BIOLOGY
LIBRARY

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GENETICS

AN INTRODUCTION TO THE STUDY
OF HEREDITY

BY

HERBERT EUGENE WALTER

ASSISTANT PROFESSOR OF BIOLOGY
BROWN UNIVERSITY

WITH 72 FIGURES AND DIAGRAMS

Nefo gork

THE MACMILLAN COMPANY
1913

All rights reserved

, BIOLOGY

LIBRAE**

G

COPTEIGHT, 1918,

BY THE MACMILLAN COMPANY.

Set up and electrotyped. Published February, 1913.

J. 8. Gushing Co. — Berwick & Smith Co.
Norwood, Mass., U.S.A.

THIS VOLUME

IS AFFECTIONATELY DEDICATED
TO

MY MOTHER

259926

PREFACE

THE following pages had their origin in a course
of lectures upon Heredity, given at Brown Univer-
sity during the winter of 1911-1912, which were
amplified and repeated in part the following sum-
mer at Cold Spring Harbor, Long Island, before the
biological summer school of the Brooklyn Institute
of Arts and Sciences.

An attempt has been made to summarize for the
intelligent, but uninitiated, reader some of the more
recent phases of the questions of heredity which
are at present agitating the biological world. It is
hoped that this summary will not only be of interest
to the general reader, but that it will also be of serv-
ice in college courses dealing with evolution and
heredity.

The subject of heredity concerns every one, but
many of those who wish to become better informed
regarding it are either too busily engaged or lack the
opportunity to study the matter out for themselves.
The recent literature in this field is already very
large, with every indication that much more is about
to follow, which is a further discouragement to non-
technical readers.

It may not be a thankless task, therefore, out of
the jargon of many tongues to raise a single voice

viii PREFACE

which shall attempt to tell the tale of heredity.
There may be a certain advantage in having as
spokesman one who is not at present immersed in the
arduous technical investigations that are making
the tale worth telling. The difficulties in under-
standing this complicated subject may possibly be
realized better by one who is himself still struggling
with them, than by the seasoned expert who has
long since forgotten that such difficulties exist.

Among others I am particularly indebted to Dr.
C. B. Davenport for many helpful suggestions, to
my colleague, Professor A. D. Mead, for reading the
manuscript critically, to Dr. S. I. Kornhauser who
gave valuable aid in connection with the chapter
on the Determination of Sex, and to my wife for
assistance in final preparation for the press.

I wish to thank Professor H. S. Jennings and Dr.
H. H. Goddard, who have given generous permission
to copy certain diagrams, as well as The Outlook
Company and The Macmillan Company for the use
of figures 24 and 66, respectively.

The fact that all the suggestions which were at
various times offered by my kindly critics have
not been incorporated in the text, absolves them
from responsibility for whatever remains.

H. E. W.

PROVIDENCE, R. I.,
September, 1912.

CONTENTS

CHAPTER

I. INTRODUCTION.

1. The triangle of life 1

2. A definition of heredity 4

3. The maintenance of life 5

4. Somatoplasm and germplasm ..... 10

II. THE CARRIERS OP THE HERITAGE.

1. Introduction 14

2. The cell theory 14

3. A typical cell 15

4. Mitosis 18

5. Amitosis . .20

6. Sexual reproduction 20

7. Maturation 22

8. Fertilization 24

* 9. Parthenogenesis ....... 26

10. The hereditary bridge 27

11. The determiners of heredity 28

12. The chromosome theory 29

13. The enzyme theory of heredity ..... 33

14. Conclusion 35

III. VARIATION.

1. The most invariable thing in nature .... 36

2. The universality of variation 37

3. The kinds of variation with respect to their —

a. Nature 38

b. Duplication 39

c. Utility . 39

d. Direction in evolution 39

e. Source . . • • • • .40

/. Normality 40

g. Degree of continuity 40

h. Character 41

i. Relation to an average standard ... 41

j. Heritability 41

ix

x CONTENTS

CHAPTER PAGE

4. Methods of studying variation 42

5. Biometry 42

6. Fluctuating variation 43

7. The interpretation of variation curves ... 47

a. Relative variability 47

b. Bimodal curves ...... 47

c. Skew polygons . . . . . . .50

8. Graduated and integral variations .... 52

9. The causes of variation . . . . . .52

a. Darwin's attitude . . . . .52

6. Lamarck's attitude 53

c. Weismann's attitude ..... 55

d. Bateson's attitude 55

IV. MUTATION.

1. The mutation theory 56

2. Mutation and fluctuation 57

3. Freaks . .58

4. Kinds of mutation 59

5. Species and varieties 60

6. Plant mutations found in nature .... 63

7. Lamarck's evening primrose 64

8. Some mutations among animals . . .67

9. Possible explanations of mutation .... 69
10. A summary of the mutation theory .... 72

V. THE INHERITANCE OF ACQUIRED CHARACTERS.

1. Summary of preceding chapters 74

2. The bearing of this chapter upon genetics ... 75

3. The importance of the question ..... 75

4. An historical sketch of opinion . . . . .76

5. Confusion in definitions ...... 77

6. Weismann's conception of acquired characters . . 78

7. The distinction between germinal and somatic charac-

ters 79

8. What variations reappear ? . . . . .80

9. What may cause germplasm to vary or to acquire

new characters ? 81

10. Weismann's reasons for doubting the inheritance of

acquired characters ..... 84

CONTENTS xi

CHAPTER PAGE

11. No known mechanism for impressing germplasm with

somatic characters ..... 84

12. Evidence for the inheritance of acquired characters

inconclusive 86

a. Mutilations ....... 87

6. Environmental effects ..... 88

c. The effects of use or disuse . . . .91

d. Disease transmission ..... 92

13. The germplasm theory sufficient to account for the

facts of heredity ...... 94

14. The opposition to Weismann 95

15. Conclusion 96

VI. THE PURE LINE.

1. The unit character method of attack ... 97

2. Galton's law of regression ..... 98

3. The idea of the pure line 102

4. Johanssen's nineteen beans 103

5. Cases similar to Johanssen's pure lines . . . 107

6. Tower's potato-beetles 108

7. Jennings' work on Paramecium .... 110

8. Phenotypical and genotypical distinctions . .113

9. The distinction between a population and a pure line 115
10. Pure lines and natural selection .... 118

VII. SEGREGATION AND DOMINANCE.

1. Methods of studying heredity 120

2. The melting-pot of cross-breeding .... 120

a. Blending inheritance . . . . .121

6. Alternative inheritance ..... 121

c. Particulate inheritance 121

3. Johann Gregor Mendel ...... 123

4. Mendel's experiments on garden peas . . .124

5. Some further instances of Mendel's law . . . 128

6. The principle of segregation . . . . .130

7. Homozygotes and heterozygotes .... 131

8. The identification of a heterozygote .... 132

9. The presence and absence hypothesis . . . 132
10. Dihybrids 133

xii CONTENTS

CHAPTER PAGE

11. The case of the trihybrid 140

12. Conclusion 143

13. Summary 144

VIII. REVERSION TO OLD TYPES AND THE MAKING OF NEW
ONES.

1. The distinction between reversion and atavism . 146

2. False reversion ....... 149

a. Arrested development . . . . . 149
6. Vestigial structures 149

c. Acquired characters resembling ancestral ones 150

d. Convergent variation

e. Regression . . . . « .

3. Explanation of reversion . . ... .151

4. Some methods of improving old and establishing new

types 152

a. The method of Ballet . . . . .152
6. The method of Rimpau . . . . .153

c. The method of de Vries . . . .154

d. The method of Vilmorin 155

e. The method of Johanssen .... 155
/. The method of Burbank . . . .156
g. The method of Mendel 157

5. The factor hypothesis ...... 159

a. Bateson's sweet peas ..... 160

6. Castle's agouti guinea-pigs . . . . 163

c. Cuenot's spotted mice . . . . . 164

d. Miss Durham's intensified mice . . .165

e. Castle's brown-eyed, yellow guinea-pigs . . 166

6. Rabbit phenotypes 169

7. The kinds of gray rabbits 171

8. Conclusion . . 173

IX. BLENDING INHERITANCE.

1. The relative value of dominance and segregation . 174

2. Imperfect dominance ...... 175

3. Delayed dominance 177

4. "Reversed" dominance . 178

xiv CONTENTS

CHAPTER P\GE

7. The control of defects 235

8. Inbreeding 238

9. Experiments to test the effects of inbreeding . . 240

10. The influence of proximity 241

11. Inbreeding in the light of Mendelism . . 242

XII. HUMAN CONSERVATION.

1. How mankind may be unproved .... 244

2. More facts needed 245

3. More application of what we know necessary . . 247

4. The restriction of undesirable gennplasm through —

a. The control of immigration .... 248

b. More discriminating marriage laws . . . 250

c. An educated sentiment ..... 251

d. The segregation of defectives .... 252

e. Drastic measures 254

5. The conservation of desirable germplasm . . . 255

a. By subsidizing the fit .... 256

6. By enlarging individual opportunity . . 258

c. By preventing germinal waste .... 258

1. Preventable death 258

2. Social hindrances 259

6. Who shall sit iniudgmfiPJL? .' 260

XIII. BIBLIOGRAPHY 263

XIV. INDEX 265

CONTENTS xiii

CHAPTEK PAGE

5. Potency .179

a. Total potency 179

6. Partial potency 180

c. Failure of potency ...... 180

6. Blending inheritance ...... 182

7. The case of rabbit ears 183

8". The Nilsson-Ehle discovery 186

9. The application of Nilsson-Ehle's explanation to the

case of rabbit ear-length . . . .193

10. Human skin color 196

X. THE DETERMINATION OF SEX.

1. Speculations, ancient and modern .... 197

2. The nutrition theory 198

3. The statistical study of sex 200

4. Monochorial twins ....... 201

5. Selective fertilization ...... 202

6. The neo-Mendelian theory of sex .... 205

a. Microscopical evidence ..... 207

1. The " x " chromosome .... 207

2. Various forms of x chromosomes . . 208

3. Sex chromosomes in parthenogenesis . 210

b. Castration and regeneration experiments . . 210

c. Sex-limited inheritance ..... 213

1. Color-blindness 214

2. The English currant-moth . . .216

d. Behavior of hermaphrodites in heredity . . 220

7. Conclusion 222

XI. THE APPLICATION TO MAN.

1. The application of genetics to man .... 224

2. Modifying factors in the case of man . . . 225
j~j 3. Experiments in human heredity .... 227

a. The Jukes 227

6. The descendants of Jonathan Edwards . . 228
c. The Kallikak family . . - . . .229

4. Moral and mental characters behave like physical

ones 230

5. The character of human traits ..... 231

6. Hereditary defects 232

GENETICS

CHAPTER I

INTRODUCTION

1. THE TRIANGLE OF LIFE

WITHIN a generation the center of biological inter-
est has gradually been swinging from the origin of
species to the origin of the individual. The nine-
teenth century was Darwin's century. His monu-
mental work "On the Origin of Species by Means of
Natural Selection," which appeared in 1859, not only
dominated the biological sciences but also influenced
profoundly many other realms of thought, partic-
ularly those of philosophy and theology.

Now, at the beginning of the twentieth century,
a particular emphasis is being laid upon the study
of heredity. The interpretation of investigations
along this line of research has been made possible
through the cumulative discoveries of many things
that were not known in Darwin's day. Trained
students have been patiently and persistently bend-
ing over improved microscopes, untangling the
mysteries of the cell, while an increasing host of in-
vestigators, inspired by the Austrian monk Mendel,
have been industriously devoting their energies to

2 GENETICS

breeding animals and plants with an insight denied
to breeders of preceding centuries.

The study of the origin of thajndividual, which
has grown out of the more general consideration of
the origin of species, forms the subject-matter of
heredity, or, to use the more definitive word of Bate-
son, of genetics.

It is not with the individual as a whole that

RE

HERITAGE
FIG. 1. — The triangle of life.

genetics is chiefly concerned, but rather with char-
acteristics of the individual.

Three factors determine the characteristics of an
individual, namely, environment, training, and heri-
tage as expressed diagrammatically in Figure 1. It
may indeed be said that an individual is the result
of the interaction of these three factors since he
may be modified by changing any one of them.
Although no one factor can possibly be omitted, the
student of genetics places the emphasis upon heritage
as the factor of greatest importance. Heritage, or

INTRODUCTION 3

"blood," expresses the innate equipment of the indi-
vidual. It is what he actually is even before birth.
It is his nature. It is what determines whether he
shall be a beast or a man. Consequently in the
diagram (Fig. 1), the triangle of life is represented
as resting solidly upon the side marked "heritage"
for its foundation.

Environment and training, although indispensable,
are both factors which are subsequent and secondary.
Environment is what the individual has, for example,
housing, food, friends and enemies, surrounding aids
which may help him and obstacles which he must
overcome. It is the particular world into which he
comes, the measure of opportunity given to his
particular heritage.

Training, or education, on the other hand, repre-
sents what the individual does with his heritage and
environment. Lacking a suitable environment a
good heritage may come to naught like good seed
sown upon stony ground, but it is nevertheless true
that the best environment cannot make up for
defective heritage or develop wheat from tares.

The absence of sufficient training or exercise even
when the environment is suitable and the endowment
of inheritance is ample will result in an individual
who falls short of his possibilities, while no amount
of education can develop a man out of the heritage
of a beast. Consequently the biologist holds that,
although what an individual has and does is un-
questionably of great importance, particularly to the
individual himself, what he is, is far more important

4 GENETICS

in the long run. Improved environment and educa-
tion may better the generation already born. Im-
proved blood will better every generation to come.

What, then, is this "blood" or heritage? Ex-
actly what is meant by heredity ?

2. A DEFINITION OF HEREDITY

Professor Castle, in his recent book on " Heredity
in Relation to Evolution and Animal Breeding," has
defined heredity as " organic resemblance based on
descent." The son resembles his father because he
is a "chip off the old block." It would be still
nearer the truth to say that the son resembles his
father because they are both chips from the same block,
since the actual characters of parents are never trans-
mitted to their offspring in the same way that real
estate or personal property is passed on from one
generation to another. When the son is said to have
his father's hair and his mother's complexion it
does not mean that paternal baldness and a vanish-
ing maternal complexion are the inevitable conse-
quences.

Biological inheritance is more comparable to the
handing down from father to son of some valuable
patent right or manufacturing plant by means of
which the son, in due course of time, may develop an
independent fortune of his own, resembling in charac-
ter and extent the parental fortune similarly derived
although not identical with it.

So it comes about that "organic resemblance"

INTRODUCTION 5

between father and son, as well as that which often
appears between nephew and uncle or even more
remote relatives, is due not to a direct entail of the
characteristics in question, but to the fact that the
characteristics are "based on descent" from a
common source. In other words, an "hereditary
character" of any kind is not an entity or unit which
is handed down from generation to generation, but
is rather a method of reaction of the organism to the
constellation of external environmental factors under
which the organism lives.

To unravel the golden threads of inheritance
which have bound us all together in the past, as
well as to learn how to weave upon the loom of the
future, not only those old patterns in plants and
animals and men which have already proven worth
while, but also to create new organic designs of an
excellence hitherto impossible or undreamed of, is
the inspiring task before the geneticist to-day.

3. THE MAINTENANCE OF LIFE

So far as we know, every living thing on the earth
to-day has arisen from some preceding form of life.

How the first spark of life began will probably
always be a matter of pure speculation. Whether
the beginnings of what is called life came through
space from other worlds on meteoric wings, as Lord
Kelvin has suggested ; whether it was(sgontarieouslv^
generated on the spot out of lifeless components;
or whether life itself was the original condition of

6 GENETICS

matter, and the one thing that must be explained is
not the origin of life, but of the non-living, no one
can say. Leaving aside the first speculation as un-
tenable and the third as irrational, since it jars so
sadly with what astronomers tell us of the probable
evolution of worlds, the theory of (spontaneous)gener-
ation seems to be the last resort to which to turn.

In prescientific days this idea of spontaneous
generation presented no great difficulties to our
imaginative and credulous ancestors. John Milton,
with the assurance of an eye-witness, thus described
the inorganic origin of a lion : - ±

" The grassy clods now calved ; now half appears
The tawny lion, pawing to get free
His hinder parts — then springs as broke from bonds,
And rampant shakes his brindled mane."

(" Paradise Lost," Book VII, line 543.)

Ovid also in his "Metamorphoses," not to mention a
more familiar instance, easily succeeded in creating
mankind from the humble stones tossed by the
juggling hands of Deucalion and Pyrrha.

Although under former conditions on the earth
it might have been possible for life to have originated
spontaneously, and although it may yet be possible
to produce life^from inorganic materials in the labora-
tory or elsewhere, the exhaustive work of Pasteur,
Tyndall and others effectually demonstrated a genera-
tion ago that to-day living matter always arises from
preceding living matter and this conclusion is gener-
w ally accepted as an axiom in genetics. ,

INTRODUCTION 7

There are various methods of producing more life,
given a nest-egg of living substance with which to
start. Any organism, whether plant or animal, is
continually transforming inorganic and dead material
into living tissue. Through the process of repair,
for example, an injury to a form as highly developed
even as man is frequently made good, if it is not too
extensive, as in the case of a skin wound.

When the intake of non-living material is in excess
of the outgo, growth results, with the consequence
that more living substance is built up than existed
before. Thus a fragment of a living sponge or a
piece of a begonia leaf are each sufficient to restore a
duplicate of the original organism.

A process similar to the repair of the begonia leaf
is that employed so effectively in the great groups of
the one-celled animals and plants, the Protozoa and
Protophyta, by means of which their numbers are
maintained. These one-celled organisms multiply
by fission, that is, by equal division into halves, and
each half then grows to the size of the parent organism
from which it sprang. When two daughter pro-
tozoans are thus formed, they are essentially orphans
because they have no parents, alive or dead. The
parental substance in such a process, along with the
regulating power necessary to reorganization, goes
over bodily into the next generation in the forma-
tion of the daughter-cells, leaving usually no re-
mains whatever behind.' In primitive forms of this
description, continuous life is the natural order,
and death, when it does occur, is, as Weismann has

8 GENETICS

pointed out, accidental and quite outside the plan of
nature.

In these cases it is easy to see the reason for "or-
ganic resemblance" between successive generations.
Parent and offspring are successive manifestations
of, the same thing, just as the begonia plant, restored
from a fragment of a begonia leaf, is simply an ex-
tension of the original plant.

Many modifications of the process of multiplica-
tion by fission occur, all of them, however, agreeing
in the fundamental principle that the progeny re-
semble the parents because they are pieces of the
parents.

Thus the greening apple maintains its individuality
although coming from thousands of different trees,
because all of these trees through the asexual process
of grafting are continuations of the one original
Rhode Island greening tree grown by Dr. Solomon
Drowne in the town of Foster, nearly a century ago.

Again, certain fresh- water sponges and bryozoans,
quite unlike any of their marine relatives, keep a
foothold from year to year within their particular
shallow fresh-water habitats by isolating well pro-
tected fragments of themselves in the form of
gemmules and statoblasts. These structures may
drop to the muddy bottom and live in a dormant
condition throughout the icy winter when it would
not be possible for the entire organism to survive
near the surface.

In order to meet the conditions imposed by winter,
however, these fragments have become so modified

INTRODUCTION £

as temporarily to lose their likeness to the parent
generation, although readily regaining that likeness
when springtime brings the opportunity. The unity
of two succeeding generations, although interrupted
by the temporary interposition of something ap-
parently different in the form of gemmules or stato-
blasts, is thus essentially maintained. The bryozoan
colonies of two successive seasons in a fresh-water
pond may be regarded as parts of the same identical
colony, since they present an "organic resemblance
based on descent," although the sole representatives
of the parent colony during midwinter may be the
sparks of life locked up within the statoblasts buried
in the mud.

Similarly, the asexual spores of many plants, such
as molds, mosses and ferns, may be regarded as
gemmules reduced to the lowest terms, namely, to
single cells. As in the preceding cases so in this
instance the resemblance of the offspring which may
arise from these spores, to the parents which pro-
duced them, is due to the essential material identity
of two generations.

These illustrations of heredity in its simplest mani-
festations give the key to "organic resemblance"
higher up in the scale. Sexual reproduction is no
less plainly the direct continuation of life though in
this instance two sporelike fragments out of one
generation contribute to form the new individual of
the next generation instead of one fragment. In all
cases there is a material continuity between succeeding
generations. Offspring become thus an extension of

10 GENETICS

a single parent or of two parents, while heredity is
simply "organic resemblance based on descent."

4. SOMATOPLASM AND GERMPLASM

In forms that reproduce sexually there theoretically
occurs a differentiation of the body substance into
what Weismann terms somatoplasm and germplasm.

The somatoplasm includes the body tissues, that
is, the bulk of the individual, which is fated in the
course of events to complete a life-cycle and die.
The germplasm, on the contrary, is the immortal
fragment freighted with the power to duplicate the
whole organism and which, barring accident, is des-
tined to live on and give rise to new individuals.

The germplasm thus carries potencies for develop-
ing both germplasm and somatoplasm, while the
somatoplasm, according to this conception, has only
the power to reproduce more of its own kind. More-
over, the germplasm is not formed afresh in each gen-
eration, neither does it arise anew when the individual
reaches sexual maturity, but it is a continuous sub-
stance present from the beginning. Although this
theory of the continuity of the germplasm has been
actually demonstrated in comparatively few instances,
all the facts we know concerning the behavior of the
germinal substance are consistent with it.

In many of the Protozoa the entire organism is
possibly comparable to germplasm, but in all forms
of life that are compounded of several cells the germ-
plasm is probably set aside early in the development

\

INTRODUCTION

11

of the individual, and this remains undifferentiated,
or in reserve, like a savings-bank account put by
for a rainy day,
while the somato-
plasm is expended
in the immediate
demands of the
tissues that make
up the individual.
In one instance at
least, that of the
nematode worm
Ascaris, as con-
firmed by Boveri,
this splitting off
or isolation of the
germplasm occurs
with the very first
cleavage of the
fertilized egg into
the two-celled
stage, when one
of the two cells
forms the future
germplasm, while
the other differen-
tiates by succes-
sive divisions into
the animal itself.

Thus there results a continuous stream of germ-
plasm, receiving contributions from other germ-

GE RMPLASM \ SO^ATOPLASM

FIG. 2. — Scheme to illustrate the continuity of
the germplasm. Each triangle represents an
individual made up of germplasm (dotted) and
somatoplasm (undotted) . The beginning of the
life cycle of each individual is represented at
the apex of the triangle where germplasm and
somatoplasm are both present. As the indi-
vidual develops each of these component parts
increases. In sexual reproduction the germ-
plasms of two individuals unite into a common
stream to which the somatoplasm makes no
contribution. The continuity of the germ-
plasm is shown by the heavy broken line into
which run collateral contributions from suc-
cessive sexual reproductions.

12 GENETICS

plasmal streams at the time of sexual reproduction,
as shown diagrammatically in Figure 2, in which
individuals are represented by triangles. From
this continuous stream of germplasm there split off
at successive intervals complexes of somatoplasm, or
"individuals," which go so far on the road of speciali-
zation into tissues that the power to be " born
again" is lost, and so after a time they die, while
the germplasm, held in reserve, lives on.

This is what is meant by saying that a father and
son owe their mutual resemblance to the fact that they
are chips off the same block rather than by saying
that the son is a chip off the paternal block. Both
somatoplasm s are developments at different inter-
vals from the same continuous stream of germplasm
instead of one somatoplasm being derived from a
preceding one. As a matter of fact the germplasm
from which the son arises is modified by the addition
of a maternal contribution, so that father and son in
reality hold the same relation to each other that half-
brothers do.

From the point of view of genetics, then, the real
mission of the somatoplasm, which is so marvelously
differentiated into all the various forms that we call
animals and plants, is simply to serve as a temporary
j domicile for the immortal germplasm. Thus the
parent becomes as it were the "trustee of the germ-
plasm, " but not the producer of the offspring.

In the light of these preliminary explanations it is
plain that the hopeful point of attack in the science
of genetics must inevitably be the germplasm which

INTRODUCTION 13

is the source, or point of departure, in the formation
of each new individual, rather than the somatoplasm,
which represents the end stages of the hereditary
processes.

This has not been the method of the past. The
resemblances of the visible father and son have
usually been traced instead of the character of their
unseen germplasms. By following this old method,
investigators have often been misled because the
visible or apparent is not always the true index of
what lies behind it. A gray and a white rabbit, for
example, may produce some offspring that are
entirely black just as two white-flowering sweet peas
when crossed may sometimes produce purple blos-
soms. Consequently it is a great fallacy to affirm
that in heredity "like produces like," since the op-
posite is quite often the case.

The new heredity, embodied in the science of
genetics, attempts to go deeper than the surface
appearance of the somatoplasm. It aims to get at
the source or origin of organisms, that is, the germ-
plasm which is the only connecting thread between
succeeding generations of living forms. It is con-
cerned not so much with somatoplasm, which repre-
sents what the germplasm has done in the past, as
with the germplasm and what it can do in the future.

CHAPTER H

THE CARRIERS OF THE HERITAGE
1. INTRODUCTION

HEREDITY, as has been shown in the introductory
chapter, is essentially a matter of continuity between
succeeding generations of living organisms. This
continuity may be direct, as when a mother protozoan
divides into two daughters, or it may be indirect, as
illustrated by the relationship of a father and son,
an uncle and nephew, or any other relatives of varying
degrees of kinship which, taken singly or collectively,
are somatoplasms derived from a common stream of
germplasm.

It is the purpose of the present chapter to consider
this material continuity between succeeding genera-
tions and to discover, if possible, just what are the
carriers of the heritage from one generation to another.
To this end it will be necessary in the first place
to take up what is meant by the "cell theory."

2. THE CELL THEORY

In 1838-1839 the "cell theory" of Schleiden and
Schwann, which affirms that all organisms, both
plant and animal, are made up of cellular units,
had its birth.

14

CHE CARRIERS OF THE HERITAGE 17
•p

«,. mighout all the various cells that make up the

T. duals of any one species these chromosomes

j ir to be practically constant in number with some

', >tions to be mentioned later in connection with
wri
rpi This law of the constant chromosome number

any species was first stated by Boveri in 190Q.

he chromosomes of different organisms vary in

iber from two in the worm Ascaris up to perhaps

.0, according to Haecker ('09), in certain radiolaria.

3cies which apparently are closely related may

fer widely ^with respect to the number of their

romosomes, while species of unquestionably re-

ote relationship may have an identical number of

iromosomes in each of their cells. The number of

iromosomes characteristic for a species, therefore,

j in no way an index to the complexity or degree of

iifferentiatiori of the species.

Besides the nucleus there may often be identified
tn the cytoplasm of the animal cell a tiny body known
as the centrosome. At certain times in the life-cycle
of a cell the centrosome becomes the focal point of
peculiar radiating lines, which play an important
part in the behavior of the cell, particularly during
the period of division. ^

Every cell passes through a cycle of life which may
be compared with that common to individuals. It
is born from another cell ; passes through a vigorous
youth characterized by growth and transformation ;
attains maturity when the metamorphoses of its
earlier life give place to a considerable degree of
stability ; and finally, after a more or less extended

CHAPTER H

cell
nay
nee
ind

THE CARRIERS OF THE HERITAGE

>r-

1. INTRODUCTION of

?e
HEREDITY, as has been shown in the introductolg

chapter, is essentially a matter of continuity bet wet r
succeeding generations of living organisms. Thj
continuity may be direct, as when a mother protozoa
divides into two daughters, or it may be indirect, a
illustrated by the relationship of a father and son
an uncle and nephew, or any other relatives of varying
degrees of kinship which, taken singly or collectively,
are somatoplasms derived from a common stream of
germplasm.

It is the purpose of the present chapter to consider
this material continuity between succeeding genera-
tions and to discover, if possible, just what are the
carriers of the heritage from one generation to another.
To this end it will be necessary in the first place
to take up what is meant by the "cell theory."

2. THE CELL THEORY

In 1838-1839 the "cell theory" of Schleiden and
Schwann, which affirms that all organisms, both
plant and animal, are made up of cellular units,
had its birth.

14

THE CARRIERS OF THE HERITAGE 17

Throughout all the various cells that make up the
individuals of any one species these chromosomes
appear to be practically constant in number with some
exceptions to be mentioned later in connection with
sex. This law of the constant chromosome number
for any species was first stated by Boveri in 1900t.

The chromosomes of different organisms vary in
number from two in the worm Ascaris up to perhaps
1600, according to Haecker ('09), in certain radiolaria.
Species which apparently are closely related may
differ widely with respect to the number of their
chromosomes, while species of unquestionably re-
mote relationship may have an identical number of
chromosomes in each of their cells. The number of
chromosomes characteristic for a species, therefore,
is in no way an index to the complexity or degree of
differentiation of the species.

Besides the nucleus there may often be identified
in the cytoplasm of the animal cell a tiny body known
as the centrosome. At certain times in the life-cycle
of a cell the centrosome becomes the focal point of
peculiar radiating lines, which play an important
part in the behavior of the cell, particularly during
the period of division. ^

Every cell passes through a cycle of life which may
be compared with that common to individuals. It
is born from another cell ; passes through a vigorous
youth characterized by growth and transformation ;
attains maturity when the metamorphoses of its
earlier life give place to a considerable degree of
stability; and finally, after a more or less extended

18 GENETICS

period of normal activity old age ensues, and death
completes the cycle. In most instances, however,
before this final phase is reached, the cell gives place
to daughter-cells through fission, after the manner
of most protozoans, and a new cell cycle is begun.

Sometimes the road of differentiation has been
traveled so far that it is apparently impossible, as
in the case of the complicated brain-cells, to retrace
these steps of differentiation and begin again. In
such instances the outfit of cells provided in the em-
bryo determines the numerical limit of the cells
available throughout life. When this supply is ex-
hausted no more cells appear to replace those which
have been worn out.

4. MITOSIS

The ordinary process by which two cells are made
out of one is termed mitosis. It occurs constantly,
and particularly during growth, in all cellular organ-
isms. A series of diagrams, modified from Boveri,
illustrating the typical phases of mitosis is given
in Figures 4 to 13.

The resting cell (Fig. 4) is characterized by the
presence of a nuclear membrane, a single centrosome,
and by a chromatin network within the nucleus. In
the beginning of the prophase (Fig. 5) the centrosome
has divided into two parts, while in the early prophasi
(Fig. 6) the two centrosomes have moved farther
apart and definite separate chromosomes have formed
out of the chromatin network. The prophase proper
(Fig. 7) is marked by the vanishing of the nuclear

THE CARRIERS OF THE HERITAGE 19

membrane and the more compact form of the chromo-
somes. At the end of the prophase (Fig. 8) the chro-
mosomes have come to lie at the equator of the cell,

F7g.4. The nesting cell FJo,.5. Beginning Prophase fl£.6. Earlij Prophase

Fi£. 7 Prophase F7£. 8. End of Prophas* F?£.9. Metaphase

FiC> 10. Beginning Anaphaje

Rg.ll. Anaphase

R£.I2 Beginning Telophase Ro,.l3. End of Telophase
FIGS. 4-13. — Diagrams illustrating mitosis. After Boveri.

being connected by the mantle fibers with the cen-
trosomes, each of which has now come to occupy a
polar position. In the metaphase (Fig. 9) the chromo-
somes split lengthwise, and at the beginning of the

20 GENETICS

anaphase (Fig. 10) these half chromosomes commence
to separate from each other and to move toward
the poles, while the mantle fibers shorten. During
the anaphase (Fig. 11) the cell body lengthens and
begins to divide, while the migration of the half
chromosomes toward the poles is completed. In
the beginning of the telophase (Fig. 12) the half
chromosomes grow until they attain full size and
the division of the cell body into two parts becomes
complete. The mantle fibers have disappeared and
the nuclear membrane begins to re-form around the
chromosomes. Finally, at the end of the telophase
(Fig. 13) the nuclear membrane becomes complete,
the chromosomes break up into a chromatin network,
and two resting cells take the place of the single one
with which the process began (Fig. 4).

5. AMITOSIS

Amitosis, or the formation of two cells from one
without the machinery of mitosis, is comparatively
rare. It occurs in certain rather isolated instances
among animals and plants, particularly in old cells
late in their life-cycle or in cells that are on the road
to degeneration. When amitosis takes the place
of the more elaborate process of mitosis it is fre-
quently, though not always, a signal of the dea^h-
warrant for that particular cell.

6. SEXUAL REPRODUCTION

The mechanism by means of which two cells unite
to make one in sexual reproduction is quite as com-

k

THE CARRIERS OF THE HERITAGE 21

plicated as that of mitosis by which one cell is trans-
formed into two.

In sexual reproduction there are two kinds of germ-
cells, the egg and the spermatozoan respectively,
which take part in producing a new organism. These
cells are structurally unlike each other in nearly
every particular, but each is a true cell, which von
Kolliker made clear as early as 1841, and each has
typically the same number of chromosomes in its
nucleus, a fact more recently determined by van
Beneden in 1883.

The egg-cell is often supplied with one or more
envelopes of protective or nutritive function, and it is
usually distended with stored up yolk, in consequence
of which it is comparatively large and stationary.
The 'result is that whatever locomotion is necessary
to bring the two cells together for union devolves
upon the sperm -cell. Consequently the sperm-cells
are practically nuclei with locomotor tails of cyto-
plasm, and frequently, in addition, with some struc-
tural modification for boring a way into the egg-cell.
They are, moreover, much more numerous than the
egg-cells, so that although many go astray, never
fulfilling their mission, the chances are nevertheless
good that some one of them will reach the egg and
« JFect fertilization.

Ordinarily only one sperm enters the egg, but
when several succeed in penetrating into the cyto-
plasm only one proceeds to combine with the egg
nucleus, that is, only one sperm nucleus is normally
concerned in the essential process of fertilization.

22 GENETICS

It was formerly thought by the school of "ovists"
that in fertilization the essential process is a stimu-
lation of the all important egg by the sperm. The
opposing school of "spermists," on the other hand,
regarded the egg simply as a nutritive cell the func-
tion of which is to harbor the all important sperm.
It is now known that both the egg- and the sperm-cell
are equally concerned in fertilization, which consists
in the union of their respective nuclei within the
cytoplasm of the egg.

7. MATURATION

Certain preliminary changes of a preparatory
nature, termed maturation, regularly precede the
union of the nuclei of the two sex-cells in fertiliza-
tion.

These maturing changes result in reducing the
outfit of chromosomes in each sex-cell to one half the
original number, a process which is necessary in
order to maintain the chromosome count which is
characteristic for any particular species and which is
known to exist unbroken from generation to genera-
tion. If there were no such reduction, then the
fertilized egg, formed by the union of egg and sperm
nuclei, would contain double the characteristic
number of chromosomes, and during the formation of
a new individual, the number in all the cells arising
by mitosis from such a fertilized egg would like-
wise be double. When the germ-cells of such indi-
viduals unite in fertilization, the original number of
chromosomes would be quadrupled, and so on in

THE CARRIERS OF THE HERITAGE 23

geometric progression throughout subsequent genera-
tions. In 1883, too late for Darwin to learn of it,
van Beneden discovered the important fact that the

Primordial Sex Cells

Mani) similar cell divisions

MATURATION

Spermatocyte 1- OoqU
<-Spermalid5 2.-Oocijte-

^

FIG. 14. — Scheme to illustrate maturation of germ-cells.
j

nature germ-cells, as expected, actually contain only
naif the normal number of chromosomes.

'"lie mature egg- or sperm-cell, with half its normal
number of chromosomes, is termed a gamete (marry-

24 GENETICS

mg cell), while the fertilized egg which is formed by
the union of two gametes (mature egg- and sperm-
cell), and which consequently has the characteristic
number of chromosomes, is called a zygote (yoked
cell).

A diagrammatic representation of the process of
maturation is shown in Figure 14.

The number of chromosomes (not shown in the
diagram) remains constant in each germ-cell respec-
tively until the division of spermatids into sperma-
tozoa, and of the second oocytes into mature eggs aid
second polar cells, when it is reduced to one half the
normal number. As spermatozoan and mature egg
unite in fertilization, the original number of chromo-
somes is restored in the fertilized egg (zygote).

V

8. FERTILIZATION

The stages concerned in a typical case of fertiliza-
tion, according to Boveri, are illustrated in Figures
15 to 23.

In Figure 15 the "head" and the "middle piece"
of the sperm-cell have penetrated into the egg ctyto-
plasm, while in Figure 16 the tail of the sperm-cell
has become lost and the middle piece, which furnished
the centrosome, has rotated 180° so that it ,is
between the nucleus, or head, of the sperm-cell anl
that of the egg-cell. Figure 17 shows an increase in
the size of the sperm nucleus and a division of the
centrosome into two parts which begin to migrate
towards the poles. This process of polar migration
of the centrosomes is carried further in Figure 18 as

THE CARRIERS OF THE HERITAGE 25

fi^.16. Approach of 5per«
Nucleus

Fig.15. Entry of Sperm FI&16. Loss of SpermTdll R$17. Division of Centrosome

ffg.ia Increase of Sperm F/(J.ao. Formation of
Nucleus Chromosomes

F7&2.1. Splitting of Chromosomes

. Anaphase

Ff^.SS.Two-celled Stap,e

FIGS. 15-23. — Diagrams illustrating fertilization. After Boveri.

26 GENETICS

well as the increase in the size of the sperm nucleus,
until in Figure 19 the process is complete so that the
centrosomes have assumed a polar position and the
sperm nucleus is equal in size to the egg nucleus
and lies in contact with it. In Figure 20 the chro-
matin network of the two nuclei has formed into an
equal number of chromosomes which in each case
is half the number characteristic for the species.
Figure 21 shows the complete disappearance of the
nuclear membrane, a process that had already begun
in the preceding figure, and also the arrangement of
the chromosomes, connected with mantle fibers, in the
equatorial plane where the former split longitudinally.
In Figure 22, when the half chromosomes thus formed
pull apart and migrate toward the poles, the segmenta-
tion of the fertilized egg has begun, and there finally
occurs, as shown in Figure 23, the two-celled stage
following fertilization in which each cell contains the
normal number of chromosomes, half of which came
from the egg and half from the sperm.

9. PARTHENOGENESIS

Fertilization is by no means an essential process in
the formation of a new individual, even in those ani-
mals which produce both eggs and sperms. Many
animals and plants reproduce parthenogenetically,
that is, the egg-cell may develop without first uniting
with a sperm-cell. In these instances the chromo-
somes of the egg are not halved during maturation,
and the offspring, therefore, have the same number

THE CARRIERS OF THE HERITAGE 27

>«*

v*

of chromosomes as the parent, since they are simply
fragments of the parent.

Professor Loeb, by the use of certain chemicals,
has succeeded in doing artificially what apparently
is never accomplished in nature, namely, making an
egg that normally requires fertilization develop par-
thenogenetically.

10. THE HEREDITARY BRIDGE

Whatever may ultimately prove to be deter-
miners of the hereditary characters which appear in
successive generations, it is obvious that, in any
event, such determiners must be located in the zygote,
that is, in the fertilized egg. This single cell is the
actual bridge of continuity between any parental
and filial generation. Moreover, it is the only
bridge.

In the majority of animals the egg develops enT
tirely outside of and independent of the mother^
thus limiting to the egg-cell itself all possible~mater-
nal contributions to the offspring. Although there
is abundant evidence that half of the filial char-
acteristics come from the male parent, the only
actual fragment of the paternal organism given over
to the new individual is the single sperm-cell, which
unites with the egg in fertilization, and the whole
of this even is not usually concerned in the process
of fertilization. The entire factor of heritage is
packed into the two germ-cells derived from the re-
spective parents and, in all probability, into the
nuclei of these germ-cells, since the nuclei are ap-

28 GENETICS

parently the only portions of these cells that in-
variably take part in fertilization. To the new
individual developing by mitosis from the fertilized
egg into an independent organism, the factors of
environment and training referred to in Figure 1
are subsequently added.

When it is remembered that the human egg-cell
is only about ^ih of an inch in diameter, a gigantic
size as compared with that of the human sperm-cell,
and, furthermore, when one passes in rapid review
the marvelous array of characteristics which make
up the sum total of what is obviously inherited in
man, the wonder grows that so small a bridge can
stand such an enormous traffic. A sharp-eyed patrol
of this bridge as the strategic focus of heredity is
proving to be one of the most effective points of
attack in the entire campaign of genetics.

It is not desirable at this time to discuss possible
ways in which the determiners of the heritage, what-
ever they may be, are originally packed into the
germ-cells, for this question can be more conven-
iently considered in a later connection. It is im-
portant at present, however, to emphasize the ob-
vious conclusion that determiners of heredity must
inevitably be present in the germ-cells in order to
account for the fact of "organic resemblance based
on descent" between parents and their progeny.

