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TEXTBOOK OF EVOLUTION AND GENETICS

THE MACMILLAN COMPANY

NEW YORK • BOSTON • CHICAGO • DALLAS
ATLANTA • SAN FRANCISCO

MACMILLAN & CO., Limited

LONDON • BOMBAY • CALCUTTA
MELBOURNE

THE MACMILLAN COMPANY
OF CANADA, Limited

TORONTO

TEXTBOOK OF
EVOLUTION AND GENETICS

BY
ARTHUR WARD LINDSEY

PROFESSOR OF ZOOLOGY IN DENISON UNIVERSITY

Npm fork

THE MACMILLAN COMPANY

1929

ALL RIGHTS RESERVED, INCLUDING THE RIGHT OF REPRODUCTION
IN WHOLE OR IN PART IN ANT FORM

Copyright, 1929,
By the MACMILLAN COMPANY

Set up and electrotyped.
Published January, 1929.

SET UP AND ELECTROTYPED BY T. MOREY A SON

PRINTED IN THE UNITED STATES OF AMERICA BY

THB BERWICK & SMITH CO.

TO MY WIFE
WINIFRED WOOD LINDSEY

PREFACE

For nearly three quarters of a century the occurrence of evohi-
tion among organisms has been widely accepted by scientists.
It is only natural that the work of Darwin, which made possible
this acceptance, should have colored the beliefs of the period
thi'ough which we have since been passing, and that the apparent
opposition of his views and those of Lamarck should establish
the basis for theoretical consideration of the processes of evolution.
Initial views in any subject are likely to determine the trend of
human thought upon that subject.

In the case of evolution it was obvious even when the Origin of
Species was published that there were many difficulties to be
surmounted before we could know even approximately how the
wonderful adjustments of evolution were consummated. It was
possible then as now to recognize the existence of natiu-al selec-
tion, and to some a wider application of this principle appeared
logical then than now seems justified. The Lamarckians offered
their explanations as opposed to the Darwinian point of view
and vice versa, and there is nothing so stimulating as confhct, al-
though it is of doubtful productiveness.

When Mendel's discoveries were taken up early in the twentieth
century and the new science of genetics arose it seemed that we
might expect new concrete information regarding evolution, for
in genetics we come as near as possible to the raw materials of
evolution. The bearing of genetics on the larger problem of evo-
lutionary processes is, indeed, of the utmost importance, but in
the first quarter of the century it has made little if any unpression
upon the established treatment of evolutionary problems. The
Lamarckian and Darwinian points of view still determine the
course of a vast majority of writings on this subject.

It is difficult for most people to depart entirely from a point
of view once learned as true. The printed page is probably the
most potent influence in establishing an initial belief. Give a
class a textbook which leans ever so slightly toward one opinion
and no matter how vigorously an instructor may assert the op-

vii

viii PREFACE

posite view, a majority of students will accept the word of the
textbook.

These facts have impressed themselves so strongly upon me
during my years of teaching that they have led to the production
of this book. Our treatment of evolutionary processes and our
methods of investigation have been at an impasse for many years.
If we continue to teach the old point of view, can we expect to
progress with reasonable rapidity beyond our old limits of knowl-
edge? We have taught with unwarranted emphasis upon some
of the factors in evolution, in spite of the fact that modern biology
shows very clearly that many things must enter into the evolu-
tion of organisms. Such emphasis can hardly lead to great dis-
coveries.

In this volume I have attempted first to present the materials
of evolution in such a way that their true logical relationship is
clear to the student, second to give a concise account of the funda-
mental principles of genetics, and finally to sum up the theoretical
matter of the subject and to present a logical analysis of the
factors bearing upon evolutionary theory. Since nothing is so
interesting to man as himself, the bearing of all material upon the
human species has been treated as fully as seems warranted.

The book has been written for students who desire a sound in-
troduction to the subject and not merely such an elementary ac-
count as is presented very adequately in most textbooks of biology.
The material contained in it has been used in my own classes for
students who have previously completed a course in general zoology
or biology and for a few students of marked ability without such
prior training. While the facts presented must often be unfamiliar
to such students, their significance should be evident with the
brief treatment given to them here. The work is not designed for
entertainment but for serious instruction in a difficult field of
biology, although I have yet to find a student to whom the sub-
ject is not intensely interesting.

Acknowledgments for the use of illustrations are made where
the figures appear, but I wish to extend my thanks again to all
who have assisted in this way. My deepest gratitude is also due
to my wife, Winifred Wood Lindsey, for her intelligent criticism
of many scientific points, for assistance in the formulation of the
manuscript, and for invaluable aid in the laborious work of proof
reading.

PREFACE ix

If the book shall succeed in imparting to other adventurers in
science the impartial attitude toward the problems of evolution
which I believe to be essential to future progress it will have
justified its preparation.

A. W. LiNDSEY
Granville, Ohio,
December, 1928.

CONTENTS

CHAPTER PAGE

I. Introduction 1

II. The History of Evolution 5

RELATIONSHIPS OF ORGANISMS

III. The Relationship of Existing Organisms

1. Classification 19

IV. The Relationship of Existing Organisms {Con-

tinued)

2. Embryology of Vertebrates ... 39

V. The Relationship of Existing Organisms {Con-
tinued)

3. Comparative Anatomy of Vertebrates . 68

VI. The Relationship of Existing Organisms {Con-
tinued)

4. Physiology ....... 90

VII. Evidences of Evolution

1. Existing Organisms 105

VIII. Evidences of Evolution {Continued)

2. The Geological Record . . . .119

IX. Evolution of the Vertebrates .... 144

X. Elephants, Horses, and Camels .... 164

XI. The Evolution of Man ..... 187

THE PROCESS OF EVOLUTION

A. The Foundation
XII. Adaptation , . 210

XIII. The Basis of Adaptation ..... 247

B. Genetics

XIV. The Foundations of Genetics .... 264
XV. Mendelian Heredity 276

XI

^oji-/

Xll

CONTENTS

CHAPTER

XVI. The Chromosome Theory of Heredity
XVII. Genes and Characters
XVIII. The Determination of Sex .
XIX. The Practical Value of Genetics
XX. Heredity in Man ....
XXI. Eugenics . . . „ .

PAGE

289
305
325
341
359
374

C. Theories of Evolution

XXII. Natural Selection ....

XXIII. Other Theories of Germinal Selection

XXIV. The Lamarckian Theory

XXV. Evolution Today .....

Index

385
401
413

427

445

TEXTBOOK OF EVOLUTION AND GENETICS

TEXTBOOK OF EVOLUTION
AND GENETICS

CHAPTER I
INTRODUCTION

Whatever may be the attitude of the individual toward evolu-
tion, memory will tell him that there was a time in his life when he
did not think. Experience with others or the words of his parents
will show him that he was a living, active organism, carrying on in
his small body all of the fundamental life processes which continue
in it today, but without consciousness of these processes or of the
world about him, or of his individual existence. There is a distinct
resemblance between this past oblivion of infancy and the normal
condition of the lower animals. We cannot say that they are
entirely devoid of the processes of mind which are so highly de-
veloped in ourselves ; rather it seems that they do the same things
in lesser degree, handicapped as they are by inferior brain develop-
ment and by lack of that convenient means of storage and ex-
change, language and articulate speech.

If we look back through the long ages of recorded history we
cannot fail to note another analogy in the gradually increasing
complexity of society, in the development of mankind from
savagery to primitive cultures and finally to the great civilizations
which have come and gone. Each stage has contributed to the
greatness of its successors, each has added to the complexity of
human knowledge, each has made man a httle more independent
of his environment, each has turned his thoughts a little more
keenly inward until race consciousness has become an active factor
in the shaping of human destiny. Behind this long record we find
a few remnants which tell us of the infancy of the human race.
Crude drawings on the walls of caverns and the implements which
these primitive peoples used disclose something of their limited
culture. Bones associated with these eloquent legacies tell us
much of the characteristics of the people who left them. Every-
thing points to gradual change, but behind these records — what?

1

2 EVOLUTION AND GENETICS

Man has first of all a heritage, in common with all other organ-
isms, a thing without which he cannot exist. He lives surrounded
by conditions of various kinds which, in the aggregate, we call his
environment. Environment is a second essential; to it the heritage
responds within the limits of its possibility. The combination
means life. We know from our own experience that hfe can exist
without consciousness. We know too that at some time in the
ascending complexity of individual development consciousness
dawns, and the individual responds to the world about him not
merely as a series of reactions to environmental stimuli, but with
an awakening realization of other entities about him, and at last a
consciousness of self. Where this point hes in the organic world
we cannot say with certainty; it may be that man alone is more
than an organic automaton. The light of personal experience
clarifies its significance. Have we always, as a species, possessed
this quality which must develop in each individual? In view of
the records just mentioned this seems unlikely. Back of that
crude beginning of our record of man's progress there must have
been something. Consciousness must have had a beginning.

Through Beebe's striking powers of description we may share his
imaginative conception of this process as he watched the monkeys
in his jungle laboratory. "A little monkey climbed down a sway-
ing vine, hand over hand, until his face was close to a quiet pool of
sweet water. The day before at evening, he had done the same
thing. His mother and his ancestors for generations had done
hkewise. And always they chattered at the monkey they saw in
the water, and finally in anger snatched at him, and their little
fingers troubled the water and the monkey vanished. Then they
drank eagerly, turned quickly, and clambered swiftly up to rest.

"Today the little monkey began to chatter, then stopped. He
moved, and the monkey in the water moved. He brushed away
some hairs from his face and the water monkey. Then something
happened. He stopped chattering and peered again and again at
the face in the water. He put his little paw over his eyes and
slowly took it away. Then he forgot his thirst, raised his head and
gazed fixedly before him, wrinkling his forehead and remaining
very quiet. And the more distant his gaze, the less he seemed to
observe, and the deeper became the wrinkles.

"... Something introspective had come to pass — a glimpse
of the ego — a momentary flash of self-consciousness. The little

INTRODUCTION 3

face in the water was not really another monkey. And the end of
this realization was to be man."

The dawn of consciousness alone can have been the beginning of
that curiosity which has led man for ages to attempt the explana-
tion of the world about him, and himself. Nothing in the world
has been more baffling in this pursuit than the thing called life.
Is it some force or quality distinct from all else, or is it merely the
product of other forces? Is it divine, or is it an earthly thing?
Shall we ever be able to explain it, or must it alwaj^s remain a great
mystery? Whatever may be the answer. Philosophy will continue
its attempts to explain and Biology its investigations, and if
nothing more accrues, they will at least have clarified our under-
standing and increased our store of facts.

It is only natural that in other lives, particularly in very different
lives, this spirit of curiosity should find a major stimulus. Animals
were competitors of primitive man for the bounties of the earth
in various ways. Thej^ must often have used food which he him-
self desired, and others must have been ready at any moment to
use man himself as food. They must have contributed to his diet
early in his existence, and when domesticated they became not
only a more important l3ut a more intimate part of his life. They
must, as a result of these varied contacts, have impressed them-
selves upon him as a conspicuous part of his environment. We can
imagine a first scientist pioneering in comparative anatomy as he
picked the bones of game at his fireside. He might note that both
fish and bird have that peculiar jointed axis of bones which we call
the spinal column, and might wonder why they should be so dif-
ferent in other ways and so nearly alike in this. He might see the
same thing in a rabbit, and the evident resemblance between its
legs and the wings of the bird, superficially so different. Or in his
chance contacts afield he might wonder why the deer, so different
in many ways, should have hair like man and the rabbit, while the
bird has feathers and the fish, scales. Out of an infinite accumula-
tion of such observations, leading step by step to greater powers of
observation and increasing possibilities for inteq^retation, has
developed the science of Biology, and out of an insatiable desire
to explain these relationships of different organisms, all united by
the possession of that unknown thing called life and in varying de-
grees by peculiarities of organization, has come our recognition of
that process of nature which we call evolution.

4 EVOLUTION AND GENETICS

From the first realization of evolution as a natural process by
which species are developed from preexisting species, and from
the first sound attempts to explain this process there arose a
series of theories which we can still believe in part. It was only
a natural outcome of scientific progress that a reaction to this
method should take place. Science needs working hypotheses,
but sooner or later these must be soundly rooted in fact and the
twentieth century has seen a vigorous attempt to discover the
underlying principles of all types of development. The most
significant field of investigation has been the relationship of
individuals of different generations, the process of heredity. The
science of genetics has explained many phenomena of heredity.
It is still impossible to correlate genetics wholly with other fields
of biology and to determine just how the transition from species
to species is brought about in evolution but we no longer lack
a foundation of soundly organized facts for interpretation.

However willing we may be to refer ultimate causes to faith in
God or some mystic force, we cannot fail to admit that in man's
knowledge of the living things al)Out him there is much that is
within his power to explain on a basis of natural laws. That in-
quiry into these things need conflict with other fundamental
beliefs is a product of the imagination of those who do not, will not,
or cannot understand the findings of science; if faith without
understanding is beautiful, then faith with understanding is tran-
scendent. We can conclude no better than with the ideas expressed
by Erasmus Darwin in his Zoonomia: "The world has been evolved,
not created; it has arisen little by little from a small beginning
and has increased through the activity of the elemental forces
embodied in itself, and so has rather grown than suddenly come
into being at an almighty word. What a sublime idea of the in-
finite might of the great Architect! the Cause of all causes, the
Father of all fathers, the Ens entium! For if we could compare
the Infinite it would surely require a greater Infinite to cause the
causes of effects than to produce the effects themselves.

"All that happens in the world depends on the forces that pre-
vail in it, and results according to law; but where these forces and
their substratum. Matter, come from, we know not, and here we
have room for faith."

CHAPTER II

THE HISTORY OF EVOLUTION

It is often difficult to know what is cause and what is effect in
past occurrences and it is therefore not easy to decide whether the
innate curiosity of developing intelligence first led man to set
down records in primitive form and thus stimulated his desire for
the accumulation of knowledge, or whether the increase of knowl-
edge led to a conscious desire for some way to record it. In either
case facility of written expression increased rapidly with the de-
velopment of more complex social systems and the earliest civili-
zations found man al)lc to make permanent records with great
accuracy of detail. There is abundant evidence that he took note
of the organic world very early in his existence beyond the mere
need of supplying himself with food and clothing, but we find noth-
ing like an organized natural science until the Greek and Roman
civilizations arose. Several men of those periods are entitled to
rank as pioneers in the field of natural history.

The Greek Philosophers. Among the Greek philosophers
Anaximander (611-547 b.c), Empedocles (495-435 b.c), Democ-
ritus (460?-357 b.c.) and Aristotle (384-322 b.c.) and his pupil and
associate, Theophrastus (370-286 b.c), produced works which
show a surprising clarity of interpretation for a period when so
little was known of the world of nature. In an examination of the
beliefs of these men a salient feature is seen to be that striving for
an explanation of hfe and living things which has led gradually up
to our modern ideas of evolution. The pioneer work which estab-
lishes the early Greeks as the founders of natural history is, indeed,
largely lacking in the observation and recording of facts, with the
exception of that contributed by Aristotle and Theophrastus; it
neglects experimental methods and original investigation, but it
strikes at once into the problems which have remained forever
since open to investigation. While these men saw but vagu(4y and
expressed themselves fantastically in the light of modern knowl-
edge, we must remember that their investigations and inquiries

5

6 EVOLUTION AND GENETICS

had no precedent and no foundation in recorded science. This
very lack was probably in part responsible for the accuracy of the
generahzations which pay such high tribute to their mental powers.
"The spirit of the Greeks was vigorous and hopeful. Not pausing
to test theories by research, they did not suffer the disappoint-
ments and delays which come from our own efforts to wrest truths
from Nature. Combined with great freedom and wide range of
ideas, independence of thought, and tendencies to rapid generaliza-
tion, they had genuine gifts of scientific deduction, which enabled
them to reach truths, as it were, by inspiration" (Osborn).

Anaximander is conspicuous among these philosophers for his
idea of an actual transformation of living organisms from one
state into another, particularly from aquatic to terrestrial. He
even included man in this theory. Although vague in detail, his
work foreshadows our modern idea of the adaptation of or-
ganisms.

Empedocles has been called the father of the evolution idea be-
cause he first expressed theories to account for the gradual develop-
ment of different kinds of organisms. These theories were founded
on some erroneous and fantastic ideas, but they embody the germ
of the evolution conception.

Empedocles believed in the spontaneous origin of living creatures
from inorganic matter, but when we consider that this belief was
commonly accepted for many centuries thereafter, and was not
completely overthrown until late in the nineteenth century,
Empedocles' acceptance of it seems less remarkable. His belief
that independent parts of organisms arose spontaneously and later
became associated to form entire animals seems little short of
ridiculous. The thought of heads, bodies and legs wandering about
and finally combining at random is contradictory to the simplest
biological knowledge of today, but here again, when we remember
the centaurs and satyrs of Greek mythology, we realize that there
was reason for Empedocles' belief. He had been taught that such
anomalous creatures actually existed and it was no more than
natural for him to attempt to account for their occurrence along
with that of normal animals. He added to this fantastic portion
of his theories the belief that some of the random combinations
were unable to maintain themselves and so were replaced by more
perfect individuals which were able to live and to perpetuate their
kind. This view is very close to the idea of competition in nature

THE HISTORY OF EVOLUTION 7

and the survival of the fittest which persists even today as a logical
interpretation of some evohitionary proc(\sses.

In spite of tlie vagueness of his theories, Enipcdocles therefore
dealt with logical ideas of evolution including th(^ gradual develop-
ment of existing species, the necessity for adaptation, competition
among organisms, and the extinction of less perfect creatures
which accompanies the persistence of those better fitted for life.

Aristotle. Other Greeks contributed ideas likewise vaguely
suggestive of modern scientific beliefs, but Aristotle is generally
admitted to be the Outstanding thinker of the times. He worked
on the same basis as his predecessors, for they had accumulated no
dependable facts, but in spite of such limitations he expressed
most of the fundamental principles of evolution. Although this
phase of his work is of chief interest to us in such a study as this,
it is important for a full understanding of Aristotle's place in bio-
logical science to note that he did not limit himself to philosophical
considerations, but made extensive and in many cases accurate
observations of natural phenomena. At least one of his observa-
tions, that of parthenogenesis in the honey-bee, is commonly
credited to a scientist of the nineteenth century. In the science of
botany Theophrastus shares Aristotle's eminence as an accurate
and original observer.

Aristotle's ideas in the field of evolution may be summed up as
follows :

1. He believed in natural law as the source of evolutionary
change.

2. He believed in intelligent design as the ultimate cause of all
nature.

3. He did not accept the idea of survival of the fittest.

4. He believed in the development of modern organisms from a
primordial soft mass of living substance, essentially as we believe
today.

5. His works suggest a phylogenetic series such as we now recog-
nize in living organisms.

6. He recognized rudimentary organs as an evidence of relation-
ship and the unity of groups of related forms.

7. He believed in epigenesis in ontogeny.

8. He recognized fundamental principles of heredity.

9. He believed in prenatal influences and in the inheritance of
acquired characters, the former a fallacy and the latter still un-
proved.

8 EVOLUTION AND GENETICS

In spite of the inaccuracies of some of these views and certain
other erroneous opinions which he held, Aristotle's work eclipses
that of all other ancient scientists in this field and he was not sur-
passed until the beginning of modern scientific methods several
centuries later.

Through the Dark Ages. For years after Aristotle's life the
contributions which can be said to have any bearing on the prob-
lems of organic development and the origin of life have no more
than minor biological significance. Pliny (27-79 a.d.) and Galen
(131-200 A.D.) are the most conspicuous figures of the few suc-
ceeding centuries; the former did little or nothing of sound scientific
value, but Galen was a remarkable observer, clear thinker and
excellent writer. He was the foremost anatomist of antiquity.
Finally the influence of the early Christian church, favoring "tra-
ditional knowledge and the special-creation idea in its most literal
form", so hindered independent thought that not until the six-
teenth century was progress again resumed. It is gratifying to
note that even during this dark period three theologians, Gregory
of Nyssa (331-396 a.d.), Augustine (353-430 a.d.), and Thomas
Aquinas (1225-1274 a.d.) expressed belief in the symbohc nature
of the Biblical story of the creation.

Development of Scientific Methods. An inevitable step in
the development of true natural science was the departure from
unsupported or poorly supported philosophical reasoning and
reference to authority, which took place soon after the renewal of
scientific thought. During the sixteenth century great strides
were made in the development of modern scientific methods, and
since then there has been no interruption of progress. Vesalius
(1514-1564) in anatomy and Harvey (1578-1667) in physiology
are outstanding figures in this period. Each applied to his work
sound principles of observation and experiment, and each is known
for the accurate contril)utions to science which resulted from these
methods. A little later the microscope was introduced, and in-
vestigation of fields hitherto ])arred from human vision began.
Hooke (1635-1703), Malpighi (1628-1694), Swammerdam (1737-
1680) and Leeuwenhoek (1632-1723) were among the pioneers in
microscopic work, which has been destined to play such a large
part in the biological sciences.

Philosophy was not neglected during this period. The names
of Bacon (1561 1626) and Kant (1724-1804) especially are cited in

THE HISTORY OF EVOLUTION 9

connection with the maintenance of the primitive idea of evohition.
Their work was destined, however, because of the very nature of
purely philosophical limitations, to add nothing more than corol-
laries to the points so w(41 c^xpn^ssed by the Greeks.

Results of the New Methods. The accumulation of scientific
data by observation and experiment could hardly fail to give a
different impetus to scientific progress. The old desire to explain
life and the relationship of living things was maintained, but new
methods of study disclosed such a storehouse of accurate informa-
tion to be had for the seeking that the observation and recording
of material facts came to be, for the time, the prevailing tendency.
We find that knowledge of natural facts accumulated rapidly
while philosophical interpretation entered a fallow period which
lasted, with a few interruptions of importance, for many years.
Finally Darwin, at the middle of the nineteenth century, placed
the old evolution idea on a basis of sound scientific data, and thus
brought it for all time into the realm of ])iology.

Early Evolutionists. Among the scientists of the eighteenth
and nineteenth centuries prior to Darwin, Linnaeus (1707-
1778), Buffon (1701-1788), Erasmus Darwin (1731-1802), La-
marck (1744-1829), and Saint-Hilaire (1772-1844), made notable
contributions to biology. None of these was destined to bring the
theory permanently before the world, but their theories were
valuable and show increasing accuracy in the interpretation of
natural phenomena.

Linnaeus' chief contribution to biology was the plan of classifi-
cation, which still prevails, together with the same binomial
system of nomenclature now employed. Even his classification of
organisms left its impress on that still in use, although it has been
almost completely concealed by the corrections and amplification
of the intervening years. In spite of the fact that in working out
his classification of plants and animals he did much to illustrate
their phylogenetic relationships, he did it unknowingly. He be-
lieved firmly in special creation as the origin of primary forms, al-
though to this belief he appended a theory of development of the
various species from a limited number of such forms.

Buffon "was not a true investigator," although "of a more
philosophical mind than many of his contemporaries" (Locy).
Buffon believed in the gradual evolution of species, but in spite
of the fact that he retained this belief throughout his life he was

10 EVOLUTION AND GENETICS

hesitant in expressing it. His writings are noted for their excellent
diction, but on the point of evolution they are vague and obscure.
Some writers have attributed this reticence to the weight of ecclesi-
astical authority for special creation which then obtained, and to
this we may add the knowledge that "he was a man of elegance,
with an assured position in society." (Locy.) Such standing
would hardly be conducive to militant opposition to the church.
In spite of the vagueness which he displayed on evolution, there
is a general agreement that he was the first to believe in the direct
modification of organisms by their environment. He also antici-
pated Malthus in the idea of struggle for existence as a compensa-
tion for overproduction in maintaining the balance of nature, and
expressed other opinions which are strongly suggestive of Darwin's
theory of Natural Selection.

Erasmus Darwin, the grandfather of Charles, also believed in
the inheritance of acquired characters, or environmental effects,
but instead of emphasizing the formative power of the environ-
ment he recognized the activity of forces within the organism
responding to environmental conditions as the basis of change.
He, too, recognized the occurrence in nature of a struggle for ex-
istence, and carried the idea one step further than Buffon by sug-
gesting its ultimate beneficial results. His works vaguely sug-
gest sexual selection and the idea of protective coloration. Some
biologists have speculated on the possible influence of his work on
that of Lamarck, l^ut Packard's vigorous defense of the integrity of
Lamarck's contributions leads to the conclusion that he did not
know of Darwin's writings. It is certain, however, that Erasmus
Darwin's work received some contemporary recognition, and since
he was a physician and naturalist, it was probably sound enough to
deserve even more.

Lamarck (Fig. 1) later and apparently independently developed
the ideas of his predecessors to such a degree that he ranks second
only to Darwin as the founder of one of the schools of modern
evolutionary theory. The available accounts of his life afford an
interesting evidence of the adverse conditions under which valu-
able scientific work may be produced. Lamarck was born in 1744,
the eleventh child in a military family. All of his brothers entered
the army, so Jean Baptiste was placed in training for the clergy.
This was so little to his taste that he followed the army into Ger-
many and in his short period of service displayed "the courage

THE HISTORY OF EVOLUTION

11

and independence that characterized his later years." He was
found physically- unfit for a military life and took up the study of
medicine in Paris, later becoming a naturalist. He devoted him-
self for years to the study of botany, earning a scanty living by
filling various positions as instructor and curator. During his
connection with the Koyal Garden in Paris, which was named the
Jardin des Plantes at his suggestion, he became associated with
Cuvier, who was to have such an important influence on his work.
When fifty years of age, in
1794, Lamarck turned to the
study of invertebrate animals,
for which he developed a
greatly improved classifica-
tion. What effect this work
may have had on his philo-
sophical conclusions it is diffi-
cult to say, but six years after
undertaking it he departed
from his previous idea of the
fixity of species, and in 1809
published the Philosophic Zo-
ologique which formulated his
theory of evolution (Locy).
After the publication of his

views on evolution, which he ^ ^ t, ■ ^

, , ,11, 1 Fig. 1. — -Jean Baptiste Lamarck,

elaborated later, he was

strongly opposed by Cuvier. Cuvier's position was superior to
that of Lamarck and his influence greater; his scientific conclu-
sions were, however, much less accurate. The resulting unfair
disregard of Lamarck's theories, together with poverty and blind-
ness, contributed to the sadness of his declining years, and in
1829 he died, his true greatness for the time unrecognized.

Lamarck's contributions to science include th(^ proposal of the
term "biology" and the tree of life, representing the phylogenetic
relationships of existing organisms, in addition to his actual theory
of evolution. This, when first pubUshed in 1809, consisted of two
laws, translated as follows:

"First Law: In every animal which has not exceeded the term of
its development, the mon^ fretjuent and sustained use of any organ
gradually strengthens this organ, develops and (mlarges it, and

12 EVOLUTION AND GENETICS

gives it a strength proportioned to the length of time of such use,
while the constant lack of use of such an organ imperceptibly
weakens it, causing it to become reduced, progressively diminishes
its faculties, and ends in its disappearance.

"Second law: Everything which nature has caused individuals
to acquire or lose by the influence of the circumstances to which
their race may be for a long time exposed, and consequently by the
influence of the predominant use of such an organ, or by that of
the constant lack of use of such part, it preserves by heredity and
passes on to the new individuals which descend from it, provided
that the changes thus acquired are common to both sexes, or to
those which have given origin to these new individuals."

To these he added later the idea that necessity in the organism
gives rise to new organs. Other corollaries expressed his belief
in various modifying factors, but essentially his theory involves
the belief that change springs from within the organism, in response
to definite conditions of the environment, and that such changes,
once initiated, are transmitted ])y heredity. The last point has
been a frequent subject of dispute, and was further complicated
because Lamarck added to these points the assumption that the
environment acted directly on plants.

Saint- Hilaire was a contemporary of Lamarck who went back to
the belief of Buff on in the direct effect of environment. His chief
claim to distinction is that he believed in the occurrence of sudden
changes in organisms, giving rise to new species, an idea later
developed by deVries.

Charles Darwin (1809-1882), (Fig. 2), is preeminent in the
field of evolutionary thought, as is well shown by the common use
of the word Darwinism as a synonym of evolution. While this
is an erroneous use of the term, his eminence is justified by his
works, not because he was the first man to believe in evolution as a
natural process, but because he brought to the support of the
theory so much evidence, accumulated and prepared with such
painstaking care, that he may rightly be termed the first to place
it upon an adequate and permanent foundation of scientific fact.
Since the appearance of his Origin of Species in 1859 there has been
no doubt among scientists of the reality of evolution as a process
in nature, although Darwin's theory of method has remained,
like all other such theories, a subject of dispute. His work
is essentially responsible for reforms in all fields of biology

TITK HISTORY OF EVOLUTION

13

Fig. 2. — Charles Darwin.

14 EVOLUTION AND GENETICS

which have made possible the modern development of the
science.

Darwin's early training included the study of medicine at Edin-
burgh and preparation for the ministry at Christ's College, Cam-
bridge. During the three years at the latter place he became in-
terested in science, and when H. M. S. Beagle was devoted to an
expedition from the years 1831 to 1836, he accompanied the survey
party as naturalist. His account of this period, under the title
The Voyage of the Beagle, shows a remarkable capacity for observa-
tion of facts of great variety. Much of the time the ship was
absent from England was spent in South America, but the brief
stop which Darwin was enabled to make at the Galapagos Islands
seems to have been a particularly productive part of the trip. The
remarkable conditions prevailing in these islands, recently brought
to the attention of the world in inimitable style in Beebe's Gala-
pagos, World\ End, appear to have been a stimulus to his inquiring
mind, and later to have furnished him with valuable data in con-
nection with his work on evolution, although he did not begin his
first notebook on the development of species until 1837.

After returning to England, Darwin devoted himself to his
scientific investigations. Although he was financially well able to
do this, he was handicapped for the rest of his life by ill health, and
was forced to limit his periods of work to less than two hours each.
His first idea of natural selection came as a result of reading
Malthus on Population, a work which set forth the part played by
overproduction and the consequent struggle for existence in the
human race. This was destined to disclose to him the idea of the
survival, under similar conditions in the organic world, of those
individuals best fitted for life under the existing conditions, and
the destruction of those less favored. With the aid of his own ex-
tensive knowledge of variation in organisms he was al)le to formu-
late the theory which has carried the name natural selection or
survival of the fittest. Darwin records that he first set down this
theory in June, 1842, and later extended it in 1844, but it was not
until 1858 that it was finally made public under circumstances
which are a fine example of individual generosity. During the
year 1858 Alfred Russel Wallace (1822-1913), as a result of reading
the same work which had given Darwin his first idea of natural
selection, conceived a theory of the origin of species which was
identical with that of his countryman. He communicated his

THE HISTORY OF EVOLUTION 15

theory to Darwin, who was about to give up his own claim to it
when dissuaded by two friends, Hooker and Lyell. The theory
thus independently formulated by the two men was presented to
the Linnaean Society of London in a joint paper on July 1, 1858,
and during the succeeding few months Darwin wrot(^ and pub-
lished the Origin of Species which appeared in 1859. With gener-
osity no less than Darwin's, Wallace recognized the more extensive
studies of his fellow scientist on the subject and insisted on relin-
quishing his own claim to credit. The Origin of Species was sup-
posed to be an outline of the subject but in his subsequent work
Darwin failed to produce anything which equalled the first in
effect. It is interesting as this account is being written to note
that the first publication of the work aroused a storm of opposi-
tion, much of it similar to that of the present day, against which a
vigorous defense was conducted by such men as Thomas Henry
Huxley (1825-1895).

After Darwin. The period immediately following Darwin's
productive work witnessed much speculative thought on the sub-
ject of evolution, but before the close of the nineteenth century
scientific activity also showed a fortunate trend toward the accu-
rate examination of the more tangible related subjects. Indi-
vidual development and individual relationships became an object
of careful attention and experimentation. Heredity was investi-
gated by several biologists and the foundations of the modern
science of genetics were laid. The statistical method of handling
biological data was introduced, out of which biometry has de-
veloped.

Herbert Spencer (1820-1903) was the outstanding philosopher
among the evolutionists of this period. His work had considerable
influence, but in general the hypotheses which he advanced have
failed to stand the test of scientific progress.

August Weismann (1834-1914), a German biologist, also con-
tributed notably to the interpretation of facts bearing on evolu-
tion. With a thorough knowledge of biological principles as then
understood, including some facts of cell structure, he was much
better equipped for his work than the earlier scientists and, al-
though his conclusions are now partially disproved, his keen under-
standing played an important part in the development of modern
ideas.

Weismann's work dealt largely with inheritance. It was ap-

16 EVOLUTION AND GENETICS

parent even before his time that the germ cells were the carriers
of hereditary characters, and in dealing with the fundamental prob-
lems of evolution it was inevitable that he should enter this field.
His idea of the distinctness of the germ cells from the body has
come down to the present and is even now a prominent factor in
evolutionary thought. It furnished the basis for other theories
which were necessary to harmonize known facts with his idea of
the germinal origin of characters. None of his theories are now
regarded as adequate explanations of the process of evolution.

Sir Francis Galton (1822-1911), a cousin of Charles Darwin,
carried on extensive studies of human heredity and published three
books, of which the two best known, Hereditary Genius and Natural
Inheritance, appeared in 1869 and 1889 respectively. He is re-
garded as the founder of biometry, for the nature of his material
made necessary some statistical treatment. The complexity of
human heredity is so great and its inadaptability to experimental
methods so complete that Galton could not approach the results of
his contemporary, Mendel, but his work is even now of great value.

Johann Gregor Mendel (1822-1884) (Fig. 3) must be given the
credit for laying the foundation of our modern knowledge of hered-
ity. He was an Austrian monk, and a botanist. In his monastery
garden at Briinn he experimented with inheritance in garden peas
and formulated from his results the laws of inheritance that bear
his name. By using peas of several varieties, hybridizing the
various kinds, and rearing them through several generations he
learned definitely how characters may behave in heredity, and in
1866 published his conclusions in the proceedings of the natural
history society of Briinn. This epoch-making paper was lost to the
scientific world until the beginning of the twentieth century. Its
rediscovery at that time found a number of biologists ready to
accept and verify the conclusions which it expressed, and progress
in the study of heredity has since been rapid.

Modern Evolution. During the twentieth century many fa-
mous names have been linked with progress in our knowledge of
evolution. Darwin's and Lamarck's theories have come to be the
basis for two leading schools of thought on the subject, and there
is an abundance of literature which deals with their extension and
verification. Darwin himself, in the later years of his work, indi-
cated his belief that natural selection was not a sufficient explana-
tion for evolution, ])ut that there was also much evidence for the

THE HISTORY OF EVOLUTION

17

action of environment as a formative influence. The opinions of
later writers have carried on varying degrees of controversy to a
final recognition of the unproved state of both theories. In addi-
tion new theories have shown us that there are probably many
different processes of evolutionary change, as might be expected
in view of the great comphwity of living organisms. Natural selec-
tion and the inheritance of acquired characters must take? their
places with such
theories as muta-
tions and kineto-
genesis among the
numerous proba-
ble processes.

Through the de-
velopment of the
science of genetics
accurate data
have been accu-
mulated on the
mechanism of
transmission of
characters which
must be involved
in the origin of
species as well as
in the origin of
individuals. The
facts available are
not yet wholly
correlated with
other fields of
l)iology but they

are sufficient to furnish a sound foundation for future progress.
Best of all they encourage the broad thinking which alone can
arrive at great truths. Neither the philosophical nor the purely
materialistic aspects of evolution seem complete in themselves.

Late years show an increased emphasis on the philosophical
aspects of the problem, and purposive evolution is now popular as
a modification of the older and more mechanistic th(^ories. The
chief problem of modern evolution, however we approach it, is

Fig. 3. — Johann Gregor Mendel. (From Locy's Bi-
ology and It.s Makers, witli the permission of Henry
Holt and Company.)

18 EVOLUTION AND GENETICS

always, how it occurs. The fact of its occurrence is accepted by
all schools of thought and the biologist can scarcely avoid the
conviction that there is still much to be explained on the basis of
known facts before recourse must be had to purely philosophical

assumptions. • r, u • •

That inherent curiosity which prompted man m the beginning
to investigate the conditions of life still persists, and with a vast
store of accurate knowledge, improved equipment and methods,
and probably a gradual increase in his own mental capacity, he
may one day solve the problem whose pursuit has already met with
such a gratifying degree of success. All of this work must deal with
processes, for with Darwin's contribution man became convinced
that the relationships which he had striven for centuries to explain
were the result of orderly natural development— of evolution.

Summary. The history of biology shows that ideas of the
relationship and evolutionary development of organisms are by
no means of recent origin. The ancient Greeks foreshadowed
many of our modern discoveries and Lamarck, at the beginning
of the nineteenth century, formulated a valuable theory of evolu-
tion Between the time of Lamarck and Darwin, numerous con-
tributions appeared on the subject of evolution, but it remained for
Darwin to express the theory which gained a permanent place for
evolution in biological science. This he did in such a masterly way
that his eminence is well deserved. Even though his theories ol
evolutionary method are no longer regarded as an adequate expla-
nation of the way in which species are formed, they are still
accepted as an accurate explanation of natural processes which
play a part in evolution. Since Darwin's time a major tendency
has been the examination of natural phenomena by the exact
methods of observation and experiment. Theoretical contnbu-
tions have been made, but genetics and other branches of biology
have become the most important fields of progress m our knowl-
edge of evolution. The fact of evolution is now generally ad-
mitted but there is still much to be known of its processes.

REFERENCES

Packard, A. S., Lamarck, 1901.

OsBORN, H. F., From the Greeks to Darwin, 1905.

LocY, W. A., Biology and Its Makers, 1910.

Lull, R. S., Organic Evolution, 1917.

Newman, H. H., Readings in Evolution, Genetics and Eugenics, 1921.

CHAPTER III
THE RELATIONSHIP OF EXISTING ORGANISMS

1. CLASSIFICATION

In any primary study of the organic world such as we have
assumed to be the starting point of evohitionary thought, the
things which an individual might see about him or the largcn- aggre-
gate which might be collected through the efforts of many ob-
servers in different places must necessarily^ have been the whole
source of facts. The accumulated knowledge of modern science
has extended this field and added to it records of extinct organisms
whose fossilized remains are the material of palaeontology. To-
gether with the information worked out by geologists concerning
the relative ages of the rock deposits in which fossils occur, palae-
ontology gives us in many cases an accurate idea of the past
histories of existing plants and animals, and shows within certain
limits of accuracy from what forms they spring and through what
changes they have proceeded to their present state. The periods
covered by this natural record are often unbelievably vast. They
show us conclusively that all of our written records together are
no more than a page out of the history of the world of nature, and
that in the field of evolution, all of our observations of living things
are of the present. The few centuries during which we have been
making and setting down exact scientific observations are so in-
significant in relation to all time that they are but as a moment,
and all of the records so slowly and laboriously accumulated are
little more than a fairly complete catalogue of the things present
in the world at any moment.

Species. In the examination of living things in this brief span
of human experience a resemblance is noted at once between cer-
tain individual organisms. We see birds in the trees and call them
robins or bluel^irds or crows. The individuals thus grouped to-
gether have rather definite features in common which enable us
to associate them easily in the smallest groups commonly used in
scientific classification, the species. In spite of the fact that such

19

20

EVOLUTION AND GENETICS

Ectoderm /
Entoderm
Coelenteron

Perlsarc

r.iL— Blastostyle

species as those mentioned are easily recognized and sharply de-
limited, a closer scrutiny of individual characteristics shows that

in many cases it is ex-
tremely difficult to say
exactly what characters
define the species and
how they are separated
from each other. We
have never been at loss
for examples of species,
yet there has never
been unanimity of
opinion among scien-
tists as to what species
really are.

The belief has been
-Gonotheca pxpresscd that there is
'— Meuusa-buj no real group in nature,
but that only individ-
uals are real entities
and the groups into
which we gather them
for our scientific rec-
ords nothing more
than conveniences. In
contrast the other ex-
treme has been urged
from time to time, that
as some individuals re-
semble each other more
than they do any other
organisms, they must
constitute a natural
group with definite
limits, even though
they may vary within
these limits. As is us-
ually the case, the opinion has gradually developed that there
is sound value in both interpretations. It is now supposed
that there are such things as natural groups which may aptly

;^^...Statocyst

Radial canals

^,. Reproductive organs

Tentacles

Mouth

Fig. 4. — Obelin. A, stalk bearing hydranths; B,
medusoid. (From Parker and Haswell.)

EXISTING ORGANISMS— CLASSIFICATION

21

be called species, but that many of our named species blend into
each other so gradually that it is difficult or impossible to place
certain individuals accurately. These may not be species, and
ofti'n an abundance of material shows that such is the case. A

Worker

Queen
Fig. 5. — Honey-bees.

Drone

(From Hegner.)

scientist may work with specimens from widely separated regions
and find them different, while the later acquisition of specimens
from intermediate regions shows that there is a gradual transition
from one to the other, or he may be confronted with great varia-
tion in a single local-
i t y, with blood
brothers bearing little
resemblance to each
other. The only possi-
ble conclusion of prac-
tical value is that
species are not all in
the same state; that
while many show
marked uniformity of
characters, others are
apparently unstable
and in all probability even now undergoing change.

Subspecific Forms. The lack of fixed characters as a universal
basis for species is further complicated by the occurrences in many
of them of very different types of individuals which nevertheless
bear a definite and intimate relationship to each other. This rela-

FiG. 6. — The Codling Moth Carpnrap.sa pnm-
onella. a, adult; b, larva in an apple; r, pupa
or chrysalis. (From Farmer's Bulletin 2S3,
U. S. Dept. Agriculture.)

22

EVOLUTION AND GENETICS

tionship may be in the form of constant association, as in the many-
colonial animals (Fig. 4) and the social forms (Fig. 5), or it may be
in a succession such as the alternation of different reproductive
types, and the succession of stages in metamorphosis (Fig. 6).
In all of these cases, the ability of the various forms to produce each
other is abundant evidence of specific unity, but mere lack of in-
formation has often led to the sepa-
ration of subspecific forms.

Sexual Differences. Sex is a com-
mon and sometimes a conspicuous
example of intraspecific difference
(Fig. 7) . In addition to those differ-
ences which arc essential to comple-
mentary reproductive functions,
others of an apparently unrelated
character often appear. These are
called secondary sexual characters.
The long tail feathers of male
turkeys, chickens, peacocks and
pheasants, bright colors in the male
sex of many species of birds, the
mane of the lion, and other charac-
ters are well known examples.
Sensory organs in many male insects
and the horns of bucks and rams
are more evidently useful to the
animals. Even in the human race
the sexes differ fundamentally, for
accumulations of subcutaneous
adipose tissue give the body of the female characteristic round-
ness of outHne, while the growth of whiskers and of the vocal
cords brings about equally characteristic male development.
Among the invertebrates sexual dimorphism sometimes involves
the general structure and appearance, as in the common prome-
thea moth, in which color, pattern and shape of the wings differ,
as well as the form of the antennae and development of sense
organs. The sexes of some of the parasitic worms are even more
diverse. While this is true of both flat- and roundworms it is
probably nowhere more conspicuous than in Schistosoma haemato-
bium, a blood fluke of the eastern Mediterranean region (Fig. 8).

Fig. 7. — Enlheus pdcua Linn.
A, male; B, female. The male
is dark brown with an orange-
red band and transparent
orange-yellow spots. The
female is brown with white
markings.

EXISTIXG ORGANISMS— CLASSIFICATION

23

#-gy»i.C.

Colonial Forms. Such forms are highly developed in the
Coelenterata (Fig. 4), among them such species as the Portuguese
man-of-war, which hves in free swimming colonies made up of
numerous modified individuals which carry on limited activities
for the common good. Many of the simpler hydroids display
three forms simultaneously, viz.,
polyps, asexual reproductive in-
dividuals and sexual medusoids
similar to small jellyfishes, which
later detach themseh'cs from
the colony. All of these forms
are connected structurally, yet
in degree their differentiation
is not unlike that of the castes
of social insects. In the honey
bee colony there are three
forms, the functional sexes or
queen and drones, and the
workers, which are imperfectly
developed females with certain
modifications peculiar to their
own caste. Among the ants
division of labor is accompanied
by the development of distinct
forms by the mochfication in
various directions of the three
fundamental types. That these
differences of individuals are
closely associated with division
of labor in all cases is obvious,
and the two are generally sup-
posed to have developed to-

Fio. 8. — Blood fluke, Schistosoma hae-
malohium. Male (cf ) carrying female
( 9 ) in ventral groove; int, intestine;
gyn. c, ventral groove or gyneco-
phoric canal; m, mouth; v. s., ventral
sucker, x 8. (Reprinted by permis-
sion from Animal Parasites and
IhoiKin Disease by Asa C. Chandler,
]:)ublished by John Wiley and Sons,
Inc.)

gether.

Alternation of Generations. Still another type of polymor-
phism is alternation of generations such as that found in many
plants and some of the lower animals. In Obelia (Fig. 4), for ex-
ample, a member of the phylum Coelenterata, asexual individuals
produce the medusoids by budding, while the whole colony is
nourished by the polyps. The medusoids swim away from the
colony, mature their germ cells, and by this process of sexual repro-

24

EVOLUTION AND GENETICS

duction give rise to individuals
which develop into new colonies.
Ferns are asexual plants which pro-
duce many spores, small reproduc-
tive bodies which are able to de-
velop under favorable conditions
into plants of entirely different
appearance. The fern of common
parlance is the sporophyte (Fig. 9),
and the plant produced from its
spores the gametophyte or pro-
thallus (Fig. 10). The latter, hke
the medusoids of Obelia, produces
germ cells which fuse to give rise
by sexual reproduction to a new
sporophyte.

Metamorphosis resembles this
process only in that the different
forms appear in succession; all
forms are a part of a single genera-

Fio. 9.— The sensitive fern, Ono-
clea seiiaibilis, showing a vegeta-
tive leaf (left) and a spore-
bearing leaf (right). (From
Woodruff, after Bergen and
Davis.)

Fig. 10.. — Gametophyte or
prothallus of a fern,
Aspidi\imftlix mas, from
below. (From Stras-
burger, after Schenck.)

tion. Holometabolous insects show a maximum degree of meta-
morphosis in their transition from egg to larva, to pupa and thence
to adult (Fig. 6). The larva is essentially a growing stage, the

EXISTING ORGANISMS— CLASSIFICATION 25

pupa a resting stage through which transition from the larval to
the very different adult structures is accomplished, and the adult
is primarily the reproductive stage.

What Are Species? In the face of such diversity of form
within many species, it is impossible to lay down definite criteria
for the limitation of this unit of classification. The ability of indi-
viduals to produce fertile offspring when mated has been adopted
by some scientists, l)ut it has been shown that not only do many
closely related species cross, but in some cases they produce fertile
offspring which maintain themselves within the range of variation
represented by their diverse parents. Criteria of morphology fail
when great diversity occurs, and rigid delimitation on this basis
covers a latitude in single colonial species greater than the differ-
ences between some obviously distinct species. Differences of
physiology are difficult to judge, yet species exist between which
no other distinctions are known. We are forced to the conclusion
already briefly expressed, that there are specific entities in nature,
although the conditions of their existence are variable. Since the
species is the unit whose occurrence evolution proposes to ex-
plain, this very instalMlity is significant. Those species which are
variable, and apparently undergoing change, seem about to give
rise to several different species, while those which are fixed within
relatively narrow limits seem to be more definitely established.
By referring to the past we see that still other species have come
and gone, apparently after passing through a period of senility
characterized by inability to adapt themselves to changing condi-
tions. In this, again, palaeontology clarifies our understanding
of the condition of modern species, although the modern species
alone show that distinctness of kind is relative.

Major Groups. Beyond those relationships which enable us
to group individuals together as species, we find other points of
similarity which indicate broader associations. Our robins and
bluebirds, for example, have definite structural characteristics in
which both differ from the crow and the hawks, so we call both
thrushes, yet the thrushes and hawks are more closely related to
each other than to our domestic animals, because they are birds.
Step by step these resemblances proceed through groups of in-
creasing extent, each based on more fundamental characters than
the one below it, and conse(iuently embracing a wider range of
species. The system of classification developed by Linnaeus and

26 EVOLUTION AND GENETICS

still used with modifications employs for this succession of groups
the following terms, beginning with the initial subdivision of
living things into plant and animal kingdoms and proceeding
through those of decreasing extent:

Kingdom
Phylum
Class

Order

Family
Genus

Species

Thus man is Homo sapiens — the species sapiens of the genus
Homo, which ])clongs to the family Hominidae in the order Pri-
mates. The Primates are members of the class Mammalia, in the
phylum Chordata of the animal kingdom. To indicate finer dis-
tinctions in classification such modifications as suborders and
superfamilies are sometimes used. In the example given every-
thing up to the ordinal name indicates man's exalted opinion of
himself, for he stands alone! The order, however, acknowledges
his association with the apes and monkeys, the class, to all animals
that have hair and nourish their young with milk, the phylum to
those which have a backbone and to certain other remote rela-
tives, and the kingdom, finally, to all animals.

Mimicry. Superficial resemblance is usually but not always
a dependable index of relationship. We have mammals which may
be mistaken for fish, beetles and flies which look like wasps, flies
that resemble bumble bees, and a variety of lesser resemblances.
Such abnormal superficial similarity has been recognized as play-
ing a definite part in the lives of organisms, and the gradually
accumulated knowledge represented by modern classification has
relegated these types of resemblance to their proper places and
expressed the fundamental relationship which they often obscure.
The superficial resemblance of one species to another is called
mimicry. This is well illustrated by the resemblance of certain
harmless species of insects to others which are either unpalatable
to bird or animal enemies, or able to defend themselves. The
common eastern butterfly, Basilarchia archippus (Cram.), while
superficially very different in appearance from its congeners, is
much like the milkweed butterfly, Danaus menippe (Hbn.), which

EXISTING ORGANISMS— CLASSIFICATION 27

is apparently unpalatable because of the bitterness of its food
plant.

Convergence. Resemblance between animals of different
groups is usually due to convergence as fundamentally different
organisms become adapted to the same conditions of environ-
ment. The fishes are aquatic organisms, admirably adapted to
the conditions of their environment. Whales, seals and dolphins
arc fish-like in many ways, but they show their terrestrial origin
in that they must breathe air. Their points of reseml)lance to the
fishes are due solely to the fact that life in the water is possible only
if certain conditions of form and locomotion can be met, and the
physical conditions of the ocean are such as to limit the ways in
which they can be met. The tail of the dolphin is somewhat like
that of the fish, and serves the same purpose^; likewise the wings of
insects and of liirds are superficially similar, and have the same
function, although they are fundamentally different structures.
This resemblance of unlike structures which comes about
through the adaptation of different organisms to similar con-
ditions is known as analogy, and is a common corollary of conver-
gence.

Divergence. Relationship may also be obscured by the differ-
ences in development of closely related animals. Man is a mammal,
the squirrel is a mammal, and the seal is a mammal, but man has
assumed the erect posture for terrestrial life, the squirrel lives in
trees, and the seal is almost wholly aquatic. The result is a com-
plete overshadowing of their fundamental similarity, yet the flip-
pers of the seal and the front paws of the squirrel are the same
vertebrate structures as the hands of man. The term adaptive
radiation is applied to this divergence of related forms, and the
related structures which thus assume superficial difi^erences are
said to be homologous.

Accuracy of Classification. Such accuracy is wholly dependent
upon the ability of the taxonomist to go beyond the superficial
characteristics of the organism and to interpret fundamental con-
ditions. This has been accomplished to a marked degree in bring-
ing our classification to its present state of reasonable perfection.
Studies of evolution have been invaluable in this development in
that they have brought about a keener realization of relationship
between organisms and the progressive nature of such relation-
ship, and now that our classification is in such an excellent state,

28 EVOLUTION AND GENETICS

it exerts a reciprocal influence of great value as an illustration of
the results of evolution.

Classification of Plants. The most common classification of
plants is based upon four major groups. The third has been di-
vided into three, which are indicated here merely as components
of the one division.

1. Thallophyta. The algae, fungi and hchens.

2. Bryophyta. Liverworts and mosses.

3. Pteridophyta. Ferns, horse-tails or scouring rushes, and club

mosses.

4. Spermatophyta. The seed-bearing plants.

Classification of Animals. Some variation occurs in the classi-
fication of animals as various writers estimate differently the
importance of certain characters. The following outline includes
the major subdivisions, or ph3da, which are commonly recog-
nized.

1. Protozoa. Single-celled animals.

2. Porifera. The sponges.

3. Coelenterata. The hydroids, jellyfishes, sea anemones and

corals.

4. Ctenophora. Comb-jeUies or sea walnuts.

5. Platyhclminthes. Flatworms: free-living forms and para-

sitic flukes and tapeworms.

6. Nemathelminthes. Roundworms, including many parasitic

forms found in man as well as free-living forms.

7. Rotatoria. Wheel animalcules.

8. Bryozoa. The moss animals.

9. Brachiopoda. Tongue or lamp shells.

10. Echinodermata. Starfishes, sea urchins, sea cucumbers.

11. Annelida. Jointed worms: earthworms and leeches.

12. Arthropoda. Crustacea, including lobsters, etc. Myria-

poda, spiders, insects, etc.

13. MoUusca. Snails, mussels, squids, etc.

14. Chordata. Several obscure worm-like animals, the tunicates

and salpians, lancelets, round-mouthed eels, fishes, am-
phibia, reptiles, birds, and mammals.
The tree of life, adapted to modern classification from the
original conception of Lamarck, shows graphically the relation-
ship of these phyla and some of their more important subdivisions
(Fig. 11).

EXISTING ORGANISMS— CLASSIFICATION

29

Relationships. Protoplasm. A complete dissertation on the
relationship of minor groups of animals would involve more details
than could be set down in a single volume, but many [)oints of
fundamental relationship are visible in organisms whicii can
readily be appreciated in a brief outline. Broadest in scope is the
common basis of all life, plant and animal, the substance proto-
plasm. Protoplasm is
made up of three chem- ^=>s.^ vs"^"
ical compounds, pro-
teins, carbohydrates
and fats, associated
with various inorganic
compounds such as
water and common salt,
which are not changed
by the bod3\ The fats
and carbohydrates are
made up of the ele-
ments carbon, hydro-
gen and oxygen, while
proteins include these
three together with ni-
trogen, sulphur and in
some cases phosphorus Aesozoa'

and iron. The proper- Infi^jonajis
ties of protoplasm are
the same in all organ-
isms. In addition to
its definitive chemical
composition, it is en-
abled to continue its
existence and activity,
and to grow and reproduce, by taking in other substances, chang-
ing them chemically through the process of digestion and incorpo-
rating them into itself. This process is called intussusception.
After the substances have become an integral part of the body,
their potential (nicn-gy is liberated by oxidation and thus activates
the organism. The waste products of oxidation are then passed
out of the body l)y the excretory system. The constructive part
of the entire interchange between the organism and its environ-

FiG. 11. — Diagram, the tree of life. (From Out-
lines of Zoology, by J. Arthur Thomson, with
the permission of D. Appleton & Company.)

30

EVOLUTION AND GENETICS

ment is called anabolism, the destructive part katabolism, and the
whole process metabolism ; this is one of the most striking charac-
teristics of living matter. Another distinctive quality is the
power to reproduce itself in the organized form which is charac-
teristic of the various species, and finally this remarkable sub-
stance has properties which enable it to receive stimuli of light,

Centrosomes

Golgl bodlea-

PlaeiiQOBome or
nucleolus

Chromatin
Llnln

Karyosome

True wall or membran9
Plasma-membrane

Cortical layer
' Plastids

Mitochondria, eto«
Vacuole

Metaplasm

Fig. 12. — General diagram of a cell. (From Woodruff, after Wilson.)

heat, contact, sound waves and chemical substances in its en-
vironment, to conduct these stimuli to various parts of the body
which it forms, and to respond to them in various ways with the
result that the organism fits into, or is adapted to its environment.
The Cell. Protoplasm as we see it in living organisms is found
in only one form, the cell (Fig. 12). No animal or plant exists
which is simpler than this unit, and all more complex forms are
built up of many such units. Cells may undergo great differentia-
tion of form as they become specialized for various tasks, and so
we find in the l^ody of man the flat, horny cells of the cuticle, tall
cells wiih waving cilia lining the trachea, long and contractile cells

EXISTING ORGANISMS— CLASSIFICATION

31

in the muscles, nerve cells with long fibers which coordinate the
various organs, and many more (Fig. 13). Wherever found and

Fig. 13. — Various kinds of cells. A, ovum of cat; B, spermatozoon of a snake;
C, ciliated epithelium from the digestive tract of a mollusc; D, cartilage of a
squid; E, voluntary or striated muscle fil)er from an insect; F, involuntary
or smooth muscle fibers from the bladder of a calf; G, nerve cell from the
human brain; H, white blood cell of frog; I, red blood cell of frog; J, same,
edge view; nu, nucleus. (From Woodruff, after Parker and Haswell, H, I,
J; and Dahlgren and Kepner, A-G.)

32 EVOLUTION AND GENETICS

however specialized, cells are made up of two fundamental parts,
the nucleus and cytoplasm, the former embedded in the latter.
Each is complex, as will be seen in the diagram, and the two are
essential to each other. The nucleus appears to exert a controlling
influence over the cytoplasm, while the differentiation of the cyto-
plasm determines the chief characteristics of the various types of
cells. Some cells, such as the red blood corpuscles of most mam-
mals, are without nuclei, but after the loss of the nucleus their
lives are short and replacement occurs frequently.

Plan of Animal Structure. Single-Celled Organisms. Although
it is necessary to call on our knowledge of individual development
for accurate interpretation of the conditions found in many
organisms, many others show a simple plan of structure; for ex-
ample, the fact that living matter cannot exist in units less com-
plete than the cell leads at once to the conclusion that single-celled
plants and animals are the most simple of all organisms (Fig. 14).
For that reason, if all life has really come from such a lowly origin,
they must represent the oldest forms now extant. They have
existed longer than any other forms, and have had opportunities to
become fitted for life under various conditions; hence we find many
species differentiated in various ways. Some are parasites in man
and other animals. If man is of recent origin, as we suppose, or
even if his origin followed that of the other animals, as all admit,
this is in itself evidence that the Protozoa have departed from
their original condition to become fitted for life in the other bodies;
they have evolved.

The Germ Layers. During the development of many multi-
cellular animals a definite plan is followed in which the first step
is the repeated subdivision of the original germ cell to form a
hollow sphere, in the simplest state. This hollow sphere caves in
on one side until the two halves are in contact, thus forming a sac
with two layers of cells in its wall, called the gastrula. The inner
layer is associated primarily with nutrition and respiration, the
outer with the nervous system, sense organs, and protective skin.
Lastly, a third layer or mass of cells forms between the two, from
which develop the muscular and skeletal systems, circulatory
system, excretory system, reproductive system, and many sup-
porting and accessory parts of other structures. These three
layers of cells are called the germ layers. A group of similar cells
is obviously simpler than a sac with a two-layered wall of which

EXISTING ORGANISMS— CLASSIFICATION

33

[uplotes

Spirostomum

lionotus

Fig. 14. — Some common fresh water protozoa, greatly magnified. (From

Woodruff, after Curtis.)

the cells of the different layers carry on different functions, and
this structure is simpler than one in which a third layer is differ-
entiated.

On this basis we are able to group the fourteen phyla. Among

34

EVOLUTION AND GENETICS

the Protozoa most are independent single cells, but some are asso-
ciated together in groups (Fig. 15), and most of them remain un-
differentiated. The next three phyla are
fundamentally simple two-layered sacs
although they are modified in various
ways, and are called diploblastic. One
little fresh-water animal. Hydra, shows
this arrangement very simply (Fig. 16).
All other phyla have three fundamental
layers, and are called triploblastic. Their
relationships are indicated by the de-
velopment of other structures.

Triploblasts. Symmetry. Complexity

Fig. 15.— a simple colony ^^ ^^^^ environments to which these ani-

of unicellular organisms mals are adapted is here reflected in a

(^poudi^lnmornm). (From variety of different tendencies in their

Hegner, atter Oltmanns.) . , t.t . i, ,. , , . ,,

structure. Not all of them have followed

the same course of development beyond this acquisition of three
fundamental germ layers. Thus, the flat worms have merely
added to the complexity
of the original sac-like
digestive cavity, while
the rest have developed
a tubular structure, open-
ing cephalad at the
mouth and caudad at the
anus. Yet the flat worms
share with the remaining
phyla the development
of a head in which nerv-
ous control is concen-
trated, and accompany-
ing this, their bodies are

formed of two similar

halves, flanking the me- Fig. 1G. — Hydra. Transverse section, highly
dian axis. Below this magnified. (From Woodruff, after Shipley

.'. . , and McBride.)

group, animals are either

asymmetrical or made up of parts radiating from a common
center, as in Hydra. Such a form is known as radially symmet-
rical, while the bilateral arrangement is called bilateral symmetry.

EXISTING ORGANISMS— CLASSIFICATION 35

The Echinodcrms have returned ahnost completely to the older
form, although they begin life as bilaterally symmetrical larvae.

The Body Cavity. Triploblastic animals, even in the lower phyla,
develop another characteristic, the body cavity, or coelom, formed
by a splitting of the middle layer. This cavity is important in
many animals in connection with circulation and excretion, but
in the higher phyla is littk^ more than a cradle for the visceral
organs. It occurs in a modified form in roundworms, and is found
in all succeeding phyla (Fig. 17).

Metameric Structure. The Annelids, particularly the common
earthworm, illustrate the development of repeated similar parts,
called metameres. These are indicated by the ring-like sub-
divisions on the outside of the earthworm, but involve also a sub-
division of the coelom and the arrangement of parts of various
organic systems. This arrangement is modified in »^^any ways,
but in the earthworm not to such an extent that the b accession of
similar organs is obscured. Many segments show a portion of the
alimentary tract, a ganglion of the nerve cord, lateral nerves,
portions of longitudinal blood vessels, a pair of transverse blood
vessels, a pair of nephridia, and other structures. Such a plan is
the basis of development in all higher phyla, even in man, al-
though in the adult there is little visible evidence of it save in the
spinal column, ribs, and associated muscles and nerves. In the
fish, one of the lowest vertebrates, everyone is familiar with the
V-shaped bands of muscle which lie along the sides, each repre-
senting a metamere.

Appendages. While none of the preceding phyla have paired
jointed appendages for walking, grasping and other functions, the
Arthropods develop a long scries, so characteristic that they have
given the name to the order from the Greek apdpov, a joint, and
TTovs, TToSos, a foot. Metameric structure is conspicuous in the
phylum, although modified by the division of the body into three
distinct regions, head, thorax and abdomen, and each metamere
appears to have been fundamentally capable of producing a single
pair of appendages. In the nineteen pairs present on the lobster
or crayfish, there is a fine lesson in the possibilities of such struc-
tures (Fig. 18). Developed on a common plan of structure, they
are formed for swimming, walking, grasping, accessory organs
of generation, accessory mouth parts, mandibles, and sensory
organs.

EXISTING ORGANISMS— CLASSIFICATION 37

The Mollusca. The Mollusca are highly specialized, and dif-
ferent from the other organisms. In the class Cephalopoda, in-
cluding the squids, cuttle-fish, octopus, and other forms, we see
chiefly an illustration of different ways of obtaining the same re-
sult, for the blood carries oxygen, but not through the same me-
dium as ours, and the eyes are well developed, but different in
fundamental structure from ours.

The Chordata. These organisms have departed so widely from
the other phyla that they stand alone. While th(>ories are avail-
able to explain their relationship to the remaining phyla, nothing
affords more exact information than the facts of general structure
which have been mentioned. Their isolation is so marked that
the term invertebrate is commonly applied to all other phyla,
and that of vertebrate to the better-known members of this one.
They are characterized by the possession of an inner stiffening
structure, the notochord, in contrast to the exoskeleton, or ex-
ternal stiffening structure developed by the skin of invertebrates;
the central nerve cord is dorsal instead of ventral in position; and
the pharynx, corresponding to the human throat, is at some stage
provided with a series of paired lateral openings, the gill slits or
pharyngeal clefts, which communicate with the exterior. The
vertebrates make up the greater part of the phylum, and include
the more familiar animals of every day experience. Six classes
are recognized, as follows:

1. Cyclostomata — Round-mouthed fishes.

2. Pisces — True fishes, with hinged jaws.

3. Amphibia — Newts, salamanders, frogs and toads.

4. Reptilia — Lizards, turtles, crocodiles, snakes.

5. Aves — Birds.

6. Mammalia — Animals which have hair, and which secrete

milk for the nourishment of their young.
Relationships among the vertebrates are best emphasized in con-
nection with their embryological development and comparative
anatomy, and are of sufficient interest to us to deserve special
consideration in later chapters.

The whole great field of classification involves more than
700,000 known species of existing animals and almost 300,000 of
plants. One class of Arthropoda, the Insecta, alone includes about
600,000 of these. Detailed knowledge of any limited group, for
detailed knowledge is possible only of hmited groups in the capacity

38 EVOLUTION AND GENETICS

of one individual, is a constant repetition of relationships of species
with species and group with group. It is impossible to go into the
minor relationships in such a work as this, but in the major points
of similarity mentioned in tliis chapter we have an outline which
greater elaboration merely amplifies.

Summary. The time involved in evolution is so vast that our
observations of organisms cover, in proportion, only a moment.
Nevertheless our records contain evidence of relationship. These
are expressed in the classification of organisms into species and
the association of species into successive groups of gradually
increasing scope. Within this system relationship is evident be-
cause of similarities of structure, of which protoplasm and the cell
are common to all organisms. Among the animals a definite plan
of structure leads to a secondary grouping of the various phyla.
Relationship of the various groups is based on the fact that they
always present some expression or modification of the funda-
mental plan, however different they may be in details.

REFERENCES

Montgomery, T. H., Proc. Acad. Nat. Sci. Phil, 1902.

Powers, E. B., Am. Nat. XLIII, 1909.

Coulter, J. M., Barnes, C. R., and Cowles, H. C, Textbook of Botany, 1910.

Parker, T. J., and Haswell, W. A., Textbook of Zoology, 3rd edition, 1922.

Thomson, J. A., Outline of Zoology, 6th edition, 1914.

LiNDSEY, A. W., Denison U. Bulletin, Jn. Sci. Lab. XX, 289-305, 1924.

Hegner, R. W., College Zoology, revised edition, 1926.

Woodruff, L. L., Foundations of Biology, 3rd edition, 1927.

CHAPTER IV

THE RELATIONSHIP OF EXISTING ORGANISMS

{Continued)

2. EMBRYOLOGY OF VERTEBRATES

The relationships of no animals are more striking ban those
of the vertebrates, and no phase of vertebrate relationship is more
fascinating than the similarity of their embryonic development.
This field has a twofold bearing on evolution, for it illustrates
actual relationships in the formative stage of the individual and
at the same time points strongly to the probable succession of
changes which has brought about the transition from lowest to
highest.

Our knowledge of embryology has contributed so conspicuously
to the science of comparative anatomy that it is difficult in many
cases to separate the two; indeed, the latter cannot adequately
be treated without the inclusion of a considerable body of facts
from embryology. For this reason the matter presented here must
partake somewhat of both fields. It falls into two categories, (1)
the resemblance of embryos of the different classes and (2) the
resemblance of embryos to the adults of lower classes.

The Resemblance of Embryos of Different Classes

The individual first becomes an entity when the single cell from
which it is to develop is matured, whatever may be the nature of
this cell. From this point to the completion of its body it is an
embryo. Completion does not necessarily mean birth, for most
embryos are complete organisms long before birth. Nor does it
mean the realization of all the possibilities of differentiation in-
herent in the individual, for many of these are not normally ex-
pressed until long after birth. It means rather the formation of
those organs which constitute a complete individual, whether or
not they may later atrophy or undergo further development as life
proceeds.

Cleavage. The first step in development has already been
mentioned as cleavage, or the splitting of the fertilized ovum into

39

40

EVOLUTION AND GENETICS

successive generations of cells. Such division always occurs in the
vertebrates, but its results may be superficially very different
according to the amount of food included in the cell. The mini-
mum is found in the eggs of the lower chordates and many fishes,

B

D

E

F

d

GUI

Fig. 19. — Early stages in the development of the egg of a sea urchin. A-F,
cleavage and formation of the blastula; G, section of blastula showing the
beginning of gastnilation; H-I, early and late gastrula stages, a, ectoderm;
b, endoderm; c, blastocoele; d, blastopore, leading into the enteric cavity;
e, cells arising from the endoderm, destined to form the mesoderm. (From
Woodruff.)

the maximum in the reptiles and birds in whose eggs the entire yolk
is the fertilized ovum. In true mammals, whose young are nour-
ished by a placental connection with the mother, such a concen-
tration of food to supply the developing embryo is unneces.sary,
henc(^ the ovum is small. The stored food or yolk is inert matter
which delays the process of subdivision.

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41

42 EVOLUTION AND GENETICS

Eggs of the first type give rise by a succession of cleavages to a
hollow sphere called the blastula, made up of cells very nearly
alike in size (Fig. 19). The presence of a moderate amount of
yolk gives rise to a slight modification of this form as shown m the
blastula of the frog (Fig. 20), while the relatively enormous amount
of yolk in the egg of a bird crowds all of the cytoplasm to one pole,
with the result that cleavages cannot pass entirely through the
sphere. In these eggs the protoplasm alone is cut up into a cap of
cells underneath which a small space is equivalent to the cavity of
the spherical blastula or blastocoele, so conspicuous in the other
forms (Fig. 21).

Gastrulation. Following cleavage the simplest blastulas (Fig.
19) cave in on one side until the concave layer of cells is in
contact with the convex side, thus establishing a sac with a two-
layered wall by whose formation the blastocoele has been ob-
literated. The enclosed cavity, or archenteron, opens to the ex-
terior of the embryo by the blastopore. If we compare the
series of figures of frogs' eggs (Fig. 20) we note at once that the
blastocoele is relatively so small that this process cannot be accom-
plished. Instead, a crescentic lip evaginates from the thinner part
of the blastula, grows down and around the structure until its ends
meet and form a rounded opening, and the same result is obtained.
The blastocoele is obliterated, while the cavity present is enclosed
by a two-layered wall and opens to the exterior of the embryo.
That this archenteron is almost completely filled by the remaining
yolk mass which bulges from the inner cells is merely incidental
to the accumulation of yolk. In the bird's egg the cap of cells turns
under at one point, and l)y proliferation from this point finally
becomes d ,,ible (Fig. 22). The yolk is then capped by two layers
of cells in place of one, although it is so bulky that these layers do
not enclose it until much later. For homologies here we must note
that the blastopore has another definite characteristic : it is bounded
by the united outer and inner layers, or ectoderm and endoderm,
and this is also characteristic of the point at which the cells on the
hen's egg turn under to form the second layer.

The Neurenteric Canal. In order to compare the early devel-
opment of the mammal it is necessary to pass on to other struc-
tures. The mesoderm in all vertebrates arises between the two
original layers, and from the three all structures of the body de-
velop. As an early step a region along the mid-dorsal line of the

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44

EVOLUTION AND GENETICS

body becomes depressed, and flanked by two ridges (Fig. 23).
This process of involution finally results in a meeting and fusion
of the two ridges, now become folds, so that an inner longitudinal
tube of ectoderm is formed, surmounted by a continuous layer of

LV PH

^T

Fig. 24.^Longit,udinal sections of developing embryo of frog. A, earlier, B,
later stage. N, notochord; Fb, Mb, and Hb, fore-, mid-, and hind-brain;
PT, pituitary anlage; PD, proctodaeum; A, anus; PH, enteron; NT, neural
tube at the neurenteric canal. (From Holmes.)

superficial ectoderm. As a result of the relation of the folds to the
blastopore, the cavity of the tube communicates with that of the
archenteron (Fig. 24). The tube is destined to become the central
nervous system, and is called the neural tube, hence this con-
tinuous passage is called the neurenteric canal. The connection is
later broken, the dorsal tube persisting throughout life in the

EXISTING ORGANISMS— EMBRYOLOGY

45

brain and spinal cord, while the ventral part forms anterior and
posterior connection with the exterior and gives rise to the ali-
mentary tract and its derivatives.

The Foetal Membranes. Associated with terrestrial life the
vertebrates above the Amphibia develop a remarkable series of
structures called the foetal membranes, which compensate the
differences between the heavy water as a medium in which the
lower forms de- .

velop and the light,
dry air which sur-
rounds the eggs of
the reptiles and
birds (Fig. 25).
Two of these mem-
branes are formed
in the hen's egg by
the growth of folds
from the layers of
tissue surrounding
the young embryo.
The folds consist
of an outer layer
of ectoderm and
an inner layer of
mesoderm. By a
process of involu-
tion, similar to the
formation of the
neural tube, these
folds unite to form

B

A mnion folds

A mnion
Chorion

Chorion

Fig. 2.5. — Diagram illustrating the development of
the foetal membranes Al, allantois; Am, amniotic
cavity; Ys, yolk sac. The chorion of this figure is
the serosa of the text. (After Gegenbaur in McMur-
rich; taken from Arey's Developmental Anulomy with
the permission of the W. B. Saunders Company.)

two layers covering the embryo; the outer consists of ectoderm
outwardly and of mesoderm inwardly, and is called the serosa,
while in the inner, called the amnion, the order is reversed.
The amnion is filled with fluid which protects the delicate
tissues of the growing embryo in every way as effectively as
the ocean protects the developing embryo of a fish. As the
outer edges of the cap of growing tissue extend, they finally
meet to enclose the yolk. The serosa is then a hollow sphere,
which must be involved in any contact with the outer world.
Inside of it another sac is then completed, lined with endoderm

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46

EXISTING ORGANISMS— EMBRYOLOGY 47

and covered with mesoderm. The intimate association of this
sac with the yolk has given rise to the name yolk sac. Last of
all an evagination similar to the yolk sac in structure is formed
from the hind gut, expands, and in contact with the serosa
provides for respiration in the chick. This is the allantois.

Mammalian Homologies. In the mammalian em])r3'o all of these
things are found. The first sul^divisions in man produce a hollow
sphere which later corresponds to the outer ectodermal layer of
the serosa. A mass of cells buds off inside of this, which in turn
produces a second mass ; these are ectoderm and endoderm respec-
tively. Mesodermal tissue fills in the spaces between all three
(Fig. 26A). Finally the masses of ectoderm, endoderm and meso-
derm split, forming cavities which increase in size until the original
ecto- and endodermal masses are connected by a slender stalk

A B C

Fig. 27. — Outlines of the limb buds of embryos. A, pig; B, rabbit; C, man.
Each shows five rounded prominences which are the first evidence of digits.

with the outer sphere, now obviously similar to the serosa pre-
viously described (Fig. 26B, C, D). In the stalk a diverticulum
from the hind gut of the embryo runs out toward the serosa, small,
it is true, but with exactly the relationships of the allantois, which
it is. The embryo develops from the layers of tissue between the
cavities of the ectodermal and endodermal masses, and comes to
lie in the first of these, which thus corresponds in all particulars
with the amnion of the birds and reptiles, and lastly, although the
absence of yolk robs it of its original function of absorbing nourish-
ment, the endodermal sac has the connections and structure of the
yolk sac. In addition to these homologies we find that in the
embryo itself, the neurenteric canal is as well developed as in the
lowest chordates.

Organogeny. General Similarity. After these early steps in
the development of vertebrate embryos, the definite structures

48

EVOLUTION AND GENETICS

which appear show an equally close resemblance. The brain is
first a series of three expansions of the neural tube at the anterior
end of the body. Limbs first bud out from the body wall as
rounded projections, and digits in turn bud from them in those
animals which possess such structures (Fig. 27). The mouth and
anus form as invaginations of ectoderm, the stomodaeum and
proctodaeum, which meet the endoderm and break through to
form the continuous tube of the alimentary tract. The nostrils

Postcardinal veins
Vitelline artcrv,

Precardinal veins
Descending aortce

Umbilical arteries,

^Aortic arches i and 2

Body stalkf 1/ \ ^^==^ / \ \ H^"^^

Umbilical veins'' \^__^^ \ Sinus venosus

Vitelline veins

Fig. 28. — Lateral aspect of the circulatory system in a human embryo of
2.6 mm. Diagrammatic. (From Arey's Developmeidal Analomy, after
Felix-Prentiss, with the permission of the W. B. Saunders Company.)

are at first merely depressions of ectoderm which become asso-
ciated with the central nervous system as organs of smell, while
in the terrestrial forms they also play a part in respiration, and for
this purpose join the stomodaeum. In terrestrial forms the
trachea and its subdivisions in the lungs are first only evaginations
from the primitive gut. By later subdivision of the stomodaeum,
they are more definitely associated above the amphibia with the
olfactory part of the respiratory system. The swim bladder of a
fish corresponds in origin to the lungs. Glands, such as the liver
and pancreas, in all vertebrates bud out from the alimentary tract.
The Circulatory System. Particularly noteworthy is the cir-
culatory system, for when it once develops tubular arteries and
veins and a chambered heart, it is similar in all vertebrate em-
bryos. A pair of veins enters the body from the yolk sac and a
pair from the region of the allantois. These join the heart, whence

EXISTING ORGANISMS— EMBRYOLOGY

49

the blood passes forward into a pair of aortae. The aortae curve
dorsad and pass along the body, giving off branches which supply
both the body and the extraoni])ryonic regions whence the two
pairs of veins drain ])lood. The blood which these branches dis-
tribute to the body is collected by a pair of anterior or precardinal
veins, and a pair of postcardinals. On each side of the body these
vessels unite, to enter the heart as a common trunk. Such a plan
is common to all vertebrate embryos early in their development
(Fig. 28). Most significant of all is the series of aortic arches
formed between the ventral and descending aortae anterior to the

Aortic arch 3
Aortic arch 2

Aortic arch i

Pulmonary artery

Dorsal aorta

Aortic arch 4
Aortic arch 6

Esophagus
Trachea

Ventral aorta Bulhus cordis

Fig. 29. — Reconstruction of the aortic arches and pharyngeal pouches of a
5 mm. human emljryo. The pouches are indicated by Roman numerals.
(From. Arey's Developmental Annlomy, after Tandler, with the permission
of the W. B. Saunders Company.)

heart. The connections by which they are joined originally are
the first pair of six. The remaining five pairs form as outgrowths
of the two aortae of the same side which meet, forming dorso-
ventral connections. While these arches are very different in the
adults of the several classes, they appear in the embryos with very
similar form, and the embryo of man is not excepted (Fig. 29).

The Eye. The development of the eye in vertebrates is initiated
by the appearance of an evagination of the lateral walls of the
forebrain, the anterior of the three primitive expansions of the
neural tube (Fig. 30). The evagination expands at its outer end,
and here invaginates in turn to form a double cup, the optic cup,
connected to the forebrain by the more slender optic stalk. This
cup is destined to form the pigmented and nervous layers of the
retina in the adult Qye. The lens develops from an invagination
of ectoderm opposite to the optic cup, which is later freed from the

50

EVOLUTION AND GENETICS

outer ectoderm as a hollow vesicle. The optic nerve grows back
to the brain from the inner layer of the optic cup, and the entire

eye is enveloped by the
hard sclerotic coat de-
rived from the meso-
dermal layer. Later
on, in all forms which
have eyelids, the skin
in front of the eye
forms two folds which
grow until they meet
and fuse, later on to
separate again as the
eyelids. In some ani-
mals the lids remain
fused until after birth,
hence the young are
said to be born blind.
The skin continues
around the edges of
these folds and joins
the delicate conjunc-
tiva which covers their
inner surfaces and ex-
tends across the ex-
posed portion of the
eyeball. In this proc-
ess the eyes of all
vertebrates correspond
in so far as their adult
structures correspond.
Accompanying these
_ and many other points

Fig. 30.— Diagrams illustrating the formation of of similarity in the

the eye in an invertebrate (A) and a vertebrate parts of vertebrate

(B, C, D, E, successive stages), a ectoderm; gj^^ryog ^ striking
b, retinal layer; c, future position of the optic ^ ■ i ui

nerve; d, cavity of the brain; e, optic vesicle; superhcial resemblance

/, optic stalk, later replaced by the optic nerve; has been noted in the

g, cavity of the optic cup, later the post^erior ^^^^^ embryos of the

h

chamber of the eye; /;, developing lens. (From
Woodruff.)

five highest classes of

EXISTING ORGANISMS— EMBRYOLOGY 51

vertebrates. Explanation is inadequate to give an idea of this
resemblance, which is shown in Fig. 31. The structures which
have been described show that this resemblance is not merely
superficial, but is based on fundamental structure.

Resemblance of Embryos to Adults of Lower Forms

The resemblance of very early stages of vertebrate develop-
ment to the lowest animals has already been brought out. All
have their origin in a single cell, the fertilized ovum, morphologi-
cally equivalent to the single cell which makes up the entire body
of a protozoon. The blastula is quite similar to such colonial
protozoa as Volvox (Fig. 32), although we must recognize that
there is some differentiation in the cells of the blastula which is not
present in the cells of a Volvox colony. The simple gastrula has
been compared with coelonterates, such as Hydra (Fig. 33) and the
modifications of gastrulation brought about by the accumulation
of yolk in the ovum result in structures which can readily be
homologized with those of simpler forms. Beyond this point we
must seek reseml)lance of vertebrate with vertebrate as their
structures appear.

The Notochord. Immediately after gastrulation a structure
called the notochord appears in all chordates (Fig. 34). It is a
rod of tissue whose origin is similar to that of the mesoderm al-
though their development is not directly associated. The noto-
chord extends through the body longitudinally between the
neural tube and the alimentary tract, and just ventral to the
former so that it corresponds in position to the main axis of
the spinal column. In the cyclostomes it persists throughout life;
in the remaining vertebrate classes it is well developed onl}^ in the
embryo and is either vestigial or absent in the adult. One striking
evidence of relationship is the similarity of development of the
notochord in reptiles and mammals. Even the birds, which are
in general rather closely related to the reptiles, show a marked
modification of the process. As the bones of the axial skeleton
develop, centra of the vertebrae replace the notochord at least
in part.

The Skeleton. Lower Fishes. If we examine the most primi-
tive fishes, the sharks, we find that the skeleton consists of cartilage
alone. It is divided into axial, appendicular, and visceral parts
as in the rest of the vertebrates but the structure of each part is

52

EVOLUTION AND GENETICS

A-^ B C D

Fig. 31. — Series of vertebrate embryos at three comparable and progressive
stages of develoi)ment. A, fish; B, salamander; C, tortoise; D, chick. {Con-
tinued on next page.)

EXISTING ORGANISMS— EMBRYOLOGY

53

Fir;. 31. — (continued). E, hog; F, calf; G, rabbit; H, human. (From
Romanes' Darwin and After Darwin, after Haeckel, with the permission
of the Open Court Pubhshing Company.)

54

EVOLUTION AND GENETICS

relatively simple. The head contains no bony shell encasing the
brain but does include a mass of cartilage of peculiar form in which
the brain rests. This structure is called the chondrocranium (Fig.
35). The vertebral column consists of a series of vertebrae with
hollow ends, hour-glass-shaped in longitudinal section, between

Eeproductlve ,\V
ceU

— Stlgmar

'» Contractile
vacuole

Hacrogametes -

Microgametes

Fig. 32. — Volvox globator, a colonial unicellular organism. A, a sexually ripe
colony showing reproductive cells in various stages; B, a portion of the edge
of the colony highly magnified. (From Hegner, after Bourne and Kolliker.)

which small remnants of the notochord persist. Associated with
the head and pharyngeal region is the visceral skeleton, consisting
of the upper jaw (pterygo-quadrate cartilage), lower jaw (Meckel's
cartilage) and six pairs of cartilages behind the mouth, the first
constituting the hyoid arch and the remainder supporting the
gills and called branchial arches. The appendicular skeleton con-
sists of two horseshoe-shaped pieces of cartilage in the pectoral
and pelvic regions respectively, each bearing a pair of fins based on
cartilage supports.

EXISTING ORGANISMS— EMBRYOLOGY 55

Higher Fishes. In the higher fishes similar parts appear, but
the cartilage is largely replaced by bone. To the chondrocranium,
which also becomes bony, are added other bones which form a dorsal
shell to enclose the head (Fig. 36) . These bones are called dermal
bones, and are not formed of cartilage at any stage. The vertebral

^

^/^^v

Mouth

^

1 Hypostome

Tentacle.

■" >^^^

^

1 !

ematocyst

5

^^

M

^

...^^ .A

Y/QSs^ /^JLjgy ^^at Hypostome

Gastrovascular cavity t^^Ctt^ Ptch^p «^«^ltr^ ...

^aaj/ir^^ Al^^^T J^ ^ Tentacle

Polar bodies ^r-''^^^0 /-^^^ .^^^^^^^

Mature egg -|? ^ l~~'Z) W»>i3j /"/K-Xr^xif^

Blastula ,

Gastrula 1

-Young eggs

■ Basal disc

Fig. 33. — A longitudinal section of Hydra. Not all of the structures shown
occur on one animal at the same time. (From Hegner.)

column may remain much the same in form, although ossified.
The jaws, like the skull, are enclosed and strengthened by dermal
bones, and similar additions to the pectoral girdle occur. The
chief differences are the general tendency to ossification of the
cartilaginous structures and the addition of dermal bones.

Other Classes. As the embryo develops in classes above the
fishes, the first evidence of skeletal structure after the notochord
is the formation of cartilages in various regions. In the head, for

56

EVOLUTION AND GENETICS

example, a mass of cartilage appears similar to the chondrocranium
of the dogfish. This mass later ossifies to form most of the occipi-
tal, sphenoid, ethmoid and temporal bones, to which are added
the other bones of the skull by development directly from mem-
branous regions, without a primary cartilaginous stage (Fig. 37).

Spinal cord

Mesodermal segment

Entoderm

Coslom

Fore-gut
L. vitelline vein-
Splanchnic mesoderm

Posterior cardinal vein

Splanchnic mesoderm
Liver anlage

Vitelline vein

Fig. 34. — Sections of vertebrate embryos,
of a chick embryo of two days.

A, section through the hver anlage
{Continued on next page.)

The vertebrae arise first as a series of hour-glass-shaped struc-
tures (Fig. 38), surrounding and finally obHterating the notochord
as they take on the definitive form of the adult. The visceral
skeleton remains largely cartilaginous, and contributes to the
formation of the larynx and upper part of the trachea, the bones
of the middle ear, and as in the fishes, the hyoid apparatus and
lower jaws. The last is a striking duplication of the state illus-
trated by the various fishes. In early embryos sections of the
lower jaw show a rod of cartilage on each side. This is Meckel's
cartilage which persists as the sole support of the mandible in the
sharks. In slightly older embryos a condensation of mesenchymal

Spinal ganglion

Spinal nerve

Spinal cord

Posl. cardinal vein
Mesonephros — -^

Pleural cavity
Upper limb bud

Liver

Esophagus
Right king

Coronary appendage oj
liver

Vena cava inferior

yotochord

Dorsal aorta
Esophagus

Bifurcation of trachea
Inferior vena cava

Pleural cavity

Pleuro-peritoneal
membrane

Phrenic nerve in septum
transversum

Fig. 34. — (continued). B, section through the Hver and anterior Hmb ])uds of
a 10 mm. pig embryo; C, section tlirough hver and one limb bud of a 10 mm.
human embryo (after Prentiss). The notochord is visible in C but is not
labelled; compare with B. (From Arey's Developmental Anatomy, with the
permission of the W. B. Saunders Company.)

57

58

EVOLUTION AND GENETICS

Fig. 35. — Diagram of an early elasmobranch chondrocranium in side view;
the brain outlined, al, alisphenoid plate; bp, basal plate; gc, gill clefts;
h, hyoid; hm, hyomandibular; /, upper labials; U, lower labials; ?n, Meckel's
cartilage; nc, nasal capsule; oc, otic capsule; oi>, occipital vertebrae; ptgq,
pterygoquadrate; si, suspensory ligaments; sp, spiracle; tr, trabeculae; v,
vertel)rae; I-VII, visceral arches; 1-5, branchial arches. (From Kingsley's
Comparative Anatomy of Vertebrates, with the permission of P. Blakiston's
Son & Co.)

Fig. 36. — Dorsal (A) and ventral (B) views of the skull of Cryptobranchus
allegheniensis, a primitive salamander. Dermal bones: PMX, premaxil-
lary; MX, maxillary; N, na.sal; F, frontal; PrF, prefrontal; P, parietal;
Sq, squamosal; Pt, pterygoid; VP, vomero-palatine; PB, paraliasal. Carti-
lage bones: OS, orbit osphenoid; Q, quadrate; ExO, exoccipital; Op, opercu-
lum. Other parts: Col, columella; Nas, nasal capsules; ec, eye capsule; ot,
otic capsule. In both figures the dermal bones are removed from the left
half. (From Wilder's History of the Human Body, with the permission of
Mrs. H. H. Wilder and Henry Holt and Company.)

EXISTING ORGANISMS— EMBRYOLOGY

59

tissue appears near this cartilaso, bone spicules form in it, and a
definite bony structure makes its appearance. This bone envelops

3ftS!'-0CCfPiTrtL, jJ

Fig. 37. — Diagram of the bones of the mammalian skull. Cartilage bones
dotted, membrane bones lined. 2-12, nerve exits. (From Kingsley\s
Comparative Anatomy of Vertebrates, with the permission of P. Blakiston's
Son & Co.)

Meckel's cartilage (Fig. 39), and finally replaces it, but for a time
a relationship exists between the two similar to that in the bony
fishes. In the appendicular skeleton a similar change takes place
during embryonic development,
but most of the bones are pre-
formed in cartilage. The clav-

^otochordal sheath with
^, invading cartilage

icle in the pectoral girdle of
man is added as a membrane
bone. In the pelvic girdle the
most striking changes arc due to
the increased stresses incidental
to terrestrial life ; one of them is
the connection of this girdle
with the spinal column by the
two dorsal bones, the ilia.

The Circulatory System. Fishes.

Extent of one vertebra
Fig. 38. — Diagram of a longitudinal
section through a developing verte-
I)ral column to show the invasion of
the notochord by the cartilages from
which the centra of the vertebrae
develop. (From Woodruff, after
Walter.)

