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Monographs on Experimental Biology

EDITED BY

JACQUES LOEB, Rockefeller Institute

T. H. MORGAN, Columbia University

W. J. V. OSTERHOUT, Harvard University

INBREEDING AND OUTBREEDING

THEIR GENETIC AND SOCIOLOGICAL
SIGNIFICANCE

BY
EDWARD M. EAST, Ph.D.

AND

DONALD F. JONES, Sc.D.

MONOGRAPHS ON EXPERIMENTAL

BIOLOGY

PUBLISHED

FORCED MOVEMENTS, TROPISMS, AND ANIMAL

CONDUCT

By JACQUES LOEB, Rockefeller Institute

THE ELEMENTARY NERVOUS SYSTEM

By G. H. PARKER. Harvard University

THE PHYSICAL BASIS OF HEREDITY

By T. H. MORGAN, Columbia University

INBREEDING AND OUTBREEDING: THEIR GENETIC
AND SOCIOLOGICAL SIGNIFICANCE

By E. M. EAST and D. F. JONES, Bussey Institution. Harvard Universit

THE NATURE OF ANIMAL LIGHT

By E. N. HARVEY. Princeton University

SMELL, TASTE AND ALLIED SENSES IN THE

VERTEBRATES

By G. H. PARKER, Harvard University

BIOLOGY OF DEATH
By R. PEARL, Johns Hopkins University

INJURY, RECOVERY, AND DEATH IN RELATION TO

CONDUCTIVITY AND PERMEABILITY

By W. J. V. OSTERHOUT, Harvard University

IN PREPARATION

PURE LINE INHERITANCE

By H. S. JENNINGS, Johns Hopkins University

LOCALIZATION OF MORPHOGENETIC SUBSTANCES

IN THE EGG

By E. G. CONKLIN, Princeton University

TISSUE CULTURE

By R. G. HARRISON. Yale University

THE EQUILIBRIUM BETWEEN ACIDS AND BASES IN
ORGANISM AND ENVIRONMENT

By L. J. HENDERSON, Harvard University

CHEMICAL BASIS OF GROWTH

By T. B. ROBERTSON, University of Toronto

COORDINATION IN LOCOMOTION

By A. R. MOORE, Rutgers College
OTHERS WILL FOLLOW

Monographs on Experimental Biology

INBREEDING AND
OUTBREEDING

THEIR GENETIC AND SOCIOLOGICAL

SIGNIFICANCE

BY ^^

EDWARD M. EAST, Ph.D.

HARVARD UNIVERSITY, BUSSEY INSTITUTION

AND

DONALD F. JONES, Sc.D.

CONNECTICUT AGRICULTURAL EXPERIMENT STATION

i6 ILLUSTRATIONS

PHILADELPHIA AND LONDON
J. B. LIPPINCOTT COMPANY

i

t

COPYRIGHT, IQIQt BV J. B. LIPPINCOTT COMPANY

Eleclrolyped and printed by J. B. Lippincoti Company
The Washington Square Press, Philadelphia, U. S. A.

LIBRARY

N. C. State College

EDITORS' ANNOUNCEMENT

The rapidly increasing specialization makes it im-
possible for one author to cover satisfactorily the whole
field of modern Biology. This situation, which exists in
all the sciences, has induced English authors to issue
series of monographs in Biochemistry, Physiology, and
Physios. A number of American biologists have decided
to provide the same opportunity for the study of
Experimental Biology. '

Biology, which not long ago was purely descriptive
and speculative, has begun to adopt the methods of the
exact sciences, recognizing that for permanent progress
not only experiments are required but that the experi-
ments should be of a quantitative character. It will be the
purpose of this series of monographs to emphasize and
further as much as possible this development of Biology.
Experimental Biology and General Physiology are one
and the same science, by method as well as by contents,
since both aim at explaining life from the physico-chemical
constitution of living matter. The series of monographs
on Experimental Biology will therefore include the field
of traditional General Physiology.

Jacques Loeb,
T. H. Morgan,
W. J. V. Osteriiout.

6

i ^ ^ Of)

PREFACE

It is inevitable that each work planned as a member of
a series of biological monographs should be somewhat
technical. Of necessity each must be concise. In view of
the difficulties these limitations involve, one may hardly
expect to escape the criticism that the subject matter
often tends to be esoteric in its nature, for few can say in
Shaw's odd fancy, ^'I tried to do too much — and did if
Nevertheless, there has been a serious effort to avoid a
mere record of the development of a specific problem in
Genetics as an aid to the general biologist. No one could
have a professional interest in a subject of this kind with-
out the desire that there be some practical application
of the results to agriculture and to the many phases of
sociology where a knowledge of the laws of heredity is a
first requisite. Though such applications of the genetic
conclusions are touched but lightly here, there is the hope
that the non-biological worker interested in problems of
human welfare will find some new thoughts and pertinent
suggestions in the compelling logic of the controlled
experiments described throughout the pages. At least
it was with this idea in mind that the authors prepared
the first four chapters. For the zoologist and botanist
the well-known facts and elementary principles there dis-
cussed would have been unnecessary.

7

8 PREFACE

The manuscript has been the product, as it purports
to be, of a very intimate collaboration, and the authors
join in acknowledging their indebtedness to their fellow
biologists for the privilege of copying several illustrations
as noted in the legends, to Professor T. H. Morgan for
helpful and suggestive criticism, and to Mr. L. C. Dunn
and Mr. E. S. Anderson for assistance on the proofs.

E. M. E.
D. F. J.

Boston, September, 1919

CONTENTS

CHAPTER PAGE

I. Introduction 13

II. Reproduction Among Animals and Plantb 20

III. The Mechanism of Reproduction 36

IV. The Mechanism of Heredity 50

V. Mathematical Considerations of Inbreeding 80

VI. Inbreeding Experiments with Animals and Plants 100

VII. Hybrid Vigor or Heterosis 141

VIII. Conceptions as to the Cause op Hybrid Vigor 164

IX. Sterility and Its Relation to Inbreeding and Cross-breeding 188

X. The R6le of Inbreeding and Outbreeding in Evolution... 195

XI. The Value of Inbreeding and Outbreeding in Plant and

Animal Improvement 210

XII. Inbreeding and Outbreeding in Man: Their Effect on

THE Individual 226

XIII. The Intermingling of Races and National Stamina 245

Literature 266

ILLUSTRATIONS

FIG. PAGE

1. Asexual Reproduction, an Amoeba in Division 21

2. Asexual Reproduction by Means of Runners 22

3. Hermaphroditism in the Tapeworm Proglottid 24

4. Rhopalura, an Example of Extreme Sexual Dimorphism 26

6. Sexual Reproduction in Fucus 26

6. Ulothrix, a Primitive Type of Sexual Reproduction 28

7. An Adaptation for Self-pollination 30

8. An Adaptation for Cross-pollination 34

9. Diagram of Gametogenesis 38

10. Diagram to Illustrate Fertilization 39

11. Formation of Pollen Grains in the Lily 40

12. Fertilization in the Embryo Sac of the Lily 41

13. Entrance of Spermatozoon through Membrane of Egg of Star-fish ... 42

14. Diagram Showing the Distribution of Sex Chromosome in Protenor . . 43

15. Identical Quadruplets in Nine-banded Armadillo 44

16. Diagram to Illustrate Inheritance of Sex-hnked Character 48

17. Diagram Showing Union of like Gametes 52

18. Diagram to Illustrate Mendelism in a Cross between Long-Spiked and

Short-Spiked Wheat 54

19. Diagram to Illustrate Gamete Formation in a Dihybrid in Indepen-

dent Inheritance 59

20. Diagram to Illustrate Gamete Formation in a Dihybrid in Linked

Inheritance 63

21. Diagram to Illustrate Crossing-over 64

22. Curves Showing the Limiting Values of the Coefficients of Inbreeding

with Various Systems of Matings 84

23. Graphs Showing the Total Inbreeding and Relationship Curves for

the Jersey Bull, King Melia Rioter 14th 86

24. Graphs Showing the Reduction of Heterozygous Individuals and of

Heterozygous AUelomorphic Pairs in Successive Generations of

Self-fertihzation 90

25. Graphs Showing the Increase in the Body Weight with Age for the

Males of Inbred Albino Rats 107

26. Graphs Showing the Increase in Weight of Body with Age for DifTer-

ent Series of Male Albino Rats 108

11

12 ILLUSTRATIONS

27. Graph Showing the Average Size of Litters Produced in Successive

Generations of Inbreeding Albino Rats by Brother and Sister
Matings 109

28. Goliath, an Albino Rat, the Product of Six Generations of the Closest

Possible Inbreeding 110

29. Representative Samples of Inbred Strains of Maize after Eleven

Generations of Self-fertilization 130

30. Graphs Showing the Reduction of Variability and Segregation of

Ear Row Number in Selfed Strains of Maize 132

31. Plants of Maize after Eleven Generations of Self-fertiUzation and

Their Fi Hybrid 150

32. Ears of Maize after Six Generations of Self-fertihzation and Their

Fi Hybrid 150

33. Graphs Showing Growth Curves of Two Inbred Strains of Maize and

Their First and Second Generation Hybrids 152

34. James River Walnut, Hybrid Between Persian Walnut and Butternut 154

35. Growth Curves of Parent Races and Fi and F2 Hybrids of Guinea Pigs 160

36. Diagram to Show How Factors Contributed by Each Parent May

Enable the First Generation of a Cross to Obtain a Greater
Development than Either Parent 175

37. Cattaloes, the Product of Crossing the Cow and the Bison 180

38. Sterile Hybrid Between Radish and Cabbage 192

39. Tassels of an Almost Sterile Strain Obtained by Inbreeding Maize . . 196

40. Representative Ears of a Cross Between Two Inbred Strains of Maize 202

41. Plants of a Cross Between Two Inbred Strains of Maize 202

42. Diagram Showing a Method of Double Crossing Maize to Secure

Maximum Yields, Illustrated by Actual Field Results 203

43. First Generation Cross of Shropshire by Delaine Merino 212

44. First Generation Cross of Hereford by Shorthorn 216

45. "Big Jim" the Product of a Pure Bred Percheron Stallion Mated

with a Grade Mare of the Same Breed 220

46. First Generation Cross of Chester White and Poland China 224

INBREEDING AND
OUTBREEDING

CHAPTER I
INTRODUCTION

Interest in the effects of inbreeding and of outbreed-
ing is not confined to the professional biologist. Histori-
cally these are old, old problems, practical problems of
considerable significance bound up with man's gravest
affairs, his marriage customs and his means of subsist-
ence. In these matters, moreover, the passing of time
has not diminished the value to be attached to their solu-
tion. The questions involved belong to theoretical biol-
ogy, it is true, and the professional biologist may lay claim
to the first satisfactory analyses ; but relatively his inter-
est is that of yesterday, stimulated by the work of Dar\s^in
in establishing the doctrine of Evolution.

The intimate relation which the effects of various sys-
tems of mating bear to these three subjects will be seen
more clearly from the following brief explanation.

Anthropological investigations have shown that many
primitive peoples established rigid customs of exogamy —
marriage outside the family or the clan. Such practices,
after their identification with totemic systems by Mac-
Lennan, became the subject of much notable speculation.
In particular may be mentioned the works of Frazer, Lang
and Freud. Yet these writers have thrown little light on

13

14 INBEEEDING AND OUTBREEDING

the origin of outbreeding as a social habit, and have con-
tributed nothing whatever toward the solution of the ques-
tions of inbreeding and outbreeding in the sense in which
they will be treated here. It is probable, indeed, that these
customs usually originated without regard to matters of
physical inheritance. The tribes concerned had seldom
risen to a state of culture where the welfare of their de-
scendants might be expected to cause anxiety, since in few
cases had there been that development of animal hus-
bandry necessary for the first glimpse into the mysteries
of heredity.

These observations do not necessarily apply to the
marriage folkways which developed in western Asia and
Europe and were passed on to the United States. Our
laws preventing marriages between certain degrees of
kinship have been moulded by the touch of various civil-
izations, but in the main they are a legal heritage from the
code of Hammurabi through the Hebraic Tahnud. Since
they are based largely upon the customs of pastoral na-
tions, it may be they had some foundation in experience,
half-truths drawn from casual and fragmentary observa-
tions of the shepherd and the cattleman. There is no his-
torical record of such rational basis, however. Many of
the conventionalisms rigidly stabilized by the hand of re-
ligious authority have not the slightest biological justifi-
cation. Witness the English laws preventing marriage
with a deceased wife's sister. On the other hand, if there
had not been a dim but real fear of evil consequences aris-
ing from inbreeding, there would be something extraordi-
nary in the frequencies with which taboos against
consanguineous matings have persisted. Among the
peoples contributing to European civilization, caste sys-

INTRODUCTION 15

terns have been common, and the logical outcome of a
caste system is marriage between near relatives. Tnclo
of race encourages inbreeding among the ruling class, and
power within that ruling class prompts the perpetuation
of a serving class in the same manner. Why, then, should
exogamy have been continued so commonly throughout
epochs marked by rational thought and a high degree of
culture? It is true, there are exceptions to this general
rule. Rather intense inbreeding was practiced both in
Egypt and in Greece when they were at the height of their
power and influence. Nevertheless, exogamic customs
have prevailed. They exist in Europe and America at the
present day, and it is natural to wish to know whether
there is any biological justification for them.

Let us propose three questions which will show the
sociological bearing of the problems under consideration.

1. Do marriages between, near relatives, wholly by rea-
son of their consanguinity, regardless of the inheritance
received, affect the offspring adversely?

2. Are consanguineous marriages harmful through the
operation of the laws of heredity?

3. Are hereditary differences in the human race trans-
mitted in such a manner as to make matings between
markedly different peoples desirable or undesirable,
either from the standpoint of the civic worth of the indi-
vidual or of the stamina of the population as a whole?

Correct answers to these questions are a matter of
more importance than a superficial consideration indi-
cates. Settled in accordance with the biological facts,
they aid in establishing a concrete scientific basis for
marriage, divorce and immigration laws; they give
grounds for predicting the changes to be expected in the

16 INBEEEDING AKD OUTBREEDINQ

body politic due to differential fecundity, birth control,
and other agencies by which the character of the popula-
tion is shifted; they even have some relevancy to many
problems which one might suppose were wholly of an
economic nature, such as minimum wages and mothers'
pensions.

The second series of phenomena arousing interest in
the results of inbreeding and outbreeding comes from ob-
servation upon domestic animals and cultivated plants.
Plants are included by courtesy, though in reality intelli-
gent plant breeding hardly began until the nineteenth cen-
tury, and the methods adopted were taken from the pro-
cedures in use by animal breeders, with such modifications
and improvements as the peculiarities inherent in vegeta-
tive propagation made necessary. Animal breeding, on
the other hand, is a very ancient occupation, and more or
less accurate data on the effects of interbreeding near
relatives as compared with the effects of crossing differ-
ent strains must have been collected by all of the old
agricultural peoples. Since there is no question that
under certain circumstances inbreeding does produce un-
desirable results — defectives, dwarf -forms, sterile indi-
viduals, etc. — it may be that their experience was at the
base of some of the antagonism toward close-mating in
the human race. Or, it is possible that early breeders ob-
served the phenomenon, common to both animals and
plants, that when two unrelated stocks are crossed the hy-
brids thus produced are often more vigorous than either
parent — the phenomenon of hybrid vigor or heterosis, as
it is called at the present time. There is no proof of such a
sequence of ideas, but it seems to be a logical hypothesis.
At any rate, the views of the animal raisers regarding

INTRODUCTION 17

inbreeding and the traditions regarding marriage of near
kin are very similar. The great majority of breeders have
an ineradicable fear of evil consequences if their matings
are too close. Only here and there a few fearless ones
have used systems of extremely close mating to perpetu-
ate their breeds, and by such methods have built up in-
valuable races of horses, cattle, swine and poultry. But
here a dilemma appears. Inbreeding has deplorable re-
sults in certain cases, yet in other instances the returns
have been gratifying. What is to be the future practice!
To be more than mere trial and error, it must be founded
upon a cogent analysis of the whole subject.

Finally, interest in the effect of various systems of
mating as natural phenomena has been stimulated by the
study of organic evolution. The circumstantial data of
comparative morphology show that in nature problems
similar to those of man have arisen. If these problems
are investigated some light may be thrown upon his diffi-
culties. Sexual reproduction has been the most success-
ful method of providing for the propagation of animals
and plants. Does sexual reproduction, therefore, possess
an advantage over other methods? Would it otherwise
have persisted as it has in both kingdoms? It probably
was not the original method of reproduction. Asexual
reproduction, reproduction by simple vegetative division,
appears to have held the stage when animals and plants
were simple and unspecialized. Then, in all probability,
came sexual reproduction with separation of the sexes.
Secondarily, however, numerous species arose in both
kingdoms wherein the sexes are united, male and female
cells being produced in the same individual. Thus a sys-
tem of mating entailing the greatest possible amount of

18 INBREEDING AND OUTBREEDINa

inbreeding was established. But this system appears to
have been deticient. {Some evolutionaiy advantage asso-
ciated with separation of the sexes was lost. There is
reason for assuming that this advantage was connected
with cross-fertilization, for tertiary developments in each
kingdom brought about numerous mechanisms whereby
cross-fertilization was established in hermaphroditic or-
ganisms. Still, in spite of the obvious success of bisexual
and of cross-fertilized species, as shown by their fre-
quency, numerous self -fertilized species, and even species
reproducing exclusively by asexual methods have kept
their places in the struggle for existence. Both inbreed-
ing and outbreeding systems have developed side-by-side
under natural conditions. The data of comparative mor-
phology, therefore, seem as contradictory as those from
anthropology and agriculture.

The puzzles presented by these general facts taken
from history, husbandry and biology have one common
feature. They centre on the problem of inheritance. For-
tunately, though less than two decades have passed since
the application of quantitative experimental methods to
biology became somewhat general, the mechanism of
heredity is no longer a riddle; and to-day the effect of
inbreeding and outbreeding on plants and animals can
be described in considerable detail and interpreted with
singular precision. Having this interpretation it may be
applied to the three fields of interest we have described.

In the ensuing pages the important controlled experi-
ments in inbreeding and outbreeding necessary for an
orderly and consistent interpretation of the facts are dis-
cussed. Uncontrolled experiments, casual observations
of stock breeders, data on human marriages between near

INTRODUCTION 19

relatives, have been omitted designedly. Numerous data
of these types have been available for many years, but
they have been of little service in clarifying the situation.
This is not altogether due to their fragmentary character
in point of time, or even to the fact that they usually lack
the precision necessary in data to be used in the analysis
of such complex phenomena. Data for a limited number
of generations are often useful, and precision is a relative
matter. The truth is, the majority of these records was
collected without regard to the type of fact required, and
without reduction to concrete numerical terms. In other
words, in records otherwise accurate, critical data are
omitted ; and those given are relatively useless on account
of their form.

A detailed application of our conclusions to sociology,
agriculture and evolutionary theory has not been at-
tempted. It is hoped that the suggestions along these
various lines are sufficient to show how such application
can be made; but human direction of evolution either in
man or in the lower organisms is beset with difficulties so
numerous and so prodigious that each problem must have
its individual solution.

CHAPTER II

EEPEODUCTION AMONG ANIMALS AND PLANTS

In order to obtain a proper orientation of the problem
of inbreeding and outbreeding, one must consider first
some of the general facts regarding reproduction among
animals and plants and their relation to inheritance.

The significant changes in both kingdoms have been
remarkably similar. The differences are differences in de-
tail, and for this reason they are additional arguments in
favor of the idea that there are special advantages asso-
ciated with the coincidences found in the general processes
involved. For example, asexual propagation is more gen-
eral in the simpler, sexual reproduction in the higher or-
ganisms. But sexual reproduction in animals has largely
supplanted the asexual method, in plants sexual reproduc-
tion was merely added. Is this not evidence of an im-
portance to be attached to the sexual method, apart from
a simple provision for multiplication? Again, the diver-
sity of sex organs which has arisen among the various
groups of animals and plants is highly surprising, yet this
dissimilarity may be wholly of a superficial nature. When
examined solely with the object of inquiring what systems
of mating these variations entail, the parallelisms in each
history stand out impressively. If these facts be kept in
mind throughout the short discussion of heredity and re-
production which follow, their probable evolutionary sig-
nificance is not difficult to grasp ; but if the profusion of
variation in detail, or even the general mechanism of ac-
complishing a particular result is allowed to distract

20

ANIMAL AND PLANT REPEODUCTION 21

attention, the end may be lost to sight through admiration
of the ingenuity of the means,

(^Tiiere seems to be no question but that sexual repro-
duction is a more recent means of propagation than asex-
ual reproduction. ' Although asexual reproduction in the
narrow sense, that is, by means of simple division or by
budding, is common among the
protozoa, the sponges, the coelente-
rates and the fiat worms, it becomes
sporadic in the molluscoids and
annelids, and is found in only
one or two isolated instances in
forms as highly specialized as the
arthropods and the chordates. If
fragmentation succeeded by regen-
eration of the lost parts be conceded
to be a true means of reproduction,
however, echinoderms and nematode
worms are included. Thus of all
the great groups of animals only
certain worms (TrocJielminthes)

FiQ. 1. — Asexual reproduc-
tion. An amoeba in division.

and the molluscs have no asexual cv. contractile vacuole; ek, coto-

, Bare; en, entoaarc; u, nurleua.

reproduction m the usual sense or (Kingsiey after sehuize. cour-

^ ^ teay Henry Holt & Co.).

the word, and zoologists would
hardly feel safe in maintaining its absence in these two
phyla since the life history of so many forms is unknowai.
But since asexual reproduction is replaced by sexual
reproduction to a greater and greater extent as the higher
forms are reached one cannot avoid the conclusion that
the latter has proved to be the really successful means
of propagation. Nevertheless, variations appeared in
highly specialized forms which permitted return to an

22 INBEEEDINa AND OUTBREEDING

asexual type of reproduction. In the arthropods, as well as
in some other forms, mechanisms arose by which the eggs
developed without fertilization. This parthenogenetic
reproduction has been relatively successful, but only as a
stop-gap. Sexual reproduction persists and is used as an
occasional means of propagation. It would seem that it
possessed advantages too great to be given up entirely.
Even as sexual reproduction is a later method of

propagation than asexual reproduc-
tion, hermaphroditism appears to be
a secondary development from forms
in which the sexes were separate
(gonochorism or dioecism). Omitting
the protozoa in which it is difficult to
decide such sexual differences, gono-
chorism is present in every great
animal group but the sponges, and
hermaphroditism everywhere except
in the Trochelminthes, although in
Nemathelminthes, Echinodermata and
FiQ. 2.— Asexual repro- Avthropoda it is rare. An extended

ductioa by means of runners . . . , ^ • j a t i

in the hawkweed. (After experiment ou the subject or hermaph-
roditism certainly was made, but that
it was an experiment, that hermaphroditism is from the
evolutionary standpoint a secondary institution, is clear
if one considers the anatomical evidence, as is shown by
Caullery.23 Generally, hermaphroditism is a condition
associated either with parasitism or with a sedentary life.
Furthermore, hermaphroditic organisms do not have a
truly simple organization. They have a superficial simplic-
ity, due to an adaptation to their mode of life, but if one
compares hermaphroditic and gonochoristic species group

ANIMAL AND PLANT REPRODUCTION 23

by group, for example unisexual land or fresh-water
worms with their bisexual marine cousins, he finds the for-
mer to be the more complex, particularly as to their sex
organs. The fact that the sponges are hermaphroditic
might be considered as weighing against this argument,
but it is not without the bounds of probability that the
sponges are further along in specialization than is gen-
erally admitted, for to find the substance nearest chem-
ically to the so-called skeleton of the sponges, one must
search among the arthropods — the product of the spin-
ning glands of certain spiders and insects.

Hermaphroditism, pure and simple, however, was not
a success. Only a few degenerate forms retained self-
fertilization and persisted. Among them may be men-
tioned the tapeworms, certain crustaceans {Sacculina)
parasitic on crabs, and the colonial forms, bryozoans and
tunicates, the latter being perhaps the most degenerate of
all animals since they are wholly unrecognizable as rela-
tives of the vertebrates except at one short stage of their
life history. In most of the hermaphroditic types new
characteristics appeared which enabled them to exercise
one of the important functions of bisexuality, cross-fer-
tilization, without giving up the obvious energy conserva-
tion attainable through the production of both sex cells
in a single individual.

In nearly all of these forms, this was made possible by
the development of the eggs and of the sperm at different
times. In a few isolated cases among the turbellarians
and the tunicates the eggs develop first and then the
sperm; the animal is first a female and later a male (pro-
togyny). But in a greater number of species, the indi-
vidual is first a male and afterwards a female (pro-

24

INBREEDING AND OUTBREEDING

tandry). In the tapeworm (Fig. 3), for example, each
segment contains a complete reproductive system, testes,
ovaries and accessory glands ; when young the testes func-
tion, when older the testes atrophy and the ovaries de-
velop. In some of these protandrous species there is
even a change in the whole structure of the body, includ-

0000 o.

J O do o

0 0 qOOq

O °oOO

o ooaoo
,o "

'°a.§§

^°o?,oO.»5^o?o8.'^

O 0 0 n^A»:
ftOoooO*??

Fig. 3. — Hermaphroditism in the tapeworm proglottid. K, genital pore; ov, ovary; re,
receptaculum seminalis; t, testes; u, uterus; vd, vas deferens. (Kingsley after Sommer).

ing the sexual orifices. The isopods of the family Cynio-
thoidcd, a group of crustaceans parasitic on fish, furnish
a beautiful illustration. In the male stage the animal is a
typical crustacean and would be recognized as such by
any layman with a very slight knowledge of zoology ; but
when the animal passes over into the female stage it be-
comes merely a great ^gg sac many times the previous
size. One would hardly suppose the two stages belonged

pWPfRTT UBMRT
^ C. State CoUeg*

ANIMAL AND PLANT REPRODUCTION 25

to the same order, not to mention a transformation of the
same individual.

A few other mechanisms which promote cross-fer-
tilization have been found in isolated cases. They are not
as widespread as the one just described, but are peculiarly
interesting nevertheless. Among certain of the cirripedes,
the normal individuals are hermaphroditic, but in addition
a few tiny degenerate males are developed. They are little
more than bags of sperm and are calculated to make some-
what amusing any generalization as to the ** stronger
sex.'' Darwin, who discovered them, called them com-
plemental males. Another means of preventing continued
self-fertilization is self-sterility, a condition in which self-
fertilization is very difficult or even impossible through
some physiological impediment which is not clearly under-
stood. It was demonstrated by Castle ^^ for the American
race of Ciona mtestinalis.

In what appear to be the essential features, the
vicissitudes of reproduction have been similar in tlie
vegetable kingdom. The problems were solved in differ-
ent ways, but the gross results are largely the same. The
most striking difference is the varied success of certain
mechanisms. In the animal kingdom sexual reproduction
wherever instituted practically always displaced asexual
reproduction. Only in a few forms which are either fixed
or parasitic in their mode of life did the two methods per-
sist side by side. In plants, however, where the sessile is
the common condition, asexual and sexual reproduction
have continued harmoniously side by side clear up
through the angiosperms. Again, there is a marked dif-
ference in the success of hermaphroditism. In plants
hermaphroditic forms became the dominant types in the

26

INBREEDING AND OUTBREEDING

highest and most specialized group, the seed plants, while
in the highest group of animals, the mammals, only an
occasional individual showing rudimentary hermaphro-
ditism is found.

Just when sexual reproduction first originated in the
vegetable kingdom is even more of a question than among
animals. Only a few very simple types, the schizophytes
(bacteria) and myxomycetes, have passed it by. Perhaps

FiQ. 4.

FiQ. 6.

\ ^\' - T . .V' • T*"-' ' -^ y^-' "»>; I..- ^•- -.■■If I

-v.

(

Fia. 4. — Rhopalura, an example of extreme sexual dimorphism. (After Caullery.)
Fia. 5. — Sexual reproduction in Fucus, giving some idea of the difference in size of

egg and sperms. Sperms should be about one-tenth the size shown. (Bergen and Davis

after Thuret.)

it is for this reason these forms have remained the sub-
merged tenth of the plant world. It is tempting, as
Coulter ^2 says, to see the origin in the Green Algce,
There, in certain species, of which Ulothrix is an example
(Fig. 6), spores of different sizes are produced. The
large ones having four cilia are formed in pairs in each
mother cell, the smaller ones usually having two cilia
occur in groups of eight or sixteen in each spore-produc-
ing cell. Those largest in size germinate immediately
under favorable conditions and produce new individuals.

ANIMAL AI^D PLANT REPKODUCTION 27

Those of lesser size also germinate and produce new
individuals, but these are small and their growth slow.
Only the smallest are incapable of carrying on their vege-
tative functions. These come together in pairs and fuse.
Two individuals become one as a prerequisite to renewed
vigor. Vegetative spores become gametes. Something
valuable — speed of multiplication — is given up that some-
thing more valuable in the general scheme of evolution
may be attained.

This is indeed an alluring genesis of sex. It is rather a
genesis of sex, however, than the genesis of sex. Various
manifestations of sex are present in other widely sepa-
rated groups of unicellular or simple filamentous plants,
the Peredinew, the Conjugatce and the Diatomece — the
Conjugated being indeed the only great group of plants in
which there is no long continued asexual reproduction.
In these forms one cannot make out such a good case of
actual gametic origin, but the circumstantial evidence of
sex development in parallel lines is witness of its para-
mount importance.

After the origin of sex, many changes in reproductive
mechanisms occurred in plants, but most of them resulted
merely in better protection for the gametes, in increased
assurance of fertilization, in provision for better distri-
bution, or in greater security for the young plant.

First, perhaps, there was physiological differentiation
of the gametes. At least such an interpretation may be
given to the form of conjugation found in Spirogyra and
other Conjugates, where, either by solution of the wall
separating them, or by the formation of a tube-like out-
growth of one or both cells so that the ends touch, tlie
contents of one cell pass over to the other. We may

28

INBREEDING AND OUTBREEDING

tliink of the stationary cell as female and the other
as male.

Another line of development, however, became the
dominant one in the plant kingdom just as it did in the
animal world. A morphological differentiation of the sex
cells occurred. One became a large inactive cell stored

FiQ. 6. — Ulothrix, a primitive type of sexual reproduction. A, B, filaments; C, «o-
ospores; Z), germination of zoospore; £, gamete formation in filament; F, gametes and their
fusion; G, germination of zygospore. (Bergen and Davis after Dodel.)

with food, the egg ; the other became small and motile, the
sperm. This change is well illustrated in Fucus (Fig. 5),
one of the brown algae. It is clear that such a change in-
creased the probability of fertilization, since many
sperms could be produced without utilizing a great deal
of energy, and since the attraction of the egg for the sperm
was presumably augmented.

A further stage in the evolution of sex was reached
when the cells producing the eggs or the sperms were

ANIMAL AND PLANT REPKODUCTION 29

differentiated, thus providing for protection of the
gametes. Such organs of various types and known
by different names have persisted throughout all the
higher plants. One may call them ovaries and spermaries
and thus keep in mind that in animals the same types of
change occurred.

The final step in the general development of sexuality
is the restriction of the formation of sex organs to a par-
ticular phase in the plant's life, which on this account is
known as the gametophyte. The remaining stages are
known as non-sexual or sporophytic, because they are
characterized by the production of non-sexual reproduc-
tive cells, the spores. The liverworts, the mosses, the
ferns and the seed plants are thus set apart.

Since these two phases alternate with each other, pairs
of reproductive cells of the gametophyte producing the
sporophyte, and the non-sexual spores of the latter giving
rise to the gametophyte, the sequence has retained the
name of alternation of generations.

In the higher liverworts and mosses the gametophyte
carries on the greater part of the nutritive work, but in
the ferns the sporophyte becomes the dominant structure ;
while in the seed plants the gametophyte has degenerated
until it consists of but two or three cell divisions.

There is no question but that all of these numerous
changes are merely insurance against the future, some-
thing that may be said of seed production as a whole, since
the seed is but the younger generation nourished on the
parent stem. And it is interesting to note that just as
animals and plants paralleled each other in cramote pro-
tection and provisions for assuring fertilization, so also
the final step in each, the mammals and the seed plants,

30 INBREEDING^ AND OUTBREEDING

was the protection of the young. In certain particulars,
however, the higher plants did not simulate the higher
animals in their reproductive evolution, and it is not diffi-
cult to see the reason for the divergencies. Plants re-
tained asexual reproduction as an alternative method of
propagation, and made a success of hermaphroditism.
The obvious necessity for both was their fixed condition,
their slavery to the soil ; but if hermaphroditism with its
simplest implication, self-fertilization, had become domi-

FiG. 7. — Adaptation for aelf-pollination by means of spiral twietings of stamens and style.