11. THE DETERMINERS OF HEREDITY

What are the determiners of hereditary qualities ?
Do they actually exist in the germ-cells as visible

THE CARRIERS OF THE HERITAGE 29

entities, and is there such a thing as a mechanical
basis for heredity as the German embryologist
Wilhelm His suggested years ago when he wrote:
"It is a piece of unscientific mysticism to suppose
that heredity will build up an organism without
mechanical means " ? Can we find these determiners
by the aid of microscopes and differential stains,
or are they some sort of intangible entities, such as
enzymes or hormones or the like, which only the
chemist can detect ?

Whatever the answer to these questions, it may at
least be affirmed that the determiner represents the
adult structure without resembling it. It is something
which controls the unfolding of the developing or-
ganism with respect to both quantity and quality,
and which also governs the time and rate of appear-
ance of its various characteristics so that certain
combinations rather than others shall come about
in definite sequence. To use the words of Conklin :
"The mechanism of heredity is the mechanism of
differentiation."

12. THE CHROMOSOME THEORY

Certain investigators, who seek a morphological
basis for heredity, regard the chromosomes as the car-
riers of the heritage; in other words, as the source
of the determiners of ontogeny or the effective
factors in the process of differentiation.

A few of the grounds for this theory are briefly
indicated below.

First: In spite of the great relative difference in

30 GENETICS

size between the egg-cell and the sperm-cell, in hered-
ity the two are practically equivalent, as has been
repeatedly shown by making reciprocal crosses be-
tween the two sexes. The only features that are
apparently alike in both the germ-cells are the
chromosomes. The inference is, therefore, that they
contain the determiners which are the causal factors
for the equivalence of adult characters in heredity.
The existence of an extra chromosome in probable
connection with the matter of sex is, as will be pointed
out later, an exception to the exact chromosome
equivalence of the two sexes, which only goes to
strengthen the supposition that the chromosomes are
the carriers of hereditary qualities since extra chromo-
somes are always associated with the character of sex.

Second: The process of maturation, which always
results in halving the chromosome material of the
germ-cells as a preliminary step to fertilization, is
a series of complicated manceuvers not practised by
other cells. During this process no other part of
the cells appears to play so consistent and important
a role as the chromosomes. Provided they act as
hereditary carriers, their peculiar behavior during
maturation is just what is needed to bring together
an entire complement of hereditary determiners out
of partial contributions from two parental sources.

Third : Sometimes abnormal fertilization occurs, as
in the case when two or more sperm-cells, instead of
one, enter the egg cytoplasm and unite with the egg
nucleus. This unusual performance has been artifi-
cially induced by chemical means in the case of sea-

THE CARRIERS OF THE HERITAGE 31

urchins' eggs. The fertilized egg, or zygote, thus
formed with an excess of male chromosomes, re-
sults in the development of abnormal larvae. It is
thought that a causal connection may exist, there-
fore, between the additional male chromosomes in
the fertilized ovum and the abnormalities of the
progeny.

Fourth: The fact that chromosomes may retain
their individuality throughout the complicated phases
of mitosis, as has been proven in some instances,
agrees with the corresponding fact that certain
characteristics of the somatoplasm maintain their
individuality from generation to generation.

Moreover, certain chromosomes in the fertilized
egg have been identified with particular features in
the adult developing from that egg. Tennent sum-
marizes his recent work on Echinoderms (1912)
by the statement that from a knowledge of the
chromosomes in the parental germ-cells, particular
characters in the adult hybrids may be predicted,
and, conversely, that from the appearance qf sexually
mature hybrids the character of certain chromosomes
in their germ-cells may be predicted.

Again, the correlation of a particular chromosome
in the germ-cells with a definite adult character,
namely sex, has been repeatedly demonstrated in
connection with the so-called "extra chromosome"
to which reference has already been made.

Fifth: Finally, excellent evidence of a definite
causal connection between certain chromosomes of
the germ-cells and particular somatic characters has

32 GENETICS

been furnished by certain critical experiments upon
the eggs of sea-urchins. Boveri found that he was
able in some instances to shake out the nuclei bodily,
chromosomes and all, from the mature eggs of the
sea-urchin, Sphcer echinus, and when there was added
in sea water to such enucleated eggs the sperm-cells
of an entirely different genus of sea-urchin, namely,
Echinus, the Echinus sperm-cells entered the Sph&r-
echinus eggs, which had been robbed of their nuclei,
and from this peculiar combination larvae developed
which exhibited only Echinus characters!

Such cumulative circumstantial evidence as the
foregoing has convinced many that in the chromo-
somes we have visibly before us the carriers of
heredity.

Several biologists, however, raise an objecting
voice to this theory, protesting against the mo-
nopoly of the heritage by the chromosomes. They
point out that there always exists an intimate
physiological relationship between the nucleus and
the cytoplasm, and that it is unreasonable to expect
the isolation of one from the other /'since the two
mjist-jdways act toge^he^_as_4J^j±s__gf jm organic cell
unit^ c^te^-^^**^50"5 "Hy*''"1 ff""'f et^E***^

In sexual reproduction, moreover, some small
amount at least of spermatic cytoplasm in the form
of the so-called "middle piece," which is situated
between the head and the tail of the sperm-cell
(Fig. 15), may enter the egg about -to be fertilized
along with the sperm " head " or nucleus, containing
the chromosomes. In this way the cytoplasm of the

THE CARRIERS OF THE HERITAGE 33

male sperm-cell may not necessarily be entirely
excluded from taking part in the formation of the
zygote. As a matter of fact, this extra-nuclear part
of the sperm-cell sometimes apparently forms the
centrosome of the fertilized egg and in consequence
may have a hand, as well as the nucleus with the
chromosomes, in determining what follows.

13. THE ENZYME THEORY OF HEREDITY

It is not unlikely that the key to this whole prob-
lem will be furnished by the biochemists and that the
final analysis of the matter of the heritage-carriers
will be seen to be chemical rather than morpho-
logical in nature.

It has been found that the blood of greyhounds
and dachshunds is chemically different, although
from a morphological point of view it is apparently
identical.- The idea of "individual albumen" or
"protein specificity" for each animal of a species,
to say nothing of the animals of different species,
has been advanced as not improbable.

Miescher has shown that an albumen compound
having only forty carbon atoms, a number by no
means unusual, would make possible a million com-
binations of atoms or isomers.

The possibilities in this direction seem to be un-
limited if we take into consideration those invisible
actuators of chemical processes, the enzymes, which
the chemist brings forward with the prodigality of
an astronomer dealing in star-dust, to explain dif-
ferent chemical .reactions.

34 GENETICS

-"Montgomery has suggested that the chromosomes
themselves may be masses of enzymes although, ac-
cording to the chemist, enzymes are not morpho-
logical entities, since they seem to be able to flourish
and maintain their identity while bringing about
chemical reactions in their neighborhood without
being visibly demonstrable.

As said before, it is quite likely that in the final
analysis heredity will be reduced to a series of chemi-
cal reactions dependent upon the manner in which
various enzymes initiate, retard, or accelerate suc-
cessive chemical combinations occurring in the pro-
toplasm. When the same enzymes act upon the
same ' chemical combinations in successive genera-
tions, they bring about that "organic resemblance"
known as heredity.

E. B. Wilson, whose brilliant work in the entire
field of cell activity makes it possible for him to
speak with authority, has recently said : "The es-
sential conclusion that is indicated by cytological
study of the nuclear substance is, that it is an ag-
gregate of many different chemical components
which do not constitute a mere mechanical mixture,
but a complex organic system and which undergo
perfectly ordered processes of segregation and dis-
tribution in the cycle of cell life. That these sub-
stances play some definite role in determination is
not mere -assumption, but a conclusion based upon
direct cytological experiment and one that finds
support in the results of modern chemical research."

THE CARRIERS OF THE HERITAGE 35

14. CONCLUSION

The supposition that 'the chromosomes, with cer-
tain chemical reservations, are the morphological
carriers of the heritage forms an excellent working
hypothesis, and this chapter may suitably be closed
with a second quotation from Professor Wilson.
" In my view studies in this field are at the present
time most likely to be advanced by adopting the
comparatively simple hypothesis that the nuclear
substances are actual factors of reaction by virtue
of their specific chemical properties; and I think
that it has already helped us to gain a clearer view
of some of the most puzzling problems of genetics."

CHAPTER III

VARIATION

1. THE MOST INVARIABLE THING IN NATURE

IN the introductory chapter it was shown that
"organic resemblance based on descent," by which
is meant heredity, is due principally to the fact that
offspring are material continuations of their parents
and consequently may be expected to be like them.
The fact that this is the case in the great majority of
instances has given rise to the popular formula,
"like produces like," as a rule of heredity.

But this formula by no means always fits the facts.
Like often produces something apparently unlike.
For instance, two brown-eyed parents may produce
a blue-eyed child, although brown-eyed children are
more usual from such a parentage. It is a common
experience, indeed, for breeders of plants and animals
to meet with continual difficulties in getting or-
ganisms to "breed true."

On the other hand, it is exactly these variations
which so constantly interfere with breeding true
that furnish the sole foothold for improvement. Jf
all organisms did breed strictly true, one generation
could not stand on the shoulders of the preceding
generation, and there would be no evolutionary
advance.

36

VARIATION 37

The most invariable thing in nature is variation.
This fact is at once the hope and the despair of the
breeder who seeks to hold fast to whatever he has
found that is good and at the same time tries to
find something better. When the similarities and
dissimilarities between succeeding generations are
clear, then heredity can be explained. The entire
subject of variation is intimately and inevitably
bound up with any consideration of genetics.

2. THE UNIVERSALITY OF VARIATION

Much of the variation in nature is patent to the
most casual observer, but it requires a trained eye to
see the universal extent of many minor differences.
A flock of sheep may all look alike to a passing stran-
ger, but not to the man who tends them. A dozen
blue violet plants from different localities might
easily be identified by the amateur botanist as be-
longing to the same species when, to a specialist
on the genus Viola, unmistakable differences would
doubtless be clearly apparent.

The fact that every attempt at an intimate ac-
quaintance with any group of organisms whatsoever
invariably reveals previously unrecognized varia-
tions, indicates that variability is much more wide-
spread in nature than is commonly believed.

The key to Japanese art, as pointed out by Dr.
Nitobe, consists in being natural and in faithfully
copying nature. It is for this reason that the Jap-
anese artist makes each object that he produces

38 GENETICS

unique, because nature herself, whom he strives to
follow, never duplicates anything.

The Bertillon system of personal identification is
based upon the constancy of minor variations found
in each individual. Its importance is shown in
Figure 24. The faces of the criminals there pictured
would be easily confused by the ordinary observer, but
an examination of their thumb prints shows unmis-
takable differences between these three individuals.

3. KINDS OF VARIATION

A brief enumeration of some of the kinds of varia-
tion will reveal their diverse character.

a. With respect to their nature variations may be
morphological, physiological, or psychological. Under
morphological variations are included differences in
shape, size, or pattern as well as differences in number
and relation of constituent parts.

Differences in activity are of a physiological nature.
Many animals in captivity are less fertile than when
free, while different individuals are well known to
vary widely with respect to their susceptibility to
disease. Nageli, for example, reports the presence of
tubercles in 97 per cent of the cases in five hundred
autopsies, although a majority of the deaths in ques-
tion was not due to tuberculosis at all, — a fact which
indicates a great diversity in the resistance of differ-
ent individuals to the tubercle bacillus.

Psychological variations in man, such as those
which determine the disposition or mental traits of
individuals, are apparent to every one.

VARIATION 39

b. With respect to their duplication variations may
be single or multiple. A legless lamb 1 is an ex-
ample of a single variation or "sport." Four-leaved
clovers, on the contrary, are multiple for the reason
that this variation, although not common, neverthe-
less occurs frequently.

c. With respect to their utility variations may be
useful, indifferent, or harmful to the organism possess-
ing them. Useful variations are of the kind empha-
sized by Darwin as being effectively made use of in
natural selection. Indifferent variations, on the
other hand, are those which apparently do not play
an important part in the welfare of their possessor,
as, for example, the color of the eyes or of the hair.
Finally, the degree of degeneration in certain organs
may be cited as an illustration of harmful variations.
The amount of closure of the opening from the in-
testine into the vermiform appendix in man is an ex-
ample of a harmful variation, since the larger the
opening, the greater is the liability to appendicitis.

d. With respect to their direction in evolution varia-
tions may be either definite (orthogenetic) or indefinite
(fortuitous).

Paleontology furnishes numerous instances of the
former category, such as the series of variations
from a pentadactyl ancestor, all apparently tending
in one direction, which have culminated in the one-
toed horse. The fact that the paleontologist deals
historically with a completed phylogenetic series in"
which the side lines lack prominence, while the suc-

1 " A Peculiar Legless Lamb." Stockard. Biol. Bull, xiii, p. 288.

40 GENETICS

cessful line stands out with distinctness, makes it easy
for him to view successive variations as orthogenetic,
that is, as definitely directed in one course either
through intrinsic (Nageli) or extrinsic (Eimer) causes.

Fortuitous or chance variations in all possible
directions furnish the repertory of opportunity,
according to Darwin, from which natural selection
picks out those best adapted to survive in the strug-
gle for existence.

e. With respect to their source, variations may be
somatic or germinal. Somatic, or body variations,
arise as modifications due to environmental factors.
They are individual differences which may be quite
transitory in nature, while germinal variations may
arise without regard to the environment, are deep-
seated, and of racial rather than of individual sig-
nificance.

/. With respect to their normality variations may
fall within expected extremes and thus be considered
normal, or they may be outside of reasonable expec-
tations and consequently be reckoned as abnormal,
as in the case of a two-headed c#lf .

g. With respect to the degree of their continuity varia-
tions'may form a continuous series, grading into each
other by intermediate steps, or they may be discon-
tinuous in character. An example of continuous
variation is the height of any hundred men one might
chance to meet, which would probably represent all
intermediate grades from the highest among the
hundred to the lowest.

The number of segments in the abdomen of a

VARIATION 41

shrimp, on the other hand, which may, for instance, be
either eight or nine but cannot be halfway between,
illustrates what is meant by discontinuous variation.
The widespread occurrence of this later category of
variations has been pointed out by Bateson in his
encyclopedic volume "On Materials for the Study
of Variation."

h. With respect to their character variations may be
quantitative or qualitative. A six-rayed starfish
represents a quantitative variation from the normal
number of five rays, whereas a red variety of a flower
may differ chemically from a blue variety, or a bitter
fruit may differ from a sweet fruit in a qualitative
way dependent upon the chemical constitution of
the fruit in question.

i. With respect to their relation to an average stand-
ard variations may have a fluctuating distribution
around an arithmetical_mean, as when some of the
offspring have more and some less of the parental
character, or the variations in the progeny may all
center about a new average quite distinct from the
parental standard and consequently come under the
head of mutations.

j. Finally, and most important in the present
connection, with respect to heritability, variations may
possess the power to reappear in subsequent genera-
tions, or they may lack that power. It is this aspect
of variability which bears most directly upon genetics.

Other possible categories might be mentioned, but
a sufficient number have been cited to show the
great diversity of variations in general.

42 GENETICS

4. METHODS OF STUDYING VARIATIONS

Roughly stated, there are three ways of studying
variations : first, Darwin's method of observation
and the description of more or less isolated cases ;
second, Galton's biometric method of statistical
inquiry ; and third, Mendel's experimental method.
The second of these methods will be considered in
this chapter.

5. BIOMETRY

The new science of biometry, that is, the applica-
tion of statistical methods to biological facts, has
been developed within recent years. Sir Francis
Galton, Darwin's distinguished cousin, may be re-
garded as the pioneer in this field of research, while
Karl Pearson and his disciples constitute the modern
school of biometricians.

Although mathematical analysis of biological
data when not sufficiently ballasted by biological
analysis of the same facts may sometimes lead the
investigator astray, yet often the only way to for-
mulate certain truths or to analyze data of some
kinds is by resort to statistical methods. Biome-
tricians are quite right in insisting that it is frequently
necessary to go further than the fact of variation,
which may be apparent from the inspection of an
individual case, and to deal with cumulative evidence
as presented through statistical analysis.

In matters of heredity, however, facts as they
occur in single cases and definite pedigrees seem to

VARIATION

47

mean of all the groups making up the polygon, the"
fewer will be the individuals that represent it.

7. THE INTERPRETATION OF VARIATION POLYGONS
a. Relative Variability

The statistical determination of the relative vari-
ability of two lots of organisms with respect to a
certain character may be illustrated by the case of
the oyster-borer snail, Urosalpinx cinereus, as seen in
the accompanying table.

ATLANTIC AND PACIFIC SHELLS COMPARED

LOCALITY

NUMBER
OF SHELLS

A.M.

<T

PROB-
ABLE
ERROR

r West Shore . .

1,001

58 928

2.339

± 0352

Penzance Point ....

1,002

61.718

2.737

±.0412

Nobska Point ....

1,002

61.737

2.152

±.0324

Woods

Nobska Point ....

1,001

61.944

2.234

±.0337

Hole

Nobska Point ....

496

66.944

2.366

±.0507

Barnacle Beach ....

998

63.932

2.604

±.0393

BigWepecket . . . .

1,006

57.426

2.052

±.0308

Mid-Wepecket ....

500

57.606

2.098

,±.0447

Average for Mass. . .

61.066

2.335

±.0386

Cali-
fornia

[ Belmont Beds ....
[ San Francisco Bay . . .

1,008
520

59.051
60.892

3.023
3.361

±.0454
±.0703

Average for Cal. . . .

59.664

3.138

±.0538

Difference

803

The obvious conclusion to be drawn from this
table is that the snails which were unintentionally
carried from the Atlantic coast to California in the

48 GENETICS

transplantation of oysters show more variation in
their new habitat than they did in the old one with
respect to the particular character measured, namely,
the relative size of the mouth aperture compared with
the height of the entire shell.1

b. Bimodal Curves

Sometimes two conspicuous modes, make their ap-
pearance in a frequency polygon, as Jennings found,

20

10

20 24 2.Q 32 36 40 44 48 52 56 60 64

FIG. 26. — The body width of a population of the protozoan Paramecium,
showing a polygon with two modes. A, Paramecium aurelia. B,
Paramecium caudatum. After Jennings.

for example, in measuring the body width of a popu-
lation of the protozoan Paramecium (Fig. 26) .

1 " Variation in Urosalpinx." Walter. Amer. Nat. 1910, Vol. XLIV,
pp. 577-594.

VARIATION

49

It was subsequently found that the two modes in
this polygon were due to the fact that the material
in question was a mixture of two closely related
species, Paramecium aurelia and Paramecium cauda-
tum, the individuals of which arranged themselves
around their own mean in each instance.

Number ol J 3
ribs

15 16 17 16 19

El

FIG. 27. — The ribs of leaves from two beech trees. When put together
they form a polygon which does not reveal its double origin. From
data by Pearson.

Although such an explanation does not always
turn out to be the right one, the biometrician is
led to suspect when a two or more moded polygon
appears that he is dealing with a mixture of more
than one kind of material, each of which fluctuates
around its own average.

Heterogeneous material, it should be noted, does
not always give a bimodal curve. For example,
if Pearson's two lots of beech leaves mentioned

50 GENETICS

above are mixed together, they form a regular
series from the inspection of which no one could infer
their double origin. (See the heavy line in Figure 27.)

c. Skew Polygons

The direction in which variations are tending
may sometimes be determined by the statistical
method. As an illustration of this may be cited the
number of ray florets on 1000 white daisies (Chrys-
anthemum leucanthemum) , 500 of which were col-
lected at random by the writer from a small patch
in a swampy meadow in northern Vermont, while
the other 500 were selected in the same random
manner upon the same day from a dry hillside pas-
ture hardly more than a stone's throw distant.
Among these two lots of daisies the number of ray
florets varies from twelve to thirty-eight and their
frequency polygons, as shown in Figure 28, form
what are termed "skew polygons," because the mode
in each case lies considerably to one side of the arith-
metical mean.

It will be seen that lot A from the swampy meadow,
which in spite of the greater fertility of the soil and
tKe unquestionably greater luxuriance of the plants
themselves, produced heads with fewer florets,
fluctuates around the number 21, while the dry
pasture population J5, characterized by blossoms
which were in general noticeably smaller, fluctuates
around the number 34.

VARIATION

51

The habitats of the two lots were so near together,
however, that there was probably a considerable
intermixture of the two types, as shown by the
tendency of each polygon to produce a second mode.

13 H 15 16 17 18 19 ZO 21 22 23 Z*

26 11 28 Z9 SO 31 82 55 34 »S 36 37 38 39

FIG

28. — Variation in the ray florets of the white daisy (Chrysanthe-
mum leucanthemum) . A, from a swampy meadow. B, from a hillside
pasture near by. Both the polygons are " skew " because in each case
there is an admixture of the other type. The distinction between
the two types is due to heredity rather than to environment.

Thus the A polygon shows that there is an increasing
tendency or variability in the twenty-one floret
type toward the thirty-four floret type, due probably
in this particular instance to invasion resulting
from the proximity of the B colony.

52 GENETICS

8. GRADUATED AND INTEGRAL VARIATIONS

It is comparatively simple to treat statistically
integral variations, illustrations of which have been
given in the case of beech-leaf ribs, starfish rays, and
daisy florets, all of which are characters that can be
readily counted. In the same way any measurable
character, such as the size of snail shells, may fall
into easily limited groups, as, for example, 10 to
11 mm., 11 to 12 mm., 12 to 13 mm., etc. It is
somewhat more difficult to classify variations when
color or pattern is the character in question, since it
then becomes necessary to define certain arbitrary
limits for each class of the series within which to
group the individual variants.

Tower, in his famous researches on potato-beetles,
encountered variations in the pigmentation of the
pronotum all the way from entire absence of color
to complete pigmentation. By cutting up - this
continuous series of variations into arbitrary groups
of equal extent, however, it was quite possible to
arrange the data so that they could be statistically
treated just as conveniently as the integral variations
mentioned above. Groups or classes of this kind
are termed graduated variations.

9. THE CAUSES OF VARIATION

With respect to the causes of variation authori-
tative biologists have taken different points of view.

a. Darwin considered variations as axiomatic.
An axiom is self-evident, requiring no explanation.

VARIATION

53

The absence of variations in organisms rather than
the occurrence of variations is, from this point of
view, the phenomenon requiring an explanation.
Although Darwin himself spent some time in point-
ing out the universal occurrence of variability, he
accepted it as a primary fact and proceeded from it
as a starting point without attempting to seek its
causes.

b. Lamarck and his followers have regarded the
causes of variation either as extrinsic, that is, refer-
able to external factors making up the environment of
the organism, or as intrinsic or physiological, that
is, based upon the efforts which an organism puts
forth to fit into its particular environment success-
fully. The causes of variation are to be sought ac-

SO 35 40 45 50 55 60 65 70 75 80 85 9O 95 100 /o
< Ratio of height of head to length of shell >

FIG. 29. — Schematic curve of the head height of Hyalodaphnia under
various conditions of nourishment. Adapted from Woltereck.

cording to the Lamarckian school, in the "environ-
ment" and "training" sides of the triangle of life
rather than in the "heritage" side (Fig. 1).

For example, Woltereck, by controlling the single

54

GENETICS

extrinsic factor of food supply, was able to modify
the height of the "head" of the microscopic fresh-
water crustacean, Hyalodaphnia, in the remarkable
manner indicated in Figure 29. When poor food

Number of
Fl

FIG. 30. — Variations in the number of stamens in the flowers of the " live-
for-ever" (Sedum spectabile) under various controlled conditions.
For detailed description, see text. After Klebs.

was supplied, the percentage of the head height to
that of the body averaged hardly forty, while with
rich food it was increased to over ninety.

Similarly Klebs succeeded in changing at will the
number of stamens in the common " live-for-ever,"
Sedum spectabile, by manipulating the environment
in which the plants were kept. Some of his results
are shown in Figure 30. Polygon A combines the
data for 4260 flowers which were raised in well-fer-
tilized dry soil under bright light ; polygon B repre-
sents 4000 flowers grown in a moist greenhouse
under red light ; and polygon C includes 4390 flowers

VARIATION 55

from well-fertilized soil in moist hotbed conditions
under a weak light.

c. Weismann, on the contrary, believes that the
causes of variation, at least of heritable variations,
are intrinsic or inborn in the germplasm. His con-
ception of sexual reproduction is that it is a device
for doubling the possible variations in the offspring
by the mingling of two strains of germplasm (am-
phimixis). By far the greater number of observa-
tions recorded go to substantiate this theory.

Tower found among his potato-beetles, for exam-
ple, that two strains reared in the same environment
showed striking differences in variation, — a fact
necessarily due to intrinsic rather than to extrinsic
factors. Similar cases may be recalled by any one.

d. Lastly, Bateson, whose work "On Materials
for the Study of Variation" already cited is a classic,
takes the agnostic attitude that it is rather futile
to guess at the causes of variation before the facts
are well in hand. He consequently discourages such
attempts by saying : "Inquiry into the causes of
variation is, in my judgment, premature."

In conclusion, the words of Darwin written half
a century ago — "Our ignorance of the laws of
variation is profound" -may still be appropriately
quoted, notwithstanding the fact that in biometry
we have at least an excellent analytical method by
means of which considerable insight into variation
is being gained.

CHAPTER IV
MUTATION

1. THE MUTATION THEORY

AMONG the possible kinds of variation already
hinted at are so-called mutations which are clearly
defined from the fluctuating variations to which
reference has just been made.

Darwin was fully aware of the existence of muta-
tions or "sports" and incidentally gave time to
their consideration, but the great task which he
accomplished in such a masterly manner was to
overthrow the widespread and deep-seated belief
of his day in a sudden special creation of distinct
species. To this end he marshaled evidence in
support of the gradual transition of one species into
another, emphasizing fluctuations rather than muta-
tions which seemed to him to play a minor role in
the origin of species.

It remained for the Dutch botanist Hugo de Vries
to analyze the character of mutations. There is
something distinctly suggestive of Darwin's method
in the fact that de Vries worked in silence for twenty
years before he gave to the world the " Mutations-
theorie" with which his name will forever be con-
nected.

56

MUTATION 57

2. MUTATION AND FLUCTUATION

A mutation is something qualitatively new that
appears abruptly without transitions and which breeds
true from the very first. To use the musician's
phraseology, it is not a variation elaborated upon an
old theme, which would correspond to a fluctuating
variation, but it is an entirely new theme. The
difference between mutations and fluctuating varia-
tions is generally not one of quantity or magnitude,
although it sometimes may be so, — since muta-
tions are often much smaller than fluctuations.
Mutations are discontinuous in the same sense that
chemical combinations, such as carbon monoxide
(CO) and carbon dioxide (CO2), are discontinuous,
but the leap from one to the other may be so small
that frequently it is difficult to ascertain by inspec-
tion alone whether the difference is due to a mutation
or a fluctuation. The test comes in breeding, for the
progeny of a fluctuation will vary around the old
average of the parental generation, while the progeny
of a mutation will vary around a new average, set
by the mutation itself.

When a series of mutations is treated statistically,
it does not arrange in frequency polygons as readily
as a series of fluctuations do. The latter mass
around the average standard according to the laws
of chance much in the same way that a hundred shots
by a good marksman may center around a bull's-
eye. Mutations never act in this way. They find
no correspondence even with wild shots at the bull's-

58 GENETICS

eye. They are shots directed at a different target
altogether.

To the student of heredity there are two distinc-
tions of prime importance with respect to mutations.
First, that they usually appear full-fledged without
preparatory stages, and second, that they breed true
from the start. Fluctuations, on the contrary, ordin-
arily "revert" to the parental type in subsequent gen-
erations. The great practical importance to the
breeder of a knowledge of these heritable mutations
is at once apparent.

3. FREAKS

A further distinction should be made between
mutations and so-called freaks or monstrosities,
namely, that the former breed true, while the latter
do not. A human physical deformity, such as a
club-foot, for example, or a humped back, is not a
mutation, because it does not reappear as a heritable
character. Variations of this kind are not predeter-
mined in the germplasm, but are usually instances of
something that went wrong during the development
of the individual somatoplasm.

Thus, among normally "right-handed" snails "left-
Jianded" individuals have occasionally been dis-
covered which, when bred, were found to produce
all normal "right-handed" progeny. They are
therefore not mutations at all, but freaks or mon-
strosities due probably to some unusual occurrence
early in the cleavage stages of the embryo.

MUTATION 59

4. KINDS OF MUTATION

De Vries has classified mutations according to
their component units into three categories : pro-
gressive, regressive, and degressive.

Progressive mutations are signalized by the addi-
tion of a new character to the sum of complex char-
acters making up the individual. If rumor may be
believed, Anne Boleyn, the second in the interesting
series of wives of Henry VHI, was a progressive mu-
tant with respect to at least three characters, for she
is said to have been possessed of an extra finger on
each hand, supernumerary mammae, and extra teeth.
Evidences that each of these three characters occur
as heritable mutations is presented in Davenport's
"Heredity in Relation to Eugenics."

Regressive mutations are characterized by the
dropping out of something. Thus albinism is caused
by the absence of pigment or color. Albinic mutants
which breed true are well known, particularly among
mammals, such as rats, mice, rabbits, cats, guinea-
pigs, and even man himself.

Degressive mutations include cases of the return
of a character which was formerly present in the
past history of the race, but which has for generations
been absent or latent. Castle's four-toed race of
guinea-pigs furnishes an example of this class of
mutations. In 1906 Professor Castle discovered a
newly born guinea-pig in one of his pens with four
toes on each hind foot, from which he has successfully
established a four-toed race. The hypothetical an-

60 GENETICS

cestor of the rodents probably had five toes on each
foot, but the normal number in modern guinea-pigs
is four on each of the front feet and three on the
hind feet. The individual .from which Castle has
bred a four-toed race exhibited a degressive muta-
tion, tending toward the ancestral type.

5. SPECIES AND VARIETIES

The doctors have always disagreed regarding a
definition of species. What determines the exclusive
boundaries that shall isolate from their fellows any
particular group of animals or plants has long been
a mooted question, and still remains so.

The Linnaean concept of a species was that of an
exclusive caste of individuals, inflexibly demarked,
over whose high barriers no nondescript tramps
would dare attempt to climb. When an entomolo-
gist of the old Linnsean school encountered an insect
which did not conform to the morphological tradi-
tions of its fellows, the frequent fate of such a non-
conformist was to perish under the boot-heel rather
than to find sanctuary in the cabinet of the pre-
served. Since it was an exception, and a violator
of the divine law of the fixity of species, it deserved
to be annihilated! Those were hard days both for
heretics and for mutations.

The method of the older school of systematists
may be described as one which emphasized differences
and put up barriers that should keep the unlike
apart, at the same time allowing only "birds of a
feather to flock together." It was a brave and sue-

MUTATION 61

cessful attempt to bring order out of chaos by classi-
fying the living world, and it served its purpose well
until Darwin's idea of half a century ago, that the
origin of all species is from preceding species, put an
entirely new face upon the whole matter. Organ-
isms of different species were found to be related to
one another, and even man could no longer escape
acknowledging his poor animal relations. As a
consequence, likenesses rather than differences there-
after claimed the most attention.

During the reconstruction of phylogenetic trees,
which seized the imagination and became the prin-
cipal business of biologists as soon as the " Origin of
Species " was made common property, the crotched
sticks in the woodpile of organisms, that had hitherto
been largely discarded, were most eagerly sought after.
It was just these scraggly sticks, that were neither
trunk nor limb-wood but combinations of both,
which told the story of continuity and were indis-
pensable in building up a reunited whole.

As the analysis of the living world gradually came
to shift from species to individuals, it was shown
that individuals may be regarded simply as ag-
gregates of unit characters which may combine so
variously that it becomes more and more diffi-
cult to maintain constant barriers of any kind be-
tween the groups of individuals arbitrarily called
"species."

The old species of the systematist, upon analysis
into their respective unit characters, dissolve into
numerous "elementary species" and "varieties" dif-

GENETICS

fering from species perhaps only by the addition
or subtraction of a single character, and thus the

5PECIETS /\

FIG. 31. — Diagram to illustrate various ideas about "species." Under
Species A are represented two groups of individuals which are near
enough alike to be placed within a single species, but which are suffi-
ciently unlike each other to constitute the " sub-species " or "varie-
ties " of Darwin. Under Species B are various groups of individuals
distinguished from each other by the addition or loss of one or more
characters. These groups represent the "elementary species" and
"varieties" of de Vries. "The "barrier of Linnaeus" attempted to
separate species absolutely from each other. Darwin sought to find
loopholes in this barrier. To-day attention is directed rather to the
relation between individuals than to the boundaries between species.

possibilities of analytical classification have become
almost limitless.

An elementary species, according to de Vries, is a
progressive mutation differing from the type species

MUTATION 63

by the addition of at least a single character, while
varieties are regressive mutations distinguished from
the parent type by the loss of at least one character.
Both breed true to their respective modifications.

These different concepts of what constitutes a
species, illustrated diagrammatically in Figure 31,
pave the way for a better understanding of muta-
tions in connection with heredity.

6. PLANT MUTATIONS FOUND IN NATURE

The oldest known authenticated case of a plant
mutation is the often cited instance of the "fringed
celandine," Chelidonium laciniatum, which made its
appearance in the garden of the Heidelberg apothe-
cary Sprengel in 1590 among plants of the "greater
celandine," Chelidonium majus. The fringed cel-
andine bred true at once and is now a widespread
and well-known species.

The purple beech has appeared historically as a
mutant among ordinary beeches upon at least three
occasions in widely separated localities, and it has
always given rise to a constant progeny.

The "Shirley poppy," notable for its remarkable
range of color, originated from a single plant of the
small red poppy, Papaver rhoeas, which is commonly
found in English cornfields.

Instances are known of double flowers among
roses, azaleas, stocks, carnations, primroses, petunias,
etc., arising from single flowering plants, the seeds of
which in turn produce double flowers.

64 GENETICS

7. LAMARCK'S EVENING PRIMROSE

The most widely known plant mutations are the
progeny of Lamarck's evening primrose, CEnothera
lamarckiana, because it was these plants that led
de Vries to formulate his mutation theory.

It is believed by botanists in general that this
plant is a native of the southern United States, al-
though it is now, so far as is known, extinct as a
wild species in America, and native specimens are
included in but few American herbaria.

It was exported to London as a garden plant about
1860, and from thence it spread to the continent,
where, escaping from gardens, it became wild in at
•least one locality near Hilversum, a few miles from
Amsterdam. Here, in an abandoned potato field,
it fell under the seeing eye of Hugo de Vries in 1885,
and now both botanist and primrose are famous.

De Vries found among these escaped plants not
only 0. lamarckiana, but- also two other kinds
or mutants, 0. brevistylis, characterized by short-
styled flowers, and 0. Icevifolia, which has smooth
leaves. These two were entirely new species hitherto
unknown at the great botanical clearing-houses of
Paris, Ley den, and the Kew Gardens.

Since the seeds of the CEnothera are produced by
self -fertilized flowers, de Vries felt safe in regard-
ing these plants as mutants rather than hybrids,
and he continued to study them with especial care.
Transplanting the mutants along with representa-
tives of 0. lamarckiana to his private gardens in

MUTATION

65

Amsterdam, where it was possible to maintain them
in normal healthy condition, de Vries was able to
follow their individual histories with certainty.

He found that, out of 54,343 plants of the species
0. lamarckiana grown during eight years, there ap-
peared 837 mutants comprising seven different ele-
mentary species, all of which, with the exception of
0. scintillans, bred true. See table.

MUTANTS OF (ENOTHERA LAMARCKIANA

•

3

00

GENERATION
\

2
O

ALBIDA

OBLONGA

RUBRINER

LAMARCKI

1

i

SCINTILLA

TOTAL

I

1886-7

9

II

1888-9

15,000

5

5

III

1890-1

1

10,000

3

3

IV

1895

1

15

176

8

14,000

60

73

1

V

1896

25

135

20

8,000

49

142

6

VI

1897

11

29

3

1,800

9

5

1

VII

1898

9

3,000

11

VIII

1899

5

1

1,700

21

1

1

56

350

32

53,509

158

229

8

54,343

Some explanatory comment on this table may be
of value.

The seeds in each generation were self-fertilized
lamarckiana.

The mutant gigas occurred once, in 1895. From
the seeds of this one plant were produced 450 true
gigas offspring in the first year, and the strain con-
tinues to breed true.

Albida was first noted in 1895, but de Vries remem-

F

66 GENETICS

bered having seen it before and dismissing it as
pathological. Because of its poverty in chlorophyll
it is a mutant which probably would not maintain
itself successfully in nature, but it breeds constant
under cultivation.

Oblonga always bred true with the exception of
throwing an albida in 1895 and a rubrinervis in 1899.

Of rubrinervis over 2000 invariably bred true,
while nanella bred true in over 20,000 offspring, with
but three exceptions when oblong a characters appeared.

Lata, since it produces only female flowers and so
cannot be self-fertilized, had constantly to be crossed
back with the parent lamarckiana, when it produced
from 15 to 20 per cent lota and 80 to 85 per cent
lamarckiana.

Finally, scintillans which appeared at three separate
times proved constant only in its inconstancy be-
cause it invariably produces a heterogeneous progeny.
The 1895 plant gave 53 per cent lamarckiana, 35
per cent scintillans, 10 per cent oblong a, and 1 per
cent lata. One of the 1896 plants gave 51 per cent
lamarckiana, 39 per cent scintillans, 8 per cent
oblonga, 1 per cent lata, and 1 per cent nanella, while
another 1896 plant gave only 8 per cent lamarckiana,
but 69 per cent scintillans, 21 per cent oblonga, and
2 per cent of nanella and lata together.

These seven elementary species are distinguished
from each other by features which are unmistakable
even to the uninitiated. The old-time systematist
would undoubtedly have regarded them as distinct
species.

MUTATION 67

De Vries' experiments and observations have been
repeated on a large scale and extended, notably by
MacDougall in the New York Botanical Gardens
and by Shall at the Carnegie Institution for Ex-
perimental Evolution, Cold Spring Harbor, Long
Island, and his conclusions have been confirmed in
all essential points. The mutability of 0. lamarcki-
ana is as unmistakable and as diverse in America as
it is in Holland.

A parallel case of a plant caught in the act of giving
rise to mutations is that of the roadside weed Lychnis,
reported by Shull, and the phenomenon is probably
by no means as unusual as is generally believed.
The chief reason why such definite examples of mu-
tation are so infrequently noted and recorded is
because the attention of the investigator has generally
been directed, not to them, but to gradual fluctuating
variations which, according to Darwin's conception,
furnish the material for the operation of natural
selection. Mutations are doubtless much more com-
mon than has been generally supposed, and it is likely
that they will receive more attention in the future
than they have in the past. De Vries rather pointedly
says: "The theory of mutations is a starting-point
for direct investigation, while the general belief in
slow changes has held back science from such in-
vestigations during half a century."

8. SOME MUTATIONS AMONG ANIMALS

In 1791 a Massachusetts farmer, by name Seth
Wright, found in his flock of sheep a male lamb with

68 GENETICS

long, sagging back and short, bent legs resembling
somewhat a German dachshund. With unusual
foresight he carefully brought up this strange lamb
because it was an animal that could not jump fences.
It occurred to this hard-headed Yankee that it would
be much easier to get together a flock of short, bow-
legged sheep, unable to negotiate anything but a low
hurdle, than to labor hard at building high fences.
So it came about that this mutating lamb, in the hands
of a man who appreciated labor-saving devices, be-
came the ancestor of the Ancon breed of sheep.
Later on this breed gave place in public favor to an-
other mutant, the Merino, which produces a superior
grade of wool.

Hornless cattle suffer fewer injuries from one an-
other than horned cattle. It has consequently be-
come quite a general practice among farmers to
"dehorn" their stock surgjcally. It is an obvious ad-
vantage to have cattle born hornless, and many breeds
having this character are now established. In 1889
a mutant among horned stock appeared at Atkinson,
Kansas, in the form of a hornless Hereford. From
this mutant has descended the well-established race
of polled Hereford cattle, constituting a bovine aristoc-
racy with registry books and blue blood all their own.

Taillessness in cats, dogs and poultry, as well as
hairlessness in cattle, dogs, mice and horses, are
further instances of mutations.

Davenport,1 writing of his experiments with poultry,

1 Davenport, C. B., 1909. " Inheritance of Characteristics in Domestic
Fowl." Carnegie Institution of Washington, Publication No. 121.