Although any organic sys-
tem in the body shows similar evidences of relationship, the circu-
latory system surpasses all others in its completeness. Returning
to the fishes again for the primitive type, we find a heart consist-

60

EVOLUTION AND GENETICS

B

ing of two fundamental chambers, but preceded and followed by-
two others (Fig. 41A, B). The blood enters this heart from the
body through several large veins which join the posterior cham-
ber, the sinus venosus. From the sinus venosus it passes into the
atrium, thence into the ventricle whose muscular walls contract
rhythmically and drive the blood out to the body. As it leaves the
ventricle it passes first into the conus arteriosus, which tapers into
the truncus arteriosus, a part of the tubular portion of the circu-
latory system. In some fishes
a muscular bulbus follows the
conus. From the truncus sev-
eral pairs of afferent branchial
arteries are given off (Fig. 43B)
— five in some existing fishes — ■
which pass into the branchial
arches and Ijreak up into capil-
laries wherein the blood, laden
with wastes, gives up its car-
bon dioxide and receives a
fresh supply of oxygen through
the delicate tissues of the gills.
It is then collected into other
Fig. 39. — Section of the jaw of an em- arteries, the efferent bran-
bryo kitten A Meckel's cartilage; chials, which unite to form the
B, bone trabeculae of the developing , , , i i • i ,i i

mandible. From a photomicrograph, dorsal aorta behmd the bran-
chial region. Through this
vessel and its branches the pure blood is carried to all parts of the
body and distributed to the tissues by another system of capil-
laries (Fig. 40).

After it has served the tissues the blood is collected again, this
time into veins. The veins from the caudal region return their
blood to the kidneys, where it is again distributed in capillaries.
Such a system is called a portal system. Blood from the alimen-
tary tract is conveyed in a similar manner to the liver, and from
these organs other veins convey the blood to the heart, the pos-
terior cardinals that from the kidneys, and the hepatic vein that
from the liver. These portal systems are called the renal and
hepatic portal systems, respectively (Fig. 40). Blood from the
body is also returned to the heart directly through other veins.
In the fish, therefore, we find that the muscular contractions

61

62

EVOLUTION AND GENETICS

of one chamber force the blood through a single cycle about the
body and back again to the heart, with a maximum of three and
a minimum of two capillary systems interpolated in the various
divisions of this cycle.

Terrestrial Vertebrates. In all vertebrates above the fishes air
breathing is the rule, and in the very lowest of these classes,
the Amphibia, another cycle is interpolated. Here we find a
three-chambered heart, the atrium divided into two auricles, but

Fig. 41. — Stages in the development of the vertebrate heart, as illustrated by-
various classes. A, elasmobranch fishes; B, teleost fishes; C, amphibia; D,
lower reptiles; E, alligator; F, birds and mammals, a, atrium; ao, aorta; b,
bulbus arteriosus; c, conus; cd, duct of Cuvier or common cardinal vein;
h, hepatic veins; pa, pulmonary arteries; pc, precaval and postcaval veins;
pv, pulmonary veins; s, sinus venosus; sa, interatrial septum; v, ventricles.
(From Kingsley's Comparative Anatomy of Vertebrates, with the permission
of P. Blakiston's Son & Co.)

the ventricle still a single chamber (Fig. 41C). The blood is
driven out through the truncus arteriosus into a series of paired
vessels, aortic arches, but they are only three in number (Fig.
43C). The posterior pair leads to the lungs and skin, both of
which serve as respiratory organs in this class. In the Amphibia
which have no gills, no capillary system is interpolated in the aortic
arches but the blood is aerated by being forced to the lungs or skin.
A limited portion of the body is therefore burdened with a function
dealing with all of the blood, and hence a second cycle is established
so that blood now courses from right auricle to ventricle to lungs
and back to the heart, where it enters the left auricle and is then
passed into the ventricle and pumped to the bod^^ This results

EXISTING ORGANISMS— EMBRYOLOGY

63

A trium

{behind ventricle)

trial
canal

Ventricle

D

Intervent.
I sulcus

Ventricle

Fig. 42. — External form of the human heart during development. A, from
an embryo of 2.15 mm; B, 3 mm; C, 4.3 mm; D, 10 mm. (From Arey's
Developmental Anatomy, after His, with the permission of the W. B. Saun-
ders Company.)

64

EVOLUTION AND GENETICS

in some mixing of pure and impure blood in the one ventricle, but
apparently maintains a fairly effective separation. In the reptiles
the two cycles are almost completely separated by the subdivision
of the ventricle into right and left chambers (Fig. 41D and E),
while the aortic arches are similar to those of the Amphibia (Fig.

D

E

Fig. 43. — Diagram showing the fate of the six pairs of aortic arches in the
vertebrate classes. A, the primitive condition; B, fish;.C, ami)hil)ian (frog);
D, reptile; E, bird; F, mammal, a, dorsal aorta; b, ventral aorta, leading
from heart; c, internal carotids; (/, external carotids; e, e', right and left
aortic arches;/, pulmonary arteries; g, g', subclavian arteries to fore limbs.
(From Woodruff.)

43D). In the birds and mammals the two cycles are definitely
separated (Fig. 41F), and the great arch which carries blood back
to the body is no longer paired. In the birds the left arch of this
pair disappears, and in the mammals the right is eliminated, so
that in the one class blood is conveyed to the body through a
great aorta deztra, and in the other through a similar aorta sinistra
(Fig. 43E, F).

EXISTI XG ORGANISMS— EMBRYOLOGY

65

In the embryos of birds and mammals the heart is at first a
simple tube, as in such simple chordates as Amphioxus. The first
dill'crcntiation of this tube is its separation into four regions
similar to those found in the hearts of adult fishes, viz., the; sinus
venosus, atrium, ventricle and Imlbus arteriosus. Later the first
and last are absorlwd into the adjacent regions, the auricle is
divided by a partition, and finally the ventricle is similarly divided,

post, cardinal
vein

coclom

intermediate
mesoderm

coelom

aephrostoxn&

glomus

notochord

somite

dorsal aorta

post, cardinal
vein

ncphrostome

coelom

somite

dorsal aorta

capsule

D

mesonepbric
tubuie

mesonepbric
duct

ncphrostome
' gfomerulus

Fig. 44. — The structure of nephric tubules. A, pronephric tubule from a
16-somite chick embryo; B, diagram of functional pronephric tubule; C,
primitive mesonephric tubule with rudimentary nephrostome, from a
3()-somite chick embryo; D, diagram of functional mesonephric tubule of
the primitive type. (P'rom Patten, A after Lillie, B and D after Wieder-
sheim; with the permission of P. Blakiston's Son & Co.)

SO that the organ goes through a transition approximating that
represented by the several classes (Fig. 42).

While the heart is developing, a series of aortic arches appear,
six pairs in all, which are at first symmetrical and unlike those of
the fish chiefly in the lack of capillaries (Fig. 29). As the ventricle
divides, the truncus arteriosus splits so that the separation of that
portion leading into the last pair of arches, and thus to the lungs,

66 EVOLUTION AND GENETICS

is isolated with the right ventricle, while the remainder carries
blood from the left ventricle into the other arches. Of these the
fifth pair is never large and soon disappears, along with the first
and second. The third pair, and its connections with the first and
second, persists to carry blood to the head, while the fourth differs
in development in the birds and mammals, but develops into the
great aorta in each class. The approximation of the adult struc-
tures of lower forms is not limited to these parts of the circulatory
system, for the veins also show a gradual transition, but the heart
and aortic arches are no less striking, and are more easily under-
stood.

The excretory system develops in vertebrates from paired
mesodermal masses, called the nephrotomes, extending from the
cervical to the caudal region. In its primitive form it consists of a
series of tubules opening into the coelom. By ciliary action these
tubules carry wastes from the coelom to the exterior. The pres-
ence of a knot of arteries, or glomerulus, projecting into the coelom
near each tubule suggests an association with the circulatory
system (Fig. 44). Such structures as these arise from the anterior
part of the nephrotomes, and make up a pair of bodies called the
pronephroi, or anterior kidneys. The association of the excretory
tubules with the coelom is soon supplanted by an intimate asso-
ciation with the glomeruli, so that wastes are removed directly
from the blood (Fig. 44B). A second pair of kidneys made up of
such structures, arise behind the first, and are called the meso-
nephroi. These become the functional kidneys of adult fishes and
amphibia, while the pronephroi occur only in embryos and larvae.
At a later stage of development the mesonephroi are supplanted
by a third pair of excretory organs, the metanephroi. These are
found only in reptiles, birds and mammals, which develop during
their ontogeny first rudimentary pronephroi, then functional
kidneys of the embryo, mesonephroi, and finally the metanephroi
which are to persist throughout life.

Nothing short of a thorough study of embryology can ade-
quately disclose the marvelous resemblances which occur one after
the other as the embryos of vertebrates pass through the successive
stages of development. In the nervous system, the respiratory
system, the development of the pharynx, and in countless other
details of structure involving various systems these indications of
relationship of the several classes are present. In presenting this

EXISTING ORGANISMS— EMBRYOLOGY G7

brief account the most conspicuous examples have been selected
and shorn of all unnecessary details. Such an account is neces-
sarily imperfect, but in this field even scattered facts are striking.
Summary. The relationship of vertebrates is strikingly evident
in their embryology. A study of the developmental stages of the
various classes shows that animals pass through similar steps up
to the point where the divergence of adult structure appears.
Even in the initial stages of cleavage and gastrulation modifica-
tions occur, but these are incidental to the storage of yolk in the
ovum and do not destroy the homologies. After gastrulation such
structures as the neurenteric canal and the foetal membranes show
other definite relationships. Not only do the embryos resemble
each other, but the embryos of higher classes also pass through
stages similar to the maximum development of classes below them.
The early stages of embryonic development are similar to some of
the invertebrates. Such structures as the notochord, the skeleton,
and the circulatory and respiratory systems show a gradual transi-
tion in adults from the cyclostomes to the mammals which is also
followed during ontogen3\ The entire development of the indi-
vidual is a story of relationships which are illustrated by these
selected examples.

REFERExXCES

Newman, H. H., Vertebrate Zoology, 1920.

Wilder, H. H., History of the Human Body, revised edition, 1923.
McEwEN, R. S., A Textbook of Vertebrate Embryology, 1923.
Arey, L. B., Developmental Anatomy, 1924.

CHAPTER V

THE RELATIONSHIP OF EXISTING ORGANISMS

(Continued)

3. COMPARATIVE ANATOMY OF VERTEBRATES

The preceding consideration of embryological relationship has
necessarily touched upon some of the salient features of compara-
tive anatomy. The transition of skeletal development, the struc-
ture of the heart in the several classes, development of the aortic
arches and the venous system may as well be treated in one field
as in the other, since they cannot be made clear without reference
to both. Comparative anatomy discloses, however, a great many
details of homology without reference to development. Many
vestigial structures which anatomists have found in man, for
example, are shown to have the same relations as functional
structures in the bodies of other animals, while the functionally
active systems are built up of the same tissues and organs, arranged
according to the same plan. While the similarity of functional
parts is in itself significant, the occurrence of vestigial organs,
particularly those which are found only in occasional individuals,
is doubly so. We may content ourselves with necessity as a
reason for the presence of useful organs, but obviously useless
structures can be explained only on a very different basis.

The Skull. In the skulls of vertebrates from the fishes to the
mammals a large number of bones are found, some present in all
forms, some in only the lower forms. We have already noted that
these are of two different kinds, those originating in cartilage as
parts of the chondrocranium and those which develop directly
from the embryonic mesoderm. The latter are of particular
interest at this point because of the completeness of their history
as shown by existing forms. The chondrocranium, or primordial
skull, is well developed in the elasmobranch fishes as a supporting
structure extending forward from the spinal column beneath the
brain, which it does not enclose dorsally or anteriorly except by
the development of secondary membranous structures. The same
primordial skull develops in forms above the elasmobranchs, but

68

EXISTING ORGANISMS— ANATOMY

69

to it is added the series of dermal bones which encloses the brain
on all other sides and form the greater part of the skull (Fig. 37).

The Ganoid Stage. The stage in which the dermal bones of
the skull are well developed is nicely represented by existing bony
fishes, or ganoids, and hence is called by Wilder the ganoid stage.
The sturgeons illustrate the origin of these bones as dermal plates,
or scutes, which are derived like scales
from the corium, the under layer of
the skin, and instead of forming an
inner bony box, remain an outer bony
armor (Fig. 45). Wilder describes
these scutes as follows: "The snout,
or rostrum, is covered by a series of
small rostrol plates, which extend back
as far as the nostrils; back of these
openings may be found a pair of nasals;
behind these again, and between the
eyes, is a pair of frontals, often ac-
companied by prae- and post-frotitals.
Behind these is a pair of parietals,
and one or more supra-occipiials. On
the sides of the head, at about the
level of the parietals, are the squamo-
sals, and around the eye are several
orbitals, distinguished as pre-, supra-,
post-orb itals, etc. The operculum, or
gill-flap, which is present in these
fishes, is covered and augmented by
supra-, sub-, and pre-operculars."

The occurrence of bones similar in
arrangement to these bony scutes is
dependent to some extent, of course,
on their functional importance in the
various fishes. Terrestrial animals, for
example, would not be expected to
have bones corresponding to the opercular series, and the shorten-
ing of the face and great enlargement of the brain in man must
necessarily be accompanied by differences in the skeletal parts
involved. Due to these variations in importance, exact duplica-
tion in widely different species is not found.

Fig. 45. — Dorsal view of the
skull of a sturgeon {Acipen-
ser), showing dermal bones.
ROS, rostral plates; N, nasal;
F, frontal; PrF, pre-frontal;
Post Fr, post-frontal; Post
Orb., post-orbital; P, par-
ietal; SQ, squamosal; OP,
opercular; OCLa, lateral oc-
cipital; SO, supra-occipital;
SCL, supra-clavicle. (From
Wilder's Hislory of the Hu-
man Body, with the permis-
sion of Mrs. H. H. Wilder
and Henry Holt and Com-
pany.)

70 EVOLUTION AND GENETICS

Amphibia. In the skulls of Amphibia the dermal bones are no
longer external as in some fishes at any stage of development, but
have become definitel}^ incorporated with the cartilage bones as
parts of the internal skeleton. In addition the characteristically
piscine elements, like the rostrals, the orbitals and those associated
with the operculum, have been lost, and the remaining bones

Fig. 46. — Dorsal view of schematic skull, the chondrocranium dotted, mem-
brane bones outlined, premax, premaxilla; pref, prefrontal; postfr, post-
frontal; postor, postorbital; squamos, squamosal; quju, quadratojugal; inp,
interparietal; exocci, exoccipital; supratem, supratemporal; supraoc, supra-
occipital; other names in full. (From Kingsley's Comparative Anatomy of
Vertebrates, with the permission of P. Blakiston's Son & Co.)

more nearly approximate the number and relationships of the
higher terrestrial forms. We find among them a pair of small
bones behind the anterior nares, behind which a similar pair of
larger bones reach the orbits on the sides. Still another median
pair lie behind the orbits. The first correspond in their orientation
with the nasals of the fishes, and are called the nasal bones. The
next are the frontals, representing these and some of the smaller
scutes of the sturgeon and the last are the parietal bones. A pair of
prefrontal bones lie behind the nasals and lateral to the frontals,
as in the fishes. Caudad this portion of the skull is associated,
through the supra-occipital, with the basal or occipital part of

EXISTING ORGANISMS— ANATOMY

71

the chondrocranium, from which develops the occipital bone,
Laterad and ventrad other bones occur, but these are primarily
visceral in their associations (Figs. 36 and 46).

Above the Amphibia. In the reptiles, birds and mammals the
same essential parts and relationships persist, with some slight

./-Ai-^

Fig. 47. — Morphology of ribs, a, ganoid fish; b, dipnoid fi.sh; c, teleost fish;
d, shark; e, Polypterus, a special case among ganoids; f, urodele a^nphibian.
In the first three the condition in trunk (left) and tail (right) is given. In
all figures the "fish rib" is striped and the true rib is black. (From Wilder's
History of the Human Body, after Wiedersheim, with the permission of
Mrs. H. H. Wilder and Henry Holt and Company.)

variation in the accessory frontals and in the proportionate sizes
of the various bones.

The Human Skull. The skull of man differs in a few conspicuous
points, although it is in general similar. The nasals persist as

72 EVOLUTION AND GENETICS

small, paired bones, which may fuse to form a single bone. The
frontals also are ordinarily fused to form a single large bone, but
in a very small percentage of cases they remain separate. The
supraoccipital is usually fused with other elements in the occipital
bones, but sometimes persists as a separate bone between the
parietals and occipital, as is normally the case in some other
mammals. Both frontals and parietals are proportionally much
greater in size than in the lower animals because of the large size
of the brain, which they enclose.

The Spinal Column. In the remainder of the axial skeleton
relationships are quite obvious. Vertebrae in all of the classes
above the Cyclostomes consist of a solid centrum from which a
dorsal neural arch arises, enclosing and protecting the spinal
cord (Fig. 47). This arch is surmounted by a spinous process
which furnishes attachments for muscles. Above the fishes the
neural arch bears two anterior and two posterior articular processes
which aid in preserving a firm articulation of successive vertebrae,
and a pair of transverse processes. The centrum may also bear
short transverse processes and a ventral arch, called the haemal
arch, which is well developed in the fishes and forms a conspicuous
appendage of some reptilian vertebrae. The haemal arch termi-
nates in a haemal spine.

Ribs in many fishes are merely the halves of incomplete haemal
arches. In some fishes, amphibians and reptiles, ribs of this type
are found attached to the same vertebrae that bear other ribs,
more dorsal in position (Fig. 47e, f). The former are commonly
called fish ribs, and the latter true ribs. True ribs in their typical
form have two heads, one of which articulates with the lateral
process of the centrum, and the other with that of the neural
arch.

The Sacrum. Terrestrial animals have the pelvic girdle
attached to the spinal column, and one or several vertebrae are
modified for this attachment (Fig. 48). In Necturus, a urodele
amphibian, only one vertebra is involved, usually the 19th, some-
times the 20th, and rarely the 18th, although obUque attachments
are on record in which the pelvic girdle joined the left side of
one of these vertebrae and the right side of another. Such verte-
brae are called sacral vertebrae, and when more than one is
involved a fusion often occurs, resulting in the development of
a composite bone, the sacrum. Wilder notes that "this anchylosis

EXISTING ORGANISMS— ANATOMY

73

of adjacent sacral vertebrae is the most complete in birds and in
man, and for the same reason, namely the employment of the
hind limbs alone for the support of the body, although in the two
cases the number and arrangement of the associated parts differ
very considerably." He adds that "variation in the sacral region
is not confined to the lower forms, although it is more frequent
in these latter (e.g., Necturus) and becomes relatively stable in
the higher and more specialized classes."

The Visceral Skeleton. In the visceral skeleton and associated
membrane bones relationships are in some cases more obscure,
but as worked out by comparative anatomists and checked by

Fig. 48. — Variations in the composition of the human sacrum. (From Wilder's
History of the Human Body, after Gegenbaur, with the permission of Mrs.
H. H. Wilder, and Henry Holt and Company.)

the facts of embryology, they are well established and remarkable
(Fig. 49).

The Jaws. The upper and lower jaw cartilages (pterygo-
quadrate and Meckel's, respectively) have already been mentioned.
These belong to the visceral skeleton, and in the elasmobranch
fishes are the only skeletal structures about the mouth. In higher
groups each is associated with membrane bones which finally
supplant it entirely as the skeleton of a jaw. The membrane
bones of the upper jaw include the anterior premaxillaries, which
lie at the tip of the skull, just below the anterior nares. Behind
them are the larger maxillary bones, followed along the outside
of each cartilage by a zygomatic (malar or jugal), a quadratojugal
and a squamosal bone. On its mesial surface it is associated with
an anterior palatine and a posterior pterygoid which form part
of the roof of the mouth. The posterior part of the cartilage
itself gives rise to the quadrate bone which sometimes intervenes

VII "

'■"^yuV^

Tim IV V vn'^^viii'

Fig. 49. — Morphology of. the visceral skeleton. A, shark; B, amphibian;
C, reptile; D, mammal; the successive arches are indicated by Roman
numerals; the dorsal and ventral parts of arches I and II are indicated by
exponent letters, d and v. lb, labial cartilage; s, spiracular cartilage; o,
operculum; VII -^ and VII ^ are the arytenoid and tracheal cartilages, re-
spectively. (From Wilder's History of the Human Body, with the permis-
sion of Mrs. H. Hs Wilder and Henry Holt and Company.)

74

EXISTING ORGANISMS— ANATOMY 75

between the lower jaw and the skull. The changes in the lower
jaw involve in the lower classes the addition of an outer series
of bones including the anterior dentale, which usually bears teeth,
followed by a splcnial and angulare, above which lies the surangu-
lare. The coronoid runs back from the dentale above the surangu-
lare. The posterior end of the cartilage itself may develop into
an articular bone by which the jaw is articulated with the quadrate.
All of these parts are present in the existing reptiles.

The modification of these bones in the mammals leaves the
upper jaw with the same parts, excepting the quadrate bone.

Fig. 50. — Pectoral fin and girdle of dogfi.sh. s, scapula; ss, .suprascapula; c,
coracoid; p, propterygium; ms, mesopterygium; mt, metapterygium; rad,
radials. (From Wilder's History of the Human Body, with the permission
of Mrs. H. H. Wilder and Henry Holt and Company.)

The lower jaw consists of two mandibles, firmly united in man to
form a single bone, consisting of the dentales and possibly the
splenial and coronoid bones. Other homologies are obscure, but
the angulare is said by Kingsley to be apparently the tympanic
bone of the skull. The articulation of the lower jaws with the
skull is thus shifted in the mammal to the mandible (dentale?)
and squamosal. The articulare and quadrate, freed from this
function, are found in the middle ear, the former being certainly
the malleus, while the quadrate is possibly the incus.

The remaining parts of the visceral skeleton form the hyoid
apparatus and embrace the branchial region in the fishes, furnishing
support for the gill arches, while in terrestrial classes their first
function continues and they are otherwise represented by the
cartilages of the lar>'nx and upper part of the trachea.

The Appendicular Skeleton of Fishes. The appendicular
skeleton is simplest in the fishes. Since their bodies are buoyed
up at all points by the heavy medium in which they live, the

76

EVOLUTION AND GENETICS

pectoral and pelvic fins and the girdles to which they are attached
do not require the strength and rigidity of supporting structures.
In the primitive elasmobranchs the pectoral girdle consists of a
V-shaped piece of cartilage (Fig. 50) to which is articulated a
large basal cartilage of the fins, the mesopterygium. To the
mesopterygium are attached two other pieces, the propterygium
and metapterygium. These basal cartilages, of which only one
is found in the pelvic fins, bear a fan-like series of radial cartilages
to which the thin terminal portion of the fin is attached. The
V-shaped cartilage obviously consists of the ventral portion that

Fig. 51. — Right pectoral fin of Sauripterus laylori from the Upper Devonian,
cl, cleithrum; co, coracoid; cv, clavicle; sc, scapula; s.cl., supracleithrum;
H, humerus; R, radius; U, ulna. (From Lull, after W. K. Gregory.)

runs between the fins and a dorsal portion extending dorsad from
each fin. Each dorsal piece is surmounted by another small
cartilage. The ventral part is called the coracoid, the dorsal piece
the scapula, and the ultimate dorsal cartilage the suprascapula.

Transitional Fins. Homologies of the structures described in
the preceding paragraph are not entirely clear, but in one group
of bony fishes, the Crosso-pterygii, a significant modification of
structure and use of the pectoral fins is found. These lobe-finned
ganoids have the strange habit of resting on the bottom of the
water in which they live, and supporting themselves by the front
fins in a manner distinctly similar to the use of the front legs
of the terrestrial organisms.

The fins of some extinct forms of Crossopterygii are well pre-
served, and in one of these, Sauripterus taylori, from the Upper
Devonian, the entire skeletal structure of the pectoral girdle and
fin is shown (Fig. 51). In this species it is at once evident that

EXISTING ORGANISMS— ANATOMY

77

the principal bony framework of the fin is not unhke that of the
appendages of terrestrial vertebrates. The fin is articulated to
the girdle by a single basal bone, to which are attached two other
bones, and following these is a series of bones of less regular
arrangement. The fringe of the fin is, of course, a structure
adapted to aquatic life, and hence plays no part in comparison
with terrestrial forms, but the rest of it foreshadows both in struc-
ture and use the conditions found in higher animals.

The Appendicular Skeleton of Terrestrial Vertebrates. Above
the fishes we have to deal only with terrestrial animals and those

c ^:::^'

t

Fig. 52. — Diagram of girdles and appendages from the posterior side; upper
letters, fore limb; lower, hind limb. «, acetabulum; c, carpus; co, coracoid;
/, femur; _^, fibula; g, glenoid fossa; /;, humerus; i, ilium; is, ischium; ss supra-
scapula; mc, to<, metacarpals and metatarsals; p, pubis; pc, procoracoid; ec,
epicoracoid; p/i 1-3, phalanges; r, radius; s, scapula; i, tarsus; Ih, tibia; w,
ulna; I-V, digits. (From Kingsley's Comparative Anatomy of Vertebrates,
with the permission of P. Blakiston's Son & Co.)

which have become secondarily aquatic, of which there are exam-
ples in all classes except the birds. Among birds aquatic habits
are always associated with terrestrial and usually also aerial life.
Throughout these four classes the girdles still consist of a ventral
part extending between the appendages, and lateral parts extending
dorsad, which may be attached to the spinal column, as in the
pelvic, or supported by muscles as in the pectoral girdle (Fig. 52).
The Pelvic Girdle. The pelvic girdle is regarded as probably
more conservative (Fig. 53). It consists in the primitive sala-
mander, Necturus, of a broad ventral plate of cartilage, bearing
two bones, the ilia, running dorsad. The acetaljulum, a cavity
in which the hind limb articulates, is located at the junction of
the two parts. At the posterior angles of the cartilage plate
are two centers of ossification. In higher Amphibia two anterior

78

EVOLUTION AND GENETICS

centers also appear. The bones formed from these four centers
are the anterior pubic bones and the posterior ischia, between

Fig. 53. — Series illustrating a theory of the development of the pelvic girdle,
a, sturgeon, Acipenser; b, a ganoid fish, Scaphyrhynchus; c, a ganoid, Polyp-
ierus; d, a primitive salamander, Necturus; e, a South African frog, Daclyl-
elhra; f, turtle. In a the part m is formed by a fusion of the anterior rays.
The pieces kk, segmented off from m in b, form in c a rhomboidal plate.
In d this i)late has grown large and bears a pair of ossified ilia, i, and a pair
of centers of ossification, the is(;hia, h. In e two more centers of ossifica-
tion, the pubes, g, have appeared. ' f, is a t /pical pelvic girdle with all its
parts. The epipubis, e, is incidental and unimportant. (From Wilder's
History of the Human Body, with the permission of Mrs. H. H. Wilder and
Henry Holt and Company.)

which in reptiles develop the obturator foramina which sometimes
join to form a single large opening. While pronounced modifica-
tions of these bones occur in higher forms, such as the ventral
separation of the ischia in man and of both ischia and pubes in

EXISTING ORGANISMS— ANATOMY

79

birds, and the broadening of the ilia in man to form the basin-
shaped pelvis correlated with his erect posture, the three bones
retain characteristic fundamental relations.

The Pectoral Girdle. This girdle is further modified. The
scapula remains in Amphibia, surmounted by the cartilaginous
supra-scapula, as part of the girdle extending dorsad from the
anterior limb. The coracoid develops a pair of distinct bones,
corresponding to the ischia of the pelvic girdle, while anterior
cartilaginous strips, the procoracoids, separated from these bones
by the coracoid foramen, correspond to the pubes. Three mem-
brane bones, the clav-
icle, interclavicle and
cleithrum, are associ-
ated with the cartilag-
inous parts, but the
two last are rare above
the fishes (Fig. 54).
A median element, the
interclavicle, is some-
times present between
the ends of the clav-
icles, and epicoracoid
cartilages may join the
clavicles and coracoids.

Fig. 54. — Diagram of the shoulder girdle of a
primitive reptile, showing the complete series of
elements found in the vertebrates above the
fishes. Dermal bones: CLTH, cleithrum; CL,
clavicle; IC, interclavicle; Cartilaginous ele-
ments: PC, procoracoid; C, coracoid; S, scapula.
(From Wilder's History of the Human Body,
with the permission of Mrs. H. H. Wilder and
Henry Holt and Company.)

The coracoid and scapula at least take
part in the formation of the glenoid cavity, in which the fore
limb is articulated. The ventral parts are complicated by associa-
tion with the median epicoracoid cartilages and with the sternum.
Of these bones the dorsal scapula persists as a more or less blade-
like bone, the shoulder blade of man. All ventral parts are found
in reptiles and both coracoid and clavicle in birds, but only the
clavicle in man. In the primitive Prototheria all parts of the
primitive girdle are found, and in some of the higher mammals
the scapula is the only bone present (Fig. 55).

The Pentadactyl Appendage. The appendages of terrestrial
forms, both pectoral and pelvic, are invariably attached to their
respective girdles by a single bone, the humerus of the anterior
limb and femur of the posterior. To these are articulated two
bones, the inner radius and tibia, and the outer ulna and fibula
of the anterior and posterior limbs respectively. These are fol-
lowed by a series of small bones, the carpals and tarsals, which

80

EVOLUTION AND GENETICS

include a proximal pair, a distal transverse row of five, and two
median in position. The five distal bones are followed by five
long bones, the metacarpals and metatarsals, and these by five
series of shorter phalanges which form the skeleton of the digits.
This primitive type is called the pentadactyl appendage (Fig. 52).
Specialized Appendages. Modifications are present in such
highly specialized structures as the wing of the bird and the foot

Fig. 55. — Sternum and shoulder girdle of mammals, a, the duck-mole,
Ornilhorhynchus, a primitive oviparous mammal; b, human embryo, c, oor-
acoid; d, epicoracoid; e, episternum; f, clavicle; g, scapula; h, suprascapula;
m, manubrium; stb, sternebrae; x, xiphisternum. (From Wilder 's History
of the Human Body, after W. K. Parker; with the permission of Mrs. H. H.
Wilder and Henry Holt and Company.)

of the horse, involving the reduction and fusion of parts, and in
the marine mammals the phalanges are multiplied for the support
of the flippers; but in a large majority of terrestrial vertebrates
the fundamental plan of the pentadactyl appendage is distinctly
traceable (Fig. 56). It is well represented in a primitive state in
the salamander, Necturus, and is not highly modified in lizards
and Crocodilia; in mammals its variations are numerous but often
not extreme. Conspicuous examples of modifications in mammals
are found in the fore limb of the bat, in which the second, third,
fourth and fifth digits are greatly proi nged to support the wing
membrane; in the horse, with its single p Tsisting third digit; and
in the dolphin, which has some digits prolo. i;ed by the multiplica-

81

82

EVOLUTION AND GENETICS

tion of phalanges and others shortened, even to a rudimentary
state.

The Exoskeleton. The integumentary structures of verte-
brates inckide, in addition to the skin itself, glands, claws, hoofs,
nails, horns, teeth, scales, feathers and hair, among which several
homologies are evident.

Placoid Scales. If we return again to the elasmobranchs, we
find that their skin is studded with minute scales of a type quite

a V 1)

1-^ V 1 1 H 1 1 1 1 W

''■•^m

Fig. 57. — Comparison of the development and structure of a placoid scale
and a tooth, a, b, and c represent the scale; d, e, and f represent the tooth.
The corneous layer of the epidermis is dotted, the germinative layer is
represented as a single layer of large cells with nuclei. The dermal layer of
the skin is represented as a filirous layer with scattered cells, x, enamel-
producing cells; y, mesodermal papilla; e, enamel; d, dentine; p, pulp cavity.
(From Wilder's Histori/ of the Human Body, with the permission of Mrs.
H. H. Wilder and Henry Holt and Company.)

different from those of other fishes. These are called placoid
scales, and consist of a flattened base formed of dentine, a fine,
compact bony substance, from which projects an oblique cusp.
The whole is covered by a layer of enamel which is thickest at the
tip of the cusp. A mesodermal papilla projects into the hollow
under surface of the scale and furnishes it nourishment. A study
of the development of these scales discloses that the dentinal
portion is derived from the inner, or dermal layer, of the skin, while
the enamel is formed from the enveloping epidermal layer.

Scales and Teeth. The jaws of these fishes are armed with
numerous teeth arranged in rows, which bear a conspicuous re-

EXISTING ORGANISMS— ANATOMY 83

semblance to the much smaller placoid scales in form. Unlike the
teeth of higher vertebrates, these are superficially attached by a
broad base. Their structure is exactly similar to that of the
placoid scales, consisting of an inner core of dentine surroimding
a mesodermal papilla and covered by a hard enamel layer. In all
respects, these teeth are so like the placoid scales that the transition
of development is almost obvious. They are larger, but since they
are found in a region where transition from skin to buccal epithe-
lium occurs, it is conceivable that even the slight roughness caused
by placoid scales might be useful in holding prey and that useful-
ness might account for the greater development of the teeth.

The teeth of higher vertebrates differ in l^eing deeply seated
in sockets in the bones but when we remember that the bones
which support them are not derived from primitive cartilages like
those of the elasmobranch jaws but are developed from the dermal
layer, this relationship is not surprising. During development
the teeth are differentiated from portions of the ectoderm which
grow into the underlying tissues (Fig. 57).

In structure teeth consist, like the placoid scales and teeth of
elasmobranchs, of an inner layer of dentine surrounding a meso-
dermal core, the pulp, and an outer enamel layer which is thickest
on the exposed points. Cement, a substance which covers the
root and is conspicuous in the teeth of some animals is added by
outer mesench>Tnal tissue. The two principal parts of the tooth
have exactly the same origin as the primitive scales and teeth of
the sharks, for the enamel is epidermal and the dentine, dermal.
Teeth vary greatly in form according to the habits of animals.
Some are pointed for grasping and tearing, others are sharp edged
for cutting, and still others have broad rough surfaces for grinding.
All three types are present in the human mouth. Rodents have
highly developed chisel-like incisors and some snakes have fangs
for injecting poison. Such animals as the anteaters have no use
for teeth, and they are accordingly reduced or absent.

Scales, Feathers, and Hair. The scales of fishes above the elas-
mobranchs have no enamel covering and so are wholly dermal in
origin; nor are they Uke the scales of reptiles and birds, which
consist of epidermal folds, cornified to some extent and nourished
by dermal papillae. The feathers of birds are of exactly the same
origin as the scales of this group and the reptiles, though they are
much more complex in structure. Scales in the mauunals are

84

EVOLUTION AND GENETICS

relatively rare, but are commonly found on the feet and tails of
such animals as rodents, and in the armadillo and a few other

a

I • •«.«

e

• • •

'•'• •

• • •

•'•'•

• '•

' • • .• •

i.y '■:■)

m mm

^ - -r. • *t«

Fig. 58. — Hair pattern in mammals, diagrammatic, a, tail of Myopotamus,
a South American rodent, with scales and hairs; b, back of Midas, a Bra-
zilian monkey; c, back of pig, Sus vittatus; d, back of Coelogenys paca, a
South American rodent; e, back of Dasyurus viverrinus, an Australian
marsupial; f, back of Loncheres cristata, a South American rodent. (From
Wilder's History of the Human Body, after de Meijere; with the permission
of Mrs. H. H. Wilder and Henry Holt and Company.)

mammals the dorsal surface is covered with them. These scales
are also similar in form and derivation to those of the birds and
reptiles, but are usually less horny. Hair is very different from

EXISTING ORGANISMS— ANATOMY

85

scales, but the two are associated in some mammals and the pat-
tern of arrangement persists in some that have lost all trace of
scales (Fig. 58).

Claws, Hoofs, and Nails. These structures are likewise similar
in origin (Fig. 59). The first consist of approximately equal con-
vex dorsal and concave ventral plates in the more primitive birds
and reptiles. Mammalian claws have the ventral plate greatly
reduced and the tip of the toe covered ventrad by a terminal pad
which is scarcely evident in the other groups. In hoofs the dorsal

Fig. 59. — Diagrammatic longitudinal sections through digits of various mam-
mals to illustrate the morphology of claws, hoofs and nails, a, Echidna, a
primitive oviparous mammal; b, a typical clawed mammal (unguiculate);
c, horse; d, monkey; e, man. The dorsal plate is in black, ventral plate
striped, bones stippled. (From Wilder's Hislorij of the Human Body, a after
Gegenliaur, b-e after Boas; with the permission of Mrs. H. H. Wilder and
Henry Holt and Company.)

plate is enlarged and thickened, while the ventral plate is also
extensive and horny, though still a rather soft structure. The
terminal pad is lacking. In the monkeys nails are found which
have a broad dorsal plate extending little if any beyond the tip
of the digit; the ventral plate is reduced to a transverse strip
beneath the tip of the dorsal plate. In man this reduction is carried
still further and the ventral plate is vestigial. The terminal pad
is evident in the primates only by the persistent friction ridges
with definite pattern which occur on the tips of the digits.

Vestigial Structures in Man. For complete homologies of the
remaining systems, reference must necessarily be made to embryo-
logical development, involving greater detail than can be included
here, although the resulting comparisons are as conclusive as those

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86

EXISTING ORGANISMS— ANATOMY 87

already considered. The circulatory system is among the best to
demonstrate relationships, and has already been considered under
embryology. The nervous system is very evidently formed by
the modification of the same primitive parts in the various classes,
as is shown in the figure comparing the brain structure of
vertebrates (Fig. 60). In all systems there are evidences of struc-
tures having persisted beyond their period of usefulness in the
form of vestiges. These are especially interesting in man.

Supernumerary Mammae. Wiedersheim records numerous in-
stances of the occurrence of supernumerary mammary glands.
These glands develop embryonically in a ventrolateral milk line, of
which all traces are obliterated except the normal adult mammae.
In such animals as dogs, cats and pigs which bear several young at
one time, these are several in number and lie in two rows indicat-
ing the position of the embryonic milk line, while in animals
whose young are usually limited to one or two at a birth a more
extreme localization limits the functional adult mammae to a
pectoral pair (Primates, etc.) or a pelvic pair or group (domestic
animals). In rare cases individuals, both male and female, have
in addition to the two normal pectoral teats of man a series of
additional rudimentarj^ teats in a row, as in lower mammals. In
females supernumerary mammae may be functional or mere ves-
tiges, while in males, since the teats are normally vestigial, the
same is true of others which may appear. Wiedersheim cites data,
dealing largely with soldiers, which show a surprisingly high per-
centage of polymasty. One observer whose records he uses re-
corded this condition in more than 5 per cent of cases.

Persistent Hair. This peculiarity has also been recorded in
many individuals, such as Jeftichjeff, the "Russian Dog-Man"
and Julia Pastrana, a woman whose face was almost completely
covered by hair. These cases are probably due to the persistence
of the lanugo, a coat of hair which covers the body of the embryo
but is normally shed before birth and replaced by the restricted
hair of the adult. More significant evidences of relationship of
man with the lower animals are found in the resemblance of the
hair tracts on his body to those of quadrupeds.

The Tail. While the tail in man is normally reduced to a series of
fused vertebrae, the os coccyx, completely enclosed within the
body, it too may develop occasionally as an external appendage.
Wiedersheim cautions against interpreting all such appendages as

88 EVOLUTION AND GENETICS

true tails, but he records two cases in which they contained verte-
brae and were very probably vestigial remnants of the tails nor-
mally found in quadrupeds.

The Third Eyelid. The eye is the seat of another vestigial
structure which normally persists. This is the thin pink fold at the
inner corner called the plica semilunaris, supposed to be a remnant
of the third eyelid of birds and amphibia. The full development
of the structure can easily be seen by watching an owl close its
eyes. As the upper and lower lids approach each other, the third
lid, a filmy, grayish membrane, moves across the eyeball behind
them.

Vestigial Muscles. In a few parts of the human body muscles
persist which are usually useless and only rarely functional, among
them the muscles which move the ears and scalp. These are usu-
ally vestigial, but a few individuals retain the ability to contract
them at will.

The Appendix. One of the most familiar vestiges is the vermi-
form appendix. Essentially the same in structure as the intestine,
it varies in different individuals from a tubular diverticulum to a
solid structure, and in length from three quarters of an inch to
nine inches. It is attached to the caecum, a blind end of the large
intestine which extends beyond the union of that tract and the
small intestine. Both caecum and vermiform process are found in
animals below man, and in some species, particularly herbivorous
animals, the caecum often attains relatively enormous dimensions
and is a correspondingly important organ. According to Wieder-
sheim it may be longer than the entire body, although in Carniv-
ora and several other orders it is reduced as greatly as in man.

Wisdom Teeth. The third molars, or wisdom teeth, are like-
wise vestigial. Although the total number of thirty-two teeth is
less than the primitive number, we are losing four more, as is
apparent from the variable development of the wisdom teeth and
their usual ineffectiveness.

The Significance of Vestigial Structures. In these and many
other structures a condition is visible in man which can be inter-
preted only as the persistence of parts for which he has lost or is
losing all need. Throughout the various organic systems of the
body more or less conspicuous examples of this type of resem-
blance may be found, and in all systems a very evident similarity
of minute structure, arrangement of tissue, and general plan of

EXISTING ORGANISMS— ANATOMY 89

gross morphology. The resulting adaptations often appear to be
but a poor makeshift. They are effective, since their effectiveness
is necessary to existence of the species, but they seem crude in
comparison to organs of other animals which are used in the same
way. Enough examples can be observed among the many species
of animals of very different ways of attaining the same end effec-
tively, such as the wings of birds and insects, to make possible
only one interpretation of the existing conditions of resemblance
among vertebrates, and among invertebrates as well. We can
conclude only that the fundamental similarity of their often dif-
ferent structures in any system is evidence of definite relationship;
that the dolphin's flippers and the human hand have evidently
homologous structure not because such structure is the only
possible foundation for a swimming organ and a prehensile ap-
pendage but because their ancestors were related in possessing
just such a foundation as the pentadactyl appendage, of which
they have made different uses.

Summary. The anatomy of adult animals of the vertebrate
classes shows many points of similarity. All parts of the skeleton
are evidently based on the same plan of structure. The differences
which appear are easily correlated with special habits. Exo-
skeletal structures of different kinds also show fundamental simi-
larity. Relationship of other systems is closely linked with
embryology, but the presence of vestigial organs is significant,
especially in man. Since these organs are useless or nearly so the
only possible explanation of their presence is that they are the
remains of once useful organs which the animal has not yet entirely
lost.

REFERENCES

Romanes, G. J., Danoin and After Darwin, 1892.

WiEDERSHEiM, R., The Structure of Man (translated by Bernard), 1895.

, Vergleichende Anatomie der Wirbeltiere, 7th edition, 1909.

Reynolds, S. H., The Vertebrate Skeleton, 2nd edition, 1913.

Walter, H. E., The Human Skeleton, 1918.

Cunningham's Textbook of Anatomy, 5th edition, 1921.

Newman, H. H., Readings in Evolution, Genetics and Eugenics, 1921.

Wilder, H. H., History of the Human Body, revised edition, 1923.

Kingsley, J. S., Comparative Anatomy of Vertebrates, 3rd edition, 1926.

CHAPTER VI

THE RELATIONSHIP OF EXISTING ORGANISMS

(Continued)

4. PHYSIOLOGY

We have so far dealt primarily with the structure of living
things but the action of the parts composing an individual organ-
ism, the processes which are even more evidently essentials of life,
are no less significant in evolution. The structure of organisms is
a more tangible evidence of relationship but it is important to
remember that it is only one of the three essential factors in the
existence of living things. While environment, response, and
hereditary structure are distinct, we cannot have life or individual
lives without the intimate correlation of all three. The response
of the organism or its functional activity constitutes the subject
matter of physiology.

Fundamental Physiological Properties. Among the character-
istics of living matter we find that a certain few l^elong to this
category. While chemical composition and definite form are
distinctive, the qualities of irritability and contractility are no
less so, yet they are of an entirely different nature since their
existence depends both on the things which the organism has re-
ceived from its ancestors and upon the conditions under which it
lives. These physiological properties include irritability, con-
ductivity, contractility, metabolism and reproduction. They may
be known equally well as the adaptive properties since it is through
them that the organism is adjusted to its environment. With the
exception of reproduction alone all are involved in the correlation
of the individual and its environment, and the excepted property
is, of course, essential to the adaptation of species.

It is in this group of characters that the kingdoms of organisms
are chiefly distinct from each other, although there are species
which are both plant and animal even in physiological processes.

In the single-celled organisms, where we can observe the proc-
esses of life at their simplest, it is obvious that all of these things
are active in any cell, be it plant or animal. Such cells as the little

90

EXISTING ORGANISMS— PHYSIOLOGY 91

green alga, Sphaerella, and the Protozoon, Paramecium, are in
many respects the same. Each moves about actively, an evidence
of its inherent contractility. Each, if it comes in contact with some
object or substance in the water, indicates that it has received a
stimulus from that object or substance. The indication may be
in the form of a movement involving the end of the body opposite
to that which received the stiuuilus, hence we know that some
impulse has passed through the organism, an evidence of irrita-
bility and conductivity, and in addition of response through con-
tractility, which produces motion. Finally, if we watch the organ-
isms long enough, and conditions in the environment are favorable,
we note that by the acquisition of substances from the environ-
ment the individual grows and reproduces itself in some way.
Most of these properties can be equally well observed in a complex
animal, such as man, but irritability, conductivity, and contrac-
tility are so little emphasized in the higher plants as to be totally
obscured except in rare cases and in normally inconspicuous
phenomena.

Plant and Animal Metabolism. In these two simple one-celled
organisms, however, we see the fundamental manifestation of life
processes, but even here there is a strange difference. The animal
is practically colorless, but the body of the plant is green. The
animal, as it swims about in the water, is constantly occupied in
sweeping into its gullet minute organisms which are massed to-
gether and passed into the cytoplasm of its body as food vacuoles,
which in a few minutes are changed into a part of the animal itself.
The plant does none of this. We know through the investigations
of scientists that the two differences are closely related. The green
substance in the plant is called chlorophyll, meaning literally the
green of leaves, and its presence is the basis for an entirely different
metabolism, the foundation of all life.

Photosynthesis. Chlorophyll acts in the plant as a catalytic
agent, a substance by means of which a chemical action goes on,
although the catalyst is not changed in the process. Plants which
possess it, that is green plants, secure from their environment the
inorganic substances, carbon dioxide, water and certain salts
such as nitrates and phosphates, which may be called their food.
In the presence of sunlight as a source of energy, the plant com-
bines the first two, carbon dioxide and water, through the agency
of its chlorophyll, to form sugars and starches. A different arrange-

92 EVOLUTION AND GENETICS

ment of component elements produces fats, and the addition of
elements derived from inorganic salts forms proteins. While the
complexity of the processes involved is infinitely greater than this,
the essential result is the formation of these three fundamental
compounds of living matter through synthesis of inorganic sub-
stances and the addition of energy derived from sunlight, the whole
depending on the plant's possession of chlorophyll.

The plant's activity is thus chiefly constructive from the point
of view of living things. Careful study has shown that this is not
its entire activity, however, for while it uses carbon dioxide in the
synthesis of carbohydrates, and liberates oxygen as a result,
exactly the converse of the process which liberates energy by
oxidation in the animal body, it also oxidizes some of the substances
which it has elaborated, for the lil^eration of energy. The skunk
cabbage, familiar to anyone who has studied nature in the eastern
part of the United States, furnishes an excellent example. It
blooms in swampy places as early as February, sometimes before
the ice has entirely disappeared, and its release of energy in these
frigid surroundings is so great that a temperature difference of six
degrees has been recorded between the inside of its curious spathe
and the cold outside air.

Animal Metabolism. The animal is in general a much more
dynamic organism, for it takes the substances synthesized by the
plants, with their abundant potential energy, and carries on its
own activities solely by releasing this energy by oxidation, after
breaking down the substances into their simpler components and
resynthesizing these into the similar compounds of its own body.
The animal is thus dependent either directly or indirectly upon
the plant for its existence. It is wholly unable to build up the
substances that it needs from inorganic materials.

In four of the five physiological properties then, as well as in
morphological characteristics, the plants and animals are defi-
nitely related. In the fifth, metabolism, we see that there is funda-
mental difference due to the ability of green plants to carry on
photosynthesis. However there is also a degree of similarity that
is even more striking in parasitic plants like the fungi, which are
as devoid of ability to utilize inorganic compounds, as dependent
on the green plants, as are the animals.

Plant- Animals. In a few organisms both of these powers are
resident. These are called plants and animals, and as a matter of

EXISTING ORGANISMS— PHYSIOLOGY 93

fact are both. The genus Euglena includes a number of species
not unUke Sphaerella to the extent that they are green with chloro-
phyll and move about by means of a slender flagellum. It has
been found that cultures of Euglena kept away from light lose
their green color, and after a few generations become more defi-
nitely animals in appearance through the lack of chlorophyll. Such
colorless Euglenae can be kept alive by the addition of soluble
organic foods to the water in which they live, and since they are
kept away from sunlight, they carry on, obviously, the metabolism
of animals. Their production of chlorophyll is, however, merely
interrupted, for they again become green if brought into the light,
and carry on the metabolism of plants. They are definite connect-
ing links between the otherwise different kingdoms.

Other Tjrpes of Metabolism. Although the method by which
both animals and plants liberate energy is the one most widely
prevalent in the organic world, it is not the only possil)le means.
Some of the bacteria carry on a fundamentally different process of
metabolism in which energy is secured by the oxidation of very
different substances. Beggiotoa and Thiothrix, the sulphur bac-
teria, for example, oxidize hydrogen sulphide and store up sulphur
as a by-product; this is later oxidized and excreted as sulphuric
acid. At least one of the iron bacteria, Spirophyllum ferrugineum,
is equally dependent upon ferrous carbonate as a source of energy.
Still other plants, the yeasts and anaerobic bacteria, do not need
any free oxygen to liberate energy, but accomplish this result by
modification of food substances in a way illustrated by ordinary
fermentation. In this process yeasts break down sugars, forming
alcohol and carbon dioxide and releasing the energy which the
organisms require.

Specialization. It is of little or no use to consider the similarity
of details of physiological processes, for it is difficult to compare
the functions of different organs in animals which are not closely
related, and in similar organs similarity of function would be a
natural consequence. However, some generalizations are possible
in connection with the fundamental physiological properties of
matter and their distribution in the complex organism. The
ectoderm of Metazoa, since it retains direct contact with the
environment, might be expected to display a greater development
of the quality of irritability and this is true even in the highest
phylum, where it is the source of the highly developed nervous

94 EVOLUTION AND GENETICS

system. The function of conductivity is naturally retained in some
degree by all cells, but it too is highly developed in the specialized
ectoderm of complex nervous systems. Contractility, the means
of accomplishing immediate response to environmental stimuli,
likewise becomes an ectodermal function in the Coelenterata, but
with the development of the third germ layer, this function shifts
almost completely to the third layer, the mesoderm.

Inasmuch as the body form in the Coelenterates almost com-
pletely removes the endoderm from environmental contacts and
protects it by the ectoderm from all necessity for direct response
for protection or the securing of food, it is not to be expected that
this layer would retain the same qualities as the ectoderm. The
additional fact that it lines a cavity of sufficient size to contain
particles of food too large for ingestion by single cells, suggests its
logical association with digestion, and we find that the initial steps
of metabolism are henceforth functions of endodermal tissue.
The development of large and complex glands such as the liver and
pancreas, involving quantities of mesodermal tissue, still involves
the endoderm as the source of the epithelium — the glandular
portion — of these organs.

The Endocrine Glands. One evidence of similarity in the func-
tions of similar organs is sufficiently striking to be worthy of com-
ment, inasmuch as valuable therapeutic results have been ob-
tained on this basis. The several ductless or endocrine glands of
vertebrates produce secretions known as hormones which exert
specific correlating influences on the body through their power to
activate or inhibit the development and functions of various
parts. These glands include the pituitary body, thyroid, thymus,
gonads and various other parts. Because of their importance in
the human body they have been made the subject of extensive
study in lower animals, and have been found to exert the same
influence upon individuals of the species producing them and
upon others, even of different classes.

The secretion of the thyroid, whose effect is evident through
modification in the various types of goiter and in the congenital
insufficiency of cretinism, is now extracted from domestic animals
for therapeutic use. Its effect on the normal course of physical
development is exerted not only on man, a member of the same
class, the mammals, but also on tadpoles and larval salamanders.
Tadpoles to which the extract (thyroxin) is administered undergo

EXISTING ORGANISMS— PHYSIOLOGY 95

metamorphosis before attaining the normal growth, while the
Axolotl, a salamander which normally remains aquatic, may be
caused to develop into a terrestrial animal tlirough the usual am-
phibian metamorphosis by the administration of the same sub-
stance. Likewise, the symptoms of cretinism may be corrected and
normal development induced in infants by supplying thyroid ex-
tract secured from domestic animals.

The part played by insulin, a hormone secreted by the islands
of Langerhans in the pancreas, has recently been given great pub-
licity because of its isolation and the discovery of its effect on the
metabolism of carbohydrates. Insufficiency of this secretion
results in the disease diabetes, whose effects are now minimized or
completely abated by the administration of the commercially
extracted secretion of other animals.

These two are familiar examples, but are duplicated by the
behaviour of all the other endocrine glands. The interspecific
effectiveness of hormones is so dependable that a considerable
number are now commercially available, all extracted from the
glands of domestic animals, and are used extensively in endocrine
therapy. The one limitation is not their lack of potency, but our
incomplete knowledge of their action.

Blood Tests. One evidence of physiological relationship has
received much attention in the literature of evolution, namely
Nuttall's famous precipitin tests for blood. The physiological
basis for these tests is treated in Nuttall's book from which we
ma}' draw.

Immune Sera. Immunizing properties of l)lood serum removed
from animals which had developed natural immunity from such
diseases as diphtheria and tetanus were noted late in the nine-
teenth century. Ehrlich then experimented with the toxic sub-
stances ricin and abrin. He found that animals treated with in-
creasing doses of these poisons developed increased tolerance, or
immunity, and that the blood sera of these animals neutralized
the poisons in vitro, and, of course, when injected into other indi-
viduals rendered them immune from the effects of the poisons.
He proved also that a serum capable of neutralizing ricin had no
effect upon abrin and vice versa. EhrUch concluded that definite
compounds were produced in the blood in response to the poisons.
These he called antitoxins, or antibodies. As Nuttall points out
"we now know that normal serum contains a number of anti-

96 EVOLUTION AND GENETICS

bodies having similar actions to those artificially produced as a
result of immunization with this or that substance, we know of
normal agglutinins, haemolysins, bacteriolysins, antitoxins, anti-
ferments, etc., all of which go to prove the correctness of Ehrlich's
views in this respect."

The theory of antibody formation and structure is complex in
its details. For our purpose it is sufficient to note that the various
types of antibodies are antitoxins, antiferments, cytotoxins of
various kinds, agglutinins and precipitins, each named for the
substance whose presence in the serum gives rise to it, or for its
action upon that substance.

Precipitins. Thus the precipitins have the power to form a pre-
cipitate when mixed with the substance which has produced them.
Little is known of their nature, but the precipitates produced by
their reaction with proteins of blood sera show characteristics of
proteins in several recorded cases.

Preparation of Precipitins. Nuttall's procedure involved the
use of rabbits, chiefly, as the source of antisera (i.e., sera contain-
ing antibodies). Injections of the blood or sterilized blood serum of
other animals were made at intervals of several days until three to
twenty had been administered. After the completion of this
treatment six to fifteen days were allowed to elapse, the rabbit
was then killed and bled, and the blood serum extracted and pre-
served with the necessary precautions to maintain its sterility.
In this way antisera were developed for the blood of a number of
species of animals, as well as for other substances, such as cows'
milk and egg albumen.

Nuttall states that "we have sufficient evidence to show that
precipitins are not formed in the serum of closely related animals."
He cites the experiments of Bordet and Hamburger, in which
precipitins were not produced when rabbit serum was injected into
guinea-pigs. Nolf likewise found it impossible to produce antisera
in pigeons treated with fowl serum. This indicated a sufficient
similarity in the bloods of the animals concerned to account for
tolerance of the one species for the blood of the other, an evidence
of blood relationship which harmonizes with the fact that the
animals are related in a morphological way.

Precipitin Tests. In using the antisera thus prepared, samples
of sera were collected in two ways, viz., fluid and dry, although
the latter method proved to be the more practicable. Dilutions

EXISTING ORGANISMS— PHYSIOLOGY 97

or solutions in salt solution wore prepared in test tubes, and a
drop or two of antiserum introduced. The result in the case of
related sera and antisera is the formation of a precipitate; un-
related sera do not react when the antiserum is added. Varia-
bility in the results is explained by Nuttall's statements that
"where a powerful antiserum is added to its homologous blood
dilution, the reaction is almost instantaneous, in other cases it
takes place more slowly. In the case of a strong antiserum, the
reaction takes place as a rule rapidly in related bloods, more
slowly in distantly related bloods. The rate at which the reaction
takes place may depend also upon the concentration of the blood
dilution, the more concentrated dilutions, within limits, reacting
earlier than higher dilutions. A weak antiserum will act more
slowly than a powerful one." Thus we might expect, all other
factors being equal, that the reaction would correspond to the
nearness of relationship as determined on the basis of classifica-
tion, and that in cases of doubtful relationship precipitin tests
might furnish a valuable corollary to the usual evidences.

A few examples from Nuttall's extensive tables of results are
given in the table on page 98. In the horizontal line are listed the
antisera, in the vertical column, the blood tested. Only easily
interpreted examples are given, and the symbols have been
modified from the original to indicate great reaction, marked
reaction, moderate reaction, slight reaction, and no reaction, in
order as follows: 1,2,3,4,0. Blank spaces indicate the lack of a test.

Even in the few cases here presented, it is obvious that the most
pronounced reaction is usually obtained with so-called homologous
sera, and that animals of different species react in a degree similar
to the degree of relationship determined in other ways. This is,
of course, subject to error, like all pioneer procedure in science,
but in the first three cases two show a maximum reaction of human
blood with human antisera, one a slightly greater reaction with
the antiserum of an anthropoid ape, all some reaction with anti-
sera of other primates and with some antisera of more distantly
related mammals, but none with antisera of the other classes.
In cases 5 and 6 the bloods of the orang and chimpanzee show a
maximum reaction with antisera of man and the anthropoids, but
none with other classes. However, in cases 12, 13, 14, and 15, the
bloods of various species of reptiles and birds show some reaction
with antisera of these two classes but with no others.

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EXISTING ORGANISMS— PHYSIOLOGY 99

Quantitative studies of the precipitates produced in these reac-
tions serve as a measure of finer degrees of relationship, but all
tests are subject to modification by various conditions. The
following excerpts from Nuttall's work are a partial consideration
of these factors.

He writes: "Uhlenhuth agrees with me in finding that the zoologi-
cal relationships between animals are best demonstrated by means of
powerful antisera. He judged from reactions with such antisera
that the ox is not so closely allied to the sheep, as the sheep is to
the goat. He found that weak anti-sheep serum produced no
reaction in ox blood. In my paper of 21, XI. 1901, I wrote 'The
more powerful the antiserum the greater is its sphere of action upon
the bloods of related species'. For instance, a weak anti-human
serum produced no reaction with the blood of the Hapalidae,
whereas a powerful antiserum did produce a reaction and proved
what I may be permitted to call the 'blood relationship' in the
absence of a better expression. ... I also noted that reactions
took place 'to a lesser extent in the bloods of allied animals, than
in the homologous blood.' " In the following paragraph he shows
that the serum may react with a remotely related antiserum if
allowed to stand for some time, but he notes the facts that have
been brought out here, "that anti-mammalian sera only produce
these later reactions in mammalian bloods, anti-avian sera simi-
larly in avian sera alone."

Reproduction. Finally, the reproductive processes of organ-
isms afford unsurpassed evidence of relationship. Many cells
retain the property throughout their lives; others are formed as
end products in the highly specialized organisms and normally
lose it. Some cells of the latter type are capable, however, under
the stimulus of abnormal conditions in the organism, of repro-
ducing during the process of regeneration of tissues or parts even
though reproduction is not a constant function. A degree of
specialization which completely eliminates this function is rela-
tively rare.

Amitosis. In its simplest state cell reproduction apparently
consists of a simple elongation, constriction, and separation of the
parent cell, resulting in the production of two daughter cells. This
process is known as amitosis. It occurs in certain parts of metazoa,
as for example the well-known laboratory illustration, the ovary
of the cricket, apparently in highly specialized cells.

100

EVOLUTION AND GENETICS

Fission. Single-celled organisms often reproduce by a super-
ficially similar process called fission which also produces two similar
individuals at the expense of parental loss of individuality, but
this process foreshadows a more complex type of reproduction
called mitosis. It is said by some authorities to be a true mitosis.

A y^ "-\ B /r-^"~7>x C

Fig. 61. — Typical stages of mitosis in which the chromosome number is
assumed to be eight. A, beginning of prophase: chromatin in a reticular
form, centrosome divided and astral fibers formed about it; B, early pro-
phase: chromatin in spireme, centrosomes moving apart and spindle form-
ing between them; C and D, later prophase: chromosomes forming and
remainder of nucleus breaking down; E, metaphase: the chromosomes ar-
ranged in the equator of the spindle and split longitudinally; F, G, anaphase:
the chromosomes migrating toward the centrosomes; H, telophase: a
gradual return to the original state of the nuclear constituents and centro-
somes, accompanied by constricti'on of the cytoplasm; I, the two cells formed
at the completion of mitosis. (From Woodruff.)

Mitosis. This process involves primarily the chromatin of the
nucleus (Fig. 61). During mitotic division of animal cells the
centrosome divides, the two halves move away from each other,
and about each a series of radiating fibers or apparently fibrous
structures appears. Between the two a series of connecting fibers
develops. The entire structure is not unlike the field of a bar

EXISTING ORGANISMS— PHYSIOLOGY 101

magnet, with the two centrosomes representing the poles and the
fibers lines of force. For obvious reasons the radiating fibers are
called astral and the others spindle fibers. During the formation
of the spindle the chromatin becomes condensed into a compact
thread called the spireme, the nuclear membrane breaks down, and
all nuclear structures except the spireme become a part of the
cytoplasm. The spireme breaks up into a number of parts called
chromosomes, and these bodies migrate into the equator of the
spindle. The number of chromosomes varies in different species;
there are four in some worms, forty-eight in man, and even more
in some animals. This much of the process is called the prophase.
During the next stage, the metaphase, each chromosome splits
longitudinally into apparently equal halves. Following this the
halves migrate toward the nearest aster in the anaphase, and the
final telophase includes the constriction and splitting of the cyto-
plasm into two parts, each including one centrosome and one set
of chromosomes, and the reconstruction of the nuclei as in the
original parent cell.

Reproduction of Individuals. In its simplest state the repro-
duction of individuals is no more than cell reproduction. In the
lower Mctazoa, however, it passes beyond this simplicity since
it must involve many specialized cells, but even here a process of
fission or budding is common which is little more than the repro-
duction of numerous cells of the parent to form a new, similar
individual. In all Metazoan phyla, an additional specialization
occurs, cither associated with the process of fission or budding or
the sole method of reproduction. In it the reproductive power
of certain cells of the body is emphasized to the extent that they
are developed solely for the purpose of producing new individuals,
so that the complete differentiation of the species from a single
cell occurs anew with every generation. To this is added gametic
or sexual reproduction, involving the union of two independent
germ cells. The modification of cells accompanying sexual repro-
duction is a further addition to the intricacies of mitosis, but it has
such an important bearing on other phases of our subject that it
will be taken up in connection with the laws of genetics.

Accessory Reproductive Functions. With the establishment of
sexual reproduction the germ cells, or gametes, are formed from
ectoderm or endoderm in diploblastic animals, but in the triplo-
blasts they are consistently mesodermal as far as can be determined.

102 EVOLUTION AND GENETICS

It is certain that the gonads, the organs in which they are devel-
oped, originate from that layer. The gonads are commonly
formed in the wall of the body cavity, and the gametes discharged
into that cavity. Openings in some animals connect the coclom
with the exterior apparently solely for the escape of the germ cells,
but the excretory tubules or nephridia with the same relations
provide another convenient means of egress, which results in a
common association of reproductive and excretory systems.

The formation of two types of gametes, the ova and spermatozoa,
demands different provisions for their escape and union, but the
association of nephridia and gonads remains obvious throughout
the higher phyla.

Reproduction of Vertebrates. In the vertebrate classes the
transition of reproductive functions illustrates first in the cyclo-
stomes, fishes and amphibia the simpler stages of sexual repro-
duction. The gametes of most species are discharged from the
body and united in the water, where reproduction occurs in all
three classes, with a very few exceptions among. the amphibia.
Terrestrial reproduction in the Sauropsida, including the reptiles
and birds, demands additional modifications which are chiefly
expressed in the foetal membranes treated under embryological
relationship. Internal fertilization of the ovum is, however, a
functional modification of equal importance, since the fluid medium
is essential for the union of the germ cells.

The addition to the ovum in oviparous species of enough food
to carry the young animal through its development to a point
where it is capable of functioning as a more or less independent
terrestrial organism is accomplished by the assumption of the
function of secreting these substances by the tissues associated
with the passage of the egg from the body.

The structural and functional modifications associated with
the development of large eggs containing much food are not
changed in the lowest mammals, but here a new function, with
accompanying modification of structures, makes its appearance;
viz., the secretion of milk for the nourishment of the young. These
mammals are the lowly oviparous Prototheria, the duck mole and
spiny anteater of Australia. The next division of the mammals,
the Metatheria, includes such species as the kangaroos and opos-
sums. In these the highly developed ducts of the female gonads
assume a new function, that of providing nourishment through

EXISTING ORGANISMS— PHYSIOLOGY 103

the circulation to the developing embryo, instead of secreting it
as an addition to the egg. Here development is carried to a state
of partial perfection, and the young animal is then carried in a
pouch on its mother's abdomen and nourished with milk until
able to shift for itself. Finally, the Eutheria, or true mammals,
illustrate an elaboration of the same process. The connection of
embryo with adult is so intimate that it is looked upon as essen-
tially a parasitic relationship, and development proceeds not only
to morphological completeness, but through a considerable degree
of growth before parent and offspring are separated l:)y ])irth.

Throughout the multitude of functions of the living organism
of which these are only a f(nv illustrations, occur the fundamental
relationships which have here been emphasized. From simplest to
most complex they are no more than the five fundamental func-
tions, yet in these and in the complexities of their distribution
among the parts of the specialized organism we see one more evi-
dence of common relationship of all living things, and of lesser
relationships of various kinds among the different groups.

Structure, development, function, and the classification which
we have derived from the study of these and other things, all
point to the truth which is so well established in modern science,
that all living things are related. This much cannot be logically
doubted; it is the function of evolution to explain their relation-
ship.

Summary. The relationship of organisms is as well shown by
their physiological processes as by their structure and develop-
ment. Plants in general have one fundamental process of metab-
olism, animals another. Some organisms carry on both types
and others, the colorless plants, form a connecting link between
the green plants and animals. Exceptions to these normal proc-
esses make it all the more evident that similarity means funda-
mental relationship. The activity of the endocrine glands is a
striking evidence of relationship among animals. Blood tests,
based on the immunizing reaction of the blood of an animal to
that of another species, have been worked out so extensively that
they not only illustrate the fact but also the varying degrees of
relationship. The process of cell reproduction, mitosis, links all
organisms, and in the reproduction of individuals we find indica-
tions of less extensive relationship within phyla. The vertebrates,
for example, show a gradual transition of reproductive functions.

104 EVOLUTION AND GENETICS

REFERENCES

NuTTALL, G. H. F., Blood Immunity and Blood Relationship, 1904.

LoEB, J., Studies in General Physiology, 1905.

Howell, W. H., Textbook of Physiology, 9th edition, 1920.

Dercum, F. X., Biology of Internal Secretions, 1923.

Jordan, E. O., General Bacteriology, 8th edition, 1924.

Robertson, T. B., The Chemical Basis of Growth and Senescence, 1926.

Woodruff, L. L., Foundations of Biology, 3rd edition, 1927.

CHAPTER VII
EVIDENCES OF EVOLUTION

1. EXISTING ORGANISMS

Relationship. The facts set forth in the four preceding chap-
ters are such as to leave no logical doubt of the relationship of the
various kinds of organisms now extant. It is obvious that there is
some degree of resemblance between any species which we may
choose, no matter how remote they may seem, even though extreme
cases force us back to the cell as a common basis of structure and
protoplasm as the one living substance. Other things may be
fundamentally different, but these at least connect all living
creatures.

The entire body of scientific facts is merely an elaboration and
extension of such obvious things as we have assumed to be man's
first observation of organic relationship. While some organisms
are very obviously related, others are less evidently so, but through
his gradual accumulation and interchange of knowledge, man has
arrived at an understanding of organic unity. Science makes
clear the fact that relationship among organisms is universal.

The Significance of Relationship. The interpretation of such
facts beyond this point is somewhat different. To the scientific
mind they can have only one meaning, but it is evident that there
are others who find it possible to believe in the independent origin
of these related organisms. When men were struggling to explain
such natural phenomena as are in accord with evolution, even the
contemporary origin of new organisms from inorganic sources did
not seem absurd to them. We are now in possession of abundant
evidence of the non-existence of spontaneous generation as a
source of organisms, but the dim records of the past are less easily
demonstrated by empirical methods. The nature of organic rela-
tionship must therefore be based on logical interpretation of the
available facts. Whether or not the processes which have come
to be accepted on this basis will some day be demonstrated in the
laboratory, it remains for the future to prove. Many things are

105

106 EVOLUTION AND GENETICS

accepted with less definite proof than that now available for evo-
lution.

Community of Origin. When relationship is mentioned, the
immediate thought aroused is of similarity. Further analysis
shows that we cannot have similarity, i.e., relationship, without
some degree of community of origin. Thus among inanimate
objects we speak of sedimentary rocks, igneous rocks, of bricks,
of porcelain, of automobiles and radio sets. In any of these
categories similarity is evident, although it is not merely likeness
which has given us similarity, but the fact that in the one category
all things have a common origin. Sedimentary rocks must be
laid down by water and igneous rocks must be cooled from a
molten state. Bricks are produced by burning clay, and porcelain
by a similar process from a different kind of clay. Automobiles of
many kinds are the various developments of a single idea, and all
radios have been produced by the elaboration of the original
mechanism of wireless transmission.

Within the same category we find that the relationship of things
is again directly proportional to community of origin. There are
various sedimentary rocks. Limestones differ from sandstones
and both from shales. Automobiles are the same to a certain
degree, but only those produced by the same maker are even
approximately identical.

Forces and Materials. This analogy is faulty in more than one
way, but it serves to emphasize a fact easily overlooked, viz., that
similarity is due to a common origin. It is conceivable, of course,
that similar things should be independently produced, but sig-
nificant that they rarely are. Moreover when similar things are
independently produced, either by man or nature, we can be cer-
tain that at least the same forces or materials or l^oth have entered
into their production. These factors may be widely disseminated
if independent from the product, but in living organisms we see
that they are concentrated wholly within organic matter, so that
this alone is a demonstrable source of living things. It is an old
biological principle that all life comes from preexisting life.

Relationship of Individual Organisms. Such a relationship is
even more evidently a matter of origin. Organisms are brother
and sister because they are produced by the same parents, or
cousins because their parents were so related. The more remote
this common source, the more distant the relationship, until the

EVIDENCES— EXISTING ORGANISMS 107

limitations of human records lose sight of it altogether and we look
upon our friends and associates as wholly unrelated. Supposedly
unrelated individuals marry. The union of any more closely
related than cousins is frowned upon, yet after all these are only
degrees of relationship.

The relationship of these apparently unrelated individuals
serves as an excellent illustration of the course of development of
a population. Assuming that one individual is produced by un-
related parents, and that his parents are derived from equally
distinct lines, only a few generations back we find his ancestors
multiplied to a ridiculous point. The number of ancestors doubles
with each generation in geometrical progression. Allowing twenty-
five years to each generation, a reasonal^le average for the period
covered by recorded history, we find that seventy-seven genera-
tions have passed during the Christian era. By carrjdng the an-
cestry of our one individual back through one-half of that period —
238 — ^g reach the stupendous total of 274,877,906,944 ancestors.
The present population of the world is approximately 1,748,-
000,000 and it has increased constantly. Or on the basis of the
closest union, that of cousin marriages, assuming even the im-
possible constant of siblings marrying cousin siblings, we find that
the total ancestry of one hundred unrelated individuals vastly
exceeds the total population of the world fifty generations back.
The absurdity of these results is ample evidence that all members
of a given species are related in some degree.

By carrying out such computations of ancestry and comparing
them with the increasing population of the world, it is obvious
that the individuals of any species have sprung from a very
limited number within the period of recorded history. It is neither
difficult nor illogical to carry the idea back to an initial unit, an
individual or pair. Both biologically and through other sources
this view becomes available.

Relationship of Species. As we apply this analysis of relation-
ship to the species which make up the organic world, we find that
the direct connection of one with another which is evident in indi-
vidual relationship is not apparent. The period covered by human
records is so brief that it does not afford an opportunity to view
the transition of one to another. That this transition may yet
be seen is strongly suggested by the production of distinct forms
by mutation, a process which will be considered in detail in a later

108 EVOLUTION AND GENETICS

chapter, but even this has not resulted in the undisputed origin
of species under human observation.

Since relationship as indicated by structural and functional
similarity is evidence of development through similar processes
from similar things in all cases which we can examine, however,
it is indicative of a similar origin of related species. Through the
concentration of all factors in the production of living things in
organic matter alone, we may logically conclude that robins and
bluebirds, or butterflies and moths, are related because they came
from common beginnings, and that birds and Lepidoptera are
likewise related in more fundamental particulars because of a
more remote derivation from a common source. This view is
logically tenable, but in the evaluation of details it is likely to be
confusing if not substantiated by extensive knowledge.

Examination of the several evidences of relationship supplies
the needed support for details of evolution. The field is so vast
that complete analysis is not to be expected, but as many examples
are available as the individual may care to seek.

In such species as are found in the genus Euxoa, of the moths,
the most intimate degree of relationship is apparent. Some have
been named from one region, others from another, and material
from intermediate regions has later resulted in their union. Those
individuals which occur in the Rocky Mountain region have no
direct association with those which fly in the Mississippi valley.
They are related in structure, pattern, color and habits. Why?
Careful evaluation of the facts leaves only one possible conclusion.
They must have been derived from the same source, and since
they are individuals of the same species, such derivation is easily
understood.

In connection with this case, one step leads us to that of ob-
viously different species belonging to the same genus. Suppose
that the two widely separated lots just mentioned should have
proved to be actually different, no matter how much material
from intervening regions might have been secured. They are no
more independent in fact than the extremes of the one species, yet
we are unable to demonstrate any connection between them other
than a certain similarity. Why should they display this simi-
larity? Again the only possible answer is because of a similar
origin. Through the characters of the genus, the two are the same.
They must then, have had a common origin a little more remote

EVIDENCES— EXISTING ORGANISMS 109

than that of the forms of the one variable species, and so, step by-
step, the varying degrees of relationship made evident by our
classification are evidences of more and more remote common
sources.

Ontogenetic Succession. For an illustration of the succession
of changes which may have passed in the development of existing
forms, it is possible to look to the actual record, which is now
availa])le in sufficient extent to be here treated in(lei)endently.
But the things to which man first had to turn in his attempts to
explain the relationship of living creatures are entirely within the
organism.

In studying the common characters of the vertebrates we have
noted the succession of forms characteristic of the several phyla.
That these forms may be looked upon as a chronological succession,
and not merely a succession in degrees of complexity is made
evident by the combination of anatomical and embryological
facts. In the skeleton of vertebrates, for example, the occur-
rence in some fishes of a cartilaginous cranium alone, in others of
such a cranium partly ossified together with external bony plates
with characteristic arrangement, and in the higher classes of a
skull which embryology shows to be made up of a similar cartilag-
inous portion in the beginning, from which certain bones are
derived by ossification and to which others are added by develop-
ment directly from mesodermal tissue, is indicative of relation-
ship in chronological succession. The same applies to other bones
of the body. This gradual succession of stages during embryo-
logical development which correspond to those represented by
adults of the several classes is clear evidence that the higher forms
have come from lower in this phylum. We see in the few points
mentioned that the formation of cartilage is not an essential step
in the formation of bone, so the transition of some bones can mean
only that they still pass through the stages which they have
followed in the past.

The circulatory system, especially in the development of the
aortic arches, is a similar case. While they are symmetrically
paired in the fishes, amphibia and reptiles, although reduced in
number in the last two, they are still further reduced and asym-
metrically developed in the birds and mammals. Superficially the
resemblance is slight, but when we consider the embryological
succession of parts, and see even in the human embryo the develop-

no

EVOLUTION AND GENETICS

ment of six symmetrical pairs of arches, of which some are later
resorbed to bring about the adult condition, we can conclude only
that the six are there because they were the original source of those
that persist. Nature is not in the habit of producing useless struc-
tures, and the appearance of unnecessary structures such as these,
which are so very similar to the adult arches of the fishes, points
strongly to the derivation of the higher forms from fish-like
ancestors.

The Recapitulation Theory. The same interpretation can be
applied to any of the evidences of relationship previously brought

out. Pharyngeal clefts,
Meckel's cartilage, ves-
tigial structures, all such
apparently useless
things, can be inter-
preted only as remnants
of an ancestral condition,
and since these things
resemble functional parts
of existing organisms,
those organisms may
logically be interpreted
as near that ancestral
condition.

This resemblance of

embryonic stages to a

of adults of

classes has

to the rcca-

succession
different
given rise

Fig. 62. — Diagram showing the structure of a
primary ocelkis. c, cornea; c.hy., corneal
hypodermis; rel., retina; n, ocellar nerve; p,
accessory pigment cell; r, rhabdom. (From
Comstock's Introduction to Entomology, with
the permission of the Comstock Publishing
Company.)

pitulation theory, the
belief that in its individual development an organism repeats
the steps of phylogenetic development which gave rise to its kind.
The repetition is undoubtedly modified in many cases according
to the conditions of existence of the various species, but there is
every reason to believe that it is in general true. In the transition
from a single cell to a small group, to the hollow spherical blas-
tula, the sac-like gastrula, and on through such details of develop-
ment as have been mentioned, it is highly probable that in em-
bryological development, or ontogeny, we have before us a partial
record of past changes.

EVIDENCES— EXISTING ORGANISMS

111

The Significance of Homologies and Analogies. Those who
find special creation an adequate explanation of diverse living
things see nothing more in these
facts than the will of the Creator
to produce such organisms as now
exist. The activating force, in
other words, is regarded as inde-
pendent of the necessary materials,
a belief which is not liorne out
by observed facts. If this were
the foundation of life, there is every
reason to suppose that every crea-
ture would be given the best possi-
ble equipment for its mode of life.
Instead, organisms often have
structures which show definite
resemblance to those of other
species living under very different
conditions. They cannot logically
be supposed to have been made
from unspecialized raw materials.

Fins and Flippers. Such homol-
ogies, very common among living
things, do not indicate that the
same end cannot be met by
different organs, for analogous
structures are also fairly common.
The whale has flippers which show
definite homology with the fore
limbs of terrestrial vertebrates,
although they are more like the
fins of fishes in function. Its res-
piration is carried on by the same
organs as those used by terrestrial
species. It would obviously be
better fitted for purely aquatic

life if it could breathe without rising to the surface, but this it
is unable to do. On the other hand its need for such an organ as
the tail of the fish is met by a very similar structure which is
merely analogous.

Fig. 63. — An ommatidium of Ma-
chilis, c, cornea; hy, corneal hy-
podermis; cc, crystalline cone
cells; i, iris pigment cells; r, re-
tinula; rh, rhabdom; b, basement
membrane; n, nerve; ap, acces-
sory pigment cell. (From Com-
stock's Introduction to Entomology,
with the permission of the Com-
stock Publishing Company.)

112

EVOLUTION AND GENETICS

Surface of lens

Cornea \
Iris

Lens.

Retina '

Optic ganglion

Anterior
• optic cliamber
Cornea

s. Posterior
optic chamber

Eyes. In the eyes of Arthropoda, Mollusca and vertebrates we
find a remarkable example of this kind. All are special sense organs
for the reception of light stimuli, and in their highest development,
for the formation of visual images, yet they are very different
structures.

Insect eyes are of two types, simple and compound; the former
may be composed of numerous visual cells grouped beneath a
transparent lenticular cornea, developed from the hypodermis
(Fig. 62). In the compound eye similar visual cells form units with
accessory cells and a separate cornea. These units are called om-

matidia (Fig. 63) and are associ-
ated in large numbers in the most
highly developed eyes. Their
action is explained by Miiller's
theory of mosaic vision. Accord-
ing to this theory each omma-
tidium records a point of light,
not a complete image. The result
of numerous points of light re-
FiG. 64. — Diagrammatic section of corded by reflection of rays from
the eye of a squid, Loligo. (From different parts of an object is an
Heener, after Grenadier.) , . . ™i • •

erect mosaic miage. Ihis rniage

would depend for its resemblance to the original on the numljer of
ommatidia in the eye, and the resulting completeness of repro-
duction of details.

Eyes of Molluscs and Vertebrates. Molluscan eyes as de-
veloped in the Cephalopoda and vertel^rate eyes are very different
in optical function from the insect eye. Each is provided with a
lens whjch forms on the retina a complete image of any object
within the field of vision. This is, of course, an inverted image.
In visual function the two eyes are similar. In structure and
origin, however, they are different (Figs. 64 and 65).

Both cephalopod and vertebrate eyes have an outer cornea,
behind which is an anterior space or chamber. Between this and
a larger posterior chamber lies the lens. In front of the lens the
iris governs the size and shape of the pupil, and at the back of
the posterior chamber the light-sensitive retina is located. In the
vertebrates, however, the posterior chamber of the eye, and con-
sequently the retina, are derived from the first brain vesicle, while
in the Cephalopoda they develop directly from the outer ecto-

EVIDENCES— EXISTING ORGANISMS

113

dermal layer. In the former the lens is derived independently
from the outer ectoderm, but in the latter it comes from the optic
vesicle. The outer chamber of the eye of the squid is never com-
pletely closed. In the vertebrates it is never open to the exterior,
but forms by a splitting of mesenchyme outside of the lens. The
iris in the vertebrate eye is inside of the sclerotic layer, while in the

.^-f

Fig. 65.— The vertebrate eye (human), vertical section in situ, a, eyelash;
b, lid; c, bony orbit; d, e and g, muscle.s;/, optic nerve; h, anterior chamber
filled with aqueous humor; i, pupil; j, conjunctiva, a transparent membrane
continuous with the lining of the eyelid; A:, cornea; I, iris; m, lens; n, sus-
pensory ligament of lens; o, retina; p, choroid coat; q, sclerotic coat; r, mu.s-
cles to ligament suspending lens; s, posterior chamber containing vitreous
humor; t, point of entrance of optic nerve; u, fatty tissue. (From Woodruff.)

squid it is a projection from the margin of this layer. They are
remarkable analogous organs, and an e.xcellent example of con-
vergence.

Blood. In the blood of the same three classes a similar diversity
prevails. Insect blood consists of a fluid plasma in which float
leucocytes similar to those of the vertebrates. The blood does not
carry oxygen to the tissues, however, and there are consequently
no red corpuscles. Oxygen reaches the tissues through the inde-
pendent tracheal respiratory system. The blood of the mollusca,
unlike that of the insects, must carry the animal's supply of oxy-
gen, hence it contains a blue pigment, haemocyanin, which con-

114 EVOLUTION AND GENETICS

tains copper and iron, the former in relatively large quantities.
Haemoglobin, the iron compound which gives the red color to
vertebrate blood and serves as a vehicle for transporting oxygen
in the body is also present in mollusca to a limited degree. Thus
the blood of the three classes is similar as a conveyor of absorbed
food and blood cells, but in only two cases is it a conveyor of oxy-
gen, and in these two it accomplishes its respiratory function by
means of different substances.

Metabolism. For a more generally applicable case we have only
to refer to the several metabolic processes mentioned in Chap-
ter VI. All animals must have energy. The response of the organism
to its environment invariably involves the controlled release of
energy in some form. Transformation or release of energy is nob
limited in the physical world to any one process, and the fact that
the iron bacteria and sulphur bacteria secure their energy through
two processes while most organisms carry on a third entirely
different process is sufficient evidence that living organisms are
not necessarily limited to one source of energy. It is very strongly
suggestive of similarity of function through similarity of origin.

The Cause and Process of Change. In these examples organ-
isms are seen to differ in degrees corresponding to the remoteness
of their relationship. If all species were wholly independent in
origin, we might expect a variety of structures and functions no
less than the number of species involved. That similar results
would be accomplished in many cases l)ecause of similar needs we
cannot doubt, for examples are before us, but it is equally im-
possible to believe that these independent species would follow
in their individual development unnecessarily tortuous paths, or
that they would produce even temporarily structures which were
of no use.

Factors in Existence. The questions naturally occur, why
should these differences have come about, and how? Evolution-
ists have attempted to answer these questions, although in these
attempts we still find the purely theoretical aspects of the subject.
The interpretation of relationships leaves no doubt of the reality
of evolution, but the exact forces through which it is expressed are
not yet proved. These problems must be considered in detail, but
to establish completely the evidences of evolution it is necessary
to consider in brief the possibility of such changes.

Returning to the analogy of individual relationship, the three

EVIDENCES— EXISTING ORGANISMS 115

determiners of individual "existence are seen to apply to any phase
of individual activity. Even the production of a new individual
demands first of all inherent fitness, second, the proper environ-
ment, and third, the proper response. Some individuals are con-
genitally unable to produce germ cells. In most species repro-
duction occurs only under definite conditions. Finally some
individuals reproduce in response to certain conditions while others
do not. When once produced the individual has a certain range of
possibilities. Within the same species these possibilities may vary,
but in the main they fall between certain extremes in most indi-
viduals. The different responses of various individuals within this
range are known as variation. It is so prevalent that no two indi-
viduals of any species are exactly alike, and so obvious that it
has been called the most invariable thing in nature.

Most species come into contact with environment common to
various other species. Not all species respond to the same condi-
tions in the environment, or like individuals, they may respond
to them in different ways, depending on their inherent powers.
In some cases the possibilities of the species are such that any indi-
viduals may accomplish the same end in either of two different
ways. So it is with Euglena, which can, if light is lacking but
organic food is available, live as an animal, although under normal
conditions it is equally able to live as a plant. The inherent possi-
bilities of an organism may be such as to enable it under different
environmental conditions to live in very different ways.

Variability of Organisms. That difference of response results
in qualitative differences in the organisms is evident in the same
organism, Euglena, which loses its chlorophyll when it lives in the
dark as an animal. Within our own lives we see similar evidences
in the development of calluses through constant friction or
pressure, and the increase in size of muscles through use. The
converse is equally true, that disuse of a part results in diminution
of its powers. These effects of use and disuse were emphasized by
Lamarck as a part of his theory of evolution.

The degrees in which organisms are able to respond to varying
conditions are in themselves variable. It has been recorded, for
example, that in streams where dipnoid fishes are found, the cessa-
tion of flow and stagnation of the pools remaining in the river bed
are sometimes fatal to ordinary gill breathing fishes, while the
Dipnoids, because of their ability to secure abundant oxygon at the

116 EVOLUTION AND GENETICS

surface, are able to live. Some species live under extremely limited
favorable conditions, like the myrmecophilous insects; others
seemingly thrive almost without restriction, like the ubiquitous
English sparrow. If the characters of these species are such as
actually to enable them to live only if the environment is favorable
within narrow limits, they are said to be specialized; if such that
they can live equally well under any of a variety of conditions, they
are called generalized species. Fitness for life under definite con-
ditions of all degrees is known as adaptation.

Variability of Environment. The second factor in existence,
environment, is no less variable. From day to day and from sea-
son to season conditions of temperature, light and moisture change,
and with them come changes in the food supply. Change in envi-
ronment demands a change in the organism, hence mammals
acquire thicker fur in the winter, and certain aquatic snails seal
their shells with a mucus plug and lie dormant when drought robs
them of their normal habitat. However, many of the more highly
developed animals are able to exist in spite of extreme fluctuations,
and to remain in the same locality in spite of seasonal or other
changes.

The Time Factor. Although environmental change, or exposure
to different environments, may result in different responses among
the individuals of a given species, it has been found impossible
until recently to produce such changes experimentally as a per-
manent modification of the species. Midler has succeeded in
producing permanent modifications of fruit flies by subjecting
adults to the action of X-rays. After this treatment mutations
appeared in the progeny of the flies much more abundantly than
in control cultures which had not been subjected to the rays. The
results depended upon the length of treatment with X-rays, and
in the most heavily treated lots of flies reached a maximum of 150
times the frequency of mutation in the controls. While these
results are interesting and significant, especially in the field of
genetics, they do not refute the fact that the manipulation of con-
ditions such as occur in a natural environment has not yet pro-
duced permanent modifications. We must remember in judging
experiments of the latter type, however, that even a factor operat-
ing since the beginning of recorded history would have had rela-
tively little time to effect a permanent change in comparison to
the vast periods which have come and gone during the existence

EVIDENCES— EXISTING ORGANISMS 117

of some species, and that the duration of experiments has been an
infinitesimal part of this period. The mere fact that even indi-
viduals are capable of some degree of adaptive adjustment leads
one to believe that the species which they constitute, since their
fundamental qualities are rooted in adaptations, are also plastic
in that respect. But since the species is a more permanent unit
than the individual, it is not difficult to understand its obviously
slighter susceptiliility to change.

