Marine Biological Laboratory Library

Woods Hole, Massachusetts

Gift of F.R. Lillie estate - 1977

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EVOLUTION AND GENETICS

London: Humphrey Milford
Oxford Universitij Press

EVOLUTION AND
GENETICS

BY
THOMAS HUNT MORGAN

PROFESSOR OF EXPERIMENTAL ZOOLOGY
IN COLTTMBIA UNIVERSITY

PRINCETON
PRINCETON UNIVERSITY PRESS

1925

Based on lectures delivered at Princeton University

Under the Louis Clark Vanuxem Foundation

February 24, March 1, 8, 15, 1916

COPYRIGHT, 1916, BY PRINCETON UNIVERSITY PRESS
PUBLISHED OCTOBER, 1916, AS "A CRITIQUE OF THE THEORY OF EVOLUTION'

SECOND PRINTING, SEPTEMBER, 1917
THIRD REVISED PRINTING, FEBRUARY, 1919

COPYRIGHT, 1925, BY PRINCETON UNIVERSITY PRESS
SECOND EDITION, SEPTEMBER, 1925

PRINTED AT PRINCETON UNIVERSITY PRESS, PRINCETON, N.J., LT.S.A

PREFACE

^HE third reprinting of the Vanuxeni Lectures
. for 1915-16, entitled A Critique of the Theory
of Evolution, having been exhausted, the pubhshers
have asked for a revised edition. The revision is no
less an attemj^t at a critique of the evolution theory
than its predecessor, but, as the change in title sug-
gests, greater attention is here paid to one of the
most debated questions among evolutionists today,
namely, the bearing of the recent discoveries in
genetics and in mutation on the theory of evolution.

While in a general way Darwin's theory of Natu-
ral Selection is independent of the origin of the new
variations that furnish it with its materials, yet the
scientific formulation of the theory is intimately con-
nected with the origin and inheritance of suitable vari-
ations. For instance, if most of the observed variabil-
ity of animals and plants were due directly to the
environment, and if the effects thus brought about
were not inherited, such variability could no longer
be appealed to as material for natural selection.

Again, if the variations that aj^pear as mutants are
alwaj^s defective types, they could not, even though
they are inherited, be appealed to as furnishing ma-
terial for progressive evolution.

vi EVOLUTION AND GENETICS

A discussion of these two problems in their histor-
ical setting is one of the princij^al themes treated in
the following pages.

The four original lectures (chapters) have been
subdivided and enlarged into thirteen chapters. Two
of these are entirely new, one dealing with the non-
inheritance of acquired characters ( copied with slight
changes from the Yale Review for July 1924), the
other a criticism of the evidence of human inherit-
ance. The somewhat acrimonious discussion taking
place at the present time concerning racial differ-
ences in man, a discussion in which "nature" and
"nurture" are often confused, mav furnish an excuse
for the addition of this final chapter.

T. H. ]\I0RGAN

3Iarch 1925

CONTENTS

Preface v

I. Different Kinds of Evolution 1

II. The Four Great Historical SjJeculations 7

THE ENVIRONMENT 7

USE AND DISUSE 10

THE UNFOLDING PRINCIPLE 12

NATURAL SELECTION 14

III. The Evidence for Organic Evolution 17

THE EVIDENCE ITIOM COMPARATIVE

ANATOMY 17

THE EVIDENCE FROM EMBRYOLOGY 23

THE EVIDENCE FROM PALEONTOLOGY 32

THE EVIDENCE FROM GENETICS 35

IV. The Materials of Evolution 41

V. MendeVs Two Latvs of Heredity 55

mendel's first law 57

mendel's second law 66

• •
Vll

viii CONTENTS

VI. Tlic Chromosoiiies and MendeVs Two

Laws 73

THE CELLULAR BASIS OF HEREDITY AND

DEVELOPMENT 73

THE MECHANISM OF MENDEl's TWO

LAWS 82

VII. Tlie Linkage Groups and the

Chromosomes 87

THE FOUR GREAT LINKAGE GROUPS OF

DROSOPHILA MELANOGASTER 89

VIII. Sejc-Linkcd Inh eritance 101

IX. Crossing -over 111

X. Natural Selection and Evolution 119

THE THEORY OF NATURxVL SELECTION 119

THE MEASUREMENT OF VARIATION 121

SELECTION AND VARIATION 129

PURE LINES 130

GENETIC VARIABILITY 133

CONCLUSIONS 135

CONTENTS ix

XI. The Origin of Species by Natural

Selection 137

THE DIAGNOSTIC CHARACTERISTICS OF

SPECIES AND THE ORIGIN OF SPECIES

BY NATURAL SELECTION 142

CHANCE AND EVOLUTION 144

PROGRESSIVE EVOLUTION 148

THE DOMINANCE OF THE WILD TYPE

GENES lol

XII. TJie N on -Inheritance of Acquired

Characters 155

XIII. Human Inheritance 179

GENERAL STATEMENT 179

INHERITANCE OF PHYSICAL DEFECTS 184
THE FOUR BLOOD GROUPS AND THEIR

INHERITANCE 194

INHERITANCE OF OTHER TRAITS 198

THE INHERITANCE OF MENTAL TRAITS 200

Chapter I

DIFFERENT KINDS OF EVOLUTION

We use the word evolution in many ways — to in-
clude many different kinds of changes. There is
hardly any other scientific term that is used so care-
lessly— to imply so much, to mean so little.

We S23eak of the evolution of the stars, of the evo-
lution of the horse, of the evolution of the steam
engine, as though they were all part of the same
process. What have they in common? Only this,
that each concerns itself with the history of some-
thing. When the astronomer thinks of the evo-
lution of the earth, the moon, the sun and the
stars, he has a picture of diffuse matter that has
slowly condensed. With condensation came heat;
with heat, action and reaction within the mass until
the chemical substances that we know today were
produced. This is the nebular hypothesis of the as-
tronomer. The astronomer explains, or tries to ex-
plain, how this evolution took place, by an appeal to
the physical processes that have been worked out in
the laboratory, processes which he thinks have ex-
isted through all the eons during which this evolution
was going on and which were its immediate causes.

When the biologist thinks of the evolution of ani-
mals and plants, a different picture presents itself.
He thinks of series of animals that have lived in the

2

EVOLUTION AND GENETICS

past {fig. 1) whose bones and shells have been pre-
served in the rocks {fig. 2) . He thmks of these ani-
mals as having in the past given birth, through an

Ecjuus

'liohippus

Nerychippus

Mesohippus Orohippus Ppotopohippus

Fig. 1. — Outline of horses of different geological periods, showing
their relative sizes and the decrease in the number of toes. {After Lull.)

DIFFERENT KINDS OF EVOLUTION

3

unbroken succession of individuals, to the living in-
habitants of the earth today. He thinks that some of
the simpler types of the past have in part changed
over into the more complex forms of the j^resent
time.

Orohippus Miohippus Hipparmn Equus

Fig. 2. — Forefeet of horses, showing the progressive loss of the lateral
toes. (After Marsh, from Lull.)

He is thinking as the historian thinks, but he runs
the risk of thinking that he is explaining evolution
when he is only describing it.

A third kind of evolution is one for which man
himself is responsible, in the sense that he has
brought it about, often with a definite end in view.

His mind has worked slowly from stage to stage.
We can often trace the history of the stages through
which his creative j^rocesses have passed. The evolu-
tion of the steam-boat, the steam engine, paintings,
clothing, instruments of agriculture, of manufac-

4 EVOLUTION AND GENETICS

ture, or of warfare {fig, 3) illustrates the history of
human progress. There is an obvious and striking
similarity between the evolution of man's inventions
and the evolution of the shells of molluscs and of the

IHON HAT 1-iNINO
-^•YJ-^ POT

THi- pakt.s of a hklaict

boo A-jD

HELMETS

THCIR KIXDS AND DE\ El.OPMENT DURING

THE CENTURIES

SPANGENHELM

A.r> 600

Fig. 3. — Evolution of helmets. {After Dean.)

DIFFERENT KINDS OF EVOLUTION 5

bones of mammals, vet in neither case does a knowl-
edge of the order in which these things arose explain
them. If we appeal to the psychologist he will prob-
ably tell us that human inventions are either the
result of hapj)y accidents, that have led to an un-
foreseen, but discovered use; or else the use of the
invention was foreseen. It is to the latter process
more especially that the idea of pur pose is applied.
When we come to review the four great lines of
evolutionary thought we shall see that this human
idea of purpose recurs in many forms, suggesting
that man has often tried to explain how organic evo-
lution has taken place by an appeal to the method
which he believes he makes use of himself in the
inorganic world.

What, I repeat, has the evolution of the stars in
common with the evolution of the horse, and what
have these in common with the evolution of human in-
ventions? Clearly no more than that from a simple
beginning through a series of changes something
more complex, or at least different, has come into
being. To lump all these kinds of changes into one
and call them evolution is only to assert that you be-
lieve in consecutive series of events (which is history)
causally connected (which is science). It is the aim
of science to find out ^/^^cZ/zca//?/ what kinds of causes
were at work when the stars evolved in the sky, when
the horse evolved on the earth, and the steam engine
was evolved from the mind of man.

Chapter II

THE FOUR GREAT HISTORICAL

SPECULATIONS

Looking' backward over the history of the evolution
theory we recognize that during the hundred and
odd years that have elapsed since Buff on, there have
been four main lines of specuJation concerning evo-
lution. We might call them the four great cosmogo-
nies or the four modern epics of evolution.

The Environment

GEOFFROY ST. HILAIRE

About the beginning of the last century Geoffroy
St. Hilaire, protege, and in some respects a disciple,
of Buffon, was interested as to how living species are
related to the animals and plants that had preceded
them. He was familiar with the kind of change that
takes place in the embryo if it is put into new or
changed surroundings, and from this knowledge he
concluded that as the surface of the earth slowly
changed — as the carbon dioxide contents in the air
altered — as land appeared, and as marine animals
left the water to inhabit it, they or their embryos re-
sponded to the new conditions and those that re-
sponded favorably gave rise to new creations. As the
environment changed the fauna and flora changed —
change for change. Here we have a picture of pro-

8 EVOLUTION AND GENETICS

gressive evolution that carries with it an idea of
mechanical necessity. If there is anything mystical
or even improbable in St. Hilaire's argument it does
not appear on the surface ; for he did not assume that
the response to the new environment was always a
favorable one or, as we say, an adaptation. He ex-
2)ressly stated that // the response was unfavorable
the individual or the race died out. He assumed that
sometimes the change might be favorable, i.e., that
certain species, entire groups, would respond in a
direction favorable to their existence in a new envi-
ronment and these would come to inherit the earth.
In this sense he anticipated certain phases of the
natural selection theory of Darwin, but only in part ;
for his picture is not one of strife within and without
the species, but rather the escape of the species from
the old into a new world.

If, then, we recognize the intimate bond in chem-
ical constitution of living things and of the world in
which they develop, what is there improbable in St.
Hilaire's hypothesis? Why, in a word, is not more
credit given to St. Hilaire in modern evolutionary
thought ? The reasons are to be found, I think, first,
in that the evidence to which he appealed was meagre
and inconclusive; and, second, in that much of his
special evidence does not seem to us to be applicable.
For example the monstrous forms that development
often assumes in a strange environment, and with
which every embryologist is only too familiar, rarely

HISTORICAL SPECULATIONS 9

if ever furnish combinations, as he supposed, that
are capable of Hving. On the contrary, they lead
rather to the final catastrophe of the organism. And
lastly, St. Hilaire's appeal to sudden and great
transformations, such as crocodile's egg hatching
into a bird, has exposed his view to too easy ridicule.

But when all is said, St. Hilaire's conception of
evolution contains elements that form the back-
ground of our thinking today, for taken broadly,
the interaction between the organism and its envi-
ronment was a mechanistic conception of evolution
even though the details of the theory were inade-
quate to establish his contention.

In our own time the French metaphysician Berg-
son in his Evolution Cr^eat^iee has proposed in mys-
tical form a thought that has at least a superficial
resemblance to St. Hilaire's conception. The re-
sponse of living things is precise, exact, yet not
mechanical in the sense at least in which we usually
employ the word mechanical. For Bergson claims
that the one chief feature of living material is that
it responds favorably to the situation in which it
finds itself, at least so far as lies within the possible
physical limitations of its organization. Evolution
has followed no preordained plan; it has had no
creator; it has brought about its own creation by
responding adaptively to each situation as it arose.

But note: the man of science believes that the
organism responds today as it does, because at pres-

10 EVOLUTION AND GENETICS

ent it has a chemical and physical constitution that
gives this response. We find a specific chemical com-
position and generally a specific physical structure
already existing. We have no reason to suppose that
such particular reactions v^ould take place until a
specific chemical configuration had been acquired.
Where did this constitution come from? This is the
question that the scientist asks himself. I suj^pose
Bergson would have to reply that it came into exist-
ence at the moment that the first specific stimulus
was applied. But if this is the answer we have passed
at once from the realm of observation to the realm of
fancy — to a realm that is foreign to our experience ;
for such a view assumes that chemical and physical
reactions are guided by the needs of the organism
when the reactions take place inside living beings.

Use and Disuse

FROM LAMARCK TO WEISMANN

The second of the four great historical explana-
tions appeals to a change not immediately connected
with the outer world, but to one within the organism
itself.

Practice makes perfect is a familiar adage. Not
only in human affairs do we find that a part through
use becomes a better tool for performing its task,
and through disuse degenerates; but in the field of
animal behavior we find that manv of the most essen-
tial types of behavior have been learned througli

HISTORICAL SPECULATIONS 11

repeated associations formed by contact with the
outside.

It was not so long ago that we were taught that
the instincts of animals are the inherited ex^^erience
of their ancestors — lapsed intelligence was the cur-
rent phrase.

Lamarck's name is always associated with tlie
application of the theory of the inheritance of ac-
quired characters. Darwin fully endorsed this view
and made use of it as an explanation in all of his
writings about animals. Today the theory has few
followers amongst trained investigators, but it still
has a j^opular vogue that is widespread.

To Weismann more than to anv other single in-
dividual should be ascribed the disfavor into which
this view has fallen. In a series of brilliant essays he
laid bare the inadequacy of the sup23osed evidence
on which the inheritance of acquired characters
rested. Your neighbor's cat, for instance, has a short
tail, and it is said that it had its tail pinched off by a
closing door. In its litter of kittens one or more is
found without a tail. Your neighbor believes that
here is a case of cause and effect. He may even have
known that the mother and grandmother of the cat
had natural tails. But it has been found that short
tail is a dominant character; therefore, until Ave
know who was the father of the short-tailed kittens
the accident to its mother and the normal condition
of her maternal ancestry are not to the point.

12 EVOLUTION AND GENETICS

Weismann appealed to common sense. He made
few experiments to disprove Lamarck's hypothesis.
True, he cut off the tails of some mice for a few
generations but got no tailless offspring and while
he gives no exact measurements with coefficients of
error, he did not observe that the tails of the descen-
dants had shortened one whit. The combs of fighting
cocks and the tails of certain breeds of sheep have
been cropped for many generations and the practice
continues today, because sheep's tails are still long,
and cocks still grow combs.

The Unfolding Princijjle

NAGELI

I have ventured to put down as one of the four
great historical explanations, under the heading of
the unfolding principle, a conception that has taken
protean forms. At one extreme it is little more than
a mystic sentiment to the effect that evolution is the
result of an inner driving force or principle which
goes under many names such as Bildungstrieb, nisus
formativus, vital force, and orthogenesis. Evolu-
tionary thought is replete with variants of this idea,
often naively expressed, sometimes unconsciously
implied. Evolution once meant, in fact, an unfold-
ing of what pre-existed in the egg, and the term still
carries with it something of its original significance.

Nageli's speculation may be taken as a typical

HISTORICAL SPECULATIONS 13

case. JVageli thought that there exists in hving ma-
terial an innate power to grow and expand. He
vehemently protested that he meant only a mechan-
ical j^rinciple but, as he failed to refer such a princi-
ple to any properties of matter known to physicists
and chemists, his view seems still a mvsterious affir-
mation as difficult to understand as the facts them-
selves which it purports to explain.

Nageli compared the process of evolution to the
growth of a tree, whose ultimate twigs represent the
living world of species. Xatural selection plays only
the role of the gardener who prunes the tree into
this or that shape but who has himself lyroduced
nothing. As an imaginative figure of speech Xageli's
comparison of the tree might even today seem to
hold if we substituted propagation and variation for
"growth," but although we know so little about what
causes variation there is no reason for sujjposing it
to be due to an inner vague impulse.

In his recent presidential address before the Brit-
ish Association, Bateson has inverted this idea. I
suspect that his effort was intended as little more
than a tour de force. He claims for it no more than
that it is a possible line of speculation. Perhaps he
thought the time had come to give a shock to our too
confident views concerning evolution. Be this as it
may, he has invented a striking paradox. Evolution
has taken place through the steady loss of inhibiting

14 EVOLUTION AND GENETICS

factors. Living matter was stopped down, so to
s]3eak, at the beginning of the world. As the stops
are lost, new things emerge. The germinal material
has changed only in that it has become simpler.

Natural Selection

DARWIN

Of the four great historical sj^eculations about
evolution, the doctrine of Xatural Selection of Dar-
win and Wallace has met with the most widespread
acceptance. Later the theory will be examined more
critically. Here only its broadest aspects will be
considered.

Darwin appealed to chance variations as supply-
ing evolution with the material on which natural se-
lection works. If we accept, for the moment, this
statement as the cardinal doctrine of natural selec-
tion it may appear that evolution is due, ( 1 ) not to
an orderly response of the organism to its environ-
ment, (2) not in the main to the adjustment of the
animal through the use or disuse of its parts, (3)
not to any innate non-physical principle of living
material itself, and (4) above all not to purpose
either from within or from without. Darwin made
quite clear what he meant by chance. By chance he
did not mean that the variations were not causal.
On the contrary he taught that in science we mean
by chance only that the particular combination of
causes that bring about a variation is not known.

HISTORICAL SPECULATIONS 15

They are accidents, it is true, but they are causal
accidents.

In his famous book on Animals and Plants under
Domestication, Darwin dwells at great length on
the nature of the conditions that bring about varia-
tions. If some of his views seem to us today at times
vague, at times problematical, and often without a
secure basis, nevertheless we find, in every instance,
that Darwin was searching for the ijhysical causes
of variation. He brought, in consequence, conviction
to many minds that there are abundant indications,
even if certain proof is lacking, that the causes of
variation are to be found in natural processes.

Today the belief that evolution takes place by
means of natural processes is generally accepted. It
did not seem probable that we should ever again have
to renew the old contest between evolution and spe-
cial creation.

But this is not enough. We can never remain satis-
fied with a negative conclusion of this kind. We must
find out what natural causes bring about variations
in animals and plants; and we must also find out
what kinds of variations are inherited, and how they
are inherited. If the circumstantial evidence for or-
ganic evolution, furnished by comparative anatomy,
embryology and paleontology is cogent, we should
expect to observe evolution going on at the present
time, i.e., w^e should be able to observe the occurrence
of variations and their transmission. This has actu-

16 EVOLUTION AND GENETICS

ally been done by the geneticist. Certain kinds of
new characters have been seen to arise by a process
called mutation and their inheritance is now known
to follow Mendel's laws.

Chapter III

THE EYIDEXCE FOR ORGANIC

EVOLUTIOX

Four branches of study have furnished the evidence
of organic evolution: Com^^arative anatomy; Em-
bryology; Paleontology; and Experimental Breed-
ing or Genetics.

The Evidence from Comparaiive Anatomy

When we study animals and plants we find that
they can be arranged in groups according to their
resemblances. This is the basis of comparative anat-
omy, which is only an accurate study of facts that
are superficially obvious to everyone.

The groups are based not on a single difference,
but on a very large number of resem])lances. Let us
take for example the group of vertebrates.

The hand and the arm of man are similar to the
hand and arm of the ape {jig. 4) . The legs of man,
monkey, dog, sheep, and the horse are made up of
similar bones {jig, 5) . The same parts are found in
the leg of the lizard, the frog, and, even though less
certainly, in the fins of fishes. Comparsion does not
end here. We find similarities in the skull and back-
bones of these same animals ; in the brain ; in the di-
gestive system; in the heart and blood vessels; in
the muscles.

Each of these systems is very complex, but the

18

EVOLUTION AND GENETICS

same general arrangement is found in all. Anyone
familiar with the evidence will, I think, probably
reach the conclusion either that these animals have
been created on some preconceived plan, or else that
they have some other bond that unites them ; for we

uo

Chfmpanzee

Ma.n

Fig. 4. — Arm of chimpanzee and of man,
drawn to scale. (After Haeckeh)

ORGANIC EVOLUTION

19

find it difficult to believe that such complex, yet
similar, things could have arisen independently.

Because we can often arrange the series of struc-
tm-es in a line extending from the very simple to the

Me.n

Monkey Dog Sheep Horse

Fig. 5. — Legs of five mammals, drawn to scale, to show homologous
parts. (After Leconte, from Romanea.)

more complex, we are Si\)i to become unduly im-
pressed by this fact and conclude that if we found
the comjilete series we should find all the interme-
diate steps and that they have arisen in the order of
their comj^lexity. For example, there have appeared
in our cultures of the vinegar fly, Drosophila mel-
anogaster {fig. 6) over four hundred new types that
breed true. Each has arisen independently and sud-
denly. Every part of the body has been affected by

20

EVOLUTION AND GENETICS

one or another of these mutations. Many different
kinds of changes have taken place in the wings and
several of these involve the size of the wings. If we
arrange the latter arbitrarily in the order of their
size there will be an almost complete series begin-

FiG. 6. — Male and female vinegar fly, Dro-
sophila melanogaster.

ning with normal wings and ending with those of
aj^terous flies. Several of these types are re^^resented
in figure 7. The order in which these mutations oc-
curred bears no relation to their size ; each originated
independently from the wild type.

JNlutations have occurred involving the pigmen-
tation of the body and wings. The head and thorax
of the wild Drosophila melanogaster are grayish yel-
low, the abdomen is banded with yellow and black,
and the wings are gray. There have appeared in our
cultures several kinds of darker tyj)es ranging to
almost black flies and to lighter types that are pale
yellow. If put in line a series may be made from
the darkest flies at one end to the light yellow flies

ORGANIC EVOLUTION 21

at the other. These types, with the fluctuations that
occur within each tyj)e, furnish a complete series
of gradations ; vet historically they have arisen inde-
pendently of each other.

JNIany changes in eye color have appeared. As

*

'«

•SLi^

fl

"msg^i^^

A

-^-m

**%

^M^:

' d

¥

-^^*EV-

FiG. 7. — Mutants of Drosophila melanogaster, arranged in
order of size of wings; rr, cut,; 6, beaded; c, stumpy; c/, an-
other stumpy; e, vestigial; j, apterous.

many as fifty or more races differing in eye color are
now maintained in our cultiu'cs. Some of them are
so similar that they can scarcely be separated from
each other. It is easily possible beginning with the
darkest eye color, sepia, which is deep brown, to
2)ick out a perfectly graded series ending with pure
white eyes. But such a serial arrangement would

22 EVOLUTION AND GENETICS

give a totally false idea of the way the different
types have arisen; and any conclusion based on the
existence of such a series might very well be entirely
erroneous, for the fact that such a series exists bears
no relation to the order in which its meni])ers have
a|)peared.

Suppose that evolution "in the open" had taken
place in the same way, by means of discontinuous
variation. What value then would the evidence from
comparative anatomy have in so far as it is based
on a continuous series of variants of any organ?

No one familiar with the entire evidence will doubt
for a moment that these four hundred races of Dro-
sojihila belong to the same species and have had a
common origin, for while they may differ mainly in
one thino; they are extremely alike in a hundred
other things, and in the general relation of the parts
to each other.

It is in this sense that the evidence from compara-
tive anatomy can be used, I think, as an argument
for evolution. It is the resemblances that the animals
or plants in any group have in common that is the
basis for such a conclusion ; it is not because we can
arrange any j)articular variations in a continuous
series. In other words, our inference concerning the
common descent of two or more species is based on
the totality of such resemblances that still remain
in large part after each change has taken place. In
this sense the argument fi'om comparative anatomy.

ORGANIC EVOLUTION 23

while not a demonstration, furnishes circumstantial
evidence too strong to be disregarded.

The Evidence from Emhryology

In passing from the egg to the adult the individ-
ual goes through a series of changes. In the course
of this development we see not only the beginnings
of the organs tliat gradually enlarge and change
into those of the adult animal, but also see some or-
gans appear and later disappear before the adult
stage is reached. We find, moreover, that the young
sometimes resemble in a most strikin"' wav the adult
stage of groups that we place lower in the scale of
evolution.