MUTATION 69

says: "During the past four years I have handled
and described over 10,000 poultry of known ancestry.
Of striking new characters I have observed many,
some incompatible with normal existence; others in
no way unfitting the individual for continued life.
In the egg unhatched I have obtained Siamese twins,
pug jaws, and chicks with thigh bones absent. There
have been reared chicks with toes grown together
by a web, without toenails or with two toenails to
a toe ; with five, six, seven, or three toes ; with one
wing or both lacking ; with two pairs of spurs ; with-
out oil-gland or tail; with neck devoid of feathers;
with cerebral hernia and a great crest; with feather
shaft recurved, with barbs twisted and dichoto-
mously branched or lacking altogether. Of comb
alone I have a score of forms. All of these characters
have been offered to me without the least effort or
conscious selection on my part, and each appeared in
the first generation as well-developed peculiarities,
and in so far as their inheritance was witnessed, each
refused to blend when mated with a dissimilar form."
Bateson (1894), in his "Materials for the Study of
Variation," gives a detailed list of 886 cases of "dis-
continuous variations" among animals, many of which
doubtless belong to the category of mutations, al-
though several must be placed in the non-inherit-
able class of "freaks."

9. POSSIBLE EXPLANATIONS OF MUTATION

It is apparent that the causes of mutations, since
they occur regardless of the environment, are prob-

70

GENETICS

ably of an intrinsic or germinal nature. Evening
primroses display the same mutants whether in
Holland or America, in a wild state or under culti-
vation. Mutations, like poets, are born, not made.
It has been suggested by Standfuss that species
may go through the same kind of a life-cycle that in-
dividuals do, only
taking infinitely
more time to do
it. As shown in
Figure 32, they
are born of other
species and enter
the prodigious
growth period of
infancy and youth,
both of which are

FIG. 32. — Diagram of the relation of repro- characterized by

much fluctuation.

With maturity they gradually become comparatively
stable until the reproductive period is reached, when
they throw off their progeny, as on a tangent. They
finally pass into the excessively differentiated period
of old age, from which there is no recall, although
it approaches in many features the infantile condition,
and end in death or extinction. This cycle is repeat-
edly illustrated by phylogenetic lines of fossil forms
which have long since become extinct.

Beecher has pointed out that, in paleontological
times just before they became extinct, species often
underwent extreme specialization in the form of

MUTATION 71

fantastic shapes, an excessive number of spines or
elaborate sculpturings on the shells as seen among
the ammonites, belemnites, and trilobites, or of
gigantic size as in the dinosaurs, plesiosaurs, and
theromorphs. All of these facts indicate a species-
cycle in which these abnormal features were the un-
mistakable signs of old age.

The reproductive period of a species when mutants
are being thrown off, as of an individual, may ex-
tend over a considerable period of the whole cycle,
or it may be confined to a relatively small segment.
It is possible that in the evening primrose de Vries
may have caught a plant passing through the crucial
period of species-reproduction.

Another reason why so few mutations have as yet
been seen, is because the majority of organisms are
not, during the short span of human observation,
in the reproductive part of their cycles. When it is
remembered that accurate observation with this
object in view has extended over only a brief period,
insignificant in comparison with the vast geologic
stretches of time concerned in species-building, the
marvel is that so much, rather than that so little, has
been seen.

A further suggestion in connection with the pos-
sible sources of mutation is that mutations may be
the results of hybridization, appearing as Mendelian
recessives after crossing. As a matter of fact, Spren-
gel's Chelidonium laciniatum, already cited, when
crossed with Chelidonium majus, behaves according to
such an expectation. This phase of the question,

72 GENETICS

however, may be more suitably considered later after
what is meant by a "Mendelian recessive" has been
made clear.

It is extremely doubtful, however, whether any
recombination of parental characters in a hybrid may
properly be called a mutation, since no character
strictly new is thus produced.

10. A SUMMARY OF THE MUTATION THEORY

The main features of the mutation theory of de
Vries may be indicated as follows : —

a. New species arise abruptly regardless of environ-
ment without transitional forms, and at present they
are not known to arise in any other way.

6. New forms arise as unusual deviations from the
parent form, which itself remains unchanged, al-
though it may repeatedly give rise to similar devia-
tions.

c. New mutations are, from the first, constant, that
is, they produce their like. They do not become
gradually established as the result of natural selection.

d. Among mutations there may occur forms
characterized by the addition of something new, —
progressive elementary species, — as well as forms
lacking something present in the parental type, —
regressive varieties.

e. The same mutation may arise simultaneously in
many individuals instead of as a single '* sport."

/. Mutations do not vary around an arithmetical
mean with respect to the parent form, as is the case

MUTATION 73

with fluctuating variations, but they fluctuate around
a new average of their own, thus forming a discon-
tinuous series with the parent form.

g. Mutations may occur in all directions, that is,
they are not necessarily definite or orthogenetic.

h. Mutations probably appear periodically >

i. Every mutation means a possible doubling of the
species.

j. Useless or insignificant fluctuating variations are
not necessarily the material from which natural selec-
tion must sift out new species.

The bearing of the whole matter of mutation upon
heredity lies in the fact that, contrary to Darwin's
belief, it is apparently mutations, and not fluctuations,
that make up heritable variations. If this supposition
proves to be true, mutations furnish the essential
material in the study of heredity. Consequently,
whatever knowledge we may gain of them has a direct
relation to the entire problem of genetics. (

CHAPTER V

THE INHERITANCE OF ACQUIRED CHARACTERS
1. SUMMARY OF PRECEDING CHAPTERS

HEREDITARY resemblance is due to the derivation of
offspring from the same stock as the parent, and suc-
cessive generations, therefore, are simply periodic
expressions of the same continuous stream of germ-
plasm.

Perfect inheritance, or uniformity of generations,
does not exist, since variations always occur in suc-
cessive generations. It is upon these variations that
evolution depends. Without them there would be
no change of type and consequently no possibility
of evolutionary advance.

Some variations are fluctuating or continuous in
character and may be detected and analyzed by
statistical methods, while others are mutations, or
discontinuous variations, representing qualitative
differences which do not lend themselves readily to
statistical analysis.

Mutations are more common than was formerly
believed, and since they are germinal rather than
somatic in character, they play an important role
in heredity.

74

INHERITANCE OF ACQUIRED CHARACTERS 75

2. THE BEARING OF THIS CHAPTER UPON GENETICS

Only those variations which reappear in succeeding
generations have to do with heredity. Hence it be-
comes important to inquire as to what kind of varia-
tions actually reappear. Can variations that are not
inborn, but which are acquired during the lifetime of
the individual, be inherited ? Does the experience of
the parent become a direct part of the child's heritage,
or can the environment of the one enter in any way
into the heredity of the other ? Can changes wrought
in the somatoplasin be so impressed upon the germ-
plasm as to change it in such a way that it, in turn,
will give rise to similarly modified somatoplasm in the
next generation ? Can nurture as well as nature be
transmitted ?

In answering these questions we are of course con-
cerned solely with biological inheritance and not at
all with those extra-biological accumulations in the
way of arts, literature, tradition, invention, and the
like which constitute civilization and which make
us the "heirs of the ages." Such benefits are entailed
upon us much in the same way as property is "in-
herited," but they form no part of the personal bio-
logical heritage into which we are now inquiring.

3. THE IMPORTANCE OF THE QUESTION

This inquiry concerning the inheritance of acquired
characters, which Professor Brooks has called "the
interminable question," is not simply an academic
matter. Its solution is of vital importance from

76 GENETICS

several viewpoints. For breeders, who are trying to
maintain or improve particular strains of animals or
plants; for physicians, who, in fighting disease, are
honestly seeking to substitute an ounce of prevention
for a pound of cure; for sociologists and philanthro-
pists, who have at heart the permanent bettering of
human conditions ; for educators, who cherish hopes
that their life-work of unfolding the youthful mind
may prove cumulative and lasting rather than tran-
sitory ; for religious workers, who want their faith
strengthened that conquests in character-building
may outreach the individual and so enrich the race ;
for parents, who entertain hopes that their own efforts
may give their children a better biological start in
life, — for all. these and many more, it is important
to know the answer to the question : Can acquired
characters be inherited ?

4. AN HISTORICAL SKETCH OF OPINION

That the personal accumulations of a lifetime are
heritable was generally believed all through the
credulous ages. A century ago Lamarck made this
idea the corner-stone of his theory of evolution. It
was all very simple. The reason evolution occurs
in nature is because individual acquirements are
being continually added to the onflowing stream
of living forms. This cumulation of characters
indeed is evolution. How else can the present stage
of adaptation of organisms to their several niches in
nature be explained save by seeing in it the final results
of generations of gradually inherited adaptations ?

INHERITANCE OF ACQUIRED CHARACTERS 77

Darwin also believed in the inheritance of acquired
characters, although he differed from Lamarck with
respect to how such characters are acquired.

Francis Galton in 1875 was one of the first to ex-
press skepticism regarding this generally accepted
belief but the man who, in a masterly manner, fo-
cused the growing doubt, and who did more than
any other to inspire thought and investigation upon
the subject, was August Weismann, for nearly fifty
years professor in the University of Freiburg in
Baden. Weismann made the issue so clear that the
heritability of acquired characters became the parting
of the ways which divided biologists into the two
camps of Neo-Lamarckians who affirm, and Neo-
Darwinians who deny, such inheritance. If the
question could be decided by a vote or by an expres-
sion of opinion, the result would be doubtful, since
each column contains the names of men whose scien-
tific accomplishments entitle them to a respectful
hearing. Geneticists and embryologists, represent-
ing the two lines of study which furnish the most
immediate approach to this problem, are well-nigh
agreed, however, that acquired characters are not
inherited.

But just what are the facts of the case ?

5. CONFUSION IN DEFINITIONS

The source of much of the lack of agreement in
this controversy lies in the definition of what con-
stitutes an "acquired character." One is reminded
of the two knights who fought so bitterly over the

78 GENETICS

color of a shield, one maintaining that it was red,
the other that it was black. So they hacked away
at each other, as all good knights should do in the
defense of the truth, until they both fell down dead
beside the shield which was black on one side and
red on the other.

Of course actual characters are never inherited, but
only the determiners or potentialities which regulate
the way in which the organism reacts to its environ-
ment or training with respect to the characters in
question. Reid has pointed out that in one sense
every adult character is "acquired" because it has
no expression at first. For instance, there is no
beard on the face of a male infant, but it will presum-
ably be "acquired" later on in the life-cycle.

It is plain that every new character which repre-
sents a forward evolutionary step must have been
"acquired," or added, sometime and somewhere, else
it would not be present, as there is evidence that it is.
Perhaps the question, as Montgomery has suggested,
ought to be changed to read : " What kinds of acquired
characters are inherited?" It is obvious that dis-
cussion is futile until a common denominator in the
shape of a definition of acquired characters shall be
accepted.

6. WEISMANN'S CONCEPTION OF ACQUIRED
CHARACTERS

Weismann defines an acquired character as any
somatic modification that does not have its origin in the
germplasm.

INHERITANCE OF ACQUIRED CHARACTERS 79

Of course those somatic modifications which are
phases of the developing individual, such as the
acquisition of a deeper voice at puberty or the sub-
stitution of the permanent dentition for the milk-
teeth, are somatic variations which have their rise
and control in the germplasm and consequently
cannot properly be included under the head of ac-
quired characters.

Examples of acquired characters in the Weis-
rnannian sense are mutilations, the effects of environ-
ment, the results of function as in the use or disuse of
certain organs, and such diseases as may be due either
to invading bacteria or to the neglect or abuse of
the bodily mechanism.

7. THE DISTINCTION BETWEEN GERMINAL AND
SOMATIC CHARACTERS

Redfield has recently thrown light on the classi-
fication of the characters which make up the indi-
vidual by quoting the familiar lines : —

" Some are born great,
Some achieve greatness,
Some have greatness thrust upon them."
^SoWt^wKo v€U€r <xeX\COfc ^reofneSV
"Born" characters are constitutional, having

their origin in the germplasm itself. They are
never Weismannian acquired characters and may be
illustrated by eye-color, mental disposition, or facial
features. Lightning calculators and musical prodi-
gies may have their gifts developed and enlarged,

80 GENETICS

but the fact that their talent is nevertheless an
unmistakable gift, and not an acquisition, remains
true.

" Achieved" characters are functional and are
gained by exercise. Many things are achieved,
however, which are not acquired characters, as,
for instance, wealth, reputation, or an education. Not
any of these are biological characters, and therefore
we are not concerned with them in this connection,
although in the case of education it should be noticed
that the mental exercise necessary to bring about a
trained mind, if not the subject matter of the edu-
cation itself, is distinctly an acquired character of
the "achieved" type.

"Thrust" characters are the results of environ-
ment. They are acquired without functional activity
on the part of the organism and usually in spite of
anything the organism can do to prevent. Some-
times these characters are thrust upon the individual
before birth, as in the case of blindness caused by
parental gonorrhoea or tuberculosis arising from
uterine infection, in which case they are termed con-
genital characters.

Congenital or prenatal characters, however, are in
no way the same as germinal characters, for they fall
just as truly into the category of acquired variations
as do those which make their appearance in later life.

8. WHAT VARIATIONS REAPPEAR ?

Returning now to Montgomery's question, — " What
kinds of acquired characters are inherited?" — it

INHERITANCE OF ACQUIRED CHARACTERS 81

is apparent that only the "born" ones can be, which
have their roots in the germplasm whence the new
individual arises, and that "achievements" and
"thrusts," in order to reappear in the succeeding gen-
eration, can do so only by first becoming incorporated in
the germplasm.

Any character that is not acquired must have been
present in the germplasm from which the organism
arose, as there is no transfer of characters between
organisms except through the germ-cells. Thus it is
evident that the only inherited acquisitions are those
which, either primarily or secondarily, bring about
variation in the germplasm. Such temporary acquisi-
tions as a coat of tan or a display of freckles do not
impress the germplasm, for when the cause that in-
cites their appearance is removed, they soon vanish.

9. WHAT MAY CAUSE GERMPLASM TO VARY OR TO
ACQUIRE NEW CHARACTERS?

The causes which bring about changes in the germ-
plasm may be either internal or external.

Of possible internal causes may be mentioned first
the "amphimixis" of Weismann, that is, the mixture
of two nearly related strains of germplasm in sexual
reproduction within a species, or secondly, the mixture
of two more remotely related strains resulting in hy-
bridization. In either case the strain of germplasm
undergoes a shake-up that may result at least in new
combinations of characters, if not in the production
of entirely new characters. This recombination of

82 GENETICS

characters in amphimixis and hybridization will re-
ceive further attention in a later chapter.

The fact that successive parthenogenetic genera-
tions, in which amphimixis does not of course occur,
may show a larger degree of variability than sexually
produced generations, indicates that amphimixis in
itself is by no means sufficient to account for all kinds
of variations.

The abrupt way, for instance, in which mutations
appear in apparent independence of external influences
suggests that there may be some internal facipr, as
yet unknown, acting directly through the germplasm,
regardless of external causes.

The assumption of an unknown factor does not
necessarily imply a return to "vitalism," which is so
elusive of experimental test and hence so unsatisfac-
tory to the scientific mind, nor does it admit, simply
because this factor is at present an unknown quantity,
that it is consequently doomed to remain so.

It is easily conceivable that the external factors
acting upon the germplasm may be grouped into two
alternative classes : first, external factors that act upon
the somatoplasm and through the agency of the
somatoplasm affect the germplasm ; and second, those
that act directly upon the germplasm without neces-
sarily at the same time influencing the somatoplasm.

The first category, ' that of somatic modifications
which leave their impress upon the germplasm, in-
cludes true acquired characters according to our
definition, while the second, which includes cases of
the direct influence of external stimuli upon the

INHERITANCE OF ACQUIRED CHARACTERS 83

germplasm, regardless of any simultaneous modifica-
tion of the somatoplasm, must be excluded as irrele-
vant to a discussion of the heritability of acquired
characters in the Weismarinian sense, since they are
not somatic modifications at all.

Many instances of direct influence of external
stimuli upon germplasm are known in biological
literature, and these have led to some of the misunder-
standings concerning the "interminable question" of
the inheritance of acquired characters.

MacDougall, for example, was able by injecting
certain salts into the carpels of plants to stimulate
the germplasm of the forming seeds so directly tliat
a progeny of modified character was produced which,
in succeeding generations, bred true to the newly in-
duced character.

Sitkowski, also, fed the caterpillars of the moth
Tineola biselliella with an aniline dye (Sudan red III),
obtaining therefrom, instead of the normal whitish
ones, moths that laid colored eggs, and these in turn
hatched into caterpillars still tinged with the color of
the red dye. Riddle, with guinea-pigs, and Gage,
with poultry, obtained quite similar results. This is
an instance of what has been termed " parallel induc-
tion " where somatoplasm and germplasm are affected
together by an external factor, as opposed to "somatic
induction " or Weismannian acquired characters in
which the germplasm is secondarily influenced through,
or by the agency of, the somatoplasm.

84 GENETICS

10. WEISMANN'S REASONS FOR DOUBTING THE INHER-

ITANCE OF ACQUIRED CHARACTERS

Weismann's reasons for questioning the popularly
accepted view that acquired characters are inherited
may be briefly stated as follows : —

First, there is no known mechanism whereby
somatic characters may be transferred to the germ-
cells.

Second, the evidence that such a transfer actually
does occur is inconclusive and unsatisfactory.

Third, the theory of the continuity of the germ-
plasm is sufficient to account for the facts of heredity
without assuming the inheritance of acquired somatic
characters.

Let us examine these three statements a little more
closely.

11. No KNOWN MECHANISM FOR IMPRESSING THE

GERMPLASM WITH SOMATIC CHA-RACTE,R^S

The somatoplasm is something that has traveled
out from the original fundamental germplasm along
the paths of differentiation and elaboration. The
more complex the body cells become, that is, the
more .successive modifications they undergo, the more
difficult it is for these somatic cells to return to their
original primitive estate.

In many lower forms of life where cell elaboration
is not so great, a part lost by amputation is often
regenerated, but this process is not possible in higher

INHERITANCE OF ACQUIRED CHARACTERS 85

forms where the parts represent cell complexes too
hopelessly differentiated to begin anew the unfolding
sequences of their elaboration. This difficulty was
a very real one in the mind of that famous nocturnal
inquirer Nicodemus when he asked: "Ho wean a man
be born when he is old ? Can he enter a second time
into his mother's womb and be born?"

Not only the development of the race which we
call evolution, but also the determination of the
individual in heredity, is a chain of onward-moving
sequences like the succession of events in history. It
is hard to see how recent events can influence pre-
ceding events. It is hard to see how the water that
has gone over the dam can return and affect the flow
of the river upstream in any direct way. It is like-
wise hard to see how differentiated somatoplasm, which
represents the end stage of a successive series of
modifications, can make any definite impress upon
the original germplasmal sources from which it arose.

Darwin felt this difficulty and presented with apolo-
gies his provisional hypothesis of pangenesis in which
he assumed that every bodily part sends contributions
to the germ-cells in the form of "gemmules." These
gemmules, or iypothefical somatic delegates, then
reconstruct in the_ germ-cells the characters of the en-
tire body, including acquired modifications as well as
all others, and thus there is no reason why acquired
characters cannot readily be transmitted. Unfortu-
nately there is no tangible basis in fact for this de-
lightfully simple explanation to rest upon. It is a
theory assuming that all parental somatic cells take

86 GENETICS

part in the formation of the new individual, hence
it was called "pangenesis," or origin from all.

Nothing we have subsequently learned of minute
cell structure favors this hypothesis, while many
facts go quite against it. Moreover, it is directly
opposed to the theory of the continuity of germplasm
so convincingly set forth later on by Weismann.
Darwin indeed advanced it only in the most tenta-
tive way, being entirely ready to see it abandoned at
any time for something better. It at least per-
formed one valuable service to science, namely, that
of demonstrating how far investigators were from
an adequate conception of any means by which
somatic modifications might become incorporated in
the germ-cells.

We must acknowledge, however, with Lloyd
Morgan that the fact that a mechanism for the trans-
fer of somatic characters to the germ-cells has not
been discovered, is not proof that such a mechanism
does not exist. It may simply be beyond our present
powers of penetration.

12. EVIDENCE FOR TRANSMISSION OF ACQUIRED
CHARACTERS INCONCLUSIVE

The evidence for the inheritance of acquired
characters was, for a long time, taken for granted.
This theory was the most obvious explanation of
many facts and so was accepted without question.
An obvious interpretation, however, is not always the
correct one. The sun appears to go around the
earth, but astronomers assure us that it does not.

INHERITANCE OF ACQUIRED CHARACTERS Z7

When Weismann began to sift the evidence for
the inheritance of acquired characters, he found that
it was largely based upon opinion rather than fact,
much like the popular belief with regard to prenatal
influences and birthmarks, or the causation of warts
by handling toads.

The supposed evidence for the inheritance of ac-
quired characters falls chiefly into four categories : -

a. Mutilations ; >

b. Environmental effects ;

c. The effects of use or disuse ;

d. The transmission of disease.

a. Mutilations

It is fortunate that the sons of warriors do not
inherit their fathers' honorable scars of battle, else
we would now be a race of cripples.

The feet of Chinese women of certain classes have
for centuries been mutilated into deformity by band-
aging, without the mutilation in any way becoming
an inherited character. The same result is also
true of circumcision, a mutilation practised from
ancient times by the Jews and certain other Eastern
peoples. The progressive degeneration or crippling
of the little toe in man has been explained as the
inheritance of the cramping effect of shoes upon
generations of shoe wearers, but, as Wiedersheim
has pointed out, the fact that Egyptian mummies
show the same crippling of the little toe is unfavorable
to this hypothesis, for no ancient Egyptian could

88 GENETICS

ever be accused of wearing shoes or of having had
shoe- wearing ancestors.

Sheep and horses with docked tails as well as dogs
with trimmed ears never produce young having the
parental deformity. Weismann's classic experiment
with mice, an experiment subsequently confirmed
by others, is additional negative evidence upon this
same point.

What Weismann did was to breed mice whose
tails had been cut off short at birth. He continued
this decaudalization through twenty-two generations
with absolutely no effect upon the tail-length of the
new-born mice. One may see in the catacombs of
the Zoologisches Institut at Freiburg, filed carefully
away on shelves, as a " document," long rows of labeled
bottles containing the fifteen hundred and ninety-two
martyrs to science which made up the twenty-two
generations of mice in this famous experiment.

Conklin has hit the nail upon the head with respect
to mutilations by saying: "Wooden legs are not in-
herited, but wooden heads are."

b. Environmental Effects

Trees deformed by prevailing winds, like the
willows that line the canals in Belgium and Holland,
or storm-crippled trees along the exposed seacoast
are not known to produce a modified progeny
when their adverse environmental conditions are
removed. Similarly, the persistent sunburn of Eng-
lishmen long resident in Indiajoes not reappear m
their children born in England.

INHERITANCE OF ACQUIRED CHARACTERS 89

^ Sumner kept mice in a constant but abnormally
high temperature of 26° C. with the result that the
ears, tail, and feet grew noticeably larger than in
control animals kept in ordinary lower temperatures,
while at the same time the general hairiness of the
body decreased. It should be remembered, however,
that mice are mammals which pass through an ex-
tended uterine existence, so that it is easy to see how
the offspring in this case were subjected to the same
excessive temperature as the parents for a period
sufficient to amply account for their subsequent
variation when removed to a normal environ-
ment.

>- Lederbaur finds that the wayside weed Capsella,
which in the course of many years has gradually
crept along ,the roadside up into an Alpine habitat
and there "acquired" Alpine characters, upon being
transplanted to the lowlands retains its Alpine
modifications. Although this case has been cited
as an authentic instance of the inheritance of ac-
quired characters, is it not possible that the conquest
of the Alps by Capsella has been due, in the course
of time, not to the inheritance of acquired characters
at all, but to a gradual natural selection of just those
germinal variations which best fitted it to cope with
Alpine conditions until, finally, a strain of germplasm
producing somatoplasm suitable to Alpine conditions
has been isolated in the form of an elementary species
derived from the original type ? If this is what has
happened, of course such germplasm would give
rise to Alpine plants whether individual plants grew

90 GENETICS

to maturity near the snow-line or in the warm valleys
at a lower altitude.

Marie von Chauvin was able, by decreasing the
amount of water in an aquarium, to transform the
gill-breathing salamander Axolotl into the land form,
Amblystoma, which in its adult form has no gills, but
breathes by means of lungs. Both of these forms*
are sexually mature, reproducing their like, and had
long been recognized by systematists as distinct
species.

More recently Kammerer, by similarly reducing
the water supply, succeeded in transforming Sala-
mandra maculosa, a salamander that normally pro-
duces about seventy eggs which, when hatched in
water, become gill-breathing tadpoles, into a sala-
mander producing only two to seven young which
are born alive without gills and are able to live out
of water entirely, in damp situations. These land-
adapted offspring, moreover, when supplied with
abundant water, produce in turn tadpoles which spend
days only, instead of months, in the water under-
going their metamorphosis, thus showing an appar-
ent inheritance of an acquired character.

It should be pointed out, however, that in these
cases the gill-breathing forms in each instance rep-
resent a case of arrested development.. Axolotl is
simply a larval form of Amblystoma that, under nor-
mal conditions of an abundant water environment
and high temperature, gets no further in its meta-
morphosis than the tadpole stage, when it produces
eggs and sperms and finishes its life story. A change

INHERITANCE OF ACQUIRED CHARACTERS 91

in environment simply permits the life-cycle to go
on further. Changing from gill-breathing to lung-
breathing is not, therefore, an acquired character,
but a purely germinal character that may be either
blocked or released by changing conditions in the
environment.

c. The Effects of Use or Disuse

The callosities on the end of a violinist's left-hand
fingers are acquired by use, but they are not inherited.
There are callosities on the knees of the wart-hog,
Phacechcerus, which are also apparently the result
of use, for these animals kneel as they root for a living
in the African forests, and have done so for untold
generations. It has been noticed that young wart-
hogs as soon as they are born possess the callosities,
so that this instance looks like one of inheritance of
a character acquired through use or exercise.

The skin on the soles of human feet is thicker than
the skin elsewhere, and by use it becomes still thicker.
This is apparently another instance of the same sort.
The writer has observed, however, that a cross sec-
tion through the foot of a "mud puppy," Necturus
maculatus, shows a much thickened sole. Necturus,
it should be noted, is a very primitive salamander
living always under water and never using the soles
of its feet in any way to bear its weight, nor is it
reasonable to suppose that it ever had any ancestors
who did so, for the hands and feet of the Amphibia
are the most primitive and ancient hands and feet to
be found in the animal kingdom without any known

92 GENETICS

ancestral types. The thickening of the skin on the
sole of the mud puppy's feet must be due, therefore,
to germinal determiners and is in no way an acquisi-
tion through use. The same may also be true of the
wart-hog's knees and of human soles.

The strong arm, the skilled hand, and the trained
ear are not inherited. They have always to be
reacquired in each succeeding generation just as
surely as the ability to walk, or to read and write.

Herbert Spencer has defined instinct as "inherited
habit." But surely those instincts which determine
a single isolated action during the lifetime of the
individual, such as the spinning of a peculiar cocoon,
cannot be the result of habit, since habits are formed
only through repeated action. If, then, some in-
stincts require a different explanation from that of
"inherited habit," may it not be likely that all in-
stincts do ? Dr. Hodge, who succeeded in hatching
tame quail chicks out of "wild" eggs, asks the perti-
nent question: "How can & fear hatch out of an
egg?" The habit of wildness, particularly with
precocial chicks like quails, may, under an inciting
environment, be very soon established, but it is diffi-
cult to see how caution, gained by the experience of
the parents, can find its way into the fertilized egg.

d. Disease Transmission

Many diseases, like tuberculosis, have their im-
mediate cause in invading pathogenic bacteria.
Bacteria themselves cannot be inherited for the
reason that it is not possible for them to become an

INHERITANCE OF ACQUIRED CHARACTERS 93

integral part of the fertilized egg and thus cross the
"hereditary bridge" which joins two generations.
A general predisposition to bacterial disease, that is, a
lack of resistance to bacterial invasion due to de-
fectiveness in physical or physiological equipment,
may be present as a combination of characters in
the germplasm, or an individual, as the result of
disease, may "acquire" a generally weakened germ-
plasm and so produce a progeny exhibiting general
liability to disease ; but it is doubtful if such a con-
dition can properly be termed the inheritance of
an acquired character, since the particular definite
disease in question is not demonstrably heritable.

When alcoholism "runs in a family," its reappear-
ance in the son is probably due to the fact that he
is derived from the same weak strain of germplasm
as his father. The fact that the father succumbed
to the alcohol habit is not the determining cause of
drunkenness in the son. The same thing that caused
the father to become an alcoholic, namely, weak
germplasm, and not the resulting drunkenness in the
parent, is the causal factor for alcoholism in the son.

At the same time it is entirely probable that hered-
itary alcoholism may in some cases arise through
"parallel induction," that is to say, acquired alco-
holism may end in the simultaneous poisoning and
consequent modification of both the somatoplasm and
germplasm of the parent, with the result that the
germplasm has less resistance to alcoholism in a suc-
ceeding generation. The offspring are consequently
more likely to succumb to the disease. This, how-

94 GENETICS

ever, is not the inheritance of an acquired character
or of a definite somatic modification.

When a man of the present generation has rheu-
matic gout, it is a severe stretch both of patriotism
and of the powers of heredity to trace the origin of
the affliction back to a revolutionary ancestor who
acquired sciatic rheumatism by sleeping on the
ground at Valley Forge, yet this is quite as direct as
many alleged instances of the inheritance of disease.

In the majority of instances, apparent cases of
disease inheritance are merely instances of reinfec-
tion. This reinfection of the offspring may occur
very early in embryonic life, or it may happen after
birth, provided the offspring are exposed to the same
environment as that in which the parent acquired the
disease, but in any case reinfection is not heredity .

13. THE GERMPLASM THEORY SUFFICIENT TO AC-
COUNT FOR THE FACTS OF HEREDITY

Weismann holds that the theory of the continuity
of the germplasm, already considered in a previous
chapter, is sufficient in itself to account for the facts of
heredity. Hence it is quite unnecessary to fall back
upon the inheritance of acquired characters as an ex-
planation, since this theory is at least difficult, if not
impossible, of satisfactory proof.

To prove the inheritance of acquired characters,
according to Weismann three things are necessary :
first, a particular somatic character must be called
forth by a known external cause ; second, it must be
something new or different from what was already

INHERITANCE OF ACQUIRED CHARACTERS 95

exhibited before, and not be simply the reawakening
of a latent germinal character; and third, the same
particular character must reappear in succeeding
generations in the absence of the original external
cause which brought the character in question forth.
As yet these conditions have not been convincingly
met in the evidence which has been brought forward
in support of the inheritance of acquired characters.

14. THE OPPOSITION TO WEISMANN

The opponents of Weismann point out, as a weak
place in his argument, the assumption that the germ-
plasm is so insulated from the somatoplasm as not to
be influenced by it. Weismann assumes, of course,
that the germplasm is isolated from the somatoplasm
very early in the development of the fertilized egg
into an individual, and that when once isolated it there-
after takes no active part in, nor is in any way affected
by, the vicissitudes through which the somatoplasm,
or the body itself, passes. The somatoplasm is thus
merely a carrier of the germplasm and unable to
affect the character of it any more than a rubber hot-
water bag, although capable of assuming a variety
of shapes, can affect the character of the water within
it.

In opposition to this view it is urged that every
organism is a physiological as well as a morphological
unity, and that cells entirely insulated within such a
unity would be a physiological miracle.

There is abundant evidence that germ-cells, or
rather the sexual organs producing the germ-cells, do

96 GENETICS

affect the somatoplasm under particular conditions,
as, for example, in cases of castration when those
somatic features called "secondary sexual charac-
ters" undergo profound modification.

If the germplasm thus exercises a constant influ-
ence on the somatoplasm, why, it seems legitimate to
ask, may not the reverse be true and acquired somatic
characters leave their impress upon the germ-cells ?

15. CONCLUSION

But even granting the reverse to be true, that is,
that the somatoplasm affects the germ-cells, the in-
heritance of acquired characters is by no means
thereby established.

In order to do this, the precise acquired character
in question, which indirectly exercised its influence
upon the germ, must be redeveloped, and, although
the germplasm might conceivably receive an influ-
ence from the somatoplasm and be affected by it in
a general way, it is a different matter entirely to
develop anew the verisimilitude of the character itself
which is supposed to have been acquired.

It will be seen in subsequent pages, under the dis-
cussion of data furnished by experimental breeding,
that the weight of probability is decid^dl^a^ajnst^
the time-honored belief in the iim"efitance of acquired
characters.

CHAPTER VI

THE PURE LINE

1. THE UNIT CHARACTER METHOD OF ATTACK

IN reducing any body of facts to a science, it is
first necessary to determine the underlying units
out of which the facts are made up.

Chemistry was alchemy until the chemical ele-
ments were identified and isolated. Histology was
terra obscura until the cell theory brought forward
"cells" as the units of tissues. In the same way
there could be no science of genetics until the con-
ception was developed that the individual is a bundle
of unit characters rather than a unit in itself. So it
has come about that we now speak of inheritance as
applied to unit characters rather than to individuals
as a whole.

Incidentally the fact that an organism is a com-
bination of many units makes it easy to account for
the wide diversity of forms found in nature, since the
addition of a single unit greatly increases the pos-
sible combinations in successive generations.

Thus if three unit characters, A9 B, and C, are

present in each parent, for example, there would be

six possible double combinations in the offspring,

namely, A A, AB, AC, BB, BC, and CC. If now a

H 97

98 GENETICS

fourth unit D is added by one parent, there would be
not only the original six double combinations, but in
addition to these, AD, BD, and CD, that is, as many
more as there are unit characters with which the
new one may combine.

Obviously, when individuals are made up of very
many unit characters, as, for instance, a thousand, the
addition of one new unit character will increase the
possible double combinations a thousand fold.

2. G ALTON'S LAW OF REGRESSION

Galton was one of the first 1 to attempt to express
mathematically the relationship between parents
and offspring by means of treating statistically a
single unit character. According to Galton, a mathe-
matical expression of the relationship between two
generations should serve as a corner-stone of heredity.

What Galton did was to take human stature as a
unit character in comparing 204 English parents and
their 928 adult offspring, because human stature is
not complicated by environmental influences and is,
consequently, a purely hereditary matter.

Since female height is normally less than male
height, the two were reduced, for purposes of
comparison, to a common male denominator by
multiplying each female height by 1.08, which is
the average amount that the male exceeds the
female in height. There are always two parents con-
cerned in the sexual production of every offspring,

1 " Hereditary Genius," 1869.

THE PURE LINE

99

therefore Gallon reckoned a "midparent" in each
case, according to the formula

1.00 a + 1.08 $

in order to represent the double parental generation
by a single number for the purpose of easy compari-
son with the filial generation.

Inches

TO
69
68
67
66
65
64

t

V

FIG. 33. — Scheme to illustrate Gallon's law of regression. The circles
represent graded groups of parental height while the arrowpoints in-
dicate the average heights attained by the respective offspring. The
offspring of undersized parents are taller, and of oversized parents are
shorter than their respective parents* Based on data from Galton.

The results of his ^measurements expressed in
inches are shown in the following table, in which the

100

GENETICS

offspring are, in each instance, arranged under their
respective midparents.

Midparental

height . .

64.5

65.5

66.5

67.5

68.5

69.5

70.5

71.5

72.5

Average

height of

offspring .

65.8

66.7

67.2

67.6

68.3

68.9

69.5

69.9

72.2

The mean group for all the midparents, it will be
seen, is 68.5, and the offspring of this group average
68.3. The table is expressed graphically in Figure
33 in which the circles connected by the diagonal
line represent the graded parental heights, while the
arrowpoints indicate the average heights of the off-
spring in each group. In order to compare these
two series of numbers more readily, they may be
reduced to a common basis in which the mean class
in each instance is made equal to 100, as follows : —

Midparental

height . .

94

95.5

97

98.5

100

101.5

103

104.5

106

Average

height of

offspring .

96

97.5

98.5

99

100

101

101.5

102

105.5

The same series may be expressed in terms of
amount of deviation from the mean or middle classes,
as shown below. The deviation of each group in the
is marked by the signs + or — according as

seres

the heights given are greater or less than 100.

THE PURE LINE

101

Midparental

height, . .

- 6

-4.5

-3

-1.5

0

+ 1.5

+ 3

+ 4.5

+ 6

Average

height of

offspring .

- 4

-2.5

-1.5

-1

0

+ 1

+ 1.5

+ 2

+ 5.5

Finally, the relation between the midparent and
the average offspring may be expressed in fractional
form by taking the average height of the offspring
for the numerator and the height of the midparent
for the denominator in each instance. The minus
deviations are thus seen to be

4 2.5 1.5 1
6 4.5 2 1.5

which, added together and reduced to a decimal,
equal .60.

Similarly, the plus deviations are

J_ _L5 2 5.5
1.5 3 4.5 6

which reduce to .63. The average of the minus
deviations (.60) and the plus deviations (.63) is
nearly .62, or about two thirds.

That is to say, the fraction f represents the
amount of resemblance or "inheritance" between two
generations, as determined by the foregoing series,
while the remaining I is the measure of "regression"
from the general type.

This illustrates Galton's Law of Regression or the
tendency in successive generations toward medioc-
rity. The law may be stated as follows : -

102 GENETICS

Average parents tend to produce average children ;
minus parents tend to produce minus children ; plus
parents tend to produce plus children ; but the progeny
of extreme parents, whether plus or minus, inherit
the parental peculiarities in a less marked degree than
the latter were manifested in the parents themselves.

3. THE IDEA OF THE PURE LINE

It was Galton's law of regression that suggested
to the Danish botanist Johanssen a possible means of
controlling heredity. In his mind arose the ques-
tion whether it would not be possible by continually
breeding from plus parents, granting that plus par-
ents produce plus offspring and making allowance
for some regression to type, to shove over the off-
spring more and more into the plus territory and so
to establish a plus race.

To test this hypothesis, Johanssen selected beans,
Phaseolus, with which to experiment, since this
group of plants is self-fertilizing, prolific, and easily
measurable. Somewhat to his surprise, his beans
refused to shove over as much as expected. That
is, big beans did not yield principally big offspring,
nor little beans little offspring, according to the ex-
pectation, although they each produced offspring
that varied in the manner of fluctuating variability
around an average unlike the parental type. This
gave Johanssen the idea that he was using mixed
material, so he next isolated the progeny of single
beans, which, being self-fertilized, each constituted
unmistakably a single hereditary line. In this

THE PURE LINE 103

nineteen beans, now famous, became the known
ancestors of Johanssen's original nineteen "pure
lines," a further study of which has led the way to
some of the most brilliant biological discoveries of
recent years.

A pure line has been defined by Johanssen as "the
descendants from a single homozygous organism
exclusively propagating by self-fertilization," and
more briefly by Jennings as "all the progeny of a
single self -fertilized individual."

4. JOHANSSEN'S NINETEEN BEANS

It was found by Johanssen that the progeny of
each of these pure lines of beans varied around its
own mean, which was different in each of the nine-
teen instances. When, however, extremes from any
pure line series were selected and bred from, the prog-
eny, instead of showing two thirds inheritance and
one third regression with respect to the extremeness
of a particular character, as Galton found was true in
the case of human stature, showed no inheritance and
complete regression away from the extreme condition
of the parent bean back to the type for the entire
pure line in question. That is, selection within a
pure line is absolutely without effect in modifying a
particular character in the offspring of the line in
question.

This is illustrated in Figure 34 in which the results
of selecting for size in the year 1902 is shown for four
pure lines only. The average for each pure line is
given at the top of its column. When, for example,

104

GENETICS

beans weighing 60 eg. were selected from pure lines II,
VII, and XV, the average weights of their progeny
were 56.5, 48.2, and 45.0 eg. respectively, which in
each instance is nearer to the average for the pure
line than to the weight of the parental seed.

20

10 10 JO 40 » 60 70

Pure tine number

10 ZO 30 40 SO 60 TO 10 20 JO 40

FIG. 34. — The result of selection in four pure lines of beans. The verti-
cal columns, representing the average progeny from different sized
parents all derived from the same pure line, contain groups nearer
alike than the horizontal columns, representing progeny from the
same sized parents, but different pure lines. All the numbers indicate
centigrams. Data from Johanssen.

It will be seen at once that the averages in the
vertical columns are nearer alike than the averages
in the horizontal columns. In other words, the beans
bred true to their pure line rather than to their
fluctuating parent.

