ANIMAL LIFE

mechanical aids to observation it is also easy to see that the
smaller animals, the microscopic organisms, come into ex-
istence only as they are produced by the division of other similar
animals, which we may call their parents. But in the days of
the earlier naturalists the life of the microscopic organisms,
and even that of many of the larger but unfamiliar animals,
was shrouded in mystery. And what seem to us ridiculous
beliefs were held regarding the origin of new individuals.

The ancients believed that many animals were spontane-
ously generated. The early naturalists thought that flies
arose by spontaneous generation from the decaying matter of
dead animals; from a dead horse come myriads of maggots
which change into flesh flies. Frogs and many insects were
thought to be- generated spontaneously from mud. Eels were
thought to arise from the slime rubbed from the skin of fishes.
Aristotle, the Greek philosopher, who was the greatest of the
ancient naturalists, expresses these beliefs in his books. It
was not until the middle of the seventeenth century — Aristotle
lived three hundred and fifty years before the Christian era —
that these beliefs were attacked and began to be given up. In
the beginning of the seventeenth century, William Harvey, an
English naturalist, declared that every animal comes from an
egg, but he said that the egg might "proceed from parents or
arise spontaneously or out of putrefaction." In the middle of
the same century Redi proved that the maggots in decaying
meat which produce the flesh flies develop from eggs IrJd on the
meat by flies of the same kind. Other zoologists of this time
were active in investigating the origin of new individuals. And
all their discoveries tended to weaken the belief in the theory
of spontaneous generation.

Finally, the adherents of this theory were forced to restrict
their belief in spontaneous generation to the case of a few kinds
of animals, like parasites and the animalcules of stagnant water.
It was maintained that parasites arose spontaneously from the
matter of the living animal in which they lay. Many parasites
have so complicated and extraordinary a life history that it was
only after long and careful study that the truth regarding their
origin was discovered. Bi*t in the case of every parasite whose
life history is known, the young are offspring of parents, of
other individuals of their kind. No case of spontaneous genera-
tion among parasites is known.

(1KNERATION, SEX AND ONTOGENY 213

The same is true of the animalcules of stagnant water. If
some water in which there are apparently no living organisms,
however minute, be allowed to stand for a few days, it will come
to be swarming with microscopic plants and animals. Any
organic liquid, as a broth or a vegetable infusion exposed for
a short time, becomes foul through the presence of innumerable
bacteria, infusoria, and other one-celled animals and plants,
or rather through the changes produced by their life processes.
But it has been certainly proved that these organisms are not
spontaneously produced by the water or organic liquid. A few
of them enter the water from the air, in which there are always
greater or less numbers of spores of microscopic organisms.
These spores (embryo organisms in the resting stage) germinate
quickly when they fall into water or some organic liquid, and
the rapid succession of generations soon gives rise to the hosts
of bacteria and Protozoa which infest all standing water. If
all the active organisms and inactive spores in a glass of water
are killed by boiling the water, "sterilizing" it, as it is called,
and this sterilized water or organic liquid be put into a sterilized
glass, and this glass be so well closed that germs or spores can-
not pass from the air without into the sterilized liquid, no living
animals will ever appear in it. It is now known that flesh will
not decay or liquids ferment except through the presence of
living animals or plants. To sum up, we may say that we know
of no instance of the spontaneous generation of organisms, and
that all the animals whose life history we know are produced
from other animals of the same kind. "Omne vivum ex vivo,"
"All life from life."

The method of simple fission or splitting — binary fission it
is often called, because the division is always in two — by which
the body of the parent becomes divided into two equal parts
— into halves — is the simplest method of multiplication. This
is the usual method of Amwba (Fig. 120) and of many other of
the simplest animals. In this kind of reproduction it is hardly
exact to speak of parent and children. The children, the new
Amoebce, are simply the parent cut into halves. The parent
persists; it does not produce offspring and die. Its whole body
continues to live. The new Amoebae take in and assimilate food
and add new matter to the original matter of the parent body;
then each of them divides in two. The grandparent's body is
now divided into four parts, one fourth of it forming one half
15

214

EVOLUTION AND ANIMAL LIFE

of each of the bodies of the four grandchildren. The process
of assimilation, growth, and subsequent division takes place
again and again and again. Each time there is given to the
new Amoeba an ever-lessening part of the actual body substance
of the original ancestor. Thus an Amoeba never dies a natural

FIG. 120. — A multiplication of Amoeba by simple fission.

death, or, as has been said, "no Amoeba ever lost an ancestor
by death." It may be killed outright, but in that case it leaves
no descendants. If it is not killed before it produces new
Amcebce it never dies, although it ceases to exist as a single
individual. The Amoebce and other simple animals which
multiply by direct binary fission may be said to be immortal,
and the " immortality of the Protozoa " is a phrase which will

GENERATION, SEX AND ONTOGENY

215

often 1)e mot with in the writings of Weismann and certain other
modern philosophical biologists. There is a fallacy, however,
in the phrasing, because,
as a matter of fact, the
protoplasm of a given
protozoon gradually loses
its vitality with con-
tinued division until it
ultimately is unable to
divide further or indeed
to perform the other life
functions: it dies of old
age.

Hardly less simple is
generation by budding,
which in its simplest
character is the breaking
off from one individual
of a part smaller than a
half, often, indeed, only
a vory small fractional part, which
capacity of growing and developing

FIG. 121. — Stentor reproducing by fission.
(After Stein.)

budded off part has the
into a new individual like
its parent.

A still other
mode of generation
of simple type is
that of sporulation,
or where the body
of one individual
subdivides into
more than two
parts (as in binary
fission), these parts,
each of which is
usually subspherical

I «;. \'22.~IIolophrya multifiliis, an infusorian parasitic
on fishes reproducing by sporulation.

Or ellipsoidal,
bering perhaps

many hundreds.

A condition known as parthenogenesis is found among
rrrtain of the complex animals. Although the species is repre-
sented by individuals of both sexes; the female can produce

216

EVOLUTION AND ANIMAL LIFE

young from eggs which have not been fertilized. For example,
the queen bee lays both fertilized and unfertilized eggs. From
the fertilized eggs hatch the workers, which are rudimentary
females, and other queens, which are fully developed females;
from the unfertilized eggs hatch only males — the drones.
Many generations of plant lice are produced each year parthe-
nogenetically — that is, by unfertilized females. This subject
, , will be discussed at

greater length later
in this chapter.

The modes of
generation, or re-
production, or mul-
tiplication, as this
making the begin-
nings of new indi-
viduals may be
variously called, so
far referred to, may
be grouped into a
category called asex-
ual generation.
In an examination
of the lives of the
simplest and but
slightly complex
kinds of animals we
find that even
among almost the
very simplest of or-
ganisms another
mode of reproduction obtains, at least occasionally, which
demands for its carrying out the mutual action of two distinct
individuals. The essential thing in this mutual action is tke
exchange of nuclear material from one of these individuals to
the other ; with some of the simplest organisms there is a mutual
exchange of nuclear material.

Paramcecium, for example, reproduces itself for many gen-
erations by fission, but a generation finally appears in which
a different method of reproduction is followed. Two individu-
als come together and each exchanges with the other a part of

FIG. 123. — Gregarinidse. A , A gregarinid, Actinocephalus
oligacanthus, from the intestines of an insect (after
Stein); B and C, spore-forming by a gregarinid, Coc-
cidium oviforme, from liver of a guinea-pig (after Leuc-
kart); D, E, and F, successive stages in conjugation of
spore-forming by Gregarina polymorpha. (After Kol-
liker.)

GENERATION, SEX AND ONTOGENY 217

its nucleus. Then the two individuals separate and each
divides into two. The result of this conjugation is to give to
the new Paramcecia produced by the conjugating individuals
a body which contains part of the body substance of two distinct
individuals. The new Paramoccia are not simply halves of a
single parent, they are parts of two parents.

Among the colonial Protozoa the first differentiation of
the cells or members composing the colony is the differentia-
tion into two kinds of reproductive cells. Reproduction by
cimple division, without preceding conjugation, can and does
take place, to a certain extent, among all the colonial Protozoa.
Indeed, this simple method of multiplication, or some modi-
fication of it, like budding, persists among many of the com-
plex animals, as the sponges, the polyps, and even higher and
more complex forms. But such a method of single-parent
reproduction cannot be used alone by a species for many gener-
ations, and those animals which possess the power of multiplica-
tion in this way always exhibit also the other more complex
kind of multiplication, the method of double-parent reproduc-
tion. Conjugation takes pla.ce between different members of
a single colony of one of the colonial Protozoa, or between
members of different colonies of the same species. These
conjugating individuals in the simpler kinds of colonies, like
Gonium, are similar; in Pandorina they appear to be slightly
different, and in Eudorina and Volvox the conjugating cells are
readily seen to be very different from each other. One kind of
cell, which is called the egg cell, is large, spherical, and inactive,
while the other kind, the sperm cell, is small, with ovoid head
and tapering tail, and free-swimming. In the simpler colo-
nial Protozoa all the cells of the body take part in reproduction,
but in Volvox only certain cells perform this function, and the
other cells of the body die. Or we may say that the body of
Volvox dies after it has produced special reproductive cells
Which shall fulfill the function of multiplication.

Beginning with the more complex Volvocince, which we
may call either the most complex of the one-celled animals or
the simplest of the many-celled animals, all the complex
animals show this distinct differentiation between the repro-
ductive cells and the cells of the rest of the body. Of course,
we find, as soon as we go up at all far in the scale of the animal
world, that there is a great deal of differentiation among the

218

EVOLUTION AND ANIMAL LIFE

cells of the body: the cells which have to do with the assimila-
tion of food are of one kind ; those on which depend the motions
of the body are of another kind; those which take oxygen and
those which excrete waste matter are of other kinds. But
the first of this cell differentiation, as we have already often
repeated, is that shown by the reproductive cells; and with
the very first of this differentiation between reproductive cells
and the other body cells, appears a differentiation of the re-

FIG. 124. — Spermatozoa of different animals: 1, Man; 2, Vesperugo; 3, pig; 4, rat;
5, finch; 6, triton; 7, ray; 8, beetle; 9, mole cricket; 10, snail. (After Ballowitz,
Kolliker, and Rath.)

productive cells into two kinds. These two kinds, among all
animals, are always essentially similar to the two kinds shown
by Volvox and the simplest of the many-celled animals—
namely, large, inactive, spherical egg cells, and small, active,
elongate or " tailed " sperm cells.

In the slightly complex animals one individual produces
both egg cells and sperm cells. But in the Siphonophora, or
colonial jelly fishes, certain members of the colony produce only
sperm cells, and certain other members of the colony produce
only egg cells. If the Siphonophora be considered an indi-
vidual organism and not a colony composed of many individ-

GENERATION, SEX AND ONTOGENY

219

uals, then, of course, it is like the others of the slightly com-
plex animals in this respect. But as soon as we rise higher in

A

FIG. 125. — Fertilization of Petromyzon fluviatilis: A, Sperm nucleus in periphery of the
egg plasm; B, sperm nucleus in periphery of the egg plasm, and egg nucleus ap-
proaching; C-E, fusing of the egg and sperm nuclei, and appearance of the asters;
F, cleavage of nucleus. (After Herfort.)

the scale of animal life, as soon as we study the more complex
animals, we find that the egg cells and sperm cells are almost

220

EVOLUTION AND ANIMAL LIFE

always produced by different individuals. Those individuals
which produce egg cells are called female, and those which
produce sperm cells are called male. There are two sexes.
Male and female are terms usually applied only to individ-
uals, but it is evidently fair to call the egg cells the female
reproductive cells, and the sperm cells the male reproductive
cells. A single individual of the simpler kinds of animals
produces both male and female cells. But such an individual
cannot be said to be either male or female, it is sexless —

that is, sex is some-
thing which appears
only after a certain
degree of structural
and physiological
differentiation is
reached. It is true

FIG. 126. — Conjugation of Noctiluca, a one-celled ani-
mal : A, Two individuals just fusing; B, the same
with cytoplasm wholly fused and nuclei lying closely
together; C, the two nuclei in closer fusion; D, the be-
ginning of fission. (After Ischikawa.)

that even among
many of the higher
or complex animals
certain species are
not represented by
male and female in-
dividuals, any indi-
vidual of the species
being able to pro-
duce both male and
female cells. But
this is the exception.
Among almost all the complex animals it is necessary that
there be a conjugation of male and female reproductive cells
in order that a new individual may be produced. This neces-
sity first appears, we remember, among very simple animals.
This intermixing of body substance from two distinct individuals
and the development therefrom of the new individual is a
phenomenon which takes place through the whole scale of
animal life. The object of this intermixing seems to be the
production of variation; at least it would seem that variation
must result from such a mode of generation. By having the
beginnings of an organism's body, the single cell from which
this whole body develops, composed of parts of two different
individuals, a difference between the offspring and the par-

GENERATION, SEX AND ONTOGENY

221

ents, although it may be slight and imperceptible, is in-
sured. Sex is a condition of nature which is one, at least,
of the causes of variation.

FIG. 127. — Conjugation of the infusorian, Vorti^eUa nebuUfera; the smaller individual
at the right may be regarded as the male. (After Weismann.)

As we have seen, almost every species of animal is repre-
sented by two kinds of individuals, males and females. In the
case of many animals, especially the simpler ones, these two

FIG. 128. — Male bird of paradise.

kinds of individuals may not differ in appearance or in structure
apart from the organs concerned with multiplication. But
with many animals the sexes can be readily distinguished.

222 EVOLUTION AND ANIMAL LIFE

The male and female individuals often show marked differences,
especially in external structural characters. We can readily
tell the peacock, with its splendidly ornamental tail feathers,
from the unadorned peafowl, or the horned ram from the
bleating ewe. There is here, plainly, a dimorphism — the
existence of two kinds of individuals belonging to a single
species. This dimorphism is due to sex, and the condition
may be called sex dimorphism. Among some animals this sex
dimorphism, or difference between the sexes, is carried to ex-
traordinary extremes. This is especially true among polyga-

FIG. 129. — Cankerworm moth: the winged male and wingless female.

mous animals, or those in which the males mate with many
females, and are forced to fight for their possession. The male
bird of paradise, with its gorgeous display of brilliantly colored
and fantastically shaped feathers (Fig. 128), seems a wholly
different kind of bird from the modest brown female. The
male golden and silver pheasants, and allied species with their
elaborate plumage, are very unlike the dull-colored females.
The great, rough, warlike male fur seal, roaring like a lion, is
three times as large as the dainty, soft-furred female, which
bleats like a sheep.

Among some of the lower animals the differences between
male and female are even greater. The males of the common
cankerworm moth (Fig. 129) have four wings; the females are
wingless, and several other insect species show this same
difference. Among certain species of white ants the females
grow to be five or six inches long, while the males do not ex-
ceed half an inch in length. In the case of some of the para-
sitic worms which live in the bodies of other animals the male
has an extraordinarily degraded, simple body, much smaller

GENERATION, SEX AND ONTOGENY

223

than that of the female and differing greatly from it in structure.
In some cases even — as, for example, the worm which causes
"gapes " in chickens — the male lives parasitically on the female,
being attached to her body for its whole lifetime, and draw-
ing its nourishment from her blood (Fig. 130).

Some of the complex animals are hermaphroditic — that is,
a single individual produces both egg
cells and sperm cells. The tapeworm
and many allied worms show this con-
dition. This is the normal condition for
the simplest animals, as we have already
learned, but it is an exceptional condition
among the complex animals.

However the beginnings of the new
organisms are produced, whether asexu-
ally or bisexually (whether, that is, by
simple division, budding, sporulation, or
as true but unfertilized eggs, or as eggs
with a nucleus made by the fusion of two
germinal nuclei from male and female in-
dividuals respectively, or from an her-
maphroditic individual), this new organism
in embryo has a shorter or longer course
of development and growth to undergo,
before it, in turn, is in condition to pro-
duce new individuals of its kind.

Certain phenomena are familiar to us
as recurring inevitably in the life of every
animal which we familiarly know. Each
individual is born in an immature or
young condition; it grows (that is, it in-
creases in size) and develops (that is,
changes more or less in structure) and dies,
occur in the succession of birth, growth, and development, and
death. But before any animal appears to us as an independent
individual — that is, outside the body of the mother and outside
of an egg (i. e., before birth or hatching, as we are accustomed
to call such appearance) — it has already undergone a longer
or shorter period of life. It has been a new living organism
hours or days or months, perhaps, before its appearance to us.
This period of life has been passed inside an egg, or as an egg,

FIG. 130. — The parasitic
worm, Syngamus trache-
alis, which causes the
gapes in fowls. The
male is attached to
the female and lives
as a parasite on her.

These phenomena

224

EVOLUTION AND ANIMAL LIFE

or in the egg stage, as it is variously termed. The life of an
animal as a distinct organism begins in an egg. And the true
life cycle of an organism is its life from egg through birth, growth
and development and maturity to the time it produces new
organisms in the condition of eggs. The life cycle is from
egg to egg. Birth and growth, two of the phenomena readily
apparent to us in the life of every animal, are two phenomena

FIG. 131. — Leptodera hyalina, showing sex dimorphism: A, Head of male; B, head of

female.

in the true life cycle. Death is a third inevitable phenomenon
in the life of each individual, but it is not a part of the cycle;
it is something outside.

The single cell formed by the fusion of two germ cells is
called a fertilized egg cell, and its subsequent development
results in the formation of a new individual of the same species
writh its parents. Now, in the development of this cell into
a new animal, food is necessary. So with the fertilized egg
cell there is, in the case of most animals that lay eggs, a greater
or less amount of food matter — food yolk, it is called — gath-
ered about the germ cell, and both germ cell and food yolk
are inclosed in a soft or hard wall. Thus is composed the
egg as we know it. The hen's egg is as large as it is because
of the great amount of food yolk it contains. The egg of a
fish as large as a hen is much smaller than the hen's egg; it
contains less food yolk. Eggs (Fig. 132) may vary also in
their external appearance, because of the different kinds of
membrane or shells which may inclose and protect them.
Thus the frog's eggs are inclosed in a thin membrane and

GENERATION, SEX AND ONTOGENY

225

imbedded in a soft, jellylike substance; the skate's egg has a
tough, dark-brown leathery inclosing wall; the spiral egg of
the bullhead shark is leathery and colored like the dark-olive
seaweeds among which it lies; and a bird's egg has a hard
shell of carbonate of lime. But in each case there is the essen-
tial fertilized germ cell; in this the eggs of hen and fish and
butterfly and crayfish and worm are alike, however much they
may differ in size and external appearance.

There is great variation in the number of young produced by
different species of animals. Among the animals we know
familiarly, as the mammals, which give birth to young alive,

a

FIG. 132. — Eggs of different animals showing variety in external appearance : a, Egg
of bird; 6, eggs of toad; c, egg of fish; d, egg of butterfly; e, eggs of katydid on leaf;
/, egg case of skate.

and the birds, which lay eggs, it is the general rule that but
few young are produced at a time, and the young are born or
eggs are laid only once or perhaps a few times in a year. The
robin lays five or six eggs once or twice a year; a cow may
produce a calf each year. Rabbits and pigeons are more

226

EVOLUTION AND ANIMAL LIFE

prolific, each having several broods a year. But when we ob-
serve the multiplication of some of the animals whose habits
are not so familiar to us, we find that the production of so few
young is the exceptional and not the usual habit. A lobster
lays ten thousand eggs at a time; a queen bee lays about five
million eggs in her life of four or five years.
A female termite of a certain species, after it is
full grown, does nothing but lie in a cell and
lay eggs, producing eighty thousand eggs a day
steadily for several months. A large codfish
was found on dissection to contain about eight
million eggs.

If we search for some reason for this great
difference in fertility among different animals,
we may find a promising clew by attending
to the duration of life of animals, and to the
amount of care for the young exercised by
the parents. We find it to be the general rule
that animals which live many years, and which
take care of their young, produce but few
young; while animals which live but a short
time, and which do not care for their young,
are very prolific. The codfish produces its mil-
lions of eggs; thousands are eaten by sculpins
and other predatory fishes before they are
hatched, and other thousands of the defense-
less young fish are eaten long before attaining
maturity. Of the great number produced by
the parent, a few only reach maturity and
produce new young. But the eggs of the
robin are hatched and protected, and the help-
less fledglings are fed and cared for until able
to cope with their natural enemies. In the
next year another brood is carefully reared, and so on for the
few years of the robin's life.

Under normal conditions in any given locality the number
of individuals of a certain species of animal remains about the
same. The fish which produces tens of thousands of eggs
and the bird which produces half a dozen eggs a year main-
tain equally well their numbers. In one case a few survive
of many born ; in the other many (relatively) survive of the few

FIG. 133.— Eggs
of lace-winged
fly, Chrysnpa.
The eggs are
fastened sepa-
rately, for pro-
tection from
predaceous in-
sects, on the
tips of erect
slender pedi-
cles.

GENERATION, SEX AND ONTOGENY

227

born; in both cases the species is effectively maintained. In
general, no agency for the perpetuation of the species is so
effective as that of care for the young.

Some animals do not lay eggs, that is, they do not deposit
the fertilized egg cell outside of the body, but allow the develop-
ment of the new individual to go on inside the body of the
mother for a longer or shorter period. The mammals and
some other animals have this habit. When such an animal
issues from the body of the mother, it is said to be born. When
the developing ani-
mal issues from an
egg which has been
deposited outside
the body of the
mother, it is said
to hatch. The ani-
mal at birth or at
time of hatching is
not yet fully devel-
oped. Only part
of its development
or period of im-
maturity IS passed FJG 134 _Firat ,,tages in the embryonic development of
Within the egg Or the pond snail, Lymnceus: a, Egg cell; b, first cleavage;

c, second cleavage; d, third cleavage; e, after numerous
cleavages; f, blastula — in section; g, gastrula just form-
ing— in section; h, gastrula completed — in section.
(After Rabl.)

within the body of

the mother. That

part of its life thus

passed within the

egg or mother's body is called the embryonic life or embryonic

stages of development; while that period of development or

immaturity from the time of birth or hatching until maturity

is reached is called the postembryonic life or postembryonic

stages of development.

The embryonic development is from the beginning up to a
certain point practically alike, looked at in its larger aspect,
for all the many-celled animals. That is, there are certain
principal or constant characteristics of the beginning develop-
ment which are present in the development of all many-celled
animals. The first stage or phenomenon of development is
t lie simple fission of the germ cell into halves (Fig. 1346). These
two daughter cells next divide so that there are four cells

228 EVOLUTION AND ANIMAL LIFE

(c) ; each of these divides, and this division is repeated until a
greater or lesser number (varying with the various species or
groups of animals) of cells is produced. These cells may not
all be of the same size, but in many cases they are, no struc-
tural differentiation whatever being apparent among them.

The phenomenon of repeated division of the germ cell is
called cleavage, and this cleavage is the first stage of develop-
ment in the case of all many-celled animals. The germ or embryo
in some animals consists now of a mass of few or many undif-
ferentiated primitive cells lying together and usually forming a
sphere (Fig. 134, c), or perhaps separated and scattered through
the food yolk of the egg. The next stage of development is
this: the cleavage cells arrange themselves so as to form a
usually hollow sphere or ball, the cells lying side by side to
form the outer circumferential wall of this hollow sphere (/).
This is called the blastula or blastoderm stage of development,
and the embryo itself is called the blastula or blastoderm.
This stage also is common to all the many-celled animals.
The next stage in embryonic development is formed by the
bending inward of a part of the blastoderm cell layer, as shown
in (g) (or the splitting off inwardly of cells from a special part
of the blastula cell layer). This bending in may produce a
small depression or groove; but whatever the shape or extent
of the sunken-in part of the blastoderm, it results in distinguish-
ing the blastoderm layer into two parts, a sunken-in or inner
portion called the endoblast and the other unmodified portion
called the ectoblast. Endo- means within, and the cells of the
endoblast often push so far into the original blastoderm cavity
as to come into contact with the cells of the ectoblast and
thus obliterate this cavity (h). This third well-marked stage
in the embryonic development is called the gastrula stage, and
it also occurs in the development of all or nearly all many-
celled animals.

In the case of a few of the simple many-celled animals the
embryo hatches — that is, issues from the egg at the time of
or very soon after reaching the gastrula stage. In the higher
animals, however, development goes on within the egg or
within the body of the mother until the embryo becomes a
complex body, composed of many various tissues and organs.
Almost all the development may take place within the egg,
so that when the young animal hatches there is necessary

GENERATION, SEX AND ONTOGENY 229

little more than a rapid growth and increase of size to make
it a fully developed, mature animal. This is the case with
the birds: a chicken just hatched has most of the tissues and
organs of a full-grown fowl, and is simply a little hen. But in
the case of other animals the young hatches from the egg
before it has reached such an advanced stage of development;
a young starfish or young crab or young honeybee just hatched
looks very different from its parent. It has yet a great deal
of development to undergo before it reaches the structural
condition of a fully developed and fully grown starfish or crab
or bee. Thus the development of some animals is almost
wholly embryonic development — that is, development within
the egg or in the body of the mother — while the development
of other animals is largely postembryonic, or larval develop-
ment, as it is often called. There is no important difference
between embryonic and postembryonic development. The
development is continuous from egg cell to mature animal,
and whether inside or outside of an egg it goes on regularly
and uninterruptedly.

The cells which compose the embryo in the cleavage stage
and blastoderm stage, and even in the gastrula stage, are ap-
parently all similar; there is little or no differentiation shown
among them. But from the gastrula stage on, development
includes three important things: the gradual differentiation of
cells into various kinds to form the various kinds of animal
tissues ; the arrangement and grouping of these cells into organs
and body parts; and finally the developing of these organs and
body parts into the special condition characteristic of the species
of animal to which the developing individual belongs. From
the primitive undifferentiated cells of the blastoderm, develop-
ment leads to the special cell types of muscle tissue, of bone
tissue, of nerve tissue; and from the generalized condition of
the embryo in its early stages, development leads to the special-
ized condition of the body of the adult animal. Development
is from the general to the special, as was said years ago by
von Baer, the first great student of development.

A starfish, a beetle, a dove, and a horse are all alike in
their beginning — that is, the body of each is composed of a
single cell, a single structural unit. And they are' all alike, or
very much alike, through several stages of development; the
body of each is first a single cell, then a number of similar un-
1G

230 EVOLUTION AND ANIMAL LIFE

differentiated cells, and then a blastoderm consisting of a single
layer of similar undifferentiated cells. But soon in the course
of development the embryos begin to differ, and as the young
animals get further and further along in the course of their
development, they become more and more different until
each finally reaches its fully developed mature form, showing
all the great structural differences between the starfish and
the dove, the beetle and the horse. That is, all animals begin
development apparently alike, but gradually diverge from each
other during the course of development.

There are some extremely interesting and significant things
about this divergence to which attention should be given.
While all animals are apparently alike structurally l at the
beginning of development, so far as we can see, they do not
all differ noticeably at the time of the first divergence in de-
velopment. The first divergence in development is to be
noted between two kinds of animals which belong to different
great groups or classes. But two animals of different kinds,
both belonging to some one great group, do not show differences
until later in their development. This can best be understood
by an example. All the butterflies and beetles and grass-
hoppers and flies belong to the great group or class of animals
called Insecta, or insects. There are many different kinds of
insects, and these kinds can be arranged in subordinate groups
(orders) , such as the Diptera, or flies, the Lepidoptera, or butter-
flies and moths, and so on. But all have certain structural
characteristics in common, so that they are comprised in one
great class — the Insecta. Another great group of animals is
known as the Vertebrata, or backboned animals. The class

1 We can say that they are alike structurally, only when we consider
the cell as the unit of animal structure. But, that the egg cells of different
animals differ in their fine or ultimate structure, seems certain. For each
one of these egg cells is destined to become some one kind of animal, and
no other; each is, indeed, an individual in simplest, least developed con-
dition of some one kind of animal, and we must believe that difference in
kind of animals depends upon difference in structure in the egg itself.
Indeed Wilson, the foremost American student of egg structure, believes
himself able to perceive in many eggs a structural differentiation within
the egg protoplasm itself, corresponding, in some measure, with the struc-
tural differentiation of the embryonic animal as revealed in early develop-
mental stages.

GENERATION, SEX AND ONTOGENY 231

Vertebrata includes the fishes, the batrachians, the reptiles,
the birds, and the mammals, each composing a subordinate
group, but all characterized by the possession of a backbone,
or, more accurately speaking, of a notochord, a backbonelike
structure. Now, an insect and a vertebrate diverge very
soon in their development from each other; but two insects,
such as a beetle and a honeybee, or any two vertebrates, such
as a frog and a pigeon, do not diverge from each other so soon.
That is, all vertebrate animals diverge in one direction from
the other great groups, but all the members of the great group
keep together for some time longer. Then the subordinate
groups of the Vertebrata, such as the fishes, the birds, and the
others, diverge, and still later the different kinds of animals
in each of these groups diverge from each other.

That the course of development of any animal from its
beginning to fully developed adult form is — in all its essentials
—fixed and certain is readily seen. All rabbits develop in
the same way; every grasshopper goes through the same de-
velopmental changes from single egg cell to the full-grown,
active hopper as every other grasshopper of the same kind —
that is, development takes place according to certain natural
laws: the laws of animal development. These laws may be
roughly stated as follows: All many-celled animals begin life
as a single cell, the fertilized egg cell; each animal goes through
a certain orderly series of developmental changes which, ac-
companied by growth, leads the animal to change from single
cell to the many-celled, complex form characteristic of the
species to which, the animal belongs; this development is from
simple to complex structural condition; the development is
the same for all individuals of one species. While all animals
begin development similarly, the course of development in
the different groups soon diverges, the divergence being of the
nature of a branching, like that shown in the growth of a tree.
In the free tips of the smallest branches we have represented
the various species of animals in their fully developed con-
dition, all standing more or less clearly apart from each other.
But in tracing back the development of any kind of animal
we soon come to a point where it very much resembles or
becomes apparently identical with the development of some
other kind of animal, and, in addition, the stages passed through
in the developmental course may very much resemble the

232

EVOLUTION AND ANIMAL LIFE

fully developed, mature stages of lower animals. To be sure,
any animal at any stage in its existence differs absolutely from
any other kind of animal, in that it can develop into only its
own kind of animal. There is something inherent in each
developing animal that gives it an identity of its own. Al-
though in its young stages it may be hardly distinguishable
from some other kind of animal in similar stages, it is sure to
come out, when fully developed, an individual of the same
kind as its parents were or are. A very young fish and a very
young salamander are almost indistinguishably alike, but one
is sure to develop into a fish and the other into a salamander.
This certainty of an embryo to become an individual of a
certain kind is called the law of heredity. Viewed in the light
of development, there must be as great a difference between
one egg and another as between one animal and another, for
the greater difference is included in the less.

The significance of the developmental phenomena is a
matter about which naturalists have yet very much to learn.

FIG. 135. — Stages in the development of the prawn, Peneus potimirium : A, Nauplius
larva; B, first zoea stage; C, second zoea stage. (After Fritz Miiller.)

It is believed, however, by practically all naturalists that many
of the various stages in the development of an animal cor-
respond to or repeat, in many fundamental features at least,
the structural condition of the animal's ancestors. Naturalists
believe that all backboned or vertebrate animals are related
to each other through being descended from a common ancestor,
the first or oldest backboned animrJ. In fact, it is because all

GENERATION, SEX AND ONTOGENY

233

these backboned animals — the fishes, the batrachians, the
reptiles, the birds, and the mammals — have descended from
a common ancestor that they all have a backbone. It is
believed that the descendants of the first backboned animal
have in the course of many generations branched off little by
little from the original type until there came to exist very
real and obvious differences among the backboned animals —

FIG. 136. — Later stages in the development of the prawn, Peneus potimirium : D, Mysis
stage; E, adult stage.

differences which among the living backboned animals are
familiar to all of us. The course of development of an in-
dividual animal is believed to be a very rapid and evidently
much condensed and changed recapitulation of the history
which the species or kind of animal to which the developing in-
dividual belongs has passed through in the course of its descent
through a long series of gradually changing ancestors. If this is
true, then we can readily understand why a fish and a salaman-
der, a tortoise, a bird and a rabbit, are all much alike, as they
really are, in their earlier stages of development, and gradually
come to differ more and more as they pass through later and
later developmental stages. A crab has a tail in one of its

234

EVOLUTION AND ANIMAL LIFE

developmental stages, so that at that time it looks like and
really is like the mature stage of some tailed crustacean like
a crayfish. A barnacle, which looks little like a crayfish or
crab in its mature stage, is hardly to be distinguished in its
immature life from a young crab or lobster. Sacculina, which
is a still more degenerate crustacean, is only a sort of feeding
sac with rootletlike processes projecting into the body of the
host crab on which it lives as a parasite, but the young free-
swimming Sacculina is essentially
like a barnacle, crayfish, or crab
in its young stage.

However, it is obvious that
this recapitulation or repetition
of ancestral stages is never per-
fect, and it is often so obscured
and modified by interpolated
adaptive stages and characters
that but little of an animal's
ancestry can be learned from a
scrutiny of its development.
The fascinating biogenetic law
of Miiller and Haeckel summed
up in the phrase, "ontogeny is
a recapitulation of phylogeny,"
must not be too heavily leaned
on as a support for any specula-
tions as to the phyletic affinities of any species or group of
species of organisms. " Embryology is an ancient manuscript
with many of the sheets lost, others displaced, and with
spurious passages interpolated by a later hand."

While a young robin when it hatches from the egg or a
young kitten at birth resembles its parents, a young starfish
or a young crab or a young butterfly when hatched does not
at all resemble its parents. And while the young robin after
hatching becomes a fully grown robin simply by growing larger
and undergoing comparatively slight developmental changes,
the young starfish or young butterfly not only grows larger,
but undergoes some very striking developmental changes; the
body changes very much in appearance. Marked changes in
the body of an animal during postembryonic or larval de-
velopment constitute what is called metamorphic development,

FIG. 137. — Metamorphosis of a bar-
nacle, Lepas: a, Larva; 6, adult.

GENERATION, SEX AND ONTOGENY

235

or the animal is said to undergo or to show metamorphosis in
its development.

This metamorphosis is familiar to all in insects; to zoologists,
it is familiar among numerous other kinds of animals. Fig. 138

/

FIG. 138. — Metamorphosis of the Monarch butterfly , A nosia plexippus: a, Egg; b, larva;
c, pupa; d, imago, or adult.

shows the different stages in the metamorphic development
of the common large red-brown milkweed butterfly, Anosia
plexippus. From the egg hatches a crawling, wormlike larva,
wingless, without compound eyes, and with strong jaws and
other mouth parts fitted for biting. This creature develops
into the winged butterfly with different eyes, different antennae,
different mouth parts, different almost everything. And, by
the intervention of a curious quiescent stage called the pupal

236

EVOLUTION AND ANIMAL LIFE

or chrysalid stage, the changes seem to be made by sudden
leaps. Of course, this is not so. It is all done gradually,
although there are certain periods in the course of the develop-
ment when the changing is more rapid and radical than at
other times. The changing is masked by the outer covering
of larva and pupa, and although it is indeed startlingly radical
in its character, it is wholly continuous.

The metamorphosis of frogs and toads also is familiar.

FIG. 139. — Stages in development of silkworm moth.

The eggs of the toad are arranged in long strings or ribbons
in a transparent jellylike substance. These jelly ribbons
with the small, black, beadlike eggs in them are wound around
the stems of submerged plants or sticks near the shores of
the pond. From each egg hatches a tiny, wriggling tad-
pole, differing nearly as much from a full-grown toad as a
caterpillar differs from a butterfly. The tadpoles feed on the
microscopic plants to be found in the water, and swim easily
about by means of their long tails. The very young tadpoles
remain underneath the surface of the water all the time, breath-
ing the air, which is mixed with the water, by means of gills.
But as they become older and larger they come often to the
surface of the water. Lungs are developing inside the body,
and the tadpole is beginning to breathe as a land animal,
although it still breathes partly by means of gills, that is, as

GENERATION, SEX AND ONTOGENY

237

an aquatic animal. Soon it is apparent that although the
tadpole is steadily and rapidly growing larger, its tail is grow-
ing shorter and smaller instead of longer and larger. At the
same time, fore and hind legs bud out and rapidly take form
and become functional. By the time that the tail gets very
short indeed, the young toad is ready to leave the water and
live as a land animal. On land the toad lives, as we know,
on insects and snails and worms. The metamorphosis of the
toad is not so striking as that of the butterfly, but if the tad-
pole were inclosed in an unchanging opaque body wall while
it was losing its tail and getting its legs, and this wall were
to be shed after these changes were made, would not the meta-
morphosis be nearly as extraordinary as in the case of the

FIG. 140. — Metamorphosis of the toad: At left, strings of eggs; in water, various tad-
pole or larval stages; and on the bank, the adult toads. (Partly after Gage.)

butterfly? But in the metamorphosis of the toad we can see
the gradual and continuous character of the change.

Many other animals, besides insects and frogs and toads,
undergo metamorphosis. The just-hatched sea urchin does
not resemble a fully developed sea urchin at all. It is a minute

238

EVOLUTION AND ANIMAL LIFE

wormlike creature, provided with cilia or vibratile hairs, by
means of which it swims freely about. It changes next into
a curious bootjack-shaped body called the pluteus stage. In
the pluteus a skeleton of lime is formed, and the final true sea-
urchin body begins to appear inside the pluteus, developing
and growing by using up the body substance of the pluteus.
Starfishes, which are closely related to sea urchins, show a

similar metamorphosis,
except that there is no
pluteus stage, the true
starfish-shaped body
forming within and at
the expense of the first
larval stage, the ciliated
free-swimming stage.

A young crab just
issued from the egg
(Fig. 141) is a very
different appearing
creature from the adult
or fully developed crab.
The body of the crab
in its first larval stage
is composed of a short,
globular portion, fur-
nished with conspicuous

long SPineS .and a Pela-
tively long, jointed tail.

This is called the zoea

stage. The zoea changes into a stage called the megalops,
which has many characteristics of the adult crab condition,
but differs especially from it in the possession of a long, seg-
mented tail, and in having the front half of the body longer
than wide. The crab in the megalops stage looks very much
like a tiny lobster or shrimp. The tail soon disappears and
the body widens, and the final stage is reached.

In many families of fishes the changes which take place in
the course of the life cycle are almost as great as in the case
of the insect or the toad. In the ladyfish (Albula vulpes) the
very young are ribbonlike in form, with small heads and very
loose texture of the tissues, the body substance being jelly-

FIG. 141.-Metamorphosi& of a crab: a, The zoea
stage; 6, the megalops; c, the adult.

GENERATION, SEX AND ONTOGENY

239

like and transparent. As the fish grows older the body oecomes
more compact, and therefore shorter and slimmer. After
shrinking to the texture of an ordinary fish, its growth in size
begins normally, although it has all the time steadily increased
in actual weight. Many herring, eels, and other soft-bodied
fishes pass through stages similar to those seen in the ladyfish.
Another type of development is illustrated in the swordfish.
The young has a bony head, bristling with spines. As it
grows older the spines disappear, the skin grows smoother, and,

a

FIG. 142. — Three stages in the development of the swordfish, Xiphias gladius : a, Very
young; b, older; c, adult. (After Liitken.)

finally, the bones of the upper jaw grow together, forming a
prolonged sword, the teeth are lost and the fins become greatly
modified. Fig. 142 shows three of these stages of growth. The
flounder or flatfish (Fig. 143) when full grown lies flat on one
side when swimming or when resting in the sand on the bottom
of the sea. The eyes are both on the upper side of the body,
and the lower side is blind and colorless. When the flounder
is hatched it is a transparent fish, broad and flat, swimming
vertically in the water, with an eye on each side. As its de-
velopment goes on it rests itself obliquely on the bottom, the
eye of the lower side turns upward, and as growth proceeds it
passes gradually around the forehead, its socket moving with
it, until both eyes and sockets are transferred by the twisting

240 EVOLUTION AND ANIMAL LIFE

of the skull to the upper side. In some related forms, called
soles, the small eye passes through the head and not around
it, appearing finally in the same socket with the other eye.

Thus in almost all the great groups of animals we find
certain kinds which show metamorphosis in their postem-
bryonic development. But metamorphosis is simply develop-
ment; its striking and extraordinary features are usually due
to the fact that the orderly, gradual course of the development
is revealed to us only occasionally, with the result of giving

FIG. 143. — Young stages of a flounder, Platophrys pedas. The eyes in the young
flounder are arranged normally; that is, one on each side of the head. (After Emery.)

the impression that the development is proceeding, by leaps and
bounds from one strange stage to another. If metamorphosis
is carefully studied it loses its aspect of marvel, although never
its great interest.

After an animal has completed its development it has but
one thing to do to complete its life cycle, and that is the pro-
duction of offspring. When it has laid eggs or given birth to
young, it has insured the beginning of a new life cycle. Does
it now die? Is the business of its life accomplished? There are
many animals which die immediately or very soon after laying
eggs. Some of the May flies — ephemeral insects which issue
as winged adults from ponds or lakes in which they have spent
from one to three years as aquatic crawling or swimming larvse,
flutter about for an evening, mate, drop their packets of fertil-
ized eggs into the water, and die before the sunrise — are ex-
treme examples of the numerous kinds of animals whose adult
life lasts only long enough for mating and egg-laying. But
elephants live for two hundred years. Whales probably live
longer. A horse lives about thirty years, and so may a cat or
toad. A sea anemone, which was kept in an aquarium, lived
sixty-six years. Crayfishes may live twenty years. A queen
bee was kept in captivity for fifteen years. Most birds have

GENERATION, SEX AND ONTOGENY 241

long lives — the small song birds from eight to eighteen years,
and the great eagles and vultures up to a hundred years of
more. On the other hand, among all the thousands of species
of insects, the individuals of very few indeed live more than a
year; the adult life of most insects being but a few days or weeks,
or at best months. Even among the higher animals, some are
very short-lived. In Japan is a small fish (Solaux) which prob-
ably lives but a year, ascending the rivers in numbers when
young in the spring, the whole mass of individuals dying in the
fall after spawning.

Naturalists have sought to discover the reason for these
extraordinary differences in the duration of life of different
animals, and while it cannot be said that the reason or reasons
are wholly known, yet the probability is strong that the dura-
tion of life is closely connected with, or dependent upon, the
conditions attending the production of offspring. It is not
sufficient that an adult animal shall produce simply a single
new individual of its kind, or even only a few. It must produce
many, or if it produces comparatively few it must devote great
care to the rearing of these few, if the perpetuation of the
species is to be insured. Now, almost all long-lived animals
are species which produce but few offspring at a time, and
reproduce only at long intervals, while most short-lived animals
produce a great many eggs, arid these all at one time. Birds
are long-lived animals; as we know, most of them lay eggs but
once a year, and lay only a few eggs each time. Many of the
sea birds which swarm in countless numbers on the rocky
ocean islets and great sea cliffs lay only a single egg once each
year. And these birds, the guillemots and murres and auks,
are especially long-lived. Insects, on the contrary, usually
produce many eggs, and all of them in a short time. The May
fly, with its one evening's lifetime, lets fall from its body two
packets of eggs and then dies. Thus the shortening of the
period of reproduction with the production of a great many
offspring seem to be always associated with a short adult life-
time; while a long period of reproduction with the production
of few offspring at a time and care of the offspring are asso-
ciated with a long adult lifetime.

At the end comes death. After the animal has completed
its life cycle, after it has done its share toward insuring the
perpetuation of its species, it dies. It may meet a violent

242 EVOLUTION AND ANIMAL LIFE

death, may be killed by accident or by enemies, before the life
cycle is completed. And this is the fate of the vast majority
of animals which are born or hatched. Or death may come
before the time for birth or hatching. Of the millions of eggs
laid by a fish, each egg a new fish in simplest stage of develop-
ment, how many or rather how few come to maturity, how
few complete the cycle of life!

Of death we know the essential meaning. Life ceases
and can never be renewed in the body of the dead animal. It
is important that we include the words " can never be renewed,"
for to say simply that "life ceases/' that is, that the perform-
ance of the life processes or functions ceases, is not really death.
It is easy to distinguish in most cases between life and death,
between a live animal and a dead one, yet there are cases of
apparent death or a semblance of death which are very puzzling.
The test of life is usually taken to be the performance of life
functions, the assimilation of food and excretion of waste, the
breathing in of oxygen, and breathing out of carbonic-acid
gas, movement, feeling, etc. But some animals can actually
suspend all of these functions, or at least reduce. them to such
a minimum that they cannot be perceived by the strictest exam-
ination, and yet not be dead; that is, they can renew again the
performance of the life processes. Bears and some other
animals, among them many insects, spend the winter in a state
of deathlike sleep. Perhaps it is but sleep; and yet hibernat-
ing insects can be frozen solid and remain frozen for weeks and
months, and still retain the power of actively living again in
the following spring. Even more remarkable is the case of
certain minute animals called Rotatoria and of others called
Tardigrada, or bear animalcules. These bear animalcules live
in water. If the water dries up, the animalcules dry up too ;
they shrivel into formless little masses and become desiccated.
They are thus simply dried-up bits of organic matter ; they are
organic dust. Now, if after a long time — years even — one of
these organic dust particles, one of these dried-up bear ani-
malcules, is put into water, a strange thing happens. The body
swells and stretches out, the skin becomes smooth instead of
all wrinkled and folded, and the legs appear in normal shape.
The body is again as it was years before, and after a quarter
of an hour to several hours (depending on the length of time
the animal has lain dormant and dried) slow movements of

GENERATION, SEX AND ONTOGENY 243

the body parts begin, and soon the animalcule crawls about,
begins again its life where it had been interrupted. Various
other small animals, such as vinegar eels and certain Protozoa,
show similar powers. Certainly here is an interesting problem
in life and death.

When death comes to one of the animals with which we
are familiar, we are accustomed to think of its coming to the
whole body at some exact moment of time. As we stand
beside a pet which has been fatally injured, we wait until
suddenly we say, " It is dead ! " As a matter of fact, it is diffi-
cult to say when death occurs. Long after the Heart ceases
to beat, other organs of the body are alive — that is, are able to
perform their special functions. The muscles can contract for
minutes or hours (for a short time in warm-blooded, for a long
time in cold-blooded animals) after the animal ceases to breathe
and its heart to beat. Even longer live certain cells of the
body, especially the amceboid white blood corpuscles. These
cells, much like the Amoeba in character, live for days after the
animal is, as we say, dead. The cells which line the tracheal
tube leading to the lungs bear cilia or fine hairs which they
wave back and forth. They continue this movement for days
after the heart has ceased beating. Among cold-blooded ani-
mals, like snakes and turtles, complete cessation of life func-
tions comes very slowly, even after the body has been literally
cut to pieces.

Thus it is essential in denning death to speak of a complete
and permanent cessation of the performance of the life processes.

CHAPTER XIII

FACTORS IN ONTOGENY AND EXPERIMENTAL
DEVELOPMENT

Many biologists find their greatest triumph in the doctrine that the
living body is a "mere machine," but a machine is a collocation of
matter and energy working for an end, not a spinning toy, and when
the living machine is compared to the products of human art the
legitimate deduction is that it is not merely a spinning eddy in a
stream of dead matter and mechanical energy, but a little garden in
the physical wilderness.

What the distinction (between vital and non vital) may mean in
ultimate analysis, I know no more than Aristotle or Huxley, nor do
I believe that anyone will know until we find out. — BROOKS.

WHILE in the foregoing chapter there is outlined in some
detail the general facts and processes and so-called "laws" of
ontogenetic development, we purposely omitted any reference
to what is known or guessed concerning the causes and control
of this development. Only less wonderful than life itself is
the unfolding and changing of a single tiny apparently homo-
geneous speck of life substance (a fertilized egg cell), into a
great myriad-celled, extraordinarily heterogeneous, but per-
fectly organized fully developed plant or animal body. And
only second in point of insistence to man's queries about the
whence and whither of life itself are his demands to be informed
concerning the causes and control of development. It is indeed
strongly felt by most biologists that the study of development,
that is, the study of the initiating and guiding factors of de-
velopment, is more likely to reveal to us the basic factors and
mechanism of evolution than any other kind of study. It is
plain that evolution, its causes and method, are intimately
bound up with the general primary phenomena of life, as

244

FACTORS IN ONTOGENY 245

assimilation, growth, differentiation, adaptation, heredity,
variation, etc., and it is also plain that these fundamental life
phenomena are to be most effectively studied in their relations
to the development of individual organisms.

The most casual analysis of development shows that nu-
merous and various influences play their parts in determining
its course; it satisfies no one any longer to say that the course
and character of an animal's development is determined by
heredity. No influence or "force" of heredity can make up
in any degree in the case of the development of a chick, for
example, for the absence of a proper temperature. This purely
external factor of heat is as indispensable to the development of
the new chick creature as is the mysterious inherent capacity of
the tiny protoplasmic mass to unfold or change so radically
that it (and what it adds to itself) may become a peeping
chicken. And temperature is but one of a number of other
external factors that contribute to the creation of the new
chicken, as indeed the inherent capacity of the protoplasm
of a hen's egg cell to rearrange itself chickwise and no other
wise during development is but one among a number of neces-
sary intrinsic factors whose correlated influence or working is
part of the developmental mechanism.

The influences or factors which determine the initiation,
course, and outcome of development, then, may be roughly
classified into intrinsic and extrinsic factors. And as in our
search for rational mechanical explanations of vital phenomena
we look on factors as causal, we may use the word "causes"
in place of "factors" or "influences" if we like. The intrinsic
causes we must believe to be dependent on or incident to the
protoplasmic structure of the germ stuff and to be largely the
guiding and determining factors in development, while the
extrinsic causes are largely such as supply stimulus and energy
for the development. Among intrinsic developmental factors
are included assimilation, growth, division, differentiation,
etc., all constituting what His calls the "law of growth"; under
extrinsic factors may be listed heat, light, moisture, food,
gravitation, osmosis, etc., composing, according to His, the
conditions under which the " law of growth " operates.

In order to understand just what part each one of the vari-
ous developmental factors or causes plays, there is necessary
a most thorough analytical study of development, and an
17

246 EVOLUTION AND ANIMAL LIFE

attempt o determine in measurable or quantitative degree just
what specific effects each factor produces. Obviously the most
reliable way to effect this analysis and this determination of
the specific cause and effect relations is to appeal to experi-
ment. But biology has always been looked on as, and until
recently has actually been, almost wholly a science of observa-
tion. It is now becoming, in part at least, a science of experi-
ment as chemistry and physics have long been (these are now
becoming more and more sciences of calculation, that is, exact
sciences like mathematics), and this change and advance — for
it is truly an advance when a science formerly relying for its
facts on observation begins to base its foundations on the
results of experiment — is due primarily to the modern interest
and work in the problem of developmental causes. The search
for a rational, causomechanical explanation of the complex
and at first sight wholly baffling phenomena of development
has been a great stimulus to the bold questioning of many
other vital phenomena heretofore looked on as to be explained
only by the assumption of a mystic vital force or capacity
wholly beyond and foreign to the physicochemical world of
matter and force. Mechanism versus vitalism is one of the
greatest present-day battles in biology, and nowhere is the
struggle keener or are the mechanists more bold in their posi-
tion than in the particular field of the processes and factors of
development. To the mechanists the play of familiar physico-
chemical forces through the complex and unique structure of
germ plasm and living tissues has for result all the extraor-
dinary outcome of developmental course and outcome; to the
vitalists this course and outcome are far too complex and pur-
poseful to be explicable without the assumption of an extra-
physicochemical force, with a capacity beyond any single or
any combination of several physicochemical forces, which
they call vitalism.

There is little need of discussing the great mechanism
versus vitalism problem here: it is too difficult a subject, and
one as yet too little illuminated by known facts, to introduce
into any elementary discussion of evolution matters. But
it may not be amiss to call the attention of even the most
elementary student of evolution and general bionomics phenom-
ena to the obvious fact, that the moment one indulges a
penchant for assuming a mystic, extra-physicochemical force

FACTORS IN ONTOGENY 247

to explain a particularly hard problem, one has simply removed
his problem from the realm of scientific investigation. It is
no longer a problem. It is explained — that is, it is explained
for whoever accepts the vitalistic assumption.

The varying behavior of things in the inorganic world, the
functions and capacities of these things, depend on the varying
physical and chemical make-up of these things acted upon by
the various kinds of energy, such as heat, motion, electricity,
and what not, which we are more or less familiar with as a part
of the physicochemical world. Varying energy acting upon,
or better, through varying structure: this is the causomechanical
explanation of all the phenomena in the inorganic world.
Should we not in any open-minded consideration of the phe-
nomena in the organic world strongly incline to hold to this
same explanation until it is definitely proved incompetent,
untenable? Answering the question with a hearty "Yes/'
the mechanists look first of all in their study and analysis of
the so-called vital phenomena to the matter of structure of
the vital masses and to the play of energy through the masses,
to discover, if possible, a tangible clew to the "mysteries" of
the life process. In the study of development, then, we strive
first to see and to understand the intimate structure of the
germ plasm, this protoplasmic stuff with its wondrous endow-
ment of potentiality.

In Chapter III we have already stated summarily what is
known of the chemical and physical make-up of protoplasm.
What is actually known, by chemical analysis and earnest
microscopic peering, of this structural make-up is wholly in-
sufficient to serve as a satisfactory basis of any causomechani-
cal explanation of protoplasmic properties. Although some
of the simpler capacities of protoplasm, as its motion, its
taking up of outside substances (feeding), etc., have been to
some degree explained by seeing in them direct physicochem-
ical reactions to external stimuli or conditions, practically
nothing has been really accomplished as yet toward a mechani-
cal explanation of such more complex or unusual capacities
as irritability, assimilation, and reproduction. This last func-
tion of protoplasm is in a way its most apparently hope-
lessly inexplicable property. And this is especially so when
the reproduction is of the sort peculiar to the germinal proto-
plasm; that is, where the reproducing protoplasmic mass does

248

EVOLUTION AND ANIMAL LIFE

not simply divide and thus make two masses each capable of
the growth and change necessary to make it like the parent
mass, but where the parent mass (a fertilized egg cell, or a sexual
.egg or bud cell) can grow and develop into a highly complex
many-celled new organism of type like that from which the
parent germ plasm was derived. The special capacities, there-
fore, of germ plasm have furnished for centuries, and do to-day,

the great problem of biology
(next to that provided by the
existence of life itself).