Interspecific Relationships. Interspecific relationships among
organisms offer another convincing proof that such change has
occurred. According to the doctrine of special creation, a succes-
sion of forms appeared ranging from simple to complex, with man
as the last. It is not surprising, therefore, according to this or
any other view, that many lesser beings were in existence when
man and related mammals appeared. Parasitic species, however,
of many phyla are dependent upon the human race for food. If
all of these were created in a brief span of time, granted that they
could exist until the creation of man, it has been suggested that
Adam and Eve must have l^een l)eset with all of the creatures that
now live on or in the human ])ody. Such a ludicrous picture is
contrary to reason. Obviously change in the ancestors of these
organisms has led to their association with man since his appear-
ance on earth. If independently created, there is, as already
pointed out, no reason to expect them to resemble so closely other
less specialized forms.

Cause and Effect in Variation. One other phase of organic
change is that of so-called fortuitous variation. It is doubtful that
any variation occurs independently of response to definite condi-
tions, but a complex organism consists of many parts, any one of
which may vary. The entire body of the organism is coordinated,
and each part through its functions affects other parts. This is a
primary result of specialization, but it is conspicuous in such effects
as the endocrine glands produce. Persistence of the thymus results
in gigantism; thyroid insufficiency is responsible for cretinism;
removal of the gonads results in the absence of the normal second-
ary sexual characters. The over-development of parts, or their
failure to develop, is in all cases a definite response to a definite
condition. When, as in these cases, a condition appears without
evident relation to the environment, it is said to be fortuitous.

While a stable condition persists, there is no reason to expect a

118 EVOLUTION AND GENETICS

change of response. The persistence of a response removed from
the normal is a modification of the internal environment, and as
such is capable of initiating further modifications in the organism,
so that, whatever may be their source, modifications established
in an entire species or in a large majority of the individuals of a
species are at least a potential stimulus to permanent change. The
possible behaviour of such factors is another of the problems of
evolutionary theory.

Summary. We have seen that relationship implies not merely
resemblance, but similarity of origin, and that consequently the
various degrees of relationship which are found among the existing
species of animals point to common derivations of varying remote-
ness. The details of relationship indicate definite successions
which represent the probable ancestry of different groups. Change
of individuals is possible in response to change of environment, the
environment is known to change, and single species exist even now
which are capable of responding in ways fundamentally different.
It is especially significant that while existing organisms show the
possibility of merely analogous structures meeting the same need,
in many cases the same fundamental structure is modified to meet
many needs. Evidences of the past occurrence and the present
possibility of evolution are abundant, and the factors mentioned
of variation, adaptation and environment furnish the materials
for further inquiry into the processes involved.

CHAPTER VIII
EVIDENCES OF EVOLUTION (Continued)

2. THE GEOLOGICAL RECORD

If the changes which have occurred in organisms during past
ages were recorded completely, the record would show us exactly
how one species has given rise to others, and how the gradual
change from group to grouj) through increasing degrees of com-
plexity has been accomplished. The result would be a phylo-
genetic tree rooted in the beginning of life, every l)ranch complete
in its connection with the whole. In the chronology of the various
steps would be evidence of the infinite slowness of phylogenetic
change, and in the characteristics of the organisms would be re-
flected the conditions of environment under which they lived.

Fortunately some twigs from this tree are preserved, and
through the study of these fragments of the record and the many
evidences of physical change in the earth's surface, the sciences of
geology and paleontology give us a satisfactory, if incomplete,
account of past evolution. In the findings of geologists there are
evidences of great transformations of the earth, of the elevation
of land masses and mountain ranges, the inundation of great areas
by the oceans, frigid climates resulting from extension of polar
ice caps and tropical conditions following their recession. The
evidences of these changes involve almost incomprehensible
movements of material which could not have been brought about
in less than millions of years. Paleontology correlates witli these
facts the records of organic remains, and in the whole we find frag-
mentary evidence of phylogenetic change which is a valuable
corollary to that shown in existing organisms.

Rock Formation. From evidence which we need not consider
here, geologists have concluded that the earth was once in a molten
state. The first rocks which appeared in the crust as it cooled
were therefore igneous. By weathering and erosion of these pri-
mordial rocks soils were formed and sediments of various kinds
were carried down to the oceans. Here they settled in layers of
mud and sand. Minute organisms living in the sea also settled to

119

120 EVOLUTION AND GENETICS

the bottom as they died, and the calcareous parts of their bodies
formed other layers. Through long periods of time these layers
of sediments were transformed into rocks, — shales, sandstones and
limestones, respectively, — which are called sedimentary rocks in
contrast to the basaltic and granitic igneous rocks formed by the
cooling of molten masses.

Succeeding readjustment of the unstable crust resulted in the
elevation of these stratified sedimentary rocks above the waters
in which they were laid down. They were repeatedly folded and
broken by great cataclysms which left jagged remnants projecting
thousands of feet above the level of the surrounding land masses,
and thus formed our mountain ranges. When once elevated, the
sedimentary rocks themselves were exposed to the forces which
weather and erode, and were in turn cut away and carried down to
the sea to play their part in the formation of new sedimentary
strata lying upon those previously deposited.

Geological Time. The lengths of time involved in this succes-
sion of constructive and destructive processes is inconceivable,
but some slight comprehension is possible when we consider the
enormous thickness of the aggregate deposits and the slowness with
which materials are now being removed from the land masses.
In all, geologists estimate sedimentary deposits to have reached a
mean thickness of fifty-three miles. As Schuchert graphically
expresses it, "this means the more or less rapid wearing away
almost to sea-level, one after another, of more than twenty ranges
of mountains like the present European Alps or the American
Rockies. During the incredibly long intermediate time, when the
lands were planed to a low relief, there was very little erosion."
And yet the continent on which we live is said to be undergoing
denudation to the extent of only one foot in 8,600 years. Since the
earliest record of history, North America has lost, according to
these figures, less than a foot of altitude on the average, yet the
strata found in the Appalachian mountains indicate that they
once reached twenty thousand feet above sea level. More than
fifteen thousand feet of rocks worn away and washed into the sea
at a rate which is imperceptible within human experience!

Even in view of the fact that erosion proceeds at a much more
rapid rate where rainfall is abundant and slopes are steep, these
enormous movements of material prior to the coming of modern
man are impressive. The Appalachian Mountains are the same

EVIDENCES— GEOLOGY 121

to us as to our ancestors of Revolutionary times, yet they were
once as great a range as the Rocky Mountains.

The hill on which Denison University stands is one hundred
feet above the nearest stream and a thousand above the sea, l)ut
at its top there may be found the fossil remains of Brachiopods
and Crinoids, animals which once lived in the ocean. The greatest
cataclysms of modern times have produced no such movements of
material, yet in the terrific forces of a great earthquake we may
find some slight conception of the vastness of geological processes
in the past.

The evidence is before us that these things have occurred. We
may see in a mountain range the twisted, folded remnants of
rocks which could only have been produced by sedimentation.
We may walk in deep grooves ground by the glaciers in solid rock
and select from undisturbed masses above them the remains of
corals which once lived in the ocean. And with these facts we
may consider the visible modifications of the earth's surface to-
day, infinitesimal but relentless.

Time Divisions. On the basis of all changes including climate,
organic development, and modifications of the earth's crust, the
earth's history has been divided by geologists into eras, these into
periods, and these finally into epochs. The first are named from
the prevailing types of life, the second largely from regions where
their characteristic deposits are found, and the last with various
descriptive terms. Each division is determined by characteristic
sedimentar}^ rocks and the fossils included in them, and by the
vertical relationship of these strata the relative chronology of the
various periods has been determined with reasonable accuracy.
It is obviously impossible to estimate such periods in terms of
years with anything approaching exactness. Estimates of the
age of the earth run up to 1,000,000,000 and more years, within
which a few thousands are a slight margin of error. All of these
facts are briefly expressed in the following geological table :

122

EVOLUTION AND GENETICS

Geological Table for North America
(Modified, from Lull's Organic Evolution)

Eras

% OF

Total
Time

Major
Divisions

Periods

Epochs

Advances
In Life

Dominant
Life

?

very
small

Recent
(Post Gla-
cial)

Era of Men-
tal devel-
opment.

Age of Man

Quater-
narjr

Glacial

Pleistocene

Periodic Gla-
ciation.

Extinction of
great mam-
mals.

'o

5

Tertiary

Late
Tertiary

Pliocene

Transforma-
t i 0 n of
man-ape
into man.

Age of Mam-
m a 1 s and
Modern Flo-
ras

o

a

O

Miocene

Culmination
of mammals.

Early
Tertiary

Oligocene

Rise of high-
er mam-
mals.

Eocene

Vanishing of
archaic
mammals.

12

Late
Mesozoic

Epi-Meso-
zoic Inter-
val

Rise of
archaic
mammals.

Cretaceous

Lance

Extinction
of great
reptiles.

"o

0)

Montanian
Coloradian

Extreme
specializa-
tion of rep-
tiles.

Age of Rep-
tiles

^

Coman-
chean

Rise of
flowering
plants.
Oaks and
grasses.

Early
Mesozoic

Jurassic

Rise of birds
and flying
reptiles.

Triassic

Rise of dino-
saurs.

EVIDENCES— GEOLOGY

123

Eras

%OF

Total
Time

Major
Divisions

Periods

Advances
IN Life

Dominant
Life

Epi-Pale ozoic

Extinction of an-

interval

cient life.

Permian

Rise of land verte-

brates.

Rise of modern in-

sects.

Periodic glaciation.

Late Paleozo-

Cycads, ginkgoes,

Age of

ic or Car-

pines.

Amphibians

boniferous

and
Lycopods

Pennsylvanian

Rise of primitive

reptiles and insects.

Mississippian

Rise of ancient

sharks.

Rise of echino-

derms.

Devonian

Rise of amphibi-

ans.

First known land

floras, club

'o

28

Middle Paleo-

mosses, horse-

Age of

zoic

tails and ferns.

Fishes

■2

Earliest ammo-

C3

noids.

Silurian

Rise of lung-fishes

and scorpions.

Ordovician

Rise of land plants

and corals.

Rise of armored

fishes.

Rise of nautilids.

Earliest annelida.

Earliest sea-

urchins.

Cambrian

Rise of shelled ani-

Early Paleo-

mals.

zoic

Dominance of tri-
lobites.

First known ma-
rine faunas, in-
cluding sponges,
coelenteratcs
and echino-
dorms.

Thallophyta.

Age of
Higher
Inverte-
brates

124

EVOLUTION AND GENETICS

Eras

% OP

Total
Time

Major
Divisions

Periods

Dominant Life

(Inferred)

10

Algonkian

Great Epi-Proterozoic in-
terval

o
o

Ke ween a wan

S
2

Major mi conformity

PL,

Animikian

d o fl i i 3

Major unconformity

0) ^ m 2 m «
^ oj 2 ;i: OJ '^,
■^ t, ^ is ^

Huronian

■ss si's t-B

<

o
'o

g

2

10

Neolaurentian

Epi-Neolaurentian inter-
val

Peneplanation of moun-
tains and continents

Neolaurentian Revolu-
tion

Sudburian

o
'o

N

9.

35

Paleolaurentian

Epi-Paleolaurentian in-
terval

Profound erosion of
mountains and conti-
nents

^ 1

= - it

oj o >> S

O o N _ 43 ^

Paleolaurentian Revolu-
tion

a 1

Keewatin Grenvillc
Coutchiching

The Unrecoverable Beginning of Earth History

Fossils. The fossils which enable us to determine the type of
organisms in existence during any of these periods are buried re-
mains of organisms or deposits which record in some other sub-
stance the original form of the organism.

In order that fossilization may occur, it is necessary that the
organism first be buried in the medium which is to preserve it. An
animal may be mired in swampy ground or quicksand, or trapped
in asphalt deposits such as those near Los Angeles. In any of
these cases, it would be quickly engulfed and protected from dis-
integrative forces. Failing such accidents, any body which is
preserved must necessarily have been covered quickly enough by

S

3
O

a

in

^

-^

u

o

-C

a

-f-*

rA

C

CO

o

'^

p

£

^

3

O

a:

&

o

CO

6

c3 — ' K

o

I-

3

126 EVOLUTION AND GENETICS

sediments or wind-borne material to afford it an effective degree
of preservation. Once buried the original tissues of the organism
may be preserved, their form may be reproduced by the addition
or substitution of minerals (petrifaction), or the mold of the
body in the material which surrounded it may persist after the
tissues have disintegrated. Molds are sometimes filled in with

Fig. 67. — Bercsuvka mammoth, Eicpnas pritiujciuus, discovered frozen in
the soil. Specimen as it now appears in Petrograd. (From Lull.)

other mineral matter, forming casts of the original (Fig. 66). In
view of these processes, it is only natural that the sedimentary
rocks contain the majority of the known fossils, since they are
formed from materials which are the most likely to preserve
organic remains.

The perfection of fossils of the several kinds varies greatly.
Records are available of the preservation of animals in Siberia by
a natural cold storage in ice or soil. The remains of a mammoth
(Fig. 67) preserved in this way were found in 1901 at Beresovka,
Siberia, 60 miles north of the Arctic Circle. ''This creature evi-
dently slipped into a natural pitfall of some sort, possibly an ice
crevasse covered with soil and vegetation. A fractured hip and
fore limb, a great mass of clotted blood in the chest, and un-

EVIDENCES— GEOLOGY

127

swallowed grass between the clenched teeth all point to the violence
and suddenness of its passing. Almost all of the animal was pre-
served, though the hair of the back has disappeared and the trunk
had been eaten off by dogs before the specimen was discovered "
(Lull). P'ossils of this type, and any others buried in substances
with preservative qualities, are, of course, not unlike preserved
specimens. The most numerous are the aml)er fossils of the
Oligocene (Fig. 68). The ambers are formed by the transforma-
tion of resinous exudates from coniferous trees, and in their initial
state were of such consistency that even the most delicate insects

A B C

Fig. 68. — Amber fossils. A, an ant; B, a mayfly; C, a flea. (From Zittel.)

could be caught and embedded by them with little damage. As the
resins hardened and turned to amber the contained organisms
were preserved as in a modern microscopic mount. Thousands of
specimens of Arthropoda, chiefly insects, have been discovered in
the Baltic amber deposits of Germany, along the coast of the Baltic
Sea. Although even such delicate bodies as those of insects have
also been fossilized in shales, the amber fossils and frozen and pre-
served remains of animals are naturally more complete and perfect
than any other kind.

Fossils preserved in rocks are often broken and distorted by
the subsequent movements of the strata in which they lie. Con-
sequently they are more often incomplete or imperfect, and de-
mand more careful study and interpretation.

Interpretation of fossils. Environment. The value of fossils
is not alone the record of past life which they give us, but also
evidence of climatic conditions. In living organisms we can see
that certain structures are correlated with certain qualities of

128

EVOLUTION AND GENETICS

Fig. 69. — An ancient climate. (From Knowlton's Plants of the Past, with
the permission of the Princeton University Press.)

EVIDENCES— GEOLOGY 129

environment. When fossils possess similar structures we can be
certain that they met similar needs, and that the environment of
the extinct animal was therefore similar to that of the one now
living. Plants are very closely linked with the physical environ-
ment, hence fossil plants are valuable evidence of the temperature
and rainfall of the period to which they belong (Fig. G9). The
teeth of an animal are equally indicative of the kind of food which
it eats, and if a fossil has shearing teeth, we know at once that it
was carnivorous, while broad, grinding teeth indicate grazing
forms. Thus aridity, forestation, temperature, and various haljits
are disclosed by these remains, and a careful study of all available
evidences has pieced out the record of prehistoric life to a remark-
able degree.

Succession of Forms. The general illustration of evolution de-
rived from such sources is adequately expressed in the geological
table. If evolution has occurred, the most primitive creatures
would necessarily have existed in the earliest periods of the proc-
ess, and successive degrees of complexity would become evident
as time passed. This proves true to such a high degree that we
are justified in looking upon it as a further proof of evolution, and
in adding to the chronological succession the earliest stages, which
can only be inferred, since the organisms must have been too small
and too delicate to be preserved readily as fossils. Fossils have
been reported even from the Archeozoic, but they are open to
doubt. With this initial step alone based only upon estimate, we
find that the Proterozoic rocks contain only primitive inverte-
brates, and that a gradual ascension of forms proceeds through the
remaining eras. Just as we see in the species now living the order
proceeds through the higher invertebrates to the fishes, the most
primitive of vertebrates, thence through the Amphi!)ia and the
reptiles to the birds and mammals, and finally man. Since ani-
mals are entirely dependent on the green plants for their food
supply, it is significant that in this record the first known land
floras precede the true terrestrial animals in development, and that
in many other known details the various parts of the record coincide.

By reference to the table, it will be seen that the late Protero-
zoic, while it is evidently the period during which the marine
invertel^rates were the dominant form of life, is characterized by a
scarcity of fossils. For this reason it is impossii)le to judge the
phylogenetic association of species which must have existed.

130 EVOLUTION AND GENETICS

The next geological division, the early Paleozoic, records so
many forms of invertebrates that we are able to see in the fossils
many evidences of phylogenetic succession. However, this sudden
appearance of many forms deprives us of the opportunity to see
how and from what they arose. We are forced to begin in the
middle of the record, and by the steps which are clearly disclosed in
its more complete portion to judge as best we can what processes
took place in the periods which are forever closed to us.

Through the remaining periods we are confronted by similar
difficulties. It seems that while hard parts are necessary for
effective fossilization, the development of such parts is character-
istic of specialized organisms, and not of those generalized species
which might be expected to give rise to specialized forms. The
record of detailed phylogeny is therefore confined to minor groups
of relatively high development. We are able to trace descent from
genus to genus in the ammonoids, for example, in the Devonian
and later periods, and in the related nautiloids, which probably
arose in the Cambrian. In the Ordovician, however, there are
fossils of armored fishes of whose ancestors we have no exact
record, and in the Devonian, we find evidences of amphibia, al-
though no intermediate form connecting them with the fishes is
known.

However, the major succession is established even by these
abrupt transitions in the geological record. The lung fishes, able
to breathe air, are known to have occurred in the Silurian, prior to
the Amphibia, the Amphil)ia precede the wholly terrestrial verte-
brates, reptiles precede mammals and man is the last species of all
to appear. While a chapter is gone here and there, the entire
record, together with such details as are well preserved, is con-
vincing evidence of gradual succession.

Succession Within Animal Phyla. Among the invertebrate phyla,
the status of individual groups also shows definite succession.
The Protozoa, now recognized as the most primitive existing ani-
mals, are too small and delicate to be readily fossilized. Some are
specialized, however, in the production of calcareous, siliceous, or
chitinous tests, and consequently are readily preserved (Fig. 70).
The Porifera, also very low in the systematic scale, are preserved
because of their hard spicules, and are found among the earliest
Paleozoic fossils in the Cambrian. The same is true of such Coelen-
terates as are preserved. Worms, because of their soft ])odies, are

EVIDENCES— GEOLOGY

131

not abundant among fossils, but tho protective tubes and hard
teeth of marine AnneUda occur even as early as the Ordovician.
The Bryozoa, now rela-
tively imimportant, were
very numerous during the
upper Cretaceous. The
genera then living are now
extinct, however, while
those which followed dur-
ing the Pliocene are still
represented, in some cases
l)y the same species which
then existed. The Brach-
iopods show a very similar
course in their develop-
ment, but reached the
climax of diversity in the
Ordovician and Silurian,
and have since become
less numerous. Most of
the Tertiary species are
congeneric with those now
living. The Echinodermata
reached their climax in the
Paleozoic. They were
represented by three classes
which are now entirely
extinct. One, the Crinoidea
(Fig. 71), is represented
by a few living species
but was once well diver-
sified, and four, including
the sea cucumbers, sea
urchins, starfishes and
brittle stars have come
down from the early Pa-
leozoic. All of these

are among the lower phyla of animals, and it is to be noted
that they extend well back toward the beginning of the fossil
record.

Fig. 70. — Skeletons of typical Protozoa.
B, siliceous skeleton of a radiolarian,
Stauraspis stauracantha, x 170; C,
calcareoas skeleton of a typical fora-
minifer, Globigerina hidloidcs, x 30.
(From The Origin and Evolution of
Life bj' Henry Fairfield Osborn,
courtesy of Charles Scribner's Sons.)

132

EVOLUTION AND GENETICS

The Mollusca. This is the highest group of unsegmented
invertebrates that has yet appeared. They are characterized

by (1) the absence
of appendages, (2) a
ventral ' ' foot ' ' which
is usually associated
with locomotion, (3)
the mantle, a dorsal
fold of the body
wall which often
secretes a shell and
encloses a space in
which lie the gills,
(4) a heart consisting
of two auricles and
a ventricle, (5) a
tubular vascular
system associated
with spaces called
lacunae in which the
blood also circulates,
and (6) develop-
mental stages of
which one resembles
some larval annelids.
The Cephalopoda
are apparently the
most highly devel-
oped class. They
include the four
existing species of
Nautilus, the cuttlefishes, squids, Octopus, Argo7iauta, etc., all
of which have the highly developed eye described in a pre-
vious chapter.

The shell is, of course, the part of a molluscan body most
easily preserved as a fossil. Consequently it is on the change
of structure of the shell that our knowledge of phylogeny is
based. The phylogeny of the group is indicated in the follow-
ing diagram, which has been modified to indicate the change of
shell structure.

Fig. 71. — A crinoid, Pentacrinus madearanus.
(From the Cambridge Natural History.)

EVIDENCES— GEOLOGY

133

Octopoda
8 armed
Argonauta. Male
shell-less,
female with
external shell,
unchambered.
Octopus. Shell-
less.

Belemnoidea
10 armed
Belemnites. Internal

shell, partially
chambered, extinct.
Spirula. Internal
shell, wholly
chambered.

Sepioidea
Internal vestigial
shell. Squids &
cuttlefish.

Nautiloidea
Shell pri-
mitive with

simple
sutures, not
highly ornamental.

Mollusca,

primitive radicle

(Modified, after Lull)

By comparing this diagram with the table it will be noted that
Mollusca of other groups occurred in the Cambrian. The earliest
Nautiloidea, derived at some unknown
point from the moUuscan stem, are the
oldest group of cephalopods, and appear
first in the Ordovician. From this stem
arise the more highly specialized Am-
monites, which first appear in the De-
vonian (Fig. 72). The Ammonites then
become extinct, while the nautiloids con-
tinue to the present, although with de-
creasing abundance and diversity. A
divergence of the ancestral stem gives
rise to three other forms, the Belem-
noidea, Octopoda, and Sepioidea. Of
the main branch, the belemnoids, many
fossils are found in the Jurassic and
Lower Cretaceous rocks, apparently de-
rived from another group during the
early Triassic. The squids, however, F^^- 72.-A shell oi 11 ete-
"^ 1 T • u-i iu roceras, a cephalopod

appear first m the Jurassic, while the moll use. (From Lull,

origin of the Octopoda is not indi- after Schuchert.)

134

EVOLUTION AND GENETICS

cated by fossils. The belemnoids persist in the one genus,
Spirilla.

The Arthropoda. Another significant group of invertebrates is
the phyhnn Arthropoda (Fig. 73). In this phyhim a chitinous
exoskeleton is well developed, favoring fossilization. Another
distinctive characteristic, the presence of jointed appendages,
gives the phylum its name. The body is segmented. Because of
the high development of the group, it is looked upon as the culmi-
nation of evolution of seg-
mented invertebrates.

The arthropods are at pres-
ent more successful than
any other group of animals
for three reasons; the same
may be said with equal truth
of one of the included classes,
the insects. Of these there are
three times as many species
as of all other animals to-
gether. Many of the species
produce immense numbers of
individuals, and finally, there
are almost no habitats on
earth of which the arthropods
have failed to avail them-
selves.

Arthropod Ancestors. The
earliest organisms which show
arthropod characters of segmented body and jointed appendages,
with a hard exoskeleton, are the trilobites (Fig. 74). During the
Cambrian they were numerous, exceeding in abundance and diver-
sity all other kinds of animals. In the Devonian they began to de-
cline, and in the Permian became extinct. They were aquatic ani-
mals, as is shown by their association with marine deposits. They
are characterized by the division of the dorsal shield by two longi-
tudinal grooves into three parts, hence the name trilobite. The
body is divided transversely into an anterior head called the
cephalon, a series of segments forming the thorax, and a tail piece
or abdomen. The jointed appendages are biramous, as in the
Crustacea, with the exception of the anterior pair, the antennae.

Fig. 73. — The crayfish, an arthropod.
(From Comstock's Introduclion to En-
tomology, with the permission of the
Comstock Pubhshing Company.)

EVIDENCES— GEOLOGY

135

The outer branch of the appendage, or cxopodite, has a long
fringe which apparently converted it into a swinniiing and
respiratory organ. In all of these characters and in others,
the trilobite corresponds closely with fundaniontal crustacean

Fig. 74. — A trilobite, Triarthrus hecki, restored. 1, dorsal aspect; 2, ventral
aspect. X 2. (From Lull, after Beecher.)

characters which may be expected in ancestors of the latter
organisms.

Aquatic Arthropoda and Their Descendants. On this basis the
Crustacea are judged to have arisen from the trilobites very
early in the Paleozoic. Some species long extinct show unmis-
takable similarity to the trilo])ites. One closely related group of
primitive Arthropoda, the Palaeostraca, is still represented bj^ a
single species, the horseshoe cral). This group included forms,
now extinct, from which developed the primitive Arachnida in
the Silurian. The class Arachnida persists in the nimierous
modern spiders, mites, ticks, scorpions, etc., all terrestrial
species.

136 EVOLUTION AND GENETICS

Terrestrial Arthropoda. The source of the Chilopoda and
Diplopoda (centipedes and milUpedes) is obscure, but they seem
to have come from trilobites.

Handlirsch interprets the insects as descendants of a trilobite
whose second and third thoracic segments bore extensions of the
lateral lobes which later developed into wings.

Several more obscure classes have also been worked out by
Handlirsch from his studies of fossils. Of these the Onychophora
are significant as the most primitive air-breathing or tracheate
Arthropoda. They are worm-like animals which show definite
evidences of segmentation in the paired appendages and nephridia.
They show evident annelid affinities.

Handlirsch incorporates his analysis of arthropod phylogeny
in a diagram which is here reproduced in part, with the group
names translated into familiar terms where possible (Fig. 75) .

Insects. The two insect groups indicated are looked upon
as subclasses. The Apterygota include such famiUar forms as the
fish moths or silver fishes and the spring-tails. They are primitive
wingless forms which apparently have an origin associated with
that of the more abundant Pterygota. While many of the latter
are wingless, most of them are winged, and all show evidences of
relationship with winged ancestors. It is in this subclass that
most well-known insects belong.

Metamorphosis. The two subclasses differ further in that the
Apterygota are without metamorphosis. When they hatch from
the egg they resemble the adult, and the only conspicuous change
is growth. The Pterygota when hatched may resemble the adult
to some degree but important changes always take place before
maturity is reached. Three types of metamorphosis are recog-
nized in the Pterygota. The Paurometabola are not unlike the
adult when hatched although they lack wings. They live like the
adults and develop gradually into mature insects, the chief trans-
formation being in the development of wings. The Hemimetabola
are wingless naiads when hatched and are adapted for a mode of
life very different from that of the adults. After a period of growth
their last transformation brings them suddenly into the adult
stage which is normally winged (Fig. 76). The Holometabola
emerge from the egg as larvae, very different in appearance from
the adults (Fig. 6). They are wingless and often have differently
formed mouths. Some are caterpillars, some grubs, some maggots;

EVIDENCES— GEOLOGY

137

Id

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Mesozoic

Permian

Carboniferous

Devonian

Silurian

Cambrian

Archeozoic

Fig. 75. — Diagram of the evolution of the major groups of Arthropoda.

(Modified after Handlirsch.)

138

EVOLUTION AND GENETICS

all represent the growing stage of the insect and all transform
ultimately into a more or less different resting or pupal stage.
During the pupal stage the transition from larval structure to the
very different adult structure is accomplished. The pupa trans-
forms in due course into the adult, which undergoes no change of
form and does not grow. The development of metamorphosis
is evidently a factor which enabled the insects to meet the in-
creasing rigors of climate during the period of their evolution,

Fig. 76. — A may-fly, Ephemera varia. A, adult; B, naiad. (After Needham,
from Comfstock's Inlroduclion to Entomology, with the permission of the
Comstock Publishing Company.)

since it provides for periodical growth and dormant stages in a
very effective manner.

Diversity of Insects. Structural modifications of insects are,
as might he expected in a class containing more than 500,000
species, extremely diverse. Every part of the body displays some
modification, but the main changes are in the mouth parts and

wmgs.

The Mouth Parts. As in all Arthropoda, the mouth parts of the
insects are apparently modified jointed appendages. In the primi-
tive state they are formed for biting and chewing, but various
modifications give rise to suctorial and lapping mouths of the kinds
found in true bugs, flies, butterflies and moths, and bees. The
biting mouth consists of a pair of mandibles behind which lies a
pair of maxillae, and behind the maxillae there is a labium, origi-
nally paired. The mouth parts of a cockroach illustrate this type
(Fig. 77). Suctorial mouths consist of a trough-like structure

EVIDENCES— GEOLOGY

139

formed of the labium fin bugs
and flies) or a more or less com-
plete tube formed of the maxillae
(butterflies and moths ; the honey-
bee) in which other modified parts
may slide back and forth. The
long, slender, bristle-like mandi-
bles and maxillae operate in this
way in the bugs (Fig. 78). In the
flies a similar modification occurs.

In both
orders the

Labrum

_.Mandlble

f „ , Glossae or lleula
f Galea : , , , ,

SLuclnia i labial palp

Stipes. ••

Maxillary

palp

Paraglossa

Cardo A V ^""^^ maxilla

MentumVT' V/"Submentum
Second maxUlffi

slender Fig. 77.-

parts may

roach.

-The mouthparts of a cock-
(From Hegner, after Kerr.)

be developed into sharp piercing organs.
The maxillae of butterflies form a closed
tube through which liquids are drawn by
muscular suction. In the honey-bee they
form a partial sheath for the hairy ligula,
a slender part of the labium, which slides
back and forth between them and raises
liquids to the mouth.

The Wings. Some of the primitive an-
cestors of the insects had two pairs of
well developed wings on the second and
third thoracic segments and a pair of broad
lobes on the prothorax which appear to
have been rudimentary or vestigial wings
(Fig. 79A). In the existing species the pro-
thorax has no trace of wings but the persist-
ing wings of the remaining divisions supply
some of the finest evidences of evolution.
They vary in texture from thin membranous
structures with heavier supporting veins to
heavy chitinous shields, in some cases so
hard that they must be drilled before a
stock's Introduction to pin oan be passed through the insect. In
Entomology, with per- gome insects they are reduced to two, usu-
ITpubLhin'gS™: S'lly *c anterior pair (Fig. 80). Otlier
pany.)

Fig. 78. — Last segment
of the beak of Letho-
cerus, a bug. md, man-
dibular setae; mx, max-
illary setae; t, tactile
hairs. (From Com

insects have lost them entirely.

140

EVOLUTION AND GENETICS

The venation of the wings is based on a definite plan which
shows modification in several ways toward greater complexity or
simplicity (Fig. 136).

In all of these structures and in their metamorphosis insects
show the possibility of almost infinite modification to fit varied

environmental conditions.
Their diversity is exceeded
by no other animals and
consequently furnishes an
excellent illustration of an
actual phylogenetic series.

Since Handlirsch's studies
of insect phylogeny are
more nearly complete than
those of any other ento-
mologist, his table is here
reproduced with the popu-
lar equivalents for the or-
dinal names indicated
wherever such terms are
available (Fig. 81). It will
be noted that the extensive
fossil remains of the Car-
boniferous and Permian are
in only four cases referable
to modern orders. All of
these creatures are insects,
beyond a doubt, but they
have almost completely dis-

FiG. 79.— Palaeozoic insects. A, Sienodichja appeared, leaving a more
lobata; B, Eubleptus danield. Both be- or less different fauna to
long to the primitive Palae<)dictyoptera. ^ake their places. As the
(From Lull, after Handhrsch.) » ,, ...

names oi the pnmitive or-
ders indicate, some resemble modern orders. Others are wholly
unlike existing insects except in fundamental structure.

Such diagrams as this are necessarily based on all available
knowledge. Comparative anatomy, embryology, and palaeon-
tology alike contribute to the formulation of a reasonably complete
result. Since the actual record of evolution is broken we must be
content with such information as we can piece together from the

EVIDENCES— GEOLOGY

141

available fragments, and it is gratifying that it is so often adequate.
The mere fact that fossils prove the past existence of forms no
longer living although often related to existing forms is in itself an
evidence of the actual course of evolution, and the transition from
form to form through the ages is even more significant.

In the phylum Chordata the record covers a briefer span and
is more complete. Both for these reasons and because we our-

FiG. 80. — A male stylopid, Oplhalmochlus duryi. The front wings are short
club-shaped appendages while the hind wings are used for flight. Most
two-winged insects retain the front wings. (After Pierce, from Comstock's
Inlroduclion to Entomology, with the permission of the Comstock Publishing
Company.)

selves are vertebrates it is of great interest and deserves special
treatment in succeeding chapters.

Summary. The present state of the earth is the result of a
succession of physical phenomena. The ancient igneous rocks have
been supplemented by layers of sedimentary rocks formed from
the hard parts of minute organisms under water and from
materials washed down from land masses. These changes have
occupied an enormous span of time. Organisms of the various
periods have been Ijuried and preserved as fossils. From the com-
bined study of the rocks and fossils it is possible to learn of the
conditions prevailing at different times, as shown by the compari-
son of organic remains with existing organisms and the conditions
under which they live. The succession of organic forms is disclosed
in the same study. The result is a relative and incomplete record
of evolution, but it is sufficiently complete to show that a gradual

142

EVOLUTION AND GENETICS

•s

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u S

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5

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Q S H

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Fig. 81. — Diagram of the evolution of the insects. (Modified after Handlirsch.)

EVIDENCES— GEOLOGY 143

addition of more and more complex organisms to the population
of the earth has occurred as time has passed. The succession cor-
responds to the major sequence of existing phyla and even within
minor groups is often a valuable indication of phylogeny.

REFERENCES

Von Zittel, K. A., Textbook of Palaeontology, translated by Eastman, C. R.,

1900.
Scott, D. H., The Evolution of Plants, 1911.
Thomson, J. A., Outline of Zoology, 6th edition, 1914.
Lull, R. S., OrgaJiic Evolution, 1917.

Lull, R. S. (editor). Evolution of the Earth and Its Inhabitants, 1918.
Handlirsch, a., Schroder's Handbuch der Entomologie, 1925.
Knowlton, F. H., Plants of the Past, 1927.

CHAPTER IX
EVOLUTION OF THE VERTEBRATES

In no phylum is the course of evolution more clearly or com-
pletely indicated than in the phylum Chordata, particularly the
subphylum Craniata, commonly called vertebrates. As has
already been shown, the details of comparative anatomy and em-
bryology in this group are remarkably clear evidences of relation-
ship of the various classes in a succession of stages which point
clearly to a chronological as well as merely comparative series.
In addition the geological record is excellent. The bony endo-
skeleton and many hard integumentary structures, such as teeth,
scales and protective armor are well fitted for preservation as
fossils, and conditions have often been favorable for the fossiliza-
tion of many individuals.

The Origin of the Vertebrates. In spite of these conditions
within the phylum, we are faced at the outset by a break in the
paleontological record second to no other. The vertebrates appear
in the Ordovician as well developed fishes without any indication
of their invertebrate ancestors. In the contemporary invertebrate
fossils and those of the Cambrian are found many forms that may
have been ancestral or derived from the same source, but the gulf
is great. We still have only theories to explain the origin of the
vertebrates. While each theory finds facts to support it both in
the study of living forms and in the geological record, as might be
expected of organisms which have some degree of common relation-
ship, none establishes that completeness and certainty of phylog-
eny which is to be desired.

The three leading theories of vertebrate descent are the Am-
phioxus theory, the annelid theory, and the arthropod theory.
Several minor theories have been formulated which are of interest
only to the special student.

Since Amphioxus is a chordate, the probability that it is similar
to the ancestor of the vertebrates is merely one step in establishing
their origin and does not indicate a connection with the inverte-
brate phyla. The annelid theory brings out facts which, with the

144

EVOLUTION OF THE VERTEBRATES

145

Amphioxus theory, afford a very satisfactory hypothetical con-
nection of vertebrates and invertebrates.

The arthropod theory is at fault chiefly in tracing the vertebrates
to invertebrates which were themselves highly specialized and
has not been given general support. The fishes through which the
connection is traced are looked upon as specialized types and such
resemblance as they show with fossil arthropods may therefore
be due to convergence.

Hypothetical Ancestors. It is generally agreed that the ances-
tors of the vertebrates must have been free-swimming, active,

Fig. 82. — Am-phioxus lanceolatus. (PYom Newman, after Wille3^)

aquatic animals. They are equally certain to have been bilaterally
symmetrical with well developed heads, and definitelj^ metameric
in structure. They must also have possessed the three funda-
mental characters of the chordates, viz., tubular dorsal nervous
system, notochord, and pharyngeal clefts.

The fishes are recognized as the most primitive of the true
vertebrates, and such an ancestor could not have been very dif-
ferent from the typical fishes. The existing lancelets {Branchios-
toma or Amphioxus) of all animals below the vertebrates best fit

146 EVOLUTION AND GENETICS

these specifications. They are small fusiform creatures with a
dorsal tubular nerve cord, well developed segmental musculature,
a notochord, and numerous pharyngeal clefts. They are able
either to lead a sedentary life or to swim about.

Amphioxus has some characters which are not primitive, but it
corresponds so nearly in its general structure with the hypothetical
ancestor of the vertebrates that we are justified in considering
it an approximation of their true progenitors. In the course of
time the lancelets of the present have apparently taken on only a
few new characters while the vertebrates have been attaining their
remarkable diversity (Fig. 82).

The Amphioxus Theory of Vertebrate Origin. The possibilities
of development of lancelets or similar ancestral forms is suggested
by the habits of the existing species. They live along sandy shores
and pass some time buried in the sand, but in response to tidal
action they are able to swim rapidly against the currents. Their
food concentrating mechanism is of an unusual type, well adapted
to sedentary life. The animal is one of those peculiar forms men-
tioned in the last chapter, able to respond with equal facility to
widely different conditions of environment. As pointed out be-
fore, the constant action of one environmental condition would
be expected to call forth only one response. Hence a shift into
the still waters of the ocean might develop its sedentary characters
and produce something akin to the tunicates, while the opposite in-
fluence, constant motion of the waters in swift flowing streams,
might bring out the characters which fit Amphioxus for active
resistance to such motion. Here the geological record supplies the
interesting information that the earliest fossil fishes show associa-
tion with fresh waters, and not with marine deposits.

The Annelid Theory of Vertebrate Origin. The annelid theory
is based on the resemblance of embryonic characters of vertebrates
to certain annelid structures, and on the resemblance of the adult
vertebrates in general plan to an inverted annelid, with a different
mouth and anus (Fig. 83). Embryology shows that the mouth
and anus of vertebrates actually develop secondarily, and are not
at the original ends of the primitive gut, as in the annelid. More-
over, several highly developed invertebrates of the present live
normally in an inverted position. The theory is thus less peculiar
than it seems at first thought.

Reversal of the annelid results in the nerve cord lying above the

EVOLUTION OF THE VERTEBRATES 147

alimentary tract as in the vertebrate. Formation of a ventral
mouth would then cut off the anterior end of the alimentary tract,
which in the annelid passes through the C(>ntral nervous system.
The entire central nervous system woukl then lie on the same side
of the alimentary tract. In the annelid the blood flows forward in a
dorsal vessel, down through paired connectives, and back through
a ventral vessel; by reversal this results in a flow similar to that
of the vertebrates, forward from the hc^art through tlie ventral
aorta, dorsad through the paired aortic arches, and back through

OI'raMHV

Fig. 8.3. — Reversible diagram illustrating the annelid theory of vertebrate
origin. Index letters applying to both forms: S, brain; X, nerve cord; H,
alimentary canal, applying to annelid only; m, mouth; a, anus; applying to
vertebrate only; st, stomodaeum; pr, proctodaeum; nt, notochord. (From
Wilder 's History of the Human Bodij, with the permission of Mrs. H. H.
Wilder and Henry Holt and Company.)

the dorsal aorta. The typically vertebrate notochord finds .its
counterpart in the Faserstrang, a fibrous structure with similar
anatomical relations which appears in the annelid.

While the idea is no more than a theory, we cannot fail to see in
it the possibility that vertebrates may have arisen through a form
similar to Amphioxus from some invertebrate with annelid char-
acters.

Origin of Vertebrate Classes. Given the existing classes of
vertebrates, there is much evidence to explain their common
sources. The cyclostomes are probably derived from remote and
unknown ancestors. The remaining vertebrates are known as the
Gnathostomata, or hinge-mouthed animals, and come undoubtedly
from a common source. The sequence from fishes to amphibia,
thence to reptiles, and from this class by different lines of descent
to the birds and mammals is well marked. Whether we trace
entire groups with all available evidence, or single organs and sys-
tems alone, the succession is in most cases clear and well sub-
stantiated.

Emergence of Terrestrial Vertebrates. From the most primi-
tive vertebrates, the fishes, the initial step is one of the greatest

148 EVOLUTION AND GENETICS

to be taken anywhere in the development of the higher classes.
The transition from aquatic to terrestrial life between fishes and
amphibia, or better still between fishes and reptiles since the am-
phibia are still in a transitional state, involves fundamental
changes of no slight degree.

Water as an environmental medium contrasts with the air in
several important particulars. (1) Because of its higher specific
gravity it buoys up the organism with greater force. (2) It pre-
vents the loss of water from the body by evaporation. (3) It
offers greater resistance to motion on the part of immersed bodies.
(4) It presents different conditions of visibility.

The Demands of Terrestrial Life. While removal from the
water to the air demands relatively slight modifications in response
to some conditions, to others extreme adjustments are necessary.
The body is no longer buoyed up completely by the surrounding
medium, but is of so much higher specific gravity that it must
rest on the ground. This demands a different type of locomotion
involving even in primitive forms limited points of support for the
body, and consequently a more rigid skeletal structure. The in-
tegument must be modified to conserve moisture within the body
in proportion to the dryness of the air. While aquatic animals need
streamline bodies if they are to move rapidly, in the air the
resistance is so much less that body form is of relatively little im-
portance. The difference of visibility in the air makes possible
some modifications of the eye. In addition, the two media demand
entirely different respiratory organs, and correspondingly different
circulation.

For complete separation from the water as a habitat, the de-
velopment of fetal membranes is an apparent essential, due no
doubt to the delicacy of eml)ryonic tissue and the resulting neces-
sity for protection against dessication and mechanical injury.

Transitional Forms. A first step in tracing the theoretical
portion of vertebrate evolution is to decide what characters would
be present in a species capable of developing into terrestrial forms.
To substantiate the hypothetical ancestor, it is necessary to de-
termine whether or not such organisms have ever existed. Finally
we must determine whether environmental conditions during the
geological period of the transition were favorable for the change.

Since the earliest vertebrates were fishes, the ancestors of the
terrestrial forms must have belonged to this class. In order to

EVOLUTION OF THE VERTEBRATES 149

accomplish the shift of habitat, these ancestral forms must have
been able to breathe air, and because of the extreme changes in
locomotion, they must have had structures adequate to move
their bodies over a solid substratum. The other conditions of
change need not have been mot at once. If the primitive terres-
trial vertebrates remained in a moist environment, as would be
expected, protection against dcssication would be unnecessary.
Changes in the special senses and in the skeleton would be valu-
able, but in the absence of competition with other terrestrial
species, would not necessarily be of vital importance. This transi-
tional state is nicely illustrated by the existing Amphibia, par-
ticularly the tailed species, the newts and salamanders.

When we seek fishes with some capacity for terrestrial life, we
find that two groups are capable of breathing air by a diverticulum
of the alimentary tract fundamentally similar to a primitive verte-
brate lung. While other fishes are able to exist out of water,
the blennies even to the extent of leaving it to escape their enemies,
they are adapted in ways different from the usual terrestrial
forms. One of the two significant groups is the subclass Dipnoi,
the lung-fishes. The other is the order Crossopterygii of the sub-
class Teleostomi. The Dipnoi are an interesting group but for
the purposes of this study they may be set aside. Their paired fins,
which alone are in a position to aid in locomotion on a solid sub-
stratum, are very different in structure from the pentadactyl
appendage and cannot be looked upon as precursors of such ap-
pendages.

The Crossopterygii. In the order Crossopterygii, on the other
hand, the structure of the pectoral fins shows a surprising degree
of resemblance to the terrestrial liml). The fishes of this order use
the pectoral fins not merely as l^alancing organs, but also as paddles
in swimming, and when resting on the bottom as support for the
anterior part of the body. Like the pentadactyl appendage they
have a single proximal bone, two bones distal to it, and several
series of smaller bones distal to the two. To the small bones is
attached the fringe of dermal rays which support the aquatic part
of the appendage. However the fins are not the only point of
resemblance. The fishes breathe air at the surface of the water
by means of a double air sac connected by a single tube to the
ventral side of the pharynx, a condition resembling the lungs of
Amphibia and the early embryonic lungs of other classes. The

150 EVOLUTION AND GENETICS

larvae are said to resemble those of Amphibia. In short, the
Crossopterygii have a number of the characters of the hypo-
thetical ancestor of terrestrial vertebrates.

The Period of Transition. In the geological record a very
significant fact is the abundance of these fishes in the Devonian,
the period which also saw the rise of the Amphibia.

During this time, the paleontologists tell us, climatic conditions
were such as to force upon animals the change from aquatic to
terrestrial life. As Lull expresses it, "Diastrophic movement
during the Silurian period initiated a widespread aridity which
culminated in the latter part of the period, continued with varying
intensity into and through Devonian time, and rose again to greater
severity in the latter part of that period. This meant, as in Aus-
tralia today, the reduction of rivers and other bodies of fresh
water and the entailed concentration of their fauna, which is
borne out by the mode of occurrence of the Lower Devonian (Old
Red Sandstone) fishes — innumerable specimens in very restricted
areas. Add to this the diminution of aeration of these waters and
it will be seen that a high premium would be placed upon powers
of air breathing or of aestivation. Still further dessication would
necessitate some sort of activity during the increasingly long
droughts, for the periods of torpor would otherwise bear too great
a ratio to the creature's Hfe span. Thus a premium would be
placed upon ability to crawl ashore and maintain an active life,
while the less fit would sleep the sleep that knows no waking, to
their racial extinction."

The Amphibia. Once able to exist on land there would be
abundant reason for animals to continue their development toward
greater fitness for the new mode of Hfe. In the waters would be
concentrated all of their ancient enemies; on land would be freedom
up to the limits imposed by their own structure. The story of
vertebrate succession, already touched upon many times, shows
by what means this has led up to the maximum development of
the class. The geological record shows how many species, once
successful, have fallen before unfavorable conditions because they
were unable to meet the requirements of a changing environment.

The development of the Amphibia as terrestrial organisms
probably l^egan in the Middle Devonian period and extended
through the Carboniferous when the drying of the earth's surface
produced vast swampy areas. Restriction of bodies of water and

EVOLUTION OF THE VERTEBRATES

151

the persistence of abundant wet regions were ideal conditions for
amphibian evolution. It is recorded of the earliest transitional
types that they retained both lungs and gills, and had both limbs
and a tail fin. During the Carboniferous they attained a consider-
able variety of forms, but the Permian brought an extension of
continental areas, with relatively dry surfaces and seasonal

-=:.^:=5^.j=z^

O F

Fig. 84. — Group of extinct Amphibia. A, B, D and E from the Carboniferous;
C and F from the Permo-Carboniferous. A, Pylonius; B, Ampliihamus;
C, Cacops; D, Cricotus; E, Diplocaulus; F, Erynps. (From Newman, after
Osborn, based on restorations by Gregory and Deckert.)

changes in the bodies of fresh water, which were unfavorable to
amphibian life. They have been able to persist, but beset by the
limitations of two environments and the narrowness of a transi-
tional life zone they have been unable to rise above a corresponding
limitation of forms.

During their ascendency some time in the early Carboniferous
the Amphibia developed a number of terrestrial forms which were
made possible by the altc^rnation of arid seasons with periods when
the streams were full to overflowing and herbaceous flood-plain

152 EVOLUTION AND GENETICS

vegetation was Abundant. Among such Amphibia, the Stego-
cephalia or "solid-headed" forms, were the ancestors of the rep-
tiles (Fig. 84). Already well adapted in many ways to the con-
ditions of terrestrial life, they were forced to make use of these
adaptations or perish when the increasing aridity of the later
Carboniferous deprived them even of the seasonal opportunity to
return to the water.

The Reptiles were abundant and well diversified in the Permian.
Some of them developed peculiar structures which had no use that
we can now distinguish, and all of the Permian species are now
extinct. Among the orders represented by Permian reptiles, how-
ever, are probal)ly the Chelonia or turtles, and the Rhynchoce-
phaha, both still in existence. Primitive crocodiles also probably
appeared, but the other great orders of extinct reptiles and
the snakes, lizards and crocodiles of the present arose much
later.

Among the adaptations by which the reptiles are differentiated
from their amphibian ancestors are the resistant integument and
the foetal membranes, both associated with life away from the
water. The integument even of the early species was tough, and
provided with scales or bony exoskeletal structures. Through this
adaptation the animals were able to remain constantly in dry air
without losing more water from the body than could be replaced.
The foetal meml^ranes, as already noted, are apparently essential to
reproduction elsewhere than in water.

Beyond these essential structures the reptiles also show a great
variety of adaptive possibilities which resulted in the development
of fish-like aquatic forms, flying forms, the dinosaurs, cynodonts,
and a few of less importance (Fig. 85). They attained their
greatest diversity in the Mesozoic, where they were represented
by some of the greatest animals that the world has ever seen. The
aquatic ichthyosaurs and plesiosaurs included highly specialized
animals, some enormous in size. Newman graphically describes
one, Trmacromerion, as "a creature with all the earmarks of an
aquatic speed demon, and doubtless as much of a terror to the
fishes as were the dinosaurs to the smaller denizens of the dry
land." These latter were of two fundamental types, the car-
nivorous and herbivorous dinosaurs. They ranged from small
species to creatures as large as the modern whales, and from heavy,
sluggish forms, protected by massive bony armor, to active preda-

EVOLUTION OF THE VERTEBRATES

153

cious bipedal giants which must have been a terror to the other
inhabitants of the earth. The pterosaurs flew by means of folds
of skin extending from the hind limb to one enormously elongated
digit of the fore limb. In the cjmodonts, or dog-toothed reptiles,
we find the probable ancestors of the mammals, and in some an-

FiG. 85. — Group of Mesozoic reptiles. A, long-necked plesiosaur, Elasmo-
saurus; B, short-necked plesiosaur, Trinncromerion; C, ichthj'osaur, Bap-
tanodnn; D, pterodactyl; E, ostrich dinosaur, Slrulhiomimus; F, carnivorous
dinosaur, Tyrannosaurus; G, giant herbivorous dinosaur, Brachiosaurus;
H, hooded duck-bill dinosaur, Corythosaurus. (From Newman, after
Osborn.)

154 EVOLUTION AND GENETICS

cestral dinosaur or an even more primitive form, the forerunner
of the birds.

The Origin of Birds. The origin of the birds is another lost
step in the evolution of the vertebrates. That they came from
some extinct reptilian form is certain, but paleontology has no
definite information to offer concerning these ancestors. Some of
the dinosaurs show characteristics similar to those of birds in
skeletal structure, bipedal locomotion, and the development of a
beak. While these facts do not point to such dinosaurs as an-
cestors of the birds, they do harmonize with the paleontologists'
theory that both birds and dinosaurs came from the same stock.
According to this theory the ancestor of the birds must have
been either a primitive dinosaur or an unknown common an-
cestor.

Flight Adaptations. The chief characteristics of the birds are
associated with flight. By comparing them with the reptiles we
find that no other distinctive characters are present. The avian
skeleton is highly specialized for lightness and rigidity by the
development of hollow bones and fusion of separate parts. The
scales are restricted to the legs and feet, where they are the same
as reptilian scales; elsewhere the l^ody is covered with feathers
which serve the double purpose of light planes for flight and a
warm covering for protection against the low temperature of the
upper air. Although much more complex, feathers are seen by
their development to be closely related to scales. The digestive
system is specialized to provide the al^undance of energy required
by flight. The lungs are so constructed that air enters the alveoli
through one passage and leaves through another, so that every
breath brings an entirely fresh supply of air into contact with the
respiratory epithelium. The vascular system is completely
divided into pulmonary and systemic circulations. High and
constant body temperature is maintained by a vaso-motor system.
It serves to promote the rapid metabolism demanded by flight
and to protect the bird against low temperature.

Theories of the Origin of Flight. In view of the high degree
of specialization indicated by these characters, and the lack of
definite fossil ancestors, it is not surprising that all explanation
of the origin of birds should rest upon theories of the origin of
flight. These theories include cursorial origin, arboreal origin, and
diving origin. All assume the development of broadened limbs

EVOLUTION OF THE VERTEBRATES

155

through extension of the scales, and the use of these broadened
limbs as aids to locomotion. Many birds now flap their wings
for this purpose while running. Such primitive wings would also
have been of aid in jumping from branch to branch or from tree

Coracold ■
Humerus

• Furcula, ; ;

^r4^-'^^, ;; /m

' ' It

Radius

Fig. 86. — Archaeopteryx lithographica, as it appeared in the fossil specimen.
I-IV, digits. (From Hegner after Steinmann and Doderlein.)

to tree, or in sailing out over the water to dive for fish. None of
the theories has any particular advantage over the others, although
the one fossil pro-avian, Archaeopteryx, was probably a climber.
However, this may indicate either that it climbed trees or that it

156 EVOLUTION AND GENETICS

climbed rocks or cliffs in order to glide or flap out over the water
as it dived for fish.

Archaeopteryx. This pro-avian is represented by well preserved
fossils which give an excellent idea of its structure (Fig. 86). It

Fig. 87. — Nestling hoatzin, climbing with thumb and forefinger. (From
Jungle Peace by William Beebe, published by Henry Holt and Company,
with the permission of author and publisher.)

lacked a number of the characteristic flight adaptations of birds.
The bones, for example, were not hollow, and fusion had not oc-
curred to the extent noted in modern birds. The sternum lacked
the deep keel which provides attachment for the powerful flight
muscles of birds. The tail was not reduced to the rudder-like
condition found in modern birds, but consisted of a series of

EVOLUTION OF THE VERTEBRATES 157

vertebrae and bore lateral feathers. The wings were w(^ll de-
veloped, but the first three digits remained free as long clawed
grasping appendages. Finally the jaws bore teeth, though this
condition was found in other extinct birds as well. Archaeoptcryx
is therefore not truly a bird, and yet is so definitely bird-like as to
be beyond the reptilian ancestor whose discovery is still to be
made.

The Hoatzin. One existing species of bird is reminiscent of this
ancient pro-avian condition. This is the hoatzin of tropical South
America. In young hoatzins the first and second digits, unlike the
vestigial remnants found in other birds, are functional and are used
effectively as grasping organs in climbing (Fig. 87). The young
hoatzin is also an able swimmer.

Mammalian Evolution. The mammals differ less conspicu-
ously from the reptiles, since their development is not associated
with any extreme specialization. The skeleton shows certain
peculiarities, including triple centers of ossification in long bones
and paired occipital condyles. Hair, a characteristic mammalian
structure, has been definitely associated with scales in proof of
transition between the two (Fig. 58). The teeth undergo great
differentiation and are of several kinds, i.e., the dentition is
heterodont. Both the process of reproduction and the nourish-
ment of the young by milk, a glandular secretion, are generally
distinctive, but the lowest mammals, the Prototheria or Mono-
tremata, lay eggs. In many ways the mammals are similar to the
birds in degree of development, and they correspond closely in the
four-chambered heart and vaso-motor apparatus, but in most
respects the two classes are divergent.

Mammalian Ancestors: The Cijnodonts. Because of structural
peculiarities the mammals were once supposed to be more closely
related to the amphibia than to the reptiles, but the evidence of
paleontology now points definitely to the Cynodontia, the dog-
toothed reptiles, as their ancestors.

The cynodonts are distinctly reptilian animals in most particu-
lars but they agree with the mammals in two distinctive ways.
The skull is articulated with the first vertebra V)y two occipital
condyles instead of one as in other reptiles, and the dentition is
heterodont, while in other reptiles all of the teeth are primitive
conical structures.

Among the cynodont characteristics which are favorable for

158

EVOLUTION AND GENETICS

development may be included the evolution of the four limbs for
rapid locomotion and the differentiation of the teeth. The first

is supposed to have been as-
sociated with the develop-
ment of intelligence and
change of location through
migration. Differentiation
of the teeth would fit the
animal for eating different
kinds of food, which would
tend to stimulate the devel-
opment of powers of observa-
tion and choice, and con-
sequently intelligence. We
are unable to associate the
warm-blooded condition of
the mammals so directly with
external factors, but Osborn
supposes that it may have
appeared in some of the
cynodonts; if so, it would no
doubt also have favored the
evolution of mental powers
by maintaining a high and
constant rate of metabolism
and thus freeing the animals
partly from their dependence
upon the physical environ-

IV V F i- .7

. ment.
Fig. 88. — Diagrammatic sections oi vari-
ous forms of teeth. I, tusk or incisor The Course of Mammalian
of elephant; II, human incisor during Evolution. With independ-
development; III^^ completely formed f.^^^ ^f ^j-^g water for repro-
duction and the maintenance
of body moisture, came
greater dependence for ex-
istence upon the varied
factors of the terrestrial en-
vironment. Since food habits estaV)lish the most important con-
tact of the individual with his (environment, and the teeth of
mammals are specialized for various food habits (Fig. 88), the

human incisor; IV, human molar; V,
molar of ox. In all figures the enamel
is black, the dentine shaded with hori-
zontal lines, the pulp white, and the
cement stippled. (From Parker and
Haswell, after Flower and Lydekker.)

EVOLUTION OF THE VERTEBRATES 159

evolution of the teeth is important in mammalian evolution.
On the food habits of himself or his neighbors also depends an
animal's need for keen senses, defensive or aggressive structures,
and the development of powers of locomotion for escape from
enemies or for the capture of prey. Mammalian reproduction,
while it is distinctive, and to a limited degree varied within the
class, is by no means as important an indication of the course of
evolution as these other characters.

Teeth. The fairly primitive teeth of man are an excellent illus-
tration of the possibilities of hcterodont dentition. The incisors
are sharp edged for cutting through tough tissues or biting off
pieces of food; they are effective in cutting flesh. The canines, on
the other hand, are of little or no use for anything but holding and
tearing. The broad molars are effective grinding structures by
which tissues of all kinds may be crushed and reduced to smaller
parts before swallowing. Carnivorous animals obviously have
greater need for cutting and tearing than for grinding teeth, hence
we find that even their molars are sharp edged shearing teeth.
Herbivorous species, on the other hand, have need for grinding
teeth in proportion to the harshness of their food; the grazing ani-
mals therefore have broad teeth with hard ridges for chewing
grains and grasses. Still other animals, like the anteaters, have no
need for teeth, which are correspondingly reduced or lacking.

Structures and Habits. Correlated with food habits are other
structural characters. The carnivores have keen senses and speed
in order to detect and reach their prey, and powerful jaws and
sharp claws to aid in its capture. Herbivores find their teeth
poor defensive structures so their limbs are highly specialized for
speed without limitations imposed by other needs. If an ani-
mal finds food or protection in a different environment, its
limbs show the need of that environment, like the powerful fos-
sorial front legs of the mole, flippers of aquatic mammals, wings
of the l)ats, and arboreal adaptations such as the sharp claws of
the squirrels, hooked claws of the sloth, and prehensile append-
ages of the primates (Fig. 56).

Classification. The classification of the Mammals is based
largely on such structural differentiation, although they are first
divided into the Prototheria, Metatheria and Eutheria partly on a
basis of reproductive functions. Members of the first group lay
eggs. The second are marsupials, including such animals as the

160 EVOLUTION AND GENETICS

kangaroo and the opossum, and are viviparous. The young are,
however, usually without a placental connection with the mother,
and are in an imperfect state of development when born. They are
then carried in an abdominal pouch and are temporarily attached
to the long tubular teats by a special oral sucker. In the last
group occur those animals whose young are connected to the uter-
ine wall of the mother by a placenta. Through this the developing
animal receives nourishment from the blood of the parent until
birth.

The Euthcria include most mammals. They are supposed to
have originated from the cynodont stem later than the Metatheria,
while the Prototheria are supposed to have sprung from an even
more primitive reptile. During the Tertiary they attained the
differentiation which they have since maintained. Some species
of immense size appeared, although in this direction the mammals
were surpassed by the reptiles ; the whales of the present are among
the largest animals ever developed. While this period in mam-
malian development produced some species belonging to existing
genera, all are now extinct. Most of the Tertiary mammals were
less closely related to existing species.

Dispersal. In writing of the rise of the mammals during the
Tertiary, Lull calls attention to the similarity of the North Ameri-
can and European faunas, and to the existence of a land bridge
across what is now Bering Strait as indicative of their circumpolar
origin. He adds that the climate of this region was at first warm
and favorable and speaks as follows of the influences of the
period :

"There is always a tendency on the part of every group of
animals, as their numbers increase, to spread from their ancient
home along lines of least resistance, provided no climatic or other
insuperable barriers are to be overcome, and that may well have
been one very potent cause for the southward migration of the
modernized hordes. But there was an additional incentive, for
throughout the early Tertiary there is evidence of climatic varia-
tion and of a very gradual cooling of the northern climate and a
consequent southward retreat of the higher plants and mammals
which occurred as a succession of migratory waves. In this way
there came, first, the least hardy like the insectivores and primates,
the latter especially depending so largely upon the tropical forests
for their sustenance that any change cither in extent or character

EVOLUTION OF THE VERTEBRATES

IGl

162 EVOLUTION AND GENETICS

of their habitat would be reflected in their distribution at
once ..."

So definite is the geological record of vertebrate evolution that
these changes of fauna, correlated with the varying climates of the
world, tell us of the influences which were active in shaping the
group to which we belong. To these influences we owe not only
the present state of the fauna of the world, but even our own ex-
istence.

Step by step, through the successive conditions imposed upon
the phylum by swift waters, increasing aridity, the drying up of
streams and bodies of water, and finally changes of climate we have
come from these remote invertel^rate ancestors to our present
state (Fig. 89), not suddenly, but through a wonderful inherent
power to meet varying conditions successfully. By such easy
steps have the vertebrate classes originated. Each in itself has
made some adjustment to the various possibilities of the modern
world, but only in the whole do we find the maximum power of the
phylum as it is thus expressed. Upon future possil^ilities we can
only speculate.

Summary. The evolution of the vertebrates is characterized
by the completeness of all kinds of evidence and is therefore well
estabUshed. The origin of the phylum is, however, obscure. It is
explained by several theories, among which the annelid and A7n-
phioxus theories combine to suggest a logical explanation of the
transition from invertebrate to vertebrate. Within the phylum
the origin of the terrestrial classes from their primitive aquatic
ancestors is a major step, but the existence of modern fishes with
some adaptations for l^oth hal)itats is proof of the possibility of
such a transition. The most primitive terrestrial forms were
Amphibia. Conditions of increasing aridity favored the develop-
ment of the wholly terrestrial reptiles which later became highly
diversified. The birds and mammals probably originated from
different groups of reptiles and independently acquired the im-
portant vaso-motor apparatus and circulatory system in which
they are so similar. Development of the birds, however, has been
concentrated upon the perfecting of flight while development of
the mammals has resulted in structural diversity involving a
considerable degree of evolution.

EVOLUTION OF THE VERTEBRATES 163

REFERENCES

Von ZitteLj K. A., Textbook of Palaeontology, translated by Eastman, C. R.,

1902.
Patten, W., The Evolution of the Vertebrates and Their Kin, 1912.
Scott, W. B., A History of Land Mammals in the Western Hemisphere, 1913.
Lull, R. S., Organic Evolution, 1917.

Lull, R. S., (editor) Evolution of the Earth and Its Inhabitants, 1918.
OsBORN, H. F., The Origin and Evolution of Life, 1918.
Newman, H. H., Vertebrate Zoology, 1920.

CHAPTER X
ELEPHANTS, HORSES, AND CAMELS

The fossil remains of these three groups of vertebrates and their
ancestors are so abundant that they afford us an almost complete
record of the evolution of modern species since the Eocene. The
record is so remarkable that it has deservedly been described many
times, and has had unsurpassed influence in the establishment
of the theory of evolution. The first extensive series of fossils of
any of these animals was that assembled at Yale University,
largely through the efforts of Professor Marsh, to illustrate the
evolution of the Horse. This collection has been characterized
as the "first documentary record of the evolution of a race " (Lull).
It was regarded by Huxley as conclusive evidence of evolution,
and would have been visited by Darwin had his health permitted.
The fossil records now available afford many such examples of an
actual phylogenetic series, but none surpass that of the horses.
The records of elephants, horses, and camels are among the most
striking and complete that we have.

Systematic Position. The three forms of animals are included
in three orders of Eutheria, the Pro]:)Oscidea, Perissodactyla and
Artiodactyla respectively. All belong to the section Ungulata,
the hoofed animals. The first order is characterized by the elonga-
tion of the nose and upper lip, the occurrence of five functional
digits on all feet, the development of the upper incisors into tusks,
and the highly developed grinding molars. The second order
represents a very different type of adaptation, although the animals
of both orders are herbivorous. The Perissodactyla have not
more than four digits on the front feet and not more than three on
the hind feet. In all species the third digit is the most important
and in the family Equidae, including the horses, asses and zebras,
the third alone is functional. Their teeth, like those of the ele-
phants, are ridged grinding structures, but are less extremely
speciahzed. The camels are speciahzed for life in arid regions.
They are even-toed ungulates, i.e., the feet retain hoofs on two or

164

ELEPHANTS, HORSES, AND CAMELS

165

Fig. 90. — Modern elephant.s. A, Afrit-an; B, Indian. (From Lull, after
photographs from the New York Zoological Society.)

166

EVOLUTION AND GENETICS

four toes, and they are herbivorous animals like those of the two
preceding orders.

Elephants. Size and Structure. The elephants are in general
more primitive animals. While highly specialized in the develop-
ment of the ridged grinding molars and tusks, and in a number of
ways associated with their great bulk, they retain several distinctly
primitive characters, including the five functional digits. Their
speciaKzations are mostly associated with their size.

The existing elephants belong to two species, the African and the
Indian (Fig. 90). A height of thirteen feet has been reported for

some African individuals, and eleven feet
actually recorded, while a weight of six and
a half tons for the famous "Jumbo" is the
maximum on record. In these points they
rival all but the largest of the extinct species.
Lull states that a skeleton of the extinct
Elephas meridionalis in the Paris Museum
measures about fourteen feet in height at the
shoulder.

Limbs. To support this enormous bulk,
the limbs of the elephants are developed in
such a way that stresses are placed on the
longitudinal axes of the bones. Throughout
the limb the bones are aligned with each other
in such a waj^ that at no point is the stress
applied ol^liquely or transversely. The result
is an appendage aptly characterized as pillar-
like. Anyone who has watched a circus parade .
is familiar with the peculiar shuffling straight-
legged gait which is the result of this structure, and with the strange
feet which seem little more than the blunt termination of the limbs
themselves, with five nail-like hoofs set along their anterior
margins. The skeletal strength is supplemented by this com-
pactness of the feet, whose digits are scarcely divergent (Fig. 91).
The greater part of the sole of the foot is made up of a thick pad
which lies behind and below the digits and receives most of the
weight. The hoofs, unlike those of the horse, are unimportant
in this respect.

The Trunk. While in most animals with long limbs the neck
is correspondingly long, a compensation evident in the horse,

^ ^ z

3

Fig. 91.— Forefoot, of
the Indian elephant,
anterior aspect,
showing the com-
pact skeletal struc-
ture. (From Lull,
after Flower.)

ELEPHANTS, HORSES, AND CAMELS

167

the elephants' proboscis, or trunk, serves the same purpose and the
short neck permits an efficient arrangement of structures for the
support of the massive head. The proboscis, composed of the
elongated nose and upper lip, is a powerful yet delicate prehensile
organ. Because of its relative lightness and flexibility it is superior
to the jaws for handling objects, and with the head and neck
formed as they are becomes an exceedingly strong and efficient

Fig. 92.— Skull of Indian elephant, in longitudinal section. B, brain cavity;
i, incisor or tusk; m 3-5, the third to fifth molars. (From Lull, after Owen.)

organ. In contrast the elongation of the neck in other animals to
compensate length of limb is of limited use.

The Skull. In the elephant the skull is much shorter and higher
than in most mammals (Fig. 92). The change in form is accom-
plished by the thickening of the l)ones, whose lightness is preserved
by the formation of large enclosed spaces. The skull of an animal
acts as a lever of which the occipital condyles are the fulcrum, the
longitudinal axis the work arm, and the vertical height above the
condyles the power arm. In most animals the power arm is rela-
tively short, but in the high skull of the elephant it is much longer
and the leverage availal)le for the support of the heavy head and

168

EVOLUTION AND GENETICS

trunk is correspondingly increased. By this adaptation the animal
is also freed from the necessity of supporting the weight of its head
at the end of a long neck, although it has an ample reach and great

power in the combination of short,
powerful neck, short skull, and long,
flexible proboscis.

Teeth. The molar teeth of ele-
phants are made up of relatively
thin plates of dentine surrounded
by enamel and connected by cement.
They lie in such a position that
their ends form ridges across the
grinding surface (Fig. 93). Since
the teeth grow obliquely toward the
plane of contact with those of the
opposed jaw, successive parts come
into use as they grow, with the result
that a limited number of teeth or
parts are in use at any time. The
maximum number of teeth is twenty-
eight, but these come into use and
are worn out and shed in such a way
that not more than two molars in each half-jaw are functional at
the same time, making eight molars in all. In addition to these
the tusks or upper incisors are present from their first appearance;
they are preceded by a pair of small
milk tusks which are shed early in
life. Lull records a pair of tusks of
an African elephant which were 10
feet ^ inch and 10 feet 3}/^ inches
long respectively, and weighed 224
and 239 pounds, an almost incredi-
ble weight for an animal to carry in Fig. 94.— Skull of MoeriiheHum
two teeth alone. The tusks are com- ^■'/"«'^'*"' ^"e tenth natural size,
posed of dentine, excepting a small
enamel tip, and grow throughout life.

Specialization in three directions is shown by this dentition: (1)
departure from the primitive numl)er of sixty teeth by reduction
to a total of twenty-eight, (2) modification of the primitive mam-
malian molar to form a finely ridgc^l grinding tooth, and (3) de-

FiG. 93.— Molar tooth of Indian
elephant. A, crown; B, lon-
gitudinal section. Enamel is
black, dentine shaded with
oblique lines and cement stip-
pled. (From Lull.)

From the Eocene, Africa.
(From Lull, after Andrews.)

ELEPHANTS, HORSES, AND CAMELS

169

velopmcnt of two incisors into enormous tusks, used in fighting
and digging.

Elephant Phylogeny. Remote Ancestors and Divergent Forms.
The earliest fossils of proV)oscidean ancestors an^ found in th(^ upper
Eocene deposits of Africa. They belong to the genus Moeritherium,
which is characterized by the elongation of incisors in both jaws,
enlargement and recession of the nasal openings in the skull, forma-
tion of air cells at the back of the skull, and transversely ridged
molars, although it shows little further resemblance to the Probos-
cidea (Fig. 94). It is
supposed to have existed
until some time in the
Oligocene, when it be-
came extinct.

The Oligocene also
produced the genus Pal-
aeomastodon (Fig. 95),
of which fossil remains
are found in Africa and
Asia. The members of
Palaeomastodon show a
marked advance in the development of the characters mentioned,
and were probably descendants of Moeritherium. The molars are
little more advanced, but the incisors of the upper jaw are well
developed tusks, and the skull is higher, with a greater develop-
ment of cancellated bone.

In the Miocene another probable descendant of Moeritherium
appeared in Europe, existing until the Pliocene, when it became
extinct. This genus, Dinotherium, displays a peculiar develop-
ment of deflected lower jaw and tusks, while the tusks of the
upper jaw appear to have been lacking. The size of these animals
was about that of the mastodons. While they show evidences of
the development of a proboscis, and are in general well advanced
over Moeritherium, they gave rise to no existing forms.

The Miocene, however, saw the rise of three other genera which
appear in Europe, Asia, Africa and North America. The first,
Trilophodon, is supposed to have descended directly from Palaeo-
mastodon. Its distribution includ(\s the four continents men-
tioned and it persisted "until the extinction of the mastodon in
post-Glacial time." Aside from the elongation of the lower jaw,

Fig. 95. — Head of Palaeomastodon, restored by
Lull. (From Lull.)

170

EVOLUTION AND GENETICS

and the presence of lower tusks, it was distinctly like the elephant
in appearance (Fig. 96). Its teeth and skull show a greater de-
velopment of the typical proboscidean character than any of its
predecessors.

Mastodon and Its Contemporaries. During the same period
Mastodon arose (Fig. 97), to persist in America until its extinction

Fig. 96. — Restoration of Trilophodon. (From Lull, after the British Museum

Guide to Elephants.)

in the Pleistocene, and Tetralophodon marked the beginning of
another line which later produced Dihelodon. Dibelodon migrated
to South America and became extinct at about the same time as
the mastodons.

TetralopJwdon (Fig. 98) represents the extreme development of
the lower jaw and the four-tusked condition in the Prohoscidea,
although its descendant, Dihelodon, had the lower jaw shortened
and without tusks. The two are different from the main line of
descent in the more complex structure of the molars.

Mastodon included a number of different species, and has been
subdivided into other genera by some paleontologists. In general
these animals had the form of elephants, but they were more

4-

%^

o

p

o

O

m

c3

c3

o

CO

d

171

172 EVOLUTION AND GENETICS

primitive. The molars were transversely ridged, but the ridges
were made up of associated cusps (Fig. lOOC). They were so small
that several were probably in use at the same time. Tusks were
sometimes present, although poorly developed, in the lower jaw.
Writing of the American Mastodon, Scott says that there is evi-
dence to show that "it had a covering of long, coarse hair, and that
it fed upon the leaves, shoots and small branches of trees, espe-
cially of conifers." The genus became extinct in the Old World
before the end of the Pliocene, but migrants which entered America

by way of the land bridge
between Asia and Alaska
persisted until the middle
of the Pleistocene, and
were probably contem-
poraneous with the early
human inha])itants of the
Fig. 98.— Head of Tefralophodon lulli. The continent. Remains in-
lower jaw, the longest recorded in any pro- dicate that the heavy
boscidean, measured at least six feet in . , „, . ,

length. (From Lull, after Barbour and Kunz.) annuals were often mu'cd

in sloughs and marshes.
An unusually fine skeleton was taken from such a situation in
the summer of 1926 at Johnstown, Ohio. The bones were very
near the surface and in a well settled region, conditions which
emphasize one reason for incompleteness of our knowledge of
extinct species, viz., the element of chance in the discovery of
fossils.

From Mastodon to the Elephants. Mastodon produced another
genus, Stegodon, which appears in Asiatic deposits of the early
Pliocene. Stegodon was much like the modern elephants. Its
molars bore more and finer ridges than those of the true mastodons,
and the derivation of these ridges from rows of conical eminences
is less evident (Fig. lOOB). The worn surface shows dentine
plates surrounded by enamel, but the cement is not abundant as
in the true elephants. The lower jaw is short and without tusks,
and other characters are so close to those of the elephants that
Stegodon has been looked upon as congeneric with them. Fossil
remains of Stegodon have been found only in southern and south-
eastern Asia, which may therefore have been the region in which
the true elephants developed.

From Stegodon it is only a step to the genus Elephas, including

ELEPHANTS, HORSES, AND CAMELS

173

many extinct species and the existing Indian elephant. The
African elephant is included by some authorities in the same genus
and by others in the g(>nus Loxodonta. All may be considered as
true elephants. From Asia they probably reached North America
during the Pliocene, and Africa during the Pleistocene. In all
regions except Africa and Asia they became extinct during the
latter period.

Through the frozen mammoths of Siberia we have detailed
knowledge not only of the skeleton but also of the soft parts of

Fig. 99. — The Woolly Mammoth {Elephas primigenius). (From Lull.)

these great mammals. The species preserved in this way is
Elephas primigenius, the hairy mammoth (Fig. 99). It was pro-
vided with a coat of coarse hair covering a close woolly vestiture
which enabled it to resist the cold of high latitudes. It ate grasses
and the tender parts of trees. Other extinct species include Elephas
antiquus and Elephas meridionalis of Europe, and Elephas imperator
and Elephas columbi (Fig. 208) of North America, all of which
inhabited warmer regions. Primigenius inhal^ited the northern
parts of both continents as well as Asia. The chief differences in
fossil remains of these animals are the development of the molars
and the degree of curvature of the tusks. In Elephas antiquus the
latter are nearly straight, while in the Columl^ian elephant they
spiralled to such an extent that in old age the tips crossed. Draw-
ings on th(» walls of cavei-ns in Europe show that early man was
familiar with the mammoths.

174

EVOLUTION AND GENETICS

The chief features of the direct line of evolution of the elephants
are graphically illustrated in Figure 100. Here the steps leading
gradually from teeth with distinct cusps to those with fine trans-

Fig. 100. — Evolution of the head and molar teeth of proboscideans. A, A';
Elephas, Pleistocene; B, Stegodon, Pliocene; C, Q', Mastodon, Pleistocene;
D, D', Trilophodon, Miocene; E, E', Palaeoviastodon, Oligocene; F, F,'
Moeritherium, Eocene. (From Lull.)

ELEPHANTS, HORSES, AND CAMELS

175

verse ridges are shown as they occur in the fossil remains of ex-
tinct species. Here also the transition from the elongate head of
Moeritherium to the high short head of the elephant is illustrated
from actual remains of a chronological succession of species. Such
a series is not merely interpreted as evi-
dence of evolution; it is the actual record
of evolutionary change. •"- ^'"^~

Adaptive Structure of the Horse. Loco-
motion. The evolution of the horse is in a
number of ways more extreme than that
of the elephants. With respect to speed,
the legs are elongated and slender, retain-
ing only one functional digit. As in many
other animals, the feet no longer rest flatly
on the ground, but in no other animal is
the elevation to the toes more extreme
than here, for not merely the tip of the
toe, but the hoof alone, the homologue of
claws and nails, comes in contact with the
ground. This lifting of the body and
elongation of the lower parts of the ap-
pendages results in a relative shortness
and concentration of the leg muscles, and
consequent rapid movement and lengthen-
ing of stride. The function of propulsion
is largely relegated to the hind limbs, in
which elongation of the foot and con-
centration of leg muscles is extreme.

Elongation of the legs is compensated in
the horse by lengthening of the neck, so
that the head of the animal can reach the
ground.

Teeth. The food hal^its of the horse
and other grazing types demand special
development of the teeth for chewing harsh
vegetation. Grazing habits demand no
canines, hence they are reduced. The
incisors are important for cropping low
vegetation and are elongated. The molars
and three premolars are similar. All are

I I e'yvayyvel

H den-tvyie-

@ cernertt

■ ■natvtr'a-l cavity

Fig. lOL — The grinding
surface of horses' teeth.
A, worn surface of milk
molar of colt about six
months old; B, unworn
surface of milk molar
before birth; C, pre-
molar of horse eight or
nine years old ; e, enamel ;
i, natural cavity in ce-
ment; d, cavity, later
fi 1 1 e d with (; e m e n t.
(From Lull, after
Chubb.)

176 EVOLUTION AND GENETICS

high crowned. The worn surfaces show a complex pattern of
dentine enclosed by enamel, which is in turn surrounded by
cement (Fig. 101). Such a structure presents the same funda-
mental characteristics as the teeth of the elephants, but in the
horse wear is compensated by the height of the crowns, while in
the elephants the teeth are used successively. Growth of horses'
teeth continues for a little more than five years. At the end of
this time all of the permanent teeth are in use. Subsequently the
teeth are pushed farther out of the jaws as the crowns are worn
down, and when of no further use, are shed (Fig. 102). The total
number of teeth which develop in the horse is forty, but canines
are not found in mares, and the first molar is vestigial or lacking.
Two incisors, three molariform premolars, and three molars are
normally produced in each half-jaw.

Phylogeny of the Horses. The Direction of Change. The evo-
lutionary changes which are made evident by the known fossil
horses are summarized by Lull under the following heads:

1. Increase in size.

2. Lengthening of the limbs.

3. Reduction of ulna and fibula, with a consequent limitation
of the range of movement.

4. Change of the foot posture from plantigrade to unguligrade.

5. Reduction and loss of digits from five to one.

6. Perfection of the hoof.

7. Perfection of the dental battery in elongation and complexity
of teeth.

8. Premolars becoming molariform.

Eohippus, the American genus, and Hyracotherium, the Euro-
pean genus, belong to the Eocene epoch and are the oldest known
ancestors of the horse. The connection of the two is un-
certain. Htjracotherium is more primitive, but no ancestral forms
are known to indicate the origin of the closely related forms.
While the European genus has left no known descendants, there is
a succession of genera from Eohippus to Equus in North America,
although the Hne at last became extinct in the Pleistocene, and
modern horses were introduced from Europe by the early ex-
plorers. From time to time ancient forms migrated from America
into other continents, where they either became extinct or gave
rise to the limited number of modern species of Equidae.

ELEPHANTS, HORSES, AND CAMELS

177

Fig. 102. — Dental battery of horse, to show growth and wear of tne teeth.
A, five years old, permanent teeth all in use. B, eight years old, crowns
reduced by wear and roots longer. Vestigial upper premolar, pm', jjresent.
C, thirty-nine years old, lower molars incline forward. Canines absent
(female). (From Lull, after Chubb.)

178

EVOLUTION AND GENETICS

Eohippus, the ''Dawn Horse," was a small animal, about the
size of a fox terrier (Fig. 103). Its head was elongate and rather
horse-like, although the eye was much farther forward than in
modern horses. The feet were digitigrade, and the legs onl}^
moderately elongated, hence the neck was also moderate. The
fore limbs had four functional digits, the second to fifth, while the

Fig. 103. — Restoration ul' Kulnppu-i .sp. ul ilit* luuei li,oceiie. (I'Vom Scolt.)

first was completely lost. The hind limbs were three-toed, with
minute vestiges of the first and fifth (Fig. 104). Each lower limb
retained the two primitive bones, the radius and ulna of the fore
limbs and the tibia and fibula of the hind limbs. The teeth were
relatively advanced, foreshadowing the modifications to come,
but the premolars were still distinct from the molars and the first
premolars were present (Fig. 105). Correlated with this primitive
condition of the teeth is the relative shallowness of the jaws and
the position of the eye. The conditions under which the species
lived were such as to encourage the development of grazing habits
because of the abundance of meadows and grassy plains. In spite
of such facts, however, these little creatures are conspicuously
unhke the modern horses. It is only through the intermediate
forms that the two extremes can be associated.

ELEPHANTS, HORSES, AND CAMELS

179

Orohippus, the next genus to appear, followed Eohippus in the
Eocene and shows only slight changes. The vestigial bones of
the first and fifth digits of the hind limb had disappeared, the fifth
digit of the fore limb was shorter,
and the third premolar was molar-
iform. It included somewhat larger
species than its predecessors.

Epihippus, a third genus, occurred
later in the Eocene. Complete
skeletons have not been found, but
the last two premolars were molari-
form, the first still persisted, and
the teeth were somewhat higher
crowned than in the older genera.

Mesohippus, an Oligocene genus,
included several species which
varied in size but scarcely attained
the size of a sheep. The upper
incisors differed from those of earlier
species in the "mark," an enamel
ridge behind the cutting edge,
which is characteristic of true
horses. The premolars were molar-
iform with the exception of the
fh'st, which shows a tendency to

disappearance. The teeth had not yet acquired the high-crowned
form of the true horses (Fig. 106). The eye was set farther
back than in Orohippus, and its orbit was closed behind, unlike
the preceding forms. The bones of the feet of Mesohippus are

still further reduced (Fig. 107).
All four feet had three functional
digits, and only the anterior pair

Fig. 105.-Upper teeth of Eohippus. stained a vestige of the fifth.

Premolars visibly smaller and sim- The ulna of the fore limb and

pier than molars. (From Lull, after fibula of the hind limb were very

^ ^^ slender, but complete. The same

epoch produced Miohippus, a genus of similar forms.

In his recent review of the evolution of the horse Matthew

records changes in the relationships of the carpal and tarsal bones

which are of importance in the development of the one-toed foot.

Fui. 104. — Feet of Eohippus ven-
iicolus. (From Matthew.)

180

EVOLUTION AND GENETICS

In the earlier species the cuboid bone received only the meta-
tarsal of the fourth digit. As the third digit enlarges the cuboid
articulates in part with it, and so gives the functional toe greater
lateral support and tends to prevent rocking. This change first
appeared in Miohippus.

Environmental Conditions. This epoch was a time of increas-
ing aridity due partly to continental uplift. Such conditions re-

FiG. 106. — Upper teeth of Mesohippus. Three premolars Hke the molars.

(From Lull, after Matthew.)

suited in the decrease of bodies of water and the extension of
prairie areas, although the persistence of forests and meadows, as
well as dryer areas, favored the development of several types of
primitive horses which have since disappeared. The continuation
of climatic change in the Miocene gave rise to
the great prairies of North America and was
accompanied by the development of large
numbers of grazing species. These animals
must necessarily have had the characters
which we see in the modern horse, viz., teeth
adapted to harsh grasses and legs adapted
for rapid locomotion over hard open ground.

Miocene Horses. The divergence which be-
gan in the Oligocene culminated in two lines
during the Miocene in North America. These
were Parahippus and Hypohippus. Both had
low crowned teeth, obviously developed for
browsing on soft herbage rather than for graz-
ing, and spreading feet which must have been
fitted for walking on soft ground. They were
undoubtedly forest animals. Most of the
species were fairly large, a little more than three feet in height.
Migrants from the divergent types also gave rise to an Asiatic
genus, Anchitherium, which likewise became extinct.

In the direct ancestry of the modern horses, the period produced

Fig. 107.— Feet of
Mesohippus. A,
anterior; B, pos-
terior. (From Lull,
after Marsh.)

ELEPHANTS, HORSES, AND CAMELS

181

two significant genera, Merychippus (Fig. 108) and Protohippus.
The molars of Merychippus mark the transition from the browsing
to the grazing type, for paleontology records that the milk denti-

FiG. 108. — Restoration of the prairie horse, Merychippus, from the Miocene.

(From Lull.)

tion is of the low crowned primitive type while the permanent
molars are high crowned grazing teeth (Fig. 109). The feet
were three-toed, but the middle toe was so much more highly

developed than the others that it alone
supported the body, while the others
did not reach the ground. Protohippus
differed from Merychippus in the fact
that both milk and permanent molars
were high crowned. These genera in-
clude the first highly specialized grazing
horses.

From the same stem another genus,

Hipparion, originated during the late

Fig 109.-lTpper premolars Miocene in North America, whence it

of Merychippus. A, milk . -^^ •

teeth, without cement; B, migrated mto Asia and Europe. During

permanent teeth, with ce- the Pliocene it became extinct. Matthew
ment. (From Lull.) states that Hipparion whitneyi, from

South Dakota, was a very slender and graceful horse, except for its
large head, and was adapted by the complexity of the enamel
ridges of its teeth to eat the harshest herbage. It is therefore
looked upon as a very fleet species, fitted to live in semi-desert

182 EVOLUTION AND GENETICS

country. It was of about the same size as the other Miocene
species.

Pliohippus, a North American genus of the early PHocene, was
the last progenitor of the modern genus Equus. The included
species were no larger than the few immediate ancestors, i.e.,
about forty inches high. They difTered conspicuously, however,
in the reduction of the second and fourth digits on all feet to
splints; thus Pliohippus is the earHest known one-toed horse.
The third digit was highly developed and ])ore a well formed hoof.
This genus gave rise to Plesippus, from which it was only a step to
Equus.

South American Horses. Either Pliohippus or Protohippus gave
rise to the genus Hippidion of South American horses which ex-
isted during the Pliocene. This genus, its derivate Onohippidion
of the Pleistocene, and some migrants from North America be-
longing to the genus Equus are all of the horses known to have
occurred in South America. All were extinct by the end of the
latter epoch. (See diagram, page 183.)

The Genus Equus in North America. Equus of North America
is not well known. Scott's statement quoted below, gives the
paleontologists' opinion of these species.

"In the latest Pliocene, and no doubt earlier, species of the
modern genus Equus had already come into existence ; and in asso-
ciation with these, at least in Florida, were the last survivors of
the three-toed horses which were so characteristic of the early
Pliocene and the Miocene. However, little is known about these
earliest recorded American species of Equus, for the material so
far obtained is very fragmentary. In the absence of any richly
fossiliferous beds of the upper Pliocene generally, there is a pain-
fully felt hiatus in the genealogy of the horses ; and it is impossible
to say from present knowledge, whether all of the many species of
horses which inhabited North America in the Pleistocene were
autochthonous, derived from a purely American ancestry, or how
large a proportion of them were migrants from the Old World,
coming in when so many of the Pleistocene immigrants of other
groups arrived. It is even possible, though not in the least
hkely, that all of the native American stocks became extinct in
the upper Pliocene and that the Pleistocene species were all
immigrants from the eastern hemisphere ; or the slightly modified
descendants of such immigrants; but, on the other hand, it is

ELEPHANTS, HORSES, AND CAMELS

183

Africa

Eurasia

NORTH AMERICA

SOUTH AMERICA

Ethiopian

Palearctic
+ Oriental

NEARCTIC

NEOTROPIC

Equus

!?«,,,,,,

H

(^uua

w
u

Zebras

Tarpan

and ass

kiang

\

and

asses

\

1

o

\

Extinct

Extinct

£2 z

Equus

Equus

Equus

^

k

t

'^ Onohippidion

a,

/ 1

E

> Equus

u

quus< —

2

Plesippus
t

o
o

t— (

Extinct

HipF

idion

Hipparion

^^

X

Pliohippus

^^

Jiipparion

^

\ Pro to

lippus

Extinct

H

Extinct

>

Z

W

. i.nchitheriun

.