As a further example of this law, take the result

THE PURE LINE

105

of selection for six years in pure line I as shown in
the accompanying table and in Figure 35.

MEAN WEIGHT OP SELECTED
PARENT SEED

MEAN WEIGHT OF OFFSPRING

Minus

Plus

From Minus
Parent

From Plus
Parent

1902 . . . .

60

70

63.15

64.85

1903 . . . .

55

80

75.19

70.88

1904 ....

50

87

54.59

56.68

1905 ....

43

73

63.55

63.64

1906 . . . ,

46

84

74.38

73.00

1907 ....

56

81

69.07

67.66

It is evident, for instance, that in 1907 the smallest
beans, weighing an average of 56 eg., gave an average
progeny weighing 69.07 eg, while the largest ones
for the same year, weighing an average of 81 eg.,
produced nearly the same average in their progeny
as did the smallest beans, that is, 67.66 eg.

Incidentally all the progeny from both large and
small parents averaged notably less in 1904 than all
the progeny from large and small parents in 1906,
a result due to a "poor year" when certain factors
of environment were unfavorable. Such unfavor-
able conditions, however, are known to influence in
no way the hereditary qualities of the beans. Thus it
appears that, although the progeny of a pure line
present plenty of variations of the fluctuating type,
due probably to environmental differences in nutri-
tion, moisture, etc., such variations are quite inef-

106
O

GENETICS

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- - —

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THE PURE LINE 107

fectual so far as inheritance is concerned, and it
makes no difference whether the largest or the small-
est beans within a pure line are selected from which
to breed, the result will be the same, in that there is a
complete return to mediocrity or type with no "in-
heritance " of the parental modification. As a
matter of fact in 1903, 1906 and 1907 the lighter
parents gave a heavier progeny than the heavier
parents.

It will be seen at once that here is a discovery of
far-reaching importance which may require us to
reconstruct certain cherished ideas about the part
played in the evolution of species, as well as in
heredity, by natural selection.

5. CASES SIMILAR TO JOHANSSEN'S PURE LINES

Although according to Johanssen pure lines are
"the progeny of a single self -fertilized individual,"
it is plain that in at least three other possible cases
something quite similar to "pure lines" may be
obtained.

First, when two similar organisms identical in
their germinal determiners with regard to a particular
character inbreed, their progeny will form a pure line
50 far as this particular character is concerned just as
truly as two parents that are united in a single in-
dividual produce a pure line progeny as the result of
self-fertilization.

Second, in cases of parthenogenesis, the progeny
arising from a single female individual without the
customary qualitative reduction of chromosomes that

108 GENETICS

accompanies sexual reproduction, constitute a pure
line or an unmixed strain.

Third, in cases of asexual reproduction where the
progeny are simply the result of continued fission of
the original individual, a pure line may be said to
continue from generation to generation.

In the second and third categories it should be
pointed out that the "pure line" is assured only so
long as asexual reproduction continues. It is quite
possible for an organism, heterozygotic in composi-
tion, to continue to breed true or to produce an ap-
parently pure line so long as asexual methods are
employed. As soon as such an organism, however,
changes to the sexual method of reproduction, seg-
regation of characters may occur and different
combinations result.

6. TOWER'S POTATO-BEETLES

As an illustration of the effect of selection within
pure lines of the first category may be mentioned a
case given by Tower in his exhaustive experiments
on the Colorado potato-beetle Leptinotarsa decem-
lineata. Among the numerous cultures of this
beetle which were under control, a considerable
variation in color made its appearance. For con-
venience in classification these variations were
graded into arbitrary classes or graduated variants
(see p. 52) ranging from dark to light.

When a male and a female from the extreme class
at the dark end of the series were allowed to breed
together, their progeny were not dark, but fluctuated

xzr
xr

21

i/s

FIG. 36. — Diagram showing the ineffectiveness of selection through twelve
generations within a homozygous strain in the case of the Colorado
potato-beetle (Leptinotarsa) . In each generation extreme dark speci-
mens were selected as the parents of the succeeding generation but the
progeny always swung back to the type. After Tower.

110

GENETICS

in color around the original average of the entire
series. This process of selecting each time an ex-
treme pair of dark parents was continued for twelve
generations, as shown in Figure 36, without in any
way increasing the percentage of brunette potato
beetles in the progeny.

Thus in a pure line formed by the breeding of two
individuals alike with respect to color, the selection
of an extreme variant was quite without effect in
modifying the color of the progeny.

7. JENNINGS' WORK ON PARAMECIUM

An instance of the third category of pure lines
is furnished by Jennings' remarkable work on the
protozoan Paramecium, which was published in 1909.
Jennings carried on his experiments quite independ-

206 2OO

FIG. 37. — Eight pure races of Paramecium. The actual mean length of
each race is given in micra below the corresponding outline. Magni-
fied about 230 diameters. After Jennings.

ently of Johanssen, but he nevertheless arrived at
the same general conclusion, namely, that selection
within a pure line is without effect.

Jennings found that Paramecia differ from each

THE PURE LINE 111

other in size, structure, physical character, and rate
of multiplication as well as in the environmental
conditions required for their existence and, further-
more, that these differences, in an hereditary sense,
are "as rigid as iron."

With respect to the character of mean length he
was able to isolate eight races, or pure lines, whose
average size, drawn to scale, is shown in Figure 37.

256 < MICRA > 80

FIG. 38. — Diagram of a single race (D) showing the variation in the size
of the individuals. Magnified about 230 diameters. After Jennings.

Each of these pure lines produced a progeny
which exhibited a considerable range of fluctuating
variation. The offspring of pure line D, for example,
varied from 256 to 80 micra l in length with an aver-
age of 176 micra, as shown in Figure 38, where samples
of the different classes of variants in pure line D are
arranged in a series.

A single representative of each of the different
classes of variants out of all of the eight pure lines
bred by Jennings is shown in Figure 39.

Each horizontal row represents a single race or

1 A micron is th of a millimeter.

112 GENETICS

pure line, the average size of which is indicated by the
sign -f. The mean length of the entire lot, as shown

FIG. 39. — Diagram of the species Paramecium as made up of the eight
different races shown in Figure 37. Each horizontal row represents a
' single race. The individual showing the mean size in each race is in-
dicated by a cross placed above it. The mean for the entire lot is at
the horizontal line. The magnification is about 24 diameters. After
Jennings.

by the vertical line, is 155 micra. The total number
of individuals belonging to each size is not indicated,
but in every horizontal line their number is more

THE PURE LINE 113

numerous near the average for that line and less
numerous at the extremes, thus forming the typical
normal frequency polygons of fluctuating variability.
The significant fact about these series is this, that
extreme individuals selected from any pure line do
not reproduce extreme sizes like themselves, but
instead, a progeny varying according to the laws of
chance around the average standard of the particular
line from which it came.

8. PHENOTYPICAL AND GENOTYPTCAL DISTINCTIONS

From the foregoing it will be seen that the be-
havior of an organism in heredity cannot always
be determined by an inspection of its somatic char-
acters alone.

For example, six Paramecia, each 155 micra in
length and apparently identical, could be selected
from the six upper pure lines in Jennings' table given
in Figure 39 which would produce six progenies
definitely unlike, whereas in the case of pure line D,
twenty-four Paramecia, all measurably different
from each other in size, would be found to produce
twenty-four progenies practically identical.

Organisms that appear to be alike, regardless of
their germinal constitution, are said by Johanssen
to be identical phenotypically •, or to belong to the
same phenotype.

On the other hand, organisms having identical
germinal determiners such as those of the varying
members of pure line D, are said to be genotypically
alike or to belong to the same genotype.

114 GENETICS

Organisms belong to the same phenotype with
respect to any character when their somatoplasms
are alike. They belong to the same genotype when
their germplasms are alike.

The word "genotype" was suggested by Johanssen
in honor of Darwin and his theory of pant/ercesis,
although there are certain objections to its use in
this connection for the reason that systematists have
already appropriated it in a different sense.

Natural history and common usage deal prin-
cipally with phenotypes, that is, with organisms as
they appear. The older theories of heredity were
likewise concerned with phenotypes, but we are now
coming to see more clearly than before that heredity
must always be a case of similarity in origin, that is,
in germinal composition, and that similarity in ap-
pearance by no means always indicates similarity
in origin or true relationship.

The assumption that similarity in appearance does
indicate relationship has been made the foundation
of many conclusions in comparative anatomy and
phylogeny, but to the modern student of genetics
who places his faith in things as they are, rather than
in things as they seem to be, conclusions based upon
phenotypical distinctions alone have in them a large
source of error which must be taken into account.

In a museum of heredity, should such a collection
ever be assembled, the specimens would not be ar-
ranged phenotypically as they are in an ordinary
museum where things that look alike are placed
together as if in bonds of relationship, but they

THE PURE LINE 115

would be arranged historically from a genetic point
of view to show their true origin one from another.

9. THE DISTINCTION BETWEEN A POPULATION
AND A PURE LINE

A mixture of pure lines has been called a popula-
tion.

It is not possible to distinguish a pure line from a
population by inspection, since both may be pheno-
typically alike. Fluctuations about the average
occur in both cases with no appreciable difference
in character, although such fluctuations, when they
occur within a pure line, are simply somatic differ-
ences caused in general probably by modifications
in nutrition or some other external factor of environ-
ment, while fluctuations in a population include not
only modifications of this transient nature, but also
permanent hereditary differences due to germinal
differences in the various pure lines of which the
population is composed.

Johanssen has made the distinction between pure
lines and populations clear by the following figure
(Fig. 40), in which five pure lines of beans are com-
bined artificially to form a population.

The beans which make up the pure lines noted in
this figure are represented inclosed within inverted
test tubes. The beans in any single tube are all of
one size. Tubes vertically superimposed upon each
other also contain only beans of one size.

Thus it is seen that what may be a rare size of
bean in one line, for instance that in the left-hand

116

GENETICS

tube of pure line 3, may be identical with the com-
monest size in another line, as pure line 2. The

PURE LINE

POPULATION

FIG. 40. — Diagrams showing five pure
and a population formed by their union,
beans of each pure line are represented as as-
sorted into inverted test tubes making a curve
of fluctuating variability. Test tubes contain-
ing beans of the same weight are placed in the
same vertical row. After Johanssen.

five pure lines
represented in
Figure 40 are
combined in a
population at the
bottom of the
figure, making
a phenotype
that marks the
five phenotypes
above, which are
also five geno-
types. In the
population, how-
ever, the five
genotypes are
hidden within
one phenotype.
Hence, while
selection within
a pure line has
no hereditary in-
fluence, it is evi-
dent that selec-
tion within a

Lines

The population may
shift or move
over the type
of the progeny

THE PURE LINE 117

obtained, in the direction of the selection simply
by isolating out a pure line of one type. Thus
beans chosen from the extreme left-hand test tube
in the population cited would belong only to pure
line 2, while .those taken from the extreme right-
hand test tube could belong only to pure line 3.

Galton's "law of regression," namely, that minus
parents give minus offspring and plus parents plus
offspring, with a tendency to reversion from genera-
tion to generation, depends simply upon a partial
but not complete isolation of pure lines out of a
population.

From this distinction between pure lines and popu-
lations it is clear why breeders in selecting for a
particular character out of their stock need to keep
on selecting continually in order to maintain a cer-
tain standard. As soon as they cease this vigilance,
there is a "reversion to type" or, as they say, "the
strain runs out," which means that the pure lines
become lost in the mixed population which inevi-
tably results as soon as selective isolation of the pure
line ceases.

Such reversion must always be the case in dealing
with a population made up of a mixture of pure
lines, for only by the isolation of pure lines can
the constancy of a character be maintained. When,
however, a pure line is once isolated, then all the mem-
bers of it, large as well as small, are equally efficient
in maintaining the pure line in question, regardless
of their phenotypical constitutions.

118 GENETICS

10. PURE LINES AND NATURAL SELECTION

From the foregoing statements it appears that by
means of selection within a population, such as occurs
normally in nature, it is not possible to get anything
out that was not already there to begin with. If
this is so, the origin of species cannot have come
about, as Darwin thought, through natural selection
by a gradual accumulation of slight favorable varia-
tions. The best that selection can do is to isolate
pure lines. Within pure lines it is quite powerless
to change the genotypical characters. In other
words, natural selection can only maintain and
strengthen the frontier posts that are already es-
tablished. It cannot break into the wilderness and
create new centers.

Since the extreme members of a pure line, having
the same genotypical constitution, always tend to
backslide to mediocrity within the limits of the line
in question, the crucial question is : How can the
critical step from one genotype to another, a step
indispensable in the evolutionary derivation of
species, ever occur ? That it has repeatedly oc-
curred in the course of time is amply proven by the
fact that somehow or other we have gone from
Ameba to man.

At present the only loophole of escape seems to lie
either in the unlikely inheritance of acquired char-
acters, or in mutations which make the leap from
one character to another, and^so eventually from one
type to another, without the aid of selection.

THE PURE LINE 119

It is interesting to note that Johanssen himself,
who has been so prominently concerned in erecting
this barrier in the way of the evolutionary derivation
of species by natural selection, has recently reported
mutations arising within his pure lines of beans.
It must be admitted that to the skeptical there is a
vicious circle here, for when a variation fails to re-
appear in a subsequent generation, it may be ex-
plained as the failure of natural selection to act
within a pure line, but when a variation does reap-
pear it is hailed as a mutation !

In any event the way of experiment lies open,
and the evidence of investigators in this critical
field will be awaited with keen interest.

CHAPTER VII

SEGREGATION AND DOMINANCE
1. METHODS OF STUDYING HEREDITY

MODERN studies in heredity have been pursued
principally in three directions : first, by microscopical
examination of the germ-cells ; second, by statistical
consideration of data bearing upon heredity ; and
third, by experimental breeding of animals and
plants.

The first two of these methods of approach have
already been touched upon as well as experimental
breeding with reference to "pure lines." In the
present chapter attention will be directed to a con-
sideration of experimental breeding__with reference
to hybridization, that is, breeding from unlike par-
ents, a process which Jennings characterizes by the
expressive phrase, "the melting-pot of cross-breed-
ing."

2. THE MELTING-POT OF CROSS-BREEDING

Hybridization, or cross-breeding, as formulated
by Galton (1888), results in one of three kinds of
inheritance, namely, blending, alternative, or par-
ticulate.

120

SEGREGATION AND DOMINANCE 121

Of these, blending inheritance may be called the
typical "melting-pot" in which contributions from
the two parents fuse into something intermediate
and different from that which 'was present in either
parent. Galton illustrated this process by the
inheritance of human stature in which a tall and
a short parent produce offspring intermediate in
height. A more thorough consideration of this type
of inheritance will be presented in Chapter IX.

By the method of alternative inheritance the pa-
rental contributions do not meTPupon union, but
retain their individuality, reappearing intact in the
offspring. J In inheritance of human eye-color, for ex-
ample, the offspring usually have eyes colored like
those of one of the parents when the parental eye-
color is unlike in the two cases, rather than eyes
intermediate in color between those of both parents.

According to Galton particulate inheritance results
when the offspring present a mosaic of the parental
characters, that is, when parts of both the maternal
and paternal characters reappear in the offspring
without losing their identities by blending or without
excluding one another. Piebald races of mice arising
from parents with solid but different colors have
been cited as illustrations of this sort of inheritance,
although it will be seen later in connection with the
44 factor hypothesis" that another interpretation of
this phenomenon is not only possible but probable.

The distinctions between these three categories
of inheritance are diagrammatically represented in
Figure 41.

122 GENETICS

In blending inheritance the offspring are seen to
be unlike either parent, because the parental deter-
miners fuse into a new thing. In alternative in-
heritance, on the contrary, the offspring may be
like either parent, since the characters in question do
not lose their individuality upon union, as shown in
the diagram. Only one or the other of the two

BLENDING ALTERNATIVE PARTICIPATE

Characteristics /^*\
of parental I \

FIG. 41. — Three .kinds of inheritance described by Galton.

mutually exclusive characters thus becomes effective
in determining the nature of each offspring.

Finally, in particulate inheritance the double
germplasm which determines a new individual may
be imagined to undergo a diagonal rather than a
vertical cleavage upon maturation, thereby causing
unblended fragments of both parental characters
to become effective at once, in this manner producing
a mosaic offspring.

SEGREGATION AND DOMINANCE 123
i

3. JOHANN GREGOR MENDEL

Our understanding of the working of inheritance
in hybridization we owe largely to the unpretentious
studies of an Austrian monk, Jflhami Gregor Mendel,
who, although a"e*ontemporary of Darwin, was prob-
ably unknown to him. For eight years Mendel
carried on original experiments by breeding peas in
the privacy of his cloister garden at Briinn and then
sent the results of his work to a former teacher,
the celebrated Karl ^Nageli, of the University of
Vienna. At the timeNageli's head was full of other
matters, so that he failed to see the significance of
his old pupil's efforts. However, in 1886 Mendets
results appeared in the Transactions of the Natura}
History Society of Briinn,1 an obscure publication
that reached hardly more than a local public. Here
Mendel's investigations were buried, so to speak,
because the time was not ripe for a general apprecia-
tion or evaluation of his work.

At that time neither the chjomO^pme theory nor
the germplasm theory had been ToSnulated. More-
over, much of our present knowledge of cell structure
and behavior was not even in existence. Weismann
had not yet led out the biological children of Israel
through the wilderness upon that notable pilgrimage
of fruitful controversy which occupied the last two
decades of the nineteenth century, and the attention
of the entire thinking world was being monopolized

1 Verhandlungen naturf. Verein in Briinn. Abhandl. IV, 1865 (which
appeared in 1866).

124 GENETICS

by the newly published epoch-making work of Charles
Darwin.

Mendel died in 1884, and his work slumbered on
until it was independently discovered almost simul-
taneously by three botanists whose researches had
been leading up to conclusions very much like his
own. These three men were de Vries of Holland,
von Tschermak of Austria, and Correns of Germany.
Their papers were published only a few months apart
in 1900 and were closely followed by important
papers from Bateson in England and Davenport
and Castle in America, with a rapidly increasing
number from other biologists the world over. To-
day the literature upon this subject has grown to
be very large, and the end is by no means yet in
sight.

Concerning Mendel, Castle has well said :_T Mendel
had an analytical mind of the first order which en-
abled him to plan and carry through successfully
the most original and instructive series of studies
in heredity ever executed."

4. MENDEL'S EXPERIMENTS ON GARDEN PEAS

What Mendel did was to hybridize certain varie-
ties of garden peas and keep an exact record of all
the progeny, in itself a simple process but one that
had never been faithfully carried out by any one.

Before examining Mendel's results it may be well
to state the difference between normal and artificial
self-fertilization. Self-fertilization occurs when from
the pollen and ovule of the same flower are derived

.

SEGREGATION AND DOMINANCE 125

the two gametes which uniting produce a zygote
that develops into the seed and subsequently into
the adult plant of the next generation. In artifi-
cially crossing normally self-fertilized flowers it is
necessary to carefully remove the stamens from one
flower while its pollen is still immature, and later, at
the proper time, to transfer to it ripe pollen from
another flower.

Mendel's cross-breeding experiments on peas
showed certain numerical relations which gave rise
to what has come to be rather indefinitely known as
"Mendel's law." This law may be temporarily
formulated as follows : —

When parents that are unlike with respect to any
character are crossed, the progeny of the first gen-
eration will apparently be like one of the parents
with respect to the character in question. The
parent which impresses its character upon the off-
spring in this manner is called the dominant. When,
however, the hybrid offspring of this first generation
are in turn crossed with each other, they will produce
a mixed progeny, 25 per cent of which will be like the
dominant grandparent, 25 per cent like the other
grandparent, and 50 per cent like the parents resem-
bling the dominant grandparent.

An illustration will serve to make plain the man-
ner in which this law works out.

Mendel found that when peas of a tall variety
were artificially crossed with those of a dwarf variety,
all the resulting offspring were tall like the first
parent. It made no difference which parent was

126 GENETICS

selected as the tall one. The result was the same
in either case, showing that the character of tallness
is independent of the character for sex.

When these tall cross-bred offspring were subse-
quently crossed with each other, or allowed to pro-
duce offspring by self-fertilization which amounts
to the same thing, 787 plants of the tall variety and
277 of the dwarf kind were obtained, making approx-
imately the proportion of 3 to 1.

On further breeding the dwarf peas thus derived
proved to be pure, producing only dwarf peas, while
the tall ones were of two kinds, one third of them
"pure," breeding true like their tall grandparent,
and two thirds of them "hybrid," giving in turn the
proportion of three tall to one dwarf like their parents.

These crosses may be expressed as follows : —

Tall, T, X dwarf, /, = tall, T(t).

That is, tallness crossed with dwarfness equals
tallness with the dwarf character present but latent.

Mendel termed the character, which became ap-
parent in such a hybrid, in this case tallness, the
dominant, and the latent character which receded
from view, in this instance dwarfness, the recessive.

When now the hybrids, T(t)9 were crossed to-
gether, the result algebraically expressed was as
follows : -

T + t (all possible egg characters)
T + t (all possible sperm characters)

TT+ Tt

Tt +tt

TT+2 T(t) + ft

SEGREGATION AND DOMINANCE 127

MALE GAMETES

T i

That is, one out of four possible cases was dwarf, t,
in character and the other three were apparently
tall, although only one out of the three was pure tall,
T, while the remaining two were tall with the dwarf
character latent, T (t).

The same thing may be expressed more graphically
by the checkerboard plan,
which Punnett suggested
(Fig. 42). Each square
of the checkerboard rep-
resents a zygote which,
having received a gamete
from each of the two par-
ents, may develop into a
possible offspring. The
character of the gametes
of the parents is shown
outside of these squares,
while the arrows repre-
sent the parental source

from which the offspring have received their heredi-
tary composition.

The essential feature of Mendel's law is briefly
this: hereditary characters are usually independent
units which segregate out upon crossing, regardless of
temporary dominance.

Mendel carried on further experiments with garden
peas, using other characters. He obtained practically
the same result as in the instance already given, for
the actual progeny in the second generation of the
cross-bred offspring figured up, as seen in the table

— $—

>TT

/

—4.

T;i

•|T

1 1

FIG. 42. — Diagram to illustrate
theoretically the formation of the
four possible zygotes in the second
filial generation of a monohybrid.

128

GENETICS

below, very nearly to the 'expected theoretical ratio
of 3 to 1.

CHARACTER

NUMBER OF
DOMINANTS

NUMBER OF
RECESSIVES

RATIO

Form of seed ....

5474 smooth

1850 wrinkled

2.96 to 1

Color of seed coat . .
Color of flowers . . .

6022 yellow
705 colored

2001 green
224 white

3.01 to
3.15 to

Form of pods ....
Color of unripe pods . .
Position of flowers . .

882 inflated
428 green
651 axial

299 constricted
152 yellow
207 terminal

2.95 to

2.82 to
3.14 to

Length of vine . . .
Total

787 tall

277 dwarf

2.84 to
2 98 to 1

Darbyshire repeated the yellow-green cross with
garden peas, obtaining in the second generation the
large total of 139,837 individuals of which 105,045
were yellow and 34,792 green, which is very close to
3 to 1.

5. SOME FURTHER INSTANCES OF "MENDEL'S
LAW"

Since the rediscovery of Mendel's law the ratio of
3 to 1 in the second generation has been found by
a number of different investigators to be constant in
a large array of characters observed both in animals
and plants of diverse kinds when these are cross-bred
with reference to the characters in question.

Botanists have the advantage perhaps in this
matter, as they deal with forms which usually produce
a large number of offspring from a single cross, —
a very desirable thing in estimating ratios. On the

SEGREGATION AND DOMINANCE

129

other hand, they are handicapped by being unable
usually to obtain more than one generation in a year,
while zoologists may secure from many animals like
rabbits and mice several generations in a year, al-
though ordinarily the number of progeny is much

ORGANISM

AUTHOR

DOMINANT

RECESSIVE

Q

Nettles

Correns

Serrated leaves

Smooth-margined leaves

'03

Sunflower

Shull

Branched habit

Unbranched habit

'08

Cotton

Balls

Colored lint

White lint

'07

Snapdragon

Baur

Red flowers

Non-red flowers

'10

Wheat

Biffen

Susceptibility to

Immunity to rust

'05

rust

Tomato

Price and

Two-celled fruit

Many-celled fruit

'08

Drinkard

Maize

de Vries

Round, starchy

Wrinkled, sugary kernel

'00

kernel

Silkworm

Toyama

Yellow cocoon

White cocoon

Cattle

Spillman

Hornlessness

Horno

Pomace fly

Morgan

Red eyes

Horses

Bateson

Trotting habit

Land snail

Lang

Unbanded shell

Mice

Darbyshire

Normal habit

Guinea-pig

Castle

Short hair

j

Canaries

Bateson and

Crest

I

Saunders

Poultry

Davenport

Rumplessness

Lc

Man

Farrabee

Brachydactyly

Nc

Barley

von Tschermak

Beardlessness

Bet

'01

Salamander

(Amblystoma)

Haecker

Dark color

Light color

'08

smaller and the ratios obtained have a larger chance
of error than is the case with the more numerous
plant offspring.

Semi-microscopic animals, as, for example, the
pomace fly, Drosophila, which produces a large progeny
every two weeks or so, may combine the general
advantages mentioned for the two groups of organisms

130 GENETICS

indicated above, but they have the disadvantage of
being so small that the detection of their distinctive
phenotypic characters is attended with considerable
technical difficulty.

What the modern experimenter in genetics desires is
an organism, first, that possesses conspicuous distinc-
tive somatic characters, and, second, which will come to
sexual maturity early and breed either in captivity
or under cultivation both numerously and frequently.

The preceding table, compiled chiefly from Bateson 1
and Baur,2 might easily be much extended. It shows
from what diverse sources confirmatory evidence of the
truth of Mendel's law has been derived within the
past lew years

JE OF SEGREGATION

hich Mendel demonstrated
in cases at least, the deter-
ged from diyejcsgj>arental
^ in a common stream of germplasm
from which, in subsequent generations, they may
segregate out apparently unmodified by having been
intimately associated with each other. This "law of
segregation" depends upon the conception that the
individual is made up of a bundle of unit characters.
It may be illustrated by the separate flowers picked
from a garden which, after being made into a nose-
gay, may be taken apart and rearranged without in

1 "Mendel's Principles of Heredity," 1909.

2 " Einf iihrung in die experimentelle Vererbungslehre," 1911.

SEGREGATION AND DOMINANCE 131

any way disturbing the identity of the separate
blossoms.

The general formula of segregation that covers
all cases of organisms cross-bred with respect to a
single character, that is,, monohybrids, is given in
Figure 43.

(Domlnsnt)

D(

DD ffi(R) RR

D 2DCR) RR RR

DD DD Dr. *D(R) RR RR ^ RR

FIG. 43. — General Mendelian formula for a monohybrid.

7. HOMOZYGOTES AND HETEROZYGOTES

A character which is present in the offspring in
double quantity because it was present in both parents
is said by Bateson to be homozygous, while an or-
ganism which is homozygous with respect to any
character is called a homozygote so far as that particu-
lar character is concerned.

In contrast to the homozygous condition, an organ-
ism is said to be heterozygous when it derives the
determiner of a character from one parent only.
Such an organism is described os a heterozygote with
respect to the character in question. A homozygous
and a heterozygous dominant may appear alike,

132 GENETICS

although not necessarily so, that is, they may have
the same phenotvpicjd constitution, but their geno-
typical composition is always different.

8. THE IDENTIFICATION OF A HETEROZYGOTE

"Homozygote" and " heterbzygote " are terms then
descriptive solely of the genotypical constitution of
organisms, and, as has been said, it is not always
possible to distinguish one from the other by inspec-
tion, although it may frequently be done, as will be
pointed out later. The only sure way to identify a
heterozygote is by breeding to a recessive and observing
the kind of offspring produced**£^_

Peas of the formulae TT and T(t), for example,
both look alike, since a single determiner for the tall
character, T, is sufficient to produce complete tallness.
When, however, these two kinds of tall peas are
each bred to a recessive dwarf pea, of the formula it,
the progeny will differ distinctly in the two cases as
follows : — 1 7 _ f

" — """ *

Case I. T + T X t + t = 100 per cent T(f).
Case II. T + t X t + t = 50 per cent T(t) + 50 per cent «.

That is, if the dominant to be tested is homozygous
(Case I), the entire progeny will exhibit the dominant
character, but if the dominant to be tested is heterozy-v
gous (Case II), then only one half of the progeny will
show the character in question.

9. THE PRESENCE AND ABSENCE HYPOTHESIS

Mendel's conception that every dominant character
is paired with a recessive alternative is now being

SEGREGATION AND DOMINANCE 133

largely replaced by the presence and absence hypothesis
which was first suggested by Correns but later logi-
cally worked out by others, particularly by Hurst,
Bateson, and Shull. According to this latter inter-
pretation, a determiner for any character either is,
or is not, present. When it is present in two parents,
then the offspring receive a double, or duplex, "dose,"
to use Bateson 's word, of the determiner. When it
is present in one parent only, then the offspring have
a single, or simplex, dose of the character. When it
is present in neither parent, it follows that it will not
appear in the offspring. In this case the offspring
are said to be nulliplex with respect to the char-
acter in question. Take the case of tall and dwarf
peas, the determiner for tallness when present pro-
duces tall peas, even if it comes from one parent
only, but if this determiner for tallness is absent from
both parents, the offspring are nulliplex, that is, the
absence of tallness results and only dwarf peas are
produced.

The difference between the presence and absence
theory and the dominant and recessive theory is that
in the former case the "recessive" character has no
existence at all, while in the latter instance it is
present, but in a latent condition.

10. DIHYBRIDS

So far reference has been made exclusively to mono-
hybrids, any two of which are supposed to be similar
except with respect to a single unit character. Mono-
hybrids are comparatively simple, but when two

134 GENETICS

organisms are crossed which differ from each other
with respect to two different unit characters, the situa-
tion becomes more complicated.

Mendel solved the problem of dihybrids by crossing
wrinkled-green peas with smooth-yellow peas. He
found that smoothness S is dominant over wrinkled-
ness W and that yellow color Y is dominant over
green G, or, as it would be stated according to the
presence and absence theory, smoothness is a positive
character which fills out the seed-coat to plumpness
while its absence leaves a wrinkled coat, and yellow-
ness is a positive character due to a fading of the green
which causes the yellow to be apparent. In the
absence of this green fading factor or determiner the
green, of course, appears.

If smooth-yellow SY and wrinkled-green WG are
crossed, all the offspring are smooth-yellow, but
they carry concealed the recessive determiners for
wrinkledness and greenness according to the formula
S(W)Y(G). When the determiners of these cross-
breds segregate out during the maturation of the
germ-cells, they may recombine so as to form four
possible double gametes, namely, smooth-yellow SY
and wrinkled-green WG, which are exactly like the
grandparental determiners from which they arose,
and in addition, two entirely new combinations,
smooth-green SG and wrinkled-yellow WY.

Since the male and the female cross-breds are each
furnished with these four possible gametic combina-
tions, the possible number of zygotes formed by their
union will be sixteen (4X4 = 16). That is, the

SEGREGATION AND DOMINANCE 135

monohybrid proportion of 3 to 1 in dihybrid com-
binations is squared, (3 + I)2 =16.

It of course does not follow that the offspring in
dihybrid crosses will always be sixteen in number, or
that they will always conform strictly to the theoreti-
cal expectation of (3 + I)2. The offspring obtained
undoubtedly obey the laws of chance, but the greater
the number of offspring, the nearer they come to fall-
ing into the expected grouping.

The sixteen possible zygotes resulting from a
dihybrid cross will give rise to sixteen possible kinds
of individuals which in turn, as will be demonstrated
directly, present four kinds of phenotypic and nine
kinds of genotypic constitutions.

A dihybrid mating, using the same symbols em-
ployed in the case just described, would be expressed
algebraically as follows : —

SG+ WY+ SY+ $£G = all the possible egg gametes
SG.+ WY+ SY+ W8 = all the possible sperm gametes
SQSG+ SGJ%Y+ SGJSY+ SGffG

SQWY +WYWY+ &YSY+ WiYWG

SGSY + WYSY +SYSY+ SYWg

+ 8YWB+WGWG
SY

SGSG+2 SGWY+2 SGSY+2 SGWG+WYWY+2 WYSY+ 2 $YWG+SySY+ 2SYWG+WGWO

The second and the ninth items in this result are
alike ; f by combining' them the revised result reads": —

SGSG+1SGWY+2SGSY+2SGWG+WYWY+2 WYSY+2 WYWG+SYSY+WGWQ

There are then these nine different combinations
of germinal characters or nine different genotypes
in any dihybrid cross. By placing the recessive char-
acters in parentheses, whenever the corresponding
dominant is present to indicate that the dominant

136

GENETICS

causes the former to recede from view, these nine
genotypes may be combined into four phenotypes as
follows : —

Phenotypes . .

9SY

3SG

SWY

1WG

Gedotypes •/<

4S(G)(JF)F
2S(G)SF
ZSY(W}Y
SYSY

SGSG

2SG(W)G

WYWY

2 WYW(G}

WGWG

From this analysis it may be said that the Mendelian
ratio for a typical dihybrid is phenotypically 9:3:3:1,
while that for a monohybrid, as we have already seen,

(?-- SG WY SY WG

4

SG

WY

SY

WG •

SG

•>SG!

SG

SG

**©

©

©

®

SG

WY

SY

WG

WY->

WY

WY

*WY

WY

©

©

©

©

SG

WY

SY

WG

SY-*

SY

fSY-

SY

SY^v

" ®

" ©

©

®

IX

SG

WY •

SY

WG

WG-*

WG

WG

WG

WG

' @

©1

©

©

FIG. 44. — Diagram to illustrate the possible combinations arising in the
second filial generation (Fa) following a cross between yellow -smooth
YS and green- wrinkled GW peas.

is 3:1. This expected ratio corresponds essentially
with the actual results Mendel obtained in crossing
smooth-yellow and wrinkled-green peas.

SEGREGATION AND DOMINANCE 137

Figure 44 presents a graphic representation of the
different combinations resulting from a dihybrid cross
following the checkerboard plan used in Figure 42
to illustrate monohybrids.

The nine genotypes and four phenotypes which
result from a dihybrid cross are shown in the following
squares.

Number in
Each Class

GENOTYPE

Number of Squares
in Fig. 44

PHENOTYPE

SY

Number in
Each Class

1 /

'8Y8Y.

11 »

9

2 V

(W)YSY

7-10

2 1

S(G)SY

3-9

4

S(G)(W)Y

2-5- 12- 15

1

SGSG

1

SG

3

2

&G(W}G

13-4

1

WYWY

6

WY

3

2

WYW(G)

8-14

1

WGWG

16

WG

1

16

16

Another illustration of dihybridism is shown in
Figures 45 and 46 which is based upon data fur-
nished by the Davenports.1 In the matings given
here, dark or pigmented hair, represented by the solid
black circles, is dominant over light-colored, that is,
unpigmented or slightly pigmented hair, symbolized
by the open circles, while curly hair is dominant

^'Heredity of Eye-color in Man," Science, N. S. 26, p. 589, 1907;
" Heredity of Hair Form in Man," Amer. Nat. 42, p. 341, 1908. Daven-
port, C. B. and G. C.

138

GENETICS

over straight, represented by crooked and straight
lines respectively in the diagram. In other words,
the presence of pigment is dominant over the ab-
sence of pigment, while the factor that causes curli-
ness is dominant over the absence of this factor,
with respect to human hair.

KEY TO SYMBOLS

• = Dark
O = Li&ht '
3 = Curlq

- = Strait

FIG. 45. — The heredity of human hair according to data by C. B. and
G. C. Davenport. The arcs represent the somatoplasms of four indi-
viduals. Within the arcs are the gametes formed by these individuals.
The dominant character is placed on the outside of the arc where it
will be visible.

SEGREGATION AND DOMINANCE

189

When a homozygous individual with dark curly
hair crosses with a homozygous individual with light
straight hair, all the offspring have dark curly hair.

The dark curly-haired individuals of this second
generation, however, are heterozygous with respect
to each of these two hair characters. When any two
individuals having this particular genotypic compo-
sition mate, therefore,

they may produce any N£«* [GENOTYPE PHENOTYPE
one of four possible
phenotypes — dark

curly, dark straight, I ^ I ^ |/k/*/://r/y
light curly or light
straight haired individ-
uals. These four phe-
notypes in turn will
present nine different
genotypic combina-
tions out of sixteen pos-
sible cases, as shown in
Figure 46.

Figure 45 further- / $£) \Lgfit stni^t I J
more serves to. make I le \ \ \ J6 ,

dear, first, the distinc- FIG. 46. — Diagrams showing the pos-

tion between somato-

plasm and germplasm ;

second, the maturation

of germ-cells ; third, the

segregation of gametes ; and fourth, the formation of

zygotes in sexual reproduction.

The cells of the somatoplasm are represented as

Dark strviQht

Light curly

sible genotypic and phenotypic com-
binations resulting when two hetero-
zygous individuals, with dark curly
hair, mate. Symbols are the same
as in Figure 45.

140 GENETICS

making up the arcs within which are inclosed the
germ-cells after their reduction through maturation,
which results in giving to each germ-cell half the
number of determiners that are present in the soma-
tic cells.

It will be remembered that when two gametes,
or mature germ-cells, unite, they form a zygote
having the proper number of determiners normal to
the species in question instead of double that number.
Symbols for dominant characters in the diagram are
placed on the outside of the somatic arcs, because
these are the characters that are visible or pheno-
typic, while the non-apparent recessives are placed
on the inside out of sight.

11. THE CASE OF THE TRJHYBRID

Mendel went even further and computed the
possibilities which would result when two parents
were crossed differing from each other with respect
to three unit characters. He found that the results
actually obtained by breeding closely approximated
the theoretical expectation.

This expectation in the case of a trihybrid cross is
that the cross-breds resulting will all exhibit the
three dominant characters, while their ' genotypic
constitution will include six factors, namely, these
three dominant characters plus their corresponding
recessives or "absences."

Cross-breds of the first generation will, therefore,
have eight possible kinds of triple gametes and when
interbred may form a possible range of sixty-four

SEGREGATION AND DOMINANCE 141

(8 X 8) different zygotes, which corresponds to a
monohybrid raised to the third power (3 + I)3.
These sixty-four zygotes group together in eight

cf-

?1

R«p

rSP

rsp

RSP RsP RSp Rsp rSP rsP rSp r«p

i i i i i i r i

RSP
RSP

RsP
RSt>

RSp
RSP.

RlP
RSP

rSP
RSIJ.

r.P

RSP

rSp
RSP,

rsp
RSP

RSP
Rsp

R*P
RsP

RSP
R.Pt

R^P
RsP

rSP
Rs£-

rs4P
RsP

rSp
RsP,

^SP
RsP

RSP
RSP

R*P
RSP

RSo
RSP

*P
RSP

rSP
RS'R,.

rsP
RS&

rSp
RSP

r.sp
RSp

RSP

R'sp

RsP
Rsp

RSp
Rsp

•F^sp
Rep

oS'P-
Rsp

rs^P
Rsp

rSp
Rsp

rsp

Rsp

RSP

rSP

f}sP
rS£

RSp
rSP

R/P
rSP

nSP
rSP

rsP
rSP

rSp
rSP

rsp
rSP

R^,
rsP

FfeP
rsP

RSp
-P

FUP
r«P

rSP
rsP

rsP
rsP

rSp
rSP

r*p
rsP

RSP

r Sp

R|?
rSp

RSp

rSp

Rap
rSp

rSP
rSp

rsP
rSp

r§P
rSp

rgp
rSp

RS£
rap

RsP
r*p

RSp

rsp

Rsp
rsp

rSP
rsp

rsP
rsp

rSp
r^p

rsp
rap

FIG. 47. — Diagram showing the possible combinations in a guinea-pig
trihybrid of the Fa generation. R, rosjsttejicoat ; r, non-rojetted coat
(absence of R) ; S, short hair ; s, angdra h^|r (absence of S) ; P, pig-
mcnted ; p, albino (absence of pigment). The eight possible triple
gametes of each parent are placed in the upper and left hand margins.
Each of the sixty-four squares represents a possible zygote or ferti-
lized egg, having received a triple gamete from each parent.

different phenotypes and twenty-seven different
genotypes.