If we cling to a belief that
in some way, after all, -the ex-
planation of the general proto-
plasmic and special germ plasm
capacities lies in an unusual
combination of structure and
play of familiar form of energy
through the structure, we are
at once forced to assume a
structural make-up of proto-
plasm and germ plasm beyond
the highest powers of our mi-
croscopes to detect. And this
assumption actually is made
by most biologists. No agree-
ment, however, exists among biologists as to this assumed
structure. Biology does not have its atomic theory as chemistry
does, to explain the ultramicroscopic make-up of the sub-
stances with which it has to deal, but has its atomic theories,
a score of fairly well-marked theories as to the ultimate struc-
ture of germ plasm having been advanced in the last couple of
centuries of biologic study.

Almost all of these theories assume a micromeric structure
of protoplasm; a few are antimicromeric. By micromeric is
meant simply that the plasm which appears to us as a viscous
colloidal substance, somewhat differentiated into denser and
less dense parts, appearing as fibrils or grains or alveoles in a
ground substance of different density, is assumed to be com-
posed of myriads of minute, ultramicroscopic units of the
general nature of combinations of chemical molecules. These
unit combinations are given, in the theories of various authors,

FIG. 144. — Egg cell of a sea urchin, Toxo-
pneustes lividus, showing cytoplasm,
nucleus, and nucleolus, and network
or alveolar appearance of the proto-
plasm. (After Wilson.)

FACTORS IN ONTOGENY 249

various names, endowed with various particular properties,
and attributed, as to their origin, to varying sources.

In the seventeenth century and early part of the eighteenth
century, before the time of the microscope, many naturalists
and physicians believed that in each germ cell (or, according to
some, in each egg cell, according to others, in each sperm cell)
there existed, preformed and almost complete, a new organism
in miniature, and that development was simply the expanding
and growing up of this tiny embryo man, or monkey, or chick.
Also they were forced to believe, if this first assumption were
true, that in each preformed embryo still smaller replicas of
their particular kind must exist to be the children of this child,
and so on, ad infinitum. Like the nests of Japanese boxes, the
outer one encasing a smaller and this still a smaller, and this
yet a smaller and so on, the young and future young of any
kind of organism were, according to this encasement theory
of the germ cell structure, nested in the egg and sperm cells of
any organism.

But the invention and use of the microscope soon put this
theory aside. The germ cells were found to contain no pre-
formed embryo. Indeed, they seemed to the earlier micro-
scopists to be utterly homogeneous little specks or masses of
protoplasm, and the pendulum of speculative explanation
tended to swing well away from any preformation theory
toward the speedily formulated epigenetic theories, which
assumed that all germ cells were practically alike except as
to their paternity and maternity, and that the development of
these homogeneous specks of protoplasm must be determined
chiefly by external conditions and influences.

However, it was obvious that there was no logical or
even fair reason for believing that the lack of structural
differentiation in the germ plasm revealed by the micro-
scope was a proof of the actual absence of such organiza-
tion. The first microscope magnified but a few hundred
diameters, revealing structure invisible to the unaided eye;
but later microscopes, magnifying objects a thousand and
more diameters, revealed structure and organization which
were quite invisible to the lower-powered instruments. And
so, although to-day we examine germ plasm with lenses
magnifying three thousand times, and yet fail to discover
more than threads, rods, grains, or droplets in a viscous

250 EVOLUTION AND ANIMAL LIFE

ground substance, we do not believe at all that this struc-
tural differentiation is the ultimate physical make-up of the
mysterious substance protoplasm. We readily believe there
may exist an ultramicroscopic structure of great complexity.

Buffon suggested that the living stuff is composed ultimately
of tiny structural units, which he called organic molecules;
these molecules are universal and indestructible; they do not
increase in number or decrease; when united in groups they
form organisms; when an organism dies its organic molecules
are freed but not destroyed, and later may help compose other
organisms. Bechamp believed in similar living micromeric
units called microzymes, created directly by the Supreme Being,
indestructible and strewed everywhere in earth, air, and water.

Herbert Spencer postulated the existence of so-called phys-
iological units: living units all of the same structure, active
because of their polarity of form and of molecular vibrations,
in size and character midway between molecules and cells,
small but 'complex and possessed of a delicate and precise
polarity analogous to that of the molecules of crystalline sub-
stances, a polarity which gives them the capacity to group
themselves into organic parts and wholes. Other theories
similar to Spencer's assume a special physicochemical en-
dowment of the chemical molecules in the organic body (Ber-
thold) , or a special electrical endowment of the life units (Fol) ,
or a special chemical one (Altmann and Maggi), or, finally, a
special vital one (Wiesner).

Darwin proposed a theory to explain how the germ plasm
could unfold into the whole body, called the theory of the
pangenesis of gemmules. Darwin postulated the existence in
the body of a host of life units called gemmules to be found
in all the various body cells, capable of rapid self-multiplica-
tion and of a migratory movement through the body, the direc-
tion and goal of which movement is determined by delicate
affinities existing among the various gemmules. When a gem-
mule enters an undifferentiated or developing cell, as yet
gemmuleless, it controls the development of that cell. Thanks
to the delicate and precise affinities of the gemmules, they
always get to just where they should, to produce harmonious
development; but in the germ cells lodge gemmules from all
over the body, so the development of these cells results in a
new whole body.

FACTORS IN ONTOGENY 251

Nageli, a philosophical botanist, proposed a theory of germ-
plasm structure and behavior which may be called the theory
of micella, nutritive plasm and idioplasm. When the complex,
life-characterizing albuminous substances took their birth in an
aqueous liquid, they were precipitated as tiny particles called
micella1, which attracted other micellae to themselves and thus
produced aggregates of primitive life stuff, or protoplasm. The
micella? are all separated from each other by thin envelopes
of water, thus making water an integral part of protoplasm,
and making growth by intercalation of new micellae possible;
this primitive protoplasm becomes arranged in two ways,
resulting in producing two kinds, one called nutritive proto-
plasm, and the other idioplasm or germ plasm, extending all
through the nutritive protoplasm as a fine network.

Finally, the most recent micromeric theory of germ-plasm
structure is that of Weismann, the modern champion of natural
selection. According to him the protoplasm of the nucleus is
made up of units called biophors, which are the bearers of the
individual characters of the cell; the biophors are complex
groups of molecules, capable of assimilating food, growing,
and reproducing; the number of biophors is enormous, as it
must equal the possibilities of cell variety. The biophors are
united into fixed groups called determinants, each determinant
containing all the biophors necessary to determine the whole
character of any one cell; in each specialized cell there need
be but one determinant, but in the germ cells every kind of
determinant must be represented.

In connection with the postulation concerning the ultimate
make-up of the plasm of the germ cell, Weismann has formu-
lated a theory of germinal selection to account for the obvious
fact that a certain cumulation of variation of a certain kind or
along fixed lines may take place without the aid of natural
selection: this variation cumulation often being indeed of a
degree too slight to give any opportunity for interference by
natural selection. To account for this fact, which has been
much used by adverse critics of natural selection, Weismann
assumes a competition of the determinants in the germ cells
for food, hence for opportunity to grow, to be vigorous, and to
multiply; the initially slightly stronger or more favorably
situated determinants will get the most food, lessening, at the
same time, the food supply of others. Now, when the germ cell

252 EVOLUTION AND ANIMAL LIFE

begins development the kind of cells or tissues or organs will
be best developed whose determinants happen to be the better
fed, stronger ones, while other parts of the body may be made
smaller or even not appear at all on account of the starvation
of their determinants; also the stronger determinants in the
better developed parts of the body will produce by multiplica-
tion more and stronger daughter determinants for the germ
cells of the new individual than the weak determinants in the
ill-developed body parts, and thus this disparity in develop-
ment of body parts will be passed on, cumulatively, to suc-
cessive generations: which is nothing more nor less than de-
terminate variation.

All the speculations about the ultimate structure of the
germ plasm are interesting, but none of them of course is really
convincing. As Delage has well said, the chances are too many
to one against the probability of anyone's guessing correctly
the actual facts concerning the complex structural detail of
the protoplasmic make-up. The structural or inherent factors
in ontogeny, then, are to be understood only in so far as ob-
vious results or effects may reveal them. Now there is one set
of phenomena in ontogeny, to which we have not as yet called
attention, which does seem to throw some light on certain
essential features or facts of germ-cell structure which other-
wise would not be obvious to us. This set of phenomena is
that called mitosis or karyokinesis, and occurs in connection
with each division or cleavage of the egg cells, and of their
daughter cells or blastomeres. It occurs also in the division
or multiplication of cells in all the tissues of the body, and is
a phenomenon normal to cell increase anywhere in the body
at any time in the life of the organism.

Direct or amitotic cell division is much less common and
seems to be restricted to certain kinds of tissues or to certain
periods in the history of the life of certain tissues. However,
the recent investigations of Child and others show that cell di-
vision without mitosis is more common than is usually thought.
In this kind of division, the process consists simply of the con-
striction and equal (or unequal) splitting of the cell body into
twro parts, the dividing of the nucleus usually being slightly
in advance of that of the cytoplasm. Each half of the parent
cell has then but to increase in size to become the counterpart
of its progenitor. In the mitotic or indirect division, on the

FACTORS IN ONTOGENY 253

contrary, the process is more complex. It has been described
by F. M. McFarland * as follows:

"One of the earliest results of the study of cell multiplication was
the discovery that division of the nucleus precedes the division of the
cell body. Furthermore, a careful examination of the different phases
of the process offers the strongest proof that the most important
feature of this division, an end to which all the other processes are
subsidiary, is the exact halving of a certain nuclear substance, the
chromatin, between the two daughter cells which result from the
division. To gain a clear conception of this process of indirect cell
division, called 'mitosis' or ' karyokinesis,' let us consider the changes
which take place in typical cell multiplication. Two parallel series
of changes occur nearly simultaneously, the one affecting the nucleus,
the other the cytoplasm. In the so-called 'resting' nucleus — i. e.,
the nucleus not in active division — the chromatin, as we have seen,
exists usually in the form of scattered granules arranged along the
linin network, and does not color readily with nuclear stains. As
division approaches, these chromatin granules become aggregated
together in certain definite areas, forming usually a convoluted thread
or skein, which now readily takes up the nuclear stains which may be
used. In some nuclei this skein is in the form of a single long filament,
in others the chromatin is divided up from the first into a series of
segments, a condition which soon follows in the case of a single fila-
ment. By transverse fission the latter breaks up into a series of seg-
ments, the 'chromosomes,' the number of which is constant for each
species of animal or plant. Thus in the common mouse there are
twenty-four, in the onion sixteen, in the sea urchin eighteen, and in
certain sharks thirty-six. The number may be quite small, as, for
example, in Ascaris, a cylindrical parasitic worm inhabiting the alimen-
tary canal of the horse. Here the number is either two or four,
depending upon the variety examined. In other forms the number
may be so large as to render counting exceedingly difficult or im-
possible. In all cases, however, one fact is to be especially noted,
viz., the number is always an even one, a striking fact which finds its
explanation in the phenomena of fertilization to be discussed later on.

"While the chromatin is collecting into the form of the chromo-

1 Most of the discussion in the following twenty pages, whether indicated
by quotation marks or not, is taken from McFarland's essay on "The
Physical Basis of Heredity " in Jordan's " Footnotes to Evolution " (1902).

254 EVOLUTION AND ANIMAL LIFE

somes the nuclear membrane has disappeared. The chromosomes
soon reach their maximum staining capacity, and appear usually as
a collection of rods or bands of deeply staining substance lying free
in the cytoplasm.

" While this is taking place in the nucleus, another series of changes
has been gone through by the centrosome and the cytoplasm im-
mediately surrounding it. We have already indicated the presence
of the centrosome as a minute spherical structure lying at one side
of the nucleus. This body assumes an ellipsoidal form, constricts
transversely into a dumbbell-shaped figure, and divides into two
daughter centrosomes, which at first lie side by side but soon move
apart. Around each of them is gradually developed a stellate figure
composed of a countless number of delicate fibrils radiating out in all
directions from the centrosome as a center. This 'aster' or 'astro-
sphere ' is at first small in extent, but grows in size progressively as the
two centers move apart, apparently being derived from a rearrange-
ment and modification of the threadlike network of the cytoplasm
under the influence of the centrosomes.

"Between these two asters, which lie a short distance apart and
at one side of the nucleus, a spindle-shaped system of delicate fibrils
may often be made out, stretching from the center of one aster to that
of the other. This fusiform figure is termed the 'central spindle.'
The two asters, together with the central spindle, form what is termed
the 'amphiaster' or the 'achromatic' portion of the karyokinetic
figure. The two series of changes in nucleus and cytoplasm, which
have thus far gone on apparently independently of each other, now
become closely interrelated in that, as the nuclear membrane dis-
appears, a system of fibrils grows out from each astrosphere, which
attach themselves to the individual chromosomes. These 'mantle
fibers' insert themselves along the chromosomes in such a way that
each segment receives a series of fibrils from each pole of the amphi-
aster, the two series being attached along opposite sides of the chromo-
somes. Under the influence of these fibers, probably by direct pulling,
the chromosomes, now bent into V- or U-shaped loops, tend to place
themselves in a circle around the center of the spindle, transversely
to its long axis, and form the 'equatorial plate.'

"The changes thus far constitute the 'prophases' of the division.
The 'metaphases' following these consist primarily in the longitudinal
splitting of each chromosome and the moving apart of the halves.
This longitudinal splitting of the chromosome into two equivalent
parts forms the most important act of the whole cell division, and is

FACTORS IN ONTOGENY

255

FIG. 145. — Cell fission in the salamander: A, Resting nucleus stage, centrosome partly
developed; B, skein stage, chromatin visible as a convoluted hand, the centrosomes
having separated; C, the nuclear membrane having disappeared, and a few of the
chromosomes lying free in the cytoplasm; D, central spindle complete, the chro-
mosomes on splitting being drawn to the spindle; E, metaphase; F, anaphase, the
chromosomes being drawn to the poles. (After Druner.)

256 EVOLUTION AND ANIMAL LIFE

of the greatest theoretical significance. By it the chromatin substance
of the original nucleus is equally distributed between the two daughter
nuclei, so that each receives a half of each original chromosome. The
elaborate mechanism and consequent expenditure of energy involved
in this careful longitudinal division of each chromosome, rather than
a simple mass division, such as might be brought about by far less com-
plicated means, indicates clearly that the distribution of the definite
organization of the chromatin to the daughter cells is of primary
importance, a conclusion which is further strengthened by much
evidence too extended to be entered upon here.

"In the 'anaphases' and 'telophases/ which include the closing
stages of division, the daughter chromosomes migrate along the fibers
of the central spindle toward its poles, perhaps through the direct
contraction of the mantle fibers under the influence of the centro-
some, though this and many other points regarding the forces at work
must be left for future investigation to decide. Arrived at the poles,
V-shaped chromosomes become grouped in a star-shaped figure, the
'aster/ their outer ends become again joined together in the form of a
tangled skein, the individual chromatin granules separate somewhat
along the threads of the linin network, their deeply staining quality
is decreased, and a new nuclear membrane develops around each
group of chromosomes. Simultaneously with this the cytoplasm
constricts across the middle of a somewhat elongated cell, resulting
in complete division in the equatorial plane of the spindle, and two
separate daughter cells result. Each of these is made up of cytoplasm
containing a centrosome and a nucleus, similar in all respects to the
parent cell from which it has arisen.

"A simple tabulation of the changes just described is as follows:

PHASES OF CELL DIVISION BY KARYOKINESIS

( 1. Resting nucleus.

I. Prophases -s 2. Skein stage of chromatin.

(. 3. Segmented skein.

( 4. Equatorial plate and splitting of

II. Metaphase ]

( chromosomes.

C 5. Movement of chromosomes to poles

III. Anaphases -j and formation of

I 6. Segmented daughter skeins.

( 7. Reconstruction of nucleus.

IV. Telophases i 0 ^. . . - ,

( 8. Division ot cytoplasm.

FACTORS IN ONTOGENY 257

"It is readily seen that the culmination of the process lies in the
splitting of the chromosomes and the separation of their component
halves to form the two new daughter nuclei."

The obvious distinction in capacity of development shown
by the various cells which compose an animal's body leads us
to ask whether we can distinguish differences associated with
these different potentialities in the fine structure of the cells
themselves, and especially in their behavior during the process
of multiplication. For the fate or future character of any
cell must largely depend on the nature of its origin, the character
of its inheritance. Now in some cases this difference in poten-
tiality of the undifferentiated dividing cells is plainly shown
by differences in the details of the process of division. A
conspicuous and important instance of this, and one bearing
directly on our subject of the relation of the structure and
character of the germ plasm to the fully developed organism,
is the distinction, usually easy to make, between the body or
so-called somatic cells and the reproductive or germ cells of
any organism.

" Every multicellular organism arises by a process of division from
a single cell, the fertilized germ or egg cell, which in turn has been cut
off from the cells of a preexisting individual. Out of the group of
cells which result from the continued division of the germ cell and its
descendants are differentiated the various tissues and organs of the
body through which the vital functions are carried on. Those tissues
and organs which perform functions pertaining directly to the existence
of the individual have been termed 'somatic/ and their constituent
cells the 'somatic' or body cells, in contradistinction to the repro-
ductive tissues or cells whose function concerns the continuance of
the species. In some forms these groups of cells, the somatic and the
reproductive, become isolated from each other quite early in develop-
ment; in one case, indeed, the differentiation of reproductive cells
from the somatic ones has been traced by Boveri back to the first
division of the egg. This case of Ascaris megalocephala is so striking
and of such fundamental theoretical importance that it must not be
passed without notice, for in it we find marked differences between
the somatic and reproductive cells in their nuclear structure, their
relative amount of chromatin, and mode of division. The egg of
Ascaris has been the classical object for cytological studies on account

258

EVOLUTION AND ANIMAL LIFE

of its small number of chromosomes (two in variety univalens, four in
bivalens), their large size, and the diagrammatic clearness of the

FIG. 146. — Reduction of the chromatin in the cleavage of the egg of Ascaris megalo-
cephala var. univalens. (After Boveri.)

changes which take place in division. In the division of the fertilized
egg cell we have two (in univalens) long chromosomes handed over to
each daughter cell. As these two cells in turn divide, a striking

FACTORS IN ONTOGENY 259

difference is seen in the karyokinetic figures. In Fig. 146, A, such a
two-celled stage is seen from the pole; in B, a slightly later stage in
side view of the spindle. In the upper cell of A , the division is of the
usual form, the two chromosomes split longitudinally, and their two
halves travel to opposite poles of the spindle (B). But in the lower cell
this is not the case. The central portion of the two chromosomes is
broken up into a large number of minute chromatin granules which
divide, and, as shown in B, form the only portion of the chromosomes
drawn up to the poles and entering into the structure of the resting
nuclei after the division is complete. The large swollen outer ends of
the chromosomes are cast off into the cytoplasm and are eventually
absorbed, playing no further part as nuclear structures. C shows the
four-celled stage, in which a marked difference in the size of the nuclei
of the upper and lower cells is visible. Lying near the margins of the
lower cells are the remnants of the ends of the chromosomes which have
been cast off in the division. In D the four-celled stage is shown with
the karyokinetic figures of the next division. In the lower cells the
spindles are seen from the pole, the chromatin is present in the re-
duced amount, in the form of small granules. In the upper left-hand
cell the two full chromosomes are seen, each split longitudinally, while,
the upper right-hand cell shows a repetition of the reduction phenome-
non— viz., the central portion of the two chromosomes, broken up into
granules, alone "enters into the spindle figure, the outer ends being
cast off into the cytoplasm, where they suffer a similar fate to those of
the lower cell in the previous division. The next division repeats the
process, one cell retaining the two full chromosomes, while all the
others have the reduced amount. This takes place for five successive
divisions and then ceases; from the one cell having the two full chro-
mosomes the reproductive tissues develop, the others with reduced
chromatin form the somatic tissues. Thus is accomplished a visible
structural differentiation of the nuclei of the reproductive cells which
distinguishes them sharply from all the somatic tissues in Ascaris.
We shall see further on that there is abundant evidence in favor of
the theory that the nucleus — i. e., the chromatin — is the bearer of
hereditary influences from one generation to the next, and that the
specific development and functions of each individual cell are de-
pendent upon the specific changes which take place in the chromatin
of its nucleus. In this light the almost isolated case of Ascaris pos-
sesses a value and interest that cannot be overestimated.

"While in the higher forms of animals and plants we find a sharp
differentiation of their tissues into somatic and reproductive or germ

260 EVOLUTION AND ANIMAL LIFE

cells, we must bear in mind that not in all forms is this power of the
reproduction of the whole organism so sharply limited to the germ
cells alone. The familiar propagation of plants by cuttings, the re-
generation of complete animals from small portions of their somatic
tissues in many lower forms, and numerous other considerations such
as these, show clearly that the difference between the powers of
somatic and germinal cells is but one of degree; that while in higher
organisms the two seem sharply defined from each other, a series
of lower forms may be taken which will show the intermediate steps
in this gradual specialization of function.

"In the unicellular organisms we have most interesting examples
of the fundamental facts of reproduction, and through an examina-
tion of these we may gain an insight into the more complicated processes
of the Metazoa. Each of these lowest forms consists of a single cell in
which are carried out in a generalized way the complex physiological
functions which, in many-celled animals, are divided up among cell
groups. In reproduction the animal simply divides into two, the
division of the nucleus preceding that of the cytoplasm, and the method
is usually a more or less modified karyokinetic one. This mode of
multiplication continues in most forms for a certain number of genera-
tions, and then the necessity for conjugation — i. e., a temporary or
permanent fusion with another individual — sets in. If this conjuga-
tion be prevented, the animal soon shows increasing signs of de-
generation which result in death. This 'senescence' of the powers of
growth and multiplication can only be checked by the admixture of
new nuclear substances from an entirely different individual b}r con-
jugation. In its simplest terms this process is found in Chilodon,
according to Henneguy. Chilodon is a minute fresh- water infusorian,
which multiplies for a considerable period of time by transverse divis-
ion. After a time, however, the physiological necessity for conjugation
ensues. The animals having placed themselves side by side in pairs
and partly fused together, the nucleus of each individual divides
into two portions, one of which passes from each infusor into the other
to unite with the half remaining stationary. The two then separate,
each having received a half of the nucleus of the other. After thus
trading experiences, as it might be termed, a period of renewed vigor
and activity for each sets in, manifested in rapid growth and multi-
plication by division, producing a large number of generations, which
continues until weakening vital activities indicate the periodically
recurring necessity for conjugation. In general, among the Infusoria
we find the same process taking place in regular cyclical order, with

FACTORS TX ONTOGENY

261

more or less complicated variations of the phenomena just outlined
for ('l/ilo</on. In all of them the aim of the conjugation is the same,
the exchange of a certain amount of nuclear substance between the
two conjugating individuals, and the same physiological effect is
reached, a rejuvenescence, as it were, of the two organisms which
manifests itself in renewed vigor of growth and multiplication.

" In some of the lowest forms of unicellular life — for example, the
Sehizomycetes or bacteria and their allies-— this necessity for con-
jugation does not
appear to exist, but
for the vast ma-
jority of forms this
cyclical law of de-
velopment holds
good. In the Pro-
tozoa no division
into somatic and
germinal cells is
found, both func-
tions being united
in the one cell which
forms the whole
body of the or-
ganism. In the Met-
azoa, however, this
differentiation has
taken place ; the germinal cells are set apart for the preservation of the
race; the somatic cells carry on their various functions for a time,
grow old, die, and disappear, certain of the germ cells alone surviving
in the production of new individuals. On the borderland between
the unicellular and the multicellular organisms, however, stand cer-
tain colonial forms, which show an exquisitely graded series of steps,
from the conditions of unicellular multiplication to those of the multi-
cellular forms." (McFarland.)

In the many-celled animals the egg is a single cell laden
with a large amount of food yolk, and made up of nucleus and
cytoplasm as the living elements. For the normal development
of this egg, conjugation with another germ cell, derived from a
different individual, is usually necessary. This germ cell is the
spermatozooid, a minute cell consisting of nucleus and centro-
18

FIG. 147. — Gonium pectorale, a simple colonial Protozoan,
composed of sixteen cells holding together in a single
layer or plate: A, The whole colony; B, a single cell; e,
eye spot; cl, chloroplast; n, nucleus; v, vacuole. (After
Campbell.)

262 EVOLUTION AND ANIMAL LIFE

some with a small amount of cytoplasm modified primarily into
an organ of locomotion, the tail. A physiological division of
labor is here met with which admirably meets two diametrically
opposed requirements. The one of these demands that the
conjugating cells be highly motile, and consequently small,
in order that they may be able to come together in the water
in which they are usually set free. The second requires that
there be furnished a sufficient amount of nutritive material
for the nourishment of the embryo until it arrives at a stage of
growth in which it can shift for itself. These two necessities
have been met by the physiological division of labor between
the two conjugating cells. The one, the sperm cell, has become
reduced in size with a corresponding gain in motility, the
other, the egg cell, has had food yolk stored up in it, and its
consequent increased size prevents any more than a very slight
degree of independent movement, if any. Different stages of
these modifications may be met with among unicellular forms,
as illustrated in Pandorina, Eudorina, and Volvox, to which
might be added many others. In Pandorina the conjugating
cells are of nearly equal size, in Eudorina an intermediate con-
dition is reached, while in Volvox the egg and sperm cells are
sharply differentiated in size and motility. Again, in the first
two and their allies all of the cells are at first vegetative and
afterward reproductive, while in Volvox the definite separation
into vegetative or somatic, and reproductive or germinal cells
makes its appearance.

We arrive then at the conclusion, from the consideration
of these and many other lines of evidence, that the germ cells
were primitively exactly alike, and that the differences between
them have arisen in the process of differentiation along two
separate lines. Furthermore, it is clear that the differences
between the two sexes, which become strongly characterized
in the higher vertebrates, are all of a purely secondary nature.

In their early development the germ cells are indistinguish-
able from each other, and both pass through certain stages,
preliminary to their union, which are essentially alike. The
animal egg is a large, more or less spherical cell, enveloped
usually by certain membranes, containing a large nucleus and
cytoplasm. The vast bulk of the egg cell, however, is made
up of inert food material in the form of yolk- granules, which
are stored up in it as nourishment for the developing embryo.

FACTORS IN ONTOGENY

263

FIG. 148. — Maturation of the egg of Cyclops (the full number of chromosomes is not
shown): A, Chromosomes already split longitudinally; B, chromatin masses with
indication of transverse fission to form the tetrads; C, the young tetrads arranging
themselves on the first polar body spindle; D, tetrads in first body spindle; E,
separation of the dyads in the same; F, position of the dyads in the second polar
body spindle, the first polar body being really above the margin of the egg. (After
Riickert.)

264

EVOLUTION AND ANIMAL LIFE

The nucleus, or germinal vesicle, is large, and contains a net-
work of chromatin together with one or more conspicuous
nucleoli.

There are three periods usually recognized in the develop-
ment of the egg cell, viz.: (1) The period of multiplication; (2)
the period of growth; and (3) the period of maturation. The
first period is characterized by a continued series of divisions
of the primitive reproductive cell and its descendants, which

FIG. 149. — Formation of the chromatins and tetrads in the spermatogenesis of Ascaris
megalocephala. In A-C, the whole cell is shown; in D-H, only the nucleus. (After
Brauer.)

produces a large number of "ovogonia." Succeeding this is
a period of growth in which the ovogonia increase greatly in
size, mainly through the production and storing up of food
yolk. At the close of this period the germ cell, now termed a
"primary ovocyte," enters upon the maturation period, in
which it undergoes two divisions in rapid succession, by means
of which two minute cells, the polar bodies, are cut off from
the egg. Through these two divisions the number of chromo-
somes in the egg nucleus is reduced to one half that which is
found in the other cells of the body. The first polar body also
usually divides, and thus, at the close of the period of matura-
tion, four cells result: one large mature egg cell, ready for the
fertilization which initiates the development of the embryo,
and three minute polar bodies, which are to be regarded simply
as rudimentary eggs. The nuclei of these four cells are exactly

FACTORS IN ONTOGENY

205

alike in that they all contain the same number of chromosomes,
i. e., one half the number in the somatic cells of the individual.
The difference in size is due simply to the concentration of the
food yolk and most of the cytoplasm in one of the cells; the
other three degenerate, being sacrificed to the production of
an egg cell with the largest possible supply of nutritive sub-
stance in it.

In the development of the sperm cell (Figs. 149, 150), we find
an exactly parallel series of stages, the end results, however,
differing much in size.
The mature sperma- /\
tozoon is an exceed-
ingly minute cell, con-
sisting typically of a
cylindrical or conical
"head" containing a
nucleus, a short cyto-
plasmic "middle
piece/7 and a long
vibratile "tail," an
organ of locomotion
differentiated out of
the cytoplasm of the
cell from which the
spermatozoon is de-
rived. The stages of

multiplication, growth, and maturation are passed through in
the development of the spermatozoon in the same order as
in the egg development, save that the period of growth does
not include the storage of food yolk in the primary sperma-
tocyte, and the two divisions of the maturation stages are
equal ones, resulting in the production of four cells of the
same size, each of which develops into a complete sperma-
tozoon.

The accompanying diagrams of Fig. 151, taken from Boveri,
illustrate clearly the homologies existing between the life
histories of the two sorts of germ cells. The earlier stages of
ovogonia and spermatogonia are indistinguishable from each
other; later in the period of growth the increase in size of an
ovocyte marks it off from the minute spermatocyte, but this
distinction is merely one due to nonliving food material, and

FIG. 150. — First (A-O and second (D-H) matura-
tion of the sperm atocytes of Ascaris megalocephala.
(After Brauer.)

266

EVOLUTION AND ANIMAL LIFE

in nowise affects the fundamental identity of the two. In the
maturation period the number of chromosomes in the nuclei
of both egg and sperm is reduced one half — on the one hand,
the ripe egg cell and three rudimentary egg cells (the polar
bodies) being formed; on the other, four equal "spermatids"
are produced, which develop into four mature spermatozoa.
The contrast in size which exists between the two mature re-
productive cells is enormous, the spermatozoon in some cases
containing less than TTMHTOF (Wilson), and in extreme cases less
than To"ooWo -Q (Hertwig) of the volume of the egg cell.

FIG. 151. — At left, diagram illustrating the development of the spermatozoon; at right,
diagram illustrating the development of the egg. (After Boveri.)

A discussion of the method by which the reduction of the
chromosomes in the germ nuclei is brought about, may profit-
ably be deferred until the essential features of fertilization
have been examined. The phenomena of the fusion of egg and
sperm can best be studied in some such form as the sea urchin,
in which the egg is very small, and, in some species, quite
transparent. As fertilization takes place free in the sea water,
the germinal cells being cast out from the parents, it is possible
to collect the eggs and sperm separately from mature in-
dividuals and bring them together in small dishes of sea water,
and at such times as may suit one's convenience. Then in the
living egg much of the process may be followed under the
microscope, and properly prepared sections of the eggs, killed

FACTORS IN ONTOGENY

267

by reagents at the various stages, enable conclusions to be
drawn as to matters of minute detail.

Fig. 153, A to F, presents a series of diagrams, taken from
Boveri, illustrating the principal facts in the process of ferti-
lization. In A, the egg is represented with its clear nucleus
in the center, surrounded by the egg membrane. Clustered
around the periphery are a number of spermatozoa endeavoring
to find their way into the substance of the egg. On the right-
hand side in the figure one has penetrated the membrane and
is shown passing into the egg cytoplasm, which puts forth a
small conical prominence to meet
it. As soon as the head of one
sperm enters the egg cytoplasm a
new membrane is formed around
the egg which effectually prevents
the entrance of any others. The
head and middle piece penetrate
into the egg, the tail usually re-
maining imbedded in the mem-
brane, where it soon degenerates.
A few moments after the sperm has
entered, a system of radiations
appears around the middle piece

\\\ . . FIG. 152.— Egg of the worm, Myzo-

Which develops mtO an aster SUr- 8toma glabrum, being fertilized by

rounding the CentrOSOme of the spermatozoon. (After Wheeler.)

sperm (B). The sperm nucleus

swells up and rapidly increases in size, its chromatin changing
from the compact condition in which it is arranged in the
sperm head to a reticulate condition (C). The chromatin re-
ticulum of the egg nucleus becomes also more clearly visible.
Sperm aster and sperm nucleus now move in toward the
egg nucleus, the aster usually preceding. As the nuclei ap-
proach, the sperm nucleus increases still more in size until
it becomes indistinguishable from the egg nucleus (C). The
chromatin network of each now. breaks up into a number
of chromosomes, one half of the number found in the som-
atic cells, and the nuclei come into contact, fusing together
,in some cases. As in the sea urchin, Echinus, the number of
chromosomes is eighteen, nine would therefore be found in
the germ nuclei; for the sake of clearness and simplicity
but two are represented in the diagram, those of the sperm

268

EVOLUTION AND ANIMAL LIFE

[\

Fio. 153. — Diagrams illustrating the fertilization of the egg: A, Egg surrounded by
spermatozoa; on the right, one has just penetrated the egg membranes and is en-
tering the egg cytoplasm; egg nucleus in the center. B, Egg nucleus with chromatin
reticulum on the left; on the right, the sperm nucleus preceded by its centrosome
and attraction sphere. C, Egg nucleus on the left, sperm nucleus on the right of
the center of egg; aster fibrils preceding the fission of the centrosome. D, The
centrosome has divided, the two attraction spheres separate to form the first
cleavage of the spindle; the chromosomes of the egg and sperm nuclei clearly visible
and distinguishable (in the figure the egg chromosomes are black, the egg sperm
chromosomes shaded). E, First cleavage, showing spindle with splitting chromo-
somes. F, Completion of first cleavage, two-celled stage, each nucleus showing four
chromosomes, two from the egg and two from the sperm. (After Boveri.)

FACTORS IN ONTOGENY 2b'<)

nucleus being slightly shaded while those of the egg nucleus
are black.

The centrosome divides together with its aster (Z)), the two
daughter centrosomes move apart to opposite poles of the egg,
and the typical amphiaster of cell division is formed (E), the
nuclear membranes disappearing and the chromosomes being
drawn together into the equatorial plate where each splits
longitudinally. The halves are drawn by the mantle fibrils
toward the opposite poles, and the egg divides transversely
into two cells (F). This process of division is repeated con-
tinuously in each of the resulting generations of cells, and
from the mass of cells thus formed develops the new organism.
Each cell in the two-celled stage has received half of its
chromosomes from the egg nucleus and half from the sperm,
thus containing equal amounts from each parent. The centro-
some, which, as we have seen, is to be regarded as the dynamic
center of the cell division, comes from the spermatozoon alone;
the egg, on the other hand, furnishes the yolk and practically
all of the cytoplasm.

After this preliminary outline of the facts of fertilization, we
are in a better position to understand the significance of a process
which occurs in the development of both egg and sperm cells,
namely, the reduction of the chromosomes. The necessity for
such a reduction is evident from a moment's reflection. We
have seen that the number of chromosomes in the nucleus is
a constant and typical one for each animal and plant species
so far as known. As fertilization consists in the union of two
cells in one, from which the young organism develops, it is
plain that, were there no reduction, the number of chromo-
somes would be doubled in each succeeding generation. How-
ever simple this necessity for reduction may appear, the mi-
nutiae of the processes through which it is brought about, and
the theoretical significance of these facts, form one of the most
involved problems of biology to-day. In a few forms, especially
among the lower Crustacea, the facts of the reduction are clear
and relatively simple; in other forms they thus far stand in
direct contradiction, and, for the present, a comprehensive
explanation applicable to all forms must be left to further
investigation.

The significance of reduction turns upon the conception
of a definite organization and individuality in the chroino-

270

EVOLUTION AND ANIMAL LIFE

somes and the assumption that they represent the physical
basis of heredity — i. e., that they influence and determine into
what the fertilized egg shall develop. Fifteen years ago
Wilhelm Roux showed with convincing clearness that the
complicated facts of nuclear division, the careful longitudinal
halving of the chromatin thread and its equal distribution
between the two daughter cells, can be explained only on the
basis that the chromosomes possess different structure in dif-
ferent parts of their extent, and that these structures, repre-
senting tendencies in development, are distributed in definite
ways to the daughter cells. Were this not the case, a simple
direct mass division of nucleus and cytoplasm instead of the

complicated process of
karyokinesis with its
consequent much greater
expenditure of energy
would serve all pur-
poses.

In the light of this
probable individuality
and morphological or-
ganization of the chro-
mosomes, the method of
their reduction in num-
ber, preparatory to the fusion of their germ cells, becomes
of the greatest significance; to those who may deny this in-
dividuality and definite architecture, the phenomena can have
no great importance save as concerns a general mass reduc-
tion in the amount of the chromatin present in the germ nu-
clei. It may be assumed as true, in the majority of cases
now accurately known, that the reduction takes place some-
where in or near the last two divisions of the germ cells previous
to their fusion — that is, in the egg, in the divisions forming
the polar bodies, and in the sperm, in the last two divisions of
the spermatocyte which produce the four spermatids out of
which develop as many mature spermatozoa. The phenomena
are exactly homologous in both cases, as has already been
pointed out, differing only in the minor details which do not
affect the end result.

Two peculiar features mark these divisions off from all the
others which precede and follow them. One of these is the

FIG. 154. — Formation of the polar bodies shown
diagrammatically. (After Korschelt and Heider.)

FACTORS IN ONTOGENY

271

absence of an intermediate resting stage between them, the
second division following immediately upon the first without
the reconstitution of the chromosomes into the skein stages.
The second peculiarity lies in the fact that the chromatin (now
the individual chromosomes) appears in one half the typical

. 155. — Reduction of chromosomes in the spermatogenesis of A scaris megalocephala
var. biralens: A, Nucleus of a spermatogonium, the typical number of chromo-
somes (4) shown, each split longitudinally preceding the nucleus fission; B, young
splitting primary spermatocyte, two tetrads present, each with body double from
longitudinal splitting of a chromatin thread; C, the tetrads in the equatorial stage
of fission; D, separation of the dyads; E, the dyads in succeeding fission of the
secondary spermatid; F, completion of the fission of the same, each cell (spermatid)
contains the reduced number of chromosomes (2) . (After Brauer.)

number of the chromosomes in the first division, and is
usually arranged in "tetrads," or groups of four rounded,
deeply staining bodies connected by linin fibers. These tetrads
are always one half the number of the original rod- or thread-
like chromosomes. Thus in Fig. 155, A represents a sperma-
togonium nucleus of Ascaris with the four chromosomes,
showing the longitudinal splitting preparatory to division.

272 EVOLUTION AND ANIMAL LIFE

B represents an early spindle stage in the division of the
primary spermatocyte, in which not four bandlike chromo-
somes, but two tetrads, or chromatin groups or four rounded
bodies are found. C to F show clearly the further steps in
the spermatogenesis. In C the tetrads are grouped in the
equatorial plate, and in D, in the closing stages of the first di-
vision into two spermatocytes, each tetrad has divided into
two "dyads," which are drawn to the poles, and the division
of the cell body follows. Without an intervening rest stage
each spermatocyte now divides again, as in E and F ', each
dyad being separated into halves, so that in the spermatids of
F but two chromatin masses are present. Thus the tetrads
of the primary spermatocyte are divided up among the four
spermatids, so that each of the latter receives one fourth of
each tetrad. Since later stages show that the two chromatin
masses in each spermatid of F represents two chromosomes, we
see that the number of chromosomes has been reduced from
the four in A to the two in F.

Manifestly the key to the explanation lies in the relations
which exist between the four chromosomes of A and the tetrads
of B. The two divisions consist merely in the distribution of
the already separated parts of the tetrads ; in the rearrange-
ment of the four chromosomes into the two tetrads lies the
possibility of the reduction which is carried out by the fol-
lowing divisions. The problem thus resolves itself into the
question, What is the nature of each tetrad? Is it made up
of a single chromosome? of two? of four? or have the constituent
parts of the original four chromosomes become so completely
rearranged and redistributed that their identity as such is
completely lost?

Turning for a moment to the lower Crustacea, we find among
the Copepods forms admirably suited for the careful following
out of the changes taking place in the rearrangement of the
chromosomes into the tetrads. To Riickert we owe the clearest
account of the process as exhibited in the egg maturation of
Cyclops. Here the normal number is twenty-two, or perhaps
twenty-four, the minute size rendering counting difficult. Fig.
148, A to F, taken from Riickert, gives the essential points of
the formation of the tetrads and their following divisions, not
all the chromosomes being represented. In A the chromatin
filament has broken up into one half the usual number of

FACTORS IN ONTOGENY 273

segments (chromosomes) , and each shows the precocious longi-
tudinal splitting. These segments shorten up into the double
rods of B, which in C are being arranged in the developing
spindle. A comparison of these three figures will show clearly
that each chromatin segment has divided both longitudinally
and transversely, its parts shortening and arranging them-
selves in tetrad formation of D. The first division following
separates the tetrad along the longitudinal plane of its former
splitting (E) and the second division along the transverse
plane (F).

In Cyclops then, the tetrads are formed by the chromatin
thread of the resting nucleus breaking up into one half the
usual number of segments, and each of these in turn dividing
longitudinally and transversely. A tetrad here is made up of
two chromosomes slightly united end to end and split longi-
tudinally. Thus if abcdef . . . n represent the unsegmented
filament of the resting nucleus, a-b-c-d-e-f would show its
breaking up into the normal number of chromosomes which

split lengthwise, forming -> T> -> -v -» v in the equatorial plate.

a o c d e j

In the Cyclops nucleus of Fig. A the filament has separated
into the segments ab-cd-ef . . . n, each of which has split longi-
tudinally into — > — ;> •*> etc., and its transverse division, sub-
ab cd ef

sequently becoming more apparent, gives to each tetrad the

a

b cd ef

composition --T-J--V-V etc. By the first division in the
ab cd ef

longitudinal plane, each daughter cell receives a half of each
chromosome; in the second, however, in the vertical plane,
this is not the case, as can be readily seen. This is clearly a
qualitative division, and the daughter cells receive unlike
chromosomes. This forms the "reducing division" in Weis-
mann's sense, and as such is a most beautiful demonstration
of his postulated reduction of the ancestral plasm.

In Ascaris, however, the evidence is just as clear that no
reducing division in Weismann's sense takes place, though
the actual number of the chromosomes is also reduced.

Boveri has shown for the egg and -Brauer for the sperm
that the tetrads arise by a double, longitudinal, splitting of
the chromatin filament which later breaks into two segments.
Thus abed would again represent the unsegmented filament,

274 EVOLUTION AND ANIMAL LIFE

a-b-c-d the individual chromosomes, and -> r> -> -. their splitting

abed

longitudinally in ordinary division. In the maturation of
the egg and in spermatogenesis, however, the thread segments

. , , , ,., , ., ,. „ abab cdcd

into ab, cd, and splits twice longitudinally into — r-r» - — ;»

^^^^ ao ao ca cd

the two tetrads of B in Fig. 155. The^B ^ion of chromatin
here is only a reduction in mass and « Balitative one, in
Weismann's sense, as in the Crustacea ^^^Piects. In Ascaris
the actual reduction in number of chromosomes takes place
in the nucleus previous to the maturation divisions of the
ovocyte and spermatocyte ^pectively. In Cyclops the for-
mation of the tetrads is merely a pseudo-reduction, the actual
reduction taking place in the second division, which gives rise
to the mature egg on the one hand, or the spermatids, which
develop into the spermatozoa, on the other.

One fundamental fact is clear in these divergent accounts.
The number of chromosomes is reduced in both sorts of the
germinal cells as a preliminary to their union. Whether there
is likewise a qualitative distribution of the chromatin elements
remains for future investigation to decide. From the facts of
ordinary cell division we have seen that the chromatin of
the nucleus is to be regarded as the bearer of hereditary qualities
in the cell. The phenomena of fertilization greatly increase
this probability. The offspring resembles both of its parents,
and the paternal tendencies can be conveyed in the minute
spermatozoan head alone, which is constituted almost entirely
of chromatin. The scrupulous exactitude with which, in both
germ cells, the chromosomes are reduced to one half the normal
number preparatory to the union of the pronuclei in fertiliza-
tion, and the distribution of the paternal and maternal chro-
matin equally to the resulting cells of cleavage, lend added
weight to the theory.

The development of the fertilized germ cell into the com-
plete organism is discussed in the preceding chapter as also
is the significance of sex. This significance in the light of actual
processes of germ-cell formation, maturation, and fertilization
is seen to be very important in relation to the phenomenon
of variation, a phenomenon or fact which we have already
learned to recognize as the absolutely essential basis of all
organic evolution.

FACTO US IX OXTOUKXY

275

f\

B

D

FIG. 156. — A, Normal larva of Echinus microtuberculatua. front view; B, the same, side
view; C. normal larva of Sphaerechinus granularis, front view; D, the same, side
view. (After Boveri.)

276 EVOLUTION AND ANIMAL LIFE

Whether the new individual to be exists in the germ cell
as a more or less nearly completely preformed embryo needing
only to expand, unfold, and grow to be the fully developed
new creature, or whether the fertilized egg cell is a bit of prac-
tically undifferentiated protoplasm, endowed with a limited
and specific potentiality, but depending for its marvelous out-
come chiefly on extrinsic imposed influences — this question
has been a matter of contention since the beginning of the
study of generation and development.

From our scrutiny of the phenomena of mitosis, it is ap-
parent that, while the germ cell is certainly considerably
differentiated as regards its fine structure, on the other hand it
as certainly contains no preformed embryo of the individual
into which it is to develop, as the old school of preformationists
held. But the testimony from mitosis by no means settles the
controversy between the modern preformationists and the
modern epigenesists. This rages hotly, and furnishes a great
incentive to the pushing on of the study of development.

What is most interesting, perhaps, about this present-day
embryological study is, perhaps, its method. Where hereto-
fore the study of development has been almost purely descrip-
tive and comparative, as, indeed, all biological study has, the
modern embryologist is an experimenter. Experiment, the
method of the study of inorganic nature, is being resorted to
and relied on for the determination of biological problems, and
in particular that one that has for its subject the seeking of
the factors and actual causes of individual development.
This has been aptly named preformation versus epigenesis.
It might also pertinently be called intrinsic versus extrinsic
factors or, more broadly, vitalism versus mechanism.

The new phase or mode of the study of development has
been variously called developmental mechanics, experimental
development, or, more broadly, experimental morphology,
because the experimental method has been extended to the
study of phenomena not strictly, or at least not usually in-
cluded in the immature, or developing stage of the animal's
life; the study of regeneration, of reactions to stimuli, and of
reflexes and movements in general, has all been illuminated
by the decisive results of the substitution of experiment for
haphazard observation in nature. And the further extension
of experimental and statistical modes of investigation to the

FACTORS IN ONTOGENY

277

FIG. 157. — A, Hybrid larva (Sphcerechinus $ and Echinus i), front view; B, the same
from side view; C, hybrid larva, Sphcerechinus $ (nonnucleated egg formation;
and Echinus £ , of the type; D, the same larva in side view. (After Boveri.)

19

278

EVOLUTION AND ANIMAL LIFE

"grand problems" of heredity and variation, already well
entered upon, bids fair to produce the most rapid and real
advance that has yet been made toward the goal of solving
some of the mystery which has so far enwrapped these funda-
mental phenomena of life.

To return to our special problem of preformation or epi-
genesis, it must be said at the outset that the evidence touching
it, which has so far been derived from experiment, is distinctly
conflicting. For example the frog's egg (which has been a

classic V ersuchs object in this
study), when treated after its
first cleavage so that one of its
two blastomeres (daughter cells
of the original fertilized egg cell)
is killed, develops half a frog,
which would indicate that the
embryo was preformed in the
egg cell, or at least that each
part of the egg cell had its fate
predetermined, so that the loss
of part of the egg would produce
a loss of a definite part of the
embryo.

But in the hands of other
investigators diametrically op-
posed results were got. Hertwig managed to separate entirely
the two first cleavage cells and got from each of these half
eggs a complete embryo but of dwarfed size, which would
indicate that any part of the egg stuff is able to produce any
part of the embryo. Other investigators have succeeded in
separating blastomeres of later cleavage stages, and have
variously got either miniature but complete embryos from
these fractional egg parts, or on the other hand parts of embryos
representing apparently the predetermined developmental
fate of the various parts of the egg. To list briefly a few of
these cases, we may refer to the development of partial embryos
from separated (2-16 cell stage) blastomeres of various Cten-
ophora, and the similar results with the molluscs Patella, Den-
talium, and Ilyanassa: to the production of defective larvae by
the mutilated eggs of Beroe, also of ascidians and of Echinus:
to Driesch's distinction between ectoderm and endoderm after

FIG. 158. — Lithium larva of the sea
urchin, Sphccrechinus granularis:
A, Elongated blastula; B, evagi-
nated gastrula. (After Herbst.)

FACTORS IN ONTOGENY

279

the first cleavage of Synapta, and the cell lineage studies of zur
Strassen on the eggs of Ascaris in which it was shown that a
definite status of outcome for each blastomere was determined
after successive early cleavages. All these results seem to be

A

B

FIG. 159. — A, Normal gastrula of sea urchin, Echinus microtuberculatus; B, gastrula of
sea urchin, Sphver echinus granularis, from a lithium culture. (After Herbst.)

good evidence for preformation, that is, for a predetermination
of the role each part of the egg cell is to play in development.
Indeed, Wilson is convinced that an obvious structural differ-
entiation (bands, zones, delimited regions) can be seen in the
undeveloped eggs of numerous animals, a differentiation corre-
sponding to structural di-
vergence in development.

On the other hand, nu-
merous results of experi-
ment speak just as loudly
against preformation or pre-
determination. Such are
Herlitzka's half-sized Triton
embryos from the two sepa-
rated first cleavage cells,

Driesch's two half-sized and four quarter-sized sea-urchin plutei
from the cells of the first and second cleavages, respectively,
his eight and sixteen small gastrulse, and thirty-two tiny blas-
tulae from the separate blastomeres of the third, fourth, and
fifth cleavages respectively; also Zoya's medusa embryos from

FIG. 160. — Abnormal larval stages of the sea
urchin, Sphcerechinus granularis, produced
by heat. (After Driesch.)

280

EVOLUTION AND ANIMAL LIFE

FIG. 161. — A, Lateral view of pluteus larva of
Echinus; B, lateral view of pluteus larva of
Sphtcr -echinus; C, hybrid pluteus of the female
Sphcerechinus and male Echinus. (After Boveri.)

separated blastomeres of the two, four, eight, and even sixteen-

cell stages of developing hydro-medusa eggs. Loeb was able

to effect the bursting of

n rl\ " _ c ^ the membrane of sea-

urchin eggs and the con-
sequent partial escape
or protrusion of parts of
the egg plasm forming
so - called extra - ovates.
Each of these extra-
ovates began develop-
ment as a distinct bias-
tula, the remainder of
the egg forming another
blastula (Fig. 163).
Thus we see that experimental work has, so far, not afforded

a positive answer to the general query proposed by the pre-

formation versus epigen-

esis problem. But at

the same time it is

obvious that the results

of the experimental

method are of extraor-
dinary interest and of

brilliant promise. What

seems to be revealed so

far, is that the animal

egg is certainly not

rigidly preformed; that

there is no absolute

predetermination of the

fate in development of

each part of the egg

stuff. But that nor-
mally in most eggs a

given part of the egg

does have a prospective Fl?' '^.--Cleavage of Echinus eggs in water free
1 from calcium. Note that the cleavage cells tend

definitive fate, SO that to separate entirely. (After Herbst.)

one-half of the egg may

be looked on as corresponding to one particular half of the

future organism. However, the actual potentiality of any part

FACTORS IN ONTOGENY

281

of the egg is not limited by its prospective fate. If accident
in nature or ruthless handling in the experimenter's labora-
tory destroy or remove part of the egg, the remainder has a
power of regulation which is in some respects the highest
and most important kind of organic adaptation that we know.

The same data derived from the
experimental study of development, to-
gether with data got from the experi-
mental study of mature and even
senescent stages of various organisms,
constitute our chief evidence touching
the problem of mechanism versus vi-
talism. This problem may be posed in
question form as follows: In how far
can so-called vital phenomena be ana-
lyzed into physicochemical, or mechan-
ical phenomena? Is life simply an in-
teraction, very complex to be sure, and
so far largely unanalyzed and hence not
directly referable to specific physico-
chemical causes, between substances of
particular chemical and physical struc-
ture and those familiar forms of energy
known to us in the physicochemical
world, or is it the result or manifestation
of an extra physicochemical force and
set of conditions?

When the sunflower bends its face
always toward the sun, we do not at-
tribute this behavior either to the in-
telligence or the instinct of the plant.
But when young spiderlings or moth
caterpillars or green aphids just from
the egg move with one accord toward
the light side of the glass jar, we do attribute this behavior to
animal instinct or to the exercise of a preference or choice.
When iron filings rush toward a magnet brought sufficiently
near them, we have on our tongues' end the sufficient explana-
tion of this behavior in the single word "magnetism." Now
the biological mechanist, observing that in all these cases
there is a certain apparent definite relation between a cause

FIG. 163. — Extra - ovates
from the eggs of the sea
urchin, Arbacia. By di-
luting the sea water the
osmotic pressure bursts
the egg membranes so
that part of the egg plasm
issues and forms an ex-
tra-ovate. (After Loeb.)

282

EVOLUTION AND ANIMAL LIFE

and an effect, presumes
to say that all these
phenomena may be
much more nearly of
the same sort than we
are accustomed to con-
sider them to be. The
biological mechanists
believe, in a word, that
all vital phenomena
will in last analysis
prove to be truly phys-
icochemical phenom-
ena; that organisms
show in their reactions
no new forces or prin-
ciples, but that their
behavior is only an im-
mensely complex interplay of the same forces and activities
already known to us in the inorganic world.

And their belief is not wholly without some basis of observed
or experimentally proved fact. Many of the simpler so-called
vital phenomena, especially the movements of the simplest

FIG. 164. — Regeneration in Hydra viridis: A, Nor-
mal hydra (lines show where piece was cut out) ;

B, 1-4, changes in a piece of A as seen from side ;

C, 1-4, same as seen from end ; D, E, F, later
changes in same piece. (After Morgan.)

FIG. 165. — Regeneration of Stentor ccrruleus: A, Cut in three pieces; B, row showing
regeneration of the anterior piece ; C, regeneration of middle piece ; D, that of
posterior piece. (After Morgan.)