(After Kerner.)

nant, there would have been little from their life histories
upon which to base an argument regarding the respective
virtues and defects of inbreeding and outbreeding. This,
however, was not the case. Many plants characterized by
autogamy persisted and flourished. They even developed
numerous devices promoting self-fertilization (Fig. 7),
such as pollination before the flower opens, inclination
of the anthers toward the pistil or the pistil toward the
anthers, rapid elongation of the pistil through a ring of
stamens, or various torsions of the accessory floral parts ;
but it seems perfectly clear from the exhaustive investi-
gations on the fecundation of plants made in recent years

ANIMAL AND PLANT REPRODUCTION 31

that only an extremely small percentage of the species of
flowering plants which have held their own to the present
day in the struggle for existence, have adopted a method
of fertilization which permits no crossing. Some of our
most vigorous cultivated plants — tobacco, wheat, peas and
beans — are naturally and usually self -fertilized, but they
each and every one have their flowers so arranged as to
pennit an occasional cross.

At the same time, one would be too hasty if he con-
cluded from these facts that continuous self-fertilization
or other means of reproduction which result in a single
line of descent is incompatible with inherent racial vigor.
At least, there is evidence that various species which seem
well able to hold their own seldom resort to crossing as a
means of propagation, yet one could hardly use them as
examples of degeneration. As illustrations, there is no
need to go below the flowering plants, either, although if
one desires an example of a long-continued evolution of
species and genera without any form of sexual reproduc-
tion he is forced to look to the Basidiomycetes. In this
large group the fungi are not only asexual themselves, but
appear to have been developed in a purely asexual manner
from asexual ancestors. But in the flowering plants,
many of our most useful types — the potato, the banana,
hops and sugar cane — seldom have recourse to sexual re-
production. It is true many agriculturists insist that
these species sooner or later degenerate for this very
reason, but they have never been able to bring forward
one atom of critical evidence to uphold their view. Vari-
eties of potatoes or of sugar cane do indeed degenerate,
but it is probably because of disease which from their
method of propagation is difficult to eradicate, and not

32 INBEEEDINa AND OUTBBEEDINa

because of the method itself. Again, if one desires further
evidence of descent in a single pure hereditary line con-
sistent with high specialization and inherent vigor
through long periods of time, there is the phenomenon
of apomixis to be cited. Apomixis is a general term for
certain reproductive anomalies in plants which are really
a return to vegetative reproduction. In a broad way it is
synonymous with parthenogenesis in animals; but par-
thenogenesis in animals includes only reproduction from
an unfertilized egg, while apomixis takes in reproduc-
tion from certain cells which are not eggs. Some twenty
or thirty species of vascular plants have already been
found to reproduce in this manner, and unquestionably
the list is very incomplete. Examples from Polypodiacece,
Ranunculacece and Rosacece are not uncommon, but in
particular it is the Composited, the highest group of flow-
ering plants, which seem inclined to make this method of
reproduction a habit. Of course, one cannot insist that
such a return to primitive reproductive methods even by
a more modem labor-saving route is wholly for the good
of the species concerned. No one in possession of all of
the facts could maintain the change to be progressive, or
argue that the species adopting it will have a great future
as future is measured by the evolutionist. This is not
the contention. We merely cite the adoption of apomixis
by flourishing genera of the most specialized and highly
developed plants as examples of asexual reproduction
over long periods without visibly harmful effects. We do
this because we believe the emphasis put by Darwin and
his followers on supposed ill effects following any type
of inbreeding or asexual propagation was misplaced.
Certainly the majority, the great majority, of the higher

ANIMAL AND PLANT REPRODUCTION 33

plants returned to a type of reproduction which held all
the advantages of bisexuality by evolving means for pro-
moting cross-fertilization. But it is the advantage of
cross-fertilization and not the assumed disadvantage of
self-fertilization that should be stressed. The Knight-
Darwin Law, ^'Nature abhors perpetual self-fertiliza-
tion/' should read Nature discovered a great advantage
in an occasional cross- fertilization.

The higher plants made a success of hermaphroditism
because there was a return to the advantages of gono-
chorism through the development of almost innumerable
devices tending to promote frequent crossing between
plants of the same and nearly related species. Some
species did actually return to true structural gonochorism,
but in most cases other means of obtaining cross-fertiliza-
tion were developed. There was no advantage, consider-
ing their sessile mode of life, in relinquishing the
possibility of self-fertilization.

Some of the various cross-fertilization mechanisms
utilized are very reminiscent of those of animals. Monce-
cism, the production of male and of female flowers on the
same plant, and dichogamy, the maturation of the male
and female organs at different times, have their counter-
parts in the other kingdom. So also the physiological phe-
nomenon self-sterility of which only one instance is known
among animals is very common among plants. Some
hundred or so species distributed throughout thirty-five
or more families have been shown to be self-sterile, al-
though the true number is probably many times this
figure. Again polygamy, where, in addition to hermaph-
roditic flowers, either male or female flowers are devel-

3

34

INBEEEDINQ AND OUTBREEDING

oped, has its analogue in the complemental males charac-
teristic of the Cirripedes.

But by far the most numerous and most iateresting
adaptations for cross-pollination are characteristic of
plants alone. These are the thousands of structural modi-
fications which utilize external agencies. Wind and water
have not been despised, but the real servants — ^they are

FiQ. 8. — Adaptation for oroBa-pollination, transference of pollen by insects. (After Kerner.)

not slaves for they are paid for their services — are the
lower animals and in particular the insects.

The ideas of Darwin resulting in the tremendous labors
of Miiller,Delpino, Kerner, Knuth and others have made it
no longer necessary to describe the facts concerning the
dispersal of pollen by animals.^^** ^^'^ The subject has
been so fascinating that it is common knowledge how the
insects are attracted to flowers by odor and by color ; how
they are rewarded for visits by nectar and by pollen ; how

ANIMAL AND PLANT REPRODUCTION 35

provisions are made to use them as pollen carriers
through numberless modifications of calyx, corolla, sta-
mens and pistil; how the animals themselves have devel-
oped organs for extraction of food or for attachment to
the blossoms (Fig. 8). Perhaps some of these mutual
adaptation mechanisms are a little fanciful, but the fact
remains that actually an occasional or a frequent cross-
poUination is secured by a majority of our 100,000 or more
species of flowering plants by means of insects, and the
hundreds of mechanisms by which it is obtained are wit-
ness of its paramount importance.

The thesis of this chapter, then, is simple. The whole
trend of evolution in both animals and plants as regards
all the mechanisms in any way connected with reproduc-
tion, has been such as to provide effectively for continuous
descent. In the midst of strenuous competition for place,
those organisms which were able to cross with others, at
least occasionally, held such an advantage over those
which were compelled to continue through one single line
of descent, that their descendants have persisted in greater
numbers. They have dominated the organic world. Any
satisfactory interpretation of the effects of inbreeding
and outbreeding must permit a reasonable explanation of
this situation.

CHAPTER III
THE MECHANISM OF EEPEODUCTION

Theee is a division of labor in all the higher plants and
animals, the result of setting apart definite tissues for
producing germ cells. In addition, another important
matter is accomplished. The germ cells are insulated
from ordinary environmental changes, and are enabled to
go through a very exact routine of processes in prepara-
tion for the formation of the new organism — the zygote.

In general the animal body or the sporophyte of the
higher plants can be considered as a double organization.
Various parts make up each of the cell units ; but of them
all the nucleus, and within the nucleus the chromosomes,
seem to be the most important. Each species has a char-
acteristic and constant number of these bodies, and it is
their distribution which parallels — and probably regu-
lates— ^the distribution of the hereditary differences
within a species. The double organization of the bodies
of the higher organisms is dependent upon the receipt of
one set of these chromosomes from each parent. And it
is the peculiar method by which these chromosomes are
apportioned to the gametes, together with experiments on
the actual distribution of characters in the generations
succeeding a cross, which have given us a fairly clear
idea of heredity as a mechanical process.

In ordinary cell division during growth each chromo-
some divides longitudinally so that both daughter cells
apparently receive an exact half of the chromatin, al-
though possibly some sort of a special apportionment is

36

THE MECHANISM OF REPRODUCTION 37

made in the segregation of particular tissues. But when
the germ cells are formed, at spermatogenesis and oogen-
esis, the chromosomes unite in pairs, a process technically
known as synapsis, and at division one member of each
pair passes entire to one of the two new daughter cells,
thus reducing the number of the chromosomes in the
gamete to one-half of those possessed by the body cells.
Subsequently there is an equating or halving division
similar in appearance to the cell divisions in ordinary
growth. Four gametes are thus formed. Leaving out of
account the behavior of certain chromosomes believed to
control the distribution of sex, there is good evidence that
this union of chromosome pairs at synapsis always takes
place between two chromosomes, one of which had been
received from the father and one from the mother. In
other words, it seems clear that each gamete obtains one
of each hind of chromosome, although it is a matter of
chance whether the cell receives the maternal or paternal
representative of any type. Thus, if the chromosomes
of the body cells of a particular species are six in number,
and we represent them as ABC ahc, regarding A, B, and
C as of maternal and a, b, and c as of paternal origin, at
synapsis A only pairs with a, B with b and C with c.
This procedure, however, will yield eight types of gam-
etes, ABC, ABc, AbC, aBC, Abe, aBc, abC, and abc, siuce it
is a mere matter of chance which daughter cell receives
either member of any pair.

In spermatogenesis four sperms are formed from each
immature germ cell, but in oogenesis — the maturation of
the egg — only one functional gamete is produced, the
other three being aborted. Nevertheless, the two processes

38

INBREEDINa A^D OUTBREEDING

are similar in all essential features, as may be seen in Fig.
9, the elimination of three out of four of the oocytes taking

SPSRMATOOKNBSIS

I

$pj5.jto.

Spermatocyte

Secondary
— rwato-
cytes

Spertnato

imr

Spera-
uosoa

Multiplication
p*riod

056EN£SIS

i

.^

(^ h.

Odgonia

•■ Growth period -*

Pairing of Chrorsosomes
Reducing division J

\

/ \

Primary oocyte

Secondary oocyte

(cvoft) and f iret

polar body)

Mature ovup ^,
and poiar bodies

Mature ovin

WUn* 0 f) 1^^

. mmber cS

Fio. 9. — Diagram of gametogenesis showing the parallel between maturation of the aperm
cell and maturation of the ovum. (After Guyer.)

place in order that their store of nutritive materials may
go to make one large egg.

Fertilization consists in the fusion of one egg with one
sperm, thus bringing back the double number of chromo-

THE MECHANISM OP REPKODUCTION 39

somes characteristic of the body cells (Fig 10). and since
it is a matter of chance what gametes unite, such gametic
differences as we have illustrated would give a possibility
of obtaining 8 x 8 or 64 types of zygotes.

FiQ. 10. — Diagram to illustrate fertilization; & , male pronucleus; 9 , female pro-
nucleus; observe that the chromosomes of maternal and paternal origin, reepectively, do
not fuse. (After Guyer.)

The mechanism of the process of gametogenesis and
fertilization in animals need not concern us further here.
We must speak of the process in the seed plants, however,
for a rather odd phenomenon occurs there to which there
will be occasion to refer later.

40

INBEEEDING AND OUTBREEDING

Reduction of the chromosomes takes place in plants
just as it does in animals, but the introduction of a gamete
generation, the gametophyte, complicates matters. In
the seed plants, pollen mother cells are produced in the
anthers of the flower which go through precisely the
same divisions as in animal spermatogenesis (Fig. 11).
But each of the four nuclei thus produced divides during

Fig. 11. — Formation of pollen grains in the lily. B, stages in the formation of pollen
grains in a group of four (tetrad) within the pollen mother cell; C, mature pollen grain
with early stages in the development of the male gametophyte; t, tube nucleus; g, generative
nucleus. (After Bergen and Davis.)

the formation of the pollen grain, forming a generative
and a tube nucleus. The tube nucleus it is that germinates
and passes down the style when the pollen grain falls on
a ripe stigma. During this period of pollen tube growth
the generative nucleus passes down through it toward the
ovule, and while so doing divides again, leaving two nuclei
each wdth a function to perform. One fuses with the egg
and the other with the so-called endosperm nucleus, com-

THE MECHANISM OF REPRODUCTION 41

pleting in this manner the peculiar double fertilization
characteristic of the angiosperms.

In the meantime, the egg and the endosperm yiucleus
have been prepared by the
accessary cell divisions of the
female gametophyte. The
reduction division occurs in
the usual manner, and as in
animals three of the cells are
absorbed, leaving a single
one to provide for the
hereditary succession. Its
container enlarges and be-
comes the embryo sac, while
the cell itself typically goes
through three cell divisions
resulting in the formation of
eight nuclei. Any of these
nuclei may become the egg,
but generally the egg can be
recognized by its position
(Fig. 12). Two others from
among these nuclei fuse to-
gether and become the endo-
sperm nucleus, which in turn
fuses with the second male
nucleus and by succeeding
cell divisions forms the en-
dosperm of the seeds, the function of which is to furnish
food for the young plant, the embryo. Thus, if we
represent the chromosome complex of the gametes by x,
the embryo is 2x, and the endosperm 3a;.

Fertilization in the embryo sac
e, egg; fs, first ppcrm; pv,
fused polar nuclei ; ss, second sperm.
(After Bergen and Davis.)

Fia. 12
of the lily

42

INBEEEDING AND OUTBREEDING

It is clear from this short description of gametogenesis
and fertilization that the processes in plants and in ani-
mals are identical in what we deem to be the essential
features, the behavior of the chromosomes. If one visual-
izes the behavior of hereditary characters in crosses in
which the parents differ as the result of the operation of
potential factors carried by these bodies, he can correlate

i

J

•V

'■iW T

Fig. 13. — Entrance of the spermatozoon through the membrane of the egg of a etar-
fish giving an idea of the difference in size between the sperm and the egg. (Wilson's "The
Cell." Courtesy Macmillan Co.)

every fact thus far discovered, with the exception of a
few isolated cases found in plants where particular char-
acteristics appear to be distributed by the cytoplasm lying
outside of the nucleus. Not only can the distribution of
ordinary characters be interpreted as functions of the
chromosomes, but the distribution of the sexes as well.
There is reason to think the behavior of the sex-control-
ling chromosomes may perhaps occasionally be influenced
by external conditions, but sex itself is determined by the

THE MECHANISM OF REPEODUCTION

43

behavior of particular chromosomes of which we have not
hitherto spoken (Fig. 14).

The evidence in favor of this view of the determination
of sex at the time of fertilization through the chromosome
complex is from several very different sources.

First, there is the phenomenon of multiple births

Protevior cf

•••

B

••••••

Fig. 14. — Diagram showing the distribution of the sex chromosome in Protenor. (After

Morgan.)

among mammals. In general, animals in which this is
the rule, bear both males and females, through all of the
individuals must have been under the same environmental
conditions. There are multiple births, however, in which
the young are invariably of the same sex. Such is the
case with those remarkably similar human twins kno^^^l as
identical twins. Such is the case with the four young in
each litter of the nine-banded armadillo (Fig. 15). Now

44

INBREEDINa AND OUTBREEDING

it can be shown that in these two and other similar in-
stances, the several young are the product of a single
fertilized egg which so develops as to form two or four
complete individuals. If sex was determined after fer-
tilization, one might expect a random sample of the two
sexes here, but this is not the case.

I

FiQ. 15. — Identical quadruplets in the nine-banded armadillo.

Doncaster.)

(After Newman from

The chief support of this idea of sex determination,
however, comes from the microscope and the breeding
pen. In an ever increasing number of species, possibly
including man himself, it has been found that besides the
regular paired chromosomes, the autosomes, there is a
single chromosome or possibly a chromosome group com-
monly known as the x-chromosome, whose behavior in cell
division is somewhat different from the others, and whose

THE MECHANISM OF REPRODUCTION 45

distribution absolutely parallels the distribution of sex.
There are two types. In the males of animals of Type A,
which includes numerous flies, beetles, grasshoppers and
bugs from among the insects, as well as representatives
from several orders of mammals, a single x-chromosome
is present in addition to the regular chromosome pairs,
and for this reason two kinds of spermatozoa are pro-
duced at spermatogenesis in equal numbers, those pos-
sessing the extra element and those without it. In the
females, on the other hand, two of these elements are
present and the eggs, therefore, always possess it. Thus,
on fertilization, half of the resulting young have two
x-chromosomes and these become females, while half ha^^e
but one and become males.
Diagrammatically it is this :

Ovum with x fertilized by sperm mth x = female.
Ovum with x fertilized by sperm without x - male.

In some other cases (Type B), the eggs are di-
morphic, while the sperm are all alike, but the result
is the same ; the sex distribution follows the chromosome
differentiation.

In dioecious plants there is some evidence of a similar
condition. Strasburger ^^^ found in one of the liverworts,
Sphaerocarpus, where the four spores produced by a single
spore mother cell hang together and each such tetrad
could be planted separately, that invariably two males and
two females were produced. More recently Allen ^ has
presented evidence of an x-chromosome in this genus. His
discovery was made with material of the species Spliwro-
carpus Donnellii, but it has been corroborated by one of
his students working with Sphccrocarpus texanus.

f^

46 INBREEDING AND OUTBREEDING

Again, in the dioecious moss, Funaria, the Marchals;^^^
by a remarkable series of regeneration experiments, have
proven the determination of sex at the reduction division.
Each spore was found to contain the potentialities of but
one sex, but in the sporophyte they demonstrated the po-
tentialities of both sexes by inducing direct aposporous
development of gametophytes, which proved to have both
antheridia and archegonia, the organs of both sexes.

The situation in hermaphroditic plants and animals
is not so clear. Particularly in plants the peculiar life
history with the introduction of alternation of generations,
makes experimental work exceedingly difficult. Further-
more, there are many species of animals where the sex
ratio is nowhere near equality and where both external
and internal conditions undoubtedly do have marked in-
jfluence, but in such a fundamental phenomenon we can
hardly believe these difficulties are insurmountable or
will lead to any radically different interpretation of the
problem. "Where there is such clear evidence fromvery dif-
ferent modes of attack and upon species so unrelated one
is constrained to believe the obstacles to a unified theory
are only superficial. This is particularly true since there
is another line of experimental evidence in favor of the
determination of sex by the chromosomes. Our whole
evidence on inheritance, in f a-ct, is linked up with chromo-
some distribution, so that the easiest way to visualize the
process is by supposing that the individual potentialities,
the factors, which cooperate in the development of plant
and animal characters, are disposed in a definite manner
in the chromosomes, as we shall see in the next chapter.
The particular discoveries which demand our attention in
connection with the phenomenon of sex, however, are

THE MECHANISM OF REPKODUCTION 47

those regarding characters commonly known as sex-
linked, whose distribution can be accurately predicted
if we assume they are definitely coupled with the
sex determiner.

Such a character is hereditary color-blindness in maji,
a condition in which the affected individual cannot dis-
tinguish between red and green. It is far commoner in
man than in women, and its inheritance is so peculiar that
it often seems to skip a generation.

A color-blind man married to a normal woman will
have only normal children of either sex. The sons will
never have color-blind progeny by women with nonnal
vision, but the daughters, though married to normal men,
will transmit color-blindness to one-half of their sons.
If, moreover, a daughter mates with a color-blind man,
as might frequently happen in marriage between cousins,
on the average one-half of her daughters as well as one-
half of her sons will be abnormal.

This interesting and apparently complicated inheri-
tance is really very simple if we merely assume that the
sex chromosomes of the color-blind individuals also carry
the determiner for color-blindness. Fig. 16 shows what
would be expected. Eepresenting the normal vision by
boldface type and color-blindness by outline we see first
the result of mating a normal woman with a color-blind
man. Since all of her sex-cells, when matured, contain
one normal x-element, and since the sex-cells of the
male are of two kinds, half containing an abnormal or
color-blind determining x-element and half containing no
x-element whatever, it is obvious that the sons must re-
ceive their x-element only from their mother and the
daughters must receive one of their x-elements from their

48

INBEEEDING AND OUTBREEDING

father. The sons, therefore, cannot be color-blind and
cannot transmit color-blindness, but the daughters, though

(Normal)

VaU tin*
(Color bUDd)|

of parenVs

o( paresU

fiodjr-cellt
ot chlldreo

Sex-cellt
ot cbildroa.

Body-cells
of ^rand-
«bildrea

FiQ. 16. — Diagram illustrating the inheritance of a sex-linked character such as
color-blindness in man on the assumption that the factor in question is located in the sex
chromosome. The normal sex chromosome is indicated by a black X, the one lacking
the factor for color perception, by a light X. It is assumed that a normal female is mated
with a color-blind male. (After Guyer. Courtesy Bobbs Merrill Co.)

they will not be color-blind themselves because one normal
x-element is sufficient to determine normal vision, will
produce defective x-elements in one-half of their ova,

THE MECHANISM OF REPRODUCTION 49

and for this reason will transmit color-blindness to one-
half of their sons by a normal man, as will be seen by
following out the fourth and fifth columns in the diagram.
An egg containing the normal x-element can meet a sper-
matozoon carrying an x-element and thus produce a
daughter, or it may meet a spermatozoon with no x-ele-
ment and thus produce a son; but in either case the chil-
dren will have normal vision. On the other hand, an egg
containing a defective x-element will by similar fertiliza-
tions result either in a normal-visioned daughter, who will
carry color-blindness in half of her ova, or in a son who
will be color-hlind.

Such a scheme of interpretation might seem quite
visionary were it not for the fact that similar types of
inheritance occur in many of the lower animals. By care-
fully controlled experiments with them it has been proven
beyond a doubt.

CHAPTER IV
THE MECHANISM OF HEEEDITY

The scientific era in the investigation of heredity be-
gan in the latter half of the nineteenth century with the
57ork of Galton and of Mendel. Both enthusiastic and
competent investigators, their efforts made with differ-
ent material and from diverse points of view, did not
fare the same. Galton measured the inheritance of groups
of individuals by their resemblance to their progenitors
and failed because his method could not take into account
the true relationship between the germinal constitution
and the body characters of an individual ; Mendel deter-
mined the inheritance of a single organism by making the
characters of its progeny the criterion and succeeded.
Without knowledge of the cell mechanism of gameto-
genesis and fertilization, Mendel described the results of
his hybridization experiments in terms which agreed pre-
cisely with these later discoveries in the field of cytology.
Mendelian heredity has proved to be the heredity of sex-
ual reproduction: the heredity of sexual reproduction
is Mendelian.

Progress in the study of heredity through investiga-
tions patterned after Mendel's model has been so great
that the subject now forms an important sub-division of
general biology — Genetics. The details of the subject
have outgrown the limits of a single volume, and a knowl-
edge of the generalities is no longer confined to the pro-
fessional biologist. For such reasons we propose to

60

THE MECHANISM OF HEREDITY 51

discuss here only the broader relationships of Mendelian
heredity to the behavior of the chromosomes, since this
phase must be emphasized as a basis for correlating the
facts from Nature's experiments on inbreeding and out-
breeding with the results from the experiments made
by man.

The Mendelian method of studying heredity consists
essentially in crossing forms which differ by well-defined
characteristics and in following the distribution of these
characteristics separately and quantitatively in the suc-
ceeding generations. If a wheat with a long lax head or
spike is crossed with one having a short dense spike the F,
(first filial) generation bears intermediate spikes. The
Fi generation, self-fertilized, however, yields all three
types — long, intermediate and short spikes — in the F2
generation ; and in large numbers these types bear a con-
stant ratio to each other in the proportion 1 long spike :
2 intermediate spikes : 1 short spike. Nor is this all. The
long-spiked plants all breed true to long spikes, the short-
spiked plants all breed true to short spikes, while tlv
plants bearing intermediate spikes again produce the
ratio exhibited by the F^ generation. Diagrammatically
the result of the cross is as follows :

Pj Long spikes x Short spikes

I
Fj Intermediate spikes

^-^ II

F„ Long spikes Intermediate spikes Short spikes

I ^ W ^^ I

Fg Long spikes Long spikes Inter- Short spikes Short spikes

mediate
spikes

52

INBREEDING AND OUTBREEDING

If the description of the dual nature of the cells of
plants and animals and the result of gametogenesis is
recalled, the reason for the production of the ratio of 1
long spike : 2 intermediate spikes : 1 short spike in the F2
generation is plain. Furthermore, it is clear why the
types like the grandparents breed true and the type like
the hybrid F^ generation does not breed true.

The long-spiked wheat has received the factor for long
spikes, the something in the germ cell that stands for the
production of long spikes, from hoth of its parents ; there-

Feviale gantete

\likU gamete

Fia. 17. — Diagram showing the union of like gametes.

fore it breeds true to long spikes. The gametes which it
produces all bear the factor for long ears.

The diagram illustrating the fusion of the parental
gametes, shows why the long-spiked wheat produces
gametes, each of which bears the factor for long spikes.
If the letter 8 is substituted for the letter L in the dia-
gram, the same illustration holds for the short-spiked
wheat. But what happens when the long-spiked variety
is crossed with the short-spiked variety? A gamete bear-
ing L fuses with a gamete bearing ^S' and a zygote LS is
formed. The interaction of the factors L and S produces
an F^ plant bearing intermediate ears. "When this hybrid

THE MECHANISM OF HEREDITY

53

comes to produce gametes they bear either the one or the
other — and never both — of these factors. In other
words, the germ cells (both male and female) of the hy-
brid are half of them L and half of them S. When the
F^ generation is selfed, therefore, it is a matter of
chance which of these geiTa cells meet to form zygotes.
If a large progeny is produced, there will be a ratio of

1|L|L| : 2|L|/S'| : IIaS'IiS'I, and since the formulas L\L\ and

S\S\ are like the zygotic formulae of the long-spiked and
short-spiked parents, respectively, the plants that they
produce will be long-spiked and short-spiked, as the case
may be, and will breed true to that character. The inter-
mediates, however, having been produced by zygotes

L S lilie the F^ generation, will behave in the same man-

ner when selfed.

That the ratio will be approximately 1 L\L : 2 L S\

1 8 S is plain if one thinks for a moment what the result

would be if a thousand tickets bearing the letter L and a
thousand tickets bearing the letter S were shuffled up in a
hat and drawn out in pairs, replacing the pair each time
after drawing and recording. Suppose the first member
of the pair represents the egg cell; the chances are %
that it will be L or S. The second member of the pair
represents the male cell and the chances are likewise %
that it will be L or ^S'. Therefore, when L is the first mem-
ber of the pair, half of the time the zygote formed will be

L L and half of the time it will be L S , Likewise,

when 8 is the first member of the pair, zygotes \S\L\

and S S| will be formed in equal quantities. Combining

these possibilities, the ratio 1|-^|L| : 2|L|5'| : 1|<S'|<S'| is

54

INBREEDING AND OUTBREEDING

obtained. Diagrammatically the expected production of
gametes and zygotes is as follows :

^ gametes

Jj zygote

Eggs

R gametes

L S

s '

S S L S

L S

Pa 2ygote

LL LS SL SS

I I I 1 1 I 1 —

Fia. 18. — Diagram to illuatrate Menddiam in a crosa between long-spiked and short-spiked

wheat.

Let us express the whole matter mathematically.
The chance of an event happening in an infinite num-
ber of trials is expressed by a fraction of which the
numerator is the number of favorable ways and the de-
nominator the whole range of possibilities, both favor-
able and unfavorable, if each is equally likely to occur.
Hence, certainty is expressed in the figure 1. Further-
more, the chance that two or more independent events
will happen together is the product of their respective
chances of happening. The chance of an L egg meeting

THE MECHANISM OF HEREDITY 55

an L sperm is % x % = 14, and the chance of an L egg

meeting an 8 sperm is % x 1/2 = %• Similarly, the chance

of an S egg meeting an L sperm is % ^^^ of an L egg

meeting an S sperm %. The sum of the possibilities

is then

L +L = 1/4
6'4-L = l/2
S-{-S =1/4

One important thing to be remembered, however, is
that the law of chance expresses the probability when in-
finitely large numbers are concerned. Wlien small num-
bers are dealt with, departures from the expected ratios
are obtained. It is a matter of common sense that al-
though the chances of throwing heads with a penny are 1/2,
yet in a small mmiber of throws exactly % of them may
not be heads. A regular ratio of these departures is to be
expected which accords with the law of error.

Typically, experiments such as this are the basis for
MendePs Laws of Inheritance, the first of which may be
stated as follows : U-nit factors contributed by tivo parents
hailing definite roles in the development of characters,
separate in the germ cells of the offspring without having
influenced each other. Although Mendel himself knew of
no mechanism by which such a process could take place,
although his theory was evolved wholly as a fitting inter-
pretation of the facts obtained by breeding, what can be
more reasonable than to suppose that the germ cell factors
reside in the chromosomes and that the separation of the
chromosomes at the reduction division in gametogenesis
furnishes the segregation of factors required?

The interaction of a pair of homologous factors in a
hybrid — allelomorphs they are called — does not always

56 INBEEEDING AND OUTBREEDING

result in the production of an intermediate. Often the
action of one factor dominates the action of the other,
either by masking it or by inhibiting its operation. When
this occurs the dominated character recedes from sight in
the Fi generation and the ratio in the F2 generation is
3 dominant : 1 recessive. But since only one-third of these
dominants breed true * and two-thirds behave as did the
Fi generation, the results are, therefore, comparable with
those illustrated by the wheat, and the phenomenon of
dominance is a mere detail.

Mendel was not content with experiments in which
only one pair of differentiating characters was concerned.
He made crosses between varieties of the garden pea
which differed by two and by three allelomorphic pairs of
characters, and was rewarded for his perseverance by
discovering a second law, usually known as the Law of
Recombination. This laiv states that two or more allelo-
morphic pairs of factors may segregate independently
and may recomhi<ne in all the combinations possible gov-
erned by chance only.

Though thousands of characters have been investi-
gated since MendePs time one cannot improve on the
original classic as illustrative material for explanation
of dihybrid heredity.

With two pairs of characters he designated the factors
representing the dominant characters as A and B and the
factors representing the recessive characters as a and b.
The characters in the varieties crossed were as follows :

Seed parent ( A form round Pollen Parent J a form wrinkled

AB } B cotyledon yellow ab } b cotyledon green

a When the factor for a character has been received from both parents
the organism is said to be homozygous for it; if it has been received from
only one parent the individual is heterozygous or hybrid for it.

THE MECHANISM OF HEREDITY 57

When these two forms were crossed all of the hybrid's
seeds appeared round and yellow (AB) like those of the
seed parent; that is, the round character and the yellow
character were each dominant.

When these F\ seeds were sown and the plants self-
fertilized, four kinds of seeds appeared in the progeny
in the four combinations that were possible, with the
following numbers of each :

AB round and yellow 315

aB wrinkled and yellow 101

Ab round and green 108

ab wrinkled and green 32

These figures stand approximately in the relation of
9AB to SaB to SAb to lah. The forms appeared to
belong to but four homogeneous classes (phenotypes).
This was due to the phenomenon of dominance masking
the difference between homozygotes and heterozygotes.
By their behavior in the next generation they were found
to belong to nine really different classes.

From the round yellow seeds (apparently AB) were obtained by
self-fertilization :

(1) A ABB seeds all round and yellow 38

(2) AABb seeds all round, yellow and green 65

(3) AaBB seeds all yellow, round and wrinkled 60

(4) AaBb seeds round and wrinkled, yellow

and green 138

From the round and green seeds (apparently Ab) were obtained:

(5) AAbb seeds all round and green 35

(6) Adbb seeds all green, round and wrinkled 67

From the Tvninkled and yellow seeds (apparently aB) were obtained :

(7) aaBB seeds all wrinkled and yellow 28

(8) aaBb seeds all wrinkled, yellow and green 67

From the wrinkled and green seeds (apparently ab) were obtained:

(9) aabb seeds all wrinkled and green 30

58 INBREEDING AND OUTBREEDING

The total number of plants in tlie F^ generation is not
quite the same as the F2 generation due to seed not
germinating or plants dying, but it is plain that the ratio
of 9AB : 3aB : 3Ab : lab obtained in the F^ generation
is made up of the following actual classes :

9 apparently AB made up of

3 apparently Ab made up of

fl AABB
2 AaBB
2 AABh
4 AaBb

1 AAhb

2 Aabb

a auBB
3 apparently aB made up of jo oaBb

1 apparently ah made up of 1 aabb

The four classes AABB, aaBB, AAhh and aabh, hav-
ing each factor in the duplex condition bred true; the
heterozygous condition of one or both allelomorphs
in the remainder was shown by the character of the
F^ progeny.

In order to visualize these facts, or in truth the facts
from any number of pairs of allelomorphs in independent
Mendelian inheritance, one has only to recall that pairs
of homologous chromosomes — one paternal and one ma-
ternal— meet during the process of gametogenesis and
one or the other of each pair passes to either daughter
cell. The factors A and a lie in corresponding loci in
one pair of chromosomes, the factors B and b lie in cor-
responding loci in a second pair from among the seven
paired chromosomes of the garden pea. Thus gametes
bearing the factors AB, Ab, aB and ab will be formed in

THE MECHANISM OF HEREDITY

59

equal quantities in both, the egg cells and the pollen cells,
as is shown by the accompanying diagram.