The trihybrid cross with its resulting combinations
is well illustrated by Castle's work on guinea-pigs
which confirms the Mendelian hypothesis on an

142

GENETICS

Number in
each class

GENOTYPE

PHENOTYPE

Number in
each class

1

SS PP RR

2

SS Pp RR

2

Ss PP RR

4

Ss Pp RR

SPR

271

2

SS PP Rr

Short, pigmented, resetted

»/j

4

SS Pp Rr

4

Ss PP Rr

8

Ss Pp Rr

1

SS pp RR

2

Ss pp RR

SpR

2

SS pp Rr

Short, albino, resetted

4

Ss pp Rr

1

ss PP RR

2

ss Pp RR

sPR

2

ss PP Rr

Angora, pigmented, resetted

4

ss Pp Rr

1

SS PP rr

2

SS Pp rr

SPr

9

2

Ss PP rr

Short, pigmented, non-rosetted

4

Ss Pp rr

1

ss pp RR

spR

3

2

ss pp Rr

Angora, albino, resetted

1

SS pp rr

Spr

3

2

Ss pp rr

Short, albino, non-rosetted

1

ss PP rr

sPr

3

2

ss Pp rr

Angora, pigmented, non-rosetted

1

ss pp rr

spr
Angora, albino, non-resetted

1

64

64

SEGREGATION AND DOMINANCE 143

extensive scale. In Figure 47 dominant characters
are represented by capital letters, while recessives or
absences are indicated by corresponding small letters.

When a smooth, or non-rosetted (r), short-haired
(S), pigmented (P) guinea-pig is crossed with a
rosetted (R), long-haired (s), albino (p) guinea-pig,
all the offspring appear to be of one phenotypic
constitution, namely, rosetted, short-haired, and
pigmented (RSP). Their genotypic constitution is
represented by the formula RrSsPp. These six
factors may form eight possible triple gametes, as
follows : RSP, RsP, RSp, Rsp, rSP, rsP, rsp. When
two germ-cells each made up of these eight triple
gametes unite in sexual reproduction, they will give
rise to sixty-four (8 X 8) possible zygotes as dis-
played in Figure 47.

An analysis of Figure 47 shows among the off-
spring eight different phenotypes in the ratio of
27:9:9:9:3:3:3:1 and 27 different genotypes in
the proportions indicated on the opposite page. The
order of the three pairs of symbols is changed from
that in Figure 47 to emphasize the fact that with
independent unit characters the order is immaterial.

12. CONCLUSION

Although the ratios for more than a trihybrid
were computed by Mendel, the experimental test
has never been carried out, since it involves such
large and complicated proportions.

In the case of four differing unit characters in the
parental generation, the offspring of the quadruple

144 GENETICS

hybrids derived from such an ancestry would in-
clude 256 or (3 + 1) 4 possibilities instead of 64 or
(3 + 1) 3, as in the case of trihybrids. When ten
differing characters are combined in the parental
generation, there would result over a million possible
kinds of offspring among the hybrids of the second
generation, (3 + 1) 10 = 1,048,576.

From the foregoing it is apparent that in practical
breeding the only hope lies in dealing with not more
than one or two characters at a time.- Since unit
characters usually behave independently of each
other, one may breed for a single character until it
is segregated out in a homozygous, that is pure,
condition, and then in the same way obtain a second
character, a third, and so on.

Thus in a few generations of properly directed
crosses there can be obtained combinations of char-
acters united in one strain that formerly were never
obtained at all or were only hit upon by the merest
chance at long intervals. Herein lies the scientific
control of heredity which the trinity of Mendelian
principles <^namely, independent unit characters, seg-
regation, and dominance, has placed in human hands)

13. SUMMARY

Three principles are concerned in Mendel's law :
independent unit characters, dominance, and seg-
regation.

a. Independent Unit Characters. An organism,
although acting together as a physiological and mor-
phological whole, may be regarded from the point

SEGREGATION AND DOMINANCE 145

of view of heredity as consisting of a large number
of independent heritable unit characters.

b. Dominance. In the germplasm there are cer-
tain determiners of unit characters which dominate
others during the development of the somatoplasm.
In other words, they determine the apparent charac-
ter of the organism by causing that character to
become visible.

The alternative recessive characters, although they
may be present in the germplasm, are unable to be-
come manifest in the somatoplasm so long as the
dominant characters are present. When, however, a
dominant character is absent, its recessive alterna-
tive becomes manifest.

c. Segregation. Unit characters, although they
may be intimately associated together in the indi-
vidual, during the complicated process of maturation
that always precedes the formation of a new indi- i
vidual, separate' or segregate out as if independent
of each other and thus are enabled to unite into
new combinations^ •

CHAPTER VIII

REVERSION TO OLD TYPES AND THE MAKING
OF NEW ONES

1. THE DISTINCTION BETWEEN REVERSION AND
ATAVISM

THERE are two ways in which types of animals or
plants that are different from the present ones may
be conceived to arise, namely, by the reappearance
of old types and by the formation of new ones. In
the reappearance of old types & distinction may be
drawn between reversion and what has been termed
atavism.

Atavism, or "grandparentism," may be defined as
skipping a generation with the result that a particu-
lar character in. the offspring is unlike the corre-
sponding character in either parent, but instead,
resembles the character in one of the grandparents.

In reversion, on the contrary, a character reappears
which has not been manifest perhaps for many gen-
erations, although it was actually present in some
remote ancestor. J. Arthur Thomson's definition
of reversion is : "All cases where through inheritance
there reappears in an individual some character
which was not expressed in his immediate lineage,
but which had occurred in a remoter, but not hypo-
thetical, ancestor."

146

OLD TYPES AND NEW

147

This distinction between atavism and reversion
becomes clearer by illustration.

If heterozygous brown-eyed individuals mate,
there is one possibility in four that their offspring

OF/U

GRANDMOTHER

HO^OZYSOTC HOMOZ^TE '$«*£
\\ '

\ '

BOTHER

,'HETERC/ZYGOfE

f/Stofkx\ \y*^ ,' Simplex \

/ 1 s\ ^' I \

1 I s \ /N^ / v

HETEROZYGOTE HOMOZYGOTE HETEROZYGOTE
Simplex Duplex Simplex

FIG. 48. — Three generations of a Mendelian monohybrid. The outlines
represent the somatoplasms with the phenotypic character on the out-
side. The black symbols inclosed within the somatoplasm stand for
the germplasm in the form of gametes. The short dotted arrows indi-
cate the relation between germplasm and somatoplasm. The long
dotted arrows indicate possible recombinations of germplasms.

will have blue eyes unlike their own,' but like the two
blue-eyed grandparents. Such a blue-eyed child
would be an instance of atavism. The explanation

148 GENETICS

of this apparently inconsistent hereditary behavior is
perfectly simple in the light of the Mendelian ratios,
as shown diagrammatically in Figure 48, in which the
circles represent the blue-eyed and the squares the
brown-eyed character.

This figure also illustrates what typically occurs in
the formation of Mendelian monohybrids of the first
and second filial generations. The squares are
symbols for the dominant characters, while the circles
are symbols for the recessive characters. When the
two are superimposed, the circle recedes from view,
The large outside figures indicate the somatoplasm,
therefore the phenotype. The small inclosed figures
indicate the germplasm, therefore the genotype. The
short dotted arrows indicate what it is that deter-
mines the somatoplasm in each case, while the long
dotted arrows show what possible recombinations of
germplasms can be made. Child No. 4 is an " ex-
tracted recessive" derived from dominant parents,
but with one recessive grandparent on each side. It
is a case of "atavism," or taking after the grand-
parent. Notice that atavism can occur only by
alternative inheritance.

To quote Davenport: "In the majority of cases
atavism is a simple reappearance in one fourth of the
offspring of the absence of a character due to the
simplex nature of the character in both parents."

An illustration of reversion would be the reappear-
ance of the ancestral jungle-fowl pattern in domestic
poultry or of the slaty blue color of the ancestral
rock-pigeon among buff and white domestic pigeons,

OLD TYPES AND NEW 149

for the ancestral character or characters in this type
of hereditary behavior, as said before, reappear only
after a lapse of many generations.

2. FALSE REVERSION

"Around the term ' reversion,' " Bateson observes,
"a singular set of false ideas have gathered them-
selves . " In proof of this statement there may be cited
at least five categories of apparent reversion which
properly ought not to be classed as true reversion.

a. Arrested Development

Feeble-mindedness is not reversion to ancestral
forms of less intelligence, but an instance of arrested
development when, for some reason, the individual
fails to accomplish his normal cycle of development.

Likewise harelip in man is not a case of reversion
to rabbit-like ancestors in which harelip is the nor-
mal condition, but it is ordinarily due to an arrest or
failure of certain embryonic steps that are essential
to the development of the usual form of human lip.

b. Vestigial Structures

These are the vanishing remains of characters that
were formerly of significance. They do not represent
something latent that is now reappearing, for they
have never yet disappeared phylogenetically, and con-
sequently they cannot be regarded as true reversions.

The muscles under the scalp which enable those
persons possessing them to wiggle the ears; the
palatine ridges in the roof of the mouth of many

150 GENETICS

babies and some adults which resemble the ridges
in the roof of a cat's mouth ; the vermiform appen-
dix, a necessary part of the digestive apparatus of
many animals but fraught so often with evil conse-
quences to man ; f these and scores of similar charac-
ters, which, 'taken together* make man in the eyes
of the comparative anatomist a veritable old curi-
osity shop of ancestral relics, are the last traces of
characters which formerly had a significance in some
of man's forbears. Having lost their usefulness,
these structures still hang on to the anatomical
household as pensioners. They have not been re-
called from the past, but have always been with us,
although of diminishing importance. In no sense,
therefore, can they be called reversions.

c. Acquired Characters resembling Ancestral Ones

Sometimes the drunken descendant of a drunken
great-grandparent has acquired this characteristic
through his own initiative quite aside from any an-
cestral contribution to his germplasm. This is not
reversion. It is a reacquisition which resembles
the ancestral condition.

Again, tame animals that run wild acquire habits
resembling those of their wild ancestors, but this is
not necessarily reversion. It is the natural response
of feral animals to the conditions of wild life.

d. Convergent Variation

The European hedgehog, Erinaceus, an insecti-
vore, the American porcupine, Erithizon, a rodent,
and the Australian spiny anteater, Echidna, a

OLD TYPES AND NEW 151

monotreme, are all mammals which have developed
in a similar manner the very peculiar device of der-
mal spines. There is no reason, however, for regard-
ing this character as due to descent from a common
spiny ancestor. It is not reversion to an ancestral
type, but rather a case of convergent variation.
Similarity does not always indicate genetic continuity.
In the case of birds albinism, melanism and fla-
vism are modifications of ordinary pigmentation which
appear irregularly among many different species as
pathological " sports," but no one of these conditions
can be regarded as reversions to ancestral white,
black, or yellow types.

e. Regression

Galton's "law of regression" refers to the wide-
spread phenomenon already explained of a constant
swinging back to mediocrity which the breeder must
oppose with continual selection in order to maintain
the standard of any particular strain. We have
seen that within a "pure line," regression is complete
and that in populations made up of a mixture of
pure lines it is a factor always to be reckoned with.
Regression, however, has to do with fluctuating varia-
tions and does not bring about a permanent change
of type. It should, therefore, not be confused with
reversion.

3. EXPLANATION OF REVERSION

Darwin, who did not always differentiate between
reversion and atavism, suggested that reversion was

152 GENETICS

due sometimes to the action of a more natural en-
vironment, as in the case of animals set free after
having been in captivity, and sometimes to hybridi-
zation, since there seems to be a general tendency of
hybridized organisms to "revert" to ancestral types.
It is now known that reversion, like atavism, is
simply a case of latent characters becoming apparent
according to the Mendelian principle of segregation.
To quote Davenport: "There is nothing more mys-
terious about reversion, from the modern standpoint,
than about forming a word from the proper com-
bination of letters."

4. SOME METHODS OF IMPROVING OLD AND ESTAB-
LISHING NEW TYPES

a. The Method of Ballet

This method, which was formulated by the English
wheat-grower Hallet in 1869, has been in common
use for a long time. It consists in placing the organ-
isms to be bred in the very best possible environment
and then choosing those individuals which make the
best showing as the stock from which to breed further,
a procedure based upon the deep-seated belief that
acquired characters are inherited.

For example, in a field of wheat, plants near the
edge of the field which, from lack of crowding or by
reason of proximity to an extra local supply of fer-
tilizer or any other favorable environmental factor,
make a more vigorous growth than their neighbors,

OLD TYPES AND NEW 153

are selected in the hope that the gains made by them
will be maintained in their offspring.

We have seen that it is very questionable whether
acquired characters which are due to environmental
conditions play any role whatever in heredity. The
phenotypic character does not always indicate what
the germplasm will subsequently do, and when the
true genotypic constitution of the germplasm is still
further masked by the temporary fluctuations caused
by a modified environment, it is increasingly difficult
to select wisely from the display of variants those
which will produce the best ancestors for the future
stock.

That this common procedure of selecting the best-
appearing animal in the flock and the biggest ear of
corn in the bin, has met with a large degree of success
in the past is due entirely to the fact that in many
instances the phenotypic character is an actual ex-
pression of the genotypic constitution. This is not
always the case, however, and we cannot now fail
to see that the method is blind and full of error. Its
successes are due to the indirect results of chance
rather than to a direct control of the factors of hered-
ity. The great proportion of failures resulting from
this procedure now find a reasonable explanation from
the standpoint of Mendelism.

b. The Method of Rimpau

Contrasted with the Hallet method of augmenting
acquired characters and then selecting the best display
of them, is the method of Rimpau, who experimented

154 GENETICS

for two decades with various grains and, finally,
among other results, produced the famous Schand-
stedt barley.

Rimpau's method is to sow grain under ordinary
conditions with a minimum rather than a maximum
amount of fertilizer and then to select individuals,
neither from the rich spots nor from the edges of
the field where there is little crowding, but from situa-
tions where the environmental conditions are ordi-
nary or even unfavorable. Individuals making a
good showing under such usual, or even adverse,
conditions are worthy by nature rather than by nur-
ture and are consequently most desirable as progeni-
tors of future stock. By this method the attempt is
not to keep the progeny of single individuals sepa-
rate, but to mass together the best as they appear
under ordinary normal environment.

This again is an indirect method of procedure,
although the character of the germplasm is more
nearly hit upon in this way than by Hallet's method,
since the mask of temporary accessory modifications
is stripped so far as possible from the somatoplasm,
and the phenotype made to approximate the geno-
typical constitution.

c. The Method of de Vries

The method of de Vries has already been in part
described in Chapter IV. It depends upon the pres-
ervation and exploitation of the mutations occurring
in nature. It recognizes clearly the fact that change
of type is dependent upon a germplasmal variation

OLD TYPES AND NEW 155

which is largely, if not entirely, independent of environ-
mental factors.

Accordingly, the work of the successful breeder
consists in simply taking what nature spontaneously
furnishes to him rather than in attempting to force
nature into producing something new. These muta-
tions, when isolated, may become the progenitors of
desirable new lines.

d. The Method of Vilmorin

This is an isolation method which has been success-
fully applied to the sugar-beet industry. The seeds
from each plant to be tested are sown in separate beds
from which upon maturity samples are taken and
tested for sugar content. The plants from the bed
furnishing the sample which contains the highest per-
centage of sugar are then used as the seed producers
for the next generation. In this way by continual
selection an improved strain may be maintained.

e. The Method of Johanssen

The method of isolating pure lines or homozygotes
out of a mixed population has been considered in
Chapter VI. As in the method of de Vries of isolating
mutations so, too, in the pure line method it is recog-
nized that the germplasm is the source of initiatory
changes and that the technique of establishing new
types consists in sorting out homozygous strains of
this germplasm.

The method of Johanssen is quite different from
those of Hallet and Rimpau in that the ideal organi-

156 GENETICS

zation is not sought for among phenotypes, but
among genotypes. It is not the somatoplasm, but
the germplasm that is selected.

/. The Method of Burbank

This is a method of greatly increasing the number of
variants by promiscuous hybridization and then of
eliminating all except those of a desired phenotypic
combination. Indirectly it depends upon the princi-
ple of the segregation of unit characters which makes
possible rearrangements of these characters according
to the laws of chance. The characters themselves
remain unchanged, since nothing new is produced
by hybridization except new arrangements of existing
characters.

The spectacular success of Luther Burbank in
"creating" new plant forms is due largely to his very
extensive hybridizations, his skill in detecting among
the varying progeny the winning phenotype and his
ruthless elimination of the great majority of varia-
tions that do not quite fill his requirement.

The successful combinations must be propagated
in most instances asexually by grafting, cuttings,
bulbs, etc., rather than sexually through the medium
of seeds, because new genotypes which will breed true
are not necessarily isolated by this procedure. The
consequence is that Burbank's method cannot be
utilized in animal breeding to any great extent where
the maintenance of a desirable strain by asexual prop-
agation is out of the question.

It will be seen that this method, like the first two, is

OLD TYPES AND NEW 157

fortuitous and to a certain extent unscientific in that
no one can repeat the exact conditions of the experi-
ment and arrive at the same results. It depends upon
the chance mixing up of a large number of possibilities
and then in not being distracted or blinded by the
good while selecting the best. In the hands of a skilful
plant breeder with unlimited resources at his com-
mand it may result in much practical achievement, but
it does not particularly illuminate the path of other
breeders who wish to repeat the experiment. It is
after all a selection of phenotypes and, therefore,
forever open to error, since phenotypes do not always
indicate what the behavior of their constituent geno-
types will be in heredity.

g. The Method of Mendel

The method of Mendel, like the foregoing, depends
upon hybridization with the difference that the
desired combination is sought directly by definite
predetermined crosses, according to the expectations
of the Mendelian ratios, rather than through the
random result of fortuitous combinations. This
method has been, rendered possible by the determina-
tion of Mendel's laws of dominance, and of the inde-
pendence and segregation of unit characters which
give to the experimental breeder definite expectations
and a method of procedure.

If, upon hybridization, the desired character be-
haves like a recessive, then all that is necessary to
establish a pure stock exhibiting the character in
question, is to breed two recessives together, because

158 GENETICS

recessives are always homozygous and, regardless of
their ancestry, breed true.

On the other hand, if the desired character proves
to be a dominant, then it is necessary to determine
whether it is present in a duplex or a simplex condi-
tion; in other words, whether it is homozygous or
heterozygous, for only homozygous organisms breed
true. Establishing a strain consists, consequently,
in making an organism homozygous.

The test to determine whether a dominant character
is homozygous or heterozygous, that is, whether
it will breed true or not, can be made by a single
cross according to the procedure outlined in para-
graph 8 of Chapter VII. If, upon crossing the
individual to be tested with a recessive, it produces
an entirely dominant progeny, then its germplasm is
duplex for this character, and it will always reproduce
the character in either duplex or simplex condition
according to what it may be crossed with. When
crossed, for instance, with another duplex dominant
like itself, a pure homozygous strain of the character
in question will be perpetuated.

If, on the contrary, the dominant character to be
tested proves to be simplex or heterozygous, as de-
termined by the fact that, when crossed with a re-
cessive, 50 per cent of the progeny are recessive, then
it requires more than a single generation to establish a
homozygous dominant strain.

In random inbreeding of diverse strains if the re-
cessives are constantly eliminated as they appear, a
population is gradually obtained which is composed

OLD TYPES AND NEW 159

of an increasing number of dominants so that after
only a few generations the chances are much reduced
that recessives will appear, which means the practical
purity of the strain.

5. THE FACTOR HYPOTHESIS

It has been ascertained within the last decade
that some characters require more than a single de-
terminer to bring them to expression. The idea of
compound determiners for a single character may be
termed the factor hypothesis of heredity. The con-
verse is also true, that certain single determiners
may control more than one character. For instance,
the determiner for gray hair in rats also produces a
lighter color on the belly.

Mendel, whose experiments led him to believe that
each character depends upon only a single deter-
miner for the reason that he worked on characters
severally belonging to different parts of the plant,
was apparently unaware of the existence, in certain
cases at least, of compound determiners.

These compound factors may be arranged in va-
rious categories. For example, there may be, —

(1) A complementary factor which is added to a
dissimilar factor in order that a particular character
may appear ;

(2) A supplementary factor which is added to a%
dissimilar factor with the result that a character is
modified in some way ;

(3) A cumulative factor which, when added to

160 GENETICS

another similar factor, affects the degree of expression
that a character is given ;

(4) An inhibitory factor, which prevents the action
of some other factor, and so on.

It will be profitable to consider the factor hypothe-
sis in some detail, since it helps to explain both rever-
sion and the formation of new types.

a. Bateson's Sweet Peas

In the course of numerous breeding experiments
Bateson obtained two strains of white sweet peas,
Lathyrus, which, when normally self-fertilized, each
bred true to the white color. When these two strains
were artificially crossed, however, the progeny all
had purple flowers like the wild ancestral Sicilian
type of all cultivated varieties of sweet peas.

Here was apparently a typical ins^|ce of reversion,
but according to the factor hypofcnesis the explanation
is this. The character of purple color is dependent
upon two independent factors which, though sepa-
rately heritable, are both required to produce it. Each
of these white strains of sweet peas possesses one of
these factors which can produce colored flowers only
when united with its complement, a proof of which
appeared upon interbreeding hybrid purples from such
a cross. In short, the color purple depends upon the
action of two complementary factors which follow
the behavior of a dihybrid. (See Chap. VII, par. 10.)

The gametic formulae for the two strains of white
sweet peas used in this experiment are Cp and cP,
respectively. C stands for a color factor without

OLD TYPES AND NEW

161

which no color can appear, even though pigment for
color may be present, and c is the absence of this
factor, while P represents a purple pigment factor
which only finds expression in the somatoplasm when

FIG. 49. — Diagram to illustrate the possible progeny from two hetero-
zygous purple sweet peas according to data from Bateson. C, color
factor (large circles) ; c, absence of C (small circles) ; P, pigment fac-
tor (large crosses) ; p, absence of P (small crosses). In the zygotes
within the checkerboard squares the gametic symbols are superimposed.

taken together with the color factor C. The small
letter/ p stands for the absence of the purple pigment
factor^ — "It will be seen that each of the white sweet
peas whose formulae are given above lack one of the
two essential factors for purple color. When the

162 GENETICS

two are crossed, however, all the progeny are purple
with the formula CcPp.

These hybrid sweet peas upon gametic segregation
theoretically produce four kinds of gametes, CP, Cp,
cP, and cp which may combine as any other dihybrid
in sixteen different ways. In this case, however,
these combinations group themselves into only two
phenotypes, purple and white, as indicated in the
accompanying diagram (Fig. 49) in which C and c
are represented by large and small circles respec-
tively, while P and p are correspondingly indicated
by large and small crosses. The gametic symbols are
superimposed to form the zygotes.

The theoretical expectation here shown was closely
approximated in the actual results.

It may be noted in passing that the seven kinds of
white sweet peas resulting from the above cross, while
phenotypically alike, that is, in the zygotic symbols
of Figure 49, lacking either the large circle (color) or
the large cross (pigment), belong to three distinct
genotypes as follows : -

NUMBER OF
ZYGOTE IN
FIGURE 49

Without the pigment factor (large cross)
Without the color factor (large circle)
Without either pigment (large cross) or color (large circle)

6- 8-14
11 -12- 15
16

Among the purple peas are the following four geno-
types : —

OLD TYPES AND NEW 163

NUMBER OF
ZYQOTE IN
FIGURE 49

Duplex for both color (large circle) and pigment (large cross)
Duplex for color (large circle) but simplex for pigment

(large cross)
Simplex for color (large circle) but duplex for pigment

2-5
3-9

(large cross)
Simplex for both color (large circle) and pigment (large

cross) 4 -7 -10 -13

b. Castle s Agouti Guinea-pigs

An illustration of a supplementary factor that acts
only in conjunction with some other to bring about
a modification, is the pattern factor demonstrated by
Castle in his guinea-pigs.

The wild gray, or "agouti, " color of the hair of cer-
tain guinea-pigs is due to the fact that pigment is
distributed along the length of each hair in a definite
pattern. The tip of a single hair is black followed by a
band of yellow, while most of the proximal part which
is more or less concealed by overlapping hairs is a
leaden color. The distribution of pigment in such a
pattern gives the characteristic gray, or agouti color
to the coat when taken as a whole.

Castle demonstrated the separate nature and be-
havior of such a pattern factor when he discovered
that it is transmitted independently of pigment, which is
necessary to bring it to expression. He showed that
upon crossing a solid black guinea-pig, unquestionably
possessing pigment but no "pattern," with a white

164 GENETICS

albino guinea-pig having no pigment, some of the
offspring "reverted" to the ancestral agouti, or
"pattern" type, thus proving that the pattern must
be carried in this case by the white or albino guinea-,
pig as a factor independent of the color which is
necessary for its expression.

c. Cuenofs Spotted Mice

Another instance of the interaction of supple-
mentary factors is seen in the spotting of piebald
mice. Cuenot discovered that such spotting is due
to the absence of a uniformity factor which if present
causes color to be uniformly distributed over the
entire coat.

Both of these independent factors, spotting and
uniformity, are real and not imaginary, since they may
be separately transmitted through albino animals in
the same way as the pattern factor mentioned above,
notwithstanding that in albinos both are hidden
through the absence of pigment, upon the presence of
which their visibility depends.

Whenever piebald or spotted animals appear in a
progeny derived originally from self-colored stock,
it is evidently due to the absence of such a "uni-
formity" factor as has just been described.

Gallon's theory of "particulate inheritance"
(page 121) is now satisfactorily explained as fr^£ al-

rnative inheritance in which the mosaic app^Jplice
is caused by a Mendelian determiner, in this instance
a spotting factor or, in other words, the absence of
a factor for uniformity.

OLD TYPES AND NEW 165

d. Miss Durham s Intensified Mice

Miss Durham, in her work with mice, has demon-
strated an intensifying factor, the absence of which
she calls a diluting factor. The action of the former
produces, as its name implies, intensity of color,
while that of the latter serves to lessen the degree of
intensity in which color appears.

These factors of intensity and diluteness, it should
be observed, do not in any way correspond to the
duplex and simplex condition of a dominant color
character, either of which would straightway appear
if crossed with an albino. The factors of intensity
and dilution of color are of an entirely different
nature, as they have been proven to be indepen-
dently transmissible through albinos where a color
character could not appear because of the absence of
pigment.

The following illustration of this kind of sup-
plementary factors taken from Miss Durham's
experiments will serve to make the case clear. The
symbols employed are : —

B = black pigment which masks brown, or chocolate.

b = the absence of B, consequently chocolate'. .

I = intensity factor. .

i = dilution factor or absence of intensity.
C = a complementary color factor acting with P.

#= a complementary pigment factor acting with C.
= black.

BiCP = blue or maltese (dilute black).
bICP = chocolate.
biCP = silver-fawn (dilute chocolate).

166

GENETICS

The crosses which were made are represented in
the table below, in which the expectation according
to the Mendelian dihybrid ratios is given in paren-
theses after the actual results of each cross.

BLACK

BLUE

CHOCO-

SILVER-

(BICP)

[BiCP)

LATE
(bICP)

FAWN
(WCP)

Black (BICP) X Silver-fawn (biCP)

9(9)

4(3)

3(3)

2(1)

Blue (BiCP) X Chocolate (bICP)

42(45)

16(15)

14(15)

8(5)

Blue (BiCP) X Silver-fawn (biCP)

0(0)

33(36)

0(0)

12(12)

It will be seen that the actual results, even when
such small totals are concerned, approximate very
closely the expectation and are entirely consistent.

e. Castle's Brown-eyed Yellow Guinea-pigs

Recently Castle has shown that in guinea-pigs
there is an independent factor for extension of pig-
ment distinct from the uniformity factor already
mentioned. The absence of this extension factor
("restriction") is manifested by a lack of black
or brown pigment everywhere except in the eyes
and to a slight extent in the skin of the extremities,
while the distribution of yellow is wholly unaffected
by it.

That such "extension" and "restriction" factors
really exist, is proven in the following way : —

When a brown (chocolate) guinea-pig is crossed
with an ordinary black-eyed yellow one, the young
are all black pigmented, but by cross-breeding

OLD TYPES AND NEW

167

these hybrid young four varieties are obtained in
the next generation, viz., black, brown, black-eyed
yellow, and brown-eyed yellow, the latter a variety
unknown before Castle's experiment in breeding
was made.

For the sake of clearness the formation of the
brown-eyed yellow is shown below in Figure 50.

Y

BE Be

Black

10

Black

H

b e

Be bE

Black

11

bE||bE

Chocolate

be||bE

Chocolate

Y.

BE||be

Black

8

BeMbe

bE||be

Chocolate

16

be| |be

3rown-eyed Yellow

FIG. 50. — Diagram to illustrate the origin of a brown-eyed yellow guinea-
pig from two heterozygous black parents based upon Castle's experi-
ments. The factor for yellow (Y) is present in every gamete and is
consequently duplex in every zygote but is hidden whenever the fac-
tor B is present. B, black pigment hiding brown or chocolate ; 6,
chocolate (absence of B) ; E, extension of B over the entire body
hiding Y ; e, restriction of B to eyes alone thus exposing Y over the
entire body.

168 GENETICS

Symbols

B = black pigment, hiding brown or chocolate.
b = absence of B, or chocolate.
Y = yellow pigment, hidden by B.
E = extension of B over entire body, hiding Y.
e = restriction of B to eyes alone, thus exposing F over

the entire body.
C = complementary color factor acting with P to produce

color.
P = complementary pigment factor acting with C to

produce color.
(The factors C and P may be omitted for the sake of

simplicity, since they are present in each instance.)

First Cross

"Extended" chocolate (bEY) X black-eyed yellow
(BeY) = black (BbEeYY).

Second Cross

When these cross-breds are mated with each other,
they each form four kinds of gametes, BEY, BeY,
bEY, and beY, which unite into sixteen theoretical
genotypic possibilities, shown in Figure 50. These
fall into four phenotypes, nine black (BEY), three
black-eyed yellow (BeY}, three chocolate (bEY),
and one brown-eyed yellow (beY). The actual
results in Castle's experiments gave all four kinds
in close numerical agreement with this expectation.
The action of extension and restriction factors is,
therefore, plainly a case of Mendelian dihybridism
in which two independent pairs of alternative char-
acters are concerned.

OLD TYPES AND NEW

169

6. RABBIT PHENOTYPES

Perhaps no better application of the factor hy-
pothesis may be found than the case of the color
of rabbits.

There are many varieties of rabbits so far as color
is concerned, particularly among domesticated races.
These varieties are now quite explainable by the
factor hypothesis, as indicated in the table below.
The sixteen kinds of rabbits there catalogued have

THE FACTOR HYPOTHESIS APPLIED TO COLORS OF RABBITS

CONSTANT
FACTORS

ALTERNATIVE
FACTORS

GAMETIC
FORMULA

PHENOTYPIC CHARACTER
WHEN CROSSED WITH THE
SAME KIND OP
GAMETIC COMBINATION

1

2

3

4

5

6

7

8

Br

B

Y

C

E
e

I
i

u

u
U

A

AUIEC[YBBr

Gray

a

aUIEC [YBBr

Black

A

a

AuIEC [YBBr

Gray spotted

auiEC (YBBr

Black spotted

A

AUiEC [YBBr

Blue-gray

a

aUiEC [YBBr]

Blue (Maltese)

u
U

A

a

A

AuiEC [YBBr]

Blue-gray spotted

auiEC [YBBr]

Blue spotted

I

AUIeC (YBBr]

( Yellow (with white belly
I and tail)

a

aUIeC (YBBr

( Sooty yellow (with yellow
I belly and tail)

u

A

AuleC [YBBr]

Yellow spotted

a

auleC [YBBr]

Sooty yellow spotted

i

U
u

A
a
A
a

AUieC (YBBr]

Cream

aUieC (YBBr]

Pale sooty yellow

AuieC (YBBr

Cream spotted

auieC [YBBr] | Pale sooty yellow spotted

170 GENETICS

been obtained by Castle and other experimental
breeders as well as many of the albino types that
would double this list if c, or the factor for absence of
color, should be substituted for C, the presence of
color, in column 4 of the table on page 169.

Explanation of Symbols in the Foregoing Table

Br = a factor acting on C to produce brown pigmentation.

B = a factor acting on C to produce black pigmentation.

F = a factor acting on C to produce yellow pigmentation.
The three factors, F, B, Br, are present in every
rabbit gamete and up to date have not been sepa-
rable as independent unit characters, although they
have been separated out in guinea-pigs and mice.
There are no brown rabbits, because black always
goes linked with brown covering the brown factor.
Yellow rabbits result, as explained below, through
the action of factor e.

C = a common color factor necessary for the production
of any pigment. It was discovered in 1903 by
Cuenot.

c = the absence of C which results in albinos, regardless
of whatever pigment factors may be present.
By changing C to c, sixteen kinds of albinos would
be added to this catalogue, an addition of one
phenotype and sixteen genotypes, all looking alike
but breeding differently.

E = a factor governing the extension of black and brown

pigment, but not of yellow.

e = the absence of extension or restriction of black and
brown pigment to the eyes and the skin of the
extremities only, while yellow remains extended
and visible. Demonstrated by Castle in 1909.

OLD TYPES AND NEW 171

/ = an intensity factor which determines the degree of
pigmentation. It can be transmitted indepen-
dently of C through an albino. Discovered by
Bateson and Durham in 1906.

i = the absence of intensity or dilution. Dilute black =
blue. Dilute yellow = cream. Dilute gray =
blue-gray.

U = a factor for uniformity of pigmentation or "self-
color" discovered by Cuenot in 1904.

u = the absence of uniformity which results in spotting

with white.

A = a pattern factor for agouti, or wild gray color, which
causes the brown and black pigments to be ex-
cluded from certain portions of each hair, resulting
in the gray coat. When present in the rabbit, it
is also associated with white or lighter color on
the under surfaces of the tail and belly. It was
demonstrated by Castle in 1907.

a = the absence of the agouti or pattern factor.

7. THE KINDS OF GRAY RABBITS

Each of the apparent kinds of gray rabbits indicated
in the foregoing table may be made up of various
genotypes. For instance, there are thirty-two differ-
ent genotypes, each of which is phenotypically a gray
rabbit. The zygotic formula for each of these thirty-
two possibilities is displayed in the next table, and it
will be seen that these range all the way from rabbits
homozygous in all their variable characters (No. 1)
to those homozygous in none (No. 32).

The progeny of these various types of gray rabbits
when inbred will consequently vary from the pure

172 GENETICS

THE KINDS OF GRAY RABBITS (Color only)

GENOTYPE
ZYQOTIC FORMULA.

NUM-

EROF

HET-

ROZY-
GOTIC

FAC-
TORS

PHENOTYPES
When inbred, these kinds are produced

I

AAUUHEECC [YBBr] [YBBr]

None ><

2

AAUUHEECc [YBBr][YBBr]

One >

3

AAUUIIEeCC [YBBr][YBBr]

One >

4

AAUUHEECC [YBBr][YBBr]

One >

5

A A UuIIEECC [ YBBr} [ YBBr}

One >

6

AaUUIIEECC [YBBr] [YBBr]

One >

C X

7

AAU UliEeCc [ YBBr} [ YBBr]

Two >

<

8

AAU UliEECc [ YBBr} [ YBBr}

Two >

(

9

AAUuIIEECc [YBBr][YBBr]

Two >

( >

10

AaU UIIEECc [ YBBr} [ YBBr}

Two >

< X

11

AAU UliEeCC [ YBBr} [ YBBr}

Two >

<

X

12

AA UuIIEeCC [YBBr} [YBBr}

Two )

< >

13

AaU UIIEeCC [ YBBr} [ YBBr]

Two >

< X

X X

14

A A UuIiEECC [ YBBr} [ YBBr}

Two )

(XX

15

AaUUIiEECC [ YBBr] [ YBBr]

Two )

< X

X X

16

Aa UuIIEECC [ YBBr} [ YBBr]

Two )

rpi \

< X >

( X

17
18

Aa UuIiEECC [ YBBr] [ YBBr]
Aa UuIIEeCC [ YBBr} [ YBBr]

I nree /
Three )

< X >

s X

X X X >

19

Aa UuIIEECc [ YBBr} [ YBBr]

Three )

< X >

\ x

x

20

Aa U UliEeCC [ YBBr} [ YBBr}

Three )

< x

X X

x x

x x

21

Aa U UliEECc [ YBBr} [ YBBr}

Three :

< X

X X

X

22

AaUUIIEeCc [YBBr][YBBr]

Three ;

< x

X X

v/

23

A A UuIiEeCC [ YBBr} [ YBBr}

Three ,

>(. )

< X X

X X

x

A

24

A A UuIIEeCc [YBBr] [ YBBr}

Three

X )

x x

X

25

AAUuIiEECc [ YBBr] [ YBBr}

Three

X )

X

26

AAU UliEeCc [ YBBr} [ YBBr

Three

x

X

X

x

X • X

27

AAUuIiEeCc [YBBr\[YBBr

Four

X )

28

AaU UliEeCc [ YBBr] [ YBBr]

Four

X X

29

Aa UuIIEeCc [ YBBr] [ YBBr]

Four

X X )

XXX)

x

30

AaUuIiEECc [YBBr][YBBr

Four

X X )

< X X X X

x

31
32

Aa UuIiEeCC [ YBBr] [ YBBr
Aa UuIiEeCc [ YBBr] [ YBBr

Four
Five

X X )

X X )

XXX

o w

U-

* 8 g g g

l^g »

'• 3 2.

S£g

1

•n ^* W OR

= a s :

s i'

1

S o w

ft

II 1

1

(

1

:

It

OLD TYPES AND NEW 173

gray, as in No. 1, to a gray from which sixteen pos-
sible types of young may be expected as in No. 32.
Up to the time when Castle's paper upon the factor
hypothesis 1 was published in 1909, nine genotypic
kinds of gray rabbits had been obtained in his ex-
periments, whose genotypic formulae correspond to
the following numbers in the list : 1, 3, 6, 10, 13, 20,
22, 28, 29.

8. CONCLUSION

That a relatively small number of factors may pro-
duce an extensive array of combinations is evident
from this data.

The analysis of germplasm by the factor hypothesis
is now being generally applied by geneticists to the
particular organisms with which they are concerned.
It has been carried out notably in detail by both
Bateson and Davenport for poultry and byBaur for
the snapdragon, Antirrhinum.

Finally, the elucidation of the factor hypothesis
makes any further explanation of reversion super-
fluous. It is now easy to see how a particular char-
acter may remain latent for generations and at last
come to expression only when the missing factor
necessary to its activity is supplied by some cross.

It is also clear how hybridization, in which many
characters are concerned, is bound to furnish far more
new combinations than would, at first thought, be
expected.

1 " Studies of Inheritance in Rabbits ." Carnegie Institution Publica-
tions, No. 114, 1909. W. E. Castle in collaboration with Walter, Mul-
lenix and Cobb.

CHAPTER IX

BLENDING INHERITANCE

1. RELATIVE VALUE OF DOMINANCE AND SEGRE-
GATION

OF the three fundamental principles which underlie
"Mendel's law," namely, segregation, independence of
unit characters, and dominance, the principle of
dominance has been found to hold true in a surpris-
ing number of cases and in relation to very diverse
organisms, notwithstanding the fact that the time
spent in the investigation of dominance, as that term
is now understood, has been comparatively short.
Doubtless future experimentation will demonstrate
the existence of dominance to a far greater extent
than has at present been discovered.

Its universal application is by no means assured,
however, since the mathematical precision with which
it works, that following its discovery in 1900 has so
captivated the biological world, is beginning to give
way in the face of many exceptions which have been
steadily accumulating.

Even Mendel himself noted certain exceptions to
the law of dominance, and his followers have pointed
out with increasing emphasis that it is subject to
many modifications. It is now understood, indeed,

174

BLENDING INHERITANCE 175

that segregation, not dominance, is the most essential
factor in the Mendelian scheme.

2. IMPERFECT DOMINANCE

It frequently occurs that dominance is so imperfect
that a heterozygous, or simplex, dominant may be
distinguished at once by simple inspection from a
homozygous, or duplex, dominant, whereas the test
of crossing with a recessive is necessary whenever
dominance is complete, as has been previously ex-
plained. The single dose of the determiner in such a
case has plainly, then, less phenotypic effect than a
double dose.

There are many cases of imperfect dominance among
flowering plants. Correns has shown that when
plants of a white-flowering race of the "four-o'clock,"
Mirabilis jalapa, are crossed with those of a red-
flowering race, all the offspring in the first filial genera-
tion, unlike either parent, exhibit rose-colored flowers.
When, however, these rose-colored flowers are crossed
with each other, they produce red, rose, and white
in the Mendelian ratio of 1 : 2 : 1 ; that is, three colored
to one white. The red-flowering race thus proves
to be homozygous and the rose-flowering race hetero-
zygous. Here color dominates the absence of color,
or white, but the degree of the color depends upon
whether the dose of pigment is duplex or simplex.