Hypohippus

o

Meryc"hippu3 /

/

^ /

i

/Extinc
/Parahipp

IS

w

V

y

z

u

Miohippua

o

J

\

o

o

3

Mesohippus

o

?A

' 1

Epihippus

w

t

w
o

Orohippua
t

o

Ilyracotherium

ippus

H

Phylogeny of the horses. (Modified, after Lull.)

184 EVOLUTION AND GENETICS

altogether probal^le that some of these numerous species were
intruders."

Whatever may have been their source, all species living in the
Americas became extinct during the Pleistocene, and from the
Palearctic stock developed the zebras, asses, wild horses and
domesticated races of the present. Conditions are now so favor-
able for their existence that feral horses are found in the new
world. On the western ranges of our own country they have
even multiplied to such an extent that they are becoming a prob-
lem to stock raisers. What may have been the cause of their
ancient extinction becomes a difficult problem. Lull offers as a
theory the introduction of some virulent insect-borne disease, but
of course no such theory can be proved with evidence now avail-
able. All that we can know is that they died, and in some places
immense numbers are preserved as fossil skeletons.

Camels. In still another group, the camels, an extraordinarily
complete series of fossil forms are available. These animals are
now represented only by domestic and feral individuals, and do
not occur in North America. They are Artiodactyla, or even-toed
ungulates and represent the course of adaptation for speed and for
subsistence on harsh and scanty herbage. The modifications of the
legs, however, show clearly the animal's fitness for progress over a soft
substratum such as the loose sand of many deserts, and in existing
forms the nostrils and eyes are equally eloquent of ability to resist
desert conditions. The ability of camels to store water beyond
their immediate needs, and the storage of fat in the hump, are
probably the best known adaptations.

The evolution of the camels and their relatives, the llama and
alpaca of South America, is so similar to that of the horses that
Figure 110 will be an ample presentation for the purposes of this
work. It is significant that their development occurred at the
same time as that of the horses and in response, apparently, to the
same conditions of gradually increasing aridity. Although they no
longer occur in North America the remains of ancestral species
show that they were once common over the greater part of the
western United States and it was here that the true camels were
evolved. The camels of the Old World were apparently derived
from ancestors which migrated from North America over the
Bering Isthmus, leaving the main stock to perish.

Such cases as these are the last word in evidence for evolution.

ELEPHANTS, HORSES, AND CAMELS

185

That we can see relationship in hving forms, particularly relation-
ship which can be interpreted only on the basis of common deriva-

EVOLUTION OF THE CAMELS

g

CD

s

Pleistocene

I

fe

;3i

Recent

Pliocene

Miocene

Auchenia
(Llama)

Skull

Feet

Procamelus

Poebrotheriixm

Oligocene

Eocene

Protylopus

Teeth

Mesozoic or Age of Reptiles

Hjjpotheticol five-toed Ancestor

Fig. 110. — Evolution of the camels, as indicated by the skull, feet, and teeth.

(From Lull, modified after Scott.)

tion, is conclusive in itself, but the discovery of actual n^nains of
creatures now extinct in such series as these admits of no other

186 EVOLUTION AND GENETICS

logical explanation. A transition by gradual steps from a rela-
tively primitive state, such as that of Eohipjms, to the highly
specialized modern horse, correlated with geological time succes-
sion and evidence of chmatic change, is a part of that natural
record which, if complete, would be the whole story of evolution.
Summary. The fossil remains of many vertebrates show a
gradual transition in structure from primitive ancestors to existing
species. The evolution of the elephants, horses, and camels is
especially well demonstrated by these records. Evolution in the
elephants is chiefly linked with the development of great bulk and
browsing habits, and is shown by the development of pillar-like
limbs, finely ridged grinding teeth, and characteristically high
short heads. Their length of liml? is accompanied by the develop-
ment of a trunk. Horses are more lightly built. Their feet are
elongated, their leg muscles bunched, and their toes reduced to
one on each foot. Every characteristic of the limbs shows adapta-
tion for rapid locomotion over hard ground. Length of limb is
compensated by elongation of the neck and the teeth are adapted
for grazing. The camels are differently adapted for life in arid
regions. In all three groups primitive ancestral species of the
Eocene are known which can be linked with the existing animals
through a chain of other species occurring in the intervening
epochs. The structures of these species in all cases show a gradual
transition leading up to the highest stage. Divergent species are
also recorded which became extinct without leaving known de-
scendants. The correlation of structural transition, chronology,
and environmental conditions is significant evidence of evolution.
These cases are, in fact, fragments of the actual record of past
evolution.

REFERENCES

Scott, W. B., A History of Land Mammals in (he Western Hemisphere, 1913.
Lull, R. S., Organic Evolution, 1917.

Matthew, W. D., "The Evolution of the Horse. A Record and Its Interpre-
tation." Quarterly Review of Biology, 1, 139-185, 1926.

CHAPTER XI
THE EVOLUTION OF MAN

The differences between man and the lower animals are such
that we can hardly avoid being prejudiced judges of our place in
the world. We know relatively little cither of the world or of our-
selves and there are many factors in our lives as human beings
which tend to influence our evaluation of such things as we do
know. Science tries to set aside these prejudices and to judge the
characteristics of man as impartially as those of other organisms.
It has succeeded to some degree and therefore gives us a more
logical account of ourselves than any other field of knowledge,
but in this as in other fields we must be constantly aware that our
information is incomplete.

Man's Systematic Position. Of man's position among other
organisms, fortunately, we need not be in doubt. Although he
stands well al)ove the other animals in some ways, he is animal in
structure and in functions and shows in his anatomy as definite
relationships as are displayed by the forms already studied. Among
his animal structures he has the dorsal tubular nervous system,
the vertebral column which replaces the embryonic notochord,
and at one stage evidence of the pharyngeal clefts which stamp
him a chordate and a vertebrate. Within this phylum he has the
hair, the circulatory system, and the mode of reproduction of the
mammals, and in the highly specialized connection of parent
and embryo he shows the most conspicuous character of the Eu-
theria.

The Eutheria include four groups, the Unguiculata or clawed
animals, the Primates or animals with nails, the Ungulata or
hoofed animals, and finally the Cetacea, made up of such highly
specialized marine creatures as the whales, dolphins and por-
poises. Obviously man is most nearly like those species which
have nails on the digits, and so we find him a member of the order
Primates, in the group of the same name.

The Primates. Characteristics. In general the primates are
arboreal animals. They have prehensile appendages with the

187

188 EVOLUTION AND GENETICS

thumb and great toe more or less opposable to the remaining
digits. Such structures are effective in the plantigrade position
for walking but are especially fitted for locomotion in trees. In-
stead of claws or hoofs they have nails. They are further char-
acterized by having the body covered with hair except the palms,
soles and parts of the face; the mammae are reduced to a single
pectoral pair, the eyes directed forward and the orbit surrounded
by bone, a clavicle always present, and the brain relatively large
and well convoluted. The body of every individual verifies man's
possession of most of these characters.

Classification. Classifications of the Primates differ. The fol-
lowing is that of W. K. Gregory, which has been widely used.
Recent publications dealing with the phylogcny of the group do
not agree with it in detail, but as an indication of the general sub-
divisions it is wholly adequate:

Suborder 1. Lemuroidea. Lemurs or "half-apes."
Suborder 2. Anthropoidea.

Series 1. Platyrrhini. New World apes.
Family 1. Hapalidae. Marmosets.

Family 2. Cebidae. Capuchins, howler monkeys, spider
monkeys, etc.
Series 2. Catarrhini. Old World apes and monkeys.
Family 3. Cercopithecidae. Monkeys, baboons, ma-
caques, etc.
Family 4. Simiidae. Man-like or anthropoid apes.
Family 5. Hominidae. Man.

The series Platyrrhini is distinguished by a broad nasal septum,
a reduced and non-opposable thumb, and other characters. In
contrast, the Catarrhini have a narrow nasal septum, and as the
name suggests, the nostrils point downward. They have thirty-
two teeth, opposable thumb, and non-prehensile tail, which is often
rudimentary and not developed as an external appendage. The
latter group corresponds in these things with the structures of
man but the two lower families differ from man in the opposable
great toe. In general, then, man harmonizes closely in structure
with the other members of the Catarrhini. The differences which
exist are traceal)le almost entirely to differences in habits, since
man is an erect terrestrial species while the others are arboreal,
and at the most only semi-erect.

THE EVOLUTION OF MAN

189

The Man-Like Apes. The family Simiidae includes four existing
genera: Hylobates, the gibbons; Simia or Pongo, the orang; Pan,
the chimpanzees (Fig. Ill); and Gorilla, the gorilla (Fig. 112).
All of these animals are tailless. Though they can walk in a semi-

FiG. HI. — Chimpanzee, Pun pyqmaem. (From Lull, photograph from the

New York Zoological Society.)

erect position, touching the knuckles to the ground to aid their
progress, they are predominantly arboreal with the exception of
the gorilla. The opposable thumbs and great toes provide them
with four grasping appendages which are very effective for moving

190

EVOLUTION AND GENETICS

through the trees. So much has been said of the habits of these
apes, particularly of the chimpanzee and orang, that their intelli-
gence and imitativeness are almost common knowledge. The

Fig. 112. — Gorilla. (From a specimen in the American Museum of Natural
History mounted by Carl E. Akeley. Through the courtesy of the Museum.)

half-human antics of certain simian performers in the movies has
probably done more than anything else to acquaint man with these
interesting relatives, and there are numerous excellent accounts
of their structure and habits in print.

THE EVOLUTION OF MAN 191

The anatomy of the man-hkc apes is much Hke that of man
(Fig. 113). The apes have stronger jaws and teeth and a relatively
low cranial capacity, their mouths are not formed in such a way
as to permit articulate speech, their hands and feet alike are grasp-
ing appendages, and the skeleton is not sufficiently modificnl to
allow a fully erect posture. Within the range of their own group,
however, they show nnich greater anatomical differences than are
evident between the highest ai^es and man. Man shows all of the
modifications incidental to intellectual dcwelojMiient and erectness.
These structures and the pro])al)le reasons for their development
are treated in the following paragraphs; detailed comparison with
the existing apes is unnecessary for these animals are, at the most,
merely similar to some of the remote ancestors of man.

The Arboreal Origin of Man. The fact that man is in so
many ways like these great apes, and that they are highly de-
veloped arboreal animals, suggests that man himself is derived
from arboreal ancestors. In analyzing this possibility it is first
necessary to consider how the assumption of arboreal habits
might affect the normal quadrupedal form characteristic of the
lower vertebrates. With the condition of the arboreal primates
established, it is then necessary to inquire into the possibility of
return to the ground, and into the effects of such a return upon
the arboreal organism.

Arboreal Quadrupeds. The assumption of arboreal life by
quadrupeds is not at all uncommon. Squirrels are a familiar ex-
ample. In all of the many mammalian orders represented by
such species, however, sharp curved claws are the effective means
of locomotion except among the lemurs, the lowest primates.
Here the thumb and great toe are opposable, and the animal is
able to grasp the limbs of trees instead of merely clinging to them
or hanging from them as by a series of hooks.

Hands versus Claws. The result of opposal)ility is a much more
effective appendage for arboreal progress. Claws, if sufficiently
long, as in the sloth, are most effective for suspension of the body
from branches, but they are a hindrance to any other use of the
appendages. If short and sharp, they provide sure footing, but as
organs of prehension they are of little or no use, and for suspending
the body from branches, of very limited use. A grasping struc-
ture, however, as we can determine from personal experience, is use-
ful in many ways. It provides sure footing for locomotion above

192

EVOLUTION AND GENETICS

the substratum; it is an effective organ for suspending the body
from branches within the limits of muscular strength; lastly, the

Fig. 113.— Skeletons of man (A) and gorilla (B). (From Lull.)

aliility to grasp objects enables the animal to move them about and
bring them before its eyes for examination.

THE EVOLUTION OF MAN 193

The Effects of Brachiation. Two important changes may re-
sult from the use of such appendages. Physically, the ability to
hang from a limb by one or more appendages, makes possible the
type of locomotion, known as ])rachiation, i.e., swinging from
branch to branch by the fore-limbs, as is done by the arboreal
primates. The effect of gravity would then be felt by the body as
a straightening pull, while the hind-limbs would be resolved
functionally into supports for the body while resting upon branches
or moving about on the ground. Such division of locomotion
would be functional specialization of great importance to arboreal
animals. Jones writes, "As arboreal life becomes more complete,
the search for a new foothold will become a far more exacting
business than it is in the mere clambering we have pictured (of
quadrupedal animals). The more exacting the search becomes,
the more will there tend to be developed that most important
factor — the specialization of the functions of the fore- and hind-limbs.
While the animal reaches about with its fore-limb, the hind-limb
becomes the supporting organ. With the evolution of this process
there comes about a final liberation of the fore-liml) from any such
servile function as supporting the weight of the body; it becomes a
free organ full of possibilities, and already capable of many things.
This process I am terming the emancipation of the fore-limb, and its
importance as an evolutionary factor appears to me to be enor-
mous." Restriction of the supporting function to the hind-limb
and the straightening of the extended l^ody alike would tend to
develop the erect posture from the quadrupedal. The transition
is, of course, not a slight one, but in the peculiar locomotion of
the apes we see that such transitional development may exist.

Brachiation and Mind. The "emancipation of the fore-limb"
could not fail to be an active stimulus to nervous development.
Swinging from branch to branch high above the ground would
require keenness of the senses, especially of vision, nice coordina-
tion, and exactness of judgment of contributing physical factors,
such as wind. We can best understand these things by consider-
ing the feats of acrobats. Since the penalty of inaccuracy would be
death or serious injury in most cases, there would be no question
of perpetuation of the unfit. In this way and through the freedom
of the fore-limbs for handling and examining any object within
reach the development would react upon the senses. Not only
would inherent keenness of the mind and senses be emphasized; a

194 EVOLUTION AND GENETICS

wide range of stimuli would also be brought to bear upon the ani-
mal by its increased ability to investigate other things.

Mind and Environment. Conditions at present show us that
only tropical forests are favorable to the existence of such arboreal
types. Elsewhere they would find food too scarce, and would be
forced to seek it on the ground as well as in the trees. In the tropi-
cal forests, however, there is a constant supply of fruits, which
are well adapted to mastication by their rather primitive teeth.
Insects are also available as a part of their diet. We have already
seen in the heterodont dentition of more primitive mammals and
of the cynodont reptiles an initial stimulus to development of in-
telligence through the variety of reactions in securing food. Such
food habits as those of the primates might well be correlated with
this type of development, but in other ways as well the tropical
forests would provide diversity of contacts. Their richness in
all forms of Ufe is unsurpassed by any other terrestrial environ-
ment.

From the Trees to the Ground. Anything which might force
these arboreal creatures to the ground, such as scarcity of food or
the thinning of the forests, would find them well able to meet com-
petition. They would be less specialized for attack or defense
than the carnivores, less specialized for flight from their enemies
than the herbivores, and consequently unable to meet either of
these groups in direct competition within their limited fields of
activity. However, they could easily escape from terrestrial
enemies by climbing, and through their omnivorous habits would
find an abundance of food without the limitations imposed upon
the more specialized animals.

On the ground the animal would have a new set of tools at his
disposal. His hind-limbs, developed for perching upon branches,
would find the ground a more stable substratum, and his hands
would then be freed from all but occasional use in locomotion.
Their grasping power, turned to new uses, might readily accom-
plish many things. The use of stones and clubs as weapons sug-
gests itself as a simple result of his inferiority in competition with
carnivores, but whatever might be the beginning of his use of im-
plements it could hardly fail to open up new possibilities.

All of the things which might logically follow upon descent from
the trees point toward ever increasing diversity of activities and
stimuli. Once the ability is acquired to make use of other things

THE EVOLUTION OF MAN 195

than those provided by the body itself, the possibihtics are without
hmit. Inventiveness in the human race has gradually brought us
to our present state. We cannot yet see the limits of our powers
in this direction, but our attainments are only the gradual accu-
mulation of the ages.

Evolution of Terrestrial Primates. Such diversity of activities
and its resulting diverse reactions upon the organism suggest
that subsequent physical modification would be slight. With ade-
quate locomotion, a free pair of grasping appendages, and the
ability to make use of inanimate objects to compensate inherent
deficiencies, everything would favor continued development along
the same lines, and increasing mental power would be the result.

One physical modification begun during arboreal life might be
expected to continue. Brachiation does not emancipate the fore-
limbs to the extent that is possible in terrestrial life, and conse-
quently does not favor the development of completely erect pos-
ture. Terrestrial life supplies a dependable support on which
the hind-limbs are adequate for locomotion. Increase in stability
of equilibrium and the consequent freedom of the arms from acces-
sory locomotor functions would make possible the maximum
maintenance of the erect position, and would favor any changes in
structure dependent upon it.

The Results of Erectness. Man is the only available example
of a wholly erect animal. In his body are found all of the modifica-
tions which depend upon the change from quadrupedal to erect
posture. Such a change involves primarily a shifting of the hori-
zontal axis of the body to a vertical position with concomitant
changes in anatomy, but Jones logically cautions against accept-
ance of this shift as an explanation. It seems to him rather an
outcome of "an arboreal apprenticeship." "Walking upright
upon the surface of the earth," he points out, "has produced its
changes in the human body, of this there is no doubt; but we must
be careful to distinguish between these 'finishing touches' and
those other changes which are so much older and so much more
important — the adaptations to arboreal life."

These "finishing touches," since they include a fundamental
change in the axis of the body, bring about compensating changes
in axial structure (Fig. 113). The spinal column of a quadruped is
arched between the supporting appendages, and has flexible ante-
rior and posterior parts, the neck and tail. Rotation through 90°

196 EVOLUTION AND GENETICS

from a fixed attachment to the pelvic girdle brings about some
curvature immediately above the point of attachment; this lumbar
curvature is incipient in monkeys, and well developed in man.
The head, supported without change of position upon this new
axis, would point upward rather than forward. Its articulation
with the vertebral column in quadrupeds allows only partial in-
clination, since flexibihty of the neck provides a greater latitude of
movement. The vertical position of the axis is compensated by
a shift in position of the occipital condyles, which are ventrocaudal
in quadrupeds. In arboreal primates, and to a greater degree in
man, they have shifted along the formerly ventral surface of the
skull to such a degree that this surface has also swung through 90°
and become caudal instead of ventral. The face is thus brought
forward.

Changes in the skull are not, however, entirely referable to the
changed axis of the body. Reduction of the prognathous form is
more directly correlated with the ability of the organism to use
its hands for grasping, breaking or tearing, and bringing food to
the mouth. The mouth in an animal of even semi-erect form is
not its only facility for handling objects, and consequently is not
used in the same way as the prognathous mouths of other
animals.

Of the remaining parts of the skeleton the pelvic girdle and limbs
alone undergo great change. The pectoral girdle and limbs are
more primitive than in many other mammals. In comparison
with the horse, for example, the hands of man are still in the
primitive pentadactyl state, while the fore-limbs of the horse have
lost four digits. The forearm in man contains the primitive bones,
the ulna and radius, and they retain their primitive flexibility of
movement. In the horse the ulna is reduced to a vestige which is
combined with the radius, and the articulations are so modified
that the limb moves in one plane, forward and back. Man also
retains the clavicle, which is no longer present in the horse.

The pelvic girdle in man is compact and firmly articulated with
the spinal column, as in all bipedal animals. Shift of the body axis
places the stress of body weight differently upon its articulation,
however, and the result is an elongation of the articular surfaces
cranio-caudally, in contrast to the dorso-ventral elongation found
in quadrupeds. The shift of stresses also throws the weight of the
viscera toward the pelvis instead of toward the ventral body wall,

THE EVOLUTION OF MAN 197

a change which is compensated in part by the broad, basin-hke
pelvis.

The leg bones of man are modified to a relatively slight extent,
but the femur is much straighter than in arboreal primates or
quadrupeds. The foot is completely without the grasping power
found in the primates, and is therefore much different from the
hand which retains an opposable thumb. Its most interesting
character is the predominance of the big toe. Specialization of
the appendages usually n^sults in emphasis upon one digit, but
the most prominent digit of the primitive pentadactyl appendage is
the third. In the horse we have noted the development of this one
to the exclusion of all others. IVIan therefore is unusual in the
great development of his first toe. A logical explanation is found
in the significance of this digit in an arboreal animal. Develop-
ment as a digit opposable to all others would find it already the
most specialized of all when its possessor became terrestrial, and
would provide the basis for further emphasis upon it during ter-
restrial life. In connection with its dominance, all other toes are
reduced, and the little toe is even rudimentary in some races.
Jones cites the Malays and Nubians as extreme examples. In these
peoples the fifth toe is said to be stumpy and often without a
nail. The foot has an arched skeleton, apparently for the absorp-
tion of shocks which would otherwise be transmitted through the
entire longitudinal axis of the body.

A final specialization of man, the loss of hair from most of the
body, and his delicacy of skin are probably associated with the
development of intelligence. Matthew points out that man's
retention of hair on the ventral surface of the body, where it is
thinnest in other animals, is exactly what might be expected of
long use of protective clothing. A simple garment, such as a skin
thrown over the shoulders and tied around the waist, would pro-
tect just those parts of the body where the hair is most completely
lost. It is significant that monkeys in our zoos make use of cover-
ing in this way. A number were wintered in outdoor cages as an
experiment, and were provided with gunnj^sacks as protection
against the cold. When evening came, each monkey helped him-
self to a sack, climbed to his perch, threw the sack over his shoul-
ders and settled down for the night.

In most of his structures man is a primitive animal. In those
mentioned, however, he is definitely specialized, and his specializa-

198 EVOLUTION AND GENETICS

tions coincide very well with the effects of arboreal life. Or on a
basis of known facts entirely we may say that anatomical condi-
tions which fit existing primates for arboreal life point strongly
toward the higher development of similar structures in man as a
terrestrial descendant of arboreal ancestors.

The Geological Record. Inquiry into the fossil record of
man's development shows first of all that primates were abundant
during the Eocene, even in North America. They became extinct
on this continent, but continued their existence in Eurasia, where
the remains of several interesting genera have been discovered.
Wilder describes two European species, Pliopithecus antiquus and
Dryopithecus fontani, as Miocene apes. In Asia Palaeopithecus
sivalensis and Pithecanthropus erectus are significant.

Palaeopithecus. This primate has been called a chimpanzee,
but against this identification we are told: "In comparison with
the chimpanzee its canine and lateral incisor teeth are much
reduced, and the two lines formed by the lower molars converge
anteriorly, this character lying midway between the condition in
the chimpanzee, in which the two rows are parallel, and that found
in Man, where marked anterior convergence of the rows of lateral
teeth results in the formation of a gentle curve" (Wilder).

Pithecanthropus. Pithecanthropus was found in central Java in
1891 by a Dutch army surgeon, Eugcn Dubois. The remains first
uncovered consisted of a single upper molar tooth and the top of a
skull, separated by about a meter in the same deposits. Later a
second tooth, also a molar, and a left femur were discovered about
fifteen meters away but also in the same deposits. These parts
have been literally bones of contention. There seems little reason
to doubt that they belonged to the same individual, although that
possibility must be admitted. However they are of great im-
portance, whatever our opinion of their relationship with each
other, for conclusions based upon the single parts are in themselves
significant.

The age of the deposits in which the bones were found has been
placed at the early Pleistocene, but Osborn interprets the remains
of mammals found in the same strata as late Pliocene. All evi-
dence points to the fact that Java was connected with Asia at that
period, and contemporary researches are being centered upon the
search for fossils in the SiwaHk Hills of India and central Asian
regions.

THE EVOLUTION OF MAN 199

Pithecanthropus has been called the Trinil race and the Java
ape-man. He is looked upon generally as more than ape, and yet
less than man, so the latter term is apt.

The skull cap is fortunately complete enough to give an expert
anthropologist data for the reconstruction of the cranium, while
the teeth are indicative of additional characters of the jaws
(Fig. 1 14) . The cranial capacity was about two-thirds that of man,
the forehead low, and the l)row ridges prominent. The centers of
touch, taste and vision were well developed in the brain, according
to Osborn, and the "central area of the brain, which is the store-
house of memories of actions
and of the feelings associ-
ated with them . . . but
the prefrontal area, which
is the seat of the faculty of
profiting by experience or
of recalling the conse-
quences of previous re-
sponses to experience, is
developed to a very limited

degree." The known teeth fjo. 114.— Skull of the Java ape-man,
are larger than human teeth, Pithecanthropus erectus, restored. (From
and differ in some particu- L^"' ™«dified after Dubois.)
lars, but are manhke. The femur is not very different from that
of man, indicating, if the bones belong together, that the owner
of the skull cap was erect, and consequently that he had free use
of his arms.

From the many discussions of the standing of Pithecanthropus
in relation to man we may conclude that the species is undoubtedly
a transitional form representing a very early stage in the evolu-
tion of the human beings of today. It is uncertain whether it is
in the direct line of human descent or shghtly removed from that
line, but in either case it has many characters of our prehuman
ancestors (Fig. 115A).

A more recent discovery of prehuman remains was made in 1925
in Bechuanaland, South Africa, by Professor Raymond Dart.
This consisted of a brain cast bearing the bones of the face and
part of the skull of a child of six years. The complete association
of parts indicates that the creature was a transition form perhaps
even more important than Pithecanthropus. Wilder gives a brief

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THE EVOLUTION OF MAN 201

account of the discovery in his book on The Pedigree of the Human
Race. This species has been named Australopithecus ofricanus.

A single molar tooth discovered in Nebraska which bears the
name Hesperopithecus haroldcooki has caused much comment.
This tooth is said to indicate a stage intermediate between Pithe-
canthropus and modern man, but its standing is in dispute. Such
a stage of human development is not to be expected in North
America and so it demands very accurate analysis.

With these limited remains of prehuman species we cannot, of
course, establish a continuity of descent comparable to the phylo-
geny of the horse. They are significant, however, for they verify
many points in the theory of derivation from arboreal ancestors
by the physical "finishing touches" of erect posture and the de-
velopment of intelligence. All show greater brain capacity than
the apes. All show the reduction of the jaws and teeth. Finally,
within limits, all indicate increasing erectness. Such checks, how-
ever incomplete, can only strengthen our belief in evolution.

Climatic Factors. Not only are the structural features of
these remains of prehuman species in accordance with the theory
of descent, but also the climatic conditions under which they must
have lived. Geologists tell us that the late Pliocene witnessed the
first glaciation. Over the northern hemisphere the extension of
ice sheets from the north in some regions accompanied a general
lowering of temperature which marked the culmination of a proc-
ess long in operation. The gradual cooling of the climate brought
about a change of flora. Tropical forests could no longer flourish,
and the trees which existed were more like those of the north tem-
perate zone of the present, finally to be replaced by conifers in
regions far south of their present range.

These changes tended first of all to destroy the arboreal habitat
of the great anthropoids. They were faced with two alternatives
as the tropical forests disappeared and the climate became more
severe, namely, to migrate southward with the retreat of condi-
tions favorable for their continued arboreal existence, or to remain
where they were and meet the changed conditions with changed
habits. As in the case of other animals which we have considered,
there is every reason to believe that they could do either; very
probably they did both. Those which migrated had no reason to
change, but those which remained in the north must have been
subjected to exactly those stimuli which we have considered.

202

EVOLUTION AND GENETICS

Forced from their arboreal homes by the search for food and by
inadequacy of the thinning forests for a wide range of movement,
they would immediately encounter the conditions of a semi-
terrestrial life, with its manifold advantages and demands. To
these they must have responded if they were to exist, and by their
remains we see that they existed.

Subsequent to the first glaciation a second occurred in the early
Pleistocene, and a third and fourth later in the same epoch. These
resulted in fluctuations of climate, but a relative alnmdance of
fossil remains of man shows that his development had reached a
point where changing conditions could be met by his improving
intelligence. The remains with which we deal are not as highly
developed as modern man; we are forced to look upon them as
different species; but all authorities agree that they are definitely
above the status of the apes. Moreover they are associated with
implements of stone and other evidences of culture which have
never been acquired by other animals.

Three species of fossil men are recognized in addition to remains
referable to Homo sapiens. These are Homo heidelhergensis, Homo

neanderthalensis and Eoan-
thropus dawsoni. Of these
the second is represented by
many specimens, while the
other two are based on limited
material.

Heidelberg Man. Homo
heidelhergensis is based on a
jaw found near Heidelberg in
1907. The bone was in ex-
cellent condition and con-
tained a full set of lower
teeth (Fig. 116). Osborn, quoting from Schoetensack, the
author of the species, gives the following account of the speci-
men: "The mandible shows a combination of features never
before found in any fossil or recent man. The protrusion of the
lower jaw just below the front teeth which gives shape to the
human chin is entirely lacking. Had the teeth been absent it
would have been impossible to diagnose it as human. From a
fragment of the symphysis of the jaw it might well have been
classed as some gorilla-like anthropoid, while the ascending ramus

Fig. 116. — Jaw of Heidelberg man, Homo
heidelhergensis. (From Lull, after
Schuchert and Woodward.)

THE EVOLUTION OF MAN 203

resembles that of some large variety of gibbon. The absolute
certainty that these remains are human is based on the form of the
teeth — molars, premolars, canines, and incisors in form, show no
trace of being intermediate between man and the anthropoid apes,
but rather of being derived from some older common ancestor.
The teeth, however, are somewhat small for the jaw; the size of
the border would allow for the development of much larger teeth;
we can only conclude that no great strain was put on the teeth, and
therefore the powerful development of the bones of the jaw was not
designed for their benefit. The conclusion is that the jaw, regarded
as unquestionably human from the nature of the teeth, ranks not
far from the point of separation between man and the anthropoid
apes. In comparison with the jaws of Neanderthal races, as found
at Spy, in Belgium, and at Krapina, in Croatia, we may consider
the Heidelberg jaw as pre-Neanderthaloid; it is, in fact, a gener-
alized type." The race probably lived between the second and
third glacial stages, i.e., during the second interglacial stage,

Piltdown Man. Eoanthropus dawsoni, the Piltdown man, may
have been contemporaneous with the Heidelberg man but prob-
ably Hved later. The remains
were found near Piltdown,
Sussex, in a gravel pit, and in-
cluded enough fragments of a
skull to make possible its recon-
struction, half of a jaw, nasal
bones, and a canine tooth (Fig.
117). The jaw has been inter-
preted as of a species of primate
lower than primitive man, but

the later discovery of additional

, , 1 • 1 .1 X Fig. 117.— Skull of Piltdown man,

bones shows conclusively that Homo (Eoanlhropus) dawsoni. (From

it belonged with the skull. Such Lull, modified after Woodward.)

ape-like characters as the jaw

displays may therefore be regarded as proof of the primitive

nature of the species. The remains are generally assigned to the

third interglacial stage, with an estimated age of 100,000 to

150,000 years.

The Piltdown race coml)ined both primitive and fairly advanced
characters. The jaw is distinctly ape-like, as shown by the single
canine tooth and the mandible, yet the brows lack such prominent

204

EVOLUTION AND GENETICS

Neanderthal
man

Heidelberg
man

ridges as are found in the apes and most prehuman species. Even
the Neanderthal race had much more prominent supraorbital
ridges. The cranial capacity was probably 1100 c.c. or slightly
less, in contrast with a range of approximately 1200 to 1500 c.c. in
modern races.

Interpretations of this species vary greatly. It has been sup-
posed that Piltdown man represents an ancestor of the Heidelberg
and Neanderthal races, which must then be looked upon as de-
generate. On the other hand, Osborn interprets the race as a
side branch, while still other scientists look upon it as ancestral in
varying relationships to other forms. There seems to be no ade-
quate reason for interpreting the race as ancestral to Neanderthal
man, but the development of the brows points rather definitely

Modern man ^^ ^^^^ ^ relationship to Ho7no
sapiens. It is therefore highly
probable that Eoafithropus di-
verged from a remote ancestral
stage of modern man, and that
it is derived from the same Hne
as Homo neanderthalensis, of
which the Heidelberg race is
supposed to be ancestral (Fig.
118).

A most interesting circum-
stance regarding the Piltdown
race is the association with it
of crude chipped flints, which
indicate a very primitive cul-
ture. The reader should con-
sult Osborn's Men of the Old
antri^id anSL Stone Age for an account of the
Fio. n8.-Diagram showing the ap- development of primitive Indus-
proximate relationships of the chief try and art.

fossil species of man, modern man, Neanderthal Man. The Nean-
and the modern apes. ^^^^^^j ^^^^^ ^^ ^^^.^^^y ■^^^^_

cated, is represented by numerous skeletons from various localities
in Europe. These people Uved after the third interglacial stage, and
are looked upon by some scientists as degenerate. The skull is char-
acterized by large orbits and heavy, prominent supraorl:»ital ridges
(Fig. 119). The cranial capacity varies, quite naturally, but is in

Modem
apes

THE EVOLUTION OF MAN

205

most cases estimated as well above the minimum of Homo sapiens
and in some well above the average for modern man. The lower
jaw was powerful, but less so than that of Heidelberg man. There
was no chin.

Other parts of the skeletons show that the race varied in height
from a little less than five feet to over five and one-half. The chest

Fig. 119. — Skull of Neanderthal man, Homo
neanderthalensis, from Chapelle-aux-Saints.
(From Lull, after Boule.)

was large, the shoulders and arms were power-
ful, and the hands large. The thigh bones
are curved in such a way that the race could
not have been fully erect, a conclusion which
is borne out by the absence of a cervical
curvature of the spine and by the form of
the knee joint (Figs. 120 and 115B).

The Neanderthals were cave dwellers. Their
bones have been found associated with worked
flints, bones of animals, and evidences of the
use of fire. Skeletons have been found which
indicated formal burial.

All known facts indicate that the Neander-
thal race was human. While they were very primitive both in
anatomy and in mental development, they had, no doubt, the
power of articulate speech. Their burial customs indicate
reverence for the dead, and therefore probably belief in some form
of future existence. These things can hardly have failed to

Fig. 120.— Skeleton
of Neanderthal
man. (From Lull.)

206

EVOLUTION AND GENETICS

accompany departure from purely objective mental processes, a
step in evolution limited to man. Their well-worked flints and
use of fire indicate a degree of control over the environment not
previously approached. No longer subject to the untempered
vicissitudes of life in the open, they could make themselves
reasonably comfortable in their caves during severe weather.
Food and protection were assured by their powerful bodies, aided
by the use of weapons, and their mental development was such
as to guarantee gradual improvement of the means at their dis-
posal.

The race lived for several thousands of years, but finally became
extinct. Some authorities have believed that they developed into

Fig. 121. — Reindeer, cave bear, and two horses, from rock engravings in the
Grotte de la Mairie, Dordogne. (From Men of the Old Stone Age by Henry
Fairfield Osborn, after Capitan and Breuil, courtesy of Charles Scribner's
Sons.)

the lower races of Homo sa-piens, but the opinion generally held
is that they were exterminated through the arrival of a more highly
endowed race, the Cro-Magnon.

The Cro-Magnon Race. Unlike the other fossil men, this was a
race of Homo sapiens, physically and mentally equal to many ex-
isting peoples. It was first made known to science through the
discovery of five skeletons at Cro-Magnon, France. They were
tall people, males averaging over six feet and females almost five
and one-half, and were fully erect. The forehead was high but
narrow, the face broad, the chin well developed. The brain was
large. Osborn says that the facial characters are most suggestive
of Asiatic races of the present (Fig. 11 5C).

THE EVOLUTION OF MAN

207

The Cro-Magnon people are supposed to have produced the
drawings and paintings on the walls of European caverns which
are so beautifully reproduced in Osborn's book (Fig. 121). It is
certain that they were mentally developed to a point which would
have made this attainment in art possible. In addition there is
evidence that their art included crude sculpture. Industrially
they worked flint and bone, and probably developed weapons
such as the spear and harpoon {¥\g. 122). They were easily on a
par in these respects with existing savage peoples.

What may have been the fate of the Cro-IMagnon race we cannot
know with certainty. Their head form is so nearly reproduced in

Fig. 122. — Chipped stone implements such as were made and used by the
Cro-Magnon race. Numbers seven and eight are supposed to have been
used for sculpture. (Fi-om Men of the Old Stone Age by Henry Fairfield
Osborn, after Breuil, courtesy of Charles Scribner's Sons.)

the people of Dordogne that Osborn emphasizes the possibility
that the ancient race gave rise to these modern inhabitants of
their old land, and correlates with this the theory that the Basque
language, different from all other European tongues and the most
primitive of all, bears the stamp of early association with the Cro-
Magnon tongue.

Recent Human Evolution. The fact that the ancient Cro-
Magnon race was so highly developed anatomically places addi-
tional emphasis on the course of evolution in an intelligent species
as already outhned. Although these magnificent representatives
of our species lived at least twenty-five thousand years ago, they
were structurally similar to ourselves. Mentally the gulf between

208 EVOLUTION AND GENETICS

them and modern man is equivalent to that between Neohthic
cultures and modern civihzation with its highly developed arts and
complex industries. Cro-Magnon man had a brain apparently
adequate for the gradual development of such things, but the
difference is evident. Whether modern civilizations represent
merely a gradual accumulation of individual experiences and dis-
coveries, or in addition to this form of progress an actual increase
in mental power it is difficult to say, but it is highly prol^able that
the change in mental development which is so well demonstrated
by primitive species of man has continued ever since. Although
the ancient Greeks were intellectually our equals in so far as we can
judge, we must remember that they lived only two thousand years
ago, while primitive species of man occurred one hundred thousand
years ago according to geological estimates. The first seventy-
five thousand years of this period, leading up to the Cro-Magnon
race, show evidences of physical change in our ancestors, but only
those finishing touches incidental to the attainment of erect pos-
ture. In the last twenty-five thousand years there is no evidence
of significant physical evolution. Such changes as have come about
in man are associated with his intelligence; the development of
writing and other facilities for the exchange of ideas, perfection of
social organization, the control of food supply and other phases of
enviromnent have all contributed to the attainment of our modern
state, and none has required any different physical equipment from
that of the earliest members of our species.

In this record, fragmentary though it be, is the story of Homo
sapiens. One hundred thousand years ago he did not exist. The
mute remains of that period show us that other species did occur,
half ape and half man, and that they were succeeded by still others
which we can definitely call human, although much more primitive
than any known race. By comparing the anatomical structures
disclosed by these remains with existing arboreal primates — the
great apes, — and man we find reason to believe that all came from
a single source, a great arboreal primate in many ways hke the
apes of the present. This ancestral species probably existed in
Asia, but change of climate and consequent change of flora resulted
in his becoming in part terrestrial, and with his adaptation to
terrestrial life he developed migratory powers which account for
the appearance of fossil remains in Europe, and the later occur-
rence of man in all the world.

THE EVOLUTIOX OF MAN 209

Like the more complete record of the development of the horse,
man's progress has Ijeen due to changing environment, range of
inherited possibilities and the interaction of the two. Unlike
the other animals, his development has involved a shift from
physical to mental modifications. Through his intelligence he is
now able to control to some degree one of the fundamental factors
of existence, environment, and on that biological foundation rests
his future.

Summary. Man occupies a systematic position among the
highest mammals, the Primates. He is ordinarily included in a
separate family, the Hominidae. The anthropoid apes of the
family Simiidae are the nearest relatives of the human species and
so furnish the only indication to be found among existing species
of his probable origin. Structurally man and the apes are similar
but man differs in details associated with his intelligence, articu-
late speech, and erect posture. The arboreal habits of the apes
and their structural resemblance to man suggest that he may be
derived from arboreal ancestors, and a consideration of the effects
of arboreal life show that this may well be true. Descent from the
trees, which must have preceded the development of terrestrial
man, was favored by the climatic conditions of the time when
primitive man arose. Of these early creatures we have only scanty
records but they are sufficient to show that species existed which
were higher than the apes and lower than man, and that a gradual
transition occurred leading up to the structural characters of
modern man. Even the development of culture is indicated by
artefacts associated with the remains of extinct species of man.

REFERENCES

Haeckel, E., The Evolution of Man, 1905.

Drummond, H., The Ascent of Man, 14th edition, 1911.

Geikib, J., Antiquity of Man in Europe, 1914.

Jones, F. W., Arboreal Man, 1916.

OsBORN, H. F., Men of the Old Stone Age, 1916.

Lull, R. S., Organic Evolution, 1917.

Wilder, H. H., The Pedigree of the Human Race, 1926.

OsBORN, H. F., "Recent Discoveries Relating to the Origin and Antiquity
of Man," Science LXV, 481-488, 1927.

Gregory, Wm. K., "How Near is the Relationship of Man to the Chimpanzee-
Gorilla Stock?" Quarterly Review of Biology, II, 549-560, 1927.

CHAPTER XII
ADAPTATION

In previous chapters we have considered the correlation of in-
heritance and environment in the determination of the organism,
and various specific instances of the resulting adaptation. These
things are of particular importance in the theories of evolutionary
processes. If species change and give rise to other species, the
results of their modification must be adaptations fitting the later
generations to some definite type of environment. We can hope to
understand the process of change only through extensive knowl-
edge of changes which have already come aliout, since the duration
of science has been too brief to afford us an actual view of evolu-
tion in progress.

Adaptations : Process and Result. The process of adaptation,
for it is a process as well as a result, is visible in the lives of indi-
viduals, and is experienced by each of us. We spade the garden,
and blister our hands, but if we continue such work day after day
our palms form calluses which no ordinary amount of friction can
blister. We train for sports, and our strength or endurance or
skill increases day by day. After an athletic career in college we
return gradually to the less active round of business or professional
life, or suffer from a sudden change of habits. All of these things
are adaptive processes.

In the individual adaptations can be observed as readily as the
activities which give rise to them. They fit each being into the
environment which he occupies, and whether they are mental or
physical, they are no less real.

Adaptations are as conspicuous in all species, moreover, as they
are in its component individuals, but the processes which brought
them about are no longer evident. All animals of common ex-
perience show peculiar fitness for the lives which they lead. Squir-
rels have chisel-like teeth which are effective for opening nuts.
Similar teeth serve the beaver for cutting down trees, but the
beaver is otherwise fitted for swimming and the squirrel for climb-
ing trees. The fitness of the horse's teeth for grazing has been

210

ADAPTATION 211

mentioned, and the effectiveness of teeth and claws of carnivorous
animals for seizing and tearing their prey. Such niceness of adap-
tation is universal. Several attempts have been made to explain
its attainment; these we shall consider later.

Caenogenesis, Neoteny, and Paedogenesis. In some organisms
metamorphosis enables the individual to occupy different habitats
at different periods of its life. The adaptations of earlier stages
may be wholly different from those present at maturity and are
sometimes even more wonderful examples of fitness for a given
environment. The development of such temporary adaptations
has been called caenogenesis. In some cases caenogenetic modifica-
tions have apparently been of greater benefit to the species than
adult adaptations and have been carried over into the adult stage.
This condition is known as neoteny. A more extreme emphasis
upon the value of caenogenetic adaptations is found in species
which attain sexual maturity while still in an immature stage
morphologically. This phenomenon is called paedogenesis.

The development of familiar insect larvae, such as the cater-
pillars, maggots, and hellgramites, is caenogenetic. Of these the
caterpillar at least is familiar to everyone, and the complete lack
of resemblance between it and the adult butterfly or moth into
which it develops. Neoteny is illustrated by the axolotl, a sala-
mander found in Mexico, which remains an aquatic form and
retains its larval gills throughout life. Salamanders usually de-
velop into terrestrial adults and this metamorphosis can be arti-
ficially induced in the axolotl. Excellent examples of paedo-
genesis have been reported in a few species of insects from the more
primitive families of two-winged flies. Both larvae and pupae
have been observed to produce young in these families.

The Environment. The environment to which organisms are
adapted is complex. We recognize that every organism has an
association with the surrounding world, from which it receives
stimuli of a chemical and physical nature, as well as the materials
of which its body is composed. To this environment it responds
by more or less complex reactions. By its intricate responses and
by the ultimate return of the substances which it has used during
its life, every organism contributes to the complexity of the en-
vironment of every other organism. The trees shade other plants,
and so modify their relations to sunlight. They also give a home
to arboreal animals, and when they die, furnish food for insects,

212 EVOLUTION AND GENETICS

bacteria, and fungi. The green plants make other forms of life
possible. Thus the organism as a whole is related to an inorganic
and an organic environment, but within itself there are related
parts which show that we must consider further an internal en-
vironment of any organ. Many organs, indeed, respond only to
stimuH from this internal environment, and are reached directly
from without only by accident, if at all. To all of these phases of
environment the organ or organism must be adjusted if it is to
live successfully. Lines of demarcation are not necessarily sharp
for an organ may be effective in more than one way. It is possible,
however, to note definite adaptations to definite conditions in all
organisms.

Non-Adaptive Characters. Following the publication of Dar-
win's Origin of Species there was a marked tendency among scien-

FiG. 123. — Skull of woodcliuek, showing an upper incisor that had grown in
an arc of a circle until it entered the roof of the mouth, after the opposing
lower incisor had been broken off. (From Reese's Economic Zoology, with
the permission of P. Blakiston's Son and Company.)

tists to seek and describe marvellous adaptations. It is not sur-
prising that this should have occurred, since Darwin showed how
wonderfully species are associated with their environments and
how important the usefulness of adaptations may be, but the ten-
dency to regard any character as adaptive to the external environ-
ment must be regarded as extreme.

We now recognize that an organism may possess many char-
acters which cannot be construed as having any value in meeting
the conditions of adaptation. Such characters have been called
non-adaptive, and it is evident that organisms are made up of

ADAPTATION

213

both this type and distinctly adaptive characters, both of which
must be explained by any theory of evolution if the theory is to
be adequate.

We cannot, however, avoid the belief that there is stimulus and
response in all organic conditions. Non-adaptive characters
therefore serve to emphasize the importance of the internal environ-
ment, in response to which parts of an organism may attain even
harmful development with regard to external conditions. But
even in these cases, inherent powers of development can be real-
ized only if the proper conditions surround the part.

The chisel-like incisors of rodents are an evidence of these rela-
tionships. Ordinarily those of the upper and lower jaws are ex-
actly opposed to each other
so that they wear away equally
and maintain a constant
length and position. Several
cases are on record of the
serious effects of loss of one
incisor; one of these is illus-
trated in Figure 123. The
lower incisor in this case was
broken and the opposed upper
incisor, continuing its normal
growth without an}^ com-
pensating wear, finally penetrated the brain and caused the death
of the animal. This case involves definitely adaptive structures
but the fate of the animal was due entirely to the power of growth
inherent in its own body and to the removal of an influence
normally supplied by its body. The abnormal growth of the
upper incisor was in no way adaptive, but it was due to definite
responses no less than truly adaptive structures.

Adaptation to the Physical Environment. Such adaptation is
closely linked with the three major habitats, water, earth and air.
An organism may be adapted to two or to all three, but in any
case it shows its fitness b}^ structural characters. The loon, for
example, is highly developed for aquatic life. Its feet are effective
for swimming, but they also enable it to move about on land
as a terrestrial organism. Like most birds, it is also able to
fly, and the volant adaptations of birds surpass those of all other
animals.

Fui. 124. — Gonionemus, a hydrozoan
jelly-fish. (From Hegner, after Har-
gitt.)

214

EVOLUTION AND GENETICS

Aquatic Adaptation. Purely aquatic organisms are safe from
dessication, consequently they have no need of a moisture con-
serving integument. They are buoyed up by the water in which
they live so effectively that they have no need of rigid structure.
The result is that the most delicate organisms are aquatic. Jelly-
fishes are made up mostly of water (Fig. 124). Their beautiful,
filmy bodies, if removed from the water, fall into a shapeless heap
and quicldy dry into a small organic residue.

Benthos. Some of the aquatic animals remain on a solid sub-
stratum, and are either attached to immersed objects or move
from place to place over the bottom (Fig. 125). Some rigidity is

Fig. 125. — Sea anemones. (From Hegner, after Coleman.)

obviously necessary in these animals as protection, not only
against their enemies, but also against the motion of the water in
which they live. It is also an advantage for the sedentary forms
to be able to reach out in all directions, since their food must come
within reach instead of being sought. Consequently radial sym-
metry is a common character. Radially symmetrical animals are
made up of similar parts arranged about a common center, in con-
trast to the more common bilaterally symmetrical forms, which are
made up of similar halves flanking the longitudinal axis. Regard-
less of their powers of motion, these bottom forms are called the
benthos. The shallow seas are rich in benthonic forms, such as the
sponges, sea anemones, corals, barnacles and many others. '

ADAPTATION 215

Plankton. In contrast to benthonic animals are those which
merely float in the water, drifting about with its movements. This
group is called plankton, and comprises innumerable species both
of animals and plants. Single-celled organisms are very numerous.
The Coelenterata are also well represented, and molluscs of some
classes. The plankton includes the most extreme aquatic organ-
isms, such as the jelly-fishes. Radial symmetry is common in this
group as in the sedentary benthos, and for the same reason, lack
of locomotion. Transparency is also common in the plankton.

Nekton. The transition from plankton to the third division of
aquatic life, the nekton, is gradual. The essential characters of
organisms belonging to the nekton are correlated with their ability
to move freely through the water, resisting all motion of the me-
dium in which they live. This necessitates a non-resistant body
form, well illustrated by the spindle-shaped bodies of common

Fig. 126. — The herring, Clupea harengus. (From Hegner, after Jordan and

Evermann.)

fishes (Fig. 126), characteristic organs of locomotion, broadened to
offer the necessary resistance to a fluid medium, and bilateral sym-
metry, a common corollary of well-developed powers of locomo-
tion. Such invertebrates as aquatic insects and some moUusca,
and most aquatic vertebrates are included here. The fishes are
excellent examples.

The Abyssal Realm. The ocean affords still other examples of
adjustment to the physical environment in the peculiar fauna of
the deep sea. This environment includes depths below 100 fath-
oms, where light does not penetrate. The pressure is great, in-
creasing at the rate of one ton per square inch with every thousand
fathoms, and the temperature is low, near the freezing point in the
open ocean. Because of the absence of light plants do not grow,
and animals must subsist by eating each other and the things
which settle from above. It seems only natural that conditions so

216

EVOLUTION AND GENETICS

different from those in which we Hve should produce organisms
which are, from our point of view, bizarre (Fig. 127). Strange

Fig. 127. — Deep-sea fishes. A, Photoslomias guernei, length 1.5 inches, taken
at a depth of 3500 feet; B, Idiacanthus ferox, 8 inches, 16,500 feet; C, Gas-
trostomns bairclu, 18 inches, 2300-8800 feet; D, Cryptopsaras coucsii, 2.25
inches, 10,000 feet; E, F, Linophryne lucifer, 2 inches. (From Lull, after
Goode and Bean.)

body forms prevail, and luminescence is common. The purpose of
luminous organs seems usually to be the attraction of prey or the
provision of Ught for their possessor, but Beebe, in The Arcturus

ADAPTATION 217

Adventure, has recently reported a striking variation in a deep-sea
prawn. The cephalopods Hving in shallow waters conceal them-
selves when molested by discharging a cloud of brown secretion
into the water. The prawn behaved likewise, but the darkness of
the abysses could hardly be darkened, so the discharge of this
animal was a luminous cloud, which in the general absence of
light would conceal its movements and often obtain its escape.

Terrestrial Adaptation : Locomotion. Lull very logically applies
the classification of aquatic organisms to those living in the air,
so that volant forms may be looked upon as aerial nekton, and
ordinary terrestrial forms as aerial benthos. Aerial plankton is
very limited. Bacteria are known to float in the air, but they
probably remain there only temporarily. The same is true of the
spores and pollen of plants. Consequently, while there is an
aerial plankton, no organisms can be said to belong to it perma-
nently.

The differences between terrestrial and aquatic life have already
been considered under the emergence of the terrestrial vertebrates.
A conspicuous feature of the adaptation of terrestrial forms is the
modification of supporting and locomotor organs. Since the air
by which the animal is surrounded does not support it like the
water, its points of contact with the rigid substratum must serve
both for support and locomotion. The pentadactyl appendage
(Fig. 54) in vertebrates and the jointed appendage in Arthropoda
are the outcome of this need while in other forms the body lies on
the ground and locomotion is accomplished by creeping, aided some-
times, as in the annelids, by setae or other projections to increase
the hold of the organism on the surface which supports it.

While creeping is a very simple process, in general the same in
all groups which move about in this way, the development of
jointed appendages of either type paves the way for a variety of
modifications. Thus we find the vertebrate limb modified for
walking, running, jumping, burrowing, and climbing and the in-
vertebrate appendage for most of these functions.

Ambulatory Adaptations. The ambulatory, or walking type
obviously involves the least change of form. Such locomotion re-
quires in addition to supporting function no further power than
successive shifting of position of the several limbs in relation to the
ground. Without sufficient development of muscles for this pur-
pose, the structures could not function even as supports for the

218

EVOLUTION AND GENETICS

body, consequently the ambulatory condition is the most primi-
tive stage of the terrestrial appendages.

Cursorial Adaptation. Cursorial animals must move their
limbs rapidly, and for a maximum rate of speed must also be able
to move by long strides. Rapidity of movement is in part a
physiological adaptation, but it is aided by the structural modi-
fications which result in lengthened stride. These include
lengthening of the limb and slenderness, so that the greatest
reach is attained with minimum bulk. The hmb is a lever of the
third order in which the point of articulation to the body is the
fulcrum and the insertion of a muscle the point of application of
power. By lengthening of the entire limb, particularly its distal

Fig. 128. — Types of insect legs. A, grasshopper, a jumping leg; B, tiger
beetle, a running leg; C, gyrinid beetle, a swimming leg; D, mantis, a rap-
torial leg; E, mole cricket, a burrowing leg. (From Sanderson and Jack-
son's Elemenlary Enlomology, with the permission of Ginn and Company.)

segments, the ratio of work arm to power arm is increased, conse-
quently greater range of movement is acquired at the expense of
power. In cursorial Arthropoda this lengthening and slenderness
is general (Fig. 128A). In vertebrates it involves the distal seg-
ments of the appendages (Fig. 56), while the proximal muscles
remain short and bunched, making for quick and powerful con-
tractions.

Correlation with flight lessens the necessity for cursorial adap-
tation among the Arthropoda, but such insects as the tiger beetles
(Cicindelidae) are able to run very rapidly for their size. Their
legs are long and slender.

In the vertebrates the first step in cursorial adaptation is change
of posture. The animal rises to its toes, becoming digitigrade, in-
stead of plantigrade, and thus adds to the free length of the hmb

ADAPTATION 219

the length of the foot. Later the foot and lower limb elongate
and the change of posture becomes more extreme, giving rise to
the unguligrade habit, in which only the extreme tips of the digits
touch the ground. With high specialization in this direction the
final step, reduction of the digits, occurs. Such a change becomes
possible through emancipation of the appendages from other
functions than locomotion; it is an advantage in reducing the
weight of the appendage with the reduction of its efficiency as a
lever.

Vertebrates of the classes Reptilia, Aves and Mammalia have
developed cursorial powers, but none show any higher specializa-
tion than the horse, which we have already considered in detail.
Some existing lizards are cursorial, but the extinct dinosaurs
attained the highest speed adaptations in this class. Their adapta-
tions included also bipedality. The flightless birds, such as the
ostriches, and some of our common birds including the quail, are
excellent runners, although the power of flight, when not com-
pletely lost, is always a last resort when speed is necessary. Such
adaptations as are shown by the horse are developed in lesser
degrees in many mammals, consequently we are most familiar
with cursorial animals in this class. The ungulates including
dogs and cats are highly developed cursorial animals.

Saltatory Adaptations. Saltatory, or jumping animals, are more
conspicuously different in the two groups. In the vertebrates the
hind legs are always predominant appendages for propulsion,
while the front legs act as supports for the anterior end of the
body, serve to catch the body at the termination of a leap, and in
a minor degree aid in propulsion. This predominance of the hind
legs is well illustrated by their earlier specialization in fossil
horses. Locomotion in cursorial vertebrates is therefore asso-
ciated with saltatory power, since a gallop is essentially a succes-
sion of jumps. In the rabbits and the kangaroos and wallabies
the development of the hind legs is carried to such a point that the
animals may be called truly saltatory (Fig. 129). This is espe-
cially true of the kangaroos, since they are able to move by a
series of jumps, without the aid of the fore-limbs.

The insects show saltatory adaptations of a very different type.
The basis for such development is probably twofold. Structur-
ally, the presence of six legs affords an opportunity for walking,
even though one pair be modified to such an extent that they are

220

EVOLUTION AND GENETICS

practically useless for that purpose. In addition, the size of
insects is so small in relation to objects about them that a single
powerful leap is often a guarantee of safety, since it may carry
them readily into concealment. The suddenness of this type of
locomotion is its most valuable feature.

The structural modification of a saltatory insect leg includes
great enlargement of the femur, which contains large extensor

■^imi&

il^--;

■ !^.,,.-r-^:.

iA'«g^^;^«?,i>iirc/j^y*-*;-'ii^

Fig. 129. — The Rock Wallaby Petrogale xanlhopus. (From Parker and Has-

well, after Vogt and Specht.)

muscles (Fig. 128B). Elongation is common, but not essential,
for among the beetles and the fleas are included very powerful
jumpers with hind legs as short as is normally the case in other
insects. The Orthoptera include more familiar jumping insects,
however, and in this order the long hind legs of the grasshoppers
and crickets are familiar to everyone. In these legs another
specialization is evident. The foot is not a dependable support,
but heavy spines are developed at the tip of the tibia, analogous

ADAPTATION

221

to the lower limb of a vertebrate, and these guarantee a non-sldd
take-off from any surface providing the slightest of holds. For
obvious reasons only the hind legs of jumping insects are special-
ized.

Fossorial Adaptations. In contrast to cursorial and saltatory
adaptations, fossorial species have the front legs most highly
developed, since they must open a way for the body through the
earth. Among the invertebrates without appendages burrowing
is accomplished by the simple means of forcing the slender, tapering
body through relatively loose soil, or by passing earth through
the alimentary tract as the animal progresses. Arthropoda, how-
ever, make use of the mouth parts and of the legs in digging. An
extreme specialization of this type is seen in the mole-cricket,

Fig. 130. — Common mole, Talpn europnea, showing skeleton and outline of
body. (From Lull, after Pander and D'Alton.)

whose front legs are strong, notched, shovel-like appendages with
which the insect digs rapidly and effectively (Fig. 128C). These
strange httle creatures are also covered with moisture-resisting
down.

The mole is the most highly specialized fossorial vertebrate
since it lives entirely undergrovmd (Fig. 130). Its pectoral girdle
and fore-limbs are massive, and the forefeet are very broad and
provided with strong claws. In addition to this elaborate mech-
anism it has an elongate pointed snout which aids it in forcing its
way rapidly through soft earth. Its progress may be a combina-
tion of digging and spreading of the earth before it. The mole,
like its namesake, the mole-cricket, has extremely fine vestiture,
resistant to moisture. The eyes are vestigial in the mole, and the
external ears are lacking. Either organ would be liable to injury
in burrowing, and neither could be of use to an animal whose hfe
is spent in darkness surrounded by solid earth.

Many other vertebrates representing all classes above the fishes
are fossorial, though they do not remain altogether underground.

222 EVOLUTION AND GENETICS

Such animals, as might be expected, show adaptations similar to
those of the mole but less extreme. The fore-hmbs and claws are
well developed, but not extremely. The eyes are, of course, useful
above ground, and remain functional, but they are reduced in
proportion to the amount of time spent in burrows. The same is
true of the ears. Tapering of the body may occur, but is no more
extreme than in many animals which do not burrow.

In addition to the animals which form burrows for conceal-
ment there are some that dig for food. The elephant uses his
tusks for this purpose. The snout of hogs and of some snakes is
turned up at the tip, forming an effective organ for burrowing to
slight depths in soft ground.

Scansorial Adaptations. Such adaptations are chiefly related
to the organic environment, but since they are for the purpose of
locomotion they may conveniently be treated here. Whether an
animal climbs trees or cliffs, the demands upon its body are the
same. The appendages and girdles must be strong, to support the
weight of the body. In addition to this there must be provided
some means of maintaining a safe grip upon the supporting object.
Finally the proximal segments of the limbs are seen to be elon-
gated in some arboreal vertebrates. This reversal of the condition
noted in cursorial forms is due, no doubt, to the fact that intrinsic
strength is necessary in the distal parts which are in immediate
contact with the support, so that reach must be gained elsewhere
if at all.

Appendages are modified in several ways to enable animals to
cling to branches. The claws are usually involved, but in some
cases the appendages themselves are prehensile. Animals which
climb by clinging to the bark of tree trunks, and run along the
upper surface of branches have sharp claws which give them an
adequate grip on the surfaces which support them. Squirrels are
perhaps the most familiar example, but many birds and some
reptiles are similar in habits. The sloths normally suspend them-
selves from branches, and are provided with great hook-like claws
as an aid to this habit (Fig. 131). The development of the ap-
pendages is so extreme that they are scarcely able to move about
on the ground. Prehensile appendages are best developed in the
primates, where opposability of the thumb and locomotion of the
type known as brachiation are found. Adaptations of the last
kind have already been discussed under the evolution of man.

ADAPTATION

223

Adhesive organs are found in insects and in such vertebrates as
the tree frogs. Adhesion may be accomphshed by the secretion
of fluids or by the vacuum-cup principle, and is especially effective
for climbing on smooth surfaces. The ability of a fly to walk up a
window pane or across the ceiling is due to such organs.

Some animals use other organs than the limbs for climbing.
The true chameleon of Africa and some monkeys have prehensile

Fig. 131. — The two-toed sloth, Choloepus didadylus. (From Parker and Has-

well, after Vogt and Specht.)

tails which are used to grasp branches, and parrots use their power-
ful beaks for the same purpose.

Adaptations to Light. In addition to adaptations of the appen-
dages terrestrial animals are adapted to conditions of light and
moisture, although to a lesser degree than the green plants. Since
sunlight plays so large a part in the metabolism of the latter organ-
isms, they are nicely adjusted to it. Shade-loving species are of
more delicate texture, for various reasons, and have broader
leaves than those which live in the open. Violets afford a familiar
illustration; most species have entire leaves, but in those which
are found on the dry, brightly lighted prairies the leaves are finely

224 EVOLUTION AND GENETICS

divided. Animal adaptations concern visual functions chiefly,
and in animals which have no eyes, the sensitiveness of the skin
to light rays. Burrowing animals may retain light sensitiveness,
as is true of the earthworm, but vertebrates like the mole tend to
lose their eyes. Such elaborate organs are obviously of no use in
darkness, and consequently they disappear or lose their functions
in cave-inhabiting animals as well as fossorial species. Sala-

— ,..___^ manders and in-

^v>^-^Z^~~^^nSv sects without eyes

.-.,_^__^ ^lVIJ ^^^^ been recorded

^^^'TT' 'JiiUi^^^ / / , ; /"7", ■", '""'^ frOm caves, and

y'^'^'^ '^^ni ""^^^^^^nN""^^**^ some blind fishes

(^,_-. — ^^^^ V from subterranean

Fig. 132. — Blind salamander, Proteus anguinus, from waters (Fig. 132).
underground waters. A European species. (From Another charac-

Lull, after Gadow.) , v.- i • j- -i

ter which IS directly

correlated with absence of light is reduction or loss of pigmenta-
tion. The presence of pigment in the skin acts as a protection
against the rays of the sun, as everyone has experienced through
sunburn followed by tanning. It is also of value in the develop-
ment of color and pattern for interrelations with other animals.
Where light does not penetrate, obviously neither use exists.

Adaptation to Aridity. Animals. Water is essential to all life,
and is almost universally abundant. Only in desert regions is it
scanty, and animal adaptations to meet this lack are invariably cor-
related with adaptation to other conditions of desert life. Adapta-
tions for the conservation of water in animals are of three types:
(1) storage reservoirs, such as are found in the stomach of the
camel; these have already been mentioned ; (2) lack of the power
to perspire; (3) ability to absorb water through the skin.

Plants. Plants require water as well as sunlight for photo-
synthesis, and since they cannot move about in search of it, but
must depend upon rainfall, their adaptation to lack of moisture is
not limited to desert species. If one month of the plant's life
must be passed without rain, it must be able to withstand this
lack of moisture or perish, no matter how great the abundance of
moisture at other times. Plants so adapted are known as xero-
phytes. They meet conditions of dryness in several ways, viz.,
(1) by extensive root systems, which draw moisture from a large
volume of ground; (2) by reduction of leaf surface through small

ADAPTATION 225

size or subdivision of leaves, or in extreme eases by complete loss
of leaves and shifting of their functions to the branches; (3) by
the development of thick cuticle; and (4) by scaly, hairy and
spiny surfac(^s (Fig. 133). The last three serve to prevent rapid
transpiration, and so conserve the moisture which the plant con-
tains. Desert plants, such as the cacti, are extreme adaptations.
They are without leaves and are provided with a moisture con-
serving cuticle. Some contain large quantities of water. A desert
fern, Notholaena, has wiry stipes and waxy fronds, densely scaly
below. More common illustrations are found on the western
prairies, where harsh grasses and other plants with harsh, hairy
and finely divided leaves are common. The roots of many of these
plants penetrate far into the ground, although their branches may
extend less than a yard above the surface.

Flight. Volant animals, since the air is too light a medium to
buoy them up, must combine their flight adaptations with others.
They may be entirely aquatic and still possess some slight power
of flight, but ordinarily they are in some degree terrestrial. Thus a
combination of terrestrial, aquatic and volant adaptations in one
individual is common, and no animals are exclusively volant.

Structure. The fundamental requirements for flight are cor-
related with the fluid quality and lightness of the air. They in-
clude lightness of structure, broad planes for support, steering,
maintenance of equilibrium and propulsion, great muscular power
and power of endurance. Newman's treatment of "The Bird as an
Automatic Aeroplane" is a graphic account of these adaptations,
since the birds are the most highly developed flying animals.

Lightness is secured by the development of hollow bones.
Rigidity and strength are maintained, in spite of the relatively
fragile bone structure, by their form. Many of the bones are
formed like the T and I beams used in structural steel work. The
sternum, which bears the great stresses of flight, is an especially
fine example of T beam. Fusion and overlapping of bones also
add to rigidity of the bird skeleton, while the presence of air at
the relatively high body temperature adds to the buoyancy of the
entire animal. The feathers form the lightest broad supporting
surface known in the organic world, and enclose air about the body;
"nearly half of the contour volume of a bird is air-filled" (New-
man).

Organs for supporting, steering, propelling and balancing are

Fig. 133. — A group of Xerophytes. The ca(;tu.s is Cereus giganteus; absence
of leaves and the development of spines are conspicuous. At the right are
an agave with broad fleshy leaves and a yucca with harsh, narrow leaves;
both have moisture-conserving epidermis. The small plants are Mesembry-
unthemum and Sedum, both of which have fleshy leaves for the storage of
water and some protective covering, such as hairs or thick epidermis, to
aid in its conservation. (From Campbell, after a photograph by Dr. F. M.
MacFarland.)

226

ADAPTATION

227

formed of feathers in birds. Both tail and wings arc inchidcd, the
latter alone as propelling organs. In other flying animals, such as
the bats, similar organs are formed of folds of skin extending from
limb to limb along the sides of the body or stretched between
elongated bones (Fig. 134). The fins of flying fishes express the
same adaptive tendency as fins in general, but carried to an ex-
treme correlated with the Ughtness of the air (Fig. 135). Insect
wings are entirely different; since the exoskeleton provides rigidity

Fig. 134.— Bats.

A, Vesper lilio noctula, an insectivorous species; B, Pteropus
sp., frugivorous. (From Lull.)

throughout the body, the wings are simple broad flat evaginations
of the body wall, stiffened by local thickenings, the veins, between
which the cuticula is extremely thin and fight (Fig. 136).

Power and endurance are closely correlated. Enlargement of
the muscles that move the organs of flight is necessary, and is ex-
treme in birds. The large pectoral muscles, attached along the
keel of the sternum, make up the masses of breast meat with which
everyone is familiar. The rhythmic contraction of these muscles
during long flights demands an abundant supply of food and oxy-
gen, and rapid removal of wastes. These needs are met by modi-
fications of the alimentary tract, providing for the storage of a

228 EVOLUTION AND GENETICS

supply of undigested food in the crop, and for rapid digestion.
The respiratory system is so modified that air enters the alveoU
of the lungs through one system of tubules and passes out through
another, so that fresh air passes constantly over the respiratory
epithelium. These things result in rapid metabolism, which is
correlated with high body temperature. Many birds maintain a

Fig. 135. — Flying fish, Dadijloplerus volilans. (From Lull.)

temperature of 105° F., and the best fliers reach 110° to 112°. High
temperature is of value not only in promoting rapid metabolism
but also as a protection against the low temperature of the upper
air.

The extent to which animals are adapted for flight varies greatly
however. Some forms are able to move through the air to a
limited degree only, by gliding, while others are capable of true
flight.

Gliding is a self-explanatory term. If an animal is provided
with sufficiently extensive membranes, it is buoyed up by the air
pressure induced by gravitation, and is able to coast from higher
to lower levels on the air. The same principle enables birds to soar
on air currents without moving the wings, but this demands a
nice adjustment of equilibrium of which less highly adapted organ-

ADAPTATION

229

isms are incapable. Gliding animals include the remarkable flying
dragon, a reptile of the Indo-Malayan region (Fig. 137), the flying
squirrel (Fig. 138), and a tree frog, Rhacophorus. In the flying
dragon the supporting planes are developed as membranes on the
sides of the body, covering elongated ribs. The flying frog has
broad webbed feet which serve as gliding planes, and the flying

MA

C D

Fig. 1.36. — Wings of insects, showing the supporting veins. A, honey-bee;
B, Osmylus; C, Evaniellus; D, fore-wing of Anosia, a butterfly. (From
Constock's Wings of Insects with the permission of the Comstock Pubhshing
Company.)

squirrel has folds of skin extending along the sides of the body
from limb to limb.

True flight of a very limited degree is possibly found in the
flying fishes. These animals have greatly enlarged pectoral fins,
which are flapped during their long jumps through the air. How-
ever, some biologists construe their progress as gliding.

The power of sustained flight has been developed independently
in three classes of vertebrates. Among extinct forms the reptiles
are represented by the pterodactyls. The birds as a whole are
flying animals or derived from flying ancestors. The mammals are
represented by Galeopithecus, the "flying lemur," which is inter-

230

EVOLUTION AND GENETICS

mediate between the insectivores and bats, and by the entire order
Chiroptera, the bats.

The Pterosaurs had wings formed of sldn folds stretched between
the hind-limb and one greatly elongated digit of each fore-limb
(Fig. 139). In Galeopitheciis the membrane stretches from limb
to limb, and to the tail, and in the bats a similar condition pre-
vails, but the elongation of several digits of the fore-hmb extends
the membrane into a wing-like form.

Adaptation to the Organic Environment. The Web of Life.
The relationship of organisms with each other is infinitely com-
plex. All organisms depend
directly or indirectly upon
photosynthesis for food.
The green plants carry on
the process for themselves,
and some animals feed upon
the green plants, but other
animals secure their food by
eating animals. Some or-
ganisms depend upon others
for protection or conceal-
ment, for tempering the
intensity of hght, for shelter
from the weather and for
many other things. The
resulting complexity has
given rise to the conception
of the web of life, in which
organic associations are
likened to a woven fabric.
The disturbing of one
thread affects the relation-
ships of others. The more
intimate the association with the source of disturbance, the greater
the change, but step by step the effect may be transmitted
throughout the whole fabric.

Darwin called attention to a striking illustration of such inter-
action. He noted that red clover was fertilized by bumble bees,
and that protected flower heads did not produce seed. Since field
mice destroy the nests of bumble bees, they are a check on the

Fig. 137.

—The flying dragon, Draco volans,
a lizard. (From Lull.)

ADAPTATION

231

production of red clover, and since cats destroy mice they are
beneficial to its production.

INIodcrn economic biology is full of such illustrations. We spray
plants to get rid of aphids, but it is often necessary to destroy a

*/"/.,iiii'""

Fig. 138. — Flying squirrel, Sdiiropterus volucella. (From Lull.)

neighboring ants' nest before a cure can be accomplished, for the
ants may bring other aphids to replace those killed by spraying.
We also poison, trap and swat the fly, but if horse manure is piled
in the neighborhood, we can destroy more flies with less effort by
having it removed. We raise fruit, and many orchardists have
found it to their advantage to rent bees to be placed in their
orchards during blossom time, in order that cross fertilization

232

EVOLUTION AND GENETICS

may be more certainly accomplished. Pages could be filled with a
mere listing of these known associations.

The Nature of Organic Relations. A primary reason for the
association of organisms is the securing of food, but since this in-
volves the destruction or injury of other organisms except in the
case of the green plants, there must always be adaptations for the
protection of those subject to attack. Finally there are associa-
tions of convenience, wherein animals of the same or different

Fig. 139. — Pterodactyl, Rhamphorhynchus phyllurus. (From Lull.)

species find it possible to meet the requirements of life better
through the assistance of others.

Food- Securing. The Structures Involved. In securing food the
adaptation of the mouth is necessarily important. We have
already noted a striking example in the evolution of the teeth in
elephants and horses, and another in the heterodont dentition
of the primates in connection with omnivorous habits. The lips
of animals also show adaptation. Grazing species have a prehen-
sile upper lip which is useful in gathering tufts of grass into the
mouth, while rabbits nibble at leaves and fruits without the neces-
sity for securing a quantity of pieces at one mouthful. Whatever
may be the food, the teeth at least are formed so that it may be
effectively chewed. The appendages are often modified as acces-
sory structures.

Anteaters. Such animals as the anteaters are extreme adapta-
tions to a limited diet. The great ant-bear has strong, hooked
claws with which it tears open the nests of ants (Fig. 140). Its
snout is slender and elongate, its tongue long and sticky, and it is

ADAPTATION

233

entirely toothless. The tongue serves to gather the multitude of
tiny insects which are necessary to nourish its seven-foot body,
and since ants are so small and soft, teeth are quite unnecessary.
The claws are incidentally effective weapons.

Carnivorous Animals. Such animals, while they have special-
ized teeth, are no less dependent for food upon adaptations of the
appendages. The ability to stalk prey silently, to pursue it rap-
idly, and to spring upon it quickly must be brought into play
before the teeth are necessary to hold and kill, and the shearing

.J \

Fig. 140. — Ant-bear, Myrmccophaga jubata. (Through the courtesy of the

New York Zoological Society.)

molars to cut through the relatively tough flesh as the prey is
eaten.

Insects. In all forms of animals such adaptations are found.
Insects have mandibles whose strength is in proportion to the
harshness of their food. Species which live on fluids have the
primitive mandibulate mouth highly modified to form suctorial
structures, and in some cases piercing structures to enable
them to reach their food (Figs. 141 and 78). Both types of
mouth are found in both phytophagous and carnivorous in-
sects. In the latter some type of powerful grasping leg is also
present.

Birds. Birds likewise have powerful hooked beaks and strong
claws if carnivorous and hooked beaks but weak claws if they eat

234

EVOLUTION AND GENETICS

carrion. Seed eaters have thick, powerful beaks, but in in-
sectivorous species the beak is more slender (Fig. 142).

Protective Adaptation. Reproduction. A simple protection
for species which are preyed upon by others is the production of
enough offspring to maintain the species in spite of its constant
loss. Among many species this adaptation of reproduction takes
the place of more active defenses. The oyster, for example, lays
approximately 16,000,000 eggs each year, and many fishes are said
to lay millions. In other animals this enormous rate of repro-
duction is supplanted by the production of less young, with paren-

Ir-e

mx

Fig. 141. — The head and mouthparts of a mosquito, Anopheles sp. a, antennae;
Ir-e, labrum-epipharynx; h, hypopharynx ; m, mandibles; mx, maxillae; I,
labium; mp, maxillary palpi. (After Nuttall and Shipley, from Comstock's
Introduction to Entomology, with the permission of the Comstock Publish-
ing Company.)

tal care to aid them in reaching maturity. Such adaptations
often involve structural modifications of the organism. They may
result in viviparity, i.e., the production of living young, so that
the inert egg and tender embryo are not subjected to the vicissi-
tudes of independent existence. In extreme degrees viviparous
species bear young when sufficiently developed to care for them-
selves to a marked degree, while in others Httle more than the
embryonic period is eliminated from the independent existence of
the individual. Mammals are the best known viviparous ani-
mals, and in this group parental care is also highly developed.
Normally oviparous groups, such as the fishes and insects, may
also include viviparous species. Viviparity is not essential to

ADAPTATION

235

parental care, however; the birds are oviparous, but their care of
their young is a matter of common knowledge. In all cases, of
course, the rate of reproduction must be great enough to offset
destruction or the species must ultimately become extinct.

Fig. 142. — Beaks and feet of birds. A, foot of bald eagle; B, head of turkey
vulture, a carrion eater; C, beaks of seed-eating birds; D, beak and foot of
swallow, an insect eater. (From Chapman's Handbook of Birds of Eastern
North America, with the permission of D. Appleton and Company.)

During the independent life of the individual it escapes various
dangers by running away, by resisting them through combat, or by
various conceahng or repellent means. Some animals are ar-
mored, and are therefore almost immune from attack by others,
and still others are protected by such peculiar powers as autot-
omy.

Armor. Armor was highly developed in some of the extinct
reptiles, such as Stegosaurus and Triceratops (Fig. 143). These

236

EVOLUTION AND GENETICS

animals were sluggish creatures, and were well protected by their
bony plates in such encounters as they must have had. Such

Fig. 143. — Restoration of the dinosaur, Triceratops. Length 20-25 feet.
Upper Cretaceous of western North America. (From Lull.)

modern species as the alligators are no mean illustrations of pro-
tective armor, but the armadillo is among the best (Fig. 144).

Fig. 144. — Nine-banded armadillo, Tatii novcmcinctus. (Through the courtesy
of the New York Zoological Society.)

These little mammals are so completely covered with bony dermal
plates that when they roll into a ball little else is exposed.

Speed. This quality as an adaptation is too well known to need
discussion. It is conspicuous in such animals as the horse and
deer as a primary protective adaptation.

ADAPTATION 237

Defensive Weapons, These may be the same as aggressive in
vertebrates. The teeth and claws of many herbivorous animals
are excellent weapons. They also include structures in no way
correlated with securing food. The tail of Stcgosanriis, for ex-
ample, with its enormous bony spines, must have been a terrible
weapon, and the tails of modern Oocodilia are effective clubs.
Horns of ungulates should be mentioned here, although they are
also organs of aggression in individual combat within the species.
The stings of insects an^ among the most highly developed de-
fensive weapons (Fig. 145).

Glandular Secretions. Some animals are provided with glands
which secrete poisonous substances or repulsive scents. Many
insects have such glands. The stink-luigs derive their popular
name from their unpleasant secretions, and many other true bugs
secrete more or less unpleasant substances. The larvae of the
common swallow-tail butterflies have an eversible scent organ
just behind the head which has been shown to be repulsive to
birds. The scent produced by the skunks requires no detailed
discussion, nor does the venom of poisonous snakes.

Electrical Organs. A few animals are able to defend them-
selves by discharging electricity from special organs. The electric
rays and eels are capable of giving severe shocks.

Concealing Discharges. Such discharges have already been
mentioned under adaptations to deep-sea life. The discharges of
Cephalopoda and of the deep-sea prawn are the only familiar ex-
amples of this adaptation.

Autotomy. One of the most striking of all defensive adaptations
is autotomy. In some lizards the tail is brightly colored and so
is most likely to be the part seized by a carnivorous species, but
if grasped it breaks away from the body and the lizard is able to
scamper away to raise a new appendage. Crabs are said to drop
off claws if seized by them. Such powers are usually correlated
with ability to regenerate the lost appendages. The term autot-
omy is sometimes a misnomer in this connection since the loss of
parts may apparently be due more to fragility of structure than to
the animal's power to drop appendages at will, although the latter
is sometimes present. The term is also, and more accurately,
applied to the process of fission.

Animal Associations. Gregariousness. Many animals obtain
by association with others of the same species or with different

238

EVOLUTION AND GENETICS

Fig. 145. — Sting of honey-bee, x30. A, sting separated from its muscles;
ps, poison sac; pg, poison gland; 5lh g, fifth abdominal ganglion; n, n,
nerves; e, external thin membrane joining sting to last abdominal segment;
i, k, I and i , A; , Z , levers to move the darts; sh, sheath; v, vulva; p, sting
palpus or feeler, with tactile hairs and nerves. B and C, sections through
the darts and sheath, x300; sh, sheath; d, darts; b, barbs; /;, poison chan-
nel. D, end of a dart, x200; o, o, openings for escape of poison. (From
Packard, after Cheshire.)

ADAPTATION 239

species the benefits which free-living individuals must secure for
themselves. Association within the species may be limited to the
mere banding together of individuals as in herds of grazing animals;
such association does not reduce the potential indc^pendcnce of
the individual but gives him the protection of numbers. Carnivo-
rous species may run in packs, like wolves. Lull characterizes
this arrangement as "a sort of armed truce, the idea of mutual aid
for defense evidently being foreign to their code of ethics, for they
will at once turn upon, destro\^, and devour one of their own band
who happens to be wounded, even though it delay the chase."
Crows flock together apparently for no other end than companion-
ship, although they probably derive some measure of benefit in
the possibility of more certain warning of danger.

Communal Associations. Such associations are accompanied
by a division of labor and in most animals by some structural
specialization of the different castes. Among the insects the ter-
mites (Order Isoptera) and ants, bees and wasps (Order Hymen-
optera) include many highly developed communal forms. The
condition found in the honey-bee colony is a relatively simple
illustration of communal association, since the number of forms
involved is only three (Fig. 5), many less than in the ants and
termites, although the division of labor is intricate and no indi-
vidual can long exist independently.

The honey-bee colony is made up of a single queen, many
workers, and during the breeding season some drones. The queen
is a perfect female. She alone mates and lays eggs under normal
conditions, and she must even be fed by the workers. The worker
caste in a strong colony of Italian bees may include 100,000 or
more individuals, although a wild colony is likely to be much
smaller. During the winter under natural conditions there are
less workers present. These insects are females, but they are im-
perfectly developed and never mate. They are capable of laying
eggs under abnormal conditions, but these eggs can develop only
into drones, which always come from unfertilized eggs. The
drones are males, and are tolerated only during the breeding
season. In the fall they are killed by the workers or thrust out of
the colony to die. There is definite structural difference between
each of the three forms and in addition a division of labor not
wholly correlated with structural differences. Workers, when
newly emerged from the pupa, are idle for a few days. They then

240 EVOLUTION AND GENETICS

care for the young brood and build comb for a longer period, and
finally fly out to gather nectar and pollen for food, and propolis
to stop up crevices in the hive. When there are no young workers
available, as in the spring, the older ones take up the work of the
hive in addition to their own duties afield.

Communal life is nowhere more complex than in the human
species, but here it depends on differences of training, and not on
structural adaptation. This is made possible by the high develop-
ment of intelligence.

Commensalism and Symbiosis. Different species are sometimes
associated for mutual benefit. If the association is not indis-
pensable, it is known as commensalism. A classic example is that
of the hermit crab and the hydroid which grows on its shell. The
crab is more or less protected by the disguise afforded by the
hydroid, and pro])ably by the latter's stinging cells, while the hy-
droid is carried from place to place and is able to benefit by the
scraps of food discarded by the crab (Fig. 146). If the association
is indispensable, it is known as symbiosis. A familiar example is
the association of fungi and algae to form lichens. Among animals
the white ants and their intestinal fauna of protozoa have recently
been proved equally necessary to each other. The termites eat
wood, but cannot digest the cellulose of which it is composed;
the protozoa digest the wood for the termites, and in turn receive
a favorable environment and plenty of food.

Parasitism. Not all associations are mutually beneficial, how-
ever. Some organisms live upon or within others, and entirely
at their expense. Such organisms are called parasites, and those
upon which they live are known as hosts. Parasitism is extremely
varied and complex. It cannot be treated adequately in such an
account as this, but in all cases certain tendencies are to be noted.
In proportion to the dependence of the parasite, it degenerates
structurally and becomes incapable of independent life. Obligate
parasites are thus our most degenerate organisms. In proportion
to the risk involved in changing from host to host during repro-
duction or during the life of the individual, compensating adapta-
tions must be acquired. These are usually reproductive and are
either in the form of a very high rate of increase to meet enormous
destruction, or the elimination of the more vulnerable stages.
In proportion to the constancy of the association, the powers
of locomotion may be reduced.

ADAPTATION

241

Internal parasites include many protozoa, flatworras and round-
worms, Crustacea and insecta. Such forms are, of course, the
most extremely modified. External parasites are chiefly arachnida
(mites and ticks), and insects. The latter are often adapted in
texture, vestiture and structure to aid locomotion on the body

:i^S^.'

rz^,

Fig. 146. — A hermit crab occupying the shell of a Gasteropod mollusc which
is almost concealed by four sea anemones, ac, ac' are acontia of the anem-
ones; sh, shell of Gasteropod. (From Parker and Haswell, after Andres.)

of the host and to resist capture, but they retain all of the struc-
tures of independent organisms and must be looked upon as
highly specialized rather than as degenerate creatures.

Coloration and Mimicry. One of the most remarkable phases of
adaptation to the organic environment is coloration and mimicry.
The complexity of such adaptations is well indicated by the fact
that they can be classified under the following diverse heads:

242 EVOLUTION AND GENETICS

1. Resemblance to surroundings, involving both color and

form, for the concealment of both hunter and hunted.
This has been called protective and aggressive colora-
tion.

2. Alluring colors to attract prey.

3. Warning colors indicating noxious qualities.

4. Signal and recognition marks.

5. Confusing colors.

6. Sexual colors.

7. Mimicry of other organisms.

Protective and Aggressive Colors. The efficacy of these adapta-
tions has been questioned, and we may well imagine that ani-
mals often perish in spite of them. However we cannot doubt
that the white winter coat of a prairie hare is more of a protection
when the ground is covered with snow than its brown summer
coat would be. Nor can we doubt that the similar resemblance
of the arctic fox to its environment enables it to approach its
prey with greater certainty of success than if it were red. The
writer and a friend once took a picture of a whip-poor-will resting
on dead leaves, and could never decide which object in the picture
represented the bird! As in other cases, the effectiveness of these
adaptations, even if limited, is sufficient to account for their
continuation, although not necessarily responsible for their
origin.

Alluring Colors. Such colors involve a degree of mimicrj^, in
that the animal must resemble something attractive to its prey.
In approaching the supposedly dcsiral^le object, the prey comes
within the reach of the aggressor.

Warning Colors. Many poisonous or unpalatable animals are
brightly colored, and are usually avoided by other animals.
Among these are the Gila Monster of southwestern deserts, the
red-banded coral snake, and the brightly colored or conspicuously
marked insects such as the wasps and bees and many unpalatable
species. Marshall records definite evidence that a few trials of
such insects result in their refusal by birds and monkeys. There is,
of course, nothing intrinsically repellent about their colors, but
they serve to impress upon the memory of a bird or animal, that
the insect which wears them is not good food, and to bring about
ready recognition upon later encounters.

ADAPTATION

243

Signal Marks. Signal marks include the white tails of deer
and rabbits, which are shown only when the animal runs or when
the tail is raised.

Recognition Marks. The red lateral spots of trout have been
interpreted as recognition marks, since they are visible to fishes

^

B

Fig. 147. — Catocala ilia Cramer. A, on the bark of a tree with wings folded;
B, the same moth with wings spread. The hind wings are orange with
black bands.

at their own level, while above and below trout have the char-
acteristic dark and light protective colors so common in fishes.
The colors of moths and Ijutterflies have been added to this
category, but there is abundant evidence pointing toward the
sense of smell as the chief means of recognition among such insects.
Confusing Colors. Colors of this nature are undoubtedly
effective. The Catocala moths and many grasshoppers have

244 EVOLUTION AND GENETICS

brightly colored underwings which are conspicuous during flight
(Fig. 147). Their other colors are dull. When the insect flies, its
bright colors attract attention, but when it settles again thej'- are
so suddenly concealed that it seems to disappear, and it is very
difficult to determine its exact location. Another effect of these

bright parts is that they are
hkely to be seized by a bird
if it overtakes the insect in
flight, so that the insect
escapes with no more injury
than a torn wing.

Sexual Colors. Sexual
colors are again complex.
Many birds have brilliant
colors in the male sex, and
dull in the female. To what
extent the colors are attrac-
tive to the female, we cannot
say. It is almost certain
that they afford some degree
of protection to the nesting
birds, since marauders would
be more easily attracted to
the conspicuous male than to
the brooding female. We can
judge no better the value of
sexual coloration than by
Beebe's story of the tinamou
(Crypturus variegatus), in
which sexual behaviour is
completely reversed. In this
species the male raises the
young, and the female does
Fig. 148. — Larva of a geometric! moth, the courting. Through song
resting extended from a twig (From ^^^ ^^ ^^^ ^^^ display

Woodruff, after Jordan and Kellogg.) , , , i i

of such color as she possessed,

he observed one of these birds carrying on her courtship in the
jungle of British Guiana. The bright colors are usually con-
cealed beneath the rudimentary tail, and then the bird blends
wonderfully into its jungle background.

ADAPTATION 245

Mimicry. This characteristic is common in insects. Some cater-
pillars can scarcely be distinguished from a dead twig; in form,
color, and behaviour when they are disturbed the imitation is
very nearly perfect (Fig. 148). Other insects resemble leaves,
still others the bark of trees, and some, flowers or other insects.
The effect is either concealment, for protection or aggression, or
safety through the mimic's power to profit by some repulsive
quality of the imitated.

Concealment through mimicry is often imperfect. The bilateral
symmetry of many moths resting on tree trunks discloses their
presence rather easily to the practiced eye, though it is impossible
to know how many are passed by unnoticed. One species, Stenoma
schlaegeri and probably other related species of the same genus,
conceals its symmetry by lapping one front wing over the other;
this moth resembles a small l)ird-dropping so closely that either
maj' easily be mistaken for the other.