Many years before Darwin advanced his theory
of evolution through natural selection, the resem-
blance of the young of higher animals to the adults
of lower animals had attracted the attention of zo-
ologists and various views, often very naive, had
been advanced to account for the resemblance.
Among these speculations there was one practically
identical with that adopted by Darwin and the jDost-
Darwinians, namely that the higher animals repeat
in their development the adult stages of lower ani-
mals. Later this view became one of the cornerstones
of the theory of organic evolution. It reached its cli-
max in the writings of Haeckel, and I think I may
add without exaggeration that for twenty-five years
it furnished the chief inspiration of the school of

24

EVOLUTION AND GENETICS

descriptive embryology. Today it is taught in many
textbooks of biology. Haeckel called this interpre-
tation the Biogenetic Law. The parallel between the
historical development of antlers of deer {fig. 8)
and their postnatal development from year to year
(fig. 9) is most striking. Historically we may sup-
pose that the develojiment was due to the appear-

FiG. 8. — Fossil deer antlers. The first two
to the left are from the niid-eosine; the third
is from the iip})er miocene; the fourth is from
the pliocene, as is also the fifth figure. The
figure to the right is from the "forest-bed of
Norfolk." (After RoTrwnes.)

ance of hereditary variations in the germ material.
At the present time, the stages in their development
are closely correlated with age, size, and especially
with the yearly increase in the internal secretion of
the testes ; for, after castration the antlers no longer
develop. In living deer all the hereditary factors that
appeared in the past are present at each stage, but the
extent to which the antlers develop depends on the
physiological conditions mentioned above. In other
words, while there is a close relation in both cases

ORGANIC EVOLUTION

25

between the hereditary factors present, the causes
that led to the development of the more complicated
stages in the past (the accumulation of hereditary
factors) are different from the causes that, at pres-

1<"lt«*H^

Fig. 9. — Antlers of stag',
showing successive additions
of brandies in successive
years. (After Romanes.)

ent, bring about a similar sequence as the individual
gets older.

It was early recognized that many embryonic
stages could not possibly represent ancestral animals.
A young fish with a huge yolk sac attached {fig. 10)
could scarcely ever have led a free life as an adult
individual. Such stages were interpreted, however,
as emhryonic additions to the original ancestral

26 EVOLUTION AND GENETICS

type. The embryo bad done sometbing on its own
account.

In some animals tbe young bave structures tbat
attacb tbem to tbe motber, as does tbe placenta of tbe

Fig, 10. — Young trout, six days after hatching.
{After Zieyler.)

mammals. In otber cases tbe ^^oung develop mem-
branes about tbemselves — like tlie amnion of tbe
cbick {fig. 11) and mammal — tbat would bave sbut
off an adult animal from all intercourse witb tbe out-

FiG. 11. — Diagram of chick, show-
ing relations of amnion, allantois,
and yolk. {After Wilder.)

ORGANIC EVOLUTION 27

side world. Hundreds of such embryonic structures
are known. These were explained as embryonic
adaptations and hence falsifications of the ances-
tral records.

At the end of the last century Weismann injected
a new idea into our views concerning the origin of
variations. He urged that variations are germinal,
i.e., they first appear in the egg and the sperm as
changes that later bring about modifications in the
individual. The idea has been fruitful and is gener-
ally accepted by most biologists today. It means
that the offspring of a j^air of animals are not af-
fected by the structure or the activities of their par-
ents, but tlie gerni material is the unmodified stream
from which both the parent and the young have
arisen. Hence their resemblance. Now, it has been
found that a variation arising in the germ material,
no matter what its cause, may affect any stage in
the development of the next individuals that arise
from it. There is no reason to suppose that such a
change produces a new character that always sticks
itself, as it were, on to the end of the old series. This
idea of germinal variation therefore carried with it
the death of the older conception of evolution by
sujierposition.

In more recent times another idea has become cur-
rent, mainlv due to the work of Bateson and of
de Vries — the idea that variations are discontinu-
ous. Such a conception does not fall easily into line

28 EVOLUTION AND GENETICS

with the statement of the biogenetic "law"; for
actual experience with discontinuous variation has
taught us that new characters that arise do not add
themselves to the end of the line of already existing
characters but if they involve adult characters they
change them without, as it were, passing through
and bevond them.

I venture to think that these new ideas and this
new evidence have played havoc with the biogenetic
"law." Nevertheless, there is an interpretation of
the facts that is entirely compatible with the theoiy
of evolution. Let me illustrate this by an example.

Fig. 12. — Diagram of head of chick, A and B, showing
gill slits and aortic arches; and head of fish, C, showing
aortic arches. {After Hesse.)

The embryos of the chick (fig. 12) and of man
(fig. 13) possess at an early stage in their development
gill slits on the sides of the neck like those of fishes.
No one familiar with the relations of the parts will

ORGANIC EVOLUTION 29

for a moment doubt that the gill slits of these em-
bryos and of the fish represent the same structures.
When we look further into the matter we find that

Gill slits

Fig. 13. — Human embryo, showing gill slits
and aortic arches. {After His, from Marshall.)

young fish also possess gill slits (fig. 14) — even in
young stages in their development. Is it not then
more probable that the mammal and bird possess

^-GUl slits -

Fig. 14. — Side views of head of embrvo shark,
showing gill slits. {After Sedgwick.)

this stage in their development simply because it
has never been lost? Is not this a more reasonable
view than to suppose that the gill slits of the em-
biyos of the higher forms represent the adult gill

30 EVOLUTION AND GENETICS

slits of the fish that in some mysterious Avay have
been pushed back into the embryo of the bird?

Many similar examples con Id be given. All can
be interpreted as embryonic survivals rather than as
phyletic contractions. Not one of them calls for the
latter interpretation.

The stud}" of the cleavage i^attern of the segment-
ing egg furnishes the most convincing evidence that
a different explanation from the one stated in the
biogenetic law is the more probable explanation.

It has been found that the cleavage pattern has
the same general arrangement in the early stages of
flat worms, annelids and molluscs (fig. 15). Ob-
viously these stages have never been adult ancestors,
and obviouslv if their resemblance has any historical
meaning at all, it is that each group has retained the
same general plan of cleavage, possessed by their
common ancestor.

Accepting this view, does the evidence from em-
brvoloffv favor the theory of evolution? I think that
it does very strongly. The embryos of the mammal,
bird, and lizard have gill slits today because gill slits
were present in the embryos of their common ances-
tors. There is no other view that explains so well
their presence in the higher forms.

It may be asked whether this is not all that the
"biogenetic law"claims. Has not the old conclusion
been reached in a roundabout wav? I think not. To
my mind there is a wide difference between the old

ORGANIC EVOLUTION 31

statement that the higher animals Hving today have
the original adult stages telescoped into their em-
hrvos, and the statement that the resemblance be-

FiG. 15. — Cleavage stages of four types of eggs; a,
Planarian; 6, Annelid; c, Mollusc (Crepidula); d.
Mollusc (Unio).

tween certain characters in the embryos of higher
animals and corresponding stages in the embryos of
lower animals is most plausibly explained by the as-
sumption that they have descended from the same
ancestors, and that their common structures are em-
bryonic survivals.

32 EVOLUTION AND GENETICS

The Evidence from Paleontology

The direct evidence furnished by fossil remains
is by all odds the strongest evidence that we have in
favor of organic evolution. Paleontology holds the
incomparable position of being able to point directly
to fossil remains showing that the animals and
plants living in past times are connected with those
living at the present time, often through an un-
broken series of stages {jig. 16). Paleontology has
triumphed over the weakness of the evidence, which
Darwin admitted was serious, by filling in many of
the missing links.

Paleontology has been criticised on the ground
that it cannot pretend to show the actual ancestors
of living forms, because, if in the past genera and
species were as abundant and as diverse as we find
them at present, it is very improbable that the bones
of any individual that happened to be ]3reserved are
the bones of just that species that took part in the
evolution. Paleontolooists freelv admit that in manv
cases this is probably true, but even then the evi-
dence is still, I think, just as valuable and in exactly
the same sense as is the evidence from comparative
anatomy. It suffices that there lived in the past a
particular "group" of animals that had many points
in common with those that preceded them and with
those that came later. Whether these are the actual
ancestors, or not, does not so much matter ; for, the
view that, from such a group of species, the later

ORGANIC EVOLUTION

33

%limf

Fig. 16. — Evolution of the horse, showing the changes in the
skull and in the bones of the legs. {After Matthews.)

34 EVOLUTION AND GENETICS

species have been derived is far more probable than
any other view that has been proposed.

With this unrivalled material and splendid series
of gradations, paleontology has constructed many
stages in the past history of the globe. But paleon-
tologists have sometimes gone beyond this descrip-
tive phase of the subject and have attempted to for-
mulate the "causes," "laws" and "principles" that
have led to the development of their series. It has
even been claimed that paleontologists are in a much
better position than are zoologists to discover such
principles, because they know both the beginning
and the end of the evolutionar}^ series. The reply is
obvious. In his sweeping and poetic vision the pale-
ontologist may fail completely to find out the nature
of the pigments that have gone into the painting of
his picture, and he may confuse a familiarity with
the different views he has enjoyed of the canvas, with
a knowledge of how the painting was done.

When the modern student of variation and he-
redity— the geneticist — looks over the different
continuous" series, from which certain "laws" and
principles" have been deduced, he is struck by two
facts : that the gaps, in some cases, are enormous as
compared with the single changes with which he is
familiar, and (what is more important) that they
involve numerous parts in manv wavs. Since the
paleontologist does not know, and from the nature
of the case cannot know whether the differences are

a

a

ORGxWIC EVOLUTION 35

due to one change or to a thousand he cannot with
any certainty tell anythino^ ahout the hereditary
units which haye made the process of eyolution j)os-
sible. Without this knowledge there can be no satis-
factory understanding of the origin of new heredi-
tary yariations through which eyolution is possible.

The Evidence from Genetics

In recent years a study of the orioin of new
characters has led to the discoyery that sudden

a.

changes appear at times in the germinal elements —
changes that haye an effect on one or more charac-
ters of the organism. The ^^rocess by which the
change takes place is called mutation, and the new
kind of indiyidual it produces is called a mutant.
The mutants breed true to the new type. How far,
and in what sense, the occurrence of mutants suf-
fices to supply the material for eyolution in animals
and plants is one of the important biological prob-
lems of today.

While in a strict sense genetics deals only with
the mode of transmission of the hereditary elements
(the genes) from parent to offspring, the scientific
study of the origin of new yariations is undertaken
today almost exclusiyely by students of o^enetics,
and is now recognized as a part of their Avork. I shall
include, therefore, under the "eyidence from genet-
ics" the origin of new characters by mutation.

In many groups of animals and plants new kinds

36 EVOLUTION AND GENETICS

of individuals have been seen to suddenly appear.
The new characters that they show are transmitted

ft'''

to their descendants. In some cases the new charac-
ters depart widely from those of the stock from
which the new individual has sprung; in other cases
the departure may be very slight and pass unnoticed
unless one is very familiar with the parent stock. In
fact, it is owing, in large part, to a minute examina-
tion of plants and animals, that many of the new
characters studied by geneticists have been discov-
ered. So small are some of the observed changes that
they are not greater than the fluctuating changes
due to the environment. The inheritance of the new
character, when it is as slight as this, can only be
determined by a careful study of successive genera-
tions under controlled conditions both genetic and
environmental. It is the realization of these require-
ments that has enabled genetics to make its contri-
bution to the theory of evolution.

While it is true that most of these mutant changes
have been observed in animals under domestication,
or in species that have recently been brought into
the laboratory, there is no evidence that they owe

ft -^ ft'

their appearance to cultivation or to domestication.
On the contrary similar mutants have been found in
wild species. Cultivated plants and domesticated
animals are more familiar to us, and more carefully

-^ ft-

scrutinized, than are wild species. Hence in large

ORGANIC EVOLUTION 37

part the more frequent discovery of new types in
these forms.

Many, probably most, of the extreme mutant
types could not compete with the native types from
which they have sprung, and this has been urged as
an argument against the view that mutants could
furnish the materials for evolution; because, it is
said, only better adapted types coidd survive that
have an advantage of some sort over the parent
types so that they can replace the original types, or
else find a new environment to which they are
better suited than were the original forms. This ar-
gument, if it could be substantiated, would be a fatal
blow to the mutation theorv of evolution. It calls,
therefore, for careful and impartial consideration.
That it is not justified is shown, I think, by the fol-
lowing evidence.

While it is true that manv of the mutant charac-
ters that are preserved by geneticists for a study of
heredity are extreme departures from the original
type, these are utilized rather than smaller differ-
ences because thev are easilv observed and their
classification readilv determined. Those characters
that depart little from the jjarent type, or are diffi-
cult to distinguish from the fluctuating variations of
that type, are more difficult to study and are neg-
lected. Hence has arisen in some quarters the er-
roneous idea that all mutant types are defectives

38 EVOLUTION AND GENETICS

and incaj)able of competing with the original form.

In this connection it is important to keep in mind
the fact that animals and plants are extremely com-
plex machines that are highly adapted to the condi-
tions of life in which they live. Any change, and
especially any great change in them, is far more
likely to throw them ont of balance with their envi-
ronment than to bring to them an advantage, but it
is possible, nevertheless, that some of the changes,
however slight, might be beneficial, especially those
that add to or diminish slightly some important char-
acter or function already present. These changes
might furnish materials for evolution.

Darwin rested his case for evolution on the ob-
served small differences that all animals and plants
show. Now, while we realize today that many of these
slight differences are not inherited, we also know
that amongst them there are some that are inher-
ited, and that these, so far as we know, have arisen
as mutations. At present there is no evidence that
will stand the test of criticism in favor of any other
origin than that all known variations owe their ap-
pearance to the same process of jiiutation that also
produces the larger differences; and, I repeat, that
there is much explicit evidence to show that very small
differences, that add to or subtract from characters
already j)resent, do appear by mutation.

Finally there is evidence that the differences
shown by individuals in nature are inherited in the

ORGANIC EVOLUTION 39

same way as are the observed mutant characters of
domesticated forms. While this identity in tlie
method of inheritance may not })e a conclusive argu-
ment in itself in showing that in both cases the dif-
ferences have arisen by the same process, still the
probability is so strong that it would be short-sighted
to reject it, when the alternative assertion that the
differences in the wild form have had a different
origin, has nothing to support it. These questions
will come up again for further consideration, but
enough has been said to show that the discoveiy of
the way in which new characters appear and are in-
herited marks a distinct advance in our study of
the evolutionary process.

Chapter IV

THE MATERIALS OF EVOLUTION

The apparent permanence of the types of animals
and plants living at the present time is a common
fact of observation. If this were the whole story it
would appear that evolution had come to an end.
If living things at the present time were really
stable, we could give no good reason why it has not
always been so in the past. On the other hand, if this
stability is deceptive, we might expect to find evolu-
tion still taking place at the present time as in the
past, and if this is true we might hope by a careful
study of what is happening in living things about us
to be able to get some information as to the way in
which the process has taken place in the past. It is,
of course, also conceivable that, even if evolution
went on in the past, it has actually come to an end at
the present time, or at least, having reached its cli-
max, a declining process may be the order of the
day — a process the reverse of that by which the up-
ward trend of evolution went on in the past. It is
also conceivable that the process of evolution is so
slow that we may not be able to detect or measure it
with means at our disposal. It may be true, further-
more, that certain species at least have become so
far adjusted to the present conditions of the earth,

42 EVOLUTION AND GENETICS

that they are no longer advancing, and that only a
few species are producing new or better types.

While it is well to keep these possibilities in mind,
an appeal to the actual evidence furnishes no grounds
for the belief that the process of evolution has come
to an end. The conditions of land, water, and atmos-
jihere have in all probability changed slowly since
life first aj^peared, as they are changing today, and
if in the past, evolution has progressed while the ex-
ternal world has been so slowly changing, it is a fair
presumption that, to some extent at least, we may
exjDect to find evolution taking place at the present
time. It should also not be forgotten that the re-
adjustment of animals and plants to each other may
be as important a condition of evolution as their
adjustments to the external world. From the latter
point of view, the extraordinary complexity of the
relation of living things to each other may even seem
to furnish ample ojiportunities for further adjust-
ments. It is certain, at least, that with the advent of
man's interference with the natural conditions es-
sential to other animals and plants, widespread
changes may be expected to take place in part de-
structive but possibly also constructive. His domes-
tication of several kinds of j)lants has produced
changes in them that are very striking, but whether
by the same kind of processes that take place in
nature is a matter of dispute.

There is then, on the whole, a fair a priori expec-

MATERIALS OF EVOLUTION 43

tatioii that evolution may still be taking place, and
that it may not be so slow as to be beyond our powers
of observation and even experimentation. The study
of living things has, I think, confirmed this expecta-
tion despite the fact that there exist at present rather
wide differences of opinion as to the legitimate con-
clusions from the evidence.

A great deal of the discussion about evolution has
centered about the Ongin of Species. Historically,
the question as to what constitutes a species goes
back at least as far as Linne (1707-1778) who clas-
sified all plants and animals known to him into such
groups. The systematic arrangement of living things
into species, genera, families, etc., still attracts the
attention of a large number of naturalists.

JNIany difficulties arise Avhen an attempt is made
to arrange animals into groups. It is generally rec-
ognized that, in some groups, species have a different
value from that in others. While some taxonomists
prefer to arrange individuals into large species,
other systematists split these large species into sev-
eral or many smaller ones, still calling them species.
It is generally admitted that the classification into
species is often an arbitrary procedure, one that is
useful, of course, in order to indicate the resem-
blances and differences that a study of animals and
plants reveals. A few students of the subject still
attempt to arrange their species in such a way as to
indicate their relationship by descent, but the at-

44 EVOLUTION AND GENETICS

tempt, while desirable from the standpoint of evo-
lution, has proven so difficult that it has been tacitly
abandoned or ignored by many taxonomists.

It is unfortunate, in my opinion, that ever since
the time of Darwin the question of the origin of spe-
cies has occupied so much of the forefront of specu-
lation concerning the evolution of living things ; for
if, as I have stated, "sj^ecies" are to a large extent
arbitrary arrangements of animals and f)lants — -
arrangements that may be essential for the purpose
for which they are made, but entirely unsuited for
evolutionary study — many unnecessary difficulties
may arise in an attempt to explain their origin by
natural processes, if some species are only groups
of individuals arranged by taxonomists for con-
venience.

Nevertheless, there are certain important con-
siderations connected with attempts to classify liv-
ing things into species that cannot be dismissed by
the foregoing treatment of the subject.

In the first place, experience has shown that most
animals and plants do fall into groups that are more
like each other than like other grouj^s; and in the
second place, that within each group the individuals
freely cross and leave fertile offspring, while be-
tween most species crossing is rare or impossible,
and when it occurs the offspring are generally ster-
ile. When two types are infertile with each other
they are generally admitted to be "good species"

MiVTERIALS OF EVOLUTION 45

and even when they j)i'oduce offspring, if the latter
are sterile, the two types are recognized as separate
species. It becomes necessary, therefore, to examine
further into the significance of these relationships.

There are found in nature many interbreeding
mdividuals that are sharply defined from other
groups. It is convenient to have a name for all of the
individuals of such a group, even when, as in the
human species, the individuals living in the different
parts of the world may present striking differences
in structure, color, temperament, and social be-
havior. There are other types that are different from
all others within a circumscribed region, but in other
regions are rej)resented by individuals that show
slight but constant differences from the former.
Here it becomes a more difficult matter to decide
whether to call them all one species or whether to
make two species. In ^^ractice it is generally agreed
amongst taxonomists to give one specific name to
such groups, if intermediate forms between the
grouj)S are found, but, if intermediate types are
absent, the two separated groups are called differ-
ent species. It is obvious, therefore, that tlie dis-
tinction between such species is largely arbitrary
and artificial, especially since in the great majority
of cases no tests of cross-breeding are made to de-
termine whether the extreme types will cross and
leave fertile offspring, or whether they are infertile
or if fertile leave sterile offspring.

46 EVOLUTION AND GENETICS

That the visible structural differences between
groups of individuals is not a safe guide on which to
construct a real distinction, is exemplified by numer-
ous cases in which groups may exist side by side, that
are so closely similar that only an expert can sep-
arate them, yet are either infertile when crossed or
else leave offspring that are sterile. For example,
two species of Drosophila, often found together, are
so similar that complete separation is extraordi-
narily difficult. They can be made to cross only after
many matings, but the offspring are completely
sterile. With this information at hand no one would
hesitate to call them different species, while without
it the two would be placed in the same species.

It is true, in general, that the more "different"
in structure two groups are, the smaller the chance
that they will be found to cross, and still smaller the
chance that they will have fertile offspring. This
brings us back to the second question concerning
the relation of infertility between species and the
sterility of species hybrids. The infertility may ap-
pear at first sight to be due to the observed differ-
ences, but there mav be some other less obvious
relation that is responsible for their failure to pro-
duce offspring. Darwin was familiar with this prob-
lem and has written about it extensively. He pointed
out that there are no sharp lines with respect to
infertility between species, and gave many exam-

MATERIALS OF EVOLUTION 47

pies, especially in plants, in which cross fertilization
between individuals belonging to groups, unmis-
takably species according to ordinary standards,
takes place. Darwin was also impressed by the fact
that even self-sterility often occurs in hermaphro-
ditic sjDecies, not only because self-fertilization is
often made difficult in one way or another, but also
because it does not actuallv occur even when the
spenn has access to the egg, as has been sho\^^l by
suitable tests.

Darw^in was also familiar with the sterility of
hybrids from species-crosses, and here again he em-
jjhasized the lack of any sharp distinction; for in
some cases the sterility is complete, in others partial,
and in still others it is not present. All this is in har-
mony with his conception of the gradual breaking
vip of a species into new groups or varieties which as
they become more and more different may show,
when crossed, various degrees of fertility, and of
sterility in their offspring.

Since Darwin, the subject has not advanced much
further, although genetics has contributed a little
more information and holds out promise of furnish-
ing more. It has been shown in one or two hermaph-
roditic species that genetic factors are present that
are concerned with self-sterilitv, and in a few other
cases it has been shown that similar conditions are
explicable on the assumption of more than one such

48 EVOLUTION AND GENETICS

factor. It has been found both hi plants (tobacco)
and in one animal (Ciona) that the failure to self-
fertilize is not due to incompatibility of any sort be-
tween the egg and the sperm, but to a physiological
block to the penetration of the sperm into the egg. It
has also been shown in the case of several marine
animals (sea urchins and fishes) that the eggs may
be entered by spermatozoa of widely separated spe-
cies— belonging even to different families — and
start development. The failure to produce a normal
embryo is due in some cases to the failure of the
sperm to develop normally in the foreign egg; in
other cases to the failure of the chromosomes derived
from the two sources to become normallv distributed
in the cleavages of the egg, and in still other cases
to the inability of the introduced chromosomes to
function in the cytoplasm of the foreign egg.

All this is satisfactory and carries us a step fur-
ther in an understanding of the problem of the infer-
tility between species. We may add a further con-
sideration in line with what genetics and embryology
lead us to exjiect, namely, that the genetic factors
present in the chromosomes of the fertilized egg
derived in part from the egg, in part from the sperm,
are acting on the cytoj)lasm throughout the process
of development. So long as these jDairs of factors are
alike or identical (one maternally derived, one pater-
nally) the course of development runs smoothly,
but if one member of the pair acts in a different way

MATERIALS OF EVOLUTION 49

from the other member it is easy to see why sooner
or later the result is disastrous owing to a conflict of
competing factors.

The egg's cytoplasm that has been formed under
the dual influence of the maternal set of chromo-
somes appears to determine the early stages of de-
velopment so that even if the sperm introduces
factors that w^ould act disastrously on these stages
their influence does not at first show itself, but as
development proceeds the influence of the paternal
chromosomes comes more and more into play and
further progress is arrested. This is, in fact, what is
seen, in a way, when widely different species of
echinoderms or of fish are crossed. The earlv stages
of cleavage run smoothly and follow the maternal
type, but as the embryo develops further, irregulari-
ties and delays occur that bring progress to an end.

Thus while some advance has been made in the
study of infertility between species, the other prob-
lem, namely the sterility of the hybrids, when such
are produced between species not too different, is
not so well understood, but in some cases at least
the immediate cause of the sterility has been found.
It has been shown, in recent vears, to result from
changes that take place in the ripening of the germ-
cells. There comes a time when the pairs of chromo-
somes unite throughout their length and subse-
quently separate to go into sister cells. This is the
so-called conjugation process. Now in certain hy-

50 EVOLUTION AND GENETICS

brids, the mule for example, it has been shown that
the maternal and paternal chromosomes fail to pass
successfully through this ordeal with the result that
later they are separated very irregularly. In conse-
quence the germ-cells contain all kinds of assort-
ments of the chromosomes and become abnormal.
The result is that the individual is sterile.

While these observations do not explain why the
chromosomes fail to unite, thev do account for the
sterility of the hybrid. Until we learn more concern-
ing the conditions that bring about the union of the
chromosomes, it may be unsafe to offer any expla-
nation of the process ; nevertheless, for the present
at least, it is not irrational to ascribe the failure to
the differences in the hereditary factors carried by
the chromosomes of the two species.

Now while all this will, I think, be conceded as
theoretically possible, the fact remains that in no
case has a mutant type been seen to arise that has
produced individuals that are fertile inter se, but
sterile with the parent species.^ Bateson has recently
emphasized this point and has insisted that until it
can be met we are not justified in assuming that new
species are formed by mutation.