A classic illustration of imperfect dominance among
animals is the "blue Andalusian fowl," the hereditary
behavior of which is illustrated below (Fig. 51). It
will be seen that when two blue Andalusian fowls,

176

GENETICS

characterized by a mottled plumage, are bred together,
they produce three kinds of offspring in the ratio of
1:2:1. Twenty -five per cent are clear black, 50 per
cent are blue Andalusian, and 25 per cent are white
"splashed" with black. Both the black and the
splashed white fowls from this cross prove, upon
further breeding, to be homozygous, while the blue
Andalusian itself is heterozygous and can, therefore,

Andalusian

Andalusian

r~

Black

I

Andalusian

i l

Andalusian Splashed White

BlacK

Black Andalusian

FIG. 51. — The heredity of the blue Andalusian fowl, an -illustration of
"imperfect dominance."

never be made to breed true. In order to produce
100 per cent of blue Andalusian chicks, it is necessary
simply to cross a splashed white with a black Anda-
lusian.

There is nothing in this case to indicate whether the
black or the splashed white should be regarded as
the homozygous dominant, since dominance is im-
perfect. In either case the heterozygous blue Anda-
lusian is at once evident in the first filial generation
without further crossing.

A similar case of imperfect dominance is furnished
by the roan color of cattle which results when red
and white are crossed. If two roans are mated, they

BLENDING INHERITANCE 177

produce red, roan, and white offspring in the propor-
tion of 1:2:1, thus showing that roan is a heterozy-
gous character in which the dominance of red is
imperfect.

Even in cases of apparently perfect dominance it
is sometimes possible by close inspection to detect
differences between a pure dominant (DD), Figure 43,
and a heterozygous dominant .(DK) when a superficial
examination is not sufficient to distinguish them.

For instance, in the cross between smooth and
wrinkled peas, a microscopic examination of the
starch-grains in the cotyledons of the hybrid peas
shows that they are of two kinds. Darby shire calls
attention to the fact that, in the power of absorption,
hybrid smooth peas (DR) are intermediate between
their pure dominant smooth (DD) and pure recessive
wrinkled (RR) parents.

3. DELAYED DOMINANCE

A character which is really dominant is sometimes
so late in manifesting itself in the individual growth
of the offspring that it may properly be termed a
delayed dominant.

Dark-haired individuals often do not acquire their
definitive hair color until adult life, and it is common
knowledge that the eyes of an infant for a consider-
able period provoke no little speculation among ador-
ing relatives as to " whose eyes " they are.

According to Davenport, when a white Leghorn
fowl is crossed with a black Leghorn, white being
dominant in this case, chicks are produced that are

178 GENETICS

white with black flecks in their plumage. These black
flecks, however, disappear at the time of the first
molt. The complete dominance of white is, there-
fore, simply delayed.

4. "REVERSED" DOMINANCE

In certain instances there seems to be a reversal
of dominance, as may be illustrated by Lang's results
with snails (Helix) . He has proven in his experiments
that red snails are generally dominant over yellow
snails, although in certain cases there is apparently
an exception to the rule, for snails with yellow shells
dominate those with red shells.

Davenport also has shown that although extra
toes are usually dominant over the normal number
in poultry, yet, in something like 20 per cent of the
cases, the normal number is dominant.

To speak of these cases as instances of "reversed
dominance," is open to serious objection, since such
an explanation does not agree with the generally
accepted "presence and absence" idea of heritable
characters. It is difficult to see how the presence of
a certain determiner can dominate in a part of the
offspring of any cross and the absence of the same
determiner be able to dominate the remainder.

It is perhaps nearer the truth to conceive that in
cases of apparent "reversal" of dominance there is
an insufficient amount of a particular determiner
available to bring the character concerned into
expression. In other words, although a dominant
character may be present in two cases, yet in one

BLENDING INHERITANCE 179

it fails, for some reason, to become effective. This
interpretation agrees with the facts brought out by
subsequent breeding in cases of this sort.

It sometimes occurs that a character which is
dominant in one species may be recessive in another.
Horns are dominant in sheep, but recessive in cattle.
White color is recessive in rodents and sheep, but
dominant in most poultry and in pigs.

5. POTENCY

Davenport seeks to explain modifications in typical
dominance as variations in the potency of determiners.
He defines potency as follows: "The potency of a
character may be defined as the capacity of its germi-
nal determiner to complete its entire ontogeny."

That is, if the potency of a determiner, for some
reason, is insufficient, there may be either an incom-
plete or delayed manifestation of the character in
question, or it may fail entirely to develop.

The variations of potency may be grouped into
three general categories according to the degree of
their manifestation ; namely, total potency, partial
potency, and failure of potency.

A further word of explanation for each of these
three kinds of potency seems desirable at this point.

a. Total Potency

This is complete Mendelian dominance in which
even the heterozygotes produced by a simplex dose
of a character are indistinguishable phenotypically,
that is, by inspection, from the homozygotes produced

180 GENETICS

by a duplex dose of the same character. It is as if
a single bottle of black ink poured into a jar of water
was just as effective as two bottles of ink, in forming
an opaque fluid.

b. Partial Potency

Partial potency covers all cases of incomplete
dominance, such as those of the four-o'clock (Mirabilis)
and blue Andalusian fowls, where a simplex dose of
a determiner does not produce the same visible effect
as a double dose.

The dominant prickly Jamestown weed (Datura),
when crossed with a recessive glabrous variety of
the same plant, produces cross-breds in the first
generation which show only a few prickles (Bateson)
(Baur), following the law of partial potency.

Banded and uniformly colored snails also, when
crossed together, produce snails with shells showing
only a pale banding (Lang).

Numerous further instances of incomplete domi-
nance could be cited.

c. Failure of Potency

If for any reason a determiner fails to accom-
plish its possibilities in whole or in part, then the
character in question may never become evident, and
the result, so far as appearances go, is the same as
if it was a recessive lacking the determiner entirely.

That the failure of potency is not identical with
the absence of a determiner can usually be demon-
strated by further breeding, because dominants failing

BLENDING INHERITANCE 181

in potency, which are either of the formula DD or DR,
may, if bred inter se, give a various progeny among
which the dominant character D is likely to again
become manifest, while recessives, of the formula
RR, on the contrary, will always give offspring which
all agree in the entire absence of the character in
question.

Davenport cites an extreme case of failure of potency
in one of two rumpless cocks from the same blood.
The character of rumplessness is due to an inhibitor
of tail development. That these two cocks both
possessed this character was demonstrated by the
entire absence of any tail in either case. The in-
hibiting determiner for tail growth was so weak in
cock No. 117, however, that, to quote Davenport's
exact words : "In the heterozygote the development of
the tail is not interfered with at all, and even in ex-
tracted dominants it interfered little with tail develop-
ment, so that it makes itself felt only in the reduced
size of the uropygium and in-bent or shortened back.
But in No. 116 the inhibiting determiner is strong.
It develops fully in about 47 per cent of all the
heterozygotes and in extracted dominants may pro-
duce a family in all of which the tail's development
is inhibited."

Here were two birds of the same blood, pheno-
typically alike and presumably genotypically . alike,
which because of an individual difference in the
potency of the determiner for rumplessness produced
quite different results in their offspring although bred
to precisely the same array of hens.

182 GENETICS

6. BLENDING INHERITANCE

In the instances of imperfect dominance given
above, where the progeny of unlike parents present
an intermediate condition, it is found that, upon
cross-breeding these offspring, segregation into the
grandparental types occurs just as truly as in instances
of complete dominance.

In poultry, for example, when Cochins, which are
"booted," and Leghorns, which are clean-shanked,
are crossed, booting of an intermediate grade of four
results, on a scale in which ten represents complete
booting, and zero no booting or clean shank (Daven-
port). The character of booting and its alternative
absence, however, segregate out in true Mendelian
fashion when these hybrids are subsequently crossed
together. It is evident that dominance plays only
a secondary role in such cases, and that the all-im-
portant factor is segregation.

Are there, then, any cases where true fusion of hered-
itary parental traits occurs, in other words, where
segregation in the second filial generation does not
appear? Does the "melting-pot of cross-breeding"
ever "melt" the characters thrown into it?

It was formerly believed that diverse parents
generally produce intermediate offspring, and that
this intermediate condition continues without any
segregation at all in the form of "blending inheritance,"
but within the last decade apparent cases of blending
inheritance have been thrown out of court one after
the other by the Mendelians. Bateson, in an inaugural

BLENDING INHERITANCE 183

address at Cambridge University in 1908, stated that
what was once believed to be the rule has now be-
come the exception. He goes on to say : "One clear
exception I may mention. Castle finds that in a cross
between the long-eared lop rabbit and a short-eared
breed, ears of intermediate length are produced ; and
that these intermediates breed approximately true."

Let us examine this "one clear exception" a little
more closely.

7. THE CASE 'OF RABBIT EARS

As a typical example of blending inheritance in
rabbit ears may be cited the following case : —

A female Belgian hare with an ear-length of 118 mm.
was crossed with a male lop-eared rabbit with an
ear-length of 210 mm. The average of these ear-
lengths is 164 mm. Five offspring from this pair
had ear-lengths, when adult, approximating this aver-
age as follows: 170, 170, 166, 156, 170, of which
two were females and three were males. When from
this litter one of the females measuring 170 mm. in
ear-length was subsequently crossed with her brother
having an ear-length of 166 mm., two litters were
produced in which the individuals when adult at-
tained ear-lengths of 170, 166, 168, 160, 172, and
168 mm. These results are represented diagram-
matically in Figure 52.

This illustration is typical of many other breed-
ing experiments made by the same investigators1

1 Castle, in collaboration with Walter, Mullenix and Cobb. " Studies
of Inheritance in Rabbits." Carnegie Institution Publications, Wash-
ington, No. 114, 1909.

184

GENETICS

34 56 7

Offspring of Sand 5

E 1

1 i

i?

'?

> 3

5 «

> IT

> E

• «

1 s

c

1

> <i

= i
r «

52

i

= i

?5

- c
= i

1 S

<

i

J S

>

Ofepnngof land?

FIG. 52. — A case of three generations of ear-length in rabbits, a-b,
average ear-length of the first filial generation (F^. a'-b', average
ear-length of the Fg generation derived from 1 and 7. Data from
Castle, in collaboration with Walter, Mullenix and Cobb.

upon the ear-length of rabbits which included 70
different litters of rabbits containing 341 individuals.
In none of these experiments could the blend in the

BLENDING INHERITANCE 185

second filial generation be called perfect, but it may
at least be said that evidence of segregation, that is, a
return to one or the other of the parental types, was
much less apparent than evidence of blending.

Furthermore, crosses were made in which lop ears
of various fractional lengths were obtained as desired,
including £, J, f , J, f , }, and f lengths. Not one of
these fractional lengths apparently segregated in
subsequent generations after the Mendelian fashion,
but all bred approximately true.

Moreover, ears of one half lop length, for instance,
were obtained in three ways : first, by crossing full-
length lops with short-eared rabbits as indicated in
the first cross of the case cited above; second, by
crossing one half lop lengths together, demonstrated
by the second cross in the illustrative case given, and
third, by mating J and f lop lengths. Theoretically,
J and | as well as f and f lop lengths would also pro-
duce J lop lengths, for in all of the crosses that were
made the length of ear behaved in a blending fashion.

These results were based, not upon a single measure-
ment of each specimen, which might be open to
considerable error, but upon daily measurements
from the time the rabbits were two weeks old until
their ears ceased to grow at about twenty weeks. The
growth curves drawn from these daily measurements
showed continually an intermediate or blending condi-
tion in progeny derived from diverse parents.

A Mendelian explanation of this apparently excep-
tional case of blending inheritance has been suggested
by Lang based upon the result of Nilsson-Ehle's

186 GENETICS

discoveries while breeding wheats at the Agricultural
Experiment Station of Svalof in Sweden.

8. THE NILSSON-EHLE DISCOVERY

Nilsson-Ehle found in breeding together different
strains of wheat that a certain wheat with brown
chaff crossed with a white-chaffed strain yielded only
brown-chaffed wheat in the first generation. These
heterozygous or hybrid brown-chaffed wheats when
crossed with each other produced, not the expected
proportion of three brown to one white, but fifteen
brown to one white. This was not explainable as the
chance result of a single cross, but was the conclusion
drawn from fifteen different crosses all of the same
strains that yielded a total progeny of 1410 brown-
chaffed to 94 white-chaffed plants, which happens
to be exactly the proportion of fifteen to one.

In other experiments it was discovered that although
dominant red-kerneled strains of wheat crossed
with white-kerneled varieties usually gave the three-
to-one proportion upon segregation in the second
filial generation, yet one particular strain of red-
kerneled Swedish wheat in the second generation
gave approximately sixty-three red to one white-
kerneled strain.

The explanation of these two unexpected results
is this. In the case of brown-chaffed wheat there are
two independent determiners for the character of
brown color, and these simply follow the Mendelian
laws for a dihybrid, while in the case of the red-
kerneled wheat there are three independent deter-

BLENDING INHERITANCE

187

miners for the character of red color, each of which
is able to give red color to the wheat. Taken together,
these three determiners behave cumulatively, follow-
ing the law of a trihybrid.

For example, if a brown-chaffed wheat with the for-
mula BB', in which B and E' each represent a brown-
chaffed factor, is crossed with a white-chaffed wheat of
the formula bb1 ', in which b and b' each represent the
absence of B
and B' respec-
tively, then all
the progeny of
this cross will be
brown-chaffed,
having the zy-
gotic formula
BB'bV. When
upon matura-
tion the gametes
form out of the
germ-cells from
such hybrids,
the following
four combina-
tions are pos-
sible, and no
others: BB', Bb', bB', bb'. These represent, there-
fore, the possible gametes present in each sex of the
first filial generation, and upon intercrossing they can
combine into sixteen possible zygotes to form the
second filial generation, as shown in Figure 53.

BB'

Bb'

bB'

b b'

BB

BB'
BB

©

Bb',
BB

©

b B;

b b'

B®B'

Bb

BB;

Bb;

b B'
Bb'

©

b b'
Bb'

©

bB'

BB'

b B'

©

©

b B'
b B'

©

b b'
bB'

©

bb'

BB'
b b'

B b'

b b'

©

b B'
b b'

©

b b'
b b'

©

FIG. 53. — Diagram of the possible combinations
in the F2 generation of brown-chaffed wheat
according to experiments of Nilsson-Ehle. B
and B1 are cumulative factors for the brown-
chaff character. 6 and 6' denote the absence of
B and B' respectively.

188 GENETICS

The numbers in the squares indicate how many
times a brown determiner is present in each zygote.
It will be seen that only one out of the sixteen possi-
bilities lacks a brown-chaff factor, and this one will

f 3 £ 7 0

Number of doses of the brown determiner

FIG. 54. — The distribution of the sixteen possibilities resulting when two
similar determiners (brown-chaff) act together as a dihybrid.

consequently produce only white chaff, while the re-
maining fifteen possibilities, each of which has at least
a single determiner for brown, will all yield brown
chaff.

The brown-chaff factor, moreover, is present in

BLENDING INHERITANCE

189

varying doses among these fifteen possibilities, as indi-
cated by the numbers in the squares. It is evident,
therefore, that several shades of brown will be rep-

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FIG. 55. — Diagram to illustrate Nilsson-Ehle's case of trihybrid red
wheat. The large screwheads each represent a single determiner for
the red character. The small screwheads symbolize the absence of
the red character, or white. The number in each square indicates
how many doses of the "red" determiner is present. For further
explanation see text.

resented depending upon the number of doses of the
brown determiner in each instance.

Figure 54 shows how these different shades of
brown arrange themselves in the manner of a fre-
quency polygon of fluctuating variation with the
greatest number in the halfway class and the least

190 GENETICS

numbers at the two extremes. In this instance
six out of sixteen individuals of the second gen-
eration theoretically present a perfect "blend" be-
tween the original brown- and white-chaffed grand-
parents, although complete segregation has actually
occurred.

The same explanation holds true as displayed in
Figure 55 for the trihybrid case of red- and
white-kerneled wheats in which only one white-
kerneled to sixty-three red-kerneled individuals ap-
pear in the second filial generation. The number
of red determiners in each zygote is indicated by the
figure at the bottom of each square. The large screw-
head symbols with vertical, horizontal and diagonal
slots each represent an independent determiner for
red kernel, while the small screw heads symbolize
the absence of each of these determiners, or white
kernel. When the pure strain of red-kerneled wheat
is crossed with a pure strain of white-kerneled wheat,
the first generation is all a heterozygous red of a

(D00 + ® e o = (D<DGe00
Pure red + ouhite = Hybrid r«d

FIG. 56. — The result of crossing white wheat with trihybrid red wheat.

somewhat lighter shade than the original pure red
strain.

When plants of this heterozygous sort are crossed
together, they yield plants producing red-kerneled
and white-kerneled wheats in the proportion of sixty-
three to one. The sixty- three kinds of red wheats are

BLENDING INHERITANCE

191

of varying degrees of redness and may be classified
after the manner of fluctuating variations with the
greatest number of kinds
at the intermediate degree
between pure red and pure
white. (See Figure 57.)

In order to test whether
the sixty-four kinds of
wheats produced in the
second filial generation, as
theoretically displayed in
Figure 55, really contain
separable, though indistin-
guishable, determiners for
red-kernel, Nilsson-Ehle
produced families of the
third filial generation by
self-crossing plants of the
second generation. It was
to be expected that, if
these hybrid wheats of the
second generation carried
one, two, three, or more
determiners for a red kernel
as the theoretical tables in
Figures 55 and 57 demand,
their progeny would be
distributed with reference
to the number of red- and
white-kerneled individuals,
in the following ratios : —

#

FIG. 57. — The distribution of the
sixty-four possibilities in the F2
generation when three similar
determiners act together as a
trihybrid.

192 GENETICS

3 red to 1 white when 1 determiner for red is present.

15 red to 1 white when 2 determiners for red are present.

63 red to 1 white when 3 determiners for red are present.

All red to no white when 4 or more determiners for red

are present.

Among seventy-eight sample families of the third
generation inbred to test this theoretical conclusion,
the actual results were: —

8 families giving the ratio of 3 red to 1 white.
15 families giving the ratio of 15 red to 1 white.

5 families giving the ratio of 63 red to 1 white.
50 families giving the ratio of all red to no white.

It has been actually demonstrated therefore, in '
the case of this particular strain of wheat: (1) that the
factors producing red kernel are several in number;
(2) that they act independently of each other in
heredity; (3) that these several independent factors
segregate; and (4) that any one red factor acting
alone produces a "red" result.

The Nilsson-Ehle principle of cumulative determin-
ers has been confirmed in America by East in a mas-
terly series of breeding experiments upon maize.

In connection with the Nilsson-Ehle principle, it
will be seen that the possible number of intergrades
between the two extremes increases rapidly as the
number of duplicate determiners increases. Thus
with six duplicate determiners for the same character
present, the ratio of possible dominants to recessives
in the second filial generation would be 4095 to 1.
The reappearance of this single recessive among 4095

BLENDING INHERITANCE 193

dominants would be extremely unlikely, and it might
easily be mistaken for a mutation or a freak. Appar-
ent blends of all intermediate degrees, however, would
be sure to appear. Yet these are not blends in the
"melting-pot" sense at all, but strictly cases of Men-
delian dominance and segregation.

9. THE APPLICATION OF THE NILSSON-EHLE EX-
PLANATION TO THE CASE OF RABBIT EAR-
LENGTH

The so-called blending rabbit ears, along with
other similar cases, can now be made to fall into line,
as pointed out by Lang, with the Mendelian law of
segregation.

If we assume that the long ear of the lop rabbit
has only three independent but equal determiners for
excess length, the case becomes one of Mendelian
trihybridism with cumulative factors, which works
out like Nilsson-Ehle's red-kerneled wheat in the
following manner: —

In general the average for full lop ear-length may
be placed at 220 mm. and for the ordinary short-
eared rabbit 1 at 100 mm. The difference, or the ex-
cess length of the lop ear, is 120 mm., which, according
to the trihybrid formula, corresponds to the six doses
of the character symbolized in the upper left-hand
square in Figure 55 by six large screw heads, three

1Not the Belgian hare, as cited in the illustration given in Figure
52. The Belgian hare has typically a somewhat longer ear than
the ordinary short-eared rabbit.

194

GENETICS

coming from each parent respectively. If all of these
independent determiners are equal as regards excess
ear-length, each factor would represent an excess of
20 mm. above the normal ear-length found in short-
eared rabbits, that is, —

220 mm. - 100 mm.
6

= 20 mm.

When according to this computation a lop (20
mm. X 6 + 100 mm. = 220 mm.) and a pure short-
eared rabbit (20 mm. X 0 + 100 mm. = 100 mm.)
are crossed, if imperfect dominance occurs, which
is a very common phenomenon, it is true that
the offspring might present a "blended" appear-
ance. If now these cross-breds of the first gen-
eration prove to be trihybrids with respect to excess
ear-length, there would be sixty-four possibilities
in their progeny segregating out just as in the red-
kerneled wheat.

These possibilities would be arranged in the fol-
lowing frequencies : —

NUMBER OF EXCESS EAR-
LENGTH DETERMINERS

NUMBER OF CASES OCCUR-
RING OUT OF 64

TOTAL LENGTH IN MILLI-
METERS OF EARS RESULTING

6

1

220

5

6

200

4

15

180

3

20

160

2

15

140

1

6

120

0

1

100

BLENDING INHERITANCE 195

Since the average litter among rabbits is about
five, the chances that these five rabbits will breed
true to their hybrid parents and form a perfect
blend between their grandparents is 20 out of 64,
while the chance of their being like either grand-
parent is only one out of 64.

It should be noted further that 50 out of 64, or 77
per cent, of these hybrids of the second filial gen-
eration would have an ear-length between 140 and
180, thus approximating a "blend" closely enough
to be so classified upon a casual inspection.

Moreover, if it should be found that excessive
ear-length in rabbits is due to more than three dupli-
cate determiners, the possibilities of getting anything
but an apparent blend would be much decreased.

The fact, furthermore, that the fractional ear-
lengths of the hybrid rabbits in Castle's experiments
bred approximately true in the second and subse-
quent filial generations, may also be explained by
the Nilsson-Ehle hypothesis.

For example, half lop lengths, according to this
explanation, are those with three doses of the deter-
miner for excess ear-length. It follows that the
progeny of two rabbits each carrying three doses of
a determiner will likewise, after the reduction during
the maturation of the germ-cells, have three doses

/3 _i_ 3 \
of the determiner ( — — — = 3 J.

It would be interesting to breed rabbits having
ears of one eighth lop length in which, according to
the foregoing hypothesis, there would presumably be

196 GENETICS

present only a single determiner for excess ear-length,
with ordinary short-eared rabbits having no excess
ear-length, in order to see if the expected Mendelian
three-to-one proportion for a monohybrid would ap-
pear in the progeny.

10. HUMAN SKIN COLOR

Finally, although accurate published data is
wanting, it is probably true that skin color in all
kinds of hybrids resulting from crosses between
negroes and whites is not a case of blending inherit-
ance, as commonly supposed, but rather of true
Mendelian segregation. In fact, there is frequently
visible evidence that segregation does occur, as shown
by many authentic instances where the offspring
of diversely colored parents produce children with
skin color of different shades.

If human families included hundreds of offspring
in a single generation instead of the usual number,
the problem of skin color in man could doubtless
be quickly solved since ratios could then be obtained
large enough to reveal the underlying laws of inher-
itance.

CHAPTER X

THE DETERMINATION OF SEX
1. SPECULATIONS, ANCIENT AND MODERN

FROM the earliest times the desirability of con-
trolling the sex of an unborn child, in particular
instances at least, has seemed very great. Likewise
the wish to be able to predetermine sex among do-
mesticated animals has made breeders quick to grasp
at every clue that promised success.

There has been no want of speculations concerning
the determination of sex. J. Arthur Thomson,
who with Professor Geddes wrote "The Evolution
of Sex" in 1889, says : "The number of speculations
as to the nature of sex has been well-nigh doubled
since Dreylincourt, in the eighteenth century,
brought together 262 * groundless hypotheses' and
since Blumenbach caustically remarked that nothing
was more certain than that Dreylincourt's own theory
formed the 263rd. Subsequent investigators have
long ago added Blumenbach's theory of 'Bildungs-
trieb' or formative impulse, to the list." It maybe
added in passing that the hypothesis of the deter-
minative action of external factors upon developing
germ-cells which Geddes and Thomson elaborated
in the book just referred to, has, in its turn, accord-
ing to most biologists, joined the long roll.

197

198 GENETICS

Hippocrates thought that sex of the offspring
depends upon the relative "vigor" of the parents,
while Sadler (1830) concluded that the relative ages
of the two parents is the determining factor.
Other writers, on the contrary, have thought that
the age of the mother at the time of childbirth deter-
mines the sex of the offspring, and Thury (1863), in
the days before the facts of maturation were known,
ascribed the determinative factor to the relative
degree of "ripeness" of the egg when fertilized. It
was once assumed also that the right ovary or the
right testicle is the seat of one sex and the left ovary
or left testicle of the other. Galen, who did the
biological thinking for several centuries of mankind,
asserted that the right side of the body, "being
warmer" than the left, consequently produces males.
Schenk cites a most amazing bit of folk-lore to the
effect that: "In Servia if a man has a stye on his
eyelid he comes to the conclusion that his aunt is
pregnant. If the stye is on the upper eyelid, the
child will be a male ; if on the lower, a female."

Modern theories of sex determination, like the
earlier speculations, may be resolved into two groups,
namely, those which depend upon controllable ex-
ternal or environmental factors such as food, climate,
chemical dosage and will power, and those which de-
pend upon internal factors at present beyond control.

2. THE NUTRITION THEORY

Of external factors which may exert a moulding
influence upon the sex of the offspring, nutrition is

THE DETERMINATION OF SEX 199

possibly the most potent. This factor may be
conceived to act either upon the parent previous
to the maturation of the germ-cells, upon the germ-
cells themselves, or upon some susceptible embryonic
stage of the life cycle subsequent to that of the fer-
tilized egg.

It has been suggested that since the egg is char-
acterized by possibly a more advanced metabolic
condition than the sperm due to the presence of the
nutritive yolk, consequently the more yolk or nutri-
tion there is, the more femaleness will characterize the
egg. In other words, femaleness is a nutritive condi-
tion associated in the egg with the presence of yolk.

A generation ago Professor Schenk of Vienna, by
controlling the nitrogenous diet of certain royal
prospective mothers, gained a soothsayer's reputa-
tion as a prophet of sex which was based upon several
correct predictions.

Of course, any prediction of sex is bound to turn
out correct in 50 per cent of the cases, regardless of
what it is based upon, since in man the two sexes
are approximately equal in numbers. Adherents
of all sorts of theories, therefore, have always been
able to produce considerable "evidence" to sub-
stantiate their speculations, however crude the latter
have been.

Statisticians have pointed out that in times of
unusual hardship, like famine or war, when the
amount of available nutrition for pregnant mothers
is presumably reduced, there seems to be a prepon-
derance of males born.

200 GENETICS

%

' A series of nutrition experiments upon frogs per-
formed by Born ('81), Pfluger ('82), and Yung ('85)
showed that the percentage of female offspring, which
normally is slightly over fifty, could be changed to
over 90 per cent by regulating the food supplied to
the mother before the egg-laying period. Cuenot
and King, however, working independently, repeated
these experiments with great care, taking into account
all the eggs that were laid and not simply the ones
that developed, and both obtained negative results.
They concluded, therefore, that the high percentage
of female tadpoles appearing in the initial experi-
ments was due to a greater mortality among the males
and not to the transformation of possible males into
females.

There seems to be no doubt that nutrition may
affect the percentage of those which reach maturity.
If one sex requires a greater amount of nutrition
than the other to carry out successfully the more
strenuous metabolic changes in its life-cycle, then
unequal percentages between the sexes of the sur-
vivors resulting from modified nutrition do not in
any way help to solve the problem of determining
the sex of the individual. In other words, the elim-
ination of one sex through modified nutrition does
not "determine" the other sex.

3. THE STATISTICAL STUDY OF SEX

From statistical sources it has been ascertained
that ordinarily there is produced a practical equality
in the numbers of the two sexes.

THE DETERMINATION OF SEX 201

Oesterleben in Europe summarized the data for
nearly sixty million human births and found that
an average of 106 males are born to every 100 fe-
males.

According to various authorities, the relative num-
ber of males per 100 females is given for horses as 99,
for cattle 94, and poultry 95, while in pigs, rabbits,
pigeons, and greyhounds the corresponding number
of males is slightly over 100.

This practical equality of the sexes in all sorts of
natural environments indicates the improbability of
the assumption that external conditions determine
sex.

4. MONOCHORIAL TWINS

There are two kinds of twins, namely, ordinary
twins, which come from two separately fertilized
eggs each inclosed in its own chorion, and "identical
twins, " that have their origin in one egg which is in-
closed in one chorion. Of the former, something like
30 per cent in man are reported as being of two sexes,
thus showing that it is neither nutrition nor envi-
ronment which determines sex. Usually when twins
are of the same sex, they exhibit as great a range of
difference in mental and physical traits as do ordinary
children of the same fraternity born at different
times, but occasionally "identical twins" are born,
and such monochorial twins. are always of the same
sex. This is evidence that sex, like other somatic
characters, is determined in the germplasm at the
time of fertilization.

202 GENETICS

Similarly, in the chalcid fly, Ageniapsis, a chain of
embryos is formed from a single egg, and these, ac-
cording to Marschal, are all of the same sex.

Newman and Patterson also have shown that in
the armadillo, Tatusia, there are customarily produced
four young within a single chorion, all of which are
of the same sex.

These facts point toward the conclusion that the
determination of sex takes place at the time of fer-
tilization.

5. SELECTIVE FERTILIZATION

Within the last ten years considerable evidence
has been collected in support of the supposition that
sex is a Mendelian character. Mendel himself,
without elaborating this idea into a definite hypothe-
sis, suggested the probability that sex is a heritable
character behaving in the same way as other herit-
able characters.

In 1903 Castle published a paper 1 in which a
tentative explanation, since abandoned, of the phe-
nomenon of sex determination was advanced, based
upon three assumptions : first, that all germ-cells are
heterozygous for sex and, therefore, upon maturation
there are formed both male and female eggs as well
as male and female sperms ; second, that in fertili-
zation the gametes always unite with their opposites
so far as sex is concerned and never with their like,
with the result that each fertilized egg must carry

1 Castle, W. E., "The Heredity of Sex." Bull. Mus. Comp. Zool.,
Harvard, Vol. XL, No. 4, 1903.

THE DETERMINATION OF SEX

203

determiners for both sexes and be heterozygous, as
indicated in Figure 58 ; and third, that the character
of sex follows the law of alternative dominance, ac-
cording to which in the male offspring the male
determiner dominates M(F), while in the female
the female dominates (M)F.

This hypothesis is simply an attempt to explain
the numerical ^equality of the sexes, and also the fact
that the determiner for the opposite sex may be car-

MALC
GAMETES M + (F)

ZYGOTES

FEMALE:

k

M(F) M(M) (F)F

MALE - DO NOT OCCUR -

F(M)
FEMALE

FIG. 58. — Diagram to show Castle's 1903 theory of the heredity of sex.

ried by either parent, but it leaves unanswered the
question of what causes "selective fertilization"
and "alternative dominance."

There appears to be some evidence that selective
fertilization, which was assumed in Castle's 1903
theory, ma'y actually occur under certain circum-
stances. For example, homozygaus or pure yellow
mice, that is, mice with a duplex determiner for
yellow color, are not known. In breeding, all kinds
of yellow mice behave as if heterozygous or simplex
with respect to yellow color, for when any two yellow
mice are bred together, they produce a certain per-
centage of recessives which would not happen if they

204 GENETICS

i

were pure yellow. In a Mendelian monohybrid cross,
as has been previously pointed out, the expectation is
that in the second generation one fourth of the offspring
will be recessives (DR XDR = IW + <2DR + RR) , but
when yellow mice are bred together, the percentage of
recessives approximates one third instead of one fourth.
This apparent exception to the Mendelian ratio finds
an explanation, however, when it is assumed that selec-
tive fertilization takes place in such a cross, and thus,
since a D gamete never unites with another D gamete,
but always with its opposite, R, pure yellow mice are
unknown.

This supposition is further supported by the fact
that the litters of young from yellow mice are, on an
average, only three fourths as large as normal litters
of mice, which is exactly what would be expected
if one fourth of the possible gametic combinations
(DD) fail to produce offspring.

Castle's tentative explanation of the determina-
tion of sex at least breaks away from the old concep-
tion that the sperm-cell produces male offspring and
the egg-cell, females. It agrees, too, with Darwin's
idea that both sexes are present in each individual
with one sex latent. In certain parthenogenetic
rotifers, aphids and daphnids, both sexes are plainly
present in the female, since two kinds of easily dis-
tinguishable eggs are produced, one of which develops
into males and the other into females without fer-
tilization or any kind of a union with a sperm-cell.

THE DETERMINATION OF SEX

205

TYPE

ZYGOTES

GAMETES

I

(Pemale) Qtf
(Male) Otf

9 + cf

rf 4- cf

60* gc? .

(Female! (Male)

n

(Female) ?V
(Male) ?O*

? -H V

2 -H d*

92 VcT

(Female)' <Ma>e)

FIG. 59. — Diagram to show the neo-Men-
delian theory of the heredity of sex, using
sex symbols.

6. THE NEO-MENDELIAN THEORY OF SEX

Correns (1906) avoids the difficulties of alternative
dominance, which Castle's hypothesis offers, by sup-
posing that one par-
ent only is hetero-
zygous with respect
to sex, and this sup-
position is becom-
ing more and more
probable as evidence
accumulates. Ac-
cording to this idea,
there are two types
of cases, one when the female is the heterozygous
parent and the other when the male is the hetero-
zygous parent, as represented in Figure 59.

The formulae for these types may be expressed in
the nomenclature of the presence and absence theory,

as follows (Fig. 60),
in which the sym-
bol x represents the
female determiner
in the heterozygous
case of type I, and
xx the female de-
terminer when the
male is the hetero-
zygous parent.
The formulae may be still further modified, accord-
ing to Morgan, for the satisfaction of those who ob-

TYPE:

ZYGOTES

GAMETES

I

(Female) x O
(Male) o O ,

X -f O

O -4- O

xo oo

(Female) (Male)

n

(Female) x X
(Male) x O

X + X

X + O

XX XO

( Female ) ( Male /

FIG. 60. — Diagram to show the neo-Men-
delian theory of the heredity of sex accord-
ing to the presence and absence hypothesis.

206

GENETICS

ject to regarding the male factor as nothing positive,
but simply the absence of femaleness, by assuming
that a universal factor of maleness (ra) is present in
all cases, as shown in Figure 61.

Thus in type I of this scheme it is only when the
dominant female factor F is entirely absent that male-
ness becomes expressed in the somatoplasm, while in

type II it is neces-
sary to have a double
dose of the factor F
in order to produce
a female, since a
single dose results in
a male.

All of these three
theoretical schemes

TYPE

ZYGOTES

GAMETES

i

(Female) Frofm
(Male ) fmfm

Fm 4- fm

fm -1- fm

Fmfm Mm
(Fema\ej (Male)

n

(Female) FmFm
[Mole) Fmfm

Fm -j- Fm
Fm + fm

FmFm Fmfm
(Female) (Male)

FIG. 61. — Diagram to show the neo-Men-
delian theory of the heredity of sex with
Morgan's modification, making male-
ness (m) present as a positive character
in every gamete.

agree in assuming
that one sex is hetero-
zygous, while the other
is homozygous and that femaleness is the result of
an added factor in excess of maleness.

The evidence for these conclusions has been ob-
tained chiefly from four sources : first, from a mi-
croscopical examination of the germ-cells ; second,
from castration and regeneration experiments ; third,
from the results of. hybridization in "sex-limited
inheritance"; and fourth, from the behavior of
hermaphrodites in heredity.

THE DETERMINATION OF SEX 207

a. Microscopical Evidence
A, 1. The "X" Chromosome

In 1891 Henking called attention to the presence
of two kinds of spermatozoa in the firefly, Pyrrho-
coris, and later McClung (1901), in studying the
spermatogenesis of the grasshopper, discovered a
similar phenomenon with respect to the chromosomes
of its spermatozoa. Soon after, Stevens and Wilson

FIG. 62. — Diagram to show how numerical equality of the sexes results
when one parent is homozygous (the female in this instance) and the
other is heterozygous for the sex character.

working independently on various species of insects,
and Boveri, on sea-urchins, found that when the
male is characterized by two kinds of sperm-cells,
one of which has an "extra" chromosome (the so-
called "accessory" or (tx" chromosome), while the
other does not, the female of the same species, upon
maturation of the eggs, produces mature eggs, all of
which possess one "x" chromosome. The result
of this heterozygous condition of the male and homo-
zygous condition of the female with respect to the
x chromosome is the theoretical equality of the
sexes among the individuals formed by their union, as

208 GENETICS

shown in Figure 62 or in type II of Figures 59, 60
and 61.

It will be seen that when a male gamete bearing
an x chromosome unites with a female gamete also
bearing an x chromosome, the outcome is a fertilized
egg containing xx chromosomes. Such an egg is con-
sequently homozygous for sex, and will develop into
a female individual. In the same way when a male
gamete lacking an x chromosome, as half the gametes
derived from a heterozygote do, unites with a female
gamete bearing an x chromosome, as all gametes
from a homozygote must do, then the fertilized egg
will be heterozygous, carrying only one x chromosome,
and will develop into a male individual.

The chromatin difference between the two sexes
may be qualitative, as Wilson holds, or quantitative,
as Morgan assumes, but in either case it seems cer-
tain that, with difference in sex, there is invariably
associated a definite difference in the character of
the chromosomes present in the germ-cells.

These conclusions have been abundantly con-
firmed in various species by a large number of inde-
pendent workers, and are now well established as
a part of biological science. In fact, it is not at
all unusual to find the technical confirmation of the
x chromosome theory given as a part of the routine
class work in university courses.

A, 2. Various Forms of X Chromosomes

The extra chromosome in different species may
assume various forms or degrees of complexity. It

THE DETERMINATION OF SEX 209

may be either single or multiple. It may be paired
before maturation with its absence, or with an unlike
("y") chromosome. It may be linked inseparably
with some one of the ordinary chromosomes (auto-
somes), or resemble the autosomes so closely that its
presence can only be assumed from analogy with
other cases, and not definitely determined at all.

In all of these cases, however, there is one point
of likeness, and that is that there always seems to
be additional chromatin material associated with the
female sex.

The reason for this may lie in the more highly
metabolic requirements of the female, who must
produce yolk or provide in some way for the main-
tenance of the young in addition to furnishing half
of the germinal heritage.

In the microscopical evidence on this point there
is one apparent exception to the rule that females
are homozygous and males heterozygous with respect
to sex. Baltzer (1910) found that in one of the sea-
urchins an extra sex chromosome is associated with
the female sex, so that two kinds of mature eggs are
produced upon maturation and only one kind of
sperm-cells. In other words, in this case the female
is heterozygous for sex and the male homozygous,
instead of the reverse which is true for all other
forms thus far microscopically investigated.

Such cases as this of the sea-urchin are theoreti-
cally provided for in the formulae under type I given
above in Figures 59, 60 and 61.

210 GENETICS

A, 3. Sex Chromosomes in Parthenogenesis

The behavior of the chromosomes in cases of
parthenogenesis, where the union of an egg-cell and
a sperm-cell are not necessary for the production
of a new individual, throws additional light upon
the relation between chromosomes and sex determi-
nation.

For instance, among the social Hymenoptera, bees,
ants, wasps, etc., the "queen" produces eggs which
upon maturation, if unfertilized, develop into males
or drones, all of whose cells contain a reduced amount
of chromatin (Fig. 63). It is only when sexual repro-
duction occurs through the union of a mature egg-
cell and a mature sperm-cell or spermatozoan, that
the full complement of chromatin is restored to the
fertilized egg and females are again produced.