The explanations of adaptation are many. Whether we shall
ever know how all of them have come about is very doubtful, for
the details of the problem are infinite. They are universal, how-
ever, and afford us unlimited material for the study of evolution-
ary processes. From such studies theories have been formulated
to explain the modification of organisms and in these theories we
find the probable foundations of this varied and intricate associa-
tion of organisms and environments. It is certain that some
process or processes of change have given rise to the modern
condition since evolution points to a common origin for all living
things.

Summary. Adaptations are the characters of organisms which
fit them for hfe in a given environment. In the individual we
see both the process of adaptation and the adaptations resulting
from it, while in species we see only the latter. It is the function
of evolutionary theory to account for the wonderful adaptations
which are characteristic of species. When we consider the environ-
ment with which they are associated we find that it involves
external factors, both physical and organic, and in addition the
internal environment which the individual body furnishes for any
of its parts. Adaptations which are due to response to conditions
of the internal environment have been called non-adaptive char-
acters but they must be due to cause and effect no less than
characters directly related to the external environment. The

246 EVOLUTION AND GENETICS

more evidently adaptive characters in the latter category have
been classified in detail. They include adaptations for aquatic
and terrestrial life and for flight, adaptations to varying degrees
of moisture and light, and adaptations for association with other
organisms. Organic relations necessitate special structures for
securing food and for protection. Protective adaptations are
exceedingly varied, including structural adaptations for speed and
many other characters. Both ends are gained by more or less
permanent associations of animals and by coloration and mimicry.
The idea of the web of life expresses the complexity of organic
relationships from which is derived much of the subject-matter
of evolution.

REFERENCES

Evans, A. H., Cambridge Natural History, Vol. IX, Birds, 1900.

Beddard, F. E., Cambridge Natural History, Vol. X, Mammals, 1902.

Weismann, a., The Evolution Theory, 1904.

Banta, a. M., "The Fauna of Mayfield's Cave," Carnegie Inst, of Washing-
ton, Publication No. 67, 1907.

Gadow, H., Cambridge Natural History, Vol. VIII, Amphibia and Reptiles,
1909.

Thayer, G. H. and A. H., Concealing Coloration in the Animal Kingdom, 1909.

Roosevelt, T., African Game Trails, Appendix E, 1910.

Wheeler, W. M., Ants, 1910.

Mast, S. O., Light and the Behaviour of Organisms, 1911.

Henderson, L. J., The Fit?iess of the Environment, 1913.

Lull, R. S., Organic Evolution, 1917.

Newman, H. H., Vertebrate Zoology, 1920.

Chandler, A. C, Animal Parasites and Human Disease, 3rd edition, 1926.

CoMSTOCK, J. H., An Introduction to Entomology, 1924.

Beebe, Wm., The Arctnrus Adventure, 1926.

CHAPTER XIII
THE BASIS OF ADAPTATION

Variation. We have already noted that organisms vary and
that variation itself is the most invariable thing in nature. The
possibility of change in organisms is directly associated with this
phenomenon.

If an individual were incapable of varied responses change of
environmental conditions beyond its power to respond could only
result in death. In the latitude of response within the individual
we find the foundation of individual success. Individuals differ
in their power of response, however, even within the same species
because of their variation, so that conditions which make life
impossible for one may be met easily bj^ another.

Since species are merely aggregations of individuals these things
are also the basis for success of species in meeting varied conditions
and in becoming fitted for life in a limited environment. Extinc-
tion may occur and has occurred in many known cases, but it
can result only from a change so extreme that no individuals can
meet it. Any change less extreme could be met by some; these
survivors would continue to perpetuate the species which would
differ from its former state through the loss of the unsuccessful
individuals. The responses of the species as a whole would then
be those of the surviving individuals; it would be changed to
meet the changed environment.

Environment. Although change of external environmental
relationships must always accompany the evolution of organisms,
change of the external environment need not always be the
inciting cause. Non-adaptive changes in organisms produce struc-
tures correlated wholly with the internal environment, although
their establishment may later exact adjustments to the external
environment as the price of existence. It is probable, at least,
that any change in an organism will modify its ability to avail
itself of surrounding conditions in some degree.

Whatever may be the source of change in organisms, it involves
changed response to conditions in the complex environment.

247

248 EVOLUTION AND GENETICS

Ultimately, as species give rise to other species, this change of
response must find expression in change of structure and function
as heritable properties.

Many recognizable forces may contribute to the initial shift
of an organism from one environment to another. Change of
climate due to geological factors might readily cause readjustment.
Change of food supply, involving either climatic factors and soil
conditions, or interspecific relationships probably occurs even
more often than marked geological changes. Overpopulation is
commonly regarded as a potent factor in evolution. In connection
with all three forces, an animal's power of migration may deter-
mine the nature of its response, and in some cases it is probably a
primary factor in itself.

Change of Climate. Geology and paleontology together show
that from tiine to time in the remote past climatic conditions
have varied greatly. Our own continent has passed through
tropical and arctic conditions, as well as temperate periods with
climates like that of the present. "Geologists now know of seven
periods of decided temperature changes (earliest and latest Late
Proterozoic, Silurian, Permian, Triassic, Cretaceous-Eocene, and
Pleistocene) and of these at least four were glacial climates (both
Proterozoic times, Permian and Pleistocene) . The greatest inten-
sity of these reduced temperatures varied between the hemispheres,
for in the earliest Late Proterozoic and Pleistocene it lay in the
northern, while in latest Proterozoic and Permian time it was
more equatorial than boreal. Cooled climates occur when the
lands are largest and most emergent, during the closing stages
of periods and eras, and cold climates nearly always exist during
or immediately following revolutions, when the earth is undergoing
marked m^ountain making " (Schuchert) (Fig. 149).

Factors Which Influence Climate. The things which modify
temperature may also be instrumental in determining the amount
of precipitation, the relation of seasons, and minor fluctuations.
A high range of mountains, for example, may cause abundant
rainfall on one side and desert conditions on the other by cooUng
moisture laden currents of air as they blow toward it from the
nearest ocean. This is true of mountains of the western United
States. The same effect may be responsible for localization of
rainfall. High altitudes are also subject to fluctuations of many
degrees in temperature between day and night, so that organisms

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249

250 EVOLUTION AND GENETICS

must be adapted to both extremes. The same quahty is expressed
in short summers and long winters at high altitudes even in lower
latitudes. Snow may be expected throughout the year at 15,000
feet in latitudes where it never falls at sea level.

A geological change such as the elevation of continents would
change the climate of a region completely. If local, in the form
of elevation of a mountain range, it would have a similar effect
on the region involved, but would exert various influences on
other parts of the same land mass. Its modification of precipita-
tion might be extreme. Drainage might be affected in such a
way as to change the distribution of fresh waters and the char-
acter of the streams. These changes could not fail to modify the
environment of many organisms.

Climatic Factors and the Food Supply. As a result of geo-
logical changes and their indirect influences, the flora of a continent
is likely to be modified. One of the most conspicuous indications
of the climate of past geological periods is, in fact, the nature
of the plant remains. This is a natural outcome of the intimate
association of plants with the physical environment. Change of
climatic factors to them means direct modification of the food
supply, and their response, in turn, means modification of the
food supply of all other organisms, directly or indirectly. Through
the web of life a change may be transmitted to unsuspected lengths.

Food supply is one of the most important contributions of
the environment to individual life. Its modification is therefore
a matter of deep concern, perhaps more vital than the modification
of other factors, but it is easily seen to be intimately linked with
climatic factors on one hand, as a causative factor in evolution.

Rate of Reproduction. No less intimately associated with
modification of the food supply is the factor of overproduction,
although in this case it is not the actual supply that is modified,
but its availability to individuals. The principle of overproduction
was emphasized by Malthus in his work on population, which
furnished the inspiration for Darwin's and Wallace's theories
of natural selection. In man it seems less conspicuous than in
animals, although man was Malthus' subject and the same process
was later recognized in animals. It is obvious, however, that all
organisms produce more individuals than can survive, and that
only constant destruction prevents overpopulation. Interruption
of the natural balance by transportation of animals to foreign

THE BASIS OF ADAPTATION 251

countries has given us illustrations in numerous cases. Rabbits
introduced into Australia years ago have become a nuisance.
The English sparrow has been equally successful in North America.
Such insects as the San Jose scale, gipsy moth and Japanese
beetle have been even more serious. The first named threatened
to wipe out the citrus fruit industry of the west until it was
traced to its oriental home, whence natural enemies were secured
to hold it in check. The others have cost millions in the eastern
United States, and are still a problem.

The production of an excessive number of offspring is essential
to the maintenance of the food supply of all organisms, since
those which are destroyed are either killed for food or become
food incidentally. It is also closely linked with the food supply
of the species concerned, for the persistence of an abnormal number
of individuals might well carry the needs of the species beyond
the limits of the available food supply and necessitate adjustments
of some kind.

Response of Organisms. The response of organisms to changes
in any of the above factors might be accomplished by change of
location. The power of locomotion is developed in some animals
to such a high degree, however, that it may well be a primary
factor in evolution itself, since a species may gradually extend
its distribution from an original center into regions which make
different demands upon it.

The Interaction of Factors. The entire series of factors interact
in a complex way, for the operation of one may well bring another
into action. Geological changes alone are beyond the influence
of the others.

Migration. Whenever environmental conditions change, two
courses are open to the organism. It may either migrate from the
region in which it lives in search of more favorable surroundings,
or it may remain and carry on its existence by taking advantage
of conditions which were formerly of no importance to it. Either
of these courses depends to some extent upon its latitude of
response, but the latter especially. To migrate may carry the
animal into an environment but little different from that to
which it is accustomed. To remain in a changing environment
demands change inevitably. The two responses are in some
degree associated, for the former is certain to contribute to the
adaptive modification of some organisms, and the latter is equally

252 EVOLUTION AND GENETICS

certain to result in interchange between divisions of the region
concerned which amount to a hmited migration.

The opportunities available to a species within a limited region
depend partly upon its established habitat. A terrestrial form
is intennediate between fossorial and arboreal habitats, and
between aerial and aquatic. Any terrestrial form may avail itself
of the advantages of partial occupation of one or more of these
neighboring habitats without relinquishing its primary terrestrial
associations. A species living in the shallow sea is in a position
to enter the abysses or fresh water streams, or to move into the
intertidal zone of the littoral fauna. Such species are also subject
to transition between the three stages, benthos, nekton and plank-
ton. The more specialized forms, such as fossorial, abyssal and
benthonic animals, have more limited contacts and less ability
to respond to varied conditions. Specialization is fundamentally
a reduction of possibilities, since it demands particular fitness
of the organisms' equipment for some limited mode of life. In
spite of this limitation, however, even specialized forms retain
some latitude of response and diversity of contacts. It is only
in the most extreme forms, such as the endoparasites, that com-
plete inability to modify the mode of life in some slight degree is
likely to be found, and even some of these develop immunity to
drugs used for their destruction.

Adaptive Radiation. The results of animal response to the
various possibilities of a limited environment have been expressed
in Osborn's law of adaptive radiation (Fig. 150). This law was
formulated in connection with studies of mammalian evolution,
but is generally applicable with minor modifications. As Lull
notes, "adaptive branching" is a more satisfactory term, since
the process is not always one of radiation from a common central
point. The principle involved is, in any case, much as it was
formulated by Osborn in his original law. It may be stated in
terminology as near as possible to the original as follows: Each
isolated region, if large and sufficiently varied in its topography,
soil, climate, and vegetation, will give rise to a diversified fauna
in any group of animals. The larger the region and the more
diverse the conditions, the greater the resulting variety of forms.
From a primitive stem form, new lines of adaptation will go out
into associated habitats. One result is divergence of form in
related animals, which has already been mentioned.

THE BASIS OF ADAPTATION

253

Although it is generally applicable, there is probably no better
illustration of the process than the mammals. Originally terres-
trial, they have radiated into four habitats. On the basis of
hmb modifications, from the original ambulatory stem fossorial,
volant, cursorial and aquatic species have arisen. Such animals
as the shrews represent the primitive type. Scansorial animals
like the squirrels may well be transitional to volant typ(>s. The
bats are the most nearly perfect volant mammals (Fig. 134).
The mole is the most nearly perfect fossorial form (Fig, 130).

Cursorial-
Unguligrade

Limbs

Volant
(Aerial)

Cursorial-Digitigrade

Scansorial
(Arboreal)

( Short-limbed, plantigrade,
(TS^reitr?aB\ P^ntadactyl, unguiculate
' ^ [stem

Natatorial
(Amphibious)

Fossorial
(Subterranean)

(Aquatic)
Fig. 150. — Diagram of adaptive radiation. (After Lull.)

From ambulatory forms digitigrade runners like the carnivores
arose, and through similar limb posture came the highly specialized
unguligrade appendages of animals like the horse. Among aquatic
mammals the whale is extreme; many amphiljious forms suggest
the possibility of gradual change in this direction. The teeth also
show adaptive radiation from the primitive insectivorous stem
to herbivorous, carnivorous, omnivorous and ant-eating types, the
last with greatly reduced dentition.

Adaptive Branching. Among insects the bugs (Order Hemip-
tera) show many evidences of adaptive branching (Fig. 151).
They were originally, no doubt, terrestrial ambulatory species
with volant powers. We now have highly aquatic species, which

254 EVOLUTION AND GENETICS

still retain the power of flight {Belostomatidae, Corixidae, Notonec-
tidae, Nepidae). These insects have the legs highly modified as
swimming organs. Other species (Gelastocoridae) are found along
shores, while still others (Veliidae, Gerridae) skate about on the
surface film of fresh waters. One genus of the Gerridae is marine.
This genus, Halobates, may be found far from land, and Beebe
has recently confirmed the belief that its eggs are deposited on
floating feathers. Some are fossorial (Gelastocoridae, Saldidae,
Cydninae). In food habits they vary greatly. Since they have

Fossorial Aquatic-Volant

Fossorial-Volant

Water-striders
(Volarxt:

Ambulalory-Volant

' Stem Water-striders

(Non-volant^

Ambulatory
(Non-volant)

Fig. 151. — Diagram of adaptive branching in the Hemiptera.

suctorial mouths, they are limited to liquid foods, but they include
both phytophagous and carnivorous forms. Some are highly
predacious, some parasitic, and some probably scavengers.

Geographical Distribution. Animals that migrate, whether in
response to the pressure of necessity or to the inherent ability and
disposition to roam, have the world and its varying conditions
at their disposal. Within certain limits of time and space they
are restricted by the physical characteristics of the earth's surface,
but these features have changed repeatedly during the past, so
that what is now impossible may once have been easy. Migration
is likely to carry animals unchanged into favorable regions or to
carry them by gi-adual steps into regions which would demand
new adaptations. It has resulted in the population of the earth
with a unified fauna and flora. In similar habitats, however
remote, we may expect similar organisms, and only those land
masses whose isolation extends back beyond the origin of modern
species are peopled with organisms fundamentally different from
those of other similar regions.

THE BASIS OF ADAPTATION 255

Zoiigeographical Realms. The earth is now divided into several
zoogeographical regions, indicative of the surficial distribution
of animals (Fig. 152). Lydekker proposed three chief divisions,
each including territory which has been sufficiently isolated in
the past to produce a characteristic fauna. These arc the Arcto-
gaeic realm, including America north of the Mexican plateau,
Europe, Asia, Africa, and all adjacent islands; the Neogaeic realm,
including South America and adjacent islands, Central America
north to the Arctogaeic, and the West Indies; the Notogaeic
includes Australia and the islands of the Pacific. The Notogaeic
realm has been isolated since the beginning of the Tertiary, and
has consequently developed a peculiar fauna including a number
of primitive forms. The Neogaeic also enjoyed a long period of
isolation during the Tertiary, and has a characteristic fauna as a
result, but the greater area of the Arctogaeic has been open more
or less constantly to intermigrations, and is populated by a
remarkably homogeneous fauna.

Within these realms various subdivisions are recognized in
which the faunae are distinguished by minor characteristics. The
subdivisions commonly used are:

1. Palaearctic, including Europe, Asia north of Persia, the
Himalayan Mountains and the Nan-ling mountains of China, the
adjacent islands, and Africa north of the Sahara desert.

2. The Nearctic, including the American portion of the Arcto-
gaeic realm.

3. The Neotropical region is the same as the Neogaeic realm.

4. The Ethiopian region includes Africa, Arabia and adjacent
islands, south of the Palaearctic region.

5. The Oriental region includes India, the Malay peninsula and
other parts of Asia south of the Palaearctic boundary, and in
addition the Malayan archipelago.

6. The Australian region is the same as the Notogaeic realm.
There is some transition between it and the last in the Malayan
archipelago.

Few places on the earth are completely devoid of fife, although
such limited areas as the Great Salt Lake desert appear to be so.
In this region the earth is a glistening mass of salt for miles. Not
a plant is visible within the outer borders of the desert, where a
few stunted shrubs are able to live. Excepting such relatively
small areas, every region has its living forms. No matter how

Pm

256

THE BASIS OF ADAPTATION 257

cold, how hot, how wet or dry, or how rocky, some organisms can
be found cHnghig precariously to life, unless some deleterious
substance or influence prevents for a time.

Barriers. When any of the factors mentioned at the beginning
of the chapter make it necessary or desirable for animals to seek
new homes, migration is apt to be one of the chief means of adjust-
ment. It may be accomplished through the animal's own powers
of locomotion with considerable rapidity, but in some parts of the
world there are insuperable barriers. The oceans are impassable
to terrestrial species, high mountain ranges are often so, and
climatic conditions in some regions may hinder the passage of
animals not adapted to them. In addition new contacts with
other species may hinder migration.

Mountain ranges are effective barriers when parallel to the
equator. The effect of a few thousand feet of altitude on such
a range is equivalent to a change in latitude of many degrees.
"This is notably true," says Lull, "of the great Himalayan Range
in northern India, which rears its mighty summits far beyond the
limits of perpetual snow. On the south we have the hot, moist
plains of India, with a very distinct tropical fauna which in many
respects resembles that of Africa. North of the barrier, conditions
of climate, both in temperature and degree of moisture, are
entirely changed, and with them appear animals, with some
notable exceptions, of a totally different sort, more nearly com-
parable to those of Europe."

The difference between these ranges and mountains that run
north and south, like the Sierra Nevada and Rocky Mountains,
is easily understood. The temperature fluctuations, and resulting
climatic conditions, at an altitude of eight to ten thousand feet
in the latitude of central Colorado are much like those at normal
levels far north of the United States. Consequently there is a
zone of similar climatic conditions extending gradually upward
from north to south in these mountain ranges, and migration may
be accomplished along this zone without sudden change. The
northern hemisphere contains several species of insects which
illustrate the condition admirably. Two of these, related to the
butterflies, are found in northern Europe, Asia, and North Americ?^
In the central part of Canada they range southward nearly to th<»
international boundary, but in the mountains of the east and west
they extend far to the south. One has been taken at an altitude

258 EVOLUTION AND GENETICS

of 13,000 feet in southern Colorado and in the Appalachian range
as far south as North Carolina. The other is found at lower
altitudes in the western mountains and extends as far south as
Arizona. Such irregularity of distribution shows clearly how
transition from one side of a range to the other may be accom-
plished through the compensating influences of latitude and
altitude.

Oceans are effective barriers to the migration of terrestrial
animals, as also are the climatic conditions of extensive desert
areas and to some extent, perhaps, other climatic extremes. To
volant species, however, such areas are not obstacles. Butterflies
have been observed three hundred miles from land and even the
normal seasonal migration of birds may carry them a much longer
distance over unbroken ocean.

Aids to Dispersal. The passage of such areas is not impossible
to other organisms when aided by favorable circumstances. In
the geological past oceans have been bridged by narrow isthmuses
such as the Isthmus of Panama which now connects North and
South America. There was no such connection during most of the
Tertiary, consequently the relationships between the Nearctic and
Neotropical species are either older or more recent than that time.
On the other hand we know that Alaska was once joined to Asia
by an isthmus extending from the Alaskan peninsula to Kamchatka
which allowed free communication between the Nearctic and
Palearctic regions (Fig. 153). All that now remains above the
ocean is the chain of the Aleutian Islands, but the water between
them is nowhere very deep. The faunae are very similar on the
two continents because of the early association. Apparently the
remarkable Galapagos Islands were also once connected to the
Americas, but their isolation must have been remote, and even
the early occurrence of a connection is disputed.

The land bridge between North America and Asia furnished a
convenient passage for many mammals during the Tertiary.
Extinct proboscideans and horses, as well as more obscure forms
which we have not considered, passed from continent to continent
over this route, and it is very probable that the ancestors of the
American Indian entered the continent from Asia in the same
way.

Drifting debris and ice floes may also carry terrestrial animals
many miles over the ocean. Floating logs and masses of tropical

259

260 EVOLUTION AND GENETICS

vegetation have been observed far from shore. Beebe records in
The Arcturus Adventure a population of fifty-four species on a
single log, and in his interesting chapter An Island of Water, in
the same work, lists four species of plants, five of birds, eight of
shore fishes, four of shore crabs, and eight of insects observed
within a space of ten days which might have become established
had his "island" been a new-born bit of land. If this could
occur in ten days, what might not pass by in a year? With such
evidence before us we can hardly imagine land remaining unin-
habited.

Elsewhere the transportation of polar bears on ice floes has
been recorded, and the observation of animals of considerable
size on drifting masses of vegetation, which are apparently of
common occurrence in tropical waters.

Wind is an important factor in the dispersal of animals of
small size, and an aid to the dispersal of volant forms in particular,
whatever their size. Insects are especially likely to be transported
in this way, for their bodies are so small and light that they are
unlikely to fly strongly enough to resist high winds. It is not
uncommon to find battered specimens of butterflies a hundred or
more miles to the north of their normal range, often after a period
of southerly winds. These individuals, because of highly special-
ized food habits, do not usually establish the species in the region
to which they are carried. In some cases, however, a species
appears occasionally on cultivated plants which is not normally
seen. Its intermittent occurrence may well be due to gravid
females carried by flight and wind. Such forces have no doubt
been responsible for the peopling of oceanic islands with organisms
similar to those of the nearest mainland.

Plants are in many cases specially adapted for wind dispersal
through the production of buoyant seeds. Wing-like expansions
and tufted appendages, such as those of maple and milkweed seeds,
are familiar examples.

Flowing waters, either ocean currents or streams, act in a similar
way, but they can act only on aquatic organisms or in connection
with some buoyant object. The seeds of plants may be trans-
mitted in this way usually without special adaptation, since many
of them are incidentally light enough to float. Some are aided,
however, by buoyant accessory structures, such as the fibrous
husk of the cocoanut and the receptacle of water liUes.

THE BASIS OF ADAPTATION 261

The agency of other organisms is probably of greater importance
than we reahze in dispersal. The transportation of plant seeds
by birds and other animals is familiar. It may come about through
use of fruits as food and failure to digest the seeds, or through
accidental means. The transportation of such specialized fruits
as the cockleburs, beggar's-ticks and Spanish needles by animals
is furthered by their special adaptations for adhesion to fur or
skin. Small animals may also be transported to some degree in
this way. Parasitic, commensal and symbiotic species are so
distributed, beyond doubt, but the activities of men are probably
of greater importance in the dispersal of free living animals than
the incidental associations of lower forms. The railroads and
other vehicular traffic are known to have carried organisms
accidentally for long distances, and some of our most troublesome
pests, the San Jose scale, gypsy moth, Japanese beetle, European
corn borer, English sparrow and starling have been imported,
intentionally or accidentally, from other continents.

Agencies of this kind may well transport animals from one land
mass to another accidentally, but the results would be similar to
those arising from forced migration. Accidental transportation,
however, would be much less likely to carry organisms into regions
favorable for their development than intentional migration.

For these reasons no part of the world is able to remain long
without a population of living organisms if it provides the bare
necessities of life. Sooner or later, through accident or the pressure
of competition in other regions it receives pioneers which may or
may not be able to maintain themselves. The repetition of such
occurrences is bound in time to result in the establishment of
some species and with every addition the possibility of adaptive
branching increases. Ultimately the region is itself a source of
migrants to other places.

The Constancy of Change. It must not be supposed that
these are abrupt transitions. Only such limited land masses as coral
and volcanic islands, lava flows, and inundated and glaciated re-
gions can have been utterly devoid of life at any time since life be-
gan. The transitions of most of these have been gradual, and with
their gradual changers must have come gradual development of
flora and fauna as opportunity arose.

In most regions life has been continuous, as it is now, and as
now, fluctuating in details. Emigration and immigration go on

262 EVOLUTION AND GENETICS

constantly in these regions, and minor adjustments are as con-
stantly taking place. In an area of less than a square mile the
writer once watched within a decade a transition from a beauti-
fully wooded valley to a weedy patch of dead and dying trees,
of prairie land to a field of sweet clover, and of a grassy meadow
to a slough, overgrown with coarse sedges and with water standing
in its lower spots, all without the interference of man. In the
same area it was often possible to see walnut trees defoliated by
larvae of a moth. The overpopulation of a tree resulted in the
destruction of the food supply, and the caterpillars were forced
to leave the tree in search for another, some perhaps successfully,
some perhaps not. Such cases are examples on a small scale of
the factors which bring about adaptation.

Adaptations are the result of change, change in the organism
through influence of its internal environment, change brought
about by factors in the external inorganic environment, or change
due to competition within the species or with other organisms,
but always change. Static conditions do not demand fluctuating
response, and static organisms, if such could exist, would be
incapable of responding in more than one way. The organism
varies and everything about it varies. In varied responses, whether
migrations into new regions or reassociations within the same
limited region, lies the beginning of the many adaptations which
characterize the various creatures of the earth. Adaptation is a
process and that process is evolution.

This much it is easy to say, but the fact remains that adaptation
as a process is visible to us only in the individual. In order to
accomplish the evolution of species it must be a process in the
entire aggregates of individuals that constitute species, and here
we face the complex problem of chronological as well as immediate
association of individuals. The unity of successive generations is
the field of heredity. It became evident many years ago that
the linldng of generation with generation was an important point
of attack for the solution of the problems of evolution and the
attention of many biologists has been turned upon it. Out of
their efforts has grown the science of genetics, whose importance,
both practical and purely scientific, is inestimable.

Summary. The components of individual existence, viz.,
heredity, environment and response, are variable. Change of
environment in the broad sense necessitates change of response

THE BASIS OF ADAPTATION 263

which is possible within certain hereditary limits of variation,
and so furnishes a basis for the modification of the organism.
Differences of heredity make it necessary for organisms to seek
different conditions of environment. Since the individuals making
up a species are not all the same, changing environment may
favor some and destroy others, so that the limits of variation
of the species may be narrowed. Response of organisms to
changing conditions may be accomplished in various waj^s, but
one important result is interchange between various hal)itats and
regions. The result is a constant dispersal of organisms from
their centers of development. This dispersal is hindered and
aided by various factors, and in turn causes changes which may
further influence the behaviour of other living things. Dispersal
has brought about a definite geographic distribution of organisms
and has been accompanied by adaptive radiation or branching
within limited groups. The complexity of these interactions is
the immediate explanation of adaptation. The transfer of changes
from the individual to the species as a permanent component of
the heritage is yet to be explained.

REFERENCES

Lydekker, R., a Geographical History of Mammals, 1896.

Pagenstecher, a., Die Geographische Verbrcitung der Schmetterlinge, 1909.

Clarke, W. E., Studies in Bird Migration, 1912.

Meek, A., The Migrations of Fish, 1916.

Lull, R. S., Organic Evolution, 1917.

Dahl, Fr., Okologische Tier geographic, 1921.

CoMSTOCK, J. H., Introduction to Entomology, 1924.

Holdhaus, K., Schroder's Handbuch der Enlomologie, II (7), 1927.

CHAPTER XIV
THE FOUNDATIONS OF GENETICS

The transmission of definite characters from parents to offspring
is an obvious phenomenon, but it was not until early in the
twentieth century that scientific knowledge of the subject was
sufficient to be regarded as a division of science. Under the name
genetics, coined by the English scientist, Bateson, it has since
taken its place among the important biological sciences.

Genetics deals with the origin of individuals, and in this is
closely connected with embryology. They differ in that the latter
science is concerned solely with the gradual differentiation of a
complete individual from the germ cells, while genetics attempts
to explain the appearance in every individual of the characters
previously found in its progenitors, and to account for the differ-
ences which occur. The correlation of these two fields of study is
a problem whose solution will be very valuable to biology.

Heredity and Adaptation. The facts of adaptation considered
in the last chapter are too orderly for explanation on any other
basis than that of correlation of the organism and its environment.
Correlation involving change in any factor cannot fail to bring
about different results, which in the organism would necessitate
modification of structure and function. In order to affect a species,
these modifications must not only affect individuals belonging to
it, but must reappear generation after generation. The constancy
of the species, such as it is, must be preserved, and yet somewhere
in the succession of generations change must be introduced if
evolution is a reality.

It is the difficult task of genetics to unravel this paradoxical
situation and to determine the limits of permanence of hereditary
characters. Fortunately there has been no lack of available
material. Although working hypotheses still constitute a large
part of the fabric of evolution and genetics, they are supported
by an ever increasing mass of facts. Evolutionary change has
not yet been ])rought under control so that it can be produced in
the laboratory, but more and more evidences of its existence

264

THE FOUNDATIONS OF GENETICS 265

become known as time advances and genetics in the meanwhile
has given us a convincing account of the undc^rlying mechanism
of the entire process.

Variation, one of the fundamental factors in adaptation, is
no less important in the science of genetics. Were it not for
variation we could have no knowk^dge of the methods of inherit-
ance. The fact of heredity would be no less evident; indeed,
absolute likeness of all individuals of a species through successive
generations would be even more definite evidence of its occurrence
than the partial resemblance which is known to occur. It is in
the variability of individuals, however, that the behaviour of
individual characters can be traced. When differences are mixed
in one generation and reassorted in the next they furnish a con-
trast in which the course of any one character can be seen and
traced.

Kinds of Variations: Nature. In variation lies the range of
possibility of change within the organism. Extensive studies have
shown how universal variation is, and have given rise to a classi-
fication based on the nature, degree, heritability and evolutionary
tendency of variations.

According to their nature variations are of three kinds:

Morphological. Variations in structure may involve differences
in either the form or the size of parts, or in the case of duplicated
organs they may involve differences in the extent of duplication.
Differences in number are very common in plants. The petals
of flowers, lobes of leaves, leaflets, and other structures which
are usually duplicated may vary widely in number. In animals
such variations are less commonly available, but the radially
s.ymmetrical forms often have more or less than the normal
number of parts. Hydra with six or even seven tentacles in place
of the usual five and Asterias forbesii with four or six rays instead
of five are frequently encountered in the laboratory. In man
extra fingers or toes sometimes occur, a condition known as
Polydactyly (Fig. 154). Differences in form and size of parts
are evident to all of us in everyday contacts. It is more difficult
to find human beings with approximately the same appearance
than with very different appearances.

Physiological. Variations in function are a necessary corollary
of morphological variations, since functions are merely the activi-
ties of structures. In the varied capacity of human beings to

266

EVOLUTION AND GENETICS

resist the same disease, to digest the same food, or to respond to
the same stimulant we have common examples of physiological

-^i^si?*^

fc>^

Fig. 154. — Radiograph showing Polydactyly in a child's hand. (From
Guyer's Being Well Born, copyright 1916-1927, after Dr. W. B. Helm;
used by special permission of the publishers, The Bobbs-Merrill Company.)

variation. Everyone is familiar with the difference in individual
response to caffeine. Some people cannot drink a demi-tasse in
the evening without enduring a sleepless night, while others drink

THE FOUNDATIONS OF GENETICS 267

as much coffee as they desire without unpleasant effects. Walter
cites an interesting case in th(^ kea parrot of New Zealand. This
bird was herbivorous but became carnivorous after the introduc-
tion of sheep into its native home. Thus physiological variations
may occur in individuals as well as among the different individuals
making up a species.

Psychological. Variations in mental qualities and processes
are evident in all of our associates. They too may be witnessed
in the individual from day to day as well as among the various
individuals of a group or species.

To what extent they are separable from a morphological and
physiological basis is less evident than the association of those
types of variation; there are still many persons who look upon
mind as an independent phenomenon. Biologically they are the
variable expression of brain functions. That they are intimately
associated with other bodily processes is amply attested by the
effects of indigestion or any other slight illness upon mental
efficiency, which most human beings have unwillingly experienced.
In spite of our relatively meager knowledge of nervous functions
it is therefore logical to believe that our mental processes and
their variations are as definitely and completely associated with
them as are any other phenomena of vital activity with underlying
organic causes.

Variations of Degree. According to degree two kinds of varia-
tions are commonly recognized:

Continuous. Such variations grade through long series of
individuals without an apparent Ijreak or abrupt transition. The
dimensions of any individuals of a given species, for example,
usually vary gradually from minimum to maximum. The curli-
ness of human hair also varies by minute degrees from the straight
hair of the Mongolian races to the kinky hair of the negro. Curli-
ness is based on the form of hair ; straight hair is cylindrical while
kinky hair is distinctly flattened, and various intergrades occur.

Discontinuous. Discontinuous variations, on the other hand,
exhibit abrupt transitions. Variations in the number of parts
of an organism are always of this type, for there can be no gradation
between six and seven petals or four and five rays. The term
has also been applied in an entirely different sense to mutations;
these are mentioned in the following pages under a different
category.

268 EVOLUTION AND GENETICS

Evolutionary Tendency. The effect of variations in evolution
becomes evident only through the study of other related phe-
nomena. It is impossible to determine this quality of variations
by the examination of an existing species alone, but complete
information about a phylogenetic series often discloses that varia-
tions in the past have followed either of two tendencies through
successive generations. They have been either fortuitous or
orthogenetic.

Fortuitous Variations. These are the ordinary fluctuating
variations which appear generation after generation apparently
always within the same limits and about the same mean. There
is no available evidence to prove that they contribute to the
evolution of species, although positive proof that they take no
part in evolution is equally lacking.

Orthogenetic Variations. Such variations are evident in the
field of paleontology. Their trend is in a definite direction through
a phylogenetic series, toward the ultimate modification of the
species in which they occur. A striking case of orthogenetic
variation is the gradual succession of changes in the foot of the
horse from Eohippus to the modern genus Equus as described in
chapter X.

Heritability. A more fundamental classification of variations
from the point of view of genetics is based on their heritability.
They are divided into three groups.

Modifications. Modifications are changes which appear in the
individual during the ordinary course of its life. They are usu-
ally looked upon as a product of environment, but should be
regarded, with very few exceptions, as the product of inherited
powers responding to environmental conditions. Such are tanning
of the skin and muscular development. It is evident that these
characters are not inherited as they develop in the individual,
but must develop anew with each generation if the proper con-
ditions are encountered. For the purposes of genetics they may
therefore be looked upon as not heritable.^

Combinations. Since no two individuals are exactly alike, it
follows that biparental reproduction will mix in the second genera-

1 Modifications are usually regarded as the effects of environment on organisms.
Few characters can be caused wholly by the environment, however, and most of the
so-called modifications are as described above. For this reason it seems desirable
to retain the term as it is used here. The question is considered further in chap-
ter XXIV.

THE FOUNDATIONS OF GENETICS

269

tion some of the different qualities of the first. A guinea pig may
have the black color of its father and the angora coat of its mother.
Such variations are based on the rearrangement of heritable
components and are consequently heritable. They are an impor-
tant source of change in organisms, but are necessarily Umited
by the range of characters present in the species. They have
been widely exemplified in the development of domestic races.

Mutations. When an individual appears with characters dis-
tinctly different from those of preceding generations it is said
to be a sport if its offspring return to the parental condition.
In some cases, however, differences which appear thus suddenly
are heritable and constitute permanent characters of the succeeding
generations. The
characters in ques-
tion are then said to
be mutations, and
the individual pos-
sessing them is a
mutant (Fig. 155).
Such variations are
due to an abrupt
qualitative change in
the organism and
because of their her-
itability they play an
important part in the
modification of
species. Their value
as a cause of evolu-
tion is treated else-
where in this volume.

Source of Varia-
tions. Walter sum-
marizes this question

with four opinions: (1) Darwin considered variations as axiomatic;
(2) Lamarck and his followers looked upon them as either produced
by the environment or by the organism in response to environ-
mental conditions; (3) Weismann regarded them as purely an
intrinsic product of the body; (4) Bateson regards incjuiry into
the causes of variation as premature.

a'

b'

Fig. 155. — A mutation in Drosophila melanogaster
compared with the normal character, a, a', normal
eyes; b, b', bar eyes. (From Morgan et al.,
Mechanism of Mendelian Heredity, with the per-
mission of Henry Holt and Company.)

270 EVOLUTION AND GENETICS

Modern emphasis upon the inseparability of heritage and
environment in the existence of organisms seems to dispose of
the question rather effectively. Although it does not explain in
absolute terms, it at least makes possible a logical interpretation
in place of pure opinion. If this emphasis is well placed, the
cooperating factors in existence can hardly fail to be jointly
responsible for the resulting expression of organic characters.
Any organism is infinitely complex in its inherited qualities, and
its environment is no less complex and variable. Either through
changes emanating from the organism, such as locomotion, or
through such factors as climatic fluctuations, the balance between
organism and surroundings is likely to be in a state of very delicate
and constantly shifting adjustment. It seems reasonable to sup-
pose that the result would be variation.

To this extent the interpretation is a combination of the opinions
of Lamarck and Darwin. With variation before us in its com-
plexity, however, we are forced to the belief of Weismann as
well, who saw in the mingling of diverse parental qualities in
each generation a provision for the diversification of the species.
It is necessary to bear in mind that an organism as complex as a
vertebrate or an arthropod is independent of the outer world to
the same degree that it possesses within itself the proper mechanism
for the maintenance of normal living conditions. In such a state,
with a constant shifting of stimuli through any of the changes
which have been considered as causes of adaptation, differences
may readily be expected to arise from conditions wholly within
the organism or partly without. There is sound reason in both
Lamarck's and Weismann's views, and Darwin expressed the
gist of the matter in his terse analysis. If we seek exact relations
between cause and effect in this field, however, we can only follow
Bateson's view; we do not know, and at present cannot expect to
know, exactly what condition will cause a given variation, nor is
it necessary for the purposes of genetics.

Importance of Heritability. Since the science of genetics deals
with the resemblance of different generations, it is obvious that
variations in any of the above categories must be heritable if
they are to furnish material for the study of problems of genetics.
Only combinations and mutations, therefore, are available sources
of information in this science. The latter, when they appear, can
be manipulated for the production of new combinations so as to

THE FOUNDATIONS OF GENETICS 271

indicate the fundamental methods of transmission of heritable
characters.

Methods of Study. The superficial facts of inheritance were
known long before there was a scientific foundation for their
interpretation. It requires no profound knowledge to establish
the fact that children have the traits of their parents, that some-
times they resemble more remote generations, and that individual
peculiarities are Hkely either to b(^ transmitted through many
generations or to appear onl}^ intermittently. In a practical
way man has taken advantage of this knowledge for the production
of various l)recds of domestic animals and many varieties of
plants. Hound and dachshund, mastiff and pekingese, are prod-
ucts of the same wolf-ancestors of many generations ago. Sweet
corn, flint, and dent varieties, as well as the primitive maize of
the American Indian, have been traced back to the wild teosinte
grass which now resembles them so slightly. Selection of animals
and plants showing the desired qualities and propagation exclu-
sively from these individuals have been responsible at least in
part for such changes. In other cases man has taken advantage
of the useful qualities of two species or varieties and combined
them by hybridization. The mule, a cross between the horse
and the ass, is such a hybrid.

Galton's Laws. Before these principles became a recognized
part of scientific procedure attempts were made to formulate laws
of heredity, with partial success. Best known are the two laws
of Sir Francis Galton, who worked extensively with statistical
data on human characters. They are :

The Law of Ancestral Inheritance. Each parent of an or-
ganism contributes one quarter of its inherited qualities, each of
its four grandparents one sixteenth, and so on. In other words,
the resemblance of an individual to all of its ancestors of any
generation is inversely proportional to the remoteness of the
relationship.

The Law of Filial Regression. Variation of parents from the
racial mean is transmitted to offspring in a lessened degree (Fig,
156). Tall parents tend to produce tall offspring and short
parents short offspring, but the normal expectation is that these
offspring will be nearer average h(>ight than their parents. This
law is of practical value, but is subject to modification according
to facts discovered since Galton's time.

272

EVOLUTION AND GENETICS

Influence of the Discovery of Cells. Although the celhi-
lar structure of organisms was recognized long before any impor-
tant contributions to the study of heredity appeared, detailed
knowledge of cell structure and behaviour was not available for
many years. Mendel's important discoveries described in the
next chapter were made without such knowledge, but they have
much greater significance with the background of modern cytology
than when they stand alone. The importance of cell behaviour

Fig. 156. — Diagram to illustrate Galton's law of filial regression. The circles
represent graded parental heights while the arrow points indicate the
average height of offspring descended from each group. The offspring of
short parents are taller and of tall parents shorter than their respective
parents. (From Walter.)

is strongly suggested by the methods of reproduction in organisms
of various degrees of development.

Reproduction of Unicellular Organisms. Single-celled plants
and animals reproduce in many cases by an equal subdivision
which gives rise to two new individuals with complete loss of
individuality on the part of the parent (Fig. 157). Such offspring
are apparently as nearly as possible identical with each other
and with the parent; there is abundant reason to believe them at
least potentially the same.

Occasionally these single-celled forms undergo a complex process

THE FOUNDATIONS OF GENETICS

273

of nuclear reorganization accompanied by an exchange of nuclear
material l)etween individuals. Such a process is akin to sexual
reproduction of higher forms and suggests the importance of the
nucleus in the vital processes of the cell. It also indicates the
possibility of combination of parts of two organisms in a single
individual even in the lowest groups.

Reproduction of Multicellular Organisms. Above the single-
celled organisms a process of budding or fission occurs which can

micronucteus

gullet

micronucleus
gullet:

Contractile vacuole

macronucleuS

Contractile vacuole

contractile vacuole

macronucleuS

contractile vacuole

Fig. 157. — Paramecium aurelia dividing by binary fission.

after Lang.)

(From Newman,

also be expected to produce as nearly as possible identical indi-
viduals. Hydra, a common fresh-water coelenterate, frequently
reproduces in this way (Fig. 158). Among the more highly organ-
ized forms, however, sexual reproduction is l^y far the most common
process, although sporulation still persists in such highly developed
plants as the ferns and parthenogenesis is not rare among animals.
All of these cases differ from the more primitive asexual processes

274

EVOLUTION AND GENETICS

in the minuteness of the matter from whicli the new individual
develops. This consists of a single cell, sometimes formed by
the union of two independent germ cells derived from opposite
sexes.

The Hereditary Bridge. Whatever the source of the single
cell which gives rise to a new individual, it constitutes a link

between generations which is of
the greatest importance to the
geneticist. All hereditary parts of
an organism must necessarily come
from its parents. In many cases
the production of the original germ
cells is the sole connection of the
parents with the new generation.
Obviously then everything which

Fig. l^S.-Hydra reproducing appears in the developing individ-
asexually by dividing length- ual must be based upon something
wise. (From Woodruff, after present in the cell from which it

arises. It is the most compact ex-
pression of the complete organism and well deserves Walter's
appellation, the hereditary bridge.

The value of studies of the behaviour of germ cells was recognized
long ago and cytological investigations correlated with observa-
tions of the behaviour of characters in heredity are now a funda-
mental part of genetics.

Hybridization. Unscientific breeders have not been alone in
utilizing this process. Crossbreeding of different strains within
a species is now the most important source of material for the
study of inheritance, for in the sharp contrast of characters thus
obtained the behaviour of any one is usually conspicuous.

Hybridization demands first of all, known strains or varieties,
which are best secured by careful breeding under observation to
determine the normal course of their heredity. Once isolated,
strains with contrasting characters can be crossed at will, and in
the phenomena of reproduction through successive hybrid genera-
tions is found the source of important laws of heredity.

Selection. In the establishment of pure strains for hybridiza-
tion selection is a necessary part of the methods of genetics. It
has been important in other ways, but in this science it is chiefly
an accessory of hybridization.

THE FOUNDATIONS OF GENP]TICS 275

Biometry. Many phenomena arc of interest to geneticists
whicli are not readily available for laboratory study. Such phe-
nomena can be studied by the collection of statistical data. Still
other phenomena involve variations which demand much more
refined methods of analysis than the mere recording of visible
differences, whether studied in the laboratory or in the fic^ld.
These conditions have given rise to methods of exact mathemat-
ical measurement and analysis which constitute the science of
biometry. A knowledge of biometrical methods is not necessary
to an understanding of the principles of genetics and the subject
will not be treated here. An excellent introduction may be found
in Chapter III of Babcock and Clausen's Genetics in Relation to
Agriculture.

An important application of biometry is the determination of
the amount of evolutionary change which may have occurred in
breeding experiments. Examples may be found in the work
cited.

Summary. The phenomena of adaptation, in order to be a
property of species and not merely of individuals, must be handed
down from generation to generation. The variations on which
they are based are also the materials from which our knowledge
of hereditary processes is derived. For the purposes of genetics
variations have been classified according to their nature, degree,
evolutionary tendency and heritability. Their source is not
definitely known, but their immediate origin seems adequately
explained by the complexity of the various factors of organic
existence. An understanding of the superficial facts of inheritance
has long been put to practical use; Galton formulated laws based
on such knowledge. The discovery of cells and their behaviour
in reproduction has since made possible the correlation of the
superficial phenomena of heredity with the morphological basis.
Modern genetics also makes use of the established principles of
hybridization and selection to provide materials for accurate
study. Out of these methods have come definite laws of heredity.

REFERENCES

Bateson, Wm., Materials for the Study of Variation, 1894.

Babcock, E. B. and Clausen, R. E., Genetics in Relation to Agriculture, 1918.

Castle, W. E., Genetics and Eugenics, 1921.

Walter, H. E., Genetics, revised edition, 1923.

Shull, a. F., Heredity, 1926.

CHAPTER XV
MENDELIAN HEREDITY

Johann Gregor Mendel, an Austrian monk, is the greatest figure
in the history of genetics. With less formal training in biology
than many college graduates of today he was destined through
his keen powers of analysis and his painstaking experimental
methods to give the world the first valualjle laws of transmission
of characters through heredity. These laws have stood the test
of years and have been proved over and over again by the
researches of the twentieth century. Cytological studies have
established with reasonable certainty the physical foundation for
them, and they now form an accepted basis for the usual procedure
of genetic investigation.

Mendel's work was carried on in the peaceful atmosphere of
his monastery garden at Briinn, where experiments covering
many years led at last to the formulation of the laws that bear
his name. His results were presented to the Natural History
Society of Briinn and in 1866 appeared in the transactions of
that society. They were then lost to the scientific world until
1900, perhaps l^ecause of the obscurity of the publication and
perhaps because the world was not ready to receive them. Both
causes have been suggested and both are probably true.

In 1900 three scientists are said to have revived Mendel's paper
independently. They were De Vries of Holland, Correns of Ger-
many and Tschermak of Austria. Suffice it to say that dis-
coveries in which these men participated disclosed the sound
value of Mendel's pioneer work and it became an accepted part
of science. The succeeding quarter of a century has witnessed a
constant increase in contributions to genetics, of which practically
all are based on Mendel's laws. Mendel himself, like so many dis-
tinguished men, died long before his work was recognized, in 1884.

Mendel's Materials. The variable characters of garden peas
constituted the material used by Mendel. These plants lent
themselves equally well to crossbreeding and inbreeding, they
were easy to raise under cultivation without forcing a change

27G

MENDELIAN HEREDITY 277

of environment upon thoin, as must happen when wild plants
are used for experiment, and they were sufficiently variable to
afford abundant confirmation of his conclusions. He could hardly
have made a happier choice for pioneer genetic research. The
characters with which he dealt were :

1. Shape of seeds. Some peas are round or nearly so and smooth
when ripe. Others have their seeds shrunken into deep wrinkles.

2. Color of cotyledons. These seed leaves make up the greater
part of the pea seed, and may be yellow or green.

3. Color of seed coat. Some seeds have a white coat and are
produced by plants with white flowers, while others have gray to
brown coats and are developed from purplish flowers.

4. The form of ripe pods. These may be either inflated or
wrinkled and deeply constricted between the seeds.

5. Color of unripe pods. Green or yellow.

6. Position of flowers. Axillary or terminal.

7. Length of stem. Plants vary considerably according to envi-
ronment, but under similar conditions peas fall into two classes.
There are tall or climbing peas which may be several feet in
height and dwarf peas which attain a maximum length of about
eighteen inches.

Methods. By carefully controlled cross-fertilization Mendel
produced hybrids of these varieties. For example peas bearing
smooth seeds were crossed with those bearing wrinkled seeds.
The result was the production of smooth seeds only, and in order
to determine the fate of the other character these were planted,
the plants self-fertilized, and a second generation of seeds produced.
In other experiments plants bearing y(>llow seeds were crossed
with those bearing green seeds, tall plants with short plants, plants
with yellow cotyledons and those with green, and a number of
others. The behaviour of these characters in the hybrid was noted,
and the nature of their reappearance in successive generations.

Monohybrids. In the hybrids which Mendel produced only
one of the two characters involved was evident. The hybrid peas
of the smooth-wrinkled cross, for example, were all smooth. When
the plants developed from these smooth seeds were self-fertilized,
thus com])ining only the types of gametes which a hybrid could
produce, the missing character appeared again. Approximately
one quarter of the resulting seeds were wrinkled. On raising yet
another generation the three quarters which were smooth were

278

EVOLUTION AND GENETICS

found to include two kinds of peas. One of these three quarters
produced only smooth peas while the remainder produced a three
to one ratio like their parents. The wrinkled peas bred true.

Mendel used a simple algebraic expression of his results. If
we let S represent the character of smoothness and s the alterna-
tive, wrinkled, then in the original smooth and wrinkled strains
only S and s respectively could be handed on to the next genera-
tion. In the hybrid there would be a possibility of either character
being handed down. The total possibilities to be derived from
either hybrid parent may therefore be represented by S+s, and
all possible combinations in the second hybrid generation by the
following computation :

S+s
S+s

SS+Ss
Ss+ss

SS+2SS+SS

Later Punnett suggested a simple checker-board plan commonly
called the Punnett square which expresses the same result (Fig.
159).

In the case of height all offspring of the hybrid seeds were tall.
Here of course the development of the plant was necessary for
the expression of the character. The tall peas proved to be mixed,
however, for when self-fertilized they produced three tall offspring
to one short. The short plants bred true, one third of the tall
bred true, and the remainder again produced the three to one
proportion. All of the seven characters behaved in the same way.
Tables of Mendel's results have been prepared by several writers.
In the one which follows the ratios are computed on the basis of
unity for the smaller group. The numbers cited are those actually
secured in breeding experiments.

Character

Dominants

Recessives

Ratio

1

2
3

4
5
6

7

Form of seed
Length of stem
Color of cotyledons
Color of seed coat
Form of pod
Color of pod
Position of flowers

5,474 smooth
787 tall

6,022 yellow
705 dark
882 inflated
428 green
651 axillary

1,850 wrinkled

277 dwarf
2,001 green

224 white

299 constricted

1.52 yellow

207 terminal

2.95 +
2.84 +
3.00 +
3.14 +
2.94 +
2.81 +
3.14 +

Totals

14,949

5,010

2.98 +

MENDELIAN HEREDITY

279

M

0)

+-)

0)

B

a

15
S

Homozygous
dominant

SS

Smooth

Heterozygous
dominant

S3

Smooth, with

latent determiner

for wrinkled

Heterozygous
dominant

Ss

Smooth, with

latent determiner

for wrinkled

Homozygous
recessive

SS

Wrinkled ,

Terminology. The examples given above represent in all cases
hybridization for a single pair of characters and are called mono-
hybrids. Dihybrids and trihtjbrids for two and thnn^ pairs of
characters respectively will be treated later. Any singles character
is called a unit character and the two alternative characters of a
pair are called allelomor'phs. It is customary to refer to the
original parents as the parental or P generation, to the hybrid

as the first filial generation

-r, , ,1 £f ■ f. Male gametes

or Fi, to the otfsprmg of S s

the hybrid as the F2 genera-
tion or second fihal and so
on. In any example certain
differences among individ-
uals are found which are
expressed in Figure 159.
Here it will be seen that of
the three similar F2 indi-
viduals, in two the charac-
ter wrinkled is represented
even though it does not ap-
pear. Such a character is Fig. 159.
said to be recessive, while
the allelomorph which con-
ceals it is dominant. The

three individuals look the same, and are therefore said to belong to
the same phenotype, while the characters actually represented
within them determine their genotypes. When pure, like the SS
individual, they are homozygous, when mixed hke the two Ss indi-
viduals, heterozygous. Obviously where dominance is perfect only
the test of breeding will disclose the genotype, except in the case
of a homozygous recessive.

Fundamental Principles. The results of these experiments and
the many others conducted since demonstrate three cardinal
principles of inheritance, viz:

Unit Characters. Organisms are composed of many characters
which may be combined in the same individual or distri])uted
among different strains without losing their distinctive properties.
These are commonly known as unit characters.

Segregation. The reappearance of the original unit characters
in the progeny of a hybrid indicates that association does not

Diagram to illustrate the F2
generation of Mendel's hybridized smooth
and wrinkled peas. Ratio of phenotypes
3:1; ratio of genotypes 1:2:1.

280 EVOLUTION AND GENETICS

modify characters but that during the process of reproduction
they may be separated or segregated anew.

Dominance. The fact that of the two characters present in a
hybrid one may completely conceal the presence of the other
illustrates the principle of dominance.

Behaviour of Allelomorphic Characters. It is now known that
many allelomorphic characters are not completely dominant and
recessive to each other, but that both may be expressed in a
hybrid. Three kinds of inheritance of allelomorphs are recognized,
viz., alternative, mosaic and blending.

Alternative Inheritance. This is the type described above,
in which one character completely dominates the other and the
recessive makes its appearance only in homozygous recessive indi-
viduals.

Mosaic Inheritance. Mosaic inheritance differs from alterna-
tive inheritance in that both of the allelomorphic characters may
be fully expressed, but in different parts of the body. Black and
white spotted offspring of self-colored parents may be of this
type, although they are sometimes explained in another way
(see Chapter XVII).

Blending Inheritance. This type of inheritance differs from
both of the preceding types in that neither character is fully
expressed in any part of the body when both are represented.
Such inheritance produces a character in hybrids which is inter-
mediate between the parental characters. Many flowers inherit
color in this way. The offspring of a red and white cross, for
example, may have pink flowers. In the Fo generation derived
from the pink flowers, however, red, pink and white individuals
appear.

Regardless of the manner of inheritance the integrity of the
unit characters is preserved and their segregation may occur anew
with every generation.

Effects of Allelomorphic Relationship. We have seen that in
Mendelian monohybrids the ratio 3:1 appears in the F2 generation
when one character dominates the other, although breeding tests
indicate that this is based on a 1:2:1 genotypic ratio. When
characters are inherited in a blended or mosaic conditions, however,
heterozygous individuals belong to a different phenotype as well
as to a different genotype from the homozygous, and consequently
the 1:2:1 ratio is evident without further breeding.

MENDELIAN HEREDITY

281

In the four-o'clock white flowers crossed with red produce in
the Fi generation only pink flowers. Inbreeding of this generation
produces all thrtn^ colors in the ratio of 1 white: 2 pink: 1 red
(Fig. 160). Animal characters are well exemplified by the classic
case of the blue Andalusian fowl. These chickens are produced
by crossing black and splashed white Andalusian types, and never

Ft

• •

%

Fig. 160. — Diagram to illustrate the results of crossing white and red flowered
races of four o'clocks (MirahilU jnlapn.) The somatic condition or pheno-
type is shown graphically; the small circles represent the genes involved.
(From Woodruff.)

breed true. They are the heterozygous individuals of any genera-
tion and the race can be maintained only by constant hybridizing
or by the elimination of the homozygous individuals of every
generation. The condition of the blue Andalusian has been inter-
preted as a mosaic of finely divided colors.

Dihybrid Ratios. When two pairs of characters are associated
in a hybrid the possibilities of reassociation in the F2 generation
are greatly increased, since not only the expression of each is
variable but the association of the unrelated characters as well.
There are four possibilities of distriljution of the characters as

282

EVOLUTION AND GENETICS

they are handed down to the next generation from each hybrid
parent, and the number of possible combinations received from
the two parents is therefore sixteen. Out of the sixteen combina-
tions some are duphcated. The phenotypic ratio is 9:3:3:1.
Nine are dominant for both characters, three for one, three for
the other and one for neither (Fig. 161). In this diagram it is
evident that the number of genotypes in the dihybrid is greater
than the number of phenotypes, as was the case in the monohy-
brid.

In the F2 generation many individuals are homozygous for one
character and heterozygous for the other. Such individuals, when

inbred, are capable

SY

Male gametes

Sy

s Y

sy

m

(D
+->

£

bid

s

1

SY
SY

1

1
Sy
SY

2

1
s Y
SY

3

1

sy

SY

4

1

SY
Sy

2

2

Sy
Sy

5

1
sY

Sy

4

2

sy
Sy

6

1
SY
s Y

3

1

Sy

sY

4

3
s Y
sY

7

3

sy
sY

8

J
SY

sy

4

2

Sy
sy

6

3

sY

sy

8

4

sy
sy
9

of producing off-
spring in which the
homozygous char-
acter always ap-
pears while the
heterozygous char-
acter behaves as in
monohybrids. An
SsYY pea vine, for
example, can pro-
duce only yellow
seeds but they will
appear in the ratio
of three smooth to
one wrinkled.

In experiments
with animals simi-
lar results have
been obtained. It
has been found that a guinea-pig with short black hair crossed
with a long-haired albino produces a short-haired black Fi genera-
tion. When these Fi guinea-pigs are inbred they produce an F2
generation with four t3^pes of animals, viz., short-haired black,
short-haired white, long-haired black and long-haired white. Of
these the short-haired black animals may belong to four different
genotypes, the long-haired all)inos to only one and the others to
two each. Figure 162 illustrates the kinds of guinea-pigs based
on the reassortment of three different characters.

SY

Sy

sY

sy

Fig. 161. — Diagram to illustrate the F2 generation of
Mendel's hybridization of smooth yellow and green
wrinkled peas. Ratio of phenotypes 9:3:3:1; ratio
of genotypes 1:2:2:4:1:2:1:2:1. The phenotypes
are numbered in the upper right hand corners of
the squares, the genotypes in the lower corners.

MENDELIAN HEREDITY

283

Trihybrids. As shown in Figure 1G3, a hybrid between indi-
viduals bearing the allelomorphs of three different characters

SPR

SpR

J

M

P

jiyH-"v

^^ 1

sPR

SPr

sp R

Spr

sPr spr

Fig. 162. — The eight guinea-pig phenotypes in the F2 generation of a trihy-
brid. S, short hair; s, long hair or angora; P, pigmented coat; p, aIi)ino;
R, rough or rosetted coat; r, smooth coat. (From Walter, after drawings
from Castle's photographs by C. J. Fish.)

affords possibilities for eight different com])inations in the gametes
of the Fi generation. The same number of combinations is
present in the gametes of each parent (Fig. 163), so that sixty-four

284

EVOLUTION AND GENETICS

combinations from the two parents result in the individuals of
the F2 generation. Among these sixty-four the eight phenotypcs ex-
press the trihybrid ratio 27:9:9:9:3:3:3:1. The diagram shows
both the phenotype and genotype of each individual and illustrates
the rapid increase in complexity of the F2 generation as the number

TSY

Male gametes
TSy TsY Tsy tSY tSy

tsY tsy

TSY

TSy

TsY

to

s

Tsy

"i tSY

TSY
TSY

TSy
TSY

TsY

TSY

Tsy
TSY

tSY
TSY

tSy
TSY

tsY
TSY

tsy
TSY

TSY

TSy

TSy
TSy

TsY
TSy

Tsy
TSy

tSY

TSy

tSy

TSy

tsY

TSy

tsy
TSy

TSY
TsY

TSy
TsY

TsY
TsY

Tsy
TsY

tSY
TsY

tSy
TsY

tsY
TsY

tsy
TsY

TSY

Tsy

TSy
Tsy

TsY
Tsy

Tsy
Tsy

tSY
Tsy

tSy
Tsy

tsY
Tsy

tsy
Tsy

TSY
tSY

TSy
tSY

TsY

tSY

Tsy
tSY

tSY
tSY

tSy
tSY

tsY

tSY

tsy
tSY

TSY

tSy

TSy
tSy

TsY

tSy

Tsy
tSy

tSY

tSy

tSy
tSy

tsY

tSy

tsy
tSy

TSY
tsY

TSy
tsY

TsY
tsY

Tsy
tsY

tSY
tsY

tSy
tsY

tsY
tsY

tsy
tsY

TSY

tsy

TSy
tsy

TsY

tsy

Tsy
tsy

tSY

tsy

tSy
tsy

tsY
tsy

tsy
tsy

tSy

tsY

tsy

Fig. 163. — Diagram to illustrate the F> generation of a hybrid between
tall smooth yellow and dwarf wrinkled green peas. Ratio of phenotypes
27:9:9:9:3:3:3:1.

of characters is increased. Any further examples would merely
repeat the same process without adding to the facts already
disclosed.

Theoretical Calculations. We have seen that a single pair of
allelomorphs make possible four combinations according to the
law of chance, that two make possible sixteen combinations, and
three sixty-four combinations. It is therefore apparent that the
number of possible combinations is equal to four raised to a
power indicated by the number of different characters involved,

MENDELIAN HEREDITY 285

and if we are concerned with ten characters, a modest portion of
those present in any complex animal, the result is the astonishing
total of 1,048,576. When we considcn* the number of unit charac-
ters which must be present in a human being it is easy to under-
stand why no two individuals are alike.

The Back-Cross. In cases of complete dominance in multiple
hybrids it would be an endless task to attempt the determination
of genotypes by inbreeding. Self-fertilization would lighten the
task for the plant breeder, since he could be certain of mating
like with like, but the animal l^reeder would be at a loss to know
which of his many dominants were genotypically the same, so
that their crossing would be equivalent to self-fertilization. To
meet the difficulty of this situation it is customary to test an
individual by crossing it with a known recessive. Since recessive
characters are always apparent in the phenotype, such individuals
can be selected without difficulty.

As an example let us assume that we have the F2 progeny of
the hybrid represented in Figure 163. We know that each of the
twenty-seven dominants contains the characters tall, smooth and
yellow, but whether homozygous or heterozygous for these charac-
ters must be determined. If the individual in question is mated
with a ttssyy , or homozy- j^^^^ ^^ ^^^^^^^ ^^^^^^ ^^

gous recessive, and gives dominant

only tall smooth yellow
and tall smooth gi-een Gametes of -i
offspring we can be cer- reTesJive"^) ^^

tain that it was homozy- Yig. 164.— Diagram illustrating the back-cross
gous for l)oth of the first of a tall-smooth-yellow pea of the F2 genera-
two characters and heter- tion with a short-wrinkled-green homozy-

» ,, ,1 • 1 gous recessive,
ozygous tor the third.

For every heterozygous character it would give to one-half of its
offspring the recessive and to one-half the dominant. Those which
received the recessive, since they could receive only that character
from the homozygous recessive parent, would necessarily express
the recessive condition. The individual cited would therefore
belong to the genotype TTSSYy. The results are expressed in
Figure 164.

Linkage. Although these facts justify the belief that unit
characters are independent of each other, it has been found that
some always appear tog(>ther under normal conditions. Such

TSY

TSy

TSY
tsy

TSy
tsy

286 EVOLUTION AND GENETICS

characters are said to be linked. Linkage does not imply that
the characters are in any way dependent upon each other or that
they are related in any way other than simple association in the
same individuals. They may involve very different parts of the
body, as in the case of bar eyes and small wings in Drosophila,
the fruit fly. Characters may also be definitely associated with
sex, and are then said to be sex-linked. Among these are color-
blindness in man and various characters in birds, insects and
other organisms.

The effect of linkage is a simplification of the ordinary hybrid
ratios, for a group of linked characters give only a monohybrid
ratio because of their normal inseparability.

Modem Investigations. During the twentieth century Men-
del's discoveries have been repeatedly verified and considerably
extended by breeding experiments. Both plants and animals
have been used as materials, and in all cases satisfactory results
have been secured, although accuracy is most nearly attained in
plants and such animals as produce large numbers of young.
Laboratory animals like the guinea-pig, rabbit and rat are suffi-
ciently prolific to be useful, but not to give approximately accurate
Mendehan ratios unless several litters of the same cross are used.

According to the law of chance, if a given phenomenon may
occur in a number of ways and is allowed to occur a sufficient
number of times, it will occur in all possible ways. The allelo-
morphs in a monohy])rid can combine in only four ways, three
of which are different. If animals produce only single young,
obviously one litter cannot express the monohybrid ratio. There
is a possibility that a litter of four may do so. A dihybrid ratio
cannot be completely expressed by less than sixteen individuals,
which is beyond the size of litters produced by most mammals,
although the four phenotypes represented in this ratio may
readily be produced in snialler litters.

Corn has come into use in the last few years as a laboratory
illustration of Mendclian ratios, and because of the large number
of grains on a single ear it usually approximates dihybrid ratios
fairly well and monohybrid ratios very closely. Since each seed
is a potential plant, the seed characters of a field of corn can be
studied in a few ears. The characters available include purple
and white aleurone and starchy and sugary endosperm, as well
as other color characters. A single ear representing the F2 genera-

MENDELIAN HEREDITY 287

tion of a purple-white starchy-sweet cross shows purple starchy,
purple sweet, white starchy and white sweet grains. The sweet
grains are shrunken and so stand out in sharp contrast to the
smooth, plump starchy grains (Fig. 165).

The pendulum has swung strongly toward the use of fruit flies
of the genus Drosophila in this work since the first important
contributions of Morgan and his associates. These insects have
several advantages for genetic research. They reproduce rapidly
and in large numbers; they are easily reared in the laboratory;
they are so organized as to facilitate cytological examination, and
best of all they give rise to many mutations which furnish the

Fig. 165. — An ear of corn showing the F> dihybrid ratio. Four kinds of grains
are present, viz., purple starchy, purple sweet, white starchy and wnite
sweet, in the proportion of 282:80:74:27; this is very near to the expected
ration of 9:3:3:1. The starchy grains are smooth and the sweet grains
wrinkled.

character contrasts so necessary for accurate observation (Fig.
155). Drosophila has been so productive that almost any principle
of genetics can be illustrated with it alone.

Practical Importance. From the foregoing account it is appar-
ent that the transmission of characters, whatever their number,
goes on according to definite laws. By taking advantage of these
laws desired combinations of characters can be secured through
the proper control of propagation without awaiting the accidental
occurrence of the right individual. If a plant breeder has purple
sweet corn and white field corn, for example, and wishes to
produce white sweet corn, he can do so by hybridizing the two
and selecting the homozygous recessives of the Fo g(meration.
Or if any other characters arc desired the homozygous condition
will be found in some individuals of the F2 generation, and such

288 EVOLUTION AND GENETICS

individuals will breed true. One thing alone cannot be stabilized
as an independent strain through ordinary methods of reproduc-
tion, and that is a variety based on the heterozygous condition,
such as the blue Andalusian fowl. These must be maintained })y
repeated crossing or by breeding heterozygous individuals and
selecting their heterozygous offspring.

Summary. Mendel established the fact that organisms are
made up of unit characters which are segregated during repro-
duction. These characters may be alternative, blending or mosaic
in the hybrid, but in the offspring of the hybrid they are reassoci-
ated in definite ratios, so that the characters of the original
parents as well as of the first hybrid reappear. The ratios are
fixed according to the law of chance for every number of characters.
They are only approximated in breeding experiments unless large
numbers of individuals are considered. Many organisms are now
used for experimental studies in this field, but fruit flies of the
genus Drosophila are the most important. The laws are of great
practical importance to breeders of plants and animals.

REFERENCES

Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B.,

The Mechanisyn of Mendelian Heredity, revised edition, 1922.
Morgan, T. H., The Physical Basis of Heredity, 1919.

All general works on heredity contain more or less detailed accounts of Men-
delian inheritance. Those cited above are especially valuable for detailed
evidence of the various processes as illustrated in Drosophila.

CHAPTER XVI

THE CHROMOSOME THEORY OF HEREDITY

The rcmarkal)le behaviour of the chromatin during mitosis,
described in Chapter \T, is fundamental evidence that this sub-
stance plays a peculiar and important part in the history of cells.
During mitosis all other portions of the nucleus become merged
with the cytoplasm, the centrosome plays a part which seems to
be limited entirely to cell reproduction, and the cytoplasm and
chromatin are equally divided between the daughter cells. The
division of the cytoplasm is attended by no definite phenomena

b ^ d T f ^ T :

f

Fig. 166. — Examples of one of the chromosomes of Phrynotetlix sp. taken
from thirteen individuals. The dotted line passes through homologous
granules, x 2200. (From IMcClung, in Cowdry's General Cytology, after
Wenrich; with the permission of the University of Chicago Press.)

which would indicate both quantitative and qualitative equality
of distribution. The chromosomes, however, are developed in
many cases as slender structures showing longitudinal differentia-
tion which is evident in well prepared specimens (Fig. 166) and
the longitudinal splitting that occurs in the metaphase is appar-
ently an equation division. Moreover in the many successive
mitoses intervening between the union of the germ cells and the
completion of embrj'ological development extensive cytoplasmic
differentiation occurs. Although it is accompanied by some
nuclear differentiation evidence shows that the same chromosome
complex persists even in highly specialized cells. This complex
is characteristic of the different parts of the body of all individuals
of the species, and in some cases of related species. McClung's

289

290

EVOLUTION AND GENETICS

data on the Orthoptera show striking chromosomal similarity
among related species of these insects.

The entire body of facts relating to chromosomal behaviour is
so significant that it has given rise to the chromosomal theory

-.1,. I, -,->j;i«-^

C D

Fig. 167. — Various cells. A, from peritoneum of salamander; 5, spermato-
gonium of frog; C, spinal ganglion cell of frog; D, spermatocyte of Proteus,
nucleus in spireme stage. (From Wilson, A after Flemming, B and D after
Hermann, C after Lanhossek.)

of heredity which postulates that these minute bodies are the
seat of the substances or particles which are handed down from
one generation to the next as the foundation of hereditary resem-
blance.

THE CHROMOSOME THEORY OF HEREDITY 291

This theory was preceded by the hypothesis that the body
contained some substance which acted in a similar capacity.
This hypothetical sul)stance was called the idioplasm. The term
is reminiscent of Weismann's theories in which the terms id and
idant were prominent, Ijut before Weismann's contributions
appeared, Roux (1883) had recognized the similarity of chromatin
to idioplasm. Modern science has established a large body of facts
which are little short of actual proof that the chromosomes are the
material basis of heredity.

The Organization of Chromosomes. During the resting stage
of a cell its chromatin is visible in stained preparations as a network
of uneven texture or as an aggregation of granules (Fig. 167).

9

d

/I. /,«.

Fig. 168. — Diagram of female and male diploid groups of chromosomes of
Drosophila melanogasler. The hook on the y chromosome is characteristic.
(From Morgan et al., Mechanism of Mendelian Heredity, with the permis-
sion of Henry Holt and Company.)

This arrangement is lost when the nuclear membrane breaks down
at the beginning of cell reproduction and the chromatin condenses,
sometimes into a heavier thread which later breaks up into chi'omo-
somes and sometimes directly into chromosomes. In its condensed
state the chromatin appears to be a homogeneous material with a
marked affinity for many biological stains.

According to cytologists, however, the chromatin is actually
composed of minute bodies called chromomeres which retain the
same relationships in all stages where they are visible. Wenrich
has shown that these chromatin units differ in size, so that their
arrangement in chromosomes can sometimes be traced readily.

292 EVOLUTION AND GENETICS

He has also made comparisons of many cells in which their arrange-
ment appeared to be constant (Fig. 166).

Characteristics of Chromosomes. While the inner organization
of the chromosomes is a subject for cytological studies of the
utmost refinement, the bodies themselves are more readily ob-
served and exhibit several phenomena of interest.

1. Chromosomes appear in the cells of a given species in the
same number and form, with the exception of a common discrep-
ancy between the sexes (Fig. 168).

2. Within the chromosome complex of a given species there
are chromosomes of different sizes and shapes. In species which
show chromosomal differences sufficient for accurate observation
it is evident that these characteristics are constant for different
individuals.

3. In the cells of the body the chromosomes are duplicated, i.e.,
there are two of each kind, with the sole exception of those associ-
ated with sex.

The Chromosomes in Reproduction. No single phase of the
behaviour of chromosomes is as significant as the series of changes
that take place during the formation and union of the germ cells.
In the higher organisms these are of two distinct types, the female
gamete or ovum and the male gamete or spermatozoon of animals.
Both are fundamentally similar in that they possess one half of
the total number of chromosomes characteristic of the species,
usually called the haploid number, and in the process of gameto-
genesis by which they are produced. By the union of two germ
cells the full or diploid number of chromosomes is restored.

Gametogenesis. The body of an organism during development
contains many primordial germ cells which are directly descended
from the original fertilized ovum with which its development
began. These primordial germ cells multiply by mitosis, and by
the completion of embryonic life have produced many other cells
which lie in the gonads. Those of the female are called oogonia
and those of the male spermatogonia (Fig. 169A). After a period
of growth they become the primary oocytes and primary spermato-
cytes respectively, and in this stage a significant step occurs in the
behaviour of their chromosomes, known as synapsis (Fig. 169C).

Synapsis. In brief, synapsis consists of a pairing of similar
chromosomes from the diploid complex. It is attended by a
considerable degree of complexity in some animals, but in its

THE CHROMOSOME THEORY OF HEREDITY 293

Fig. 169. — Diagram of the general plan of spermatogenesis and oogenesis in
animals. The somatic, or diploid, number of chromosomes is assumed to
be eight. Male to the left, female to the right. A, primordial germ cells;
B, spermatogonia and oogonia, many of which arise during the period of
multiplication; C, primary spermatocyte and oocyte, after the growth period,
with chromosomes in synapsis; D, secondary spermatocytes and oocytes
with haploid number of chromosomes. These cells are the product of the
reduction division; E, spermatids, which develop into spermatozoa; in the
female, one egg and three polar bodies. The cells of this stage are produced
by the equation division. The order of the reduction and equation divi-
sions is variable in different species; the two together are called the first
and second maturation divisions. F, union of ovum and spermatozoon
(fertilization); G, the diploid number of chromosomes in the daughter cells
formed by the first cleavage of the zygote. This number persists in all cells
of the body and in the germ cells until maturation. (From Woodruff.)

294 EVOLUTION AND GENETICS

essential features is apparently always the same. Omitting con-
sideration of the chromosomes associated with sex, we find that
the fully developed primary spermatocyte or oocyte contains as
many pairs of chromosomes as there are kinds. In preparation
for the two maturation divisions which follow close upon synapsis,
each of the chromosomes has split longitudinally, so that each
synaptic pair is in reality a group of four halves of chromosomes
called a tetrad. Tetrads are not represented in Figure 169, which
is designed to show in the simplest possible way the fundamental
features of the process. In Figure 170E is shown a primary oocyte
of Ascaris megalocephala containing two tetrads. The diploid
number of chromosomes in this species is four.

Maturation Divisions. Closely following the formation of tet-
rads each primary cell undergoes two successive divisions which
distribute the four parts of each tetrad among the four resulting
daughter cells. One of these divisions separates the two synaptic
mates, or entire chromosomes, and produces daughter cells with
only the haploid number. This is called the reduction division.
The other merely separates halves of chromosomes, so that
daughter cells and parent cell are similar. It is called the equation
division (Fig. 169C-E). The order in which the two divisions
occur may vary among different species.

The cells derived from primary spermatocytes are called second-
ary spermatocytes, and those of the next generation spermatids.
Spermatids undergo a process of differentiation and become motile
spermatozoa.

The maturation divisions in the female differ in that only one
primary and one secondary oocyte are produced. Each of the
maturation divisions results in the concentration of cytoplasm in
this one cell, while the necessary chromosome reduction is accom-
plished l)y the formation of abortive polar bodies. The first polar
body (Fig. 169D) is similar in nuclear organization to the secondary
oocyte and the second is similar to the mature ovum, with the
exception of such qualitative differences as may result from reduc-
tion. The first polar body sometimes divides as shown in Figure
169E.

The Mature Germ Cells. Although the mature germ cells
of both sexes contain the haploid number of chromosomes they
may be very different in other ways. The ovum is a large cell
containing an abundance of cytoplasm and often a large quantity

THE CHROMOSOME THEORY OF HEREDITY 295

of stored food. It is developed to an extreme in the eggs of birds,
in which the yolk is the true ovum but the active cytoplasm is

Fig. 170. — Spermatogeneses in Ascaria megalocephala. A-G, suece.ssive
stages in the division of the primary spermatocyte. D shows the two
tetrads in profile and E shows an end view of them; F, G, H, division of
the primary spermatocyte to form two secondary spermatocytes, each con-
taining two dyads; /, secondary spermatocyte; /, K, the same dividing;
L, spermatids, each with two chromosomes and a centrosome. The normal
diploid numl^er of chromosomes in this .species is four. (From Wilson,
after Brauer.)

restricted to a small mass on one side. The remainder is yolk
in the strict sense, and is food for the developing (Miibryo. The
white is an env(>lope of albumen of similar importance outside
of the cell.

296

EVOLUTION AND GENETICS

The spermatozoon, on the other hand, is highly differenti-
ated. It consists of several regions, particularly the head, neck
and tail. The head is a very compact nucleus, and the tail

B

Fig. 171. — Human germ cells. A, ovum, x 415, surrounded by follicle cells
from the ovary; B, sperm cell x 2000. (From Woodruff, after Retzius.)

is an organ of locomotion. The human germ cells are shown in
Figure 171.

Fertilization. This is the process of union of the two kinds of
germ cells. When a sperm cell comes into contact with an ovum
which has not previously been fertilized it penetrates the membrane
surrounding the ovum. The head, at least, enters the cytoplasm
of the ovum, while the tail remains outside. The surface of the

THE CHROMOSOME THEORY OF HEREDITY 297

ovum then undergoes changes which normally prevent the entrance
of other sperms.

The head of the spermatozoon then undergoes changes which
transform it into a t7iale pronucleus, resembhng a resting nucleus,
which is similar to the female pronucleus already present. A
centrosome appears, divides, and forms a mitotic spindle. The
source of this centrosome is uncertain. It was once thought
that it was introduced by the male cell, and this may sometimes
be the case, but there is evidence to show that it may arise from
the cytoplasm of the oviuu. After the formation of the mitotic
spindle the pronuclei undergo changes similar to those which
occur in the prophase of an ordinary mitotic division and the
resulting chromosomes group themselves together in the equa-
torial plate. From this point the process is similar to the
three final stages of mitosis, viz., the metaphase, anaphase
and telophase, and its completion is the formation of two
daughter cells. This is the first cleavage of embryonic develop-
ment.

The significant fact of fertilization is the combination in one
cell of two similar groups of chromosomes derived from different
parents. The diploid complex is maintained in the new individual
by mitosis. By referring to Figure 169F and G, in which the
chromosomes derived from the female parent are outlined and
those from the male are black, the possible results may be seen.
There is apparently no definite association between the different
kinds of chromosomes, hence the reduction division in the indi-
vidual derived from this cell may place some maternal and some
paternal chromosomes in each resulting germ cell. Such a reas-
sortment of parental chromosomes is of the greatest importance.
The number of combinations possible is obviously hmited by the
number of chromosomes present.

Sex Chromosomes. The foregoing account omits consideration
of a common contrast in the chromosome complex of males and
females of the same species. Sometimes a difference is discernible
between two synaptic mates when the number is constant, and
in other cases there is a numerical difference, one sex having an
odd number of chromosomes and the other an even number due
to the presence of a synaptic mate for the odd chromosome. These
phenomena have given rise to a theory of chromosomal determina-
tion of sex which appears to be largely sound, although recent

298

EVOLUTION AND GENETICS

investigations have indicated that the chromosomes are not alone
responsible.

In some species the male is characterized by an odd numlier of
chromosomes and the female by an even number. The odd

MALE

FEMALE

Secondary
Oocyte

Matured
Ovum

Fig. 172. — Sex determination in species having heterogametic males. (From

Walter.)

chromosome in the male is without a synaptic mate and conse-
quently the reduction division produces two types of gametes.
Such species are said to have heterogametic males. The single
chromosome is called the x chromosome. Since the females have
an even number including two x chromosomes all female gametes
are the same. The sex of the individual is therefore determined

THE CHROMOSOME THEORY OF HEREDITY 290

by the type of male gamete with which an ovum unites. Inherit-
ance of sex in these species is graphically indicated in Figure 172.

Some heterogametic males have an even diploid number of
chromosomes, but the synaptic mate of the x chromosome, called
the y chromosome, is at least different in function. The chromo-
some complex of Drosophila is of this type (Fig. 168).

Other animals, including the birds and Lepidoptera, are known
to have heterogametic females. The behaviour of the chromo-
somes is similar but two kinds of ova arc produced and only one
kind of spermatozoa. The sex chromosomes arc called z chromo-
somes in such cases.

Sex chromosomes of all kinds arc sometimes called allosomes,
and the remaining chromosomes autosomes.

Abnormal Behaviour of Chromosomes. The phenomena of
chromosome behaviour just described are the normal. In such a
complex series of changes it would he surprising to find that
abnormalities never occur. Several modifications have, in fact,
been demonstrated, including (1) interchange of parts between
synaptic mates and (2) changes in the number of chromosomes.
The latter may result from multiple fertilization or from the
failure of chromosomes to separate during the reduction division
so that the chromosome complex of some of the daughter cells
is increased and of others decreased.

Interchange of parts between synaptic mates cannot actually
be seen, but during certain stages of synapsis the thread-like
chromosomes have been found twisted together, and some of the
phenomena of heredity indicate that the separation of these mates
may be attended by rupture at one or more points of crossing.
This would result in the formation of new chromosomes, each
formed of parts of both of its predecessors (see Chapter XVII).

The multiplication of chromosome numbers does not change the
nature of the chromosomes involved, and so may not change the
nature of the individual. Haploid, triploid and tetraploid com-
plexes are on record as well as the usual diploid complex and in
some cases they have been produced experimentally. It is worthy
of note that gigantism is a common result of the tetraploid con-
dition as recorded by DeVries in the evening primrose, Marchals
in mosses and Winkler in tomato and nightshade, yet in cases
of abnormal numbers of sex chromosomes no abnormality is
apparent except the modification of sex ratios.

300

EVOLUTION AND GENETICS

The inheritance of sex in Drosophila in a case involving non-dis-
junction of the X chromosomes in the female parent illustrates
the effect of abnormal allosome number. Females of Drosophila
normally produce only one kind of egg; non-disjunction results in

kinds I I kinda
of \X\ of
egga

0 @@©

5© e© c© ©

1234 6678

Fig. 173. — "Non-disjunction and its results in Drosophila. The two large
circles in the first row represent male and female flies producing sperms and
eggs respectively. Non-disjunction in the female gives two kinds of eggs,
with XX and no sex chromosomes, instead of the normal single kind with one
X. At fertilization there are possible four combinations rather, than two,
as shown in the large circles of the second row. Owing to the several ways
in which her three sex chromosomes may be distributed at maturation, the
female represented by the third circle produces four kinds of eggs. When
mated to a normal male (below the horizontal line) with two kinds of sperms,
eight combinations are possible (last row). Numbers 1, 4 and 5 are normal
flies and give the usual type of progeny. Numbers 2, 6 and 7, owing to the
presence of three sex chromosomes, give exceptional results when bred.
Types numbers 3 and 8 do not appear in the cultures, probably because
they die very early. The original male has red eyes and the original
female white eyes. Red eyes (represented by the dots) appear in every fly
bearing the x-chromosome of the original male." (From Walter, diagram
by Sharp based on data from Bridges and Morgan.)

the production of two kinds, one with no x chromosome and one
with two. In combination with normal spermatozoa they give
the results shown in Figure 173.

Evidence for the Chromosome Theory

The Contributions of the Sexes. Nuclear Components. In

the phenomena of Mendelian inheritance it is evident that the
male and female parents contribute equally to the heritage of

THE CHROMOSOME THEORY OF HEREDITY 301

their issue with the sole exception of sex and associated char-
acters.

Since the germ cells are actually a bridge between the genera-
tions and must therefore contain the potential equivalent of all
that is to appear in the new individual it is rcasona])le to expect
similarity of those which combine to the same extent that we
find similarity of the characters which they transmit. This simi-
larity of male and female germ cells we have but briefly considered.

The development of sexual reproduction begins with the union
of obviously similar gametes. Differentiation such as that exhib-
ited by the germ cells of mammals and birds is an extreme develop-
ment. In the transition between the two stages it appears that
anything which remains constant is fundamental, and the chro-
matin of the nucleus alone fails to undergo marked change. The
highly specialized spermatozoon conveys little or no cytoplasm
into the ovum which it fertilizes; the ovum contributes much.
The nuclear material, especially the chromatin, is equivalent with
the exception of sex chromosomes. If cytoplasm played a large
part in the determination of the qualities of the new individual the
preponderance of maternal characters would seem to be inevitable.

The Role of the Cytoplasm. The cytoplasm is apparently only
a plastic material for the expression of the heritage. Lillie
and Just make the significant statement that "the materials of
the cytoplasm are . . . being constantly consumed in the metab-
olism, and the process of renewal and increase of such materials
involves interaction of nucleus and cytoplasm; therefore the
purely maternal cytoplasm soon disappears, and is replaced by
cytoplasm formed under the influence of the biparental zygote
nucleus." There is certainly some increase, if not replacement,
of material in the chromosomes as development proceeds, but it
is evidently only to meet the demands of growth and multiplica-
tion, while cytoplasmic interchanges are at the root of the intricate
vital processes in the living Ijody.

A most convincing experiment is that conducted by Boveri on
sea urchins. Enucleate cytoplasmic fragments from the eggs of
one species were fertilized by the sperms of another species.
These fragments developed sufficientl}^ to show definite charac-
ters, and the characters produced were those of the species from
which the sperms were derived. The wholly cytoplasmic maternal
contribution was apparently without (effect.

302

EVOLUTION AND GENETICS

The cytoplasm of some eggs shows marked differentiation.
The eggs of Stijela -partita, for example, are reported by Conldin
as having an orange pigment distributed over the surface before
maturation, while during fertilization this pigment retreats to the
vegetal pole. Above the orange area there appears a layer of
clear cytoplasm and in the remainder of the egg gray yolk. Such
regional differentiation is sometimes associated with the initial

B £

Fig. 174. — Diagram of zones of cytoplasmic differentiation and their distribu-
tion at the first cleavage. A, immature egg with no visible cytoplasmic
differentiation; B, mature egg with four zones; C, division of the egg;
D and E, two types of two-cell stages; D, Dentnlium or Styela, in which
one cytoplasmic zone passes completely into one cell; E, sea urchin or
Amphioxus type, with equal distribution of cytoplasm. (From Woodruff,
after Wilson.)

cellular differentiation of the developing embryo but it has not
been shown to influence hereditary characters (Fig. 174).

It is necessary to draw the conclusion that cytoplasm is merely
a material of construction. It is as essential in reproduction as
in individual life ])ut is apparently entirely under the control
of the nucleus, and particularly of the chromosomes. The two are

THE CHROMOSOME THEORY OF HEREDITY 303

mutually dependent; cytoplasm alone can give expression to the
chromosomes and the chromosomes alone can activate the cyto-
plasm.

Mitochondria. These are bodices included in the cytoplasm, to
which functions in heredity have been attributed. A paragraph
from Lillie and Just will serve as an ample refutation of this pro-
posal: "Whatever may be the function of the mitochondria in cell
physiology, it must be admitted that the study of fertilization
has shown no reason for the assumption that their introduction
into the egg by the sperm in certain species is concerned in the
transmission of paternal characteristics. The variable quantity
in different cases and the distribution to single blastomeres in
certain cases exclude the hypothesis that they have any specific
paternal hereditary effect. There is no reason to deny that sperm
mitochondria function in the egg when present, but if so it is prob-
able that they are not differentiated in their chemical composition
or genetic behaviour from the mitochondria of the egg itself."

Mendelism and the Chromosomes. There is no stronger evi-
dence for the chromosome theory of heredity than the close resem-
blance between the behaviour of Mendclian unit characters and
that of the chromosomes. The points of resemblance may be
summarized as follows:

1. Unit characters are duplicated in the organism, as is shown
by the behaviour of allelomorphic units when they are combined
in a hybrid. The monohybrid ratio indicates that there are two
and no more than two of these units present, since it expresses
almost exactly the mathematical possibilities of such a number.
The same duplication prevails in the chromosomes of the body,
where the diploid condition is normal.

2. Segregation of the determiners for unit characters occurs
during reproduction. The reappearance of recessive characters
in the Fo generation after their complete concealment in the
hybrid parents is evidence of this segregation. This is a close
parallel of the separation of synaptic mates during the reduction
division in gametogenesis.

3. The characters derived from hybrid parents are a recombina-
tion of characters represented in some way in both parents. In
the formation of gametes and their union to form ncnv individuals
the possil^ility of similar rearrangement of parental chromosomes
is apparent.

304 EVOLUTION AND GENETICS

In other details of inheritance such as hnkage, crossing over and
sex-linkage the parallel is found to hold true. Behaviour of unit
characters, however intricate, is found to be closely in accordance
with the behaviour of the chromosomes as established by cyto-
logical studies.

Every known fact concerning the structure and function of
the chromosomes singles them out as the seat of the hereditary
properties of the organism. They are not independent of the
cytoplasm, but are rather a controlling center. As the artist needs
a medium of expression, so must they use the cytoplasm as a
medium for the production of structures in the individual. There
is nothing to show that other parts of the cell exert a shaping
influence in themselves.