He says: "The production of an indubitably ster-
ile hybrid from completely fertile parents which
have arisen under critical observation from a com-

1 Plough has recently reported a case that comes very near to fulfilling
these conditions.

MATERIALS OF EVOLUTION 51

mon origin," — this "is the event for which we wait."
Bateson has worded his requirements in such a
way as to render the demonstration well-nigh im-
possible, but a somewhat different view, of the origin
of species through mutation may put the j)roblem in
another form where theoreticallv at least the diffi-

ft

culty is lightened even if not entirely removed.

Suppose on Bateson's supposition that a germ-
cell has been produced in a male or in a female that
contains a single mutant gene having the required
property of forming a sterile hybrid. In order to
perpetuate itself, this germ-cell would have to meet
a normal germ-cell. The single individual that re-
sulted would by the very conditions imposed be itself
sterile ; for, since there are no other individuals of the
kind in existence there would be no means of finding
out that the single individual would have been fer-
tile with one of its kind. It would, therefore, be lost
because of its sterilitv.

ft'

The possibilities are not much better even if we
assimie that the mutation occia-red early in the

ft'

germ-track, so that several or many germ-cells came
to contain the mutant gene. When crossed back to
the parent stock several sterile individuals would
appear, which brings the experiment to a disastrous
end as before. If on the other hand two such individ-
uals should by chance mate then a new race miffht be
started which tested to the original race would be
found to produce offsj^ring that are sterile. Here the

52 EVOLUTION AND GENETICS

conditions are fulfilled, but the chance of recording
such a result at the time would be small indeed unless
the sterile gene itself carried with it some other
landmark, some new character, that would direct
attention to it from the beginning. Something of
the sort apj^ears, in fact, to have happened in a
stock of Drosophila studied by Plough where a new
race appeared whose individuals are more fertile
inter se than with the parent stock.

The conditions for producing a hybrid sterile race
may appear more favorable in a monoecious form,
as in a plant with stamens and pistils in the same
flower. While a mutation in a single germ-cell would
again not improve the situation, yet if mutation
occurred early in the germ-track, and both pollen
grains and ovules came to contain the gene, self-
fertilization would start a race that fulfils the re-
quirements. These if inbred would then be found to
be fertile infer se and produce fertile offspring, and
with the parent type they would produce sterile
hybrids. •

It is obvious, as I have said, that tlie chance of
detecting such a mutant type would be veiy small
unless its mutation involved some other character
than the one under discussion so that it could be at
once recorded.

The necessity of putting the Mutation Theory
to the test that Bateson calls for, seems to me very
doubtful, for while this is one of the possible ways in

MATERIALS OF EVOLUTION 53

which a mutant type might spht off at once from tlie
parent type it is by no means the only way, or even,
I think, the most j^robable way in which species have
become separated.

I ventiu'e also to question the imj^ortance ascribed
to the sterility of the hybrid as a criterion of the
origin of species. The test is arbitrary and not called
for by the evidence at hand relating to sterile hybrids
between species. There are no such sharj) distinc-
tions, as implied in the test, between groups found
in nature that are called species.

The interpretation of the infertility between spe-
cies and the sterility of hybrids that seems to me
more j^robable is very different from that suggested
by Bateson. Both phenomena, as I interpret them,
are the result of many kinds of differences that have
arisen in two species that have been separated for
a long time. Each has taken on new characters due
to mutational changes of different sorts. There is no
one problem of infertility of species and no one
problem of the sterility of hybrids, but many prob-
lems, each due to differences that have arisen in
the germinal material. One or more of these dif-
ferences may affect the mechanism of fertilization
or the process of develof)ment, producing some
incompatibility.

In order that a species may split up into one or
more new species in the way suggested, isolation is
implied. Isolation may be due to difference in local-

54 EVOLUTION AND GENETICS

ity, but it may also come about by individuals within
the same area ripening their germ cells at different
times, etc.

If species arise in this way we avoid the difficulty
raised by Bateson concerning the origin of sterilit}^
of hybrids w^hose parents have arisen through muta-
tion. Moreover the difficulty is not one 23eculiar to
the mutation theory, but applies in large part to any
theory that postulates the origin of one species from
another.

If then we dismiss the problems that have grown
out of the historical definitions of species, and turn
directly to an examination of the origin of new vari-
ations we shall find that some progress has been
made since Darwin wrote, and that this new knowl-
edge supports Darwin's view that the variations
shown by animals and plants furnish materials for
a theor}^ of evolution. There is evidence that new
characters suddenlv arise bv mutation both in do-
mesticated and in wild types, and that these varia-
tions are inherited in the same way as are the differ-
ences present in wild tyj^es that distinguish them
from one another. In addition there are variations
(fluctuations) due to the action of the environment
on the developing individual. These are not inherited
and cannot therefore take part in evolution. These
statements will be discussed in later chapters after
Mendel's laws, and some other laws of hereditv dis-
covered since his time, have been examined.

Chapter V

MENDEL'S TWO LAWS OF
HEREDITY

Gregoe Mendel studied the heredity of certain
characters of the common edible pea, in the garden
of the monastery at Briinn. In the account of his
work pubhshed in 1865, he said:

It requires indeed some courage to undertake a
labor of such a far-reaching extent ; it apj^ears, how-
ever, to be the only right way by which we can finally
reach the solution of a question the importance of
which cannot be over-estimated in connection with
the history of the evolution of organic forms.

He tells us also why he selected peas for his work :

The selection of the plant group which shall serve
for experiments of this kind must be made with all
possible care if it be desired to avoid from the outset
every risk of questionable results.

The exjDerimental plants must necessarily

1. Possess constant differentiating characters.

2. The Iwbrids of such plants must, during the
flowering period, be protected from the influence
of all foreign pollen, or be easily capable qf such
protection.

Mendel succeeded not only because of his fore-
sight in planning the experiments, and in keeping

56 EVOLUTION AND GENETICS

an exact record of the numbers of individuals of
different kinds that ajDpeared in the second and third
generations, but also because of his insight in inter-
preting the results that he obtained.

Others had made crosses before Mendel. In fact,
a great deal of work had been done in making crosses
between wild species and also between cultivated
varieties. We realize today that the earlier crosses
between species failed to reveal the laws of heredity
because so many characters were involved that the
relationship of contrasted characters was obscured,
and that the crosses between domesticated varieties
failed either because the materials were not well
chosen, or else because the numerical relations in
the second generation were not observed. Some of
the earlier hybridologists, who worked before Men-
del's time and at about the same period, had ob-
served that the parental types may reappear in the
second and later generations. ISTaudin (1863, 1868)
had even suggested that this reappearance is due to
the separation of the parental types in the hybrid
but he failed to detect the numerical relations in-
volved and he did not make out the independent
inheritance of the members of different pairs of
characters.

About twenty years after Mendel's results were
first announced, but before his results became gen-
erally known, Galton (1889-) formulated several
laws of inheritance based in part on data from plants

MENDEL'S TWO LAWS 57

and animals, and also from man. He studied the
problem from a statistical standpoint, as Mendel
had done, but in the material that Galton used, the
effects due to the environment were not separable
from those due to inherited factors. Moreover he
did not give sufficient weight to the fact that the
character of an individual is not a suitable index of
its hereditarv constitution.

The latter difficulty was present in Mendel's
material also, and one of his chief merits is that he
detected this fact by making tests which revealed
the hereditary constitution of each individual.

Mendel deduced two laws of heredity that may be
called the Law of Segregation and the Law of Free
Assortment.

Menders First Law

Mendel's first law can be more strikingly illus-
trated today by examples other than those he gave.
The inheritance of the red and white colors of the
flowers of the common garden plant Mirabilis ja-
lapa or four o'clock furnishes an excellent example.
If the pollen from a plant with white flowers is
placed on the pistil of a plant with red flowers, the
seeds that are produced give rise to a plant with
pink flowers {jig. 17). The hybrid may be said to
be intermediate in the color of its flowers between
the two parents. If the hybrid is self-fertilized it
produces white-, pink-, and red-flowered plants in

58

EVOLUTION AND GENETICS

the proportion of 1:2:1. All of these had the same
ancestry, yet they are of three different kinds. If
we did not know their history it would be quite im-
possible to state what the ancestry of the white or of

r

9'

Vr

"'A-

Fic. 17. — Inheritance of color in the four o'clock (Mira-
hilis jahipa). The figure above and to the left stands for
a red flower, that above and to the right for a white flower.
The next two generations are shown below.

the red had been, for they might just as well have
come from pure white and pure red ancestors re-
spectively as to have emerged from the j^hik hybrids.
Moreover, when w^e test them we find that they are
as pure as are white- or red-flowering plants that

MENDEL'S TWO LAWS 59

have had all white- or all red-flowering ancestors.

Mendel's Law explains the results of this cross
as follows:

The egg-cell from the white parent carries a fac-
tor for white, the pollen-cell from the red parent
carries a factor for red. The hybrid formed by their
union cames both factors. The results of their com-
bined action is to produce flowers intermediate in
color.

When the hybrids mature and their germ-cells
(eggs or pollen) ripen, each carries only one of these
factors, either the red or the white (fig. 18) , but not
both. In other words, the two factors that have been
brought together in the hybrid separate in its germ-

f<39S • O
Pollen 9-^6

r.

^9 •

o

o
o

o

Fig. 18. — Diagram illustrating the transmission of the
factors for red color (here black) and for white (here the
open circles) in a cross between red and white flowered
four o'clocks.

60 EVOLUTION AND GENETICS

cells. Half of the egg-cells are white-bearing, half
red-bearing. Half of the pollen-cells are white-
bearing, half red-bearing. Chance combinations at
fertilization give the three classes of individuals of
the second generation.

The white-flowering plants should forever breed
true, as in fact they do. The red-flowering j)lants
also breed true. The pink-flowering plants, having
the same composition as the hybrids of the first gen-
eration, should give the same kind of result. They
give, in fact, this result, i.e., one white-, to two
pink-, to one red-flowered offsj^ring (fig. 18).

Another case of the same kind is known to breed-
ers of poultry. One of the domesticated breeds is
known as the Andalusian. It is a slate-blue bird
shading into blue-black on the neck and back. Poul-
try men have known for a long time that tliese blue
birds do not breed true but ^^roduce white, black, and
blue offspring. The explanation of the failure to
produce a pure race of Andalusians is that they are
like the pink flowers of the four o'clock, i.e., they
are a hybrid type formed by the meeting of the white
and the black germ-cells. If the whites produced by
the Andalusians are bred to the blacks, all the off-
spring will be blue (fig. 19) .

When two such blue colored hvbrid birds are
bred to each other, chance fertilization of any egg
by any sperm (fig. 20) will give one pure white, to
two hybrid blues, to one pure black.

MENDEL'S TWO LAWS

61

w

if w
^S7

/;

■^ft.*

Fig. 19. — Cross between s])lashed white and black fowls, giving in F ^
blue Andalusian. In F., there is one splashed white, to two blues, to one
black.

In the two cases just given the character of the
hybrid is intermediate between the two contrasted
characters of the parents ; but in the seven pairs of
contrasted characters studied bv INIendel the char-
acter of the hybrid is hke that of one of the parents.
This character is said to be dominant. Two of Men-
del's cases will serve to illustrate this relation. If a
tall pea is crossed to a short pea the offspring are
tall [fig. 21), i.e., not intermediate in height. If

62

EVOLUTION AND GENETICS

Fig. 20. — Diagram to show the distribution of the genes
for white and for blaclv in the Andalusian cross. (See
iig. 19.)

these hy'brids self -fertilize (or are bred to each
other) there are two kinds of offspring, tall and
short, produced in the ratio of three to one. The short
peas, if self-fertilized, breed true {jig. 22), but if
the tall peas are self-fertilized they are found to be
of two kinds — one-third of them breed true and two-
thirds of them produce three tall to one short off-
spring {jig. 22) . It is obvious that here, as in the four
o'clock and in the Andalusian fowl, there are pres-
ent in the second generation three kinds of offspring
in the proportion of one j^ure tall, to two hybrid
tails, to one pure short. It is the discovery by Men-
del of the existence of these three kinds of indi-
viduals in the second generation that enabled him
to deduce his first law. He discovered that there

MENDEL'S TWO LAWS

63

S^ort Q Q

Fig. 21. — Diagram to illustrate the inheritance of tall and short,
edible peas. The small letter, s, stands for the short-producing gene.
The large letter, 8^ for its normal allelomorph or tall. Only the first
and second generations are here shown. The second and third genera-
tions are shown in fig. 22.

61 EVOLUTION AND GENETICS

were three such kinds of second generation indi-
viduals by following them into the next (third) gen-
eration. Had he been fortunate enough to have
worked with a form like the four o'clock where the
1:2:1 ratio is apparent in the second generation
this relation would probably have been seen without
carrying the experiment one generation further.

T.ll Tall

Fig. 22. — Diagram illustrating the results of self-pollination of the
F^ plants of fig. 31, The results show that one of the tall F., plants is
pure for tall (tall-tall), that two of them are hybrid or heterozygous,
and that one is pure for short (short-short).

Mendel also crossed yellow and green peas (fig.
23 ) . He crossed a plant belonging to a race having
yellow peas with one having green peas. The hybrid
plants had yellow seeds. These hybrids inbred gave
three yellows to one green seed. The explanation of
the results with peas is the same as that given for the

MENDEL'S TWO LAWS 65

four o'clock. In the germ-cells of the hybrid — the
so-called first fillial generation, or Fi — the elements
(or factors) that come from the two j^arents separ-
rate and half of the egg-cells come to carry one of

Fig. 23. — Diagram of cross between
a yellow and a green pea.

the original elements and half the other element.
Chance fertilization of any egg by any sperm (pol-
len) gives the numerical relation j^resent in the next
generation.

These four cases serve to illustrate an important
fact. The character of the individual is not a measure
of the nature of the mature germ-cells that it will

66 EVOLUTION AND GENETICS

produce. The pink J^i hybrid four o'clock, that is
intermediate, in a sense, between the white- and tlie
red-flowered parents produces only white- and red-
bearing germ-cells; and the yellow Fi hybrid pea,
w^hose color is exactly that of one of the parents, also
produces two kinds of germ-cells in equal numbers,
yellow- and green-producing. It is obvious from this
that the character of the individual is not a reliable
index of its ancestry, or of what it transmits to the
next generation.

MendeVs Second Law

Besides his discovery that members of each pair
of elements disjoin in the germ-cells of the hybrid
(law of segregation) Mendel made a second dis-
covery (the law of free assortment) which also has
far-reaching consequences. The following case il-
lustrates this second law.

If a pea that is yellow^ and round is crossed to one
that is green and wrinkled {jig. 24), all of the off-
spring are yellow and round. Inbred, these give 9
yellow-round, 3 green-round, 3 yellow-WTinkled,
1 green- WTinkled. All the yellows taken together are
to the green as 3:1. All the round taken together
are to the wrinkled as 3:1; but some of the yellows
are wrinkled and some of the green are round. There
has been a recombination of characters, w^hile at the
same time the results, for each pair of characters
taken separately, are in accord witli ]Menders Law

MENDEL'S TWO LAWS 67

of Segregation. The second law of JNIendel may be
called the Law of Free Assortment of different
character pairs. A character from one organism can,
as it were, be transferred to a different organism.
The numerical results obtained, when two pairs

Fig, 24. — Diagram to show cross between a round-yellow
and a green-wrinkled pea.

of characters are, as here, involved in the same cross,
may be accounted for as follows. The element for
green color may be represented by g and its con-
trasted color vellow by G, and the element for
wrinkled by w and its contrasted character smooth
by W. The egg of the yellow round parent pea is
GW and the pollen of the green wrinkled pea is
gw. The fertilized egg, that becomes the hybrid

68

EVOLUTION AND GENETICS

(Fi), contains both sets of these elements; it is
GgJVw. These represented in pairs are:

G W

g w
Now when the germ-cells of this hybrid mature the
members of one of the pairs behave independently
of the members of the other pair. Consequently four
kinds of germ-cells are possible, namely, GW , Gw,
gJVj gw {fig. 25) . There will be four kinds of egg-

Yellov/ Round

green wrinkled

Yel]ow(^reen) Round(wrinkled)

.o.

0

Yellov/
green

Round
wrinkled

01*

Yellow
green

wrinkled
Round

o

Q

Fig. 25. — Diagram to shoM- the independent segregation of the
two pairs of factors, yellow-green and rouncl-wrinivled.

cells in the hybrid and four kinds of f)ollen grains.
Chance fertilization of any egg-cell by any pollen
grain will give sixteen classes of individuals, as
show^n in figure 26.

There will be, as inspection of the table shows, 9

MENDEL'S TWO LAWS 69

kinds of individuals that contain at least one G and
one W ; 3 kinds that will contain at least one G and
two w's ; 3 kinds that will contain at least one W and
two (/'s ; and one kind that will contain neither G nor

/^

GW

G w

gW

gw

/

GV

Gw

p-

gw

W

Fig. 26. — Diagram to show the sixteen combinations in 7^,
when two pairs of factors are involved, namely, green, ^, and
yellow, G \ wrinkled, w, and round, W.

W but two ^'s and two w's. Since yellow {G) domi-
nates green {g) , and round {W) dominates
wrinkled ( to ) there will be

9 yellow-round : 3 yellow- wrinkled : 3 green-
round : 1 green-wrinkled.

'0

EVOLUTION AND GENETICS

vv

EE

VV

ee

/^

^A^

^

Fig. 27. — Diagram to show the indejiendent inheritance of two
pairs of factors, namely, gray (EE), vestigial (vv), and ebony
(ee), long (T^l"). The lower group of 16 flies represents the 16
recombinations in the second, F.„ generation, in the ratio of
9:3::3:1.

MENDEL'S TWO LAWS 71

Thus the independent assortment of the mem])ers
of the two pairs of elements in the hybrid accounts
for the kinds of individuals that appear in the next
generation and also the proportion in which they
occur.

Another illustration of Mendel's second law — ■
one from the animal kingdom — is given in figure 27,
for the vinegar fly. xVn ebony colored fly (e) with
long wings (F) is crossed to a gray colored fly
(E) with vestigial wings (v) . The offspring (F^)
are gray flies with long wings, EeVv. If two of
these hybrids (jPi's) are mated they f)roduce 9 gray-
long: 3 ebony-long: 3 gray-vestigial : 1 ebony-vesti-
gial. The explanation (fig. 28) is the same as in the

^ggs VE Ve vE ve

sperm

VE

VE
VE

Ve
VE

vE
VE

ve
VE

Ve

VE

Ve

Ve
Ve

vE
Ve

ve
Ve

vE

VE

vE

Ve
vE

vE
vE

ve
vE

ve

VE

ve

Ve
ve

vE
ve

ve
ve

Fig. 38. — Diagram to show the composition of the IG
clashes of individuals represented in F^ of fi(/. 27.

72 EVOLUTION AND GENETICS

peas when two pairs of contrasted characters are
present.

The possibihty of interchanging characters might
be illustrated by hundreds of examples. It holds not
only for two pairs of characters but when three, four,
or more enter the cross. It is as though two individ-
uals were taken apart and their characters were put
together again by substituting one part for another.

Not only has this power to make whatever com-
binations we choose great practical importance, it
has even greater theoretical significance; for it fol-
lows that the individual is not in itself the unit in
heredity, but that within the germ-cells there exist
smaller units concerned with the transmission of
characters.

The older mvstical statement of the individual as
a unit in heredity has no longer any interest in the
light of these discoveries, except as a past phase of
biological history. We see, too, more clearly that
the sorting out of factors in the germ plasm is a very
different process from the influence of these factors
on the development of the organism. There is today
no excuse for confusing these two j^roblems.

Chapter YI

THE CHROMOSOMES AND MENDEL'S

TWO LAWS

The discoveries that ]Mendel made with peas have
been found to ap2)ly everywhere throughout the
plant and animal kingdoms — to flowering plants, to
mosses, to insects, snails, Crustacea, fishes, amphi-
bians, birds, and mammals (including man).

There must be something that these widely sepa-
rated groups of plants and animals have in com-
mon— some simple mechanism perhaps — to give
such definite and orderlv series or results. There
is, in fact, a mechanism, possessed alike by animals
and plants, that fulfils the requirements of ^lendel's
principles.

The Cellular Basis of Hereditij and Development
In order to aj^preciate the full force of the evi-
dence, a few familiar facts, that became known be-
fore the discovery of the mechanism in question,
may be briefly reviewed.

Throughout the greater part of the last century,
while students of evolution and of hereditv were
engaged in what may be called the more general
aspects of the subject, there existed another group
of students who were engaged in working out the
minute structure of the material basis of the living

74 EVOLUTION AND GENETICS

organism. They found that organs such as the
brain, the heart, the hver, the lungs, the kidneys,
etc., are not themselves the units of structure, but
that all these organs can be reduced to a simpler
unit that repeats itself a thousand-fold in every or-
gan. We call this unit a cell.

The egg is a cell, and the spermatozoon is a cell.
Fertilization is the union of two cells. Simple as the
process of fertilization appears to us today, its dis-
covery swept aside a vast amount of mystical specu-
lation concerning the role of the male and of the
female in the act of procreation.

Within the cell a new microcosm was revealed.
Every cell was found to contain a spherical body
called the nucleus {fig. 29). Within the nucleus is
a network of fibres ; a sa^) fills the interstices of the

Fig. 29.— Diagram of a "typical cell,"
showing cell-wall, cytoplasm (with solid
and fluid inclusions) and centrosome
witli astral rays (doubtfully present in
resting stage). In the center is the
nucleus with its network of chromatin,
and its nuclear sap.

THE CHROMOSOMES 75

network. The network resolves itself into a definite
number of threads or rods at each division of the cell
{fig. 30). These rods we call chromosomes. Each-
species of animals and plants possesses a character-
istic number of chromosomes which have a definite
size, and sometimes a specific shape, and even char-
acteristic granules at different levels. Beyond this
point our strongest microscopes fail to penetrate.

*A *v~. «

Fig. 30. — Diagram, slightly modified from Agar, to show a
ty])ic'al cell division (lvaryo]<inesis). The chromosomes are
represented as black threads and rods, which pass onto the
spindle fil)res and then move to the poles of the spindle where
they sul)sequently ])ecome vacuolated to form the resting nuclei
of the two daughter cells.

76

EVOLUTION AND GENETICS

Observation has reached, for the time being, its

limit.

Certain evidence relating to inheritance through
the sperm led to the conclusion that the chromosomes
are the bearers of the hereditary units. If so, there
should be many such units carried bv each chromo-

v.-'ii i^

\

■r--

&i§> @

Fig. 31. — Diagram to show stages in fertilization of an
egg by a spermatozoon. The three polar bodies lie at one
pole, and the spermatozoon is represented as entering near
the opi^osite side of the egg in 1 and 2. The head of the
sperm swells up and moves towards the egg-nucleus, that
has reformed after the polar bodies have been given off.
A centrosome forms near the sperm-nucleus. It divides
into two centrosomes, which begin to se])arate as a central
spindle appears between them. Around each centrosome
astral rays develop. The two nuclei come together in the
middle of the egg to become the segmentation nucleus. A
sjDindle develops around the nucleus.

THE CHROMOSOMES 77

some; for, the niiniber of chromosomes is hmited
while the nmiiber of independently inherited char-
acters is large. In Drosophila melanogaster it has
been demonstrated not only that there are exactly

r

Fig. 32. — Diagram showing the segmentation
of an egg into two, four, eight cells, etc. The
cells become arranged over the surface of a
sphere whose interior is filled with fluid. (After
Selenka.)

as many groups of characters that are inherited to-
gether as there are pairs of chromosomes, but even
that it is possible to locate the hereditary elements
in particular chromosomes and to state the relative
position there of the factors for the characters. If

78 EVOLUTION AND GENETICS

the validity of this evidence is accepted, the study of
the cell leads to the ultimate units about which the
whole process of the transmission of the hereditaiy
factors turns.

Before considering this somewhat technical mat-
ter, certain facts, which are familiar for the most part,
should be recalled, because, on these, rests the whole
of the subsequent explanation.

The thousands of cells that make up the cell-state
that we call an animal or plant come from the fer-
tilized egg (fig. 31) . An hour or two after fertiliza-
tion the egg divides into tw^o cells (fig. 32). Then
each half divides again. Each quarter next divides.
The j^rocess continues until a large number of cells
is formed and, out of these, organs mold themselves.

At every division of the cell the chromosomes also
divide. Half of these have come from the mother,
half from the father. Every cell contains, therefore,
the sum total of all the chromosomes, and if these
are the bearers of the hereditary qualities, every cell
in the body, whatever its function, has a common
inheritance.

At an early stage in tlie development of the ani-
mal certain cells are set apart to form the organs of
reproduction. In some animals these cells can be
identified early in the cleavage (fig. 33).

The reproductive cells are at first like all the other
cells in the body in that they contain a full comple-
ment of chromosomes, half paternal and half mater-

THE CHROMOSOMES

79

nal in origin. They divide as do the other cells of the
body for a lonff time. At each division each chromo-
some splits lengthwise and its halves migrate to op-
posite poles of the spindle.

^«-

cMp

cR

•cad

Fig. 33. — o, Section of egg of Calligrapha bigsbyana,
showing "germ-cell determinants" (granules), g c d,
at posterior end of egg; b, posterior end of a later
stage of same, showing primordial germ-cells; c, Sec-
tion of egg of Miastor, showing single primordial
germ-cell at posterior end. (After Hegner.)