I H

a
B

A

b

FiQ. 19. — Diagram to illuBtrate gamete formation in a dibybrid in independent inberitanoe.

It is clear that if this process occurs in both male and
female germ cells, and these germ cells unite by chance,
the result is that obtained in the breeding experiments.
The posssible matings can be shown graphically as follows :

A B

A b

Eggs

a B

a b

A B

Sperms
A b a B

a b

A B

A B

A B

A B

A B

A b

a B

a b

A b

A b

A b

A b

A B

A b

a B

a b

a B

a B

a B

a B

A B

A b

a B

a b

a b

a b

a b

a b

A B

A b

a B

a b

60 INBREEDING AND OUTBEEEDING

A large series of results on independent Mendelian
heredity obtained during the past eighteen years by
numerous biologists can be interpreted in the same man-
ner. At first glance many of their results appeared to be
either very complex or very irregular, but one by one
they were shown to be just as simple as the cases given.
Considering only instances which may be interpreted by
two factors, for example, we have i^g ratios of 12: 3: 1,
9:7, 9:3:4 and 13 : 3. But these are not difficult to ana-
lyze. They are simply the ordinary 9:3:3:1 ratios in
which part of the terms are combined in various ways.
For example, the F2 ratio, when certain black beans are
crossed with certain white beans, is 12 black : 3 yellow : 1
white. Clearly this is because black + yellow {AB) is in
appearance not different from black {A) alone. A purple
sweet pea crossed with a certain white variety segregates
in a ratio of 9 purple : 7 white. This result is easily ex-
plained by assuming that the purple color only appears
when a color base factor, A^ is present in connection with
a color-producing factor, B, The last three terms of the
dihybrid ratio, '^AB : ?taB : lah, therefore, are alike in
appearance. This assumption has been proved to be cor-
rect in other ways. Again if a black variety of rat, mouse,
guinea-pig or rabbit be crossed with a white variety car-
rying a factor for ticking the hair with yellow, known as
agouti, the segregating ratio is 9 agouti : 3 black : 4 white.
The reason for the combination of classes Ah and ah is
because the agouti factor does not show except in the pres-
ence of color {B), Finally, a factor A which inhibits the
action of B and therefore makes AB and Ah resemble ah,
gives the peculiar ration 13 : 3. Crossing a certain race of

THE MECHANISM OF HEREDITY 61

white fowls with colored races, for instance, gives the
ratio 13 white : 3 colored.

It is evident, if one runs over these examples and ^vorks
out all the possibilities involved, he will find that two
white races of sweet pea, when crossed, will give purples
in the F^ generation, a white race of guinea-pigs, crossed
with a black variety, will give all agouti, etc. Such curi-
ous results are actually obtained. They are quite simple,
and their whole heredity may be visualized by the use of
the same chromosome scheme as given above. Of course,
some of them require the assumption of differences in
more than two allelomorphic factors, but this can be done
by remembering that additional factor pairs follow the
same mathematical scheme as do one or two pairs.

No matter how satisfactory it would be to have all
biological facts interpreted with a primer simplicity,
the truth is that animals and plants are complex organ-
izations. Probably only the tiniest fraction of the germ
cell constitution of any organism has ever been analyzed
through Mendelian methods, yet in the pomace fly
Drosophila melanog aster, in which there are only four
pairs of chromosomes, Morgan and his associates have
traced the hereditary transmission of well over one hun-
dred factors, each of which has one or more functions to
perform in the development of characters in the adult.
It is obvious that with such a large number of characters
and such a small number of chromosomes, a single chro-
mosome must carry many factors. This conclusion
granted, it would seem as if any of these groups of fac-
tors carried by a single chromosome would necessarily
behave as single factors ; in other words, they would enter

62 INBREEDING AND OUTBREEDING

a cross in a group and be segregated in a group in the
F2 generation.

Such cases have appeared in breeding experiments,
but they are very rare and are probably not what they
seem, because of an insufficient number of individuals
from which to draw conclusions. What usually happens
is for these sets of factors to tend to hang together at the
reduction division in the F^ generation. They tend to be
linked, but the linkage is often broken.

An example from Morgan's work on the pomace fly
will make this clear. If a female fly with black body and
vestigial wings be crossed with a wild male having a gray
body and long wings, the result is offspring like the male,
gray body and long wings being dominant. Now, since
these Fi individuals have one chromosome containing the
factors for black body and for vestigial wings, and a
homologous chromosome containing the factors for gray
body and for long wings, one would expect gametes of
only two kinds to be formed at the maturation of the eggs
and sperms. If this were true, an F^ generation ob-
tained by mating a male and a female from the F^ genera-
tion should consist of 3 flies having long wings and
gray bodies to 1 fly having vestigial wings and a black
body. But this is not the result obtained. In addition to
large numbers of flies of this type, there are smaller num-
bers of flies characterized by long wings and black bodies,
and by vestigial wings and gray bodies. Such a result,
on a chromosome basis, could only be obtained through the
homologous chromosomes interchanging their factors at
the reduction division.

There is good cytological evidence that such an inter-
change of chromosome parts does take place at game-

THE MECHANISM OF HEREDITY

63

togenesis. At the time when the homologous pairs of
chromosomes approach each other just previous to the
reduction of the chromosome number to half, they twist
around each other. Often they retain their individuality
as they pass to the daughter cells; but sometimes they
break at various places, join their parts in a different
combination and pass to the daughter cells in their new
guise. The diagram will make this clear.

A

<-♦

a

will givd

A

and

a

B

i-*

b

B

b

op

A

\ /

a

A

a

\y.\/ wiJI A{„»

and

b

X

\

9

b

6

FiQ. 20. — Diagram to illustrate gamete formation in a dihybrid in linked inheritance.

The easiest way to determine the frequency with which
these breaks in linked characters occur, and a way which
gives it in terms of chromosome crossovers is to mate Fi
individuals back to the double recessive type. When the
Fi male in the cross just described is mated with a black
vestigial female, only two classes of offspnng are pro-
duced ; half are black vestigial and half are gray long in
type. The F^ male produces only two kinds of gametes.
There is, therefore, no crossing over between the chromo-
somes of the male.

On the other hand, when the F^ female is mated with
a black vestigial male, four types of offspring are
produced :

Non-crossovers Crossovers

Black vestigial Gray long Black long Gray veslig-ial

41.5 per cent. 41.5 per cent. 8.5 per cent, 8.5 per cent.

64

INBREEDINa AND OUTBREEDING

^yn

Gametes of Pj

QZ^^

Individuals of Fj
Gray Long

Gametes of Male
All direct segregates

C-ametes of Feinal(
Direct sigre^ates

S^Z3 r^"^

Double recessive

blacVt vestigial

used m bacV< crosses

black
Long
50. OJ^

Gray
vestigial

so.oji

black

Lone

41.5^

Gray

black

vestigial vestigial
41.5^ B,5%

Gray

Long
8.5%

FiQ. 21. — Diagram to illustrate linkage breaks or "crossing-over" between the loci
containing the factors for the allelomorphic pairs. Gray body, black body and long wing —
vestigial wing in Drosophila. (After Morgan.)

A complete visualization of this series of matings is
given in Fig. 21.

THE MECHANISM OF HEREDITY 65

The peculiar fact tliat there is no crossing over in
the males need not concern us here. In other species
there is evidence of crossing over in both sexes. What is
important is that crossing over does occur with a definite
frequency and this frequency is constant for any par-
ticular pair of characters except when modified in various
ways which can be given concrete explanations. It does
not matter, moreover, whether the two factors enter the
cross as ^'^ or as ^j^ , crossing over is the same in each
case; that is, the tendency is just as great for Ah to stay
together after they are in that combination as for AB to
stay together when that particular combination charac-
terizes the individual.

The gross result of several thousand experiments of
this character, therefore, is to associate Mendelian in-
heritance more definitely than ever with the chromosomes.
What seemed to be an exception, furnished striking evi-
dence in support of the theory. It is true, a few isolated
instances of characters which appear to be carried by
cell substances other than the chromosomes have been
discovered ; but it is pretty clear that in the main all of the
varied characters which differentiate the individuals
within a single species — and these are the only ones
which can be studied profitably through crosses — are con-
trolled by the distribution of factor units which lie
within the chromosomes.

We can visualize the whole process of heredity by
means of chromosome diagrams just as we can visualize
the whole process of chemical recombination through
models of the atoms constructed to fit the facts furnished
by chemical reactions. The result is that we can do much
toward predicting what will happen under given con-

5

66 INBEEEDING AND OUTBREEDING

ditions because we know wliat has happened under sim- !>
ilar conditions. For example, it having been determined
that in the pomace fly; many characters are linked to chro-
mosomes whose distribution parallels that of sex, we
know it to be much more than a guess to say that the
color-blindness of man of which the hereditary distribu-
tion was described in Chapter III, is controlled by a factor
lying in the sex chromosome and recessive to the normal.
Though the whole mechanism in the higher plants and
animals can thus be pictured as one of sexual reproduc-
tion, in its details the results are still too complex to
analyze as concretely as the cases given for illustration.
Several thousand concrete differences between plants of
the various angiosperm families and between animals in
at least three different phyla have been followed through
pedigree cultures sufficiently carefully to make possible
a definite factorial analysis of their hereditary transmis-
sion. This has been possible, first, because variation has
taken place in these factors, enabling one to follow the
transmission of each member of an allelomorphic pair,
and, second, because this variation has been somewhat
qualitative in nature. Unfortunately for the peace of
mind of the biologist, however, the more numerous differ-
ences between animals and between plants are the quanti-
tative differences, the variations which make organs a
little larger or a little smaller. Now it is a great deal
easier to determine the transmission of the factor differ-
ences which determine that one flower shall be red and
another white than it is to trace the distribution of the
factors which determine that one flower shall be one inch
and another two inches long. Nevertheless, through the
efforts of numerous investigators it has been possible to

THE MECHANISM OF HEREDITY 67

show that such hereditary differences behave as should
be expected if their inheritance follows the same laws
as do the simpler characters. The basis, as one might
say, of the Mendelian interpretation of size differences is
the proof that practically aU qualitative characters are
affected by numerous factors. Sometimes there are two
or more factors which produce nearly identical visible
results, but more often the character complex is affected
in different ways and in various degrees by particular
factors. Whether the character develops at all or not
seems to be due to the presence or absence of one or more
main factors, but given the presence of these factors the
degree of development may be influenced by many sub-
sidiary factors or modifiers. Now these modifiers being
transmitted independently of one another and of the prin-
cipal factor or factors, an individual carrying certain
modifiers and lacking the principal factor may be crossed
with an individual carrying the main factor and lacking
the modifiers. The result is a series of recombinations
among the germ cells of the F^ generation which produces
F2 individuals carrying various groups of modifiers and
therefore developing the character complex under con-
sideration in different degrees.

If one studies carefully such crosses as the one just
described, he finds that a number of general conditions
are fulfilled.

1. Wlien pure or homozygous races are crossed, the
jPi populations are similar to the parental races in uni-
formity. This conclusion devolves from observations
that if any particular factors AA and an are homozygous
in the parental races, they can only form Aa individuals
in the Fj generation.

68 INBEEEDING AND OUTBREEDING

2. If the parental races are pure, F2 populations are
similar, no matter what F^ individuals produce them,
since all variability in the F^ generation is the result of
varying external conditions.

3. The variability of the F2 populations produced
from such crosses should be much greater than that of the
JPi populations, and if a sufficient number of individuals
are produced the grandparental types should be recov-
ered. The fulfillment of this condition comes about from
the general laws of segregation of factors in F^ and their
recombination in F2.

4. In certain cases F^ individuals should be produced
sho^ving a greater or a less extreme development of the
character complex than either grandparent. This is
merely the result of recombination of modifiers, as was
explained above.

5. Individuals of different types from the F2 genera-
tion should produce populations differing in type. The
idea on which this statement is based is, of course, that all
F2 individuals are not alike in their inherited constitution
and therefore must breed differently.

6. Individuals either of the same or of different types
chosen from the F2 generation should give F^ populations
differing in the amount of their variability. This con-
clusion depends on the fact that some individuals in the
F2 generation will be heterozygous for many factors and
some heterozygous for only a few factors.

Such are the conditions which must be fulfilled by
crosses exhibiting size differences if we are to visualize
their inheritance in the same way as we visualize the in-
heritance of qualitative characters such as color. If the
size differences are controlled by numerous germ-cell f ac-

THE MECHANISM OF HEEEDITY 69

tors, the distribution of the latter cannot be followed with
the same ease as one would follow the distribution of
cotyledon colors in the garden pea. This is time because
the visible effects of certain factors is sure to be very
small, and because varying external conditions obscure
the effects of inheritance. For example, a plant which
through its inheritance should become 6 feet tall under
average conditions may become only 4 feet tall if planted
in a sterile soil, but a plant which under average condi-
tions would become only 4 feet tall might become 5 feet
tall if grown in a very fertile soil.

Nevertheless, in spite of these drawbacks, one can
select size characters for study which are influenced but
slightly by external conditions, and by studying large
numbers through several generations, and by applying
mathematical tests to determine the uniformity or the
variability of the resulting populations, he can find out
whether quantitative characters satisfy the six require-
ments seen to be fulfilled by qualitative characters. This
has been done in numerous cases, and the results firmly
convince all unprejudiced investigators that the inheri-
tance of all types of characters is the same.

Table I, from crosses between two varieties of Nico-
tiana longiflora ^^ differing in the size of their flowers,
illustrates the point. One does not need any refined
mathematical methods to see that when the small variety
having flowers about 40 mm. in length is crossed ^^dth the
large variety having flowers about 94 mm. in length; the
result is a uniform F^ population having flowers about 64
mm. in length. The two Fn populations which it produced
are much more variable ; and one can easily calculate that
if several thousand plants had been gro^^^l instead of

70

INBEEEDING AJ^D OUTBREEDING

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THE MECHANISM OF HEREDITY 71

about 200, the grandparental sizes probably would have
been obtained. Furthermore, if one studies the results
obtained in the F^, F^ and F^ generations, considering
only the range of their variability, it is dear they differ in
both type and extent of variation.

No assumptions unproved for the inheritance of quali-
tative characters are necessary for thus visualizing the
inheritance of quantitative characters, and no facts dis-
covered in tracing the inheritance of other characters —
such as those involving linkage — are overlooked. But in
order to picture the situation easily, let us assume that
dominance is usually absent (often the case), that two
doses (i.e., the homozygous condition) of a factor have
twice the effect of one dose (true for all practical pur-
poses), that independent factors cumulative in their oper-
ation are allelomorphic to their absence in the hybrid
(linkage though it complicates matters, does not change
our reasoning).

Let us assume a case of ^* blended'' inheritance where
all fluctuations due to environment are eliminated. A
plant 12 inches tall is supposed to be crossed with a plant
28 inches tall. The difference between them is 16 inches.
If this difference is due to one allelomorphic pair in which
dominance is absent, the F^ generation is all intermediate
— about 20 inches — and the JPg generation falls into three
classes in which two represent the grandparental forms
and one represents the F^ form. Twenty-five per cent,
are 12 inches tall, fifty per cent, are 20 inches tall and
twenty-five per cent, are 28 inches taU.

But suppose this 16-inch difference between the
parents is represented by two allelomorphic pairs instead
of one. The F^ generation is again 20 inches tall, but

72

INBEEEDING AND OUTBREEDING

instead of there being three classes in F2, there are five
classes, viz., 12, 16, 20, 24 and 28 inches, and they appear
in the ratio 1:4:6:4:1. Each grandparental type
appears once out of sixteen times.

The way this ratio is obtained is by simple recombina-
tion, but as dominance is absent, each time a single
** presence'' factor is added, the height is increased four
inches.

9

1 AABB

2 AaBB
2 AABh
4 AaBh

1 AAhh

2 Aahh

1 aaBB

2 aaBh

1 aahh

28 inclies

24 inches

24 inclies

20 inches

20 inches

16 inches

20 inches

16 inches

12 inches

If three independent size characters instead of two
were involved in this cross, the F^ individuals would fall
in the same class as before, but the Fn classes would be
seven in number and the grandparental sizes would each
be recovered only once out of sixty-four times. For four
factors there would be nine classes of F^ individuals, and
the grandparental t^^oes would each occur only once out
of two hundred and fifty-six times; while with only
eight factors, the forms of the grandparents would each
appear only once out of 65,536 times, and it would be
quite remarkable if they were ever recovered from an
ordinary cross.

The entire scheme of this type of inheritance can be
expressed in mathematical form just like ordinary Men-
delian inheritance with full dominance. Let us recall that

THE MECHANISM OF HEREDITY 73

the F2 Mendelian expression for N allelomorphic pairs
when dominance is complete is the expanded bionominal:

(3 + l)» or (3/4 4-1/4)**
N==l (3 + 1)1 = 34-1

iV = 2 (3 + 1)2 = 32 4-34-34-1=- 94- 34-3 + 1
iV = 3 (3 + 1)3 = 33 + 3(32) 2+3(3)41 = 27 + 9 + 9 + 9
+ 3 + 3+3 + 1

Likewise, the expanded bionomial (V2 + %)^" gives the
numerical relationships when dominance is absent and N
represents the number of allelomorphic pairs. The ex-
pression is (% + i/2)^" instead of {1/2 + 1/2)" because it is
supposed that the presence of any allelomorphic pair in
the heterozygous condition produces one-half the visible
effect on the character that is produced when the heredi-
tary factors are present in the homozygous condition.
When N is very large the frequencies with which the dif-
ferent classes occur form a regular curve called the nor-
mal curve of error. This is the curve that is produced
when the errors in any physical measurement are sim-
ilarly plotted, using as classes any constant deviation
from the average, as a^ 2a, 3a, etc. This same curve is
also produced when one plots the fluctuations of many
organic characters produced by the infinite complexity of
external conditions.

If no non-heritable fluctuations intervened to obscure
the class to which any particular zygote belongs, there-
fore, one should expect the following classes in Fn when
parents of different sizes differing in N allelomorphic
pairs are crossed. The extremes represent the grand-
parental types in each case, and the intermediate classes
theoretically divide the difference between the parents
into aliquot parts. It should be noted, however, that this

74 INBEBEDING AND OUTBREEDING

is theory only ; in reality the influence of one factor may
be somewhat different from that of another factor.

N=l 12 1

iV==2 14 6 4 1

N = 3 1 6 15 20 15 6 1

Ar = 4 1 8 28 56 70 56 28 8 1

N = 5 1 10 45 120 210 252 210 120 45 10 1

Let us now note a few of the practical difficulties in
interpreting results that may follow this method of in-
heritance. In the theoretical example that we have used
for the sake of clearness, it was assumed that there were
no non-heritable fluctuations due to environment. Unfor-
tunately this is not the case in nature. Fluctuations are
everywhere present. They would obscure the classes to
which individuals belong even if these class differences
were quite large. And since they are usually small, the
change of individual form due to environmental causes
makes it impossible to separate an Fg population into the
true classes to which they belong gametically. Nor is this
the whole trouble. If the table showing the expected re-
sults with two pairs of size characters is examined, it is
found that not all the individuals that belong to a par-
ticular size class have the same zygotic formula. For this
reason one cannot pick out zygotes of a certain size and
expect them to breed the same. Their potentialities are
likely to be different. Furthermore, practical breeding
results are undoubtedly complicated by cases of correla-
tion. This correlation need not be gametic, though such
cases in all likelihood do occur; it may be merely physi-
ological. For example, a maize plant might have the
gametic possibilities of small plant size and large ear
size, but it would be foolish to expect that a plant capable

THE MECHANISM OF HEREDITY 75

of only a limited amount of development could bear as
large an ear as if it were as a whole capable of greater size
development. Thus it must not be expected that theoreti-
cal possibilities are always expressed perfectly in nature,
any more than, it should be expected that theoretical
piiysical calculations concerning known laws should agree
perfectly with experimental data. Plants and animals do
indeed seem to have in their reproductive cells a mosaic
of independently transmissible factors, but a plant or
animal is certainly not to be described as a mosaic of in-
dependent unit characters. These factors that appear to
be independent in heredity act and react upon one another
in complex ways during their development.

Hundreds of studies on quantitative characters have
been made. They all have the same result. Mendelian
inheritance rules. Plants appear to be less complex than
animals. A size complex in an animal seems to be the
result of the interaction of a large number of factors, a
size complex in a plant appears to be the result of the
interaction of a small number of factors. But the mode
of inheritance is always the same. It is the result of the
behavior of the chromosomes, and one can picture it with
the greatest ease with the simple diagram of the reduc-
tion division at gametogenesis if he fancies to himself that
the chromosomes are carrying bodies for the unit factors
of heredity, that the arrangements within them are a near
approach to perfection, that exchanges of contents may
be made only with regularity and precision so no essen-
tial feature of the mechanism shall break down.

In thus visualizing the process of heredity, one must
not be so overcome by the beauty of the picture that he is
unable to realize just what has been done. He must not

76 INBEEEDING AND OUTBREEDING

forget which part of this diagrammatic representation
of the heredity mechanism is fact and which part is
theory, for confusion between the two has led to a regret-
table controversy over a point which is of paramount im-
portance in any discussion of inbreeding and outbreeding
— the stability of inherited factors.

The relation between fact and theory in the Mendelian
conception of inheritance is this: Various kinds of ani-
mals and of plants were crossed and the results recorded.
With the repetition of experiments under comparatively
constant environments these results recurred with suf-
ficient regularity to justify the use of a notation in which
theoretical factors or genes located in the germ cells re-
placed the actual somatic characters found by experiment.
Later, the observed behavior of the chromosomes justi-
fied localizing these factors as more or less definite physi-
cal entities residing in them. Now the data from the
breeding pen or the pedigree culture plot and the ob-
servations on the behavior of the chromosomes during
gametogenesis and fertilization are facts. The factors
are part of a conceptual notation invented for simplifying
the description of the breeding facts in order to utilize
them for purposes of prediction, just as the chemical
atom is a conception invented for the purpose of simpli-
fying and making useful observed chemical phenomena.
As used mathematically, both the genetical factor and
the chemical atom are concepts, but biological data lead
us to believe that the term factor represents a biological
reality of whose nature we are ignorant, just as a molecu-
lar formula represents a physical reality of a nature yet
but partly known.

With this distinction in mind, one may treat the fac-

THE MECHANISM OF HEREDITY 77

tor — or the atom — from two points of view, either as a
mathematical concept or a physical reality. As a mathe-
matical concept it is the unit of heredity, and a unit in
any notation must be stable. If one creates a hypothetical
unit by which to describe phenomena and this unit varies,
there is really no basis for description. He is forced
to hypothecate a second fixed unit to aid in describing
the first.

The point at issue in this connection may be explained
as follows : Characters do vary from generation to gen-
eration, and the question to be decided is, how much of
this variation is due to the recombination of factors (con-
sidered now as physical entities) and how much is due to
change in the constitution of the factors themselves. The
obvious way to determine such a matter is first to appeal
to Nature and see whether it is possible for characters
to have a long* period of stability under any conditions ;
and, second, to investigate the stability of characters
when the environment is comparativeily constant and
change due simply to recombinations of heterozygous
factors is eliminated.

Of the results of the appeal to Nature only one need
be mentioned. Wlieeler ^^^ has found that ants preserved
in amber of the Oligocene period, fossils which are better
preserved than any others, and which are thought to be at
least 3,000,000 years old, are practically identical with
living species. The only points of variance to be observed
are slight differences in shade of color, something prob-
ably due to the mode of preservation. Thus it is clear that
organic characters may remain stable for periods of time
so great as to be beyond our powers of realization.

Investigations as to the effect of selection on homo-

78 INBREEDING AND OUTBREEDING

zygous hermaphroditic plants which are self-fertilized
naturally — the only material having a critical value with
this mode of attack, if we except unicellular organisms
reproducing asexually — have been made by a number of
biologists following the lead of Johannsen/^® who opened
up the possibilities of this type of experiment. The re-
sults have always been the same. Characters are remark-
ably stable. They do change, but they change so rarely
that a more useful purpose is served by identifying the
physical unit factor with the mathematical factor unit,
than to assume without justification that the physical
factor is constantly changing and must be described by
complex mathematical formulae using other hypothetical
units having no warrant for a physical existence. It is
true, two investigations by Jennings ^^"^ and by Middle-
ton ^*2 have shown a seemingly more unstable condition in
the infusorians Difflugm coronata and StylonycMa pustu-
lata. But there are several reasons for not believing con-
clusions derived from data on these animals applicable to
the higher plants and animals in which our real interest
lies, without mentioning several technical points which
might lead to interpretations different from those given
by the authors. First, they are cases of asexual, not sex-
ual, reproduction. Second, the germ plasm of the infusoria
may not be insulated from the effects of environment as is
the germ plasm of the higher organisms. Third, measur-
able differentiation in these experiments sometimes took
such a number of generations that in man it would take
some 3000 years to produce like results.

For these and other reasons which might be given,
could further space be devoted to the subject, we believe
there should be no hesitation in identifying the hypotheti-

THE MECHANISM OF HEREDITY 79

cal factor unit with the physical unit factor of the germ
cells. Occasional changes in the constitution of these
factors, changes which may have great or small elfects on
the characters of the organism, do occur; but their fre-
quency is not such as to make necessaiy any change in
our theory of the factor as a permanent entity. In this
conception biology is on a par with chemistry, for the
practical usefulness of the conception of stability in the
atom is not affected by the knowledge that the atoms of
at least one element, radium, are breaking down rapidly
enough to make measurement of the process possible.

CHAPTER V

MATHEMATICAL CONSIDEEATIONS OF

INBREEDING

The term inbreeding can be used in a relative sense
only, except when dealing with hermaphroditic organ-
isms. To say that one individual of a bisexual species is
inbred and another not is as indefinite as saying one is
short, the other tall. Strictly speaking, inbreeding refers
only to the way in which individuals are mated together.
This fact is well expressed by Pearl,^^^ who says: '' It is
clear that underlying all definitions of inbreeding is to be
found the concept of a narrowing of the network of
descent as a result of mating together at some point in
the network of individuals genetically related to one an-
other in some degree. Let us take this as our basic
concept of inbreeding. It means that the number of po-
tentially different germ-to-germ lines or *^ blood-lines"
concentrated in a given individual is fewer if the individ-
ual is inbred than if he is not. In other words, the inbred
individual possesses fewer different ancestors in some
particular generation or generations than the maximum
possible number for that generation or generations.^*
Thus, according to the evolutionist's conception of the
origin of species by natural selection, not only are all
members of a species related in some degree, however
remote, but all members of all species from any one
original life-sparh presumably are members of one inbred
liyie. This wholly ridiculous conclusion follows, because
the lines of descent terminating in any one individual,
though they radiate back in widening angles for a time,

80

MATHEMATICAL CONSIDERATIONS 81

would be seen to gather together again in a comparatively
few individuals if the pedigree of the species could be
traced in its entirety.

Such a reductio ad ahsurdum is not altogether value-
less. It shows how essential it is for one to recognize the
unavoidable limitations, the desirability of definite analy-
sis, the necessity of precise methods of attack, in any
consideration of the proposition he may undertake.

There are three distinct phases of the inbreeding prob-
lem, as Pearl has pointed out :

1. The system of mating with regard to the relation
of the actual number of ancestors making up the pedigree
of an individual to the total possible number.

2. The constitution of each individual with respect to
Mendelian unit factors which results from the continued
operation of a given system of mating which is inbreeding.

3. The physiological effect produced upon the indi-
vidual by the constitution derived from this system
of mating.

The first two phases of the problem are capable of
abstract mathematical treatment. The third can be
solved only by experimental investigation.

Precise methods of measuring and comparing sj^stems
of mating have been devised by Pearl by the use of a
Coefficient of Inbreeding and a Coefficient of Relation-
ship." The first is a measure of the actual number of

o Pearl has made a somewhat more precise analysis of tlie Inbrecdinf]j
and Relationship Coefficients in later papers, 175, 17G, and has siiijj?ostc<i
a Partial Inbreeding Index, in a percentage which one-half of the ReUition-
ship Coefficient is of the Inbreeding Coefficient. This constant is a moaaure
of the amount of inbreeding due to relationship between the sire and dam.
Further, he has described a single numerical measure of inbrwtling for
bisexual organisms, in the ratio of the area of the inbreetling curve in any
pedigree to the area of the maximum (brother X sister) curve.

For our purposes, it is unnecessary to consider these ext4?n8ion9 of
Pearl's studies in detail, though technically they are very valuable.

6

82 INBEEEDINa AND OUTBREEDING

ancestors compared with the possible nimiber. It is
derived from the formula:

lOO/p -q \
\ n + 1 n + 1/

Z ^

n p

« + l

where pn^^ denotes the maximum possible number of
different individuals involved in the matings of the n + 1
generation, and gn+i the actual number of different indi-
viduals involved in these matings. As an illustration, any
individual in bisexual matings has two parents in the first
ancestral generation, four grandparents in the second
ancestral generation, and so on, according to the following
symbolical representation

X -^-> (1) 2<-> (2) 4-^— > (3) 8<— >(4) 16-<— > (5) 32<-^ (»)2'» . . .,

in which the enclosed numbers represent the ancestral
generations (1 = parents, 2 = grandparents, 3 = great-
grandparents, etc.), and the other figures the number of
ancestors. In the second or earlier generations the an-
cestors may not all be different individuals, so that in any
generation previous to the parental the actual number of
ancestors may be less than the possible number. For ex-
ample, in brother and sister mating, any individual in-
stead of having four different grandparents, has only two.
Expressed symbolically, as above, the representation for
this type of mating would be

X <-^ (1)2 -^-> (2)4-2/1 -^-> (3)8-7/2 <-> (4) 16-1/3 <-> (5)32-2/4. . . ,

where 2/1 = 2, 1/0 = 6, y.^ = 14, 2/4 = 30.

In this case y has the value of 2" - 2, and this is the highest

value it can have in any system of mating where two indi-

MATHEMATICAL CONSIDERATIONS 83

viduals are necessary for reproduction. Applj^ng the
formula given above, the Coefficients of Inbreeding for
each generation in brother and sister mating are :

Zo = 100 (2-2)= 0

2
Z^ = 100 (4-2) =50

4

^2 = 100 (8-2) =75

8
Z3 = 100 (16-2) = 87.5

16

The figures obtained are the differences between the
possible number of ancestors and the actual number ex-
pressed as percentages of the former. By plotting these
percentages for successive generations on the generation
number as a base, a curve of inbreeding is obtained which
can be compared to the curves obtained by other systems
of matings. This comparison is shown in Fig. 22 for the
common types of matings as worked out by Pearl.

From these curves it is evident that continued brother
by sister and double first-cousin matings have the same
effect, although the latter is one generation behind the
former. Also the curves for parent by offspring and sin-
gle first-cousin matings are similar in type, but show the
same differences in position. In any case the concentra-
tion of the lines of descent in these systems of inbreeding
is rapid, until after fifteen generations no individual can
have more than a fraction of one per cent, of the number
of ancestors theoretically possible.

The Coefficient of Inbreeding alone tells us nothing
as to the relation between the different lines of descent.

84

INBEEEDINa AND OUTBEEEDING

Two individuals may have the same Coefficients of In-
breeding when considered for any given number of gen-
erations, but differ greatly in germinal constitution. This
is due to the fact that the two lines brought together in
the immediate production of any individual may or may
not be related. For example, a closely inbred animal of
one breed may be mated to another closely inbred animal

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FiQ. 22. — Curves of inbreeding showing (o) the limiting case of continued brother
X sister breeding, wherein the successive coefficients of inbreeding have the maximum
values; (b) continued parent offspring mating; (c) continued first cousin X first cousin
mating where the cousinship is double (C^ XC^), and (d) continued first cousin X first
cousin mating where the cousinship is single (C* XC^). The continued mating of uncle X
niece gives the same curve as C^ X C^ (After Pearl.)

of an entirely different breed. The two lines of descent
would then be totally unrelated as far as the known pedi-
grees are concerned, but the resulting individual would
have a high Coefficient of Inbreeding, due to the concen-
tration of ancestry separately in the two ancestral lines.
To give some measure of the inter-relation of the lines of
descent, Pearl has devised the Coefficient of Eelationship,
Ky which is essentially the per cent, of the individuals in

MATHEMATICAL CONSIDERATIONS 85

each of the descending lines which are also represented
in the other line. To give an adequate mathematical esti-
mation of the degree of inbreeding, both constants are
necessary. There is, generally, some correlation between
them, although the Coefficient of Relationship may be
zero, and the Coefficient of Inbreeding still be high, as in
the illustration just given in which the progeny comes
from a pair of individuals from two distinct inbred lines.
The application of these methods of determining the
amount of inbreeding is illustrated by Pearl from the
pedigrees of two Jersey bulls as follows :

Inbreeding" " Z" and Relationship '' (K) '' CoeOicients

of
King Melia Rioter 14th and Blossom's Glorene

^1

Z, {KJ

0

(0)

0

(0)

A,

Z, (K,)

25

(0)

0

(0)

^3

Z. {K,)

25.00

(50.00)

12.50

(0)

^4

Z\ (K,)

37.50

(G2.50)

12.50

(0)

^5

Z, {K.J

50.00

(75.00)

25.00

(0)

^6

Z, (K,)

71.88

(87.50)

29.C9

(0)

^T

Z, (K,)

81.25

(92.19)

35.94

(0)

^8

Z, {K,)

90.63

(92.97)

40.23

(0)

The method of making the calculations is explained
clearly and concisely by the originator and we shall not
undertake to repeat it here. What we are interested
in is the genetic meaning of the figures after they have
been obtained.