Castle says : "In all known cases of parthenogenesis
the female is in the duplex (2 n) condition, and the
male is in the simplex (n), or partially duplex (2 n — I
condition. The female in all cases has the greater
chromatin content."

b. Castration and Regeneration Experiments

Certain characters which are known as "secondary
sexual characters," such as the ornamental plumage
in male birds, the beard in man or the sting in worker
bees, are often associated with a definite sex. When an
individual is castrated, it is quite common not only
for these peculiar secondary sexual characters to dis-
appear, but also for the secondary sexual characters of

THE DETERMINATION OF SEX

9 SOMA (Queen)

POLAR BODY
£* POLAR BODY

SPERM CCLLS

FERTILIZED
EGG

FIG. 63. — Diagram of the heredity of sex in bees, ants and wasps. The
outline chromosomes represent sfmple somatic chromosomes. The
solid black chromosomes stand for sex. The female has two sex
chromosomes while the male has but one.

the opposite sex to develop to a certain degree in
their stead. This indicates that the determiners for
sex are intimately associated with those for the sec-

GENETICS

ondary sexual characters, and also that the determin-
ers for the opposite sex are often present in a latent
condition, or, in other words, that the organism,
either male or female, is heterozygous with respect to
sex.

If a female of the annelid worm Ophiotrocha, for
example, is cut in half, it is effectually castrated, be-

M a 1 6 Parasiticallij castrated male

Female

FIG. 64. — The crab, Inachus, parasitized by the cirripede, Sacculina.
Evidence that a Mendelian sex determiner is correlated with "sec-
ondary sexual characters " and that the male is heterozygous for sex
while the female is homozygous. After Smith.

cause the ovaries are in the posterior part of the body.
It has the power of regeneration, however, but when
a new posterior part is formed, it contains, not female,
but male reproductive organs. The worm is, there-
fore, now a male, as shown by the presence of testes
instead of ovaries, proving that it was originally
heterozygous with respect to sex, carrying one sex
latent.

THE DETERMINATION OF SEX 213

According to Smith, parasitic castration is performed
on the crab Inachus, which is found in the Bay of
Naples, by a cirripede, Sacculina. The male crab
of this species has one large claw and a narrow abdo-
men, while the female has no large claw, but a broad
abdomen. When Sacculina parasitizes the female, the
secondary sexual characters of the female are stunted,
but not materially changed. When, on the contrary,
the male is parasitized, it not only loses its distinctive
large claw in subsequent molts, but it also takes
on the broad abdomen of the female (Fig. 64). This
apparent anomaly is quite explainable upon the as-
sumption that the female is homozygous for sex
and the accompanying secondary sexual characters,
while the male is heterozygous. When maleness is
destroyed in the male by the castrating parasite,
therefore, the femaleness that is latent in this sex
becomes manifest through the appearance of female
secondary sexual characters ; but when the female is
castrated, no other secondary sexual characters than
those already present make their appearance, since
only femaleness is present in the homozygous female
sex.

c. Sex-limited Inheritance

Additional evidence that sex is a character depend-
ing upon determiners which behave in Mendelian
fashion is furnished by what is called sex-limited
inheritance. There are certain characters known as
sex-limited characters that are in no sense to be con-
fused with secondary sexual characters which appear
to be always linked with the determiner for either one

214

GENETICS

sex or the other. They are, therefore, well described
by the term "sex-limited."

(1) Color-blindness

This phenomenon may be illustrated by the in-
heritance of human color-blindness, a character which
appears to be linked with the determiner for sex. It
requires a duplex, or homozygous, dose of the deter-
miner for color-blindness to produce a color-blind
female, while only a simplex, or heterozygous, dose is

FIG. 65. — General diagram for sex-limited inheritance. The underscored
symbol (X.) represents a sex determiner with some other character
(as color-blindness) linked with it.

needed to produce a color-blind male. These facts
agree perfectly with the idea that the female is homo-
zygous and the male heterozygous with respect to

THE DETERMINATION OF SEX 215

sex, and that the factor for color-blindness is linked
with the determiner for* sex. Sex-limited inheritance,
as shown in this case, may be illustrated by the dia-
gram on the opposite page (Fig. 65) in which, for the
sake of simplicity, only sex chromosomes and the de-
terminers for color-blindness are represented. Under-
scored >< represents a color-blind determiner linked
to a sex chromosome.

From this diagram, which agrees substantially
with the facts, it is apparent that a color-blind male
mated to a normal female will produce no color-blind
offspring, although the females will be "carriers"
of color-blindness, that is, will possess the factor in
simplex form and will, therefore, carry it for the female
in a latent condition.

The sons of such a mating having a normal mother
and a color-blind father will be absolutely free from
the defect and cannot produce color-blindness in any
of their offspring when mated with a normal strain.
If, however, the "carrier" daughters from such a
parentage, who are genotypically heterozygous for
color-blindness but phenotypically normal, mate with
normal individuals, the expectation is that one half
of the sons, and none of the daughters will.be color-
blind, but that one half of these daughters wilPcarfy

4K . - •.-• . - f '••.-

the color-blind determiner in simplex form, that is, in
a condition ineffective for producing color-blindness
in female individuals.

All of the various possibilities in the inheritance of
color-blindness according to the sex-limited interpre-
tation are indicated in the following table : -

216

GENETICS

PARENTS

EXPECTED OFFSPRING

* '

Normal •

9

Color-blind

$

Color-blmd

9

Carrier " ^

Normal

- Carrier

\ color-blind
\ normal

£ carrier
\ normal ^

Color-blind

Normal

Normal

Carrier ..

Color-blind

Color-blind

Color-blind

Color-blind

Color-blind

Carrier

\ color-blind
\ normal
_f I

^color-blind.
\ carrier . ."""

(2) The English Currant-worm

A famous case of sex-limited inheritance is that of
the English currant- worm, Abraxas, which occurs in
two varieties, viz., Abraxas grossulariata and Abraxas

FIG. 66. — Abraxas grossulariata, the English currant-moth, and (on the
right) its paler lacticolor variety. From Punnett's " Mendelism."

lacticolor (Fig. 66). The lighter-colored lacticolor
is recessive to the darker-colored grossulariata variety
and has been found in nature associated only with the
female sex.

THE DETERMINATION OF SEX 217

Doncaster and Raynor, in 1908, published the results
of various crosses between these two varieties which
demonstrate clearly that sex is a Mendelian character
and that, in this instance, maleness is homozygous
and femaleness heterozygous with the determiner

KEY TO SYMBOLS

PHENOTYPE

GROSS.C?

Gposs.d1

LACT.C?

Constitution with
respect to the

GROSSULARIATA

Duplex

v

Simplex

Multiplex

Simplex

Multiplex

GENOTYPE

floi Ooi

Ooi Ooi

0oi

Oof

floi Do

GAMETES

a-/-^ 2).

Ooi f) (Ooi

FIG. 67. — Key to the symbols employed in Figures 68-71. The outline
symbols represent samples of the autosomes or somatic chromosomes.
The black symbols stand for the "extra" or sex chromosomes. O
above a black symbol indicates the grossulariata factor linked with a
sex chromosome. The variety lacticolor occurs whenever the grossu-
lariata factor is absent.

for maleness linked with the factor producing the
variety grossulariata. A study of Figures 67-71
will make this case clear. Outline symbols represent
ordinary chromosomes or autosomes, several of which
are omitted for sake of clearness. The black sym-
bols represent sex chromosomes. The letter G placed

218 GENETICS

above a black symbol represents the grossulariata
factor linked with a sex chromosome. The variety
lacticolor occurs whenever the factor for grossulariata
is absent. In this case two sex determiners are neces-
sary to produce a male, and only one to produce a
female. In the following theoretical diagrams the
actual number of offspring obtained by Doncaster
and Raynor in each cross is indicated outside the circles
that represent the zygotes, and the parenthetical
numbers refer to the five kinds of individuals cata-
logued in Figure 67.

In the first cross (Fig. 68) where a lacticolor female
(5) and a grossulariata male (1) were bred together,
the entire progeny was grossulariata in character with
an approximate equality between the sexes, that is,
45 males (2) to 50 females (4).

When these hybrid grossulariata individuals, (2) and
(4), were mated with each other in Cross 2 (Fig. 69),
the character of grossulariata appeared again in both
sexes, (1), (2), and (4), while the character lacticolor
was confined as usual to females alone (5). It was
only when grossulariata hybrid males (2) were crossed
back to lacticolor recessive females (5) in Cross
3 (Fig. 70) that individuals of both varieties and
both sexes appeared, (2), (3), (5), (4), in practically
the expected equal numbers, namely, 63, 65, 70, 62.
The lacticolor male (3) obtained by bringing together
the two sex determiners necessary for maleness, each
of which had been dissociated through the foregoing
. crosses from the sex-limited grossulariata factor, was en-
tirely new to science, never having been found in nature.

THE DETERMINATION OF SEX
CRO SS 1

219

GAMETES fflfli

ZYGOTES

(2)

FIG. 68. — The formation of heterozygous grossulariata individuals, both
male and female, by crossing pure grossulariata males with lacticolor
females.

CRO SS 2.

GRoss.d1 GROSS.C?
(/) U)

FIG. 69. — The cross-breeding of heterozygous grossulariata individuals.

LACT. 9 GROSS.
(5) (4)

220 GENETICS

Finally, when these newly made lacticolor males (3)
were crossed with heterozygous grossulariata females
(4) (Fig. 71), the proportion of sexes was approxi-
mately equal, as expected, that is, 145 males to 130
females, but all of the males were of the heterozygous
grossulariata type (2) and all of the females of the re-
cessive lacticolor type (5), showing a return to the sex-
limited condition. All of these curious results find a
satisfactory and complete explanation in the assump-
tion, first, that sex is a Mendelian character carrying
two determiners for maleness and one for femaleness ;
and, second, that the determiner for the character of
grossulariata when present is always linked to the sex
determiner.

This case is of particular interest, since it agrees
with the microscopical evidence already referred to
in connection with the chromosomes of Baltzer's sea-
urchins, in which the male was likewise homozygous
and the female heterozygous with respect to sex.

The chromosomes of Abraxas present certain techni-
cal difficulties which at present have not been over-
come, so that we do not yet know whether the evi-
dence of the heterozygous character of one sex and the
homozygous character of the other, obtained from the
breeding experiments of Doncaster and Raynor, will
be confirmed upon a microscopic examination of the
chromosomes in the germ-cells.

(3) The Behavior of Hermaphrodites in Heredity

Certain plants occur in monoecious form, that is, as
hermaphrodites, and also in dioecious form, that is, with

THE DETERMINATION OF SEX
CROSS 3

65 "v ' 65

GROSS, d1

(3)

LACT.$> GROSS
(5) (4)

FIG. 70. — Heterozygous grossulariata male crossed with lacticolor female.
One fourth of the progeny are lacticolor MALE, not known to occur in
nature.

CROSS 4

GROSS d1 LACT 9

(2) (5)

FIG. 71. — Back cross of lacticolor male with grossulariata female produc-
ing the original sex-limited condition in which all the females are of
the lacticolor type. Data for Figures 68-71 from Doncaster and
Raynor.

222 GENETICS

the sexes on separate plants. Among such dimorphic
plants, Bryonia in particular has been investigated by
Correns and Lychnis by Shull. Without describing
the crosses made in their experiments in detail, it
may be stated that when dioecious types are recipro-
cally crossed with hermaphroditic forms, the result-
ing progeny indicate plainly that one sex is homozy-
gous while the other is heterozygous with respect to
the sex character. This confirmatory evidence is
quite in line with that already brought forward that
sex is a Mendelian character the determiners of which
are carried in the germplasm.

7. CONCLUSION

The evidence thus far obtainable from all sources
points to the conclusion that sex is unalterably fixed
at the time the egg is fertilized, by definite deter-
miners which act in the same way as other Mendelian
determiners. Dr. Shull, whose exhaustive studies in
sex determination place him in the front rank as an
authority on the subject, makes this conservative
statement: "Nearly all the recent investigations
indicate that sex is at least predominantly dependent
upon the genotypic nature of the individual."

If this is so, while it furnishes the best of confirma-
tory evidence in support of Mendel's law, it shows
that it is not possible for man to predetermine the
sex of his offspring, which he has long hoped to be
able to do. The following quotation from Castle
may suitably close this chapter: "Negative as are
the results of our study of sex control, they are perhaps

THE DETERMINATION OF SEX 223

not wholly without practical value. It is something
to know our limitations. We may thus save time
from useless attempts at controlling what is un-
controllable and devote it to more profitable employ-
ments."

CHAPTER XI

THE APPLICATION TO MAN
1. THE APPLICATION OF GENETICS TO MAN

HUMAN civilization goes hand in hand with the
degree of successful interference which man exerts
upon the natural forces surrounding him.

Primitive man was overwhelmed and outmastered
by his environment, but civilized man harnesses nature
to do his will. Savages are not proficient in the
arts of cultivating plants and domesticating animals,
while these are the very things upon which human prog-
ress fundamentally depends. The degree of civiliza-
tion of any people is closely correlated with the degree
of their success in exercising a conquering control
over plants and animals. Any knowledge of the
laws of heredity, therefore, as applied by man, either
directly to himself or indirectly to animals and plants,
is a distinct contribution to human progress.

In 1900 the National Association of British and
Irish Millers, as Kellicott points out, being dissatis-
fied with the quality and quantity of the annual
wheat yield, engaged Professor Biffen to apply his
knowledge of heredity to the practical problem of
improving their wheat crop. The characters desired
were a short full head, beardlessness, high gluten

224

THE APPLICATION TO MAN 225

content, immunity to rust, strong supporting straw,
and a high yield per acre. In the short time that
has elapsed, Professor Biffen has succeeded in pro-
ducing strains of wheat that combine all these de-
sirable characters to a remarkable degree.

Such an immediate result would not have been pos-
sible before 1900, when the rediscovery of Mendel's
law revolutionized man's knowledge of the action of
heredity in nature.

This same knowledge which has made possible the
improvement of wheat may be applied to the breed-
ing of man, for there is no reasonable doubt that
man belongs in the same evolutionary series with all
other animals, as Darwin showed, and is consequently
subject to the same natural laws to a considerable
degree.

It must be admitted that thus far in the progress
of civilization more attention has been directed to
the scientific breeding of animals and plants, little
as that has been, than to the scientific breeding of
man. Let us hope that the future will have a dif-
ferent story to tell !

2. MODIFYING FACTORS IN THE CASE OF MAN

There are certain qualifying factors which make
the problems of genetics somewhat different iri\the
case of man than of other organisms.

For example, mankind has come to be partially

exempt from some of the natural laws that affect

other organisms. Thus with respect to the workings

of natural selection man is partially under "grace"

Q

GENETICS

rather than "law." Nature no longer "selects"
good eyes in man by long, patient, and devious
processes when poor eyes are made good almost in-
stantly by a visit to the oculist. She has long since
given up providing natural weapons of defense for
those who have the wits to supply themselves more
efficiently with artificial means of self-preservation,
and she no longer attempts to improve the natural
powers of locomotion of those who are able to tame
a horse to ride upon, or who build steamships, rail-
roads, automobiles and aeroplanes, thus accom-
plishing at once what would require ages at least to
evolve.

Neither does the law of the survival of the fittest in
its original sense apply equally to man and to other
organisms. Human society to-day protects its unfit
in hospitals, asylums, and through various philan-
thropies, while physicians devote themselves to the
art of prolonging life beyond the period of usefulness.

We do not desire these results of our modern civili-
zation to be otherwise, but the fact remains that some
of the most inflexible and universal "natural laws"
are ineffective in the case of man, and.it is profitable
to bear this in mind when applying the laws of ge-
netics to man.

The laboratory for human heredity is the wide
world, but it is obvious that the experimental method
which has proven so effective in studying the heredity
of animals and plants is impracticable in the case of
man. The consideration of human heredity, there-
fore, must always be largely from the statistical side,

THE APPLICATION TO MAN 227

consisting in an analysis of experiments already per-
formed rather than in initiating new experiments.

Such institutions as insane asylums, prisons,
sanitariums, and homes for the unfortunate are
excellent foci for studying certain phases of human
heredity, because they are simply convenient places
where the results of similar experiments in genetics
have been brought together.

3. EXPERIMENTS IN HUMAN HEREDITY
a. The Jukes

A classic example of an experiment in human hered-
ity which has been partially analyzed by the statisti-
cal method is that furnished by Dugdale in 1877 in
the case of "Max Jukes" and his descendants. It
includes over one thousand individuals, the origin of
all of whom has been traced back to a shiftless, illit-
erate, and intemperate backwoodsman who started
his experiment in heredity in western New York
when it was yet an unsettled wilderness.

In 1877 the histories of 540 of this man's progeny
were known, and that of most of the others was
partly known. About one third of this degenerate
strain died in infancy, 310 individuals were paupers
who all together spent a total of 2300 years in alms-
houses, while 440 were physical wrecks. In addition
to this, over one half of the female descendants were
prostitutes, and 130 individuals were convicted crim-
inals, including 7 murderers. Not one of the entire
family had a common school education, although

228 GENETICS

the children of other families in the same region
found a way to educational advantages. Only 20
individuals learned a trade and 10 of these did so in
state's prison.

It is estimated that up to 1877 this experiment in
human breeding had cost the state of New York
over a million and a quarter dollars, and the end is
by no means yet in sight.

b. The descendants of Jonathan Edwards

In striking contrast to the case of Max Jukes is
that of Jonathan Edwards, the eminent divine,
whose famous progeny Winship describes as follows:
"1394 of his descendants were identified in 1900, of
whom 295 were college graduates ; 13 presidents of
our greatest colleges, besides many principals of
other important educational institutions ; 60 physi-
cians, many of whom were eminent; 100 and more
clergymen, missionaries, or theological professors;
75 were officers in the army and navy; 60 were
prominent authors and writers, by whom 135 books
of merit were written and published and 18 impor-
tant periodicals edited; 33 American States and
several foreign countries and 92 American cities and
many foreign cities have profited by the beneficent
influence of their eminent activity ; 100 and more
were lawyers, of whom one was our most eminent
professor of law; 30 were judges; 80 held public
office, of whom one was vice-president of the United
States; 3 were United States senators; several were
governors, Members of Congress, framers of state

THE APPLICATION TO MAN 229

constitutions, mayors of cities, and ministers to for-
eign courts; one was president of the Pacific Mail
Steamship Company ; 15 railroads, many banks, in-
surance companies, and large industrial enterprises
have been indebted to their management. Almost
if not every department of social progress and of
public weal has felt the impulse of this healthy,
long-lived family. It is not known that any one of
them was ever convicted of crime."

c. The Kallikak Family

A more convincing experiment in human heredity
than the foregoing, since it concerns the descendants
of two mothers and the same father, is furnished by the
recently published history of the " Kallikak " family.1

During Revolutionary days, the first Martin Kalli-
kak,— the name is fictitious, — who was descended
from a long line of good English ancestry, took
advantage of a feeble-minded girl. The result of
their indulgence was a feeble-minded son who be-
came the progenitor of 480 known descendants of
whom 143 were distinctly feeble-minded, while most
of the others fell below mediocrity without a single
instance of exceptional ability.

" After the Revolutionary war, Martin married a
Quaker girl of good ancestry and settled down to
live a respectable life after the traditions of his
forefathers. From this legal union with a normal
woman there have been 496 descendants. All of

1 " The Kallikak Family." H. H. Goddard. The Macmillan Co.

230 GENETICS

these except two have been of normal mentality and
these two were not feeble-minded. . . . The fact
that the descendants of both the normal and the
feeble-minded mother have been traced and studied
in every conceivable environment, and that the re-
spective strains have always been true to type, tends
to confirm the belief that heredity has been the
determining factor in the formation of their respec-
tive characters."

4. MORAL AND MENTAL CHARACTERS BEHAVE
LIKE PHYSICAL ONES

These instances of human breeding show unmis-
takably that "blood counts" in human inheritance,
even though the hereditary unit characters that lead
to these general results have not yet been analyzed
with the clearness that is possible in dealing with the
characters of some animals and plants.

There is of course no question of moral and mental
traits in plants, and the role that these play in animals
is not easy to determine ; but in man the case is
undoubtedly much more important and complex,
since mental and moral characteristics have a large
share in making man what he is. There is, however,
no fundamental scientific distinction which can be
drawn between moral, mental, and physical traits,
and they are undoubtedly all equally subject to the
laws of heredity.

For instance, as an illustration of the heritability

THE APPLICATION TO MAN 231

of non-physical traits, in the Jukes pedigree three
of the daughters of Max impressed their peculiar
moral and mental characteristics in a distinctive
way upon their offspring. To quote Davenport :
"Thus in the same environment, the descendants of
the illegitimate son of Ada are prevailingly criminal ;
the progeny of Belle are sexually immoral; and the
offspring of Effie ar&cgt^z^rs. The difference in the
germplasm determines the difference in the prevailing
trait."

5. THE CHARACTER OF HUMAN TRAITS

Of the mental, moral, and physical traits which
are heritable in man, some must be regarded as
generally desirable, some as indifferent, and others
as defects to be avoided if possible. In general the
majority of human traits, those which together make
up man as distinguished from other animals, do not
particularly claim the attention because they are so
universal. Some which stand out from the mass,
such as the physical traits of eye-color and the color
and character of hair, may be regarded as indifferent
so far as the welfare of the individual is concerned,
while others like skin color and certain racial features
that characterize particular strains of "blood" may,
under certain circumstances, work a social handicap
upon their possessors according to the traditions of
the community in which they appear.

A long list of desirable mental traits might be
enumerated that seem in a general way to be subject
to the laws of inheritance, although they have not

282 GENETICS

yet undergone the careful analysis demanded by
modern genetics which deals in unit characters rather
than in lump inheritance.

Musical, literary, or artistic ability, for example,
mathematical aptitude and inventive genius, as well
as a cheerful disposition or a strong moral sense are
probably all gifts that come in the germplasm.

They may each be developed by exercise or re-
pressed by want of opportunity, nevertheless they
are fundamentally germinal gifts.

A genius must be born of potential germplasm.
No amount of faithful plodding application can com-
pensate for a lack of the divine hereditary spark at
the start.

6. HEREDITARY DEFECTS

Undesirable hereditary traits are usually defects
due to the absence of some character. For instance,
albinism, which occurs in several kinds of animals
and also in man in one out of every 20,000 individuals
(according to Elderton), is due to the absence of pig-
ment in the skin, hair and eyes. Albinic individuals
have poor eyesight because they are unable to stand
strong light, being without protective pigment in the
eyes. This peculiarity of albinism behaves as a
recessive character both in man and in other animals.
An albinic individual may, therefore, marry a normal
individual without fear of producing albino children,
although the children of such a mating would carry
heterozygous germplasm with respect to albinism,

THE APPLICATION TO MAN 233

and in cousin marriages might subsequently produce
some albino children.

Davenport, in his recent work on "Heredity in Re-
lation to Eugenics," brings together a long catalogue
of human hereditary defects, although in most
instances they are extremely difficult of accurate
analysis. This is the case, first, because these defects
so often probably depend upon a combination of
determiners rather than upon a single one, and, sec-
ond, because the available data are usually scattered
and incomplete.

Deafness, for example, is a defect which is heredi-
tary though exactly to what degree, it is at present
impossible to state. The following table taken from
the extensive work of Fay (1898) upon "Marriage of
the Deaf in America" gives some idea of the results
of different matings lumped together statistically.

CONDITION OF PARENTS

PERCENTAGE OF DEAF
OFFSPRING

Both, born deaf ... . . ...

25.9

One born deaf, one with acquired deafness

6.3

One born deaf one normal

11.9

Both with acquired deafness

2.3

One with acquired deafness, one normal . .

2.2

That two parents born deaf do not produce more
than 26 per cent of deaf children is probably due to
the fact, first, that each parent is in all likelihood heter-
ozygous for deafness and that, second, the same com-

234 GENETICS

bination of factors which is the cause of the parental
defect on either side of the pedigree does not happen to
recombine after segregation to form the new individ-
ual. Deafness will be produced in the offspring only
when matings occur in which the proper factors are
combined. Such an undesirable result is much more
likely to happen if both parents come from the
same, or related, hereditary strains than if they are
derived from families in no way connected by blood.

Herein lies the biological objection to cousin
marriage which tends to bring together, and thus
to perpetuate, like defects. Outcrossing, on the
contrary, through the law of dominance, tends to
conceal defects and to prevent their expression.

Many other cases of human defects, such as im-
becility or insanity, are extremely difficult of analysis
from the standpoint of heredity because, in the first
place, the defective conditions descriptively included
under these vague terms are made up of a multitude
of diverse conditions each of which must have a
different array of determiners and, in the second
place, because any one definite sort of insanity or
imbecility may be conditioned by a variety of
factors.

However, the difficulty of the problem is no
reason for abandoning the attempt to reach its solu-
tion and to learn, if possible, "whence come our
300,000 insane and feeble-minded, our 160,000 blind
or deaf, the 2,000,000 that are annually cared for by
our hospitals and Homes, our 80,000 prisoners and
the thousands of criminals that are not in prison,

THE APPLICATION TO MAN

235

and our 100,000 paupers in almshouses and out "
(Davenport).

7. THE CONTROL OF DEFECTS

The method of possible control of human defects
depends upon whether they are positive or negative,
that is, dominant or recessive. In those cases where
a given defect is due to a single determiner the
Mendelian expectation for the possible offspring
arising from various matings is indicated in the fol-
lowing table in which D stands for the defect and d
for its absence : —

THE MENDELIAN EXPECTATION FOR DEFECTS

IF THE DEFECT is POSITIVE
(dominant)

IF THE DEFECT is NEGATIVE
(recessive)

When both
parents show
the defect

1
2.

•3

DD X DD = all DD

dd X dd = all dd

DD X Dd = i DD + * Dd

Dd XDd = ±DD+$Dd+$dd

When one
parent only
shows the
defect

When neither
parent shows
the defect

4
5

DD X dd = all Dd

dd XDD = all Dd

Dd X dd = * Dd + $ dd

dd X Dd = \Dd + % dd

G

dd X dd = all dd

DD XDD = all DD

7

Dd X DD = $ DD + | Dd

8

DdXDd = lDD+$Dd+$dd

If the defect is positive and in a duplex or homo-
zygous condition in one parent, as in 1, 2, and 4
above, all the offspring will possess it regardless of
the germinal constitution of the other parent. In

236 GENETICS

two cases only, namely, in 3 and 5, where the de-
fective parent is heterozygous, is there any chance
of unaffected offspring, and even in these cases the
defect is quite as likely to appear as not. It is ob-
vious that the only way to rid germplasm of a
dominant defect is by continued mating with
recessive individuals. By this method it is possible
in time to shake off the defect. When it once dis-
appears in any individual, it will never return unless
crossed back to a similar defective dominant strain.

In other words, such a recessive extracted from a
heterozygous ancestry will breed just as true as a
recessive which was pure from the start. In both
instances there is an entire absence of the character
in question, and it is clear that this character can
thereafter never again reappear, since something
cannot be derived from nothing.

On the other hand, if a defect is negative, depending
upon the absence of a normal dominant determiner,
as is usually the case with defects, it behaves as a
Mendelian recessive, that is, it is always apparent in
individuals developing from the homozygously de-
fective germplasm.

It is certain, for example, that an imbecile which
has arisen from homozygous defective germplasm
carries only the determiner for imbecility in his own
germplasm, and when two such recessives mate, noth-
ing but imbecile offspring can result, for recessives
breed true. Nothing plus nothing equals nothing.

An illustration of this principle is given in the fol-
lowing pedigree (Fig. 72) furnished by Goddard, 1910.

THE APPLICATION TO MAN

237

The result is quite different, however, when one
parent only shows the defect. If the other parent is a
normal homozygote, as in case 4 of the accompanying
table, all the offspring will be normal in appearance,
but with the bar sinister of defectiveness in their
germplasm, while if the other parent is heterozygous
(Case 5), one half of the progeny will be defective.
Finally, when neither parent shows defectiveness

N|

F

N N N N N N

FIG. 72. — Pedigree chart illustrating the law that two defective parents
have only defective offspring. A, alcoholic ; C, criminalistic ; d, died ;
F, feeble-minded ; T, tubercular. After Goddard.

but one carries the defect as a heterozygote (Case 7),
then there will be no defective children, while if
both parents are heterozygous there is one chance in
four that the offspring will be defective.

As a matter of fact, defectives usually mate with
defectives for the simple reason that normals ordi-
narily avoid them, so it comes about that streams of
poor germplasm naturally flowing together tend to
the inbreeding of like defects.

238 GENETICS

Davenport l lays down the following general eugenic
rules for the guidance of those who would produce
offspring wisely : "If the negative character is, as in
polydactylism and night-blindness, the normal char-
acter, the normals should marry normals, and they
may be even cousins. If the negative character is
abnormal, as imbecility and liability to respiratory dis-
eases, then the marriage of two abnormals means prob-
ably all children abnormal ; the marriage of two nor-
mals from defective strains means about one quarter of
the children abnormal ; but the marriage of a normal
of the defective strain with one of a normal strain will
probably lead to strong children. The worst possible
marriage in this class of cases is that of cousins from
the defective strain, especially if one or both have
the defect. In a word, the consanguineous marriage
of persons one or both of whom have the same
undesirable defect, is highly unfit, and the mar-
riage of even unrelated persons who both belong to
strains containing the same undesirable defect is un-
fit. Weakness in any characteristic must be mated
with strength in that characteristic; and strength
may be mated with weakness."

8. INBREEDING

The whole matter of inbreeding and the part it
plays in emphasizing defects has received a fresh
interpretation in the light of Mendelism.

There is a widespread popular belief that inbreed-
ing is injurious and that it is necessary to outcross

1 Davenport. Rep. of Amer. Breeders Assoc., Vol. VI, p. 431, 1910.

THE APPLICATION TO MAN 239

in order to maintain the vigor and avoid the defects
of any line. In the case of mankind, consanguineous
marriage of various degrees has long been forbidden
by law or custom in many races, particularly among
the Jews, Mohammedans, Indians and Romans. On
the other hand, the Persians, Greeks, Phoenicians and
Arabs have freely practised inbreeding, while one of
the longest of known human pedigrees, a royal line
of Egypt, was notorious for close inbreeding, even to
the mating of brother and sister.

There has been a greater degree of inbreeding in
the Puritan stock of New England than is commonly
realized. David Starr Jordan points out that a
child of to-day, supposing no inbreeding of relatives
had occurred, would have had in the time of William
the Conqueror, thirty generations ago, 8,598,094,592
living ancestors. If this theoretical supposition
were really so, it would seem quite possible for every
New Englander to-day to have had at least one an-
cestral representative who won glory under William.

The difference between the unthinkable number
given above and the actual number of probable
ancestors alive thirty generations ago emphasizes
the fact that inbreeding must have occurred freely.

There are, indeed, various well-known provisions
in nature to insure inbreeding. The majority of
plants are probably self-fertilized while hermaphro-
ditic animals, which sometimes at least are self-
fertilized particularly among the lower forms, are
very common.

Nature has secured, on the other hand, often by

240 GENETICS

elaborate devices, a separation of the sexes, especially
among the higher * organisms, and in consequence
there has arisen an unavoidable necessity of out-
crossing. The intricate adaptations existing between
insects and flowers, for example, seem to be directed
entirely toward insuring out crossing among plants.

9. EXPERIMENTS TO TEST THE EFFECT OF IN-
BREEDING

Numerous experiments to test the effect of in-
breeding have been carried out upon various or-
ganisms.

Darwin, for instance, planted morning-glories,
Ipomcea, derived from the same stock of seeds, in
two beds which were laid out side by side, that is,
in an environment as nearly the same as possible,
but with half of the beds screened from insects
which usually transfer pollen from flower to flower.
In the screened half where all insects were excluded
the flowers were of necessity self-fertilized, while in
the exposed half they were presumably cross-pol-
linated by the insects which had free access to them.
The seeds produced in the two beds were kept sep-
arate and the experiment was continued for ten years,
so that at the end of that time two lots of morning-
glories, one self-fertilized for ten generations and the
other presumably cross-pollinated for the same length
of time, were obtained for comparison. The crite-
rion Darwin used was the vigor of the plants as
shown by the length of the vine. He found that the

THE APPLICATION TO MAN 241

cross-pollinated plants were to the self-pollinated
ones as 100 to 53, and his conclusion was conse-
quently, that cross-pollination is beneficial and self-
pollination is detrimental.

Ritzema-Bos inbred rats for twenty generations.
For the first ten generations the average number of
young per litter was 7.5, while for the last ten genera-
tions it fell to 3.2.

Weismann inbred mice for twenty-nine genera-
tions and obtained a parallel result. For the first
ten generations the average number per litter was
6.1, for the second ten generations 5.6, and for the
last nine generations 4.2.

Shull found in growing Indian corn that loss of
vigor results from continual self-fertilization, and
many breeders have had similar experiences with
other plants and animals.

On the other hand, in the case of the pomace fly,
Drosophila, Castle inbred brother and sister for
fifty-nine generations without diminishing the fer-
tility of the line. No arbitrary law with respect to
the effects of inbreeding upon vigor and fertility
can be laid down, therefore, which will apply equally
to all cases.

10. THE INFLUENCE OF PROXIMITY

Inbreeding is often the result of proximity. In-
sular or isolated communities, slums in cities, where
those of one language herd together, or hovels in
the backwoods, where degenerates of a kind are kept
in intimate association, as well as asylums of various

B

GENETICS

sorts in which similar defectives are promiscuously
housed under the same roof, are all potent agencies
to insure human inbreeding.

Similarly, localities which have been devastated
by migrations of the most effective blood, as, for
example, parts of Ireland or many rural villages
in New England, are frequently characterized by a
population showing a large percentage of defective-
ness. The able-bodied and ambitious go forth into
the world to seek their fortunes, while the deficient
in body or spirit are left behind where, under the
spell of proximity, they perpetuate their deficiencies.

The part that improved transportation has played
in mixing up populations and in counteracting the
effects of stagnation on human heredity, through in-
breeding under the inertia of proximity is very great.
Before the days of railroads, cousin-marriages were
much more frequent than they are now.

From a biological point of view there is something
to be said in favor of the rape of the Sabines in the
past and for the pursuit of American heiresses by
European nobility to-day.

11. INBREEDING IN THE LIGHT OF MENDELISM

Inbreeding in itself may not necessarily be injuri-
ous. The consequence of inbreeding as shown by
the working of Mendelian laws is that latent or
recessive characters tend to become homozygous and
so brought to the surface, while outcrossing brings
about the formation of heterozygous traits which

THE APPLICATION TO MAN 243

mask recessive characters and render them ineffec-
tive.

Cousin-marriages, although producing a high per-
centage of defects, do not necessarily reproduce un-
desirable traits. They simply bring out latent or
recessive characters for the reason that under these
conditions defect meets defect instead of the opposite
normal condition which would dominate the defect
and cause it not to appear.

Since a recessive trait is properly regarded as the
absence of a positive dominant character, it more
frequently stands for an undesirable feature than
otherwise. Thus it comes about that inbreeding,
by combining negative features, may "produce" a
defective strain.

Outcrossing always increases heterozygous com-
binations in the germplasm and covers up undesirable
recessive traits through the introduction of addi-
tional dominant traits. Inbreeding, on the contrary,
tends to simplify the germplasm, that is, to make it
more homozygous, and so to bring recessive defects
to the surface.

CHAPTER XII

HUMAN CONSERVATION

1. How MANKIND MAY BE IMPROVED

THERE* are two fundamental ways to bring about
human betterment, namely, by improving the in-
dividual and by improving the race. The first
method consists in making the best of whatever
heritage has been received by placing the individual in
the most favorable environment and developing his
capacities to the utmost through education. The
second method consists in seeking a better heritage
with which to begin the life of the individual. The
first method is immediate and urgent for the present
generation. The second method is concerned with
ideals for the future, and consequently does not usu-
ally present so strong an appeal to the individual.

The first is the method of euthenics, or the science
of learning to live well. The second is eugenics,
which Galton defines as "the science of being well
born."

These two aspects of human betterment, however,
are inseparable. Any hereditary characteristic must
be regarded, not as an independent entity, but as a
reaction between the germplasm and its environment.
The biologist who disregards the fields of educational
endeavor and environmental influence, is equally at

244

HUMAN CONSERVATION 245

fault with the sociologist who fails sufficiently to real-
ize the fundamental importance of the germplasm.

Without euthenic opportunity the best of heri-
tages would never fully come to its own. Without
the eugenic foundation the best opportunity fails
of accomplishment. The euthenic point of view,
however, must not distract the attention now, for
the present chapter is particularly concerned with
the program of eugenics.

2. MORE FACTS NEEDED

Since the point of attack in human heredity must
be largely statistical, it is of the first importance to
collect more facts. Our actual knowledge is con-
fused with a mass of tradition and opinion, much of
which rests upon questionable foundations. The
great present need is to learn more facts ; to sift the
truth from error in what is already known; and to
reduce all these data to workable scientific form.
Much progress is being made in this direction, owing
to the impetus given by the revival of Mendel's
illuminating work, but as yet the science of eugenics
is in its infancy.

The most systematic and effective attempt in this
country to collect reliable data concerning heredity
in man has been initiated by the Eugenics Section
of the American Breeders' Association under the
secretaryship of Dr. C. B. Davenport. In 1910 the
Eugenics Record Office, with a staff of expert field
and office workers and an adequate equipment of
fire-proof vaults, etc., for the preservation of records,

246 GENETICS

was opened at Cold Spring Harbor, Long Island, New
York, with Mr. H. H. Laughlin as superintendent.
"The main work of this office is investigation into
the laws of inheritance of traits in human beings and
their application to eugenics. It proffers its serv-
ices free of charge to persons seeking advice as to
the consequences of proposed marriage matings. In
a word, it is devoted to the advancement of the
science and practice of eugenics." The publica-
tion of results from the Eugenics Record Office has
already been begun.

The Volta Bureau, founded about twenty-five
years ago in Washington by Dr. Alexander Graham
Bell, is collecting data with reference to deafness
and has now systematically arranged particulars con-
cerning the history of over 20,000 individuals. In
England, also, the Galton Laboratory for Eugenics,
founded in 1905, is systematically collecting facts
about human pedigrees and publishing the results in
a compendious " Treasury of Human Inheritance."

Besides these special bureaus of investigation,
innumerable facts about the inheritance of particular
traits are being incidentally brought together and
made available in various institutions and asylums
throughout the world which are immediately con-
cerned with the care of defectives of different types.
It is in connection with such institutions for defec-
tives that much of the most successful "field work"
of the Eugenics Section of the American Breeders'
Association is being accomplished in the United
States.

HUMAN CONSERVATION 247

3. FURTHER APPLICATION OF WHAT WE KNOW
NECESSARY

Human performance always lags behind human
knowledge. Many persons who are fully aware of
the right procedure do not put their knowledge into
practice. It follows, therefore, that any program of
eugenics which does not grip the imagination of the
common people in such a way as to become an effec-
tive part of their very lives is bound to remain largely
an academic affair for Utopians to quarrel and theo-
rize over.

It is not enough to collect facts and work out an
analysis and interpretation of them, for, important
as this preliminary step is, it must be followed by a
convincing campaign of education.

The lives of the unborn do not force themselves
upon the average man or woman with the same
insistency as the lives already begun. In the midst
of the overwhelming demands of the present, the
appeal of posterity for better blood is vague and
remote. If every individual regarded the germ-
plasm he carries as a sacred trust, then it would be
the part of an awakened eugenic conscience to restrain
that germplasm when it is known to be defective or,
when it is not defective, to hand it on to posterity
with at least as much foresight as is exercised in
breeding domestic animals and cultivated plants.

The eugenic conscience is in need of development,
and it is only when this becomes thoroughly aroused
in the rank and file of society as well as among the

248 GENETICS

leaders, that a permament and increasing better-
ment of mankind can be expected.

4. THE RESTRICTION OF UNDESIRABLE GERM-
PLASM

A negative way to bring about better blood in the
world is to follow the clarion call of Davenport and
"dry up the streams that feed the torrent of de-
fective and degenerate protoplasm." This may
be partially accomplished, at least in America, by
employing the following agencies : control of immi-
gration; more discriminating marriage laws; a
quickened eugenic sentiment ; sexual segregation of
defectives ; and finally, drastic measures of asexuali-
zation or sterilization when necessary.

a. Control of Immigration

The enforcement of immigration laws tends to
debar from the United States not only many unde-
sirable individuals, but also incidentally to keep out
much potentially bad germplasm that, if admitted,
might play havoc with, future generations.

For example, during the year of 1908, 65 idiots,
121 feeble-minded, 184 insane, 3741 paupers, 2900
individuals having contagious diseases, 53 tuber-
culous individuals, 136 criminals, and 124 prostitutes
were caught in the sieve at Ellis Island alone and
turned back from this country by the immigration
officials. These 7000 and more individuals probably
were the bearers of very little germplasm that we
are nationally not better off without.