But there comes a time when a new process ap-
pears in the germ-cells {jigs. 34 and 3.5). It is
essentially the same in the ^gg- and in the sperm-
cells. The discovery of this process w^e owe to the
laborious researches of many workers in many coun-

80

EVOLUTION AND GENETICS

Fig, 34. — Diagram illustrating the two maturation divisions of the
germ cells in the male. In a the chromosomes appear as thin threads
(leptotene stage). These conjugate in pairs, b, beginning at the two
ends of each loop. The threads contract, and a spindle appears, d, near
the nucleus. The conjugating chromosomes enter the spindle, d. There
they separate, e, moving to opposite poles of the spindle. The cell pro-
toplasm begins to constrict, /. The chromosomes may without entering
upon a resting nuclear stage pass onto a new spindle that has de-
veloped by the division of each of the centrosomes of each daughter
cell, g. Each chromosome now splits throughout its length (equational
division) ; half of each goes to one or the other pole. The two daughter
cells then divide, giving four cells, each of which differentiates into a
spermatozoon.

THE CHROMOSOMES

81

tries. The chromosomes come together in pairs (fig.
34^). Each maternal chromosome conjugates with a
paternal chromosome of the same kind.

Then follow two rapid divisions (fig. 34, e-i) . At
one of the divisions the double chromosomes separate
(fig. 34, d-f) so that each resulting cell comes to con-
tain some maternal and some paternal chromosomes,
i.e., one or the other member of each pair. At the
other division each chromosome simply splits as in
ordinary cell division. In the male four sj)ermatozoa

&'

M^r

- -^-^ '

ia

■'. :V-^l:?-'

x?i1^

-feii?^

\\-^m

fe-;-- ••'-■■

■■■;'■■'■■■'■:'

^;v

^W

r- d

A)°tf

■•■"s5J*.-N; ■-

'•:~'5<^

•■^."Ji &■•:■■■.■■

Fig. 35. — Diagram illustrating the two maturation divisions of the
egg. In a the polar spindle is present at the periphery of the egg. The
three pairs of chromosomes (bivalents) are represented in black and
white; the white being the paternal and the black the maternal. In b
the conjugating chromosomes have separated and are moving to the
poles. In c the first polar body has been given off, leaving three single
chromosomes in the egg. In c these have split lengthwise and lie off the
equator of a new spindle. In e the daughter chromosomes have sepa-
rated and moved to opposite poles. In / the second polar body has
been given off and the first polar body has divided. Three single
chromosomes are left in the egg.

82 EVOLUTION AND GENETICS

are produced (by these two divisions) from each cell
of the testis {jig. 34, i) .

In the female the two divisions of the egg-cell are
very unequal {jig. 35), although the chromosomes
are distributed equally to all the cells. Thus at the
first division one cell is very small {jig. 35, c) and is
called the polar body. At the next division the polar
body divides again, and at the same time the egg
divides again also, producing another polar body
{jig. 35, d, e, /) . The three polar bodies and the egg-
cell are equivalent to the four spermatozoa, but only
the egg-cell undergoes further development — the
polar bodies disappear. Although only one cell sur-
vives, nevertheless there will be as many kinds of
mature eggs as there are kinds of sperm cells (with
respect to the distribution of the chromosomes), if,
as we now know to be the case, the distribution of
the chromosomes in the two final divisions (matura-
tion divisions) are the same in the eggs and in the
sperm-cells. When the eggs are fertilized, each by
one spermatozoon, the whole mimber of chromo-
somes is restored.

The 3Iechanis?}i of McndeVs Two Laws
The behavior of the chromosomes at the time of
maturation of the egg- and sperm-cells furnishes a
mechanism for ^lendelian hereditv if the chromo-
somes are the bearers of the hereditary elements, and
if they maintain their integrity both during the rest-

THE CHROMOSOMES 83

iiig stages of the nucleus and during their period of
active division. There is a great deal of evidence from
direct observation in favor of this view and there is
more evidence from the modern work in hereditv
that points in the same direction. This evidence can
not be considered here, but if it is granted that these
relations hold, then the behavior of the chromosomes
during maturation furnishes, as stated above, an ex-
planation of Mendel's laws.

An example will illustrate this statement. If in
the four o'clock the elements for red flower color are
carried in the red parent by the two members of the
same pair of chromosomes and the elements for
white flower color are carried in the white parent by
two members of the same pair of chromosomes, the
germ-cells (ripe egg- and sperm-cells) will each
carry one of these chromosomes {fig. 36) . If the red
plant is crossed to the white, the pink hybrid will
have a red- and a white-bearing chromosome.

When in the hybrid the germ-cells ripen, these
two chromosomes, being mates, will come together
as a pair and then separate at one of the two matura-
tion divisions, and half of the eggs will contain the
red-bearing chromosome and half will contain the
white-bearing chromosome. Similarly for the pollen
grains. Chance fertilization of any egg by any sperm
will give the combinations of chromosomes that
JNIendel's law of segregation requires. In other
words the known behavior of the chromosomes is

84 EVOLUTION AND GENETICS

exactly the same as Mendel's postulated elements.

Mendel's second law for the inheritance for two
or more characters also finds its explanation in the
behavior of the chromosomes, provided the members

Red

II

Parents

White

I

Germ cells
of parents

10

Hybrid (F^)

Sperm

(Fp) red

wnite

Fig. 36. — Diagram to illustrate the distribution of the chromo-
somes in a cross between a red and a white four o'clock (see fig.
17). The chromosomes that carry the gene or factor for red are
here black, and those that carry the gene for white are w^hite.

of the pairs of chromosomes are sorted ont indepen-
dently of each other (fig. 37). For example, in the
cross between yellow-round and green-wrinkled

THE CHROMOSOMES

85

peas, if one pair of chromosomes in the hybrid (Fi)
carries the contrasted elements yellow and green
and another pair of chromosomes of the same hybrid
carries the round and wrinkled elements, then, if these
chromosomes at the maturation period behave inde-
pendently, there will be four kinds of germ-cells pro-
duced. These four kinds will carry a yellow-bearing
and a round-bearing chromosome, or a yellow-bearing
and a w^rinkled-bearing chromosome, or a green-
bearing and a round-bearing chromosome, or a
green-bearing and a wrinkled-bearing chromosome.

Yellow pound
00

II

\

I

Po^rents

Gpeen wrinkled

OO

Germ cells

O

Fig. 37. — Diagram to illustrate the distribution of two pairs of chro-
mosomes carrying two pairs of Mendelian factors, namely yellow-green
and round-wrinkled. The chromosome carrying the gene for yellow is a
black rod, that for green is a white circle; that carrying the gene for
round is a circle with a dot, that for wrinkled is a circle without the
dot.

86 EVOLUTION AND GENETICS

Only these four kinds of germ-cells are possible on
the chromosome mechanism. Self-fertilization of
such a hybrid will give the same recombinations of
chromosomes that Mendel's second law requires for
the hereditary elements.

Chapter VII

THE LINKAGE GROUPS AND THE

CHROMOSOMES

If the hereditary elements, the genes, are carried by
the chromosomes and if the chromosomes are per-
sisting structures, there should be as many groups
of hereditary characters as there are kinds of chro-
mosomes. In only a few cases has a sufficient number
of characters been studied to show whether there is
any correspondence between the number of heredi-
tary groups of characters and the number of chro-
mosomes. In the yinegar fly, Drosophila, there are
about four hundred characters that fall into four
groups. On page 88 {fig. 38) some of these are
giyen arranged according to groups. The charac-
ters are arranged in four grou2)s, Group I, II, III
and IV. Three of these grou23S are equally large or
nearly so; Group IV contains only three characters.
The characters are put into these grou2)s because, in
heredity, the members of each group tend to be in-
herited together, i.e., if two or more enter the cross
together they tend to remain together through sub-
sequent generations. On the other hand, any mem-
ber of one group is inherited entirely independently
of any member of the other groups ; in the same way
as Mendel's yellow-green pair of characters is in-
herited independently of the round-wrinkled pair.

88

EVOLUTION AND GENETICS

n mi;

0.0 ^ YELLOW
1.5 -- WHITE

6.6-- ECHINUS
7.5^^ RUBY

13.7 - - CROSSVNL'SS 14.0

20.0 -: CUT

27.5 - ■ TAN

33.0
3a 1

44.4

56.5..
57.0''

65.0 ■
68.0 -

CO - - STAR

9.0 - - TRUNCATE

29.0 - - DACHS

VERMIUON
MINIATURE

4a0-- SABLE

GARNET

,F0RKED
'BAR

CLEFT
BOBBED

650

70.0
735

\

0.0 + ROUCHOID 0.0.,;
0.5'
0.9

STREAK

25.3 SEPIA
25.8 = =''hAIRY

465 -■ ' BLACK

524 - - PURPLE

38.5
42.0
455

54.0

59.0 4- CLASS

635-1- DELTA
VESTIGIAL 655-- HAIRLESS

67.5 i EBONY
LOBE

720+ WHITE-OCELLI
CURVED

97.5 -ARC

95.4

A

/

985-- PLEXUS

103.0 ■ : BROWN
105.0 * SPECK

^BENT
\SHAVEN
EYELESS

DICHAETE

SCARLET

PINK

SPINELESS

865 - - ROUGH

CLARET

95.7== MINUTE

101.0 -- MINUTE-G

Fig. 38. — Chart of the genes of the chromosomes of Drosophila. The
genes are arranged in the four linkage groups, I, II, III, IV. The
name of the gene is given to the right of its locus, and the distance of
the loci from one end of the chromosome is indicated bv the numbers
to the left of each locus. The "distance" gives the cross-over value for
the genes corrected for double crossing-over.

LINKAGE GROUPS 89

In the chromosome group of Drosophila melano-
gaster {fig. 39) there are four pairs of chromosomes,
three of nearly the same size and one much smaller.
Xot only is there agreement between the number of
hereditary groups and the number of the chromo-

FEMALE MALE

IV / r

Fig. 39. — Female group and male group of
chromosomes of Drosophila melanogaster.

somes, but even the size relations are the same, for
there are three large groups of characters and three
pairs of large chromosomes, and one small group of
characters and one pair of small chromosomes.

The Four Linkage Groups of Drosophila

Melanogaster

The following description of the characters of the
wild fly may be useful in connection with the account
of the modifications of these characters that appear
in the mutants.

The head and thorax of the wild fly are grayish-
yellowy the abdomen is banded w4th alternate stripes
of yellow and black. In the male {fig. 6, left) , there
are three narrow^ bands and a black tip. In the female

90 EVOLUTION AND GENETICS

there are five black bands {fig. 6, right) . The wings
are gray with a sin*face texture of such a kind that
at certain angles thev are iridescent. The eves are a
deep, brick-red. The minute hairs that cover the body
have a characteristic arrangement that is most ob-
vious on the head and thorax. There is a definite
number of larger hairs called bristles which have a
characteristic position and are used for diagnostic
purposes in classifying the species. On the foreleg
of the male there is a comb-like organ formed by a
row of bristles ; it is absent in the female. The comb
is a secondarv sexual character.

Some of the characters of the mutant types are
shown in figures 40, 41, 42, 43. The drawing of a
single fly is often used here to illustrate more than
one character. This is done to economize space, but
of course there would be no difficulty in actually
bringing together in the same individual any two or
more characters belonging to the same group (or to
different groups) . Without colored figures it is not
possible to show many of the most striking differ-
ences of these mutant races ; at most, dark and light
coloring can be indicated by the shading of the body,
wings, or eyes.

GROUP I

The hereditary elements of this group are carried
bv the A^- chromosomes. The characters are said to

a.

be sex-linked.

LINKAGE GROUPS 91

111 the six flies drawn in figure 40 there are sliown
five different wing characters. The first of these
types (a) is called cut, because the ends of the wings
look as though they had been cut to a point. The an-
tennae are displaced downward and appressed and
their bristle-like aristae are crumpled.

The second figure (h) represents a fly with a
notch in the ends of the wings. This character is
dominant, but the same factor that produces the
notch in the wings is also a recessive lethal factor;
because of this latter effect of the character, no males
of this race exist, and the females of the race are
never pure but hybrid. This same figure (b) is used
here to show two other sex-linked characters. The

^»VK

0:^ ^

tS v;

y \

0i^

r'^ I

Fig. 40. — Some of the charaeters of the first chromo-
some of Drosophila melanogaster. See text.

92 EVOLUTION AND GENETICS

spines on the thorax are twisted or kinky, which is
due to a factor called "forked." The effect is best
seen on the thorax, but all spines on the body are
similarly modified; even the minute hairs are also
affected. The lighter color of the body and antennae
is intended to indicate that the character tan is also
present. The tan flies are interesting because they
have lost the positive heliotropism. As this peculiar-
ity of the tan flies is inherited like all the other sex-
linked characters, it follows that when a tan female
is bred to a wild male all the sons inherit the reces-
sive tan color and indifference to light, while the
daughters show the dominant sex-linked character
of their father, i.e., they are "gray," and go to the
light. Hence when such a brood is disturbed the
females fly to the light, but the males remain behind.

One of the first mutants that appeared was called
rudimentary on account of the condition of the wings
(c) . The same mutation has apj^eared indepen-
dently several times. In the drawing (c) the dark
body color is intended to indicate "sable."

In the fourth figure (d) the third and fourth lon-
gitudinal veins of the wing are fused into one vein
from the base of the wing to the level of the first
cross-vein and in addition converge and meet near
their outer ends.

In the fifth figure (e) the wings are shorter and
more pointed than in the wild fly. This character is

LINKAGE GROUPS 93

called miniature. The light color of the drawing may
be taken to represent yellow body color.

In the last figure (/) of "club," the wings are
pads, essentially in the same condition that they
are in when the fly emerges from the pupa case.
Not all the flies of this stock have the wings in this
condition ; some have fully expanded wings that ap-
23ear normal in all respects. Nevertheless, about the
same percentage of offspring show the pads irrespec-
tive of whether the parents had pads or expanded
wings. The flies of this stock show, however, another
character, which is a product of the same factor, and
which is constant, i.e., repeated in all individuals.
The two bristles on the sides of the thorax are con-
stantly absent in this race.

There are many different eye colors in Group I,
ranging from a pure white eye to vermilion, which
could be shown onlv bv the use of colored drawings.

GROUP II

The hereditary elements of members of Group
II are carried by one pair of the two large bent
chromosomes.

In the first drawing (a) of figure 41 which contains
members of Group II, the wings are almost entirely
absent or "vestigial." This condition arose at a single
step and breeds true, although it appears to be influ-
enced to some extent by temperature, also by modi-
fiers that sometimes appear in the stock.

9^ EVOLUTION AND GENETICS

In the second figure ( b ) the wings turn up at the
end. The mutant is called jaunty.

In the third figure (c) the wing is long and nar-
row and sometimes bent back on itself, as shown
here. In several respects the wing resembles strap
(d) but seems to be due to another factor, called
antlered.

In the fourth figure (d) the wings are long and
narrow and several of the veins are unrepresented.
This character, "strap," is very variable. On the
thorax there is a deep black mark called trefoil. In the

'^h^

"■^^^'ft-- 1

r¥K

\ '

fB

Fig. 41. — Some of the characters of the second chromo-
some of Drosophila melanogaster. See text.

LINKAGE GROUPS 95

wild fly there is a three-pronged mark on the thorax
present in many individuals. Trefoil is a further
development and modification of this mark and is
due to a special factor.

The fifth figure (e) is called dachsoid; the body
is shortened and the wings are broad and held out.
The legs are short. The absence of cross-veins in the
wings is characteristic.

In the sixth figure (/), apterous, the wings are
entirely absent, not even the base remaining as in
vestigial. The apterous flies are almost completely
sterile.

The seventh figure (g) shows the wings "curved."
In addition there is j^resent a minute black speck at
the base of each wing, due to another factor called
speck.

In the eighth figure (k) the wings are arched.
The factor is called arc. The dark color of the body,
and especially of the wings, indicates the factor for
black.

There are also a number of different eye colors in
this group — one of which, brown, is darker in old
flies than the red of the wild type.

GROUP III

The hereditary elements of Group III are carried
by the other pair of large bent chromosomes.

In figure 42 (a) , a mutant type called bithorax is
shown. The old metathorax is replaced by another

96 EVOLUTION AND GENETICS

mesothorax thrust in between the normal meso-
thorax and the abdomen. It carries a pair of wings
that do not completely unfold. On this new meso-
thorax the characteristic arrangement of the bristles
is shown. Thus at a single step a typical region of the
body has doubled. The character is recessive.

In the second figure ( h ) the dark color of the fly
is due to a factor called ebony.

The size of adult flies varies according to the
amount of nourishment obtained by the larva. After
the fly emerges its size remains nearly constant, as in
many insects. Two races have, however, been sepa-
rated that are different in size as a result of a genetic
factor. The first of these, called dwarf, is represented
by figure 42 (c). The race is small but variable in
size, depending on food and other conditions. The
same figure shows the presence of another factor,
"sooty," that makes the fly dark.

In the fourth figure (d) another mutation in size
is shown. It is called "giant." The flies are twice the
size of wild flies.

In the fifth figure {e) the mutant dichaete is
shown. It is characterized bv the absence of two of
the bristles on the thorax. Other bristles may also be
absent, but not so constantly as the two just men-
tioned. Another effect of the same factor is the
spread-out condition of the wings.

In the sixth figure (/) the wings are curled up
over the back ; the character is called curled.

LINKAGE GROUPS

97

In the seventh figure {g) the wings are beaded,
i.e., the margin is defective at intervals, giving a
beaded-Hke outhne to the wings. This condition is
very variable and much affected by other factors
that influence the shape of the wings.

There are many eye colors in this Group — one of

\

/

-*«*%

^

eS^^ b

y % Q ^

</"^ Q'^

^i>

;d;

'4^:^^''

v\\

n f

Fig. 43. — Some of the characters of the third chromo-
some of Drosophila melanngaster. S€e text.

these, sepia, becomes very dark in old flies. Pink and
peach eye colors are modifications of the same gene
(a case of multiple allelomorphs). Two other eye
colors in this group, scarlet and cardinal, are almost
indistinguishable, but the genes for these characters
lie in quite different parts of the chromosome.

98

EVOLUTION AND GENETICS

GROUP IT

The hereditary elements of Group IV are carried
by the pair of very small chromosomes. Only three
mutants have been obtained. One of these, called
"eyeless" (fig. 43 a, a"), is variable — the eyes are
often entirely absent or represented by a few or by
several ommatidia (h, h') . On the sides of the head

'^j

1-*

a

a

w

Fig. 43. — The three characters of the foiirth chromo-
some of Drosophila melanogaster. See text.

where the normal eve lies there is, in "eyeless," a
corresponding empty area in the more extreme con-
dition {a, a^) , and even when a piece of the eye is
present it lies in this area but failing to fill all of it,
the outline of the full sized eye is, so to speak, still
present. These parts of eyes {b, h') might be spoken
of as rudimentary organs.

LINKAGE GROUPS 99

Drawing (c) in figure 43, represents "bent," so
called from the shaj^e of the wings. This mutant is
likewise very variable, often indistinguishable from
the wild type, yet when well developed strikingly
different from any other mutant.

The third mutant (d) is called shaven. The
bristles and hairs are extremely short, and the thorax
esj^ecially, appears as though shaven.

This brief account of a few of the mutant races
that can be most easily represented by uncolored
figiu'es will serve to show how all parts of the body
may change, some of the changes being so slight that
they would be overlooked except by an ex^^eii:, others
so great that the characters affected depart far from
the original one.

It is important to note that the m utant genes in the
X -chromosomes are not limited to anij part of the
body, nor do theg affect more frequently a particu-
lar part. The same statement holds equally for all
of the other chromosomes. In fact, since each factor
may affect visibly several parts of the body at the
same time there are no grounds for expecting any
specicd relation between a given chromosome and
special regions of the body. It cannot too insistently
be urged that when tee say a character is the product
of a particular factor uce mean no more than that it is
the most conspicuous effect of that factor.

If, then, as these and other results to be described

100 EVOLUTION AND GENETICS

point to the chromosomes as the bearers of the JNIen-
delian factors, and if, as has been shown, these fac-
tors have a definite location in the chromosomes, it is
clear that the location of the factors in the chromo-
somes bears no spatial relation to the architecture
of the body.

Chapter VIII

SEX-LIXKED IXHEKITAXCE

When we follow the history of pairs of chromo-
somes we find that their distribution in successive
generations is paralleled by the inheritance of ]Men-
delian characters. This is best shown in the sex
chromosomes (fig. 44) . In the female of Drosophila
there are two of these chromosomes that are called

Diploid Nuclei XX

Gametes

X Y

Fertilization
Zygotes

Fig. 44. — Diagram showing the di.stri])uti()ii
of the sex chromosomes from parents to off-
spring.

X-chromosomes ; in the male there are also two, but
one differs from those of the female in its shape, and
in the fact that it carries none of the ordinary genetic
factors. It is called the K-chromosome.

The inheritance of a pair of characters whose
genes lie in the A^-chromosomes is shown in figures

102 EVOLUTION AND GENETICS

45 and 46, illustrating crosses between a white-eyed
and a red-eved individual.

The first of these represents a cross between a
white-eyed male and a red-eyed female {fig. 45, top
row) . The X-chromosome in the male is represented
by a bar, (w), the I^-chromosome is bent. In the fe-
male the A"-chromosomes are W and W. Each egg
of such a female will retain one X (with W)
after the polar bodies have been thrown off. In
the male there are two classes of sperm — the female-
producing, carrying JT (with w) ; and the male-
producing, carrying the I^-chromosome. Any egg
fertilized by an X-bearing sperm w^ill produce a
female with red eves, because the A"-chromosome
(W) from the mother carries the dominant fac-
tor for red. Any egg fertilized by a I^-bearing sperm
will produce a male with red eyes because he gets his
X-chromosome (W) from his mother.

When these two Fi flies (second row) are inbred
the following combinations are expected. Each egg
will contain a red-eye producing X, ( W ) , or a
white-eye producing X, (w) , after the polar bodies
have been extruded. The male will produce two
kinds of sperms, of which the female-producing
will contain a red-eye producing X. Since any
egg may by chance be fertilized by any sperm, there
will be the four classes of individuals shown in the
bottom row of the diagram. All the females will
have red eyes, because irrespective of the two kinds

SEX-LINKED INHERITANCE

103

Fig, 4-5. — Cross of a red-eyed female and a white-eyed male of
the vinegar fly, showing sex-linked inheritance.

104 EVOLUTION AND GENETICS

of eggs of the female, all the female-producing sperm
carry a (w) X, Half of the males have red eyes,
because half of the eggs had each a red-producing
X-chromosome. The other half of the males have
white eyes, because half of the eggs had each a white-
producing A^-chromosome. Evidence from other
sources shows that the I^-chromosome of the male is
indifferent, so far as these Mendelian factors are
concerned.

The reciprocal experiment is illustrated in jigure
46. A white-eyed female is mated to a red-eyed
male (top row) . Each of the mature eggs of such a
female contains one white-producingX-chromosome,
represented by the open bar in the diagram. The red-
eyed male contains female-producing X-bearing
sperm, that carry the factor for red-eye color, and
male-producing I^-chromosomes. Any egg fertilized
by an A"-bearing sperm will become a red-eyed
female because the X-chromosome that comes from
the father carries the dominant factor for red eye
color. Any egg fertilized by a I^-bearing sperm will
become a male with white eyes because the only X-
chromosome that the male contains comes from his
mother and is white-producing.

When these two F^ flies are inbred (middle row)
the following combinations are expected. Half the
eggs will contain each a white-producing X-chromo-
some and half a red-producing. The female-producing
sperms will each contain a white X- and the male-

SEX-LINKED INHERITANCE

105

producing sperms will each contain an indifferent
I^-chromosome. Chance meetings of eggs and sperms
will give the four F2 classes (bottom row) . These

w

w

w

•

Fig. 46. — Cross of a white-eyed female and a red-eyed male
of the vinegar fly, showing sex-linked inheritance.

106 EVOLUTION AND GENETICS

consist of white-eyed and red-eyed females and
white-eyed and red-eyed males. The ratio here is
1 : 1 and not three to one (3:1) as in other Mendelian
cases. But Mendel's law of segregation is not trans-
gressed, as the preceding analysis has shown; for,
the chromosomes have followed strictly the course
laid down on Mendel's principle for the distribution
of factors. The peculiar result in this case is due to
the fact that the F^ male gets his single factor for
eye color from his mother only, and it is contained in
a body (the X-chromosome) that is involved in sex-
determination, while the mate of this body, the Y-
chromosome, is indifferent with regard to these fac-
tors.

In human inheritance there are characters that
show this same kind of transmission. Color-blindness,
or at least certain kinds of color-blindness, appears
to follow the same scheme. A color-blind father
transmits through his daughters his peculiarity to
half of his grandsons, but to none of his grand-
daughters {fig. 69). The result is the same as in the
case of the white-eyed male of Drosophila. Color-
blind women are rather unusual, which is expected
from the method of inheritance of this character,
but in the few known cases where such color-blind
women have married normal husbands all thel^i sons
inherit color-blindness from the mother (fig. 70).
Here again the result is the same as for the similar
combination in Drosophila.