The Coefficient of Inbreeding, Z, has to do solely with
total relationship, and shows the intensity of inbreeding
in the stockman's sense of the word by measuring pre-
cisely **the proportionate degree to which the actually
existent number of different ancestral individuals fails to

86

INBREEDING AND OUTBREEDING

reach the possible number, and by specifying the location
in the series of the generation under discussion. * ' King
Melia Rioter 14th had less than 10 per cent, of the maxi-
mum number of ancestors in the 7th ancestral generation,
while in the same generation Blossom's Glorene had nearly
60 per cent. From these figures it is evident that King
Meha Rioter 14th is a much more inbred animal than

100

80

2 60

a

®

m

<S 40

20

/

^

:r:s:r=i

1
*

r/

/

/

/

/

llA

•

f

/

f
1

6 8

Generations

10

12

14

^5' Fig. 23. — Graphs showing (a) the total inbreeding (heavy solid line) and (b) the rela-
tionship (heavy broken line) curves for the Jersey Bull, King Melia Rioter 14th. The
high order of the inbreeding and relationship between the sire and dam in this case is evident
by comparison with the lighter lines, which give the maximum values for continued brother
X ^sister and^parentlXioff spring breeding. g (AftertPearl.)

Blossom's Glorene. A clearer demonstration of the mat-
ter, however, is found in Fig. 23, where the curve of the
total inbreeding of King Melia Rioter 14th plotted from
the figures just cited is compared with the curve of maxi-
mum values for continued brother x sister and parent x
offspring matings.

The Coefficient of Relationship, K, might better be
called the Coefficient of Cross-Relationship to distinguish

MATHEMATICAX. CONSIDERATIONS 87

its function from that of the Coefficient of Inbreeding,
since it is a measure of the community of ancestry of the
dam and the sire.

These two coefficients taken together, then, give us
the first quantitative measure of inbreeding as a system
of mating, but obviously they do not tell anything con-
cerning the actual germinal constitution of any individual
resulting from a given system of inbreeding. This fea-
ture of the relationship coefficients is nicely illustrated
by one of PearPs examples. Clearly, a Holstein cow pro-
duced by continued brother x sister matings (K =: 100) is
very different in its germinal constitution from a cross-
bred animal obtained by mating this cow vdih. a Jersey
bull, the product of a similar system of inbreeding (^ = 0) .
Yet the Coefficients of Inbreeding in each case form iden-
tical series, with the maximum possible value of Z when
K=0 one generation farther removed than when A"=100.

Without question the germinal (or may one call it the
Mendelian?) composition of any individual can be deter-
mined only by actually testing its breeding qualities, its
transmissive powers ; and the effect this composition may
have had upon its development can be measured only by
comparison with other individuals of known genetic con-
stitution. But an indication of the germinal constitution
of an individual produced by any long-continued system
of inbreeding, as far as the degree of heterozygosity or
homozygosity is concerned, can be obtained by applyinc:
the laws of probability to Mendelian formula?. In other
words, the laws of probability applied to Mendelian
formulae show the probable homozygosity or heterozygos-
ity of the generation as a whole for any number of Men-
delian allelomorphic pairs with any given system of

88 INBEElEDING AND OUTBREEDING

inbreeding, and some idea of the composition of the indi-
vidual may often be had from a careful consideration of
the composition of the generation to which it belongs.

In an endeavor to demonstrate the effect of various
systems of inbreeding upon Mendelian constitution, and to
appraise the effect of this constitution upon develop-
mental vigor, let us approach the problem from the op-
posite direction.

It has been established that the effect produced by
crossing depends more or less closely upon the genetic
diversity of the types which produce the hybrid. The
usual result of crossing organisms which differ in many
characters is a first generation which is no more variable
than the parental types. The second generation, how-
ever, may be expected to show a greater variability be-
cause of Mendelian segregation. The amount of such
variability is a measure of the diversity of the parents
which produce the cross. It is in crosses which show
greater variability in the second generation that hybrid
vigor is expected in the generation immediately following
the cross. When such hereditary combinations are com-
posed of unlike elements, hybrid vigor is commonly
shown; when all the combinations are composed of like
elements hybrid vigor is absent. Hence, in crossed species
of wild or domesticated animals and plants part of their
vigor may be the result of dissimilar hereditary factors
acting together. If conditions are brought about by which
this dissimilarity in aUelomorphic combinations is re-
duced or lost completely a partial diminution of develop-
mental energy will occur. Since there is a constant
tendency for inbreeding of whatever kind to bring about
similarity in germinal construction, inbreeding will,

MATHEMATICAL CONSIDERATIONS 89

therefore, frequently cause a general reduction in vigor.
It has never been held that all hereditary factors
are equally involved in this effect on vigor. Some
are considered to be wholly without effect. The fact
remains, however, that the increased growth and extra
vigor commonly resulting from hybridization as a mass
effect is intimately associated with Mendclian plie-
nomena, and its expression is roughly proportional to the
number of heterozygous factors present and disappears
when homozygosity is brought about. r.^

The reduction of the number of heterozygous allele^
morphs in an inbred population is automatic and varies ,
with the closeness of inbreeding. In self-fertilization it :
follows the well-known Mendelian formula by which any
heterozygous pair forms in the next generation 50 per
cent, homozygotes and 50 per cent, heterozygotcs in re-
spect to that pair. Since the homozygous allelomorphs
must always remain constant and the number of hetero-
zygous factor combinations is halved each generation and
one-half added to the homozygous class, the reduction in
the number of heterozygous elements proceeds as a vari-
able approaching a limit by one-half the difference in
each generation. The curve illustrating this approach to
complete homozygosity is shown as No. 1 in Fig. 24.

As lEast and Hayes ^^ have said: ^^ Mendel, in his
original paper, showed that if equal fertility of all plants
in all generations is assumed, and, furthermore, if every
plant is always self-fertilized, then in the nth generation
the ratio of any particular allelomorphic pair {A, a) would
be 2" - lAA : 2Aa: 2" - laa. If we consider only homo-
zygotes and heterozygotcs, the ratio is 2" -1 : 1. Of
course, the matter is not quite so simple when several

90

INBEEEDING AND OUTBREEDING

allelomorplis are concerned, but in the end the result is
similar. Heterozygotes are eliminated and homozygotes

100%

Percent, of Heterozygous
Individuals in Each Selfed
Generation when the Number
of Allelomorphs Concerned
Are: 1,5,10,15.

Segregating Generations

Fio. 24. — Graphs showing the reduction of heterozygous individuals and of heterozygous
allelomorphic pairs in successive generations of self-fertilization.

remain. The probable number of homozygotes and any
particular class of heterozygotes in any generation r is
found by expanding the binomial 1 + (2'' - 1)" where n rep-

MATHEMATICAL CONSIDERATIONS 91

resents the number of character pairs involved. The
exponent of the first term gives the number of hetero-
zygous and the exponent of the second term the number
of homozygous characters. As an example, suppose we
desire to know the probable character of the fifth segre-
gating generation (F^) when inbred, if three character
pairs are concerned. Expanded we get

13 + 3 [12(31)] +3 [1 (31)2] + (31)3

Reducing, we have a probable fifth-generation population
consisting of 1 heterozygous for three pairs; 93 hetero-
zygous for two pairs; 2883 heterozygous for one pair;
28,791 homozygous in all three character combinations. ' '
Of the 32,768 total number of individuals in this genera-
tion, 2977, or 9.09 per cent., are heterozygous in respect to
some characters. Of the 98,304 total number of allelo-
morphic pairs involved in all the individuals of this gen-
eration, 3072, or 3.125 per cent., are heterozygous. This
is the percentage which is obtained by halving 100 per
cent, five times. It is the per cent, of heterozygous allelo-
morphic pairs in all the individuals making up the popu-
lation as a whole that follows curve 1 in Fig. 24. The
per cent, of individuals heterozygous in any factors in any
generation inbred by self-fertilization depends upon the
number of heterozygous elements concerned at the start.
The curves where 1, 5, 10 and 15 heterozygous allelo-
morphs are present in the beginning are given in Fig. 24.
These are calculated from the formula given and illus-
trated above. The curve for the reduction in hetero-
zygous individuals where one factor only is concerned at
the start, is identical with the curve showing the reduction
in the number of heterozygous factors in an inbred popu-

92 INBREEDING AND OUTBREEDING

lation as a whole where any nuinber of factors are con-
cerned. In any case, almost complete homozygosity is
reached in about the tenth generation.*

It must be remembered that this reduction applies only
to the whole population, or to a representative sample of
the population, in which every member is self ed, in which
each individual is equally fertile, and in which all the
progeny are grown in every generation. In practice in an
inbreeding experiment, usually only one individual in self-
fertilization or two individuals in brother and sister mat-
ings are used to produce the next generation. Thus the rate
at which complete homozygosity is approached depends on
the constitution of the individuals chosen. Theoretically in
any inbred generation the progenitors of the next genera-
tion may either be completely heterozygous or completely
homozygous or any degree in between depending upon
chance. The only conditions which must follow in self-
fertilization is that no individual can ever be more hetero-
zygous than its parent, but may be the same or less. Thus
it is seen that artificial inbreeding, as it is practiced, may
theoretically never cause any reduction in heterozygosity,
or it may bring about complete homozygosity in the first
inbred generation. In other words, the rate at which
homozygosity is approached may vary greatly in differ-
ent lines. However, as the number of heterozygous fac-
tors at the commencement of inbreeding increases the
more nearly will the reduction to homozygosity follow the
curve shown, because the chance of choosing a completely

b Various formulae dealing with inbreeding have been proposed and
discussed by Pearson (177), Jennings (102, 104, 105, 106,) Pearl (168,
169, 170, 171, 172), Fish (69), Wentworth and Remick (213), Robbing
(186, 187) which are useful in predicting the character of inbred genera-
tions when certain conditions are fulfilled.

MATHEMATICAL CONSIDERATIONS 93

homozygous or completely heterozygous individual in the
early generations will become very much less.

TABLE II

The Theoretical Number and Ratio of Individualb in the Classes of

Different Degrees of Heterozygosity, After Recomhination,

WHEN Fifteen Mendelizing Units Are Involved.

Ratio of indi-

The number'of

The total number of

viduals in thr

factors

in rp-

The total number of hcteroay-

individuals in all

classes with

spcct to^whirh

gnus and homozygous factor

ClaBfl

the possible Mendo-

different num-

the

difforent

pairs ;in all the individuals in

No.

lian recombinations
in Ft when 15 fac-
tors are involved.

ber of hetero-
zygous and
ho moaygous

classes

are:

each class:

factors - co-

Hetero-

Homo-

Heterozygous

HomozvgouB

efficients (a-(-
a),i»

zygous

zygous!

factor pairs

factor pairs

1

32,768

1

15

0

15

0

2

491,520

15

14

1

210

16

3

3,440,640

105

13

2

1,365

210

4

14,909,440

455

12

3

5,460

1,365

5

44,728,320

1,365

11

4

15,015

5,460

6

98,402,304

3,003

10

5

30,030

15,015

7

164,003,840

5,005

9

6

45,045

30,030

8

210,862,080

6,435

8

7

51,480

45,045

9

210,862,080

6,435

7

8

45,045

51,480

10

164,003,840

5,005

6

9

30,030

45,045

11

98,402,304

3,003

5

10

15,015

30,030

12

44,728,320

1,365

4

11

5,460

15,015

13

14,909,440

455

3

12

1,365

5,460

14

3,440,640

105

2

13

210

1,365

15

491,520

15

1

14

15

210

16

32,768

1

0

15

0

15

16

1,073,741,824

32,768

15

15

245,760

245,760

n-fl

(2n)2

2"

n

n

mn:2n)

3^(n.2")

As an example, in Table II there is shown the theoreti-
cal classification of the progeny of a self-fertilized organ-
ism which is assumed to be heterozygous ^vith respect to
15 independent Mendelizing units. It can be seen that the
bulk of the individuals lie between classes 6 and 11,
where none of the members is heterozygous for more
than 10 or less than 5 factors. In other words, any indi-

94 INBEEEDING AND OUTBREEDING

vidual selected from this population to be the progenitor
of the next generation would most probably come from
the middle classes and, therefore, would be heterozygous
for only about half as many factors as its parent. The
chance that this individual would not come from the mid-
dle classes between 6 and 11 would be about 1 out of 10.
The chance that it would be completely homozygous or
completely heterozygous is 1 out of 32,768. If 20, instead
of 15, factors were involved, the chance would be 1 out of
1,048,576. The selection of such completely homozygous
individuals would be a remarkable event. If, for instance,
a tobacco plant, which has 24 chromosomes as the haploid
number, could be obtained which was heterozygous in one
factor pair in each chromosome and this plant were to
be self-pollinated and the progeny grown, 16,777,216
plants would have to be produced in order to provide an
even chance of securing somewhere in the lot one plant
which was homozygous in all the twenty-four factors.
This number of plants would require over 2000 acres of
land as tobacco is grown in field culture.

This condition by which the progenitor of each gen-
eration in self-fertilization tends to be half as hetero-
zygous as its parent holds true for any number of factors
and in every generation. Thus it can be seen (Table II)
that the progeny as a whole have an equal number of
heterozygous and of homozygous factor pairs in respect
to those characters in which the parent was heterozygous.
So it is that in practice the reduction in heterozygosity
accompanying inbreeding is greatest at first, rapidly be-
comes less and finally ceases for all practical purposes.
From 0 at the start the degree of homozygosity, with re-
spect to a given number of factors, increases to 99 per
cent, after 7 generations of self-fertilization; after 12

MATHEMATICAL CONSIDERATIONS

95

generations it is 99.9 per cent, and after 19 generations
99.999 per cent.

Although nearly complete homozygosis is theoretically
brought about by 7 generations of self-fertilization the
attainment of absolute homozygosity is a dillicult matter
and in practice it may never be reached. The fact that
hereditary factors are distributed by the chromosomes, so
that there is not independent recombination among all
the determiners, enters as a complicating factor. Lethal
factors, which prevent homozygotes from appearing, and
increased productivity of hybrid combinations, also tend
to prevent the complete elimination of heterozygosity.

The way in which factor linkage affects reduction to
homozygosity may be illustrated by the use of two allelo-
morphic pairs of factors. Jennings ^*^^ has calculated
the effect of three generations of self-fertilization upon
the population descending from a dihybrid when the two
pairs of factors show a linkage relation of 2 (that is, 33Vfj
per cent. ^^ crossovers'^) in both sexes; and also when the
linkage is complete in one sex, as in Drosopliila where
there is no * ' crossing over ' ' in the male. The proportions
of completely homozygous, of completely heterozygous,
and of mixed individuals — i.e., heterozygous in one pair
and homozygous in the other — obtained after three gen-
erations of self-fertilization, are compared with what is
expected when the two factors are independent as follows :

Ratio required

Two factors
independent

Linkage ratio
2 in botli sexes

Linkage

complete in

one sex, 2 in

other

Per cent, of complete homozygotes . .

Per cent, of complete heterozygotes.

Per cent, homozygous in one pair but

not in other

7G.56
1.50

21.88
87.50
12.50

77.14
2.14

20.71
87.50
12.50

78.70
•A.70

17.50

Per cent, homozygous factors

Per cent, heterozygous factors

87.50
12.50

96 INBREEDING AND OUTBREEDING

It win be seen from these figures that the proportion of
complete homozygotes and complete heterozygotes is in-
creased by linkage at the expense of the mixed class. The
proportion either of homozygous or of heterozygous fac-
tor pairs, however, is unaffected. It is evident then that
just as the reduction to homozygosity by self-fertilization
is independent of the number of factors involved, in the
same way it is independent of the way in which these
factors are linked together: but in an experiment where
particular individuals are chosen as progenitors linkage
of factors reduces the chance that these will come from
the median classes of heterozygosity; hence, the rate at
which homozygosity is attained wiU vary more widely
between dfferent lines if the factors involved are par-
tially linked than if they are all independent. This merely
means that some lines will become uniform and lose the
stimulus of hybridization in a fewer number of genera-
tions than will other lines and that this difference theoreti-
cally is increased by linkage. But the hastening of the
attainment of homozygosity in some lines is balanced by
delay in other lines, so that on the average the curve
of inbreeding shown applies equally whether linkage of
factors is involved or not.

If there were no other controlling factors the reduc-
tion in vigor resulting from inbreeding, in the majority
of cases, should approximate curve 1 in Fig. 24 on the
assumption that hybrid vigor or heterosis is associated
with heterozygosity. However, it should not be thought
that the amount of heterosis is perfectly correlated with
the number of heterozygous factors. Some have more
of an effect than others, and certain factors, when com-
bined together, may have a cumulative effect. Moreover,

MATHEMATICAL. CONSIDERATIONS 97

since the heterozygous individuals are more vigorous than
the homozygous, selection either unconscious or purpose-
ful would favor the more heterozygous so that actual
approach to homozygosity is quite likely not to proceed
at as fast a rate as the theoretical curve would indicate.

Self- fertilization is the quickest and surest means of
obtaining complete homozygosity for the reason that
whenever any pair of allelomorphs becomes homozygous
it must always remain so, as long as self-fertilization takes
place, whereas in brother and sister mating a homozygote
may be mated to a heterozygote. The approach to homo-
zygosity in self-fertilization when one pair of contrasted
characters is considered and fecundity does not vary pro-
ceeds as follows :

Generation F^ F^ F^ F^

Parent type A A 0 1/4 3/8 7/16 1/2 AA^ =.492

Parent type aa 0 1/4 3/8 7/16 1/2 aa, =.492

Hybrid type Aa 1 1/2 1/4 1/8 0 Aa^ =.008

In brother and sister mating the procedure is
as follows;

Generation F^ F^ F^ F^ F^

Parent type ^^ 0 1/4 2/8 5/16 11/32 1/2 ^^,,=.490+

Parent type aa 0 1/4 2/8 5/16 11/32 1/2 aa^, =.490+

Hybridtype^a 1 1/2 2/4 3/8 5/16 0 Aa^^=mS-\-

These figures have definite numerical relations to each
other and formulae have been obtained by Jennings ^^^ for
calculating the condition in any generation. It will be seen
from the above figures that 6 generations of self-fertiliza-
tion are more effective than 17 generations of brother and
sister matings in bringing about homozygosis.

Of parent by offspring matings there are several
7

98 INBEEEDING AND OUTBEEEDING

meth-ods which may be carried on with different results.
If the individuals are mated at random to all the differ-
ent types of parents, homozygous and heterozygous, the
effect is the same as in brother and sister mating. Cousin
matings, which may proceed either as single or double
first-cousin matings or even more distant unions, may be
equally or less effective; but all tend towards the same
end; heterozygosity is ultimately eliminated and homo-
zygosity prevails. In this way there is a difference be-
tween selective mating and random mating. Continued
selective mating is necessary to bring about homozygos-
ity. Intermittent inbreeding alternating with periods
of outcrossing which is the prevailing state of affairs
with many organisms cannot maintaiu any high degree
of homozygosity.

In self-fertilization the reduction in heterozygous
allelomorphs in a population as a whole follows curve 1 in
Fig. 24, irrespective of the number of factors concerned,
as stated before, provided that a random sample of all
the different classes of individuals are selfed and become
progenitors of the next generation and that there is
equal productiveness and equal viability. If the hetero-
zygotes are more productive, as in many cases they are,
the reduction to complete homozygosity will be delayed.

Artificial self-fertilization in naturally crossed species,
then, brings about the same condition as prevails in
naturally selfed species. The great variability of a cross-
fertilized species gives way to the more uniform and
stable condition characteristic of naturally self-fertilized
organisms. The uniformity brought about by inbreediQg
is thus due to a reduction of the genetical variability.
Inbreeding affects physiological or developmental vari-

MATHEMATICAL CONSIDERATIONS 99

ability only indirectly by changing the vigor of the organ-
isms so that they may react differently to dilTerent
environments.

Assuming, then, that the loss of the stimulation ac-
companying heterozygosity is correlated with the reduc-
tion in the number of heterozygous factors we should
expect to find the decrease of heterosis greatest in the first
generations, rapidly becoming less until no further loss
is noticeable in any number of subsequent generations
of self-fertilization, and that on the average the decrease
will become negligible from the seventh to the twelfth
generation and from then on no further marked change
will take place. Segregation of characters and appearance
of new types and reduction in variability will also follow
the same course. Some cases are to be expected in whioh
stability is reached earlier, and some cases in which it is
reached later; or, theoretically it may never be reached.
With these points in mind, let us see what are the actual
results of long-continued inbreeding.

CHAPTER VI

INBREEDING EXPERIMENTS WITH ANIMALS

AND PLANTS

Doubtless discussion has been rife since the dawn of
civilization as to the actual effect of more or less close
intermating in the various breeds of domestic animals,
since stock-raising was one of the earliest arts and was
brought to a high degree of perfection by the ancient
Semitic nations. One may surmise, from the rules they
made against the marriage of near relatives, that the pro-
ponents of cross breeding had the best of the argu-
ment ; but it is hardly likely that their practice was any-
thing more than rule-of -thumb adopted after a variety of
casual observations. At any rate, controversy is still spir-
ited, and one reads the opinion of stock-breeding authori-
ties without arriving at any definite knowledge of the
problems. Their results are confusing, and the only con-
clusion one may reach from their perusal is the wholly
unsatisfactory one that close mating, as a system of breed-
ing, has both advantages and disadvantages. Without
question, it has had great value in fixing certain desirable
types. Some breeds, as a whole, and many individual
herds, owe their uniformity in conformation and perform-
ance in a large measure to close inbreeding accompanied
by rigid selection. At the same time, it must be recog-
nized that certain evil effects may result from close inter-
mating. These effects have been frequently expressed in
lessened constitutional vigor, greater susceptibility to
disease, reduced fecundity, and, in some cases, even in

100

/

INBREEDING EXPERIMENTS 101

decreased size and in the appearance of definite abnormal
or pathological conditions.

Obviously it is not possible to formulate any definite
rule by which to judge under what condition the good
effects of inbreeding may be expected to outweigh the
evil, by generalizing from a series of isolated facts. What
is needed is controlled experimentation to determine just
what inbreeding involves, and interpretation of the re-
sults in keeping with general biological knowledge. Dar-
wiii38, 39 ^jjg ^i^g gj.g|- ^Q appreciate this. After

endeavoring rather unsuccessfully to generalize on the
subject with a collection of published records as a basis,
he himself carried on a series of inbreeding experiments
on plants extending over a period of eleven years. Plants
were probably selected as the material with which to work
primarily because the experiments could be carried out
easily and with little expense. But there is another rea-
son why plants serve the purpose better than animals in
such an investigation; the animals commonly available
are bisexual; hence, they cannot be inbred as intensively
as hermaphroditic plants. Nevertheless, Darwin's ex-
periments served to stimulate more interest in the subject
among zoologists than among botanists, and the quanti-
tative experiments carried on during the next thirty
years w^ere nearly all upon animals.

The rat has been a favorite species, it having served
as material for the extended researches of Crampe,'^*
Ritzema-Bos '^^ and King.^^**>' ^^o. 121 rpj^g grgt two investi-
gations, together with that of Weismann and von
Guaita,^^' ^'^ on mice have been the classic examples of the
adverse effects of inbreeding.

Crampe^s experiments started with a litter of five

102 INBREEDING AND OUTBREEDING

young, obtained by crossing an albino female with a wnite
and gray male. These animals were inbred in various
degrees for seventeen generations. During the experi-
ment many rats showed great susceptibility to disease,
divers kinds of abnormalities, diminished fertility, and
increased total sterility.

Similarly Ritzema-Bos started his investigations with
a litter of twelve rats obtained by crossing, this time an
albino female with a wild Norway male. This stock was
inbred in different ways for six years, during which time
he claimed to have obtained about thirty generations. His
results did not corroborate those of Crampe in so far as
susceptibility to disease or appearance of malformations
are concerned, but there was a gradual decrease in size
of litter and a gradual increase in percentage of infertile
matings, as is shown in the following table :

Year of inbreeding 1 2 3 4 5 6

Ave. number ia litter 7.5 7.1 7.1 6.5 4.2 3.2

Per cent, infertile

matings 0.0 2.6 5.617.4 50.0 41.2

These investigations, in spite of the habit biologists
have of citing them, are not calculated to settle the ques-
tion they undertook to answer. Ritzema-Bos himself
criticizes those of Crampe, because he believes them to
have been started with a weak strain. Miss King, how-
ever, thinks the weakness of these rats, as indicated by
their susceptibility to disease, the appearance of mal-
formations, and their tendency to sterility, was due to
the conditions under which they were kept. She had a
similar experience during the earlier part of her own
experiments, and found that inadequate nourishment was
largely the cause. But it is not for this reason that we

INBREEDING EXPERIMENTS 103

feel that both of these experiments should be disregarded.
Each ivas started with hybrid stock, and such experi-
ments with hybrid stock bring in an additioyial compli-
cation, Mendelian recombination. The only type of
investigation on bisexual animals calculated to offer criti-
cal evidence on the effect of inbreeding per se must be
carried on with stock which has already been inbred long
enough to reduce the genetic constitution of the animals
to an approximately homozygous condition. Then, and
then only, can the effect of more extended inbreeding be
determined without confusion as to the interpretation
of the results.

Miss King has pointed out a part of the difTiculties
involved in starting with a hybrid stock. In one of her
experiments the progeny of a cross between a wild Nor-
way rat and an albino was inbred for several generations.
She found that while the majority of the F-^ females were
fertile, at least 25 per cent, of the Fo females were com-
pletely sterile and 10 per cent, of those which did breed
cast only one or two litters. In the strains extracted from
this cross there was variation in degree of fertility, but
none was found which exhibited the high degree of fer-
tility usually existent in the albino rat. No endeavor to
select fertile strains was made and one cannot say whether
or not rigid selection would have isolated them, but the
researches of Detlefsen^' on hybrids of the genus Cavia
to which the common guinea-pig belongs indicate this to
be a probability. These investigations as well as those of
Easf^^ on the genus Nicotiana show conclusively that
various hereditary factors are involved in the partial
sterility exhibited in many species crosses, and that tliese
factors may be expected to recombine in the usual manner.

104 INBREEDING AND OUTBREEDING

It is more than a mere assumption, then, if a great part of
the sterility found by Crampe and Ritzema-Bos is attrib-
uted to the same cause.

The investigations of Weismann and von Guaita are
hardly more satisfactory. Weismann inbred a stock of
white mice for twenty-nine generations, and found the
average number of young for the three 10-gen. periods
to be 6.1, 5.6 and 4.2. Where this stock originated, and
what method of inbreeding was followed, we are not told.
Presumably the gross result was a slight decrease in fer-
tility, coincident with the amount of inbreeding, but even
this is not certain. As King points out, the average num-
ber of litters under observation in the first two genera-
tions was twenty-two ; in the last nine generations, three.
Clearly there was greater opportunity of selecting
healthy breeding stock, as well as a lower probable error,
in the earlier part of the experiment, and this might
account for the slight difference in fertility found. Von
Guaita crossed some of these highly inbred mice with
Japanese waltzers and then inbred for six generations.
He reports the average number of young in the successive
generations as 4.4, 3.0, 3.8, 4.3, 3.2 and 2.3 ; but in view of
the vigor almost always expressed in the F^ generation of
such crosses one is inclined to doubt the pertinence of
these figures to the inbreeding problem.

Although, as has been pointed out, there is good rea-
son for disallowing the claim of these much cited experi-
ments on mammals as proofs of the adverse effects of
inbreeding through consanguinity alone, there is no inten-
tion of denying the isolation of individuals characterized
by undesirable qualities from mixed strains by means of
Mendelian recombination. Perhaps it is not wise even to

INBREEDING EXPERIMENTS 105

maintain the impossibility of injury to any strain of any
species through inbreeding per se, but it is proper to say
that the evidence in favor of it is practically nil.

Doubtless we could make our case more convincing to
the stockman could the enormous number of really well-
kept herd records be cited and analyzed. But it is not
possible at present to say whether many of these records
satisfy the requirements of modern genetic research.
This is a task which must be left to the breeding organiza-
tions of the future. We can appeal at present to only two
investigations on mammals where the effect of Mendelian
recombination has been largely eliminated; and these
again are on small mammals, the rat and the guinea-pig.

The first of these investigations to be reported was
that of King.^^^' ^^o, 121 j-|- ^r^g started with a litter
of four slightly undersized but otherwise normal albino
Norway rats, two males and two females. From these
females two lines, A and J5, were carried on for twenty-
five generations by mating brother and sister. In
the earlier generations practically all of the females were
used for breeding, but in every generation after the sixth
about twenty females were selected from approximately a
thousand available young.

At first the inbred rats exhibited many of the defects
reported by Crampe. Numerous females were either ster-
ile or produced but one or two small litters. Other ani-
mals were characterized by low vitality, dwarfing, and
malformations. Stock rats exhibiting the same charac-
teristics at this time, however, led to a change in the food,
following which the **dire effects of inbreeding'^ prac-
tically disappeared. Whether this improvement in the
colony was due entirely to the change of diet or may be

106 INBREEDING AND OUTBREEDING

attributed partly to selective eUmination of the weaker
rats cannot be determined. We are inclined to agree with
Miss King in giving greater weight to the first factor,
though for a reason which she does not mention. The
general success of Miss King^s whole investigation we
believe to be due largely to the fact that the experiments
were started with stock rats which already must have
been very closely inbred and therefore in an approxi-
mately homozygous condition.

From the seventh generation on, selection was made
on the new-bom young with general vigor as the basis,
but the two lines were selected differently. In line A only
litters having an excess of males were selected to serve
as the progenitors of the succeeding generation, while in
line B the reverse was the case. The general result was
to show that the normal sex ratio in this species, 105 males
to 100 females, can be changed. At the end of nineteen
generations of selection, line A had produced litters hav-
ing a sex ratio of 122.3 males to 100 females, and line B
had produced litters having a sex ratio of 81.8 males to
100 females. From these facts there is no doubt but that
lines having an hereditary tendency to produce different
sex ratios can be isolated, but there is no evidence what-
ever in favor of the theory of Diising proposed in 1883 to
the effect that inbreeding by lessening the vitality of the
mother increases the percentage of male young. The
change in the sex ratio was made in two generations.
After that the effect of selection ceased. Such a result
not only militates against attributing the changed ratios
to inbreeding itself, but indicates that a relatively small
number of Mendelian factors are involved in the control.

The effect of continuous inbreeding on body weight is

INBREEDING EXPERIMENTS

107

shown in Fig. 25. This graph is constructed from data
collected from the records of males of line A, but graphs
constructed from the records of the females of this line
and from males and females of line B do not differ from

Growth in bo<^y wci
H S«ries A

ighl Albtno Rat
Males

PTn

120 140 160 180 200 220 240 260 260

Age in days

Fig. 25. — Graphs showing the increase in the body weight with^age for males of inbred
albino rats. (Series A.) A, graph for the males of the seventh to the ninth generati< r.u
inclusive; B, graph for the males of the tenth to the twelfth generatinns inclusive; C, graph
for the males of the thirteenth to the fifteenth generations iucluhive; D, giaph for the n.ult*
of the Grst six inbred generations. (After King.)

it in any essential feature. Curve D is further evidence
for concluding that the animals of the first six generations
suffered from malnutrition, since, as Miss King notes, it
is preposterous to suppose that these animals could have
given rise to the very large individuals represented by

108 INBREEDING AND OUTBREEDING

curve A \1 it really represented their true body weight.
How favorably these inbred strains compare with stock
animals is shown in Fig. 26.

Paralleling the results obtained for body weight were

20 40 eo

320 340 360 380 400 420 440 460 460

Fig. 26. — Graphs showing the increase in the weight of the body with age for
different series of male albino rats. A, graph constructed from Donaldson's data for stock
albinos; B, graph for males belonging in the seventh to the fifteenth generations of the
two series of inbreds combined; C, graph constructed from data for a selected series of
stock albinos used as controls for the inbred strain; D, graph for males belonging in the
first six generations of the two series combined. (AfterKing.)

those upon fertility and constitutional vigor as judged by
longevity. Neither was reduced by inbreeding; in fact,
there seems to be no doubt but that there was a significant
increase in both cases. There was a slight but definite
increase in fertility as is evident if one plots the theoreti-
cal curve which fits the experimental curve for litter size

INBREEDING EXPERIMENTS

109

throughout the course of the experiment (Fig. 27). The
whole series of inbreds compares well in this respect with
stock albinos for which the average litter size is G.7.
Further there was a notable increase in longevity in line
A and a marked increase in line B.