HUMAN CONSERVATION 249

Eugenically, the weak point in the present applica-
tion of immigration laws is that criteria for exclusion
are phenotypic in nature rather than genotypic, and
consequently much bad germplasm comes through
our gates hidden from the view of inspectors because
the bearers are heterozygous, wearing a cloak of
desirability over undesirable traits.

It is not enough to lift the eyelid of a prospective
parent of American citizens to discover whether he
has some kind of an eye-disease or to count the
contents of his purse to see if he can pay his own
way. The official ought to know if eye-disease runs
in the immigrant's family and whether he comes from
a race of people which, through chronic shiftlessness
or lack of initiative, have always carried light purses.

In selecting horses for a stock-farm an expert
horseman might rely to a considerable extent upon
his judgment of horseflesh based upon inspection
alone, but the wise breeder does more than take the
chances of an ordinary horse trader. He wants to
be assured of the pedigree of his prospective stock.
It is to be hoped that the time will come when we,
as a nation, will rise above the hazardous methods of
the horse trader in selecting from the foreign appli-
cants who knock at our portals, and that we will
exercise a more fundamental discrimination than
such a haphazard method affords, by demanding a
knowledge of the germplasm of these candidates for
citizenship, as displayed in their pedigrees.

This may possibly be accomplished by having
trained inspectors located abroad in the communi-

250 GENETICS

ties from which our immigrants come, whose duty
it shall be to look up the ancestry of prospective
applicants and to stamp desirable ones with approval.
The national expense of such a program of genealogi-
cal inspection would be far less than the mainte-
nance of introduced defectives, in fact it would
greatly decrease the number of defectives in the
country. At the present time this country is spending
over one hundred million dollars a year on defectives
alone, and each year sees this amount increased.

The United States Department of Agriculture
already has field agents scouring every land for
desirable animals and plants to introduce into this
country, as well as stringent laws to prevent the im-
portation of dangerous weeds, parasites, and organ-
isms of various kinds. Is the inspection and super-
vision of human blood less important ?

b. More Discriminating Marriage Laws

Every people, including even the more primitive
races, make customs or laws that tend to regulate
marriage. Of these, the laws which relate to the
eugenic aspect of marriage are the only ones that
concern us in this connection. "Marriage," says
Davenport, "can be looked at from many points of
view. In novels as the climax of human courtship ;
in law largely as two lines of property descent ; in
society, as fixing a certain status ; but in eugenics,
which considers its biological aspect, marriage is an
experiment in breeding."

Certain of the United States have laws for-

HUMAN CONSERVATION 251

bidding the marriage of epileptics, the insane, habit-
ual drunkards, paupers, idiots, feeble-minded, and
those afflicted with venereal diseases. It would be
well if such laws were not only more uniform and
widespread, but also more rigidly enforced.

It is quite true that marriage laws in themselves
do not necessarily control human reproduction, for
illegitimacy is a factor that must always be reckoned
with ; nevertheless such laws do have an important
influence in regulating marriage and consequent
reproduction.

Marriage laws may, however, sometimes bring
about a deplorable result eugenically, as in the case
of forced marriage of sexual offenders in order to
legalize the offense and "save the woman's honor."
To compel, under the guise of legality, two defective
streams of germplasm to combine repeatedly and
thereby result in defective offspring just because
the unfortunate event happened once illegitimately,
is fundamentally a mistake. Darwin says : "Except
in the case of man himself hardly any one is so
ignorant as to allow his worst animals to breed."

c. An Educated Sentiment

A far more effective means of restricting bad
germplasm than placing elaborate marriage laws
upon our statute-books is to educate public sentiment
and to foster a popular eugenic conscience, in the
absence of which the safeguards of the law must
forever be largely without avail.

Such a sentiment already generally exists to a

252 GENETICS

large extent with respect to incest, and the marriage
of persons as noticeably defective as idiots or those
afflicted with insanity, and also in America with re-
spect to miscegenation, but a cautious and intelligent
examination of the more obscure defective traits,
exhibited in the somatoplasms of the various mem-
bers of families in question, is largely an ideal of the
future. Under existing conditions non-eugenic con-
siderations such as wealth, social position, etc., often
enter into the preliminary negotiations of a marriage
alliance, but an equally unromantic caution with
reference to the physical, moral, and mental charac-
ters that make up the biological heritage of con-
tracting parties is less usual.

The scientific attitude is not necessarily opposed
to the romantic way of looking at things. Science
is simply "organized common sense," and romance,
that dispenses with this balance-wheel, although it
may be entertaining and always exciting at first,
is sure to be disappointing in the end. Marriages
may be "made in heaven," but, as a matter of fact,
children are born and have to be brought up on earth.
It follows without saying that it will be much easier
to stamp out bad germplasm when an educated senti-
ment becomes common among all people everywhere.

d. Segregation of Defectives

Persons with hereditary defects, such as epileptics,
idiots, and certain criminals, who become wards of
the state, should be segregated so that their germ-
plasm may not escape to furnish additional burdens

HUMAN CONSERVATION 253

to society. "We have become so used to crime,
disease and degeneracy that we take them for nec-
essary evils. That they were in the world's igno-
rance, is granted. That they must remain so, is
denied" (Davenport).

"The great horde of defectives once in the world
have the right to live and enjoy as best they may
whatever freedom is compatible with the lives and
freedom of other members of society," says Kellicott,
but society has a right to protect itself against repe-
titions of hereditary blunders.

There is one grave danger connected with the
administration of our humane and commendable
philanthropies toward the unfortunate, for it fre-
quently happens that defectives are kept in insti-
tutions until they are sexually mature or are partly
self-supporting, when they are liberated only to add
to the burden of society by reproducing their like.

Furthermore, if defectives of the same sort are
collected together in the same institutions, unless
sexual segregation is strictly maintained, they may
by the very circumstance of proximity tend to re-
produce their kind just as defectives in any isolated
community tend to multiply.

David Starr Jordan cites the interesting case of
cretinism which occurs in the valley of Aosta in
northern Italy, to prove the wisdom of the sexual
segregation of defectives. Cretinism is an hereditary
defect connected with an abnormal development of
the thyroid gland which results in a peculiar form of
idiocy usually associated with goitre.

254 GENETICS

"In the city of Aosta the goitrous cretin has been
for centuries an object of charity. The idiot has
received generous support, while the poor farmer or
laborer with brains and no goitre has had the sever-
est of struggles. In the competition of life a pre-
mium has thus been placed on imbecility and disease.
The cretin has mated with cretin, the goitre with
goitre, and charity and religion have presided over
the union. The result is that idiocy is multiplied
and intensified. The cretin of Aosta has been de-
veloped as a new species of man. In fair weather
the roads about the city are lined with these awful
paupers — human beings with less intelligence than
a goose, with less decency than the pig."

Whymper, writing in 1880, further observes: "It
is strange that self-interest does not lead the natives
of Aosta to place their cretins under such restrictions
as would prevent their illicit intercourse ; and it is
still more surprising to find the Catholic Church
actually legalizing their marriage. There is some-
thing horribly grotesque in the idea of solemnizing
the union of a brace of idiots, and, since it is well
known that the disease is hereditary and develops in
successive generations the fact that such marriages
are sanctioned is scandalous and jnfamous."

Since 1890 the cretins have been sexually segregated,
and in 1910 Jordan reported that they were nearly

all gone.

e. Drastic Measures

A fifth method of restricting undesirable germ-
plasm in the case of confirmed criminals, idiots,

HUMAN CONSERVATION 255

imbeciles, and rapists may be mentioned, namely,
the extreme treatment of either asexualization or
vasectomy. The latter is a minor operation con-
fined to the male which occupies only a few moments
and requires at most only the application of a local
ansesthetic, such as cocaine. There are no disturbing
or even inconvenient after effects from this operation.
It consists in removing a small section of each sperm
duct and is entirely effectual in preventing subse-
quent parenthood.

In the female the corresponding operation, which
consists in removing a portion of each Fallopian
tube, is much more severe, but not impracticable or
dangerous.

Eight states 1 already have sterilization laws pro-
viding for certain cases and "could such a law be
enforced in the whole United States, less than four
generations would eliminate nine tenths of the crime,
insanity and sickness of the present generation in
our land. Asylums, prisons and hospitals would
decrease, and the problems of the unemployed, the
indigent old, and the hopelessly degenerate would
cease to trouble civilization."

5. THE CONSERVATION OF DESIRABLE GERMPLASM

Not only negatively by the restriction of undesir-
able germplasm, but also positively by the conser-
vation of desirable germplasm, may the eugenic
ideal be approached.

Indiana, 1907; Washington, 1909; California, ^909; Connecticut,
1909; Nevada, 1911 ; Iowa, 1911 ; New Jersey, 1911 ; New York, 191*.

256 GENETICS

It is possible that if some of the philanthropic
endeavor now directed toward alleviating the con-
dition of the unfit should be directed to enlarging the
opportunity of the fit, greater good would result in
the end. In breeding animals and plants the most
notable advances have been made by isolating and
developing the best, rather than by attempting to
raise the standard of mediocrity through the elimina-
tion of the worst.

One leader is worth a score of followers in any
community, and the science of genetics surely gives
to educators the hint that it is wiser to cultivate the
exceptional pupil who is often left to take care of
himself than to expend all the energies of the in-
structor in forcing the indifferent or ordinary one
up to a passing standard. The campaign for human
betterment in the long run must do more than avoid
mistakes. It must become aggressive and take ad-
vantage of those human mutations or combinations
of traits which appear in the exceptionally endowed.

There are various ways in which this improvement
of society may be brought about.

a. By Subsidizing the Fit

The following unconfirmed newspaper clipping
illustrates the point of what is meant by subsidizing
the fit so far as certain physical characteristics are
concerned. "Berlin. Dec. 11, 1911. The Empe-
ror is reported to be interested in a plan proposed
by Professor Otto Hauser for the propagation of a
fixed German type of humanity, — a type which

'HUMAN CONSERVATION 257

will be as fixed as the Jewish in its characteristics,
if the suggestions of the professor can ever be carried
out. The fixed type is to be produced as follows : —
Only 'typical' couples are to be allowed to mate.
The man is to be not more than thirty years old,
the woman not over twenty-eight, and each have a
perfect health certificate. The man should be at
least five feet seven inches tall ; the woman not under
five feet six inches. Neither the man nor the woman
should have dark hair. Its tint may range from
blonde to auburn. The eyes of the pair should be
pure blue without any tint of brown. The complex-
ion should be fair to ruddy without any suggestion of
heaviness or 'beefiness.' The nose ought to be
strong and narrow, the chin square and powerful, and
the skull well developed at the back. The man and
the woman must be of German descent and must
bear a German name and speak the language of
Germany. These * mated couples' are to get a
wedding gift of $125 and an additional grant for
each child born. The couples may settle in the
United States if they prefer." This reported at-
tempt to establish a Prussian type of "Hauser
blondes" at least points the way to one sort of d
positive eugenic method that might possibly be
employed with respect to certain physical charac-
teristics.

It should be remembered, however, that the
eugenic ideal is not by any means confined to phys-
ical traits alone.

258 GENETICS

6. By Enlarging Individual Opportunity

Much good human germplasm goes to
through ineffectiveness on account of unfavorable
environment or lack of a suitable opportunity to
develop.

Every agency which contributes toward increasing
the opportunity of the individual to attain to a
better development of his latent possibilities is in
harmony with a thoroughly positive eugenic prac-
tice. Thus better schools, better homes, better
living conditions, in short, all euthenic endeavor,
directly serves the eugenic ideal by making the best
out of whatever germinal equipment is present in
man.

c. By Preventing Germinal Waste

Much good protoplasm fails to find expression in
the form of offspring because one or the other of
possible parents is cut off either by preventable
death or by social hindrances. To avoid such ca-
lamities is a part of the positive program of eugenics.

1. Preventable Death

War, from the eugenic point of view, is the height
of folly, since presumably the brave and the phys-
ically fit march away to fight, while in general the
unqualified stay at home to reproduce the next gen-
eration. When a soldier dies on the battlefield or in
the hospital, it is not alone a brave man who is cut
off, but it is the termination of a probably desirable
strain of germplasm. The Thirty Years' WTar in

HUMAN CONSERVATION 259

Germany cost 6,000,000 lives, while Napoleon in his
campaigns drained the best blood of France.

David Starr Jordan has presented this matter
very clearly. He points out that the "man with a
hoe " among the European peasantry is not the
result of centuries of oppression, as he has been
pictured, but rather the dull progeny resulting from
generations of the unfit who were left behind when
the fit went off to war never to return.

Benjamin Franklin, with characteristic wisdom,
sums up the situation in the following epigram: " Wars
are not paid for in war time ; the bill comes later."

2. Social Hindrances

There are many conditions of modern society
which act non-eugenically.

For instance, the increasing demands of profes-
sional life prolong the period necessary for prepara-
tion, which, with the "cost of high living," tends
toward late marriage. In this way much of the best
germplasm is very often withheld from circulation
until it is too late to be effective in providing for
the succeeding generation.

Certain occupations such as school-teaching and
nursing by women are filled by the best blood ob-
tainable, yet this blood is denied a direct part in
molding posterity, since marriage is either forbidden
or regarded as a serious handicap in such lines of
work. Advertisements concerning " unincumbered
help" and "childless apartments" tell their own
deplorable tale.

260 GENETICS

One of the darkest features of the dark ages from
a eugenic standpoint was the enforced celibacy of
the priesthood, since this resulted, as a rule, in with-
drawing into monasteries and nunneries much of
the best blood of the times, and this uneugenic cus-
tom still obtains in many quarters to-day.

6. WHO SHALL SIT IN JUDGMENT ?

In the practical application of a program of eu-
genics there are many difficulties, for who is quali-
fied to sit in judgment and separate the fit from the
unfit ?

There are certain strongly marked characteristics
in mankind which are plainly good or bad, but the
principle of the independence of unit characters
demonstrates that no person is wholly good or wholly
bad. Shall we then throw away the whole bundle
of sticks because it contains a few poor or crooked
ones ?

The list of weakling babies, for instance, who were
apparently physically unfit and hardly worth raising
upon first judgment, but who afterwards became
powerful factors in the world's progress, is a notable
one and includes the names of Calvin, Newton,
Heine, Voltaire, Herbert Spencer and Robert Louis
Stevenson.

Or, take another example. Elizabeth Tuttle, the
grandmother of Jonathan Edwards whose remarkable
progeny was referred to in a preceding chapter, is
described as "a woman of great beauty, of tall and
commanding appearance, striking carriage, of strong

HUMAN CONSERVATION 261

will, extreme intellectual vigor and mental grasp
akin to rapacity," but with an extraordinary deficiency
in moral sense. She was divorced from her hus-
band "on the ground of adultery and other immo-
ralities. . . . The evil trait was in the blood, for one
of her sisters murdered her own son and a brother
murdered his own sister." That Jonathan Edwards
owed his remarkable qualities largely to his grand-
mother rather than to his grandfather is shown by
the fact that Richard Edwards, the grandfather,
married again after his divorce and had five sons and
one daughter, but none of their numerous progeny
"rose above mediocrity, and their descendants gained
no abiding reputation." As shown by subsequent
events, it would have been a great eugenic mistake
to have deprived the world of Elizabeth Tuttle's
germplasm, although it would have been easy to
find judges to condemn her.

Dr. C. V. Chapin recently said with reference to
the eugenic regulation of marriage by physician's
certificate: "The causes of heredity are many and
very conflicting. The subject is a difficult one, and
I for one would hesitate to say, in a great many cases
where I have a pretty good knowledge of the family,
where marriage would, or would not, be desirable."

Desirability and undesirability must always be
regarded as relative terms more or less indefinable.
In attempting to define them, it makes a great dif-
ference whether the interested party holds to a puri-
tan or a cavalier standard. To show how far human
judgment may err as well as how radically human

262 GENETICS

opinion changes, there were in England, as recently as
1819, 233 crimes punishable by death according to
law.

One needs only to recall the days of the Spanish
Inquisition or of the Salem witchcraft persecution to
realize what fearful blunders human judgment is
capable of, but it is unlikely that the world will ever
see another great religious inquisition, or that in
applying to man the newly found laws of heredity
there will ever be undertaken an equally deplorable
eugenic inquisition.

It is quite apparent, finally, that although great
caution and broadness of vision must be exercised in
bringing about the fulfilment of the highest eugenic
ideals, nevertheless in this direction lies the future
path of human achievement.

BIBLIOGRAPHY

A few recent works of a general nature are listed below.
Several of these books contain bibliographies of technical papers
and other original sources of information.

For the past five years The American Naturalist has been
particularly devoted to papers on the problems of heredity.

BATESON, W. 1909. Mendel's Laws of Heredity. Cambridge
University Press. Cambridge.

BAUR, E. 1911. Einfiihrung in die experimentelle Vererbungs-
lehre. Gebriider Borntraeger. Berlin.

CASTLE, W. E. 1911. Heredity in Relation to Evolution and
Animal Breeding. D. Appleton and Co. New York.

COULTER, CASTLE, DAVENPORT, EAST, and TOWER. 1912.
Heredity and Eugenics. University of Chicago Press.
Chicago.

DAVENPORT, C. B. 1911. Heredity in Relation to Eugenics.
Henry Holt and Co. New York.

GALTON, F. 1889. Natural Inheritance. Macmillan and Co.
London.

GODDARD, H. H. 1912. The Kallikak Family. The Mac-
millan Co. New York.

CORRENS, C. 1912. Die neuen Vererbungsgesetze. Gebriider
Borntraeger. Berlin.

DARBYSHIRE, A. D. 1912. Breeding and the Mendelian Dis-
covery. Cassell and Co., Ltd. London.

EAST, E. M. 1907. The Relation of Certain Biological Prin-
ciples to Plant Breeding. Bull. 158. Conn. Agric.

Exp. Sta.

263

264 BIBLIOGRAPHY

GODLEWSKI, E. 1909. Das Vererbungsproblem im Lichte der

Entwicklungsmechanik. Engelmann. Leipzig.
GOLDSCHMIDT, R. 1911. Einf iihrung in die Vererbungswissen-

schaft. Engelmann. Leipzig.
JOHANSSEN, W. 1909. Elemente der exakten Erblichkeitslehre.

Fischer. Jena.
HAECKER, V. 1912. Allgemeine Vererbungslehre. Vieweg

und Sohn. Braunschweig.
KELLICOTT, W. E. 1911. The Social Direction of Human

Evolution. D. Appleton and Co. New York.
LOCK, R. H. 1909. Recent Progress in the Study of Variation,

Heredity and Evolution. Murray. London.
LOTSY, J. P. 1906-1908. Vorlesungen uber Descendenz-

theorien. Fischer. Jena.
MORGAN, T. H. 1907. Experimental Zoology. The Mac-

millan Co. New York.
MONTGOMERY, T. H. 1906. The Analysis of Racial Descent

in Animals. Henry Holt and Co. New York.
PUNNETT, R. C. 1911. Mendelism. The Macmillan Co.

New York.

REID, ARCHDALL. 1905. The Principles of Heredity. Chap-
man and Hall. London.

THOMSON, J. A. 1908. Heredity. Murray. London.
WATSON, J. A. S. 1912. Heredity. The Dodge Publishing

Co. New York.

WEISMANN, A. 1904. The Evolution Theory. London.
SCHALLMAYER, W. 1910. Vererbung und Auslese im Lebens-

lauf der Volker. Fischer. Jena.
DE VRIES, H. 1905. Species and Varieties, their Origin by

Mutation. The Open Court Publishing Co. Chicago.

INDEX

Abnormal fertilization, 30.
Abraxas, 216.
Acquired callosities, 91.
Acquired characters, definition of,

78.

Ageniapsis, 202.
Agouti, 163, 171.

Albinism, 59, 151, 232.

Alcoholism, 93.

Alternative inheritance, 121.

Amblystoma, 90, 129.

American Breeders' Assoc., 245.

Amitosis, 20.

Ammonites, 71.

Amphimixis, 55, 81.

Anaphase, 20.

Ancon sheep, 68.

Andalusian fowl, 175, 180.

Angora hair, 129.

Antirrhinum, 173.

Ants, 210.

Aphids, 204.

Appendix, vermiform, 150.

Arithmetical mean, 45.

Armadillo, 202.

Arrested development, 149.

Artistic ability, 232.

Ascaris, 11, 17.

Asexualization, 255.

Asexual spores, 9.

Atavism, 146.

Autosomes, 209.

Average deviation, 45.

Axolotl, 90.

Azaleas, double, 63.

BALLS, 129.

BALTZER, 209, 220.

Banana fly (see Drosophila).

Banded shell, 129.

Barley, 129, 154.

Barrier of Linnaeus, 62.

BATESON, 2, 41, 55, 69, 124, 129,
130, 133, 149, 160, 161, 171,
173, 180, 182.

Battle scars, 87.

BAUR, 129, 130, 173, 180.

Beans, 102, 115.

Beardless barley, 129.

Beech leaves, 43, 49.

Beech, purple, 63.

BEECHER, 70.

Bees, 210.

Begonia, 7.

Belemnites, 71.

Belgian hare, 183, 193.

BELL, 246.

Beneden, van, 21, 23.

Bertillon system of identification, 38.

BIFFEN, 129, 224.

Bimodal polygons, 48.

Biological inheritance, 75.

Biometry, 42.

Birth-marks, 87.

Blending inheritance, 121, 174, 182.

Blonde type, 257.

BLUMENBACH, 197.

Boleyn, Anne, 59.

Booted poultry, 182.

BORN, 200.

BOVERI, 11, 17, 18, 24, 25, 32, 207.

Brachydactyly, 129.

Branched habit, 129.

BROOKS, 75.

Bryonia, 222.

Bryozoa, 8.

BURBANK, 156.

Calvin, 260.
Canaries, 129.
Capsella, 89.
Captivity, effect of, 152.
Carnations, double, 63.
Carriers of color-blindness, 215.

265

266

INDEX

CASTLE, 4, 59, 124, 141, 163, 166, 170,
171, 183, 202-205, 210, 222, 241.
Castration, 210.
Cats, albino, 59.

tailless, 68.
Cattle, birth-rate of, 201.

hairless, 68.

hornless, 129.

roan, 176.

tailless, 68.
Celandine, 63.
Cell, germ, 21.

polar, 23.

resting, 18.

theory, 14.

typical, 15.

wall, 16.
Centrosome, 17.
Cerebral hernia, 69.
CHAPIN, 261.
Characters, congenital, 80.

moral and mental, 230.

unit, 61.

CHAUVIN, MARIE VON, 90.
Cheerful disposition, 232.
Chelidonium, 63, 71.
Chromatin, 16.
Chromosomes, 16.

extra, 207.

"x," 207.

"y," 209.

Chromosome theory, 29.
Chrysanthemum, 50.
Circumcision, 87.
Cirripede, 212.
Clover, four-leaved, 39.
COBB, 173, 183.
Cochins, 182.
Cocoon, yellow, 129.
Colorado potato-beetle (see Lep-

. tinotarsa) .
Color-blindness, 214.

factor, 170.
Comb characters, 69.
Complementary factor, 159.
Compound determiners, 159.
Congenital characters, 80.
CONKLIN, 29, 88.

Consanguineous marriage, 238, 239.
Constants, 44.

Continuity of germ plasm, 11.
Convergent variation, 150.
COBBENS, 124, 129, 133, 175, 205,

222.

Cotton, 129.

Cousin marriage, 234, 238, 242, 243.
Crabs, 212.
Crested head, 69, 129.
Cretinism, 253.
CUENOT, 164, 170, 171, 200.
Cumulative factor, 159, 187.
Curly hair, 137.
Currant-worm, 216.
Cytoplasm, 15.

Daisy, 50.
Daphnids, 53, 204.
DARBYSHIRE, 128, 177.
DARWIN, 1, 23, 39, 40, 42, 52, 55,
56, 61, 67, 73, 77, 85, 86, 114,
118, 123, 124, 151, 204, 225,
240, 251.
Datura, 180.

DAVENPORT, 46, 59, 68, 124, 129,
148, 152, 173, 177, 178, 179,
181, 182, 231, 233, 235, 237,
245, 248, 250, 253.
Deafness, 233.
Death, 7.

preventable, 258.
Defects, hereditary, 232.
Delayed dominance, 177.
Department of Agriculture, 250.
Dermal spines, 151.
Determiners of heredity, 28.
Deviation, average, 45.

standard, 46.
Dihybrids, 133.
Dinosaurs, 71.
Diluting factor, 165, 171.
Dioecious plants, 220.
Disease transmission, 92.
Docked tails, 88.
Dogs, hairless, 68.

tailless, 68.

trimmed ears of, 88.
Dominance, 145, 174.

delayed, 177.

imperfect, 175.

reversed, 178.

INDEX

267

Dominant, 125, 148, 158.
DONCASTEB, 217, 218.
Double flowers, 63.
DREYLINCOUBT, 197.
DRINKARD, 129.
Drosophila, 129, 241.
DUODALE, 227.
Duplex dose, 133.
DURHAM, 165, 171.

EAST, 192.

Echidna, 150.

Echinoderms, 31.

Echinus, 32.

Education, 3.

Edwards, Jonathan, 228, 260.

Richard, 261.
Egg, abortive, 23.

cell, 21.

fertilized, 24.

human, 28.

mature, 23.
EIMER, 40.
ELDERTON, 232.
Elementary species, 62, 72.
Environment, 2, 88.
Enzyme theory, 33.
Equality of sexes, 201.
Erinaceus, 150.
Erithizon, 150.
Eugenic regulation of marriage, 261.

rules, 238.

sentiment, 251.
Eugenics, 244.

Record Office, 245.
Euthenics, 244.

Evening primrose (see (Enothera).
Extension factor, 166, 170.
Extra chromosome, 207.
Extracted recessive, 148.
Extra toe, 69, 178.
Eye color in man, 121, 147, 177,231.

in Drosophila, 129.

Factor, color, 170.
complementary, 159.
compound, 159.
cumulative, 159, 187.
diluting, 165, 171.
extension. 166, 170.

hypothesis, 159.

inhibitory, 160.

intensifying, 165, 171.

pattern, 163, 171.

pigment, 161.

restriction, 166, 170.

spotting, 171.

supplementary, 159, 163.

uniformity, 164, 171.
False reversion, 149.
FARRABEE, 129.
FAY, 233.
Feathers, absent, 69.

branched, 69.

recurved, 69.

twisted, 69.

Feeble-mindedness, 149.
Feet of Chinese women, 87.
Feral animals, 150, 152.
Fertilization, 24.

abnormal, 30.

selective, 202.
Firefly, 207.
Fission, 7.
Flavism, 151.
Fluctuation, 57.
Four-leaved clover, 39.
Four-o'clock, 175, 180.
Four-toed guinea-pigs, 59. •
Franklin, Benjamin, 259.
Freaks, 58.
Freckles, 81.
Frogs, 200.

GAGE, 83.

GALEN, 198.

GALTON, 42, 77, 98, 117, 120-122,

151, 164, 244.
Galton Laboratory for Eugenics,

246.

Gamete, 23.

Garden peas, 124-128, 132, 134.
GEDDES, 197.
Gemmules, 8.

Generation, spontaneous, 6.
Genetics, 2.
Genotype, 113, 148.
Germ-cells, 21.
Germplasm, 10, 148.
GODDABD, 229, 236, 237.

268

INDEX

GOLDSCHMIDT, 44.

Graduated variants, 108.
Greyhounds, 33, 201.
Greening apple, 8.
Green peas, 134.
Growth, 7.
GRUBEK, 16.
Guinea-pig, agouti, 163.

albino, 59.

angora, 129.

brown-eyed yellow, 166.

coat character, 141.

four-toed, 59.

HAECKER, 17, 129.
Hair color, 137, 177.
Hairlessness, 68.
HALLET, 152.
Hare, Belgian, 183, 193.
Harelip, 149.
HAUSER, 256.
Hedgehog, European, 150.
Heine, 260.
Helix, 178.
HENKING, 207.
Hereditary bridge, 27.
Heredity, definition of, 4.
Hereford cattle, 68.
Heritage, 2.

Hermaphrodites, 220, 239.
Heterozygote, 131.
HIPPOCRATES, 198.
His, 29.
HODGE, 92.
Homozygote, 131.
HOOKE, 15.
Hornless cattle, 129.
Horns in sheep, 179.

in cattle, 179.
Horses, birth-rate of, 201.

hairless, 68.

pacing, 129.

trotting, 129.
Human betterment, 244.

birth-rate, 201.

egg, 28.

skin color, 196.
Human stature, 98.

traits, 231.
HTJRST, 133.

Hyalodaphnia, 54.
Hybridization, 120.
Hymenoptera, 210.

Illegitimacy, 251.

Imbecility, 234, 238.

Immigration, 248.

Immunity to rust, 129.

Imperfect dominance, 175.

Inachus, 212.

Inbreeding, 238.

Independent unit characters, 144.

Indian corn, 241.

Induction, parallel, 83, 93.

somatic, 83.

Influence of proximity, 241.
Inheritance, alternative, 121.

biological, 75.

blending, 121.

particulate, 121, 164.

sex-limited, 213.
Inhibitory factor, 181.
Insanity, 234.
Insects and flowers, 240.
Instinct, 92.

Intensifying factor, 165, 171.
Inventive genius, 232.
Ipomcea, 240.

Japanese art, 37.
Jamestown weed, 180.
JENNINGS, 48, 103, 110, 120.
JOHANSSEN, 43, 102, 110, 114, 115,

119, 155.

JORDAN, 253, 254, 259.
JUKES, 227, 231.
Jungle-fowl, 148.

Kallikak family, 229.
KAMMERER, 90.
KELLICOTT, 224, 253.
KELVIN, 5.
KING, 200.
KLEBS, 54.
KOLLIKER. VON, 21.

LAMARCK, 53, 76.
Lamarck's evening primrose, 64.
LANG, 1-29, 178, 180, 185, 193.
Lathyrus, 160.

INDEX

LAUGHLIN, 246.

LEDEBBAUB, 89.

Leghorn poultry, 177, 182.

Legless lamb, 39.

Leptinotarsa, 108.

Liability to respiratory diseases, 238.

Linnaean species, 60.

Lint, 129.

Literary ability, 232.

Live-for-ever, 54.

LOEB, 27.

Lop-eared rabbit, 183.

Lychnis, 67, 222.

MACDOUGALL, 67, 83.

Maize, 129, 192.

Mantle fibers, 19.

Many-celled fruit, 129.

Marriage, consanguineous, 238, 239.

cousin, 234, 238, 242, 243.

laws, 250.
MABSCHAL, 202.
Mathematical aptitude, 232.
Maturation, 22.
McCLUNG, 207.
Mean, arithmetical, 45.
Melanism, 151.
MENDEL, 1, 123, 130, 132, 136, 140,

143, 157, 159, 174, 202, 245.
Mendel's law, 125, 127, 144, 174,

222, 225.

Mental characters, 230.
Membrane, nuclear, 15.
Merino sheep, 68.
Metaphase, 19.
Mice, albino, 59.

decaudalized, 88.

hairless, 68.

in abnormal temperature, 89.

inbred, 241.

intensified color, 165.

piebald, 121, 164.

waltzing, 129.

yellow, 203.
Mid-parent, 99.

MlESCHEB, 33.

MILTON, 6.
Mirabilis, 175, 180.
Mitosis, 18.
Mode, 45. .

MOHL, VON, 15.

Monochorial twins, 201.
Monoecious plants, 220.
Monohybrids, 131, 147.
MONTGOMEBY, 34, 78, 80.
Moral characters, 230.
MOBGAN, LLOYD, 86.
MOBGAN, T. H., 129, 205, 208.
Morning-glory, 240.
Mud puppy, 91.
MULLENIX, 173, 183.
Musical ability, 232.
Mutation, 56.

degressive, 59.

progressive, 59.

regressive, 59.

theory, 56, 72.
Mutilations, 87.

NAGELI, 38, 40, 123.

Napoleon, 259.

Nat. Assoc. of British and Irish

Millers, 224.

Natural selection, 118, 225.
Neck bare in poultry, 69.
Necturus, 91.
Nee-Darwinians, 77.
Neo-Lamarckians, 77.
Neo-Mendelian theory of sex, 205.
Nettles, 129.
NEWMAN, 202.
Newton, 260.
Nicodemus, 85.
Night-blindness, 238.
NILSSON-EHLE, 185.
NITOBE, 37.
Nuclear membrane, 15.
Nucleus, 15.
Nulliplex dose, 133.
Nutrition theory of sex, 198.

CEnothera, 64.
OESTEBLEBEN, 201.
Oil-gland, 69.
Oocyte, 23, 211.
Oogonia, 23.
Ophiotrocha, 212.
Origin of life, 5.
of species, 1.
OVID, 6.

270

INDEX

Ovists, 22.
Oyster-borer snail, 47.

Pacing habit, 129.

Palatine ridges, 149.

Pangenesis, 85, 114.

Papaver, 63.

Parallel induction, 93.

Paramecium, 48, 110.

Parthenogenesis, 26, 107, 210.

Partial potency, 180.

Particulate inheritance, 121, 164.

PASTEUR, 6.

Pattern factor, 163, 171.

PATTERSON, 202.

PEARSON, 42, 43, 49.

Peas, garden, 124, 132, 134, 177.

sweet, 13, 160.
Petunias, double, 63.
PFLUGER, 200.
Phacechcerus, 91.
Phaseolus, 102.
Phenotype, 113, 148.
Phyrrhocoris, 207.
Plesiosaurs, 71.
Pigeons, 148, 201.
Pigment factor, 161.
Pigs, birth-rate of, 201.
Polar cells, 23, 211.
Polydactylism, 238.
Pomace fly (see Drosophila).
Poppy, Shirley, 63.
Population, 115.
Porcupine, 150.
Potency, defined, 179.

failure of, 180.

partial, 180.

total, 179.
Poultry, birth-rate of, 201.

booted, 182.

tailless, 68, 129, 181.

and the factor hypothesis, 173.

and Sudan Red III, 83.

and white color, 179.
Predisposition to disease, 93.
Prenatal influences, 87.
Presence and absence hypothesis,

132.

Preventable death, 258.
PRICE, 129.

Primitive man, 224.
Primroses, double, 63.

Lamarck's evening, 64.
Prophase, 18.
Protophyta, 7.
Protoplasm, 15.
Protozoa, 7, 10.
Proximity, influence of, 241.
Pug jaws, 69.
PUNNETT, 127, 216.
Pure line, 103.
Puritan stock, 239.
Purple beech, 63.

Quadruple hybrids, 143.
Quail, 92.

Rabbits, albino, 59.

birth-rate of, 201.

color in, 13, 169.

ears of, 183, 193.

gray, 171.

lop, 183.
Radiolaria, 17.
Rats, albino, 59.

inbred, 241.
RAYNOR, 217, 218.
Recessive, 126, 148, 157.

extracted, 148.
REDFIELD, 79.
Red flowers, 129.
Regeneration, 210.
Regression, 98, 151.
Regressive varieties, 72.
REID, 78.
Reinfection, 94.
Relative variability, 47.
Repair, 7.

Reproduction, sexual, 9, 20.
Resting cell, 18.
Restriction factor, 166, 170.
Reversed dominance, 178.
Reversion, 146.

false, 149.
RIDDLE, 83.
RIMPAU, 153.

RlTZEMA-BoS, 241.

Roan color of cattle, 176.
Rock-pigeon, 148.
Roses, double, 63.

INDEX

271

Rotifers, 204.
Rumplessness, 129, 181.
Rust, 129.

Sacculina, 212.
SADLER, 198.
Salamandra, 90.
SAUNDEBS, 129.
Scalp muscles, 149.
SCHENK, 198, 199.
SCHLEIDEN, 14.

SCHWANN, 14.

Sea-urchin, 32, 207, 209, 220.
Secondary sexual characters, 210.
Sedum, 54.

Segregation of unit characters, 130
145, 175.

of defectives, 252.
Selective fertilization, 202.
Serrated leaves, 129.
Sex-limited inheritance, 213.
Sexual reproduction, 9, 20.
Sheep, 67, 179.
Sheep, Ancon, 68.
Shirley poppy, 63.
SHULL, 67, 129, 133, 222, 241.
Siamese twins, 69.
Silk-worm, 129.
Simplex dose, 133.
SITOWSKI, 83.
Skew polygons, 50.
Skin- color in man, 196, 231.
SMITH, 212.

Smooth-margined nettle leaves, 129.
Smooth peas, 129, 134.
Snails, 58, 129*178, 180.
Snapdragon, 129, 173.
Social hindrances, 259.
Somatic induction, 83.
Somatoplasm, 10, 148.
Species, 62.

cycle, 71.

elementary, 62, 72.

Linnaean, 60.

origin of, 1.

Speculations about sex, 197.
SPENCER, 92, 260.
Spermatocyte, 23, 211.
Spermatogonia, 23.
Spermatid, 23.

Spermatozoa, 21, 23.
Spermists, 22.
Sphcer echinus, 32.
Spines, dermal, 151.
Spiny ant-eater, 150.
SPILLMAN, 129.
Sponges, 7, 8.
Spontaneous generation, 6.
Spores, 9.
Sports, 39, 56.
Spotting factor, 171.
Spotted mice, 121, 164.
Sprengel, 63.

Spurs, extra in poultry, 69.
Standard deviation, 46.
STANDFTJSS, 70.
Starchy kernel, 129.
Starfish rays, 44.
Statoblasts, 8. />
Sjentor, 16.

Sterilization laws, 255.
STEVENS, 207.
Stevenson, R. L., 260.
STOCKARD, 39.
Stocks, double, 63.
Straight hair, 138.
Sub-species, 62.
Sudan red III, 83.
Sugar beet, 155.

SUMNER, 89.

Sunburn, 88.
Sunflower, 129.

Supplementary factors, 159, 163.
Survival of the fittest, 226.
Susceptibility to disease, 93.

to rust, 129.
Sweet pea, 13, 160.

Taillessness, 68.
Tails, docked, 88.
Tatusia, 202.
Telophase, 20.
TENNENT, 31.
Theory, cell, 14.

chromosome, 29.

enzyme, 33.

mutation, 56, 72.

nutrition, 198.
Theromorphs, 71.
Thigh bones absent, 69.

272

INDEX

THOMSON, 146, 197.
Thumb prints, 38.
THURY, 198.
Tineola, 83.
Toes, crippled, 87.

extra, 69, 178.

webbed, 69.
Toenails, absent, 69.

extra, 69.
Tomato, 129.
Total potency, 179.
TOWER, 52, 55, 108.
TOYAMA, 129.
Training, 2.
Traits, human, 231.
Treasury of Human Inheritance,

246.

Trees deformed by wind, 88.
Triangle of life, 1.
Trihybrids, 140.
Trilobites, 71.
Trimmed ears of dogs, 88.
Trotting habit, 129.

TSCHERMAK, VON, 124, 129.

Tuberculosis, 38.
Tuttle, Elizabeth, 260.
Twins, 201.
Two-celled fruit, 129.
TYNDALL, 6.

Uniformity factor, 164, 171.
Unit character, 61.
Urosalpinx, 47.

Variability, relative, 47.
Variation, abnormal, 40.

continuous, 40.

convergent, 150.

definite, 39.

discontinuous, 41, 69.

fluctuating, 43.

fortuitous, 39.

germinal, 40.

graduated, 52.

harmful, 39.

hereditary, 41.

indefinite, 39.

indifferent, 39.

integral, 52.

morphological, 38.

multiple, 39.

non-hereditary, 41.

normal, 40.

orthogenetic, 39.

physiological, 38.

psychological, 38.

quantitative, 41.

qualitative, 41.

single, 39.

somatic, 40.

useful, 39.
Varieties, 62, 72.
Vasectomy, 255.
Vermiform appendix, 150.
Vestigial structures, 149.
VILMORIN, 155.
Viola, 37.
Vitalism, 82.
Volta Bureau, 246.
Voltaire, 260.

VRIES, DE, 56, 59, 62, 64, 67, 71,
124, 129, 154, 155.

WALTER, 48, 173, 183.
Waltzing habit, 129.
War, 258.
Wart-hog, 91.

and toads, 87.
asps, 210.

Weakling babies, 260.
Webbed toes, 69.
Weismann, 7, 10, 55, 77, 78;

81, 84, 86-88, 94, 95, 123,

241.

Wheat, 129, 152, 186, 224.
WHYMPER, 254.

WlEDERSHEIM, 87.

WILSON, 34, 35, 207, 208.
Wing absent, 69.
WINSHIP, 228.

WOLTERECK, 53.

Wright, Seth, 67.

Wrinkled kernel, 129, 134, 177.

X chromosome, 207.

Y chromosome, 209.
Yellow mice, 203.

peas, 134.
YUNG, 200.

Zygote, 24.

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