SEX-LINKED INHERITANCE

107

In man the sex formula appears to be XX for the
female and XO or XY for the male, and since this
is essentially the same as that in Drosophila, the ex-
planation of sex-linked inheritance is the same. Ac-
cording to de Winiwarter there are 48 chromosomes

a

'4 ■' V .

0

Fig. 47. — a, speniiatogonimn cell of white man, showing 48
chromosomes, including the small F-chromosome; 6, same of
negro; c and d. Primary spermatocytes, side views, showing
X- and F-chromosomes. {After Painter.)

in the female and only 47 in the male, the I^-chromo-
some being absent. After the extrusion of the polar
bodies there should be 24 left in the e^^. In the male

108 EVOLUTION AND GENETICS

at one of the maturation divisions the single X-
chromosome passes to one pole. In consequence there
are two classes of sperms in man ; female-producing
containing 24 chromosomes, and male-producing
containing 23 chromosomes. If the factor for color-

FiG. 48, — Primary spermatocytes of man, side views of spindle.
The X- and the F-chromosomes are separating in advance of
the others. {After Painter.)

blindness is carried by the X-chromosome its inher-
itance in man works out on the same chromosome
scheme and in the same wav as does white eve color

SEX-LINKED INHERITANCE 109

(or any other sex-linked character) hi Drosophila,
for the O-sperm in man would be equivalent to the
I^-sperm in the fly.

Painter's later evidence {fig. 47), showing that
there is a small I^-chromosome in the male that acts
as the mate of the A"-chromosome {fig. 48), is very
convincing. Whether in man the male is XO or XY
the explanation of sex-linked inheritance is the same
since the factors involved are carried bv the X-
chromosomes.

Chapter IX

CROSSIXG-OVER

If the linkage were never broken we should expect
to find that groups of characters would be inherited
together. There would be as many such groups of
characters as there are pairs of chromosomes. To a
certain extent this is true, but the study of the inher-
itance of two or more characters in the same linkage
group has revealed a further fact of great interest,
namely, that there takes place an interchange at times
between the two members of the same linkage group,
and, it may be added, only between members of the
same linkage group and never between different
linkage groups. This interchange gives rise to a new
phenomenon in inheritance that is called crossing-
over, which may be illustrated by a few typical ex-
amples from Drosophila.

There are two mutant characters, black body
color and vestigial w^ngs, whose genes lie in the sec-
ond chromosome. If a fly having these two charac-
ters is crossed to a wild type fly with normal color
and long wings {jig. 49) the offspring are like wild
type flies, because normal dominates black and long
wing dominates vestigial wing.

If one of the daughters (i^i) from this cross is
now mated to a male with black color and vestigial
wings (both recessive characters) the offsj^ring are

112

EVOLUTION AND GENETICS

of four kinds. Two of these are like the grand-
parents, one black-vestigial, the other normal-long,
but the other two kinds have, as it were, interchanged

Fig. 49. — Diagram to illustrate crossing-over. The two
mutant characters, black and vestigial, are linked, as are
their normal allelomorphs, gray and long. A black vestigial
male is mated to a gray long female. The F^ female is
back -crossed to the double recessive type, black vestigial.
Four kinds of offspring are produced.

CROSSIXG-OVER 113

these characteristics. One kind is black-long, the
other normal-vestigial.

These four kinds do not appear in equal numbers,
but 83 per cent are like the grandparents and 17
per cent are the cross-overs. The result may be
stated in another way. The mutant characters that
went in together, black-vestigial and normal-long,
liave remained together (linked) in 83 per cent
of the grandchildren, while in 17 per cent of the
grandchildren there has been an interchange or
crossing-over.

The results may be represented in terms of chro-
mosomes as follows. The gene for black is represented
by b and its normal partner by B ; the gene for ves-
tigial by V and its partner by V (fig. 49) . Black (b)
and vestigial (v) are represented in the figure as
contained in the same chromosome (here a rod) in
one parent, and gray (B) and long wing (F) by the
corresponding chromosome in the other parent, here
b}" the all-black chromosome. The daughter contains
one of each of these tw^o chromosomes. When her
eggs mature these two chromosomes separate and
some of the eggs contain one, some the other chro-
mosome. These two kinds of eggs as the sequel shows
represent 83 per cent of all the eggs. But in 17 per
cent of the cases there has occurred in some way an
interchange between these two chromosomes with
the result that the genes for black and long come to
be in one chromosome and normal and vestigial in the

114 EVOLUTION AND GENETICS

other. The genes for black and normal may be said
to have crossed over.

When such a female is mated to a black-vestigial
male, whose spermatozoa contain the chromosome
with black and vestigial, four combinations are
formed. Remembering that normal dominates black,
and long dominates vestigial, it will be seen {fig. 49)
that four kinds of offspring are expected.

A second case of crossing over, here in the X-
chromosome, is illustrated in figure 50.

If a female with white eyes and yellow wings is
crossed to a wild male with red eyes and gray wings,
the sons are yellow and have white eyes and the
daughters are gray and have red eyes. If two Fi
flies are mated they will produce the following
classes :

Yellow Gray Yellow Gray

White Red Red White

]Vot only have tlie two grandparental combina-
tions reappeared, but in addition two new combina-
tions, viz., gray-white and yellow-red. The two
original combinations far exceed in numbers the
new or exchange combinations. If we follow the
history of the A^-chromosomes we find that the
large?' elasses of grandchildren can be explained if
A^-chromosomes are transmitted in their entirety
from one generation to the next.

CROSSIXG-OVER

115

The smaller classes of grandchildren, the ex-
change combinations or cross-overs, can be explained
by an interchange taking place between the chromo-
somes in the hybrid (2^^i) female. This is indicated
in the diagram.

Non-cros5-overs 98.5 «^

Cross-overs 1.5 <^o

Fig. 50. — Cross of a female vinegar fly that has white eyes and
yellow wings to a wild type male with red eyes and gray wings,
illustrating crossing-over.

116

EVOLUTION AND GENETICS

The explanation of crossing-over rests on the
assumption that members of the same pair of chro-
mosomes may at times interchange. If the chromo-
somes are the bearers of the genes there can be no
doubt from the genetic evidence that such an inter-
change takes place. When we turn to the known
behavior of the chromosomes in the ripening of the
germ cells we find certain stages where such a pro-
cess may seem possible.

At the ripening period of the germ cell the mem-
bers of each pair of chromosomes come together. In
several forms they have been described as meeting
at one end and then progressively coming to lie side
by side as shown in figure 51. At the comj^letion of

Fig. 51. — Conjugation of the chromosomes in Batracoceps.
(After Janssens.)

the process they appear to have united along their
length. It is always a maternal and a paternal chro-
mosome that meet in this way and always two of the

CROSSING-OVER 117

same kind. It has been observed that as the members
of a pair come together they occasionally twist
aromid each other (fig. 51). If where they overlap
thev should break and the ends uHite with the cor-
responding ends of the opposite chromosomes (fig.
52), the conditions of crossing-over would be ful-
filled.

Fig. 52. — Diagram to illustrate crossing-over of two con-
jugating threads.

Unfortunately the evidence that crossing-over
takes place at the time of maturation, as the result of
overlapping of the chromosomes, is very meagre and
by no means conclusive, nevertheless, as far as it
goes, this evidence is favorable for such an interpre-
tation of genetic crossing-over as that given above.

From the genetic evidence for crossing-over it is
possible to determine the relative location of the
genes in the chromosomes. The method can not be
given here in detail but the general point of view

118 EVOLUTION AND GENETICS

may be stated. If the genes lie along the length of
the chromosomes and if crossing-over is as likely
to occur at one level as at another, then, the nearer
together two genes lie the less likely is a break be-
tween them, or conversely the further apart in the
chromosome they lie the more likely is crossinc^-over
to take place. In other words the percentage of
crossovers is an index of the distance apart of the
genes. On this basis the location of the genes, as
shown in figure 38, has been determined. From such
a chart one is enabled to calculate what the inher-
itance of any gene will be with respect to any other
gene in its group provided its relation to two other
genes is known.

The theory of crossing-over enables the geneticist
to predict the results of a given experiment with the
same precision that Mendel's two laws allow predic-
tion for a single pair of characters in the same chro-
mosome pair, or for two or more pairs of characters
in different chromosome pairs.

Chapter X

NATURAL SELECTION AND
EVOLUTION

Darwin's Theory of Natural Selection still liolds
today first place in every discussion of evolution, and
for this reason the theory calls for careful scrutiny ;
for it is not difficult to show that the expression
"natural selection" is to many men a metaphor that
carries many meanings, and sometimes different
meanings to different men. While I heartily agree
with my fellow biologists in ascribing to Darwin
himself, and to his work, the first place in evolutionary
philosophy, yet recognition of this claim should not
deter us from a careful analysis of the situation in
the light of all that has been done since Darwin's
time.

The Theory of Natural Selection

In his famous book on the Origin of Sjjecies, Dar-
win tried to do two things: first, to show that the
theory of evolution furnishes an adequate explana-
tion of the facts. No such great body of evidence had
ever been brought together before, and it convinced
most thinking men that the theory of evolution of
living things furnished a rational explanation of what
is known about their relationships and past history.

Darwin also proposed several theories as to how

120 EVOLUTION AND GENETICS

evolution has taken place. He pointed to the influ-
ence of the environment, to the effects of use and
disuse, and to natural selection. It is to the last the-
ory that his name is especially attached. He appealed
to a fact familiar to everyone, that no two individ-
uals are identical and that some of the differences
that they show are inherited. He argued that those
individuals that are best suited to their environment
are the most probable ones to survive and to leave
offspring. As a consequence, their descendants
should in time replace through competition the less
well-adapted individuals of the species. This is the
process Darwin called natural selection, and Spen-
cer called the survival of the fittest.

Stated in these general terms tliere is nothing in
the theory to which anyone is likely to take excep-
tion ; for, it may appear little more than a truism to
state that the individuals that are the best adapted to
survive have a better chance of surviving than those
not so well adapted to survive. But Darwin did much
more than appeal to any such generality. He pointed
out that variations occur in all directions; that at
least some of these variations are transmitted; and
that on an average more offspring are produced by
each pair than survive. He appealed directly to a
large amount of biological evidence in support of his
theory.

Since 1859 a great deal of work has been done that
bears on the interpretation that Darwin placed on

NATURAL SELECTION 121

the facts to which he appealed. Before deciding on
the merits of natural selection this evidence must he
examined.

The Measurement of Variation

If we measure, or weigh, or classify any character
shown hy the individuals of a po^mlation, we find
much variahilitv some of which we ascrihe to the
varied experiences that the individuals have encount-
ered in the course of their lives, i.e., to their environ-
ment, hut we also recognize that some of the differ-
ences may he due to individuals having different
inheritances. A few familiar examples will help to
hring out this contrast.

If the leaves of a tree are arranged according to
size {fig. 53), we find a continuous series, hut there

Fig. 53. — Series of leaves of a tree arranged according
to size. (After DeVries.)

are more leaves of medium size than extremes. If a
lot of beans be sorted out according to their weights,
and those between certain weights put into cylinders,
the cylinders, when arranged according to the size

122

EVOLUTION AND GENETICS

of the beans, will appear as shown in figure 54. An
imaginary line running over the tops of the piles
will give a curve (fig, 55) that corresponds to the
curve of probability (fig. 56).

Fig. 54. — Beans put into cylin-
ders according to size of beans.
The cylinders are arranged ac-
cording to the size of the con-
tained beans. (After DeVries.)

If we stand men in lines according to their height
we get a similar arrangement.

The differences in size shown by the individual
beans or by the individual men are due in part to
heredity, in part to the environment in which they
have developed. This is a familiar fact of almost
every-day observation. It is well shown in the fol-

NATURAL SELECTION

123

lowing example. In jigiire 57 the two boys and the
two varieties of corn, which they are holding, differ
in height. The pedigrees of the boys {fig, 58) make
it probable that their height is partly inherited, and

Fig. 55. — A curve resulting from
arrangement of beans according to
size. (After DeVries.)

^26 -2(5 -<5 0 +6 +2(5 +3d

Fig. 56. — Curve of probability. (After Johannsen.)

124 EVOLUTION AND GENETICS

the two races of corn are known to belong to a tall
and a short race respectively. Here, then, the chief
effect or difference is due to heredity. On the other
hand, if individuals of the same race develop in a

Fig. 57, — A short and a tall boy, each holding a stalk
of corn. The short boy holds a stalk of a race of short corn,
and the tall boy one of tall corn. (After Blakeslee.)

NATURAL SELECTION 125

favorable environment the result is different from
what their development would be in an unfavorable
environment (fig. .59) . Here to the right the corn is
crowded and in consequence dwarfed, while to the

n-r<3 n-T-o DtO

SlOi £4'

&r

EjM, M-.

6'0

D-riJ. UrrO,.

58

] U U-<J 0 O Q 0„Q D prrP. □

ii.

l3ycaTio!4

5'4" ^gj" 6;0.6" 57 ■

Fig, 58. — Pedigrees of boys shown in fig. 57.

^

^ , c -s

Fig. 59. — Corn raised under diiferent conditions. That to the
left is spaced, that to the right is crowded. {After Blakeslee.)

126 EVOLUTION AND GENETICS

left the same kind of corn has had more room to
develop and is taller.

Darwin knew that if selection of particular kinds
of individuals of a population takes place the next
generation is affected. If the taller men of a com-
munity are selected, the average of their offspring
will be taller than the average of the former popula-
tion. If selection for tallness again takes place, still
taller men will on the average arise. If selection again
makes a choice, the process may continue {jig. 60) .

Fig. go. — Curves showing (hyi)othetically) how
selection might be supposed to bring about pro-
gress in the direction of selection. {After Gold-
schmidt.)

Now while we recognize that this statement con-
tains an imj^ortant truth, it has been found that it
contains only a part of the truth. Any one who re-
peats for himself this kind of selection experiment
will find that while the average class will often at
first change in the direction of selection, the pro-
cess slows down as a rule rather suddenly {fig. 61) .
He finds, moreover, that the limits of variability
are not necessarily transcended as the process con-

NATURAL SELECTION 127

tiniies even although the average may for a while
be increased. JNIore tall men mav be produced by
selection of this kind, but the tallest men are not
necessarily any taller than the tallest in the ori^rinal
population.

Selection, then, has not produced anything new,
l)ut only more of certain kinds of individuals. Evo-
lution, however, means jDroducing new things, not
more of what already exists.

ft

Darwin's interpretation as to the effect of con-

100

10 11

Fig. 61. — Diagram ilustrating the re-
sults of selecting for extra bristles in
Drosophila melanogaster. Selection at
first produces rapid effects, which soon
slow down and then cease. {After
MacDoweU.)

128 EVOLUTION AND GENETICS

tinned selection in the same direction may seem to
imply that the range of variation shown by the off-
spring of a given individual about that type of indi-
vidual would be as wide as the range shown by the
original population, but Galton's work first made
clear that this is not the case in a general or mixed
population. If the offspring of individuals did con-
tinue to show as wide a range of variability about the
new average as did the original population, then it
would follow that selection could slide successive
generations along in the direction of selection.

Darwin himself was extraordinarilv careful, how-
ever, in the statements he made in this connection, and
it is rather by implication than by actual reference
that one can ascribe this meaning to his views. Some
of his contemporaries and many of his followers,
however, appear to have accepted this sliding scale
interpretation as the cardinal doctrine of evolution.
And in this connection we should not forget that just
this sort of process was supposed to take place in the
inheritance of use and disuse. What is gained in one
generation forms the basis for further gains in the
next generation. Now, Darwin not only believed
that acquired characters are inherited but turned
more and more to this explanation in his later writ-
ings. Let us, however, not make too much of the
matter; for it is not so important to find out whether
Darwin's ideas were as definite on this point as our
own as it is to make sure that oiu* own ideas are clear

NATURAL SELECTION 129

in the light of the more recent and extensive studies
of variation that have been made since Darwin's
time.

Selection and Variation

If, then, all that selection can do is to produce
more individuals of a given type, it may appear that
this part of Darwin's evidence fails to support his
assumption that the observed variability of animals
and plants suffices to furnish selection with its neces-
sarv material. It is here that the mutation theoiv
has a contribution to make. Since 1900 much evi-
dence has been obtained showing that new variations
may appear that transcend the extremes of varia-
tion of the original ty2)e. These, if they are inherited,
are called mutants. In some cases the new type so
far transcends the original type that the extreme
fluctuations of the two do not overlap ; but in other
cases the new type may be nearer to the original one
and the fluctuations of each may overlap.

Darwin knew^ of cases of sudden mutation and
called them sports or monstrosities. He thought that
they could seldom supply materials for evolution
because they changed a part so greatly as to throw
the organism as a whole out of harmony with its
environment. This argument for rejecting extreme
or monstrous forms seems to us todav as valid as it
did to Darwin ; but we now recognize that sj^orts are
only extreme types of mutation, and that even the

130 EVOLUTION AND GENETICS

smallest changes that add to or subtract from a part
in the smallest measurable degree may also arise
by mutation. We identify these smaller mutational
changes as the most probable variants that make a
theory of evolution possible both because they do
transcend the original types, and because they are
inherited. If there are other kinds of heritable varia-
tions than mutants, it seems scarcely possible that they
should have been overlooked; for, many thorough-
going studies of variation have now been made.

Pure Lines

The work of the Danish botanist, Johannsen, pub-
lished in 1909, furnishes the most critical evidence re-
latins' to the inheritance of variations that has as vet
been obtained. There are, moreover, special reasons
why the material that he used is better suited to give
definite information than any other so far studied.

Johannsen worked with a garden bean (Phaseolus
vulgaris nana ) , weighing the seeds or else measur-
ing them. The plant multiplies by self-fertilization.
Taking advantage of this fact Johannsen kept the
seeds of each plant separate from the others, and
raised from them a new generation. When curves
were made of these new groups it was found that
some of them had different modes from that of the
original general population {fig. 62, A-E, bottom
group) . They are shown in the upper groups {A,B,

NATURAL SELECTION

131

C, Dj E) . The general population is a composite of
all the groups.

That his conclusion is correct is shown by rearing

A-E

Fig. 62. — Pure lines of beans.
The lower figure (A-E) gives
the general population, the fig-
ures above give the pure lines
within the population. (After
Johannsen.)

132 EVOLUTION AND GENETICS

a new generation from one plant or indeed from
several plants of any one of these lines. Each line
repeats the same modal class. There is no further
breaking up into groups. Within the line it does not
matter at all whether one chooses a big bean or a
little one — they will give the same result. In a word,
the germ-material in each of these lines is pure, or
homozygous, as we say. The differences that are
found between the weights (or sizes) of the indi-
vidual beans are due to their location in the pod or
in the plant on which they have developed.

Johannsen's work shows that the frequency dis-
tribution of a pure line is due to factors that are ex-
trinsic to the germ-jjlasm. It does not matter then
which individuals in a pure line are used to breed
from, for they all carry the same germ-material.

We can now understand more clearly how selec-
tion acting on a general population brings about, at
first, changes in the direction of selection.

An individual is picked out from the population
in order to test its particular kind of germ-material.
Although the different classes of individuals may
overlap, so that one can not always judge an indi-
vidual from its appearance, nevertheless, on the
whole, chance favors the picking out of the kind of
genn-material sought. In species with separate
sexes there is the further difficulty that two indi-
viduals must be chosen for each mating, and super-
ficial examination of them does not insure that they

NATURAL SELECTION 133

belong to the same groiq) — their gerni-j^lasm cannot
be insjDected. Hence selection of bi2)arental forms is
a precarious process, now going forward, now back-
wards, now standing still. In time, however, the pro-
cess forward is almost certain to take place j)rovided
the selection is from a heterogeneous population.
Johannsen's work was simplified because he started
with pure lines. In fact were this not the case his
work would not have been essentiallv different from
that of any other selection experiment.

It has since been j)ointed out by Jennings and by
Pearl that a race that reproduces by self-fertilization,
as does this bean, automatically becomes pure in all
of the factors that make up its germ-material. Since
self-fertilization is the normal process in this bean
the purity of the germ-plasm of each line already
existed when Johannsen began to experiment.

Ge7ietic Vaiiahility

In addition to the variability due to external fac-
tors acting on the individual during its development
there are also differences in the germ-materials —
genetic factors — that are known to produce slight
differences in the extent to which some particular
part or character develops. Inasmuch as some of
these minor or modifying genetic factors produce
their results only in the presence of the chief char-
acter, thev may be concealed and only manifest their
presence when the chief character develops. The

13i EVOLUTION AND GENETICS

discovery of the occurrence of such modifying ge-
netic factors has gone a long way in making clear
some of the effects of selection — effects that have at
times led some of the neo-Darwinians to assume that
the selection process coidd hring about a change that
causes the organisms to transcend its original type.
Castle stated in 1916: "JNIany students of genetics
at present regard unit characters as unchangeable.
. . . For several years I have been investigating
this question, and the general conclusion at which I
have arrived is this, that unit characters are modifi-
able as well as recombinable. JNIany Mendelians
think otherwise but this is, I believe, because they
have not studied the question closely enough. The
fact is unmistakable that imit characters are sub-
ject to quantitative variation." That Castle was not
carelessly playing fast and loose with the term factor
(gene) and character is shown by the whole con-
text of the entire chapter in which tliis sentence
occurs. It is intended to be understood to mean that
unit characters may not only be altered by the re-
combination of modifying characters, for, Castle
has always recognized this possibility, but also
that the gene (factor) varies quantitatively and that
selection not only produces its results by selecting
larger or smaller genes but in doing so it brings
about progressive and further advances in the direc-
tion of selection. This interpretation is attributed by
Castle to Darwin himself, as another quotation from

NATURAL SELECTION 135

the same chapter shows: "Selection as an agency in
evohition mnst then be restored to the important
place which it held in Darwin's estimation, an
agency capable of producing continuous and pro-
gressive racial changes."

Now the only really critical piece of work done on
this subject, that of Johannsen, had already led to
the opposite result. It can hardly be said, therefore,
that the subject had not been studied closely enough.
The evidence from Castle's own experiments with
hooded rats when studied more criticallv has shown
that it still remains to be proven that genes are sub-
ject to quantitative variations and are amenable to
selection.

Conclusions

The evidence discussed in this chapter is consistent
with the view that the individual gene is not affected
by selection, and that the initial changes commonly
observed when selection is practised on a mixed
population are due to recombinations of the differ-
ent kinds of genes affecting the same character that
are present in most populations. Since these modify-
ing genes behave in inheritance strictly in accord-
ance with ^Mendel's laws there are no grounds for
assuming that they are different from other genes.
Selection ceases to produce any further effects after
these genes have been sorted out and the material
has become homozygous for them.

136 EVOLUTION AND GENETICS

It follows, that if new characters transcending the
extremes of the original population arise, this must
come about through a change in one or more of the
genes themselves. At present we have discovered
only one way in which such a change takes place —
by a mutation in a gene.

Chapter XI

THE ORIGIN OF SPECIES BY
NATURAL SELECTION

The question still remains whether under natural
selection the mutational changes that appear spo-
radically will suffice to sujjply the materials for new
species. Genetics has shown that in all probability
only one gene of a pair mutates at a time. If the
mutation occurs late in the history of the germ-cells,
the mutated gene might be retained in an egg or in
two sperm-cells. If the mutation occurred earlier in
the germ track the mutated gene might, if in a fe-
male, remain in several eggs after the polar bodies
are formed, or if in a male, remain in many sperm-
cells. If a germ-cell carrying the new gene happens
to combine with a germ-cell of another normal indi-
vidual, the cells of the embr^^o so produced will con-
tain the gene in only one of its chromosomes, and
such an individual will not show the character if the
gene is recessive. In this individual, half of the
mature germ-cells will now contain the gene and
half its normal partner. If such an individual mates
with a normal individual, half of its offspring will
carry the gene in only one chromosome of each cell.
Here, for the first time, the new gene is present in
many individuals — in half as many as are produced
by the mating. This process may hy chance be re-

138 EVOLUTION AND GENETICS

Ideated over and over again, bnt sooner or later two
individuals each carrying the gene may mate. One-
fourth of their offspring will then show the new
character, which now appears for the first time. Sev-
eral or many mutant individuals w^ill then suddenly
emerge, and since they are the output of the same
female, their proximity will increase the chance that
two at least mate with each other and produce
progeny with the new character.

These considerations are significant for the selec-
tion theory. They show that there is no danger of a
new mutant being lost at its inception, except in so
far as chance works against the survival of the off-
spring of any one individual. They show also that
a new gene may, by chance, become distributed in
the race before its character comes to the surface. If,
when it appears, the new character is one that better
fits the individuals to some environment at hand,
such individuals have a better chance of survival
and, other things being equal, of leaving offspring.
If one of them mates with an individual of the orig-
inal race the same process takes place all over again,
but as often as this happens, the new gene may
spread in the race at the expense of the old, and may
replace it if the character it stands for is one better
suited to the old environment ; or, if better fitted to
a new environment within reach, it will then give
rise to a new^ type leaving the original type in pos-
session of the old station.