FACTORS IN ONTOGENY

283

and even the more complex animals, have been shown to
be suggestively like motion reactions in inorganic nature. The
mechanists analyze many of the so-called instinctive perform-
ances of animals into rigorous taxic and tropic reactions to
specific external influences or stimuli. Chemotaxis, phototaxis,

FIG. 166. — Regeneration in nature of starfish, Linckia. The regenerated specimens
shown in the figure were collected as living animals on the coral reefs of Samoa.
These specimens show the great capacity for regeneration possessed by this star-
fish, a portion of an arm being capable of regenerating the disk and all the other
arms.

and oxygenotaxis, heliotropism, geotropism, thigmotropism,
etc., are the names applied to the growth or motion reactions
of organisms or their parts, conditioned by such external
stimuli or control-influences as light, gravitation, contact, the
presence of oxygen or of various other chemical substances.
And an account of the ingenious experimentation which has

284

EVOLUTION AND ANIMAL LIFE

been done to test the truth of the mechanical assumptions
is a fascinating chapter in the history of modern biological
work.

In sum we may say that there has been in recent years a
real advance on a basis of experimental work, in the analysis
of many vital phenomena long considered mysterious, or at
least too complex for human understanding, into simpler
components. And that these components are in many cases

no other than reac-
tions and motions
familiar to us in in-
organic nature. On
the other hand it
must be said that
this advance, in the
face of the immense
problem presented
by vital reactions —
that is, the behavior
of organisms — is very
small. With all our
heart we should wel-
come all attempts to
do away with ideas
of mysticism in con-
nection with biologi-
cal phenomena ; the
mechanists should
have our strong sym-
pathy and our willing
support, but to join
the more radical of
them in their claim
that the life mystery

is already solved in terms of physics and chemistry, that there
is no longer any vital problem, would be to surrender our
judgment to our inclination.

Any discussion, however brief, of experimental work in
biology should include a reference, at least, to the striking and
suggestive results that have been obtained by the application
of the experimental method to the investigation of the problems

w

FIG. 167. — Regeneration of the earthworm : A, Nor-
mal worm ; B-F, anterior ends of worms which, after
the removal of one, two, three, four, and five seg-
ments, have regenerated the same number ; G, an-
terior third cut off, only five head segments regener-
ated ; H, worm cut in two in middle, a head-end of
five segments regenerated ; /, worm cut in two be-
hind the middle, a hetoromorphic tail regenerated at
anterior end. (After Morgan.)

FACTORS IN ONTOGENY

285

. ,
V/

\J

B

D

of fertilization and parthenogenesis. Jacques Loeb has been
the most active worker in this line and his results are of ex-
treme interest. He has, by various physical or chemical treat-
ment of the unfertilized eggs of various animals, particularly
certain Echinoderms, worms and fishes, stimulated these eggs
to begin development, which development proceeds either nor-
mally or in some degree abnormally along the usual path reg-
ularly followed by the species. But in all cases this develop-
ment falls short of completion and in many cases the death of
the embryo occurs at a very early stage. Other investigators
have similarly induced a de-
velopment in parthenogenetic
eggs of animal species in
which parthenogenetic devel-
opment does not occur nat-
urally, or at least is very rare.
The significance of these
results is by no means wholly
clear. Nor do the investiga-
tors who have done the work
agree among themselves as to
the interpretation of the re-
suits. Loeb first inclined to
the belief that the stimuli
which incited the unfertilized
egg to development were

physical, OSmotiC Changes be-

ing looked On as perhaps the

i- , ,. , A,

immediate stimulus. At pres-

ent he Seems inclined tO at-

tribute the stimuli rather to

the chemical character of the media which seem to incite
the parthenogenetic development. In either case the physi-
cochemical stimulus is considered to be a substitute for the
spermatozoid. That it is a substitute in some degree, is obvi-
ous; that it is a complete substitute for it, seems equally
obviously not true. The embryos developed by artificial par-
theogenesis lack at least two fundamentally important attri-
butes which the young of bisexual parentage possess ; namely,
vigor and the heredity of the father. The lack of vigor is
shown by their death before maturity; and the chromosomes

FIG 168._Regeneration of the flatworm,

Planaria lugubris: A, shows by dotted
line where the worm was cut in two length-
wise ; B, C, D, show how a half that was

fed regenerated. Et Fi G< show how au

unfed half regenerated. (After Morgan.)

286

EVOLUTION AND ANIMAL LIFE

or other nuclear stuff that is the actual carrier of the paternal
heredity are of course actually wanting.

Another phenomenon or group of phenomena, also of much
special interest and suggestiveness to students of develop-
ment, to which the experimental method has been successfully
applied, is that known as "regeneration." The familiar repro-
duction or growth of
new plants from cut-
tings or buds is par-
alleled in the animal
world by numerous
similar cases less fa-
ff ur\ / miliar but neverthe-
less long known by
naturalists. In 1740,
Abbe Trembley made
a number of curious
experiments with Hy-
dra, whose publication
in 1744 was the begin-
ning of our knowledge
of the phenomena of
regeneration in ani-
mals. If Hydra, the
common little brown
or green fresh-water
polyp, be cut up into
many pieces, each of
these pieces has the
power to grow into a
new complete Hydra

body (Fig. 164). We know now that numerous other animals
have also this radical capacity for regeneration. Certain pro-
tozoans, hydroids, planarian worms, starfishes, etc., can re-
generate as freely or nearly so as Hydra (Figs. 165-172). And
many other animals representing almost all the great groups of
the animal kingdom possess in some degree, at least, the power
of regeneration. Some can regenerate only lost or cut append-
ages, others even less fundamental parts of the body; some
can regenerate only in their immature stages ; others only in
the earliest embryonic stages. But regeneration and "regula-

FIG. 169. — Regeneration of the tail and limbs of
the lizards, Lacerta agilis and Triton custatus :
A, Lacerta, new tail arising at place where old tail
was broken partly off; B, three-tailed form, two
tails having a common covering, all these parts
being regenerated after old tail was cut off; C,
Triton, additional leg produced by wounding femur;
D, double foot produced by tying thread over re-
generating stump; E, F, G, regenerated feet of
Triton after various mutilations. (After Tarnier. )

FACTORS IN ONTOGENY

287

tion," as certain phases
of regeneration are
called, are the property,
in some degree probably,
of most animals.

The significance of
this capacity has been
long recognized as of
much importance in our
conceptions of the germ
plasm character and dis-
position, but no general
agreement regarding it
has even yet been
reached by biologists.
More and better under-
stood facts about re-
generation are needed.
And this need it seems
to be the province of
experimental biology to
supply. By the carry-
ing on of ingeniously planned and carefully controlled series

of experiments with re-
generating animals, we
are acquiring a great
mass of important data,
and the interpretation
and generalization of
these data is certain to
be accomplished in the
near future.

We have space here
to call attention to but
one of the ways in which
an understanding of the
phenomena of regener-
ation will throw light on

FIG. 171. — Regeneration of the eye of Triton: One of the fundamental
A, Edge of iris with beginning lens; B, C, D, -,-, . , ,

later stages of same ;#, whole eve with regener- Problems in develop-

atinglens. (After Wolff and Fischel ) ment. To those biolo-

FIG. 170. — Regeneration of the flatworm, Planaria:
A, Specimen cut in two as far forward as eyes,
regenerating two half-heads ; B, cut in two at
one side of middle line, smaller piece having re-
generated a head ; C, cut partly in two, having
regenerated two heads in angle ; D, another
that produced only a single head in the angle.
(After Morgan.)

288

EVOLUTION AND ANIMAL LIFE

gists who believe with Weismann that there is a sharp distinc-
tion between the germ plasm and the somatic or body plasm,
and that this germ plasm is limited to the germ cells and
germ-cell producing tracts, the regeneration of a nearly whole
body or even a considerable part of a body from a region which
does not include a germ cell presents a serious obstacle. But

before this obstacle
can be considered as
one rendering the
germ plasm theory
absolutely untenable,
it is necessary to
prove what the re-
generated parts are
composed of. Are
they composed sim-
ply of repeated simi-
lar cells, all of one
tissue type, or do
they include other
kinds of cells or tis-
sues than those par-
ticular kinds from
which the regener-
ated part springs?
It is, of course, ad-
mitted that many,
indeed most cells of
the body, can repro-
duce other cells like themselves. Now is it a fact that regen-
erated parts are composed of different kinds of cells? As a
matter of fact this has been proved to be so by observation
and by experiment. Numerous instances are known in which
body cells arising originally from one germ layer have pro-
duced in the course of regeneration not only cells like them-
selves, but others which in normal development could only
arise from another germ layer. So it is plain that the study
of regeneration has already done much to modify our former
conceptions of the factors and conditions of development.

FIG. 172. — Regeneration of the blastula and gastrulse
of sea urchins; line indicates where the blastula or
gastrula was cut in half; the smaller figures show re-
sults of the regeneration of the two halves of each.

CHAPTER XIV
PALEONTOLOGY

This much then we have gained, that we may assert without
hesitation, that all the more perfect organic natures, such as fishes,
amphibious animals, birds, mammals, and man at the head of the list
were all formed upon one original type which varies only more or less
in parts which are none the less permanent, and which still daily
changes and modifies its form by propagation. — Goethe (1796).

IN a suggestive sentence, Haeckel speaks of our knowledge
of the line of descent in the history of any group of animals or
plants as being derived from "three ancestral documents —
morphology, embryology, and paleontology."

Of these three, paleontology is at once the most certain and
the most incomplete. Each fossil animal is a record, absolutely
authentic, so far as it goes, admitting of no doubt or question,
but for the most part yielding only a very little of the truth
involved in its existence.

For no animal whatever is preserved as a fossil except as
the result of an unusual combination of circumstances. Only
those parts which are themselves hard, calcareous, silicious, or
horny, with rare exceptions, can retain their form in the rocks,
and even these, shells, teeth, bones, and the like, are often
crushed or distorted so that their actual form or nature may
be open to question. In addition, only the minutest fraction
of the sedimentary rocks of the earth has been laid bare by
artificial excavation or by natural erosion, and thus opened
to the inspection of man, and the number of fossils actually
observed can be only the most trivial fraction of a fraction of
the organisms actually existing and preserved.

With all this, the human race has in the past shown a
singular lack of insight in the interpretation of animal remains

289

290 EVOLUTION AND ANIMAL LIFE

found in the stone. As Lyell has graphically shown, it took
one hundred and fifty years of dispute and argument to persuade
even learned men that shells and teeth in the rocks were actual
remains of actual animals, and another hundred and fifty years
to demonstrate that the shell-bearing rocks were not masses
of debris from Noah's flood. Nothing in the history of science
is more tedious than the arguments directed against the first
students of fossils, to show that these structures were mere
sports of nature, whimsicalities of creation, or freaks developed
in the fatty matter (matevia pinguis) of the earth by the en-
tangling influence of the revolving stars.

Notwithstanding all these defects in material, and this
stupidity of theory, the study of fossils has still gone on, and
by its means we are able to delineate with large certainty the
line of evolution of most groups of animals, and the nature of
faunal relations in the different periods of geological time.
If we had notv already a theory of evolution by derivation of
forms, we should be obliged to invent one in face of the facts of
paleontology. In Huxley's words, "fossils are only animals
and plants which have been dead rather longer than those
which died yesterday."

Fossils are either actual remains of bones or other parts
preserved intact in soil or rocks, or else, and more commonly,
parts of the animals which have been turned into stone, or of
which stony casts have been made. All such remains buried
by natural causes are called fossils. The process by which
they are sometimes changed from animal substance into stone
is called petrifaction.

Fossils may be of three kinds. In the case of recently
extinct animals, bones or other parts of the body may become
buried in the soil and lie there for a long time without any
change of organic into inorganic matter. Thus fossil insects
are found with the bodies preserved intact in amber, a fossil
resin from some ancient and extinct pine tree. Over eight
hundred species of extinct insects are known from amber
fossils. The bones of the earliest members of the elephant
family, the teeth of extinct sharks, the shells of extinct mollusks
and fragments of buried logs, are also often found intact, still
composed of their original matter.

In the second kind of fossils the original or organic matter
is gone, the organic form and organic structure being preserved

PALEONTOLOGY

291

FIG. 173. — Remains of Dimorphodon from the Lias of Lyme Rej?is. showing skull, neck,
and back, and some of the bones of the skeleton, (After Seeley, from a slab in the
British Museum.)

292 EVOLUTION AND ANIMAL LIFE

in mineral matter. That is, the organic matter has been
slowly and exactly replaced by mineral. As each particle of
organic substance passed away by decay, its place was taken
by a particle of mineral matter. Such fossils are called petri-
factions. This is beautifully shown in the case of petrified
wood. We can cut and grind thin a bit of petrified wood, and
see in it, with a microscope, the exact details of its original
fine cellular structure. This substituted mineral matter may
be one of several minerals, but usually it is silica (quartz) or
carbonate of lime (limestone) or sulphide of iron (iron pyrites) .
In the case of animal parts which were originally partly organic
and partly inorganic, as bones and teeth and shells, often only
the organic matter is replaced by the petrifying mineral,
although sometimes the old inorganic matter is also replaced.
Finally, sometimes the organic matter and organic structure
are both lost, only the original outline of form of the whole
part being retained. This occurs when the organic matter
imbedded in mud and clay decays away, leaving a hollow
which is filled up by some mineral different from the matrix.
In this case the fossil is simply a cast of the original organic
remains.

Some traces even of the finest organisms occasionally
appear.

"Conditions have sometimes permitted even the most delicate
structures, such as insects' wings and the impressions of jellyfishes
to become retained in the soft mud, which afterwards became solidi-
fied. Localities famous the world over for the beauty and delicacy
of their fossil remains are the lithographic stone quarries of Bavaria
and certain beds in France " (EASTMAN).

These deposits were perhaps formed in the clear, quiet waters
of a coral lagoon.

Examination and study of the rocks of the earth reveal the
fact that fossils, or the remains of animals and plants, are
found in certain kinds of rocks only. They are not found in
lava, because lava comes from volcanoes and rifts in the earth's
crust, as a red-hot, viscous liquid, which cools to form a hard
rock. No animal or plant caught in a lava stream will leave
any trace. Furthermore, fossils are not found in granite, nor
in ores of metals, nor in certain other of the common rocks.

PALEONTOLOGY 293

Many rocks are, like lava, of igneous origin; others, like
granite, although not originally in melted condition, have been
so heated subsequent to their formation, that any traces of
animal or plant remains in them have been obliterated. Fossils
are found almost exclusively in rocks which have been formed
by the slow deposition in water of sand, clay, mud, or lime.
The sediment which is carried into a lake or ocean by the
streams opening into it sinks slowly to the bottom of the lake
or ocean and forms there a layer which gradually hardens under
pressure to become rock. This is called sedimentary rock, or
stratified rock, because it is composed of sediment, and sedi-
ment always arranges itself in layers or strata. In sedimentary

FIG. 174. — Restoration of the skeleton of Dimorphodon macronyx. (After Seeley.)

or stratified rocks fossils are found. The commonest rocks of
this sort are limestone, sandstone, and shales. Limestone is
formed chiefly of carbonate of lime; sandstone is cemented
sand; and shales, or slaty rocks, are formed chiefly of clay.

The formation of sedimentary rocks has been going on since
land first rose from the level of the sea; for water has always
been wearing away rock and carrying it as sediment into rivers,
and rivers have always been carrying the worn-off lime and
sand and clay downward to lakes and oceans, at the bottoms
of which the particles have been piled up in layers and have
formed new rock strata. But geologists have shown that in
the course of the earth's history there have been great changes
in the position and extent of land and sea. Sea bottoms have
been folded or upheaved to form dry lancl, while regions, once
land, have sunk and been covered by lakes and seas. Again,
through great foldings in the cooling crust of the earth, which
20

294 EVOLUTION AND ANIMAL LIFE

resulted in depression at one point and elevation at another,
land has become ocean and ocean land. And, in the almost
unimaginable period of time which has passed since the earth
first shrank from its hypothetical condition of nebulous vapor
to be a ball of land covered with water, such changes have
occurred over and over again. .They have, however, mostly
taken place slowly and gradually. The principal seat of great

FIG. 175. — Restoration of the skeleton in probable normal position of Dimorphodon
macronyx. (After Seeley.)

change is in the regions of mountain chains, which, in most
cases, are simply the remains of old folds or wrinkles in the
crust of the earth.

When an aquatic animal dies, it sinks to the bottom of the
lake or ocean, unless, of course, its flesh is eaten by some other
animal. Even then its hard parts will probably find their
way to the bottom. There the remains will soon be covered
by the always dropping sediment. They are on the way to
become fossils. Some land animals also might, after death,
get carried by a river to the lake or ocean, and find their way
to the bottom, where they, too, will become fossils, or they
may die on the banks of the lake or ocean and their bodies may
get buried in the soft mud of the shores. Or, again, they are
often trodden in the mire about salt springs or submerged in
quicksands. It is obvious that aquatic animals are far more
likely to be preserved as fossils than land animals. This

PALEONTOLOGY 295

inference is strikingly proved by fossil remains. Of all the
thousands and thousands of kinds of extinct insects, mostly
land animals, comparatively few specimens are known as fossils.
On the other hand, the shell-bearing mollusks and crustaceans
are represented in almost all rock deposits which contain any
kind of fossil remains.

It is obvious that any portion of the earth's surface covered
by stratified rocks must have been at some time under water,
the bottom of a lake or ocean. If now this portion shows a
series of layers or strata of different kinds of sedimentary rocks,
it is evident that it must have been under water several times,
or at least under different conditions. It is also evident that
fossils found in this portion of the earth will contain remains
of only those animals which were living at the various times
this portion of the earth was under water. Of the animals
which lived on it when it was land there will be no trace,
except, possibly, a few land or fresh-water forms, which might
be swept into the sea or might be preserved in the mud of ponds.

FIG. 176. — Restoration of Dimorphodon macronyx. (After Seeley.)

That is, instead of finding in the stratified rocks of any portion
of the earth remains of all the animals which have lived on that
portion since the earth began, we shall find, at best, only re-
mains of a few kinds of those animals which have lived on this
portion of the earth when it was covered by the ocean or by a
great lake.

Thus, the great body of fossil remains of animals reveal only
a broken and incomplete history of the animal life of the past.
But the record; so far as it goes, is an absolutely truthful one,

296 EVOLUTION AND ANIMAL LIFE

and when the many deposits of fossils in all parts of the different
continents are examined and compared, it is possible to state
numerous general truths in regard to past life and the suc-
cession of animals in time. The science of extinct life is known
as paleontology.

The study of paleontology has revealed much of the history
of the earth and its inhabitants from the first rise of the land
from the sea till the present era. This whole stretch of time
—how long no one can guess — is divided into eras or ages ; these
ages usually into lesser divisions called periods, and the periods
into shorter lengths of time called epochs. Each epoch is
more or less sharply distinguished from every other by the
different species of animals and plants which lived while its
rocks were being deposited. In the earth's crust, where it has
not been distorted by foldings and breaks, the oldest stratified
rocks lie at the bottom of the series, and the newest at the top.
The fossils found in the lowest or oldest rocks represent, there-
fore, the oldest or earliest animals, those in the upper or newest
rocks the newest or latest animals.

An examination of a whole series of strata and -their fossils
shows that what we call the most specialized or most highly
organized animals did not exist in the earliest epochs of the
earth's history, but that the animals of these epochs were all
of the simpler or lower kinds. For example, in the earlier
stratified rocks there are no fossil remains of the backboned
or vertebrate animals. When the vertebrates do appear,
through several geological epochs they are fishes only, members
of the lowest group of backboned animals. More than this,
they represent generalized types of fishes which lack many of
the special adaptations to marine life that modern fishes show.
For this reason they bear a greater resemblance to the earlier
reptiles than do the fishes of to-day; in other words, they
were a generalized type, showing the beginnings of characters
of their own and other types. It is always through general-
ized types that great classes of animals approach each other.

In a later epoch the batrachians or amphibians appeared;
in a still later period, the reptiles; and last of all, the birds and
the mammals, the last being the highest of the backboned
animals. The following table gives the names and succession
of the various geological periods, and indicates briefly some
of the kinds of animals living in each. In each of these di-

PALEONTOLOGY

297

ERAS OR
PERIODS.

AGES OR SYSTEMS.

ANIMALS ESPECIALLY CHARACTERISTIC
OF THE ERA OR AGE.

Cenozoic.
Era of
Mammals.

Quaternary or Pleis-
tocene (age of man
and insects)

Tertiary : Pliocene,
Miocene, Eocene. . . .

Man; mammals, mostly of spe-
cies still living.

Mammals abundant; belonging to
numerous extinct families and
orders.

Mesozoic.
Era of
Reptiles.

Cretaceous

Birdlike reptiles; flying reptiles;
toothed birds; first snakes; bony
fishes abound; sharks again
numerous.
First birds; giant reptiles; ammo-
nites; clams and snails abun-
dant.
First mammals (a marsupial);
sharks reduced to few forms;
bony fishes appear.

Jurassic . .

Triassic

Paleozoic.
Era of
Invertebrates.

Carboniferous (age of
amphibians)

Devonian (age of
fishes)

{Earliest of true reptiles. Am-
phibians; lung fishes; fringe
fins; first crayfishes; insects
abundant; spiders; fresh-wa-
ter mussels.
{First amphibian (froglike ani-
mals); sharks; ostracophores;
first land shells (snails); mol-
lusks abundant; first crabs.
First truly terrestrial or air-
breathing animals; first in-
sects; corals abundant; mailed
fishes,
f First known fishes, ostraco-
phores, mailed and with carti-
laginous skeleton; brachio-
l pods; trilobites, mollusks, etc.
Invertebrates only.

Silurian (age of inver-
tebrates)

Ordovician or Lower
Silurian .

Cambrian

Archean.

Algonkian. Lauren-
tian

Simple marine invertebrates.

visions of geological time some one class of animals was espe-
cially numerous in species, and was evidently the dominant
group of animals through that period. The different ages are
therefore spoken of in terms of the prevailing life. Thus, the
"Silurian Age" is known as the age or era of invertebrates;
the " Devonian/' as the age of fishes. In the same way we
have the "Reptilian Age/' the "Mammalian Age/' according

298 EVOLUTION AND ANIMAL LIFE

to the great class of animals predominating at that time. Of
course, in each of the later epochs there lived animals represent-
ing the principal classes or groups in all of the preceding ones,
as well as the animals of that particular group which may
have first appeared in this epoch, or was its dominant group

FIG. 177. — Restoration of Dimorphodon macronyx, showing probable wings.
(After Seeley.)

In the study of fossils not only is it necessary for us to
consider the actual forms and structures and the s.pecies they
represent, but we should so far as possible reconstruct the con-
ditions under which the organisms were alive, and the threads
of genealogy which connect those of one period with those
which precede or follow them. By such studies as these we are
brought close to a consideration of the method of creation, and
to a knowledge not only of the origin of species but to the
causes underlying the divergence of the great trunks of animal
and plant life.

"In youth," says Dr. A. S. Packard, "the older naturalists of the
present generation were taught the doctrine of creation by sudden,
cataclysmal, mechanical creative acts, and those to whose lot it fell
to come into contact with the ultimate facts and principles of the new
biology had to unlearn this view, and gradually to work out a larger,
more profound, wider reaching and more philosophic conception of
creation."

An early paleontologist, Dr. A. Gaudry, utters these sug-
gestive words:

"We cannot refrain from looking with curious admiration upon
the innumerable creatures that have become preserved to us from

PALEONTOLOGY 299

the earth's early days and calling them to life again, and in our imagina-
tion we ask these ancient inhabitants of the earth whence they were
derived : 'Speak to us and say whether you are isolated remnants
disseminated here and there throughout the immensity of the ages,
without order more comprehensible to us than the scattering of flowers
over the prairie? Or are you in verity linked one to another so that
we may yet be able amid the diversity of nature to discover the in-
dications of a plan wherein the Infinite has stamped the impression of
His unity? ' The unraveling of the plan of creation — this is the goal
to which our efforts now aspire. Whatever our theories as to it,"
Gaudry continues, "there is a plan. A day will come when the
paleontologists will seize the plan which has presided over the de-
velopment of life.''

This plan is found in the phenomena of organic evolution,
the interrelation of the different factors or forces of heredity,
variation, adaptation, fecundity, with the conditions of isola-
tion of forms and the relations of environment. In the study
of these details, we receive great light from the investigation
of comparative structure, and the forces and processes of
individual development. These are Haeckel's ancestral docu-
ments of morphology and embryology, but all theory finds its
final verification in its accord with the facts of paleontology,
the recorded evidence of succession in time.

Among the general deductions from paleontology are the
following :

The various primary groups or branches of the animal
kingdom as well as the principal classes are all very old, most
of them, the vertebrates excepted, appearing in the earliest
known fossiliferous rocks. It is, however, evident that these
rocks, Lower Silurian or Ordovician and Cambrian, are very
far from the actual beginning of life.

In each group the earliest forms arc relatively simple,
unspecialized, and as a rule marine. Many of them are em-
bryonic types, that is, forms morphologically comparable to
the embryos of forms of later appearance. To such forms,
the less appropriate term of "prophetic types" has been ap-
plied. Many of the earlier forms are of synthetic types,
that is, embracing characters distinctive of different diver-
gent groups. Such synthetic types, where the resemblances
are shown to be indicative .of real homology, are now re-

300 EVOLUTION AND ANIMAL LIFE

garded as indicative of the actual ancestry, from which the
later types have diverged.

The persistence of heredity is the basis of the parallelism
between geological and embryonic series. By its influence
ancestral traits are repeated in the embryo, even though the
characters thus produced give way in later development to
further specialization or growth along other lines. This great
truth has been stated in these words: "The life history of the
individual is an epitome of the life history of the group to
which it belongs." This statement is only true when stated
very broadly, for there are many exceptions or modifications.
The embryonic or larval animal is subject to almost endless
secondary changes and adaptations whenever these changes
are for the advantage of the animal. In general, the simpler
the structure of the animal and the less varied its relations in
life, the more perfectly are these ancient phases of heredity
preserved in the process of development. In such case, the
more perfect is the parallelism between the development of
the individual and the succession of forms in geologic time.

It is not always true that the recent representatives of a
group are higher in a morphological sense than some or all of
the earlier members. They are, however, in all cases farther
from the original or parent stock. In many groups there is a
progress, seemingly rapid, toward a high degree of specializa-
tion followed by the disappearance of the highly organized
types, while forms of low development — sometimes even those
of primitive character — may remain in abundance. The evolu-
tion of the group of Brachiopods is an illustration of this. The
group is represented in the Lower Silurian by numerous genera
of simple structure, as Lingula, Terebratula and the like. It
culminates in the Carboniferous age with complex genera as
Spirifer, Products is, Orthis, while the modern representatives
Lingulella, Terebratulina, Waldheimia,, etc., are little more ad-
vanced than the primitive forms. Similar phases have charac-
terized the appearance, culmination, and relative extinction
of the trilobites, the crinoids, the ammonites, and other groups.
The total extinction of any large group has not usually taken
place. Usually a few species have remained, thus giving us a
better clew to the life history and development of the group
than we should otherwise possess.

One feature shown in many groups of extinct animals has

PALEONTOLOGY 301

never received accurate definition or interpretation. The
group may appear in a series of relatively simple forms, showing
affinities with some type from which it may have diverged.
These early genera will be succeeded in the rocks by others,
arranged progressively so as to form a series apparently mov-
ing in a certain direction. Each genus successively following
in time, will perhaps show a greater and greater emphasis on
some one group of characters, a greater and greater specializa-
tion in some one direction. Arranging the genera in series, it
looks as if there were a definite line of variation shown in their
gradual succession. These phenomena have been shown in
various groups of reptiles and fishes, and especially well in the
evolution of the extinct order of ammonites. These animals,
allied to the living nautilus, lived in coiled chambered shells
which gradually assumed great complication of form and orna-
mentation. The extreme of this course of evolution was fol-
lowed by corresponding progressive degeneration. In some
cases, this condition continues to the present time. More
frequently, the specialization along the original lines continues
to a certain point, to be followed by the progressive degenera-
tion and perhaps the ultimate loss of the very same structures
in which the high degree of specialization has prevailed.

To phenomena of this kind, the term determinate varia-
tion or orthogenesis has been applied. This phrase seems to
involve the theory that the evolution has gone forward toward
some predetermined end, or that in some way only variations
leading toward this end have existed or at least have been
able to maintain themselves. It is possible, however, that the
cause may be found in the influence of some phase of environ-
ment, which directs the course of natural selection continuously
along a certain line. A reversal of selection would be naturally
followed by a degeneration of the structures developed to a
point beyond the need of the animal.

It is plain that much is to be learned, especially in regard to
the relationships existing among living animals, by a study of
those of the past. A comparison of certain of the ancient
reptiles with the long-tailed Archccopteryx (Fig. 178) and other
toothed birds shows that the birds and reptiles were once
scarcely distinguishable, although now so very different. Birds
have feathers, reptiles do not; but there is scarcely any other
permanent difference. Fossils show a similar close relation

302 EVOLUTION AND ANIMAL LIFE

between amphibians and fishes. A study of these ancient
forms also throws light on many conditions of structure in
modern animals, otherwise difficult to understand. An exam-
ple of this sort is found in the splint bones of the modern
horse (see Fig. 179).

It is a fact unquestionable that a species will change on its
own grounds little by little with the lapse of time and the slow
alteration of conditions of selection. Nations change, languages
change, customs change, nothing is secure against the tooth of

FIG. 178. — Ancient bird with jointed tail, claws on wings, and teeth in jaws, Archcrop-
teryx lithographica, from the Jurassic rocks of Bavaria. (After Nicholson from
Owen.)

time. This is in general true, because with time, alteration of
environment takes place, events happen, there is an alteration
of the stress of life and with this alteration all life may be acted
upon.

That time-mutations in all forms of life do take place is
beyond question, and some have regarded these slow changes
as the chief agency in the formation of species. But the
current of life does not flow in straight lines nor in an even
current. Species are torn apart by obstacles, as streams are
divided by rocks, and the rapidity of their formation is pro-
portioned to the size of the obstacle and the alternations it
produces in the flow of life.

We have some basis for the estimate of the duration of a
species. When the great glacial Lake Bonneville occupied

PALEONTOLOGY

303

a

the basin of the Great Salt Lake, the same species of fishes
and insects were found in all its tributaries. Now that these
streams flow separately into a lifeless lake, the same species
of fishes occur in them for the most part without alteration.
One species of sucker (Catostomus ardens) and one chub (Leucis-
cus lineatus) are found unaltered throughout this region and
in the Upper Snake River (above Shoshone Falls), into which
Lake Bonneville was once drained. Other species are left
locally isolated, but one species
only (Agosia adobe), a small
minnow of the clay bottoms,
can be shown to have under-
gone any alteration. But with
the tiger beetles (Cicindelce)
a large number of species
have been produced by sepa-
ration.

From the Bay of Panama
374 species of fishes are re-
corded in the recent mono-
graph of Gilbert and Starks.
Of these species, 204 are re-
corded also from the Gulf of
California, while perhaps fifty
others are represented in the
more northern bay by closely
related forms. Comparing the
fish faunas separated by the
isthmus, we find the closest
relation possible so far as
families and genera are con-
cerned. In this respect the resemblance is far closer than
that between Panama and Chile, or Panama and Tahiti, or
Panama and southern California. On the Atlantic side, simi-
lar conditions obtain, although the number of genera and
species is far greater (about 1,200 species) in the West Indies
than at Panama. This fact accords with the much larger ex-
tent of the West Indies, its varied groups of islands isolated
by deep channels, and its near connection to the faunas of
Brazil and the United States.

But it is also noteworthy that while the families of fishes

FIG. 179. — Diagrams showing the series
of changes in geological time from a
horse's foot of four separate toes (/) to
one of one toe and a pair of splint
bones (a); a-f represent the feet of dif-
ferent horselike animals from modern
time backward.

304 EVOLUTION AND ANIMAL LIFE

are almost identical on the two shores of the isthmus of Panama,
and the great majority of the genera also, yet the species are
almost wholly different.

Taking the enumeration of Gilbert and Starks, we find that
out of 374 species, 43 are found apparently unchanged on both
sides of the isthmus ; 265 are represented on the Atlantic side by
closely related species — in most cases the nearest known relative
of the Pacific species — while 64 have no near analogue in the
Atlantic. Of the latter group, some find their nearest relative
to the northward or southward along the coast, and still others
in the islands of Polynesia.

The almost unanimous opinion of recent students of the
isthmus faunas finds expression in the following words of Gilbert
and Starks (" Fishes of Panama Bay," p. 205) :

"The ichthyological evidence is overwhelmingly in favor of a
former open communication between the two oceans, which must have
become closed at a period sufficiently remote from the present to
have permitted the specific differentiation of a very large majority of
the forms involved. That this differentiation progressed at widely
varying rates in different instances, becomes at once apparent. A
small minority (43) of the species (11 per cent of the species found on
the Pacific side; about 2.5 of the combined fauna) remain wholly
unchanged so far as we have been able to determine that point. A
larger number have become distinguished from their representatives
of the opposite coast by minute, but not 'trivial' differences, which
are wholly constant. From such representative forms we pass by
imperceptible gradation to species much more widely separated, whose
immediate relation in the past we cannot confidently affirm. . . .

"It is obvious, however, that the striking resemblances between
the two faunas are shown as well by slightly divergent as well as by
identical species, and the evidence in favor of interoceanic connection
is not weakened by an increase in the one list at the expense of the
other. All evidence concurs in fixing the date of that connection at
some time prior to the Pleistocene, probably in the early Miocene.
When geological data shall be adequate definitely to determine that
date, it will give us the best known measure of the rate of evolution in
fishes."

From this discussion, it is probable that even in isolation
some species change very slowly, that with similar conditions

PALEONTOLOGY 305

the changes within isolated groups of a species may be parallel,
and that the specific changes in different groups may progress
with very different degrees of velocity.

The earliest known vertebrate remains are found in rocks of
the Ordovician age, approximately of the epoch known as
Trenton, at Canon City, in Colorado. These remains consist
of broken bits of bony shields of mailed fishes or fishlike forms
known as Ostracophores. With these are fragments of scales,
which seem to belong to more specialized forms. It is evident
that these remains, as well as the remains of sharks which

FIG. 180. — An ostracoderm, Pterichyodes milleri, from the lower Devonian of Scotland.
The jointed appendage on the head is not a limb. (After Traquair.)

appear later in the Upper Silurian, by no means reveal the
actual first existence of vertebrates.

The sharks which appear in the Upper Silurian, although
certainly primitive, even as compared with later sharks, are
very far from the simplest even of known vertebrates. There
seems to be good reason for the view that the vertebrate type of
animal, with the nervous cord along the back and the alimentary
canal marked by gill slits, was at first soft-bodied and worm-
like, in fact, derived from a wormlike ancestry, and that, prior
to the Ordovician and Silurian time, it was devoid of hard
parts. The early sharks have teeth, and rough skin, fins, and
sometimes fin spines, all susceptible of preservation in the
rocks, even though the skeleton was soft and cartilaginous.
The Ostracophores, some of which, at least, seem to be modified
sharks, had no internal hard parts, but were protected by an
external coat of mail, perhaps formed of coalescent prickles or
scales.

From the sharks were doubtless descended the group of
Fringe-fins or Crossopterygians, which were more distinctly
fishlike. From these, on the one hand by continuous speciali-

306

EVOLUTION AND ANIMAL LIFE

zation for aquatic life, the true fishes must have been derived.
In the more primitive of these the air bladder retains the lung-
like structure characteristic of the Fringe-fins. But in the
more specialized forms this is reduced to a sac, at first with an
open tube, then to a closed sac without tube in the adult, and
finally in very many of the true fishes the air sac is altogether
lost. On the other hand, in the Amphibia, which were prob-
ably also derived from the Crossopterygia, the air bladder is
more highly specialized, fitting these animals for life outside

the water, and the
fins give place to
fingers and toes as
befitting a terres-
trial habit.

The amphibians
deposit their eggs in
damp places, and
the young are
hatched while the
external' gills are
still functional.

Among the rep-
tiles, which mark the next stage of adaptation for terrestrial
life, the gills are absorbed before the animal leaves the egg.
The reptile is therefore no longer confined to the neighbor-
hood of the water for purposes of reproduction.

The bird, derived from the reptile, and at first distin-
guishable solely by the possession of feathers, loses later
various reptilian traits and the group becomes one inhabit-
ing the air.

From the reptiles again are derived the lowest mammals.
The Monotremes of Australia lay eggs as reptiles do, these, like
reptiles' eggs, being covered with a leathery skin. The higher
mammals hatch the eggs within the body, nourish them with
milk and, in general, care for them in a degree unknown within
the class of reptiles. The traits of external hair, warm blood,
double circulation of the blood from and to a two-chambered
heart, and other characters of the mammals become fixed with
time and the group diverges into a multitude of forms living
and extinct, the last, and on the whole the most specialized of
the series being Homo, the genus of man.

FIG. 181.— The flying dragon (Draco). (After Seeley.)

PALEONTOLOGY

307

The first traces of man appear in the later geologic times
after the end of the Tertiary. Human bones have been found
in caves together with those of the cave-lion, cave-bear, and
other extinct animals. In certain lakes in Switzerland and
Austria have been found remains of peculiar dwellings, to-
gether with ancient fishing hooks and a variety of imple-
ments of stone and bronze. These houses were built on piles
in the lakes, and connected with the shore by piers or bridges.
The extinct race of men who lived in them is known as
Lake-dwellers. Relics of man, especially rough stone tools
and flint arrow and axe heads, and skulls and other bones,

FIG. 182. — Rough drawing of a mammoth on its own ivory, by a contemporary man.

(After Le Conte.)

have been found under circumstances which indicate with
certainty that man has existed long on the earth. But with
these relics very few bones are found. This has been ac-
counted for by supposing that man existed in a few wander-
ing tribes scattered widely over Europe. In Java are found
some ancient bones of manlike animals (Pithecanthropus), dif-
ferent, however, from any species or race of men living to-day,
and showing traits which indicate a close relationship with the
anthropoid apes.

The time of historic man — i. e., the period which has elapsed
since the history of man can be traced from carvings or buildings
or writings made by himself — is short indeed compared with
that of prehistoric man. Barbarous man writes no history
and leaves no record save his tools and his bones. Iron and
bronze rust, bones decay, wood disappears. Only stone im-

308 EVOLUTION AND ANIMAL LIFE

plements remain to tell the tale of primitive humanity. These
give no exact record of chronology.

So of the actual duration of man's prehistoric existence we
can make no estimate. Speaking in terms of the earth's
history, man is very recent, the latest of all the animals. In
terms of the history of man, he is very ancient. The exact
records of human history cover only the smallest fraction of
the period of man's existence on earth.

CHAPTER XV
GEOGRAPHICAL DISTRIBUTION

Is not the biological laboratory which leaves out the ocean and the
mountains and meadows a monstrous absurdity? Was not the greatest
scientific generalization of your times reached independently by two
men who were eminent in their familiarity with living beings in their
homes? — HUXLEY.

UNDER the head of "Geographical Distribution" we con-
sider the facts of the diffusion of organisms over the surface
of the earth, and the laws by which this diffusion is governed.

The geographical distribution of animals is often known as
"zoogeography." In physical geography we may prepare
maps of the earth which shall bring into prominence the
physical features of its surface. Such maps would show here
a sea, here a plateau, here a range of mountains, there a desert,
a prairie, a peninsula, or an island. In political geography the
maps show the physical features of the earth, as related to the
states or powers which claim the allegiance of the people. In
zoogeography the realms of the earth are considered in relation
to the types or species of animals which inhabit them.

Thus a series of maps of the United States could be drawn
which would show the gradual disappearance of the buffalo
before the attacks of man. Another might be drawn which
would show the present or past distribution of the polar bear,
black bear, and grizzly. Still another might show the original
range of the wild -hares or rabbits of the United States, the
white rabbit of the Northeast, the cottontail of the East and
South, the jack rabbit of the plains, the snowshoe rabbit of
the Columbia River, the tall jack rabbit of California, the marsh-
hare of the South and the waterhare of the canebrakes, and
that of all their relatives. Such a map is very instructive, and
21 309

210

EVOLUTION AND ANIMAL LIFE

FIG. 183. — Map showing the distribution of the Canadian Skipper butterfly, Erynnis
manitoba, in the United States. The butterfly is found in that part of the country
shown in the map. This butterfly is subarctic and subalpine in distribution being
found only far north or on high mountains, the two southern projecting parts of
its range being in the Rocky Mountains and in the Sierra Nevada Mountains. (After
Scudder.)

FTG. 184. — Map showing the distribution of the Clouded Skipper butterfly, Lerema
accius, in the United States. The butterfly is found in those parts of the cpuntry
shown in the map by the shading marks — the warm, moist Southern and Eastern
parts. (After Scudder.)

GEOGRAPHICAL DISTRIBUTION 311

it at once raises a series of questions as to the reasons for each
of the facts in geographical distribution, for it is the duty of
science to suppose that none of these facts is arbitrary or mean-
ingless. Each fact has some good cause behind it.

It was this phase of the subject, the relation of species to
geography, which first attracted the attention of both Darwin
and Wallace. Both these observers noticed that island life
is neither strictly like nor unlike the life of the nearest land,
and that the degree of difference varies with the degree of
isolation. Both were led fromThis fact to the theory of deriva-
tion, and to lay the greatest stress on the progressive modifica-
tion resulting from tl\e_struggle for existence.

In the voyage of the Beagle Darwin was brought in contact
with the singular fauna of the Galapagos Islands, that cluster
of volcanic rocks which lies in the open sea about six hundred
miles west of the coasts of Ecuador and Peru. The sea birds
of these islands are essentially the same as those of tne coast of
Peru. So with most of the fishes. We can see how this might
well be, for both sea birds and fishes can readily pass from the
one region to the other. But the land birds, as well as the
reptiles, insects, and plants, are largely peculiar to the islands.
Many of these species are found nowhere else. But other
species very much like them in all respects are found, and these
live along the coast of Peru. In the Galapagos Islands, ac-
cording to Darwin's notes,

"there are twenty-six land birds; of these, twenty-one, or perhaps
twenty-three, are ranked as distinct species, and would be commonly
assumed to have been here created; yet the close affinity of the most
of these birds to American species is manifest in every character, in
their habits, gestures, and tones of voice. So it is with the other
animals and with a large proportion of the plants.

"... The naturalist, looking at the inhabitants of these vol-
canic islands in the Pacific, feels that he is standing on American land."

This question naturally arises: If these species have been
created as we find them on the Galapagos, why is it that they
should all be very similar in type to other animals, living under
wholly different conditions, but on the coast not far away?
And, again, why are the animals and plants of another cluster
of volcanic islands — the Cape Verde Islands — similarly related

312 EVOLUTION AND ANIMAL LIFE

to those of the neighboring coast of Africa, and wholly unlike
those of the Galapagos? If the animals were created to match
their conditions of life, then those of the Galapagos should be
like those of Cape Verde, the two archipelagoes being extremely
alike in soil, climate, and physical surroundings. If the species
on the islands are products of separate acts of creation, what is
there in the nearness of the coasts of Africa or Peru to influence
the act of creation so as to cause the island species to be, as it
were, echoes of those on shore?

If, on the other hand, we should adopt the obvious sug-
gestion that both these clusters of islands have been colonized
by immigrants from the mainland, the fact of uniformity of
type is accounted for, but what of the difference of species?
If the change of conditions from continent to island causes
such great and permanent changes as to form new species from
the old, why may not like changes take place on the mainlands
as well as on the islands? And if possible on the mainland of
South America, what evidence have we that species are perma-
nent anywhere? May they not be constantly changing? May
what we now consider as distinct species.be only -the present
phase in the changing history of the series of forms which con-
stitute the species?

The studies of island life can lead but to one conclusion:
These volcanic islands rose from the sea destitute of land life.
They were settled by the waifs of wind and of storm, birds
blown from the shore by trade winds, lizards and insects carried
on drift logs and floating vegetation. Of these waifs few came
perhaps in any one year, and few, perhaps, of those who came
made the islands their home; yet, as the centuries passed on,
suitable inhabitants were found. That this is not fancy we
know, for we have the knowledge of specific examples of the
very same sort. We know how many animals are carried from
their natural homes. One example of this may be seen by
those who have approached our eastern shores by sea in the
face of a storm. Many land birds — sparrows, warblers, chick-
adees, and even woodpeckers — are carried out by the wind,
a few falling exhausted on the decks of ships, a few others fall-
ing on offshore islands, like the Bermudas, the remainder
drowning in the sea.

Of the immigrants to the Galapagos the majority doubtless
die and leave no sign. A few remain, multiply, and take

GEOGRAPHICAL DISTRIBUTION

313

314 EVOLUTION AND ANIMAL LIFE

possession, and their descendants are thus native to the
islands, f But, isolated from the great mass of their species
and bred under new surroundings, these island birds come to
differ from their parents, and still more from the great mass
of the land species of which their ancestors were members.
Separated from these, their individuality would manifest itself.
They would assume with new environment new friends, new
foes, new conditions. They would develop qualities peculiar
to themselves — qualities intensified by isolation. Local pecu-
liarities disappear "with wicTe" association7~ancl are intensified
when individuals of similar peculiarities are kept together.
Should later migrations of the original species come to the
islands, the individuals surviving would in time form new
species, or, more likely, mixing with the mass of those already
arrived, their special characters would be lost in those of the
majority. ^

The Galapagos, first studied by Darwin, serve to us only as
an illustration. The same problems come up, in one guise or
another, in all questions of geographical distribution, whether
on continent or island. ( The relation of the fauna of one region
to that of another depends on the ease with which barriers may
be crossed. Distinctness is in direct proportion to isolation.
What is true in this regard of the fauna of any region as a whole,
is likewise true of any of its individual species. The degree of
resemblance among individuals is in direct proportion to the
freedom of their movements, and variations within what we
call specific limits is again proportionate to the barriers which
prevent equal and perfect diffusion.

The laws governing the distribution of animals are reducible
to three very simple propositions. Every species of animal is
found in every part of the earth having conditions suitable for
its maintenance unless:

(a) Its individuals have been unable to reach this region,
through barriers of some sort ; or,

(b) Having reached it, the species is unable to maintain
itself, through lack of capacity for adaptation, through severity
of competition with other forms, or through destructive con-
dition of environment ; or,

(c) Having entered and maintained itself, it has become
so altered in the process of adaptation as to become a species
distinct from the original type.

GEOGRAPHICAL DISTRIBUTION 315

As examples of the first class we may take the absence of
kingbirds or meadow larks or coyotes in Europe, the absence
of the lion and tiger in South America, the absence of the civet
cat in New York, and that of the bobolink or the Chinese fly-
ing fox in California. In each of these cases there is no evident
reason why the species in question should not maintain itself
if once introduced. The fact that it does not exist is, in general,
an evidence that it has never passed the barriers which separate
the region in question from its original home.

Local illustrations of the same kind may be found in moun-
tainous regions. In the *i osemite Valley in California, for ex-
ample, the trout ascend the Merced River to the base of the
Vernal fall. They cannot rise above this and so the streams
and lakes above this fall are destitute of fish.

Examples of the second class are seen in animals that man
has introduced from one country to another. The nightingale,
the starling, and the skylark of Europe have been repeatedly
set free in the United States. But none of these colonies has
long endured; perhaps from lack of adaptation to the climate^
perhaps from severity of competition with other birds, most
likely because the few individuals become so widely scattered
that they do not find one another at mating time. In other
cases the introduced species has been better fitted for the con-
ditions of life than the native forms themselves, and so has
gradually crowded out the latter. Both these cases are il-
lustrated among the rats. The black rat (Mus rattus), first
introduced into America from Europe about 1544, tended to
crowd out the native wild rats (Sigmodori) , while the brown rat
(Mus decumanus), brought in still later, about 1775, in turn
practically exterminated the black rat, its fitness for the con-
ditions of life here being greater than that of the other European
species.

Of the third class, or species altered in a new environment,
examples are numerous, but in most cases the causes involved
can only be inferred from their effects. One class of illustra-
tions may be taken from island faunas. An island is set off
from the mainland by barriers which species of land animals
can very rarely cross. On an island a few waifs may maintain
themselves, increasing in numbers so as to occupy the territory,
but in so doing only those kinds will survive that can fit them-
selves to the new conditions. Through this process new species

316 EVOLUTION AND ANIMAL LIFE

will be formed, like the parent species in general structure, but
having gained new traits adjusted to the new environment.

To processes of this kind, on a larger or smaller scale, the

variety in the animal life of the globe must be largely due.

(isolation and adaptation through selection probably give the

clew to the formation of a very large proportion of the "new

species " in any group.)1

It will be thus seen that geographical distribution is primar-
ily dependent on barriers or checks to the movement of animals.
The obstacles met in the spread of animals determine the
limits of the species. Each species broadens its range as far
as it can. It attempts, unwittingly, of course, through natural
processes of increase, to overcome the obstacles of ocean and
river, of mountain or plain, of woodland or prairie or desert, of
cold or heat, of lack of food, or abundance of enemies — what-
ever the barriers may be. (Were it not for these barriers, each
type or species would become cosmopolitan or universal?)

Man is preeminently a barrier-crossing animal; hence, in
different races or species, man is found in all regions where
human life is possible. The different races of men, however,
find checks and barriers entirely similar in nature to those
experienced by the lower animals, and the race peculiarities
are wholly similar to characters acquired by new species under
adaptation to changed conditions. The degree of hindrance
offered by any barrier differs with the nature of the species
trying to surmount it. That which constitutes an impassable
obstacle to one form may be a great aid to another. The river
which blocks the monkey or the cat is the highway of the fish
or the turtle. The waterfall which limits the ascent of the fish
is the chosen home of the ouzel. The mountain barrier which
the bobolink or the prairie dog does not cross may be the center
of distribution of the little chief hare or the Arctic bluebird.

The term fauna is applied to the animals of, any region
considered collectively. Thus the fauna of Illinois comprises
the entire list of animals found naturally in that State. It
includes the aboriginal man, the black bear, the fox, and all its
animal life down to the Amoeba and the microbe of malaria.
The relation of the fauna of one region to that of another de-
pends on the ease with which barriers may be crossed. Thus
the fauna of Illinois differs little from that of Indiana or Iowa,
because the State contains no barriers that animals may not

GEOGRAPHICAL DISTRIBUTION

317

readily pass. On the other hand, the fauna of California or
Colorado differs materially from that of the adjoining regions,
because the mountainous country is full of barriers which

obstruct the diffusion of life. Distinctness is in direct propor-
tion to isolation. What is true in this regard of the fauna of
any region is likewise true of its individual species. The degree

318 EVOLUTION AND ANIMAL LIFE

of resemolance among individuals is in strict proportion to the
freedom of their movements. Variation within the limits of
a species is again proportionate to the barriers which prevent
equal and free diffusion.

The various divisions or realms into which the land surface
of the earth may be divided, on the basis of the character of the
animal life, have their boundary in the obstacles offered to the
spread of the average animal. In spite of great inequalities
in this regard, we may yet roughly divide the land of the globe
into seven principal realms or areas of distribution, each limited
by barriers, of which the chief are the presence of the sea and
the occurrence of frost. There are the Arctic, North Tem-
perate, South American, Indo-African, Patagonian, Lemurian,
and Australian realms. Of these the Australian realm alone is
sharply defined. Most of the others are surrounded by a broad
fringe of debatable ground that forms a transition to some
other zone.

The Arctic realm includes all the land area north of the
isotherm 32°. Its southern boundary corresponds closely with
the northern limit of trees. The fauna of this region is very
homogeneous. It is not rich in species, most of the common
types of life of warmer regions being excluded by the cold.
Among the large animals are the polar bear, the walrus, and
certain species of "ice-riding" seals. There are a few species
of fishes, mostly trout and sculpins, and a few insects; some of
these, as the mosquito, are excessively numerous in individuals.
Reptiles are absent from this region and many of its birds
migrate southward in the winter, finding in the Arctic their
breeding homes only. When we consider the distribution of
insects and other small animals of wide diffusion we must add
to the Arctic realm all high mountains of other realms whose
summits rise above the timber line. The characteristic large
animals of the Arctic, as the polar bear or the musk-ox or the
reindeer, are not found on the mountain tops because barriers
shut them off. But the Alpine flora, even under the equator,
may be characteristically arctic, and with the flowers of the
north may be found the northern insects on whose presence
the flowers depend for their fertilization and which in turn
depend on these for their food. So far as climate is concerned,
high altitude is equivalent to high latitude. On certain
mountains the different zones of altitude and the corresponding

GEOGRAPHICAL DISTRIBUTION 319

zones of plant and animal life are very sharply defined. Ex-
cellent illustrations are found in the San Francisco peaks of
Arizona and Mt. Orizaba in Mexico.

The North Temperate or holarctic realm comprises all the
land between the northern limit of trees and the southern limit
of forests. It includes, therefore, nearly the whole of Europe,
most of Asia, and the most of North America. While there are
large differences between the fauna of North America and that
of Europe and Asia, these differences are of minor importance,
and are scarcely greater in any case than the difference between
the fauna of California and that of our Atlantic coast. The
close union of Alaska with Siberia gives the Arctic region an
almost continuous land area from Greenland to the westward
around to Norway. To the south everywhere in the temperate
zone realm, the species increase in number and variety, and
the differences between the fauna of North America and that of
Europe are due in part to the northward extension in the one
and the other of types originating in the tropics.

Especially is this true of certain of the dominant types of
singing birds. The group of wood- war biers, tanagers, American
orioles, vireos, mocking birds, with the fly-catchers and hum-
ming birds so characteristic of our forests, are unrepresented in
Europe. All of them are apparently immigrants from the
neotropical realm where nearly all of them spend the winter.
In the same way Central Asia has many immigrants from the
Indian realm which lies to the southward. With all these
variations there is an essential unity of life over this vast area,
and the recognition of North America as a separate (nearctic)
realm, which some writers have attempted, seems hardly
necessary.