-■ttt-t: tT-T:!:tT"--

!:::::"":"":: !"V""M :■!;:;;: — "" nu: ' i :"!" !r'

-■■"ii" ::_4 _± -

_ : : : 1 : " ■ i ;;:;.! 1 It ri i : ; 1 ; !

1

~- - — 1-

I

_ _ ^ ..- _

__^ j'.i.^

8 Average Size of Litter - - - -

'

T

A -i.

• ^ " ±

■ L ML ^^Bi' ^« i \.^0ir^' ' ^^ \i.

I J ^ H L. ■ ■

^■! 1^ — ^yw 'r^r^ ^^Ni^'*'

^^ ^^^^ 1

' t M t f ■

^ *

K ^r 1

_ „ . -*«i

Im W _L-

1

*:::j| I-- "" :'_

m W

5- J «-*- _ —

_

M ^ __

1

_ _ - _ - _ _ ■ .

g_ _„__ -___..

_>_ — _ —

~ __ " _ :: : GENERATIONS :::::::: ,

L x X 1: - 3- — ^

4^x|^..|4ffl_ J..., .

tr^ii^iMMMi^lMM^II^m.l^N

11 <2 13 14 Ift 16 .17 l« 19 20 flf St fit 2« 25

Fia. 27. — Graph showing the average Bise of litters produced in Bucceseiye generatione of
inbreeding albino rats by brother and sieter matings. (After King.)

The interpretation of these experiments is wholly in
accordance with Mendelian theory. Starting with stock
rats which from previous close breeding had already been
reduced to a high degree of homozygosity, inbreeding had
the tendency to accentuate this purity of type and to
segregate slight differences. By selection vigorous uni-
form strains were built up, strains somewhat larger, more

110 INBEEEDING AND OUTBREEDING

fertile and longer lived than many strains of stock rats.
It is clear that this was the result of Mendelian recom-
bination for the two lines A and B were in the end some-
what different. The rats of line A were slightly more
fertile, attained sexnal maturity earlier, and lived longer
than those of line B, If this evidence were not sufficient, it
is supplemented by the fact that variability gradually be-
came reduced during the progress of the inbreeding.

The investigations of the effects of inbreeding on the
guinea-pig, to which we have referred, were begun in
1906 by G. M. Rommel of the United States Department
of Agriculture. In recent years the work has been in
charge of SewaU Wright, who has made a very illumi-
natiag analysis of the results obtained.

This series of experiments was started with thirty-
three pairs of stock animals which had been more or less
inbred previously. Although maintained exclusively by
mating sister with brother, sixteen of these families were
in existence at the close of 1917 after some twenty gen-
erations of the closest inbreeding.

Considered as a whole this inbred race shows distinct
evidence of having declined in every character connected
with vigor. The litters are smaller and are produced
more irregularly. The per cent, of mortality both in utero
and between birth and weaning has increased. The birth
weights are lower and the rate of growth slower than in
control stock. In spite of these facts, however, one is
forced to the conclusion that these results are not the
effect of inbreeding as a direct cause, but are to be attrib-
uted to Mendelian segregation.

There are pronounced differences between the various
families. Some are still very vigorous, comparing favor-

C
c

a:

3

o

O

3

a>
•-)
p

o'

3

3-
O

o
CO

CO

O
m

3

cr
1-1

3

INBREEDING EXPERIMENTS 111

ably with the original stock; others degenerated so rap-
idly that they soon became extinct in spite of every etl'ort
to prevent such a catastrophe. Among the families still
in existence, there is even evidence that vigor as a general
term may be divided into various causative factors and
that these factors may be combined in various ways, liy
grading each family for various characters connected
with vigor of growth and reproduction and then classi-
fying each family in numerical order for each separate
character, Wright has been able to show conclusively
that there ar^ many hereditary factors which alfect fer-
tility, growth and vitality and that almost any combina-
tion of these characters may become fixed in a family
through inbreeding.

A little later we shall have occasion to speak of sev-
eral noteworthy end results obtained by inbreeding the
larger domestic mammals, but no further discussion
seems advisable in this place because of the lack of quan-
titative data. A similar statement holds for birds.

The fruit fly, Drosopliila ynelanogaster^ is the only
insect which has been used for extended experiments on
effects of inbreeding, although there are numerous
examples on record where an importation of a rela-
tively small number of individuals has resulted in an
overwhelming increase — witness the gypsy moth in
New England.

Castle and his co-workers ^^ bred Drosophila for
many generations by continuous brother and sister mat-
ings. After fifty-nine generations of this close inbreed-
ing the fertility did not appear to be reduced below that
shown by the original stock, although it was increased
by crosses between certain inbred lines. There was some

112 INBREEDING AND OUTBREEDING

indication of reduction in size when inbred flies were
compared with random mated stock reared under the
same conditions. Far from being exterminated by in-
breeding, however, the flies at the end of the experi-
ment were apparently fully equal to those with which it
was begun,.

These experiments showed clearly that inbreeding
results in strains of unequal fertility. The less fertile
tended to be eliminated by differential productiveness, so
that only the more fertile remained. The occurrence of
absolute sterility was pronounced in the first part of the
experiment, but almost entirely disappeared in the later
generations. The figures as calculated from their table
are as follows :

Per cent.

of matings

totally sterile

Generations 6 to 24 17.80

Generations 25 to 42 18.47

Generations 43 to 59 3.37

Such a result is to be expected when it is remembered
that inbreeding produces homozygous individuals, and
these, whenever sterile, are, of course, eliminated.

Moenkhaus,^^^ Hyde,^^ and likewise Wentworth,^^^
by similar inbreeding experiments with Drosophila
found sterility, though increased in the first stages of
inbreeding, tended to be eliminated after the process
was long continued.

The only other experiments on invertebrates which
ought to be cited here are those of Whitney ^^^ and A. F.
ShuU ^^^ on the rotifer Hydatina senta. Both of these
investigators found that inbreeding had a considerable
adverse effect on the size of family, number of eggs laid

INBREEDING EXPERIMENTS 113

per day, rate of growth, and variability. The proi>er iii-
tei-pretation of their resultis is somewhat obscure, unless
one postulates the origin of frequent mutations. Thu
number of generations bred and the number of families
under observation were not sufficient to demonstrate the
segregation of differences in these characteristics, thuugh
this is to be expected since these qualities are sympto-
matic of general, vigor and general vigor was increased
by crossing. The difficulty, however, lies in the fact that
continued parthenogenetic multiplication which is possible
in Hydatina had the same effect as continued inbreeding.
Shull introduces the interesting speculation that this sim-
ilarity is due to a gradual adjustment of nucleus to cyto-
plasm during the asexual propagation — this being as-
sumed to bring about the same results as a gradual
approach toward homozygosis. We are inclined to at-
tribute both changes to environmental causes, believing
that if a proper change in diet had been made vigor would
have been maintained.

While we are not justified in concluding from these
experiments that inbreeding accompanied by rigid selec-
tion will be beneficial to bisexual animals, they certainly
show close mating is not invariably injurious. They in-
dicate that the results of inbreeding depend more upon
the genetic composition of the individuals subjected to
inbreeding rather than upon any pernicious intluenc^e in-
herent in the process itself; and, as will be emphasized
more strongly later, it is a wholly different matter
whether inbreeding results injuriously through the inlieri-
tance received, or whether consanguinity itself is respon-
sible. Yet such a status for the problem is unsatisfactory.
The experiments on animals bring to light no facts which

8

114 INBEEEDING AND OUTBREEDINa

may not be interpreted as the result of Mendelian factor
recombination ; but if one were to base his judgment on
them alone, he could not truthfully make the didactic state-
ment that inbreeding per se is not injurious. There would
ever be the uncertainty with which the additional variable
bisexuality always encumbers a genetic experiment. For-
tunately, we may turn to the numerous experiments on
hermaphroditic plants for the deciding vote.

Many wild species and cultivated varieties of plants
are almost invariably self -fertilized, and apparently lack
nothing in vigor, productiveness or ability to survive.
Amon^ wild plants many species of the family Legumi-
nosae, among cultivated plants — ^wheat, rice, barley, oats,
tobacco, beans, tomatoes — are types characterized by very
nearly continuous self-fertilization, and these plants are
in no immediate danger of extinction.

On the other hand, the majority of the higher plants
is provided with devices which promote natural cross-
pollination, and show definite injurious effects when in-
bred artificially. Even species which are characteristi-
cally self-fertilized are crossed occasionally. This, to-
gether with the fact that nearly all plants and animals
are benefited by crossing, led Knight as early as the close
of the eighteenth century to believe self-fertilization is
not a natural process and always produces more or less
injurious results. His views were summed up in the state-
ment, * * nature intended that a sexual intercourse should
take place between neighboring plants of the same spe-
cies.'* Darwin, fifty years later, basing his conclusions
upon observation of animals and direct experimentation
with plants, was even more radical, and concluded that
** nature abhors perpetual self-fertilization."

INBREEDING EXPERIMENTS 115

Darwin compared self-fertilizefl plants witli iiit<'r-
crossed plants in many different species. In tlie majority
of cases the self -fertilized plants were clearly inferior
to the crossed plants. These facts led to the belief that
the evil effects of inbreedin.2: kept on accnmnlatinsr nntil
eventually a plant or animal continuously reproducing,^ in
that manner was doomed to extinction. His own ex]>eri-
mental results came far short of proving such an assump-
tion, however. The two plants with which inhrc'Oflinir was
practiced the longest — Ipomea and Mlmnlu^ — showed
very little further loss of vigor after the first generation.
What the experiments did show, most clearly, was segre-
gation of the inbred stock into types differing in tlieir
ability to grow as well as in minor, visible, hereditary
characters. In both species plants appeared wliich were
superior to other plants derived from the same source,
some being equal or even superior in vigor to the original
cross -pollinated stock. The inbred plants differed from
the original material most noticeably in the uniformity
of visible characters. Darwin ^s gardener stated it was not
necessary to label the plants, as the different lines were
so distinct from each other and so uniform among them-
selves they could easily be recognized.

After several generations of inbreeding, Dar^vin
found it made no difference in the resulting vigor whether
the plants in an inbred lot were selfed or were crossed
among themselves. This he correctly ascribed to the fact
that the members of such an inbred strain had become
germinally alike. With less justice he attributed this
approach to similarity in inherited qualities to tlie fact
that the plants were grown for several generations under

116 INBEEEDING AND OUTBREEDING

the same conditions, but it is easy to see why he held so
tenaciously to this view if one remembers the faith he had
in the effect of environment on organisms. Such a view
he deemed supported by the fact that crosses of selfed
lines with the intercrossed lines (also inbred, but to a less
degree) did not give as great increases in growth as
crosses of selfed lines with fresh stock from other local-
ities. His crosses between inbred lines did give noticeable
increases in growth, however, in many cases equaling the
original variety. This is well illustrated by Dianthus, in
which the selfed line was crossed both with the inter-
crossed line and with a fresh stock. The ratio of each
crossed population to the selfed population in height,
number of seed capsules, and weight of seed produced is
as follows :

Selfed Selfed

X X

Intercrossed Fresh Stock

Height, comipared to selfed plants 100 :95 100 :81

No. capsules oompared to selfed plants 100:67 100:39

Weight of seed compared to selfed plants 100 :73 100 :33

With Darwin we still attribute the greater increase
of vigor in crosses of distinct stocks to a greater germinal
diversity, but we differ from him as to the way in which
that diversity is brought about. Be that as it may, great
credit is due Darwin for being the first to see it was not
the mere act of crossing which induced vigor but the union
of different germinal complexes. This he states clearly
in the following sentences ('* Cross and Self -Fertilization
in the Vegetable Kingdom,'' p. 269) : *'A cross between
plants that have been self -fertilized during several suc-
cessive generations and kept all the time under nearly

INBREEDING EXPERIMENTS 117

uniform conditions, does not benefit the offsprin^r in the
least or only in a very slight degree. Mimulus and the
descendants of Ipomea, named Hero, offer instances of
this rule. Again, plants self-fertilized during several
generations profit only to a small extent by a cross with
intercrossed plants of the same stock (as in the case of
Dianthus), in; comparison with the effects of a cross by
a fresh stock. Plants of the same stock intercrossed dur-
ing several generations (as with Petunia) were inferior
in a marked manner in fertility to those derived from
the corresponding self-fertilized pknits crossed by a fresh
stock. Lastly, certain plants which are regularly inter-
crossed by insects in a state of nature, and which were
artificially crossed in each succeeding generation in the
course of my experiments, so that they can never or most
rarely have suffered any evil from self-fertilization (as
with Eschscholtzia and Ipomea), nevertheless profited
greatly by a cross with a fresh stock. These several cases
taken together show us in the clearest manner that it is
not the mere crossing of any two individuals which is
beneficial to the offspring. The benefit thus derived de-
pends on the plants which are united differing in some
manner, and there can hardly be a doubt that it is in the
constitution or nature of the sexual elements. Anyhow,
it is certain that the differences are not of an external
nature, for two plants which resemble each other as
closely as the individuals of the same species ever do,
profit in the plainest manner when intercrossed, if their
progenitors have been exposed during several genera-
tions to different conditions.'^

Unfortunately, in Darwin's time the key to the sola

118 INBREEDING AND OUTBREEDING

tion of the problem of inbreeding was lacking. Mendel's
work was yet unrecognized ; the principles of inheritance
of separate characters, of segregation, of chance recom-
bination, Darwin was not permitted to know. Had he
realized the way in which recessive characters can be con-
cealed for many generations without making their ap-
pearance until homozygosity was brought about by in-
breeding, doubtless his views on the subject would have
been materially changed.

As we have just indicated, and as we shall have occa-
sion to emphasize again, the greatest advance in our
knowledge of the significance of inbreeding has come
through linking its effects with Mendelian phenomena.
The first experiments on the subject made in the light
of this discovery were those of G. H. ShuU and of East,
undertaken independently in 1905 with maize, an ideal
cross-fertilized species, as the subject.

Shuirs investigations were not begun with the object
of studying the effects of self-fertilization, but the studies
having involved parallel cultures of cross-pollinated and
seff-pollinated lines, it was impossible not to have noticed
the smaller stalks and ears and the greater susceptibility
to attacks of the corn-smut (Ustilago maydis) shown by
the latter. Interest thus aroused, data were collected
bearing on the subject of inbreeding, and in 1908 his first
conclusions on the subject were published.

His observation that the progeny of every self-fer-
tilized maize plant is inferior in size, vigor and productive-
ness to the progeny of a normal cross-bred plant derived
from the same source, corroborated preceding investi-
gations made by Morrow and Gardner ^^2' ^^^ and

INBREEDING EXPERIMENTS 1 1 9

Sliamel ^^^ ; but the conclusion whicli lie drew was now.
The universality of this decrease in vigor was to Shall a
proof that the injurious effect of inbreeding could not be
due to an accumulation of deficiencies possessed by the
parents since superior and inferior parents yielded sim-
ilar results. Further, Shull noted that this decrease in size
and vigor accompanying self-fertilization, instead of
proceeding at a steady or even at an increasinir rate as
might be expected from this older view, actually became
less and less in succeeding generations — ])resiiinably in-
dicating an approach to stability. The neatness with
which these observations fit a ^Nfendelian interpretation of
inbreeding did not escape notice. It was pointed out how
one might consider a com field to be a collection of com-
plex hybrids whose elementary components may lie
separated by self-fertilization through the operation
of the fundamental Mendelian laws of segregation
and recombination.

"With this working hypothesis the investigations were
continued for several years, papers on the subject ap-
pearing in 1909, 1910 and 1911. Evidence of the hybrid
nature of ordinary commercial maize plants and their de-
pendence upon hybridity for their vigor was found in the
decided differences in definite, hereditary, niorpholoia-
cal characters exhibited by self-fertilized families having
a common origin, but a further proof of the validity of the
hypothesis came in testing the conclusions to which the
view leads. Obviouslv crosses between ])lants of a single
family, which by long-continued self-fertilization has be-
come homozygous in nearly all its characters, should show
little increase in vigor over self-fertilization; but crosses

120

INBEEEDING AND OUTBREEDING

between distinct self -fertilized lines should often result
in high-yielding F^ generations possessing great vigor
and showing a high degree of uniformity. Again, crosses
between different near-homozygous strains, though uni-
form and vigorous in the Fj generations, should become
much more variable and much less vigorous in the F2
generation. These general propositions Shull tested in
a limited way in 1910 after his families had been self-fer-
tilized for ^ve generations. The variability of two such
strains and the crosses between them for a definite and
easily determined character — number of rows per ear —
is shown in the following table :

Strain

Mean

Coefficient of variation

A
B

AXB (FO
BXA (Fi)
AXB (FO
B X A (FO

8.30±.06
14.10±.15
12.71±.15
11.77±.07
11.84±.ll
13.79±.ll

8 . 50 per cent. ± . 47 per cent.

9.66 per cent. ± .74 per cent.
10.00 per cent. ± .87 per cent.

8. 13 per cent. ± .42 per cent.
14.64 per cent, i .67 per cent.
10.62 per cent. ± .56 per cent.

Clearly the F^ generation, made with either type as the
mother, is as uniform as the parent strains, but the F2
generations are both more variable.

To test the other corollaries, nine different self-fer-
tilized families of the fifth generation were compared
with families obtained by crossing two plants belonging to
each family; seven families were raised as first-genera-
tion hybrids between these different selfed strains; ten
crosses between F^ individuals were compared with ten
self-fertilizations in the same families ; and ten families
were grown in which self-fertilization had been precluded
for five years. The average height in decimeters, number

INBREEDING EXPERIMENTS

121

of rows per ear, and yield in bushels per acre of these
fifty-five families are g-iven in the following table:

Selfed

Fclfod
X Kibs

Fi

F.

F,

Holfed

Fi Sib
CroMes

Cro»-
breds

Average height . .
Average No. rows
Average yield

19.28

12.28
29.04

20.00
13.20
30.17

25.00
14.41
68.07

23.42
13.07
44 . 02

23.55
13.02
41.77

23.30
13.73
47.40

22 . 05
15.13
01.52

The sister-brother (sib) crosses give a slightly greater
height, number of rows per ear and yield per acre than
the corresponding self-fertilized families, an indication,
as Shull states, of some heterozygosis still remaining in
the selfed families ; in other particulars Mendclian expec-
tation is wholly confirmed.

The experiments of Shull on the effect of inbreeding in
maize were continued only from 1905 to 1911. We may be
pardoned, therefore, if we describe the experiments be-
gun in 1905 by East at the Connecticut Agricultural Ex-
periment Station in somewhat greater detail, for they are
still being carried on by Jones. In fact, in point of num-
bers and scope they are the most extensive experiments
on the problem of inbreeding. The general method of
procedure has been merely to self-pollinate individual
plants from different varieties of all the principal types
of maize. The seed from such self-fertilized plants has
been grown and some plants again self-fertilized. Thus
a selfed plant has been the parent of each population.
Over thirty different varieties, with several lines in
each variety, have been inbred in this way. The old-
est strains have now been self-fertilized for twelve
consecutive generations.

In every case there has been a reduction in size of
plant and yield of grain. Besides this result, to which

122 INBREEDING AND OUTBREEDING

there has been no exception, the several inbred lines
originating from the same variety have become more or
less strikingly differentiated in morphological characters.
Some of the differences which characterize the several
inbred strains in various combinations are as follows :

Colored and colorless pericarps, cobs, silks and glumes.
Profusely branched tassels and scantily branched or unbranched

tassels.
Long ears and short ears.
Round cobs and flat cob®.
Narrow silks and broad silks.
Ears with various numbers of rows.
Ears with straight rows and ears with irregular rows.
Ears with large seeds and ears with small seeds.
Ears high on the stalk and ears low on the stalk.
Stalks with many tillers and stalks with few tillers.
Leaves with straight margin and leaves with wavy margin.

Many other character differences governed by definite
inherited factors have been observed, but these may serve
as illustrations.

Along with these normal differences a number of
characters have appeared which might well be called mon-
strosities, using the term not because of any abnormality
in the method of their inheritance, but because they are
not fitted to struggle for place either in agriculture or in
nature. A common occurrence is the isolation of dwarf
plants which are rarely capable of producing seed from
their own pollen. Plants manifesting various degrees of
chlorophyll deficiency are also frequently found. This
may show in the form of an entire lack of chlorophyll, as
seen in pure albino plants which live only until the supply
of food in the seed is exhausted; or, it may appear as a
yellowish green, the plants struggling through to seed

INBREEDING EXPERBIENTS 123

production— though with some diniciilty. Some phuita
are obtained with ear malformations and thus produce
but a minimum amount of seed. Other phmts hick brace
roots and are unable to stand upright. Still others show
vaiious grades of pollen and ovule abortion, and suscepti-
bility to disease.

The variability of the inbred lines in respect to the
above characters decreased as inbreeding was continued.
After four generations they were practically constant for
the grosser characters. From the eighth generation on
they have been remarkably uniforai in all characters.

Inbreeding the naturally cross-pollinated maize plant,
then, has these results:

1. There is a reduction in size of plant and in produc-
tiveness which continues only to a certain point and is in
no sense an actual degeneration.

2. There is an isolation of subvarieties differing in
morphological characters accompanying the reduction
in growth.

3. As these subvarieties become more constant in their
characters the reduction in growth ceases to be noticc^able.

4. Individuals are obtained with such characters
that they cannot be reproduced or, if so, only with
extreme difficulty.

A large amount of data has been obtained upon which
to base these statements, but since most of them have been
published it seems desirable to include only a few illus-
trations here. The strains which have been the longest
inbred will serve to show something as to the effect which
inbreeding has had upon yield of grain, lieight of ])lant
and other maize characters.

The original experiment began ^nth four individual

124

INBEEEDING AND OUTBEEEDING

plants obtained from seed of a commercial variety grown
in Illinois known as Learning Dent. This variety was
given the number 1, and four plants which were self-
pollinated and selected for continuation of the inbreeding
experiment were numbered 1-6, 1-7, 1-9, and 1-12. These
four lines were perpetuated each year by self-pollination
and will be referred to hereafter as the Leaming strains.
In the second inbred generation two self -pollinated plants
in the 1-7 line were saved for seed and from them two
inbred lines were split off which consequently came origi-
nally from one line inbred two generations. These were
numbered 1-7-1-1 and 1-7-1-2. Many other inbred strains
coming from different material have been started from
time to time and several of them are still being continued.
There is no need to mention them specifically, except as
they bring out special features.

TABLE III
The Effect of Inbreeding on the Yield and Height of Maize

Year
grown

1916
1905
1906
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917

No. of
genera-
tions
selfed

Four inbred strains derived from a variety of Leaming dent corn

0
0

1

2
3
4
5
6
7
8
9
10
11

1-6-1-3-etc.

Yield

bu. per

acre

74.7
88.0
59.1
95.2
57.9
80.0
27.7

41.8
78.8
25.5
32.8
46.2

Height
inches

117.3

l-7-l-l-etc.

Yield
bu. per

acre

86.7

96.0

" 97.7
103.7

74.7
88.0
60.9

1908460

63.2
25.4

Height
inches

1-7-1-2-etc.

1-9-1-2-etc.

117.3

39.4
47.2
24.8
32.7
42.3

81.1

83.5

84.9
78.6

Yield

bu. per

acre

74.7
88.0

60.9
190759.3

1908597

68.1
41.3

Height
inches

117.3

90.5

58.5

19.2
37.6

88.0

86.9
83.8

Yield 1

bu. per

Height
inches

acre

74.7

117.3

88.0

42.3

51.7

35.4

47.7

26.0

76.5

191338.9

1914454

85.0

191621.6

1 "630.6

78.7

191^31.8

82.4

INBREEDING EXPERIMENTS 125

In Table III the yield of grain and heiglit of plant of
the four inbred Learning strains are given in the suc-
cessive generations of self-fertilization. In 191G seed of
the original variety, which had been gro\vn in the mean-
time in the locality in Illinois from whence it was originally
secured, was obtained and grown for comparison with the
inbred strains. This variety in Illinois in 1905 yielded at
the rate of 88 bushels of shelled grain per acre and in
Connecticut in 1916 at the rate of 75 bushels. There is no
reason for supposing that the variety had changed to any
great extent in the intervening years. Coming from Illi-
nois, it was placed at a disadvantage as compared to the
inbred strains, because it was not adapted to the local
conditions, while the inbred strains, grown for several
years, had been selected more or less unconsciously to
meet the prevailing conditions. Even with this in favor
of the inbred strains they yielded only from one-third to
one-half as much as the original variety grown under the
same conditions.

With regard to the rate of reduction in yield or the
constancy of the varieties during the later generations,
it is difficult to draw conclusions from these figures, owing
to the fluctuation in yield from year to year due to sea-
sonal conditions and to the difficulty of accurate testing in
field plot work, a fact recognized by all who have made
such tests. No yields for any of the strains were taken
in 1912. The yields for 1909 and 1915 were too low on
account of poor seasons. The yields in 1914 were too high
for the opposite reason. In 1915 the. yields were unreliable
because only a few plants were available for calculation,
most of the plants having been used for hand pollinations.

126 INBREEDING AND OUTBREEDING

In 1916 and 1917 the inbred strains were grown in some-
what larger plots and the yields are fairly reliable.

With these points in mind, an examination of the table
shows that from the beginning of the experiment to the
ninth generation there has been a tremendous drop in
productiveness, so that in that generation the strains were
approximately only one-third as productive as the variety
before inbreeding. From the ninth to the eleventh gen-
eration there has been no reduction in yield and prac-
tically no change in visible characters. Height of plant,
as far as the available figures show, followed the same
course. The reduction which has taken place occurred in
the first eight generations ; after that there has been no
appreciable change.

All along the several Learning strains have shown
considerable difiFerences in productiveness and in height.
Strain No. 1-6 has given the largest yields and the tallest
plants. It gave nearly 50 per cent, larger yields than the
poorest yielding strain in the eleventh year, and was about
30 per cent, higher than the shortest strain.

One of the strains, No. 1-12, was lost in the sixth gen-
eration. Previous to this time it had been the poorest of
the five. It was partially sterile, never produced seed at
the tip of the ear and was perpetuated only with care.
Since the difficulty of carrying along any inbred strain is
great, owing to failure to pollinate at the correct time, to
attacks of fungus on the ears enclosed in paper bags, and
to poor germination in the cold, wet weather common in
New England at com-planting time, the loss of this strain
might be easily accounted for without assuming continu-
ous deterioration. The strain probably could have been
retained if sufficient effort had been put forth; but in

INBREEDING EXPERIMENTS 127

view of the further reduction in other strains, it would
have been extremely diliicult. {Since plants are frequently
produced which cannot be perpetuated, however, it is
to be expected that some strains will also be found
which cannot survive. This is good evidence that strains,
differing markedly in their ability to grow, are isolated
by inbreeding.

Plants of the surviving strains, while smaller in size
and lower in productiveness, are perfectly healthy and
functionally normal in every way except that in many of
them there is an extreme reduction in the amount of pollen
produced. These infertile types are dependent on other
plants for pollen in order to make the yields they show
in open field culture ; when grown by themselves the yield
is less due to an inadequate supply of pollen. On the other
hand, this extreme reduction in pollen production is not
shown by all the strains, some inbred strains producing
pollen abundantly.

Prom the data given in Table III there is considerable
evidence that these plants have reached about the limit
of their reduction in size and productiveness and tluit
whatever changes have taken place in the last three years
have been slight. Further inbreeding is necessary for
one to be positive on this point. But as the crosses within
these inbred strains have given no significant increases
over the selfed lines, and as there has been no visible
change in morphological characters, in the past three
years at least, it seems apparent tliat the reduction in
vegetative vigor and productiveness is ver>' nearly, if not
quite, at an end.

Reduction is shown by inbred maize plants in other
characters. Length of ear, as well as height of plant and

128 INBREEDING AND OUTBREEDING

yield of grain, is smaller. There is also a slight reduction
in number of nodes and in rows of grain, but in contrast
to the other three characters the change is almost negli-
gible. The last two are only slightly affected by environ-
mental factors as compared with the others. A plant may
be reduced to one-half its normal height by being grown
in a poor situation, but the number of nodes will be nearly
the same in the two cases. Hence, we see that inbreeding
affects plants much in the same way as poor environmen-
tal conditions.

In all of the characters mentioned there is a reduction
in variability and change in mean differing in the several
lines. This is illustrated in Table IV, in which are given
the data for number of rows of grain on the ear of four
different plots of the origiual non-inbred variety and
four strains derived from this variety after ten genera-
tions of self-fertilization. The marked reduction in vari-
ability is apparent both in the restricted range of the
distribution of the inbred lines compared to the variety,
and in the coefficients of variability.

This reduction in variability applies only to each in-
bred line separately. If all the different lines were com-
bined together into one population the variation would
be greater than that shown by the original material. This
is readily apparent from the table ; it also follows from
the fact that many characters are produced by inbreed-
ing which are seldom seen in the regularly cross-
pollinated stock. Inbreeding reduces variability within
separate lines, but increases variability in the descend-
ants as a whole.

From the curves on inbreeding given in the preceding
chapter (Fig. 24), it was seen that the production of com-

INBREEDING EXPERIMENTS

129

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130

INBREEDINa A^D OUTBREEDING

pletely homozygous types by self-fertilization is greatest
in the generations from the third to the sixth if a large
number of factor differences are involved at the start.
The experimental results obtained from these inbred
strains of maize fit this theory well. It is not until after
about three generations of self-fertilization that extreme
types begin to appear. While there has been a reduction
in size and productiveness before this, it is at this time,
or during the next two or three generations, that the
greatest diversity of types occurs. It is here that most
of the monstrosities and plants which are unable to re-
produce themselves appear.

From Table IV we see that equally striking changes
in the mean row number also take place. The averages
have been shifted both up and down from the original
conditions. The greatest segregation has taken place
between the first and the eighth generations. In the eighth
generation the lines were again split up, but show no
marked change after this point. Differences ia the ears
of these inbred strains of com are shown in Fig. 29.

The rate of reduction in variability and rate of change
of mean are shown by the data for row number of two
of the inbred strains for successive years in Table V and
Fig. 30. These two Hues are descended from the same
plant in the second year of self-fertilization. The figures
previous to the third year are not available, and in that
year only for one of the strains, but since then a marked
change in average row number, and a reduction in vari-
ability have taken place without conscious selection one
way or the other. Though the number of plants grown in
the generations from the 7th to the 10th are too few to be
a basis for accurate conclusions, the sharp increase in

";AS<i'>

^v

:-y^

y .

."^^

m

K

"N •

Fio. 29. — Representative samples of inbred strains of maize after 11 tifncratmn.s <i| htlf.
fertilization showing cliaracteristie ciifTereuces but uniformity within each »train.

INBREEDING EXPERIMENTS

131

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132

INBREEDINa AND OUTBREEDING

average row number and decrease in variability in the
eighth generation are probably due to the favorable grow-
ing conditions of that year — ^witness the high yields for
the inbred strains in that year as given in Table III. The
apparent rise in variability after the eighth generation
is in part due to the fact that the ears had become some-

4»
C
V

■H
O

t

%4

tt)

n

20 ,

15 .

U
o

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rt C o

Ave. No. RoVS
of 1-7-1-1

Ave . No . Rows
of 1-7-1-2

Ave.C.V.

— T—

z

4 S 6 7 8 9 10 U

Generations Intred

Fia. 30. — Graphs showing reduction of variability and segregation of ear row number

in selfed strains of maize.

what more irregular in row number, so that accurate de-
termination of number of rows has been more difficult in
the later generations. However, this rise is more appar-
ent than real as the values for the coefficients of variabil-
ity in the intermediate generations are probably lower
than they would have been if an adequate number of
plants had been grown.

The effect of inbreeding upon variability is even more

INBREEDING EXPERIMENTS 133

apparent in details of plant and oar structure which are
difficult of statistical expression. The beautiful uni-
foimity of these plants in all characteristics at the pres-
ent time is one of their most striking features. This can
be seen fairly well for the ear chanicters in accompany-
ing illustrations (Eig. 29). In the minutiie of the tassels,
leaves and stalks they show the same striking uniformity.
These minor details which characterize each of these
groups of plants are difficult to describe adequat(^ly, but
are perhaps the most noticeable feature about them.
The tassels or the ears of all these four Learning
strains, if mixed together, could be separated without
the slightest difficulty.

Some characters appear so rarely in plants they have
been generally considered to be due to what might be
called physiological accidents rather than to inheritance.
An illustration of this kind is furnished in maize by the
occurrence of doubled or connate seeds. Instead of one
embryo enclosed in a pericarp, separate embryos and
endosperms are present, with the seeds arranged back
to back and the embryos facing in opposite directions.
A few seeds of this kind have been described from time
to time, but never more than one or two on an occasional
ear. From twelve inbred strains of a variety of maize
other than the ones previously described, two linos have
been obtained which produce these peculiar seeds as a
common feature. One of the strains show^s from one to
six or more on practically every ear. The second strain
shows them more rarely and the other ten strains derived
from the same variety have never been obsor\'ed to boar
them. Here, then, is a character which does not appear
except at rare intervals when the plants are crossed

134 INBREEDING AND OUTBREEDING

and in full vigor. When the plants are brought to
homozygosity and the vigor of the plants is reduced, the
doubled seeds appear in abundance in some lines, but not
in all. A character, then, may be governed in its ex-
pression by other characters and modified by the vigor
of the plant, but in the final analysis it is dependent upon
definitely inherited factors.