ORIGIN OF SPECIES i39

The integrity of a new gene j^rotects it from being
lost through crossing with the old type, because
there is no blending of the gene with the original one
each time the two are brought together. Darwin
confused here the characters that may blend in the
hybrid with the genes that do not blend. The main-
tenance of the gene's integrity overcomes a serious
difficulty in Darwin's theory of natural selection as
he first formulated it.

Soon after the appearance of the Origin, Fleming
Jenkin — a Scotch engineer — pointed out that single
variants would disappear by "swamping" even al-
though slightly beneficial, if, as Darwin supposed,
blended inheritance is the rule after crossing; for,
the new character will lose some of its advantage
each time it combines with the original type. The
chance is always greater, at first, of a mating with
an individual of the original type owing to the larger
numbers of such individuals. In later editions of the
Origin Darwin acknowledged the force of this ob-
jection and tried to meet it by postulating that the
new character must be already present in a large
number of individuals if it is going to have any-
thing like a chance of sur^dval. But one may ask if
a new adaptive character can appear in so large a
number of individuals that it swamps virtually the
original type, what becomes of the theory of natural
selection? If the adaptive change has already taken
place in so many individuals it is simpler to assume

110 EVOLUTION AND GENETICS

that it might soon appear in all as the result of what-
ever change induced its appearance in so many.

Now this dilemma is to some extent at least met
by the modern theory of the stability of the gene. A
gene is not lost by residing in the same cell with a
gene of another kind. It may, if mere chance favors
its perj^etuation, spread, and once inoculated in the
race it may produce in time enough individuals to
start a new tyj^e. Our modern knowledge of the be-
havior of the gene meets to some extent the difficulty
raised by Fleming Jenkin and reestablishes the
strength of Darwin's theory, but, on the other hand,
it should be clearlv understood that the chance of a
recessive gene becoming widely disseminated is ex-
tremely small even though the character it represents
may be a beneficial one.

The stability of the gene also enables us to under-
stand how a gene, if it is recessive, representing a
character that is even injurious to the race, may be-
come spread, localty at least, in a group without
detriment to the individuals carrviuia: one oene. This
explains the frequent occurrence, in certain re-
stricted populations, of the appearance at times of
certain abnormal types, as seen, for instance, in
night blindness, and "bleeding" in man.

There is another result, clearly established by the
genetic work on Drosophila, that is favorable to the
final establishment of a new type or character if it
is beneficial. JMost, perhaps all, of the mutations ap-

ORIGIN OF SPECIES 141

pear more than once. This improves their chances of
becoming incorporated in the species, and if the
mutation produces a character that favors snrvival
the chance of its becoming estabHshed is still further
increased. But it is also not to be overlooked that
since most of these mutational changes are not bene-
ficial, their recurrence acts as a drag on the race,
because in so far as their genes become disseminated
they give rise to defective individuals whenever two
such genes are brought together. The early death of
defective individuals in the wild state may make their
appearance less noticeable than under the more favor-
able conditions for survival under domestication.

It is sometimes implied that a mutational change
that is dominant has a better chance than a recessive.
This is not the case, however, if the dominant charac-
ter is neither advantageous nor injurious, but neu-
tral. It, then, has the same chance as a recessive. But
if the dominant is beneficial, it has a somewhat better
chance than a recessive, because, since it comes to ex-
23ression from the beginning in the hybrid type, it
improves the chances of that type in comparison
with the original type. If the dominant is injurious
it will be more quickly eliminated than a recessive
character that is injurious.

These theoretical considerations do no more than
suggest certain possibilities concerning the theory
of natural selection. Before we can judge as to its
actual efficiency we must be able to state how much

142 EVOLUTION AND GENETICS

of a given advantage each change must add to give
it a chance to become estabhshed in a population of
a given number. Since only relatively few of the in-
dividuals produced in each generation become the
parents of future generations, numbers count heav-
ily against any one individual establishing itself.
This is a most difficult problem for which we have
practically no data, and as yet only the beginning
of a theoretical analysis has been made of this side of
the selection problem. Haldane has developed a par-
tial analysis of the problem for a few Mendelian sit-
uations. He points out that the problem is extremely
complex and that there is at present not much quan-
titative information to furnish material for such a
study of natural selection by means of gene mutations.

The Diagnostic Characteristics of Species and the
Origin of Species by Natural Selection

It has often been pointed out that the characters
used by systematists to separate species have as a
ride nothing whatsoever to do with the adaptive fea-
tures of species. The latter are largely physiological.
Yet if species have originated through adaptive
modifications, it might be expected tliat physiolog-
ical characters would be the most distinctive ones
that distinguish species from one another.

The solution of this paradox is, I think, to be
found in the many-sided effects produced by each
gene. It has been shown, particidarly clearly in the

ORIGIN OF SPECIES 143

mutant types of Drosophila, that visible, superficial
changes are nearly always accompanied by other
changes tliat have a more physiological aspect, such
as the vigor, or length of life, or productivity of the
individual. From the j^oint of view of evolution
these physiological effects are those of most sig-
nificance, while the superficial changes are trivial in
comparison.

Xow it is highly probable, if definite structural
clianges have definite accompanying physiological
changes, that other nmtations that bring about phys-
iological changes produce, at the same time, superfi-
cial structural effects. If so, we mav find here an
explanation of the constancy of the latter when they
are by-products of important physiological charac-
ters. Hence their constancy and their value as diag-
nostic characters of species.

The study of the mutation process has to a large
extent also concerned itself with superficial charac-
ters, while the concomitant physiological modifica-
tions are referred to only in passing; but from the
evolutionist's point of view it is the internal phys-
iological accompaniments of the superficial effects
that are of much greater significance. It is not sur-
j^rising, therefore, that a good deal of the discussion
of the bearing of mutants on the theory of evolution
may seem rather far afield. If the mutation process
were studied as contributorv to the theorv of evolu-
tion rather than in its genetic bearings we would re-

144 EVOLUTION AND GENETICS

verse our present attitude and study minutely the
effects of each new gene on the changes that it
brings about in the Hfe of the individual and on its
23roductivity. We would then regard the superficial
characters as by-products of the invisible effects, un-
important in themselves and at best only indices of
internal modifications.

Chance and Evolution

When we consider the innumerable physiological
adjustments of any organism, and the many struc-
tural adjustments of the parts of the body to each
other and to the environment, an appeal to evolu-
tion through chance variation may seem preposter-
ous. Stated in this general way the theory of evo-
lution by chance variations seems repellent to the
traditional thinking of many persons. It is this sup-
posed difficulty, I think, that has driven some biolo-
gists and laymen, either to the acceptance of some
sort of external guiding principle responsible for
evolution, or to the assumption of an internal mys-
tical property (entelechy) of living things, or to the
cruder appeal to the inheritance of acquired char-
acters. There is, however, a well known property of
living organisms that puts the theory of chance, as
the sufficient agent in evolution, on a very different
footing from chance as generally imderstood. This
is the property of living things to multiply their
kind indefiniteh% i.e., to reproduce an indefinitely

ORIGIN OF SPECIES 145

large number of individuals with the stamp of a
lucky throw. For example, no one would maintain
that so complex a mechanism as that of a living or-
ganism could suddenly appear by the accidental
coming together of the materials of which it is at
present composed. This is as inconceivable as that
an automobile could develop through the chance
meeting of wood, iron, rubber, oil, and gasoline ; or
to use Paley's old image, that a watch could be pro-
duced by the accidental accumulation of pieces of
iron. The parts of the automobile and of the watch
have been brought together under the direction of a
human agent, but what has brought the parts of the
organism together? The implication in this question
is that there must have been a directing agent of
some sort, since bv chance such a fortuitous com-
bination is inconceivable. The statement ignores
certain properties of living materials that put the
two problems in a different light. These are tlie
property of growth by which living matter can in-
crease indefinitely in volume; the property of mul-
tiplication by which a given sam^^le may duplicate
itself without limit; and the possibility of changes
in the material that furnish new stable conditions.
We may not be able at present to explain fully how
growth takes place, but there is nothing in growth,
as far as known, that is inconsistent with chemical
processes. We may not be able to state in detail how
cells divide, but the purely physical character of the

146 EVOLUTION AND GENETICS

process can scarcely be doubted. We may not be
able to give the cause of a new variation, but we find
nothing in the occurrence of a change that produces
a variation that is inconsistent with chemical or
structural alterations in the germ material. If this
much be conceded, the problem of self-construction
of even a complicated piece of mechanism is not be-
yond our comprehension. At any rate the problem is
obviously different in kind from that of constructing
a mechanism whose materials do not possess these
properties. So long as the processes of division and
growth take place faster than the process of acci-
dental destruction or death, the living material can
maintain itself indefinitely. The stability of such an
organism is no greater, of course, than that of the
chemical material of which it is composed. If this
changes in those parts that have the property of
division and growth (without affecting these prop-
erties) something new will result, a new type, and
if this is able to maintain itself we can imagine at
least something new may be established. It is not
necessary to suppose that all changes will have a
survival value, but only that some of them may. The
alteration may bring the organism into a new relation
with its environment, or through competition with
the old type replace it, or it may make it possible for
the new type to move into an environment different
from that of the original type and hence escape com-

ORIGIN OF SPECIES 147

petition/ Such a process may or may not lead to
greater complexity, but would be an evolutionary
change in any case. It should not ])e overlooked
that only a limited number of living things are rela-
tively complicated structures. An immense world of
apparently simple organisms exists on the earth at
the present time. Evolution has not meant the sub-
stitution of the simpler by the more comj^lex; both
exist side by side today, each standing in a different
relation to the environment, but neither more capa-
ble of remaining in existence than the other.

The phrase natiu'al selection, or its equivalent,
the survival of the fittest, is generally understood to
mean that a new type that appears, being better
adapted to the same environment, displaces the old
type by competition. One new "species" replaces its
parent species. Something new has evolved, and by
implication something "better," i.e., something with
better chances of survival than the original species.
While such replacement of an old type by a new one
through competition may be one of the ways that
new types evolve, it would be erroneous to suppose
that Darwin limited the term in this wav. It would
be unfortunate to identify selection with such an

1 The situation is essentially the same if the new type is fitted to
establish a new relation with a different part of the same original
environment — as when birds developed wings to take advantage of the
air. Such a change may, it is true, lead through competition to a
substitution of the new for the old type, but at other times it may
also remove the new from competition with the old type. Birds for
example have not replaced lizards.

118 EVOLUTION AND GENETICS

interpretation. Darwin by no means restricted the
application of the term natural selection to the sub-
stitution for the parent type of a better adapted new
type. Perhaps it is owing to the various ways in
which he used natiu*al selection — often as a meta-
phor— that it has come to have so many different
meanings and is often confusedly used as synony-
mous with evolution.

Progressive Evolution

It has been pointed out that the power to reproduce
itself puts the problem of the construction of a liv-
ing organism on a different footing from the con-
struction of a complex machine out of inorganic (not
living) material. This question is so important for
the theory of evolution that its significance must
be fiu'ther indicated.

Whenever a variation in a new direction becomes
established the chance of further advance in the
same direction is increased. An increase in the num-
ber of individuals possessing a particular character
has an influence on the f utiu'e course of evolution, —
not because the new type is more likely to mutate
again in the same direction, but because a mutation
in the same direction has a better chance of produc-
ing a further advance since all individuals are now
on a higher level than before. When, for example,
elephants had trunks less than a foot long {fig. 63)
the chance of getting trunks more than one foot

ORIGIN OF SPECIES

149

y"^

il

'■li^

^^HB^^ w^^^^^^

• LLk

mSB^K&^

m

"^'ft^i^ --m

•mQ

:^v ___^

^^r

X*.>.:

*-«*Pt

Fig. 63. — Evolution of elephant's trunk; above Maeritherium,
in the middle Tetrabelodon {After Lancaster) ; below African
elephant (After Gambier Bolton).

150 EVOLUTION AND GENETICS

long would be in proportion to the length of the
trunks already present and to the number of indi-
viduals in which such a character might appear. In
other words, evolution once begun in a given direc-
tion is in a favorable position to go on in the same
direction rather than in another {fig. 64) , so long as

Fig. 64. — Evolution of elephant's trunk. (After Lull.)

the advance does not overstep the limit where fur-
ther change is advantageous.

The duality of the evolution process from the
point of view of natural selection has not always
been sufficiently emphasized. A series of events that
can be given a stricth^ causal interpretation leads

ORIGIN OF SPECIES 151

to the occurrence of a new individual, which, through
other properties inherent in living matter, can re-
produce a grouj) of individuals like itself. Another
and entirely unconnected series of events in the
outer world has produced another situation as when
the land w^as lifted above the water. If the new type
happens to come into relation with the new world
it may perpetuate itself there. This is adaptation —
the fortuitous coming together of the results of two
processes that have developed independently of each
other. The fitness of the animal or plant to an en-
vironment that it finds existing, gives the false im-
pression that its relation to the environment, its
adaptation, has come about through a response to
the environment. The central idea of natural selec-
tion, as generally understood at the present time, is
that the relation is purely fortuitous. The organism
has been produced by one series of events, the en-
vironment by another; the relation of the two is
secondary.

The Dominance of the Wild Type Genes

The genes that arise by mutation have been found
to be largely recessive to the genes already present
in the original type which are said, therefore, to be
dominant to the new genes. If the original genes also
arose by mutation there is no obvious reason why
new genes are not as often dominant as recessive to
the original ones. It may be frankly admitted that

152 EVOLUTION AND GENETICS

at present we cannot give a satisfactory explanation
of this relation if we assume that evolution has come
about by the same kind of processes that we observe
today when new mutants arise. There are, however,
certain considerations that put the situation in a
somewhat different light.

In the first place there is no such sharp contrast
as imjDlied in the statement just made between domi-
nant and recessive genes. Many genes classified as
recessive produce some effect in hybrid combination
on the character most affected.

In the second place if recessive mutant genes may
sometimes revert to the original type (for which
there is some evidence at present but not enough
perhaps to be entirely convincing) it follows that
there may be no essential difference between the
kinds of genes in question.

In the third place it is possible that some or even
many of the commonly observed mutant genes rep-
resent degradation products of the old genes (that
is, simjiler chemical bodies) that are more frequently
produced than more com^ilex bodies. Even if this is
true it does not follow that more complex genes may
not also arise by mutation and some of these might
be dominants to the old gene. At present, however,
this is purely speculative.

In the fourth place it is known that new domi-
nant genes do arise. There need be no necessary re-
lation between the dominance of a gene and an

ORIGIN OF SPECIES 153

increase in the character affected. In fact, while some
dominant mutants add something to the original
character (size or complexity), others diminish the
same character.

In the fifth place it is possible that under natural
conditions dominant advantageous characters have
a far better chance to become established than reces-
sive advantageous characters, because, by definition,
they produce a greater or less effect on the hybrid
and give it an advantage from the start. Tempting
as is such a suggestion, it would be hazardous at
present to use it to exj^lain the observed dominance
of many of the characters of the wild types as com-
pared with the recessiveness of many of the new
mutant types that aj)pear or are preserved under
cultivation.

Chapteh XII

THE NON-INHERITANCE OF
ACQUIRED CHARACTERS^

For more than a hundred years the question has
been discussed as to whether habits and physical
characteristics acquired by an individual during its
life are transmitted to its children. Lamarck's theory
of evolution rests on the assumption that adapta-
tions in the animal kingdom are brought about in
this way. Although Darwin once referred contemp-
tuously to Lamarck's nonsense, which he understood
to imply that adaptation results from the slow will-
ing of animals, he later accepted a view that is in all
essential respects really the same as Lamarck's. In
fact, Darwin went even further than Lamarck in
attempting to explain by means of his hypothesis of
pangenesis how changes in the body might be trans-
mitted to the reproductive cells and reap2:)ear in the
offspring.

Despite the high authority of Darwin's name
there has been a steady falling away from this belief
amonff biologists trained in modern methods of ex-
perimental research. It is true that among stock
breeders and farmers there has always been, and
there is still, a widespread conviction that acquired

1 From The Yale Revieio, July 19:24.

156 EVOLUTION AND GENETICS

characters are transmitted, and in the folklore, both
ancient and modern, of many peoples there are
myths that turn on a belief in the inheritance of such
characters. Phaethon driving the chariot of the sun
over Africa lost control of his father's horses and
coming too near the earth, "it is said the people
of Aethiopia became black because the blood was
called by the heat too suddenly to surface," and they
are black to this dav.

The palaeontologist Cope, an ardent Lamarck-
ian, relates a storv "from that keen observer" Pro-
fessor Eugene W. Hilgard, describing the origin of
the twisted tails of the cats in his neighborhood. A
female ("and very prolific") cat when half-grown
met with an accident that produced a compound
fracture. Her kittens inherited the maternal twist
and found favor in the eyes of their master, described
as "my Chinaman." Cope also relates the following
anecdote on the authority of an educated and reliable
breeder of game fowls : "A game-cock, in his second
year, lost an eye in a fight. Soon after, and while the
wound was very malignant (it never entirely healed) ,
he w^as turned into a flock of game hens of another
strain. He was otherwise healthy and vigorous. A
very large proportion of his progeny has the corre-
sponding eye defective. . . . The hens afterwards
produced normal chickens with another cock. Both
strains had been purely bred for ten or more years.

ACQUIRED CHARACTERS 157

and none of the fowls has been bhnd unless from
fights."

The myths relatmg to prenatal impressions are
the most pathetic of all the inventions of human
credulity, and the}^ are as old and as widespread as
the inheritance myths to which they are closely re-
lated. Jacob's slippery trick with the rods will be
long remembered. "And he set the rods which he had
pilled before the flocks in the gutters in the watering
troughs when the flocks came to drink that they
should conceive when thev came to drink. And the
flocks conceived before the rods and brought forth
cattle ring-streaked, speckled, and spotted." The
world is today filled with old wives' tales of pre-
natal influences. These mysteries, the ill-begotten
offspring of ignorance, have contributed their bane-
ful share to the social inheritance.

It is a strange commentary that, while zoologists
have never met with much success in their endeavors
to trace the origin of structural changes to the in-
heritance of acquired characters, numerous j)i'0"
posals have come from physiologists and psycholo-
gists. There was some consternation in 1923 when
the great Russian physiologist, Pawlow, reported
the results of experiments that go far beyond what
most Lamarckians have dared hope. Pawlow's con-
clusions— and as yet we have only his conclusions —
are very surprising. They can best be given in his
own words :

158 EVOLUTION AND GENETICS

The latest experiments (which are not yet fin-
ished) show that the conditional reflexes, i.e., the
highest nervous activity, are inherited. At present
some experiments on white mice have been com-
pleted. Conditional reflexes to electric bells are
formed, so that the animals are trained to run to
their feeding place on the ringing of the bell. The
following results have been obtained :

The first generation of white mice required three
hundred lessons. Three hundred times was it neces-
sary to combine the feeding of the mice with the
ringing of the bell in order to accustom them to run
to the feeding j^lace on hearing the bell ring. The
second generation required, for the same result, only
one hundred lessons. Tlie third generation learned
to do it after thirty lessons. The fourth generation
required only ten lessons. The last generation which
I saw before leaving Petrograd learned the lesson
after five repetitions. The sixth generation will be
tested after my return. I think it very probable that
after some time a new generation of mice will run
to the feeding place on hearing the bell with no
previous lesson.

Until we have a full account of Pawlow's meth-
ods it msLj be safer to wait before interpreting his
results ; but this is by no means a new topic, for al-
ready the effects of training and its possible inherit-
ance had been examined bv three American investi-
gators who used the most approved methods that
experience has taught are essential in obtaining data

ACQUIRED CHARACTERS 159

of this sort. ]Miss Vicari' has carried out for two
years a careful set of experiments with mice, extend-
ing over four generations. The records of each indi-
vidual and its pedigree were kept. The outcome
shows that no such effects as those reported hy Paw-
low appeared. INIacDowelP also carried out at Cold
Spring Harhor extensive experiments on the possi-
hle effects of alcohol in inheritance as tested by ability
to learn a maze, and, as a control, kept records of
related rats that had been trained by the same tests
used for the alcoholics. His data, recently published,
show no improvement in the offspring of trained
individuals over those not trained. Halsey Bagg^ has
published significant data on mice tested in a maze,
data that cover three generations, and here too there
is no evidence of improvement resulting from
training.

It may be objected that the methods employed
were not the same as those used by Pawlow, and,
that we must wait for his evidence. This is not to be
denied; but, on the other hand, the American data
warn us not to generalize as to the inheritance of
training. Our human experience, too, teaches cau-
tion; for how simple would our educational questions
become if our children at the sound of the school bell
learned their lessons in half the time their parents

2 Science, Vol. LIX, 1934, p. 303.

3 Science, Vol. IJX, 1924, p. 30-2.
^Archives of Psychology, Vol. XXVI, 1930.

160 EVOLUTION AND GENETICS

required! We might soon look forward to the day
when the ringing of bells would endow our great
grandchildren with all the experiences of the gen-
erations that had j)receded them.

The attempt to identify heredity w4th memory
has been made over and over again. The most bril-
liant and irresponsible undertaking of this kind was
that of Samuel Butler in his books on Life and
Habit and on U7ico7iscious Memory. His contention
was, however, neither the first suggestion of the sort,
nor was it to be the last. A few years before him a
German physiologist, Hering, had elaborated this
idea. Today this question has more than an historical
interest, since the memory-heredity theory has never
been without an advocate. Books continue to be writ-
ten about it. Orr in this country advocated something
of the kind, but was rather vague in his applications.
Semon in Germany invented a full terminology, for
his Mneme. Rignano in Italy attempted to give it a
more physical expression, as indeed had Haeckel
much earlier. Ward in England has spoken as a
philosopher in its favor, and Bernard Shaw as a
dramatist.

The comparison between heredity and memory
has taken protean forms ; none of its advocates being
able to do more than throw out suggestions as to
what sort of "identity" they were talking about.
Fantasy rather than prosaic science is the character-
istic feature of all these theories.

ACQUIRED CHARACTERS 161

That these speculations have produced ahiiost no
effect on present biological thought is not surpris-
ing, for a moment's consideration will show that, at
best, the basis for the comparison between memory
and heredity rests only on a vague analogy. In each
case something appears and reappears. In the one
case, a memory of the past in the brain as we say;
in the other case, a repetition of a similar type of
behavior in successive generations. It is tacitly im-
plied that because memory is a familiar process to
us we must know more about it than about heredity.
The fact, however, is that memory is one of the many
obscure fields of human j^sychology. It is today more
obscure to us than is heredity itself. Are we not jus-
tified, therefore, in looking askance at attempts to
account for a phenomenon taking place in one realm
of observation by an appeal to another, less w^ell
understood? It is not an exaggeration to say that
some of those who have propounded memory the-
ories of hereditv have never been in close touch with
the facts of heredity and development that are fami-
liar to students of these subjects. Our present knowl-
edge of the relations of parent to offspring is so
different from anything ever imagined by the mem-
ory advocates, that their speculations a^^pear to the
zoologist as crude as they are often grotesque.

During the last quarter of the last century, one of
the most important branches of biology came to frui-
tion. The microscopic study of cells and eggs and

162 EVOLUTION AND GENETICS

their relation to development and inheritance, made
great advances and cleared up many obscure ques-
tions. These observations were carried out in com-
plete independence of the speculations concerning
heredity that had gone before ; and the outcome has
furnished a starting point for further interpreta-
tions that have led in our own time to far reaching
discoveries. It is not possible to give here even a
summary of the evidence, because its understanding
requires familiarity with microscopic observations
covering a very wide and unfamiliar field. But, in
general, I may state that the work has led to the
conclusion that the properties of the reproductive
cells which are responsible for the characters of the
body, are inherent in these cells ; and that the trans-
mission of these properties is independent of the
body-cells, and calls for no interference from them.
This is summed up in the phrase "the isolation of the
germ-plasm." The principal idea that this familiar
phrase is intended to convey is exactly the opposite
of that implied in the inheritance of acquired char-
acters. The individual starts as an ^^^ which is itself
a cell. The ^^^ divides and ^^roduces a vast number
of cells essentially like itself. ]Most of these cells be-
come changed, as development proceeds, into the
tissues and organs of the body, but a few of them
remain as the reproductive cells of the individual in
which they live. Here they multiply to become each
in turn the beginning of a new individual with its

ACQUIRED CHARACTERS 163

contained eggs. In a word, the egg produces the
body — not the body the egg.

All this is now conceded by everj^one familiar with
the evidence ; but two further points are open to dis-
cussion. The first of these involves the possibility
that the germ-cells may be affected by the vicissi-
tudes of the body-cells, so that when their turn comes
to produce a new individual they reflect in some way
the changes that have been impressed on the body-
cells. If this takes place, the inheritance of acquired
characters would not be incompatible with the cell
theory although extraneous to the theory. The sec-
ond point relates to the possibility that the changes
in the external world that affect the body may pro-
duce a corresponding change in the germ-cells. No
amount of argument or a priori reasoning is likely
to settle these problems ; but fortunately there is at
the present time a large body of evidence, and some
of it experimental evidence, that is significant, and,
I think, convincing. Here, if anywhere, we may hope
to find proof on which to base a reasonable judgment
of the situation. To this evidence, then, I propose to
appeal.