Alfred Russell Wallace refers to this unity of northern life
in these words:

"When an Englishman travels on the nearest sea route from Great
Britain to Northern Japan, he passes countries very unlike his own
both in aspect and in natural productions. The sunny isles of the
Mediterranean, the sands and date palms of Egypt, the arid rocks of
Aden, the cocoa groves of Ceylon, the tiger-haunted jungles of Malacca
and Singapore, the fertile plains and volcanic peaks of Luzon, the
forest-clad mountains of Formosa, the bare hills of China pass suc-
cessively in review, until after a circuitous journey of thirteen thousand

320

EVOLUTION AND ANIMAL LIFE

miles, he finds himself at Hakodate, in Japan. He is now separated
from his starting point by an almost endless succession of plains and
mountains, arid deserts or icy plateaus; yet, when he visits the interior

FIG. 187. — Three species of jack rabbits differing in size, color, and markings, but be-
lieved to be derived from one stock. The differences have arisen through isolation
and adaptation. The upper figure shows the head and fore legs of the black jack
rabbit, Lepus insulariis, of Espiritu Sanctu Island, Gulf of California; the lower
right-hand figure the Arizona jack rabbit, Lepus aUeni, specimen from Fort Lowell,
Arizona; and the lower left-hand figure the San Pedro Martin jack rabbit, Lepus
martirensis, from San Pedro Martin, Baja California.

GEOGRAPHICAL DISTRIBUTION 321

of the country, he sees so many familiar natural objects that he can
hardly help fancying he is close to his home. He finds the woods and
fields tenanted by tits, hedge sparrows, wrens, wagtails, larks, red-
breasts, thrushes, buntings, and house sparrows; some absolutely
identical with our own feathered friends, others so closely resembling
them that it requires a practised ornithologist to tell the difference.
. . . There are also, of course, many birds and insects which are
quite new and peculiar, but these are by no means so numerous or
conspicuous as to remove the general impression of a wonderful
resemblance between the productions of such remote islands as Britain
and Yezo." (ISLAND LIFE.)

A journey to the southward from Britain or Japan or
Illinois, or any point within the holarctic realm, would show
the successive changes in the character of life though gradual,
to be still more rapid. The barrier of frost which keeps the
fauna of the tropics from encroaching on the northern regions
once crossed, we come to the multitude of animals whose life
depends on sunshine, the characteristic forms of the neo-
tropical realm.

The neotropical, or South American realm, includes South
America, the West Indies, the hot coast lands (Tierra Caliente)
of Mexico, and those parts of Florida and Texas where frost
does not occur. Its boundaries through Mexico are not sharply
defined, and there is much overlapping of the north temperate
realm along its northern limit. Its birds, especially, range
widely through the United States in the summer migrations, and
a large part of them find in the North their breeding home.
Southward, the broad barrier of the two oceans keeps the
South American fauna very distinct from that of Australia or
Africa. The neotropical fauna is the richest of all in species.
The great forests of the Amazon are the treasure houses of the
naturalists. Characteristic types among the larger animals
are the broad-nosed (platyrrhine) monkeys, which in many
ways are distinct from the monkeys and apes of the Old World.
In many of them the tip of the tail is highly specialized and is
used as a hand. The Edentates (armadillos, ant-eaters, etc.)
are characteristically South American, and there are many
peculiar types of birds, reptiles, fishes, and insects.

The Indo-African or paleotropical realm corresponds to
the neotropical realm in position. It includes the great part

322 EVOLUTION AND ANIMAL LIFE

of Africa, merging gradually northward into the north temper-
ate realm through the transition districts which border the
Mediterranean. It includes also Arabia, India, and the neigh-
boring islands, all that part of Asia south of the limit of frost.
In monkeys, carnivora, ungulates, and reptiles this region is
wonderfully rich. In variety of birds, fishes, and insects the
neotropical realm exceeds it. The monkeys of this district
are all of the narrow-nosed (catarrhine) type, various forms
being much more nearly related to man than is the case with
the peculiar monkeys of South America. Some of these
(anthropoid apes) have much in common with man. To this
region belong the elephant, the rhinoceros, and the hippopot-
amus, as well as the tiger, lion, leopard, giraffe, the wild asses,
and horses of various species, besides a large number of rumi-
nant animals not found in other parts of the world. It is, in
fact, in the lower mammals and reptiles that its most striking
distinctive characters are found. In its fish fauna it has much
in common with South America.

The Lemurian realm comprises Madagascar alone. It is
an isolated division of the Indo-African realm, but the presence
of many species of lemurs — an unspecialized or primitive type
of monkey — is held to justify its recognition as a distinct realm.
In most other groups of animals the fauna of Madagascar is
essentially that of neighboring parts of Africa.

The Patagonian realm includes the south temperate zone
of South America. It has much in common with the neo-
tropical realm from which its fauna is mainly derived, but the
presence of frost is a barrier which vast numbers of species can-
not cross. Beyond the Patagonian realm lies the Antarctic
continent. The scanty fauna of this region is little known,
and it probably differs from the Patagonian fauna chiefly in
the absence of all but the ice-riding species.

The Australian realm comprises Australia and neighboring
islands. It is more isolated than any of the others, having
been protected by the sea from the invasions of the character-
istic animals of the Indo-African and temperate realms. It
shows a singular persistence of low or primitive types of ver-
tebrate life, as though in the process of evolution the region
had been left a whole geologic age behind. If the competing
faunas of Africa and India could have been able to invade
Australia, the dominant mammals and birds of that region

GEOGRAPHICAL DISTRIBUTION 323

would not have been left as they are now — marsupials and
parrots.

It is only when barriers have shut out competition that
simple or unspecialized types abound. The larger the land
area and the more varied its surface, the greater is the stress
of competition and the more specialized are the characteristic
forms. As part of this specialization is in the direction of
hardiness and power to persist, the species from the large areas,
as a whole, are least easy of extermination. The rapid multi-
plication of rabbits and foxes in Australia, when introduced
by the hand of man, shows what might have taken place in this
country had not impassable barriers of ocean shut them out.

Each of these great realms may be indefinitely subdivided
into provinces and sections, for there is no end to the possi-
bility of analysis. No farm has exactly the same animals or
plants as any other, as finally in ultimate analysis we find that
no two animals or plants are exactly alike. Shut off one pair
of animals from the others of its species, and its descendants
will differ from the parent stock. The difference increases
with time and with distance so long as the separation is main-
tained. Hence new species and new fauna or aggregations of
species are produced wherever free diffusion is checked by any
kind of barrier.

In like manner, we may divide the ocean into faunal areas
or zones, according to the distribution of. its animals. For
this purpose the fishes probably furnish the best indications,
although results very similar are obtained when we consider
the mollusks or the Crustacea.

The pelagic fishes are those which inhabit the open sea,
swimming near the surface, and often in great schools. Such
forms are usually confined to the warmer waters. They are for
the most part predatory fishes, strong swimmers, and many
of the species are found in all warm seas. Most species have
special homing waters, to which they repair in the spawning
season. To the free-swimming forms of invertebrates and
protozoa, found* in the open ocean, the name Plankton is ap-
plied.

The bassalian fauna, or deep-sea fauna, is composed of
species inhabiting great depths (from 2,500 to 25,000 feet) in
the sea. At a short distance below the surface the change in
temperature from day to night is no longer felt. At a still

324 EVOLUTION AND ANIMAL LIFE

lower depth there is no difference between winter and summer,
and still lower none between day and night. The bassalian
fishes inhabit a region of great cold and inky darkness. Their
bodies are subjected to great pressure, and the conditions of
life are practically unvarying. There is, therefore, among them
no migration, no seasonal change, no spawning season fixed
by outside conditions, and no need of adaptation to varying
environme&t. As a result, all are uniform indigo-black or
purple in color, and all show more or less degeneration in those
characters associated with ordinary environment. Their bodies
are elongate, from the lack of specialization in the vertebra?.
The flesh, being held in place by the great pressure of the
water, is soft and fragile. The organs of touch are often highly
developed. The eye is either excessively large, as if to catch
the slightest ray of light, or else it is undeveloped, as if the
fish had abandoned the effort to see. In many cases luminous
spots or lanterns are developed by which the fish may see to
guide its way, and in some forms these shining appendages
are highly developed. In one form (dZthoprora) a luminous
body coyers the end of the nose, like the headlight of an engine.
Many of these species have excessively large teeth, and some
have been known to swallow animals actually larger than
themselves. Those which have lanternlike spots have always
large eyes.

The deep-sea fishes, however fantastic, have all near rela-
tives among the shore forms. Most of them are degenerate
representatives of well-known types — for example, of eels, cod,
smelt, grenadiers, sculpin, and flounders. The deep-sea crus-
taceans and mollusks are similarly related to shore forms.

The third great subdivision of marine animals is the littoral
or shore group, those living in water of moderate depth, never
venturing far into the open sea either at the surface or in the
depths. This group shades into both the preceding. The
individuals of some of the species are excessively local, remain-
ing their life long in tide pools or coral reefs or piles of rock.
Others venture far from home, becoming more or less pelagic.
Still others ascend rivers either to spawn (anadromous, as the
salmon, shad, and striped bass), or for purposes of feeding, as
the robalo, corvina, and other shore fishes of the tropics.
Some live among rocks alone, some in seaweed, some on sandy
shores, some in the surf, and some only in sheltered lagoons.

GEOGRAPHICAL DISTRIBUTION" 325

In all seas there are fishes and other marine animals, and each
creature haunts the places for which it is fitted.

There is the closest possible analogy between the variations
of species of animals or plants in different districts and that of
words in different languages. The language of any people is
not a unit. It is made up of words which have at various
times and under various conditions come into it from the speech
of other people. The grammar of a language is an expression
of the mutual relations of these words. The word as it exists
in any one language represents the species. Its cognate or its
ancestor in any other language is a related species. The words
used in a given district at any one time constitute its philo-
logical fauna. There is a struggle for existence between words
as among animals. For example the words begin and commence,
shake and agitate, work and operate (Saxon and French) are in
the English language constantly brought into competition.
The fittest, the one that suits English purposes best, will at
last survive. If both have elements of fitness, the field will
be divided between them. The silent letters in words tell their
past history, as rudimentary organs tell what an animal's
ancestry has been. This analogy, of course, is not perfect in
all regards, as the passing of the words from mouth to mouth
is not rigidly comparable with the generation of animals.

We may illustrate the formation of species of animals by
following any widely used word across Europe. Thus the
Greek aster becomes in Latin and Italian stella] whence the
Spanish estrella and the French etoile. In Germany it becomes
Stern, in Danish Stjern; whence the Scottish starn and English
star.

In like manner, the name cherry may be traced from country
to country to which it has been taken in cultivation. Its Greek
name, kerasos, becomes cerasus, ceresia, ceriso, cereso, cerise,
among the Latin nations. This word is shortened to Kirsch
and Kers with the people of the North. In England, cherys,
cherry, are obviously derived from cerise.

The study of a fauna or a flora as a whole is thus analogous
to the study of a living language. The evolution of a language
corresponds to the history of the life of some region. Philology,
systematic zoology, and botany are alike intimately related to
geography. The parallelism between speech districts and
fauna! districts has been many times noted. The spread of a
22

326 EVOLUTION AND ANIMAL LIFE

language, like the spread of a fauna, is limited by natural
barriers. It is the work of civilization to break down these
barriers as limiting the distribution of civilized man. The
dominant languages cross these barriers with the races of man
who use them, and with them go the domesticated animals
and plants and the weeds and vermin man has brought un-
willingly into relations of domination.

The profitable study of the problems of geographical dis-
tribution is possible only on the theory of the derivation of
species. If we view all animals and plants as the results of spe-
cial creations in the regions assigned to them, we have instead
of laws only a jumble of arbitrary and meaningless facts. In
our experience with the facts of science we have learned that
no fact is arbitrary or meaningless. We know no facts which
lie beyond the realm of law. We may close with the language
of Asa Gray:

"When we gather into one line the several threads of evidence of
this sort we find that they lead in the same direction with the views
furnished by other lines of investigation. Slender indeed" each thread
may be, but they are manifold, and together they bind us firmly to
the doctrine of the derivation of species."

CHAPTER XVI
ADAPTATIONS

It is a wise provision of nature that trees shall not grow up into
the sky. — GOETHE.

THE adaptation of every species of animal and plant to its
environment is a matter of everyday observation. So perfect
is this adaptation in its details that its main facts tend to
escape our notice. The animal is fitted to the air it breathes,
the water it drinks, the food it finds, the climate it endures,
the region which it inhabits. All its organs are fitted to its
functions : all its functions to its environment. Organs and
functions are alike spoken of in a half-figurative way as con-
cessions to environment. And all structures and powers are
in this sense concessions, in another sense, adaptations. As
the loaf is fitted to the pan, or the river to its bed, so is each
species fitted to its surroundings. If it were not so fitted, it
would not live. But such fitness on the vital side leaves large
room for variety in characters not essential to the life of the
animal. Thus we ascribe nonessential characters to variation,
preserved by heredity and guarded by isolation. Vital or\
adaptive characters originate in the same way, but these are!
preserved in heredity and guarded and intensified by selection./

The strife for place in the crowd of animals makes it neces-
sary for each one to adjust itself to the place it holds. As the
individual becomes fitted to its condition, so must the species
as a whole. The species is therefore made up of individuals
that are fitted or may become fitted for the conditions of life.
As the stress of existence becomes more severe, the individuals
fit to continue the species are chosen more closely. This choice
is the automatic work of the conditions of life, but it is none
the less effective in its operations, and in the course of centuries

327

328

EVOLUTION AND ANIMAL LIFE

it may be considered unerring. When conditions change, the
perfection of adaptation in a species may be the cause of its
extinction. If the need of a special fitness cannot be met,
immediately the species will disappear. For example, the
native sheep of England have developed a long wool fitted to
protect them in a cool, damp climate. Such sheep, transferred
to Cuba, died in a short time, leaving no descendants. The
warm fleece, so useful in England, rendered them wholly unfit

FIG. 188. — Nest of Vespa, a social wasp. (Photograph by A.
C. T. Brues.)

L. Melander and

for survival in the tropics. It is one advantage of man, as
compared with other forms of life, that so many of his adapta-
tions are external to his structure, and can be cast aside when
necessity arises.

The great fact of nature is adaptation. But while general
adaptation to widespread conditions is universal, there exist
also a multitude and variety of special adaptations fitting
organisms to special conditions. These special adaptations
arrest our attention to a greater degree than general adaptations
because they furnish the element of contrast.

The various types of special adaptations may be roughly
divided into five classes as follows: (a) Food-securing; (6) self-

ADAPTATIONS

329

r

defense; (c) defense of young; (d) rivalry; (e) adjustment to
surroundings.

For the purpose of capture of their prey, most carnivorous
animals are provided with strong claws, sharp teeth, hooked
beaks, and other structures familiar to us in the lion, tiger,
dog, cat, owl, and eagle. Insect-eating mammals have con-
trivances especially adapted for the catching of insects. The
ant-eater, for example, has a long sticky tongue which it thrusts
forth from its cylindrical
snout deep into the recesses
of the ant-hill, bringing it
out with its surface covered
with ants. Animals which
feed on nuts are fitted with
strong teeth or beaks for
cracking them. Strong teeth
are found in those fishes
which feed on crabs, or sea
urchins. Those mammals
like the horse and cow, that
feed on plants, have usually
broad chisellike incisor teeth
for cutting off the foliage,
and teeth of very similar
form are developed in dif-
ferent groups of plant-eating
fishes. Molar teeth are found
when it is necessary that the
food should be crushed or

chewed, and the sharp canine teeth go with a flesh diet. The
long neck of the giraffe enables it to browse on the foliage of
trees in grassless regions.

Insects like the leaf -beetles and the grasshoppers, that feed
on the foliage of plants, have a pair of jaws, broad but sharply
edged, for cutting off bits of leaves and stems. Those which
take only liquid food, as the butterflies and sucking bugs,
have their mouth parts modified to form a slender, hollow
sucking beak or proboscis, which can be thrust into a flower
nectary, or into the green tissue of plants or the flesh of animals,
to suck up nectar or plant sap, or blood, according to the
special food habits of the insect. The honey-bee has a very

FIG. 189. — The brown pelican, showing gular
sac which it uses in catching and holding
fishes for its food.

330

EVOLUTION AND ANIMAL LIFE

complicated equipment of mouth parts fitted for taking either
solid food like pollen, or liquid food Like the nectar of flowers.
The mosquito has a "bill" composed of six sharp, slender
needles for piercing and lacerating the flesh, and a long
tubular under lip through which the blood can flow into the
mouth. Some predaceous insects, as the praying horse (Fig.
38), -have their fore legs developed into formidable grasping
organs for seizing and holding their prey.

*••*>.." - »s*>St

PIG. 190. — Ant-lion larva plowing its way through the sand (upper figure), while an-
other is commencing the excavation of a funnel-shaped pit similar to one on right.
(Photograph by A. L. Melander and C. T. Brues.)

For self-protection the higher animals depend largely on
the same organs and instincts as for the securing of food. Car-
nivorous beasts use tooth and claw in their own defense as
well as in securing their prey, but these as well as other animals
may protect themselves in other fashions. Many of the higher
animals are provided with horns, structures useless in procuring
food, but effective as weapons of defense. Others defend
themselves by blows with their strong hoofs. Among the
reptiles and fishes and even among the mammals, the defensive
coat of mail is found in great variety. The turtle, the armadillo,
the sturgeon, and gar pike, all these show the value of defensive
armature, and bony shields are developed to a still greater

ADAPTATIONS

331

degree in various extinct types of fishes/ The crab and lobster
with claws and carapace are well defended against their enemies,
and the hermit crab, with its trick of thrusting its unprotected

body within a cast-off shell of a sea snail, finds in this instinct

a perfect defense. Insects also, especially beetles, are protected
by their coats of mail. Scales and spines of many sorts serve
to defend the bodies of rep-
tiles and fishes, while feathers
protect the bodies of birds
and hair those of most
mammals.

The ways in which ani-
mals make themselves dis-
agreeable or dangerous to
their captors are almost as
varied as the animals them-
selves. Besides the teeth,
claws, and horns of ordinary
attack and defense, we find
among the mammals many
special structures or con-
trivances which serve for
defense through making their
possessors unpleasant. The
scent glands of the skunk
and its relatives serve as
examples. The porcupine has
the bristles in its fur special-
ized as quills, barbed and
detachable. These quills fill
the mouth of an attacking

wolf or fox, and serve well the purpose of defense,
hedgehog of Europe, an animal of different nature, being re-
lated rather to the mole than to the squirrel, has a similar
armature of quills. The armadillo of the tropics has movable
shields, and when it withdraws its head (also defended by a
bony shield) it is as well protected as a turtle.

The turtles are all protected by bony shields, and some of
them, the box turtles, may close their shields almost hermet-
ically. The snakes broaden their heads, swell their necks, or
show their forked tongues to frighten their enemies. Some of

FIG. 191. — Scorpion showing the special
development of certain mouth parts (the
maxillary palpi) as pincerlike organs for
grasping. On the posterior tip of the
body is the poison sting.

The

332

EVOLUTION AND ANIMAL LIFE

them are further armed with fangs connected with a venom
gland, so that to most animals their bite is deadly. Besides

FIG. 192. — Cocoon enclosing a pupa of the great Ceanothus moth, Samia ceanothi,
spun by the larva before pupation.

its fangs the rattlesnake has a rattle on the tail made up of a
succession of bony clappers, modified vertebrae, and scales,
by which intruders are warned of its pre-
sence. This sharp and insistent buzz is a
warning to animals of other species and
perhaps a recognition signal to those of its
kind.

Even the fishes have many modes of
self-defense through giving pain or injury
to animals who would swallow them. The
catfish or horned pout when attacked sets
immovably the sharp spine of the pectoral ,
fin, inflicting a jagged wound. Pelicans
which have swallowed a catfish have been
known to die of the wounds inflicted by
the fish's spine. In the group of scorpion
fishes and toad fishes are certain genera
in which these spines are provided with
poison glands. These may inflict very
severe wounds to other fishes, or even to
birds or man. One of this group of poison
fishes is the nohi (Emmydrichthys) . A
group of small fresh-water cat fishes, known
as the mad torus, have also a poison gland
attached to the pectoral spine, and the
sting is most exasperating, like the sting

FIG. 193.— Larva of swal-
lowtail butterfly, Pa-
pilio cresphontes,
showing osmateria
(eversible processes
giving off an ill odor)
projected. (After
photograph by Slinger-
land.)

ADAPTATIONS

333

of the wasp. The sting-rays (Fig. 194), of which there are
many species, have a strong jagged spine on the tail, covered
with slime, and armed with broad sawlike teeth. This in-
flicts a dangerous wround, not through the presence of spe-
cific venom, but from the danger of blood poisoning aris-
ing from the slime, and the ragged or unclean cut.

The poisonous alkaloids
within the flesh of some fishes
(Tetraodon, Batistes, etc.)
serve to destroy the enemies
of the species while sacrific-
ing the individual. These
alkaloids, most developed in
the spawning season, pro-
duce a disease, known in
mariasciguatera. This is
rarely known outside of the
tropics.

Many fishes are defended
by a coat of mail or a coat
of sharp thorns. The globe
fishes and porcupine fishes
are for the most part de-
fended by spines, but their
instinct to swallow air gives
them an additional safeguard.
When one of these fishes is
disturbed it rises to the sur-
face, gulps air until its capa-
cious stomach is filled, and
then floats belly upward on

the water. It is thus protected from other fishes, though easily
taken by man. The torpedo, electric eel, electric catfish, and
star-gazer, surprise and stagger their captors by means of electric
shocks. In the torpedo or electric ray (Fig. 195), of which
species are found on the sandy shores of all warm seas, on
either side of the head is a large honeycomblike structure
which yields a strong electric shock whenever the live fish is
touched. This shock is felt severely if the fish be stabbed with
a knife or metallic spear. The electric eel of the rivers of
Paraguay and southern Brazil is said to give severe shocks to

FIG. 194. — Sting-ray, Urolophus goodei,
from Panama.

334

EVOLUTION AND ANIMAL LIFE

herds of wild horses driven through the streams, and similar
accounts are given of the electric catfish of the Nile. In
tropical seas, the tangs or surgeon fishes (Hepatus) are provided
with a knifelike spine on the side of the tail, the sharp edge
directed forward and slipping into a sheath. This is a formi-
dable weapon when the fish is
alive.

Other fishes defend them-
selves by spears (swordfish,
spear fish, sailfish) or by saws
(sawfish, sawshark) or by pad-
dles, (paddlefish). Others still,
make use of sucking disks of
one sort or another (as in the
snailfish, the clingfish, and the
goby), to cling to the under
side of rocks, or as in the
Remora to the bodies of swift-
moving sharks. Blind fishes in
the caves are adapted to their
condition, the eyes being obso-
lete, while the skin is covered
with rows of sensitive papillse.
In similar circumstances sala-
manders, crayfishes, and insects
are also blind. There are also
blind gobies which live in the
crevices of rocks and still other
blind fishes in the great depths
of the sea.

Some fishes, as the lancelet, lie buried in the sand all their
lives. Others, as the sand darter (Ammocrypta pellucida} and
the hinalea (Julis gaimardi), bury themselves in the sand at
intervals to escape from their enemies. Some live in the
cavities of tunicates or sponges or holothurians or corals or
oysters, often passing their whole lives inside the cavity of one
animal. Many others hide themselves in the interstices of
kelp or seaweeds. Some eels coil themselves in the crevices
of rocks or coral masses, striking at their prey like snakes.
Some sea-horses cling by their tails to gulf weed or sea- wrack.
Many little fishes (Gobiomorus, Carangus, Psenes) cluster under

FIG. 195. — Torpedo or electric ray,
Narcine brasiliensis, showing elec-
tric cells.

ADAPTATIONS

335

the stinging tentacles of the Portuguese man-of-war or under
ordinary jellyfishes.

Some fishes called the flying fishes sail through the air with
a grasshopperlike motion that closely imitates true flight.
The long pectoral fins, winglike in form, cannot, however, be
flapped by the fish, the muscles serving only to expand or fold
them. These fishes live in the open sea or open channels,
swimming in large schools. The small species fly for a few

FIG. 196. — Flying fishes: The upper one, a species of Cypselurus; the lower, of
Exocaetus. These fishes escape from their enemies by leaping into the air
and sailing or "flying" long distances.

feet only, the large ones for more than an eighth of a mile.
These may rise five to twenty feet above the water.

The flight of one of the largest flying fishes (Cypselurus cali-
jornicus) has been carefully studied by Dr. Charles H. Gilbert
and the senior author. The movements of the fish in the water
are extremely rapid. The sole motive power is the action
under the water of the strong tail. No force can be acquired
while the fish is in the air. On rising from the water the move-
ments of the tail are continuous until the whole body is out of
the water. When the tail is in motion the pectorals seem in
a state of rapid vibration. This is not produced by muscular
action on the fins themselves. It is the body of the fish which
vibrates, the pectorals projecting farthest having the greatest
amplitude of movement. While the tail is in the water the

336 EVOLUTION AND ANIMAL LIFE

ventral fins are folded. When the action of the tail ceases
the pectorals and ventrals are spread out wide and held at rest.
They are not used as true wings, but are held out firmly, acting
as parachutes, enabling the body to skim through the air.
When the fish begins to fall the tail touches the water. As
soon as it is in the water it begins its motion, and the body
with the pectorals again begins to vibrate. The fish may, by
skimming the water, regain motion once or twice, but it finally
falls into the water with a splash. While in the air it suggests
a large dragon fly. The motion is very swift, at first in a
straight line, but is later deflected in a curve, the direction
bearing little or no relation to that of the wind. When a
vessel passes through a school of these fishes, they spring up
before it, moving in all directions, as grasshoppers in a meadow.

Among the insects, the possession of stings is not uncom-
mon. The wasps and bees are familiar examples of stinging
insects, but many otherTonds, less familiar, are similarly pro-
tected. All insects have their bodies covered with a coat of
armor, composed of a horny substance called chitin. In some
cases, this chitinous coat is very thick ancT serves to protect
them effectually. This is especially true of the beetles. Some
insects are inedible, and are conspicuously colored so as to
be readily recognized by insectivorous birds. The birds,
knowing by experience that these insects are ill-tasting, avoid
them. Others are effectively concealed from their enemies
by their close resemblance in color and marking to their
surroundings. These protective resemblances are discussed
in Chapter XIX.

To the category of structures which may be useful in self-
defense belong the many peculiarities of coloration known as
"recognition marks." These are marks, not otherwise helpful,
which are supposed to enable members of any -one species to
recognize its kind among the mass of animal life. To this
category belongs the black tip 'of the weasel's tail, which re-
mains the same whatever the changes in the outer fur. Another
example is seen in the white outer feathers of the tail of the
meadow lark as well as in certain sparrows and warblers.
The white on the skunk's back and tail may serve the same
purpose and also as a warning. It is apparently to the skunk's
advantage not to be hidden, for to be seen in the crowd of ani-
mals is to be avoided by them. That recognition is the actual

ADAPTATIONS

337

function of such markings has never been clearly proved. The
songs of birds and the calls of various creatures may serve also
as recognition marks. Each species knows and heeds its own
characteristic song or cry, and it is a source of mutual pro-
tection. The fur-seal pup knows its mother's call, even though
ten thousand other mothers are calling on the same rookery.

In questions of attack and defense, the need of fighting
animals of their own kind, as well as animals of other races,
must be considered. To struggles
of species with those of their own
kind, the term rivalry may be ap-
plied. Actual warfare is confined
mainly to males in the breeding
season, especially in polygamous
species. Among those in which
the male mates with many females,
he must struggle with other males
for their possession. In all the
groups of vertebrates the sexes
are about equal in numbers.
Among monogamous animals, which
mate for the season or for life,
there is less occasion for destruc-
tive rivalry.

Among monogamous birds, or
those which pair, the male courts
the female of his choice by song
and by display of his bright
feathers. According to the theory
of sexual selection, the female con-
sents to be chosen by the one which pleases her. It is as-
sumed that the handsomest, most vivacious, and most musi-
cal males are the ones most successful in such courtship.
With polygamous animals there is intense rivalry among the
males in the mating season, which in almost all species is in
the spring. The strongest males survive and reproduce their
strength. The most notable adaptation is seen in the superior
sizo of teeth, horns, mane, or spurs. Among the polygamous
fur seals and sea lions the male is about four times the size of
the female. In the polygamous family of deer, buffalo, and
the domestic cattle and sheep, the male is larger and more

.

FIG. 197.— Egg case of the Cali-
fornia barndoor skate, Raja
binoculata, cut open to show
young inside. Young issues
naturally at one end of the
skate.

338 EVOLUTION AND ANIMAL LIFE

powerfully armed than the female. In the polygamous group
to which the hen, turkey, and peacock belong, the males pos-
sess the display of plumage, and the structures adapted for
fighting, with the will to use them.

/ The protection of the young is the source of many adaptive
/structures as well as of the instincts by which such structures
I are utilized. In general those animals are highest in develop-
ment, with the best means of holding their own in the struggle
for life, that take best care of their young. Those instincts
which lead to home building are all adaptations for preserving

FIG. 198. — The snake, Ichthyophis glutinosus, with egg case carried in coils of the body.
(After Goebel and Selenka.)

the young. Among the lower or more coarsely organized birds,
such as the chicken, the duck, and the auk, as with the reptiles,
the young animal is hatched with well-developed muscular
system and sense organs, and is capable of running about, and,
to some extent, of feeding itself. Birds of this type are known
as prsecocial, while the name altricial is applied to the more
highly organized forms, such as the thrushes, doves, and song
birds generally. With these the young are hatched in a wholly
helpless condition, with ineffective muscles, deficient senses,
and dependent wholly upon the parent. The altricial condition
demands the building of a nest, the establishment of a home,
and the continued care of one or both of the parents.

The very lowest mammals known, the duck bills (Mono-
tremes) of Australia, lay large eggs in a strong shell like those
of a turtle, and these they guard with great jealousy. But

ADAPTATIONS

339

with almost all mammals the egg is very small and without
much food yolk. The egg begins its development within the

FIG. 199. — Kangaroo, Macropus rufus, with young in pouch.

body. It is nourished by the blood of the mother, and after
birth the young is cherished by her, and fed by milk secreted
by specialized glands of the skin. All these features are

340

EVOLUTION AND ANIMAL LIFE

FIG. 200. — Egg case of the
cockroach.

adaptations tending toward the preservation of the young. /
In the Marsupials, which stand next to the Monotremes — the
kangaroo, opossum, etc. — the young are born in a very im-
mature state and are at once seized by the mother and thrust
into a pouch or fold of skin along the abdomen, where they are
kept until they are able to take care
of themselves (Fig. 199). This is a
singular adaptation, but less special-
ized and less perfect than the condi-
tion found in ordinary mammals.

Among the insects, the special pro- /
visions for the protection and care of

the eggs and the young are widespread and various. The eggs
of the common cockroach are laid in small packets inclosed
in a firm wall (Fig. 200). The eggs of the great water bugs
are carried on the back of the male (Fig. 201): and the spiders
lay their eggs in a silken sac or cocoon, and some of the ground
or running spiders (Lycosidce) , drag this egg sac, attached to
the tip of the abdomen, about with
them. The young spiders when hatched
live for some days inside this sac, feed-
ing on each other. Many insects have
long, sharp, piercing ovipositors, by
means of which the eggs are thrust into
the ground or into the leaves or stems
of green plants, or even into the hard
wood of tree trunks. Some of the scale
insects secrete wax from their bodies
and form a large, often beautiful egg
case attached to and nearly covering
the body in which eggs are deposited.
The various gall insects lay their eggs
in the soft tissue of plants, and on
the hatching of the larva an abnormal growth of the plant
occurs about the young insect, forming an inclosing gall that
serves not only to protect the insect within, but to furnish
it with an abundance of plant sap, its food. The young in-
sect remains in the gall until it completes its development
and growth, when it gnaws its way out. Such insect galls are
especially abundant on oak trees (Figs. 2J2 and 203).

The movements of migratory fishes are mainly controlled

FIG. 201. — Giant waterbug,
Serphus, male carrying
eggs on its back.

ADAPTATIONS

341

by the impulse of reproduction. Some pelagic fishes, especially
those of the mackerel and flying fish families, swim long dis-
tances to a region favorable for the deposition of spawn. Others
pursue for equal distances the schools of herring, menhaden, or
other fishes which serve as their prey. Some species are known
mainly in the waters they make their breeding homes, as in

FIG. 202. — Galls on the rose caused by the gall fly, Rhoditea rosce. (After Kieffer.)

Cuba, southern California, Hawaii, or Japan, the individuals
being scattered at other times through the wide seas.

Many fresh-water fishes, as trout and suckers, forsake the
large streams in the spring, ascending the small brooks where
their young can be reared in greater safety. Still others,
known as anadromous fishes, feed and mature in the sea, but
ascend the rivers as the impulse of reproduction grows strong.
Among such fishes are the salmon, shad, alewife, sturgeon, and
striped bass in American waters. The most remarkable case
23

342 EVOLUTION AND ANIMAL LIFE

of the anadromous instinct is found in the king salmon or
quinnat (Oncorhynchus tschawytscha) of the Pacific Coast.
This great fish spawns in November, at the age of four years
and with an average weight of twenty-two pounds. In the
Columbia River it begins running with the spring freshets in
March and April. It spends the whole summer without feeding
in the ascent of the river. By the autumn, the individuals

FIG. 203. — Giant gall of the white oak (California) made by the gall fly, Andricus cali-
fornicus; the gall at the right cut open to show the tunnels made by the insects in
escaping from the gall. (From photograph.)

have reached the mountain streams of Idaho, greatly changed
in appearance, discolored, worn, and distorted. The male is
humpbacked, with sunken scales and greatly enlarged, hooked,
bent, or twisted jaws, with enlarged doglike teeth. On reach-
ing the spawning beds, which may be a thousand miles from
the sea in the Columbia, over two thousand miles in the Yukon,
the female deposits her eggs in the gravel of some shallow brook.
The male covers them and scrapes the gravel over them.
Then both male and female drift tail foremost helplessly down
the stream: none, so far as certainly known, ever survives the
reproductive act. The same habits are found in the four other
species of salmon in the Pacific, but in most cases the individuals
do not start so early nor run so far. The blueback salmon or
redfish, however, does not fall far short in these regards. The
salmon of the Atlantic has a similar habit, but the distance
traveled is everywhere much less, and the hook-jawed males

ADAPTATIONS

343

drop back down to the sea and survive to repeat the acts of
reproduction.

Catadromous fishes, as the true eel (Anguilla), reverse this
order, feeding in the rivers and brackish estuaries, apparently
finding their usual spawning ground in the sea.

A large part of the life of the animal is a struggle with the
environment itself: in this struggle only those that are adapted,
live and leave descendants fitted like themselves. The fur of
mammals fits them to their surroundings] As the fur differs
so may the habits change. Some animals are active in winter:
others, as the bear, and in Northern' Japan, the red-faced mon-
key, hibernate, sleeping in caves or hollow trees or in burrows,
until conditions are favorable for their activity. Most snakes
and lizards hibernate in cold weather. In the swamps of
Louisiana, in winter, the bottom may often be seen covered
with water snakes lying as inert as dead twigs. Usually,
however, hibernation is accompanied by concealment. Some

FIG. 204. — Head of rainbow trout, Salmo irideus, with gill cover bent back to show
gills, the breathing organs.

animals in hibernation may be frozen alive without apparent
injury. The blackfish of the Alaska swamps, fed to dogs when
frozen solid, has been known to revive in the heat of the dog's
stomach and to wriggle out and escape. As animals resist
heat and cold by adaptations of structure and habits, so may
they resist dryness. Certain fishes hold reservoirs of water
above their gills, by means of which they can breathe during

344

EVOLUTION AND ANIMAL LIFE

FIG. 205. — Tree toad, Hyla regilla.

short excursions from the water. Still others (mud fishes)

retain the primitive lunglike structure of the swim bladder,

and are able to breathe air when, in the dry season, the water

of the pools is reduced
to mud.

Another series of I
adaptations is con-l
cerned with the places]
thosen by animals for
their homes. The fishes
that live in the water
have special organs for
breathing under water
(Fig. 204). Many of

the South American monkeys have the tip of the tail adapted

for clinging to limbs of trees or to the bodies of other monkeys

of its own kind. The hooked claws of the bat hold on to

rocks, the bricks of chimneys, or to the

surface of hollow trees, where the bat

sleeps through the day. The tree frogs

or tree toads (Fig. 205) have the tips of

the toes swollen, forming little pads by

which they cling to the bark of trees.

Among other adaptations relating to

special surroundings or conditions of life

are the great cheek pouches of the pocket

gophers, which carry off the soil dug up

by the large shovellike feet when the

gopher excavates its burrow.

Those insects which live underground,

making burrows or tunnels in the soil, \

have their legs or other parts adapted for \

digging and burrowing. The mole cricket

(Figs. 206 and 207) has its legs stout and

short, with broad, shovellike feet. Some

water beetles and water bugs have one

or 'more of the pairs of legs flattened and

broad to serve as oars or paddles for swim-
ming. The grasshoppers or locusts, which leap, have their

hind legs greatly enlarged and elongated, and provided with

strong muscles so as to make of them "leaping legs/' The

FIG. 206. — The mole
cricket, Gryllotalpa,
with fore legs modi-
fied for digging.

ADAPTATIONS

345

grubs or larvae of beetles which live as " borers " in tree trunks
have mere rudiments of legs, or none at all. They have great,
strong, biting jaws for cutting away the hard wood. They
move simply by wriggling along in their burrows or tunnels.

Insects that live in water either come
up to the surface to breathe or take down
air underneath their wings, or in some other
way, or have gills for breathing the air
which is mixed with the water. These gills
are special adaptive structures which present
a great variety of form and appearance. In
the young of the May flies they are delicate
platelike flaps projecting from the sides of
the body. They are kept in constant
motion, gently waving back and forth in
the water so as to maintain currents to
bring fresh water in contact with them.
Young mosquitoes do not have gills, but
come up to the surface to breathe. The
larvae, or wrigglers, breathe through a special
tube at the posterior tip of the body, while
the pupae have a pair of hornlike tubes on
the back of the head end of the body.

Many fishes, chiefly of the deep seas,
develop organs for producing light. These
are known as luminous organs, phosphor-
escent organs, or photophores. These are
independently developed in four entirely
unrelated groups of fishes. This difference
in origin is accompanied by corresponding
differences in structure. The best known
type is found in the Iniomi, including the
lantern fishes and their many relatives.
They may have luminous spots, differentiated areas, round or
oblong, which shine starlike in the dark. These are usually
symmetrically placed on the sides of the body, f hey may
have also luminous glands or diffuse areas which ar^iuminous,
but which do not show the specialized structure of the phos-
phorescent spots. These glands of similar nature to the spots
are mostly on the head or tail. In one genus, jfithoprora, the
luminous snout is compared to the headlight of an engine.

FIG. 207.— Front leg
of the mole cricket,
showing at e open-
ing of auditory or-
gan. (After Sharp.)

346

EVOLUTION AND ANIMAL LIFE

Entirely different are the photophores in the midshipman or
singing fish (Porwhthys) , a genus of the toad fishes or Batra-
choididse. These species live near the shore and the lumi-
nous spots are outgrowths from pores of the lateral line.

In one of the anglers (Corynolophus reinhardti) the com-
plex bait is said to be luminous, and luminous areas occur

on the belly of a very
small shark of the deep
seas of Japan (Etmopterus
ludfer). Dr. Peter Schmidt
of St. Petersburg has a
drawing of this shark
made at night from its
own light.

While among the
higher or vertebrate ani-
mals, especially the fishes
and reptiles, most remark-
able cases of adaptations
occur, yet the structural
changes are for the most
part external, usually not
affecting fundamentally

the development of the internal organs other than the skeleton.
The organization of these higher animals is much less plastic
than that of the invertebrates. In general, the higher the
type the more persistent and unchangeable are those struc-
tures not immediately exposed to the influence of the struggle
for existence. It is thus the outside of an animal that tells
where its ancestors have lived. The inside, suffering little
change whatever the surroundings, tells the real nature of the
animal.

FIG. 208. — Nest of the trapdoor spider.

CHAPTER XVII

PARASITISM AND DEGENERATION

Les causes de revolution regressive peuvent se ramener a une
seule, la limitation des moyens de subsistance, de la, la lutte pour
Texistence entre les organismes ou les societes, et entre leurs parties
composantes. — DEMOOR, MASSART, and VANDERVELDE.

A SPECIAL kind of adaptation is that shown by parasitic
animals. The relations of parasitic animals to their hosts
appear in many familiar examples, and the results of this para-
sitic life, or at least the conditions that seem always to attend
it, namely the degeneration, slight or extreme, of the parasites,
is also familiar to all observers of animal life. The term para-
sitism, as well as the term degeneration, cannot be very
rigidly defined. To prey upon the bodies of other animals is
the common habit of many creatures. If the animals which
live in this way are free, chasing or lying in wait for or snaring
their prey, we speak of them in general as predatory animals.
But if they attach themselves to the body of their prey or
burrow into it, and are carried about by it, live on or in it,
then we call them parasites. And the difference in habit
between a lion and an intestinal worm is large enough and
marked enough to make very clear to us what is meant when we
speak of one as predatory and the other as a parasite. But
how shall we class the lamprey, that swims about until it finds
a fish to which it clings, while sucking away its blood? It
lives mostly free, hunting its prey, clinging to it for a while,
and is carried about by it. Closely related to the lampreys
are the hag fishes (Myxine) marine eellike fishes that attach
themselves by a suckerlike mouth to living fishes and gradually
_§£rape and eat their way into the abdominal cavity of the host.
Th&se " hags " or " borers " approach more nearly to the con-

347

348

EVOLUTION AND ANIMAL LIFE

dition of an internal parasite than any other vertebrate. And
what about the flea? In its immature life it lives as a white
grub or larva in the dust of cracks and crevices, of floors and
cellars and heaps of debris; here it pupates, and finally changes
into the active leaping blood-sucking adult which finds its way
to the body of some mammal and clings there sucking blood.
But it can jump off and hunt other prey; it leaves the host body
entirely to lay its eggs, and yet it feeds as a parasite, at least
it conforms to the definition of parasite in the essential fact of

FIG. 209. — At the left, the red-tailed trichina fly, Winthemia, U-pastulata, the parasite
of the army worm, Leucania unipuncta ; at the right, the worms upon which the
fly has laid eggs. (After Slingerland.)

being carried about on or in the host body, while feeding at the
host's expense.

It is of course not particularly important that we distinguish
sharply between parasitic and predaceous animals, but as we
look on the degeneration of parasitic animals as the result of
their special habit of life, we must attempt a sort of classifica-
tion of the phases or degrees of parasitism, in order to asso-
ciate with them corresponding categories of degeneration.

The bird lice (Mallophaga), which infest the bodies of all
kinds of birds and are found especially abundant on domestic
fowls, live upon the outside of the bodies of their hosts, feeding
upon the feathers and dermal scales. They are examples of ex-
ternal parasites. Other examples are fleas and ticks, and the
crustaceans called fish lice and whale lice, which are attached
to marine animals. On the other hand, almost all animals
are infested by certain parasitic worms which live in the ali-
mentary canal, like the tapeworm, or imbedded in the muscles,
like the trichina. These are examples of internal parasites.
Such parasites belong mostly to the class of worms, and some

PARASITISM AND DEGENERATION 349

of them are very injurious, sucking the blood from the tissues
of the host, while others feed solely on the partly digested
food. There are also parasites that live partly within and
partly on the outside of the body, like the Sacculina, which
lives on various kinds of crabs. The body of the Sacculina
consists of a soft sac which lies on the outside of the crab's
body, and of a number of long, slender rootlike processes
which penetrate deeply into the crab's body, and take up
nourishment from within. The Sacculina is itself a crus-
tacean or crablike creature. The classification of parasites
as external and internal is purely arbitrary, but it is often a
matter of convenience.

Some parasites live for their whole lifetime on or in the
body of the host, as is the case with the bird lice. Their eggs
are laid on the feathers of the bird host ; the young when hatched
remain on the bird during growth and development, and the
adults only rarely leave the body, usually never. These may
be called permanent parasites. On the other hand, fleas leap
off or on a dog apparently as caprice dictates; or, as in other
cases, the parasite may pass some definite part of its life as a
free nonparasitic organism, attaching itself, after development,
to some animal, and remaining there for the rest of its life.
These parasites may be called temporary parasites. But this
grouping or classification, like that of the external and internal
parasites, is simply a matter of convenience, and does not
indicate at all any blood relationship among the members of
any one group.

Some parasites are so specialized in habit and structure that
they are wholly unable to go through their life history, or to
maintain themselves, except in a single fixed way. They are
dependent wholly on one particular kind of host, or on a par-
ticular series of hosts, part of their life being passed in one and
another part in one or more other so-called intermediate hosts.
These parasitic species are called obligate parasites, while
others with less definite, more flexible requirements in regard
to their mode of development and life are called facultative
parasites. These latter may indeed be able to go through life
as free-living, nonparasitic animals, although, with oppor-
tunity, they live parasitically.

In nearly all cases the body of a parasite is simpler in
structure than the body of other animals which are closely

350 EVOLUTION AND ANIMAL LIFE

related to the parasite — that is, animals that live parasitically
have simpler bodies than animals that live free active lives,
competing for food with the other animals about them. This
simplicity is not primitive, but results from the loss or atrophy
of the structures which the mode of life renders useless. Many
parasites are attached firmly to their host, and do not move
about. They have no need of the power of locomotion. They
are carried by their host. Such parasites are usually without
wings, legs, or other locomotory organs. Because they have
given up locomotion they have no need of organs of orientation,
those special sense organs like eyes and ears and feelers which
serve to guide and direct the moving animal; and most non-
locomotory parasites will be found to have no eyes, nor any
of the organs of special sense which are accessory to locomotion
and which serve for the detection of food or of enemies. Be-
cause these important organs, which depend for their success-
ful activity on a highly organized nervous system, are lacking,
the nervous system of parasites is usually very simple and un-
developed. Again, because the parasite usually has for its
sustenance the already digested highly nutritious food elabo-
rated by its host, most parasites have a very simple alimentary
canal, or even no alimentary canal at all. Finally, as the fixed
parasite leads a wholly sedentary and inactive life, the break-
ing down and rebuilding of tissue in its body go on very slowly
and in minimum degree, and there is no need of highly developed
respiratory and circulatory organs, so that most fixed parasites
have these systems of organs in simple condition. Altogether
the body of a fixed, permanent parasite is so simplified and so
wanting in all those special structures which characterize the
higher, active, complex animals, that it often presents a very
different appearance from those animals with which we know
it to be nearly related.

The simplicity of parasites does not indicate that they
belong to the groups of primitive simple animals. Parasitism
is found in the whole range of animal life, from primitive to
highest, although the vertebrate animals include very few para-
sites and these of little specialization of habit. But their
simplicity is something that has resulted from their mode of
life. It is the result of a change in the body structure which
we can often trace in the development of the individual para-
site. Many parasites in their young stages are free, active

PARASITISM AND DEGENERATION 351

animals with a better or more complex body than they possess
in their fully developed or adult stage. The simplicity of
parasites is the result of degeneration — a degeneration that
has been brought about by their adoption of a sedentary, non-
competitive parasitic life. And this simplicity of degenera-
tion, and the simplicity of primitiveness should be sharply
distinguished. Animals that are primitively simple have had
only simple ancestors; animals that are simple by degeneration
often have had highly organized, complex ancestors. And
while in the life history or development of a primitively simple
animal all the young stages are simpler than the adult, in a
degenerate animal the young stages may
be, and usually are, more complex and
more highly organized than the adult
stage.

In the few examples of parasitism
(selected from various animal groups)
that are described in the following pages
all these general statements are illus-
trated.

In the intestines of crayfishes, centi-
pedes, and several kinds of insects may
often be found certain one-celled animals FIG. 210.— The wingless
(Protozoa) which are living as parasites. *??**' Nycter^bia-

- . . J . (After Sharp; much en-

Their food, which they take into their larged.)
minute body by absorption, is the intes-
tinal fluid in which they lie. These parasitic Protozoa belong
to the genus Gregarina. Because the body of any protozoan
is as simple as an animal's body can well be, being com-
posed of but a single cell, degeneration cannot occur in the
cases of these parasites. There are, besides Gregarina, many
other parasitic one-celled animals, several kinds living inside
the cells of their host's body. Several kinds of these have
been proved to be the causal agents of serious human diseases.
Conspicuous among these are the minute parasitic Sporozoa
which are the actual cause of the malarial and similar fevers
that rack the human body in nearly all parts of the world.

In the class of Sporozoa (of the great branch Protozoa or
one-celled animals) is an order called Hemosporidia (or Hemo-
cytozoa) comprising numerous kinds of unicellular parasites
which live in the blood of vertebrates (with certain inverte-

352

EVOLUTION AND ANIMAL LIFE

brates as intermediate or alternate hosts). Certain kinds are
found in the cold blood of fishes, amphibians, reptiles, and
certain others in the warm blood of birds and mammals. The
genera Halteridium and Plasmodium contain certain species
that live exclusively in the blood of birds or mammals for part
of their life and in the bodies of certain arthropods for the
other part of their life. They produce a series of asexual
generations (reproduction by simple division or sporulation)
in their vertebrate hosts, and a sexual generation (reproduction

by the development
of a zygote formed
by the fusion of two
cells, called gametes)
in the arthropod host.
This arthropod host
for all the species so
far known of these
genera is exclusively
the mosquito. Three
species of the genus
Plasmodium, namely,
P. vivax, P. malarice,
and P. fakiparum, are
the specific causal
agents of the distinct
malarial fevers known
as tertian, quartan,
and tropical fever re-
spectively. (In the

literature of "mosquitoes and malaria," the name " Hcema-
moeba" will be found to be used synonymously with Plas-
modium.)

Laveran, a French surgeon in Algiers, discovered the Plas-
modium parasite in the red blood corpuscles of malarial fever
patients in 1880, and determined that the disease was actually
and solely due to the destructive and toxic effects of the growth
and multiplication of the parasite in the blood. Every forty-
eight hours in tertian fever (seventy-two hours in quartan) there
is completed a whole (asexual) cycle in the life of one of these
parasites, including its birth by the division of the body of
the mother into several small merozoites, the penetration of

FIG. 211. — The life cycle of Coccidium lithobii, Proto-
zoan parasite of the centipede, Lithobius. (After
Schaudin.)

PARASITISM AND DEGENERATION 353

a red blood corpuscle by each of these merozoites, the growth
of the parasite in the blood corpuscle at the expense of the
corpuscle, the maturing and sporulation (or division into new
merozoites) of the parasite, and the final breakdown o'f the
corpuscle and release into the blood plasma of the tiny active
merozoite. Numerous generations of this type are produced
in the blood of a patient, but finally a sort of senescence or
degeneration of the parasite sets in, and unless there is a fresh
infection of the patient from outside, the parasitic host dimin-
ishes and finally nearly disappears. If the effects of the para-
site have not been too severe during the height of its invasion
the patient now recovers.

For the continued multiplication and persistence of the
parasitic species a new process or set of conditions is necessary.
If a drop of blood drawn from a malarial patient is examined
under the microscope the parasitic individuals abundantly in
evidence in this blood will be seen to manifest a curious be-
havior within a few minutes. Some of them will move and
squirm about with great activity, and extend and retract
pseudopodiumlike processes, until finally with great rapidity
a few (usually four to six) delicate threadlike flagella or
flagellalike processes will shoot out from the body mass and
break away from it. These motile flagella are really gametes
or sexual cells of one type (the male) while other large nearly
immobile sub-spherical parasite individuals which do not be-
have as these do are gametes or sexual cells of the other (or
female) type. The flagella find and penetrate or fuse with the
larger gametes and form a zygote or resting egg cell.

While the processes just described have been taking place
in the blood droplet under our microscope, as a matter of fact
this normally takes place in the stomach of a mosquito. For
when a mosquito (at least of a certain kind) sucks blood from
a malarial patient the blood parasites are of course taken in
also and deposited in the stomach where digestion of the blood
begins. Now when the zygotes are formed in the mosquito's
stomach they do not remain lying in the stomach cavity but
move to the wall of the stomach and partially penetrate it.
As many as five hundred zygotes have been found in the
stomach walls of a single mosquito. The zygote now in-
creases rapidly in size, becoming a perceptible nodule on the
outer side of the stomach wall, but soon its nucleus and proto-

354 EVOLUTION AND ANIMAL LIFE

plasm begin to break up by repeated division (the parts all
being held together, however, in the wall of the zygote), and
by the end of the twelfth or fourteenth day the zygote 's proto-
plasm may have become divided into ten thousand minute
sporozoites. The zygote wall now breaks down, thus releasing
the thousands of active little sporozoites into the general body
cavity of the mosquito. This cavity is filled with flowing
blood plasm — insects do not have a closed but an almost com-
pletely open circulatory system — and swimming about in this
plasm the sporozoites soon make their way forward and into
the salivary glands of the mosquito. Now when the insect
pierces a human being to suck blood, it injects a certain amount
of salivary fluid into the wound (presumably to keep the blood
from clotting at the puncture) and with this fluid go many of
the sporozoites. Thus a new infection of malaria is made.
The sporozoites may lie in the salivary glands for several weeks,
and so for the whole time from twelve to fourteen days after
the mosquito has become infected with the malarial parasite
by sucking blood from a malarial patient until the sporozoites
in the salivary glands finally die, it is a means of the dissemina-
tion of the disease. There can be no malaria without mos-
quitoes to propagate arid disseminate it, and yet no mosquitoes
can propagate and disseminate malaria without having access
to malarial patients. The only mosquito species in this country
which has been proved to be a malaria disseminator is Anoph-
eles maculipennis, a spot ted- winged form spread over the
whole continent.

In the great branch or phylum of flatworms (Platyhelmin-
thes), that group of animals which of all the principal animal
groups is widest in its distribution, perhaps a majority of the
species are parasites. Instead of being the exception, the
parasitic life is the rule among these worms. Of the three
classes into which the flatworms are divided, almost all of the
members of two of the classes are parasites. The common
tapeworm (Tcenia) (Fig. 212), which lives parasitically in the
intestine of man, is a good example of one of these classes.
It has the form of a narrow ribbon, which may attain the
length of several yards, attached at one end to. the wall of the
intestine, the remainder hanging freely in the interior. Its
body is composed of segments or serially arranged parts, of
which there are about eight hundred and fifty altogether. It

PARASITISM AND DEGENERATION

355

lias no mouth nor alimentary canal. It feeds simply by ab-
sorbing into its body, through the surface, the nutritious,
already digested liquid food in the intestine. There are no
eyes nor other special sense organs, nor any organs of locomo-
tion. The body is very degenerate. The life history of the
tapeworm is interesting, because of the necessity of two hosts
for its completion. The eggs of the tapeworm pass from the
intestine with the excreta, and must be taken into the body
of some other animal in order to de-
velop. In the case of one of the several
species of tapeworms that infest man,
this other host must be the pig. In
the alimentary canal of the pig the
young tapeworm develops and later
bores its way through the walls of the
canal and becomes imbedded in the
muscles. There it lies, until it finds its
way into the alimentary canal of man
by his eating the flesh of the pig. In
the intestine of man the tapeworm con-
tinues to develop until it becomes full
grown.