In the same way such indefinite and complex charac-
ters as susceptibility and resistance to disease are shown
to be capable of segregation. In 1917 one of the inbred
Learning strains had not a siugle plant affected by the
smut fungus, although 1000 plants were grown in differ-
ent places. Other strains derived from the same variety
and grown side by side with the susceptible race showed
from 5 to 10 per cent, of plants infected. Susceptibility
of maize to smut thus seems to be dependent upon in-
herited factors. As the result of inbreeding, these fac-
tors may be segregated tato some lines and not into others.

Although there has been a striking reduction in size of
plant, general vegetative vigor and productiveness, and
in comparison with non-iribred varieties the inbred plants
are more dififtcult to grow, emphasis must be put upon the
fact that they are normal and healthy. No actual degen-
eration has occurred. The monstrosities which are com-
mon in every field of maize, such as the occurrence of
seeds in the tassels, anthers in the ears, dwarf plants,
completely sterile plants, and other similar anomalies, now
no longer appear in these inbred strains. These facts,
taken together, should be sufficient to demonstrate beyond
doubt that by far the greatest amount of the general vari-
ability found among ordinary cross-fertilized plants is
due to the segregation and recombination of definite and

INBREEDING EXPERIMENTS 135

constant hereditary factors. Some of the characters
which appear after long-continued inbreeding are seldom
seen in continually cross-pollinated plants, and never are
so many seen in combination. This is because they are
recessive in nature and complex in mode of inheritance
The most significant feature about the cliaracters wliirh
make their appearance in inbred plants is that none of
them can be attributed directly to a loss of a physiological
stimulation, although undoubtedly many of them may be
modified by the vigor of the plants upon which they are
borne. There is no one specific feature common to all
inbred strains, but simply a general loss of vigor, a gen-
eral reduction in size and productiveness accompanied by
specific characters more or less unfavorable to the
plant's best development. But these unfavorable char-
acters are never all found in one inbred strain, nor is any
one of them found in all inbred strains.

Although no systematic selection has been practiccnl
throughout these inbreeding experiments, a great deal of
selection upon many characters has been unavoidable as
is the case in any inbreeding experiment. In maize, the
difficulties of hand pollination result in the selection of
plants whose staminate and pistillate parts are matured
synchronously. Any great difference in this respect, par-
ticularly towards protandry, renders self-fertilization
difficult or impossible as the pollen is viable but a short
time. Of course, all plants wiiich are weak, steril«\ dis-
eased or in any way abnormal, tend to become eliminated
wherever these causes reduce the chance of obtainincr seed.
This unconscious selection becomes more ricrid in the lat«^r
generations of inbreedin.s: as reduction in vicror and pro-
ductiveness becomes more pronounced. Acrain, the small

136 INBREEDING AND OUTBREEDING

amount of seed produced by hand pollination under the
most favorable circumstances, necessitates the using of
the best ears obtained for planting in order to have
enough plants upon which to make any fair observations.
These factors tend to prevent the attainment of com-
plete homozygosity. Nevertheless, all the evidence at hand
indicates that the four strains of Leaming com which have
been continuously self-fertilized for twelve generations
are now very nearly, if not completely, homozygous in all
inherited characters. As stated before, this evidence com-
prises cessation of reduction in size and productiveness,
of reduction in variability, and of change of average row
number and other characters. But there are still other
ways of testing the proposition. On the theory that in-
crease in growth results from crossing when the individu-
als united differ in respect to some inherited qualities, if
no increase results, then the parents have no differences.
These strains have been tested in this way by crossing
different plants within a strain and comparing the crossed
plants with self ed plants. While some increases in growth
resulted from such crossing they were balanced by de-
creases in other cases, so that the inconsistencies are most
likely due to difficulty in securing an accurate test. At
the same time one should not shut his eyes to the possi-
bility that some of the strains have reached complete
homozygosity, while others, as yet, have not ; although no
sure evidence of such a state of affairs has been obtained.
Most of the direct experimentation to determine the
effects of inbreeding has been with cultivated plants and
domestic animals. The question will undoubtedly be asked,
therefore, as to whether the results would have been the
same had wild species been investigated. It would be

INBREEDING EXPERIMENTS 137

futile to maintain that there is every reason to supjwse
wild species should behave exactly as their domestic
cousins. Wild types, in general, nii.irht not present such
an appearance of injur>^ under inbreeding as is often
sho^vn by cultivated species. This would not be due to
differences in their method of inheritance, however, but
because wild species are usually exposed to a more rigor-
ous struggle for existence and tlic indivi(Uials are, there-
fore, less likely to differ by a large number of hereditary
factors. For such reason one should expect experiments
on different wild species to give rather varied results, and
in the comparatively small number which have ])e('n made
this is the case. Castle's experiments on the fniit fly pave
no markedly unfavorable results. Collins states that self-
fertilizing teosinte, a semi-wild relative of maize, causes
no loss of vigor. Yet Darwin compared self-fertilizod and
intercrossed plants of several species which are largely
cross-fertilized in the wild with great disadvantage to
the former.

This discussion of the effects of artificial inbreeding
in certain plants and animals has been given in some de-
tail in order to bring out the many important considera-
tions involved. There has even been repetition in order
to emphasize the most important points. Details are
merely by way of parenthesis, however. Let us now get
out of the parenthesis and into the main ar^irument.

From the preceding ohservatioyi it can he said that
inbreeding has hut one demonstrable effect on organisms
subjected to its actioii—the isolation of homozygnu^
types. The diversity of the resultmg types depends di-
rectly upon number of heterozygous hereditary factors
present in the individuals with which the process is he-

138 INBEEEDING AND OUTBEEEDING

gun; it is likely, therefore, to vary directly with the
amount of cross-breeding experienced by their immediate
ancestors. The rapidity of the isolation of homozygous
types is a function of the intensity of the inbreeding.

Take the case of maize as an example. Maize is one
of the most variable of cultivated plants, and is usually
cross-pollinated under natural conditions. In other
words, the individuals making up any commercial variety
of maize are each and every one heterozygous for a large
number of hereditary factors — a heterozygosis that is
kept up by continual crossing and recrossing. "When such
a variety is inbred there is automatic isolation of homo-
zygous combinations, following simple mathematical laws
as we have already seen. If self-fertilization is practiced,
stabilization through an approximately complete homo-
zygosis occurs after a relatively small number of genera-
tions ; if a less intense system of inbreeding is followed,
the result is the same, but it is obtained more slowly. Dur-
ing thisf process, before stabilization is reached, there is
reduction in size, vigor and productiveness following
somewhat roughly the reduction in per cent, of hetero-
zygousness. We can think of this reduction in vigor as a
change correlated with approaching homozygosis if we
wish, although as we shall see there is reason to believe it
to be a result of linked inheritance. What does occur is
a reduction in vigor of the population as a whole in each
generation associated with the isolation of individuals
more homozygous than their parents. Any particular in-
dividual may be vigorous or weak, fertile or sterile, nor-
mal or monstrous, good, bad or indifferent, depending
wholly upon the combination of characters received,
many of the characters which become homozygous will be

INBREEDING EXPERIMENTS 139

recessives or combinations of recessives which seldom are
seen under ordinary circmnstances, because they are hid-
den by their dominant allelomorphs. These recessives
are the ''corrupt i'mit" which give the bad name to
inbreeding, for they are often— very often— undesir-
able characteristics.

The homozygous inbred strains after stability has
been reached are quite comparable to naturally self -fer-
tilizing species provided they have passed as rigorous
selection as the latter have had to undergo by reason of
natural competition. And Darwin, as well as others,
found that artificial self-pollination causes no reduction in
such genera as Nicotiana, Pisum and Phaseolus where
self-fertilization is the general rule.

Are then the immediate results of inbreeding some-
times injurious? In naturally cross-fertilized organisms
they most emphatically are — nay, more, even disastrous —
when we recall the reduction to over half or one-third in
production in grain and a corresponding decrease in size
of plant and rate of growth in maize. But maize is prob-
ably an extreme case. With other organisms the results
are not so bad, and in some cases, especially when selec-
tion has been made, no evil effects are apj)arent. In fact,
there may be an actual improvement. But the truth is, we
did not set out to answer that question. It had already
received a correct answer. What ive undertook to inquire
was whether inbreeding is inj^irious merely by reason of
the consangmnity. We answer, No/ The only injury
proceeding from inbreeding comes from the inlieritance
received. The constitution of the individuals result ins<
from a process of inbreedine: depends upon tlio chance
allotment of characters preexisting in the stock before in-

140 INBREEDING AND OUTBREEDING

breeding was commenced. If undesirable cbaracters are
shown after inbreeding, it is only because they already
existed in the stock and were able to persist for genera-
tions under the protection of more favorable characters
which dominated them and kept them from sight. The
powerful hand of natural selection was thus stayed until
inbreeding tore aside the mask and the unfavorable char-
acters were shown up in all their weakness, to stand or
fall on their own merits.

If evil is brought to light, inbreeding is no more to be
blamed than the detective who unearths a crime. Instead
of being condemned it should be commended. After con-
tinued inbreeding a cross-bred stock has been purified and
rid of abnormalities, monstrosities, and serious weak-
nesses of all kinds. Only those characters can remain
which either are favorable or at least are not definitely
harmful to the organism. Those characters which have
survived this **day of judgment'' can now be estimated
according to their true worth. As we shall see later
vigor can be immediately regained by crossing. Not only
is the full vigor of the original steck restored, but it may
even be increased, due to the elimination of many unfav-
orable characters. If this increased vigor can be utilized
in the first generation, or if it can be fixed so that it is
not lost in succeeding generations, then inbreeding is not
only not injurious but is highly beneficial. As an actual
means of plant and animal improvement, therefore, it
should be given its rightful valuation.

CHAPTER VII

HYBKID VIGOR OR HETEROSIS

Whether or not inbreeding in a race of plants or ani-
mals results injuriously depends primarily, as we have
attempted to show, upon the hereditary constitution of
the organism. The beneficial effect of crossing, heterosis,
is a more widespread phenomenon. It may be expected
when almost all somewhat nearly rehited foniis are
crossed together. Even plants or animals which show no
harmful results of inbreeding are frequently improved
thus in a remarkable way. Moreover, this stimulating
effect is immediately apparent in the individuals result-
ing from the cross. It is then at its maximum.

It is natural, therefore, that the early writers on the
subject should have noticed and emphasized the good to
be derived from crossing rather than the bad which some-
times results from inbreeding. Almost without exception
the great horticultural writers of the late eighteenth and
early nineteenth centuries noted the occurrence of hybrid
vigor, and many of them described it in great detail.
Among them may be mentioned Kolreuter (1763), Knight
(1799), Mauz (1825), Sageret (1826), Borthollot (1827),
Wiegmann (1828), Herbert (1837), Lecoq (1845), Giirt-
ner (1849). In fact, in Focke^s compilation of this early
work, **Die Pflanzen-Mischlinge*' (1881), cases of heter-
osis worthy of special mention w^ere found in fifty-nine
families of the flowering plants as well as in the conifers
and the ferns. Animal husbandmen were somewhat loss

141

142 INBREEDING AND OUTBREEDING

inclined to acknowledge and discuss tlie matter, although
they had an excellent example before them in the mule —
an animal known and appreciated for over four thousand
years. But the necessity of their following the custom of
maintaining breeds true to certain fixed standards prob-
ably accounts for their conservatism in estimating the
importance of the phenomenon.

K61reuter,i25 the first botanist to study artificial
plant hybrids, made many interspecific crosses in the
genera Nicotiana, Dianthus, Verbascum, Mirabilis, Da-
tura and others, which astonished their producer by their
greater size, increased number of flowers and general
vegetative vigor, as compared with the parental species
entering into the cross. He -gives many exact measure-
ments of his hybrids and speaks with some awe of their
** statura portentosa'* and '^ambitus vastissimus ac alti-
tudo valde conspicua," Later, after some observations
on certain structural adaptations for cross-pollination
which he interpreted correctly, he made a passing re-
mark which plainly showed he thought Nature had
intended plants to be cross-fertilized and that benefit
ensued therefrom.

Some forty years after, Thomas Andrew Knight,^22 ^
horticulturist who was a very keen observer, noticed sim-
ilar instances of high vigor in his crosses : in the descrip-
tion of these experiments we note the following remarks
concerning a cross between two varieties of peas :

By introducing the farina of the largest and most luxuriant kinds
into the blossoms of the most diminutive and by reversing the process
I found that the powers of the male and female in their effects on the
offspring are exactly equal. The vigor of the growth, the size of the
seeds produced, and the season of maturity, were the same, though the
one was a very early, and the other a very late variety. I had, in this

HYBRID VIGOR OR HETEROSIS 143

experiment, a striking instance of the stimulating effects of cro6tiing the
breeds; for the smallest variety, whose height rarely exceeded two feet,
was increased to six feet, whilst the height of the large and liLxunaul
kind was very little diminLshed.

It is evideut that iu tiiLii particular cajse Krnglil was
dealing with dwarf and standard peas, and dominance of
the tali standard habit of growth is to be expected. This
is not the correct interpretation of the majority of his ob-
servations on hybrid vigor, however; a sulhcient number
of really striking manifestations of the phenomenon were
found to give adequate foundation for his anti-inbreeding
principle, elaborated by Darwin lifty years later.

Probably the most extensive series of early experi-
ments on hybridization were those of Gartner."^^ This
enthusiastic worker crossed, or attempted to cross, every-
thing available to him. According to Lindley,^-** he made
10,000 pollinations between 700 species, and produced 1250
different hybrids. Many of his attempted crosses either
failed to produce seed, or if seed was produced, gave feeble
plants; but a great number of the hybrids, where the
crosses were made between plants not too distantly re-
lated, showed distinct evidence of hybrid vigor mani-
fested in many different ways. Gartner speaks especially
of their general vegetative luxuriance, increase in root
development, height, number of flowers, the facility of
their vegetative propagation, their hardiness and early
and prolonged blooming. He says :

One of the most conspicnons and common eliaracteristics of plant
hybrids is the luxuriance of all their parts, a luxuriance that is shown in
the rankness of their growth and a prodisral development of root shoota,
branches, leaves, and blossoms that could not be induced in the parent
stocks by the most careful cultivation. The hybrids usually reach the
full development of their parts only when plante<l in the open, as

144 INBREEDING AND OUTBREEDING

Kolreuter (125) has already remarked; when grown in pots and thus
limited in food supply their tendency is toward fruit development and
seed production.

Besides possessing general vegetative vigor, hybrids are often
noticeable for the extraordinary length of their stems. In various
hybrids of the genus Verbascum, for example lychnitis-thapsus, the stem
shoots up 12 to 15 feet high, with a panicle 7 to 9 feet, the six highest
side branches 2 to 3 feet, and the stem 1 1/4 inches in diameter at the
base: in Althaea cannahino-officinalis the stem is 10 to 12 feet; in
Malva mauritano-sylvestris 9 to 11 feet; in Digitalis purpureo-ochroleuca
8 to 10 feet, with^anicles 4 to 5 feet; and in Petunia nyctaginifloro-
phoenicea and Lobelia cardinali-syphilitica 3 to 4 feet each. Prof.
Wiegmann also corroborates these observations.

The root system and the power of germination of hybrids are
highly correlated with their great vegetative vigor. Many hybrids, there-
fore, which are not so luxuriant in growth as those just described, for
example Dianthus, Lavatera, Lycimn, Lychnis, Lobelia, Geum, and
Pentstemon hybrids, put forth stalks easily and therefore are readily
propagated by layers, stolons, or cuttings. The observa4;ions of Kolreuter
(125), and of Sageret (191) agree with ours in this respect.

Luxuriation expresses itself at times as proliferation; for instance,
in Lychnis diurno-ftos cuculi the receptaculum is changed to a bud that
puts forth branches and leaves. If, moreover,, the vigor of the hybrids
especially affects the stem and the branches, particularly their length,
nevertheless the leaves take part in it by becoming larger. Hybrids in
the genera Datura, Nicotiana, Tropaeolum, Verbascum, and Pentstemon
are examples.

Naudin,^^^ the contemporary of Mendel, whose ideas
very nearly resembled modern conception of heredity,
likewise gives many excellent illustrations of hybrid vigor
from interspecific crosses which he made in Papaver, Mir-
abilis, Primula, Datura, Nicotiana, Petunia, Digitalis,
Linaria, Luffa, Coccinea and Cucumis. Out of 35 crosses
within these genera 24 show positive evidence of heter-
osis. The cross of Datura Stramonium with D, Tatula
was particularly notable in this respect. Both reciprocal
hybrids were twice as tall as either parent.

HYBRID VIGOR OR HETEROSIS 145

Even MendePs classic pea hybrids supply further in-
stances of increase in size resulting from crossing. Con-
cerning them, he says :

The longer of the two paxental stems is lUiually exceeded by the
hybrid, a fact which is possibly only attributable to the greater hixiLruince
which appears in all parts of the plants when stems of vei*y different
lengths ai"e crossed. Thus, for instance, iii repeated experiiiients, stema
of 1 foot and of b feet in length yielded without exception hybrids which
varied in length between G feet and 7 1/2 feet.

i^'ocke,'*^ in the book already cited, gives the results of
a series of experiments nearly as extensive as those of
Gartner and catalogoies his own results along with those
of his predecessors. The compilation is so careful, so
painstaking, and so complete that one may tui'n to the
linal conclusions of the author without fear of error as
far as the facts are concerned. He says: *' Crosses be-
tween different races and different varieties are distin-
guished from individuals of the pure type, as a rule, by
their vegetative vigor. Hybrids between markedly dif-
ferent species are frequently quite delicate, especially
when young, so that the seedlings are difficult to raise.
Hybrids between species or between races that are more
nearly related are, as a rule, however, uncommonly tall
and robust, as is sho^\Ti by their size, rapidity of growth,
earliness of flowering, abundance of blossoms, long dura-
tion of life, ease of asexual propagation, increased size of
individual organs, and similar characters.''

The attention of these earlier hybridizers was mainly
directed towards interspecific crosses, ])ut they also noted
a great number of instances in which crosses between
closely related forms, such as varieties or sub-variotios
of cultivated plants, gave remarkable increments in

10

146 INBREEDING AND OUTBREEDING

growth. In fact, we have found no record of intervarietal
crosses where delicate or weak progeny resulted. It
would not be useful, however, to attempt to canvass the
literature for all those cases in which crossing either did
or did not result to the advantage of the offspring. A list
of the crosses would alone fill a volume. It is only neces-
sary to point out that the value to be derived from cross-
ing thus made so evident gave great impetus to the study
of tioral structures as adaptations for cross-pollination.
So zealously was this line of investigation pursued, that
knowledge of the methods of pollination in the angio-
sperms soon exceeded that of any other phase of general
botany. The interpretation placed upon many of these
floral mechanisms was fantastic, to say the least, the en-
thusiastic claims of the workers rivalling those of zool-
ogists in mimicry and protective coloration. The net re-
sult was simply to show how widespread were means of
cross-pollination. It might be said to have proved that
cross-fertilization is an advantage ; it did not prove it to
be indispensable. There were too many naturally self-
fertilized plants for any such conclusion.

Of all the work on the effects of crossing in pre-Men-
delian times, that of Darwin is the most important. With
it we get a new insight into the meaning of inbreeding and
outbreeding. Darwin was the first to see it was not the
mere act of crossing which was beneficial. He satisfied
himself on this point by crossing different flowers on the
same plant and different plants of similar strains. In
neither case was there any positive evidence of an effect.
But crosses between different varieties or species of
plants gave unmistakable signs of invigoration. In 24
cases out of 37, cross-fertilization increased the height

HYBRID VIGOR OR HETEROSIS 147

of plant; in 5 out of 7 experiments, the weight was in-
creased. Moreover, the crossed phints frequently flow-
ered earlier and in many other ways showed their advan-
tage over the parent races.

Darwin extended his observations to the animal king
dom and his views on the whole subject are summed up
concisely in the following paragraph from ''Animals and
Plants under Domestication ^^ ''The gain in constitu-
tional vigor derived from an occasional cross between in-
dividuals of the same variety, but belonging to differ on t
families, or between distinct varieties, has not been so
largely or so frequently discussed as have the evil effects
of too close interbreeding. But the former point is the
more important of the two, inasmuch as the evidence is
more decisive. The evil results from close interbreeding
are difficult to detect, for they accumulate slowly and
differ much in degree with different species, whilst the
good effects which almost invariably follow a cross aro
from the first manifest. It should, however, be clearly
understood that the advantage of close interbreeding, as
far as the retention of character is concerned, is indis-
putable and often outweighs the evil of a slight loss of
constitutional vigor.'*

From this statement Darwin evidentlv considered the
ill effects of inbreeding and the good effects of crossincr to
be two different things. He was right in stressinc: the
benefit from crossing rather than the injury from close
mating, but wrong in thinking the evil effects accumulated
as inbreeding was continued. Such a belief is not substan-
tiated by more recent experiments, as has been shown in
the last chapter. It is true, however, that the effect of
inbreeding may not be as noticeable in the first generation

148 INBREEDING AND OUTBREEDING

as the invigoration immediately apparent after crossing.
The effects of outbreeding, unlike those of inbreed-
ing, are shown both by plants which are naturally self-
fertilized and by those which are cross-fertilized. Many
of the illustrations already given are from plants almost
go invariably self -fertilized. Crossing within a pure line of
^ such a species shows no heterosis; but if the parents
i ^ united in the cross differ more or less in minor external
a features an increase in growth is usually to be ex-

>-}

<j

pected. This has been shown to be true for peas, toma-
toes, tobacco and many other normally self-feirtilized
^ , forms among cultivated plants, as well as for several
^ wild species.

An extensive series of crosses between different Nico-
tiana species has been reported by East and Hayes.^^
The majority of these crosses were taller than the average
of the two parents and many were taller and more vigor-
ous than either parent. Some of the crossed plants were
completely sterile. In certain cases these were weak, non-
vigorous plants, but there were others in which inability
to produce seed was accompanied by increased vigor.
Thus, while occasionally the increased development of
sterile hybrids may be due in part to their having ex-
pended no energy in seed production, the fact that many
vigorous hybrids manifest greater ability to produce seed
shows this is a relatively unimportant factor and entirely
inadequate to account for the great vigor obtained where
there is full fertility.

A fair example of the way in which height is gained
by crossing is given by East and Hayes, ^^ a cross of Nico-
tiana rustica hrazilia Comes and N. rustica scahra Comes.

HYBRID VIGOR OR HETEROSIS 149

The frequency distributions of height of j^huit of the two
parents and the reciprocal hybrids are given in Tabk» VI.

TABLE yi

Hbiqht of Crosses Between Nicotiana Rustica Scabra (352) and

N. HusTiCA Brazilia (349)

Variety or

Class centers in inobefl

24

27 30 33 36 3U 42 45 48 51 54 57 60

63 66 69 72 76 78

349
352

352X349, F,
349X352, F,

4

10 22 14 7

2 1 5 11 10 17 0

13 0 5

3 5

5 5 r, 1 1

2 4 0 5 12

in both of the first hybrid generations the average height
is above the major extreme of either parent. Similar
increases in height were obtained when a commercial
variety of tobacco was crossed, first with a variety from
the same locality, then with one from the opposite side of
the world identical with the first in external appearance.
On the other hand, strains of tobacco from seed grown in
Connecticut when crossed with plants of the same vari-
eties from seed grown in Italy showed no increase in
vigor. Hence, the mere fact of residence in d liferent
parts of the world — that is, exposure to different environ-
mental conditions — has no necessar>' relation to the
phenomenon of hybrid vigor, for such individuals may
be alike in constitution. Danvin's repeated emphasis of
the good derived from crossing plants whose ancestors
were exposed to different conditions was because he
thought such differences in environment brought about
germinal changes. This attitude, therefore, does not
detract from his general position tliat it is differences
in germinal construction which bring about liybrid vigor;
and this is the principal point at issue.

150 INBREEDING AND OUTBREEDING

Tlie manifestations of heterosis are most noticeable
as increases in size. This gain in size in plants which are
more or less determinate in their number of parts is made
up of an increase in the size of parts rather than in the
number of parts. In maize the number of nodes is in-
creased much less in comparison to length of internodes.
For example, in a large series of crosses between inbred
strains of maize height of plant on the average advanced
27 per cent., whereas the number of nodes rose only 6
per cent. Corresponding to the increase in intemode
length there is an extension in diameter of stalk, length
and breadth of leaves. Root development is proportion-
ally augmented. Both the tassels and ears are larger, and
frequently two ears develop on crossed plants where
either parent produces one, the color of the foliage tes-
tifying to the greater vigor.

The greatly enhanced growth of a plant may be made
up by increase in the size of cells, as well as by a multi-
plication in the number of ceUs. However, in a cross
between different species of Catalpa no differences could
be seen in tracheid length, although the cross was con-
siderably taller and larger in diameter.

The principal effect of crossing maize is shown by an
additional production of seed. A number of crosses have
given 180 per cent, increases in yield of grain over their
inbred parents. Examples of what can be done are seen in
the accompanying illustrations (Figs. 31 and 32). Im-
provement in yield is shown by crosses between inbred
strains derived originally from the same variety, as well
as between crosses of strains derived from different vari-
eties or even from quite distinct types. The results have
been very wonderful as a whole, giving at the very least

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HYBRID VIGOR OR HETEROSIS 151

a return to the condition of the original stock before in-
breeding was commenced. Some combinations regularly
give greater increases than others, but in every case such
differences are small as compared with those between the
crosses and the inbred parents.

Although, in the main, reciprocal crosses give about
the same result, some variation in this respect is habitu-
ally shown. In general, there is a correlation between the
yield of the better parent strain and the yield of the cross.
The crosses in which strain No. l-f) has been used as the
female parent have regularly given the highest yields, and
this strain is the most vigorous and productive of the
four inbred Leaming strains used in our illustrations.

In a comparison of crosses between inbred strains of
maize with ordinary outcrossed varieties the inbred hy-
brids are handicapped because they have to start from
small, poorly developed seeds. This handicap is brought
out clearly by a comparison of second generation plants
grown from self -fertilized seed produced on vigorous
hybrid plants, with hybrid plants grown from seed pro-
duced on inbred plants. The first generation starts off
poorly, as shown in the accompanying illustration (Fiir.
33), but soon catches up and passes the second generation.
At maturity the second generation is shorter and loss
productive, although it has a much greater variability.
The third generation from selfed plants of this particular
cross has been grown, and there is still further loss of the
stimulation which is at its maximum in the first gonora-
tion. On continued inbreeding these families presumably
would exhibit a continuation of the same course of reduc-
tion in size, vigor and variability shown in the original
inbreeding experiment, until homoz^-gosify was acrain

152 INBREEDING AND OUTBREEDING

100

Growth Curves of
Two In"bred Strains
of Maize and Their
Hybrids.

75

CO

«

o

C

CO
(D

50

25

P
(1-9)

(1-7)

30

40

T

60

I
70

80

50
Numljer of Days from Planting

90

100

33. — Graphs showing growth curves of two inbred strains. of maize and thier first
and eeoond generation hybrids.

HYBRID VIGOR OR HETEROSIS 153

reached. The resulting inbred strains would have about
the same amount of development as the original inbred
strains, but would probably differ from them in appear
ance through the possession of diflerent combinations of
characters. The principal point is that the vigor and size
lost by inbreeding are immediately restored by crossing,
but lost again on further inbreeding. It is a transit^)ry
effect, for the most part, impossil)le of fixation.

Increases in yield of grain are also fre^juently ob-
tained when ordinary commercial varieties of maize are
crossed. Rarely are the increases greater than 10 per
cent., however, and even this is more commonly to be
expected when varieties of somewhat different iypa are
used; for example, flint and dent. Most varieties of corn
are now so widely crossed and furthermore are so near
the limit of production that great advances are not to be
expected. Collins ^^ has obtained especially large incre-
ments in >deld by hybridizing typos of com from different
geographical regions. Three different varieties of corn
from the southwest — Hopi, Brownsville and Hairy Mexi-
can— each gave an increase of 100 per cent, or more when
crossed with a variety from China having seeds with a
different type of endosperm.

Even before the plants are obtained there is a striking
effect of crossing in an immediate increase in the size of
seed. This was noted by Roberts, ^^''^ and established very
clearly by Collins and Kempton ^ through pollinating
ears of maize with a mixture of the plants own pollen and
of a different sort. By taking advantage of the phenom-
enon of double or ** endosperm** fertilization, the
experiment was so designed that the ontrTossed seeds
could be distinguished by differences in endosponn color.

154 INBEEEDING AND OUTBREEDING

Advances in average weight of seed ranging from 3 to 21
per cent, were obtained. With inbred strains as parents,
the increases are even greater, ranging from 5 to 35 per
cent. The seeds have a heavier embryo as well as a
heavier endosperm, yet curiously enough they mature
faster than the selfed seeds on the same ears.

It is a point of some interest, perhaps, that there is no
selective action favoring the foreign pollen when these
pollen mixtures are applied. This matter has been deter-
mined very carefully on account of its bearing on Men-
delian theory, but it also answers in the negative the
question of whether there is an effect of heterosis
manifested by a selective chemotropism before the zygote
is formed.

Darwin, in ^' Cross and Self -Fertilization in the Vege-
table Kingdom, '' compares the time of flowering of 28
crosses between different types of plants which had shown
distinct evidence of hybrid vigor. Of them, 81 per cent,
flowered before the parents. In other cases, where no
heterosis was shown in other characters there was no ac-
celeration of the blooming period. These results have
been corroborated in crosses between garden varieties of
tomatoes and of sweet corn, where a tendency to put for-
ward the time of both flowering and maturing has been
shown to accompany increases in size. Shortening the
time of growth thus seems to be one of the many ex-
pressions of an increased metabolic efSciency on the part
of the hybrid plant.

Increased longevity, viability, endurance against un-
favorable climatic conditions, and resistance to disease
have also been frequently noted as properties of hybrids.
Kolreuter ^^^ and Wiegmann "^^ both mention these points.

bt

Fig. 34. — James River ^\'alIlUt, a faiijnii.s tree cuiisidcriMl to iu- a n
tween the Persian walnut and tlic coninion liuttcmnt. .\c<M»rtlinj; to (\ S ~
of the .Arnold Arboretum, tliis is the most remarkaMc hardwood trrv in tht I in!
height 10() feet, spread of brandies 134 feet, eircumfe-renre of trunk ^ll fi-i-t. 1 1 >•

•r

HYBRID VIGOR OR HETEROSIS 155

and Gartner "^^ gives them bis especial attention. Under
the heading, "Ausdauer und Lebenstouacitiit der P^as-
tardpflanzen, ' * he makes the following statements:

There is certainly no essential difference between annual and
biennial plants and between these and perennials in regard to their
longevity, for frequently different individuals oi' the same species have
a longer life at times as, for example, Draba venia which has bolh
annual and biennial forms. The longevity of a plant thei-eby f umi^hea
no specific difference but at most only signitictj a variabihty. ILowever,
in hybrids this difference deserves special consideration. In mo^t
hybrids an increased longevity and greater endurance can bo obeerved
as compared to their parental races even if they come into bloom a
year earlier. The union of an annual, herbaceous female plant uith a
perennial, shi-ubby species does not shorten the life cycle of the forth-
coming hybrid, as the union of Hyoscyanms agrestis with niger, Nicotiunn
rustica "with perennis, Calceolaria plantaginea with rugosa shows. So
also in reciprocal crosses w^hen the perennial species furnishes the seed
and the annual species supplies the pollen, as Nicotiana glauca v^ilh
Langsdorfjiij Dianthus caryophyllus with chitiensis, Malva sylcestns
with mauritiana or biennials with perennials and reciprocally, as
Digitalis purpurea with ochroleuca or lutea, and lutea with purpurea, or
ochroleuca with purpurea. From, the union of two races of different
longevity a hybrid usually results into Avhich the longer life of one or
the other of its parent races is carried whether it comes from the male
or female parent species.

Many more instances are given by Gartner supporting
the conclusion previously reached by Kolr enter that the
longer life of hybrid plants is to be counted among their
usual properties.

Gartner also gives several examples of endurance to
unfavorable weather conditions by hybrids. Many of his
tobacco hybrids actually survived the winters in the open
field in south Germany when the parents were killed.