Summary. The chromatin contained in the nucleus of a cell
is made up of many granules called chromomeres which are
aggregated during cell reproduction to form chromosomes. The
behaviour of these bodies follows a definite plan in ordinary cell
reproduction and in the sexual reproduction of the individual.
In sexual reproduction germ cells are formed which may differ in
everything but chromosome content. This alone is the same in
all germ cells of a species, with the exception of sex chromosomes.
Since the heritage is derived equally from male and female parents,
it is logical to interpret the chromosomes as its physical basis in
the germ cells. Moreover the behaviour of the chromosomes in
reproduction is so much like the behaviour of unit characters in
the course of heredity as to establish the theory beyond reasonable
doubt. The chromosomes are now commonly interpreted as the
conveyors of determiners of hereditary characters, and there is
no evidence to show that other parts of the cell act in a similar
capacity.

REFERENCE
McClung, C. E., Cowdry's General Cytology, Section X, 1924,

CHAPTER XVII
GENES AND CHARACTERS

What Is Inherited? The resemblance of successive generations
leads us to say that certain characters are inherited. For all
practical purposes this is true and sufficiently accurate. We do
receive from our parents the character in question to the extent
that in ourselves the character develops from material received
from the preceding generation and perfected during ontogeny.
A more accurate analysis shows that the thing actually handed
down from one generation to the next is some constituent of the
germinal chromosomes which is capable of bringing about the
development in the new generation of the same character which
its progenitors had developed in the old. That some such entity
exists for every unit character is evident from the facts already
cited. The determiner in general is called a factor; the material
entities within the chromosomes are called genes.

What Are Genes? During the stages of cell reproduction
when the chromosomes are evident as distinct bodies they are
more or less compact. At other periods the chromatin is obvi-
ously granular, made up of particles called chromomeres. Thus far
the cell has refused to yield up other secrets of chromatin structure
to the cytologist, and the gene remains a theoretical, not to say
hypothetical, unit. Like the molecules, atoms, and electrons of
modern physics and chemistry, genes lend themselves admirably
to the explanation of phenomena of inheritance, and in the logical
results obtained on this basis lies the justification for the use of
the term.

Genes may l)e defined as chromatin units occupj'ing definite
parts of the chromosomes. Their function is to bring to expression
in the developing organism the unit characters for which they
stand, and to give rise to other similar genes which shall con-
stitute the link with the next generation.

Somatoplasm and Germplasm. This distinction between
genes and the characters resulting from their presence in the body
emphasizes a distinction which has attained great importance in

305

306 EVOLUTION AND GENETICS

the literature of genetics and evolution. It has been shown that
the course of development after fertilization results in the early
separation of those cells which develop into the body proper and
those which remain undifferentiated for the production of new
germ cells. In Ascaris, for example, it has been found that one
cell of the sixteen-cell stage develops into germ cells alone, while
the remaining fifteen develop into the body proper. Within the
germ cells the genes never gain expression; in the differentiated
cells of the body they reach their fullest expression.

The body has been termed the soma or somatoplasm, and the
germ cells are commonly called the germplasm. It is obvious that
in all cases of sexual reproduction the former originates anew
with every generation from the latter while the germplasm con-
tinues without evident change from generation to generation.
The association of inherited genes with their respective characters
is akin to this relation of soma and germplasm. Whether or not
those transmitted to the next generation may have been influenced
by the generation which has carried and nurtured them is one
of the great unsettled problems of heredity. Mechanism for such
influence has not been demonstrated, but neither has the lack
of such mechanism.

Unit Characters and Their Factors. The behaviour of allelo-
morphic characters in hybrids indicates that each is represented
by at least one factor. Evidently then a given unit character in a
homozygous individual is also represented by two genes which
segregate in the germ cells with the synaptic mates containing
them.

Complication of Mendelian ratios has disclosed several other
relationships which are valuable both from the practical and
the purely scientific points of view. These include linkage and
sex-linkage, crossing over, multiple allelomorphs and multiple
genes.

Linkage. Different unit characters, when combined in hybrids,
usuall}^ separate from each other in varying combinations in the
F2 generation. Thus the two characters of shape and color in
Mendel's peas, with their allelomorphs, give a 9:3:3: 1 dihybrid
ratio. Occasionally however two such characters remain nor-
mally associated and give only the monohybrid 3: 1 ratio. This
phenomenon was briefly mentioned in Chapter XV. It is called
linkage.

GENES AND CHARACTERS

307

Linkage is oasily explained by the chromosome theory of hered-
ity, for organisms may have many dozens or hundreds of unit

Fig. 175. — Back-cross of Fi female (out of black by vestigial) with black
vestigial male. B indicates normal color, b black, V normal wings and v
vestigial. An illustration of crossing over. (From Morgan et al, Mechan-
ism of Mendelian Heredity, with the permission of Henry Holt and Com-
pany.)

characters but only a few chromosomes. Each chromosome in
such cases must necessarily bear the genes of many different
characters, and since the chromosome and not the gene is the

308 EVOLUTION AND GENETICS

unit vehicle of transmission, all genes included in a given chromo-
some must necessarily retain their association with each other
during reproduction. Modification of this relationship cannot
occur without irregular behaviour of the chromosomes.

In the pomace fly, Drosophila melanogaster, many cases of
linkage have Ijeen observed. Morgan and his associates record
among these the case of l^lack bod}^ color and vestigial wings
with details of the behaviour of these characters when combined
in hybrids with their normal allelomorphs, gray body color and
normal wings.

Members of the Fi generation of such a cross are all gray and
long-winged. When the Fi males are crossed with recessive, i.e.,
black- vestigial, females, two types of offspring are produced.
One half are black-vestigial and one half gray-long. This propor-
tion is exactly the same as would lie expected if only a single
character were involved, and indicates that the genes for these
characters are located in the same chromosome. Linkage implies
no functional relationship between these genes.

The reciprocal cross of an Fi gray-long female with a black-
vestigial male, however, gives wholly different results. In this
case the offspring are of four kinds, black-vestigial, gray-long,
black-long, and gray- vestigial. The ratio in which these kinds
appear in experiments is not, however, even remotely similar to
the Mendelian dihybrid ratio. Black- vestigial and gray-long
appear in equal numl^ers, making up a total of 83 per cent of the
generation; the remaining 17 per cent are also equally divided
between the characters l^lack-long and gray- vestigial (Fig. 175).
This case involves an interchange of normally associated genes,
and implies also an interchange of substance between the chromo-
somes involved.

Crossing over is the term applied to this phenomenon and to
the accompanying chromosomal behaviour. The fact that inter-
change of the characters occurs in a majority of cases points to it
as the abnormal type of inheritance, which would also be true of
interchange of substance between synaptic mates. The equality
of distribution of the reciprocal combinations is also exactly what
would result from an interchange of chromosomal substance.

During synapsis the related chromosomes have been repeatedly
observed tightly coiled about each other. McClung states that
this coiling is readily visible at one stage of synapsis in the Orthop-

GENES AND CHARACTERS

309

tera. It is easy to see that the separation of these chromosomes
during the reduction division might be accompanied ])y rupture
at some point of crossing and the formation of new chromosomes,
each made up of parts of the two original synaptic mates (Fig.
176). If the genes for Hnkod characters arc located on the opposite
sides of such a break, as indicated by the letters AB and ab, a
reassociation would result giving Ab and aB linkage in all cells
where crossing over has occurred.

According to Morgan and his associates crossing over occurs
between many other linked characters in Drosophila, although in
varying percentages. It has not been observed in the male, but

[Bl

Fig. 176. — Diagram to illustrate the mechanism of crossing over.

occurs ordinarily in a constant percentage of germ cells in the
female, and is independent of the way in which the characters
are at first combined.

Multiple Crossing Over. Since the chromosomes during synap-
sis are twisted spirally al^out each other it is possible for a rupture
to occur at one point of crossing as easily as at another, and
perhaps at more than one point during the same cell division.
Such an interchange would result in new chromosomes made up
of alternate pieces of the two original synaptic mates. In the
case of a double crossover (Fig. 177) involving three genes, two
of the characters would remain linked as before, but the middle
one would be interchanged, so that different linkage groups would
be produced. Multiple crossing over would necessarily have a
different effect on the percentage of interchange between linked
characters, and in Drosophila it has been found adequate to
explain several departures from the expected Mendclian ratios.

310

EVOLUTION AND GENETICS

a

w

M

Br

{•

'

■ ]

tv

Br

M

Linkage in Other Organisms. Scientists have recorded Hnk-
age in other organisms. Among animals in which it is known to
occur may be mentioned poultry, pigeons, rats, mice, rabbits, silk-
worms and other insects; among plants, sweet peas, snapdragons,
primroses, corn, tomatoes, etc. The chromosome explanation would
lead to the belief that it is exceedingly common, but the accumula-
tion of evidence is a slow process. Moreover, Drosophila with only

four kinds of chromosomes is an
especially favorable species for the
illustration of this phenomenon,
since one quarter of its characters
may be associated with a single
chromosome. Man, with twenty-
four kinds of chromosomes, may
have only one twenty-fourth of his
characters in one linked group.

Localization of Genes. An im-
portant result of the theory of
crossing over, based on studies in
Drosophila, is the idea of localization
of the genes. Morgan and his co-

_,,__„. . •„ 0^ . authors have published chromosome

Fig. 177. — Diagram to illustrate r r, i i ■ i-

double crossing over. In a the maps of Drosophila mdicatmg the

positions of three genes, W, M theoretical location of genes for

and Br, are indicated When ^^^^^^ ^f characters on all four of

the chromosomes twist about .

each other between these genes its chromosomes (Fig. 178). These

as in b and break to form a new maps are based on a very logical

assoc-iation of parts, as in c W analysis of percentages of crossing
and nr remain in the same chro-
mosome while M is shifted to the over between the various characters.

other. (From Morgan et al., It is obvious that two filaments

cannot twist about each other in a
space less than their diameter,
although they can be twisted several
times within a relatively short length. Consequently if genes
are located close together on a chromosome, there is less reason
to expect crossing over between them than if they are remote
from each other. It is not illogical to suppose that the per-
centage of cases in which crossing over actually occurs is in
proportion to the distance between the genes.

Crossing over may occur between any characters of a linkage

Mechanism of Mendelian He-
redity, with the permission of
Henry Holt and Company.)

GENES AND CHARACTERS

311

group, hence a study of various combinations should indicate
the spatial relations between their genes in the chromosome. If,

13.7- - cross-veinleaa
16.7- - club

20.0
21.0

27 5

36.1
37.5
38.0

36J:

53.5

54.5_
66.5.
57.0
58.5
69.5

yellow, scute
lethal 7
broad
prune

white

notch

abnormal

echinus

bifid

ruby

cut
^singed

tan

33.0-|- vermilion

tiny-bristlea
^miniature
:~dusky
furrowed

43.0- - sable
44.4 - garnet

1- - small-iving
.rudimentary
■forked

, -bar

i-|- small-eye
fused

65.0- ■ cleft

-2.0-

0.0- - star

1.0

4.0- - expanded

9.0-1

11.0
13.0
14.0- -

22.5

28.0- -flipper
29.0- - dacha

83.
35.

44.0

46.5
46.7-
48.5-

62.5

0- -

58.0
61.0

65.0-
66.5-
67.0-

70.0-
71.0

73.5
75.0-

telegraph

aristaless

truncate

gull

pin k-uiing

streak

cream-b

ski
squat

minute-B
plack
'•jaunty
apterous

. purple

T7.0- - roof

105.0

85.0
88.0-

95.0-

97.5-

98.5-
lOO.Q.
101.0
103.0
101.5

105.E

safranin

trefoil

vestigial
telescope
••dash

lobe „,
minute 511

curved
^dachsou3

minute 2
humpty

purpleoid

^arc

plexus

lethal 11a

brown
■^blistered
•^■morula
^speck
'^balloon

CO- - roughoid

25.3

25.8-

82.0
33.5.
34.0

38.3.
38.5-
40.5
41.5-
43.0
44.0
45.0:
45.5

M n 53.8.

55.0

56.0

60.6

61.0

63.5-
65.6
67.5-
68.0-
70.0
72.0- -

sepui
hairy

divergent
cream III

-divarfoid

-scarlet

till
-dichaet

ascute
•^deformed.
~^maroun
- curled
-dwarf
^pink

,two-brisile
-spineless
^bithorax
^bithoraxoiOr

glass

kidney
^giant

spread

delta
^hairless

ebony
-band

cm

white oeellS

1.5- - rough
89.0- - beaded

95.4. . claret
95.7~|-^ minute

0.0-1-

bent
eyelesez\J.O

Fig. 178. — Chromo.some maps of Drosophila. (From Walter, after Sharp.)

for example, crossing over occurs between B and C in 5 per cent
of cases, and between A and C in 25 i)er cent, we may con-

312

EVOLUTION AND GENETICS

a

20

3J-54C

A

B
C

elude that C is five times as far from A as from B. Whether A and
B are on the same side of C or on opposite sides is not disclosed
by these data but if an additional test gives 20 per cent of
crossing over between A and B we may conclude that they are on
the same side (Fig. 179).

Some genes of Drosophila are shown to be widely separated,
yet cross over much less than is to be expected in such cases

when handled together. Double or multi-
ple crossing over accounts for these cases
without invalidating the data derived from
the study of more intimately associated
characters.

Sex-Linkage. This interesting type of
linkage results from the retention by sex
chromosomes of genes related with wholly
different characters. The inheritance of
sex-linked characters is, as the term sug-
gests, intimately associated with the sex
of the individuals in a given generation.
Our now familiar example, Drosophila,
affords numerous illustrations.

In this species one of the chromo-
somes, commonly called the x chromosome,
is associated in the male with a dissimilar y
chromosome (Fig. 168). The y chromo-
some apparently lacks the functional

powers of the x chromosome, although the
age of crossing over and , ,• , i • ,•

the location of genes in ^^o are synaptic mates, and is sometimes

the chromosomes. looked upon as a degenerate x chromosome.

In the female there are two x chromosomes.

The gene for the mutant character white eyes in Drosophila is
located in the x chromosome. This character is recessive to the
normal red eyes of the wild fly, consequently a white-eyed female
is a homozygous recessive. When mated with a red-eyed male
all females of the Fi generation are red-eyed, but all males are
white-eyed. The results in this and the F2 generation are graphi-
cally expressed in Figure 180. Fi males produce two types of
gametes, one containing the x chromosome bearing the gene for
recessive white eyes, the other bearing the y chromosome without
genes. The female produces two types of eggs, each with the x

Fig. 179. — Diagram to
illustrate the relation-
ship between percent-

GENES AND CHARACTERS

313

chromosome but one bearing the gene for red eyes and the other
for white. Any egg which receives the x chromosome of the
male will produce a female, therefore the F2 generation contains

r\ r\

Fig. 180. — Diagram of the cross between a red-eyed female and white-eyed
male of Drosophila melanogastcr. W indicates red and iv white. (From
Morgan et al., Mechanism of Mendelian Heredity, with the permission of
Henry Holt and Company.)

314 EVOLUTION AND GENETICS

one half red-ej^ed heterozygous females and one half white-eyed
recessives. Those eggs which receive the y chromosome from the
male develop into males, which are one half white-eyed and one
half red-eyed. In this case the condition of the male is determined
entirely by genes received from its mother.

The reciprocal cross of a homozygous red-eyed female with a
white-eyed male produces only red-eyed individuals. The females
are heterozygous, and so produce two types of eggs as in the
preceding example, while the males produce germ cells bearing
the X chromosome with the gene for red eyes and the y chromosome
with no genes, respectively. The F2 generation therefore contains
only red-eyed females, one half homozygous and one half hetero-
zygous, while the males arc one half red-eyed and one half white-
eyed (Fig. 181).

Sex-linkage is known in other organisms, including man, but
not all characters associated with sex are necessarily due directly
to sex-linked genes. Secondary sexual characters may result
from hormones produced by the gonads, and may appear in the
opposite sex under abnormal conditions, although there is no
reason to suppose that the chromosomes are modified by the
unusual stimulus.

Multiple Allelomorphs. Most known allelomorphic characters
appear in pairs which are independent of all other characters.
Exceptions have been discovered in Drosophila of which the
following data quoted from The Mechanism of Mendelian Heredity
by Morgan, Sturtevant, Muller and Bridges are an illustration:

"1. If a white-eyed male of Drosophila is mated to a red-eyed
female, the F2 ratio of three reds to one white is explained by
Mendel's law, on the basis that the factor for red is the allelomorph
of the factor for white.

"2. If an eosin-eyed male is mated to a red-eyed female, the
F2 ratio of three reds to one eosin is also explained if eosin and red
are allelomorphs.

"3. If the same white-eyed male is bred to an eosin-eyed female,
the F2 ratio of three eosins to one white is again explained by
making eosin and white allelomorphs."

The assumption that these three characters are allelomorphic
to each other carries with it as a necessary corollary the assumption
that their genes occupy the same position in the chromosome.
Data on crossing over between these and other characters bear

GENES AND CHARACTERS

315

out this conclusion. Under such conditions it is apparent that

only two of the characters can be represented in any one individual.

The characters that have given such a wealth of material to

the geneticist in Drosophila are mutations. There is no reason

Fig. 181. — Diapram of the reciprocal cross of Fip;. ISO, i. e., white-eyod female
and red-eyed male of D. melunogcidcr. W indicates red and w white. (From
Morgan et al., Mechanism of Mendelian Heredity, with the permission of
Henry Holt and Company.)

316 EVOLUTION AND GENETICS

to suppose that a character which has given rise to one mutation
is thereby prevented from giving rise to others. Multiple muta-
tions of a character, since they are allclomorphic to the original
character, would logically be allelomorphic to each other when
brought together in one individual. This explanation is in accord-
ance with all available data in the chromosome theory of descent.
It is adequate without further amplification, which may be found
if desired in the work cited above.

Multiple Factors. Although the laws of heredity thus far
considered have to do with characters determined by single factors,
there are many different cases on record which must be explained
on the assumption that more than one factor is concerned in the
production of a single character. Extensive data have led to the
discovery of four kinds of multiple factors, known respectively as
duplicate, cumulative, complementary, and supplementary.

Duplicate Factors. Shull discovered that the determination
of seed-capsule shape in shepherd's purse is accomplished by the
action of two pairs of factors. The seed capsules of this common
plant are usually triangular in outline but may be fusiform.
When plants of the two varieties are crossed, the Fi generation
has triangular capsules, and only one individual in sixteen in the
F2 generation reverts to the recessive spindle-shaped condition.
This is reminiscent of the dihybrid ratio, but an examination of
Figure 182 shows that the presence of a single dominant determiner
results in the appearance of the dominant character, and only the
homozygous recessive reveals its genotypic character. The be-
haviour of factors in this case is similar to that of all multiple
factors, but the appearance of the resulting characters differs in
this and the three following cases.

More than two pairs of duplicate factors may govern a character ;
their distinctive quality is the similarity of their effect, no matter
how many of the dominant determiners are present.

Cumulative Factors. Those factors which bring a character to
expression in the soma in proportion to the number of dominant
determiners present are called cumulative factors. The case of
Nilsson-Ehle's wheat is an old and excellent example.

Nilsson-Ehle found that a race of wheat with red grains and
a race with white grains, when crossed produced an Fi hybrid with
intermediate pale red grains. In the F2 generation very few white
grains appeared, so that the color was obviously not due to a

GENES AND CHARACTERS

317

single pair of determiners. In one experiment, according to
Babcock and Clausen, seven families with a total of 440 plants
produced white seeds on only one plant.

Omitting further details, it has been found that color in this
case was determined by three different factors. The genotypic
formula for the parents may be indicated as AABBCC and aabbcc,

Q (/-^Cp Cd cD

T Y I __ Y

CD-

Cd-

cD-

cd-

CD. CD

Cd-CD

1:0

cD.CD

1:0

cd 'CD

15:1

CD-Cd

1:0

Cd-Cd

1:0

cD-Cd

15:1

cd- Cd

CD'CD

1:0

Cd-cD

15:1

cDcD

1:0

cd-cD

3:1

f

CD. cd

15:1

Cd'cd

3:1

3:1

cd ■ cd

0:1

Fig. 182. — Punnett square with diagrammatic figures to illustrate the inheri-
tance of seed capsule shape in Bursa (B. bursa-pasloris x B. heegeri). The
two duplicate factors are represented l)y oblique ruling in opposite direc-
tions, and the number of factors present by the spacing of the lines. The
ratios indicate the expectation in the F3 generation from self-fertilized
individuals. (After ShuU from Babcock and Clausen's Genetics in Relation
to Agriculture by permission McGraw-Hill Book Company, Inc.)

and that of the Fi generation as AaBbCc. In the F2 generation
the recombination of these factors occurs in sixty-four waj'S
according to the law of trihybrids, but only one out of the sixty-four
has the homozygous recessive organization which produces white
grains. All heterozygous individuals, such as AABbCc, AaBbcc,
aaBbcc, etc., produce red grains, varying in depth of color according
to the number of dominant factors present. Out of these red

318

EVOLUTION AND GENETICS

n

m

m

m

m

m

m

#

m

#

m

#

m

#

m

#

m

#

m

#

m

#

grains only one of the sixty-four can be of the depth of color of
the original dominant parent. The construction of a Punnett

square for this case will indicate that
there are only six degrees of red pres-
ent, in addition to the one white
individual, and that most of the sixty-
four chance combinations fall within
the three intermediate groups of the
seven (Fig. 183).

Ear length in rabbits was studied
by Castle with similar results. The
cross between lop-eared and short-
eared parents gives an Fi generation
with intermediate ears, while the F2
generation produced no individuals
showing the ear length of either of
the grandparents. It has been pointed
out by various writers that if four
factors are involved in the determina-
tion of car length, only one individual
out of two hundred and fifty-six in the
F2 generation can be expected to re-
semble either grandparent, while with
six factors such resemblance can occur
only once in 4096 times! As Castle
concludes: "it would be remarkable if
under such conditions the extreme size
were ever recovered from an ordinary
cross."

Inheritance of this type has been
called blending inheritance, but it is
to be distinguished carefully from the
type of blending referred to in Chap-
ter XV. The pink four-o' clocks cited
as an example in that case are due to
the fact that neither the red nor the
white color factor dominates the other
in heterozygous individuals, and the
result is a complete expression of the Mendelian 1:2: 1 monohy-
brid ratio. When cumulative factors are involved, however, the

m

m

m

#

#

#

m

m

m

m

#

#

i-

-f

-f

#

#

-I-

-h

+

G R 4 3 2 1 0
Fig. 183.— The distribution of
the .sixty-four possihihtie.s in
the F2 generation when three
similar determiners act to
produce a given character.
The numbers indicate the
number of determiners pre-
sent in the individuals repre-
sented by the column above.
(From Walter.)

GENES AND CHARACTERS

319

CR

Cr

cR

cr

CR

Cr

intermediate individuals are not all the same, but fall into graded
classes whose number depends ujjon the number of factors. The
F2 ratio is always a modification of some Mendelian ratio above the
monohybrid.

Complementary Factors. Two factors that have no effect when
they occur separately l)ut produce a (U^finite character when
they are brought together in the same individual are said to be
complementary.
An example often
cited is that of
Bateson's white
sweet peas and
their red offspring.

The two factors
in this case may
be interpreted as a
color factor C and
a red factor R.
The white parents cH
are C C r r and
ccRR, individuals
which produce
gametes bearing
the genes Cr and

cR respectively, so jtk^, 184.— Diagram showing the behaviour of com-

that the Fi hybrid plementary factors. The genotypic ratio is the same

is heterozvffous for ^^ '" ^*^'^" ^''^ ^^ ^ normal dihybrid but the pheno-
, , , « , , , typic ratio is 9 : 7.

both lactors. Its

CcRr organization combines the two complementary factors and

results in the production of red flowers. The F2 generation

of this hybrid corresponds genotypically to a dihybrid, but since

the presence of both factors is necessary for the production of

color, all combinations of C and R with recessives of the opposite

type included in the second and third groups of the 9:3:3:1 ratio

are white, and a 9:7 ratio results (Fig. 184).

Similar factors have been discovered in corn, rabbits and other
organisms.

Supplementary Factors. This last group consists of those
which condition the expression of others without being essential
to the production of the character in question. In the sweet peas

cr

CR
CR

red

Cr
CR

red

cR
CR

red

cr
CR

red

CR
Cr

red

Cr

Cr

white

cR
Cr

red

cr
Cr

white

CR
cR

red

Cr
cR

red

cR
cR

white

cr
cR

white

CR
cr

red

Cr
cr

white

cR
cr

white

cr
cr

white

320 EVOLUTION AND GENETICS

just described, for example, color is produced by the factors C
and R, but a factor B occurs in some sweet peas which modifies
the color by adding l)lue.

Rabbits have a complex series of color and pattern genes which
illustrate both supplementary and complementary factors. In
them the factor C is necessary for the production of color. A cc
individual is always an albino, although it contains factors for
yellow, black and brown pigment. The albino may also carry a
factor A, or agouti, which determines the distribution of pigments
in individual hairs; this is characteristic of the cotton-tail rabbit
and is known as the wild gray type. A black rabbit crossed with
an albino bearing the agouti factor produces offspring of the
wild gray type. Other factors influence the depth of color and
its distribution on the body (Fig. 185).

Lethal Factors. Factors have appeared in a few cases whose
presence in the proper combination results in death. Among
animals these factors are usually evident through reduction of
the usual number of progeny but among plants the congenital
absence of chlorophyll is a lethal factor which does not interfere
with the development of the seedling until food stored in the seed
has been exhausted. Several kinds of plants, including corn, are
known to produce these individuals. The character is in all
cases, whether plant or animal, a recessive which can be perpetu-
ated through heterozygous individuals.

Several lethal factors have been discovered in Drosophila. The
effects of a sex-linked lethal character in these flies is shown in
Figure 186. Since the male bears only one x chromosome it
cannot be a carrier, but dies if recessive for the lethal character.
When a female carrier, the only individual capable of perpetuating
such a character, is mated, two-thirds of her offspring are females
instead of one-half as under normal conditions, and of these
females one-half are normal and one-half carriers. In the figure
L indicates the normal condition and 1 the lethal recessive.

The Effect of X-rays on Genes. In connection with the study
of heredity in Drosophila Professor H. J. Muller has recently
shown that the treatment of flies with X-rays has a direct effect
on the behaviour of hereditary characters. His experiments prove
that the rays affect germ cells at any stage, even including the
spermatozoa contained in the seminal receptacle of the female.
Mutations appeared in the flies developed from gametes so treated

GENES AND CHARACTERS

321

Constant
Genes

Alternative
Genes

Gametic
Formula

Phenotypic Character

when Crossed with

the Same Kind of

Gametic Combination

1

2

3

4

5

6

7

8

B

Y

C

E

I

U

A

AUIEC

YBBr

Gray

a

aUIEC

YBBr

Black

u

A

AuIEC

YBBr

Gray spotted

a

auIEC

YBBr

Black spotted

i

U

A

AUiEC

YBBr

Blue-gray

a

aUiEC

YBBr

Blue (Maltese)

u

A

AuiEC

YBBr

Blue-gray spotted

Br

a

auiEC

YBBr

Blue spotted

e

I

U

A

AUIeC

YBBr

Yellow (with white belly
and tail)

a

aUIeC

YBBr

Sooty yellow (with yellow
belly and tail)

u

A

AuIeC

YBBr

Yellow spotted

a

auIeC

YBBr

Sooty yellow spotted

i

U

A

AUieC

YBBr

Cream

a

aUieC

YBBr

Pale sooty yellow

u

A

AuieC

YBBr

Cream spotted

a

auieC

YBBr

Pale sooty yellow spotted

Fig. 185. — The factor hypothesis applied to colors of rabbits. (From Walter.)

Explanation: Br=a gene acting on C to produce brown pigmentation.

B = a gene acting on C to produce black pigmentation.

Y = a gene acting on C to produce yellow pigmentation. The three genes, Y, B, Br, are present
in every rabbit gamete and up to date have not been separable as independent unit
characters, although they have been separated out in guinea-pigs and mice. There are
no brown rabbits, because black always goes linked with brown, covering the brown
factor. Yellow rabbits result, as explained below, through the action of factor e.

C = a common color gene necessary for the production of any pigment. It was discovered in

190.3 by Cu^not.
c=the absence of C which results in albinos, regardless of whatever pigment gene may be
present. By changing C to c, sixteen kinds of albinos would be added to this catalogue,
an addition of one phenotype and sixteen genotypes, all looking alike but breeding
differently.

E = a gene governing the extension of black and brown pigment, bid not of yellow.
e=the absence of extension or restriction of black and brown pigment to the eyes and the
skin of the extremities only, while yellow remains extended and visible. Demonstrated
by Castle in 1909.
I = an intensity gene which determines the degree of pigmentation. It can be transmitted

independently of C through an albino. Discovered by Bateson and Durham in 1900.
i = the absence of intensity or dilution. Dilute black = blue. Dilute yellow = cream. Dilute
gray = blue-gray.

U = a gene for uniformity of pigmentation or "self-color" discovered by Cu6not in 1904.

u= the absence of uniformity which results in spotting with white.

A = a pattern gene for agouti, or wild gray color, which causes the brown and black pigments
to be excluded from certain portions of each hair, resulting in the gray coat. When
present in the rabbit it is also associated with white or lighter color on the under sur-
faces of the tail and belly. It was demonstrated by Castle in 1907.
a = the absence of the agouti or pattern gene.

322

EVOLUTION AND GENETICS

at a rate up to 150 times the normal. Among the mutations
produced from treated flies there were many lethal characters
and recessives, but such familiar things as multiple allelomorphism
were also produced.

The experiments are of great interest in evolution because they
show definitely that genes may respond directly to conditions of

/^

r\

L

1

\y

<J

r\

o

\J \J

Gametes

/^

r\

L

1

v_<'

U

r-\

(O

KJ

Fig. 186. ^Diagram to show the inheritance of a lethal factor in DrosopMla.
1 indicates the lethal factor and L its normal allelomoi-ph. The male at the
right in the Fi, generation fails to develop so that the ratio of sexes in such
a generation is two females to one male.

the external environment. The appearance of reverse mutations,
moreover, establishes the reversibility of slight evolutionary
changes.

Genes in the Development of the Individual. Beyond these
facts of inheritance, showing close association of characters in
the individual and genes in its chromosomes, we may well inquire
how the genes act during ontogeny to condition the differentiation

GENES AND CHARACTERS 323

of somatic cells. Lillic summarizes modern knowledge of this
subject in a recent article and concludes that "we have no present
working hypothesis effective in this most fundamental aspect of
the life history."

According to modern views the complex of genes which is present
in the zygote is carried into all cells of the body. The differentia-
tion of the cytoplasm which gives rise to the multitude of special-
ized adult cells is therefore not accompanied by nuclear differentia-
tion of equal degree. In spite of this fact there is evidence that
the genes are active at various periods of life in connection with
cytoplasmic differentiation for adaptation to definite environ-
mental conditions. The ability to become tanned, for example,
is inherited although it does not become operative until the proper
environmental stimulus is received.

Goldschmidt has attempted to explain this interaction by two
theories, one of which assumes that the genes are so conditioned
that they enter into activity at a definite rate and time for each,
while the other suggests that the genes act only with specific
cytoplasmic materials. As Lillie points out, the latter assumes
cytoplasmic differentiation as a condition for activity of the
genes, yet differentiation is what it attempts to explain. The
former theory also appears weak in the light of modern embryology.

Lindsey has suggested that the action of genes is not limited
to the cells containing them, but that all genes of a kind in the
body, through the coordination which is evident in more obvious
phj^siological processes, act together to bring a character to expres-
sion. Some facts are apparently contradictory to this theory but
not necessarily inimical to it.

A detailed discussion of these matters has no place in such a
work as this, especially since no theory is generally accepted and
none is more than a partial explanation. The facts given are
sufficient to illustrate our ignorance of the connection between
genes and the characters which they produce. So far we must
content ourselves with the knowledge that both exist and that
the appearance of characters is in accordance with definite laws
which are harmonious with the behaviour of the chromosomes and
the genes.

Summary. Characters of organisms are determined by definite
things in the body, called factors. These factors are based upon
hypothetical bodies called genes, located in the chromosomes, and

324 EVOLUTION AND GENETICS

occur in pairs except in cells which have only the haploid number
of chromosomes. Here a single gene may represent a character.
Characters are said to be linked when their genes occur in the
same chromosome and therefore usually remain together. Linkage
relations may be modified by interchange of material between
two chromosomes of a kind during the reduction division; this
phenomenon is called crossing over, and gives evidence of definite
localization of the genes. When genes are located in the sex
chromosomes the characters which they control are said to be
sex-linked. In addition to characters determined by a single pair
of factors many are known which involve two or more pairs.
These multiple factors behave in the normal Mendelian manner,
but the phenotypical ratios differ from those produced by the
same number of independent factors. The difference depends
upon the number of factors and the nature of their interaction.
The way in which genes act to produce characters in the individual
has been the subject of some speculation but is still unknown.

REFERENCES

Castle, W. E., Heredity of Coat Characters in Guinea-Pigs and Rabbits, Pub.

Carnegie Inst., No. 23, 1905.
Castle, W. E. and Wright, S., Studies of Inheritance in Guinea-Pigs and

Rats, Pub. Carnegie Inst., No. 241, 1916.
Morgan, T. H. and Bridges, C. B., Sex-Linked Inheritance in Drosophila,

Pub. Carnegie Inst., No. 2.37, 1916.
Babcock, E. B. and Clausen, R. E., Genetics in Relation to Agriculture,

2nd edition., 1927.
Morgan, T. H., Sturtevant, A. H., Muller, H. J. and Bridges, C. B.,

The Mechanism of Mendelian Heredity, revised edition, 1922.
Morgan, T. H., Cowdry's General Cytology, Section XI, 1924.

CHAPTER XVIII

THE DETERMINATION OF SEX

The association of chromosomes with sex has already been
mentioned in Chapter XVI. For many years their behaviour has
been regarded by many scientists as an adequate explanation of
sex determination, although anomalies of various kinds have
required ingenious explanations to harmonize them with the facts
of chromosome behaviour.

At present the state of experimental work in this field enables
us to say that the chromosomes are definitely associated with
sex determination under ordinary conditions and that sex is there-
fore inherited in many cases as a Mendelian character; we now
know, however, that this is only a part of the story and that
other factors, possibly many others, are concerned with sex deter-
mination. These other factors have been discovered in some
cases and placed under control with such accuracy that the sex
of an individual, not merely superficial but also fundamental
characters, has been determined independently of its chromosomal
complex.

What Is Sex? Among the fundamental principles of biology
reproduction is recognized as a primary characteristic of living
matter. In the simplest unit, the cell, reproduction is a common
function and in complex organisms every individual plays some
direct part in the production of new individuals unless it is special-
ized for other purposes in a colony of different kinds of individuals.

In its simplest form, the process is one of subdivision into two
new individuals as nearly as possible like each other and like
their common parent. This process of subdivision is carried to a
point where only a small portion of the parent buds off to form a
new individual ; the parent does not lose its identity in the process,
but may continue its normal vital processes even while it gives
rise to a new generation. It is only a step from this process of
budding to the development of specific reproductive cells or
spores, each of which after separating from the parent is potentially
a new organism. Between this and sexual reproduction there is

325

326

EVOLUTION AND GENETICS

a gi'eat gap, for the formation and union of gametes demand a
differentiation of reproductive cells which the simpler processes
of asexual reproduction do not require.

In the simplest forms of sexual reproduction all individuals are
apparently the same and the gametes which they produce show

Fig. 187. — Life history of Sphaerella lacuslris. a, b, c, d, asexual cycle; a, w, x,
y, z, sexual cycle, a, dormant cell; the protective cyst has ruptured to per-
mit the escape of the protoplast; b, division of the protoplast to form four
spores (c), each of which grows and takes on the adult form (d). This
process may be repeated, but ultimately the cells become dormant again.
Under some conditions the protoplast divides as in iv, forming 32 or 64
small cells. These are gametes. They escape (x) and fuse in pairs (y) to
form a zygote (z) which develops a cyst as in a. (From Woodruff.)

THE DETERMINATION OF SEX 327

few or no differences. The gametes of Sphaerella, for example,
are both motile cells of the same form, but they are not potential
individuals for two must unite to bring about the development
of a complete organism (Fig. 187). In higher organisms the
female gamete becomes more and more specialized as a passive
cell in which food for the developing embryo is concentrated,
while the male gamete is a highly specialized motile cell, unham-
pered by stored food and therefore able with maximum efficiency
to seek out the egg cell. The difference may be extreme, yet as
we have seen in Chapter XVI, the cells are potentially equal in
their contributions to the new generation.

Among the more primitive organisms and among some higher
species, differentiation of the individuals producing the two kinds of
gametes is conspicuously lacking, even to the extent of hermaph-
roditism. In this condition the same individual produces both
male and female germ cells. In the higher animals, however,
and in some of the higher plants, complete separation is accom-
plished into male and female sexes each of which produces only one
kind of gamete. The sexes are also often conspicuously differ-
entiated in superficial character. These superficial differences may
be without evident connection with the essential sexual processes.

It is evident that the one fundamental difference between the
sexes is the ability to produce the different kinds of germ cells,
yet when we consider that sex cells are, in their essential con-
stituents, the same except in connection with sex determination,
the difference seems entirely an adaptation to specific conditions
of reproduction. The demonstrated occurrence of sex reversal
in the domestic fowl and frogs further complicates the problem.
Since some birds and frogs have been both father and mother
during their lifetime, sex seems biologically much more a matter of
convenience than of necessity.

The one great contribution of sexual reproduction for which
there is no known substitute is the reassortment of characters
with each generation through the combination of those present
in two individuals to produce a third. To conclude with this
statement is to confess our entire ignorance of the development
of sex. We can only point out the existing gradations in living
organisms and the one biological phenomenon which seems to
be an adequate reason for sexual reproduction. It remains for us
to consider the peculiarities of the germ cells and of the individuals

328 EVOLUTION AND GENETICS

producing them, and the conditions which have been found to
play a part in the behaviour of these things.

Individuality of the Gametes. In most cases of sexual repro-
duction the function of each gamete is wholly complementary to
that of the other. The cytology of these cells suggests that if
the nucleus is, as supposed, the essential structure for the develop-
ment of hereditary characters, the presence of a complete haploid
set of chromosomes would enable either kind of gamete to develop
into a complete organism. The fact that the cytoplasm is necessary
for the differentiation of the somatic structures implies a limitation
which actually does prevent the development of male gametes
alone, but inheritance under certain experimental conditions and
the inheritance of sex in special life cycles show that each gamete
is fundamentally and potentially an individual.

The fertilization of enucleate eggs of sea urchins by the sperms
of other species and their subsequent development of the characters
of the male parent shows that the male germ cell is equal to the
female except in its reduced and specialized cytoplasm. This
must be regarded as incidental to the conditions of reproduction,
and not as a fundamental difference in reproductive power. It is,
of course, very effective in limiting the capacity of highly developed
male gametes.

Parthenogenesis. Development of the unfertilized ovum, or
parthenogenesis, occurs normally in many insects and other
Arthropoda, in some worms and possibly in other phjda. Arti-
ficial parthenogenesis has been brought about by the application
of various stimuli such as unusual chemical contacts and mechani-
cal manipulation. The production of males would naturally result
from parthenogenesis when this is the heterogametic sex, l^ut in
some cases females are produced.

Parthenogenesis in the Honey-Bee. Parthenogenesis here is very
well known because of the economic importance of the species. It
has been proved that all drones, or male honey-bees, are produced
from unfertilized eggs, while the queens and workers, both females,
are developed from fertilized eggs. A fertilized egg can develop into
either queen or worker according to the food received by the grow-
ing larva and queen breeders have produced intermediate individ-
uals. Some of these are said to have mated and deposited fertile
eggs, although scarcely distinguishable from the workers, which
never mate.

THE DETERMINATION OF SEX 329

Gametogenesis occurs normally in the queen bee, consequently
her eggs contain the haploid number (16) of chromosomes. It
follows that the drones developed from these eggs have only the
haploid number although the queens and workers have the normal
diploid complex of thirty-two chromosomes. The production of
normal male gametes demands some compensating phenomenon,
which is found in the abortive reduction division. At this stage
all of the chromosomes pass into one spermatocyte so that all
spermatozoa receive the normal haploid number, and at fertiliza-
tion establish the normal diploid complex of the females.

The Life Cycle of Aphids. This life cycle demands another type
of compensation in behaviour of the chromosomes. Sexual individ-
uals appear at intervals during the cycle, but they are separated by
long successions of parthenogenetic females. The stem mothers
hatch in the spring from fertilized eggs which have passed the
winter; no males appear at this season. The stem mother heads the
succession of parthenogenetic females, and in the fall males and
females appear which produce the winter eggs. The diagram
(Fig. 188) shows the behaviour of the chromosomes accompanying
these steps. The parthenogenetic eggs from which the males and
females develop are produced in one case by a normal mitotic
maturation division which results in one ovum and one polar
body. The egg in this case contains the same chromosomal com-
plex as the cells of the parent. In the other case one entire x
chromosome is extruded in the polar body, and consequently the
egg receives the odd number of chromosomes characteristic of the
male. In order that females alone may develop from the winter
eggs, maturation in the male is accompanied by degeneration of
the secondary spermatocytes which contain no x chromosome.
All spermatozoa therefore contain an x chromosome and the dip-
loid number is restored in every case by fertilization.

Elimination of the Male. It is evident that the parthenoge-
netic production of females, which occurs in some insects and roti-
fers, might in extreme cases result in the limitation of reproduction
to this method. In such an Amazonian species males would be
superfluous, and this is a possible explanation for the failure of
biologists to discover the males of certain rotifers. The evidence
available is not, however, conclusive.

Artificial Parthenogenesis. The occurrence of natural par-
thenogenesis in sexual organisms suggests that the stimulus of

330

EVOLUTION AND GENETICS

fertilization need not be an extreme one. Loeb's researches in
this field showed that a slight increase in the potassium or sodium
chloride concentration of sea water was enough to bring about
parthenogenetic development of the eggs of sea urchins and

Soma of stem-mother and
migrant females (her daughters)

Parthenogenetic eggs

of migrant females

giving rise

to

Anaphase of the single
maturation division

Primary
Spermatocyte

Secondary
Spermatocytes

i'ene'

Spermatids
a

Fig. 188.— The chromosome cycle in the parthenogenetic insects, aphids and

phylloxerans. (From Waher.)

annehd worms. He further records that the moderate agitation
caused by transferring the eggs of some starfishes from one dish
to another was suflacient to cause cleavage without fertilization.
Such cases could be multiplied from the findings of other investi-
gators, but there is little variety in them. Development proceeds
to various degrees, but in Loeb's researches the chemical stimulus

THE DETERMINATION OF SEX 331

which brought aljoiit abnormal parthenogenesis was usually so
severe as to cause early death.

The Purpose of Sexes. The foregoing data show that the
different germ cells are fundamentalh^ similar in so far as the
fact of reproduction is concerned, and different only in accord
with the parts which they play in the details of its accomplishment.
The sexes which produce them cannot, therefore, have different
capacities with respect to the fundamental fact of reproduction,
but only with respect to the degree of specialization of the process
in the species to which they belong.

This being the case, it is apparent that the differentiation of
the sexes should be commensurate with their roles in normal
reproduction. The similarit}' of sexes in the lower organisms and
the occurrence of hermaphroditic animals are therefore not sur-
prising. Nor is the extreme diiferentiation of sexes in the higher
animals and the concomitant necessity for sexual reproduction in
a great majority of species any more than a normal development
in harmony with the high degree of differentiation characteristic
of these organisms.

Differentiation of Sexes. When a phenomenon is constant in
occurrence and apparently necessary to the existence of the species
some provision for its perpetuation is to be expected, for it is
quite in harmony with the fundamental principles of biology.
The chromosome theory of sex determination therefore deals with
facts which seem entirely normal. It is not entirely clear how
the presence of two chromosomes in the cells of an individual
can cause it to belong to one sex while the presence of either
alone would cause its development into the opposite sex, but the
association of chromosomes with sex is well established.

The fact that the allosomes in one sex are the same as those
in the other indicates one important consideration. While we are
forced to the conclusion that allelomorphic characters are based
upon qualitatively different genes, there is nothing to indicate
that the sexes are in any way allelomorphic. The chromosomal
difference between them is apparently quantitative. Under ordi-
nary conditions it has a very definite effect upon the differentiation
of the sexes, but we must recognize that the same active con-
stituents are present in the allosomes of the two sexes. This
interpretation is favored by the occurrence of hermaphrodites and
abnormal individuals intermediate in sex.

332 EVOLUTION AND GENETICS

Hermaphrodites are individuals which bear functional gonads
of both sexes, and produce both male and female gametes. It
does not follow that they are capable of self-fertilization, although
this is sometimes a possibility and sometimes apparently the
normal method of reproduction. Hermaphroditism is common
among the lower phyla. Sponges and coelenterates, flatworms
and annelids include many striking examples.

Since the individual can have only one chromosome complex,
the occurrence of these bisexual forms is absolute proof that
chromosomal difference is not an essential of sex. The same
processes that produce the gametes in different individuals produce
them here in one, and the same result of recombination of parental
characters is attained as when the sexes are separate.

Abnormalities of many kinds occur in nature or can be pro-
duced by artificial means which further substantiate this view.
Wherever they occur in nature they are apparently due to accidents
of development or to abnormal behaviour of the germ cells.
Experimentally they have been produced by operative methods
and by variation of the environment.

Gynandromorphs, sometimes incorrectly called hermaphrodites,
are occasionally produced among insects of various orders and
among other animals. They are individuals which display the
characters of both male and female in different parts of the body,
and are incapable of carrying on the normal reproductive functions
of either sex.

Gynandromorphs are usually bilateral, one half being male and
the other female. In species with distinctly different sexes this
results in very striking contrasts as shown in Figure 189. This
distribution of sex characters is accompanied ])y a similar asym-
metry of chromosome distribution, the female half having the
female complex and the male half the male complex. How such
an abnormality occurs it is difficult to determine but one logical
explanation is available.

According to this interpretation the first cleavage of the ovum
may be hastened by some force so that two groups of chromosomes
are present at the consummation of fertilization. The union of
the male pronucleus with one of the female pronuclei would
establish the diploid complex in one half of the individual while
the other half would have only the maternal haploid complex.
The diploid complex might readily contain two allosomes and so

THE DETERMIXATION OF SEX 333

be of the homogametic sex while the other side would necessarily
be of the opposite sex, since it could not have more than one
allosome.

Mosaic gynandromorphs in which the characters of the two
sexes are distributed at random through the body arc baffling.
The one valid conclusion to be derived from them is found in
McClung's statement that ''like other bodily functions ... sex

Fig. 189. — A gynandromorph of PapUio turnus Linn. The left half of the
insect is of the normal yellow male sex while the right half is of the black
female form glaucus. (Through the courtesy of the Philadelphia Academy
of Sciences.)

is a matter of cell activity, and must ultimately be explained in
terms of cell performance."

The known facts concerning gynandromorphs have led to the
conclusion that in insects at least, the secondary sexual characters
are determined directly by genes. Any allosome, however, may
contribute definitely to the determination of either sex. The
chromosomal difference between the sexes is quantitative, as has
already been pointed out, unless we conclude that the y chromo-
some is an active determiner of sex and such a conclusion is not

334 EVOLUTION AND GENETICS

in harmony with the available data. The y chromosome is not
constant in occurrence ; inheritance of sex is the same whether the
y chromosome is present or absent, and finally sex-linked charac-
ters are transmitted by the x chromosomes. In view of these
facts it is impossible to accept the conclusion that genes of the
allosomes alone are directly responsible for the development of
the secondary sexual characters even in the insects.

Although no adequate explanation of the entire phenomenon
of gynandromorphism is available, we cannot fail to see that the
chromosomes are not alone in their effect upon the organism as
determiners of sex.

The secondary sexual characters of birds and mammals are
known to be dependent for their development upon hormones
secreted by various ductless glands. The gonads, although they
are more evidently cytogenic glands, also have the function of
secreting hormones which influence the development of the body,
and the other endocrine glands, such as the pituitary, may be
related so closely to sex as even to affect the development of the
genitalia. Obviously any effect produced by a hormone is likely
to be expressed in all possible parts of the individual, since the
hormones are distributed by the blood stream. The activity of
these substances would seem to be inimical to the development
of gynandromorphs.

Insufficiency of hormones results in complete or partial failure
of the individual to develop the characters dependent upon them.
Since the characters of the sexes are in many cases antithetical
expressions of the same structure, the effect of insuflficiency is
often an approximation of the secondary sexual characters of the
opposite sex.

An extreme degree of insufficiency is caused by removal of
the gonads. Since this is a common practice in the breeding of
animals for food its effects are well known. In all domestic
animals it results in the development of males which resemble
females instead of normal males. Removal of the ovaries is
almost without effect. Removal of the testes of most fowls likewise
results in the development of some female characteristics, but
early removal of the ovaries results in the almost complete develop-
ment of male characters in the female.

Males of Sebright bantams have tail feathers similar to those
of most hens, but when the testes are removed they develop the

THE DETERMINATION OF SEX 335

long curved tail feathers characteristic of most cocks. In this
case a type of cell usually found only in ovaries is present in the
testes. It apparently produces a hormone which influences the
development of the tail feathers and the removal of the testes
or ovaries in which it occurs frees the individual from this influence.

When we consider that the males of mammals are heterogametic
while in the birds this condition is true of the opposite sex, the
contrasting effects of castration are seen to be quite in harmony
with the relationship of chromosomes and sex.

Freemartins afford another striking illustration of the effect
of hormones. They are infertile female cattle, twin-born with
males. Lillie has concluded in his studies of twinning that
the condition is due to the mingling of l)lood in the two foetal
circulatory systems through their close approximation in the
maternal uterus. The hormones produced in either body would
be carried through l)oth by the fusion of the systems. Those
produced in the male appear to inhil)it the normal development
of the female, with resulting sterility, although they are not
capable of bringing about complete reversal of sex.

The indifferent stage of development through which all indi-
viduals pass, and the fact that the primary sexual organs of the
two sexes show definite homologies still further indicate that l^oth
male and female have a common l^eginning. The abnormal
cases cited show that residual capacity for the expression of one
sex may remain even after the complete differentiation of its
opposite. In these cases it is evident also that the differentiation
of a heterogametic sex is an advance over that of a homogametic
sex.

Sex Reversal. Man has not yet succeeded in determining the
sex of animals under his control, but the facts so far presented
suggest that such control might be possible, and records of sex
reversal in fishes, amphibia and fowls show that the chromosomes
are not an insurmountable obstacle. One of the most striking
instances of sex reversal on record is that published by Crew in
the Journal of Heredity. This case records that an adult hen
from whose eggs normal chicks had hatched began to show the
secondary sexual characters of the male and later became the
father of chicks (Fig. 190). As in other cases of apparent sex
reversal in fowls and pigeons this change was due to the destruction
of the ovary by tuberculosis. Crew states that fresh growths of

336 EVOLUTION AND GENETICS

sex-cords from the peritoneum enter the ovary in the fowl, and
that the absence of growing oocytes permits the development of
these cords into male structures. For a detailed consideration
of sex reversal Crew's paper should be consulted.

Schaffner's Plants. The conclusive results obtained by Schaff-
ner in his experiments on the determination of sex in plants must
be regarded as among the greatest contributions since the discovery
of the sex chromosomes. These experiments have been conducted

Fig. 190. — A Buff Orpington hen tliat became a cock. Two stages early
and late, in the reversal of sex. This hen laid eggs from which normal chicks
were hatched, and later became the father of normal chicks. (From Shull's
Heredity. McGraw-Hill Book Company, Inc., by permission.)

chiefly on hemp (Cannabis saliva L.), but corn and various other
plants have also been used. Plants have been reared under various
conditions of environment and the results show that it is possible
to determine the sex of the individual at will by changing the
food supply, or to bring about reversal in a plant which has
previously developed an inflorescence of one or the other sex.

One of the most striking of Schaffner's experiments was the
production of Siamese twins of opposite sex in the jack-in-the-
pulpit (Arisaema triphyllum L.). The result was obtained by
very careful control of the food supply of the two individuals.
Since the plants developed from a forked corm the control could
not be accomplished through quantitative treatment but only
through artificial reduction of the leaf surface of one plant and
such other factors as might influence its capacity for photosjm-
thesis and food storage. In spite of the fact that Arisaema twins
are normally of the same sex or shnilarly monoecious, the result

THE DETERMINATION OF SEX

337

■-%

^

Fig. 191. — Siamese twins of Jack-m-tlie-pulinl [Ariaaema Iriphyllum) . Male,
left; female, right. The smaller size of the male plant is the only character
readily seen in the figure. (Courtesy of Professor John H. Schaffner.)

338 EVOLUTION AND GENETICS

of the experiment was the production of distinctly male and
female individuals, still joined by the same forked corm (Fig. 191).

Conclusions. As we consider the entire succession of organic
forms it is evident that the more highly organized a species may
be, the less directly dependent it is upon the conditions of environ-
ment. The lower organisms and even such complex animals as
the insects are directly influenced by temperature but the birds
and mammals have surmounted this limitation. Because they
are homothermous they can withstand a great range of external
temperature without cons})icuous modification of their activities.
In man this process culminates, for through his intelligence he
exerts control over all phases of his environment.

To a certain extent this independence of the organism is due
to the fact that the complex body itself establishes an environment
for its several parts through which the exigencies of life are tem-
pered. This internal environment has become a definite part of
the heritage, since it is in reality the result of correlation of herit-
able structures. Since the chromosomes are the carriers of heredity
it seems wholly logical that all inherited characters should be in
some way connected with them, and the internal environment
may be regarded as a product of chromosomal activity.

No characters, however, are developed unless the proper con-
ditions of environment favor the normal expression of the chromo-
somes. An animal without air cannot live; a nucleus without
cytoplasm is equally incomplete. Since these conditions are
resident in the body in the case of characters which appear auto-
matically in every generation it is not a simple matter to determine
what they are, but it is logical to believe that when a condition
can be discovered, or when an external condition on which it
depends can be determined, the behaviour of the genes with which
it is associated can be modified. There is no reason to suppose
that sex is any different in this respect from other characters.

It is therefore possible to conclude that sex is determined by
the chromosome complex reacting to definite environmental con-
ditions which are in most cases determined by the organism
itself. The modification of these conditions, whether within or
outside of the organism, results in the modification of sexual
characters.

Modification of a sex is, however, subject to definite limitations.
In most animals sexual organs are developed only once during

THE DETERMINATION OF SEX 339

the life of an individual, instead of annually or more frequently
as in manj^ of the flowering plants and some of the lower animals.
The degree of differentiation of the primitive ind(4(M-minate
structures in the higher animals also is extreme. Although the
sexes of the birds retain a fairly ])rimitive condition of the repro-
ductive organs which offers no structural obstacle to reversal, the
highly differentiated organs of mammals can hardly be expected
to submit to such an extreme change. It is probable that if the
mammalian embryo could be subjected to the proper stimulus
while its sexual organs were still in the indeterminate stage its
sex might be controlled. The freemartin most nearly attains this
condition, but since male hormones appear to be the determining
factor it is evident that sexual differentiation must precede the
formation of these substances; the female must be definitely
female before it is subjected to the action of the hormones of its
male twin.

The entire subject is one of little practical importance. One
sex is sometimes more useful than the other among both plants
and animals and man has alwaj'S shown a desire to predict or
control the sex of his own offspring, but there is so Httle possibility
of simple and effective methods of controlling sex that at present
it has only scientific value. In this it is of primary importance
as a demonstration of chromosomal functions.

Summary. Sex is not essential to reproduction as a biological
phenomenon, although in most of the higher organisms reproduc-
tion is accompanied by sex phenomena. Both types of germ cells
and the sexes that produce them are equivalent, and their differ-
entiation must therefore be explained by the specialization of the
reproductive process. The most important result of sexual repro-
duction is the recoml^ination of the unit characters making up
the species. These conclusions are borne out by a large number
of unusual cases, both natural and artificial, including hermaph-
roditism, gynandromorphism, the effects of endocrine imbal-
ances and sex reversal. All available evidence combines to show
that sex is in many cases determined by the chromosomes but
that these require definite conditions for their normal expression.
Whether these conditions are also determined by the body or
come from without, their modification may be expected to influence
the sex of the individual, but determinate sexual structures may
Umit the extent of modification.

340 EVOLUTION AND GENETICS

REFERENCES

KoRNHAUSER, S. I., Deuison U. Bulletin, Jn. Sci. Lab. XX, 1-21, 1922.

Walter, H. E., Genetics, revised edition., 1923.

ScHAFFNER, J. H., "Siamese Twins of Arisaema Triphyllum of Opposite Sex
Experimentally Induced," Ohio Jn. Sci. xxvi, 276-280, 1926.

"Sex and Sex-Determination in the Light of Observations and Experi-
ments on Dioecious Plants," Am. Nat. LXI, 319-332, 1927.

Shull, a. F., Heredity, 1926.

RoBB, R. C, " Y-chromosome Inheritance," Am. Nat. LXL 568-571, 1927.

Crew, F. A. E., "Abnormal Sexuality in Animals," Quarterly Review of Biology,
I, 315-359, 1926; II, 249-266, 427-441, 1927.

CHAPTER XIX
THE PRACTICAL VALUE OF GENETICS

The human race was successful to a remarkable extent in shaping
other organisms to its needs and desires before the advent of
science. Many of our domestic animals and plants were developed
without the shghtest knowledge of the underlying principles of
heredity and breeders even now continue to apply the practical
principles of their craft without such knowledge. As in so many
other fields of human activity, theory in this field is not essential
to practical success.

The science of genetics has shown us not only how the results
of the past have been obtained but also why many desired results
have never been secured. By clearly establishing the fundamental
principles of inheritance it has shown what we can hope to do in
the future, how it can best be accomplished and in some cases
how human efforts in this field are hedged about by apparently
insurmountable barriers. Some things known to practical breeders
and scientists alike can only be determined by experiment, but
in general the best results can be obtained only through the com-
bination of scientific knowledge with sound practical methods.
As in all fields, limited knowledge of any kind cannot hope to
compete with broad knowledge of all phases of the subject.

The methods of plant and animal breeding are dependent upon
the two general methods, hybridization and selection. In a work
of this kind it is impossible to mention the details of method
involved; discussion must be limited to these fundamental proc-
esses, some of their results and their common limitations.

Hybridization. This process, as the beha\dour of Mendelian
unit characters shows, makes it possible to secure combinations of
characters found in nature only separately in different species or
strains. The combination desired may easily be secured if the
difference between the parents is slight, or it may involve a
multitude of difficulties if the parents belong to different species.
As a general rule any combination is possible, even though it
may not be practicable to secure it.

341

342

EVOLUTION AND GENETICS

Fig. 192. — Results of crossing two inbred strains of corn. A, at the left are
two rows, each containing an inbred strain. The tall corn at the right is the
result of crossing them. B, the basket at the right represents the average
production of the two inbred strains after three generations of inbreeding —
sixty-one bushels per acre. The basket at the left shows the first generation
yield from the hybrid — one hundred and one bushels per acre. (From
Walter, after East and Hayes.)

THE PRACTICAL VALUE OF GENETICS 343

The diagrams of Mcndolian hybrids (Fig. 159, 161, 163) show
that any combination, no matter how many unit characters are
involved, appears in a hmited number of individuals in each genera-
tion after the first fihal in the homozygous state. When once
secured, individuals of this kind bearing a new combination of
characters are the potential heads of a new race.

If, for example, one of our American roasting-ear enthusiasts
were cast on an uninhal:)itcd island with only yellow field corn and
white sweet corn in his possession he could secure at l(^ast an
imitation of his beloved yellow bantam sweet corn in the second
generation. By cross-fertilizing the two strains he would secure
a yellow-starchy hybrid of the zygotic formula YySs. When
these seeds were planted and the new generation carefully inbred,
four kinds of grains would appear, viz., yellow starchy, yellow
sweet, white starchy and white sweet. Among the yellow sweet
grains would be two thirds Yyss and one third YYss. The latter
could produce nothing but yellow sweet corn.

The sources of valuable characters are those already mentioned.
The range of fortuitous variation supplies some, as in the case
of corn which varies in starch, sugar and oil content. Mutations
useful to man have also appeared. Babcock and Clausen record
the following mutants of cultivated plants:

Early maturing varieties of the Florida Velvet Bean.

Tobacco, including one mutant of Connecticut Cuban which
showed an increase of 90 per cent in yield.

Sugar beets with increased sugar content.

Various useful mutations of cotton, hemp, rye and sunflower.

Plant Hybrids. Such hybrids are valuable for new combina-
tions and for increased vigor. Hybrids of different strains of
corn are commonly reported to show a marked increase in yield
over either of their parents, sometimes amounting to 250 per cent
(Fig. 192). Crosses have been made of many different varieties
of corn, and in most cases this increase in yield has been observed.
Such increase of vigor in hybrids, whether plants or animals, has
been called heterosis by Shull. The following table gives the
results of some of the crosses of corn.

344

EVOLUTION AND GENETICS

Yields of Maize Crosses Compared with Parental Yields
(Modified from Babcock and Clausen, after Collins)

Name of Hybrid

Maryland dent by Hopi
Tuscarora by Cinquantino
Kansas dent by Chinese
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Many other plants have been improved in the same way.
Tomatoes, cucumbers and strawberries not only give vigorous
plants in the Fi generation Ixit produce heavier yields than the
most prolific parents. Hybrid varieties of some of these plants
are commonly used, but difficulties arise if the plant must be
raised from seed because of the segregation of different combina-
tions of characters in the F2 generation. This has limited the
commercial utilization of heterosis, although plant breeders have
shown that in many cases the additional difficulty of securing
seed is more than compensated b.y the productivity of the
hybrid. The simplest method of producing seed is to maintain
the two parent strains and cross them whenever seed is required.
Since the fertility of most seeds lasts several years it is not neces-
sary to hybridize every year.

No example of hybridization for character combinations is
more striking than that of the Concord grape. "Ephraim Wales
Bull produced the Concord grape as a result of eleven years of
patient work in crossing the native species, Vitis lahrusca, with
European varieties, raising the seedlings and testing selections.
'From over 22,000 seedlings there are 21 which I consider valu-
able,' he writes. Although the hybrid nature of the Concord and
other derivatives of Vitis labrusca has been questioned, the evi-
dence from extensive tests of selfed seedlings of this and several
other standard American varieties as reported by Hedrick and

THE PRACTICAL VALUE OF GENETICS 345

Anthony seem to indicate that they are really hybrids between
American species if not between V. labrusca and V. vinifera.
Whatever the origin of the Concord may have been, its sterling
value is evidenced by its history. Introduced in 1853, ' ten years
later the Concord grape was spread over the entire northern part
of the United States and is now widely used in the temperate
regions of most parts of the earth.' Ephraim Bull's service to
his fellow men seems to have been all but forgotten while he was
still living, since 'he died neglected, in poverty, broken in spirit.'
Vast as would be the value of his contribution if it could be com-
puted, even more valualile was the inspiration he gave, 'which
has helped to make plant breeding one of the great forces in
cheaply feeding the world.' " (Babcock and Clausen.)

Nor has food supply alone been the object of plant hybridiza-
tion. An inestimable number of beautiful varieties of flowers
have been given to us through this medium, and even now fanciers
of peonies, irises, roses and many other plants find new offerings
available every year from the gardens of plant breeders who
experiment tirelessly with hyl^iids of promising varieties. Within
the last few years the beautiful yellow hybrid tea rose. Souvenir
de Claudius Fernet, has been acclaimed by lovers of flowers
throughout the world. Even more recently there has been added
to the already magnificent array of tulips a new class, late-flowering
hybrids, produced by crossing the Darwin and Cottage varieties.
The poetaz narcissus was produced by crossing poeticus ornatus
and polj^anthus varieties. It combines the large clusters of the
latter with the hardiness of the former and has an exquisite odor
of its own (Fig. 193). New varieties of Iris germ.anica are also
constantly appearing.

Animal Hybrids. The problems of the animal breeder are
very different from those of the plant breeder, but in general the
same fundamental methods are open to him. W^hile selection
plays a very large part in the development of improved strains
of animals, we are not without familiar examples of animal
hybrids whose value to the human race is permanently established.
Mules, for example, are produced b}^ crossing the male ass with
the female horse and can be produced in no other way. The
mule breeding industry attained a value of $500,000,000 in the
United States in 1915. The mule is a more vigorous animal than
either of the parent species, and is more resistant to adverse

346

THE PRACTICAL VALUE OF GENETICS 347

environmental conditions; it coml^nos morphological characters
of both parents.

Few other interspecific animal hybrids are of more than potential
value. Cattle have been crossed with the American bison, the
zebu, and other species with excellent results, but the hybrids are
not in common use (Fig. 194). Of these crosses Babcock and
Clausen say: "By long-continued selection it would be possible
to transfer many of the excellent qualities of the bison such as
superior coat, greater hardiness, resistance to tick and insect

Fig. 194. — Quinto Porto, five-eighths bison, three eighths polled Hereford.
(With the permission of the Journal of Heredity.)

infestation, and superior beef qualities to domestic cattle." The
hybrid between the zebu and our common cattle is sufficiently
resistant to tick-borne disease to be very valuable in the south-
western states according to Lush. Many other hybrids have been
recorded between domestic animals, such as the sheep and goat
and various species of fowls, l)ut they are chiefly of scientific
interest.

An interesting effect of hybridization occurs in bees. The
Italian bee has long been recognized as superior to the black or
German bee and has become the most popular and widely kept
variety in the United States. It is resistant to one of the two
serious bee diseases and is not seriously affected by the bee moth,
while the black bee is susceptible to both diseases and when

348 EVOLUTION AND GENETICS

colonies become weak they may succumb to the inroads of the
bee moth. Hybrids between the two have no proved superiority
and retain the characteristic nervousness of the black bee which
makes them difficult to handle as compared with Itahans. In
addition they are the most ill-tempered of all three. Italians
vary greatly in temper but are in general mild, while the hybrids
sting readily under anj^ conditions. This cross is usually looked
upon as intervarietal, although the systematic rank of the various
kinds of honey-bees is by no means clear.

Heterosis is sometimes as marked in animals as in plants. It
must not be thought, however, that because crossing may increase
vigor, inlDreeding must reduce it, for many strains of domestic
animals have been intensively inbred for many generations without
reduction of vigor or fertility. In some cases, in fact, these
qualities have been improved through inbreeding accompanied
by careful selection. In general, crossing results in heterosis while
inbreeding may be practiced with varied results, hence the con-
clusion has been reached that vigor and fertility depend upon
unit characters which are much more likely to be isolated in
homozygous combinations through inbreeding than through the
mixing of various strains.

Limitations of Hybridization. The crossing of different
species is attended l)y many difficulties which limit its value.
Some are insurmountable while others can be met by special
methods of procedure. Hyl)ridization within a species, whether
between varieties or with respect to single characters, is a much
simpler process.

Interspecific infertility is the most serious of these limitations,
for when normal union of the germ cells cannot take place
no hybrid can be produced. It is such a common phenomenon
that some biologists have recognized in it a criterion for the
limitation of species. While such an extreme interpretation is
not favored by the availal^le evidence, the fact remains that many
species cannot be crossed. The facility with which hybridization
may be accomplished is often, if not always, in proportion to the
degree of relationship between the species involved.

The reasons for infertility are various. In some cases the sperm
cell is unable to penetrate the surface of the ovum of a different
species. In others the spermatozoon not only enters the ovum
but also initiates development, although it makes no material con-

THE PRACTICAL VALUE OF GENETICS 349

tribution to the new individual. In no case can we conclude
that chromosomal insufficiency is responsible, for the haploid
complex of either germ cell may contain a complete set of deter-
miners for the production of a new individual. Hybrids have
been produced many times between species with different numbers
of chromosomes. The general reason for infertility between
species may therefore be stated as some lack of harmony in the
accessory phenomena of reproduction.

Since varieties and strains of the same species are similar in
all fundamental structures and processes this difficulty cannot
hinder the production of hyl^rids between such groups.

Infertility of Hybrids. Although difference in the chromosome
complexes of the parents need not affect the production of
the hybrid, it may well be expected to have serious results in
the production of the F2 generation. Since the process of matura-
tion of the germ cells hinges upon synapsis and the resulting
reduction of the chromosomes, it is easy to see that any asymmetry
which affects the consummation of this delicately adjusted series
of events may prevent the formation of normal germ cells. The
number of chromosomes found in ova of the horse is said to be
nineteen, and the number in the spermatozoa of the ass thirty-two
or thirty-three. The mule therefore has an asymmetrical chromo-
some complex. Male mules are not known to produce functional
germ cells. Cases are on record of fertile female mules, and they
may occasionally occur, although they are open to doubt. The
rarity of even doubtful cases is in itself suggestive.

When the species are closel}^ related and have similar chromo-
somes there is no reason to expect infertility in their hybrids, but
the actual occurrence of infertility even in such hybrids forces us
to the conclusion that physiological differences in the chromosomes
may exist even when visible morphological differences are lacking.
The production by some species of fertile female hybrids and
infertile males is another puzzling complication of the problem.
This is true of the hybrids of domestic cattle with the bison.

For practical purposes it is evident that the infertility of hybrids
is not insurmountable. It is, indeed, no more serious than the
inconstancy of desirable heterozygous strains. Either demands
the maintenance of pure parent stocks and the production of new
hybrids solely by repetition of the cross unless the breeder, in
the case of heterozygotes, is willing to breed from hybrids and

350

EVOLUTION AND GENETICS

discard the fifty per cent of homozygous individuals in every
generation.

Asexual Propagation. Repeated hybridization is naturally more
complicated and more expensive than the normal course of re-
production. It is avoided by commercial plant breeders through
asexual propagation of their hybrid stocks. Hybrid fruits are
propagated by grafting scions of the desirable stock onto hardy
root systems, sometimes of entirely different species. The beautiful
varieties of French lilacs are grafted onto roots of the common
lilac or privet. Chrysanthemums are easily raised from cuttings,
roses from cuttings or by grafting, peonies and other flowers by
division of the roots and crown of the plant, and bulbs through
their natural asexual increase. Plants which can be produced
only from seed are obviously subject to the same limitations as
animals.

Hybridization for the production of new combinations of char-
acters is limited only by the difficulty of isolating homozygous
strains. If the desired type is complex this difficulty is great and
it is necessary to resort to asexual propagation if possible. Many
desirable hybrids are simple, however, so this does not limit the
uses of the process to organisms which can be produced asexually.

Perhaps no useful hybrid is a better illustration of complexity
and the value of asexual propagation than Burbank's Alhambra
plum. The ancestry of this variety is incorporated in the following
diagram by Babcock and Clausen:

Nigra

^d......

Americana

nbra

e '

c ■

' Triflora
Simoni

French Prune

b ■

.a ■

Pissardi

Kelsey

What an impossible task it would be to fix the desirable combina-
tion of characters in any other way!

THE PRACTICAL VALUE OF GENETICS 351

Selection. The process of selection has been practiced for
many centuries for the production of improved strains of cultivated
plants and domestic animals. It is a logical consequence of the
fact that "like produces like" to a marked degree. Cows which
produce a large quantity of rich milk and bulls of the same strain
are much more likely to produce good dairy cattle than those
which possess other qualities. Sheep with fine and heavy fleeces
are obviously more valuable for the production of wool than
those whose fleeces are coarse and light. By the early recognition
of these facts man has produced beef and dairy cattle, draft and
race horses, dogs of many breeds and a multitude of other distinct
varieties of relatively few natural species (Fig. 195).

Methods of Selection. Before the discovery of scientific
principles of inheritance selection was necessarily based upon
observed characters, and hence may be called phenotypic selection.
It is usually known as mass selection because the best individuals
from a given group of organisms are selected as the parents of
future generations and reproduction of the poorer individuals is
prevented, but no further attention is given to the details of
parentage.

Closer attention to individual parentage brought about the
refinement of method known as line selection, which is closely
allied to the most modern and scientific method, genotypic selec-
tion.

Mass Selection. The English scientist, Hallet, associated se-
lection with environmental effects by giving plants the best
possible environment and selecting those which did best under
these favorable conditions. Rimpau, on the other hand, subjected
his grains to unfavorable or merely average conditions, and
selected those which showed the ability to do well in spite of
adverse surroundings. Either method results in the improvement
of the organism, but the latter in particular is valuable for it
discloses something of the inherent possibilities of the individual.

Selection as it has unavoidably been limited in the honey-bee
is a fine example of the effectiveness of mass selection. Since the
functional sexes are merely reproductive and the individuals
which are directly of use to man do not reproduce, the breeding
stock in this case can be judged only by its progeny. The mating
of bees occurs in flight so that only the female parentage of a
colony can be definitely known and selected. Only within the

Prjevalsky Horse, a
wild Asiatic species.
(Through the courtesy of
the New York Zoological
Society.)

B. Lou Dillon, a

trotter.

C. Benedict, a Clydesdale
stallion. (B and C from
Plumb's Types and Breeds
of Farm Animals, with the
permission of Ginn and
Company.)

Fig. 195.— The results of selection in the horse.
352

THE PRACTICAL VALUE OF GENETICS 353

last few years has a method been devised for the artificial control
of mating, and this has not yet been widely used.

Selection has been practiced for various characters in the honey-
bee, such as light color, industry based on the amount of honey
stored, color of wax produced, temper, and swarming propensity.
By rearing queens only from mothers whose colonies best expressed
the desired characters and by restricting the production of drones
(males) in other colonies as much as possible, many distinct
strains of Italian bees have been produced. Some are called
three-banded leather-colored Italians, others golden Italians be-
cause they very nearly lack the black abdominal bands and are
pale in color. Some sting readily while others are mild tempered
and sting only when conditions for handling bees are very unfavor-
able. A most desirable result of selection is the reduction or
elimination of swarming instinct, which is the bee-keeper's greatest
source of annoyance. Some strains swarm readily and often, while
others will go through a season under conditions entirely favorable
to swarming without attempting it.

Line Selection. The fundamental principles of Mendelian in-
heritance disclose the necessity of knowing the genotypic organi-
zation of an individual for accurate control of succeeding genera-
tions. Even before Mendel worked out his laws the value of
his discoveries in relation to selection was anticipated by the work
of Vilmorin, near the middle of the nineteenth century. He selected
single plants whose offspring were isolated for comparison. The
principle was later applied by various plant breeders, and in the
hands of Hjalmar Nilsson at the experiment station of the Swedish
Seed Association at Svalof it has produced many valuable strains
of wheat, peas, potatoes and other plants. The method is also
known as pedigree breeding.

The obvious value of line selection is that the isolation of
offspring of single individuals is much more likeh^ to produce a
uniform variety. Even superficially identical individuals, as we
have seen, may be genotypically different and therefore capable
of producing different offspring.

Genotypic Selection. While any method that takes into ac-
count the character of the succeeding generations in relation
to their known ancestry is to some extent genotypic, in the strict
sense, this term should apply to the type of selection which is
used in connection with known facts of Mendelian inheritancBo

354

EVOLUTION AND GENETICS

It must be used in connection with hybridization for the isolation
of desirable characters and character coml^inations in the homo-
zygous state, and if the heterozygous individuals are desired it

Pure Line

3

^yy^^H^gfgyy

may be used for the elimi-
nation of their homozygous
offspring.

The Pure Line. Dar-
win's theory of natural
selection influenced thought
in such a way that for many
years selection, through
natural or artificial means,
was supposed to result in
actual modification of the
line selected. Galton's law
of filial regression also sug-
gested very strongly the
possibility of shifting the
general character of a group
of organisms by always
selecting extreme individ-
uals as the parents of the
next generation. The
Danish botanist, Johannsen,
tested the accuracy of this
view and in doing so dis-
covered the existence of
pure lines.

Johannsen used for his

Fig. 196.-D7agmm~showmg five p«reZwes experiments a cultivated
and a -population formed by their union, bean {Phaseolus vulgaris
The beans of each pure Hne are repre- nana). The weights of seeds
sen ted as assorted into inverted test tubes, 1,1 1 i i
making a curve of fluctuating variabihty. Planted were recorded and
Test tubes containing beans of the same the entire lot of seeds pro-
weight are placed in the same vertical duced by every plant was

row. (From Walter, after Johannsen.) /. ,, , , ■, j

carefully harvested and

weighed. In general the largest beans produced the largest

offspring, but Johannsen was impressed by two important

facts. In the first place, the seeds produced by a single plant

sometimes fluctuated about a mean quite different from that

THE PRACTICAL VALUE OF GENETICS 355

suggested by the size of the parent seed, and in the second,
parent seeds of the same weight in some cases produced beans
with very different ranges of variation. In spite of the fact that
the results were reasonably harmonious with Galton's law of
regression Johannsen concluded that he was dealing with mixed
stock and conducted further experiments to explain his discoveries.

In these experiments plants produced from beans of known
weight were self-fertilized and their offspring for several genera-
tions were treated in the same way until it was certain that the
lines were homozygous. The result indicated that the original
stock was made up of nineteen different kinds of beans, each kind
varying between certain extremes of weight which might overlap
with others, but every one of the nineteen fluctuating about a
different mean. These nineteen groups were called pure lines.
Johannsen defined a pure line as the progeny of a self-fertilized
homozygous individual. An aggregation of pure lines such as
that with which he first dealt was called a population. Figure 196
illustrates graphically the difference between five of Johannsen's
pure lines and the population formed by mixing them.

It is evident that intensive line selection in any plant may
bring about the isolation of pure lines (Fig. 197). The difficulty
of maintaining such lines under ordinary conditions is obvious,
however, and it is doubtful that true pure lines often occur in
nature. They must certainly be restricted to those plants in
which elaborate adaptations for self-fertilization are present.

Equivalents of the Pure Line. The self-fertilization of a homo-
zygous individual is the same in result as the union of gametes
with the same complex of determiners from different individuals,
consequently homozygous crosses produce equally homogeneous
groups of offspring. It is also similar in effect to reproduction
without fertilization, since here there is no possibility of different
characters being brought in. Reproduction of the latter type
includes parthenogenesis and agamic reproduction, such as fission
and budding. The individuals descended from one ancestor
through a series of asexual generations collectively constitute a
clone.

Under natural conditions none of these pure-line equivalents
are more likely to be maintained than the typical pure line, but
they may exist for a considerable time in many species. At the
end of a summer, for example, the offspring of each stem-mother

356

EVOLUTION AND GENETICS

aphid constitute a clone. Such animals as the Protozoa and
Hydra also undergo a series of asexual divisions which give rise
to clones before sexual reproduction intervenes to luring about a
reassortment of characters.

Homozygous crosses are not only uncommon in nature but also
difficult to obtain in the laboratory. With respect to single char-

w

V

1,

1/1

7 H-

00

CJ

Fig. 197. — Typical heads from seven pure lines of Defiance wheat. (From
Babcock and Clausen's Genetics in Relation to Agriculture. McGraw-Hill
Book Company, Inc., by permission.)

acters this difficulty is not encountered and lines homozygous for
one or a few characters have been secured many times and in
many species.

Selection in Pure Lines. When Johannsen had isolated his
nineteen pure lines of beans he found that no matter what the
size of the parent seed, those which it produced fluctuated about
the mean for the pure line to which it belonged. Walter has
graphically indicated these results in a diagram which is repro-
duced in Figure 198. In all lands of pure lines the effects of
selection have been hkewise negative. Aphids, daphnids, Droso-

THE PRACTICAL VALUE OF GENETICS

357

phila, Paramecium and many plants have been the basis for the
conclusion b}'- many biologists that selection within the pure hne
is without effect.

The Pure Line as a Limit of Selection. The fact that selection
has proved a valuable method of improving organisms according
to most experimental evidence, rests upon the possibility of isolat-

Average of_
all progeny
Weight of parent seed]

Centigrams

70

jEU

45. U I

45.0 I

im

'''■''' ' ' ' ' L

I I t

10 2U 30 40 5U 60 70 10 20 30 4U 60 60 70 10 20 30 40 60 6070 10 20 30 40 60 60 70 10 20 30 40 60 60 70

Pure line number >II VII XV XVIII

Fig. 198. — The result of selection in four pure lines of beans. The vertical
columns, representing the average progeny from different sizes of parents
all derived from the same pure lines, contain groups nearer alike than the
horizontal columns, repre.senting progeny from parents of the same size
but of different pure lines. All of the numbers indicate weight in centi-
grams. (From Walter, after data from Johannsen.)

ing those pure lines which best express the desired character. It
has been found that environmental conditions may bring about
differences in the development of individuals within the pure line,
hence this factor may also enter into improvement by selection,
and finally supplementary MendeUan factors may act upon char-
acters in a pure line to produce an entirely different result. The
pure line is therefore only a partial limitation of selection and
even without the inconstant modifications mentioned there is a
lack of absolute evidence that it is permanently fixed in nature.
Mutations occur to change pure lines, and there is a growing
feeling that the environment may be important in the modification
of inherent qualities.

358 EVOLUTION AND GENETICS

Summary. Hybridization and selection have played an impor-
tant part in the establishment of varieties of plants and animals
of use to man. Hybridization is useful for two purposes, \az.,
increase in vigor and productiveness, called heterosis, and the
combination of useful characters. Its effectiveness in animals is
limited by the segregation which occurs during reproduction.
This necessitates repeated hybridization whenever the hetero-
zygous condition is the one desired. In plants even this limitation
is offset by the fact that asexual propagation is usually possible.
Hybridization is also hmited where sexual reproduction is unavoid-
able by the frequent infertility of hybrids. Selection is accom-
plished by several methods, all directed toward the isolation of
the most favorable individuals as the parents of the following
generation. It is practiced in connection with h5d3ridization for
the isolation of desired combinations of characters, and within
established species for the isolation of the best strains. Selection
is apparently limited by the pure lines of which a species is com-
posed, although the isolation of these lines may constitute an
effective degree of modification. Even pure lines are known to
be susceptible to some modification.

REFERENCES

Babcock, E. B. and Clausen, R. E., Genetics in Relation to Agriculture, 2nd
edition., 1927.

An extensive bibliography is published in Babcock and Clausen's work.

CHAPTER XX
HEREDITY IN MAN

The study of heredity in the human race is hindered in a number
of ways. Because of the span of generations, results approximating
those secured in the laboratory in the study of other organisms
are impossible. It is rare to hear of five generations or even four
aUve at the same time in a human family, and most of us never
know more than three, so it is necessary to fall back upon records
and these are at present pitifully incomplete. Genealogies furnish
some valuable data, the records of pul^lic institutions are also a
dependa])le source of information, and within the present century
such institutions as the Eugenics Record Office at Cold Spring
Harbor, Long Island, have begun the work of making accurate
scientific records in this field.

Even the accurate observation and recording of natural phe-
nomena cannot, however, give results like those obtained in the
study of laboratory animals. It is and will probably always be
impossible to control the reproduction of human beings except
in extreme cases which demand the action of organized society
for its own protection. Fortunately such methods, undesirable
from the normal human viewpoint, are not essential to an under-
standing of human inheritance. We are animals and there is every
reason to believe that the laws of heredity in other organisms are
equally applicable to ourselves. The corroboration of this rela-
tionship by the available data is adequate.

One unfortunate feature of our knowledge of human heredity
is that extreme cases and particularly abnormalities are most
likely to make an individual the object of scrutiny. Data are
more abundant concerning the inheritance of supernumerary digits
and mental defects than on the behaviour of valuable qualities,
although the latter are by no means lacking. Fortunately accurate
data, whatever the characters recorded, are a valuable basis for
the application of general laws as worked out in other organisms,
and at least a partial indication of the trend of heredity in general.

359

360 EVOLUTION AND GENETICS

What Is Inherited? The application of conscious thought to
the prol^lems of existence is a comphcating factor which it is
exceedingly difficult to avoid in a scientific consideration of human
behaviour. Every-day interpretations of factors in human life
must be translated into the accuracy of scientific observation.
We hear of the inheritance of drunkenness, disease and special
ability, as well as of structural characters, whereas these things
are phases of Ijehaviour, and behaviour can only be the expression
in the individual of its hereditary properties. There is necessarily
some basis in the individual for anything which it does, and that
basis is at least indirectly associated with some inherited character.
It is not enough to say that a great musician inherits musical
talent; he inherits an exceptional sense of hearing, great manual
dexterity and the wonderful nervous coordination which any
skilful performance demands. The use to which he puts these
things is response. The skilled mechanic who builds an instru-
ment of precision has an equally fine inheritance, but his ability
attracts less attention.

Man, like other organisms, has a heritage and an environment
to which the heritage responds. Ilesponscs are the things that
interest us chiefly. They are so conspicuous that they usually
obscure the heritage from which they arise but careful considera-
tion will show that a structural heritage is present for every
function. Mental activity, although it is exceedingly complex, is
no less definitely based on structure than other functions. It is
none too well understood in detail but of its anatomical source
we need have no doubts. Whatever the inherited structure, in
so far as it finds the proper environment for its expression, it will
manifest itself in the same way in successive generations.

The distinction between heritage and response, since they are
so likely to correspond in successive generations, is not essential
from the popular point of view. In efi'ect, a talent may be inherited.
From the point of view of the scientist, however, no talent is the
simple thing into which ordinary language resolves it. In no
case can exceptional ability be looked upon as a unit character,
nor can conditions of mental deficiency always be so simply
handled. These things are the result of many conditions present
in the body. They may be based upon unit characters, but are
due to complex immediate causes. The things actually handed
down from generation to generation in man as in other organisms

HEREDITY IN MAN 361

are chromosomal determiners which arc capable of a certain
response if th(> right conditions prevail.

Unit Characters. The complexity of human responses is so
baffling that it is often impossible to discover all of their under-
lying causes. In simpler conditions the behaviour of heritable
characters can often be traced through several generations, so
that the occurrence of unit characters in man is well established.
Eye color, hair color, pigmentation of the skin, curhness of hair,
polydactj^ly and symphalangism are among the well known struc-
tural characters in this category.

The chromosomes of man are also similar to those of other
animals and furnish a basis for heredity of the same type. Various
investigators have studied the cytology of human cells with the
result that the chromosome number is commonly accepted as
forty-eight. Of these, forty-six are autosomes and two allosomes.
There are an x and a y chromosome in the male and two x chromo-
somes in the female. The gametes therefore contain either
23+x or 23+y. This is only a moderately large number but con-
sidering only one determiner to a chromosome it affords the
possil)ility of 4^^ or more than two hundred thousand billion
recombinations. The diversity of human beings is not surprising!

Eye Color. The color of the eyes depends upon the presence
of two pigments, brown and blue. Brown pigment varies greatly
in quantity, so that brown-flecked blue eyes are common, but
when distributed through the entire area of the iris it masks the
blue because it lies in front. It is dominant over lack of
brown, which is, under ordinary conditions, equivalent to domi-
nance over blue. The two are not allelomorphic, but the aflelo-
morph of brown permits the blue pigment to show and only
in albinos can the absence of blue be seen. Brown-eyed parents
may be heterozygous and are therefore able to produce blue-eyed
children.

Hair Color. Hair color is also due to two pigments. Its be-
haviour is not thoroughly understood because of the occurrence
of various modifying conditions, but darker colors are dominant
over light hair and inheritance is in general similar to that of
eye color. Black-haired parents may produce blond children but
blond parents cannot produce brunettes.^

'A recent article by Hausman {Am. Nat. LXI, 545-554, 1927) contains many
interesting facts on the pigmentation of iiuraan hair.

362

EVOLUTION AND GENETICS

Pigmentation of Skin. Davenport's studies of negro-white
crosses are the chief source of information on this subject. He
reached the conclusion that pigmentation depends on two pairs
of cumulative factors which may be designated as AA and BB
(Fig. 199). Since the white race is not totally devoid of pigment

AB Ab aB ab

AB

Ab

aB

AB
AB

70

Ab
AB

55

aB
AB

53

a b
AB

38

AB
Ab

55

Ab
Ab

40

aB

Ab

a b
Ab

23

AB
aB

53

Ab
aB

38

aB

aB

36

a b
aB

21

AB
a b

38

Ab
a b

23

aB
a b

21

a b
a b

6

a b

Fig. 199. — Punnett square showing the expected shades of color in the pos-
sible offspring of two mulattoes. A = 18, B = 17, a = 2, and 6 = 1 per cent of
black pigment. (From Walter, after data from Davenport and Danielson.)

it follows that the factors aa and l)b do not stand for albinism
but only for slight pigmentation. The percentage value ascribed
to each determiner by Davenport is indicated in the diagram.
A pure African l^lack would have the formula AABB for color,
while a white would be represented by aabb. The hybrid mulatto
has the formula AaBb. Figure 199 represents the F2 generation
derived from mulatto parents. In this diagram it is evident that
there are three kinds of mulattoes with the formulae AAbb, AaBb
and aaBB, each differing slightly from the others in pigmentation,
as well as intergrades between these and the dominant and
recessive combinations. The names for these intermediates are
quadroon for individuals with only one dominant determiner and

HEREDITY IN MAN"

363

mangro or sambo for those with throe. It is also evident from the
diagram that mulatto parents have one chance in sixteen of
producing a black child and one of producing a white. Individuals
of the latter class are called by a number of names, including

Fig. 200. — Radiograph showing symphalangism in man. The two proximal
phalanges in each of the four fingers are fused. (Through the courtesy of
Professor R. A. Hefner.)

pass-for-white and octoroon. Their negroid ancestry is usually
plainly evident in other characters than color.

Polydactyly, Brachydactyly, and Symphalangism. These con-
ditions are modifications of the fingers and toes which are
dominant over the normal condition. The first is multiplication
of the usual number of five digits. An extra thumb or great toe

364 EVOLUTION AND GENETICS

is frequently the added member. Brachydactyly is extreme
shortening of the fingers, which sometimes lack one of the three
phalanges and sometimes have an extremely short terminal pha-
lanx. Symphalangism is fusion of the phalanges of toes or fingers
so that one of the usual joints is stiff.

Hefner has traced the last condition through six generations
of a family in which it behaved as a Mendelian dominant and
appeared in both males and females (Fig. 200). In his report on
this case he cites another remarkable record: "John Talbot, first
Earl of Shrewsbury, was supposed to have had fingers with stiff
joints. He was killed in battle near Bordeaux in 1453, by a blow
on the head, received after his thigh had been broken. He was
buried in Shrewsbury Cathedral. Recent alterations made it
necessary to disturb his grave, when tradition was confirmed and
his bones identified by the fused finger-joints, the cleft skull, and
the broken thigh-bone. By a strange coincidence this work was
under the direction of one of Talbot's direct descendants in the
fourteenth generation, the joints of whose fingers were fused like
those of his remote ancestor. ..."