In a lake in Yellowstone Park the
suckers are infested by one of the flat-
worms (Ligula) that attains a size of
nearly one fourth the size of the fish in
whose intestines it lives. If the tape-
worm of man attained such a compara-
tive size, a man of two hundred pounds' weight would be in-
fested by a parasite of fifty pounds' weight.

Another group of animals, many of whose members are
parasites, are the round worms or threadworms (Nemathel-
minthes). The free-living roundworms are active, well-
organized animals, but the parasitic kinds all show a greater
or less degree of degeneration. One of the most terrible para-
sites of man is a round worm called Trichina spiralis (Fig. 213).
It is a minute worm, from one to three millimeters long, which
in its adult condition lives in the intestine of man or of the pig
or other mammals. The young are born alive and bore through
the walls of the intestine. They migrate to the voluntary
muscles of the hosts, especially those of the limbs and back,

FIG. 212. — Tapeworm,
Tcenia solium. In the
upper left-hand cor-
ner is the much en-
larged head. (After
Leuckart.)

356

EVOLUTION AND ANIMAL LIFE

and here each worm coils itself up in a muscle fiber and becomes
inclosed in a spindle-shaped cyst or cell (Fig. 213). A single
muscle may be infested by hundreds of thousands of these
minute worms. It has been estimated that fully one hundred
million encysted worms may exist in the muscles of a " trichin-
ized" human body. The muscles undergo more or less de-
generation, and the death of the host may occur. It is necessary,
for the further development of the
worms, that the flesh of the host be
eaten by another mammal, as the
flesh of the pig by man, or the flesh
of man by a pig or rat. The Trichina
in the alimentary canal of the new
host develop into active adult worms
and produce new young.

In the Yellowstone Lake the trout
are infested by the larvae or young of
a roundworm (Bothriocephalus cordi-
ceps) which reach a length of twenty
inches, and which are often found
stitched, as it were, through the vis-
cera and the muscles of the fish. The
infested trout become feeble and die,
or are eaten by the pelicans which fish
in this lake. In the alimentary canal
of the pelican the worms become
adult, and parts of the worms con-
taining eggs escape from the ali-
mentary canal with the excreta. These

portions of worms are eaten by the trout, and, the eggs give
birth to new worms which develop in the bodies of the fish
with disastrous effects. It is estimated that for each pelican
in Yellowstone Lake over five million eggs of the parasitic
worms are discharged into the lake.

The young of various carnivorous animals are often infested
by one of the species of roundworms called " pup worms "
(Undnaria). Recent investigations show that thousands of
the young or pup fur seals are destroyed each year by these
parasites. The eggs of the worm lie through the winter in the
sands of the breeding grounds of the fur seal. The young
receive them from the fur of the mother and the worm de-

FIG. 213. — Trichina spiralis,
the terrible parasite of pork:
a, Male; b, cyst; c, female.

PARASITISM AND DEGENERATION

357

velops in the upper intestine. It feeds on the blood of the
you ni;- seal, which finally dies from anaemia. On the sand
beaches of the seal islands in Bering Sea there are every year

thousands of dead seal pups which have been killed by this
parasite (Fig. 214). On the rocky rookeries, the young seals
are not affected by this parasite.
24

358

EVOLUTION AND ANIMAL LIFE

Among the more highly organized animals the results of a
parasitic life, in degree of structural degeneration, can be more
readily seen. A well-known parasite, belonging to the Crus-
tacea— the class of shrimps, crabs, lobsters, and crayfishes — is
Sacculina. The young Sacculina (Fig. 215, A) is an active, free-
swimming larva much like a young prawn or young crab. But
the adult bears absolutely no resemblance to such a typical
crustacean as a crayfish or crab. The Sacculina after a short

B

FIG. 215. — Development of the parasitic crustacean, Sacculina carcinus: A, Naplius
stage; B, cypris stage; C, adult attached to its host, the crab, Carcinus mccnas.
(After Hertwig.)

period of independent existence attaches itself to the abdomen
of a crab, and there completes its development while living as a
parasite. In its adult condition (Fig. 215, C) it is simply a great
tumorlike sac, bearing many delicate rootlike suckers which
penetrate the body of the crab host and absorb nutriment.
The Sacculina has no eyes, no mouth parts, no legs, or other
appendages, and hardly any of the usual organs except re-
productive organs. Degeneration here is carried very far.

Other parasitic Crustacea, as the numerous kinds of fish
lice (Fig. 216) which live attached to the gills or to other parts
of fish, and derive all their nutriment from the body of the
fish, show various degrees of degeneration. With some of
these fish lice the female, which looks like a puffed-out worm,
is attached to the fish or other aquatic animal, while the male,
which is perhaps only a tenth of the size of the female, is per-
manently attached to the female, living parasitically on her.

PARASITISM AND DEGENERATION

359

FIG. 216.— The fish louse,
Lernaecera: a, Adult; b,
larva.

Among the insects there are many kinds that live para-
sitically for part of their life, and not a few that live as
parasites for their whole life. The true
sucking lice and the bird lice live for
their whole lives as external parasites
on the bodies of their host, but they
are not fixed — that is, they retain their
legs and power of locomotion, although
they have lost their wings through de-
generation. The eggs of the lice are
deposited on the hair of the mammal
? or bird that serves as host; the young
hatch and immediately begin to live as
parasites, either sucking the blood or
feeding on the hair or feathers of the
host. In the order Hymenoptera there
are several families, all of whose mem-
bers live during their larval stage as parasites. We may
call all these hymenopterous parasites ichneumon flies. The
ichneumon flies are parasites of other insects, especially of the
larvae of beetles and moths and butter-
flies. In fact, the ichneumon flies do
more to keep in check the increase of in-
jurious and destructive caterpillars than
do all our artificial remedies for these in-
sect pests. The adult ichneumon fly is
four-winged and lives an active, indepen-
dent life. It lays its eggs either in or on
or near some caterpillar or beetle grub,
and the young ichneumon, when hatched,
burrows into the body of its host, feed-
ing on its tissues, but not attacking such
mon fly, Pimpia con- organs as the heart or nervous ganglia,

guisitor, laying eggs ° . . . . . .. ' ' . .,

in the cocoon of the whose injury might mean immediate death
to the host. The caterpillar lives with the
ichneumon grub within it, usually until
nearly time for its pupation. In many
instances, indeed, it pupates with the
parasite still feeding within its body, but it never comes to
maturity. The larval ichneumon fly pupates either within the
body of its host (Fig. 218) or in a tiny silken cocoon outside

FIG. 217. — The ichneu-

American tent cater-
pillar moth. (After
Fiske; about natural
size.)

360 EVOLUTION AND ANIMAL LIFE

of its body. From the cocoons the adult winged ichneumon
flies emerge, and after mating find another host on whose
body to lay their eggs.

One of the most remarkable ichneumon flies is Thalessa
(Fig. 219), which has a very long, slender, flexible ovi-
positor, or egg-laying organ. An insect known as the pigeon
horntail (Tremex columba) (Fig. 220) deposits its eggs, by
means of a strong, piercing ovipositor, half an inch deep in
the trunk wood of growing trees. The young or larval Tremex
is a soft-bodied white grub, which bores deeply into the trunk
of the tree, filling up the burrow behind it with small chips.
The Thalessa is a parasite of the Tremex, and "when a female
Thalessa finds a tree infested by Tremex, she selects a place

FIG. 218. — Parasitized caterpillar from which the ichneumon fly parasites have issued,
showing circular holes of escape in skin.

which she judges is opposite a Tremex burrow, and, elevating
her long ovipositor in a loop over her back, with its tip on the
bark of the tree (Fig. 221), she makes a derrick out of her body
and proceeds with great skill and precision to drill a hole into
the tree. When the Tremex burrow is reached she deposits
an egg in it. The larva that hatches from this egg creeps
along this burrow until it reaches its victim, and then fastens
itself to the horntail larva, which it destroys by sucking its
blood. The larva of Thalessa, when full grown, changes to
a pupa within the burrow of its host, and the adult gnaws
a hole out through the bark if it does not find the hole already
made by the Tremex. )}

The beetles of the family Stylopidse present an interesting
case of parasitism. The adult males are winged, but the adult
females are wingless and grublike. The larval stylopid at-
taches itself to a wasp or a bee, and bores into its abdomen.
It pupates within the abdomen of the was]) or bee, and lies
there with its head projecting slightly from a suture between
two of the body rings of its host.

PARASITISM AND DEGENERATION

301

Almost all of the mites and ticks, animals allied to the
spiders, livo parasitically. Most of them live as external para-
sites, sucking the blood of their host, but some live under-
neath the skin like the itch
mites (Fig. 222), which cause,
in man, the disease known
as the itch.

Among the vertebrate ani-
mals there are not many ex-
amples of true parasitism.
The hagfishes or borers
(Myxine, etc.) have been al-
ready mentioned. These are
long and cylindrical, eellike
creatures, very slimy and
very low in structure. The
mouth is without jaws, but
forms a sucking disk, by
which the hagfish attaches
itself to the body of some
other fish. By means of the
rasping teeth on its tongue,
it makes a round hole
through the skin, usually at
the throat. It then devours
all the muscular substance
of the fish, leaving the vis-
cera untouched. When the
fish finally dies it is a mere
hulk of skin, scales, bones,
and viscera, nearly all the
muscle being gone. Then
the hagfish slips out and at-
tacks another individual.

The lamprey, another low

fish, in similar fashion feeds leechlike on the blood of other
fishes, which it obtains by lacerating the flesh with its rasp-
like teeth, remaining attached by the round sucking disk of
its mouth.

Certain birds, as the cowbird and the European cuckoo,
have a parasitic habit, laying their eggs in the nests of other

FIG. 219. — The large ichneumon fly,
Thalessa, with long ovipositor.

362

EVOLUTION AND ANIMAL LIFE

FIG. 220. — The pigeon horn-tail, Tremex
columba, with strong bearing ovipositor.

birds, leaving their young to be hatched and reared by their
unwilling hosts. This is, however, not bodily parasitism, such

as is seen among lower
forms.

We may also note that
parasitism and consequent
structural degeneration are
not at all confined to ani-
mals. Many plants are
parasites and show marked
degenerative characteristics.
The dodder is a familiar
example, clinging to living
green plants and thrusting
its haustoria or rootlike
suckers into their tissue to
draw from them already

elaborated nutritive sap. Many fungi like the rusts of cereals,
the mildew of roses, etc., are parasitic. Numerous plants, too,
are parasites, not on other plants, but on animals. Among
these are the hosts
of bacteria (sim-
plest of the one-
celled plants) that
swarm in the tis-
sues of all animals,
some of which are
causal agents of
some of the worst
of human and ani-
mal diseases (as
typhoid fever, diph-
theria, and cholera
in man, anthrax in
cattle). There are
also many more
highly organized

fungi like the whole family of Entomophthorse, and the genus
Sporotrichum that live in and on the bodies of insects, often
killing them by myriads. One of the great checks to the
ravages of the corn and wheat-infesting chinch bug (Blissus

FIG. 221. — Thalessa lunator boring. (After Comstock.)

PARASITISM AND DEGENERATION

363

leucopterus) of the Mississippi Valley is a parasitic fungus (Sporo-
trichum globuliferum) . In the autumn, house flies may often
be seen dead against a windowpane surrounded by a delicate
ring or halo of white. This ring is
composed of spores of the fungus, Em-
pusa aphidis, which has grown through
all the tissues of the fly while alive,
finally resulting in its death. The
spores are thrown off from tiny fruit-
ing hyphse of the fungus which have
grown out through the body wall of
the insect. And they serve to inocu-
late other flies that may come near.

Just as in animals, so in plants;
parasitic kinds, especially among the
higher groups as the flowering plants,
often show marked degeneration. Leaves
may be reduced to mere scales, roots are lost, and the water-
conducting tissues greatly reduced. This degeneration in plants
naturally affects primarily those parts which in the normal
plant are devoted to the gathering and elaboration of inor-
ganic food materials, namely, the leaves and stems and roots.

FIG. 222. — The itch mite,
Sarcoptes scabei.

FIG. 223. — The fungus, Cordiceps, growing on a caterpillar. (Natural size.)

The flowers or reproductive organs usually retain, in parasites,
all of their high development.

While parasitism is the principal cause of degeneration of
animals, other causes may be also concerned. Fixed animals
or animals leading inactive or sedentary lives, also become
degenerate, even when no parasitism is concerned. The tuni-
cata or s^a squirts (Fig. 224) are animals whose simplicity of
structure is due to degeneration from the acquisition of a
sedentary habit of life.

The young or larval tunicate is a free-swimming active tad-
polelike creature with organs much like those of the adult of

364

EVOLUTION AND ANIMAL LIFE

the simplest fishes or fishlike forms. That is, the sea squirt
begins life as a primitively simple vertebrate. It possesses
in its larval stage a notochord, the delicate structure which
precedes the formation of a backbone, extending along the
upper part of the body, below the spinal cord. It is found
in all young vertebrates, and is characteristic of the branch.

The other organs of the
young tunicate are all of
vertebral type. But the
young sea squirt passes a
period of active and free
life as a little fish, after
which it settles down and
attaches itself to a stone
or shell or wooden pier
by means of suckers, and
remains for the rest of its
life fixed. Instead of go-
ing on and developing into
a fishlike creature, it loses
its notochord, its special
sense organs, and other
organs; it loses its com-
plexity and high organiza-
tion, and becomes a "mere
rooted bag with a double
neck," a thoroughly de-
generate animal.

A barnacle is another

example of degeneration through quiescence. The barnacles
are crustaceans related most nearly to the crabs and shrimps.
The young barnacle just from the egg (Fig. 225, /) is a six-
legged, free-swimming nauplius, much like a young prawn or
crab, with single eye. In its next larval stage it has six
pairs of swimming feet, two compound eyes, and two large
antennae or feelers, and still lives an independent, free-swim-
ming life. When it makes its final change to the adult con-
dition, it attaches itself to some stone or shell, or pile or
ship's bottom, loses its compound eyes and feelers, develops
a protecting shell, and gives up all power of locomotion. Its
swimming feet become changed into grasping organs, and it

FIG. 224. — The sea squirt or tunicate.

PARASITISM AND DEGENERATION

365

loses most of its outward resemblances to the other members
of its class (Fig. 225, e).

Certain insects live sedentary or fixed lives. All the mem-
bers of the family of scale insects (Coccidae), in one sex at
least, show degeneration that has been caused by quiescence.
One of these coccids, called the red orange scale, is very

FIG. 225. — Three crustaceans and then larvae : a. Prawn, Peneus; b, Peneus, larva;
c, Sacculit:a, parasite; d, larva Sacculiiia; e, barnacle, Lepas, quiescent; /, larva
of barnacle. (After Haeckel.)

abundant in Florida and California and in other orange-grow-
ing regions. The male is a beautiful, tiny, two-winged midge,
but the female is a wingless, footless little sac without eyes or
other organs of special sense, and lies motionless under a
flat, thin, circular, reddish scale composed of wax and two
or three cast skins of the insect itself. The insect has a long,
slender, flexible, sucking beak, which is thrust into the leaf or
stem or fruit of the orange on which the " scale bug " lives and
through which the insect sucks the orange sap, which is its only
food. It lays eggs or gives birth to young under its body, under

366

EVOLUTION AND ANIMAL LIFE

the protecting wax scale, and dies. From the eggs hatch active
little larval scale bugs with eyes and feelers and six legs. They
crawl from under the wax scale and roam about over the orange
tree. Finally, they settle down, thrust their sucking beak
into the plant tissues, and cast their skin. The females lose
at this molt their legs and eyes and feelers. Each becomes a

mere motionless sac capable
only of sucking up sap and
of laying eggs. The young
males, however, lose their
sucking beak and can no
longer take food, but they
gain a pair of wings and an
additional pair of eyes. They
fly about and fertilize the
saclike females, which then
molt again and secrete the
thin wax scale over them.

Throughout the animal
kingdom loss of the need
of movement is followed
by the loss of the power
to move, and of all struc-
tures related to it.

Loss of certain organs
may occur through other
causes than parasitism and

FIG. 226.— The black scale, Lecanium olea; & £xe(J fifa Many insects
and its parasite, the tiny chalcicl fly, Scu- . .

tellista cyanea; and the ladybird beetle, llVC but a short time m

Rhizobius ventralis. (After Isaacs.) their adult Stage. May flies

live for but a few hours

or, at most, a few days. They do not need to take food to
sustain life for so short a time, and so their mouth parts have
become rudimentary and functionless or are entirely lost.
This is true of some moths and numerous other specially shprt-
lived insects. Among the social insects the workers of the
termites and of the true ants are wingless, although they are
born of winged parents, and are descendants of winged ancestors.
The modification of structure dependent upon the division of
labor among the individuals of the community has taken the
form, in the case of the workersj of a degeneration in the loss of

PARASITISM AND DEGENERATION 367

the wings. Insects that live in caves are mostly blind; they
have lost the eyes, whose function could not be exercised in the
darkness of the cave. Certain island-inhabiting insects have
lost their wings, flight being attended with too much danger.
The strong sea breezes may at any time carry a flying insect
off the small island to sea. Probably only those which do not
fly much survive, and so by natural selection wingless breeds
or species are produced. Finally, we may mention the great
modifications of structure, often resulting in the loss of certain
organs, which take place to produce protective resemblances
(see Chapter XIX). In such cases the body may be modified
in color and shape so as to resemble some part of the environ-
ment, and thus the animal may be unperceived by its enemies.
Many insects have lost their wings through this cause.

When we say that a parasitic or quiescent mode of life leads
to or causes degeneration, we have explained the stimulus or
the ultimate reason for the degenerative changes, but we have
not shown just how parasitism or quiescence actually produces
these changes. Degeneration or the atrophy and disappear-
ance of organs or parts of a body is often said to be due to dis-
use. That is, the disuse of a part is believed by many natural-
ists to be the sufficient cause for its gradual dwindling and final
loss. That disuse can so afreet parts of a body during the life-
time of an individual is true. A muscle unused becomes soft
and flabby and small. Whether the effects of such disuse can
be inherited, however, is open to serious doubt. Such in-
heritance must be assumed if disuse is to account for the gradual
growing less and final disappearance of an organ in the course
of many generations. Some naturalists believe that the results
of such disuse can be inherited, but as yet such belief rests on
no certain knowledge. If characters acquired during the life-
time of the individual are subject to inheritance, disuse alone
may explain degeneration. If not, some other immediate
cause, or some other cause along with disuse, must be found.

We are accustomed, perhaps, to think of degeneration as
necessarily implying a disadvantage in life. A degenerate
animal is considered to be not the equal of a nondegenerate
animal, and this would be true if both kinds of animals had to
face the same conditions of life. The blind, footless, simple,
degenerate animal could not cope with the active, keen-sighted,
highly organized nondegenerate in free competition. But free

368 EVOLUTION AND ANIMAL LIFE

competition is exactly what the degenerate animal has nothing
to do with. Certainly the Sacculina lives successfully; it is
well adapted for its own peculiar kind of life. For the life of a
scale insect, no better type of structure could be devised. A
parasite enjoys certain obvious advantages in life, and even
extreme degeneration is no drawback, but rather favors it in
the advantageousness of its sheltered and easy life. As long
as the host is successful in eluding its enemies and avoiding
accident and injury, the parasite is safe. It needs to exercise
no activity or vigilance of its own; its life is easy as long as its
host lives. But the disadvantages of parasitism and degenera-
tion are apparent also. The fate of the parasite is usually
bound up with the fate of the host. When the enemy of the
host crab prevails, the Sacculina goes down without a chance to
struggle in its own defense. But far more important than
the disadvantage in such particular or individual cases is the
disadvantage of the fact that the parasite cannot adapt itself
in any considerable degree to new conditions. It has become
so specialized, so greatly modified and changed to adapt itself
to the one set of conditions under which it now lives, it has
gone so far in its giving up of organs and body parts, that if
preseni^conditions should change and new ones^ome to exist,
the parasite could not adapt itself to them. The independent,
active animal with all its organs and all its functions intact,
holds itself, one may say, ready and able to adapt itself to any
new conditions of life which may gradually come into existence.
The parasite has risked everything for the sake of a sure and
easy life under the presently existing conditions.
conditions means its extinction.

CHAPTER XVIII

MUTUAL AID AND COMMUNAL LIFE AMONG

ANIMALS

More ancient than competition is combination. The little feeble
fluttering folk of God, the spinning insects, the little mice in the
meadow, the rat in the cellar, the crane in the marshes or the booming
bittern, all these have learned that God's greatest word is together
and not alone. He who is striving to make God's blessing and bounty
possible to most is stepping into line with nature. The selfish man is
the isolated man. — OSCAR CARLTON McCuLLocn.

MAN is not the only social animal, nor the only animal
species whose individuals live in mutually advantageous rela-
tions with each other, and in mutually advantageous relations
with individuals of other animal kinds. Just as man lives
communally and mutually helpfully with other men, so do the
members of a great honeybee or ant community live together:
and as we find various other animals as dogs, horses, and doves
living under the care and protection of man and returning to
him a measure of service in work, companionship, or other
helpfulness for his care and feeding, so do we know of hundreds
of kinds of other insects that live commensally with ants, each
party to this commensal or symbiotic life gaining something
from and giving something to the other party of this arrange-
ment. Indeed, the communal life of such insects as the social
bees, wasps, and ants is developed along true communistic
lines far more specialized than the communism shown by man.

Just as students of human society can trace a series of steps
from a very primitive living together or communal life among
men to the present highly specialized condition, so among vari-
ous animals we can find a long series of gradatory conditions of
social life from mere gregariousness like that of a band of wolves

369

370 EVOLUTION AND ANIMAL LIFE

or a herd of bison, to the extremely specialized, interdependent
and unified community of the honeybee, or agricultural ant.
Before taking up this series of stages in true social or communal
development among the lower animals, however, we may profit-
ably give some attention to the conditions of animal association
commonly known as commensalism or symbiosis in which in-
dividuals of one species are associated to their mutual advan-
tage with individuals of different species.

In the relations of parasite and host, discussed in the last
chapter, all the advantages of the association lie with the para-
site. The other animal involved; the host, suffers inconveni-
ence, injury, often untimely death. But in commensalism and
symbiosis both associating kinds of animals reap advantage, or

FIG. 227. — Remora Echeneis remora, with dorsal fin modified to be the sucking plate
by which the fish attaches itself to a shark.

at least neither suffers in any serious way from the effects of
the other's presence. The two kinds live together in harmony
and usually to their actual mutual advantage. The term
commensalism may be applied to denote a condition of loose
and often not obviously equally mutual advantageous asso-
ciation, while symbiosis is used to refer to a more intimate and
persistent association with maybe marked cooperation and
mutual advantage. A few examples of each are given in the
following pages. Of course, no marked line of demarcation
can be really drawn between the two conditions, any more
than we can establish a sharp distinction between the preda-
tory and parasitic modes of life.

A curious example of commensalism is afforded by the
different species of Remoras (Echenididse) which attach them-
selves to sharks, barracudas, and other large fishes by means
of a sucking disk on the top of the head (Fig. 227) . This disk
is made by a modification of the dorsal fin. The Remora thus
attached to a shark may be carried about for weeks, leaving
its host only to secure food. This is done by a sudden dash

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 371

through the water. The Remora injures the shark in no way
save, perhaps, by the slight check its presence gives to the
shark's speed in swimming.

In the mouth of the menhaden (Brevoortia tyrannus) a small
crustacean (Cymothce prcegustator) is almost always present,
always resting in the front of the lower jaw. This arrange-
ment is of advantage to the crustacean, but is a matter of in-
difference to the fish. Latrobe, who first described this fish,
compares the crustacean to the prsegustator or foretaster of
the Roman tyrants — a slave used in prevention of poisoning.

Whales, similarly, often carry barnacles about with them.
These barnacles are permanently attached to the skin of the
whale just as they would be to a stone or wooden pile. Many
small crustaceans, annelids, mollusks, and other invertebrates
burrow into the substance of living sponges, not for the purpose
of feeding on them, but for shelter. On the other hand, the
little boring sponge (Cliona) burrows in the shells of oysters
and other bivalves for protection. These are hardly true cases
of even that lesser degree of mutually advantageous associa-
tion which we are calling commensalism. But some species of
sponge "are never found growing except on the backs or legs
of certain crabs. " In these cases the sponge, with its many
plantlike branches, protects the crab by concealing it from
its enemies, while the sponge is benefited by being carried about
by the crab to new food supplies. Certain sponges and polyps
are always found growing in close association, though what the
mutual advantage of this association is has not yet been found
out.

Among the coral reefs in the South Seas there lives an
enormous kind of sea anemone or polyp. Individuals of this
great polyp measure two feet across the disk when fully ex-
panded. In the interior, the stomach cavity, which com-
municates freely with the outside by means of the large mouth
opening at the free end of the polyp, there may often be found
a small fish (Amphiprion percula). That this fish is purposely
in the gastral cavity of the polyp is proved by the fact that
when it is dislodged it invariably returns to its singular lodging
place. The fish is brightly colored, being of a brilliant vermilion
hue with three broad white cross bands. The discoverer of
this peculiar habit suggests that there are mutual benefits to
fish and polyp from this habit. "The fish being conspicuous,

372

EVOLUTION AND ANIMAL LIFE

is liable to attacks, which it escapes by a rapid retreat into the
sea anemone; its enemies in hot pursuit blunder against the
outspread tentacles of the anemone and are at once narcotized
by the 'thread cells' shot out in innumerable showers from

the tentacles, and afterwards
drawn into the stomach of the
anemone and digested/'

Small fish of the genus Nomeus
may often be found accompany-
ing the beautiful Portuguese
man-of-war (Physalia) as it sails
slowly about on the ocean's sur-
face (Fig. 228). These little fish
lurk underneath the float and
among the various hanging
threadlike parts of the Physalia,
which are provided with sting-
ing cells. The fish are protected
from their enemies by their prox-
imity to these stinging threads.
Similarly, several kinds of me-
dusae are known to harbor or to
be accompanied by the young
or small adult fishes (Caranx,
Psenes) .

In the nests of the various
species of ants and termites
many different kinds of other
insects have been found. Some
of these are harmful to their
hosts, in that they feed on the
food stores gathered by the in-
dustrious and provident ant, but
others appear to feed only on
refuse or useless substances in

the nest. Some appear to be of help to their hosts by clean-
ing the nests and by secreting certain fluids much liked by
the ants. Over one thousand species of these myrmecophilous
(ant-loving) and termitophilous (termite-loving) insects have
been recorded by collectors as living habitually in the nests of
ants and termites. Many of them (they are mostly small

FIG. 228. — The Portuguese man-of-
war, Physalia, with men-of-war
fishes, Nomeus gronovii, living in
the shelter of the stinging feelers.
(Specimens from off Tampa, Fla.)

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS ;}?:

beetles and flies) have lost their wings and have had their
bodies otherwise considerably modified, usually in such wise
that they come greatly to resemble in external appearance
the ants with which they live. The owls and rattlesnakes
which live with the prairie dogs in their villages afford another
familiar example of commensalism.

Of a more intimate character, and of more obvious and
certain mutual advantage, is the well-known case of the sym-
biotic association of some of the numerous species of hermit
crabs and certain species of sea anemones. The hermit crab
always takes for
its habitation the
shell of another
animal, often that
of the common
whelk. All of the
hind part of the
crab lies inside
the shell, while its
head with its great
claws project from
the opening of the
shell. On the sur-
face of the shell
near the opening
there is often to

be found a sea anemone, or sea rose (Fig. 229). This sea
anemone is fastened securely to the shell, and has its mouth
opening and tentacles near the head of the crab. The sea
anemone is carried from place to place by the hermit crab,
and in this way is much aided in obtaining food. On the
other hand, the crab is protected from its enemies by the
well-armed and dangerous tentacles of the sea anemone. In
the tentacles there are many thousand long, slender stinging
threads, and the fish or octopus that would obtain the her-
mit crab for food must first deal with the stinging anemone.
There is no doubt here of the mutual advantage gained by
these two widely different but intimately associated com-
panions. If the sea anemone be torn away from the shell
inhabited by one of these crabs, the crab will wander about,
carefully seeking for another anemone. When it finds it, it
25

FIG. 229. — Hermit crab within a shell on which is growing
a colony of Podocoryne cornea. This colony is composed
of several different kinds of polyp individuals, the sting-
ing ones being situated along the front margin of the
shell. (After Weismann.)

374 EVOLUTION AND ANIMAL LIFE

.struggles to loosen it from its rock or from whatever it may
be growing on, and does not rest until it has torn it loose and
placed it on its shell.

There are numerous small crabs called pea crabs (Pin-
notheres) which live habitually inside the shells of living mussels.
The mussels and the crabs live together in apparent harmon}^
and to their mutual benefit.

The relations between ants and aphids (plant lice) are
often referred to in popular natural histories and books about

FIG. 230. — The hermit crab, Pagurus bernhardus, in snail shell covered with
Hydractinia.

insects as examples of symbiosis of unusual interest. Un-
fortunately, however, not enough careful study has been given
to many of these apparently true examples of symbiosis to
enable us to be certain of the truth of the alleged care and
guarding of the ant-cows, as Linnseus called these aphids, by
their milkers, the ants. That ants do swarm about the aphids
to lap up the "honey dew" excreted by them is wholly true,
and the very presence of the sharp-jawed and pugnacious
ants must keep away many enemies of the defenseless plant
lice, toothsome morsels for the ladybird beetles, flower-fly
larva? and other predatory insects.

In the case of the interesting relations between the corn
root aphid, Aphis maidisradici, of the Mississippi Valley States
and the little brown ant, Lasius brunneus, however, we

MlTl.'AL AID AND COMMUNAL LIFE AMONG ANIMALS 375

have the careful observations of Professor Forbes to rely on.
In the Mississippi Valley, this aphid deposits in autumn its
eggs in the ground in corn fields, often in the galleries of the
little brown ant. The following spring before the corn is
planted, these eggs hatch. Now, the little brown ant is es-
pecially fond of the honey dew secreted by the corn root lice.
So when the latter hatch in the spring, before there are corn
roots for them to feed on, the ants carefully place them on
the roots of certain kinds of grass and knot weed (Setaria,
Polygonum) , and there protect them until the corn germinates.
They are then removed to the roots of the corn. It is prob-
able that the ants even collect the eggs of the aphids in the
autumn and carry them into their nests for protection and care.

The studies of Wheeler and others have revealed some
interesting cases of the living together of different species of
ants. In some cases one of the ant species may be living almost
wholly at the expense of the other species, as does the little
yellow thief-ant, Solenopsis molesta. Although this ant some-
times lives in independent nests, more often it is to be found
living in association with some large ant species — it consorts
with many different hosts — feeding almost exclusively on the
live larvae and pupae of the host. The thief-ant is so small
and obscurely colored that it seems to live in the nests of its
host practically unperceived. The Solenopsis nest may -be
found by the side of the host nest, around it, or partly in it,
the tiny Solenopsis galleries ramifying through the nest mass
of the host, and often opening boldly into these large galleries.
Through their narrower passages, too narrow to be traversed
by the hosts, the tiny thief-ants thread their way through the
host nest in their burglarious excursions (Fig. 245).

But there are numerous cases of a less one-sided advantage
in the association of different species. As an example the
conditions exhibited by the red-brown ant, Myrmica brevi-
nodes and the smaller Leptothorax emersoni (conditions made
known by Wheeler's careful observations) may be briefly
described (Fig. 246). The little Leptothorax ants live in the
Myrmica nests, building one or more chambers with entrances
from the Myrmica galleries, so narrow that the large Myrmi-
cas cannot get through them. When needing food the Lepto-
thorax workers come into the Myrmica galleries and chambers
and, climbing on the backs of the Myrmica workers, proceed

376 EVOLUTION AND ANIMAL LIFE

to lick the face and the back of the head of each host. A
Myrmica thus treated, says Wheeler,

"paused, as if spellbound by this shampooing and occasionally folded
its antennae as if in sensuous enjoyment. The Leptothorax after licking
the Myrmica's pate, moved its head round to the side and began to
lick the cheeks, mandibles, and labium of the Myrmica. Such ardent
osculation was not bestowed in vain, for a minute drop of liquid —
evidently some of the recently imbibed sugar-water — appeared on the
Myrmica's lower lip and was promptly lapped up by the Leptothorax.
The latter then dismounted, ran to another Myrmica, climbed on its
back, and repeated the very same performance. Again it took toll
and passed on to still another Myrmica. On looking about in the nest
I observed that nearly all the Leptothorax workers were similarly
employed."

Wheeler believes that the Leptothorax get food only in this way.
They feed their queen and larvae by regurgitation. The
Myrmicas seem not to resent at all the presence of their Lepto-
thorax guests, and indeed may derive some benefit from the
constant cleansing licking of their bodies by the shampooers.
But the Leptothorax workers are careful to keep their queen
and young in a separate chamber, not accessible to their hosts.
This is probably the part of wisdom, as the thoughtless habit of
eating any conveniently accessible pupae of another species is
widespread among ants.

There are numerous interesting cases of symbiosis in which
not different kinds of animals are concerned, but animals and
plants. It has long been known that some sea anemones
possess certain body cells which contain chlorophyll, that green
substance characteristic of the green plants, and only in few
cases possessed by animals. When these chlorophyll-bearing
sea anemones were first found, it was believed that the chloro-
phyll cells really belonged to the animal's body, and that this
condition broke down one of the chiefest and most readily
apparent distinctions between animals and plants. But it
is now known that these chlorophyll-bearing cells are micro-
scopic, one-celled plants, green algae, which live habitually
in the bodies of the sea anemone. It is a case of true symbiosis.
The algae, or plants, use as food the carbon dioxide which is
given off in the respiratory processes of the sea anemone, and

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 377

the sea anemone breathes in the oxygen given off by the algae
in the process of extracting the carbon for food from the car-
bon dioxide. These algae, or one-celled plants, lie regularly
only in the innermost of the three cell layers which compose
the wall or body of the sea anemone (Fig. 231) . They penetrate
into and lie in the interior of the cells of this layer, whose special
function is that of digestion. They give this innermost layer
of cells a distinct green color. Even certain amcebalike
protozoans have been found to contain individuals of a one-
celled alga, Chlordla,
in their single-celled
bodies, the tiny ani-
mal and smaller plants
living together truly
symbiotically.

Among the higher
plants and animals,
cases of symbiosis are
not rare. There lives
in the live-oak trees
in the vicinity of Stan-
ford University a CCr- FIG. 231.— Diagrammatic section of sea anemone:

tain scale insect, Cero-
coccus ehrhorni, which
differs from the other
two or three species of

its genus in not having its body covered by a heavy, thick,
protecting layer of secreted wax. But it gets the needed
protection in another way. It is always covered by a thick
feltlike fungus growth, which has been found by investiga-
tion to germinate its spores and to find a constant food
supply in the "honey dew" excreted by the scale insects.
This feltlike covering of fungus, never found to be lacking
in the scale insect, serves apparently as a sufficient sub-
stitute for the heavy waxen mass common to the related
species.

The ants show particularly well instances of interesting
symbiotic life with plants. Fig. 232, drawn from a specimen
sent to us from the Philippine Islands by the botanist Cope-
land, shows some details of one such instance. The Dis-
chidias are milkweeds of the extreme Orient. They twine

a, The inner cell layer contains alga cells, the
two isolated cells at the right being cells of this
layer with contained algae; 6, middle body wall
layer; c, outer body wall layer. (After Hertwig.)

378

EVOLUTION AND ANIMAL LIFE

upon trees by means of their flexible stems and branches and
are especially noted for possessing appendages in the form of
pitchers. These pitcherlike appendages are modified leaves:
the normal Dischidia leaf is orbicular, thick, and fleshy. Each
pitcher is the blade of a leaf folded so that the lower surface
forms the inner surface of the pitcher. Into these pitchers

FIG. 232. — Leaves of Dischidia, which contain adventitious roots of the same plant
and in which live colonies of small ants. (From specimens from Philippine
Islands.)

grow adventitious roots that spring from the leaf peduncle.
Also in these pitchers live colonies of ants. As rent for furnish-
ing these comfortable cozy little ant homes, the Dischidia gets,
by means of the adventitious roots in the pitcher, food from
the excreta and cadavers of the ants. Hundreds of ants with
larvae and pupae can be found in these Dischidia leaves, and
without doubt we have here a mutually advantageous sym-
biotic adaptation.

From Weismann's chapter on Symbiosis in his "Vortrage
iiber Descendenztheorie," Vol. I, 1902, we translate the follow-

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 379

ing account of the symbiosis of the Aztec ants and the imbauba
tree:

"In the forests of South America grow the imbauba or so-called
candelabra trees, species of the genus Cecropia, which well deserve
their name, 'candelabra,' from the curious appearance given them
by the outspringing bare branches, each bearing a tuft of leaves at the
free end. These leaves are often attacked by the leaf-cutting ants of
the genus (Ecodoma, which roam by
tens of thousands over the various
plants of the forest biting off the
leaves, that they may fall to the ground,
where they are again seized, bitten into
pieces and the pieces carried into the
nests of the ants. In the nests they
serve as a medium on which grow cer-
tain molds or fungi, much liked by the
ants. The candelabra tree protects it-
self from these leaf-robbing enemies by
an association with another ant species,
Azteca instabilis, which finds safe dwel-
ling places in the hollow trunk of the
tree and a special supply of food in a
brownish fluid secreted by it. Along
the tree trunk occur in regular order
little pits through which the female
Azteca can easily bore into the interior,
where she lays her eggs and establishes
colonies, so that soon the interior of
the whole trunk swarms with ants

which rush out whenever the tree is shaken. But this alone would not
serve to protect the imbauba from the leaf cutters, for how could the
Aztecs dwelling inside the tree know of the presence of the light-footed
leaf-cutters without? But this is arranged for by the development on
the outside of the tree, at the very points where the danger is greatest,
namely, on the petioles of the younger leaves, of peculiar little hairy
growths from which project small white grains which are very nutri-
tious and not only eagerly eaten by ants, but garnered by them to carry
into their nests, presumably as food for their larva?. Thus right where
protection is most needed the plant has developed a special organ
attractive to the fierce Aztec ants, so that their constant presence at

FIG. 233. — Piece of a branch of
the Imbauba tree, Cecropia, the
leaves cut away, showing at
the base of each petiole the
small tuft on the food ; at the
right some of this ant food en-
larged. (After Schimper.)

380 EVOLUTION AND ANIMAL LIFE

these points is an effective protection against the encroachments of
the leaf-cutters, as courage and eagerness to fight other ants is already
characteristic of the Aztecs. Not all candelabra trees live in symbiosis
with ants or possess this special protection against the ravages of the
leaf-cutter species. Schimper found in the forests of Brazil several
species of Cecropia which never shelter ants in the chambers of the
hollow trunk. Now these species do not develop the curious special
food-producing organs at the bases of the leaf petioles. These species
lack the means of attracting and retaining the ant guests. Only one
species of candelabra tree, Cecropia peltata, has developed this arrange-
ment, and it is plainly of no direct use for the tree except through the
bringing to it of the protecting ants."

There are, of course, numerous other examples known of the
symbiotic association of plants and animals; and if we were to
follow the study of symbiosis into the plant kingdom we should
find that in one of the large groups of plants, the familiar
lichens which grow on rocks and tree trunks and old fences,
every member lives symbiotically. A lichen is not a single
plant, but is always composed of two plants, an alga (chloro-
phyll-bearing) and a fungus (without chlorophyll) living
together in a most intimate, mutually advantageous associa-
tion. But. we must devote no more space to the consideration
of this fascinating subject.

The simplest form of social life, or the living together of
several to many individuals of the same species, is shown among
those kinds of animals in which many individuals of one species
keep together, forming a great band or herd. In this case there
is not much division of labor, and the safety of the individual
is not wholly bound up in the fate of the herd. Such animals
are said to be gregarious in habit. The habit undoubtedly is
advantageous in the mutual protection and aid afforded the
individuals of the band. This mutual help in the case of
many gregarious animals is of a very positive and obvious
character. In other cases this gregariousness is reduced to
a matter of slight or temporary convenience, possessing but
little of the element of mutual aid. The great herds of rein-
deer in the north, and of the bison or buffalo which once ranged
over the Western American plains, are examples of a gregari-
ousness in which mutual protection from enemies, like wolves,
seems to be the principal advantage gained. The bands of

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 381

wolves which hunted the buffalo show the advantage of mutual
help in aggression as well as in protection. In this banding
together of wolves there is active cooperation among individuals
to obtain a common food supply. What one wolf cannot do —
that is, tear down a buffalo from the edge of the herd — a dozen
can do, and all are gainers by the operation.

On the other hand, the vast assembling of sea birds
on certain ocean islands and rocks is a condition probably
brought about rather by the special suitableness of a few places
for safe breeding than from any special mutual aid afforded;
still, these sea birds undoubtedly combine to drive off attack-
ing eagles and hawks. Eagles are usually considered to be
strictly solitary in habit (the unit of solitariness being a pair,
not an individual) ; but the description, by a Russian naturalist,
of the hunting habits of the great white-tailed eagle (Hali-
aetos albicilla) on the Russian steppes shows that this kind of
eagle at least has adopted a gregarious habit, in which mutual
help is plainly obvious. This naturalist once saw an eagle
high in the air, circling slowly and widely in perfect silence.
Suddenly the eagle screamed loudly. "Its cry was soon an-
swered by another eagle, which approached it, and was followed
by a third, a fourth, and so on, till nine or ten eagles came to-
gether and soon disappeared." The naturalist, following them,
soon discovered them gathered about the dead body of a horse.
The food found by the first was being shared by all. The
association of pelicans in fishing is a good example of the ad-
vantage of a gregarious and mutually helpful habit. The
pelicans sometimes go fishing in great bands, and, after having
chosen an appropriate place near the shore, they form a wide
half-circle facing the shore, and narrow it by paddling toward
the land, catching the fish which they inclose in the ever-
narrowing circle.

The wary Rocky Mountain sheep (Fig. 234) live together
in small bands, posting sentinels whenever they are feeding or
resting, who watch for and give warning of the approach of
enemies. The beavers furnish a well-known and very interest-
ing example of mutual help, and they exhibit a truly com-
munal life, although a simple one. They live in "villages"
or communities, all helping to build the dam across the stream,
which is necessary to form the broad marsh or pool in which the
nests or houses are built. Prairie dogs live in great villages

382

EVOLUTION AND ANIMAL LIFE

FIG. 234. — Rocky Mountain, or bighorn, sheep. (By permission of the
publishers of " Outing.")

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 383

or communities which spread over many acres. They tell
each other by shrill cries of the approach of enemies, and they
seem to visit each other and to enjoy each other's society a
great deal, although that they afford each other much actual
active help is not apparent. Birds in migration are grega-
rious, although at other times they may live comparatively
alone. In their long flights they keep together, often with
definite leaders who seem to discover and decide on the course

FIG. 235. — Prairie dogs. (Adapted from photo-
graph by Merriam.)

of flight for the whole great flock.
The wedge-shaped flocks of wild
geese flying high and uttering their
sharp, metallic call in their south-
ward migrations are well known in many parts of the United
States. Indeed, the more one studies the habits of animals
the more examples of social life and mutual help will be found.
Probably most animals are in some degree gregarious in habit,
and in all cases of gregariousness there is probably some de-
gree of mutual aid.

An interesting series of gradations from a strictly solitary
through a gregarious to an elaborately specialized communal
life is shown by the bees. Although the bumblebee and the
honeybee are so much, more familiar to us than other bee kinds
that the communal life exemplified by them may have come
to seem the usual kind of bee life, yet, as a matter of fact, there
are many more solitary bees than social ones. The general
character of the domestic economy of the solitary bees is well
shown by the interesting little green carpenter bee, Ceratina
dupla. Each female of this species bores out the pith from
five or six inches of an elder branch or raspberry cane, and

384

EVOLUTION AND ANIMAL LIFE

divides this space into a few cells by means of
transverse partitions (Fig. 236). In each cell
she lays an egg, and puts with it enough food
— flower pollen — to last the grub or larva
through its life. She then waits in an upper
cell of the nest until the young bees issue
from their cells, when she leads them off, and
each begins active life on its own account.
The mining bees Andrena, which make little
burrows (Fig. 237) in a clay bank, live in large
colonies — that is, they make their nest bur-
rows close together in the same clay bank, but
each female makes her own burrow, lays her
own eggs in it, furnishes it with food — a kind
of paste of nectar and pollen — and takes no
further care of her young. Nor has she at any
time any special in-
terest in her neigh-
bors. But with the
smaller mining bees,
belonging to the
genus Halictus, several
females unite in mak-
ing a common burrow, after which
each female makes side passages of
her own, extending from the main or
public entrance burrow. As a well-
known entomologist has said, Andrena
builds villages composed of individual
homes, while Halictus makes cities
composed of apartment houses. The
bumblebee (Fig. 238), however, es-
tablishes a real community with a
truly communal life, although a very
simple one. The few bumblebees
which we see in winter time are
queens; all other bumblebees die in
the autumn. In the spring a queen
selects some deserted nest of a field
mouse, or a hole in the ground, FlG 237.-Nest of the A
gathers pollen which she molds into the mining bee.

FIG. 236.— Nest
of carpenter
bee, Ceratina
dupla.

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 385

Q

a rather large irregular mass and puts
into the hole, and lays a few eggs on
the pollen mass. The young grubs or
larvae which soon hatch feed on the
pollen, grow, pupate, and issue as
workers — winged bees a little smaller
than the queen. These workers bring
more pollen, enlarge the nest, and make
irregular cells in the pollen mass, in each
of which the queen lays an egg. She
gathers no more pollen, does no more
work except that of egg-laying. From
these new eggs are produced more
workers, and so on until the com-
munity may come to be pretty large.
Later in the summer males and females
are produced and mate. With the ap-
proach of winter all the workers and
males die, leaving only the fertilized
females, the queens, to live through
the winter and found new communities Fl°- 238-~~ Bumblebees: «.

. , . Worker; o, queen or fer-

in the Spring. tile female.

The social wasps — as with the bees,

there are many more kinds
of solitary wasps than social
ones — show a communal life
like that of the bumblebees.
The only yellow jackets and
hornets that live through
the winter are fertilized
females or queens. When
spring comes each queen
builds a small nest sus-
pended from a tree branch,
or in a hole in the ground,
which consists of a small
comb inclosed in a covering
or envelope open at the
lower end. The nest is

FIG. 239 -The yellow jacket, Vespa, a social Composed of " wasp paper,"
wasp: a. Worker; 6, queen. made by chewing bits of

386 EVOLUTION AND ANIMAL LIFE

weather-beaten wood taken from old fences or outbuildings.
In each of the cells the queen lays an egg. She deposits in the
cell a small mass of food, consisting of some chewed insects
or spiders. From these eggs hatch grubs which eat the food
prepared for them, grow, pupate, and issue as worker wasps,

FIG. 240. — At the left, nest of Vespa, a social wasp; at the right, nest of Vespa opened
to show combs within. (From photographs.)

winged and slightly smaller than the queen (Fig. 239) . The
workers enlarge the nest, adding more combs and making
many cells, in each of which the queen lays an egg. The
workers provision the cell with chewed insects, and other broods
of workers are rapidly hatched. The community grows in num-
bers and the nest grows in size until it comes to be the great
ball-like oval mass which we know so well as a hornets ' nest
(Fig. 240), a thing to be left untouched. When disturbed,
the wasps swarm out of the nest and fiercely attack any
invading foe in sight. After a number of broods of workers
has been produced, broods of males and females appear and
mating takes place. In the late fall the males and all of the
many workers die, leaving only the new queens to live through
the winter.

Honeybees live together, as we know, in large communities.

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 387

We are accustomed to think of honeybees as the inhabitants
of beehives, but there were bees before there were hives. The
"bee tree" is familiar to many of us. The bees, in Nature,
make their home in the hollow of some dead or decaying tree-
trunk, and carry on there all the industries which characterize
the busy communities in the hives. A honeybee community
comprises three kinds of individuals (Fig. 241) — namely, a
fertile female or queen, numerous males or drones, and many
infertile females or workers. These three kinds of individuals
differ in external appearance sufficiently to be readily recogniz-
able. The workers are smaller than the queens and drones,
and the last two differ in the shape of the abdomen, or hind
body, the abdomen of the queen being longer and more slender
than that of the male or drone. In a single community there
is one queen, a few hundred drones, and ten to thirty thousand
workers. The number of drones and workers varies at different
times of the year, being smallest in winter. Each kind of
individual has certain work or business to do for the whole
community. The queen lays all the eggs from which new bees
are born; that is, she is the mother of the entire community.
The drones or males have simply to act as royal consorts ; upon
them depends the fertilization of the eggs. The workers
undertake all the food-getting, the care of the young bees, the

FIG. 241. — Honeybee: o, Drone or male; 6, worker or female; c, queen or fertile female.

comb-building, the honey-making — all the industries with which
we are more or less familiar that are carried on in the hive.
And all the work done by the workers is strictly work for the
whole community; in no case does the worker bee work for
itself alone ; it works for itself only in so far as it is a member of
the community.

How varied and elaborately perfected these industries are
may be perceived from a brief account of the life history of a

388

EVOLUTION AND ANIMAL LIFE

bee community. The interior of the hollow in the bee tree or
of the hive is filled with "comb" — that is, with wax molded
into hexagonal cells and supports for these cells. The molding
of these thousands of symmetrical cells is
accomplished by the workers by means of
their specially modified trowellike mandibles
or jaws. The wax itself, of which the cells
are made, comes from the bodies of the
workers in the form of small liquid drops
which exude from the skin on the under
side of the abdomen or hinder body rings.
These droplets run together, harden and
become flattened, and are removed from
the wax plates, as the peculiarly modified
parts of the skin which produce the wax
are called, by means of the hind legs, which
are furnished with scissorlike contrivances
for cutting off the wax (Fig. 242). In cer-
tain of the cells are stored the pollen and
honey, which serve as food for the com-
munity. The pollen is gathered by the
workers from certain favorite flowers and is
carried by them from the flowers to the
hive in the "pollen baskets/7 the slightly
concave outer surfaces of one of the seg-
ments of the broadened and flattened hind
legs. This concave surface is lined on each
leg of worker honey- margin with a row of incurved stiff hairs,
bee. Concave sur- whjcn hold the pollen mass securely in place
(Fig. 242). The "honey" is the nectar of
flowers which has been sucked up by the
workers by means of their elaborate lapping
and sucking mouth parts and swallowed
into a sort of honey sac or stomach, then
brought to the hive and regurgitated into
the cells. This nectar is at first too watery
to be good honey, so the bees have to evaporate some of this
water. Many of the workers gather above the cells contain-
ing nectar, and buzz — that is, vibrate their wings violently.
This creates currents of air which pass over the exposed nectar
and increase the evaporation of the water. The violent buzz-

FIG. 242. — Posterior

face of the upper
large joint with the
marginal hairs is
the pollen basket;
the wax shears are
"•the cutting surfaces
of the angle be-
tween the two large
segments of the leg.

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 389

ing raises the temperature of the bees' bodies, and this warmth
given off to the air, also helps make evaporation more rapid.
In addition to bringing in food the workers also bring in, when
necessary, "propolis," or the resinous gum of certain trees,
which they use in repairing the hive, as closing up cracks and
crevices in it.

In many of the cells there will be found, not pollen or
honey, but the eggs or the young bees in larval or pupal con-
dition (Fig. 243). The queen moves about through the hive,
laying eggs. She deposits only one egg in a cell. In three
days the egg hatches,
and the young bee ap-
pears as a helpless soft,
white, footless grub or
larva. It is cared for
by certain of the
workers, that may be
called nurses. These
nurses do not differ
structurally from the
other workers, but they
have the special duty
of caring for the help-
less young bees. They
do not -go out for pol-
len or honey, but stay in the hive. They are usually the
new bees — i. e., the youngest or most recently added workers.
After they act as nurses for a week or so they take their
places with the food-gathering workers, and other new bees
act as nurses. The nurses feed the young or larval bees at
first with a highly nutritious food called bee jelly, which the
nurses make in their stomach, and regurgitate for the larvae.
After the larvae are two or three days old they are fed with pollen
and honey. Finally, a small mass of food is put into the cell,
and the cell is " capped " or covered with wax. Each larva,
after eating all its food, in two or three days more changes into
a pupa, which lies quiescent without eating for thirteen days,
when it changes into a full-grown bee. The new bee breaks
open the cap of the cell with its jaws, and comes out into the
hive, ready to take up its share of the work for the community.
In a few cases, however, the life history is different. The nurses
26

c^. 243. — Cells containing eggs, larvae, and pupse
o»*he honeybee. The lower, large, irregular cells
are the queen cells. (After Benton.)

390 EVOLUTION AND ANIMAL LIFE

will tear down several cells around some single one, and enlarge
this inner one into a great irregular vase-shaped cell. When
the egg hatches, the grub or larva is fed bee jelly as long as it
remains a larva, never being given ordinary pollen and honey
at all. This larva finally pupates, and there issues from the
pupa not a worker or drone bee, but a new queen bee. The egg
from which the queen is produced is the same as the other eggs,
but the worker nurses by feeding the larva only the highly
nutritious bee jelly make it certain that the new bee shall be-
come a queen instead of a worker. It is also to be noted that
the male bees or drones are hatched from eggs that are not
fertilized, the queen having it in her power to lay either ferti-
lized or unfertilized eggs. From the fertilized eggs hatch
larvae which develop into queens or workers, depending on
the manner of their nourishment; from the unfertilized eggs
hatch the males.

When several queens appear there is much excitement
in the community. Each community has normally a single
one, so that when additional queens appear some rearrange-
ment is necessary. This rearrangement comes about first by
fighting among the queens until only one of the new queens is
left alive. Then the old or mother queen issues from the hive
or tree followed by many of the workers. She and her followers
fly away together, finally alighting on some tree branch and
massing there in a dense swarm. This is the familiar phenome-
non of "swarming." The swarm finally finds a new hollow
tree, or in the case of the hive bee the swarm is put into a
new hive, where the bees build cells, gather food, produce
young, and thus found a new community. This swarming
is simply an emigration, which results in the wider distribu-
tion and in the increase of the number of the species. It is a
peculiar but effective mode of distributing and perpetuating
the species.