The evidence is of various sorts, and mav be
roughly grouped under several headings. First, that
of the supposed inheritance of use and disuse. This
takes us back to Lamarck, but while he rested his
case on generalities that were often fantastic, such as
the origin of the giraffe's long neck, there is now a

164 EVOLUTION AND GENETICS

good deal of evidence that is significant and un-
favorable. Darwin explained the eyeless condition
of many cave animals as a result of disuse. Recently
Pa3aie has bred fifty generations of flies in total
darkness and has found that their reaction to light
had been in no way impaired. Darwin suggested that
the wingless condition of some insects living on
islands was due in part to disuse. Now, there have
appeared in our laboratory cultures of flies raised
in milk bottles, of three different kinds having no
wings. These appeared as single individuals with the
wings entirely absent, from j)arents whose wings
had not decreased visibly in size in their long con-
finement. Each of the new types arose by a muta-
tion; and the inheritance of the wingless condition
shows that they owe their peculiarity to a change in
a single hereditary element, and are, in this respect,
comparable to the four hundred other mutant types
that have also arisen, whose new characters have no
conceiA^able relation to their confinement.

It is more difficult to obtain definite information
as to whether or not the use of a part that increases
its size or improves its functions is inherited. Imagi-
nary cases of this sort are abundant, but since
other explanations will cover them they do not serve
our present j^urposes. There are no measurements,
so far as I know, to prove or to disprove the claim
that the children of blacksmiths have stronger arms

ACQUIRED CHARACTERS 165

than other children, or that the children of football
players have bigger legs.

William Brewer suj)j)osed that the speed of trot-
ting horses was due chiefly "to better training but
also in part to special exercise of function." Later
Caspar Redfield insisted that the wisest sons have
been born to the more aged fathers, and that the rec-
ords of racing horses show that the fastest colts have
come from parents that have been trained for rac-
ing; but his statistics will not stand the scrutiny of
an actuary. Pearl has shown the fallacies that lie
concealed in his premises.

The loss of a part is supposed in popular tradi-
tions to lead sometimes to its absence in the off-
spring. The typical example is that of the cat whose
tail was pinched off by a closing door. Her kittens
were tailless. There are, I believe, authentic cases of
this sort, but it is also true that unpinched cats often
have tailless kittens. In fact there is a special breed of
these cats which when crossed to other cats transmit
their peculiarity, and since from the nature of things
the paternity of cats in general is always 023en to
suspicion no great weight is to be attached to an oc-
casional accident and the occurrence of tailless kit-
tens— except in so far as it illustrates a curious
faculty of the human mind to draw f)remature in-
ferences. In rebuttal to the cat anecdotes it should
be pointed out that some races of dogs and sheep

166 EVOLUTION AND GENETICS

have had their tails removed for generations and
that puppies and lambs are born still with tails. Both
Cope and Weismann cut off the tails of mice for sev-
eral generations without producing bobt ailed mice.
We do not have to go to the lower animals to get
evidence. The several kinds of mutilations and re-
movals that man has practised on his own body for
centuries have left no permanent record on the race.

From removals to distortions is a distinct step,
since it has been said by some of the Lamarckians
when pressed for evidence of the inheritance of loss
of parts, that, after all, the part is gone, and could
not be supposed to transmit its absence. This eva-
sion does not cover the case when a distortion is in
question. The stock case is the flat fish, which, ac-
cording to Cunningham, owes its asymmetry to the
habits acquired by its ancestors that came to lie on
their sides at the bottom of the sea. One eve was
thereby put out of commission, but, as a result of the
muscles pulling it over so that it could peep around
the corner of its own head and look up, the eye
slowlv shifted "in time" until todav it too lies on the
side of the head that is upj)ermost — otherwise, of
course, it would have been expected to degenerate.

We do not have to go to Eocene times for evi-
dence. Chinese women of high caste have had their
feet bound and deformed for many generations, and
now that the custom is being abandoned the children
do not appear to have feet different from those of

ACQUIRED CHARACTERS 167

other Chinamen. Nearer home we do not observe
the effects of the corsets of our grandmothers on the
size of the waists of our children.

Several years ago a famous French physiologist,
Brown- Sequard, described some interesting facts
about epilepsy and malformations in guinea pigs
that he interpreted as due to the inherited effects of
surgical operations. At the time, these experiments
aroused great interest, and were much discussed, by
zoologists at least. The operations have been re-
peated on rather a large scale and offspring ob-
tained, but with results so inconclusive that Se-
quard's work is largely forgotten, and not often
quoted by those who themselves have new^ claims to
bring to the attention of the public.

If we turn now to the experimental evidence of
more recent date, we shall find several instances
where induced changes have led to deformities and
malformations which may "reappear" in the next
generation, and hence may be said, in a sense, to be
inherited. But the storv they tell leads to a very dif-
ferent interpretation from the popular one of the
inheritance of acquired character ; and while it is not
entirely clear sailing, yet the general trend of the
work is instructive and furnishes, I think, more than
a hint as to the way in which some of these results

ft''

may have been produced.

I refer to the experiments of Stockard on the in-
fluence of alcohol, of Guyer on the influence of anti-

168 EVOLUTION AND GENETICS

lens serum ; of Griffith and Detlef sen on the effects
of long continued rotation; of Bagg and Hanson
and Little on some of the effects of radium and of
X-rays. To give a fair treatment of the interesting
results that have come out of this work would re-
quire a detailed account of the special conditions
involved in each case. To make a generalized state-
'ment that would cover them all would undoubtedly
mislead the reader. I shall attempt, therefore, a com-
promise between these extremes.

JNIany of the facts can be accounted for on the
view that the reproductive cells have been directly
injured by the treatment, and since there is evidence
that the chromosome mechanism is the basis for the
transmission of the hereditary elements, one may
even go further and suggest that the chromosomes
have been altered. Xow, embryologists have been
familiar for a good many years with the injurious
effect of alcohol, of X-rays, and of radium on the
chromosomes in causing irregularities in their dis-
tribution, and with the consequent injurious effects
on the developing embryo, so that one need not go
far afield to find evidence in support of the view that
injuries produced on the germ-cell may affect the
individual that comes from it. How far the injuries
induced by these agents are sj)ecific, and how far
general is difficult to state at present ; but since, as
Stockard has pointed out, the organs affected are

ACQUIRED CHARACTERS 169

just those that are most subject to injury when eggs
are treated by many kinds of reagents it appears
that the results are general rather than sj^ecific. The
organs affected are the most delicate j^arts or the
parts that require in their develo23ment the most per-
fect adjustments. I am also inclined to favor such a
view, which, if established, may explain why alcohol,
and X-ravs, and radium show their effects most
often in the malformations of the eve.

The more difficult task remains to attemj^t to ap-
praise those results in which a highly specific effect
is claimed to have been produced. Guyer's experi-
ment easily comes first in this respect. He removed
the lenses from the eyes of rabbits, crushed them, and
injected the mash into fowls. After a time the blood
of these birds was injected into pregnant rabbits.
The lenses of the offspring were often opaque and
other abnormalities also appeared in their eyes. The
effects were transmitted to later generations both in
the male and female line. Here we have apparently
a straightforward case of specific inheritance, unless,
indeed, the injected serum is supposed to have af-
fected not only the eves of the embryo but their
germ-cells also. Crucial experiments w^ould settle
this point, but as yet they have not been forthcom-
ing. Guyer's experiment has been recently repeated
by Finlev and also by Huxley and Carr- Saunders
with entirely nee^ative results. We can safely w^ait,

170 EVOLUTION AND GENETICS

therefore, until further and more critical evidence is
obtained as to the nature of the effect, if any, that
was induced in Guyer's experiment.

The next best case is that of Griffith and Detlef-
sen. Rats were rotated for several months in cages.
Some of the young born outside the cage showed
irregularities in their gait, and when tested gave a
different and specific response according to whether
their parents had been rotated to the right or to the
left. Detlefsen states that the disequilibrated rats
showed frequent pathological sequelae, such as dis-
charges from the ears; and this, he says, raises the
question "whether Griffith has not merely presented
us with numerous specimens of some vertebral dis-
ease." The disease once begun might be contagious,
but he adds, "It is difficult to compromise this hy-
pothesis with Griffith's contention of specificity."

This brings us finally to a point where something
more definite may be said and therefore said briefly.
Blakeslee and Belling have shown that if, during the
maturing of the reproductive cells of a flowering
plant, the common jimson weed, the jDlant is sub-
jected to cold, the germ-cells may be so affected that
the distribution of the chromosomes is on rare oc-
casions altered, and a jDlant may be produced that
has double the normal number of chromosomes.
This change carries in its wake some corresponding
changes of character. Changes of both these kinds
often take place when the egg is not treated, and

ACQUIRED CHARACTERS 171

thev are transmitted in the same wav, so that, at
best, the special environment inducing them can only
be said to make their occurrence more frequent.

Finally there is a considerable body of evidence
showing that characters, whose development is
known to be affected by environmental influences
(which therefore might be supposed to be the very
best kind of material to exhibit the effect of acquired
characters) are not affected by the changes induced
in their parents by the environment. There are sev-
eral striking cases of this kind that have been met
with in the course of our experiments with vinegar
flies. There is a race of these flies that have been
long inbred, in order to make them uniform in a
genetic sense, in w^hich the eves are entirely absent
in most individuals, but occasionally one or both
eyes may be present much reduced in size. If the
flies that have these small eves are bred to each other
they arive exactly the same results as when their eye-
less brothers and sisters are bred together. As each
stock culture gets older, more and more of the flies
that emerge have eyes, and, towards the end, an in-
creased number of the flies have both eyes present
and almost full size. If some of these are used as the
parents of a new generation, the results obtained are
precisely the same as when eyeless flies are used.
What better evidence could we hope to obtain to
show that the presence of a character in the individ-
ual has no influence on the reproductive cells i This

172 EVOLUTION AND GENETICS

case does not stand alone but is duplicated by simi-
lar evidence from other characters subject to en-
vironmental changes in these flies, namely, bar eyes,
abnormal abdomen, and extra legs, all of which are
greatly affected by the environment, but the effects
are not transmitted. Is it surprising, then, in the
light of these detailed and controlled data that we
should look askance at claims which pretend to
demonstrate the inheritance of acquired characters
from observations that are in most cases inadequate
to prove the point at issue ?

The experiments that Kammerer has carried on
for several years relate, for the most part, to the
kind of characters which I have just mentioned. He
finds that salamanders spotted with black and yel-
low change to more black or more yellow individuals
if kept on a black or a yellow background. Their off-
sj^ring reared on a neutral background show, he be-
lieves, some influence of the effects produced on their
parents, and so on. Until these results are repeated
on material that is more thoroughly controlled, or
on material where the effect produced can be stated
in measurable terms and not by pictures of selected
material, it is in my opinion better to suspend judg-
ment in respect to their interj^retation. The careful
work of Herbst that was undertaken to check up
Kammerer's evidence has so far found no justifica-
tion for Kammerer's conclusion. ^luch of the otlier
work that Kammerer has brought forward as evi-

ACQUIRED CHARACTERS 173

dence of the inherited effect of the enviroiiiiieiit is
open to the same objection — the inheritance of color
changes in hzards, the change in the breeding habits
of the midwife toad, and the development of horny
pads on the thumbs of the male. That the environ-
ment causes changes in some of these characters
need not be questioned, but that the effects produced
are transmitted to the next generation, through the
bodily changes produced, may be questioned, both
because of the inadequacy of the evidence and also
because in other cases where the materials are suit-
able for making such tests there is no evidence that
such influences produce such results. Perhaps the
most careful and thoughtful piece of analytical
work that has been done in this field is that by
Sumner, extending over five years, on the effect of
heat and cold on the length of the tail, ears, and feet
of white mice, as well as on the increase in the thick-
ness of the hair in the cold.

Some of the mice were reared from birth in a cold
room, others in a warm room. The average difference
in temperature was eighteen degrees centigrade.
The tails of the mice in the warm room series were
longer than the tails of those in the cold, for mice of
the same body length. The length of the feet and of
the ears was also greater in the warmer room, al-
thouerh the effect of the cold on the ears was incon-
stant. These two kinds of mice were then brouglit
together in a common room of intermediate tem-

174 EVOLUTION AND GENETICS

perature, where each series was bred separately,
and measurements were made of the offspring when
the mice were full grown. It was found that the tail,
foot, and ear length were greater in the mice of
warm-room jDarentage than that of cold-room par-
entage but the difference was not so great as that
between their parents.

This may be interpreted to mean that the smaller
increase shown by the tails of these mice of the sec-
ond generation from warm-room parents was due to
the intermediate temperature in which they were
reared, while their length, which was greater than
that of the mice of cold-room parentage, was in-
herited from the warm-room parents. But how?
Was it the effect of cold on the germ-cells, or did it
come from the longer tails of their parents? It is not
easy to imagine that the effect was due to the direct
influence of the cold on the germ-cells since mice are
warm blooded and maintain a nearly constant body
temperature when adult, and as young mice they
were kept warm in the nest and by the brooding of
their mothers. JMust we then conclude that the germ-
cells are so sensitiye to slight differences in the size
of the organs of the body that the effects are shown
in the next generation? If so, might we not expect
that all individual differences would reappear in the
characters of the offspring?

But this question, at least, has now been settled by
Johannsen's brilliant analysis on the non-inheritance

ACQUIRED CHARACTERS 175

of individual differences that are called forth hv the
environment. His experiments were carried out on
material that was adequate to give a crucial answer
to the question involved. In support of Johannsen's
conclusion there is an extensive hody of genetic evi-
dence which can he interpreted as meaning that
while much individual variability is due to minor
genetic factors, and this is inherited, some individual
variability is due to the environment and this is not
inherited.

Is it possible, then, that Sumner's results were due
to chance, in the sense that the two series happened
to give the averages shown ? It does not seem prob-
able that this w^as so, but we can never be certain
until the experiment is re23eated on material that is
first made pure for factors involving the length of
the parts to be studied. Sumner is himself very cau-
tious in his interpretation of his results. He says,
"At no time have I declared my results to be proof
of, or even evidence for, the inheritance of acquired
characters. Indeed, I have insisted that in the pres-
ent state of our problems this latter expression has
become ho^^elessly obsolete. As regards the various
possible interpretations of my own results I have
always expressed indecision."

Castle and Phillips j^erformed an experiment on
guinea pigs that w^ould be exj^ected to show the in-
fluence of the body on the germ-cells if such effects
are 2)ossible. The ovary from a black female was

176 EVOLUTION AND GENETICS

transplanted into a white female whose ovary had
been removed. After the transplanted ovary had
established itself, the white female was bred to a
pure white male. The offspring were black, although
the mother and the father were white. The eggs had
not been affected by their sojourn in the body of a
white individual.

It is not as widely known as it should be that
most of the assumptions of the Lamarckians contra-
dict the fundamental principles of JNIendel's law of
heredity. Mendel's law of segregation states that
the hereditary elements received from the parents
separate in the germ-cells of the offspring without
having affected each other, and, hy implication,
without having been affected by the character of the
individual in which thev were contained. An ex-
ample will make this statement clearer. Suj^pose a
white mouse is bred to a wild gray mouse. The hy-
brid offspring will be gray. If two such hybrids are
bred together they give rise to gray and to white
offspring in the ratio of three grays to one white.
This ratio is understandable if in the hj^brids half
of the reproductive cells carry the element for gray
and half that for white. Thousands of instances of
this sort are known today. To reject the evidence
would be scientific suicide; to refuse to accept the
theorv would be to throw reason to the winds. Men-
del's j^ostulates concerning the clear separation of the
elements of heredity mean that the white-producing

ACQUIRED CHARACTERS 177

elements in the gray hybrids have been unaffected
by the gray color of the hair of the animal that car-
ries them.

JNIan}^ similar tyjjes of inheritance in man tell
the same story. A blue-eved man marries a brown-
eyed woman, and if she has come from a race pure
for brown eyes, all the children will have brown
eyes. If an individual of this parentage marries an-
other with a similar parentage their children will be
brown-eyed and blue-eyed as three to one. Still an-
other case may seem more impressive since the char-
acter involved is one that dominates the normal and
may appear therefore as something more positive
in its nature. There is a tyj^e of malformed hand in
which the middle segment of each finger is missing.
If a short-fingered man marries a normal handed
woman half of the children will have short-fingered
hands and half of them will be normal. The expla-
nation here is the same as before. The man was a
hybrid (his father had short fingers and his mother
was normal) , hence he produces two kinds of repro-
ductive cells. When he marries a woman w^hose re-
productive cells are normal, two kinds of offspring
are expected and two kinds are found. It may be
added that the normal children show no trace what-
soever of the influence of the hand of their short-
fingered father and never transmit this deformity
to their descendants — the separation of the elements
in the dominant parent has been clean.

178 EVOLUTION AND GENETICS

It is scarcely necessary to elaborate this theme.
The facts are not disputed by any student of genet-
ics who is familiar with the evidence ; and they fur-
nish, in my judgment, convincing disproof of the
loose and vague arguments of the Lamarckians.

The "will to believe" in the inheritance of ac-
quired characters is widespread and an interesting
feature of human behavior. The eagerness with
which each new claim is listened to is only too fa-
miliar to those who concern themselves with evolu-
tionary controversies.

The willingness to listen to every new tale that
furnishes evidence of the inheritance of acquired
characters arises perhaps from a human longing to
pass on to our offspring the fruits of our bodily
gains and mental accumulations. While every scien-
tific investigator has sympathy for this human weak-
ness, he cannot allow it to influence him in his exami-
nation of the facts as they actually exist. In our

•- «■'

hope for the best we forget that we are invoking a
principle that also calls for the inheritance of the
worst. If we cannot inherit the effects of the train-
ing of our parents, we escape at least the inheritance
of their misfortunes. A receptive mind may be a
better asset for the child than a mind weighted down
from birth with the successes and failures of its
ancestors.

Chapter XIII

HUMAX IXHERITAXCE

A LARGE number of malformations in man have been
shown to be inherited. In the medical literature there
are hundreds of family pedigrees in which one or an-
other defect appears in successive generations, espe-
ciallv when the stock has been rather closely inbred,
or where the defect is a dominant one. The few books
in which these cases of human inheritance have been
collected may give the impression that our knowl-
edge of man's heredity is mainly concerned with the
transmission of his defects. The eugenic programme
or recommendations with which these treatises usu-
ally wind uj) may give the impression that our chief
concern with human inheritance relates to the elimi-
nation of the defective materials (cacogenics) that
have become incorporated in the s^^ecies, rather than
with the discovery of superlative human materials,
their preservation and perpetuation (eugenics) . All
this calls for comment.

General Statement

In extenuation of the depressing effect of such
literature it may be said that malformations are util-
ized for genetic work not because of their intrinsic
interest — although to the pathologist they are in
themselves important — ^but because, as in other

180 EVOLUTION AND GENETICS

species, they furnish clear-cut characters that are
sharply separated from normal structures, and hence
can be traced through successive generations. It may
properly be claimed that in studying the inheritance
of each defect we are also studying the inheritance of
a normal character that forms the other member of
the contrasted pair. This statement, however, calls
for an important reservation ; for, all that we mean
by such a contrast is that the "normal" is not the ab-
normal. We do not in reality know anything more
than this. This relation is, however, inherent in all
Mendelian contrasted character-pairs, unless mem-
bers of an allelomorphic series are somewhat more
specific.

The presence of malformations of the body in
human stocks is not supposed to be due to a greater
tendency in the human species than in other species
to produce, de novo, defective mutants, but rather
to be due in man to the preservation of individuals
having such characters and allowing them to marry,
or at least not preventing them from mating. The
higher ethical standards of man lead him to preserve
human life, and in the absence of severe competition
(through which the maladjusted would go under)
the defective child reaches maturitv.

If the new character is recessive its gene may
become widely disseminated in the human germ-
material before two individuals each with the reces-
sive gene mate. One-fourth of their offspring will

HUMAN INHERITANCE 181

then show the defect. Bv the time this has occurred
the gene may have become so widely spread that the
most that can be done — if something is to be done —
is to discourage the defective individuals that have
reappeared from further contamination — a council
of perfection that may not be appreciated.

When the defect is dominant it will appear in half
the offspring^ if marriage with a normal person oc-
curs. The other half of the children that are normal
do not transmit this dominant defect, and have, so
to speak, escaj^ed entirely from the inheritance.

There are, however, a number of cases where the
defect is not perfectly dominant. This means that its
variabilitv is so wide that a few individuals that
carry the gene may fail to show it, or show it to such
a small degree as to escape casual examination.
There are, apj)arently, cases of this sort which have
been utilized by the opponents of the modern the-
ory of heredity as justification for the statement
that such a character does not show "strictlv ]Men-
delian inheritance." Unless suitable tests are made
it is not possible to claim that extreme cases of this
sort are exceptions to Mendelian inheritance, for,
similar cases are known in other animals, and have
been shown not to be exceptions, but due to the wide
variability of the hybrid character.

Aside from the major physical defects there are
manv smaller ones that do not interfere seriouslv

1 Assuming the parent is heterozygous for the character.

182 EVOLUTION AND GENETICS

with the welfare of the individual, or which can he
corrected by modern appliances or operations. We
have far less information concerning the inheritance
of these blemishes, and at present can seldom be
certain that they are inherited or how they are in-
herited. Only by comparison with the better-known
cases can we surmise that inheritance may play a
role in many of them. Since the disadvantages that
follow are slight, or can be corrected, these defects
have little practical importance, which need not,
however, detract from their theoretical interest.
Other ways than elimination by means of artificial
selection have been found to standardize individuals
that show these slight dej^artures. Corrective surger}^
has proven a more efficacious remedy in man than
the slow process of selective breeding.

In genetic work each mutant type (defective or
otherwise ) is contrasted with the original type from
which it came, sometimes called the normal type. In
man and in some of the domesticated animals there
is no standard or original type with which to make
such a comparison ; and opinions may even differ as
to what is to be regarded as a normal type, each race
having probably a standard of its own. The situation
is the same in several domestic races of animals and
garden plants. In some, the new mutant genes have
entirely replaced the original genes, i.e., the race has
become pure for certain mutant genes. Whenever
the original genes have been replaced byinutant genes

HUMAN IXHERITAXCE 183

it is not possible to recover the old ty2)e, but if two
races have independently arisen from the same orig-
inal wild type, and have accumulated different sets
of mutant genes, it is still possible to obtain the orig-
inal type by crossing in so far as each has retained
some of the original genes. For example, there are
two white breeds of fowls which when crossed pro-
duce offspring show^ing the colored plumage of the
wild jungle fowl. Similarly there are two white races
of sweet peas which if crossed give the color of the
purple Sicilian w41d pea from which our cultivated
forms are said to have come. Some of the races of
mankind have been long separated. It might seem
possible to recover the type from which they have
departed by crossing them. Racial crosses have been
made frequently, and the hybrids described, but
there is no way of determining how far the outcome
represents in certain respects the common ancestral
type, and how far it is due to the interacting domi-
nant factors of the combination. Possibly it might
be supposed that the mulatto, w4th a yellow skin,
that results from the white-negro cross represents
the type of skin color from which both white and the
negro races have diverged. If so, a yellow race
breeding true to that color might be obtained after
the white and the black genes had been replaced by
the original yellow genes. As yet there is no certain
record of such a consummation, although the defi-
ciency of white-skinned and black-skinned offspring

184 EVOLUTION AND GENETICS

from mulattos of later generations might in some
few cases be accounted for in this way. On the other
hand the intermediate color of the mulatto of the
first generation might be due to the interaction of
the incompletely dominant genes of the two parents,
and in later generations it might not be possible to
distinguish by inspection alone such a condition from
that due to the restoration of the original normal
genes. It would require elaborate genetic tests to
settle such a question.

Meanwhile we shall have to rest content with the
admission that there is no single type of human nor-
mal individual with which to standardize the differ-
ent racial tj^pes. At best, cases of human atavism
produced by crossing, would be expected to go no
further back than the race to which the modern types
of men converge, and from an evolutionary point of
view this is a very recent event. We should antici-
pate, therefore, that all the races of mankind have
an enormous number of genes in common and only
few that are different. The latter produce the rela-
tively slight structural differences that are found in
different races.

The Inheritance of Physical Defects

A few examples of the inheritance of physical
characters in man will suffice to show that in his in-
heritance man conforms to the same laws that regu-
late the inheritance of other animals and plants.

HUMAN INHERITANCE

185

Fig. 65. — Two Brachydactyl hands. (After Farabee.)

Fig. 66. — An X-ray photo-
graph of the hones of a
Brachvdactvl hand.

There is a rare abnormalitv of the hand and foot
known as brachydact^dy, or short-fingeredness {figs.
65, 66). Farabee has recorded the history of an

186

EVOLUTION AND GENETICS

American family of this sort, and Drinkwater that
of a British family, and later of a second family that
migrated from America back to England. As seen
in figure Qo, the fingers of the brachydactyl hand
are short owing to the absence of the middle segment
of each finger. The fingers are about half normal
length. A short-fingered man marrying a normal
woman transmits the defect to half of the children.
The character is dominant {jig. 67). There are no
recorded cases of the marriage of two short-fingered
persons and the pure (or double) dominant char-
acter is unknown. There is a possibility that such an
individual might not be viable.