The hardiness of hybrids is frequently shown by a
great resistance to parasitism. Gornert'^'^ states that teo-
sinte and the first generation cross of teosinte and maize

156 INBREEDING AND OUTBREEDING

are not attacked by the apMds which damage maize. The
cross between the inbred strain of maize most susceptible
to smut, previously mentioned, and the strain not affected,
gives a hybrid which is only slightly parasitized. The
same thing has been noted in crosses between other
strains of maize, some of which are quite badly damaged
by an unidentified leaf blight organism. Radish seedlings
which were naturally cross-fertilized were much less dam-
aged by damping-off fungus than uncrossed seedlings
from the same plants. Resistance is not shown by all
first generation hybrids when the parents diif er in suscep-
tibility. Some cases are known in which the hybrids are
fully as susceptible as the less immune parent. In
the majority of crosses reported, however, in which re-
sistance to parasitism is a factor the hybrids tend to
show resistance.

Among the diverse manifestations accompanying
heterozygosity may be mentioned viability of seed. In
maize, crossed and selfed seeds from the same ears have
shown a difference of 16 per cent, germination in favor
of the crossed seeds. The crossed seedlings appeared
earlier and grew faster from the first.

Increased facility of vegetative propagation of hy-
brids was frequently noted by the early hybridizers.
Sageret ^^^ makes particular note of a hybrid tobacco
which easily propagated itself vegetatively. Many of our
cultivated fruits which are propagated by buds, grafts,
cuttings, etc., owe part of their excellence at least to the
fact that they are in a heterozygous condition. Moreover,
there is no evidence to prove that plants lose any of their
hybrid vigor in long continued vegetative multiplication
through innumerable generations.

HYBRID VIGOR OR HETEROSIS 157

In general, as noted before, there is similarity between
the effect of heterozygosis and that of a good environ-
ment. Those characters which are quickest to be modi-
fied by external factors also show the greatest change
on crossing. A good illustration of this is a Nicotiana
cross which was ahove the average of the parents in botli
height and leaf size. The length of the corolhis, on tlio
other hand, a character very slightly affected by the en-
vironment, was not increased. There is at least one
difference between the two, however; in time of maturity,
environment and heterosis have somewhat opposite ef-
fects. Generally speaking, favorable growing con<]iti()ns
tend to delay flowering and maturing, whereas conditions
which stunt the plants tend, like heterosis, to hasten them.

Each of these effects is by no means always present
when, a cross is made. The usual and of course the most
noticeable effect is the increase in size. But crossing may
have a stimulating effect upon certain part^ and a de-
pressing effect on others. This is shown in many species
crosses in which reproductive ability is greatly roducod
or even totally eliminated, while at the same time vegeta-
tive growth is enormously increased. Freeman and Sax
have independently obtained seeds from crosses between
common bread wheats and macaroni wdieats which were
shrunken in appearance and small in size, owning to a poor
development of the endosperm. The embryos were well
developed, however, and the plants produced gave dis-
tinct evidence of hybrid vigor.

In this discussion there has been a noticeable omission
of the effect of crossing on animals. Illustrations are not
lacking that crossing frequently is highly ])onori('ial to
animals; but animals do not funiish as desirable research

158 INBEEEDING AND OUTBREEDING

material for this particular problem as plants, on account
of their bisexuality, as was explained earlier, and for this
reason but few quantitative data are available. There is
no question but that animals behave the same as plants
in heredity ; therefore, one might transfer the conclusions
reached in the one kingdom to the other without apology,
for the effects of inbreeding and cross-breeding are
wholly and solely the working out of the laws of heredity.
At the same time, it will not be amiss to present some of
the results obtained by zoologists, for they strengthen the
case immensely.

In the two cultivated species of insects which form our
sole instances of domestication, bees and silkworms, there
seems to be evidence of increased vigor on crossing only
in silkworms (Toyama ^^^), In the fruit fly, however, upon
which the greatest amount of genetic work has been done,
Castle,2i Moenkhaus,^*^ Hyde^^ ^nd Muller^^* all found
size, fecundity and general constitutional vigor increased
remarkably, particularly when the strains crossed had
been inbred previously. In the rotifer, Hydatina, Whit-
ney 216 and A. F. Shull ^®® obtained similar results. Fur-
ther, Gerschler^® describes and figures first generation
crosses between different genera of fishes which show very
marked increases in size.

In birds also there is such an increase in vigor that
poultry fanciers often cross two distinct strains and sell
the progeny because of their rapid growth and large size.
No attempt is made to breed from the hybrids ; they are
simply produced because of their vigor. When very great
differences in size exist, there is not, of course, an increase
in size sufficient to throw the individual of the first hybrid
generation above the larger parent, as is shown by the

HYBRID VIGOR OR HETEROSIS 159

work of Pliillips ^•*^ on crosses between the large Freiicli
Rouen and the small domestic Mallard duck, and by the
work of Punnett ^^^ on crosses betwen the iSilver Sebright
bantam and Gold-pencilled Hamburgh breeds of poultry.
There is an increase over the average of the two parents,
but the i*'i's do not reach the size of the larger parent
race. Part of the reason for the comparatively small
sizes of the i^\'s in these crosses, however, is due to the
fact that the crosses were always made on the i>mall hens
allowing the hybrid birds to get their start in life with
only the nutriment stored in the smaller eggs.

The greatest amount of data on this subject, just aa
there is the greatest amount of interest, has been obtained
from the mammals. In the meat breeds of cattle, swine
and sheep, as in poultry, it is a common practice to cross
distinct races and sell the progeny. The increase in size
and the rapidity with which this size is obUiined are so
general a phenomenon that it bids fair partially to replace
the older method of pure line breeding. Not only are
varietal crosses thus characterized, but speciiic crosses.
We have already mentioned the mule. With the disad-
vantage attached to sterility, the mule certainly would not
have held its own throughout the past forty centuries
were it not for its tremendous capacity for work and its
remarkable resistance to disease. Crosses between the
ass and the zebra, and between the cow jmd the zebu also
give animals of considerable merit, and one can hardly
refrain from thinking that within a few years some con-
siderable use ^\dll be made of them.

For precise data on the effect of cro.^sing ditTeront

II races, however, we must turn to the small maninmls used

so constantly in experimental work, the mouse, the rat, the

160

INBREEDING AND OUTBREEDING

guinea-pig and the rabbit. One need go no further than
to cite the work of Castle and his students at the Bussey
Institution of Harvard University, the work of Miss King
at the Wistar Institute of Anatomy and that of Wright
at the Bureau of Animal Industry of the United States
Department of Agriculture. The painstaking researches
of these investigators show mthout question that the
effect of crossing on animals is the same as upon plants.

1

Weight
ini

(f F,. Cut "Band C

Oran
fififi

8

d^aeeB

X

dF,.Ci

tVBandC

600

/

^

dCullen

400
200

^

y

[^

^

Age in Daya 40

do

120

100.

200

240

sso

320

3G0

40O

FiQ. 35. — Growth curves of males of race B guinea-pigs and Cavia cutleri and their Fi

and Ft hybrids. (After Castle.)

The results from one genus is typical of them all.
Castle ^^ made a cross between a domestic guinea-pig and
a wild cavy, Cavia cutleri. The first generation hybrid
males weighed about 85 grams at birth, which is slightly
more than the young of either pure race, and retained this
lead throughout their subsequent life as is shown by the
growth curve in Fig. 35. At maturity they weighed about
890 grams, as compared with 800 grams for the guinea-
pig ancestor and 420 grams for the cutleri ancestor.

The second generation hybrids of both sexes were

HYBRID VIGOR OR HETEROSIS ICl

smaller than the first generation hy])ri<]s from birth on,
showing that some of the growtli impetus produwHi by the
hybridization had not beun retained. 13ut the growth
curve of the second generation hybrids rises rapidly at
first, showing the healthy start in life they obtained from
their vigorous F^ mothers.

Perhaps no such increase in vigor as that shown in the
species cross just described is usually found when dif-
ferent sub-races are crossed. It would not be expecti'd,
for ordinary races of mammals are contiinially iMMng
crossed within the variety and, therefore, hybriilization
would not be expected to increase heterozygosis to any
marked degree. But results similar to those obtaineil in
plants may be expected if the genetic conditions are sim-
ilar. This is proved by the data Wright obtained when ho
crossed guinea-pigs bom of unrelated inbred mothers and
fathers. The cross-breds were distinctly superior to their
inbred relatives in nearly all characters coimected with
vigor. In spite of the fact that their inbred mothers were
small and somewhat deficient in vigor, a slightly larger
per cent, of cross-breds than of inbreds were bom alive,
and a distinctly larger per cent, of those born alive were
raised. They were somewhat heavier at l)irtli in a given
size of litter and gained in weight much more rapidly
between birth and weaning. They matured earlier and
produced larger litters and produced them more regularly
than the inbreds.

Thus the results with animals are comparable to those
obtained with plants in all essential features. Brietly, in
crosses which are fertile the effects aro sueh as to con-
tribute to a greatly increased reproductive ability, making
11

ry.'

162 INBREEDING AND OUTBREEDINa

possible a larger number of offspring. The degree to
which heterosis is expressed is correlated, within limits,
with the differences in the uniting gametes. When homo-
zygous forms are crossed, it is at its maximum in the
first hybrid generation, and diminishes in subsequent gen-
erations of inbreeding as segregation occurs and homo-
zygosity is again attained. It is a widespread phenom-
enon and accompanies heterogeneity of germinal consti-
tution whether the organisms crossed are from the same
or diverse stocks, whether they have been produced under
similar or under different environmental conditions;
although it is not apparent until the zygote is formed,
from that time on it is expressed in many ways through-
out the lifetime of the individual and is undiminished
by asexual propagation.

These are the effects of cross-breeding upon develop-
ment in which we have been particularly interested, those
in which the organizations of the combining gametes are
sufficiently compatible to permit continued propagation.
But it must not be forgotten that we have dealt with only
one part of the problem. As the differences between the
forms increase limits are reached beyond which the organ-
isms neither reproduce nor flourish. One can arrange a
series in plants in which (1) the parents are so diverse
the cross cannot be made; (2) the seed obtained fails to
germinate under any set of conditions; (3) the hybrids
are so weak they are unable to reach maturity; (4) the
hybrids are extremely vigorous, but sterile except pos-
sibly in back-crosses ; (5) the hybrids are fully fertile and
more vigorous than either parent; or (6) the parents may
be so closely related no effects whatever are to be noted.

HYBRID VIGOR OR II1:TKKoSIS IG.'i

A somewhat similar series can be arranged with animals,
although usuiilly in wide crosses if the hybrids can Ix'
obtained at all they are as large or larger than the average
between parents. A satisfactory interpretation of the
vigor of hybridization must take all these facts into con-
sideration, even though tliey may not be the result of the
operation of one single law.

CHAPTER VIII
CONCEPTIONS AS TO THE CAUSE OF HYBEID

viaoR

The early plant hybridizers, although they frequently
discussed the increased size and vigor of their crosses,
seldom commented on the effect of inbreeding, and made
no speculations as to the cause of either. The animal
breeders of the period were more imaginative. Ac-
quainted with both phenomena, but more familiar with the
results of inbreeding, they unhesitatingly linked the two
— the first as an antidote for the second. They attributed
most of the injurious effects which appeared in their herds
to the concentration of undesirable traits. If unfavor-
able characters and tendencies to disease were present,
mating similar animals brought out these undesirables
more pronouncedly; whereas, if healthy animals from un-
related herds were brought in, such tendencies were
checked, the defects disappeared, and the health and vigor
of the herds returned.

Darwin, however, refused to ascribe any large part
of the effects of inbreeding to this cause. He knew of
many cases in which weakened animals from different in-
bred herds had been mated together, and gave progeny
of full health and vigor and of increased size. The unde-
sirable features induced in both herds by inbreeding dis-
appeared when animals of the different herds were mated.
Instead of a concentration of the less favorable traits of
the two parental lines the reverse seemed to have oc-
curred. Similar cases in plants were familiar to him, and

164

CAUSE OF HYBRID VIGOR 1G5

proved beyond question tlie ^a'eat advantage to be gainLxi
by crossing even when the individuals themselves were
weak. These facts, taken together with the many mar-
velous and intricate contrivances of phuits to insure
cross-pollination, led him to believe that selt'-fertilizatioii
was inherently harmful and something to be avoided it"
possible. The benefits accming Trom crossing he ascrilied,
as we have seen, to the meeting of sexual elements liaving
diverse constitutions.

After Darwin's contribution to the problem of inbree<l-
ing no progress was made until less than two decades ago,
when the Mendelian discovery opentsl up s(j many new
possibilities. The conception of an inheritance made up
of separable units aroused a new interest in the matter
and made possible a unilied and satisfactory interpreta-
tion of all the facts.

Mendel had shown that characters from one i»arent
might disappear completely in the progeny only to reap-
pear in subsequent generations in some of the offspring.
Surely here was something of importanee to the inbreed
ing problem. Unfavorable charaeters might vanish when
different organisms were crossed; but they were merely
hidden. Inbreeding revealed thom for what they were.

Shortly after Menders experiments became known.
Bateson crossed two pure white Howered varieties of tlie
sweet pea. Instead of having the whiti^ flowers oharac
teristic of two parental races, the hybrid llowers wer*^
purple. The wild progenitor of tlie sweet pea has purj*!^'
flowers. Here was a case in which crossing brought back
previously existing conditions, a return to the wild iy\^o
characters. This phenomenon had been obser\'ed long
before this time; in fact, it was so well known it had been

166 INBEEEDING AND OUTBREEDING

given a special name. Atavism^ or the reappearance of
previously existing characters, was immediately put upon
a Mendelian basis by hypothecating two separable factors
both of which were necessary for the production of the
end result. This was proved by the fact that later white
flowered plants were obtained which did not produce color
on crossing with either of the original white flowered va-
rieties and therefore lacked both factors. The light in-
creased. Some of the chaotic observations of the earlier
hybridizers began to be understood as orderly facts.

Ten years after the rediscovery of MendePs work a
symposium was held by the American Society of Natur-
alists on the ' ' Genotype Hypothesis, ' ' an indication of the
growing importance of the ideas associated with the name
of Johannsen. The basis of the genotype conception is
that individuals which are visibly alike may be germi-
nally unlike — merely an extension of the above Mendelian
concepts. Johannsen's contribution was the idea that the
unit factors of Mendel are relatively constant and stable
in whatever combinations they occur, and that the vari-
ability of a constantly cross-fertilized population is
largely due to the segregation and recombination of these
unmodified factors. When such a heterogeneous popula-
tion is continuously self -fertilized, homozygosity is ulti-
mately attained, and much of the previous variability dis-
appears. Similar individuals making a homozygous, pure
breeding population, are known as a pure line, and while
they still vary as affected by different environmental con-
ditions, such variability does not respond to selection, and
the average condition is not changed. Although there is
still a question as to the degree of stability of the Mendel-
ian unit factor, as there is to the degree of stability of

CAUSE OF HYBRID VIGOR ir.?

the atom, the principle of the pure line has been finnly
established by an ever-increasing body of evidence, and
is of the utmost importance in a proper understanding of
the facts involved in inbreeding and outbreeding.

The first application of these principles to the problem
of inbreeding was made with the results from maize al-
ready described. It was shown tliat self-fertilization
automatically brings about homozygosity, and with it a re-
duction of a great deal of the variability commonly shown.
Along with this reduction in variability, certain charac-
ters manifest themselves which are more or less unfavor-
able to the plants' best development. Plants with sterilo
tassels and sterile ears and plants which lack chorophyll
appear, cannot reproduce themselves, and are eliminated.
Other characters come to light which do not cause the
extinction of the plants directly, but which more or less
handicap them in their development; for example, partial
chlorophyll deficiency, dwarfness, bifurcated organs, con-
torted stems and deficient root systems.

All of these characters are sho\Mi in small numbers in
a cross-pollinated field of maize, but not in sufficient fre-
quency to reduce productiveness seriously. As the result
of self-fertilization some of the strains o])tained possess
certain of these unfavorable characters as regular fea-
tures. Here, then, is an explanation of part of the in-
jurious effects of inbreeding. Unfavorable characters
are segregated out, which reduce the developmental efTi-
ciency of the plants which possess them. lUit if unfavor-
able characters are concentrated in some lines, favorable
characters are concentrated in others. Some have more
of the favorable than of the unfavorable, hence some
strains resulting from inbreeding are better than others.

168 INBKEEDING AND OUTBREEDING

But all strains in maize are so greatly reduced by in-
breeding that none can be compared in productiveness to
the normal cross-pollinated plants. Something besides
ordinary segregation must be involved in this well-nigh
universal effect of inbreeding.

It was apparent that when germinal heterogeneity
was at the maximum the greatest vigor was shown. When
this heterogeneity was reduced by inbreeding, vigor was
lost. Hence, the fundamental fact that hybrid vigor
varies directly with heterozygosity was clearly estab-
lished. To account for the greater vigor and increased
development of hybrids, it was only necessary to postu-
late that a developmental stimulation was evolved when
different germ plasms were united. This hypothesis
(East and Hayes,^^ ShulP®^) satisfied all the essential
facts, and for the first time the effects of inbreeding and
cross breeding were clearly understood in their true rela-
tion to each other. Inbreeding was not a process of con-
tinuous degeneration; it was a process of Mendelian
segregation, and its effect was directly related to the
number and type of characters existing originally in a
heterozygous condition. If unfavorable characters were
covered up by favorable characters, inbreeding brought
them out whenever a simplification of the germ plasm al-
lowed them to appear. Inbreeding was in eif ect the isola-
tion of homozygous hereditary complexes from an hetero-
zygous hereditary complex. If the best of these combina-
tions failed to attain the development of the original
stock, it was thought to be because they were deprived of
a stimulus which only accompanied heterozygosity and
which seemingly was impossible to fix.

This hypothesis, by associating all the facts of inbreed-

CAUSE OF HYBRID VIOOR ](V^

ing and outbreeding with the phenomena of Mendcliaii
heredity was a great step forward. It wont as far as it
was possible to go at the time it was devised, and it is
capable of interpreting all the facts to-day. ]5ut it held
some disadvantages. The assumption of a physiologiciil
stimulation arising from the interactic^n of fliffcreni
hereditary factors was not altogether satisfactory, for it
locked the door on any hope of originating pure strains
having as much vigor as first generation hybrids. For-
tunately, the development of Mendelian heredity has been
such that this part of the hypothesis can be sui-^orseded.

The basis for this hypothetical stimulation was seen in
the fact that fertilization is usually necessary to start the
development of the Q^g. In most cases, without the union
with the sperm, the eg^ cannot divide and development
is prevented. The reaction of the different substances
HDrought together at fertilization stimulates cell division
and starts development. This made it reasonable to as-
sume that when the egg and sperm differed in here<iitary
factors stimulus to development was increased and con-
tinued throughout the growth of the resulting organism.
According to the view of G. IT. Slinll and of Fast, it was
the interaction of different elements in the nuclei that
produced the stimulation. A. F. Shull,^^^ on the other
hand, assumed the stimulation to result from the inter-
action of the new substances brought in by the s|)erm with
the maternal cytoplasm. In his opinion the stimulation
might persist for a time even after homozygosis was at-
tained, because foreign elments brought in by the original
cross would still remain to react with the cytoplasmic
matter. Moreover, it was further assumed that the stim-
ulation might decrease in long-continued asexual propa-

y"

170 INBREEDING AND OUTBREEDING

gation through the cytoplasm becoming adjusted to a
heterozygous nucleus. This theory was proposed as an
interpretation of a reduction in vigor which he had found
in parthenogenetically reproduced rotifers. The recent
facts, however, are more in accord with the former view
because (1) the stimulus actually is lost as homozygosity
is attained, and (2) the evidence of vigor being reduced
in continued asexual reproduction is not at all conclusive.

The reasons for holding the whole stimulation hypoth-
esis in abeyance at present has developed from the follow-
ing facts. In 1910, Keeble and Pellew^^^ offered a con-
crete illustration of a purely Mendelian method by which
increased growth could result from crossing. They united
two varieties of garden peas, which, as grown by them,
each ranged from 5 to 6 feet in height. The first genera-
tion grown from this cross was from 7 to 8 feet in height,
2 feet taller than either parent, a result comparable to
heterosis. The second generation showed segregation
into four classes, one class containing plants as tall as the
first generation, two classes having plants similar in
height to the two parents, and one class made up of
dwarfs shorter than either parent. The two classes of
medium tall plants, similar in height, were differentiated
in the same manner as the parental races ; one had thick
stems and short intemodes, the other had thin stems and
long internodes with fewer of them. The number of
plants falling into these four classes agreed closely with
the expectation for a dihybrid ratio (9:3:3:1) where
two factors showing dominance are concerned.

Keeble and Pellew assumed two hereditary factors to
be involved: one producing thick stems, the other long
internodes. These factors they designated T and L. One

CAUSE OP HYBRID VIGOR 171

of the parental varieties was medium in height, because
it possessed one of these factors ; e.g., that for thick stems,
but lacked the other. Such a plant had the f onnula TTil.
The other variety was of medium height, because it lacked
this T factor, but possessed the factor for long intemodes,
and was given the formula it LL. Both of these factors
showed dominance over the aUelomorphic condition;
hence, the first generation of the cross was taller than
either parent because both factors were present. Whether
or not later investigations have justified this precise in-
terpretation makes no material difference in the discussion
here. Taken as it stands, it is a beautiful instance of the
way in which complementary action of dominant factors
may increase a character in a first generation hybrid over
its expression in either parent.

The investigators attempted to generalize from this
experiment and to apply a dominance interpretation to
the many other cases in which an increase in growth is
occasioned. As the matter stood at that time, however,
it was impossible to see why recombination of all the
dominant factors concerned in the increased growth of
the first generation could not readily be obtained, and
hence some individuals be produced having maximum
size and vigor, yet unaffected by inbreeding because
of their homozygous condition. In other words, in gen-
erations after the first it ought to be possible to obtain
some strains having all the dominant factors and others
with all these dominant factors lacking. Any such race
could be rendered homozygous ; thereafter, self-fertiliza-
tion would not result in a less vigorous progeny. And
while such results may have been obtained in the peas,
investigators have not been able to duplicate them in the

172 INBEEEDING AND OUTBEEEDING

many other crosses which showed hybrid vigor. Further-
more, not only was the union of such simple factorial com-
binations inadequate to account for the frequency of the
widespread occurrence of heterosis, but there was another
seemingly insurmountable objection to the interpreta-
tion. It was pointed out that if heterosis were due solely
to dominance of independent factors, the distribution of
the second generation would be unsymmetrical in respect
to those characters in which an increase was shown in the
first generation. This criticism has its basis in the famil-
iar fact that Mendelian expectation in the second hybrid
generation where there is complete dominance its always
an expansion of the form (3 + 1) to a power represented
by the number of factors. Even with partial dominance
the criticism holds, although the lack of symmetry is not
so marked.

But in the vast amount of data accumulated upon the
inheritance of quantitative characters no such tendencies
toward asymmetrical distribution in the second genera-
tion are evident. In the majority of cases recorded where
hybrid vigor is shown in the first generation, the distri-
bution of the individuals fits the synometrical curve, com-
monly known as the Curve of Error, remarkably well.

It is evident, therefore, that the objections raised
against the hypothesis of dominance as a means of ac-
counting for heterosis, as outlined by Keeble and Pellew,
are valid. But both these objections to dominance as an
interpretation of heterosis were made before the facts of
linkage were known. With linkage these criticisms based
upon Mendelian expectancies with independent factors do
not hold.

Abundant evidence is fast being accumulated to show

CAUSE OF HYBRID VIGOR 173

that characters are inherited in groups. The different
theories accounting for this linkage of factors make no
essential difference in the use to which these facts will be
put here. It is only necessary to accept as an established
fact that characters are thus inherited and that it is these
groups of factors which Mendelize. The chromosome
view of heredity will be used, as it is the most probable,
the most useful, and permits representation in the sim-
plest manner ; but adherence to this view is not necessary
for our purpose.

The increasing complexity of Mendelism points very
strongly to the probability that the important characters
of an organism are determined, or at least affected, by
factors represented in practically all of the chromosomes
or linkage groups. This is comprehensible when it is re-
membered that height or any other size differentiation
is only an expression of an organism *s power to develop.
Hereditary factors which affect any part of the organism
may indirectly determine the maximum of any size char-
acter. For example, in plants height is governed by root
development as well as by that of the aerial parts.

The widespread occurrence of abnormalities and char-
acters which are detrimental to an organism ^s best devel-
opment are well known. It may be taken for granted,
nevertheless, that no one individual has all the unfavor-
able characters, nor, on the other hand, all the favorable
characters known to occur in the species. For the most
part, each possesses a random sample of the good and the
bad. This being true, it is only necessary to assume that
in general the favorable characters are in some degree
dominant over the unfavorable, and the normal over the

174 INBREEDING AND OUTBREEDING

abnormal in order to have a reasonable explanation of the
increased development of hybrids in the first generation
over the average of the parents or subsequent generations.
In the first hybrid generation the maximum number of
different factors can be accumulated in any one individ-
ual ; and because of factor linkage it is extemely difficult
to recombine in one organism in later generations any
greater number of homozygous characters than were
present in the parents, provided the factors are distrib-
uted at random in all of the chromosome pairs. This
view of the situation makes more understandable
why the effects of heterozygosis result in an increase
in development, and why they remain throughout
the life of the sporophyte, even though innumerable
asexual generations.

The abstract view of the dominance hypothesis may
be somewhat clearer if a concrete diagrammatic illustra-
tion is made. A case will be assumed, in which two homo-
zygous individuals, having three chromosome pairs, both
attain the same development as represented by any meas-
urable character. This development will be considered to
amount to 6 units, 2 of which are contributed by each
chromosome pair. One of these individuals, which we will
call **X,'' attains its development through the operation
of factors distributed in the three pairs of chromosomes,
each differing from the others in its contribution. Any
number of factors can be chosen, but, for the sake of sim-
plicity, only three in each chromosome will be employed.
These are numbered 1, 3, 5 ; 7, 9, 11 ; and 13, 15, 17 in the
accompanying diagram (Fig. 36). The second individual,
**Y,*' develops to an equal extent in the character meas-

CAUSE OF HYBRID VIGOR

175

ured. It attains this same development, however, by the
operation of a different set of factors distributed in the
three chromosomes and numbered 2, 4, 6 ; 8, 10, 12 ; and
14, 16, 18 in the diagram. Both individuals are homo-

X: 6

Y : 6

2

2

2

2

2

2

*A BB cc a'/C fi'e'Cri'

1

1

7

7

13

13

^■^"^

3

3

9

9

15

15

2

2

8

8

14

14

S

5

11

11

17

17

4
6

4
6

10
12

10
12

16
18

16
18

Xx Y: 12

2

2

2

2

2

2

A / B ef c d'

1

2

7

8

13

14

3

4

9

10

15

16

s

6

11

12

17

18

Fia. 36. — To show how factors contributed by each parent may enable the first generation
of a cross to obtain a greater development than either parent.

zygous ; i.e., the^ allelomorphic pairs are composed of like
elements. It is also assumed that all these nine factors
are as fully effective in the haploid as in the diploid con-
dition; in other words, they show perfect dominance over
their absence. It will be seen from the diagram that when
these individuals are crossed together the progeny de-

176

INBREEDING AND OUTBREEDING

velop to twice the extent of either parent, because there
are present eighteen different factors instead of nine.

TABLE VII

Composition of a Mendelian Tri-Hybrid in Fj Where the Development
Which Each Individual Attains Depends upon the Number of
Heterozygous Chromosomes Contained and Thereby upon the
Total Number of Different Factors Present.

Number of individ-

Contributions of

Total

uals in each

Categories

each chromosome

develop-

category

pair

ment

1

A A

BB

CC

2+2+2

6

2

A A'

B B

CC

4+2+2

8

2

A A

BB'

CC

2+4+2

8

2

A A

BB

CC

2+2+4

8

4

A A'

BB'

CC

4+4+2

10

4

A A

BB'

CC

2+4+4

10

4

A A'

BB

CC

4+2+4

10

8

A A'

BB'

CC

4+4+4

12

1

A A

BB

CC

2+2+2

6

2

AA

BB'

CC

2+4+2

8

2

A A'

BB

CC

4+2+2

8

4

A A'

BB'

CC

4+4+2

10

1

A A

B'B'

CC

2+2+2

6

2-

AA

B'B'

CC

2+2+4

8

2

A A'

B'B'

CC

4+2+2

8

4

A A'

B'B'

CC

4+2+4

10

1

A'A'

BB

CC

2+2+2

6

2

A'A'

BB'

CC

2+4+2

8

2

A'A'

BB

CC

2+2+4

8

4

A'A'

BB'

CC

2+4+4

10

1

A'A'

B'B'

CC

2+2+2

6

2

A'A'

B'B'

CC

2+2+4

8

1

A'A'

BB

CC

2+2+2

6

2

A'A'

BB'

CC

2+4+2

8

1

AA

B'B'

CC

2+2+2

6

2

A A'

B'B'

CC

4+2+2

8

1

A'A'

B'B'

CC

2+2+2

6

64 Total

Distribution of the

Fj

individuals

according

to the development attained

Classes

Frequency

6
8

8
24

10
24

12

8

= 4
= 64

Number of classes
Total population

Following this hypothetical case into the second gen-
eration by selfing or by interbreeding the individuals of

CAUSE OF HYBRID VIGOR 177

the first generation, the data given in Table VII are ob-
tained. Summing up the results of this tabulation, it will
be found that eight individuals are completely homo-
zygous and reach the same development as either parent,
six units ; eight are heterozygous in all three chromosome
pairs and duplicate the twelve-unit growth of the first
generation; the remaining forty-eight individuals fall
into equal-sized groups, developing to eight and ten units,
respectively. In other words, the distribution is sym-
metrical, and this symmetry remains, however many
chromosomes are involved.

It should also be noted that the mean development
of the second generation is nine units, which is an excess
of just half of the excess of the first generation over the
parent. The extra growth derived by crossing the two
different types has diminished 50 per cent. In the third
generation, from a representative sample of the second
generation, it can be shown that this excess again dimin-
ishes 50 per cent., so that the effect on the average is only
25 per cent, as great in this generation as in the first, and
so on, in subsequent generations, until the effect dimin-
ishes to a negligible quantity in about the eighth genera-
tion. This is in fair agreement with the actual results
obtained by inbreeding maize, as it ought to be, because
the development attained by each individual varies di-
rectly with the number of heterozygous factors.

In the preceding illustration of the way heterosis
may be brought about, perfect dominance was assumed.
Moreover, breaks in linkage with the formation of
new linkage groups were not considered. All these
things enter as complicating factors. Perfect domi-
nance, except in more or less superficial characters,

12

178 INBKEEDING AND OUTBEEEDING

rarely occurs, and even when it does occur, it may
be merely an appearance rather than a reality. The gen-
eral consensus of opiaion at the present time is that there
is no such thing as perfect dominance, that the hetero-
zygote merely approaches the condition of one or the other
parent more or less closely. When two different poten-
tialities are contributed by the parents, there results an
interaction between them and the end product is repre-
sented in the organism. Because the most striking effect
may resemble the character of one parent more than the
other, we say that this character is dominant. In reality,
in the more fundamental characters, the hybrid usually
shows a resemblance to both parents. The more common
illustrations of dominance, such as fur colors and flower
colors, probably have little to do with heterosis. Other
dominant characters, however, have a fundamental effect
upon development, nearly always being essential to great-
est vigor. Various grades of albinism are common in
maize and in many other plants. Since this affects the
amount of chlorophyll, the presence of albinism in any
form seriously retards the growth. In extreme cases the
plants are totally incapable of continuing existence be-
yond the stage made possible by food stored in the seed.
In animals, albinism does not have the physiological sig-
nificance that it has in plants, but even here it is some-
times unfavorable to the individuals showing it. In every
case, and in all degrees, true albinism is recessive to the
normal condition. In maize, the heterozygous green plants
camiot be distinguished from homozygous green plants.
Many other unfavorable characters in maize are also re-
cessive. Absence of brace roots, bifurcated ears, dwarf-
ism, susceptibility to smut all behave in this way.