There are many other interesting and suggestive things
which might be told of the life in a bee community: how the
community protects itself from the dangers of starvation
when food is scarce or winter comes on by killing the useless
drones and the immature bees in egg and larval stage; how
the instinct of home-finding has been so highly developed
that the worker bees go miles away for honey and nectar,
flying with unerring accuracy back to the hive; of the extraor-

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 391

dinarily nice structural modifications which adapt the bee so
perfectly for its complex and varied businesses; and of the
tireless persistence of the workers until they fall exhausted
and dying in the performance of their duties. The community,
it is important to note, is a persistent or continuous one. The
workers do not live long, the spring broods usually not over
two or three months, and the fall broods not more than six or
eight months; but new ones are hatching while the old ones are
dying, and the community as a whole always persists. The

a

•^_^ ^~*

FIG. 244. — Female (a), male (6) and worker (c) of an ant, Camponotus sp.

queen may live several years, perhaps as many as five.1 She
lays about one million eggs a year.

There are many species of ants, two thousand or more, and
all of them live in communities and show a truly communal
life. There is much variety of habit in the lives of different
kinds of ants, and the degree in which the communal or social
life is specialized or elaborated varies much. But certain
general conditions prevail in the life of all the different kinds of
individuals — sexually developed males and females that possess
wings, and sexually undeveloped workers that are wingless
(Fig. 244). In some kinds the workers show structural differ-
ences among themselves, being divided into small workers,
large workers, and soldiers. The workers are, as with the

1 A queen bee has been kept alive for fifteen years.

392

EVOLUTION AND ANIMAL LIFE

bees, infertile females. Although the life of the ant communities
is much lesG familiar and fully known than that of the bees, it
is even more remarkable in its specializations and elaborate-
ness. The ant home, or nest, or formicary, is, with most
species, a very elaborate underground, many-storied labyrinth
of galleries and chambers. Certain rooms are used for the

storage of food ; certain
C others as "nurseries"

for the reception and
care of the young; and
others as " stables " for
the ants' cattle, certain
plant lice or scale in-
sects which are some-
times collected and
cared for by the ants.

The food of ants
comprises many kinds
of vegetable and ani-
mal substances ; but
the favorite food, or
"national dish," as it
has been called, is a
sweet fluid which is
produced by certain
small insects, the plant
lice (Aphididse) and
scale insects (Coccidse).
These insects live on
the sap of plants; rose bushes are especially favored with their
presence. The worker ants (and we rarely see any ants but the
wingless workers, the winged males and females appearing out of
the nest only at mating time) find these honey-secreting insects,
and gently touch or stroke them with their feelers (antenna) ,
when the plant lice allow tiny drops of the honey to issue from
the body, which are eagerly drunk by the ants. It is manifestly
to the advantage of the ants that the plant lice should thrive; but
they are soft-bodied, defenseless insects, and readily fall a prey
to the wandering predaceous insects like the ladybirds and
aphis lions. So the ants often guard small groups of plant
lice, attacking, and driving away the would-be ravagers. When

FIG. 245. — The ant, Solenopsis fugax: a, Male; b,
dealated female; c, worker; d, portion of nest show-
ing broad galleries of the host ant intersected by
the tenuous galleries of Solenopsis, the thief ant.
(After Wasmann. See account on page 375.)

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 393

the branch on which the plant lice are gets withered and dry,
the ants have been observed to carry the plant lice carefully
to a fresh, green branch. On page 374 is described how the
little brown ant Lasius brunneus cares for the corn root plant
louse. In the arid lands of New Mexico and Arizona the ants
rear their scale insects on the roots of cactus. Other kinds of
ants carry plant lice into their nests and provide them with food
there. Because the aijts ob-
tain food from the plant lice
and take care of them, the
plant lice are not inaptly
called the ants' cattle.

Like the honeybees, the
young ants are helpless
little grubs or larvse, and
are cared for and fed by
nurses. The so-called ants'
eggs, little white, oval
masses, which we often
see being carried in the
mouths of ants in and out
of ants' nests, are not eggs,
but are the pupa? which
are being brought out to
enjoy the warmth and light
of the sun or being taken
back into the nest after-
wards.

In addition to the workers that build the nest and collect
food and care for the plant lice, there is in many species of
ants a kind of individuals called soldiers. These are wingless,
like the workers, and are also, like the workers, not capable of
laying or of fertilizing eggs. It is the business of the soldiers,
as their name suggests, to fight. They protect the community
by attacking and driving away predaceous insects, especially
other ants. The ants are among the most warlike of insects.
The soldiers of a community of one species of ant often sally
forth and attack a community of some other species. If suc-
cessful in battle the workers of the victorious community take
possession of the food stores of the conquered and carry them
to their own nest. Indeed, they go even further; they may

Fio. 246. — Nest of the ant, Leptothorax emer-
soni, with the nest of another ant, Myrmica
scabrinodes. (See account on page 375.)
(After Wheeler.)

394 EVOLUTION AND ANIMAL LIFE

make slaves of the conquered ants. There are numerous
species of the so-called slave-making ants. The slave-makers
carry into their own nest the eggs and larvae and pupae of the
conquered community, and when these come to maturity they
act as slaves of the victors — that is, they collect food, build
additions to the nests, and care for the young of the slave-
makers. This specialization goes so far in the case of some
kinds of ants, like the robber-ant of South America (Eciton),
that all of the Eciton workers have become soldiers, which no
longer do any work for themselves. The whole community
lives, therefore, wholly by pillage or by making slaves of other
kinds of ants. There are four kinds of individuals in a robber-
ant community — winged males, winged females, and small and
large wingless soldiers. There are many more of the small
soldiers than of the large, and some naturalists believe that the
few latter, which are distinguished by heads and jaws of great
size, act as officers! On the march the small soldiers are ar-
ranged in a long, narrow column, while the large soldiers are
scattered along on either side of the column and appear to act
as sentinels and directors of the army. The observations made
by the European students of ants, Huber, Forel, Emery and
Wasmann, and by McCook and Wheeler in America, read like
fairy tales, and yet are the well-attested actual phenomena of
the extremely specialized communal and social life of these
animals.

The bumblebees and social wasps show an intermediate
condition between the simply gregarious or neighborly mining
bees and the highly developed, permanent honeybee and ant
communities. Naturalists believe that the highly organized
communal life of the honeybees and the ants is a develop-
ment from some simple condition like that of the bumblebees
and social wasps, which in its turn has grown out of a still
simpler, more gregarious assembly of the individuals of one
species. It is not difficult to see how such a development could
in the course of a long time take place.

The termites or white ants (not true ants) are also communal
insects. Some species of termites in Africa live in great mounds
of earth, often fifteen feet high. The community comprises
hundreds of thousands of individuals, which are of as many as
eight kinds or castes (Fig. 247) viz., sexually active winged males,
sexually active winged females, other fertile males and females

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 395

which are wingless, wingless workers of both sexes not capable
of reproduction, and wingless soldiers of both sexes also in-
capable of reproduction. The production of new individuals
is the sole business of the fertile males and females ; the workers
build the nest and collect food, and the soldiers protect the
community from the attacks of marauding insects. The egg-
laying queen grows to monstrous size in some species, being some-
times four or five inches long, while the other individuals of the
community are not
more than half or
three-quarters of an
inch long. The great
size of the queen is
due to the enormous
number of eggs in
her body.

We have pointed
out elsewhere that
the complexity of
the bodies of the
higher animals de-
pends on a speciali-
zation or differentia-
tion of parts, due
to the assumption of
different functions or
duties by different

parts of the body; that the degree of structural differentiation
depends on the degree or extent of division of labor shown in
the economy of the animal. It is obvious that the same prin-
ciple of division of labor with accompanying modification of
structure is the basis of colonial and communal life. It is
simply a manifestation of the principle among individuals in-
stead of among organs. The division of the necessary labors
of life among the different zooids of the colonial jellyfish is
plainly the reason for the profound and striking, but always
reasonable and explicable, modifications of the typical polyp
or medusa body, which is shown by the swimming zooids, the
feeding zooids, the sense zooids, and the others of the colony.
And similarly in the case of the termite community, the sol-
dier individuals are different structurally from the worker in-

FIG. 247. — Termites: a, Queen ; 6, male ; c, worker ;
d, soldier.

396 EVOLUTION AND ANIMAL LIFE

dividuals because of the different work they have to do. And
the queen differs from all the others, because of the extraor-
dinary prolificacy demanded of her to maintain the great com-
munity.

It is important to note, however, that among those animals
that show the most highly organized or specialized communal
or social life, the structural differences among the individuals
are the least marked, or at least are not the most profound.
The three kinds of honeybee individuals differ but little; in-
deed, as two of the kinds, male and female, are to be found in
the case of almost all kinds of animals, whether communal
in habit or not, the only unusual structural specialization in
the case of the honeybee, is the presence of the worker indi-
vidual, which differs from the other individuals primarily in
the rudimentary condition of the reproductive glands. Finally,
in the case of man, with whom the communal or social habit
is so all-important as to gain for him the name of "the social
animal," there is no differentiation of individuals adapted
only for certain kinds of work. Among these highest ex-
amples of social animals, the presence of an advanced mental
endowment, the specialization of the mental power, the power
of reason, have taken the place of and made unnecessary the
structural differentiation of individuals. The honeybee work-
ers do different kinds of work: some gather food, some care for
the young, and some make wax and build cells, but the in-
dividuals are interchangeable; each one knows enough to do
these various things. There is a structural differentiation in
the matter of only one special work or function, that of re-
production.

With the ants there is, in some cases, a considerable struc-
tural divergence among individuals, as in the genus Atta of
South America with six kinds of individuals — namely, winged
males, winged females, wingless soldiers, and wingless workers
of three distinct sizes. In the case of other kinds with quite
as highly organized a communal life, there are but three kinds
of individuals; the winged males and females and the wingless
workers. The workers gather food, build the nest, guard the
"cattle" (aphids), make war, and care for the young. Each
one knows enough to do all these various distinct things. Its
body is not so modified that it is limited to doing but one
kind of thing.

MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 397

The increase of intelligence, the development of the power
of reasoning, is the most potent factor in the development of
a highly specialized social life. Man is the example of the
highest development of this sort in the animal kingdom, but
the highest form of social development is not by any means the
most perfectly communal.

The advantages of communal or social life, of cooperation
and mutual aid, are real. The animals that have adopted
such a life are among the most successful of all animals in the
struggle for existence. The termite individual is one of the
most defenseless, and, for those animals that prey on insects,
one of the most toothsome luxuries to be found in the insect
world. But the termite is one of the most abundant and
widespread and successfully living insect kinds in all the tropics.
Where ants are not, few insects are. The honeybee is a popu-
lar type of a successful life. The artificial protection afforded
the honeybee by man may aid in its struggle for existence, but
it gains this protection because of certain features of its com-
munal life, and in Nature the honeybee takes care of itself
well. The Little Bee People of Kipling's Jungle Book, who
live in great communities in the rocks of Indian hills, can put to
rout the largest and fiercest of the jungle animals. Coopera-
tion and mutual aid are among the most important factors
which help in the struggle for existence. Its great advantages
are, however, in some degree balanced by the fact that mutual
help brings mutual dependence. The community or society
can accomplish greater things than the solitary individuals,
but cooperation limits freedom, and often sacrifices the indi-
vidual to the whole.

CHAPTER XIX
COLOR AND PATTERN IN ANIMALS

In spite of the fluency with which so many people talk of the
meaning of color in organisms, the subject is as incomplete on the
theoretical as on the physiological side. . . . The two deficiencies are
related and a little more physiology will arm the theorists with better
weapons. — NEWBIGIN.

A CONSPICUOUS characteristic of the animal body is its color
pattern. Not all kinds of animals attract our attention by
their colors: there are even whole groups whose uniform mono-
chrome color scheme is of a sort to relieve them completely
from any imputation of flaunting showiness or of bizarre fancies
in personal decoration. But consider such a class as the insects :
the painted butterflies, the burnished beetles, the flashing
dragon flies, the green katydids and brown locusts All attract
attention first by the variety or intensity of their colors and
the arrangement of these colors in simple or intricate symmetry
of pattern. Even the small and at casual glance, obscure and
monochrome insects often reveal, on careful examination, a
large degree of color development and ofttimes amazing in-
tricacy and beauty of pattern. So uniformly developed is
color pattern among insects, that no thoughtful collector or
observer of these animals escapes the self-put question: Why
is there such a high degree of specialization of color throughout
the insect class? If he be an observer who has taken seriously
the teachings of Darwin and the utilitarian school of naturalists,
his question becomes couched in this form: What is the use to
the insects of all this color and pattern?

For the attitude of any modern student of Nature, con-
fronted by such a phenomenon, is that of the seeker for the
significance of the phenomenon. And the key to significance

398

COLOR AND PATTERN IN ANIMALS 399

in such a case is to be sought in utility. The usefulness of
color in animate nature as an inspirer and satisfier of our own
aesthetic needs and capacities, or of color patterns as means
whereby we may distinguish and recognize various sorts of
animals and plants, is a usefulness which may be answer enough
to the passing poet on the one hand, and to the old-line Lin-
nsean systematist on the other, but it is, of course, no answer
to science. Science demands a usefulness to the color-bearing
organisms themselves: and a usefulness large and serious
enough to be the sufficient cause for so highly specialized and
amazing a development.

The explanations, of some of the color phenomena of animals
are obvious: some uses we recognize quickly as certain, some as
probable, some as possible. Some colors are obviously there
simply because of the chemical make-up of parts of the insect
body. That gold is yellow, cinnabar red, and certain copper
ores green or blue, are facts which lead us to no special inquiry
after significance: at least, not after significance based on
utility. If an insect has part of its body composed of or con-
taining a substance that is by its very chemical and physical
constitution always red or blue or green, we may be content
with knowing that, and not be too insistent in our demand to
the insect to show cause, on a basis of utility, for being partly
red or blue or green. And even if this red or blue be disposed
with some symmetry, some regularity of repetition, either
segmentally or bilaterally, this we may well attribute to the
natural segmental and bilaterally symmetrical repetition of
similar body parts. Some color and some color pattern, then,
may be explicable on the same basis as the color of a mineral
specimen or of a tier of bricks.

But no such explanation will for a moment satisfy us as
to the presence of and arrangement of colors in the wings of
Kallima, the dead leaf butterfly, or in Phyllium, the green leaf
phasmid, or in the butterfly fish, Chcetodon, or in the lichen
spider, or in the chameleon with its changing tints, or in any
one of a score of other more or less familiar forms whose color
pattern makes, even on the casual observer, an insistent
demand for rational explanation.

Certain uses of color seem apparent: the colored eye flecks
or pigment spots of many of the lower animals presumably
serve their possessors as organs by which to distinguish the

400 EVOLUTION AND ANIMAL LIFE

presence or absence of light, by virtue of their capacity to
absorb light and thus stimulate the specially sensitive cells
composing them. And the pigment or absence of it (dark
or light color) in the fur and plumage of certain mammals and
birds may perhaps serve to absorb or to reflect the sun's rays
so as to help keep warm or cool the animals thus colored. But
such explanations of animal colors can obviously apply to but
few cases. Very plainly color, and especially pattern, has its
significance if anywhere in connection with certain special re-
lations of animals to other animals and to the world generally.
So, ever since the days of Darwin, two general categories
of such significance or explanation of color and pattern have
been in the minds of naturalists. One of these is the signifi-
cance attributed to color pattern by the theory of sexual
selection; the other is that attributed to it by the general theory
or group of theories of protective resemblance, ^recognition,
warning, directive, and mimetic coloration, etc. Of these two
general explanations, one has steadily lost ground since Dar-
winian and early post-Darwinian days, while the other has
slowly but steadily gained adherents and has been extended to
cover more and more cases of animal ornamentation. Of the
theory of sexual selection it must be said that it certainly can-
not explain the conditions of secondary sexual differences,
including colors and patterns, in many groups of animals, and
it has really not been proved to explain them in any single
group, although in the case of birds and mammals it seems
possible that the theory is applicable: at least no other ex-
planation of equal validity has yet been presented. Of the
specialization of color and pattern for the sake of protecting
the animal by making it so harmonize or fuse with the usual
environment as to be indistinguishable, or by making it simulate
with sufficient fidelity some particular part of its surroundings
as a green or dead leaf, a twig, the dropping of a bird, a bit of
lichen or what not, or by making it mimic some other animal
notoriously well defended by sting or fangs or ill-tasting body,
so that the otherwise defenseless mimicker is mistaken by its
enemies for the defended mimicked kind of animal — of this
specialization and utility of color and pattern, evidence for its
reality is gradually accumulating to convincing amount. And
it is of this sort of color and pattern specialization that the
brief discussion to follow will be devoted.

COLOR AND PATTERN IN ANIMALS

401

FIG. 248.— Katydid, Cyrtophyllis crepitans, from the West
Indies, with green body and wings resembling the leaves
among which it lives. (After Sharp.)

The green katydid singing in the tree-top or shrubbery is
readily known to be there by its music, but just which bit of
green that we see is katydid, and which is leaf, is a matter to
be decided only by unusually discriminating eyes. The clacking
locust, beating its
black wings in the
air, is conspicuous
enough; but after
it has alighted on
the ground it is
invisible, or,
rather, visible but
indistinguishable :
its gray and brown
mottled color pat-
tern is simply continuous with that of the soil. The green
larvae of the Pierid butterflies lying longitudinally along green
grasses simply merge into the color scheme of their environ-
ment. The gray moths rest unperceived on the bark of the

tree trunk. Hosts of insect kinds
do really harmonize with the color
pattern of their usual environ-
ment, and by this correspondence
in shade and marking, are difficult
to perceive for what they are.
Now if the eyes that survey the
green foliage or run over the gray
bark are those of a preying bird,
lizard, or other enemy, it is quite
certain — our reason tells us so in-
sistently— that this possession by
the insect of color and pattern
tending to make it indistinguish-
able from its immediate environ-
FIG. 249.-Smaii locust of the Colo- ment is advantageous to it— ad-

rado-Mohave desert on the sand. VantagCOUS to the degree often of

saving its life. Now such a use

of color and pattern is obviously one which can be widespread
through the insect class, and may be, to many species which
lead lives exposed to the attacks of insectivorous animals, of
large — even of life and death — importance. And naturalists,

402 EVOLUTION AND ANIMAL LIFE

most of them at least, believe that this kind of usefulness is
real, and that it is the principal clew to the chief significance
of color and pattern — and this not alone in the case of in-
sects, but of most other animals as well.

From this point of view, namely, that color patterns may
be of advantage in the struggle for existence, just as strength,
swiftness, and other capacities and conditions are, the speciali-
zation and refinement, all the wide modification and variety
of colors and patterns are explicable by the hypothesis of
their gradual development in time through the natural selec-
tion of fortuitous advantageous variations. On this basis,
such special instances of resemblance to particular parts of
the environment, as that shown by Kallima in its likeness
to a dead leaf, and Diapheromera in its simulation of a dry,
leafless twig, are simply the logical extremes of such a line of
specialization.

But the nature observer may be inclined to ask how such
brilliant and bizarre colors as those of the swallowtail butter-
flies and the tiger-banded caterpillars of Anosia can be included
in any category of "protective resemblance" patterns. They
are not so included, but are explained ingeniously by an added
hypothesis called that of "warning colors/' while for the strik-
ing similarities of pattern often noted between two unrelated
conspicuously colored species still another hypothesis is pro-
posed. In these cases it is not concealment that the color
pattern effects, but indeed just the opposite. Since the pioneer
studies of Bates and Wallace and Belt, naturalists have been
observing and experimenting and pondering these exposing,
as well as these concealing, conditions of color and pattern,
and they have proposed several theories or hypotheses ex-
planatory of the various conditions. These hypotheses are
plausible; but they are much more than that: they are each
more or less well backed up by observation and experiment,
and some of them have gained a large acceptance among
naturalists. Both the reasoning and observed facts on which
these hypotheses rest are based on the usefulness of the colors
and patterns to the animals in their relation to the outside world.
And the influence of advantage and natural selection is given
the chief credit for determining the present-day conditions of
these colors and patterns.

Before, however, we take up these hypotheses, defining

COLOR AND PATTERN IN ANIMALS 403

them and looking over some of the evidence adduced for their
support, as well as some of the criticism leveled at them, we
may advisedly look to the actual physical causation of color
in animals. Whatever the use or significance of color, our
understanding of this use must be based on a knowledge of the
method or modes of its actual production.

Color in organisms is produced as color in inorganic nature
is. Certain substances have the capacity of selective absorp-
tion of light rays, so that when white light falls on them,
certain colors (light waves of certain length) are absorbed, while
certain others (light waves of certain other lengths) are re-
flected. An object is red because the substance of which it is
(superficially) composed, reflects the red rays and absorbs the
others. Certain other objects or substances may produce color
(be colored) because of their physical rather than their chemical
constitution; their surface may be composed of superposed
lamellae, or it may be so striated or scaled that the various
component rays of white light are reflected, refracted, and
diffracted in such varying manner (at different angles and
from different depths) that complex interference effects are
produced, resulting in the practical extinguishing of certain
colors (waves of certain length) or the reflection of some at
angles so as not to fall on the eye of the observer, and so on.
Such colors will change with changes in the angle of observa-
tion, and are the so-called metallic or iridescent colors. These
two categories of color have been aptly called chemical and
physical: chemical color depending on the chemical make-up
of the body, physical on its structural or physical make-up.
As a matter of fact we shall find that most animal colors
are due to a combination of these two kinds.

(Substances that produce color by virtue of their capacity
to absorb certain colors, and reflect only certain others, we
may call, in our discussion of color production, "pigments";
and " pigmental " may be used as practically synonymous with
" chemical " in referring to colors thus produced, while " struc-
tural" may be used as synonymous with "physical" in
referring to colors dependent on superficial structural character
of the insect body. For colors produced by the cooperation
of both pigment and structure, "combination" or "chemico-
physical " may be used as a defining name.)

Now in all animals, color depends on the presence and ar-

404 EVOLUTION AND ANIMAL LIFE

rangement of pigments or on the fine structure of superficial
parts, as feathers, scales, skin, etc., or on a combination of the
two color-producing conditions. In birds, for example, certain
fat pigments called lipochromes (which are either actual reserve
food products or are associated with such), are abundantly
present in the feathers, bill, feet, etc., producing reds, yellows,
browns, etc., and certain other dark melanin pigments are
distributed as minute amorphous granules in the cuticular
structures or epidermis, producing pjfain gray, brown, black,
and related tints. In addition, the feathers are so constructed
that they may, and do in some cases, produce the most brilliant
iridescent and metallic colors, as familiarly shown to us by the
humming birds, the grackles, etc. Most such metallic colors
in birds, however, are produced by a combination of pigment
and structure, and not by structure alone. The colors of
mammals, of reptiles, of amphibians and of fishes might also
be referred to, and as far as they have been studied or analyzed
according to their causes, we should find that in mammals the
pigmental colors are mostly produced by so-called melanins
which seem to be waste products. In the fishes, amphibians
and reptiles, the pigments are both lipochromes and melanins,
while in all the vertebrate classes there occur cases in which
vivid physical or optical colors are produced by cuticular
structure. The most extended study of color in animals,
however, has been devoted to insect colors. Here we have a
pretty clear understanding of all the color-producing agents,
and an analysis of all the colors more usually met with, into
their proper classes, that is, whether exclusively pigmental,
exclusively structural or mixed structural-pigmental. In a
valuable paper by Tower, a table of insect colors showing the
classification and mode of production of the various colors is
given, as follows (see next page) :

The only hypothesis that gives to colors and markings a
value in the life of animals, at all comparable with the degree
of specialization reached by these colors and markings and by
the special structures developed to make them possible, is that
already referred to as the theory of protective and aggressive
resemblances, of warning and directive patterns, and of mimi-
cry. These various uses of color patterns are all concerned
with the relation of the animal to its environment: they are
means of protecting the animal from its enemies, or of enabling

COLOR AND PATTERN IN ANIMALS
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405

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406 EVOLUTION AND ANIMAL LIFE

it to capture its prey. They are uses obviously concerned
with the "struggle for existence": they are "shifts for a living."
For the sake of clearness in discussion these various uses
will be rather arbitrarily classified into several categories which
in Nature are not so sharply distinguished as the paragraph
treatment of them might suggest.

The general harmonizing in color and pattern with the color
scheme of the usual environment is a condition which every
field student of animals recognizes as widely existing. The
green color of foliage-inhabiting forms, as tree frogs and katy-
dids, the mottled gray and tawny of the mammals, birds,
lizards, and insects of the deserts, and the white of the hares
and foxes and owls and ptarmigan of the arctic and alpine
snow-covered wastes, are color tones obviously in harmony with
the general color of the environment. In the brooks most fishes
are dark olive or greenish above and white below. To the birds
and other enemies which look down on them from above, they
are colored like the bottom. To their fish enemies which look
up from below, their color is like the white light above them,
and their forms are not clearly seen. The fishes of the deep sea
in perpetual darkness are violet in color below as well as above.
Those that live among seaweeds are red, grass-green, or olive
like the plants they frequent. The difficulty of distinguishing
a quiescent moth from the bark on which it is resting, a green
caterpillar or leaf-hopper or meadow grasshopper from the
leaf to which it clings, a roadside locust from the soil on which
it alights, is a difficulty which has to be reckoned with by
every collector.

Now while there are few human collectors of insects, there
are hosts of bird and toad and lizard insect-hunters, to say
nothing of the many kinds of predaceous insects which use their
own cousins for chief food. So that where this difficulty of
distinguishing the resting insect from its environment is suffi-
cient to postpone success on the part of the insect-hunting
bird or lizard, the life of the protectively colored insect is
obviously saved, for the time, by its dress. This is a utility of
color and pattern than which there can be, from the insect point
of view, nothing higher.

One special point should be noted in connection with the
general protective resemblance, and that is, that the harmon-
izing or melting into the environment may often be accomplished

COLOR AND PATTERN IN ANIMALS 407

by a color and pattern not directly imitative of the immediate
environmental objects, but of such a kind as to be lost among
the light and shadow gradations produced by light shining
through leaves, twigs, etc. Thayer has very interestingly
shown the possibilities and actual effects of such gradatory or
light and shadow patterns among birds, and thus explains many
cases of bird patterns not apparently very closely imitative,
but nevertheless very effective in making the bird indistin-
guishable when at rest on its nest or in the bushes or grass
of its usual habitat. General protective resemblance is un-
doubtedly very widespread among animals, and is not easily
appreciated when the animal is seen in museums or zoo-
logical gardens — that is, away from its natural or normal en-
vironment.

A modification of general color resemblance found in many
animals, may be called variable protective resemblance. Certain
hares and other animals that live in northern latitudes are
wholly white during the winter when the snow covers every-
thing; but in summer, when much of the snow melts, revealing
the brown and gray rocks and withered leaves, these creatures
change color, putting on a grayish and brownish coat of hair.
The ptarmigan of the Rocky Mountains (one of the grouse),
which lives on the snow and rocks of the high peaks, is almost
wholly white in winter ; but in summer, when most of the snow
is melted, its plumage is chiefly brown. Locusts of various
species of the genus Trimerotropis show a variability in color
of individuals, ranging through gray, brown, reddish, plumbeous
and bluish, and such accompanying variability in marking as
to result in producing much variety of appearance in a single
series of collected individuals. We have noted in collecting
these locusts in Colorado and California that this variability
of coloration is directly associated with color differences in the
soil of the localities in which these locusts live; the reddish
individuals are taken from spots where the soil is reddish, the
grayish, where it is sand-colored, and the plumbeous and bluish
from soil formed by decomposing bluish rock. The same
variations in color are evident in the horned toads (Phrynosoma) ,
as found on various colors of desert soils.

On the campus of Stanford University there is a little pond
whose shores are covered in some places with bits of bluish
rock, in other places with bits of reddish rock, and in still

408 EVPLUTION AND ANIMAL LIFE

others with sand. The toad bug (Galgulus) lives abundantly
on the banks of this pond. Specimens collected from the
blue rocks are bluish in groymd color, those from the red
rocks are reddish, and those from the sand are sand-colored.
But these insects have fixed colors ; they cannot, like the
chameleon and certain other lizards, or like numerous small
fishes and some tree frogs, change color, quickly or slowly,
with changes in position — that is, movements from green to
brown or to other colored environment. Variable protective
resemblance in insects is, as far as known, a variability directly
induced, to be sure, by varying environment, but all acquired
during the development of the individual insects, and fixed by
the time they reach the adult stage. But changes of color
to suit the changing surroundings can be quickly made in the
case of some animals. The chameleons of the tropics, whose
skin changes color momentarily from green to brown, blackish
or golden, is an excellent example of this highly specialized
condition. The same change is shown by a small lizard of our
Southern States (Anolius), which from its habit is called the
Florida chameleon. There is a little fish (Oligocottus) which
is common in the tide pools of the bay of Monterey, in Cali-
fornia, whose color changes quickly to harmonize with the
different colors of the rocks it happens to rest above. Most
of the tree frogs show this variable coloring.

The well-known experiments of Trimen, Miiller, and Poulton
on the pupating larvae of swallow-tailed butterflies (Papilio),
and of Poulton on other butterflies of numerous species with
naked chrysalids, show that they take on the color, or a shade
resembling it, of the substance surrounding these larvae. They
show also that the result is due to a stimulus of the skin by the
enclosing color, and not to a stimulus received through the
eyes, and carried to the skin by the nerves. Larvae just ready
to pupate were enclosed in boxes lined with paper of different
colors ; the chrysalids when formed were found to be colored to
harmonize with that particular shade of paper by which they
were surrounded while pupating. As these chrysalids in nature
hang exposed on bark and in other unsheltered places, without
protecting cocoon or cover of any kind, the actual protective
value of this harmonious coloration is obvious. It is a familiar
fact to entomologists that most butterfly chrysalids and naked
pupae of moths (unless concealed in the ground or elsewhere)

COLOR AND PATTERN IN ANIMALS

409

FIG. 250. — Chrysalid of swallowtail
butterfly, Papilio, which closely re-
sembles the bark on which it rests.

resemble in color and general external appearance the surface

of the object on which they rest. The chrysalids of various

Papilios are indeed marvelously

faithful imitations of bits of

rough bark (Fig. 250).

The larvse (caterpillars) of

various moths, particularly Geo-

rnetrid and Sphingid species,

often appear in two color types,

one brown and the other green.

Poult on has shown by experi-
ment and observation with some

of these species that those larvse

reared among green leaves and

twigs become green, while those

on dry branches become brown.

This variable protective resemblance, like that of Trimero-

tropis, Galgulus, and the Papilio chrysalids, also is fixed after

being once acquired.

An interesting example of
color harmony which may be
classified under the head of
variable protective resem-
blance is that of the larvse of
Lyccena sp., abundant on the
flower heads of the California
buckeye, jEsculus calif or nicus ,
that blooms in May. The buds
of the buckeye are green, or
green and rose, or even all
rose externally. The quiet
sluglike Lycsenid larvse lie
longitudinally along the buds
and their short stems, and
are either green with faint

FIG. 25i.— TWO leaf hoppers or mem- rose tinge, especially along

bracids: The upper one, Xerophyllum •<-}-,« middle of the back or
simile; the lower one, Cladonotus hum-

beriianus. (After Bolivar.) are distinctly rosy all over,

depending strictly upon the

color tone of the particular branch serving as their habitat.
The correspondence in shade of color is strikingly exact: the

410

EVOLUTION AND ANIMAL LIFE

FIG. 252. — The mousefish, Pterophryne histrio, in the Sargassum or Gulf weed. The
fishes are marked and colored so as to be nearly indistinguishable from the mass of
the Gulf weed. In the lower right-hand corner of the figure are two seahorses, also
shaped and marked so as to be concealed.

COLOR AND PATTERN IN ANIMALS

411

utter indistinguishability of the larvae is something that needs
to be experienced to be fairly realized.

Far more striking are those cases of protective resemblance
in which the animal re-
sembles in color and shape,
sometimes in extraordi-
nary detail, some partic-
ular object or part of its
usual environment. Cer-
tain parts of the Atlantic
Ocean are covered with
great patches of seaweed
called the Gulf weed (Sar-
gassum), and many kinds
of animals — fishes and
other creatures — live upon
and among the algae. No
one can fail to note the
extraordinary color resem-
blances which exist be-
tween these animals and
the weed itself. The gulf
weed is of an olive-yellow
color, and the crabs and
shrimps, a certain flat-
worm, a certain mollusk,
and certain little fishes,
all of which live among
the Sargassum, are exactly
of the same shade of yel-
low as the weed, and have
small white markings on
their bodies which are
characteristic also of the
Sargassum. The mouse-
fish and the little sea-
horses, often attached to

the Gulf weed, show the same traits of coloration (Fig. 252).
The slender grass-green caterpillars of many moths and butter-
flies resemble very closely the thin grass blades among which
they live. The larvae of the geometrid moths, called inch worms

FIG. 253. — A Geometrid larva on a branch.
(The larva is the upper right-hand projection
from the twig.)

412

EVOLUTION AND. ANIMAL LIFE

or spanworms, are twiglike in appearance, and have the habit,
when disturbed, of standing out stiffly from the twig or branch
upon which they rest, so as to resemble in position as well as
in color and markings a short or a broken twig. One of the

FIG. 254.— The walking-stick insect,
Diapheromera femorata, on twig.

FIG. 255. — A twig-simulating
insect from Samoa. (From
specimen.)

most striking resemblances of this sort is shown by the large
geometrid larva illustrated in Fig. 253, which was found near
Ithaca, New York. The body of this caterpillar has a few small,
irregular spots or humps, resembling very closely the scars left
by fallen buds or twigs. These caterpillars have a special mus-
cular development to enable them to hold themselves rigidly

COLOR AND PATTERN IN ANIMALS

413

for long times in this trying attitude. They also lack the
middle proplegs of the body, common to other lepidopterous
larvae, the presence of which would tend to destroy the illusion
so successfully carried out by them. The common walking
stick (Diapheromera) (Fig. 254), with its wingless, greatly
elongate, dull-colored body, is an excellent example of special
protective resemblance. It is quite indistinguishable, when at
rest, from the twigs to which
it is clinging. Another member
of the family of insects to which
the walking stick belongs is the
famous green-leaf insect (Phyl-
Hum) (Fig. 256). It is found
in South America and is of
a bright green color, with broad
leaflike wings and body with
markings which imitate the leaf
veins, and small irregular yel-
lowish spots which mimic decay-
ing or stained or fungus-covered
spots in the leaf.

There are many butterflies
that resemble dead leaves. All
our common meadow browns
(Grapta), brown and reddish
butterflies with ragged - edged
wings, that appear in the autumn
and flutter aimlessly about ex-
actly like the falling leaves,

show this resemblance. But most remarkable of all is a large
butterfly (Kallima) (Fig. 257) of the East Indian region. The
upper sides of the wings are dark, with purplish and orange
markings, not at all resembling a dead leaf. But the butter-
flies when at rest hold their wings together over the back,
so that only the under sides of the wings are exposed. The
under sides of Kallima's wings are exactly the color of a dead
and dried leaf, and the wings are so held that all combine to
mimic with extraordinary fidelity a dead leaf still attached to
the twig by a short pedicle or leaf-stalk imitated by a short tail
on the hind wings, and showing midrib, oblique veins, and,
most remarkable of all, two apparent holes, like those made

FIG. 256.— The green-leaf insect,
Phyllium.

414

I

EVOLUTION AND ANIMAL LIFE

in leaves by insects, but in the butterfly imitated by two small
circular spots free from scales and hence clear and transparent.
With the head and feelers concealed beneath the wings, it makes
the resemblance wonderfully exact.

FIG. 257. — Kallima, the "dead-leaf butterfly."

The moths of the genus Cymatophora, and their larvae also,
mostly harmonize excellently with the gray bark on which they
rest, the moths adding to their general simulation the curious
habit of resting, often with folded wings, at an angle of forty-
five degrees with the tree trunk, head downward, with the
curiously blunt and uneven wing tips projecting, so as to imitate

COLOR AND PATTERN IN ANIMALS

415

with great fidelity a short broken-
off branch or chip of bark. Nu-
merous other moths and caterpil-
lars resemble bark and habitually
rest on it. Catocala, Schizura,
and other genera furnish ex-
amples familiar to the moth col-
lector.

There are numerous instances
of special protective resemblance
among spiders. Many spiders
that live habitually on tree
trunks resemble bits of bark or
small, irregular masses of lichen.
A whole family of spiders, which
live in flower cups lying in wait
for insects, are white and pink
and particolored, resembling the
markings of the special flowers
frequented by them. This is, of
course, a special resemblance not
so much for protection as for ag-
gression; the insects coming to
visit the flowers are unable to
distinguish the spiders and fall
an easy prey to them.

Any field student of insects,

by paying attention to the matter of special protective resem-
blance, cpn soon make up a striking list of examples. Some
of these may be more convincing to him than to persons see-
ing his specimens
in the collecting
boxes, and some
indeed will prob-
ably be ques-
tioned by closet
naturalists. But
nevertheless no
collector or field

FIG. 259.— A pipefish. Phyllopteryx, which resembles the sea- S t U Q C n t h a S
weed among which it lives. failed to note

FIG. 258. — An insect, Gongylus gongy-
loides, from the East Indies, which
rests on the branches of a bush re-
sembling the rosebush, the leaves
of which are closely simulated by
the body of the insect. (After
Sharp.)

416 EVOLUTION AND ANIMAL LIFE

many examples of this clever artifice of Nature to protect her
children.

If the field student may be relied on to note and record a
long list of insects colored and marked so as to harmonize well
with their general environment or with some specific part of it,
he may also be relied on to bring in a list of opposites : a record
of bizarre and conspicuous forms, colored with brilliant blues
and greens and streaked and spotted in a manner utterly at
variance and in contrast with the foliage or soil or bark or what-
ever is the usual environment of the insect. The great red-
brown monarch butterfly and its black-striped green and
yellowish larva, the tiger-banded swallowtails, the black and

FIG. 260. — Ladybird beetles, conspicuously colored and marked.

yellow wasps and bees, the ladybird beetles with their sharply
contrasting colors, the brilliant green blister beetles, the striped
and spotted chrysomelids — in all these and many others
there can be no talk of protective resemblance. If only such a
paradoxical theory as protective conspicuousness could be
established, then these colors and markings might well be
explained by it.

Exactly such an explanation of brilliant color and contrast-
ing markings is afforded by the theory of warning colors. It
has been conclusively shown, by observation and experiment
by several naturalists, that many insects are distasteful to
birds, lizards, and other predaceous enemies. This is so be-
cause the blood lymph or some specially secreted body fluid of
these insects contains an acrid or ill-tasting substance so that
birds will not, if they can recognize the kind of insect, make
any attempt to catch or eat one. This letting alone is un-

COLOR AND PATTERN IN ANIMALS 417

doubtedly the result of previously made trials; that is, it has
been learned by experience. Now it would obviously be of
advantage to those species of insects that are ill-tasting, if
their coloring and pattern were so distinctive and conspicuous
as to make them readily known by birds, and once learned
easily seen. A distasteful caterpillar needs to advertise its
unpalatability so effectively that the swooping bird will recog-
nize it before making that single sharp-cutting stroke or peck
that would be as fatal to a caterpillar as being wholly eaten.
Hence the need and the utility of warning colors. And indeed
the distasteful insects, as far as recognized, are mostly of con-
spicuous colors and patterns.

Such warning colors are presumably possessed not only by
unpalatable insects, but also by many that have certain special
means of defense. The wasps and bees, provided with stings,
dangerous to most of their enemies, are almost all conspicu-
ously marked with yellow and black. Many bugs, well defended
by sharp beaks, possess a conspicuous color pattern.

Numerous other animals besides insects also are believed
to have warning colors. The Gila monster (Heloderma), the
only poisonous lizard, differs from most other lizards in being
strikingly patterned with black and brown. Some of the ven-
omous snakes are conspicuously colored, as the coral snakes
(Elaps) or coralillos of the tropics. The naturalist Belt, whose
observations in Nicaragua have added much to our knowledge
of tropical animals, describes as follows an interesting example
of warning colors in a species of frog:

" In the woods around Santo Domingo (Nicaragua) there are many
frogs. Some are green or brown and imitate green or dead leaves,
and live among foliage. Others are dull earth-cojored, and hide in
holes or under logs. All these come out only at n|ght to feed, and they
are all preyed upon by snakes and birds. In conHfc||^sdth these ob-
scurely colored species, another little frog hops about^mthe daytime,
dressed in a bright livery of red and blue. He can not^B mistaken for
any other, and his flaming breast and blue stockings^lBw that he does
not court concealment. He is very abundant in tlaf damp woods, and
I was convinced he was uneatable so soon as I made his acquaintance
and saw the happy sense of security with which' he hopped about. I
took a few specimens home with me, and tried/ny fowls and ducks
with them, but none would touch them. At last, by throwing down

418 EVOLUTION AND ANIMAL LIFE

pieces of meat, for which there was a great competition among them,
I managed to entice a young duck into snatching up one of the little
frogs. Instead of swallowing it, however, it instantly threw it out of
its mouth, and went about jerking its head, as if trying to throw off
some unpleasant taste."

Certain other insects which are without special means of
defense and are not at all formidable or dangerous, are yet so
marked or shaped and so behaved as to present a curiously
threatening appearance. The large green caterpillars of the
sphinx moths have a curious rearing-up habit which seems to

simulate threatened
attack (Fig. 261).
They have, too, a
great pointed spine
or horn on the
back of the pos-
terior tip of the

FIG. 261. — A "tobacco-worm," larva of the sphinx moth, body which has a
Phlegethontius Carolina, showing terrifying attitude. most formidable

appearance, but is,

as a matter of fact, not at all a weapon of defense, being quite
harmless. Numerous stingless insects, when disturbed, wave
about the hind part of the body or curl it over or under, much
as stinging insects do, and seem to be threatening to sting.
The striking eye spots of many insects are believed by some
entomologists to be of the nature of terrifying markings. Mar-
shall tried feeding baboons a full-grown larva (about seven
inches long) of the sphinx moth, Cheer ocampa osiris. The
larva has large strongly colored eye spots and is

" remarkably snakelike, the general coloring somewhat recalling that
of the common puff-adder, Bitis arietans. The female baboon ran for-
ward expecting a titbit, but when she saw what I had brought she
flicked it out of my hand on to the ground, at the same time jumping
back suspiciously: she then approached it very cautiously, and after
peering carefully at it at the distance of about a foot she withdrew in
alarm, being clearly much impressed by the large blue eyelike mark-
ings. The male baboon, which has a much more nervous tempera-
ment, had meanwhile remained at a distance surveying the proceedings,
so I picked up a caterpillar and brought it towards them, but they

COLOR AND PATTERN IN ANIMALS 419

would not let me approach, and kept running away round and round
their pole, so I threw the insect at them. Their fright was ludicrous
to see ; with loud cries they jumped aside and clambered up the pole as
fast as they could go, into their box, where they sat peering over the
edge watching the uncanny object below." (Marshall.)

Marshall also writes concerning the markings on the wings
of the mantis, Pseudocreobotra wahlbergi :

11 They are, I think, almost certainly of a terrifying character. When
the insect is irritated, the wings are raised over its back in such a
manner that the tegmina stand side by side, and the markings on them
present a very striking resemblance to the great yellow eyes of a bird
of prey or some feline animal, which might well deter an insectivorous
enemy. It is noticeable that the insect is always careful to keep the
wings directed toward the point of attack, and this is often done with-
out altering the position of the body."

Still another use is believed by some entomologists to be
afforded by such markings as ocelli and other specially con-
spicuous spots and flecks on the wings of butterflies and moths,
and by such apparently useless parts as the " tails " of the hind
wings of the swallowtail, and Lyccenid butterflies, and others.
Marshall occupied himself for a long time with collecting butter-
flies which had evidently been snapped at by birds (in some
cases the actual attack being observed) and suffered the loss of
a part of a wing. Examining these specimens when brought
together, Poulton and Marshall noted that the "great majority
[of these injuries to the wings] are inflicted at the anal angle
and adjacent hind margin of the hind wing, a considerable
number at or near the apical angle of the fore wing, and com-
paratively few between the points." In this fact, coupled with
the fact that the apical and hind angles of the fore and hind
wings respectively are precisely those regions of the wings most
usually specially marked and prolonged as angular processes or
tails, Poulton sees a special significance in the patterns of these
wing parts. He thinks they are "directive marks which tend
to divert the attention of an enemy from more vital parts."
It is obvious that a butterfly can very well afford to lose the
tip or tail of a wing if that loss will save losing head or abdomen.
Poulton sees a "remarkable resemblance of the marks and

420

EVOLUTION AND ANIMAL LIFE

structures at the anal angle of the hind wing, under side, in
many Lyccewidas, to a head with antennae and eyes/' and recalls
that this has been independently noticed by many other ob-
servers. The movements of the hind wings by which the tails,
which appear like antennae, are made continually to pass and
repass each other, add greatly to this resemblance.

Very many species of animals, especially among the verte-
brates, possess certain distinctive and striking markings, which

FIG. 262. — The butterfly fish, Chcetodon vagabundus, from Samoa. This small fish is
most strikingly colored.

have been supposed to serve as recognition marks to other
animals of the same species. In this theory, these marks afford
a swift means of knowing friends from enemies. Of this
nature are the white tufts at the tail of the cottontail rabbit,
the black patch of the blacktail deer, the flanks of the Rocky
Mountain antelope, the concealed scarlet crest of the kingbird,
the fiery shoulder of the redwing blackbird, the blue speculum
of the duck, the black bars and eye spots of the butterfly fishes
(Chcetodon), and the peculiar marks of one form or another
on a host of mammals, birds, reptiles, and fishes.

It is very easy to indicate recognition marks. Keeler, among
others, has given an elaborate list of the principal cases among
American birds, and there is scarcely a species without one or

COLOR AND PATTERN IX ANIMALS 421

morn. Nevertheless we are not sure that many, or even any
of them, actually serve the purpose of recognition among the
animals themselves, however convenient they may be to us
who study them. Howrever plausible the theory of recognition
marks may seem, it is still not proved to have any objective
basis.

Of all the theories accounting for the utility of color and
pattern, that of mimicry demands at first thought the largest
degree of credulity. As a matter of fact, however, the observa-
tion and evidence on which it rests are as convincing as are those
for almost any of the other forms of protective color pattern.
Although the word "mimicry" could often have been used
aptly in the account of special protective resemblance, it has
been reserved for use in connection with a specific kind of
imitation; namely, the imitation by an otherwise defenseless
insect, one without poison, beak, or sting, and without acrid
and distasteful body fluids, of some other specially defended or
inedible kind, so that the mimicker is mistaken for the mimicked
form and, like this defended or distasteful form, relieved from
attack. Many cases of this mimicry may be noted by any
field student of entomology.

Buzzing about flowers are to be found various kinds of bees,
and also various other kinds of insects thoroughly beelike in
appearance, but in reality not bees nor, like them, defended
by sting. These bee mimickers are mostly flies of various
families (Syrphidce, Asilidce, Bombyliidce), and their resem-
blance to bees is sufficient to and does constantly deceive
collectors. We presume, then, that it equally deceives birds
and other insect enemies. Wasps, too, are mimicked by other
insects ; the wasplike flies, Conopidce, and some of the clear-
winged moths, Sesiidce are extremely wasplike in general
seeming.

The distasteful monarch butterfly, Anosia plexippus, wide-
spread and abundant — a successful butterfly, whose success
undoubtedly largely depends on its inedibility in both larval and
imaginal stages — is mimicked with extraordinary fidelity of detail
by the viceroy, Basilarchia archippus (Fig. 263). The Basil-
archias, constituting a genus of numerous species, are with but
two or three exceptions not at all of the color or pattern of
Anosia, but in the case of the particular species archippus,
not only the red-brown ground color, but the fine pattern

422

EVOLUTION AND ANIMAL LIFE

FIG. 263. — The mimicking of the inedible monarch butterfly by the edible viceroy.
The figure at the top is the monarch, Anosia pJexippus. The middle figure is the
viceroy, Basilarchia archippus. The lowest figure is another member of the same
genus, Basilarchia, to show the usual color pattdn of the species of the genus.

COLOR AND PATTERN IN ANIMALS 423

details in black and whitish, copy faithfully the details in Anosia;
only in the addition of a thin blackish line across the discal area
of the hind wings does archippus show any noticeable difference.
The viceroy is believed not to be distasteful to birds, but its
close mimicry of the distasteful monarch undoubtedly leads
to its being constantly mistaken for it by the birds and thus
left unmolested.

The subject of mimicry has not been studied largely among
the insects of our country, but in the tropics and subtropics
numerous striking examples of mimetic forms have been noted
and written about. The members of two large families of butter-
flies, the Danaidse and Heliconidse, are distasteful to birds,
and are mimicked by many species of other butterfly families,
especially the Pieridse, and by the swallowtails, Papilionidse.
Many plates illustrating such cases have been published by
Poulton and Marshall, Haase, Weismann, and others. Shelford,
in an extended account of mimicry as exemplified among the
insects of Borneo, refers to and illustrates many striking ex-
amples among the beetles, the Hemiptera, Diptera, Orthoptera,
Neuroptera, and moths; distasteful Lycid beetles are closely
mimicked by other beetles, by Hemiptera, and by moths;
distasteful ladybird beetles are mimicked by Hemiptera,
Orthoptera, and by other beetles; stinging Hymenoptera are
mimicked by stingless Hymenoptera, by beetles, flies, bugs,
and moths. Poulton and Marshall, in their account of mimicry
among South African insects, publish many colored plates
revealing most striking resemblances between insects well
defended by inedibility or defensive weapons, and their mim-
ickers. Our space unfortunately prevents any specific con-
sideration of these various interesting cases.

The special conditions under which mimicry exists have been
seriously studied and are of extreme interest. It is obvious that
the inedible or defended mimicked form must be more abundant
than the mimicker, so that the experimenting young bird or
lizard may have several chances to one of getting an ill taste
or a sting when he attacks an insect of certain type or pattern.
This requirement of relative abundance of mimicker and
mimicked seems actually met, as proved by observation. In
some cases only females of a species indulge in mimicry, the
males being unmodified. This is explained on the ground of
the particular necessity for protection of the egg-laden, heavy-

424 EVOLUTION AND ANIMAL LIFE

flying, long-lived, and hence more exposed females, as compared
with the lighter, swifter, short-lived males.

It has been found that individuals of a single species may
mimic several different species of defended insects, this poly-
morphism of pattern existing in different localities, or indeed
in a single one. Marshall believes that seasonal polychromat-
ism of certain butterfly species is associated with the mimicry
of certain defended butterflies of different species, these different
species appearing at different times of the year.

It is needless to say that such hypotheses and theories of
the utility of color and pattern have been subjected to much
criticism, both adverse and favorable. The necessity for limiting
results within the working range of efficient causes has been
the soundest basis, in our judgment, for the adverse criticism
of the theories of special protective resemblance, warning
colors, and mimicry. Until recently most of the observations
on which the theories are based have been simply observations
proving the existence of remarkable similarities in appearance or
equally striking contrasts and bizarrerie. The usefulness of
these similarities and contrasts had been deduced logically,
but not proved experimentally nor by direct observation. In
recent years, however, a much sounder basis for these theories
has been laid by experimental work. There is now on record
a large amount of strong evidence for the validity of the hypoth-
esis of mimicry. Certainly no other hypothesis of equal
validity with that of protective resemblance and mimicry has
been proposed to explain the numerous striking cases of similar-
ity and the significant conditions of life accompanying the ex-
istence of these cases, which have been recorded as the result
of much laborious and indefatigable study by certain naturalists.

Plateau and Wheeler have tasted so-called inedible and
distasteful insects and found nothing particularly disagreeable
about them. But as Poulton suggests, the question is not as
to the palate of Plateau and Wheeler nor of any man ; it concerns
the taste of birds, lizards, etc. Better evidence is that afforded
by actual observation of feeding birds and lizards; of experi-
mental offering under natural conditions of alleged distasteful
insects to their natural enemies. Marshall's observations and
experiments on the point are suggestive and undoubtedly
reliable. Much more work of the same kind is needed.

The efficient cause for bringing color and pattern up to such

COLOR AND PATTERN IN ANIMALS 425

a high degree of specialization has been assumed, by nearly all
upholders of the use hypotheses, to be natural selection. This
agent can account for purposefulness, which is obviously an
inherent part of all the hypotheses. And no other suggested
agent can. Weismann makes, indeed, of this fact, by inverting
the problem, one of the most effective arguments for the potency
and " Allmacht" of natural selection. He declares that the
existence of special protective resemblance, warning colors,
and mimicry proves the reality of selection. But it must be
asked, while admitting the cogency of much of the argument
for natural selection as the efficient cause of high specialization
of color and pattern as we have seen it actually to exist, how
such a condition as that shown by the mimicking viceroy butter-
fly has come to be gradually developed — gradual development
being confessedly selection's only mode of working. Could the
viceroy have had any protection for itself, any advantage at all,
until it actually so nearly resembled the inedible monarch as
to be mistaken for it? No slight tinge of brown on the black
and white wings (the typical color scheme of the genus), no
slight change of marking, would be of any service in making
the viceroy a mimic of the monarch. The whole leap from
typical Basilarchia to (apparently) typical Anosia had to be
made practically at once. On the other hand, is it necessary
for Kallima, the simulator of dead leaves, to go so far as it has
in its modification? Such minute points of detail are there
as will never be noted by bird or lizard. The simple necessity
is the effect of a dead leaf; that is all. Kallima certainly does
that and'more. Kallima goes too far and proves too much.
And there are other cases like it. Natural selection alone could
never carry the simulation past the point of advantage.

But whatever other factors or agents have played a part in
bringing about this specialization of color and pattern, exem-
plified by animals showing protective resemblances, warning
colors, terrifying manners, and mimicry, naturrl selection has
undoubtedly been the chief factor, and the basis of utility
the chief foundation, for the development of the specialized
conditions.

CHAPTER XX

REFLEXES, INSTINCT, AND REASON

We live in a world which is full of misery and ignorance, and the
plain duty of each and all of us is to try to make the little corner he can
influence somewhat less miserable and somewhat less ignorant than it
was before he entered it. To do this effectually it is necessary to be
fully possessed of two beliefs — the first, that the order of nature is
ascertainable by our faculties to an extent which is practically un-
limited ; the second, that our volition counts for something as a condi-
tion of the course of events.