OB

^ t^~i I

S^

I

II

III

cS^rtrt^mMl

V

Fig. 67. — Pedigree chart of the inheritance of Brachydactyly.
{After Farabee.)

Drinkwater has also recorded other cases of minor
brachydactvlv in which the finoers are less short-
ened. Several other cases in which one or more of

HUMAN INHERITANCE 187

the fingers are short have been descri})ed. One such
case — also a dominant — has been traced by JNIohr
and Wriedt through six generations which carries
the pedigree to the year 1764. Here the shortening
involves mainly the forefinger (fig. 68) .

Fig. 68, — Four pairs of hands showing a shortened
condition of the fore-finger. {After Mohr and Wriedt.)

Color-blindness is a sex-linked recessive character
in man. A color-blind man married to a normal
woman has only normal daughters and sons; all of
the daughters, however, transmit color-blindness to
half of their sons (fig. 69) .

Color-blind women are rare, because they can
never arise unless a color-blind man marries a
w^oman who is color-blind, or else marries a normal

188 EVOLUTION AND GENETICS

woman who had a color-bhnd father, or had a
mother heterozygous for color-blmdness {fig. 70) .

The pedigrees of color-bhnd famihes — and they
are many — leave little doubt as to the mode of in-
heritance of this character {fig. 71).

Accepting this evidence as on the whole satisfac-

XX? XY(/

XI?

XYc/

Fig. 69. — Diagram to show the inheritance of color-blindness
in man. The eye that can distinguish red from green is here half
black, half barred, while the color-blind eye is stippled. A color-
blind man mates with a normal woman. The sons and daughters
are normal. Two individuals of such i)arentage give three nor-
mals to one color-blind individual in l\. The color-blind indi-
vidual is always a male.

tory, there is still something more to be said. As is
well known, there are many grades of color-blindness.
We do not know whether these grades are due to
individual, non-genetic, variations — assuming it to

HUMAN INHERITANCE 189

be due to one gene; or whether there are several
genes that differ in the degree to which they produce
the defect. We know of a good many cases in other
animals where there are several mutations of the
same gene. For instance, in Drosophila there is a

I?

XYc/

Fig. 70. — The designations as in fig. 69. Here a color-blind
woman mates with a normal male. All of her sons are color-blind,
her daughters have normal vision (but carry a factor for color-
blindness). In the F.^ generation, half the daughters and half the
sons have normal eyes and half are color-blind.

series of ten such multiple allelomorphs for eye
colors that range from pure white to deejD wine-red.

There is still another possible interpretation of
the different kinds of color-blindness — one which
a priori would seem to be the most probable —

190

EVOLUTION AND GENETICS

namely, that the differences are due to other modi-
fying genes that affect the extent to which the
character develops.

mo

1 i 1 i L,

oAii

«

DtO ChO

i^i

aioinii

t^

Fig. 71. — Two pedigrees for color-
blindness. {After Lenz.)

Blue eyes in man behaves as a recessive to brown
eyes. Two blue-eyed parents have only blue-eyed
children {fig. 72). Pure brown-eyed individuals

HUMAN INHERITANCE 191

have only brown-eyed children {fig. 72), but a
brown-eyed mdividual, one of whose parents had
blue eyes, married to a blue-eyed individual has both
blue- and brown-eyed children in equal numbers ac-
cording to Mendelian expectation. In recent years a

Pi

brbf

Fig. 72. — Diagram to illustrate the inheritance of blue eyes (to
the left), and of brown eyes (to the right).

few cases have been recorded where two blue-eyed
parents have had some brown-eyed children, and
this has furnished the opponents of Mendelian
inheritance with an argument against the general
application of Mendel's theory. Such cases are, how-
ever, only an argument against an overstatement
of that theory as always applying to apparently
blue-eyed individuals. It is known that occasionally
blue-eyed individuals have only a speck of brown
pigment in their eyes. They may then produce some
brown-eyed children. In other words, the hybrid
brown eye-color is variable in extent, and at one ex-
treme shows almost no brown color or possibly none
at all, and yet is genetically brown-eyed. That this
is the true explanation is shown by the pedigree of

192 EVOLUTION AND GENETICS

these genetic browns, for, so far as recorded they
have had at least one brown-eyed parent. In other
words, in extreme and exceptional cases possibly
due to weakness or disease the brown eye-color may
not develop in an individual that is genetically a
brown hybrid. This failure of the somatic character
does not affect the brown-producing gene, for, such
individuals behave in inheritance as hybrid brown-
eyed individuals.

Albinism in man has been known for a long time
and the earlier records of white Indians seem, in the
light of recent discoveries, not to be mythical. In
all races, including negros, albinos are known. It
has been estimated that this occurs once in 5,000 to
once in 30,000 individuals. It is possible that there is
more than a single kind of albino due to mutation in
different genes or to allelomorphic mutations that
give different degrees of deficiency of pigment.

Albinism is a recessive character. A few cases are
on record where two albinos have had only albino
children. In true albinos the brown pigment is ab-
sent from the skin, hair, and iris. Its absence in
the iris gives the eye a pink color due to the blood
in the back of the eye, showing through the semi-
transparent iris.

A defect in vision known as stationary night-
blindness has been shown by Nettleship to be a
dominant Mendelian trait {fig. 73) . In one locality

HUMAN INHERITANCE 193

(near Montpellier in France) it has been traced to
the year 1637 and hence has been handed down for
about two hundred and fifty years. There are other
types of night-blindness that have a different in-
heritance not entirely made out.

Haemophilia in man has been shown to be trans-

T , , •

Fig. 73. — Diagram to show the inheritance of night-blindness.
(After Nettleship.)

mitted as a sex-linked character {fig. 74) . The blood
of affected individuals fails to coagulate quickly
when exposed to air, hence there is danger of the
individual bleeding to death. Several pedigrees have
been made out. It is a recessive character whose gene
is carried by the A^-chromosome. It appears in any
male whose single A^-chromosome carries the gene
for haemophilia. Its relative infrequency in women
is explained on the grounds that it can appear in
them only when the father and mother both possess
the character or when the mother herself has had a
haemophilic father — in other words when both X-
chromosomes carry the gene.

194 EVOLUTION AND GENETICS

The Four Blood Groups and their Inheritance

One of the most remarkable cases of heredity in
man is found in the so-called blood groups. As first
shown by Von Dungern and Hirschfeld in 1910 the
inheritance of the four blood groups conforms to

I

II

III

IV

V

VI

VII

mo

Dt6 OtOO

iiP ko 6n666a6 it^hi

^6hn6hni^^

ma 5a5^

d

DiO

iiiii tin

Fig. 74. — Diagram to illustrate the inheritance of bleeding or
haemophilia. {After Bulloch and Fildes.)

Mendel's laws. So consistent is this relation that,
as Ottenberg pointed out in 1921, the evidence
might be used in certain cases to determine the par-
entage of the child. The presence of two pairs of
factors will account for the results. Thus if one pair
of genes be represented by A and a and the other
pair by B and h, and if an individual with the ge-

HUMAN INHERITANCE 195

netic constitution AaBh be mated to another indi-
vidual of like constitution (AaBb), then each will
contain four kinds of germ cells, viz., AB, Ah, Ba,
and ah. The sixteen possible combinations formed,
if any sperm may fertilize any egg^ are shown in
figure 75.

These sixteen individuals fall into four groups
according to whether they have both A and B, or

Mating of blood group AaBb to same AaBb

Ege= AB

a

B Ab

a

b

jeTTa

AB

aB

Ab

ab

AB
AB

a B

aB

m.

aB

a

m

Ab
AB

a b

aB

IE

a b
a b

Fig. 73. — Diagram illustrating the sixteen classes of indi-
viduals when two members of the blood group AaBa mate.
There are four classes of individuals produced, indicated by
the circles, lines, squares, and absence of A and B.

196 EVOLUTION AND GENETICS

only A, or only B, or neither A nor B (i.e., ah) in
the proportion of 9AB:3A:3B:lah. These four
genetic classes correspond to the four recognized
blood types IV, II, III, I, as indicated in the
diagram.

Now these sixteen kinds of individuals are found
in all populations, so far studied, although in some-
what different proportions in different "races."

It is very simple to tell what the kinds of genetic
offspring will be when any one of these sixteen indi-
viduals marries any other one. These possibilities are
summarized in the following statement taken from
Ottenberg :

Unions of I and I give I

I "1 I II
II II) '

^ "M I III
III III ) ^' ^^^

Unions of II and III give I, II, III, IV
IV I I, II, III, IV

IV II I, II, III, IV

IV III I, II, III, IV
IV IV I, II, III, IV

Two actual pedigrees, one of them carried throiigh
three generations, will serve to illustrate particular
cases (fig. 76).

HUMAN INHERITANCE

197

From a knowledge of the blood group to which
the child belongs it is possible to predict to what
groups its parents may have belonged, and in certain
cases it is possible to state that an individual of a cer-

ab

aB

m

iB

Fig. 76. — Two pedigrees showing tlie inheritance of two blood
groups. See text.

tain group could not have been the parent of a par-
ticular child.

In the transfusion of blood from one individual

198 EVOLUTION AND GENETICS

to another, that is sometimes necessary, it is essen-
tial that the blood corpuscles of the donor are not
agglutinated by the serum of the recipient. Thus it
is a matter of great importance to select a donor that
does not bring about such a catastrophe. The simple
rules are that individuals belonging to the same blood
group (I, II, III, or IV) do not agglutinate each
other's blood, but the blood corpuscles of an indi-
vidual represented hj AA or A a will be precipitated
if the donor contains the agglutinin represented by
aa, and similarly the blood corpuscles of an indi-
vidual represented by BB or Bh will be precipitated
if the donor contains the agglutinin represented by
bb. Inspection of the diagram will show that group
II (with serum bb) precipitates III and IV, and
group III (with serum aa) precipitates II and IV.
Further, the serum of group I (aa bb) precipitates
all of the other groups; while the serum of group IV
precipitates none of the others.

Inheritance of Other Traits

There are numerous other physical characters of
man that are evidently inherited but where the num-
her of factors involved is uncertain or entirely un-
known. Some of these characters are present in all
races. Others to some extent are racial characters.
Thus height in man is a very variable character. It is
obviously a complex of several or many elements
little understood. Differences in length of legs, or

HUMAN INHERITANCE 199

of body, or of neck, and of different combinations of
these may be present. Height is a growth phenome-
non depending amongst other things on the time at
which the growth of the bones, especially the long
bones of the leg, stops, and this in turn is, to some
extent at least, connected with the time of sexual
maturity, which depends again on the time of func-
tioning or the amount of secretion ^^roduced by some
of the glands of internal secretion (testes or ovary,
thyroid, pituitary, etc.). These inter-relations have
made the study of growth very difficult especially
when the determination of the rate of growth or its
cessation is connected with internal organs that are
seldom seen or measured. Nevertheless there is
nothing in these complications that precludes the
possibility that the ultimate source of the variability
is due to genetic factors.

There is little accurate information at present as
to the number of factor differences that are involved
in the inheritance of hair color in man. The changes
in color that take place during the life of the indi-
vidual renders its study difficult. An individual child
may start with flaxen hair, later have brown hair,
and in old age become white-headed. Three genera-
tions living at the same time may show these differ-
ences. A complete record would then have to extend
over several years; for, hearsay evidence as to the
color of the hair of the grandparent when in middle
age may be inaccurate and the futiu'e color of the

200 EVOLUTION AND GENETICS

hair of a child would be largely guessing. Data col-
lected by the questionnaire method, that has been
used in the study of the inheritance of hair color, can
not be relied upon without some more definite stand-
ard than popular designations of shades of color.

The Inlieiitance of Mental Traits

Man's success as a social animal depends as much
on his mental qualities as on his physical character-
istics. No one will deny, I suppose, that men behave
in different ways, but who can say how far differ-
ences in human behavior depend on the physique of
the individual, how far on his early experiences and
training, and how far on differences in his sense
organs and central nervous system? Until some of
these questions are better understood it is impossible
to know how far observed differences are innate and
how far acquired.

Here again, as in the case of man's physical de-
fects, there are a few extremely abnormal conditions
where the evidence indicates that something is in-
herited, but even here there is much that is obscure.
The case most often quoted is feeble-mindedness
that has been said to be inherited as a Mendelian re-
cessive, but until some more satisfactory definition
can be given as to where feeble-mindedness begins
and ends, and until it has been determined how many
and what internal physical defects may produce a
general condition of this sort, and until it has been

HUMAN INHERITANCE 201

determined to what extent feeble-mindedness is due
to syphilis, it is extravagant to pretend to claim that
there is a single JNIendelian factor for this condition.

Family pedigrees in which an unusual number of
individuals below par are present undoubtedly give
the impression that something is inherited, but until
all the social conditions surrounding the childhood
of the individual are examined and given proper
weight, serious doubts will arise as to what form of
inheritance is producing the results. It is quite prob-
able that there are extraneous factors involved in
such pedigrees.

There is no a priori objection to the assumption
that different sense organs and different brains react
as differently as do other organs of the l)ody. Those
that react below some selected standard might be
called feeble-minded; but there are no grounds for
assuming that the results are due to one particular
defect in the nervous system, and in fact a critical
study of the cases shows that they are probably not
all due to a single factor difference or even to the
same ones. The pedigrees that have been published
showing a long history of social misconduct, crime,
alcoholism, debaucherv, and venereal diseases are
open to the same criticism from a genetic point of
view^; for it is obvious that these grouj^s of individ-
uals have lived under demoralizing social conditions
that might swamp a family of average persons. It
is not surprising that, once begun from wliatever

202 EVOLUTION AND GENETICS

cause, the effects may be to a large extent communi-
cated rather than mherited.

It is quite possible of course that an inherited de-
fective dominant character might furnish the start-
ing jDoint for these histories, but that the subsequent
events are all due to "bad blood" or "defective germ-
plasm" remains to be shown.

"Insanity" is another "psychological trait" that
is said to be inherited and the numerous pedigrees
that have been collected showing that certain types
of insanity occur more frequently in certain fami-

1

ii^

^T

Fig. 77. — Chart showing; the inheritance of Huntington's
chorea. (After Davenport.)

lies than in others seem to furnish evidence in sup-
port of such a claim. This is particularly the case
in Huntington's chorea {fig. 77) a type of insanitj^
often leading ultimately to suicide, that does not
appear as a rule until middle life or later. Since it
apj)ears to be dominant, its history is more easily
followed than in most other cases where the domi-
nance or recessiveness is in question. Huntington's
chorea has been traced in a limited group of indi-

HUMAN INHERITANCE 203

vidiials. The background of its expression appears
to be connected in some way with the sex organs but
what this connection may be is imknown, for it
appears in both sexes which makes it difficult to ac-
count for the disturbance on the basis of a sex
endocrine.

At best one can say, perhaps, that in certain
strains and perhaps under certain conditions mental
disorders appear, but so long as neither the physio-
logical background of insanity, or the external
agents, that are contributory, are known, its genetic
relations must remain obscure.

If these "best cases" are so far from being estab-
lished on a scientific footing, it is not particularly
profitable to discuss the many claims that have been
set up for other mental traits, even though it must
be conceded that defective characteristics might be
the ones, judging by analogy with mutant phys-
ical defects, that would be more likely to furnish evi-
dence of Mendelian inheritance than the less extreme
differences that distinguish "normal" individuals.
The important point, however, to be urged is that
the "mental traits" in man are those that are most
often the product of the environment which obscures
to a large extent their inheritance, or at least makes
very difficult their study.

While the inheritance of disorders relative to
human behavior are of importance to the pathologist
and to the penologist, the inheritance of individual

204 EVOLUTION AND GENETICS

differences that fall within what would be called the
"normal" is more important from the point of view
of human evolution. Here also we are on very du-
bious grounds when we discuss the inheritance of
individual mental peculiarities, and in still greater
danger of error if we attempt to discriminate be-
tween racial complexes. The similarity in behavior or
in "temperament" or mental qualities of identical
twins might be expected to furnish important infor-
mation as to how much is acquired and how much
inherited. The very close physical similarity of twins
of this kind might make such material favorable for
study. There are, however, even here two serious
drawbacks that complicate the results. In the first
place unless the twins had been separated in very early
childhood it would be difficult to decide how much
is due to similarity of nature and how much to nur-
ture. A comparison with other children in the same
family may be helpful but is not decisive, for the
ex23erience of each child from successive births is
affected by older and younger children in the family.
In the second place the so-called standard tests may
measure training rather than constitutional factors.
Until these difficidties can be overcome, the manv
anecdotes of the close similarity in temperaments, or
abilities, of identical twins do not supply the needed
evidence.

We can, by artificial selection, eliminate struc-
tural defects from a race of animals or plants and

HUMAN INHERITANCE 205

by j^roper breeding make the race more uniform and
maintain it at or near a chosen standard. Since we
have many good reasons to think that man's physical
inheritance conforms to the same jDrinciples that
apply to other animals, it follows that by elimination
and suitable mating man too could be standardized.
How far one might have to go in order to carry out
this reformation is a matter of opinion. If too strenu-
ous standards were set up the human race might be
exterminated before the reformation began. Genetic
reformers and racial propagandists do little more
than recommend cutting off a few of the most de-
fective individuals. But it is not so much the physic-
ally defective that appeal to their sympathies as the
"morally" deficient and this is supposed to apply to
mental traits rather than to physical characters.
Ruthless genetic (?) reform here might seem too
drastic and might be retroactive if pressed too far.
Social reforms might, perhaps, more quickly and
efficiently get at the root of a part of the trouble, and
until we know how much the environment is respon-
sible for, I am inclined to think that the student of
human hereditv will do well to recommend more en-
lightenment on the social causes of deficiencies rather
than more elimination in the present deplorable
state of our ignorance as to the causes of mental
differences.

Lest it appear from what has been said that I
have too little faith in the importance of breeding

206 EVOLUTION AND GENETICS

for mental superiority I should like to add that I am
inclined to think that there are considerable indi-
vidual differences in man that are probably strictly
genetic, even though I insist that at present there is
for this no real scientific evidence of the kind that we
are familiar with in other animals and in plants. I
will even venture to go so far as to suppose that the
average of the human race might be improved by
eliminating a few of the extreme disorders, how-
ever they may have arisen. In fact, this is attempted
at present on a somewhat extensive scale by the seg-
regation into asylums of the insane and feeble-
minded. I should hesitate to recommend the incar-
ceration of all their relatives if the character is
suspected of being recessive, or of their children if a
dominant. After all, these segregations are based on
humanitarian principles, or for our i^rotection rather
than for genetic reasons. How long and how exten-
sively this casual isolation of adults would have to
go on to produce any considerable decrease in defec-
tives, no informed person would, I should think, be
willing to state.

Least of all should we feel any assiu-ance in de-
ciding genetic superiorit}^ or inferiority as applied
to whole races, bv which is meant not races in a
biological sense but social or political groups bound
together by physical conditions, by religious senti-
ments, or by political organizations. The latter have
their roots in the past and are acquired by each new

HUMAN INHERITANCE 207

generation as a result of imitation and training. If
it is unjust "to condemn a whole iieople' meaning
thereby a political group, how much more hazardous
is it, as some sensational writers have not hesitated to
do, to pass judgment as to the relative genetic in-
feriority or superiority of different races.

If within each human social group the geneticist
finds it impossible to discover, with any reasonable
certainty, the genetic basis of behavior, the problems
must seem extraordinarily difficult when groups are
contrasted with each other where the differences are
obviously connected not only with material advan-
tages and disadvantages resulting from location,
climate, soil, and mineral wealth, but with tradi-
tions, customs, religions, taboos, conventions, and
prejudices. A little goodwill might seem more
fitting in treating these complicated questions than
the attitude adopted by some of the modern race-
proj^agandists.

INDEX

ACQUIRED characters, 155-78
albinism, 191
Andalusian, 60, 61
antlered, 94
antlers, 34, 25
apterous, 95
arc, 95

Bagg, 159, 168

bar eyes, 172

Bateson, 13, 27, 50, 51, 52, 53, 54

beaded, 97

beans, 121, 122, 132

Belling, 170

bent wings, 99

Bergson, 9, 10

biogenetic law, 24

bithorax, 95

black body-color, 95, 111-4

Blakeslee,' 124, 125, 170

blood groups, 194-8

blue eyes, 177, 190, 191, 192

Bolton, Gambler, 149

brachydactyly, 185, 186

Brewer, 165

brown eyes, 95, 177, 190, 191, 192

Brown-Sequard, 167

Buffon, 7

Bulloch, 194

Butler, Samuel, 160

CACOGEXICS, 179

caligrapha, 79

cardinal eye-color, 97

Carr-Saunders, 169

Castle, 134, 135, 175

chance, 144

chart of chromosomes, 88, 118

chick, 26, 28

chorea, 202

chromosomes, 73-89

ciona, 48

cleavage, 31

club wing, 93

color blindness, 106, 188
conjugation of chromosomes, 116,

117
Cope, 156, 166
corn, 124-5

crossing-over, 111, 116, 117, 118
Cunningham, 166
curled wing, 96
curved wing, 95
cut wing, 91

DACHSOIDj 95

Darwin, 8, 11, 14, 15, 23, 32, 38,
44, 46, 119, 120, 126, 127, 128,
129, 134, 135, 139, 140, 147, 148,
155, 164

Dean, 4

Detlefsen, 168, 170

dichaete, 96

disuse, 120

dominance, 151

Drinkwater, 186

Drosophila, 19, 20, 22, 46, 52, 87,
89, 140, 189

von Dungern, 194

dwarf, 96

EBOXY, 71, 96

elephants, 148-50

embryology, 23

eugenics, 179

evolution, 1-5, 12, 41-54, 119-35

eyeless, 98, 164, 171

Farabee, 185, 186

feeble-mindedness, 200, 201

Fildes, 194

Finley, 169

fishes, 28

forked bristles, 92

four o'clock, 57, 58, m^ 83

fused, 92

Galtox, 57, 128
giant, 96

210

INDEX

gill slits, 28, 29
Goldschmidt, 136
Griffith, 168, 170
Group I, 90-3
Group II, 93-5
Group III, 95-7
Group IX, 98
guinea pigs, 175, 176
Guyer, 167, 169, 170

Haeckel, 23, 24, 160

haemophilia, 193

haircolor in man, 199

Haldane, 142

Hanson, 168

Hegner, 79

height in man, 199

heliotropism, 92

Herbst, 172

Hering, 160

Hesse, 28

Hilgard, 156

Hirschfeld, 194

His, 29

horse, 2, 3, 33

human inheritance, 106, 179-206

Huntington's chorea, 202

Huxley, 169

INHERITANCE of acquired

characters, 155-78
insanity, 202

Janssens, 117
jaunty wings, 94
Jenkin, Fleming, 139, 140
Jennings, 133

Johannsen, 123, 130, 131, 132, 133,
135, 174

Kammerer, 172

Lamarck, 10, 11, 12, 155

Lancaster, 149

leaves, 121

Lenz, 190

linkage, 87-100, 111

Little, 168

Lull, 2, 3

MacDowell, 127, 159

map of chromosomes, 88, 118

Marshall, 29

Matthews, 33

maturation divisions, 80, 81

memory, 160, 161

Mendei, 55, 56, 57, 64

Mendel's laws, 55-65

miastor, 79

mice, 173, 174, 176

miniature wing, 93

Mirabilis, 57, 58

Mohr, 187

mutation, 35-7, 52, 129, 141

Nageli, 12, 13

natural selection, 14, 119-35,

137-53
Xettleship, 192, 193
night-blindness, 192, 193
notch wings, 91

ORIGIN of species, 43, 119, 137-53
Orr, 160
Ottenberg, 194

Painter, 109
jialeontology, 32
Paley, 145
Pawiow, 157-8, 159
Payne, 164
peach, 97
Pearl, 133, 166
peas, as, 61-9, 85
Phaseolus, 130, 131
Phillips, 175
j^ink eye-color, 97
Plough, 52
pure lines, 130

radium, 168
rats, 170
Kedfield, 165
Rignano, 160
Romanes, 24, 25
rudimentary wings, 92

SABLE bodv-color, 92
St. Hilaire, 7, 8, 9
scarlet eye, 97

INDEX

211

Sedgwick, L'9

selection, 1:29

self sterility, 47

Semon, 160

sepia eye-color, 97

sex-chromosomes, 101

sex in man, 107-9

sex-linked inheritance, 101-6,

107, 193
shaven, 99
Shaw, Bernard, 160
sooty body-color, 96
species, 43, 44, 46, 14;2, 143
Spencer, 120
sport, 129
sterility, 47
Stockard, 167, 168
strap wing, 94
Sumner, 173, 175
sweet peas, 183

TAX body-color, 93
tobacco, 48

twins, 304
USE, 120

VARIATION. 139

vestigial, 71, 93, 111-4

Mcari, 159

vinegar fly, 71, 87

de Vries, 27, 121, 122, 123

Wallace, 14

Ward, 160

Weismann, 10, 11, 27, 166

white eve, 114-5

Wilder,' 26

deWiniwarter, 107

Wriedt, 187

X-CHROMOSOME, 90, 114

X-rays, 168
YELLOW wings, 114-5

ZlEGLEHj 25