Sex- Linkage. Color blindness, a sex-linked character, is
inherited in man in the same way that other sex-linked characters
are inherited in Drosophila. It is recessive to normal vision. A
color-blind man and a woman with normal vision cannot produce
color-blind children but one-half of their daughters are carriers
and can produce color-blind sons even if mated with normal men.
Their daughters would be one-half normal and one-half carriers.
Color-blind females can be produced only when both parents
supply factors for this condition, since one x chromosome comes
from each parent. The following diagrams show how the char-
acter is transmitted in the four possible crosses (Fig. 201).

These diagrams make evident a number of interesting phe-
nomena. A shows that a color-blind parent may have children
with normal vision. Even a color-blind female, as shown in B,
may have some children with normal vision, and if she had only
daughters all would be apparently normal although able to
transmit the defect. Diagram D shows how parents with normal
vision may produce color-blind sons. A combination of such cases
as these shows how a sex-linked defect may be transmitted genera-
tion after generation through a female line, to crop out in an
occasional male. Because of the small size of human families

HEREDITY IN MAN

365

there is so little chance for a complete expression of Mcndelian
ratios that this may easily occur, while a high rate of reproduction
would be almost certain to bring out all possibilities within one or

Color-blind male
XY
Gametes X Y

Normal female
XX
X

generation XX XX

Carrier females

Normal male
XY
Gametes X

generation

XY XY

Normal males

Color-blind female
XX
X X

XX XX

Carrier females

B

Color-blind male
XY
Gametes X Y

XY XY

Color-blind males

Carrier female
XX
X

generation XX XX XY XY

50% Color- 50% carrier 50% Color- 50% normal

blind females females blind males males

C

"Normal male Carrier female

XY XX

Gametes X Y X X

^1
generation XX XX XY XY

50% carrier 50% normal 50% Color- 50% normal
females females blind males males
D
Fig. 201. Diagram showing the four possible combinations of color blind-
ness and the way in which the condition is inherited from these crosses.
X represents the x chromosome l)earing the determiner for color blindness
and X that bearing the determiner for normal vision which is dominant
over X.

a few generations. The small size of human families also makes
possible the elimination of the defect from a line of descent in
some cases.

366

EVOLUTION AND GENETICS

Inheritance of Defects. The lesson taught by these cases of
human inheritance is rather obvious. Many defects are heritable
which are not necessarily serious but we must recognize the
occurrence of other heritable defects which are not only a serious
handicap to the individual but a menace to society. Like the
simple morphological defects these may appear or be concealed
in an individual; if the carrier finds a normal mate, the latent
defect need never appear in his descendants, but if, as so often
occurs, similar individuals mate, there is little hope that they will
produce normal offspring. Their appearance depends, of course,
on the method of inheritance. If dominant a defect must appear
more often than if recessive. The following table indicates the
expectation for all possible crosses.

The Mendelian Expectation for Defects
(After Walter)

1
2
3
4
5
6
7
8

If

THE Defect Is Positive

(Dominant)

If the Defect Is Negative
(Recessive)

When both
parents show
the defect

DD

DD = allDD

dd

DD

Dd = h DD i Dd

dd =alldd

Dd

Dd =1 DD i Dd idd

When one par-
ent only shows
the defect

DD

dd =allDd

dd

DD=allDd

Dd

dd = h Dd A dd

dd

Dd =iDd idd

When neither
parent shows
the defect

dd

dd =alldd

DD

DD = allDD

Dd

DD = iDD iDd

Dd

Dd = i DD i Dd i dd

Individuals with determiners for dominant defects cannot be
unaware of their defectiveness. There is a possibility that defective
parentage may produce normal offspring in some cases, but it is
obviously much more certain if only one parent shows the defect.
When the defect is recessive it is impossil)le to know of its presence
in a carrier except through his ancestry or his offspring. If the
defect is evident the individual is certainly a homozygous recessive
and unless mated with a normal individual some of his offspring
are certain to be defective.

Human Pedigrees. The heritability of serious defects, such
as insanity, epilepsy, cretinism, pauperism and the socially less

HEREDITY IN MAN 367

serious but none the U^ss unfortunate (h^fects of deafness, tendency
to disease and similar conditions is illustrated by a large numl^er
of recorded pedigrees. Kellicott reproduces several from Whet-
ham's Treasury of Human Inheritance, including the following
record of inherited deaf-mutism (Fig. 202). This case is ade-
quately illustrated in the diagram, but attention should be given
especialh" to the frequent marriage of defectives in this line and
to the fact that even normal unions produced defective offspring.
In contrast to the unavoidable state of deaf-mutism other
pedigrees show the constant recurrence of tuberculosis. This
disease must be acquired by every individual. Possibly nobody

1 no

u m® mm m® ? ao ^d

mm® ? ■•■••DAAAAAD? •no«B«DAA 2m®m®m®m®

V AB ? • ?B«*B«»OAA AB B

Fig. 202. — A family hi.story showing the inheritance of deaf-muti.sm. Males

are indicated by squares, females by circles, and individuals of unknown

sex by triangles; deafness is shown by black, normal hearing by white, and

uncertain data by shaded areas. (Modified from Kellicott, after Whetham.)

goes through life without infection but some persons lack the
inherent qualities to resist the attack of the bacillus and so
develop tuberculosis. The recognition of such an inherited weak-
ness of resistence, or as we usually say, tendency to the disease,
should be a warning which would lead to the proper steps for
avoiding its serious results. Choice of location, occupation, and
recreation might well offset the inherent defect.

Inbreeding. The preceding cases are very good illustrations
of the effects of inbreeding in the human race. Defective lines,
whether the defect is dominant or recessive, are improved by
intermarriage with normal lines. Since recessive defects are not
evident in heterozygous individuals it is impossible to predict
what the results of marriage of normal individuals may be within
lines known to have such defects. These marriages are very

368 EVOLUTION AND GENETICS

likely to result in the reappearance of the defect, while marriage
of the heterozygous members of a line to members of normal lines
would be certain to keep any but a sex-linked character submerged
and might even eliminate it entirely.

The same must l^e said of desirable and indifferent qualities.
Whether evident or not a character is much more likely to be
preserved through the mating of closely related individuals than
through the crossing of different strains.

It is fortunate that modern social customs do not favor cousin
marriages, although they are tolerated as the closest permissible
inbreeding. Such an attitude tends to prevent the expression of
inherent defects. Even though good and bad alike are perpetuated
through inbreeding it is doul^tful that the benefits derived from
the marriage of closel}'^ related persons are sufficient to offset the
risks.

A moderate degree of inbreeding cannot be avoided, but the
persons involved are usually so remotely related that the effect
is practically the same as that of marriages within a social or
intellectual class. Too great contrast between individuals is
inimical to happy married life, and happiness must remain a
fundamental consideration in this important relationship. Like
will continue to seek like and to beget like. The latter process
is the one certain result, whether good or bad, of inbreeding.

The Jukes. Several families have become famous in connec-
tion with the study of human heredity, among them the Jukes.
The history of this family was first reported by Dugdale in 1875
and more recently by Estabrook in 1916. Dugdale's interest was
first aroused by the frequent recurrence of the same name (the
name Jukes is fictitious) in prison records. His original investiga-
tions covered 709 individuals of whom " 180 were paupers or had
received poor relief to the extent of 800 years, 60 were habitual
thieves, 50 prostitutes, 7 murderers, and the total cost to the
state was estimated at $1,308,000.00" (Holmes).

When Estabrook monographed the family in 1916 he was able
to report on 2,094 individuals, of whom not more than one-half
were living. The general quality of the family was the same as
in its earlier years. Criminal records, intemperance, pauperism,
and prostitution abound in the story of these people. Feeble-
mindedness is very common, especially among the criminal mem-
bers of the family, and combinations of feeble-mindedness, ille-

HEREDITY IN MAN 369

gitimacy, pauperism, and criminality arc pitifully frequent. It is
obvious that they have been reared under the poorest environ-
mental conditions, but no less obvious that their heritage is very
deficient; it is doubtful that they would be able to respond ade-
quately to the finest of surroundings.

In writing of this and similar families Holmes sums up their
significance in heredity in the following words: "People with good
stuff in them very often rise out of their vicious environment,
while others under the best of conditions seem to take instinctively
to evil pursuits. We should bear in mind in studying degenerate
famihes and their unfavorable surroundings, that bad environment
tends to be created by a bad heredity. Given stocks with an
inheritance of low mentality, feeble inhibitions, and more or less
mental disorder, in a few generations such stocks would gradually
sink into the ranks of dependent or outcast humanity, and would
soon develop traditions of vice and immorality which would make
it especially hard for an individual to rise in the social scale.
When we consider a single individual born amid such unfavorable
surroundings, we might be prone to attribute his shortcomings to
his poor opportunities. We might be al)le to point to many cases
in which members of degenerate strains have become worthy
citizens when given better chances for obtaining success. Such
cases, in fact, are not infrequent. But this fact would in no wise
controvert the assertion that heredity is primarily responsible for
the condition of these degenerate families. Under the conditions
that prevail in our civilized society, there is a general tendency
for families of good inheritance to rise into higher ranks, whatever
misfortunes may have been responsible for their inferior position
in the social scale. Families of bad inheritance, although they
may be endowed with wealth and social standing, tend after a
time to sink into the lower social strata."

Illustrious Families. After such a depressing picture as the
Jukes it is a pleasure to turn to some families of the opposite type.
Galton was one of the earhest writers to consider the inheritance
of ability, and in his work on Hereditary Genius he shows that
there is a striking tendency for th(> reappearance, generation after
generation, of high ability in the same line of descent. Superior
ability in almost every line of human endeavor has been shown
to follow this rule. A particularly appropriate example for such
a work as this is the family of Charles Darwin. His grandfather,

370 EVOLUTION AND GENETICS

Erasmus Darwin, has already been mentioned as one of the great
contributors to the early history of evolution. Two sons of
Erasmus, one the father of Charles, were distinguished men in
their chosen fields. Charles himself needs no mention; the fact
that his name is almost synonymous with organic evolution in
the popular mind is enough evidence of his greatness. Charles
Darwin's wife, Emma Wedgwood, was his cousin. Her grand-
father was the founder of the famous Wedgwood pottery works.
The four sons born to this union were prominent in as many
activities.

Winship's data on the family of Jonathan Edwards, an eminent
minister, are a similar evidence of inherited ability. Of 1394
descendants identified in 1900 there are listed 295 college gradu-
ates, 13 presidents of leading colleges and many in similar offices
of less importance, 60 physicians, over 100 clergymen and religious
workers, 75 officers in the army and navy, 60 writers, over 100
lawyers, 30 judges, 80 public officials including a vice-president
of the United States and three senators, and many officials in
business enterprises of various kinds.

The Kallikak Family. The record of this family is even more
convincing evidence of the potency of heredity in determining the
value of human beings, for it contains contrasting lines of descent
from a single ancestor, Martin Kallikak. (This name is also
fictitious.) Kalhkak, although of good family, became the father
of a feeble-minded son by a feeble-minded woman. The descend-
ants of this son have been traced, and out of several hundred
none have been above average ability, most have been below
average, and more than a quarter were feeble-minded. Later
Kallikak married a girl from a good family and the known issue
of this union, numlDering almost the same as his other descendants,
have been almost without exception respectable citizens of normal
ability (Fig. 203).

The Basis of Mental Traits. Many students of human heredity
have attempted to analyze the inheritance of mental qualities
without gratifying success. The behaviour of such characters can-
not be explained on the basis of simple Mendelian laws, although
it is impossible to avoid the conviction that a Mendelian founda-
tion is present in some degree of complexity. Even though
authorities disagree on the subject it seems that complexity is
the keynote to human ability and mental traits. So many different

HEREDITY IN MAN

371

kinds of ability combine to make a skilful surgeon, for example,
or an expert engineer or mechanic or musician, that in the absence
of exact knowledge we can only admit our ignorance of these
things. Of this fact we can be certain, that there is a heritable

Normof man
NJ /Vormat woman
Feebfe'mfndect man

m

a/ 735

-©

m

a/ 763

Feeble-minded woman

■<S)

n^

m

Undetermined man

d inf Died In infancy

D i770 %

fJame/ess jj

feeble-minded girt 1^

®-

During (he Revo/ution

^

II
35

m-

Diss 7

Normal wife
After the Pe\'o/ution /-^

aomdv-^v®®^;

d.
inf

tftisOn

Martiia

m

Deborah

Fig. 203.— The Kallikak family. Of
the 480 descendants of this branch
of the family 143 (21%) were
feeble-minded; only 46 (9%) were
normal; of the rest 189 (68%)
are still undetermined. 24 were
confirmed alcoholics; 3 were crim-
inals; 3 were epileptics; 82 died
in infancy; 41 were degenerate.

I

^H-®®® (n)(n)[n](n)

-(N)

-O

m

■%

Of the 496 descendants of this
branch of the familj-, none were
feeble-minded and all were good
citizens. Among them were edu-
cators, physicians, lawyers, judges,
traders, land-owners — men and
women prominent in every phase of
social life. Only 2 were alcoholics.
(From Coddard.)

372 EVOLUTION AND GENETICS

basis for the mental qualities involved as well as for the physical
structures.

Environment and Human Life. Writers have disagreed on
the part played by environment in the development of human
characters. We must realize that intelligence has given us a
degree of control over our environment which removes us almost
completely from the direct influence of natural conditions, and
such control cannot fail to have an effect. Through the facilities
of modern agriculture, transportation, and storage we maintain
an almost constant food supply whose seasonal fluctuations are
chiefly among the luxuries of diet. Engineering methods give us
summer temperatures in our homes during the winter, and warm
clothing protects us in proportion to our needs. Our supply of
light and water is also carefully regulated. It would be difficult
to provide more uniform and favorable surroundings than those
of civilized man.

Environment is only one of the fundamental factors of existence,
however, and we cannot expect it alone to shape the life of an
organism. If a child grows up in a musical family any musical
ability that he may possess is likely to be expressed, but if he
lacks ability there is no possibility that he can be made musical.
He may be forced through a course of training in music and liecome
a mediocre performer, but of such material true musicians are not
made.

With the breadth of opportunity available in modern society
it is difficult to avoid the conviction that as man has shaped his
own environment, so will the individual shape his. The least
that the individual can do is to seek the most favorable environ-
ment within his reach, and reach in America is a matter of inherited
capacity. The environment of earh' life may facilitate or retard
his progress but eventually the life of the individual is bound to
be an expression of his heritage. If he has not made himself a
good environment, he has shown himself deficient in a fundamental
quality of the human race.

Summary. While human inheritance cannot be studied as
readily as that of other organisms, enough is known to show that
it is based upon the same principles. There is a material basis
similar to that of other animals in the chromosome complex.
Anatomical unit characters of several kinds have been traced
repeatedly through several generations, and in some cases more

HEREDITY IN MAN 373

complex forms of Mendelian heredity have been demonstrated.
Human responses are of more interest than structures. These
are, however, based upon the structure and functions of various
organs, and for that reason have a heritable basis whose transmis-
sion from generation to generation is essentially the transmission
of the response. Mental defects and other undesirable qualities,
as well as unusual ability, are so transmitted. Many records are
available to prove these facts. While environment may have
some effect upon the progress of individuals, the power of man to
control his own environment is evidence that ultimate develop-
ment is largely the expression of the heritage.

REFERENCES

Kellicott, W. E., The Social Direction of Human Evolution, 1911.

GoDDARD, H. H., The Kallikak Family, 1912.

Davenport, C. B., "Heredity of Skin Color in Negro-White Crosses," Pub.

Carnegie Inst., No. 188, 1913.
Davenport, C. B., "The Feebly Inhibited," Pub. Carnegie Inst., No. 236,

1915.
Estabrook, a. H., "The Jukes in 1915," Pub. Carnegie Inst., No. 240, 1916.
Castle, W. E., Genetics and Eugenics, 1921.
Holmes, S. J., The Trend of the Race, 1921.
Shull, a. F., Heredity, 1926.
Gtjyek, M. F., Being Well-Born, 1927.

CHAPTER XXI
EUGENICS

It is a safe premise that man's chief interest is himself. We
are no longer at the mercy of the elements. Wild beasts have
ceased to be a daily menace. Even physical conflict among
ourselves is insignificant in comparison with the eternal struggle
of wild creatures. The instinct of self-preservation is still strong
within us but it has taken on a new significance which finds
expression in better homes, higher standards of living, and
education.

Perhaps it is only natural that we should regard ourselves as
the chosen species and feel that all else exists for our benefit.
Certainly this attitude prevails, whether natural or not, and we
go blithely through the years, making and modifying to suit our
needs. We have a right to a modest degree of conceit over our
attainments, for even the brief span of the twentieth century is
crowded with progress. But through it all thoughtful minds have
noted the very human tendency to choose pleasant rather than
useful activities, save under the pressure of necessity, and a
proneness to avoid fundamental facts of our very existence.

Civilizations have come and gone. Their contributions to our
own are of no mean value and their examples, good and bad, are
before us. We can see their errors and their greatness. W"e know
more than ever before of the foundations for both. We have
discovered how to secure and perpetuate desired qualities in other
organisms ; can we do the same for ourselves? An earnest attempt
is being made to answer this question in the science of eugenics.
Its material is complex and in many cases elusive, but there is
every reason to suppose that it will some day be an important
factor in human welfare. It is literally the science of good
birth.

The Problems of Eugenics. What Is Desirable? A funda-
mental requisite of any attempt to improve the human race is
accurate knowledge of what constitutes good birth. The com-
plexity of our social organization requires very careful judgment

374

EUGENICS 375

of this matter because the responses on which ordinary judgment
is based must be analyzed in terms of hereditary quahties by the
eugenist. It requires very httlc insight to note that the expert
mechanic has a number of quahties in common with the skilful
surgeon. The keenness of senses, manual dextcrit}^, and nice
coordination demanded of each in the performance of his duty are
not at all different fundamentally. The knowledge which each
man uses and the training which has given him the al)ility to
express these inherited powers are very different. When such
basic similarity can exist between such remote walks of life, who
can determine what qualities, what activities, are the most valuable
in our social structure from among the many heritable fundamen-
tals?

The Ultimate Goal. This is a matter which cannot fail to
concern us in a broad application of the principles of genetics
to our own future welfare. If we are to control our own destiny
our attempts should be directed toward the highest realization
of our powers and the maximum efficiency of our organization.
Such a program is at present too idealistic to be regarded as
practicable, but it is the only possible goal for eugenics. If we
are to succeed where other civilizations have failed, we must do
so through the elimination of their fatal errors from our own
racial lives and through the highest possible development of the
qualities that build civilizations.

Present Possibilities. In this quest for betterment of the hu-
man species we are fortunately not limited to idealistic pur-
suits. They are fascinating, it is true, and have claimed all too
large a place in the popular literature of eugenics, but scientific
sponsors of the movement are not blind to their dangers. A few
things are clearly valuable steps toward l)etterment of the human
heritage and from these a sound foundation for eugenic prog-
ress can be built even with the limited knowledge now avail-
able.

Since man's evolution has become almost entirely a matter of
intellectual development, mental capacity is of primary impor-
tance and the maintenance of a high level of intelligence is essential
to continued progress. Whatever may be the field of activity,
the individual with the greatest mental ability is certain to excel
in the performance of his duties, other factors being equal. The
details of such a generalization must be extremely varial)le;

376 EVOLUTION AND GENETICS

Goddard points out that the intelligence of a moron, for example,
fits him admirably for special education for the performance of
menial tasks in which he can feel a sense of accomplishment, while
a more intelligent person would find the same tasks drudgery.
We cannot avoid the fact, however, that intelligence is a valuable
asset. A man must have the physical capacity for his occupation,
but within limits he may even overcome physical inferiority by
the exercise of intelligence.

The physical basis for human activities finds its greatest expres-
sion in health, since we have now so largely replaced the need
of great physical strength with machinery. Normal bodies to
begin with and normality of functions throughout life are not only
a blessing to the individual but a real asset to society. Their
lack is not inimical to great accomplishment, as has been shown
over and over in the lives of great men, but it cannot fail to be a
handicap even to those who successfully overcome it.

Abnormalities, both of body and of mind, are fundamentally
abhorrent to the eugenist. Abnormalities of body are usually
nothing more than handicaps and so can hardly be considered as
justifiable material at present for eugenic control; they must be
relieved and corrected as far as possible but humanitarian con-
siderations forbid any further control. There are some very
serious hereditary structural defects whose perpetuation is a matter
of grave concern but these, fortunately, are not common.

Abnormalities of mind, however, are much more likely to be
insurmountable. The intelligence of a moron may not make him
a public charge. Indeed, if we follow Goddard 's teaching we must
regard morons as an asset when properly trained. Idiocy and
congenital insanity are on an entirely different plane; they are
certain to throw a considerable burden on the public and the
public should therefore have something to say about the proper
control of their lives. In 1910 there were 187,791 insane in hos-
pitals in the United States alone. Here is a case in which eugenists
can logically and humanely urge restriction. In addition to these
extremes there are many socially undesirable traits including
dipsomania, paranoia, immorality, etc., which are not necessarily
a matter of public concern in every individual displaying
them.

The amelioration of existing conditions is rather a matter of
sociology than of biology, but in many cases conditions are such

EUGENICS 377

that their perpetuation from generation to generation seems
unnecessary and undesirable. Here eugenics is attempting to
awaken humanity to the need for the sensible application to
ourselves of the principles which have been used so successfully
to shape the development of domestic animals and plants.

Limitations of Eugenics. The situation encountered in this
attempt is unique. No other animal consciously attempts the
control of its own future. Man has acquired the ability to do so
but the same qualities that have made it possible have, para-
doxically, made it difficult if not impossible. Control of environ-
ment is responsible for all of our material acquisitions; abundant
and constant food supply, physical comfort in spite of natural
conditions, and luxuries of all kinds are due to the fact that we
need not take things as they come but may shape them as we
desire. Every step in this direction has carried us farther and
farther from the primitive conditions of human existence. Social
as well as individual efforts tend to emphasize the individual and
yet the subordination of the individual alone can further the best
interests of humanity.

Strangely it is not the limitation of defectives that is the chief
source of difficulty; this is rather the most hopeful field for eugenic
progress in the near future. It is the preservation and improve-
ment of desirable classes that seems to be an insurmountable
difficulty because of the divine rights of the individual! De-
mocracy, the highest affirmation of equality of rights, puts the
matter squarely up to everyone. Will 7jou do the best thing for
humanity where you yourself are concerned? If not we can do
little to guarantee continued progress unless conditions change
enormously.

The Differential Birth Rate. Since eugenics is concerned
with the quality of the heritage a knowledge of the birth rate,
especially of different classes, is important. Moreover it is the
finest possible illustration of the facts just presented.

Decrease in Birth Rate. It is significant that the birth rate
of entire populations undergoes a gradual decrease with the ad-
vance of civilization. European countries in general show this
tendency, France to such a degree that her inadequate birth rate
is a matter of common knowledge. In the United States official
statistics on the birth rate are lacking but the following table is
indirect evidence of a similar decline.

378 EVOLUTION AND GENETICS

Decreasing Proportion of Children in the United States
(From Holmes, after Willcox)

r^ Number of Children under 5 per

^^^^ 1,000 Women 16-44 Years of Age

1800 976

1810 976

1820 928

1830 877

1840 835

1850 699

1860 714

1870 649

1880 635

1890 554

1900 541

1910 508

This is not, in itself, a serious matter. Natural conditions tend
to maintenance of species rather than to their increase, and there
is nothing in human welfare that demands any more than the
maintenance of our present population. The birth rate could
continue to decrease in the United States for many years without
bringing about a decrease in population.

Reproduction of the Unfit. A most unfortunate factor in
the decreasing birth rate is that the least desirable members of
society do not share in the common decrease. Many eugenists
have called attention to the fecundity of mental defectives, which
can probably be explained by the fact that they lead a more nearly
animal existence than persons of normal intelligence. Families
of five to eighteen children have been reported among such people
and even unmarried individuals contribute to their undesirable
stock through illegitimate unions. When associated with the
frequent transmission of their mental defectiveness to their off-
spring the fertility of these people becomes a serious prol)lem.
Whetham says: " Most of these children inherit the mental
condition of their parents, and where both parents are known to
be feeble-minded, there is no record of their having given iDirth
to a normal child. In one workhouse there were sixteen feeble-
minded women who had produced between them one hundred
and sixteen children with a large proportion of mental defect.
Out of one such family of fourteen, only four could be trained to
do remunerative work."

The Birth Rate of the Mentally Normal. Even among the
classes of desirable citizens a marked disparity may be noted.

EUGENICS 379

The studies of various investigators show a higher birth rate
among unskilled than among skilled laborers. Members of the
latter class usually marry later in life, although this is not neces-
sarily the direct cause of smaller families. Professional men
usually have smaller families than any of the classes mentioned
and consequently a lower birth rate, but the intellectual classes
are the worst offenders in this respect.

Data derived from several sources on the percentage of mar-
riages and the number of children per family among the graduates
of several colleges and universities disclose the following facts:

1. Graduates of colleges marry later than persons who have
not attended college.

2. A low percentage of college graduates marry, viz., about
75 per cent of the men and less than 60 per cent of the women.

3. The number of children in the families of college graduates
is low. "The average for Wellesley graduates between 1875 and
1899 was .83 of a child." Cattell's studies of scientific men show
that they have a little better record with almost 90 per cent
married and 1.88 children each in the completed families.

Since an average of more than three children per family is
necessary for maintenance without increase, it is obvious that the
intellectual classes are not maintaining themselves. When we
consider that college graduates include chiefly men who enter the
professions or become capable business leaders, the conclusion
becomes even more significant.

The Causes of a Differential Birth Rate. The gradual low-
ering of the birth rate in the ascending scale of ability and
intelligence and the fact that it has been found impossible to
correlate these things definitely with the fertility of individuals
suggests that it is rather the economic status and the foresight
of parents that determines the rate of reproduction. Parents who
have little or no sense of responsibility for their children produce
the most. The higher the standard of living, the less children are
produced. Many other factors enter into the problem but this
alone seems to be constantly applicable. The cost of raising
children properly is high. Education, especially higher education,
is expensive even though children of college age may be ready to
assume a part of the burden of their own maintenance. It is only
natural that these things should force parents with high ambitions
and a keen sense of responsibility for their children to limit the

380 EVOLUTION AND GENETICS

size of their families in proportion to their means. All too often
families are limited in spite of their means, and in this we must
agree with Shull's statement: "If, in some cases, selfishness leads
to a desire to avoid children, and if selfishness is inherited, as
it presumably is to a large extent, such people are probably
doing the race a service by permitting their lines to be extin-
guished."

It seems very probable that most persons do not give such
matters serious attention, and that sheer indifference is a contrib-
uting factor in their failure to reproduce. There are so many
attractive activities in the modern world to occupy the individual's
time that mere conflict of interests may result in the limitation
of family without any sense of race consciousness or economic
pressure entering the matter. Such a tendency must probably
be construed as a degree of selfishness; if so it is a very common
fault. The same indifference undoubtedly contributes to the
fecundity of the lower classes, who merely obey the powerful
instinct of reproduction without thought of the social and economic
consequences.

Immigration and the Birth Rate. Immigration is a national
problem for more reasons than one which cannot be considered
here, but it has had an important effect in the maintenance of
desirable elements in the population of the United States which
bears definitely on the problems of eugenics.

Such men as Carnegie and Steinmetz are fine evidences of our
indebtedness to foreign countries even within recent jTars for men
of exceptional ability. Our native American population must
acknowledge its foreign ancestry within a few generations of
ancestors, but even since the establishment of a fairly definite
American stock we have drawn constantly upon the European
nations. Opportunity has been the keynote of immigration; in
America lay opportunity which the Old World could not furnish
and many valuable men have taken advantage of it.

It is difficult to say what might have been the accomplishment
of such men if they had stayed in their own countries. One can
hardly imagine them contented with anything less than a leading
role in their chosen fields, yet the class hmitations of Europe are
undeniably more stringent than those of America and can scarcely
have afforded ample opportunity for the development of their
genius. Given the opportunity of America, where achievement

EUGENICS 381

is limited only by ability, they have risen to prominent places
in all walks of life.

Our leaders have increased in number with the increase of
population. Since they have not been maintaining themselves,
to what extent has immigration been responsible for the renewal
of these classes? The number of foreign names added to the
personnel of American institutions of learning since the world
war suggests that it may even now be an important source of
desirable citizens.

Immigration since the war has, however, shown an undesirable
trend toward peoples of proved inferiority. The precautions
taken in the admission of aliens are such as to prevent the entry
of undesirable individuals, Init we must expect a distinct reduction
in the valuable types that have previously entered this country
and a modification of the total effects.

It would be pessimistic to suggest that this change must have an
early effect on the general level of intelligence, or that it might
result in the extinction of those classes which are failing to repro-
duce themselves. It is possible that enough native-born citizens
rise above the standing of their parents because of increased
opportunity to offset that phase of the differential birth rate.
We must recognize in immigration, however, an important factor
which has previously exerted a valuable influence and now bids
fair to change. Moreover it is impossible to point out with cer-
tainty anything which is taking its place.

What Shall Be Done? In meeting these problems the eugenist
is confronted by the difficulty of securing a sympathetic audience.
Prosperity tends to blind a people to their future problems, and
when these problems are beyond the pale of their education, they
are doubly oblivious. Relief measures, especially when they are
of serious consequence to the individual, are not popular. It is
all too easy to leave action to the other fellow.

Reduction of Defectives. In spite of this difficulty the eugen-
ics movement has made a steady advance and offers several
proposals which are well grounded and worthy of serious con-
sideration. Among them the sterilization of mental defectives
has a prominent place. The year 1927 has witnessed a reaction
to this proposal which urges that not all mental defects are known
to be heritable and that even such a taint does not prevent the
production of valuable citizens by the defective hne. Such a

382 EVOLUTION AND GENETICS

cautious attitude is desirable but probably unnecessary. The men
in whose hands the administration of restrictive laws would be
placed would necessarily be trained scientists in whom we could
expect a maximum of ability and discrimination. The fact remains
that too many records show fecundity, pauperism, and feeble-
mindedness going hand in hand through the generations, and no
adequate reason can be given for the perpetuation of such con-
ditions.

Many states have already provided legally for the sterilization
of defectives under prescrilied circumstances, and many operations
have been performed under these laws. It is a matter of record
that some individuals have voluntarily applied for treatment.

Segregation of defective men and women is another measure
which would have the same effect. It is favored by many eugen-
ists, l^ut with the exception of cases in which confinement in
public institutions is necessary for other reasons it involves much
greater expense. Neither measure would bring about rapid reduc-
tion of defectives even if rigidly administered, but either would
be a step in the right direction.

Guyer presents the following opinions in favor of segregation:
"It has been urged against vasectomy [sterilization] that it will
work untold harm because it relieves of the responsibility of a
probable parentage. This argument does not appeal to one as
very weighty as far as the imbecile or other degenerate is con-
cerned, because one of the very traits characteristic of such indi-
viduals is lack of any sense of responsibility. By this same token,
however, we have a very good argument for sequestration as
against sterilization, for the degenerate, even though sterilized,
will not be restrained sexually and will be likely to disseminate
venereal diseases or commit rape. Furthermore, there will
be the temptation to sterilize and liberate certain types
that vv^ould otherwise have been kept permanently in cus-
tody.

Education of the Fit. The world has long cared for its depend-
ents, but if a valuable line ceases to perpetuate itself it is gone
forever. For this reason the correction of the disparity in birth
rate among desirable classes is greatly to be desired. Individuals
in these classes are responsible and self-maintaining. Together
they make up the backbone of social structure and their inter-
dependence is so complete that it is impossible to say that one is

EUGENICS . 383

more necessary than the other, however different their material
rewards may be.

Nol3ody would think of urging forcible control of these
classes, but education may ultimately succeed in awakening a
sense of racial responsibility which will bring about the desired
result. Attempts have been made to increase desirable families
by subsidizing human reproduction and various laws have been
suggested for their preferential treatment. None of these methods
have had or can be expected to have an extensive effect, and since
the classes with a low birth rate are not financially incompetent
it is doubtful that the desired adjustment can be gained in this
way. ]\Ioreover parents do not necessarily produce children as
capable as themselves, so that application of such measures might
well fail of its intended result.

Perhaps some of the educated people included in the classes
with a low birth rate are really ignorant of their responsibility
to society. Proper understanding of the prevailing conditions
would have a desirable effect in such cases, but the impression
that they do not constitute a majority is strong.

Education of the classes with a high birth rate, on the other
hand, might readily bring about the reduction which would be a
necessary step, in any case, in securing a satisfactory balance.

The Effects of Environment. Environmental conditions often
have a great deal to do with individual accomplishment, and many
agencies are directed toward the maintenance of proper conditions
of life. Child labor movements, public health, physical training
and other things are designed to offset the unnatural conditions
which result from our rapid social metamorphosis before the body
can adjust itself thoroughly to the change. They are valuable
corollaries of eugenics, although we are not yet in a position to
say that they actually contribute to the heritage. Certainly, since
they are practicable, they should be emphasized as the nearest
approach to the ideals of eugenics. They at least aim at the
maximum realization of inherited possibilities.

Practical Aspects of Eugenics. Education of a considerable
portion of any population in an unfamiliar movement is not easy.
Human beings respond emotionally with conspicuous readiness,
and to a subject like eugenics, demanding rigorous analysis of
inalienable personal rights, the response is likely to be emotional
opposition. Education must be gradual. Those who are able and

384 EVOLUTION AND GENETICS

willing to give careful consideration to such a movement should
do so; they should also learn to distinguish between the careful
decisions of well-informed scientists and the scientific quackery
which is all too common in matters concerning the human race.
No thoughtful eugenists would consider the wholesale regulation
of marriages beyond protection against unfortunate consequences,
yet the idea has gained a hold on the popular mind through
various agencies that this is proposed by eugenics. Our knowledge
of man is insufficient to accomplish the production of ideal types
even if such a course were desirable. Many of the popular ideas
of eugenics are myths which have had unjust and wholly unwar-
ranted consequences.

The whole program of eugenics at present may be summed up
as watchful waiting. Proposals of eugenic organizations recom-
mend principally extensive research in all fields related to human
heritage, education, both formal and popular, in heredity and
eugenics, and a policy of delay in attempts to secure legislation
in this field. Such a program is no more than the wise use of our
intelligence in relation to ourselves.

Summary. Eugenics recognizes the importance of the heritage
in man and proposes as an ideal goal the securing of an adequate
heritage for every individual. The movement is young but it is
even now possible to make positive proposals for the elimination
of obviously unfit strains, such as mental defectives. The differ-
ential birth rate shows the need of eugenic measures in all classes
of society, since the lowest classes are the most prolific and edu-
cated classes do not maintain themselves. In this field, however,
nothing but sound education is at present possible. The program
of eugenics therefore urges continued investigation, extensive
education, and only such legislation as progress warrants.

REFERENCES

Holmes, S. J., The Trend of the Race, 1921.

Studies in Evolution and Eugenics, 1923.

A BibliographT/ of Eugenics, 1925.

GuYER, M. F., Being Well-Born, 1927.

CHAPTER XXII
NATURAL SELECTION

Many theories of varying importance represent our attempts
to explain the evident relationship of organisms and their apparent
origin from a common source. Some of these theories were for-
mulated without knowledge of some of the important biological
principles which science now makes available for their evaluation.
It is therefore important to bring to the study of evolutionary
theory a sound understanding of the facts of organic relationship
presented in the foregoing chapters.

The theories represent, in the main, two general tendencies
which are foreshadowed by the two scientists, Lamarck and
Darwin. The former placed emphasis on the environment as a
factor in evolution, the latter upon inherent qualities. There is
less of a gulf between their views than is indicated by this common
treatment, but the fact remains that they have been regarded as
the exponents of opposed schools of thought.

Origin of the Theory of Natural Selection. No theory has
played a greater part, and none perhaps contains a greater measure
of truth than the theory of natural selection which was formulated
almost simultaneously by Charles Darwin and Alfred Russell
Wallace and later developed by Darwin in his famous book. The
Origin of Species. It was suggested to both men, strangely enough,
by Malthus' ideas of overproduction in the human race. It is
not to be supposed, however, that this alone was responsible, for
the application of the principle to plants and animals and its
elaboration to explain the origin of species with their infinite
diversity required knowledge of a wide range of scientific facts.
Both Darwin and Wallace were well informed in the field of natural
science, and the range of material presented by Darwin in his
books relating to evolution is truly remarkable.

Statement of the Theory. The theory of natural selection
is based on the fact that more individuals are produced in every
species than can survive, and on the occurrence of useful charac-
ters in the normal range of variation. It assumes that overpro-

385

386 EVOLUTION AND GENETICS

duction must result in a struggle for existence in which survival
is determined by the inherent characteristics of individuals;
those possessing useful variations will survive and those with
harmful variations or merely without the useful characters will
perish. The surviving individuals alone will perpetuate the
species and so their characters will become characters of the spe-
cies. Darwin added the belief that this process, through a suc-
cession of generations, would result in progressive development
of a character. To this simple foundation may be added other
principles which exert a similar selective action upon species, but
in the beginning it was literally a theory of the "survival of the
fittest."

Underlying Principles. Variation. The importance of varia-
tion was impressed upon Darwin's mind especially during the
voyage of the Beagle. He noticed as he travelled not only that
species varied in a given region but that as he passed from one
limit of their range to another their general characteristics also
varied according to geographical distribution. These facts showed
him that species, far from being rigidly fixed entities, showed a
tendency toward intergradation and developed the idea of change-
ability which was necessary to any thought of evolution.

In the classification of variations we have noted that some are
a part of individual life, both as process and result. These have
been called modifications and acquired characters, and are as
aptly characterized by one term as by the other. Better still they
may properly be looked upon as individual adaptations. Such
characters appear in every individual in response to definite con-
ditions of existence. Many examples are familiar to everyone,
including such common things as tanning of the skin, calluses,
muscular development, tolerance for poisons and the like. Mutila-
tions have also commonl}^ been included here, but for reasons to
be considered later they cannot properly be considered as charac-
ters of the organism. The possibility of modifications affecting
the heritage of the species has been one of the most bitterly
contested points in evolutionary theory.

In contrast to this kind of variation are two which are not
evident as processes within the individual but only as fully devel-
oped characters. To the extent that we are able to apply the
idea of heritage and response to characters of the organism we
must look upon them as a product of inherited powers responding

NATURAL SELECTION 387

to conditions within the body during development. Thc}^ include
adaptations of the spc^cies, but arc sometimes non-adaptive in so
far as external conditions are concerned. These two arc ordinary-
fluctuating variations which appear independently of external
conditions, and mutations.

Both fluctuating variations and mutations are available as
materials for natural selection, since they are known to be heritable
as they occur. The former differ from the latter in that they may
arise, and probably do arise in most cases, from the recombination
of unit characters made possible by sexual reproduction, but in
that they are both the direct result of chromosomal determiners
and inherited bodily qualities, both are definitely of the heritage.

Importance of Variations to the Individual. A further fac-
tor emphasized by Darwin in the Origin of Species is value to
the individual. Since no indifferent character can be of vital
importance to an individual, only useful or harmful characters can
play an active part in the shaping of a species. Indifferent char-
acters may, however, be incidentally or accidentally selected.

Among the indifferent characters of animals may be included
slight variations in the color and pattern of insects. Some butter-
flies have brightly colored ocelli, or eye-like spots, on the hind
wings which vary in number although not ordinarily to a sufficient
extent to cause a distinct difference in appearance, since a row of
spots is usually present. In the human race, however, variations
in resistance to or tolerance of the typhoid bacillus is of primary
importance. Some individuals suffer no ill effects from the presence
of these bacilli, and become carriers, while to others the disease
caused by their presence is fatal. A race of carriers would be, in.
effect, immune from the disease, while an ordinary population is
very variable in susceptibility.

Segregation. The last condition necessary for natural selection
is something to separate the individuals of a species into groups
possessing different variations or to preserve only a limited portion
of the individuals. The cause of segregation may be mere spatial
isolation brought about by topographic or climatic change or
migration, or it may be the relationships of organisms within a
limited region. The two causes are closely linked with distribution
and adaptive radiation respectively.

Spatial isolation may have varied effects upon the organisms
that come under its influence. It was made the basis of a theory

388 EVOLUTION AND GENETICS

of evolution by Moritz Wagner, with some reason, but in consider-
ing Wagner's work Darwin looked upon isolation as of minor
importance. He recognized, however, that it must necessarily
be a contributing factor in natural selection in some cases. While
it may perpetuate and even emphasize indifferent characters, it
may also subject the organism to specific conditions of environ-
ment or organic association which some variations may meet more
readily than others, and to this extent it is an effective stimulus
to the selection of the useful characters.

Overproduction and Crowding. Organic relationships, based
on the principle of overproduction and crowding suggested by
Malthus' work, were chiefly emphasized by Darwin as the imme-
diate cause of natural selection. Such relationships may, as we
have already seen, be due either to the competition of individuals
of the same species or those of different species, and may bring
about very different responses.

The fact of overproduction cannot be doubted. If the eggs
produced by single animals, or the seeds of single plants, were
all to mature, in a few generations one species would crowd the
earth to its own extinction. Darwin wrote: "Even slow-breed-
ing man has doubled in twenty-five years, and at this rate,
in less than a thousand years, there would literally not be standing
room for his progeny. Linnaeus has calculated that if an annual
plant produced only two seeds — and there is no plant so unpro-
ductive as this — and their seedlings next year produced two, and
so on, then in twenty years there would be a million plants."
But we have already considered cases of rapid increase of animals
of economic importance when freed from natural checks. When
we consider that oysters commonly produce 16,000,000 eggs, and
some fishes even more, we realize the vastness of overproduction
and the possible effects of interruption of the natural balance in
such species. Lull states the striking facts that if all the progeny
of one oyster survived and multiplied and so on until there were
great-great-grandchildren, these would number 66,000,000,000,-
000,000,000,000,000,000,000,000, and the heap of shells would be
eight times the size of the earth!

Competition. Such increase can only result in competition of
various kinds. Animals must have other organisms for food, and
so much of the surplus is destroyed. Plants must have sunlight
and moisture, and so they cannot be crowded beyond the available

NATURAL SELECTION 389

supply. Grass is a fine example of maximum crowding, but where
the periwinkle vine grows, it crowds out even grass, and where
the rainfall is slight, only sparse grasses can grow, since they nmst
draw water from a greater area of soil in order to live.

The struggle between organisms may thus be passive competi-
tion for the same things, or open conflict. Between animals it
may be either, between plants it is the former. Animals may
fight for the right to a favorable range, and the conflict between
carnivorous species and their prey can only be direct competition.
In many cases, however, competition implies merely that different
organisms seek the same thing. If the supply is ample, both
succeed; if insufficient, then first come, first served, and the loser
must die or change his mode of life. In all cases the relationship
is essentially the same. The organisms concerned are actually
pitted against each other, and whether the conflict is of tooth and
claw, or merely a matter of the early bird's getting the worm,
some individuals must lose out.

The Role of Useful Characters in Competition. It is in such
competition that the factor of usefulness is preeminent. Every
contact with the environment demands some power of response.
If struggle is merely in the form of passive competition for a
limited food supply, keenness of the senses involved in the
search for food may decide the winner. Some slight variation in
speed might well favor the faster animal in seeking the same
prey. Broad leaves might enable a plant in crowded ground to
secure necessary sunlight and at the same time deprive the small-
leaved neighbor in its shade of an adequate supply. The fact
that some plants vary to such an extent that they can meet the
requirements of very different environments emphasizes the
importance of such conditions. Some species of plants have finely
divided leaves when immersed in the water, but when stranded
by receding ponds, or when growing on muddy banks, they pro-
duce much less finely divided or even entire leaves (Fig. 204).
They may live either as hydrophytes or mesophytes.

Harmful Characters. In contrast to useful characters there
are some structures whose development may reach the point of
inconvenience. In one extinct species, the Irish elk, the enormous
antlers were probably a handicap. The broad wings of many
butterflies make flight in a wind impossible, while others with
narrow wings and strong flight muscles are hindered but little.

390

EVOLUTION AND GENETICS

Fig. 204. — Transition from aquatic to aerial habitat as shown by leaf struc-
ture. A, Proserpinaca paluslris, submerged leaf; B, same, aerial leaf; C,
Trapa natans, submerged leaf; D, same, aerial leaf.

The blade-like canines of the sabre-tooth tiger were apparently
so easily broken that their excessive development ultimately
became a handicap (Fig. 205). In addition to such positive char-

NATURAL SELECTION

391

tern

stm

Fig. 205. — Skulls of (A) the sabre-tooth tiger, Smilodon, and (B) a cat, Felv^,
showing contrasting skull form and musculature, c, occipital condyle;
cl. VI., cleidomastoid; dig., digastric; m, mastoid process; mas, masseter;
si. m., sternomastoid; /em., temporalis. (From Lull, after Matthew.)

acters, the extreme of variation opposite to any useful character
is hkely to be a hindrance, and is certain to operate in direct
competition to determine the loser.

392 EVOLUTION AND GENETICS

Accidental Destruction. Belief in the positive or negative value
of variation does not mean that they are absolute determiners
of individual success in all cases. Undoubtedly many well endowed
organisms are destroyed by chance, both through contact with
other organisms and through natural catastrophes. A volcanic
eruption or a great flood is an extreme condition which cannot be
met directly, but must be avoided if possible, and to many organ-
isms the chance for safety is not given. In general, however,
conditions of the environment are subject only to gradual fluctua-
tion with which normal variations may be an adequate basis of
adjustment.

Change of Habits and Migration. It is necessary to recognize
the fact that overproduction and consequent crowding, in addition
to determining the survivors in a given region, may result in
individuals utilizing their fitness for other conditions of the imme-
diate environment. If a species depends normally upon one kind
of plant for food, as is often the case among the insects, crowding
may result in the shifting to a new food plant of the individuals
which are less successful in securing a share of the available supply.
Shortage of food arising from any other cause might have a similar
effect. This response would depend, of course, upon the ability
of the animal to thrive upon the new diet. On the other hand,
such conditions might result in the species spreading through
gradual migration into adjacent regions, whereupon the factor of
spatial isolation might play a part in its development. During a
severe winter in the vicinity of Sioux City, Iowa, the writer once
witnessed a minor case of migration induced by hunger, although
it was, of course, temporary. Heavy snowfall, beginning early
in the winter, had kept the prairie chickens from their normal
supply of food. Under ordinary conditions it was unusual to see
more than two or three of these birds in a season, but during
that winter they had left their range, and on one day sixty-seven
were seen within a distance of a few miles. Hunters also reported
their abundance, but said that the birds shot were very thin.

Summary of Natural Selection. Natural selection makes use
of all of the foregoing factors. The theory may be summarized
as follows:

1. All organisms produce more young than can survive.

2. Struggle for existence results from overproduction.

NATURAL SELECTION 393

3. All organisms vary, their variations including useful and
harmful, as well as indifferent characters.

4. In the struggle for existence favorable variations are pre-
served and harmful eliminated, while indifferent variations may-
be incidentally influenced.

5. As a second result of crowding some individuals may be forced
into other habitats involving the use of other characteristics, or

6. Some may be forced into adjacent regions, in which variation
may be of immediate importance, either as a result of competition
or by geological disturbances.

7. Any variation either isolated or emphasized by any of these
processes will be preserved through heredity and made a character
of the species or variety descended from the originally isolated
progenitors.

8. Successive repetition of the process results in gradual change
leading to an ultimately wide separation from the parent
species.

Examples of Natural Selection. Hypothetical. As an example
of the operation of this process, we may assume a case of a carniv-
orous species, of which ten thousand individuals live in a given
area. The prey of these animals may be chiefly an herbivorous
species, which, because of an epidemic disease or some other
factor, is reduced in numbers to a point where it can suffice for
the food supply of only one-half of the carnivorous species.
Granting that the sole defense of the herbivorous species is speed,
it is very probable that the five thousand carnivores best adapted
for speed will l)e the survivors; they alone would be able to repro-
duce, and the inherited characters which gave them superior speed
would become a quality of the subsequent generations. The
herbivores likewise would be able to survive only if swift enough
to escape, consequently they too would attain greater speed as
a fixed quality of the species.

Wallace's Orchid. Wallace cites the case of a Madagascar
orchid {Angraecum sesquipedale) with an extremely long and deep
nectary. The flower is cross-fertilized only by long-tongued moths.
In reaching for the nectar the base of the proboscis comes into
contact with the anthers, and pollen is carried to the next flower.
Wallace's explanation of the case follows: "Now let us start from
the time when the nectary was only half its present length or
about six inches, and was chiefly fertilized by a species of moth
which appeared at the time of the plant's flowering, and whose

394 EVOLUTION AND GENETICS

proboscis was of the same length. Among the millions of flowers
of the Angraecum produced every year, some would always be
shorter than the average, some longer. The former, owing to
the structure of the flower, would not get fertilized, because the
moths could get all the nectar without forcing their trunks down
to the very base. The latter would be well fertilized, and the
longest would on the average be the best fertilized of all. By this
process alone the average length of the nectary would annually
increase, because, the short-nectaried flowers being sterile and the
long ones having abundant offspring, exactly the same effect
would be produced as if a gardener destroyed the short ones and
sowed the seed of the long ones only; and this we know by experi-
ence would produce a regular increase of length, since it is this
very process which has increased the size and changed the form
of our cultivated fruits and flowers." Wallace carries the example
on to greater lengths, but this alone is a sufficient illustration of
the logic of the theory.

Sexual Selection. As a corollary to his theory of natural
selection Darwin proposed the theory of sexual selection. This is
based upon the same fundamental principles as his chief theory,
but takes note of the choice exercised by the female in her selection
of a mate as the factor which determines the variation to be
preserved.

It is well known that animals at the mating season display
their qualities to the best advantage and parade them before the
opposite sex. Ordinarily the male alone finds it necessary
to woo a mate by these means, but Beebe's observations of
the tinamou, already mentioned, show the possibility of a complete
reversal of these instincts. Even common birds are well known
for their display of color and song during the mating season, and
a few species, such as the ruby-throat humming bird and the
yellow-breasted chat, go through wonderful evolutions in the air
which show remarkable powers of flight. Obviously the female
makes a choice from among her available suitors. Granting that
there is a definite basis for this choice, and that the things attrac-
tive to one female are those normally attractive to others of the
species, we may readily see that the variations in the selected
males would be preserved in their offspring. The characters of
rejected males would die out for lack of an equal opportunity to
be perpetuated.

NATURAL SELECTION 395

Objections to the Theory. Darwin recognized many objections
to his theory, and a few others have been added in later times.
The sahent objections may be summed up as follows:

L If species have descended from other species by fine grada-
tions, what has become of the many transitional forms?

2. Can the production of relatively simple organs and such
complex structures as the eye both be explained on this basis?

3. Can instincts be acquired and modified through natural se-
lection?

4. How can the sterility of hybrids be accounted for?

5. How can the development of neuter castes be accounted for?

6. Can natural selection account for the occurrence of vestigial
structures and overspecializations when they are of indifferent
value to the organism?

Most of these objections have been adequately met, but a few
indicate limitations of the theory.

Answers to Objections. Transitional Forms. Darwin's an-
swer to the first objection was: "As natural selection acts
solely by the preservation of profitable modifications, each new
form will tend in a fully stocked country to take the place of, and
finally to exterminate, its own less improved parent-form and
other less favored forms with which it comes into competition.
Thus extinction and natural selection go hand in hand. Hence, if
we look at each species as descended from some unknown form,
both the parent and all the transitional varieties will generally
have been exterminated by the very process of the formation and
perfection of the new form." The failure of these intermediates
to persist in abundance in the geological record is readily explained
by the incompleteness of that record; they do exist in small
numbers, and transitional individuals occur among some Uving
creatures.

Complex Organs. The production of complex organs was not
an insurmountable difficulty to him, although he recognized the
impossibility of explaining in detail just how the gradual elabora-
tion of such an organ as the eye might have occurred. As a
matter of fact, the eyes of modern insects range from a state
of development which makes possible merely the reception of light
stimuli and the determination of the direction whence they come,
to the elaborate compound eyes of higher orders (Fig. 206, compare
with Fig. 62 and 63). The very existence of stages of development

396

EVOLUTION AND GENETICS

bears out Darwin's answer to the objection. We may never be
able to explain the exact steps in the development of these organs,
but the belief that it has been gradual offers no greater difficulty
than the belief that such elaborate structures have sprung suddenly
into being.

Instincts. Instincts are difficult to explain, especially when
they involve but a single action during the lifetime of the indi-
vidual. Many larval insects produce cocoons as an exercise of
just such instinct. Darwin answered this ol^jection by pointing
out that inherent mental traits vary just as do structural charac-
ters, and that
useful variations
might be selected
as readily in the
one category as
in the other. The
selection of a def-
inite mental
tendency would
result in its be-
coming a part of
the inheritance
of the species. It
would then be
generally ex-
pressed by the
individuals of the
species, whether
only once or of-
ten within a lifetime. In support of this view Darwin mentioned
the occurrence of peculiar instinctive actions in related species
in widely separated regions; "... the Hornbills of Africa and
India have the same extraordinary instinct of plastering up and
imprisoning the female in a hole in a tree, with only a small
hole left through which the males feed them and their
young when hatched." The male wrens both of Europe
and America build several nests, only one of which is occupied
by the female. These facts point strongly to similar proc-
esses of development of the mental and physical traits of
organisms.

c

7j -

^m^

pzk

f

V

SI

Fig. 206. — Median section through a primitive insect
eye (OrchescUa rvfcscens var. pallida), x 7,50. c, cuti-
cula; hy, hypodermis; pzk, pigment cells; sz, visual
cells; C, caudal ; R, cephalic. (From Schroder's Hand-
buch der Etiloinologie, after Hesse, with the permis-
sion of the publishing house of Gustav Fischer, Jena.)

NATURAL SELECTION 397

Sterility of Hybrids. Such sterility and interspecific sterility
are probably due to many causes. Now that the minute structure
of the germ cells is known, we realize that the mechanism of
reproduction differs in various species as well as gross morphology.
Even in similar species this mechanism may differ, and the associa-
tion and separation of chromosomes may take place, if it takes
place at all, in such an irregular way that the normal process
of reproduction is prevented. The mere fact that species have
arisen from other species does not involve the maintenance of
fertility between the divergent ))ranches; if they differ in one way
they may well differ in others.

Neuter Castes. In social insects, such as the ants and ])ees,
neuter individuals do not ordinarily reproduce, but they are
merely non-functional females and may in certain cases lay eggs.
Worker bees do so when the colony becomes queenless and there
is no way to rear a new queen. In such cases the eggs laid by the
workers are not fertilized, since the insects never mate, and can
develop only into drones. There is possibility here for the trans-
mission occasionally of the specializations of the so-called neuter
caste, but it is doubtful that the conditions would occur often in a
natural state. Even discarding this possibility, however, we find
in more primitive species of the same order an explanation of the
development of worker structures and instincts. The functional
female, or queen bumblebee, not only acts as the mother of the
colony, but also carries on all of the duties of workers until she
has reared a colony to relieve her of such work. It is highly
probable therefore that the characters of workers appeared before
limitation of reproduction to one or a few individuals. This
would imply that no further change could occur in the workers,
but we must recognize that the queens vary, and that there is
possibility in this variation for the selection of characters to be
expressed in the workers. On the basis of such selection carried
on by bee-keepers, various strains have been established, as noted
in Chapter XIX. The difference between this and non-social
species is that the colony becomes the biological unit, instead of
the pair. Colonies are preserved or destroyed through the qualities
of the workers, which in turn are derived from the queen. The
individual is wholly subordinated to the welfare of the group.

Vestigial Organs. Probal)ly the most important objection to
natural selection is its failure to explain the persistence of vestigial

398 EVOLUTION AND GENETICS

organs. These organs are obviously in a state of gradual reduction
following the termination of their usefulness to the individual.
They are usually indifferent, but too uniform in occurrence to be
explained as indifferent structures incidentally retained. The
human appendix, for example, might readily be influenced by
natural selection to a point of indifference, since we see that it
is now a harmful structure through its tendency to become infected.
Beyond that point, however, its condition would be of no vital
importance, would neither eliminate nor preserve the individual,
and so could be changed only through the operation of other
forces.

Overspecialization. Overspecialization may conceivably bring a
structure to a stage of development which destroys its usefulness
without rendering it actually harmful. The tusks of the Columbian
elephant, now extinct, became very large, and in some specimens
curved so extremely that their tips must have crossed (Fig. 207).
Such organs could not be used for digging or for defense in the way
that the tusks of existing elephants are used, yet there is nothing
to show that they were sufficiently burdensome to this magnificent
creature to account for its disappearance.

The theory of natural selection is therefore widely applicable,
but is not a sufficient explanation of all evolution of species.
Darwin, in his earlier works, looked upon it as the chief process of
evolution, and some of his followers have given it even greater
emphasis, but even Darwin came to reaUze that other views were
not without a claim to serious consideration. The contributions
of the twentieth century have added definite theories no less
logical than Darwin's, if of less widespread appHcation, and to
these we may now turn our attention.

Darwin's theory may be looked upon as one of several partial
explanations of evolutionary change. It is limited in that it does
not account for the origin of characters, and only incidentally
affects indifferent characters. It may readily be seen, however,
that the useful and harmful characters of indi\'iduals play an
important part in their lives, and that natural selection logically
accounts for the preservation and elimination of such characters
as components of the species. Within these limitations it may
be applied extensively to problems of evolution, Ixit beyond them
it must give way to other theories. It is more a theory of adapta-
tion than of species-formation.

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399

400 EVOLUTION AND GENETICS

Summary. The theory of natural selection is the most impor-
tant of the great theories of evolutionary method. It is based
on the fundamental principles of variation and influences tending
to favor the development of a limited range of the variations of a
given species. These influences may be either spatial isolation or
the effects of overproduction and resultant crowding. Crowding
brings organisms into competition in which useful variations aid
in the preservation of the individuals possessing them and harmful
variations hasten destruction. The result is the preservation of a
limited number of individuals with some uniformity which is
preserved in their offspring, and the range of variation in the
species is thus shifted to that of the surviving individuals. Darwin
interpreted this as a cumulative process which would ultimately
bring about a greater development of the characters preserved.
Various objections to the theory have been proposed and many
of them have been satisfactorily answered, but they lead to the
conclusion that natural selection is an effective process of adapta-
tion rather than of species-formation.

REFERENCES

Wallace, A. R., Contributions to the Theory of Natural Selection, 1870.

MivART, St. G., On the Genesis of Species, 1871.

Darwin, C, The Origin of Species, 6th edition, 1880.

Romanes, G. J., Darwin and After Darwin, 1892.

Morgan, T. H., Evolution and Adaptation, 1903.

Kellogg, V. L., Darwinism Today, 1907.

Lull, R. S., Organic Evolution, 1917.

Newman, H. H., Readings in Evolution, Genetics and Eugenics, 1921.

CHAPTER XXIII
OTHER THEORIES OF GERMINAL SELECTION

In the preceding chapter we have seen that Darwin's theory of
natural selection, while it appeals strongly to th(^ logical mind as
having a definite bearing upon the process of evolution, is by no
means a complete explanation, as has sometimes been urged. In
recognition of its hmitations various scientists have proposed
other theories to account for those processes of evolution which
are not within the scope of natural selection. Some of these
theories have returned to the Lamarckian concept of environ-
mental influence; these will be treated in the next chapter. Others
have assumed the heritability of the factors involved and conse-
quently stand on the same basis as natural selection; these are
our present concern, and they may be known as theories of germinal
evolution since they look upon changes in the hereditary germ
plasm as causes and not results of evolution.

The failure of natural selection to account for the origin of
variations is shared in part by these other theories but this subject
involves factors which can better be treated in a succeeding chap-
ter.

The Limitations of Selection. Darwin's use of fluctuating
variations as a basis for evolution has been a source of difficulty,
for in artificial selection and the experiments of geneticists it has
been shown that the possibility of change through selection of
such characters is probably limited. Artificial selection was
emphasized by Darwin as an example of the possibility of rapid
change in organisms through selective processes. It is now recog-
nized that the aggregations of individuals making up a species
vary not only in actual range of characters, but also in the latitude
of variation which they are able to impart to successive generations,
and that the limits of artificial selection are the separation of the
various pure lines. Short and tall races may be produced, for
example, from a normal population, but the maximum and
minimum cannot be extended indefinitely by selection alone.

This question is closely related to Galton's law of filial regres-

401

402 EVOLUTION AND GENETICS

sion, and was clearly explained by Johannsen's work on pure
lines, which was described in Chapter XIX.

The Mutation Theory of De Vries. Darwin recognized the
occurrence of sudden departures from parental characters, or
saltations, but made little use of them. They later became the
basis for the mutation theory of De Vries, which disposes effectively
of the question of specific change by disclosing the possibility
of minute mutations serving as materials for natural selection.
Such variations accomplish the actual shifting of specific charac-
ters which in ordinary fluctuating variations is limited to the
isolation of pure lines.

De Vries, a Dutch botanist, conducted a long series of experi-
ments with the evening primrose, Oenothera lamarckiana, as the
basis for his mutation theory. He found that this species was
highly variable in its natural habitat near Hilversum, in the
vicinity of Amsterdam, where it was observed for several years.
De Vries speaks of some of its variations as new species. "Some
of them were observed directly on the field, either as stems or as
rosettes. The latter could be transplanted into my garden for
further observation, and the stems yielded seeds to be sown under
like control. Others were too weak to live a sufficiently long time
in the field. . . . These various methods have led to the discovery
of over a dozen new types, never previously observed or described."
(Fig. 208.)

To give an account of the differences between the species or
mutants so produced, would be beyond the needs of this work.
It is sufficient to note that the degree of difference between mutants
and the forms from which they originated was great enough to
lead De Vries to call them species; it was as great as differences
between other related species of plants, and greater than exists
between some divisions commonly called species. In addition a
mutant once produced continued to produce offspring like itself — an
essential factor which distinguishes true mutants from mere sports.

A significant feature of De Vries' results was the production of
a species sciniillans, which in turn gave rise to mutants. This
species was produced, like others, from self-fertilized seeds of
Oenothera lamarckiana, and among the mutants that it produced
were many lamarckiana. some oblonga, and a few lata, in addition
to a moderate percentage of true scintillans. Similar reverse mu-
tation has since been secured by Muller in Drosophila.

Fig. 208. — Oenothera lamarckinna and some of its mutants. A, lamnrckiarm;
B, gigas; C, oblonga; D, scintillans; E, lata; F, nanella. (From Castle's
Genetics and Eugenics, with the permission of the Harvard University Press.)

403

404 EVOLUTION AND GENETICS

Objection to the mutation theory as an explanation of species-
formation has been based chiefly on the repeated assertion that
lamarckiana was probably a hybrid, and that the mutants were
merely different strains separated from the hybrid. Such a view
is plausible, for a homozygous recessive based on two determiners
would have only one chance in sixteen of appearing, on three
determiners only one chance in sixty-four, etc. However, a
recessive once produced would be unable to produce the charac-
ters of its parents among its offspring, as was the case with
scintillans.

Examples of Mutants. Fortunately, although species very
productive of mutants are open to interpretation as hybrids, there
are many examples of apparently stable mutants. Not the least
of these are the classic examples of the Ancon sheep and polled
Hereford cattle. Each is said to have originated from a single
mutant. The sheep first appeared in the flock of Seth Wright, a
Massachusetts farmer, in 1791, and the hornless Hereford mutant
in a herd of horned Herefords at Atchison, Kansas, in 1889. The
fact that the mutant character proved to be dominant over the
normal condition in each case is another argument against the
hybrid-species interpretation, since dominant genes, if present,
are expressed. If separated as homozygous dominants from a
hybrid race there is no somatic difference from the parent unless
the hybrid character is a blending or mosaic.

In the vast amount of work done within recent years on the
fruit flies of the genus Drosophila many mutations have been
discovered which illustrate the possil^ility of very slight departure
of a mutant from the characters of its parents. Such slight depar-
tures as have been recorded in these flies have not been interpreted
as specific differences, in fact, in any case. Even a slight mutation,
however, is inherited, and if of any use to the individual or indi-
viduals in which it appears, may well be a deciding factor in the
preservation of its possessors.

Mutations in Natural Selection. It is the operation of natural
selection upon such definite even though slight departures from
the parental form, that seems most likely to account for the
formation of species according to Darwin's ideas, rather than its
operation upon fluctuating variations. If the short-legged Ancon
sheep, for example, had occurred in a natural state and not under
domestication, and if it had depended upon speed for safety, it

OTHER THEORIES OF GERMINAL SELECTION 405

would iindoubtedly have been more quickly eliminated than any
of its longer-legged relatives.

Mutations apparently do not depend on natural selection,
however, for their potency in the formation of species. We must
recognize that whether we are willing to call them species or
merely varieties, some of the forms that have arisen this way
are entitled to specific rank, provided only that they can satisfy
other criteria for the limitation of species than mere morphological
differences.

The Cause of Mutations. The possible cause of mutations
has been the subject of nuich discussion. An early view compared
the species to the individual in its development from infancy to
old age. In its period of maturity the species was supposed to
produce other species as the individual produces its offspring.
Such an analogy is attractive but hardly a soundly scientific
explanation. There is more reason to believe the conclusion of
modern geneticists that abnormal behaviour of the chromosomes
or modification of the genes by unexplained processes may be
responsible. Even this opinion, while it leaves us with a definite
basis for future investigation, also leaves us with a feeling that
we have not explained the origin of mutations; rather we have
merely paved the way for further study of the problem.

Given living organisms and their several kinds of variations
we now know something about the processes of change which go
on within them, but we have not yet penetrated beyond that
starting point. Our progress is not discouraging for there is still
much to be discovered by accurate scientific study of these matters,
and additions are ]:)eing made to our knowledge every year.

Weismann's Theories. Two other corollary or substitute
theories of evolution were proposed by Weismann, one of the
greatest followers of Darwin. One, the theory of germinal selec-
tion, is a frank attempt to meet the failure of natural selection to
explain the modification of organs beyond the range of importance
to the individual. The other, panmixia, was first developed for
the same purpose and later discarded in favor of germinal selection.
Weismann recognized that his theory of panmixia explained the
modification of indifferent organs to a hmited degree even though
he found it necessary to discard it as an explanation of evolution.

Panmixia. The principle of panmixia is that indifferent organs,
since they are of no consequence to the individual, will be mingled

406 EVOLUTION AND GENETICS

at random during reproduction and so will have an effect on the
evolution of the species. Certainly these organs can have no
effect in determining the survivors of a generation or the propaga-
tion of the species, and so they cannot be the basis for natural
selection. It is equally certain that they will be inherited, regard-
less of their state of development; the greatly reduced will be
mingled with the highly developed and the result will be an aver-
aging of development of the organ in the entire species. This is,
in effect, a reduction in the case of formerly useful organs, but
it can hardly luring about an extreme degree of modification.

In 1904 Weismann wrote of this theory: "... I was obliged
to seek for some other factor in modification, which should be
sufficient to effect the degeneration of a disused part, and for a
time I thought I had found this in panmixia, that is, in the mingling
of all together, well and less well equipped alike. This factor
does certainly operate, but the more I thought over it the clearer
it became to me that there must be some other factor at work as
well, for while panmixia might explain the deterioration of an
organ, it could not explain its decrease in size, its gradual wearing
away, and ultimate total disappearance. Yet this is the path
followed, slowly indeed, but quite surely, by all organs which
have become useless." We now recognize this process, which is
nothing more than the amphimixis of modern biology, as a means
of diversification, or distribution of the various degrees of variation
of specific characters, and of maintaining a reasonably definite
association of such characters.

Germinal Selection. Weismann's realization of the weakness
of the theory of panmixia led him to formulate under the name of
germinal selection a " theory which now seems of no more value
than the former, yet it was based on some conceptions of living
substance which are worthy of consideration. An examination of
these conceptions leads one to the feeling that Weismann very
narrowly missed some important conclusions. In judging his
work we must remember that biology was scarcely beyond its
infancy during the early years of his scientific activity, and that
the great modern discoveries in genetics were little more than
begun when he died in 1914, at the age of eighty years.

Weismann assumed as a starting point for his theory that the
germ plasm is composed of different living units or "determinants"
which control the development of different parts of the organism.

OTHER THEORIES OF GERMINAL SELECTION 407

Although his intorprctation of these determinants is not exactly
the same as our modern understanding of genes, it is a tribute to
his insight that he came so near to the fundamentals of the modern
theory. He cautions his readers that nothing can be learned
directly of the intimate structure of the germ plasm; this difficulty
has, of course, been partly removed by modcn-n cytology, although
we are still unable to see the genes in which we now believe so
firmly.

The next step in his theory assumes that, since these particles
in the germ plasm are living matter, they are nourished, grow, and
multiply. We know no more about this matter than Weismann
did, but we must still admit that his opinion is soundly logical.
However minute the gene may be, or whatever may be the nature
of its organization, we cannot fail to regard it as living matter,
subject to the same fundamental laws that have been observed in
larger units.

Weismann then assumed that irregularities occur in the nourish-
ment of the determinants and that they differ in their capacity
to make use of available food. As a result he supposed that some
would increase in vigor at the expense of others, and attain greater
expression of the structures which they produced in the body.
Such a modification of determinants would necessarily be cumu-
lative, for weakening would still further reduce the possibility
of a part's securing adequate nourishment. The result would be
a gradual modification of the characters of the organism, some
increasing and some decreasing in development.

Criticism of Germinal Selection. At this point the unsound-
ness of the theory becomes apparent. We must consider that
most of the organisms with which we are concerned are normal
and lead a normal existence. There is every reason to believe
that in such organisms every part of the body is supplied with
adequate nourishment and that competition between the various
parts does not occur. Even in starvation the whole body starves
together; distribution of the available nourishment is made for
the greatest common benefit. When we consider the capacity
of individual determinants for securing available nourishment we
are dealing with hereditary quaUties; the very existence of a living
unit would demand suflficient ability to meet existing conditions.

It is impossible to see how this theory can account for evolu-
tionary changes in organisms except on the basis of varying

408 EVOLUTION AND GENETICS

hereditary qualities in the germinal determiners. With our modern
knowledge of heredity and the theory of the gene we realize that
germ cells do contain the genetic foundations of the new individual
and that the genes are probably subject to variation like the
characters which they bring to expression. This does not, how-
ever, explain the process of change.

Three points in Weismann's theory should be borne in mind:
1. The idea of determinants foreshadows the well-established
theory of the gene, although it was necessarily vague and imperfect
in detail. 2. Weismann's emphasis upon nourishment brings out
the importance of environment. This is all the more suggestive
when we consider that he did not believe in the activity of the
environment in evolutionary processes. 3. The idea that deter-
minants, although hypothetical structures, obey the same laws as
the larger units of living matter which can be observed and sub-
jected to experiment is an important and necessary interpretation
which we can apply profitably to the gene.

Roux's Intraselection. Prior to the formulation of Weismann's
theories, Roux proposed a theory of intraselection which was
fundamentally similar in that it assumed struggle among parts of
an organism for opportunity to develop as a basis for the evolution
of its parts. The theory assumes that conditions within an organ-
ism include mechanical relationships such as contact and pressure
which exert an influence upon the development of the parts
involved. For example, pressure on a bone is supposed to result
in a greater deposition of bony tissue to meet the stress. Objection
has been made to this theory on the ground that it involves a
distinctly Lamarckian point of view, for cianges so induced would
be responses to the internal environment. Many biologists are
still unwilling to accept this point of view. In any case the theory
is of minor importance, and must be looked upon as a subsidiary
theory. It has not been wholly condemned, although there are
excellent reasons for only partial acceptance.

Coincident Selection. This theory, variously expressed, has
been widely used in an attempt to utilize Lamarckian characters
without recourse to Lamarckian principles. Proponents of the
theory recognize the value of individual adaptations, but explain
their effect as merely the preservation of the organism until the
appearance of germinal variations capable of producing similar
adaptations in the entire species.

OTHER THEORIES OF GERMINAL SELECTION 409

"It has been ol)joctcd to this thcoiy, that since the individually
acquired modifications possess the main selective value in these
instances, there is no reason why the corresponding germinal
variations should be fostered at all. The individuals with the
right, but slight, congenital variations would have no special
advantage over their fellows who show no such coincident varia-
tions. Nor is there any ground to assume that the individuals
with the greatest amount of plastic modification in a given direc-
tion will tend to exhibit similar innate variations to a greater
degree than those individuals not possessing this plasticity."
(Herbert.)

The Theory of Isolation. In the discussion of natural selection
Wagner's theory of isolation was briefly mentioned. It is without
doubt one of the valuable theories of species-formation, although,
as previously indicated, its fundamental principle of isolation may
also be an initial stimulus for natural selection.

Isolation may be due to any factor that prevents the mingling
of individuals for purposes of reproduction. Such factors may be
purely biological, and they may be purely physical. The writer
once noted the flight of a small l^utterfly, Pleheius melissa, over a
period of several years, and found that the males were abundant
six weeks before the appearance of the first females. The fortuitous
destruction of such delicate insects must be great, and the chance
of the earliest males to mate correspondingly slight. Such seasonal
distribution amounts to isolation through biological factors alone,
and must have some influence on the development of the species.

The best example of physical isolation is found in the separation
of land areas by barriers. In oceanic islands isolation of this kind
is often extreme^ and in such a group as the Galapagos Islands the
animals may show definite correlation of characters and distribu-
tion which is apparently the result of isolation. Beebe's account
of the mocking birds, in Galapagos, World's End, is an excellent
exposition of this correlation. It is as follows:

"... Without doubt we have here birds which were once living
on a single land mass — connected long ago with the mainland,
probably along the Costa Rican-Panamanian latitude, and later
insulated on a single large Galapagos island. This in turn has,
through subsidence, been reduced to a few volcanic peaks, on all
of which Nesomimus still survive, and which have begun to differ
slightly among themselves. This is due probably to causes far

410

EVOLUTION AND GENETICS

other than resulted in the general, generic wing reduction and leg
increase, and causes of relatively less importance, whether initiated
as internal variations or in response to external conditions we
know not. But however these were initiated and preserved, we
have a splendid example of evolution in the three planes of space.

Fig. 209. — The geographical distribution of song sparrows. Each number
indicates the habitat of a distinct subspecies. (From Woodruff after Chap-
man.)

Hence the facts so annoying to the taxonomist of a mocker in
Tower Island, far in the northeast of the archipelago, which has
a black moustache like the birds in Chatham, seventy miles to
the southeast, a black back like those in Bindloe, thirty miles due
west, a large beak corresponding to the beaks of the Abingdon
birds, forty-five miles northwest, and yet which in all characters

OTHER THEORIES OF GERMINAL SELECTION 411

is almost identical with the Nesomimus of little Culpepper, a
speck nearly a hundred and fifty miles to the northwest.

" . . . if we conceive that the ancestors of the birds on Wenman,
Tower and Indefatigable were identical, and gradual subsitlence
separated the three, causal variation would go on with a slightly
greater emphasis on one character or another, and with no possi-
bility of indication of specifically separate relationship. . ."

Such is the action of isolation upon the course of evolution.
Merely the separation of individuals occupying different positions
in the range of variation of the species is enough to account for
differences in succeeding generations in the separate localities. It
makes no difference whether the characters concerned are impor-
tant or indifferent; the mere fact that they are a part of the
heritage of the species is enough to make them a source of future
difference in the isolated races. The distribution of song sparrows
in North America is a similar case which has been expressed
graphically in Figure 209.

Although Darwin placed little emphasis upon this theory,
recently scientists have recognized that it is certain to be a potent
factor in the shaping of species. It almost seems axiomatic, in
view of our knowledge of geological changes, the possibility of
accidental dispersal beyond normal barriers, and the ordinary
range of variation, that this should be the case. Quoting Newman,
"If natural selection may be said to be the prime factor in pro-
ducing adaptations, isolation may be said to be the prime factor
in species differentiation, guided only within moderate limits by
natural selection."

Common Objections to Germinal Theories. In conclusion we
should note several characteristics of these theories of evolution
which place them on an equal basis with the Lamarckian theories
to be treated in the next chapter.

1. No theory of germinal evolution accounts for the origin of
variations.

2. Only the mutation theory offers an adequate explanation of
change beyond the isolation of pure lines.

3. Specific change has been accomplished very gradually in so
many cases that the mutation theor^^, in spite of its obvious value,
seems to be limited in application.

Summary. The limitations of natural selection as an explana-
tion of the origin of species were clearly indicated by Johannsen's

412 EVOLUTION AND GENETICS

study of pure lines. Other theories which have been proposed to
supplement or replace that of Darwin are the mutation theory of
De Vries, Weismann's theories of panmixia and germinal selection,
Roux's intraselection, coincident selection, and the theory of
isolation. The occurrence of mutations is well established and
seems adequate to explain the formation of species in some cases
alone and in some through a process of selection of minor muta-
tions. Weismann's theories are now given little attention but
they deserve to be considered for several valuable points even
though they must be discarded as theories of evolution. Isolation
is apparently an important factor in the development of variations
of all kinds into specific characters. The remaining theories are
interesting chiefly in the fact that they emphasize the association
of changes with external influences. Since their authors were
avowedly against Lamarckian principles this tendency is doubly
suggestive of the inevitability of such association. Even the
writings of Darwin are crowded with Lamarckian explanations, in
spite of his early antipathy to Lamarck's views.

REFERENCES

Morgan, T. H., Evolution and Adaptation, 1903.

Weismann, a., The Evolution Theory, 1904.

De Vries, H., Species and Varieties, Their Origin by Mutation, 1905.

Herbert, S., The First Principles of Evolution, 1913.

Lull, R. S., Organic Evolution, 1917.

Newman, H. H., Readings in Evolution, Genetics, and Eugenics, 1921.

Walter, H. E., Genetics, revised edition, 1923.

Eldridge, S., The Organization of Life, 1925.

CHAPTER XXIV
THE LAMARCKIAN THEORY

The Lamarckian conception of evolutionary processes has had
a more varied career than any of the other existing theories. It
has been treated by most evolutionists as the antithesis of Darwin's
theory, and most works on this subject devote themselves in some
degree to the support of one view and the denial of the other.
Under the common designation, the inheritance of acquired char-
acters, the ideas credited primarily to Lamarck have usually
suffered in these encounters, and have been dismissed with the
conclusion that they are still unproved. Common emphasis upon
other theories of descent has resulted in their being much more
widely accepted. It is true, nevertheless, that an occasional
defender has come to the support of Lamarckian views, and that
others have lent their support more or less unwittingly to them in
dealing with difficult questions of evolution. Weismann, for
example, as pointed out in the last chapter, made use of an out-
standing environmental factor in expressing a theory of germinal
evolution. At present there is a marked tendency among scientists
to feel that Lamarck's views have more to commend them than
has been admitted, but they have so far not received so convincing
support as Darwin's theory of natural selection and the mutation
theory.

Lamarck's Theory of Evolution. Lamarck was not the first
scientist to express Ijelief in the shaping of organisms ])y their
environment. He was preceded in this by Buff on and Erasmus
Darwin, l)ut like Charles Darwin, he was the first to give the
matter adequate presentation to impress it upon the scientific
world. That his theory was so largely ignored by his contem-
poraries was due to a number of factors, among them the opposition
of his influential associate, Cuvier. It is abundant testimony to
the importance of his theory that his name and Darwin's are
preeminent in the literature of evolutionary theories.

Lamarck's theory, when first published in 1809, consisted of
two laws which have been translated as follows:

413

414 EVOLUTION AND GENETICS

a

First law: In every animal which has not exceeded the term
of its development, the more frequent and sustained use of any
organ gradually strengthens this organ, develops and enlarges it,,
and gives it a strength proportioned to the length of time of such
use, while the constant lack of use of such an organ imperceptibly
weakens it, causing it to become reduced, progressively diminishes
its faculties, and ends in its disappearance.

"Second law: Everything which nature has caused individuals
to acquire or lose by the influence of the circumstances to which
their race may be for a long time exposed, and consequently by
the influence of the predominant use of such an organ, or by that
of the constant lack of use of such part, it preserves by heredity and
passes on to the new individuals which descend from it, provided
that the changes thus acquired are common to both sexes, or to
those which have given origin to these new individuals."

To these laws he added later the idea that necessity in the
organism gives rise to new organs. Other corollaries expressed
his belief in various modifying factors, but essentially his theory
involves the belief that change springs from the action of the
environment upon organisms. He supposed that this action was
direct in the case of plants, but that in animals it involved response
of the organisms to the environmental condition. His belief in
the inheritance of such changes as are obviously produced in
correlation with environment is the point which has always been
the chief subject for opposition.

Criticism of Lamarck's Laws. Use and Disuse. Lamarck's
first law concerning the results of use and disuse in organisms is
apparently sound. Every individual during his life furnishes an
example of the effects of functional activity. If we exercise a
muscle repeatedly it gains in development, usually to a visible
degree, and whatever may be the effect on its bulk, the power
exerted by its contraction is certain to increase. Conversely
failure to use muscles results in diminution of muscular strength,
A man who leads a sedentary life is not expected to have the
powerful muscles of a laborer, and a muscle which for any reason
cannot be used at all, may atrophy to an extreme degree.

In considering the effects of use and disuse we need not confine
ourselves to the obvious cases of physical activity. Tolerance for
poisons is an expression of an obscure functional power of the
body, but it is no less subject to increase or decrease. Increase

THE LAMARCKIAN THEORY 415

of individual tolerance for the habit-forming drugs is a case
commonly known. Ordinary poisons such as strychnine and
arsenic have a similar influence. They are used in minute quanti-
ties in medicine, and if these doses, tolerated by the normal
human bodj^, are gradually increased, a dose can ultimately be
administered without causing harm which would be sufficient to
kill a number of men if administered without previous preparation.
If such an extreme dosage is suddenly discontinued, the effects
are as serious as its sudden administration, but it can be reduced
gradually until the body is again able to get along without it.

Functions of such organs as the eye are usually thought of in
terms of normal exercise and excess. It is true that excessive use
of the eyes frequently results in their impairment, but not less
true that reasonable use of the eyes results in increased power
of vision. Biologists who constantly use the microscope are likely
to use one eye to the exclusion of the other, either because of
natural preference or convenience in drawing or writing while
making observations, and it has often been noted that the eye
habitually used becomes much more effective for microscopic
observation. The converse cannot be demonstrated satisfactorily
in this case, for eyes when present, cannot often be wholly unused.
Such cases as the rudimentary eyes of moles and cave animals
are suggestive, but they require interpretation.

The evidences of biology therefore support the first of Lamarck's
laws. We may definitely conclude that the use of functional
power brings about its increase, and disuse it decrease. The
matter of size is not necessarily a corollary of functional power,
and so is of fimited significance.

The Inheritance of Acquired Characters. In fornmlating his sec-
ond law Lamarck made an unjustifiably positive statement.
No matter how confident we may feel that changes which appear
in individuals in response to environmental conditions may exert
an influence over future generations, we must admit that evidence
of such action is not available. The facts of palaeontology often
show such a gradual transition of phylogcnetic changes in correla-
tion with a changing environment that they almost convince us
of the potency of the environment in bringing about the evolution
of organisms, but careful consideration of such individual acquisi-
tions as are frequently produced will show that every generation
develops them anew; at birth there is no trace of the characters

416 EVOLUTION AND GENETICS

which the parents had acquired. Such structures as heritable
callosities are obviously similar to the calluses which can be
developed in an individual, but relationship between the two has
not been proved. The association of characters of these two types
has long been one of the most vexatious problems in biology, and
has been the foundation for many experimental studies.

Necessity in the Production of Organs. The idea that need
in an organism may bring about the development of a new organ
is scarcely worthy of scientific attention. On a purely logical
basis it may be refuted, for organisms, in order to exist, must
have all of the organs necessary for successful existence. In
human experience need may be interpreted in a different way;
in the biological sense only those things are needed which enable
the organism to live successfully and to perpetuate the species.
In an examination of the facts of biology, and in particular of
adaptation, it seems rather that organisms have made use of such
things as they have possessed than that they have developed
organs to meet preexisting needs.

The Unity of the Organic World. Finally Lamarck's opinion that
the environment acts directly on plants and indirectly through
the nervous system on animal structures expresses a naive disregard
for the fundamental unity of the organic world which could hardly
fail to weaken the theory of which it is a part. A liberal inter-
pretation shows that it is probably based on the much more inti-
mate association of plants with the physical environment. We now
recognize that whatever is true of the fundamental evolutionary
processes of one kingdom is in all proba])ility true of the other.

Lamarck's theory therefore narrows down to one critical point
of controversy, viz., the inheritance of acquired characters. All
other premises are either well established or universally denied,
but this one point has baffled scientists. Of the many attempts
which have been made to prove or disprove it experimentally a
few are significant. The results obtained have generally failed to
establish even the possibility of such inheritance, but positive
disproof has also been lacking. The question still arises and still
finds ardent defenders as well as opponents.

Acceptance of the Inheritance of Acquired Characters. The
proponents of Lamarck's view include all kinds of thinkers, among
them men of philosophical mind who were content with a purely
logical analysis of the question and able biologists whose opinions

THE LAMARCKIAN THEORY 417

should have some weight as scientific deductions. We can learn
very little therefore from th(^ controversial matc^-ial now available.
One side presents as imi:)osing authority as the other; the scientists
are by no means united against the inheritance of acquired char-
acters, nor are its supporters lacking in scientific training and
ability.

Since ancient times many men have accepted the idea in con-
nection with human evolution without any critical attempt to
establish its validity. The close association of human progress
with the activities of past generations is sufficient to justify this
feeling, but far from sufficient as yet to prove that any change
has actually been produced in the heredity of the human race
as a result of human activities. If we could know that education
gradually modifies the human mind, it would be of the greatest
value to us. This is, however, merely supposed to occur by those
who favor the Lamarckian point of view. The only thing definitely
known to us in this connection is that human experience is per-
petuated through records and teaching, so that future generations
may profit by it as a part of their environment, even though they
may have received no inheritance from it.

Herbert Spencer stands out among those who have believed in
the inheritance of acquired characters, and like others, his attitude
was an outcome of such keen appreciation of the intimate relation-
ship existing between an organism and its environment that he
could not imagine an evolutionary process in which environment
was wholly passive.

Circumstantial Evidence. Much evidence has been brought
to the support of this point of view from the study of existing
animals. The similarity of such things as heritable callosities
and the callosities which appear in individuals as acquired charac-
ters in response to friction is one such case. It has been regarded
as wholly logical to conclude that inherent callosities located where
they resist extreme friction may well be the result of the long-
continued action of such a stimulus. However convincing one
may find this type of evidence, he must admit that it is not proof
that the character in question was so developed. In the one case
we deal with a definitely germinal character in the usual sense;
in the other we deal with a character which appears anew with
every generation only when the proper environmental stimulus
favors its development.

418 EVOLUTION AND GENETICS

The student should note that questions of this kind may be
very intricate and may permit apparently sound conclusions which
are, in reahty, of no value whatsoever. No better example of the
difficulty is available than a recent treatment of this same question.
The skin on the soles of the feet is thickened in individuals in
proportion to the friction between the feet and the substratum.
Walter points out that even in the mud-puppies {Necturus, a
genus of tailed amphil)ia) the skin of the soles is thickened,
although these animals always live in the water and therefore
cannot exert much pressure on the soles of the feet. He adds:
"Nor is it reasonable to suppose that it ever had any ancestor who
did so for the hands and feet of the Amphil)ia are the most primitive
and ancient hands and feet to be found in the animal kingdom
without any known ancestral types. The thickening of the skin
on the sole of the mud-puppies' feet must be due, therefore, to germi-
nal determiners and is in no way an acquisition through use."
Detlefsen quotes this passage, inserting the italics which are here
reproduced, and asks: "But is it really proven that the ancestors
never habitually applied pressure or friction to the soles of their
feet? Or do we know that the small amount of friction which
Necturus develops as it crawls along the bottom or behind rocks
and stones, would not develop some skin thickening?" These
two opinions on one point are an excellent illustration of individual
fallibility, for as we examine the evidences of evolution we find
that the pentadactyl appendage first appears as an adaptation to
terrestrial life. If this generally accepted point is true, then no
animal with pentadactyl appendages or their derivatives can fail
to have had terrestrial or amphil:)ious ancestors. At some time
in the past, therefore, Necturus must have had ancestors in which
the feet supported the body on solid earth during at least a part
of their lives, and here is the desired source of friction! It is
therefore just as inaccurate to say that the thickened soles of
Necturus cannot be due to the individual acquisitions of long ago
as to say definitely that they were so produced. Such evidence is
simply inconclusive.

Evidences of this kind may be explained just as readily by the
Darwinian theory. It is quite as accurate to say that a certain
useful character appears fortuitously in some individuals and is
perpetuated by natural selection as to say that it developed in
the individual as an acquired character and because of its value

THE LAMARCKIAN THEORY 419

finally became a part of the heritage of the species. Either view
has its hypothetical aspects; neither is an adequate explanation
of the gradual development which is so clearly shown in evolution-
ary series.

Experimental Evidence. The conditions established for experi-
mentation are that a character must appear in response to a
definite environmental condition which does not ordinarily occur
in the species under consideration, and second, that this character
must persist in some degree in later generations produced after
the cessation of the environmental influence.

Experiments dealing with the inheritance of acquired characters
have dealt with four kinds of environmental effects, viz., effects
upon the soma, effects upon the germ plasm, parallel induction or
simultaneous effects upon l)oth soma and germ plasm, and somatic
induction or effects upon the soma which are later transmitted to
the germ cells.