Each of these beliefs can be verified experimentally as often as we
like to try. Each, therefore, stands upon the strongest foundation upon
which any belief can rest, and forms one of our highest truths. If we
find that the ascertainment of the order of nature is facilitated by using
one terminology or one set of symbols rather than another, it is our
clear duty to use the former; and no harm can accrue so long as we
bear in mind that we are dealing merely with terms and symbols. —
HUXLEY.

ALL animals of whatever degree of organization show in
life the quality of irritability or response to external stimulus.
Contact with external things produces some effect on each of
them, and this effect seems to be something more than the
mere mechanical effect on the matter of which the animal is
composed. In the one-celled animals, the functions of response
to external stimulus are not localized. They are the property
of any part of the protoplasm of the body. Just as breathing
or digestion is a function of the whole cell, so are sensation and
response in action. In the higher or many-celled animals
each of these functions is specialized and localized. A certain
set of cells is set apart for each function, and each organ or
series of cells is released from all functions save its own.

426

REFLEXES, INSTINCT, AND REASON 427

In the more highly organized animals certain cells from the
primitive external layer or ectoblast of the embryo are early
set apart to record the relations of the creature to its environ-
ment. These cells are highly specialized, and while some of
them are highly sensitive, others are adapted for carrying
or transmitting the stimuli received by the sensitive cells, and
still others have the function of receiving sense impressions and
of translating them into impulses of motion. The nerve cells
are receivers of impressions. These are gathered together in
nerve masses or ganglia, the largest of these being known as
the brain, the ganglia in general being known as nerve centers.
The nerves are of two classes. The one class, called sensory
nerves, extends from the skin or other organ of sensation to
the nerve center. The nerves of the other class, motor nerves,
carry impulses to motion.

The brain or other nerve center sits in darkness surrounded
by protecting tissues or a protecting box of bone. To this brain,
nerve center, or sensorium come the nerves from all parts of
the body that have sensation — the external skin as well as the
special organs of sight, hearing, taste, smell. With these come
nerves bearing sensations of pain, temperature, muscular effort-
all kinds of sensation which the brain can receive. These nerves
are the sole sources of knowledge to any animal organism.
Whatever idea its brain may contain must be built up through
these nerve impressions. The aggregate of these impressions
constitutes the world as the organism knows it. All sensation
is related to action. If an organism is not to act, it cannot
feel, and the intensity of its feeling is related to its power to act.

These impressions brought to the brain by the sensory
nerves represent in some degree the facts in the animal's en-
vironment. They teach something as to its food or its safety.
The power of locomotion is characteristic of animals. If they
move, their actions must depend on the indications carried to
the nerve center from the outside ; if they feed on living organ-
isms, they must seek their food; if, as in many cases, other
living organisms prey on them, they must bestir themselves
to escape. The impulse of hunger on the one hand and of fear
on the other are elemental. The sensorium receives an im-
pression that food exists in a certain direction. At once an
impulse to motion is sent out from it to the muscle necessary
to move the body in that direction. In the higher animals

428

EVOLUTION AND ANIMAL LIFE

these movements are usually more rapid and more exact than
with the lower forms. This is because the organs of sense
and action, the sense cells, nerve fibers, and muscles are all
highly specialized. In the starfish sensation is slight, nervous
communication slow, and the muscular response sluggish, but

the method is apparently the
same.

But in recent years many
biologists have come to believe
that much of the behavior of
the simplest animals, and some
of the actions of the higher,
are controlled in a more rigidly
mechanical way than the above
statements suggest; that, in a
word, much of the action, and
apparent instinctive or intelli-
gent response of animals to ex-
ternal conditions, is an immedi-
ate physicochemical rather than
vital phenomenon; that the
animal body in its relation to
the external world is much
more like a passive, senseless,
although very complex, ma-
chine, stimulated and controlled
by external factors and con-
ditions, than like the percipient,
determining, purposeful creature

FIG. 264. — Diagram showing how the
Protozoan, Oxytricha fallax, reacts to
cold; slide is heated at upper end and
an Oxytricha beginning at 1 continues
to react by turning a little to right
and backing and advancing and re-
peatedly turning a little to right and
backing and advancing until posi-
tion 14 is reached. (After Jennings.) that our usual conception of the

organism m^kes it out to be.

Clever experimenters, as Loeb, Lucas, Rr,dl, Bethe, Uexkull,
and numerous others, believe themselves justified in explaining
a host of the simpler actions or modes of behavior of animals,
on a thoroughly mechanical basis, as rigorous, inevitable
reactions to the influence or stimulus of light, heat, contact,
gravity, galvanism, etc. Phototropism, stereotropism, geo-
tropism, etc., are the names given to these phenomena of re-
sponse by action and behavior to stimuli of light, contact, and
gravity respectively.

Some of these biologists are ready to carry their giving up

REFLEXES, INSTINCT, AND REASON 429

of other than mechanical behavior among animals to great
lengths. Loeb introduces a paper written in 1890 on instinct
and will in animals as follows:

"In the biological literature one still finds authors who treat the
' instinct ' or the * will ' of animals as a circumstance which determines
motions, so that the scientist who enters the region of animated nature
encounters an entirely new category of causes, such as are said contin-
ually to produce before our eyes great effects, without it being possible
for an engineer ever to make use of these causes in the physical world.
' Instinct' and * will' in animals, as causes which determine movements,
stand upon the same plane as the supernatural powers of theologians,
which are also said to determine motions, but upon which an engineer
could not well rely.

"My investigations on the heliotropism of animals led me to
analyze in a few cases the conditions which determine the apparently
accidental direction of animal movements which, according to tradi-
tional notions, are called voluntary or instinctive. Wherever I have
thus far investigated the cause of such 'voluntary' or 'instinctive'
movements in animals, I have without exception discovered such
circumstances at work as are known in inanimate nature as determi-
nate movements. By the help of these causes it is possible to control
the 'voluntary' movements of a living animal just as securely and
unequivocally as the engineer has been able to control the movements
in inanimate nature. What has been taken for the effect of 'will' or
'instinct' is in reality the effect of light, of gravity, of friction, of
chemical forces, etc."

But Jennings, a very careful and industrious student of the
behavior of the protozoa, whose studies have been perhaps more
detailed and prolonged than those of any other investigator of
the same subject, closes a fascinating volume on his work with
the following paragraph:

"The present paper may be considered as the summing up of the
general results of several years' work by the author on the behavior of
the lowest organisms. This work has shown that in these creatures the
behavior is not as a rule on the tropism plan — a set, forced method of
reacting to each particular agent — but takes place in a much more
flexible, less directly machinelike way, by the method of trial and
error. This method involves many of the fundamental qualities

430

EVOLUTION AND ANIMAL LIFE

which we find in the behavior of higher animals, yet with the simplest
possible basis in ways of action; a great portion of the behavior con-
sisting often of but one or two definite movements, movements that are
stereotyped in their relation to the environment. This method leads
upward, offering at every point opportunity for development, and
showing even in the unicellular organisms what must be considered the

beginnings of intelligence and of many
other qualities found in higher animals.
Tropic action doubtless occurs, but the
main basis of behavior is in these or-
ganisms the method of trial and error."

FIG. 265. — Diagram showing how
the motile Protozoan, Stentor,
reacts to light: A circular space
half in light and half in dark;
the animalcules collect in dark
area; 1 , 2, and 3 show the
reaction of a specimen which
came to the light line. (After
Jennings.)

Different one-celled animals show
differences in method or degree of
response to external influences.
Most protozoa will discard grains
of sand, crystals of acid, or other
indigestible objects. Such peculi-
arities of different forms of life
constitute the basis of instinct.

Instinct is automatic obedience
to the demands of external condi-
tions. As these conditions vary
with each kind of animal, so must

the demand vary, and from this arises the great variety actu-
ally seen in the instincts of different animals. As the de-
mands of life become complex, so may the instincts become so.
The greater the stress of environment, the more perfect the
automatism, for impulses to safe action are necessarily adequate
to the duty they have to perform. If the instinct were inade-
quate, the species would have become extinct. The fact that
its individuals persist shows that they are provided with the
instincts necessary to that end. Instinct differs from other
allied forms of response to external conditions in being hereditary
and continuous from generation to generation, and in being
common to the species and not characteristic of the individual.
This sufficiently distinguishes it from reason, but the line be-
tween instinct and reason and various forms of reflex action
cannot be sharply drawn.

Some writers regard instincts as "inherited habit," while
others, with apparent justice, doubt if mere habits or voluntary

REFLEXES, INSTINCT, AND REASON

431

actions repeated till they become a " second nature " ever leave
a trace upon heredity. Such investigators regard instinct as
the natural survival of those methods of automatic response
which were most useful to the life of the animal, the individuals
1 utving less effective methods of reflex action having perished,
leaving no posterity.

An example in point would be the homing instinct of the
fur seal. When the arctic winter descends on its home in the
Pribilof Islands in Ber-
ing Sea, these animals
take to the open ocean,
many of them swim-
ming southward as far
as the Santa Barbara
Islands in California,
more than three thou-
sand miles from home.
While on the long
swim they never go
on shore; but in the
spring they return to
the northward, find-
ing the little islands FlG 266 _A .,pointer,, dog in the act of ..point.

hidden in the arctic ing," a specialized instinct. (Permission of G. O.

fogS, Often landing On Shields, publisher of "Recreation.")

the very spot from

which they were driven by the ice six months before, and
their arrival timed from year to year almost to the same
day. The perfection of this homing instinct is vital to their
life. If defective in any individual, he would be lost to the
herd and would leave no descendants. Those who return be-
come parents of the herd. As to the others the rough sea
tells no tales. We know that of those that set forth a large
percentage never come back. To those that return the homing
instinct has proved adequate. This must be so long as the race
exists. The failure of instinct would mean the extinction of
the species.

The instincts of animals may be roughly classified as to their
relation to the individual into egoistic and altruistic instincts.

Egoistic instincts are those which concern chiefly the in-
dividual animal itself. To this class belong the instincts of

432

EVOLUTION AND ANIMAL LIFE

feeding, those of self-defense and of strife, the instincts of play.

the climatic instincts, and environmental instincts, those which

direct the animal's mode of life.

Altruistic instincts are those which relate to parenthood

and those which are concerned
with the mass of individuals of
the same species. The latter
may be called the social instincts.
In the former class, with the in-
stincts of parenthood, may be in-
cluded the instincts of courtship,
reproduction, home-making, nest-
building, and care for the young.

The instincts of feeding are
primitively simple, growing com-
plex through complex conditions.
The protozoan absorbs smaller
creatures which contain nutri-
ment. The sea anemone closes
its tentacles over its prey. The
barnacle waves its feet to bring
edible creatures within its mouth.
The fish seizes its prey by direct
motion. The higher vertebrates
in general do the same, but the
conditions of life modify this
simple action to a very great
degree.

In general, animals decide by
reflex actions what is suitable
food, and by the same processes

FIG. 267.— Part of branch of oak ^y rejeCt poisons Or Unsuitable
tree, showing acorns placed in , rp,, -,

holes in the bark by the Caii- substances. The dog rejects an

fornia woodpecker, Melanerpes apple, while the horse rejects a

-g of me&^ Either will turn

away from the offered stone. Al-
most all animals reject poisons

instantly. Those that fail in this regard in a state of nature
die and leave no descendants. The wild vetches or "loco-
weeds " of the arid regions affect the nerve centers of animals
and cause dizziness or death. The native ponies reject these

iormicivorus bairdii. (From
photograph taken at Stanford

University, California.)

REFLEXES, INSTINCT, AND REASON 433

instinctively: This may be because all ponies which have not
this reflex dislike have been destroyed. The imported horse
has no such instinct and is poisoned. Very few animals will
eat any poisonous object with which their instincts are familiar,
unless it be concealed from smell and taste.

In some cases, very elaborate instincts arise in connection
with feeding habits. In the case of the California woodpeckers
(Melanerpes formicivorus bairdii) a large number together select
a live-oak tree for their operations. They first bore its bark
full of holes, each large enough to hold an acorn. Then into
each hole an acorn is thrust (Figs. 267 r.nd 268). Only one tree
in several square miles may be selected, and when their work is
finished all those interested go about their business elsewhere.
At irregular intervals a dozen or so come back with much
clamorous discussion to look at the tree. When the right time
comes, they all return, open the acorns one by one, devouring
apparently the substance of the nut, and probably also the
grubs of beetles which have developed within. When the nuts
are ripe, again they return to the same tree and the same
process is repeated. In the tree figured this has been noticed
each year since 1891.

The instinct of self-defense is even more varied in its mani-
festations. It may show itself either in the impulse to make
war on an intruder or in an impulse to flee from its enemies.
Among the flesh-eating mammals and birds fierceness of de-
meanor serves both for the securing of food and for protection
against enemies. The stealthy movements of the lion, the
skulking habits of the wolf, the sly selfishness of the fox, the
blundering good-natured power of the bear, the greediness of
the hyena, are all proverbial, and similar traits in the eagle,
owl, hawk, and vulture are scarcely less matters of common
observation.

Herbivorous animals, as a rule, make little direct resistance
to their enemies, depending rather on swiftness of foot, or in
some cases on simple insignificance. To the latter cause the
abundance of mice and mouselike rodents may be attributed,
for all are the prey of the carnivorous beasts and birds, and
of snakes.

Even young animals of any species show great fear of their
hereditary enemies. The nestlings in a nest of the American
bittern when one week old showed no fear of man, but when

434 EVOLUTION AND ANIMAL LIFE

two weeks old this fear was very manifest. Young mocking
birds will go into spasms at the sight of an owl or a cat, while
they pay little attention to a dog or a hen. Monkeys that have

FIG. 268. — Section of bark of the live-oak tree, with acorns placed on it by the California
woodpecker, Melanerpes formicivorus bairdii. (From photograph taken at Stanford
University, California.)

never seen a snake show almost hysterical fear at first sight of
one, and the same kind of feeling is common to most men. A
monkey was allowed to open a paper bag which contained a

REFLEXES, INSTINCT, AND REASON '435

live snake. He was staggered by the sight, but after a while
he went back and looked again, to repeat the experience. Each
wild animal has its special instinct of resistance or method of
keeping off its enemies. The stamping of a sheep, the kicking
of a horse, the running in a circle of a hare, and the skulking
in a circle of some foxes, are examples of this sort of instinct.

FIG. 269. — Nestlings of the American bittern, two of a brood of four birds one week old,
at which age they showed no fear of man. (Photograph by E. N. Tabor, Meridian,
N. Y., May 31, 1898. Permission of Macmillan Co., publishers of "Bird Lore.")

The play instinct is developed in numerous animals. To
this class belong the wrestlings and mimic fights of young
dogs, bear cubs, seal pups, and young beasts generally. Cats
and kittens play with mice. Squirrels play in the trees. Per-
haps it is the play impulse that leads the shrike or butcher bird
to impale small birds and beetles on the thorns about its nest,
a ghastly kind of ornament that seems to confer satisfaction
on the bird itself. The talking of the parrots and their imita-
tions of the sounds they hear seem to be of the nature of play.
The greater their superfluous energy the more they will talk.
Much of the singing of birds, and the crying, calling, and howling

436

EVOLUTION AND ANIMAL LIFE

of other animals, are mere play, although singing primarily
belongs to the period of reproduction, and other calls and cries
result from social instincts or from the instinct to care for the
young.

Climatic instincts are those which arise from the change of

FIG. 270. — Nestlings of American bittern. Four birds, of which two are shown in Fig.
269, two weeks old, at which age they showed marked fear of man. (Photograph
by E. N. Tabor, Meridian, N. Y., June 8, 1898. Permission of Macmillan Co.,
publishers of "Bird Lore.")

the seasons. , When the winter comes the fur seal takes its
long swim to the southward; the wild geese range themselves
in wedge-shaped flocks and fly high and far, calling loudly as
they go; the bobolinks straggle away one at a time, flying
mostly in the night, and most of the smaller birds in cold
countries move away toward the tropics. All these movements
spring from the migratory instinct. Another climatic instinct
leads the bear to hide in a cave or hollow tree, where he sleeps

REFLEXES, INSTINCT, AND REASON

437

or hibernates till spring. In some cases the climatic instinct
merges in the homing instinct and the instinct of reproduction.
When the birds move north in the spring they sing, mate,

*t: '- • ^y*'--'

and build their nests. The fur seal goes home to rear its young.
The bear exchanges its bed for its lair, and its first business
after waking is to make ready to rear its young.
29

438

EVOLUTION AND ANIMAL LIFE

Environmental instincts concern the creature's mode of life.
Such are the burrowing instincts of certain rodents, the wood-
chucks, gophers, and the like. To enumerate the chief phases
of such instincts would be difficult, for as all the animals are
related to their environment, this relation must show itself in
characteristic instincts.

The instincts of courtship relate chiefly to the male, the female
being more or less passive. Among the birds the male in spring

is in very many species
provided with an
ornamental plumage
which he sheds when
the breeding season
is over. The scarlet,
crimson, orange, blue,
black, and lustrous
colors of birds are
commonly seen only
on the males in the
breeding season, the
young males and
the old males in the
fall having the plain
brown gray or streaky
colors of the. female.
Among the singing
birds it is chiefly the
male that sings, and
his voice and the in-
stinct to use it are
commonly lost in

great degree when the young are hatched in the nest. Among
certain fishes the males are especially brilliantly colored in the
breeding time, but there is little evidence of any personal at-
tempts to display these colors before the females.

Among polygamous mammals the male is usually much
larger than the female, and his courtship is often a struggle
with other males for the possession of the female. Among
the deer the male, armed with great horns, fight to the death
for the possession of the female or for the mastery of the herd.
The fur seal has on an average a family of about thirty-two

FIG. 272, — Horns of two male deer interlocked while
fighting. (Permission of G. O. Shields, publisher
of "Recreation.")

REFLEXES, INSTINCT, AND REASON 439

females, and for the control of his harem others are ready at all
times to dispute the possession. But with monogamous animals
like the true or hair seal or fox, where a male mates with a single
female, there is no such discrepancy in size and strength, and
the warlike force of the male is spent on outside enemies, not
on his own species.

The movements of many migratory animals are mainly con-
trolled by the impulse to reproduce. Some pelagic fishes, es-
pecially flying fishes and fishes allied to the mackerel, swim
long distances to a region favorable for a disposition of spawn.
Some species are known only in the waters they make their
breeding homes, the individuals being scattered through the
wide seas at other times. Many fresh-water fishes, as trout,
suckers, etc., forsake the large streams in the spring, ascending
the small brooks where they can rear their young in greater
safety. Still others, known as anadromous fishes, feed and
mature in the sea, but ascend the rivers as the impulse of re-
production grows strong. Among such species are the salmon,
shad, alewife, sturgeon, and striped bass in American waters.
The most noteworthy case of the anadromous .instinct is found
in the king salmon or quinnat of the Pacific coast. This great
fish spawns in November. In the Columbia River it begins
running in March and April, spending the whole surrimer in
the ascent of the river without feeding. By autumn the in-
dividuals are greatly changed in appearance, discolored, worn,
and distorted. On reaching the spawning beds, some of them
a thousand miles from the sea, the female deposits her eggs in
the gravel of some shallow brook. After they are fertilized
both male and female drift tail foremost and helpless down the
stream, none of them ever surviving to reach the sea. The same
habits are found in other species of salmon of the Pacific, but
in most cases the individuals of other species do not start so
early or run so far. A few species of fishes, as the eel, reverse
this order, feeding in the rivers and brackish creeks, dropping
down to the sea to spawn.

The migration of birds has relation to reproduction as well
as to changes of weather. As soon as they reach their summer
homes, courtship, mating, nest-building, and the care of the
young occupy the attention of every species.

In the animal kingdom one of the great factors in develop-
ment has been the care of the young. This feature is a prominent

440

EVOLUTION AND ANIMAL LIFE

REFLEXES, INSTINCT, AND REASON

441

one in the specialization of birds and mammals. When the
young are cared for the percentage of loss in the struggle for
life is greatly reduced, the number of births necessary to the

I-'ic. 274. — Nest and eggs of the Ruf us hummingbird. Trochitusrufus.
J. O. Snyder, Stanford University, California.)

(Photograph by

maintenance of the species is much less, and the opportunities
for specialization in other relations of life are much greater.
In these regards, the nest-building and home-making animals
have the advantage over those that have not these instincts.

442

EVOLUTION AND ANIMAL LIFE

The animals that mate for life have the advantage over polyg-
amous animals, and those whose social or mating habits give

1

FIG. 275. — Altricial nestlings of the blue jay, Cyanocitta cristata.

rise to a division of labor over those with instincts less highly
specialized.

When we study instincts of animals with care and in detail,
we find that their regularity is much less than has been sup-
posed. There is as much variation in regard to instinct among

REFLEXES, INSTINCT, AND REASON 443

individuals as there is with regard to other characters of the
species. Some power of choice is found in almost every opera-
tion of instinct. Even the most machinelike instinct shows
some degree of adaptability to new conditions. On the other
hand, in no animal does reason show entire freedom from
automatism or reflex action. "The fundamental identity of
instinct with intelligence/7 says an able investigator, "is shown
in their dependence upon the same structural mechanism (the
brain and nerves), and in their responsive adaptability/7

FIG. 276. — A wild duck, Aythya, family; male, female, and prsecocial young.

„ Reason or intellect, as distinguished from instinct, is the
choice, more or less conscious, among responses to external
impressions. Its basis, like that of instinct, is in reflex action.
Its operations, often repeated, become similarly reflex by
repetition, and are known as habit. A habit is a voluntary
action repeated until it becomes reflex. It is essentially like
instinct in all its manifestations. The only evident difference
is in its origin. Instinct is inherited. Habit is the reaction
produced within the individual by its own repeated actions.
In the varied relations of life the pure reflex action becomes
inadequate. The sensorium is offered a choice of responses.
To choose one and to reject the others is the function of intellect^
or reason. While its excessive development in man obscures its
close relation to instinct, both shade off by degrees into reflex

444

EVOLUTION AND ANIMAL LIFE

action. Indeed, no sharp line can be drawn between uncon-
scious and subconscious choice of reaction and ordinary in-
tellectual processes.

Most animals have little self-consciousness, and their reason-
ing powers at best are of a low order; but in kind, at least, the
powers are not different from reason in man. A horse reaches

over the fence to be
company to another.
This is instinct. When
it lets down the bars
with its teeth, that
is reason. When a
dog finds its way
home at night by the
sense of smell, this
may be instinct ; when
he drags a stranger to
his wounded master,
that is reason. When
a jack rabbit leaps
over a bush to escape
a dog, or runs in a
circle before a coyote,
or when it lies flat in
the grass as a round
ball of gray, indistin-
guishable from grass,
this is instinct. But
the same animal is
capable of reason-
that is, of a distinct choice among lines of action. Not long
ago a rabbit came bounding across the university campus at
Palo Alto. As it passed a corner it suddenly faced two hunting
dogs running side by side toward it. It had the choice of turning
back, its first instinct, but a dangerous one; of leaping over the
dogs, or of lying flat on the ground. It chose none of these, and
its choice was instantaneous. It ceased leaping, ran low, and
went between the dogs just as they were in the act of seizing it,
and the surprise of the dogs, as they stopped and tried to hurry
around, was the same feeling that a man would have in like
circumstances.

FIG. 277. — Tailor bird, Ornithotomus sutorius, and
nest.

REFLEXES, INSTINCT, AND REASON 445

On the open plains of Merced County, Cal., the jack rabbit
is the prey of the bald eagle. Not long since a rabbit pursued
by an eagle was seen to run among the cattle. Leaping from
cow to cow, he used these animals as a shelter from the savage
bird. When the pursuit was closer, the rabbit broke cover
for a barbed-wire fence. When the eagle swooped down on it,
the rabbit moved a few inches to the right, and the eagle could
not reach him through the fence. When the eagle came down
on the other side, he moved across to the first. And this was
continued until the eagle gave up the chase. It is instinct
that leads the eagle to swoop on the rabbit. It is instinct again
for the rabbit to run away. But to run along the line of a
barbed-wire fence demands some degree of reason. If the need
to repeat it arose often in the lifetime of a single rabbit it would
become a habit.

The difference between intellect and instinct in lower animals
may be illustrated by the conduct of certain monkeys brought
into relation with new experiences. At one time we had two
adult monkeys, "Bob" and "Jocko," belonging to the genus
Macacas. Neither of these possessed the egg-eating instinct.
At the same time we had a baby monkey, " Mono/' of the genus
Cercopithecus. Mono had never seen an egg, but his inherited
impulses bore a direct relation to feeding on eggs, just as the
heredity of Macacus taught the others how to crack nuts or
to peel fruit.

To each of these monkeys we gave an egg, the first that any
of them had ever seen. The baby monkey, Mono, being of an
eg^-eating race, devoured his egg by the operation of instinct
or inherited habit. On being given the egg for the first time,
he cracked it with his upper teeth, making a hole in it, and sucked
out all the substance. Then holding the eggshell up to the
light and seeing that there was no longer anything in it, he
threw it away. All this he did mechanically, automatically,
and it was just as well done with the first egg he ever saw as
with any other he ate. All eggs since offered him he has treated
in the same way.

The monkey Bob took the egg for some kind of nut. He
broke it against his upper teeth and tried to pull off the shell,
when the inside ran out and fell on the ground. He looked at it
for a moment in bewilderment, took both hands and scooped up
the yolk and the sand with which it was mixed and swallowed

446

EVOLUTION AND ANIMAL LIFE

FIG. 278. — A monkey, Cerocopithecus, in a characteristic attitude.

REFLEXES, INSTINCT, AND REASON 447

the whole. Then he stuffed the shell itself into his mouth.
This act was not instinctive. It was the work of pure reason.
Evidently his race was not familiar with the use of eggs and had
acquired no instincts regarding them. He would do it better
next time. Reason is an inefficient agent at first, a weak tool;
but when it is trained it becomes an agent more valuable and
more powerful than any instinct.

The monkey Jocko tried to eat the egg offered him in much
the same way that Bob did, but not liking the taste he threw
it away.

The confusion of highly perfected instinct with intellect is
very common in popular discussions. Instinct grows weak
and less accurate in its automatic obedience as the intellect
becomes available in its place. Intellect and instinct as well as
all other nervous processes are outgrowths from the simple
reflex response to external conditions. But instinct insures a
single definite response to the corresponding stimulus. The
intellect has a choice of responses. In its lower stages it is
vacillating and ineffective; but as its development goes on it
becomes alert and adequate to the varied conditions of life.
It grows with the need for improvement. It will therefore
become impossible for the complexity of life to outgrow the
adequacy of man to adapt himself to its conditions.

Many animals currently believed to be of high intelligence
are not so. The fur seal, for example, finds its way back from
the long swim of two or three thousand miles through a foggy
and stormy sea, and is never too late or too early in arrival.
The female fur seal goes two hundred miles to her feeding
grounds in summer, leaving the pup on the shore. After a
week or two she returns to find him within a few rods of the rocks
where she had left him. Both mother and young know each
other by call and by odor, and neither is ever mistaken though
ten thousand other pups and other mothers occupy the same
rookery. But this is not intelligence. It is simply instinct,
because it has no element of choice in it. Whatever its an-
cestors were forced to do the fur seal does to perfection. Its
instincts are perfect as clockwork, and the necessities of migra-
tion must keep them so. But if brought into new conditions
it is dazed and stupid. It cannot choose when different lines
of action are presented.

The Bering Sea Commission of 1896 made an experiment

448 EVOLUTION AND ANIMAL LIFE

on the possibility of separating the young male fur seals, or
"killables," from the old ones in the same band. The method
was to drive them through a wooden chute or runway with
two valvelike doors at the end. These animals can be driven
like sheep, but to sort them in the way proposed proved im-
possible. The most experienced males would beat their noses
against a closed door, if they had seen a seal before them pass
through it. That this door had been shut and another opened
beside it passed their comprehension. They could not choose
the new direction. In like manner a male fur seal will watch
the killing and skinning of his mates with perfect composure.
He will sniff at their blood with languid curiosity; so long as it
is not his own it does not matter. That his own blood may
flow out on the ground in a minute or two he cannot foresee.

Reason arises from the necessity for a choice among actions.
It may arise as a clash among instincts which forces on the animal
the necessity of choosing. A doe, for example, in a rich pasture
has the instinct to feed. It hears the hounds and has the
instinct to flee. Its fawn may be with her and it is her instinct
to remain and protect it. This may be done in one of several
ways. In proportion as the mother chooses wisely will be the
fawn's chance of survival. Thus under difficult conditions,
reason or choice among actions rises to the aid of the lower
animals as well as man.

The word mind is popularly used in two different senses.
In the biological sense mind is the sum total of all psychic
changes, actions, and reactions. Under the head of psychic
functions are included all operations of the nervous system
as well as all functions of like nature which may exist in organ-
isms without specialized nerve fibers or nerve cells. As thus
defined mind would include all phenomena of irritability, and
even plants have the rudiments of it. The operations of the
mind in this sense need not be conscious. With the lower
animals almost all of them are automatic and unconscious.
With man most of them must be so. All functions of the sen-
sorium, irritability, reflex action, instinct, reason, volition,
are alike in essential nature though differing greatly in their
degree of specialization.

In another sense the term mind is applied only to con-
scious reasoning or conscious volition. In this sense it is
mainly an attribute of man, the lower animals showing it in

REFLEXES, INSTINCT, AND REASON 449

but slight degree. The discussion as to whether lower animals
have minds turns on the definition of mind, and our answer to
it depends on the definition we adopt.

Most plants are sessile organisms. Each is an organic col-
ony of cells, with the power of motion in parts but not that of
locomotion. The plant draws its nourishment from inorganic
nature — from air and water. Its life is not conditioned on a
search for food, nor on the movement of the body as a whole.
Yet the plant searches for food by a movement of the feed-
ing parts. In the process of growth, as Darwin has shown,
the tips of the branches and roots are in constant motion. This
movement is a spiral squirm. The movement of the tendrils
of the growing vine is only an exaggeration of the same action.
The course of the squirming rootlet may be deflected from a
regular spiral by the presence of water. The moving branchlets
will turn toward the sun. The region of sensation in the plant
and the point of growth are identical because this is the only
part that needs to move. The tender tip is the plant's brain.
If locomotion were in question the plant would need to be
differently constructed. It would demand the mechanism of
the animal. The nerve, brain, and muscle of the plant are all
represented by the tender growing cells of the moving tips.
The plant is touched by moisture or si-nlight. It may be said,
in somewhat metaphorical language, that it " thinks " of them,
and in so doing the cells that are touched and "think" are
turned toward the source of the stimulus. The function of
the brain, therefore, in some sense exists in the tree, but there
is no need in the tree for a specialized sensorium.

The many-celled animals from the lowest to the highest,
bear in their organization some relation to locomotion. The
animal feeds on living creatures and these it must pursue if it
is to thrive. It is not the sensitive nerve tips which are to
move; it is the whole creature. By the division of labor the
whole body of the compound organism cannot be given over
to sensation. Hence the development of sense organs dif-
ferent in character: one stimulated by waves of light, another
by waves of sound; one sensitive to odor, another to taste;
still others to contact, temperature, muscular strain, and pain.
These sense organs must through their nerve fibers report to
a sensorium which is distinct from each of them. And in
the process of specialization the sensorium itself is subdi-

450 EVOLUTION AND ANIMAL LIFE

vided into higher and lower nerve centers; centers of con-
scious thought and automatic transfer of impulse into motion.
This transfer indicates the real nature of all forms of nerve
action. All are processes of transfer of sensation into move-
ment. The sensorium or brain has no knowledge except such
as comes to it from the sense organs through the ingoing or
sensory nerves. It has no power to act save by its control of
the muscles through the outgoing or motor nerves. The mind
has no teacher save the senses; no servants save the muscles.

The study of the development of mind in animals and men
gives no support to the medieval idea that the mind exists as
an entity apart from the organ through which it operates.
This " Klavier theory " of the mind, that the ego resides in the
brain, playing upon the cells as a musician upon the strings of
a piano, finds no warrant in fact. So far as the evidence goes,
\ve know of no ego, except that which arises from the coordina-
tion of the nerve cells. All consciousness is " colonial conscious-
ness," the product of cooperation. It stands related to the
action of individual cells much as the content of a poem with
the words or letters composing it. Its existence is a phenomenon
of cooperation. The " I " in man is the expression of the co-
working of the processes and impulses of the brain. The brain
is made of individual cells, just as England is made of individual
men. To say that England wills a certain deed, or owns a
certain territory, or thinks a certain thought is no more a figure
of speech than to say that "I will," "I own," or "I think."
The "England" is the expression of union of the individual
wills and thoughts and ownerships of Englishmen. Similarly,
my "ego" is the aggregate resulting from coordination of the
elements that make up my body.

That what we really know of human personality tells the
whole story of it no one should maintain. It is well, however,
not to ascribe to it, entities and qualities of which we know
nothing.

CHAPTER XXI
MAN'S PLACE IN NATURE

A sacred kinship I would not forego

Binds me to all that breathes: through endless strife

The calm and deathless dignity of life

Unites each bleeding victim to its foe.

I am the child of earth and air and sea.

My lullaby by hoarse Silurian storms

Was chanted, and through endless changing forms

Of tree and bird and beast unceasingly

The toiling ages wrought to fashion me.

Lo! these large ancestors have left a breath
Of their great souls in mine, defying death
And change. I grow and blossom as the tree,
And ever feel deep-delving earthy roots
Binding me daily to the common clay:
Yet with its airy impulse upward shoots
My soul into the realms of light and day.
And thou, O sea, stern mother of my soul,
Thy tempests ring in me, thy billows roll!

— HJALMAR HJORTH BOYESEN.

Man betrays his relation to what is below him, thick-skulled,
small-brained, fishy, quadrumanous quadruped, ill-disguised, hardly
escaped into biped, and has paid for the new powers by the loss of some
of the old ones. But the lightning which explodes and fashions planets,
maker of planets and suns, is in him. On the one side elemental order,
sandstone and granite, rock ledges, peat bog, forest, sea, and shore.
On the other part, thought and the spirit which composes and decom-

451

452

EVOLUTION AND ANIMAL LIFE

poses nature. Here they are side by side, god and devil, mind and
matter, king and conspirator, belt and spasm riding peacefully to-
gether in the eye and brain ,of every man. — EMERSON.

The ape is this rough draft of man. Mankind have their gradations
as well as the other productions of the globe. There are a prodigious
number of continued links between the most perfect man and the
ape. — JOHN WESLEY.

ONE of the most important results of Darwin's studies of
the origin of species has been the complete change in the philo-
sophical conception of man. We no longer think of the human
race as a completed entity in the midst of Nature, but apart

FIG. 279. — Skulls of man and the orang-utan: 1, skull of a seven-year-old German
child; 2, skull of an Australian from Murray River; 3, skull of young orang-utan;
4, skull of a grown orang-utan. (After Wiedersheim; one-sixth natural size.)

from it, with a different origin, a different motive, a different
destiny. Man is like the other species, an inhabitant of the earth,
a product of the laws of life; his characters are phases in the
long process of change and adaptation to which all organisms
are subject. From the point of view of zoology, the human
race is a group of closely allied species, or subspecies, undoubted-

MAN'S PLACE IN NATURE

453

FIG. 280.— Young chimpanzee. (From Welt-
all u. Menschheit; after photograph from
life by Dr. Heck of Berlin.)

ly derived from a common
stock, and each species in
its ramifications modified
by the forces and condi-
tions included under the
general heads of variation,
heredity, segregation, selec-
tion, and the impact of
environment precisely as
species in other groups are
affected. It is clear that if
there is an origin of species
through natural causes
among the lower animals
and plants, there is an
origin of species among
men. If homology among
animals and plants is the
stamp of blood relation-
ship, the same rule holds
with man as well. Man is connected with the lower animals
by the most perfect of homologies. These are traceable in
every bone and muscle, in every blood vessel and gland, in

every phase of structure,
even including those of
the brain and nervous
system. The common
heredity of man with other
vertebrate animals is as
well established as any fact
in phylogeny can be.

In working out the
details of the origin of
man, we have once more
the three "ancestral docu-
ments " of biology, compar-
ative anatomy, embryol-
ogy, and paleontology.
Considered structur-

FIG. 281.— Foot skeleton of chimpanzee at left, a^V' man forms
and of man at right. (After Wiedersheim.)

30

genus, Homo, the Sole

454

EVOLUTION AND ANIMAL LIFE

resentative among living forms of the Hominidse, the highest
family of the order of Primates. To the species, man, Lin-
naeus gave the scientific name of Homo sapiens, this being re-
garded by him as the primitive species which has diverged
into several geographical varieties or races. Of these, at least
three might well be regarded as distinct species. The form
called by LinnaBus Homo sapiens europ&us includes not only
the white men of Europe, but allied races of Africa and Asia,
as the Moors, the Jews, the Turks, the Arabs, the Hindus
and the Ainus of northern Japan. To Homo asiaticus belong

FIG. 282. — Upper teeth of man and the orang-utan: At left, of a Caucasian; in middle,
of a negro; at right, of a grown orang-utan. The condition in the negro is
between that in the orang-utan and that in the Caucasian. (After Wieders-
heim.)

the Mongolian races, probably the Esquimaux and Aleuts of
North America, and perhaps the American Indians (Homo
americanus), with the Malays, the South Sea Islanders, and the
Australians as well. Homo afer of Africa and adjacent islands
comprises the kinky-haired negroes and negritos.

Structurally the members of the genus Homo are closely
allied to the anthropoid apes. The actual differences in
anatomy are very slight. The differences in degree of mental
endowment are enormous, but it can be shown that these dis-
tinctions are, for the most part, of degree only, associated with
the greater size and greater degree of specialization of the brain
of man. Homologies of the closest sort exist, involving every
element in structure as well as every function of the organism
and every known mental attribute. The anthropoid or man-
like apes constitute the family of Simiidse. The principal
species are the following, beginning with the lowest or most
monkeylike: Hylobates, the gibbons, of several species, notable

MAN'S PLACE IN NATURE

455

FIG. 283. — Baby orang-utan.
(From life.)

for their very long arms and

erect posture; Siamanga syndac-

tyla} the siamang; Simia satyrus,

the orang-utan ; Pan gorilla,

the gorilla, sometimes called

Troglodytes gorilla (though the

name Troglodytes was first used

for the wren); and the chim-
panzees, Anthropithecus niger and

calvus. Of these the gorilla is

physically the strongest. It

reaches a height of five feet

and a weight of 200 pounds.

The chimpanzee, smaller and

more amiable in disposition,

most suggests man in appear-
ance, although the gorilla is

structurally most like him.

The order of primates has been variously classified. It is

conveniently divided into five principal groups: (a) the lemurs

(including Lemuridae, Cheiromyidae, Galeopithecidae, and still

more generalized ex-
tinct forms) ; (6) the
Platyrrhine or New-
World monkeys
(Cebidae and Arcto-
pithecidae or Mar-
mosets; (c) the
Catarrhine or Old-
World monkeys and
baboons (Cercopi-
thecidae) ; (d) the
anthropoid apes
(Simiidae) ; and (e)
man (Hominidae).

The lemurs of
Madagascar are the
most primitive.
Like other primates,
they have flat nails

FIG. 284.— Lemur, furcifer. (After Ritzema Bos.) and an

456

EVOLUTION AND ANIMAL LIFE

thumb on each foot. Monkey like in their feet and in their
general habit, yet in appearance they have little to suggest
affinity with man. In general make-up, they are superficially
comparable rather with weasels, squirrels, and bats.

The New- World monkeys differ widely from the others.
Technically they are distinguished by the diverging (platyr-
rhine) nostrils, and by the retention of the primitively larger

number of teeth. Many
of them have prehensile
tails, and in habit and
temper all are very un-
like the more hardy and
pugnacious monkeys of
t the Old World. All the

Old- World monkeys as
well as the apes and
man have parallel nos-
trils, directed downward
(catarrhine) . Their tails,
if present, are not pre-
hensile, and in their
habits and temper they
approach progressively
toward man. Catarrhine
monkeys are known to
have existed in the
Miocene period. The
anthropoid apes repre-
sent a high degree of

advancement within the same group which finds its final ex-
treme in the genus Homo.

Considering structural characters alone, it is readily con-
ceivable that man should have had an anthropoid ancestry,
that the anthropoids should have sprung from an Old- World
monkey stock, and that the Old- World monkeys in turn are
derived from the lemurs. It is not supposable that any living
species of man has sprung from any extant species of anthropoid
ape. The point of juncture is clearly far back in the earlier
Tertiary times, but morphological evidence points to the com-
mon origin of primitive man and the known anthropoids. It is,
of course, certain that the intermediate forms when known

FIG. 285.— Gorilla.

MAN'S PLACE IN NATURE

457

will not be strictly man-apes, nor ape-men, but rather primitive
creatures uniting the possibilities of both. From that condition
men and apes have since
diverged and will con-
tinue to diverge.

There is no doubt of
the truth of Huxley's
statement :

"Thus whatever system
of organs be studied, the
comparison of their modi-
fications in the ape series
leads to one and the same
result — that the structural
differences which separate
man from the gorilla and
the chimpanzee are not so
great as those which separate the gorilla from the lower apes."

FIG. 286.— Head of gorilla. (After Brehm.)

In fact, as Haeckel has observed,

"It is very difficult to show why man should not be classed with
the large apes in the same zoological family. We all know a man from

an ape, but it is quite an-
other thing to find differ-
ences which are absolute
and not of degree only."

It may be broadly
stated that man differs
from the apes in the
combination of the fol-
lowing characters: (1)
Erect walk; (2) ex-
tremities differentiated
accordingly, the great
toe not being oppos-

able, the other toes little prehensile; (3) articulate speech; (4)
higher reasoning power. The erect walk is not an absolute
character. The higher apes walk on their feet, touching the
ground at times with their knuckles. The tailed monkeys

FIG. 287.— Face of gorilla. (After Brehm.)

458

EVOLUTION AND ANIMAL LIFE

walk like a bear, four-footed, and resting on the palms of their
hands. The muscles in each case are the same, although in man
the gastrocnemius and soleus are enlarged, forming the calf
of the leg, while the expanded glutaeus maximus forms the
buttocks. Both buttocks and calf are scantily developed in
the apes and monkeys, but the muscles forming them are
essentially the same as in man.

The monkeys have been called Quadrumana, four-handed,
because the foot like the hand is fitted for grasping, and the

great toe, like the thumb, is
opposable to the other digits.
But as Huxley has clearly
shown, this modification in-
volves no real change of
structure. An examination
of the bones and muscles
involved at once shows that
the hinder limb in apes and
monkeys is truly a foot and
not a hand. Part by part
the hinder foot of the mon-
key is homologous with the
foot of man, not with the
hand (Fig. 281). The loss of
the power of opposing the
great toe, on the part of
man, is a result incident to

the use of the hinder limbs for walking alone, and not for
grasping. In some of the lower races of man the great
toe stands apart from the others to a larger extent than in the
European races.

In the apes there is a greater degree of mobility of the muscles
of the scalp and the ear than in man, but there are very many
cases of men who are able to move these muscles freely. The
muscles of the tail in man are quite useless, as are also those of
the higher apes, in which the coccyx or tail is scarcely more
developed than in man.

In man, the wisdom teeth are usually rudimentary, but in
the native Australians these teeth are the largest of the series,
as is also the case with the apes (Fig. 282).

In structure it is clear that man agrees in all large matters

FIG. 288. — A young gorilla of the Leipsig
Zoological Garden. (From Illustrirte Zei-
tung, after a photograph.)

MAN'S PLACE IN NATURE

459

with the anthropoid apes, and that he cannot be separated as
an order from other primates.

In mental attributes the differences are very great, but these
are all correlated with the large size of the human brain, and are
all psychological expressions of the high degree of specialization
of its parts. The largest recorded human brain, according to
Huxley, has a weight of sixty-five to sixty-six ounces, the
smallest of about thirty-two. The brain of the highest ape
weighs about twenty ounces.

The immense differences between the intelligence of ape and
man does not imply any corresponding physical gap between

FIG. 289. — Top of brain of a seven-to-eight months human embryo at left, and a two-
year-old female chimpanzee at right. (After Wiedersheim )

them, or any corresponding difference in their brains. Huxley
uses the illustration of a watch which keeps perfect time as
compared with a watch having imperfect machinery. The
difference is not so much in the structure of the watch as in
the fineness of the parts and the perfection of their adjustment.

" Believing as I do with Cuvier that the possession of articulate
speech is the grand distinctive character of man (whether it be abso-
lutely peculiar to him or not), I find it very easy to comprehend that
some equally inconspicuous structural difference may have been the
primary cause of the immeasurable and practically infinite divergence
of the Human from the Simian stirps."

460 EVOLUTION AND ANIMAL LIFE

Huxley further shows that

" the brain is only one condition out of many on which intellectual
manifestations depend, the others being, chiefly, the organs of the
senses and the motor apparatuses, especially those concerned in pre-
hension and in the production of articulate speech."

Selenka finds that man and the man-apes agree in the manner
and relation of the young in the mother-body to that body in
that both man and man-apes have but a single disclike placenta,
while the other apes have two opposed disc placentas. And
Friedenthal finds that while the blood serum of man is poisonous

to, and destroys the red
blood corpuscles of all
other animals experi-
mented on, these animals,
including fishes, amphib-
ians, reptiles, birds, and
mammals, among which
FIO. 290.-Right hand at left, and right foot at latter . were lemurs and

right, of a two-months-old human embryo, New-World (AteleS and
showing similar position of the first digit PltheCOSClUrUs) and Old-
(thumb, great toe) in each. (After Wieders- -ITT i j //^» T 7

heim) World (Cynocephalus,

Macacus and Rhesus)

monkeys, it does not injure the corpuscles of the man-apes
(orang, gibbon, and chimpanzee). This immunity exists only
among closely related animals as the horse and donkey, dog
and wolf, and hare and rabbit. From which it is evident that
man and the man-apes have nearly identical blood.

The second "ancestral document," embryology, emphasizes
the common origin of man with that of the higher vertebrates
and notably with that of the anthropoids. The embryos of
man and apes develop in a fashion precisely parallel. In both,
as in all other mammals, the early presence of gill slits furnishes
evidence of a descent from a fishlike ancestry. The same
evidence is given in the embryonic growth of reptiles and birds.
In the development of the human child some simian traits
appear, these being wholly or partly lost in the more advanced
stages. Among these is the lanugo or general covering of long
hairs, more or less developed in certain stages of foetal growth,
but lost entirely before birth. The curving upward of the feet,

MAN'S PLACE IN NATURE

461

characteristic of early childhood, is a simian trait. According
to Jeffreys Wyman, when the foetus is about an inch in length,
"the great toe is shorter than the others, and instead of being
parallel to them, is projected at an angle from the side of the
foot, thus corresponding with the permanent condition of this
part in the Quadrumana."

The great grasping power of young babies is well known,
and this is likewise a simian trait.
Dr. Louis Robinson has shown that
very young babies will support their
own weight, by holding to a hori-
zontal bar for a period of half a
minute to two minutes. In all cases
"the thighs are bent nearly at right
angles to the body and in no case
did the lower limbs hang down and
take the attitude of the erect posi-
tion " (Fig. 291).

The study of embryonic develop-
ment shows also that the tail in
man and ape alike is at a certain
stage of development longer than the
legs, as in the monkeys and other
lower mammals. In this stage, ac-
cording to Romanes, "the tail ad-
mits of being moved by muscles
which later on dwindle away."
Sometimes, however, these muscles
persist through life.

The vermiform appendix is like-
wise more developed in the human

embryo than in the adult, a fact which holds in regard to
vestigial structures generally. As already stated in Chapter
XX (discussion of vestigial structures), Wiedersheim has re-
corded in man 180 structural reminiscences of his descent from
the lower animals. All the facts of this class point to a common
origin of man and apes, and an earlier community of origin
with other mammals and with other vertebrates, the most
primitive traits allying all of them with the fishes.

Paleontology has comparatively little to offer, but that little
is decisive. The life habits of men and monkeys are singularly

FIG. 291. — An infant three
months old supporting its
weight for over two minutes;
the attitude of the lower
limbs is strangely simian.
(From Romanes; after an in-
stantaneous photograph.)

462

EVOLUTION AND ANIMAL LIFE

unfavorable to the preservation of their remains as fossils in
rocks. Of the hundreds of species and millions of individuals
of the monkey tribes, very few of their remains are known
anywhere. Living in thickets and underbrush there is little
opportunity for them to be preserved as fossils. With man.

FIG. 292. — End of the humerus of various animals including man, showing position of
the humerus canals. A.Hatteria; B, Lacerta; C, cat; D, man. (After Wiedersheim.)

the condition is not very different. Implements of stone, bone,
bronze, and iron mark stages in the development of primitive
tribes. Fossil remains are confined almost wholly to bones
buried in quicksand or in the drippings of caves. Of fossil
monkeys, several genera have been described. Pan sivalensis
is a species of extinct gorilla from the Pliocene of the Punjaub.

Of all the fossil primates
the one of the greatest
interest is Pithecanthropus
erectus, from the upper
Pliocene of Java, lately
described by Dr. Eugene
Du Bois. This species
has been designated by
Haeckel as "the last link"
in human genealogy. Its

FIG. 293.— The human eye showing, Ps, arrange- characters have been held
ment of the third eye, Blica semilunaris. to Correspond with those

of the hypothetical ape-
man imagined by Haeckel and named Pithecanthropus alaulus,
before these remains were found. The generic name of the
imaginary ape-man has been transferred to the actual fossil.
The discovered relics of this species are scanty enough, consist-
ing of the skullcap, a femur, and two teeth (Figs. 294 and 295).

MAN'S PLACE IN NATURE

463

In Haeckel's Cambridge lecture, "The Last Link," the facts
concerning this fossil are thus summed up:

"The remains in question rested upon a conglomerate which lies
upon a bed of marine marl and sand of Pliocene Age. Together with
the bones of Pithecanthropus were found those of Stegodon, Leptobos,
Rhinoceros, Sus, Felis, Hyaena, Hippopotamus, Tapir, Elephas, and a
gigantic Pangolin. It is re-
markable that the first two
of these. genera are now ex-
tinct, and that neither hip-
popotamus nor hyaena exists
any longer in the oriental
region. If we may judge
from these fossil remains,
the bones of Pithecanthropus
are not younger than the
oldest Pleistocene and prob-
ably belong to the Upper
Pliocene. The teeth are very
like those of man. The fe-
mur also is very human, but
shows some resemblance to
that of the gibbons. Its size,
however, indicates an animal
which stood when erect not
less than five feet six inches
high. The skullcap is very
human, but with very promi-
nent eyebrow ridges, like
those of the famous Neander-
thal cranium. It is certainly not that of an idiot. It had an estimated
cranial capacity of about 1,000 c.c., that is to say, much larger than
that of the largest ape, which possesses not more than 600 c.c. The
crania of female Australians and Veddahs measure not more than 1,100
c.c., some even less than 1,000 c.c.; but as these Veddah women stand
only about four feet nine inches high, the computed cranial capacity
of the much taller Pithecanthropus is comparatively low indeed."

The impressions left by the cerebral convolutions are also
very human, more highly developed than in the recent apes.

FIG. 294. — Remains of Pithecanthropus erectua;
the single femur shown in different aspects.
(From "The Open Court.")

464 EVOLUTION AND ANIMAL LIFE

It is stated that in the discussion at Leyden, where Dr.
Du Bois's specimens were first exhibited, "three of the twelve
experts present held that the fossil remains belonged to a low
race of man; three declared them to be those of a manlike ape
of great size, the rest maintained that they belonged to an

FIG. 295. — Cranium of Pithecanthropus erectus. (From W eltall u. Menschheit.)

intermediate form which directly connected primitive man
with the anthropoid apes/' (Haeckel.)

Of the several early relics which are distinctly human, the
Neanderthal skull, found by Professor Schaffhausen in a lime-
stone cave in the Neanderthal, near Diisseldorf, is the most
notable. This skull represents the most primitive and least
specialized of any skull type known to be distinctly human.
It has therefore been recently named as a distinct species of
man, Homo neanderthalensis.

According to Huxley, this type of man, while certainly
simple, primitive, and doubtless extremely barbarous is, never-
theless, wholly human. It shows no distinctly pithecoid char-
acters, and it belongs clearly to the genus Homo.

Another skull of great antiquity comes from a cave at
Engis in the valley of the Meuse, and is known as the Engis
skull. This was found associated with bones of the mammoth
and of the woolly rhinoceros. This also is extremely primitive,

MAN'S PLACE IN NATURE

465

suggesting the skull of an Ethiopian. It is, however, more
like that of recent man than is the Neanderthal skull.

The comparison of the different races of men through the
methods of the science of ethnology throws much light on the
relations of the races to one another, but casts little light on
the origin of the genus Homo. This study considerably in-
creases the number of primitive races beyond the three stems
usually recognized or the four named by Linnaeus. The form
of the skull, the color of the skin, the character of the hair, and
the traits of language have given rise to the technical nomen-
clature of numerous more or less well-defined groups. These

FIG. 296. — Remains of the Neanderthal man in the Provincial Museum at Bonn.
(From Weltall u. Menschheit.)

races of men limited by geographical segregation run more or
less distinctly parallel to the races or geographical subspecies
within widely distributed species of animals. Our knowledge
of the origin of man as derived from ethnology is thus summed
up by Huxley: "So far as the light is bright it shows him sub-

466

EVOLUTION AND ANIMAL LIFE

FIG. 297. — Skull of ancient man from Spy
in Belgium. (From Weltall u. Menschheit;
after Professor Fraipont's photograph of
the original in the musem at Liege.)

stantially as he is now, and when it grows dim it permits us to
see no sign that he was other than he is now."

The gradual development of man from nomadic apes;
the gradual effect of the prolonged infancy of his young in the

holding of the family to-
gether; the altruistic trans-
formation of the family into
the patriarchal and tribal
systems; the gradual in-
crease of the power of choice
among instincts, a develop-
ment which at last places
intellect above instinct, the
use of fire and the use of
tools, the growth of speech
and its reaction upon intel-
ligence, the invention of
writing, effects of the su-
premacy of the strong — all
these matters afford large

range for speculation and some opportunity for direct investi-
gation. But the essential fact, the kinship of man with the
lower forms, and his divergence from them through the opera-
tion of natural laws, forces and conditions more or less per-
fectly understood, all this must now be taken as settled by the
investigations and dis-
coveries of Darwin and his
coworkers and successors.

Assuming that the gene-
alogy of man can be traced
through the anthropoids
and the Old- World mon-
keys to the lemurs, how
much further can we go?
Apparently the lemurs rep-
resent an early offshoot
from the mammalian stock,

the nearest point of juncture being the order of marsupials, now
so largely represented in Australia. Certain extinct lemurian
genera are more distinctly primitive than any of the living
forms. The marsupials are connected with the primitive group

FIG. 298. — Diagrammatic representation of
profiles of crania of primitive types of man.
(After Lenormant.)

MAN'S PLACE IN NATURE 467

of reptile-mammals, the Monotremes (Ornithorhynchus, Tacky-
gloss us), now also represented, although scantily, in Australia.
The Monotremes may be assumed to be derived from reptilian
stock, perhaps from ancestors of the three-eyed lizards of New
Zealand, known as Sphenodon or Hatteria. Behind these
lizards we certainly find the primitive amphibians or mailed
frogs and behind these the group of lung-bearing fishes, known
as fringefins or crossopterygians. These fishes were originally
derived, no doitbt, from sharks, and the sharks may have
come from wormlike forms through intermediate groups which
find their nearest modern homologues in the lamprey and
lancelet, and possibly in the wormlike acorn-tongue or balano-
glossus, a creature which to a soft wormlike body adds the gill
slits of a vertebrate and a trace of a primitive backbone or
notochord. Haeckel goes on with confidence to show the
derivation of one type of worm from another, of all from allies
of the hydra and volvox, then from the one-celled amoeba,
and at last from the still more primitive monera, a micro-
scopic bit of protoplasm. But with every step backward the
genealogy grows more and more hypothetical. All sorts of
possibilities open at every turn and positive proof is necessarily
lacking. The gill slits and the primitive notochord of the human
embryo leave little doubt that man in common with all other
vertebrates had a fishlike ancestry. In the line of this ancestry
must have lain the extinct crossopterygian fishes, but behind
this there is room all the way for serious doubt and questioning.

This much is certain, man's place is in nature. He is part
and parcel of nature, and the forces that still act on flower and
bird and beast are the forces by which the central energy of
the universe, whatever its name or definition, each day "in-
stantly and constantly renews the work of creation."

Objections have been raised to the theory of the descent of
man from the lower primates on grounds supposed to find their
sanction in theology. Such objections have no standing in
science. In Darwin's words: "Theology and science must each
run its own course and I am not responsible if their meeting-
point be still afar off." In the long run, theology, with other
forms of philosophy, must adjust itself to harmonize with
ascertained truth. The origin of man is not a question of per-
sonal preference nor one to be decided by a majority vote. The
only question is as to what is true. "Extinguished theolo-

46S EVOLUTION AND ANIMAL LIFE

gians," Huxley tells us, "lie about the cradle of every science
as the strangled snakes beside that of the infant Hercules."
Looking along the history of human thought, we see the attempt
to fasten to Christianity each decaying belief in science. Every
failing scientific notion has claimed orthodoxy for itself. That
the earth is round, that it moves about the sun. that it is old,
that granite ever was melted — all these beliefs, now part of our
common knowledge, have been declared contrary to religion,
and Christian men who knew these things to be true have suf-
fered all manner of evil for their sake. We see the hand of the
Almighty in nature everywhere; but everywhere he works with
law and order. We have found that even comets have orbits;
that valleys were dug out by water, and hills worn down by
ice; and all that we have ever known to be done on earth has
been done in accordance with law.

Darwin says: "To my mind it accords better with what we
know of the laws impressed on matter by the Creator, that the
production and extinction of the past and present inhabitants
of the world should have been due to secondary causes, like
those determining the birth and death of an individual, f When
I view all beings, not as special creations, but as lineal descend-
ants of some few beings who lived before the first bed of Silurian
was deposited, they, seem to me to become ennobledj There
is grandeur in this view of life, with its several powers having
been originally breathed by the Creator into a few forms or
into one, arid thai wKile this~planet~has gone cycling on accord-
ing to the fixed law of gravity, from so simple a beginning,
endless forms most beautiful and most wonderful have been
and are being evolved."

^Vith the growth of the race has steadily grown our con-
ception of the omnipotence of God.J Our ancestors felt, as many
races of men still feel, that each household must have a god of
its own, for, numerous as the greater gods were, they were
busy with priests and kings. Men could hardly believe that the
God of their tribe could be the God of the Gentiles also. That
He could dwell in the temples not made with hands removed
Him from human sight. That there could be two continents
was deemed impossible, for one God could not watch them both.
That the earth was the central and sole inhabited planet rested
on the same limited conception of God. That the beginning
of all things was a few thousand years ago is another phase of

MAN'S PLACE IN NATURE 469

the same limitation of view, as is the idea of the special mechan-
ical creation for every form of animal and plant.

A Chinese sage, whose words remain though his name be
lost in mists of the ages, has said: "He cannot be concealed:
He will appear without showing Himself, effect renovation with-
out moving, and create perfection without acting. It is the
law of heaven and earth, whose way is solid, substantial, vast,
and unchanging."

81

APPENDIX

REFERENCES to general and special treatments of the subjects in-
cluded in this book, arranged according to chapters. These references
are confined to books and papers published in English (including trans-
lations of foreign works) and are intended for the assistance of general
readers and elementary students wishing to follow up, in more detail
than is offered in this book, the study of any particular phase of the
general subject of Evolution. These references are therefore mostly
not to original papers, but to manuals, summaries, and digests of
evolution subjects. The list makes no attempt, of course, to include
all of even the most general works of reference.

CHAPTER I. EVOLUTION DEFINED.

Spencer : Principles of Biology, vol. I, part iii, 1875. Conn : Evolution
of To-day, pp. 1-21, 1889. Le Conte: Evolution, Its Nature, Its
Evidences, and Its Relation to Religious Thought, pp. 3-31, 1888.
Haeckel: History of Creation, vol. I, ch. i, 1892. Marshall: Dar-
winian Theory, pp. 1-24, 1894. Williams: Geological Biology,
pp. 371-384, 1895. Romanes: Darwin and After Darwin, vol.

I, pp. 12-22, 1896. Marshall: Biological Lectures, pp. 1-26,
1897. Howison : Limits of Evolution, pp. 12-48, 1901. Huxley:
Darwiniana, 1894.

CHAPTER II. VARIETY AND UNITY OF LIFE.
Jordan: Footnotes to Evolution, ch. i and ii, 1902.

CHAPTER III. LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION.

Verworn: General Physiology, 1899. Wilson: Chapters on Evolution,
pp. 61-79, 1883. Haeckel: History of Creation, vol. I, ch. xv; vol.

II, ch. xvii and xviii, 1892. Huxley: On the Physical Basis of
Life (ch. iii, in Essays on Method and Results). Marshall: Bio-
logical Lectures, pp. 114-191, 1897. Conn: The Method of Evo-
lution, ch. ix, 1900. Jordan and Kellogg: Animal Life, ch. i, ii, and
iv, 1900. McFarland: The Physical Basis of Heredity, ch. vi in
Jordan's Footnotes to Evolution, 1902. Wilson: The Cell in
Development and Inheritance, 2d ed., 1904. Weismann: The
Evolution Theory, vol. II, Lect. 36, 1904.

471

472 APPENDIX

CHAPTER IV. THE FACTORS AND MECHANISM OF EVOLUTION.

Darwin: Origin of Species, ch. xv, 1859. Spencer: The Factors of
Organic Evolution, pp. 1-37, 1889. Gray: Darwiniana, pp. 9-61,
1889. Conn: The Method of Evolution, 1900. Cope: Primary
Factors of Organic Evolution, 1896. Jordan: Footnotes to
Evolution, ch. iii and iv, 1902. Weismann: The Evolution
Theory, vol. I, Lect. 1, 1904.

CHAPTER V. NATURAL SELECTION AND THE STRUGGLE FOR
EXISTENCE.

Malthus : Principles of Population. Darwin : Origin of Species, ch.
iii and iv, 1859. Wallace: Darwinism, pp. 14-125, 1891. Mar-
shall: Lectures on the Darwinian Theories, pp. 27-52, 1894. Ro-
manes: Darwin and After Darwin, vol. I, pp. 125-294, 1895.
Wallace: Natural Selection and Tropical Nature, 1895. Bailey:
Survival of the Unlike, pp. 13-54. 1897. Osborn : From the Greeks
to Darwin, ch. vi, 1899. Headley: Problems of Evolution, pp.
68-155, 1901. Conn: Method of Evolution, ch. ii and iii, 1901.
Morgan: Evolution and Adaptation, ch. iv and v, 1903. Weis-
mann: The Evolution Theory, vol. I, Lects. 2 and 3, 1904. De
Vries: Species and Varieties, Their Origin by Mutation, ch. xxviii,
1905. Metcalf: Organic Evolution, 1905. Kellogg: Darwinism
To-day, 1907.

CHAPTER VI. ARTIFICIAL SELECTION.

Darwin: Origin of Species, ch. i, 1859. Darwin: Animals and Plants
under Domestication, vols. I and II, 1868. Wallace : Darwinism,
ch. iv and vii, 1891. Burbank: New Creations in Fruits and
Flowers (catalogues of plant novelties), 1893 and succeeding
years. Marshall: Lectures on the Darwinian Theories, pp.
27-36, 1894. Romanes : Darwin and After Darwin, vol. I, pp. 294-
314,1896. Bailey: Survival of the Unlike, 1897. De Vries: Spe-
cies and Varieties, Their Origin by Mutation, 1905. Bailey : Plant
Breeding, 4th ed., 1906. Harwood: New Creations in Plant Life,
1906.

CHAPTER VII. THEORIES OF SPECIES-FORMING AND DESCENT

CONTROL.

Eimer: Organic Evolution, 1890. Romanes: Darwin and After Dar-
win, III, 1897. Conn : Method of Evolution, ch. vii and viii, 1900.
Packard: Lamarck, His Life and Work, 1901. Morgan: Evolu-
tion and Adaptation, 1903. Weismann: The Evolutionary The-

APPENDIX 473

ory, vol. II, Lects. 32 to 35, 1904. De Vries: Species and Varie-
ties, Their Origin by Mutation, 1905. Kellogg: Darwinism
To-day, 1907.

CHAPTER VIII. GEOGRAPHIC ISOLATION AND SPECIES-FORMING.

Wallace: Darwinism, pp. 338-374, 1891. Romanes: Darwin and
After Darwin, I, pp. 204-248, 1895. Weismann: The Evolution
Theory, vol. II, Lect. 32, 1904. Jordan: The Origin of Species
through Isolation, Science, N. S., vol. xxii, pp. 545-562, 1905.
Gulick: Evolution, Racial and Habitudinal, 1905. Montgomery:
Analysis of Racial Descent in Animals, 1906.

CHAPTER IX. VARIATION AND MUTATION.

Darwin: Origin of Species, ch. i, ii, and v, 1859. Darwin: Animals
and Plants Under Domestication, vols. I and II, 1868. Wilson:
Chapters on Evolution, pp. 121-142, 1883. Wallace : Darwinism,
ch. iii and iv, 1891. Bateson: Materials for the Study of Varia-
tion, 1894. Romanes: Darwin and After Darwin, vol. I, pp.
50-65, pp. 120-135, 1896. Bumpus: Variations and Mutations
of the Introduced Sparrow, in Biological Lectures, Wood's Hall,
pp. 1-15, 1896-97. Biometrika: A Journal of Variation, 1901—
to date. Vernon: Variation in Plants and Animals, 1903. Mor-
gan: Evolution and Adaptation, 1903. Kellogg and Bell: Studies
of Variation in Insects, 1904. Davenport: Statistical Methods,
2d ed., 1904. De Vries: Species and Varieties and Their Origin
by Mutation, 1905.

CHAPTER X. HEREDITY.

Brooks: Heredity, 1883. Weismann: Essays upon Heredity, vol. I,
Lects. 4 and 6, 1891 ; Essays upon Heredity, vol. II, ch. xii, 1892.
Marshall: Biological Lectures, pp. 159-191, 1897. Galton:
Natural Inheritance, 1899. Conn: Method of Evolution, pp.
101-156, 1901. Bateson: Mendel's Principles of Heredity, 1902.
Redfield: Control of Heredity, 1902. Wilson: The Cell in De-
velopment and Inheritance, 2d ed., 1904. Montgomery: Analysis
of Racial Descent in Animals, 1906. Woods : Heredity in Royalty,
1906.

CHAPTER XI. THE INHERITANCE OF ACQUIRED CHARACTERS.

Weismann: Essays on Heredity, vol. I, 1900. Spencer- Weismann :
Contemporary Review, May-December, 1893; September-Octo-

474 APPENDIX

ber, 1895. Cope: Primary Factors in Organic Evolution, pp.
397-473, 1896. Marshall: Biological Lectures and Addresses,
pp. 91-115, 1897. Hutton: Darwinism and Lamarckisrn, 1899.
Weismann : The Evolution Theory, vol. II, Lects. 23 and 31, 1904.
Metcalf: Organic Evolution, pp. 67-82, 1904.

CHAPTER XII. GENERATION, SEX, AND ONTOGENY.

Marshall: Lectures on the Darwinian Theories, pp. 78-115, 1894.
Geddes and Thomson: The Evolution of Sex, 1889. Romanes:
Darwin and After Darwin, vol. I, ch. iv, 1896. Marshall: Bio-
logical Lectures and Addresses, pp. 289-363, 1897. Jordan and
Kellogg: Animal Life, ch. v, 1900. Morgan: Regeneration,
1901. Weismann: The Evolution Theory, vol. I, Lects. 13 to
16; vol. II, Lects. 27 to 30, 1904. Wilson: The Cell in Develop-
ment and Inheritance, 2d ed., 1904. Montgomery: Analysis of
Racial Descent in Animals, 1906. Morgan : Experimental Zoology,
1907.

CHAPTER XIII. PALEONTOLOGY.

Wilson: Chapters in Evolution, ch. xvi, 1883. Wallace: Darwinism,
ch. xiii, 1891. Marshall: Lectures on the Darwinian Theories,
pp. 53-77, 1894. Williams: Geological Biology, pp. 1-9, 78-110,

1895. Romanes: Darwin and After Darwin, vol. I, pp. 156-203,

1896. Le Conte: Elements of Geology, part III, 1891. Smith:
Evolution of Fossil Cephalopoda, ch. ix, in Jordan's Footnotes to
Evolution, 1902. Metcalf: Organic Evolution, pp. 103-111, 1905.

CHAPTER XIV. GEOGRAPHICAL DISTRIBUTION.

Wallace: The Geographical Distribution of Animals, 1876. Wallace:
Island Life, 1880. Heilprin: Geographical Distribution of Ani-
mals, 1887. Wallace: Darwinism, pp. 338-374, 1891. Beddard,
Zoogeography, 1895. Romanes: Darwin and After Darwin, vol.
I, pp. 204-248, 1895. Jordan: Science Sketches, pp. 83-132.
Wallace: The Malay Archipelago, 2d ed., 1869.

CHAPTER XV. ADAPTATIONS.

Semper: Animal Life, 1881. Morgan: Evolution and Adaptation,

1903. Weismann : The Evolution Theory, vol. I, Lects. 5 and 7,

1904. Jordan : Guide to the Study of Fishes, vol. I, ch. xi and xii,

1905. Folsom: Entomology, with Reference to Its Biological
and Economic Aspects, 1906.

APPENDIX 475

CHAPTER XVI. PARASITISM AND DEGENERATION.

Wilson: Chapters on Evolution, pp. 342-365, 1883. Van Beneden:
Animal Parasites and Messmates, 1889. Demoor, Massart and
Vandervelde: Evolution by Atrophy, 1899.

CHAPTER XVII. MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS.

Wilson: Chapters on Evolution, ch. xiii, 1883. Van Beneden: Ani-
mal Parasites and Messmates, 1889. Kropotkin: Mutual Aid,
1902. Weismann: The Evolution Theory, vol. I, Lect. 9, 1904.
Kellogg: American Insects, pp. 490-561, 1905.

CHAPTER XVIII. COLOR AND PATTERN IN ANIMALS.

Poulton: The Colors of Animals, 1890. Beddard: Animal Coloration,
1895. Wallace: Natural Selection and Tropical Nature, ch. v,

1895. Newbigin: Colors in Nature, 1898. Weismann: The
Evolutionary Theory, vol. I, Lects. 4 and 5, 1904. Kellogg:
American Insects, ch. xvii, 1905.

CHAPTER XIX. REFLEXES, INSTINCT, AND REASON.

Darwin: Origin of Species, ch. viii, 1859. Morgan: Animal Life and
Intelligence, 1891. Jordan: Footnotes to Evolution, ch. x, 1902.
Morgan: Habit and Instinct, 1896. Morgan: Animal Beha-
vior. Loeb: Physiology of the Brain, 1900. Jennings: The Be-
havior of the Lower Organisms, 19.04. Romanes: Mental Evo-
lution in Animals, 1884. Weismann: The Evolution Theory,
vol. I, Lect. 8, 1904. Loeb: General Physiology, vols. I, II,
1906.

CHAPTER XX. MAN'S PLACE IN NATURE.

Darwin: Descent of Man, 1871. Haeckel: Evolution of Man, 2 vols.,
1883. Huxley : Man's Place in Nature, 1894. Tyler : The Whence
and Whither of Man, 1896. Calderwood: Evolution and Man's
Place in Nature, 1896. Reid: The Present Evolution of Man,

1896. Haeckel and Gadow: The Last Link, 1898. Haeckel: The
Riddle of the Universe, 1900. Le Conte : Evolution and Its Rela-
tion to Religious Thought, 1899. Fiske : Excursions of an Evolu-
tionist, 1883. Metschnikoff: The Nature of Man, 1903. Wallace:
Man's Place in the Universe, 1903.

INDEX

Actinocephalus oligacanthus (illus.),
216.

Adaptation, 56, 327; categories of,
328.

Aid, mutual, 369.

Allen, J. A., 136, 206.

Altmann, theory of protoplasmic
structure, 250.

Amaryllids, sports of (illus.), 103.

Amblystoma opacum (illus.), 20.

Amitosis, 252.

Amoeba eating one-celled plant
(illus.), 31; moving (illus.), 30;
poly podia , stages of fission
(illus.), 31; reproduction of,
213; (illus.), 214.

Amphimixis, 154.

Analogy, between variations in
species and in languages, 325;
denned, 173; in wings of ani-
mals, (illus.), 173.

Anaphase, denned, 256.

Andrena, nest of (illus.), 384.

Andricus calif ornicus, galls pro-
duced by (illus.), 342.

Anemone, sea, with algae in body
wall (illus.), 377.

Anosia plexippus and its mimic
(illus.), 422; metamorphosis of
(illus.), 235.

Antlion, larva, excavating pit
(illus.), 330.

Ants, castes of (illus.), 391; com-
mensal life with aphids, 374;
communal life of, 391; nest of
(illus ), 392; (illus.), 393; social
parasitism of, 375; symbiosis
with plants, 377.

Apes, anthropoid, classification of,

454; ears of (illus.), 174.
Aphids, commensal life with ants,

374.
Appendix vermiformis, of kangaroo

and man (illus.), 179.
Apple, hybridized mosaic (illus.),

101; seedlings of the Williams

Early (illus.), 102.
Aquila chrysdetus (illus.), 44.
Arbacia, extra-ovates of (illus.), 281;

viridis, regeneration in (illus.),

282.
A rchceopteryx lithographica (illus .),

302.
Ascaris megalocephala, spermato-

genesis of (illus.), 264; (illus.),

265; (illus.), 271; var. uni-

valens, cell division in (illus.),

258.

Ascidian (illus.), 42; (illus.), 364.
Asters, defined, 254.
Atavism, 166.
Australia, rabbit and other plagues

of, 65.
Aythya, family of (illus.), 443.

Barnacle (illus.), 365; metamor-
phosis of (illus.), 234.

Barriers, geographic, 316; influences
of geographic, 128.

Bascanion constrictor (illus.), 43.

Basilarchia archippus, mimicking
Anoxia (illus.), 422.

Bates, H. W., 402.

Bateson, Wm., 141, 147, 148, 192.

Bat tick, wingless (illus.), 351.

Beavers making nests (illus.), 437.

477

478

INDEX

Bechamp, theory of protoplasmic

structure, 250.
Bee, carpenter, nest of (illus.), 384;

mining, nest of (illus.), 384;

social, 384; solitary, 383.
Beetle, California flower, apparent

determinate variation in, 152;

polygon showing variation in

pattern of (illus.), 152; (illus.),

153; (illus.), 154; variation in

pattern of, 134.
Beetle, fore leg of male, of water

(illus.), 73; long-horned boring

(illus.), 12; (illus.), 41; scara-

beid (illus.), 72.
Bell, R. G., 85, 142, 192.
Belt, Thos., 402, 417.
Berries, Burbank's work with, 93.
Berthold, theory of protoplasmic

structure, 250.
Bethe, A., 428.
Bionomics, denned, 1.
Biophors of Weismann, 251.
Bird of paradise, male (illus.), 221.
Bittern, American, young of, show-"

ing fear of man (illus.), 436;

showing no fear of man (illus.),

435.

Blacksnake (illus.), 43.
Blastoderm, 228.
Blastula, 228.

Blindness, in fishes (illus.), 180.
Blue jay, nestlings of (illus.), 442.
Bones, homologies of, in vertebrate

hands (illus.), 169; in verte-
brate limbs (illus.), 170, 171,

172.

Boyesen, H. H., 451.
Brewer, Wm., 201.
Brittle stars (illus.), 14.
Brooks, W. K., 1, 57, 165, 197, 244.
Budding, reproduction by, 215.
Buffon, theory of protoplasmic

structure, 250.
Bumble bee (illus.), 385; communal

life of, 384.
Burbank, L., 80, 90, 91, 195;

scientific aspects of his work,
101.

Butterfly fish (illus.), 420; Monarch,
metamorphosis of (illus.), 235.

Cactus, hybrid seedlings of (illus.),
98.

Calcolynihus primigenius (illus.), 36.

Callorhinus alascanus, family of
(illus.), 440.

Calotarsa insignia, showing second-
ary sexual characters (illus.),
74.

Camponotus (illus.), 391.

Cankerworm moth, showing sexual
dimorphism (illus.), 222.

Carchesium sp. (illus.), 34.

Cardinalis Virginianus, diagram of
variation in, 136.

Castanea vesca, variation in leaves
of, 159.

Castle, W., 192.

Caterpillar, parasitized (illus.), 360.

Cattle, British breeds of (illus.), 86.

Cecropia tree, with ants (illus.), 379.

Cell, 30; egg, 224; fission of, in
salamander (illus.), 255; meto-
tic division of, described, 253.

Cells, different types of (illus.), 29;
sex, 218.

Cephalopods (illus.), 40.

Ceratina dupla, nest of (illus.), 384.

Ceratium (illus.), 207.

Cercopithecus (illus.), 446.

Chcetodon vagabundus (illus.), 420.

Characters, illustrations of acquired,
198; inheritance of acquired,
196; secondary sexual, 72.

Chestnut, variation in leaves of
(illus.), 159.

Child, C. M., 252.

Child, with six toes on each foot
(illus.), 147.

Chimpanzee, foot, skeleton of com-
pared with that of man (illus.),
453; plan of, compared with
that of man (illus.), 459.

INDEX

479

Chipmunks of California, classifica-
tion of, 117; (illus.), 118.

Chromosomes, defined, 253.

Chrysopa, eggs of (illus.), 226.

Cicada, variation in tibial spines of,
134; septendecem, variation in
tibial spines of, 136.

Cladonotus humbertianus (illus.),
409.

Classes of animals, 35.

Classification, determined by homol-
ogies, 173; of animals, 33.

Cleavage, 228.

Coccidium lithobii, life cycle of
(illus.), 352.

Coccidium oviforme (illus.), 216.

Cock, white-crested black Polish
(illus.), 81.

Cockerel, silver-laced Wyandotte
(illus.), 82.

Cockroach, egg case of (illus.), 340;
showing variation in number of
tarsal segments (illus.), 149.

Color, in animals, 398; in organisms,
how produced, 403; in protec-
tive resemblance, 400; in sexual
selection, 400; uses of, 399.

Coloration, directive, 419.

Colors, warning, 416; table of insect,
405.

Commensalism, 370.

Communism, 369, 385; advantages
of, 397; basis and origin of, 395.

Conditions, primary, of life, 38.

Conjugation, of nociiluca (illus.),
220; of Vorticella nebulifera
(illus.), 221.

Conklin, E. G., 70, 199, 201, 203.

Convergence of characters, 204.

Cope, E. D., 55, 197.

Cordyceps, growing on a caterpillar
(illus.), 363.

Corn, showing results of hybridiza-
tion, 194.

Correns, E., 192.

Coues, E., 24, 119.

Crab, fiddler (illus.), 38; hermit,

commensal life with polyps
(illus.), 373; (illus.), 374; meta-
morphosis of (illus.), 238.

Cramer, F., 11.

Cricket, mole (illus.), 344; (illus.),
345.

Crotalus adamanteus (illus.), 16.

Crustacea and their larva (illus.),
365; parasitic, 358.

Cryptomeria japonica, atavistic vari-
ation of (illus.), 160.

Cucumaria sp. (illus.), 19.

Cucumbers, sea (illus.), 19.

Cuenot, L., 185, 191, 192.

Cunningham, J. T., 201.

Cyanocitta cristata, nestlings of
(illus.), 442.

Cycle of life, 240.

Cyclops, maturation in, 272; matura-
tion of eggs of (illus.), 263.

Cypselurus (illus.), 335.

Cyrtophyllus crepitans (illus.), 401.

Dall, W., 197, 205.

Darbishire, W., 192.

Darwin, C., 9, 22, 48, 60, 63, 67, 68,
71, 76, 80, 137, 141, 196, 250,
311, 466, 467, 468.

Darwinism, defined, 48.

Davenport, C. B., 192.

Death, 241.

Decapod, showing extraordinary re-
generation (illus.), 147.

Deer, head of, showing variation in
horns (illus.), 146; horns of,
interlocked (illus.), 438.

Degeneration, 347; by quiescence,
363.

Delage, Y., 29, 46, 252.

Descent, theory of, 10; theories ot
control of, 108.

Determinants of Weismann, 251.

Development, 224; divergence in,
230; embryonic, 227; experi-
mental, 244; first stages in
embryonic (ilius.), 227; of
flounder (illus.), 240; of Mon-

480

INDEX

arch butterfly (illus.), 235; of
the prawn (illus.), 232; of silk-
worm moth (illus.), 236; of
swordfish (illus.), 239; post-
embryonic, 234; significance of,
232.

Devilfish (illus.), 40.

De Vries, H. H., 54, 111, 114, 146,
187, 192, 193; work of, on
CEnotheras, 157.

Diabrotica soror, apparent determi-
nate variation in, 152; frequen-
cy polygon of variation in
(illus.), 152; (illus.), 153; (illus.),
154; variation in pattern of
(illus.), 134.

Diapheromera femorata (illus.), 412.

Dickinson, Sidney, 65.

Didelphys virginiana (illus.), 46.

Dimorphism, sexual, 221.

Dimorphodon macronyx, restoration
of (illus.), 293; (illus.), 294;
(illus.), 295; (illus.), 296; re-
mains of (illus.), 291.

Dischidia, with ant colonies (illus.),
378.

Distribution, geographic, 309.

Dog, pointer (illus.), 431.

Dogs, prairie (illus.), 383.

Douglas, N., 79.

Draco (illus.), 306.

Dragon, flying (illus.), 306.

Drepanidse, classification and dis-
tribution of, in Hawaii, 124.

Driesch, H., 278, 279.

Du Bois, E., 462, 464.

Diirigen, 79.

Dyticus, fore leg of male (illus.), 73.

Eagle, golden (illus.), 44.

Ears of apes and man (illus.), 174.

Earthworm, regeneration of (illus.),
284.

Eastman, C. W., 292.

Echinus, cleavage in calcium-free
water (illus.), 280; microtuber-
culatus, gastrula of (illus.) 279;

microtuberculatus, normal larva

of (illus.), 275.
Ectoblast, 228.
Egg cell, 224; of sea-urchin (illus.),

248.
Eggs of Chrysopa (illus.), 226; of

various animals (illus.), 225.
Eigenmann, C., 179.
Eimer, T., 55, 197.
Embryo, human, foot of, 460.
Emerson, R. W., 452.
Emery, C., 394.
Encasement theory, 2, 249.
Endoblast, 228.
Epigenesis, 276.
Epigenetic theory, 3.
Eretmoschlys imbricata (illus.), 43.
Ergates sp. (illus.), 41.
Erynnis manitoba, distribution of

(illus.), 310.
Eutamia, in California, classification

of, 117.
Evolution, biologic, 6; cosmic, 6;

defined, 1; factors of, 48; in

philosophy, 2; Lamarckian, 55;

mechanism of, 48; orthogenetic,

55; Spencer's definition of, 5;

the unknown factors of, 115.
Existence, struggle for, 57, 60.
Exoccetus (illus.), 335.
Extra -ovates of Arbacia (illus.),

281.
Eye, human (illus.), 462; pineal, of

horned toad (illus.), 176; pineal,

of lizard (illus.), 175.

Factors, extrinsic, in development,
245; intrinsic, in development,
245; the unknown, of evolution,
115.

Fauna, 316; Bassalian, 323; littoral,
324; pelagic, 323.

Feeding, instincts of, 432.

Fertilization (illus.), 267; (illus.),
268; of Petromyzon fluviatilis
(illus.), 219.

Fishes, adaptations of, 332; blind

INDEX

481

(illus.), 180, 204; flying (illus.),
335; migrations of, 340.

Fish louse (illus.), 359.

Fission, reproduction by, 213; (il-
lus.), 214.

Fittest, survival of, 62.

Flat worm, regeneration of (illus.),
285; (illus.), 286.

Flounder, development of (illus.),
240.

Flower bug, variation in pattern of
(illus.), 135.

Flowers, varieties originated by
Burbank, 98.

Focke, 192.

Fol, H., theory of protoplasmic
structure, 250.

Foot, of horse, evolution of (illus.),
177; of human embryo (illus.),
460; of mammals, hology of
digits of (illus.), 178.

Forel, A., 394.

Fossils, 17, 290.

Fowls, skulls of (illus.), 88.

Friedenthal, 460.

Frog, male carrying eggs on back
(illus.), 73.

Galls (illus.), 341; (illus.), 342.

Galton, F., 165.

Gasterosteus, 207; cataphractus (il-
lus.), 209.

Gastrula, 228.

Gaudry, A., 298.

Gauss, 141.

Geddes, P., 1.

Gelasimus sp. (illus.), 38.

Genealogy of animals, 36.

Generation, 211; spontaneous, 42,
212.

Geography, relation of species to,
311.

Geology, table of ages of, 298.

Georgine, variation in inflorescence
of (illus.), 161.

Geranium, improvement in (illus.),
103.

Germ cells, theory of the purity of,

191.

Gilbert, C. H., 303.
Gill slits of vertebrates, 179.
Goethe, W. von, 1, 48, 163, 289.
Gongylus gongyloides (illus.), 415.
Gonium pectorale (illus.), 34; (illus.),

261.
Gorilla (illus.), 456; face of (illus.),

457; head of (illus.), 457; young

(illus.), 458.
Gray, A., 197, 326.
Gregarina polymorpha (illus.), 216.
Gregarinidae, reproduction in (illus.),

216.

Gregariousness, 380.
Grus americana (illus.), 20.
Grosbeak, cardinal, diagram of

variation in, 136.

Gryllotalpa (illus.), 344; (illus.), 345.
Gulick, J. T., 123.

Haase, E., 423.

Haeckel, E., 32, 197, 289, 457, 463,

464, 466, 467.
Hair covering of human embryo

(illus.), 174; covering of Russian

dog-man (illus.), 174.
Hawaii, classification and distribu-
tion of Drepanidae of, 124;

distribution of land snails of,

123.

Henneguy, L., 260.
Henslow, G., 124.
Heredity, 53, 163; defined, 163;

laws of, 184; Mendel's law of,

187.

Herlitzka, 279.
Hermaphroditism, 223.
Heron, flying (illus.), 24.
Hertwig, O., 278.
Heterogenesis, 54; theories of. 114;

theory of, 156.
Hippodamia convergens, aberrations

of, 130; variation in pattern of

(illus.), 132; (illus.), 133.
Hog, wild and domestic (illus.), 89.

482

INDEX

Holophnja multifiliis, reproducing
by speculation (illus.), 215.

Homologies, denned, 172; of bones
in vertebrate hands (illus.),
169; of bones in vertebrate
limbs (illus.), 160, 171, 172; of
digits of feet of mammals
(illus.), 178; of tail of man, 179.

Homo sapiens, 454.

Honey bee (illus.), 387; brood cells
of (illus.), 389; communal life
of, 387; pollen carrying leg of
(illus.), 388; wing, hooks of
(illus.), 155; wings of drone,
showing variation (illus.), 155;
(illus.), 156.

Hooker, Th., 65.

Horse, changes in foot of, in geologic
time (illus.), 303; evolution of
foot of (illus.), 177; hybrid of,
with zebra (illus.), 183.

Horses, trotting, heredity in, 168.

Huber, 394.

Human embryo, head of, showing
embryonic hair covering (illus.),
174.

Humerus, end of, of various animals
(illus.), 462.

Humming bird, male and female
(illus.), 71; Rufus, nest of eggs
of (illus.), 441.

Huxley, 7, 25, 26, 64, 197, 309, 426,
457, 459, 460, 468.

Hyatt, A., 197.

Hybrid colt of horse and zebra
(illus.), 183.

Hybridization, 88, 154.

Hydra vulgaris (illus.), 35.

Hyla regila (illus.), 344.

Icerya purchasi, 62, 64.

Ichneumon laying eggs in cocoon
(illus.), 359.

Ichthyophis glutinosus, eggs carried
by (illus.), 338.

Icterus, distribution and classifica-
tion of American species of,

128; galbula, diagram of varia-
tion in, 137.

Idioplasm of Nageli, 251.

Imbauba tree and ants (illus.), 379.

Inchworm (illus.), 411.

Inheritance, exceptions to Mendel-
ian, 193; experimental studies
of, 192; Galton's law of an-
cestral, 184; modes of, 181, 186;
of acquired characters, 196.

Insects, parasitic, 359; special adap-
tations of, 340.

Instinct, 426.

Instincts, climatic, 436; environ-
mental, 438; of care of young,
439; of courtship, 438; of re-
production, 439; terrifying, 431 ;
variations in, 442.

Intellect, 443.

Islands, relation of species to, 312.

Isolation, 53; biologic, 109; geo-
graphic, 54, 117; physiologic,
54; sexual, 54.

Jack rabbits, showing variation (il-
lus.), 320.
Jennings, H. S., 429.

Kallima (illus.), 414.
Kangaroo (illus.), 339.
Karyokinesis, details of, explained,

253.

Katydid (illus.), 401.
Keeler, C., 420.
Kelvin, Lord, 44.
Korschinsky, H., Ill, 114, 156.

Lacerta, diagram showing varia-
tion in different species of, 139.

Lacerta agilis, regeneration of (il-
lus.), 286.

Ladybird, Australian (illus.), 62, 64;
beetles (illus.), 416; convergent
variation in pattern of (illus.),
132; (illus.), 133.

Lamarck, 48, 55, 111, 196; evolu-
tionary principle of, 196.

INDEX

483

Lamarckism, 111.

Lankester, 197.

Laveran, 352.

Laws, of science, 9.

Lecanium olece (illus.), 366.

Lemur furcifer (illus.), 455.

Lepas, metamorphosis of (illus.),
234.

Lepomis megalotis (illus.), 17.

Leptodera hyalina, showing sex
dimorphism (illus.), 224.

Leptothorax emersoni, nest of (illus.),
393.

Lepus, species of, showing differ-
ences (illus.), 320.

Lerema accius, distribution of (il-
lus.), 310.

Lerncecera (illus.), 359.

Life, duration of, 240; its physical
basis, 25; origin of, 42; simplest
expression, 25.

Lilies, Burbank's work with, 100.

Lily, improved seedling with two
petals (illus.), 100.

Lina lapponica (illus.), 194.

Linckia, regeneration of (illus.), 284.

Linnaeus, 14.

Lizard, common (illus.), 18; pineal
eye of, 175; regeneration of
(illus.), 286; walking (illus.), 22.

Lizards, diagram showing variation
in, 139.

Locust, on sand (illus.), 401; red-
legged, variation in tibial spines
of, 133; (illus.), 136; seventeen-
year, variation in tibial spines
of (illus.), 136.

Locusts of Galapagos Islands, show-
ing variation (illus.), 313.

Loeb, J., 280, 428, 429.

Loligo pealii (illus.), 40.

Lophortyx californicus (illus.), 15.

Lucas, 428.

Lyell, C., 80.

Macfarland, F. M., 253.
Macropus rufus, 339.

Malaria, parasite producing, 352.

Malthus, 67, 68; law of, 67.

Mammoth, drawing of, on ivory
(illus.), 307.

Man, earliest traces of, 307, 464;
embryology of, 460; foot, skele-
ton of, compared with that of
chimpanzee (illus.), 453; gene-
alogy of, 466; Leanderthal, re-
mains of (illus.), 465; place in
nature of, 451; plan of, com-
pared with chimpanzee (illus.),
459; profiles of crania of
primitive types of (illus.), 466;
races of, 454; Russian dog, hair
covering of, 175; skull of, com-
pared with skull of orang-utan,
(illus.), 452; from Spy, skull of
(illus.), 466; structure of, 453;
teeth of, compared with orang-
utan (illus.), 454; vestigial
structures in, 461.

Man and apes, ears of (illus.), 173.

Mantis, praying, eating grasshopper
(illus.),' 61." ^

Marshall, G., 418, 419, 423, 424.

Mating, cross, 186.

Maturation in cyclops, 272; of egg of
cyclops (illus.), 263; (illus.),
265.

Mayer, A., 79.

McCook, H., 394.

McCracken, I., 192; experimental
work in heredity, 194.

McCulloch, O. C., 369.

Mechanism, 276; versus vitalism,
246.

Melanerpes formicivorus bairdii,
acorns deposited in tree by
(illus.), 432; (illus.), 434.

Melanoplus femur-rubrum, variation
in tibial spines of (illus.), 136.

Melaphase, defined, 254.

Mendel, G. J., 187; experiments in
heredity, 182.

Mercurialis annua, variation in
leaves of (illus.), 159.

4cS4

INDEX

Merriam, C. H., 117.
Metamorphosis, 235; of barnacle

(illus.), 234; of crab (illus.), 238;

of Monarch butterfly (illus.),

235; of silkworm moth (illus.),

236; of toad (illus.), 237.
Micellse of Nageli, 251.
Microzymes of Bechamp, 250.
Mid-parent of Galton, 165.
Mimicry, 421.
Mind, 448.

Mite, itch (illus.), 363.
Mitosis, 252; details of, described,

253.

Moenkhaus, 89.

Molecules, organic, of Buffon, 250.
Monkey (illus.), 446.
Morgan, 146, 157.
Mousefish (illus.), 410.
Miiller, F., 408.
Mutation, 54, 131, 147, 187; de Vries

theory of, 114.

Mutilations, not inherited, 202.
Myrmecophily, 372.
Myrmica scabrinodes, nest of (illus.),

393.
Myzostoma glabrum, fertilization of

egg of (illus.), 267.

Nageli, 47, 55, 111, 197; theory of
protoplasmic structure, 251.

Narcine brasiliensis (illus.), 334.

Neo-Darwinism, 197.

Neo-Lamarckism, 197.

Newbigin, M. I., 398.

New Zealand, plagues of, 65.

Noctiluca, conjugation of (illus.),
220.

Nomeus, commensal life with Phy-
salia (illus.), 372.

Number of young, 225.

Nycteribia (illus.), 351.

Octopus (illus.), 40; punctatus

(illus.), 40.
(Enothera Lamarckiana, de Vries'

work on, 157.

Oncorhynchus tchaytscha, egg-laying
of, 59.

Ontogeny, 211, 224; factors in, 244;
first stages of (illus.), 227.

Oogenesis, 264.

Opossum (illus.), 46.

Opuntia, hybrid seedlings of (illus.),
98.

Orang-utan baby (illus.), 455;
skull of, compared with skull of
man (illus.), 452; teeth of, com-
pared with man (illus.), 454.

Organisms, geologic age of groups of,
299; simplest, 32.

Organs, vestigial, 174.

Oriole, Baltimore, diagram of varia-
tion in, 137.

Orioles, distribution and classifica-
tion of American, 128.

Ornithotomus sutorius, nest of (il-
lus.), 444.

Orthogenesis, 113.

Ortmann, A. E., 108.

Osborn, H. F., 115, 203.

Ostracoderm (illus.), 305.

Ostrich, African (illus.), 45.

Ovocyte, defined, 264.

Ovogonium, defined, 264.

Oxytricha faUax, reacting to cold
(illus.), 428.

Packard, A. S., 197, 297.

Pagurus bernhardus (illus.), 374.

Paleontology, 289.

Palo Alto, stock farm of, 103.

Pandorina sp. (illus.), 35.

Pangenesis of gemmules of Darwin,
250.

Papilio cresphontes, larva of (illus.),
322; chrysalid of (illus.), 409.

Paramoetium aurelia (illus.), 33; re-
production of, 216.

Parasites, external, 348; facultative,
349; internal, 348; obligate,
349; permanent, 349; tempo-
rary, 349.

Parasitism, 347.

INDEX

4Sf>

Parthenogenesis, 215; artificial, 283;

variation in animals produced

by, 155.
Pasteur, 13.

Pattern, in animals, 398.
Pearson, Karl, 185.
Pelecanus erythrorhynchus (illus.),

20.
Pelican, brown (illus.), 329; white

(illus.), 20.
Peneus potimirium, development of

(illus.), 232.
Peronea cristana, variation in wing

pattern of (illus.), 148.
Pctromyzon fluviatilis, fertilization

of (illus.), 219.

Phaneus mexicanus (illus.), 72.
Pheasant, Argus, male and female

(illus.), 75.
Phlegethontius Carolina, larva of

(illus.), 418.
Phrynosoma blainevillei, pineal eye

of (illus.), 176.
Phyla, of animals, 33.
Phyllium (illus.), 413.
Phyllopteryx (illus.), 415.
Physalia, commensal life with No-

meus (illus.), 372.

Physiological units of Spencer, 250.
Piddock (illus.), 39.
Pigeons, races of (illus.), 87.
Pimpla conquisitor, laying eggs in

cocoon (illus.), 359.
Pipa americana (illus.), 43.
Pipefish (illus.), 415.
Pithecanthropus erectus, 462; remains

of (illus.), 463; (illus.), 464.
Planaria lugubris, regeneration of

(illus.), 285; (illus.), 286.
Plants, living symbiotically with

animals, 376; number of, 16;

parasitic, 362.
Plate, L., 77.
Plateau, 424.
Plum and its parent (illus.), 93;

plumcot, types of (illus.), 92;

seedlings from (illus.), 92.
32

Plums, Burbank's work with, 92.

Podocoryne carnea (illus.), 373.

Polar bodies (illus.), 270.

Polyp, fresh water (illus.), 35.

Polyps, commensal life with her-
mit crab (illus.), 373; (illus.),
374.

Poppies, Burbank's work with (il-
lus.), 99.

Potato, Burbank, 91, 92.

Poulton, E. B., 408, 419, 423, 424.

Prawn (illus.), 365; stages in de-
velopment of (illus.), 232.

Preformation, 276; theory, 249.

Prenatal influences, 167.

Pressure of atmosphere in relation to
animal life, 40.

Primates, 454; classification of, 455.

Primroses, evening, de Vries' work
on, 157.

Prophase, defined, 254.

Protista, 32.

Protophyta, 32.

Protoplasm, chemistry of, 27; phys-
ical structure of, 28, 247.

Protozoa, 32; parasitic, 354; re-
production of colonial, 216.

Pterophryne histrio (illus.), 410.

Pterichthyodes milleri (illus.), 305.

Pygosteus, 207.

Pyrus japonica, seedlings of (illus.),
95.

Quadrumana, 458.
Quail, California (illus.), 15.
Quetelet, 140, 141.
Quince, Japanese, seedlings of (il-
lus.), 95.

Radl, E., 428.

Raja binoculata, egg case of (illus.),

337.

Rambur, 13, 14.
Rat, kangaroo (illus.), 13.
Rattlesnake, diamond (illus.), 16.
Realm, arctic, 318; Australian, 322;

holarctic, 319; lemurian, 322.

486

INDEX

neo- tropical, 321; paleo-trop-
ical, 321; Patagonian, 322.

Realms, geographic organisms, 318.

Reason, 443.

Recognition marks, 420.

Record, Zoological, 14.

Redfield, C., 106.

Reflexes, 426.

Regeneration, 285; of blastula of sea-
urchin (illus.), 287; of earth-
worm (illus.), 284; of eye of
triton (illus.), 287; of flatworm
(illus.), 285; (illus.), 286; of
Hydra viridis (illus.), 282; of
lizard (illus.), 286; of starfish
(illus.), 283; of Stentor cceruleus
(illus.), 282.

Remora, commensal life with shark
(illus.), 370.

Reproduction, by budding, 215; by
sporulation, 215; excess in, 58.

Resemblance, general protective,
406; special protective, 411;
variable protective, 407.

Reversion, 166.

Rhizobius ventralis (illus.), 366.

Rhodites rosce, galls produced by
(illus.), 341.

Robinson, L., 461.

Rocks, sedimentary, how deposited,
293.

Romanes, G., 109, 461.

Roux, W., 70.

Riickert, 272.

Sacculina (illus.), 365; carcinus,

parasite of crab (illus.), 358.
Salamander, blunt-nosed (illus.), 20;

cell fission of (illus.), 255.
Salix auritax purpurea, variation

in stamens of (illus.), 160.
Salmo irideus (illus.), 343.
Salmon, quinnat, egg-laying of, 59.
Saltation, 54.
Samia ceanothi, cocoon of (illus.),

332.
Sarcoptes scdbei (illus.), 363.

Scale, black (illus.), 366; cottony
cushion, 62, 64.

Sceloporus undiilatus (illus.), 18.

Schaffhausen, 46, 464.

Schistocerca, species of, from Gala-
pagos Islands, showing varia-
tion (illus.), 313.

Schmidt, P., 346.

Scorpion (illus.), 331.

Skulls of two breeds of fowls (illus.),
88.

Scutellista cyanea (illus.), 366.

Sea anemone, anatomy of (illus.),
36,

Seal, fur, family of (illus.), 440;
killed by parasitic worm (illus.),
357.

Sea-urchin, egg cell of (illus.), 248;
larva of (illus.), 275; regenera-
tion of blastula of (illus.), 287.

Segregation, 53.

Selection, artificial, 51, 80; artificial,
its relation to evolution, 106;
claimed importance of, 50;
natural, 57; sexual, 51, 57, 71;
sexual, criticisms of, 77; sexual,
experimental stage of, 79.

Selenka, 460.

Self-defence, instincts of, 433.

Semper, C., 131.

Separation, 53.

Serpent star (illus.), 14.

Scrphus, carrying eggs (illus.), 340.

Sex, 211, 220; cells, 218; determina-
tion of, 170.

Sexes, numerical relations of, 170.

Sexual, secondary characters, 72.

Sheep, artificial selection of, 81;
Dorset (illus.), 83; Merino (il-
lus.), 85; polled Welsh (illus.),
83; Rocky Mountain (illus.),
382; Southdown (illus.), 84.

Shelford, 423.

Silkworm moth, development of
(illus.), 236.

Silkworms, experimental rearing of,
150.

INDEX

487

Silva, carmen, 167.

Skate, egg case of barn door (illus.),

339.

Slime mold (illus.), 32.
Snails, distribution of land, of

Hawaii, 123.

Solenopsis fuga, nest of (illus.), 392.
Species, changing with space and

time, 18; estimate of duration

of, 302; number of, 14; onto-

genetic, 114.
Species-forming, 117; by saltations,

156; various theories of, 108.
Spencer, Herbert, 5, 196, 197, 198;

definition of evolution, 5; theory

of protoplasmic structure, 250.
Spermatogenesis of ascaris (illus.),

264.

Spermatozoa (illus.), 218.
Spermatozoan, development of

(illus.), 266.
Spermatozoid, 261.
Spfuerechimis Echinus, hybrid larva

of (illus.), 277; hybrid pluteus

of (illus.), 280.
Sphcerechinus granularis, heating

larva of (illus.), 279; lithium

gastrula of (illus.), 279; lithium

larva of (illus.), 278; normal

larva of (illus.), 275.
Sphenodon, pineal eye of (illus.),

175.
Spider, nest of trap door (illus.),

346.

Sponge, simple (illus.), 36.
Sports, 186; examples of race formed

from, 161; species arising from,

107.

Sporulation, reproduction by, 215.
Squid (illus.), 40.
Stamens of willows, variation in

(illus.), 160.
Stanford, Leland, 103.
Starfish, regeneration of (illus.),

283; walking (illus.), 23.
Starks, E. C., 303.
Stentor cceruleus (illus.), 282; re-

acting to light (illus.), 430;

reproducing by fission (illus.),

215.
Sticklebacks (illus.), 209; onto-

gonetic variations in, 207.
Sting-ray (illus.), 333.
Structures, convergence of, 204;

parallelisms in, 204.
Struggle for existence, 57, 60.
Struthio camelus (illus.), 45.
Sunfish, long-eared (illus.), 17.
Survival of the fittest, 62.
Swift, common (illus.), 18.
Swordfish, development of (illus.),

239.
Symbiosis, 373; of plants and

animals, 376.

Syngamus trachealis (illus.), 223.
Systema Nature, 14.

Tcenia solium (illus.), 355.

Tail of man, hology of, 179.

Tailor bird, nest of (illus.), 444.

Tapeworm (illus.), 355.

Tarsal segments, variation in, cock-
roach (illus.), 149.

Taxis, 282.

Taylor, Bayard, 163.

Teeth of man and orang-utan com-
pared (illus.), 454.

Telegony, 166.

Telophase defined, 256.

Temperature in relation to animal
life, 38.

Termitophily, 372.

Termites, castes of (illus.), 395;
communal life of, 394.

Terrifying appearances, 418.

Thalessa (illus.), 361; (illus.), 362.

Theory, transmutation, of Lamarck,
111.

Timothy, heads of, improved by
selection (illus.), 91.

Toad (illus.), 344; horned, pineal
eye of (illus.), 176; metamor-
phosis of (illus.), 237.

Tobacco worm (illus.), 418.

488

INDEX

Toes, variation in number of (illus.),

147.

Torpedo (illus.), 334.
Tower, W. L., 404.
Toxopneustes libidus, egg cell of

(illus.), 248.
Toyama, 192.
Trembley, Abbe, 286.
Tremex columba (illus.), 362.
Trial and theory of behavior, 430.
Trichia favaginea (illus.), 32.
Trichina spiralis (illus.), 356.
Trimen, 408.
Triton, regeneration of eye of

(illus.), 287.
Trochilus rufus, nest of eggs of

(illus.), 441.
Tropism, 282; theory of behavior

by, 428.
Trout, ontogenetic variations in,

208; rainbow (illus.), 343.
Tschermak, 192.
Tunicate (illus.), 364.
Turner, L. J., 39.
Turtle, hawk bill (illus.), 43; with

two heads (illus.), 146.
Twig, insect (illus.), 412.
Two Ocean Pass (illus.), 317.
Types of animals (illus.), 31.

Uexkull, 428.

Uncinaria, parasite of seals (illus.),

357.

Unity, in life, 12, 22.
Urolophus goodi (illus.), 333.
Use and disuse, effects of, 206.

Variation, 131; curves of (illus.),
140; Darwin's laws of, 137;
determinate, 150; determinate,
apparent example of, 152; onto-
genetic, in sticklebacks, 207; in
parthenogenetic animals, 155;
Quetelet's law of, 141; in wings
of honeybee (illus.), 155; (illus.),
156.

Variations, acquired, 142; congeni-

tal, 142; continuous, 146; dis-
continuous, 146; in function,
19; meristic, 148; substantive,
147.

Variety, in life, 12.

Vedalia cardinalis (illus.), 62, 64.

Vegetables originated by Burbank,
98.

Vertebrates, earliest remains of, 305 ;
gill slits of, 179.

Vespa (illus.), 385; germanica, vari-
ation in pattern of (illus.), 134.

Vespa, nest of (illus.), 328, 386.

Vestigial organs, 174; explanation
of, 181; vestigial structures in
man, 461.

Vitalism, 276; versus mechanism,
246.

Volvocince, reproduction in, 217.

Von Kolliker, 111, 114, 156.

Vorticella nebulifera, conjugation of
(illus.), 221.

Vulpes pennsylvanicus argentatus
(illus.), 21.

Wagner, M., 108, 117.

Wallace, A. R., 76, 108, 197, 311,

319, 402.
Walnuts, Burbank's work (illus.),

96; (illus.), 97.
Warblers, yellow, distribution and

classification of, 120.
Wasmann, E., 394.
Wasp, social (illus.), 385; variation

in pattern of (illus.), 134.
Wasps, communal life of, 385; social

nest of (illus.), 328.
Water bug, giant (illus.), 340.
Weismann, A., 68, 154, 197, 198,

423, 425; theory of protoplasmic

structure, 251.
Wesley, J., 452.
Wheeler, W. M., 424.
Wheeler, W. W., 394.
Whitford, C. B., 168.
Whooping crane (illue.), 20.
Wiedersheim, 175, 461.

INDEX

489

Wiesner, theory of protoplasmic
structure, 250.

Willows, variation in stamens of
(illus.), 160.

Wilson, E. B., 125, 126, 279.

Wing hooks of honeybee (illus.),
155.

Wings of a drone honeybee, show-
ing variation (illus.), 155; (il-
lus.), 156; of animals, analogies
in, 173; variation in pattern of,
in Peronea cristana, 148.

Winthemia 4-pustulata (illus.), 348.

Woodpecker, California, acorns de-

posited in tree by (illus.), 432;

(illus.), 434.
Worms, parasitic, 354.
Wyman, J., 461.

Xerophyllum simile (illus.), 409.
Youatt, 80.

Zebra, hybrid of, with horse, 183.

Zirphoea crispata (illus.), 39.

Zoogeography, 309.

Zoya, 279.

Zur Strassen, O., 279.

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