While these groups are significant in the classification of experi-
mental results, it is obvious that not all of them bear directly
upon the problem in hand. The first, effects upon the soma, is
obviously of no importance unless some connection with the
germplasm can be demonstrated, for we have seen that it is only
through the chromosomes of the germ cells that hereditary char-
acters are transmitted. Effects upon the germ plasm have been
demonstrated in animals treated with X-rays, alcohol, and other
agents. Notable among these are Muller's recent results with
Di'osophila already cited. It is clear that the germ cells can be
modified directly, but characters arising from these modifications
are in no sense the acquired characters with which evolution is
concerned. Parallel induction is no more than a coml^ination of
these two. The only remaining category, somatic induction, ex-
presses the condition which nmst occur if significant acquired
characters are to be proved inherited. The character must appear
in the body of the individual organism and then affect the germ
plasm in such a way that it will be reproduced in succeeding
generations.

Weismann's Mice. One of the most famous experiments with
somatic modifications was carried out by Weismann. He cut off
the tails of young mice generation after generation, breeding the
tailless mice and removing the tails which inevitably appeared
in their offspring, without affecting in the slightest the develop-

420 EVOLUTION AND GENETICS

ment of the tail. At the end of the experiment he produced mice
of the same kind as the original generation. This experiment and
other evidence furnish abundant proof that mutilations are not
inherited.

Parallel Induction in Insects. A number of experim.ents with
insects have shown that conditions of excessively low or high
temperature or relative humidity produce melanism. The off-
spring of insects in which melanism has thus been produced have
in some cases also shown melanic color, and these have been
interpreted as due to parallel effects upon the germ plasm of the
melanic parents. The cases are open to a variety of interpreta-
tions, however, none of which is conclusive. In any case, if
parallel induction could be proved it would have little significance
since the effect on the germ plasm would be direct and the charac-
ters therefore not of the type significant in evolution. If parallel
induction were well established and frequent in occurrence its
standing would be different, for then we might conclude that the
soma and germ plasm were normally susceptible to the same
stimuli.

Schroder's Insects. Schroder experimented with the caterpil-
lars of a willow moth which usually form a case by rolling the tip
of a willow leaf. By cutting off the tips of the leaves on which they
were allowed to feed, he forced a number to roll the sides of the
leaves in order to conceal themselves. These conditions were
repeated with a second generation, and of the third generation
of caterpillars four out of nineteen continued the acquired habit
even when given entire leaves.

In another experiment he dealt with beetles which live on a
smooth-leaved species of willow. Their eggs were removed to
pubescent leaves for three generations and at the end of that
time he reported that the pubescent leaves were preferred even
when free choice of the two varieties was allowed.

Detlefsen objects to Schroder's conclusions on the ground that
they are based on too few cases and that the normal range of varia-
tion is not taken into consideration. These objections seem sound,
but another question which he raises, viz., ''whether the modifica-
tions would persist or disappear gradually if a reasonable number
of additional generations were followed under normal conditions,"
seems pointless. Either habit may be looked upon as positive.
The fact that one prevails under normal conditions may well be

THE LAMARCKIAN THEORY 421

taken to indicate that normal conditions favor it, hence a return
to the old habits might be only another case of acquisition.

The Experiments of Guyer and Smith. The experiments of
Guyer and Smith are widely cited in this connection. These
scientists produced an antiserum of fowl blood by injecting a
preparation of the lenses of rabbits' eyes. The antiserum was
injected into pregnant rabbits without affecting the eyes of the
adult, but the young when born showed several defects. Indi-
viduals selected from the defective progeny transmitted eye defect
through several generations without further treatment. The pre-
caution has been taken to test the transmission of the characters
through the male in order that no question of prenatal influence
could enter.

These experiments are the most significant yet reported on the
transmission of acquired characters. They have been subjected
to severe criticisms by several biologists and have been repeated
and confirmed by their authors. Other experimenters have failed
to secure similar results, although the significance of their failure
is reduced by the fact that they did not follow the same conditions.
The present status of the experiments is such that they must be
grouped with other evidence as inconclusive, although they are
highly suggestive.

Castle and Phillips' Ovarian Grafts. One of the most striking
experiments relating to the inheritance of acquired characters is
that performed by Castle and Phillips by transplanting the ovaries
of black guinea pigs to white individuals from which the ovaries
had been removed. One female in which the operation was
successful was later mated with a white male. Six young produced
by this union of white parents were all black.

It is known that black color is dominant over white in the
guinea pig, hence this experiment shows conclusively that the
germ cells produced by the female were produced by the "black"
ovaries and were not influenced by the white body that nurtured
them. The experiment was undertaken to determine whether the
body, as the immediate environment of the germ cells, exerts any
influence on them and has been regarded as absolute proof that
this is not the case. However it is not absolute proof. In the
first place, not merely the germ cells but ovaries of the black
female remained in the white individual, so that the environment
of the germ cells was not wholly albino. In the second place, we

422 EVOLUTION AND GENETICS

do not know enough of inheritance to say that the blood stream
contains only certain things in connection with one character and
other things for its allelomorph. It is at least possible that the
same constituents are present in all cases, hence, no matter what
the character of a body, cells introduced into it might be expected
to avail themselves of the nourishment so provided without losing
their own integrity. It has been suggested also that albinism may
be due to the absence of a factor, and that a negative condition
in the soma could hardly be expected to exert control over the
germ cells.

Neo-Lamarckian Theories. The opposition of Neo-Darwinian
evolutionists to the Lamarckian theory has l^ecn such as to con-
centrate attention upon the supposed integrity of the germ plasm.
It has come to l:)e a ver}^ commonly accepted principle that the
demonstrable continuity of the germ plasm through successive
generations is evidence that it is a thing apart from the soma,
influencing it but not influenced in turn. The soma is regarded
as a derivative of the germ plasm which develops anew in every
generation but fails to make any contribution to the heritage of
the species.

The Fallacy of Germinal Continuity. In organisms which always
reproduce by sexual processes continuity of the germ plasm is
readily demonstrable. The germ cells produced by an individual
are obviously derived from the zygote which produced it, and in
turn contribute to the zygote from which the germ cells and
bodies of the next generation arise. In roundworms of the genus
Ascaris this continuity has been demonstrated with unusual
clarity, for after a few cleavages one cell of the segmenting zygote
is set aside as the parent of all germ cells while the rest develop
into the body of the worm. Continuity of this evident kind is
not often noted, however, and in some of the plants and lower
animals we find evidence of continuity of other tissues and even
of discontinuity of the germ plasm. Many plants can be developed
from fragments of somatic tissue. In such cases we may have
continuity of root, stem or leaf tissue from generation to generation.
Animals such as Hydra display a similar continuity and in the
flatworms Child has demonstrated the development of germ cells
from somatic tissues, in addition to the somatic continuity made
evident by the asexual reproduction of these worms (Fig. 210).

The prevalence of sexual reproduction among the more famihar

THE LAMARCKIAN THEORY

423

plants and animals gives the idea of germinal continuity a plausi-
bility which it does not deserve. In the hght of the facts cited
briefly above, it is evident that continuity is merely a corollary
of reproductive function, and anything which is capable of bridging
the gap between generations will be continuous.

The idea has, nevertheless, influenced the thought of modern
supporters of Lamarckian evolution. It has appeared necessary
to explain the reciprocal association of the soma and germ plasm
whereby the in-
dividual might
make some con-
tribution to the
heritage of the
species, and va-
rious hypotheses
have been ad-
vanced with this
end in view.

Pangenesis.
Darwin, while
he was at first
vigorously op-
posed to the
views of Lamarck,
cited the effects
of use and disuse
frequently in his
writings on evolution. Like his followers he looked upon the
body as a thing more or less definitely separated from the germ
cells that it produces, and to meet the diflficulty of associ-
ating somatic characters with the heritage he proposed the theory
of pangenesis. This theory assumed that every part of the body
constantly gave rise to minute units characteristic of itself. These
units were supposed to be taken up l:)y the blood stream and
transmitted throughout the body. The units, called gemmules,
were supposed to collect in the germ cells, thus influencing them
according to the development attained by the somatic structures
so that they in turn might carry this influence to the next genera-
tion. The acquired characters of Lamarckian theory, according
to this hypothesis, would be provided with a vehicle whereby

B

Fig. 210. — Diagrams to illustrate the behaviour of the
germ plasm in a succession of generations. Black
indicates the germ plasm, the outlined triangle the
soma. The succession is from top to bottom. A,
continuity of the germ plasm in ordinary sexual re-
production, involving the confluence of two lines at
each generation; B, continuity of both germ plasm and
soma in fission; C, continuity of soma and discon-
tinuity of germ plasm in plants and flatworms.

424 EVOLUTION AND GENETICS

they might definitely influence the heredity of the species. The
hypothesis is so unsatisfactory that it is of no more than historical
interest. The assumption of purely hypothetical structures for
which absolutely no biological support is available is never a
convincing basis for scientific thought.

The Mneme Theory. More recently Semon proposed the mneme
theory, which is based on the supposition that organic activity
and conditions surrounding the organism leave definite traces in
the form of engrammes. The engrammes are supposed to accumu-
late if the condition producing them is continued, and to affect
heredity. It is similar to pangenesis in proposing a purely hypo-
thetical vehicle for the recording of the character of the organism,
and so is of no more value.

Centro-epigenesis. Rignano's theory of centro-epigenesis is
expressed in an ample and excellent treatise on the inheritance of
acquired characters. In this work, for the first time, we seem to
be very near to a logical consideration of the factors involved.
Rignano emphasizes the importance of functional activity in the
organism in connection with environmental stimuli as the cause
of evolutionary change. His analysis of characters which have
erroneously been considered as disproof of Lamarckian views and
those which are really entitled to consideration is logically admir-
able and his work seems worthy of more general consideration
than it has received.

Kinetogenesis. A third theory, proposed by Cope, is of limited
application. It is fundamentally similar to the last in that it is
based upon power of functional response to environmental con-
ditions. This theory, in brief, assumes that the effects of repeated
motion or impact, as in the striking of the feet of cursorial animals
upon the ground, finds expression in greater development of the
parts involved. It would account for the development of elongated
feet in the unguligrade animals in this way. However, it has
been shown that stimuli of pressure may result in the limitation
of bone development, as well as increased deposition of bone, so
that the theory is applicable to both of the opposed processes.
It may be looked upon as one of the valuable subsidiary theories
of evolution, although it is distinctly Lamarckian and to that
extent is without actual i:)roof.

The Value of Lamarck's Theory. The foregoing pages are a
brief statement of the variegated career of the Lamarckian theory

THE LAMARCKIAN THEORY 425

of evolution. It is fifty j^ears older than Darwin's theory, yet it
bobs up like a specter even today. In spite of the vigorous opposi-
tion of the Darwinian school of thought and the fact that the
inheritance of ac(iuired characters is devoid of sound proof, we
must recognize that many l^iologists have such a favorable attitude
toward it that they would be glad to see it proved, or at least
given as sound a basis for acceptance as the germinal theories of
evolution.

It is significant that this condition can exist. Science is not
prone to cling to useless theories when once a question is settled,
hence this situation is very suggestive. When we consider all of
the theories advanced to explain evolutionary processes, we find
that none of them is adequate. Natural selection is logical and
is commonly accepted as a factor in evolution, but as originally
stated it meets a serious oljstacle as an explanation of species-
formation. The mutation theory obviously explains many changes
in organisms, and supplies a valuable corollary for natural selec-
tion. In minute mutations we may well have the permanent
modifications for natural selection to perpetuate in the gradual
differentiation of species, but if we accept this as an adequate
explanation of species-formation we must then explain the occur-
rence of mutations. They are due to chromosomal modification,
but what modifies the chromosomes? Are all such changes hap-
hazard? Is the wonderful adjustment of organisms to their
environment entirely due to the selection and preservation of
random characters? Reason seems to tell us that there is more
to the story than this. A satisfactory explanation is not yet
available but the very persistence of the Lamarckian idea leads
us to consider evolutionary processes with open minds. Partisan
views and investigations have failed to attain the desired end,
and in their failure we may find a lesson for modern evolution.

Summary. Lamarck's theory of evolution was first stated in
1809 as two laws, known as the law of use and disuse and the law
of inheritance of acquired characters. Laws which he stated
later are of no moment. Of these two fundamental laws, use and
disuse is now commonly known to have an effect upon individual
development, but the inheritance of characters produced in this
way has never been demonstrated. For many centuries the idea
has been accepted by some individuals without proof. JNIany
biologists have also favored it, and have accumulated evidence

426 EVOLUTION AND GENETICS

of a circumstantial nature to support their views. Experimental
evidence is also abundant, but experiments have invariably given
inconclusive results. The idea of germinal continuity advanced
by Weismann has turned attention to the establishment of a
reciprocal connection between soma and germ plasm; this tendency
colors much of the literature for the Lamarckian view. In spite
of the fact that it competes in its unproved state with germinal
theories which have more or less convincing support, the idea
persists that acquired characters contribute to the heritage. This
very persistence is suggestive and valuable in that it forces us to
recognize that the question of evolutionary processes is still an
open one.

REFERENCES

Castle, W. E., and Philips, J. C, "On Germinal Transplantation in Verte-
brates," Carnegie Inst. Wash., Publication 144, 1911.

RiGNANo, E., On the Inheritance of Acquired Characters, translated by Harvey,
1911.

Child, C. M., Senescence and Rejuvenescence, 1915.

GuYER, M. F., and Smith, E., "Studies on Cytolysins. II. Transmission of
Induced Eye Defects," Jorirn. Exp. Zool. XXXI, 171, 1920.

Newman, H. H., Readings in Evolution, Genetics and Eugenics, 1921.

Walter, H. E., Genetics, revised edition, 1923.

CoNKLiN, E. G., Heredity and Environment, 6th edition, 1924.

Detlefsen, J. A., "The Inheritance of Acquired Characters," Phys. Rev. V,
244-278, 1925.

CHAPTER XXV
EVOLUTION TODAY

At no time in the history of biology has evolution occupied a
more important place in scientific thought than now. It permeates
the entire fabric of organized scientific knowledge and furnishes
the guiding principle in much, if not all, biological research. The
value of its established facts is recognized, but more than that,
the value of accurate understanding of the methods by which it
works impresses itself upon our minds. No step in biological
progress at the present time could be a greater contribution to
human knowledge.

We have now completed our survey of the field of evolution.
We have considered the growth of the idea from its earliest concep-
tion, the evidences of relationship in existing organisms, the
evidences of evolution drawn from living things and from the
sciences of geologj^ and palaeontology. We have learned of the
contributions of genetics to our understanding of the transmission
of characters from generation to generation. Finally we have
examined the theories which have been proposed to explain the
association of all of these facts in the production of the organisms
which populate the earth. And we have found that in spite of
his wonderful accumulation of facts, man still knows very little
of the forces which have placed him where he stands today.

The failure of existing theories of evolution shows very clearly
that something has been amiss in the past treatment of the subject.
It is difficult to avoid the prejudices of past training. If we bear
in mind that we are neither Neo-Darwinians nor Neo-Lamarckians,
and that we have no predilection for continuity of the germ plasm
or the inheritance of acquired characters, we can hope to learn
still more of evolutionary processes. If we remember that we are
merely scientists, seeking for clear information and logical deduc-
tions, we find that there is a firm and satisfying basis of fact in
modern biology for the examination of the problems of evolution,
and we can conclude in no more satisfactory way than at this
threshold of the evolution of the future.

427

428 EVOLUTION AND GENETICS

The Errors of the Past. Darwin's Theory. In our discussion
of the existing theories we have noted various advantages and
shortcomings which are a guidepost to future hnes of progress.
Darwin's theory has been one of the greatest of all time, yet the
investigations of modern biology have cast doubt upon its com-
plete soundness. A careful estimate of the criticisms to which it
has been subjected leaves the feeling that the theory is sound,
but by no means a complete explanation of evolutionary processes.
The occurrence of natural selection can scarcely be doubted, but
before it can operate it must have definitely useful or harmful
heritable variations with which to work and the theory does
not even attempt to explain the occurrence of such variations.
This does not rob it of scientific value, but reduces the range of
its usefulness; it becomes, as we have noted, chiefly a theory of
adaptation.

The Mutation Theory. In connection with natural selection
the mutation theory supplies an explanation of the occurrence of
such permanent changes as the latter must have to work upon.
Some examples of mutations of extreme degree are known which
suggest the possibility of even varietal or specific differences
appearing suddenly ; mutations may therefore be a sufficient cause
of the sudden evolution of species in some cases, but it is evident
that gradual evolution is more common. Can mutation be a
basis for such change? It is obvious in such species as Drosophila
melanogaster that this is at least a possibility, for here we see that
a multitude of slight mutations are constantly appearing, none
of them regarded as specific or even varietal differences, but many
potentially of importance to the individual. Natural selection,
acting upon these minute characters, may readily be supposed to
eliminate some and preserve others, to the ultimate modification
of the species.

Even when this much is granted the mutation theory, however,
we still lack an explanation of the source of evolutionary change.
Mutations occur, this nobody denies, but what causes them?
We can say that they come spontaneously from the heritage of
the species, but what does that mean? An explanation to be of
any value must make a thing clear in terms of its more familiar
causes or components. Hence the admitted occurrence of muta-
tions of both great and small degree is merely the recognition of
a process of change without explanation, just as the recogni-

EVOLUTION TODAY 429

tion of living matter and its properties is not an explanation of
life.

Lamarckian Theories. In the work of Lamarck and his sup-
porters we find the only explanation of how change; occurs in
organisms. Lamarck's law of use and disuse and the modern
recognition of its principles refer the matter of organic change to
definite causes, viz., a definite heritage in living matter, expressing
itself according to stimuU received from the surrounding environ-
ment. If this were the whole story we might happily become
Lamarckians in the strictest sense, but unfortunately it merely
carries us into another incomplete conception. We can see change
taking place in individual organisms all about us from year to
year, sometimes in direct response to environmental conditions,
but we have never been able to see the change take place in a
species. Here we encounter the serious obstacle which has been
called the inheritance of acquired characters and the theory be-
comes even less useful than the others because it fails in a vital
particular and is therefore not even a partial explanation of
evolution.

The existing theories of evolution are therefore of little indi-
vidual value as nuclei for an explanation of the process of evolution.
We cannot be selectionists or mutationists or Lamarckians without
subjecting ourselves to the limitations which accompany such
views, but we can recognize the value which exists in all of them
and avoid their limitations readily.

What Must an Adequate Theory Explain? It is improbable
that an unbiased point of view can lead at once to a thorough
understanding of evolution, but it can at least make for progress
along sound lines. Two fundamental points must be met before
the processes of evolution can be known to us: (1) The source
of variations must be explained, not in terms of axiomatic occur-
rence as properties of living matter, but in terms of intelligible
forces of a more fundamental nature; (2) The perpetuation of
some characters in organisms from generation to generation and
the elimination of others must be explained by an intelligible
process or series of processes. The various fields of biology
have contributed so many sound facts to our knowledge of the
behaviour of organisms that it seems profitable to begin in-
quiry with an examination of the fundamental factors in
Ufa.

430 EVOLUTION AND GENETICS

The Factors in the Existence of Living Organisms. This
unprejudiced analysis demands first of all an understanding of
organic existence. What lies behind the living organism? Why
are organisms definitely of this or that species, and why do they
live? We need not carry this inquiry as far as the origin of life,
for a significant answer is found in the components of existence
mentioned from time to time in previous chapters. The organism
has definite qualities because they are its heritage. They are not
absolutely fixed, for variation is universal, but within the limits
of variation they are the same throughout a given species. The
organism depends for the expression of these qualities upon things
secured from without — from its environment. Its life depends
upon its inherent power to secure and use these things. Response
to the environment, in brief, is a third factor arising from the other
two, and organic existence depends upon the interaction of these
three. Without any one of them life must cease. Modification
of any one beyond its normal range can only result in abnormal
existence or death.

Such an analysis does not explain ultimate causes, nor does it
pretend to do so, but in the realization that the three factors
are indispensable is disclosed the fallacy of many theories of
evolution. We are at present unable to do more than speculate
on ultimate causes. Whether life is due to the existence in the
organism of some cosmic influence other than those whose existence
is proved, or whether it is merely an intricate expression of familiar
matter and energy operating in space and time, we do not know.
The fact remains, however, that living things are governed by
natural laws, whatever may be the activating principle, and it
is the immediate task of evolutionists to explain acceptably how
organisms are shaped, how species are formed, on the basis of
known l)iological facts.

The heritage has often appealed to biologists as a thing
apart, which expresses itself in a definite way in spite of outer
influences. It does attain remarkably vmiform results in the many
individuals composing a species, but we may well ask whether
these results are ever, after all, an outcome of the heritage alone.
The heritage is sufficiently definite and potent that no one would
plant an acorn to raise a peach tree or buy wolves to stock a dairy
farm. The egg of a hen can produce nothing Imt a chick. Keep
it in a cool place and it remains an egg — living, but not growing.

EVOLUTION TODAY 431

Raise it to a high temperature and its substance coagulates — it
dies. But keep it at 39° C and dev(>lopnient begins. Temperature
is one of the common factors of environment, and here we see that
the correct temperature is just as essential to the initial develop-
ment of the chick as the heritage borne by the egg.

In our own bodies we may see the unfolding of hereditary char-
acters without obvious dependence upon the outer world. We grow
from day to day throughout childhood. It is true that growth de-
pends to a great extent upon the food that we cat, but some persons
fail to grow in spite of sufficient food and others grow to gigantic
size. Our mental developm(>nt increases, but in some individuals
this is not the case, although they may have a perfectly sound men-
tal inheritance. The characters of sex appear. We can see in these
cases only an indication of the fact already mentioned, that the
body itself provides stimuli for the activation and control of its
various parts. Some characters develop in response to these
stimuli alone.

The environment, therefore, is not merely that which is outside
of the individual. Any organ in the body is as truly a part of
the en\aromiient of other organs as physical factors or living
things are a part of the environment of a plant or animal. The
environment may properly be subdivided into an external envi-
ronment, made up of the physical and organic, and an internal
environment which is estal)lished by the individual's own body.

The essential effects of these environments are the same. Each
provides certain stimuli to which organs or organisms may respond
according to their inherited capacity. Glands in our bodies, food
which we secure from our organic environment and sunlight from
the physical environment all contribute to our normal growth.
The unfortunate individuals called cretins are mentally and
physically retarded because of deficiency of the thyroid gland, })ut
stunted growth and imperfect development may also come from
malnutrition and the imperfect bone development called rickets
may be due to lack of sunlight or a diet deficient in vitamines.

The Source of Organic Characters. The various parts and
functions which characterize a given species are the most definite
indication of its heritage, yet it is evident that they are a product
not merely of the heritage, but of the heritage responding to
environmental stimuli.

In the discussion of acquired characters it has usually been

432 EVOLUTION AND GENETICS

implied that they are the effects of environment impressed upon
organisms. This is true of very few characters. Mutilations are,
of course, produced in an organism in spite of its inherent qualities,
and when the woman of today has her hair curled, no inherent
quahty of the hair determines the wave, as in the kinky hair of
the negro. No one would seriously consider such characters in
connection with evolution, although it has been done in the past.
The vast majority of acquired characters or modifications are of a
very different nature. The formation of calluses, tanning of the
skin, muscular development and other so-called modifications are
due to individual response to conditions in the external environ-
ment, it is true, but can they be produced without the action of a
definite heritage?

A simple illustration will show that both heritage and environ-
mental stimulus are as essential in their production as in that
of other characters. The ability of the body to deposit pigment is
an inherited character. Some people tan readily when exposed to
strong sunlight, others burn without tanning and others freckle,
but in the case of alienism the heritage is deficient. Albinos can
deposit no pigment, therefore no amount of sunlight produces
this so-called modification in their bodies. It seems that a char-
acter which arises from the response of the heritage to a stimulus
from the internal environment may be interpreted as solely a
product of the heritage in a broad sense, since the environmental
stimulus in this case is another product of the heritage, but in
many cases we find that the internal environment merely serves
to make available in the proper form certain factors from the
external environment. Proper development of the thyroid gland,
for example, cannot be attained unless the food contains a sufficient
amount of iodine. Even the internal environment is therefore
tempered by external environmental conditions. None of the
characters which are of interest to us in the study of evolution
can be a product entirely of either heritage or environment alone.
The true distinction between the Darwinian and Lamarckian point
of view is not the source of the character, hut the source of the stimulus
which governs its appearance.

The Role of the Gene. A distinction arising from genetics
should be clearly understood at this point. The function of the
genes as determiners of hereditary characters has been so well
established that in speaking of the heritage we must think of the

EVOLUTION TODAY 433

chromosomal complex of a species and the genes which are con-
tained therein. The cj'toplasm of somatic cells is the structural
material in which the differentiation of tissues is wrought and
some biologists have questioned whether the genes might continue
to exert an effect on this substance throughout life. ExiDerimental
methods for answering such a query have not been devised, l)ut
the only logical opinion possible in the light of biological facts
is that they do remain active even in somatic cells, for living
substance is constantly being torn down and built up through
the processes of metabolism, and in enucleate cells constructive
metabolism ceases.

It is logical to conclude that the genes which are responsible
for the initial production of characters in the developing individual
are no less responsible for all of the characters which appear during
its life. It is the gene, in the end, which responds to a definite
environmental stimulus for the production of a definite character.

The Source of Change. When a certain result can be secured
by the proper association of materials and forces we find that an
accurate formula is essential to accurate results. The cake turns
out well if the proper ingredients are combined in the proper way
and baked at the right temperature. Iron and steel take a definite
temper if heated to a definite degree and properly quenched. But
vary one of these factors and the cake is ruined or the metal does
not have the desired quality. If the association of definite factors
produces a definite result, it is to be expected that different factors
would produce different results.

There is no reason to suppose that living matter is different in
relation to these fundamental truths. Every character in a living
organism is due to definite qualities of the heritage, reacting to
definite stimuli received from the environment. The two are no
more separable than batter and heat in the baking of a cake.
We must therefore recognize that change in living things cannot
fail to result from a change in either of the components.

Scientists have been prone to recognize spontaneous change in
the heritage. It would be difficult to prove the truth of such an
opinion, for we have seen that living matter is active only in
response to environmental stimuh. Moreover, in the analogies
cited, change of substance is due to outer forces — either human
interference or forces in the physical world. The environment,
however, varies according to a vast complex of physical forces

434 EVOLUTION AND GENETICS

which we can explain in part. These forces ultimately resolve
themselves into matter and energy operating in space and time,
and here we must be contented to stop.

The Tetrakinetic Theory. Osborn expressed views similar to
these more than a decade ago. Under the name of the tetrakinetic
theory he classified the factors in life as four energy complexes,
(1) the inorganic environment, (2) the life environment, (3) the
organism, and (4) the heredity-germ. In spite of his masterly
analysis the literature of evolution has unfortunately continued
along the same old lines.

The interpretation is equivalent to sa>ang that all organic
change is based upon changes in the complex environment, refer-
able ultimately to the four entities, matter and energy, space and
time. The position is logically tenable, but to satisfy the demands
of evolution we must give attention to other details.

Adaptive and Incidental Change. There is every reason to
beheve that variation of the environment is the immediate stimulus
which induces variation in organisms. This does not imply that
the environment actually shapes the organism, but it determines
the expression lohich the organic heritage will attain. Even identical
twins are never really identical. They are genetically the same
because they are derived from halves of the same fertilized ovum,
but two indi\'iduals can never be surrounded by exactly the same
conditions. They cannot eat exactly the same food nor the same
quantity; they cannot sit in the same chair at the same time; the
minute differences in their contacts may even mean life or death,
as is true of spatial relations in the traffic of city streets at any
time. Such differences as they display must therefore be due to
the reaction of their identical heritages to different stimuli.

The suggestion that change in organisms arises in response to
change in the environment docs not mean that a stimulus always
brings about a response which fits the organism to meet the
existing environmental condition; in other words adaptations do
not necessarily arise as a response to the environmental condition
which they enable the organism to meet. Adaptive changes may
appear in this way, as when the rays of the sun stimulate the
l^ody to deposit pigment for protection, but changes may occur
as incidental results. Rickets, although due to lack of sunhght
in some cases, unfits an individual for normal life, and the normal
skeletal development which is in part due to abundance of sunlight

EVOLUTION TODAY 435

durinp; growth is in no sense an adaptation to life in sunlight.
These conditions are, however, no less a result of environmental
stimulus than tanning. With respect to adaptation, all changes
appearing in response to the internal environment must be
incidental.

Incidental changes are a part of the organism just as much as
adaptive modifications, hence they are availa])le for other adjust-
ments to the environment. Any incidental modification when it
is once developed may affect the organism's relations to other
conditions than those which brought about its development.
Normal nervous development is partly a response of the heritage
to normal thyroid secretion, yet it accomplishes most of our adjust-
ments with the conditions under which we live.

How Has Evolution Progressed? When we examine the
phylogenetic series worked out by paleontologists we are able to
learn something of the superficial aspects of past evolution, even
though we may not be able to fathom the underlying forces. We
are al)le to see evidences of the heritage of long series at different
stages of development. Correlation of these characters with the
environment may be deduced from a comparison with similar
structures of living organisms, but a very definite indication of
past environmental conditions, such as climate, is afforded by the
fossil remains of other organisms, especially the green plants.
We can therefore learn from paleontology not only how the heritage
of a given hne of descent has changed from age to age but also
under what conditions these changes have taken place.

An examination of living organisms is of great value in connec-
tion with this study, for even in living things we find conditions
similar to those which have preceded evolution in the past.
From time to time in the previous chapters of this book organisms
have ]jeen mentioned which show a distinct capacity for evolu-
tionary change.

The Heritage. Whether in extinct organisms w^hose descend-
ents are before us or in living things whose possibihties can only
be estimated, we find that the heritage must not be limited rigidly
to one type of response if evolution is to occur. We cannot take
a trout out of water and expect it to live, much less to become a
terrestrial animal, but fishes have existed and still exist which
might well accomplish this transition in some degree. Fishes with
functional lungs can breathe air or secure their oxygen from the

436 EVOLUTION AND GENETICS

water. Among other cases amphibia with gills and lungs breathe
in either medium. Euglena can carry on either plant or animal
metabolism, and the quail can l^oth run rapidly and fly. With
the capacity for two or more kinds of activity an organism can
carry on either.

Evidences of this kind are so numerous that latitude of heredi-
tary possi]:>ility seems a necessary quality in organisms which
are to evolve. A species cannot be expected suddenly to acquire
something new to meet a given condition; the abundance of
extinct species shows that many an organism has failed to meet
the conditions surrounding it. If a species has a sufficient range
of possibilities to meet the conditions of a changing environment
it may survive, as in the past many species have survived. Grant-
ing its ability to survive a change of environment, the more
valuable characters of the species may persist and be developed
while others are being reduced and eliminated. The horse attained
a large third digit while losing all others.

The Environment. Like the heritage the environment has
always been variable. We witness from season to season and
from year to year fluctuations of all of the factors of the physical
environment. One year may be wet, another dry. In one region
the nights may be cold and the days hot, while in another tempera-
ture may be almost constant. Paleontology shows that this is
not the limit of fluctuation, but that gradual climatic changes
covering thousands of years have occurred. Temperate North
America has been both tropical and arctic in the past. Moreover
at any given time different parts of the earth's surface present
different climatic conditions because of their physiographic features
and their varying relations to the sun.

The effects of such fluctuations may be very great. The physical
environment is so closely associated with the metabolism of green
plants that an exceptionally dry season may mean death to a
great many, or if their seeds fall on the wrong kind of soil or in
too shady a spot they may perish. The flora of a region is there-
fore very largely an indication of its physical conditions, and by
the flora the fauna may be definitely influenced. Arboreal animals
are not found in extensive prairies, for example, nor grazing
species in dense forests. Animals may also respond directly to
physical conditions; to this relationship is due the fact that
amphibia cannot live in deserts nor in the far north.

EVOLUTION TODAY 437

Environmental change may also be due to the shifting popula-
tion, as mentioned in Chapter XIII, but whatever its cause, organ-
isms in contact with it must meet the changed conditions or die.

Duration of Environment. When biologists have experimented
with environmental effects on organisms in the past they have
had to be contented with a study covering relatively few
generations and the characters available for study have been
relatively unimportant. In long phylogenetic series we find that
the gradual development of a species is correlated with a lasting
chmate, or with a climate which is gradually changing in a way
favorable to the existence of the changing organism. The evolution
of species has not occurred in a few years nor under frequently
changing conditions.

Whether incidental changes or direct adaptive responses in
organisms have been the initial step in the evolution of species we
cannot say. This is the essence of the conflict between the views
of Darwin and Lamarck. If changes have appeared in organisms,
giving rise to an assemblage of variations, changing environment
may emphasize their relative values in such a way as to cause
natural selection. On the other hand it is obvious that some
changes come about as direct adaptive response to a new condition.
Organisms have unlimited opportunity for contact with new
environmental stimuli through migration, climatic change, and
fluctuations in the organic environment, hence there is always a
chance for the occurrence of new characters of both kinds.

Are Any Characters Permanent? Since all changes in the
organism are the product of hereditary genes responding to
stimuli from some part of the complex environment, the question
arises, how do they differ in permanence? If characters appear
in response to conditions of the internal environment, then o})vi-
ously their reappearance in each generation will be reasonably
certain. However even the internal environment is not permanent ;
it is subject to modification both from within and from without.
If the proper condition in the body fails to develop, even appar-
ently hereditary characters fail to appear. A cretin may have a
normal mental heritage, ])ut the genes responsible for mental
development do not find the proper conditions in its body for
their expression. The deficiency may be corrected by the timely
administration of thyroid extract and the heritage then responds
normally.

438 EVOLUTION AND GENETICS

Conversely an acquired character or adaptive response will
appear in every generation if the proper stimulus is available.
As long as the same conditions obtain, the character will appar-
ently be inherited because it will appear in all individuals. It
appears during the independent life of the individual, and its
appearance is directly associated with an external condition, but
these things are no less true of many of our inherent structures.
Such a character is therefore fundamentally similar to inherited
characters, save only that the heritage in no case provides the
stimulus for its development.

Permanence of a character is due to permanence of the con-
tri})uting factors, heritage and environment. Every character has
a basis in the heritage which will be handed down from generation
to generation as a part of the normal functions of the living sub-
stance, but to the same degree every character depends for its
appearance upon the presence of the proper condition in the
environment for the normal response of this heritage. How then
can a change come about in the heritage?

The Process of Evolution. In answer to this question two
interpretations may be based on the facts thus far set down:

1. The power of an organism to develop any character is a
part of its heritage. Whatever the nature of the character, it is
a product of genes derived from the germ cells of the previous
generation and some outer stimulus. Hence even so-called
acquired characters may be regarded as an expression of hereditary
qualities resident in the germ plasm from which the individual
arose. If an existing individual acquires a character, then the
germ cells contained genes with the power to produce such a
character under the proper stimulus. The parents which produced
the germ cells therefore had genes with the same power, and so,
step by step, the idea can be carried in either direction through
successive generations, even back to ancestral species.

This interpretation, carried to its logical extreme, would include
the idea that even the most primitive living matter possessed the
inherent ability to produce any of the characters of existing ani-
mals in response to the proper stimuli. To a certain extent this
must be as true as evolution, but it would require great credulity
to believe that an amoeba-like ancestor could develop the char-
acters of man in a generation if given the proper surroundings.
The view becomes acceptable only through recognition of the

EVOLUTION TODAY 439

gradual modification of the factors involved, which leads to the
second interpretation.

2. It is infinitely more logical to suppose that the heritage in
any living organism is subject to change. It has a certain range
of power which enables it to respond within limits to different
stimuli, and when it has responded in one way it seems probable
that its future behaviour and development may be modified as a
result. An individual may enter one field of activity after being
trained for another, but his earlier training can hardly fail to
have an effect on his later work.

This interpretation necessitates the wholly logical belief that
each change in an organism through response of its heritage to
conditions emanating from any part of the environment may make
possible other changes. It recognizes that departure from the
one-celled state for colonial or multicellular organization makes
possible other steps which the single-celled organism could not
directly attain, and that the attainment of triploblastic structure
makes possible other specializations impossible to diploblasts.
The primitive horses must have had a third digit strong enough to
support the weight of the body before the others could be lifted
from the ground, but once the latter ceased to bear weight, they
would have had a much better chance of reduction than before. In
all of these cases it is evident that the characters deal both with
the heritage and with environmental conditions.

In spite of the logical aspects of this interpretation we must
still meet the old distinction between heritable and acquired
characters. The heritage is always expressed through the soma,
although it is perpetuated by the germ plasm. This is true of all
characters, even those which are admittedly inherited; neverthe-
less, if so-called acquired character's have any part in evolution,
there must ])e a point of transition where the heritage ceases to
find a stimulus from the external environment necessary for its
expression and responds to a condition which is normally certain
to be supplied by the developing body itself.

Use and Disuse in the Chromosomes. The principle of use
and disuse recognizable in organic behaviour has been applied to
the chromosomes and th(Mr genes as an explanation of such cumu-
lative modification of the heritage through the generations. It
is now regarded as probable that the action of a given gene is not
limited to the cell in which it lies, but that the genes of a given

440 EVOLUTION AND GENETICS

kind throughout the body may, through the coordination of the
individual organism, exert a controlling influence over the part
which is capable of expressing them. If such is the case, the
spermatogonia and oogonia are no less subject to the coordinating
influences. Any increase or decrease in the functional capacity
of a gene would therefore be characteristic of the genes in these
germ cells as well as of those in the somatic cells expressing the
character. The transmission of the change to succeeding genera-
tions would be assured. As time went on the effect of continued use
might readily bring alwut a development of the genetic function
and an accessory development of associated parts of the internal
environment so great that the original stimulus would be a minor
matter and its cessation could not be followed by immediate loss
of the character whose appearance it had first caused. This theory
is in harmony with the known facts, but it remains untried and
is therefore only another possible explanation of evolutionary
processes.

Evolution and the Internal Environment. Regardless of the
association of germinal and somatic genes, we must recognize
that the appearance of a new character in an organism, whatever
its cause, makes a change in the internal environment. The new
character can develop only if it receives the proper support of the
remainder of the organism. Consequently if it becomes large or
important, its new relationship to the internal environment may
become so fundamental that it will be of greater importance in
the expression of the character than the stimulus which originally
favored its development.

In this secondary adjustment it is evident that structures
sometimes take on additional functions which make their per-
sistence necessary to the organism long after the loss of the original
use. Man has no further use for pharyngeal pouches as a part
of his respiratory system but the first pair have developed into
his middle ear and the remainder are associated with the formation
of a series of endocrine glands. They persist to the extent necessary
for the production of these structures.

Experimental Evolution. The foregoing explanations are obvi-
ously not an adequate solution of the process of evolution, nor
are they intended to he. They show, however, that it is possible
to go much further in a logical evaluation of the factors in evolution
than has been done in the formulation of the famous theories of

EVOLUTION TODAY 441

the past. In recognition of the universal importance of the associa-
tion of heritage and environment in all aspects of life we approach
the basis for sound future investigation.

Science cannot be wholly satisfied until its ideas have been
demonstrated in the laboratory. If evolution is to reach this
happy state of proof, it will obviously not be due to the methods
of the past. At present the possil)ility of finding methods of
experimental proof seems remote, for with the facts described in
this chapter in mind it is evident that certain conditions must be
met.

It would be foolish to expect, as has been done in the past, that
the induced change might persist after the removal of the stimulus
at the end of a few generations. Even though the heritable power
of the organism to respond to a given condition might have
changed this could not be expected, for the development of the
new response would not necessarily destroy the organism's capac-
ity for the old. Both old and new responses may be looked upon
as the positive results of different factors.

If, however, a change can l)e induced and intensified by gradual
modification of the stimulus through many generations until a
result of considerable importance in individual life is attained, we
shall have an "acquired character" worth}^ of a test. And if the
character in question should develop accessory relationships of
value within the organism it will l)e even better. Under these
conditions we may expect the removal of the stimulus, if the
organism can still live under the old conditions, to be accomphshed
without the loss of the character. Even under such conditions we
would have to recognize that the change proceeded not from the
heritage nor from the environment alone, l:)ut from the two acting
together. And if such an experiment should fail, we must then
look for some unknown property in living matter which makes
its heritage in some degree independent of environmental condi-
tions.

It is highly probable in the light of modern knowledge that the
explanation of evolution will be based upon the recognized facts
of biology. Living things as we see them now are exceedingly
varied and complex, l)ut if we could look back over the complete
record of th(Mr development we should probably find a tale of
gradually increasing complexity as each successive stage realized
the possibilities of its heritage and thereby made possible still

442 EVOLUTION AND GENETICS

other steps in evolution. Modern thought on this increase of
capacity has crystaUized under the name of emergent evolution.
The name is scarcely necessary for the belief is no more than a
logical statement of things long held true. The role of sentient
powers and inherent directive forces has also gained a prominent
place in evolutionary thought, but here we are dangerously near
to passing out of the transitional zone into purely philosophical
speculation.

Although discouraging it is not surprising that the method of
evolution should be so elusive, for it is the most complex and
baffling field of research. The genes with which the problem is
so intimately concerned are exceedingly small. They have never
been seen. Even the chromosomes must be subjected to extensive
treatment before they can be examined. Physicists may do as
they will with the atom and electron; these units are tractable in
comparison with genes, for methods can be devised for studying
them without destroj'ing them. Living matter presents more
difficulties. W hen chromosomes are made visible they are dead
and the gene remains a hypothetical unit in their densely stained
suljstance.

In spite of these limitations the fact of evolution remains an
established principle of biology through all of the investigations
and disputes concerning its methods, and, knowing this to be so,
we may feel confident that we shall some day solve the problems
which are before us today.

Summary. Considering all of the proposed theoretical explana-
tions of evolutionary method we find an almost universal lack of
explanation of the origin of variations. Darwin's theory of nat-
ural selection and the mutation theory together serve to explain
adaptation but must l^egin with variation. Lamarck's theory
offers the idea of interaction of heritage and environment but
fails to account for the extension of its results beyond the individ-
ual. An adequate theory of evolution must do both things, i.e.,
it must account for changes and explain how they become the
characters of species. A logical analysis of the available facts
shows that the organism and all significant organic characters
are a product of the heritage reacting to the complex environ-
ment, and variations in the latter seem sufficient to account for
variations in the resulting living organisms. This view is essen-
tially the tetrakinetic theory of Osborn. We are able to see varia-

EVOLUTION TODAY 443

tion as a process only in the individual, however, and it remains
for us to explain how these changes become a part of the sjoecic^s.
That they are of the heritage is easily shown. Use and disuse of
functional capacity in the hereditary bodies, the g(>nes, has been
proposed as an explanation of changes in the functional capacity
of these minute bodies. This may account for the gradual devel-
opment of characters through a long succession of generations
under continued or progressive environmental conditions, and
when characters have attained great development in response to
conditions of the external environment they may well have at-
tained such importance in the internal environment that their
persistence will be necessary. Future work in evolution must
take into account these things. The difficulties of experimental
work are great, but the fact of evolution is so well established
that we may expect confidently to solve the riddles of evolutionary
method in the future.

REFERENCES

Morgan, T. H., Evolution and Adaplaliori, 1903.

Thomson, J. A., Heredity, 1909.

OsBORN, H. F., The Origin and Evolution of Life, 1918.

Morgan, C L., Emergent Evolution, 1922.

Eldridge, S., The Organization of Life, 1925.

Kepner, W. a., Animals Looking into the Future, 1925.

Noble, E., Purposive Evolution, 1926.

LiNDSEY, A. W., "Factors in Phylogenetic Development," American Natu-
ralist XLI, 251-265, 1927.

Jennings, H. S., "Diverse Doctrines of Evolution," etc.. Science LXV, 19-25,
1927.

Washburn, M. F., "Purposive Action," Science LXVII, 24-28, 1928.

INDEX

Italics indicate pages bearing figures or diagrams.

Abyssal realm, 215; adaptations to,
216; conditions in, 215; fishes of,
216

Accidental destruction, 392

Acipenser, bony scutes, 69; pelvic
girdle, 78

Acquired characters, 7, 10, 386; in-
heritance of, 414, 415, 416

Adaptation, basis of, 247 ; and here 1-
ity, 264; process and result, 210;
and exteiTial environment, 212; and
internal environment, 213; transi-
tion from individual to species,
262

Adaptations, abyssal, 215; amliula-
tory, 217; of anteaters, 232
aquatic, 214; to aridity, 224, 236
of benthos, 214; of birds, 225, 23-5;
of carnivorous animals, 233; cur-
sorial, 218; flight, 225; for securing
food, 232; fassorial, 221; for glid-
ing, 228; for grazing, 232; to light,
223; of loon, 213; of nekton, 215;
to organic environment, 230; to
physical environment, 213; of
plankton, 215; protective, 234;
saltatory, 219; scansorial, 222

Adaptive branching, 252

Adaptive radiation, 27, 252

Adhesive organs, 223

Africa, fossil man in, 199

Agave, 226

Aggressive colors, 242

Albinos, 432

Algonkian, 124

Alhamlira plum, 350

Allantois, 4-5; human, 46

Allelomorphs, 279; multiple, 314

AJlosomes, 299

Alluring colors, 242

Alps, 120

Alternation of generations, 23

Altitude, effect on climate, 248

Amber, 127; fossils, 127

Ambulatory adaptations, 217

Ambulatory animals, 253

Amitosis, 91

Ammonites, 133

Amnion, 4-5; human, 4^

Amphibamus, 151

Amphibia, 37, 130; in vertebrate
evolution, 150; of Carboniferous,
151

Amphioxus, 145; cytoplasmic difTer-
entiation of egg, 302

Amphioxus theory of vertebrate de-
scent, 144, 146

Anabolism, 30

Analogies, significance of. 111

Analogy, 27

Anaximander, 5, 6

Ancestral inheritance, law of, 271

Aiichilherium, 180, 183

Ancon sheep, 404

Andalusian fowls, 281

Anemones, sea, 214

Angraecum sesquipedale, 393

Animal associations, see Associations

Animal hybrids, 345

Annelid theory of vertebrate descent,
144, 146

Annelida, 28, 137; artificial parthe-
nogenesis in, 330

Anopheles, head and mouth parts,
234

Anteaters, 233; adaptations of, 232

Antennae of mosquito, 234

Anthropoidea, 188

Ants, and aphids, 231

Aorta, 62

Aortic arches, diagram, 64; of human
embryo, 49

Apes, half-, 188; man-like, 189; New
World, 188; Old World, 188

Aphids, 231; and pure lines, 356; life
cycle of, 329, 330

Appalachian mountainj, 120, 258

Appendages, 35

445

446

INDEX

Appendix, human, 88, 398

Apterygota, 13G

Aquinas, Thomas, 8

Arachnida, 135

Arboreal origin of man, 191

Arboreal quadrupeds, 191

Archaeopteryx lithographica, 155, 156

Archenteron, 42

Archeozoic, 124; fossils of, 129

Arches, branchial, see Branchial
arches

"Arcturus Adventure," 260

Argonaut a, 132

Aridity, adaptations to, in animals,
224; in plants, 224

Arisaema triphyllum, sex reversal in,
336; Siamese twins, 337

Aristotle, 5, 7

Armadillo, 236

Armor, of alligator, 236; of arma-
dillo, 236; of Stegosaurus, 235;
of Triceratope, 235, 236

Arthropod theory of vertebrate ori-
gin, 144

Arthropoda, 28; adaptations of legs,
218; ancestors, 134; aquatic, 135;
burrowing, 221; terrestrial, 136

Artiodactyla, 164

Ascaris, germinal continuity, 306, 422

Asexual propagation, 350

Asphalt, 124

Asses, 164, 183, 345

Associations, animal, gregariousness,
237; communal, 239; commensal,
240; symbiosis, 240; parasitism,
240

Asterias fo7-hesii, 265

Auchenia, skull, feet, and teeth, 1S5

Augustine, 8

Australian region, 255, 256

Auslralopitliecas africanus, 201

Autosomes, 299

Autotomy, 237

Aves, see Birds

Axolotl, 95, 211

Babcock and Clausen, 275, 317, 342,

345, 347, 350
Back-cross, 285
Bacon, 8
Baplanodon, 153
Barnacles, 214
Barriers, 257

Basques, 207

Bateson, 264, 269, 319

Bats, 227, 253

Beagle, voyage of, 14

Beaks, of carnivorous birds, 233, 235;
of seed-eating birds, 234, 235; of
insectivorous birds, 234, 235

Bear animalcules, 137

Bears, polar, 260

Bechuanaland, 199

Beebe, Wm., 2, 14, 216, 244, 254, 260,
394, 409

Bee moth, 347

Bees, evolution in, 397; and fruit, 231

Beetle, Japanese, 251

Beetles, jumping adaptations, 220;
running leg of tiger beetle, 21S;
swimming leg of gyrinid, 218;
Schroder's experiments with, 420

Belemnites, 133

Belostomatidae, 254

Benedict, a Clydesdale stallion, 352

Benthos, 214; aerial, 217

Beresovka mammoth, 126

Bering isthmus, 258, 259

Biology, 11

Biometry, 15, 275

Bipedality, 219

Birds, 37; adaptations for securing
food, 233, 235; digestion, 227;
feathers, 225; flight adaptations,
225; metabolism, 228; origin of,
154; respiration, 228; secondary
sexual characters, 334; sex reversal
in, 327; skeleton, 225; tail and wing
functions, 225; temperature, 228;
and warning colors, 242; mocking,
409

Birth rate, 377; decrease in, 377; of
the unfit, 378; of mentally normal,
378; and immigration, 380

Bison, 347

Blastocoele, ^0

Blastula, jO, 41

Blending inheritance, 318

Blood, 113

Blood tests, 95

Blue Andalusian fowl, 281

Body cavity, 35

Boveri, 301

Brachiation, 193

Brachiopoda, 28, 132

Brachiosaurus, 153

INDEX

447

Brain, embryonic, 48; of primates,

188; of vertebrates, 86
Branchial arches, 54
Branchiosioma, 145
Bridges, 314

British Guiana, tinamou, 244
Brvophyta, 28
Bryozoa, 28, 132
BufTon, 9, 413
Bugs, 253

Bull, Ephraim Wales, 344
Bumble bees, and red clover, 230
Burbank, 350
Bursa, seed capsule shape, 317

Cacops, 151

Caenogenesis, 211

Callosities, 417

Cambrian, 123; coelenterates, 130;

mollusca, 133; sponges, 130; trilo-

bites, 134
Camels, 184; phylogeny, 185
Cannabis saiim, 336
Carboniferous, 123; amphibia, 151
Carnegie, 380

Castes, neuter, evolution of, 397
Castle, W. E., 318, 421
Casts, 125, 126
Cat, skull, 391
Catarrhini, 188

Caterpillar, 211; mimicry of twig, 244-
Calocala ilia, 243
Cattle, 347; selection in, 351; polled

Hereford, 404
Cave drawings, 1, 206, 207
Cebidae, 188
Cells, 30, 31; importance in genetics,

272.
Cellulose, digestion in termites, 240
Cenozoic, 122
Cephalopoda, 37, 132; concealing

discharges, 217, 237
Cercopithecidae, 188
Cereus giganleus, 226
Chamaeleon, 223
Change, of habits, 392; source of,

433; adaptive and incidental, 434
Characters, acquired, inheritance of,

386, 414, 415; evidence of, 417
Characters, organic, source of, 431;

permanence of, 437; useful and

harmful, 389; importance to in-
dividual, 387

Chat, yellow-breasted, 394

Cluld, 422

('hilopoda, 136

Chimpanzee, 189

Chlorophyll, 91

Choloepus didadylus, 223

Chondrocranium, 54, 68; of elasmo-
branch, 58

Chordata, 28, 37

Chorion, 4'5; human, ^6"

Chromomeres, 291

Chromosomes, cliaracteristics of, 292;
organization of, 291; in mitosis,
100; in gametogenesis, 292, 293;
abnormal behaviour, 299; non-
disjunction, 300; multiplication,
299; in crossing over, 309; and in-
fertility, 349; and Mendelian in-
heritance, 303; in gynandromorphs,
333; haploid and diploid number,
292; and sex, 297, 298; x, y and z,
298; of aphids, 329, 330; of Ascaris,
295; of Drosophila, 291; of honey-
bee, 329; of man, 310, 361

Chromosome theory, 289; evidence
for, 300

Cicindelidae, cursorial adaptations
of, 218

Circulation of human embryo, 48

Circulatory system, of fishes, 59; of
dogfish, 61; of terrestrial verte-
brates, 61

Class, 26

Classification, 19, 25; of animals and
[)lants, 28

Clavicle, of Sauriplerus, 76; of ter-
restrial vertebrates, 77; of prim-
itive reptile, 79; of mammals, 80;
of primates, 188

Claws, 85; of anteaters, 232; of flesh-
eating birds, 233, 235; as weapons,
237

Cleavage, 39; of frog's egg, 41 ; of
pigeon's egg, 41

Climate, changes of, 248, 249; and
food supply, 250

Climl)ing, adaptations for, 222

Clones, 355

Cocoanuts, dispersal by water, 260

Codling moth, 21

Coelenterata, 28, 130, 215

('(K'lom, 35

Colonial animals, 23

448

INDEX

Coloration, 242

Color-blindness, a sex-linked char-
acter, 364; inheritance of, 365

Columbian elej^hant, 398, 399

Comanchean, 122

Combinations, 2i58

Commensalism, 240

Communal associations, in insects,
239; in man, 240

Comparative anatomy, 3, 68

Competition, 6; and natural selec-
tion, 388; and useful characters, 389

Complementary factors, 319

Concealing discharges, 237

Concealment, 230

Concord grape, 344

Conductivity, 90

Consciousness, 1, 2

Continuity of germ plasm, 422

Continuous variations, 267

Contractility, 90

Convergence, 27

Corals, 214

Coral snake, 242

Corixidae, 254

Corn, origin of, 271; and Mendelian
inheritance, 286, 287; hybridiza-
tion and selection, 342; crosses of,
344; heterosis in, 343; sex reversal
in, 336

Correns, 276

Corythosaurus, 153

Craniata, 144

Crayfish, 134

Cretaceous, 122; Bryozoa of, lr'/2

Cretaceous Eocene, change of climate
during, 248

Cretinism, 95

Crew, 335

Cricolus, 151

Crinoidea, 131

Crocodilia, 237

Cro-Magnon race, 206; sculpture,
206, 207; implements, 207; mental
development, 208

Cross fertilization, 231

Crossing over, mechanism of, 309;
multiple, 309, 310; percentage of,
311, 312; in Drosophila, 307, 308

Crossopterygii, fins of, 76, 149

Crows, 239

Cryptohranchus allegheniensis, skull,
58

Cryptopsaras coursii, 216
Crypt urus variegalus, 244
Ctenophora, 28
Cumulative factors, 318
Cursorial adai)tations, 218
Cuttlefishes, 132
Cuvier, 11, 413 •
Cyclostomata, 37
Cydninae, 254
Cynodontia, 157

Cytoplasm, 32; role in inheritance,
301; differentiation in eggs, 302

Dactylethra, jjelvic girdle, 78

Daphnids, 356

Dark Ages, 8

Dart, Raymond, 199

Darwin, Charles, 9, 12, 13, 230, 250,
269, 369, 385, 413

Darwin, Erasmus, 9, 10, 370, 413

Darwinian theory, 385, 418, 428

Darwinism, see Darwinian theory

Davenport, C. B., 362

Deaf-mutism, inheritance of, 367

Debris, floating, as aid to dispersal,
258

Deep-sea fishes, 216

Deer, protective adaptation, 236; sig-
nal marks, 243

Defectives, inbreeding of, 367; segre-
gation of, 382; sterilization of, 382

Defects, inheritance of, in man, 366

Democritus, 5

Denlalium, 302, cytoplasmic differ-
entiation of egg, 302

Dentition, heterodont, 159; of ele-
phant, 168; of horse, 175, 177

Dermal bones, 55

Desert, Great Salt Lake, 255

Detlefsen, 418, 420

Devonian, 123, 150; amphibia of,
130; trilobite of, 134

De Vries, Hugo, 12, 276, 402

Dibclodon, 170

Differential birth rate, 377; causes of,
379

Dihybrid, 279; ratios, 281, 282

Dinosaurs, 153, 219

Dinolherium, 169

Diploblasts, 34

Diplocaulus, 151

Diplopoda, 136

Dipnoi, 149

INDEX

449

Discontinuous variation, 267

Dispersal, of nuinimals, 160; aids to,
258; barriers to, 257

Distribution, f^eographical, see Geo-
graphical distribution.

Disuse, see Use and disuse

Divergence, 27

Division of labor, 23

Dogs, ancestry of, 271

Dominance, 280

Dordogne, 207

Draco volans, 230

Drosophila, -419, 428; chromosomes
of, £91; mutation and normal eye.
269; and Mendelian inheritance,
287; multiple allelomorphs, 314;
linkage and crossing over, 30?, 308;
multiple crossing over, 309; sex-
linkage, 312, 313, 315; non-dis-
junction, 300; reverse mutations,
402; chromosome maps, 311; and
pure lines, 356

Drosophila melanogaster, see Droso-
phila.

Dryopithccus fontani, 198

Dubois, Eugen, 198

Duck mole, shoulder girdle of, 80

Dugdale, 308

Earthworm, 36

Echidna, claw of, S5

Echinodermata, 28, 132

Ectoderm, //>, 41

Edwards, Jonathan, 370

Ehrlich, 95

Elasmosaurus, 153

Electrical organs, 237

Elephants, Indian, 165; African, 165
foot, 166; skull, 167; tooth, 16S
size of, 166; limbs, 166; trunk, 166
tusks, 167; teeth, 168; Columbian,
398, 399; evolution of head and
teeth, 174; phylogeny of, 169

Elephas, 172; antiquus, 173; columbi,
173, 399; imperaior, 173; meridio-
nalis, 173; primigenius, 126, 173

Embryology, 39

Embryos, 52, 53

Empedocles, 5, 6

Endocrine glands, 94

Endoderm, 40, 41

English sparrow, 251

Enteron, 40

Entheus peleus, 22

Environment, 2, 211; modification of,
248; availability of, 252; response
to, 251; continuity of, in evolution,
436; duration of, 437; of fossils,
127; effects on soma and germ
plasm, 419; and existence, 430, 431;
and sex determination, 338; and
eugenics, 383; and human life,
372

Eoanthroputi dawsoni, 202; phylogeny,
204; skull, 203

Eocene, 122; elephants, 168; horses,
176, 178, 179; camels, 185

Eohippus, 176, 178, 183; feet and
teeth, 179

Ephemera varia., 138

Epihippus, 179, 183

Equation division, 294

Eqims, 182, 183

Eryops, 151

Estabrook, 368

Ethiopian, 255, 256

Eubleptus daniehi, I4O

Eugenics, 374; goal of, 375; limita-
tions, 377; and environment, 383;
practical aspects, 383

Eugenics Record Office, 359

Euglena, 93

Eutheria, 103, 159

Evidence, for evolution, 105, 119; for
inheritance of acquired characters,
417, 419

Evolution, 3, 4; history of, 5; emer-
gent, 442; purposive, 17; experi-
mental, 440; nature of progress in,
435; latitude of heritage in, 435;
and environment, 436, 440; of Mol-
lusca, 132; of Arthropoda, 134, 137;
of insects, 142; of vertebrates, 144;
of elephants, 164, 174; of horses,
175, 183; of camels, 185; of man
187

Evolutionists, 9

Excretory system, 65; in chick em-
bryos, 65

Existence, factors in, 430

Exoskeleton, 82

Eye, development of, 49, 50; evolu-
tion of, 395; of primitive insect,
396; of squid, 112; of vertebrate,
113; loss in burrowing animals, 224

Eyelid, third, 88

450

INDEX

Factors, and unit characters, 306;
complementary, 31.9; cumulative,
316, 318; duplicate, 316, 317; lethal,
320, 332; multiple, 316; supple-
mentary, 319; in existence, 430

Family, 26

Faserstrang, 147

Feathers, 83

Felis, skull, 391

Ferns, 24

Fertilization, 296

Filial regression, law of, 271, 272

Fin, pectoral, of dogfish, 75; of
Sauripterus, 76; transitional, 76

Fishes, 37; as vertebrate ancestors,
148; in nekton, 215

Fission, 100

Flea, amber fossil, 127; jumping
adaptations, 220

Flies, adhesive organs of, 223

Flight, theories of origin, 154; adapta-
tions of bii-ds, 154, 225; in insects,
227, 229; in vertebrates, 227-229

Flippers, 111

Fly, house, 231

Flying dragon, 229, 230

Flying frog, 229

Flying lemur, 229

Flying squirrel, 229

Foetal membranes, ^5, 102; human,
46

Food securing, adaptations, 232; of
grazing species, 232; of anteaters,
232; of carnivorous animals, 233

Food supply and climate, 250

Forelegs, skeletons of, 81

Fortuitous variations, 268

Fossils, formation of, 124, 125; inter-
pretation of, 127; succession of, 129

Fossorial ada{)tations, 221

Fossorial bugs, 254

Four o'clocks, 281

Fowls, Andalusian, 281; Sebright
bantams, 334; effects of removal
of gonads, 334; sex reversal in,
335, 336

Fox, arctic, 242

Freemartin, 335

Frog, development of egg, 4U frying,
229; sex reversal in, 327; spermato-
gonium, 290; spinal ganglion cell,
290

Functions, variations in, 265

GalApagos islands, 258
"Galapagos, Worlds End," 14, 409
Galen, 8

Galeopithecus, 229
Galton, 16, 271, 369
Galton's laws, 271
Gametes, 101; individuality of, 328;
of Sphaerella, 326, 327; see Germ
cells
Gametogenesis, 292, 293
Gametophyte, 24
Gastrostomus bairdii, 216
Gastrula, 40, 41
Gastrulation, 42; in frog, 4I; in

pigeon, 43
Gelastocoridae, 254
Gemmules, 423
Gene, theory of, see Chromosome

theory
Genes, 305; localization of, 310; in

ontogeny, 322; role of, 432
Genetics, 4, 17, 262; foundations of,

264; practical value, 341
"Genetics in Relation to Agricul-
ture," 275
Genotype, 279
Genotypic selection, 353
CJenus, 26

Geographical distribution, 254 ; realms,
255, 256; barriers, 257; aids to dis-
persal, 258
C!eological table, 122
Geological time, 120; divisions of,

122
Geology, 119
Germ cells, development, 292, 293;

human, 296; see Gametes
Germ plasm, 305; continuity of, 422
Germinal continuity, 422
Germinal selection, 406
Germ layers, 32, 40, 41
Gerridae, 254
Gibbon, 189
Gila monster, 242
Gill slits, 37
Gipsy moth, 251
Girdles, diagram, 77; pelvic, 78;

pectoral, 79, 80
Glands, endocrine, 94; and internal

environment, 431
Gliding, adaptations for, 228
Gnathostomata, 147
Goddard, 376

INDEX

451

Goldschmidt, 323

Gonads, effects of removal, 334

Goniatites, 133

Gorilla, 189, 190; skeleton, 192

Grasshopper, 220, 243; jumping leg

of, 218
Great Salt Lake desert, 255
Greek philosophers, 5
Gregariousness, 238
Gregory, W. K., 188
Gregory of Nyssa, 8
Guinea-pigs, inheritance in, 282, 283;

ovarian grafts in, 421
Guyer, 382, 421
Gynandromorphs, 332, 333

Hair, and scales, 83, 84; persistent, 87;

form of, 267
Halohntcs, 254
Handlirsch, 136, 140
Hapalidae, 188
Hare, prairie, 242
Harmful characters, 389
Harvey, 8
Heart, of vertebrates, 61; diagram,

6'2; human, development of, 63
Hefner, 364

Heidelberg man, 202; jaw of, 202
Height, inheritance of, 271
Hellgramites, 211
Helm, W. B., 266
Hemimetabola, 136
Hemiptera, adaptive branching in, 253
Hemp, sex reversal in, 336
Herbert, 409
Hereditary bridge, 274
"Hereditary Genius," 369
Heredity, 15, 264; chromosome the-
ory of, 2S0; in man, 359
Heritability, importance in genetics,

270
Heritage, 2; latitude of, in evolution,

435; and existence, 436; subject to

change, 439
Hermaphrodites, 332
Hermit crab, 240, 241
Hesperopiihecus haroldcooki, 201
Heteroceras, 133
Heterosis, in plants, 342; in animals,

348
Heterozygous, definition of, 279
Himalaj'an mountains, 257
History of evolution, 5

Jlipparion, 181, 183

Ilippidion, 182, 183

Hoatzin, 157; young, climbing, 156

Holmes, 368, 369

Holometabola, 136

Hominidae, 188

Homo, see man; heidelbergensis, 202;
jaw, 202; phylogeny 204; neander-
thalensis, 202, 204; skull and skele-
ton, 205; phylogeny, 204; restora-
tion, 200; sapiens, 26; sapiens,
Cro-Magnon race, 206; relation to
Neanderthal man, 206; restoration,
200; phylogeny, 204

Homologies, significance of. 111

Homology, 27

Homozygous, definition of, 279

Homozygous crosses, 356

Honey-bee, 21; sting, 238; organiza-
tion of colony, 239; hybridization
in, 347; selection in, 351

Hoofs, 85

Hooke, 8

Hormones, 94; and secondary sexual
characters, 314, 334; effects of in-
sufficiency, 334

Hornbills, 396

Horns, as weapons, 237

Horse, adaptive structure, 175; teeth,
177; skull, 177; evolution of, 176;
protective adaptations, 236; ortho-
genetic variation in, 268; selection
in, 351, 352; South American
horses, 182; North American
horses, 182; extinction of, 184; and
mule, 345

Humming bird, ruby-throat, 394

Huxley, 15

Hybridization, 274; practical value
of, 341; of domestic animals, 345;
limitations of, 348

Hybrids, heterosis in, 342; asexual
propagation of, 350; infertility of,
349

Hydra, 34, 51, 55; asexual reproduc-
tion in, 273, 274; clones in, 356;
and continuity of germ plasm, 422;
variation in, 265

Hylobates, 189

Hyoid arch, 54, 75

Hypohippus, 180, 183

Hypopharynx, of Anopheles, 234

Hyracotheriuin, 176, 183

452

INDEX

Ice floes, 258

Ichthyosaur, 153

Idiacanthus ferox, 216

Idioplasm, 291

Immigration, 380

Immune sera, 95

Implements, 1

Inbreeding, in man, 367

Induction, parallel, 419; in insects,
420; somatic, 419

Infertility, interspecific, 348; of hy-
brids, 349

Inheritance, of acquired characters,
414, 415; acceptance of, 416; cir-
cumstantial evidence for, 417
alternative, 280; blending, 280
mosaic, 280; in four o'clocks, 281
in Andalusian fowls, 281 ; in Guinea-
pigs, 282, 283; in peas, 278; of more
than three characters, 284; of eye
color, 361; of hair color, 361; of
skin color, 362

Insects, 136; adaptations for securing
food, 233; adaptive branching in,
253; diversity of, 138; metamor-
phosis of, 136; mouth parts, 138,
139; mimicry, 245; Paleozoic, 1^0;
Schroder's, 420; wings, 139

Instincts, and natural selection, 396

Insulin, 95

Intraselection, 408

Irises, 345

Irish Elk, 389

Irritability, 90

Isolation, biological, 409; physical,
409; theory of, 409; spatial, 387

Jack-in-the-pulpit, sex reversal in,
336; Siamese twins, 337

Japanese beetle, 251

Java, 198

Java ape-man, 199

Jaws, 73

Jelly-fishes, 213, 214

Johannsen, 354

Jones, F. W., 193, 195, 197

Jukes family, 368

Jumping, 219; value of, 220; in in-
sects, 220; in vertebrates, 219

Jurassic, 122; squids, 133

Just (and Lillie), 301

Kallikak Family, 370
Kangaroo, 219

Kant, 8
Katabolism, 30
Kea parrot, 267
Kellicott, 367
Kiang, 183
Kidneys, 66
Kingdom, 26
Kingsley, 75
Krapina, 203

Labium, of Anopheles, 234

Labrum epipharynx of Anopheles, 234

Lamarck, 9, 10, 11, 115, 385; and

variation, 269
Lamarck's laws, 11, 414
Lamarckian theory, 413; errors of,

428; value of, 424
Lance, 122
Leeuwenhoek, 8
Legs, of insects, 218, 233
Lemur, flying, 229
Lemuroidea, 188
Lethal factors, 320, 322
Lichens, symbiosis, 240
Life, 2, 3 '
Light, adaptations to, in plants, 223;

in animals, 224; pigmentation, 224;

loss of eyes, 224
Lillie, 323', 335
Lillie and Just, 301
Limb buds, 47
Limbs, of elephants, 166
Limestone, 120
Line selection, 353
Linkage, 285, 306; effect on dihybrid

ratios, 286; and sex, 286; in Dro-

sophila, 307; in man, 310; in other

organisms, 310
Linnaeus, 9, 388
Linophryne lucifer, 216
Llama, 184; skull, feet, teeth, 185
Lobster, 36
Locomotion, in terrestrial organisms,

217
Locy, 10

Loon, adaptations of, 213
Lou Dillon, 352
Loxodonta, 173
Lull, 122, 127, 133, 150, 160, 164,

166, 168, 176, 217, 257, 388
Luminous organs, 216
Lung fishes, 130
Lush, 347

INDEX

453

MacFarland, E. M., 226

Maggot, 136

Malpighi, 8

Malthus, 10, 14, 250, 385

Mammae, of primates, 188; super-
numerary, in man, 87

Mammalia, 37

Mammals, ancestors of, 157; adapta-
tions of, 159; adaptive radiation in,
253; classification of, 159; dis-
persal of, 160; evolution of, 157
secondary sexual characters of, 334

Man, 26; systematic position, 187
structural plan, 187; arboreal or-
igin, 191; structural adaptations
196; skeleton, 192; evolution of
187; fossil remains, 198, 202; sig-
nificance of geological record, 201
climatic factors in evolution, 201
phylogeny, 304; ancestral species
restored, 200; course of evolution
208; loss of hair, 197; heredity in
359; heritable characters, 360; unit
characters, 361; chromosomes, 361
symphalangism, 363; Polydactyly
266, 363; sex-linkage, 364; in-
heritance of color blindness, 365
inheritance of defects, 366; pedi-
grees, 366; inheritance of deaf-
mutism, 367; inbreeding, 367; and
environment, 372; desirable traits,
374; importance of mental develop-
ment, 375; Heidelberg, 202; Nean-
derthal, 200, 204, 205; Piltdown,
203; Cro-Magnon, 200, 206

Mandibles, of Anopheles, 234

Mangro, 363

Mantis, raptorial foreleg of, 218

Maple seeds, 260

Marsh, 164

Marsupials, 159

Mass selection, 351

Mastodon, 170, 171; skull and tooth,

174
Matthew, 179, 197
Maturation divisions, 294
Maxillae, of Anopheles, 234
Maxillary palpi of Anopheles, 234
May-fly, 138
McClung, 308, 333
Meckel's cartilage, 54, 73, 110; of

embryo kitten, 60
Mendel", 16, 17, 276

Mendelian hybrids, of plants, 342

Mendelian inheritance, 276; and chro-
mosomal behaviour, 303; funda-
mental principles, 279; monohy-
brid ratio, 277; dihybrid ratio,
281; practical importance, 287

Mental defects, and eugenics, 376;
inheritance of, 370

Mental traits, 370

Mertjchippus, 181, 183; teeth, 181

Mesemhryanthemum, 226

Mesohippus, 179, 183; teeth, 180;
feet, 180

Mesonephric tubule, 65

Mesonephros, 66

Mesozoic, 122; reptiles of, 153

Metabolism, 30, 90, 114; of animals,
92; of plants, 91; other types, 93

Metameres, 35

Metamorphosis, 24; of insects, 136

Metanephros, 66

Metatheria, 102, 159

Method, of evolution, 16

Methods, of selection, 351

Mice, field, 230

Mice, Weismann's, 419

Migration, 251, 254, 392

Milk, 102

Milkweed seeds, 260

Mimicry, 241, 245; and classification,
26

Mind, brachiation and, 193; environ-
ment and, 194

Miocene, 122; apes, 198; camels, 185;
climate, 180; elephants, 169; horses,
180

Miohippus, 179, 183

Mirabilis jalapa, 281

Mississippian, 123

Mitochondria, functions in heredity,
303

Mitosis, 100

Mocking birds, 409

Modifications, 268

Moeritherium, 168;sk\i\\ and tooth, ^74

Molds, 125, 126

Mole, 253

Mole cricket, 221; burrowing fore-
leg of, 218

Mole, European, 221; adaptations,
221

Mollusca, 28, 37, 132; in nekton, 215;
in plankton, 215

454

INDEX

Mongolian race, hair of, 267

Monkeys, and warning colors, 242;
prehensile tails, 223

Monohybrid ratio, 277, 379; modi-
fication of, 280, 281

Monohybrids, of peas, 277

Monotremata, 157

Montanian-Coloradian, 122

Moron, 376

Morphological variation, 265

Moth, gipsy, 251

Moth, geometrid larva, 244

Moth, willow, 420

Mountains, as barriers to dispersal,
257; effect on climate, 248

Mouth parts, of insects, 138, 139

Mouths of insects, 233; piercing and
sucking, 334; mandibulate, 139

Mulattos, 362

Mule, 345; fertility of, 349

Muller, H. J., 314, 320, 402, 419

Multiple allelomorphs, 314

Mutants, in cultivated plants, 342

Mutations, cause of, 405; in Dro-
sophila, 369, 404; and natural
selection, 404; and pure lines, 357;
in Oenothera, 402; theory, 428;
reverse, 402

Mutilations, 386; source of, 432

Nails, 85; of primates, 188

Narcissus, 345, 346

Natural history, 5

Natural Selection, 385; answers to
objections, 395; examples of, 393;
in Angraecum sesquipedale, 393;
objections to theory, 395; origin of
theory, 385; statement of theory,
385; summary of, 392; underlying
principles, 386

Nautilus, 132

Neanderthal man, 204; cranial capac-
ity, 204; mental development, 205;
phylogeny, 204; speech, 205; skull
and skeleton, 305; stature and
posture, 205

Nearctic, 356; realm, 255

Nebraska, fossil man in, 201

Necturus, 418; and inheritance of ac-
quired characters, 418; pectoral
girdle, 80; pelvic girdle, 77, 78;
sacrum, 73

Negro, hair of, 267; white crosses, 362

Nekton, 215; aerial, 217

Nemathelminthes, 28

Neo-Lamarckian theories, 422

Neolaurentian, 124

Neoteny, 211; in axolotl, 211

Neotropical, 255; 356

Nepidae, 254

Nesomimus, 409

Neural groove, 44; in chick embryo,

43
Neural tube, 44
Neurenteric canal, 42, 44} 47
Newman, 152, 411
Nilsson-Ehle, 316
Nilsson, Hjalmar, 353
Non-adaptive changes, 247
Non-adaptive characters, 212, 434
North America, climate of, 248
Notholaena, 225
Notochord, 37, 51, 56, 57
Notonectidae, 254
Nucleus, 32; role in inheritance, 301
Nuttall, 95

Obelia, 30, 23

Oceans as barriers to dispersal, 258

Ocellus, 110

Octopus, 132

Octoroon, 363

Oenothera lamarckina, scintillans, ob-

longa, lata, 402, 403; gigas, nanella,

403
Oligocene, 122; amber fossils of, 127;

elephants, 169; horses, 179; camels,

185
Ommatidium, 111
Onohippidion, 182, 183
Ontogeny, 109
Onychophora, 136
Oocyte, 292
Oogonia, 292
Opthalmochlus duryi, I4I
Orang, 189
Orchesella rufescens var. pallida, eye,

396
Order, 26

Ordovician, 123; fishes, 130, 144; an-
nelids, 132; Brachiopoda, 132;

vertebrates, 144
Organic environment, 230
Organic world, unity of, 416
Organisms, as agents in dispersal,

261; relationships of, 232

INDEX

455

Organogeny, 47

Organs, complex, and natural selec-
tion, 395; electrical, 237; necessity
in production of, 416; vestigial, and
natural selection, 397

Oriental region, 255, 256

"Origin of Species," 12, 15, 212, 385

Orohippus, 179, 1,S3

Orthogenetic variations, 268

Orthoptera, jumping adaptations in,
220

Osborn, H. F., 6, 198, 199, 202, 206,
207, 252

Os coccyx, 87

Ostriches, 219

Ovarian grafts, 421

Overproduction, 250

Overspecialization, 398

Ovum, SI, 40, 102, 292; of birds, 295;
human, 396

Oyster, rate of reproduction, 234, 388

Paedogenesis, 211

Palaearctic realm, 255, 256

Palaeomastodon, 169; skull and tooth,
174

Palaeopithecus sivalensis, 198

Palaeostraca, 135

Paleolaurentian, 124

Paleontology, 19, 119

Paleozoic, 123; fossils of, 130; echi-
noderms, 132; insects, I40

Pan pijgmaeus, 189

Pangenesis, 423

Panmixia, 405

Parahippus, 180, 183

Paramecium, 91; binary fission in,
273; and pure lines, 357

Parasitism, 240

Parental care, 234

Parrot, kea, 267; use of beak in climb-
ing, 223

Parthenogenesis, 328; and pure line
equivalents, 355

Pass-for-white, 363

Paurometal)ola, 136

Peas, garden, 277; sweet, 319

Pectoral girdle, 54, 79; of dogfish, 75;
of Sauriplerus, 76; of primitive
reptile, 79; of human embryo, 80;
of duck-mole, 80

Pedigrees, human, 366

Pelvic girdle, 54, 77, 78

Pennsylvanian, 123

Pentadactyl appendage, 77, 79; and

locomotion, 217
Peonies, 345
Pcrissodactyla, 164
Permian, 123; reptiles of, 152; change

of climate during, 248
Pelroqnle xanthopus, 220
Pharyngeal clefts, 37
Phaseolus vulgaris nana, 354
Phenotype, 279
Phenotvpic selection, 350
Phillips, 421

"Philosophie Zoologique," 11
Photostomias guernei, 216
Photosynthesis, 91, 230
Phylum, 26

Physiological variation, 265
Physiology, 90
Piltdown man, 203; skull, 203;

phylogeny, 204
Pisces, 37
Pithecanthropus erectus, 198; skull,

199; restoration, 200; relation to

man, 199
Pituitary gland, 94
Placoid scales, 82
Plankton, 215; aerial, 217
Plant-animals, 92
Plants, adaptation to aridity, 224,

226; dispersal, 260; heterosis in,

342; variation in, 265
Platyhelminthes, 28
Platyrrhini, 188
Plebeius nielissa, 409
Pleistocene, change of climate during,

248; glaciation, 202; elephants,

173; horses, 182
Plesiosaur, 153
Plesippus, 183
Plica semilunaris, 88
Pliny, 8
Pliocene, 122; climate of, 201; Bry-

ozoa, 132; elephants, 172; horses,

182; camels, 185
Pliohippus, 182
Pliopithecus antiquus, 198
Poehrolhcrium, skull, feet and teeth,

185
Poetaz narcissus, 3^6
Poeticus narcissus, 3^6
Polar bodies, 294
Polyanthus narcissus, 3^6

456

INDEX

Polydactyly, 265, 266

Polypterus, ribs, 71; pelvic girdle, 78

Pongo, 189

Porifera, 2S, 130

Portuguese man-of-war, 23

Prawn, deep-sea, 217, 237

Precipitin tests, 96

Prehensile appendages, for climbing,

222; of primates, 191; for handling

objects, 194; effect on evolution of

primates, 196
Prenatal influence, 7
Primates, characteristics of, 187;

classification of, 188; effects of ter-
restrial life on, 195
Primitive man, 3
Primordial germ cells, 292
Prjevalsky horse, 352
Proboscidea, 164

Procamelus, skull, feet and teeth, 185
Proctodaeum, 48
Pronephric tubule, 65
Pronephros, 66
Pronuclei, 297

Proserpinaca palustris, leaves of, 390
Protective adaptations, 234
Protective coloration, 10, 242
Proterozoic, 124; fossils of, 129;

change of climate during, 248
Proteus anguinus, 224
Protohippus, 181, 183
Protoplasm, 29
Prototheria, 102, 157, 159
Protozoa, 28, 32, 33, 130, 131; clones

in, 356; symbiosis with termites, 240
Protylopus, skull, feet and teeth, 185
Psychological variations, 267
Psychozoic, 122
Pteridophyta, 28
Pterodactyl, 153, 229, 232
Pteropus, 227
Pterosaurs, 230
Pterygoquadrate, 54, 73; of elasmo-

branch, 58
Pterygota, 136
Pure lines, 35^; equivalents of, 355;

of wheat, 356; selection in, 356; as

limits of selection, 357
Pytonius, 151

Quadroon, 362
Quaternary, 122
Quinto Porto, 3^7

Rabbits, 219; inheritance of color in,
321; inheritance of ear length in,
318; Guyer and Smith's experi-
ments with, 421

Recapitulation theory, 110

Recessive, 279, in back-cross, 285

Reciprocal cross, 308

Reduction division, 294

Regression, filial, Galton's law of,
355, 401

Relationship, significance of, 105; of
individuals, 106; of species, 107;
of organism and environment, 261

Reproduction, 90; of cells, 99; of in-
dividuals, 101; accessory functions
of, 101; in unicellular organisms,
100, 272, 273; in multicellular or-
ganisms, 273; as passive protec-
tion, 234; and change of environ-
ment, 250

Reptiles, cursorial, 219; in vertebrate
evolution, 152; of Mesozoic, 153

Reptilia, 37

Rhacophorus, 229

Rhamphorhynchus phyllurus, 232

Ribs, 71, 72

Rickets, 431, 434

Rignano, 424

Rocks, formation of, 119

Rocky Mountains, 120, 257

Roses, 345

Rotatoria, 28, 137; elimination of
male in, 329

Roux, 291, 408

Sacrum, 72; human, 73

Saint-Hilaire, 9, 12

Salamander, blind, 224; cell from

peritoneum, 290
Saldidae, 254
Saltatory adaptations, 219
Sambo, 363
Sandstone, 120 .
San Jose Scale, 251
Sauripterus taylori, pectoral girdle

and fin, 75
Scale, San Jose, 251
Scales, 83
Scansorial adaptations, 222; and

adaptive radiation, 253
Scaphyrhynchus, pelvic girdle of, 78
Schaffner, 336
Schistosotna haematobium, 23

INDEX

457

Schoetensack, 202

Schuchert, 120, 248

Scientific methods, development of,
8

Sciuropterus volucella, 231

Scott, 172, 182

Sea anemone, 214; and hermit crab,
241

Sea urchins, artificial parthenogenesis
in, 330; cytoplasmic differentiation
in ovum of, 302

Secondary sexual characters, 22, 314,
334

Secretions, repellent, 237

Sedum, 226

Seeds, dispersal of, 260

Segregation, of unit characters, 279;
and chromosomes, 303; and natural
selection, 387; of defectives, 382

Selection, 271, 274; practical value
and methods of, 351; in pure lines,
356, 357; limits of, 357; pheno-
typic, 350; natural, 385; sexual,
394; germinal, 401, 406; coincident,
408; in bees, 397; results of, in
horses, 352

Semon, 424

Serosa, Ji.5

Sex, chromosomes, 298; determina-
tion of, 298, 325; reversal, 327, 335,
338; reversal in fowl, 336; control
in Arisaema, 337; and the heritage,
327; purpose of, 331

Sexes, differentiation of, 331; con-
tributions to heritage, 300

Sex-linkage, in Drosophila, 312, 313,
315; in man, 314, 364, 365

Sexual colors, 244

Sexual forms, 22

Sexual selection, 10, 394

Shale, 120

Sheep, Ancon, 404; selection in, 351

Shepherd's purse, 316; inheritance in,
317

Shore bugs, 254

Shrews, 253

Shull, 316, 342, 380

Sierra Nevada mountains, 257

Silurian, 123, 150; lung-fishes of, 130;
change of climate during, 248

Simla, 189

Simiidae, 188

Siwalik Hills, 198

Skeleton, appendicular, 77; of lower
fishes, 51; of higher fishes, 55;
above fishes, 55; visceral, 73, 74; of
man and gorilla, 192; of Neander-
thal man, 205

Skull, ganoid stage, 68, 69; dermal
bones of, 69; of Cryptobranchus, 58;
in Amphibia, 70; above Amphibia,
71; human, 71; of elephants, 167;
of Moeritheriuiu, 168; of Probos-
cidea, 174; diagram of mammalian,
59; of horse, 177; of Smilodon, 391;
of Felis, 391; of Java ape-man, 199;
of Piltdown man, 203; of Neander-
thal man, 205

Sloth, two-toed, 223

Sloths, 222

Smilodon, skull of, 391

Smith, E., 421

Social animals, 22

Sociology, 376

Soma, 306

Somatoplasm, 305

Song sparrows, distribution of, 4IO

Sparrow, English, 251; song, distri-
bution of, 410

Specialization, 93; limitations of, 252

Species, 19, 25

Spencer, Herbert, 15, 417

Spermatids, 294

Spermatocyte, 292; of Proteus, 290

Spermatogenesis, 292; in Ascaris, 295

Spermatophyta, 28

Spermatozoa, 292; human, 296

Sphaerella, 91; life history, 326

Spinal column, 72

Spirula, 133

Spondylomorum, 34

Sponges, 214

Spontaneous generation, 6

Spores, 325; of Sphaerella, 326

Sporophyte, 24

Spy, 203

Squids, 132

Squirrels, as arboreal animals, 229;
flying, 229; in adaptive radiation,
253

Starfishes, artificial parthenogenesis
in, 330

Stegocephalia, 151, 152

Stegodon, 172; tooth of, 174

Steinmetz, 380

Stenodiclya lobata, I40

458

INDEX

Stenoma schlaegeri, 245

Sterility, 397

Sterilization of defectives, 382

Stings, of insects, 237; of honey-bee,

238
Stomodaeum, 48
Struthiomimus, 153
Sturtevant, 314
Styela partita, eggs of, 302
Stylopid, 141
Subspecific forms, 21
Supplementary factors, 319
Survival of the fittest, 7, 386
Swammerdam, 8
Sweet peas, 319
Swimming legs, in bugs, 254
Symbiosis, 240
Symmetry, 34, 214
Symphalangism, 363
Synapsis, 292, 293

Tail, in man, 87

Tails, as weapons, 237

Talpa europaea, 221

Tar pan, 183

Tatu novemcintus, 236

Teeth, 82; abnormal growth in wood-
chuck, 212, 213; adaptations, 83;
adaptive radiation in, 253; develop-
ment of, 82; of beaver, 210; of
carnivores, 159, 233; of cynodonts,
157; of elephants, 168; heterodont
dentition, 159; of herbivores, 159;
of horse, 210; of mammals, 158;
and scales, 82; of squirrels, 210

Termite, 271

Termites, sj^mbiosis with protozoa,
240

Terrestrial adaptations, 217

Terrestrial life, demands upon verte-
brates, 148; forms transitional to,
148

Tertiary, 122; climate, IGO; mam-
mals, 160; region of Bering Straits
during, 259

Tetrad, 294; in Ascaris, 295

Tetrakinetic theory, 434

Tetralnphodon, 170, 172

Thallophyta, 28

Theophrastus, 5, 7

Theories, adequate, conditions for,
429; Amphioxus, 144-146; An-
nelid, 144, 146; Arthropod, 144;

Centroepigenesis, 424; coincident
selection, 408; chromosome, 289;
Darwinian, 385, 428; Germinal con-
tinuity, 422; germinal selection,
406; Goldschmidt's, 323; intra-
selection, 408; isolation, 409, 428;
kinetogenesis, 424; Lamarckian,
413; Mneme, 424; mosaic vision,
112; mutation, 402; natural selec-
tion, 385; origin of flight, 154;
pangenesis, 423; panmixia, 405;
sexual selection, 394; tetrakinetic,
434; use and disuse in chromo-
somes, 439

Thymus, 94

Thyroid, 94

Tiger, sabre-tooth, 390; skull, 391

Tiger-beetle, running leg, 218

Tinamou, 244, 294

Trapa nutans, leaves, 390

Tree frogs, adhesive organs of, 233;
climbing, 223; flying, 229

Triassic, 122; changes of climate, 248;
mollusca, 133

Triceratops, 236

Trihybrid, 279, 283, 284; in Guinea-
pig, 283; ratios, 284

Trilobites, 134, 135

Trilophodon, 169, 170; skull and
tooth, 174

Trinacromenon, 153

Trinil race, 199

Triploblasts, 34

Trout, 243

Trunk, 166

Tschermak, 276

Tulips, 345

Tusks, 167, 168, 174

Ungulata, 164; cursorial, 219
Unguligrade appendage, 253
Unit characters, 279; and chromo-
somes, 303; and factors, 306; of
man, 361
Use and disuse, 414

Variability, of organisms, 115; of

environment, 116; cause and effect

in, 117
Variation, 247; and natural selection,

386
Variations, kinds of, 265; source of,

269

INDEX

459

Veliidae, 254

Venom, 237

Vertebrae, 56; development of, dia-
gram, 59

Vertel)rates, 37; evolution of, 144,
161; emergence of terrestrial, 147

Vesalius, 8

Vesperiilio nodula, 227

Vestigial structures, 85

Vilmorin, 353

Vitamines, 431

Yilis. labrusca, 344; V. vinifera, 345

Volant animals, in adaptive radia-
tion, 253; dispersal, 260

Volvox, 51, 54

Wagner, Moritz, 388, 409
Wallabv, 219, 220
Wallace, 14, 250, 385, 393
Walter, 274, 356, 418
Warning colors, 242
Water, as environment, 148
Water lilies, 260
Weapons, origin of, 194 *

Web of life, 230

Weismann, 15, 269, 291, 405
Wheat, 316; pure lines of, S06
Whetham, 367
Whip-poor-will, 242
Wilder, H. H., 69, 72, 198
Wings of insects, 139, 229
Winship, 370
Wisdom teeth, 88
Wolves, 239

Woodchuck, abnormal growth of in-
cisor, 212
Wrens, nesting habits of, 396

Xerophytes, 224, 226
X-rays, 116, 320, 419

Yolk, 40

Yolk sac, 45; human, 46

Yucca, 226

Zebra, 164, 183

Zebu, 347

Zoogeographical realms, 255, 256

Zygote, 326

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