CAUSE OF HYBRID VIGOR 179

Certain factors have even been recognized, and in the
case of Drosophila ^^^' ^^« have been located in the chromo-
some mechanism, which are so injurious that they cause
the death of the individuals possessing them, unless pro-
tected by the factors being in combination with a normal
allelomorph. A well-known case of this kind is the yellow
mouse. Lethal factors, in order to be recognized easily,
must be recessive in their lethal action and must show a
visible effect on the soma when in combination with their
allelomorphs, since only in that case can the heterozygotes
be detected. In the yellow mouse there is associated with
color another effect which causes the death of the animals
when they are pure for that factor. This has been demon-
strated by the altered ratios obtained. Yellow mice are
mated together ; instead of getting a ratio of 3 yellow to 1
non-yellow, the ratio is more nearly 2:1; that is, (1) : 2 : 1,
in which the pure recessives are eliminated.^^ This as-
sumption is further corroborated by actually finding the
missing number of animals in stages of dissolution in
early embryonic life. Of the more than one hundred and
twenty-five mutations which have been described in Dro-
sophila by far the greater majority of them are recessive,
and nearly all of them are less favorable to the develop-
ment of the fly than the wild-type characters. The effect
of the recessive factors even seems to be cumulative, be-
cause when many of them are combined together the flies
are extremely difiScult to maintain. Much the same con-
dition is true for domesticated animals and plants. The
majority of the variations which have occurred are reces-
sive, and are seldom beneficial and often deleterious.
/ One may be led to inquire why it is that most of the
experimentally observed mutations are recessive and less

180 INBREEDING AND OUTBREEDING

favorable to the best development of the organism. We
do not know, but we may hazard a guess. The repeated
appearance and disappearance of certain mutations is
merely a type of variability which has probably been a
constant feature of the organism for a long period and
has been subjected to natural selection in the same way as
any other character. In other words, may not the ten-
dency to produce dominant unfavorable variations have
been reduced to the minimum by natural selection? Con-
versely, a tendency to produce unfavorable recessive mu-
tations has been tolerated because the latter are pro-
tected in hybrid combinations by their dominant favor-
able allelomorphs. Whether this be true or not, there cer-
tainly is a strong tendency for dominant unfavorable
variations to be eliminated, because they are constantly
subjected to natural selection; while dominant favorable
variations, whenever they occur, replace former charac-
ters, and become part of the stock in trade of the organ-
ism. Recessive mutations, on the other hand, whether
favorable or unfavorable, cannot compete for place with
natural selection as the judge unless the proper mating
brings them into the homozygous condition. If through
continuous cross breeding this does not occur, they may
be carried on for countless generations — ^family skeletons
hidden by the phenomenon of dominance.

The relation of these reflections to heterosis is just
this : If any individual is deficient and handicapped in its
hereditary make-up, there is a good chance that this de-
ficiency will be supplied when it is crossed with other
individuals, because all are not apt to be wanting in the
same things. What one lacks is supplied by the other
and conversely. In other words, there is a poolinsr of

y CO

w P

3 EL
3 o
2 CO

»^

?3 CO

tB O
<t> CO

O 3
3 P

If

- y

o »'

4

CAUSE OF HYBRID VIGOR 181

hereditary resources, so that the combined effect is better
than either could produce alone.

This complementary action can be illustrated by as-
suming three linked factors, all of which are essential for
best development. In one organism there is AbC; in an-
other there is oBc, Dominance, either partial or complete,
is characteristic of each. Now in the former interpretation
of heterosis, where a physiological stimulation was as-
sumed, the heterozygous combination, Aa, for example,
evolved developmental energy and differed in that respect
from either AA or aa. Moreover, this stimulation was
considered to be of a general nature, affecting the organ-
ism as a whole, and was thus differentiated from the spe-
cific effect which each had as hereditary factors. With
linkage, one may consider heterosis to be due to the action
of heredity alone : the hybrid union Aa is not superior to
either of the homozygous combinations, AA or aa, but is
more or less intermediate. This view has a very great
theoretical and practical importance, because one may ex-
pect to obtain homozygous instead of heterozygous com-
binations of the factors which bring about increased vigor
in crosses and thus obtain individuals which will have a
vigor equal or even superior to the first cross and which
will not be affected by future inbreeding.

Such a happy, result was not possible on the stimula-
tion hypothesis. This hypothesis was invented to account
for the frequency of heterosis, the loss of vigor due to
inbreeding, and the extreme rarity of homozygous com-
binations approaching those of the first hybrid generation
in vigor. With a knowledge of independent Mendel i an
heredity only it was necessary. But if in our illustration
the individual AbC-aBc is vigorous because of heredity

^

182 INBEEEDING AND OUTBREEDING

alone, and if it usually segregates germ cells of the types
AbC and aBc, making the vigor thus obtained a practical
corollary of heterozygosity, there is still the chance, no
matter how closely linked these factors may be, of breaks
occurring which will bring about the production of a gam-
ete ABC. This gamete, if it meets another of the same
type, will result in a homozygous individual, and if dom-
inance is but partial, this individual, through the very
fact of its homozygous condition, will be even more vig-
orous than those of the first hybrid generation.

Practically the difficulties in the way of obtaining such
pure combinations may be very great or even insurmount-
able, but the hypothesis holds out the hope of thus ob-
taining types of great economic value. The rearrange-
ment of factors in all possible recombinations is not pre-
vented by linkage as long as there are breaks in the
linkage. But since these breaks occur with varied fre-
quency between different factor loci, and in some cases are
very rare, the problem is exceedingly complicated; and
when many linked factors are involved the chance of ob-
taining an individual which is completely homozygous
in all factors in the first segregating generation is so ex-
tremely small that for all practical purposes it may be
disregarded. In later generations the chance may become
somewhat greater because of the formation of new linked
groups. If these are combinations which are favorable to
development, natural selection will increase individuals
possessing them at the expense of those having less favor-
able combinations. In time, there is the possibility, how-
ever remote, of all the more favorable factors being
brought together in a homozygous combination which,
therefore, will not be reduced by inbreeding.

CAUSE OF HYBRID VIGOR 183

Our hypothetical iUustration of characters contrib-
uted by both parents may be supported by actual results
from a cross between inbred strains of maize. As men-
tioned before, maize strains have been obtained which lack
brace roots and are unable to stand upright when the
plants become heavy. These strains, however, have the
habit of branching freely from the base of the plant, thus
producing several main stalks from each seed. When this
strain is crossed with one which possesses well-developed
roots, but which does not branch at the base of the plant,
the result is an extremely vigorous progeny which pro-
duces several stalks from each seed and which shows no
deficiency in root development. The hybrid plants are
so large and so exceedingly vigorous that other factors
must have been involved, but these two characters can
easily be seen to have contributed something to the
luxuriant development.

An even more striking illustration was obtained by
Emerson. A dwarf race of maize which was almost com-
pletely sterile was crossed with a tall plant which was
so deficient in chlorophyll production that it was un-
able to produce seed, although it had some functional
pollen. The hybrid plants were tall, dark green and
produced well-developed ears. Here normal stature was
contributed by one parent and proper chlorophyll devel-
opment by the other, the progeny was thereby enabled to
develop well and to become highly productive.

In both these crosses the characters involved are
largely of a superficial nature, although most of them are
necessary for full development. They are characters
which are easily seen and serve as an indication of what
must have occurred in the case of other factors more in-

184 INBEEEDING AND OUTBREEDING

temal in their effect and of more fundamental importance.
These more fundamental factors are those concerned di-
rectly with metabolism and cell division. As to their na-
ture and the way in which they are inherited, we, as yet,
know little, but there is reason for supposing they are
Mendelian in mode of inheritance and operate in a way
to enable the hybrid progeny to attain a greater develop-
ment than either parent.

For a definite detailed case showing exactly how dom-
inance and linkage thus work together we must look to the
work with Drosophila melanogaster, as this is the only
material which at the present time has been sufficiently
well analyzed for our purpose. Bridges and Sturtevant
have discovered, isolated, and determined the linkage re-
lations of nearly one hundred factors distributed through-
out the four chromosomes of this little fly, in the great
majority of which the recessive condition is unfavorable.
Through this indefatigable work there is an enormous
amount of data from which to choose ; but in order to make
our illustration comparatively easy to follow, let us con-
sider only four characters which are linked together in the
second chromosome. The factors which have the princi-
pal effect on these characters may be given the name
of the character. They are long legs (D) dominant to
**dachs" legs (d)^ gray body (B) dominant to black body
(&), red eye (P) dominant to purple eye (p), and normal
wings (V) dominant to vestigial wings (v). In gamete
formation in the female there are breaks with a frequency
of 10 per cent, in the linkage between d and h, of 6 per
cent, between h and p, and of 13 per cent, between p and v,
disregarding some disturbing conditions which need not
concern us here. In the male there are no linkage breaks.

CAUSE OF HYBRID VIGOR 185

Now if a female fly with dachs legs, gray body, purple
eyes and normal wings (dBpV), be crossed with a male
having long legs, black body, red eyes and vestigial wings
{DbPv)y the resulting progeny will have the usual wild-
type characters, long legs, gray body, red eyes and normal
wings, and will be considerably more vigorous than either
parent. If these factors segregated independently, one
would expect to find one gamete out of every sixteen to be
of the constitution DBPV, and would obtain one F2 indi-
vidual homozygous for this combination of the four domi-
nants out of every two hundred and fifty-six. As a mat-
ter of fact, owing to the linkage relations found, only one
gamete of this kind is produced in two thousand and then
only in the female. It is, therefore, impossible to obtain
the type sought in the F2 generation. But males of the
all-dominant type will appear in Fj, and the pure strain
may be established in F^. The word '^may'^ is used as a
sort of forlorn hope, however. There is a possibility of
establishing the homozygous dominant strain in F^, but
when one realizes that in F2 only one such male and one
heterozygous female similar in appearance to hundreds of
her sisters will be produced in every four thousand pro-
geny, the difficulties in the situation are emphasized.

The frequency of the linkage breaks is large and the
number of factors smaU in this illustration. When it is
remembered that in other organisms there are ten, twenty,
or even forty chromosome pairs to be considered, with
possibly dozens of factor differences, instead of four in
each chromosome, some idea may be obtained of the real
difficulties involved in producing individuals of maximum
vigor unaffected by inbreeding. Practically speaking, it
is impossible unless dealing with a small number of

186 INBEEEDING AND OUTBREEDING

loosely linked factors, except when long periods of time
are available and when natural elimiriation of undesir-
ables is high.

In tracing the evolution of ideas concerning the effects
of inbreeding and outbreeding we must give great credit
to Darwin for calling attention to the importance of the
phenomena in relation to evolution and for being the first
to see that hereditary differences, rather than the mere act
of crossing, was the real point involved ; but with all due
credit to Darwin, it was not until Mendelism became
known, appreciated and applied that the first real attack
upon the problem was made possible. When linked with
Mendelian phenomena it was clearly recognized for the
first time that one and the same principle was involved in
the effects of inbreeding and the directly opposite effects
of outbreeding. Inbreeding was not a process of continual
degeneration. Injurious effects, if present, were due to
the segregation of characters. In addition to this segre-
gation of characters the fact was established that an in-
creased growth accompanied the heterozygous condition.
All the essential facts were accounted for. A decade later
the great extension of knowledge in the field of heredity
has made possible a still closer linking of the facts of in-
breeding and outbreeding with Mendelism. The hypothe-
sis of the complementary action of dominant factors is
the logical outgrowth of former views and makes the in-
creased growth of hybrids somewhat more understand-
able. The fact of a stimulation accompanying hetero-
zygosity is supplemented by a reason why such an effect
is obtained. The former view of a physiological stimula-
tion and the more recent conception of the combined ac-

CAUSE OF HYBRID VIGOR 187

tion of dominant factors are not then two unrelated hy-
potheses to be held up for the choosing of the one from
the other. The outstanding feature of the latter view is
that there is no longer any question as to whether or not in-
breeding as a process in itself is injurious. Homozygosity,
when obtained with the combination of all the most favor-
able characters, is the most effective condition for the
purpose of growth and reproduction.

I

CHAPTER IX

STEEILITY AND ITS RELATION TO INBREEDING

AND CROSS-BREEDING

Probably the most noticeable effect of inbreeding in
both animals and plants is a reduction in fertility in the
earlier inbred generations. The experiments of Ritzema-
Bos^^* with rats, of Weismann^^ with mice and of
Wright 2^^ with guiaea-pigs are all thus characterized.

Miss King/^^ on the other hand, has iabred albino
rats for twenty-five successive generations by brother and
sister matiQg without any appreciable reduction in fer-
tility. Similarly, Castle ^^ and his students maintained
the fertility of Drosophila for fifty-nine generations of
brother and sister mating by breediag from the most fer-
tile flies. Various lines were isolated, nevertheless, which
differed in the number of offspring produced, and in the
first part of the experiment many individuals appeared
which were absolutely sterile. The production of such
non-fertile flies became less in the latter part of the ex-
periment, and the average fertility of the remaining stock
was improved by this elimination.

In maize the results of inbreeding are generally quite
serious as regards fertility. In the first place the consis-
tent reduction in size and constitutional vigor of the
plants necessitates a much smaller production of pollen
and ovules. The tassels are reduced in size and have
fewer branches. The ears are smaller and shorter and
oftentimes imperfectly covered with seeds even when
abundant pollen is available. In some cases the leaves

188

STERILITY 189

enclosing the tassels do not unfold properly and the tas-
sel does not develop as it should. This is a secondary
effect, but, nevertheless, is one factor in reducing fertil-
ity. The anthers are frequently much shruuken, some-
times shedding no pollen at all and even under the best
conditions producing a very meagre supply. The amount
of pollen produced is more affected by weather conditions
in such inbred strains than in more vigorous plants. At
the same time inbred strains of maize have been obtained
which show no degeneration in the staminate parts. Their
anthers are full and produce abundant pollen. Such
strains thus far have been few in number. They are cor-
related with poor development of the pistillate parts.
Those strains which have the best developed ears as a

., rule have very much reduced tassels with a large amount
of pollen abortive. Some strains have been obtained which
are about equally well developed in both staminate and
pistillate functions, and these range all the way from
plants which are fairly productive for inbred strains down
to types which barely produce enough seed to survive, and
since many cases of failure to produce seed are met there
can hardly be a doubt that in some of them a complete

I abortion of one or both functions has taken place. As in
the many other effects of inbreeding different results are
produced in different lines, showing clearly that segrega-
tion of certain factors influencmg fertility has taken place.
On the whole, there is in this species a tendency for in-
breeding to result in a change from a monoecious condi-
tion to a functionally dioecious condition.

Sterility in the form of structural degeneration whon
it occurs gradually increases upon inbreeding until homo-
zygosity is attained, but for the most part it does not show

190 INBREEDING AND OUTBREEDING

any clear-cut segregation. Yet reduction in fertility is
noticeable only so long as there is a change in other char-
acters, constancy in visible characters being accompanied
by a constancy in the matter of fertility. In other words,
there is no more an accumulation of sterility on con-
tinued inbreeding than there is an accumulation of any
other effect. Any reduction in fertility ceases when
homozygosity is reached, but the end result may be
decidedly different in various lines coming originally from
the same stock.

Many other instances of an effect of inbreeding upon
fertility might be given, particularly the appearance of
abnormalities in the genital organs, both external and in-
ternal. But what we desire is to show their meaning
rather than to catalogue them, and for this purpose no
data have been gathered as valuable as those upon the
much cited maize. Examination of all the isolated facts
brought to light in both animals and plants shows such a
similar trend that there is no reason to believe we are not
dealing with manifestations of one and the same law, yet
only in this species do we have a critical test of the hypoth-
eses involved. And here it can be stated unequivocally
and without reservation that the effect of inbreeding on
fertility is exactly the same as its effect upon other char-
acters. Recessive combinations deleterious to the func-
tion of reproduction are brought to light. But this is not
the only conclusion to be drawn. The frequency with
which depression of fertility occurs during inbreeding, the
slowness with which it is brought to an end, the variety
of differences which is brought out, all show how complex
one must conclude the function of reproduction to be, and
how many variations affecting it must be constantly occur-

STEEILITY 191

ring. In other words, there is here a concrete illustration
of the primaiy importance of reproduction in all evolu-
tion. Since provision for succession exceeds all other
matters in import to the species, new variations are con-
stantly taking place, new processes are continuously being
tried out. The result is to have reproduction tied up with
more complications than any other physiological process,
to have in naturally cross-bred species more heterozygous
factors than in any other character complex. Repro-
duction, therefore, as we have seen, is affected more
often and more frequently than anything else when
inbreeding occurs.

When inbred strains showing reduced fertility are
crossed, on the other hand, there is almost always a return
to the original productiveness along with the return to the
original size and vigor. In fact, just as fertility is
affected adversely by inbreeding more than any other
character, so is it increased more in proportion by cross-
ing. But such increases in productiveness are the rule
only up to a certain point of germinal difference between
the individuals taking part in the cross. As dissimilarity
in the uniting germ plasms becomes greater, sterility
again manifests itself. This time, however, the sterility
shown is of a different nature. No structural abnormali-
ties appear. There are no variations such as are found in
the numerous strains differentiated by inbreeding. It is
simply a matter of non-production of functional gametes.

Based upon these germinal differences crosses be-
tween species may be classified arbitrarily as follows :

(1) The hybrid may have the same or greater vigor
and fertility than the parents. Nicotiana alata and N.
Langsdorffii, for example, are distinct species having dif-

192 INBEEEDING AND OUTBREEDING

ferences in many oliaraoters, yet their hybrids give no
indication of any lessened fertility.

(2) The hybrid may have the same or greater vigor
than the parents and at the same time show reduced fer-
tility or even total sterility. This is a common result with
many species hybrids. The increase in growth is often-
times extreme. The cross between the garden radish,
Raphanus sativus, and the cabbage, Brassica oleracea,
two species belonging to different genera, gives plants of
rampant growth which are very nearly, if not completely,
sterile, as shown by Sageret ^^^ nearly a century ago and
by Gravatt ^^ in recent times (Fig. 38). In most of these
cases no seed can be produced by self-fertilization, but
back crossing with one or the other parents is sometimes
successful. Animal hybrids frequently show sterility in
the males and partial or complete fertility in the females.
This: is the condition in Cavia species hybrids (Detlef-
sen^*^) and in crosses made between the buffalo, Bison
(MYiericanus; the yak, Bihos grumens; the gayal, Bibos
frontalis; the gaur, Bibos gaurus, and the domestic cow.
Bos taurus,

(3) The species hybrid may exhibit a reduced size and
a decline in vigor combined with complete sterility. The
phenomenon shows in various degrees. For example,
East and Hayes ^^ made several Nicotiana crosses in
which the seed would not germinate, although both em-
bryo and endosperm tissue was formed. Crosses between
Nicoticma tahacum and N. paniculata, and between N.
rustica and N. alata resulted in seed which germinated,
but the plants were weak and died before flowering, appar-
ently because of inability to utilize the starch formed.

Fig. 38. — Sterile hybrid between radish and cabbage, showing the rampant growth, aoconi
panied by steriUty, sometimes obtained in wide crosses. (Gravatt, in .Jour. Iliredity.)

STEEILITY 193

In other crosses the plants matured, but they developed
very slowly and in the end were smaller than either
of the parents.

In general, therefore, it can be said that differences in
uniting germ plasms, when not too great, may bring about
both more efficient development and increased fertility.
Beyond that critical point of difference both fertility and
vigor may be decreased, but fertility is usually the hrst
to suffer— even complete sterility often being coupled
with rampant growth. Nature thus steps in before a
germinal heterogeneity which will endanger the health of
the hybrid organism has been reached, and prevents mul-
tiplication entirely. This is an important physiological
provision, since when great germinal differences exist
there is reduced growth as well as sterihty. Groups are
thus set apart which may evolve within themselves by put-
ting to good use heterosis and Mendelian recombination.
What apparently happens is this : As germinal differences
increase a point is reached at which the precise and com-
plex machinery governing gametogenesis cannot do its
work in the normal manner and sterility results, although
under the same conditions developmental cell division
goes on as usual. Beyond this degree of difference in the
uniting germ plasms, even somatic cell division is affected.

This sterility accompanying wide crosses is an almost
untouched problem. We can throw no light upon it except
the suggestions noted in the last few sentences. For this
reason one may inquire why it is mentioned in this con-
nection at all. In spite of our comparative lack of knowl-
edge as to just what occurs in the cell divisions of vdde
crosses, however, there is an excuse for meddling. The
peculiar resemblance of the effect of inbreeding to the

13

194 INBEEEDING AND OUTBEEEDING

effect of crossing various distinct species, has led many-
writers to identify the phenomena. Further, several
critics have maintained that a theory which purports to
interpret sterility in the one case, should interpret it in
the other. Now this is a point of view which is obviously
incorrect even with our present meagre knowledge of the
facts. The sterility often accompanying inbreeding is
not the same thing as the sterility resulting from hybrid-
ization. The resemblance is superficial in the extreme.
In the one case there is the differentiation of distinct
strains differing anatomically and physiologically in their
ability to perform the act of reproduction. It is a phe-
nomenon of Mendelian heredity which stands out in the
clear-cut manner it does, because the progenitors of the
individuals thus characterized have gone through with the
mechanical process which segregates factors, in the pre-
cise manner necessary to accomplish the purpose. In the
other case, the individuals are sterile because they cannot
go through this same process in the exact and proper way
required, on account of the incompatibility of the unit-
ing cells.

CHAPTER X

THE EOLE OF INBEEEDING AND OUTBREEDING

IN EVOLUTION

In our brief consideration of the more important
changes which have occurred in the reproductive mechan-
isms of animals and plants, several features stand out
impressively. Both animals and plants have followed
modes of reproduction that are identical in what are
deemed to be the essential features, something which can
be said of no other life process. It is not enough simply
to say that sexual reproduction has become the dominant
mode of propagation among organisms. One must go
further. Cross-fertilization, either continuous or occa-
sional, is the really successful method of multiplication
everywhere. Such a parallel evolution in the two king-
doms is valid evidence of real worth in the process : a
consideration of the evidence on inbreeding and cross-
breeding permits us to state this value in concrete terms.

The establishment of methods of reproduction which
maintain variation and inheritance mechanisms on a high
plane of efficiency is naturally a fundamental requirement
in evolution. Since, however, we have seen that there is
no reason for believing sexual reproduction to be better
adapted to assure a numerous progeny than asexual re-
production, it either must be a more perfect means of
hereditary transmission, or it must offer selective
agencies a greater variety of raw material.

Fortunately we are able to eliminate the first alter-
native. There is definite evidence that sexual reprodnc-

195

196 INBEEEDING ^ND OUTBREEDING

tion does not differ from asexual reproduction in
what may be called the heredity coefficient. It holds
out no advantage as an actual means for the transmis-
sion of characters.

The majority of zoological data on this subject has
little value on account of the experimental difficulties in-
herent in the material, although zoologists have published
more on the matter than the botanists. Plants furnish the
best material because of the ease in handling large num-
bers of both cuttings and seedlings side by side, and be-
cause of the opportunity to utilize hermaphroditic species.
Even with the best plant material several undesired vari-
ables are present, and experiments with them, therefore,
are not without their disappointments ; but no one who
has had a long and intimate experience in handling pedi-
gree cultures of plants can have any doubts concerning
the correctness of our conclusion. Practically the inquiry
must take the form of a comparison between the varia-
bility of a homozygous race when propagated by seeds
and when propagated by some asexual method. The first
difficulty is that of obtaining a homozygous race and thus
eliminating Mendelian recombination. The traditionally
greater variability of seed-propagated strains is due
wholly to this difficulty, we believe. It may be impossible
to obtain a race homozygous in all factors. There may be
a physiolo.2:ical limit to homozygosis even in hermaphro-
ditic plants. The best one can do is to use a species
which is naturally self-fertilized, relying on continued
self-fertilization for the elimination of all the hetero-
zyarous characters possible. We have examined many
populations of this character in the genus Nicotiana and
have been astounded at the extremely narrow variability

\/'l-

■^ ^ ^«.

Fig. 39. — Tassels and ears of an almost sterile strain obtained by inbrcodinfi maize.

(From Emerson.)

EVOLUTION 197

they exhibit. Even though one cannot grow each member
of such a population under identical conditions as to nutri-
tion, the plants impress one as if each had been cut out
with the same die. Qualitative characters such as color
show no greater variation, as far as human vision may
determine, than descendants of the same mother plant
propagated by cuttings. Further, in certain characters
affected but slightly by external conditions, such as flower
size, the sexually produced population not only shows no
greater variability than the asexually produced popula-
tion, but it shows no more than is displayed by a single
plant. Yet one must remember that in such a test the
seeds necessarily contain but a small quantity of nutrients
and for this reason the individual plants are produced
under somewhat more varied conditions than those result-
ing from cuttings, hence it would not have been unreason-
able to have predicted a slightly greater variability for
the sexually produced population, even though the coefi5-
cient of heredity of both were the same. Similar, though
less systematic, observations have been made on wheat —
an autogamous plant almost as satisfactory for such a
test as Nicotiana — with practically identical results.

One is justified, then, in claiming there is experi-
mental evidence to show that sexual reproduction in
itself is no more than an exact equivalent of asexual re-
production in the matter of an heredity coefficient. But
is this also true for germinal variation? We believe it is.
Variations similar in size and kind arise both in asexual
and in sexual reproduction, but it cannot be maintained
they occur more frequently in the latter. There are insects
in Oligocene amber apparently identical with those of
to-day, proving constancy of type to be possible under

198 INBEEEDING AND OUTBEEEDING

sexual reproduction through millions of years ; there are
asexually reproduced species of plants just as constant
and probably stiLL more ancient. At the same time, ger-
minal variations occur to-day under sexual reproduction
in somewhat noteworthy numbers, as Morgan's work on
Drosophila shows. There has been no trustworthy esti-
mation of their frequency within even a single species, but
it cannot be said they occur in less numbers than where
asexual reproduction rules, even among organisms of a
relatively high specialization. If there are those who
doubt this statement, let them refer to the huge list of bud-
variations in the higher plants compiled by Cramer.^^
He will be able to identify there, type by type, class
by class, practically all of the variations he is able to
discover in the same species in the literature on sem-
inal reproduction.

It is rather odd that this should be the case, for what is
being discussed here is not really the frequency with
which variations occur, but rather the frequency with
which they are detected. And theoretically, the ease of
detecting variations ought not to be tlie same under the
different modes of reproduction. If it be granted that
changes in the constitution of the chromosomes are direct
causes of variations, and that such changes in constitu-
tion are equally probable in all chromosomes, it follows
that partheno genetic individuals having the haploid num-
ber of chromosomes should show a larger proportion of
germinal variations than members of the same species
having the diploid number of chromosomes, because
variations of all kinds would be recognizable in the
former case, while in the latter recessive variations could
not be detected until the first or second filial generation

EVOLUTION 199

and then only when the proper mating was made. Though
there is no direct support for this idea in the species where
the premises hold, there is some evidence that the reason-
ing is not wholly improbable. Bud-variations occur much
more frequently in heterozygotes than in homozygotes.
This simply means that bud-variations are brought to
light more frequently in heterozygotes than in homozy-
gotes : and a reason is not hard to find. Recessive varia-
tions are much more frequent than dominant variations,
and a recessive variation in a particular character shows
only when the organism is heterozygous for that char-
acter. If a recessive bud-variation arises in a homozygote
and gametes are afterwards developed from the sporting
branch, it is not at all unlikely that the variation may
show in the next generation, but it will be attributed then
to gametic mutation.

If, therefore, one is constrained to admit that the pre-
ponderance of the evidence points to practically the same
coefficient of heredity for both forms of reproduction, and
that variation in the sense of actual changes in germinal
constitution may occur with greater frequency in asexual
reproduction, if there be any difference at all between
the two forms, he is left with only one reasonable hypoth-
esis to account for everything, Mendelian segregation
and recombination.

Mendelian heredity is a manifestation of sexual re-
production. Wherever it occurs, there Mendelian heredity
will be found. Now if N variations occur in the germ-
plasm of an asexually reproducing organism, only N types
can be formed to offer raw material to selective agencies.
But if N variations occur in the germ plasm of a sexually
reproducing organism 2" types can be formed. The ad-

200 INBREEDING AND OUTBREEDING

vantage is almost incalculable. Ten variations in an
asexual species mean simply 10 types ; 10 variations in a
sexual species mean the possibility of 1024 types. Twenty
variations in the one case is again only 20 types to survive
or perish in the struggle for existence ; 20 variations, in
the other case, may present 1,032,576 types to compete in
the struggle. It is necessary to condition the argument
by pointing out that these figures are the maximum possi-
bilities in favor of sexual reproduction. It is improbable
that they ever actually occur in nature, for 2^^ types really
to be found in the wild competing for place after only 20
germinal variations would mean an enormous number of
individuals even if the 20 changes had taken place in dif-
ferent chromosomes, and if the variations were linked at
all closely in inheritance the number required would be
staggering. But there are breaks in linked inheritance,
and the possibility is as stated. Associated with this
benefit arising from the law of recombination, there is
another of great practical importance resulting from the
phenomenon of dominance. Recessive variations may
arise, which in the particular factorial complex existing
in the individuals at the time of origin, would cause the
possessors to be eliminated by natural selection. These
variations, however, may be carried an indefinite number
of generations in the heterozygous condition, thus multi-
plying the chances that they finally be combined with
other factors in complexes which as a whole are desirable.
These inestimable advantages remain even though it
should be shown later that the more fundamental and
generalized characters of an organism are not distributed
by Mendelian heredity. Loeb ^^9 suggests that the cyto-
plasm of the egg is roughly the potential embryo and that

EVOLUTION 201

the chromosomes, distributed as required by the breeding
facts of Mendelian heredity, are the machinery for im-
pressing the finer details. There is very little to be said
for this point of view, though it may have use as a working
hypothesis. But granting its truth, it does not detract
from the benefits gained by the origin of sex ; the major-
ity of variations are comparatively small, changes in
detail, the very kind which are known to be Mendelian
in their inheritance.

Yet sexual reproduction in itself does not assure these

I advantages, though they are based upon it. There must

he means for the mixture of germ plasms. This oppor-
tunity was furnished originally by bisexuality. After-
wards hermaphroditism was tried ; and, though manifestly
an economic gain, it was, on the whole, unsuccessful except

g as functional bisexuality was restored by self-sterility,
protandry, protogyny or mechanical devices which pro-
moted cross-fertilization. The prime reason for the suc-

1 cess of sexual reproduction, then, as Weismann 209 first
maintained, though he knew not the exact reason, is the
opportunity it gives for mingling germ plasms of different

I constitutions, and thereby furnishing selective agencies
many times the raw material producible through asexual
reproduction. It was not sexual reproduction per se
which triumphed, but exogamy.

While increased variability and the greater elasticity
in adaptiveness to new environments thus gained must be
given the first consideration when seeking the significance
of sex, they are not the only advantages. As we have
seen, an increased size, greater viability and increased
production of offspring commonly result from crossing
somewhat different forms. Here is a combination of

202 INBKEEDING AND OUTBREEDING

qualities unquestionably having survival value in the
great majority of oases. It is a phenomenon so universal,
so uniform in its effect, it must have played an important
role during the course of evolution. Heterosis increasing
growth and fertility immediately, segregation favoring
adaptibility in the next generation, is a partnership of
some strength. An income for life and a trust fund ma-
turing for benefit of the children, what more could
one ask?

Heterosis may even be pictured as the efficient cause of
sex survival. Some means of favoring union of dissimilar
spores occurred as a chance variation. Through the com-
bination of somewhat different qualities this new dual
product, the zygote, was better enabled to develop and to
reproduce. Its survival coefficient was high. The ten-
dency for union of spores persisted and became char-
acteristic of the species. Sex was established.

This is a pleasing theoretical picture, and we do not
believe it is overdrawn, but it must be admitted that the
concrete evidence of a sexual union being immediately
beneficial in the lower organisms is not what might be
desired. Jennings ^^^ finds a marked slowing down of
the reproductive rate in the generations immediately fol-
lowing conjugation in Paramecium, with no beneficial
effect resembling heterosis, although he suggests that re-
combination may account for the best cultures. In fact,
nothing similar to heterosis has been found in unicellular
organisms. The lowest tjipe where distinct evidence of
the phenomenon has been discovered is in Trochelminthes,
cross-fertilization increasing size, vigor, viability and re-
productive rate in rotifers. But it would be strange
indeed if no such effect did occur in low forms when it is

Fig. 40.

M ^ &

i^Mi^iJi

Fig 40. — Ears of a first generation cross between two inbred strains of maize showing

uniformity.
Fig. 41. — Plants of the first, generation cross between the sami' twi> inbred strains <>f maiip.

Note the uniformity in lieight.

■!* Vv -^sXtfifc^^v X4!i*

Fig. 42. — Diagram showing a method of double crossing maize to secure maximum yields,

illustrated by actual field results.

EVOLUTION 203

so widespread in all the higher plants and animals. Herit-
able variations are constantly arising in simple organisms,
as has been demonstrated by J ennings ^^"^ in Difflugia, and
it may be assumed that these are in part favorable and in
part unfavorable. The union of two individuals would
have the same chance of bringing together the greatest
number of favorable growth factors and the progeny
would thus be benefited, even though the mechanism fur
bringing this about is not as well organized as in the
higher forms.

Some evidence of the possible importance of heterosis
in the establishment of sex may be obtained by the con-
sideration of an analogous phenomenon, double fertiliza-
tion among the angiosperms. In the gymnosperms the
embryo develops from the fertilized germ cell, of course,
but the endosperm which nourishes the young seedling is
gametophyte tissue. In the angiosperms the endospenn
as well as the embryo develops after a fertilization has
taken place. The conditions are slightly different, as a
fusion between two maternal nuclei occurs before tin?
union with the second male nucleus, but the essential
feature is the same as in the production of the embryo —
different hereditary materials are united when cross-fer-
tilization occurs. And in the same way that the embr^^o
and the resulting plant may be greatly benefited by cross-
fertdization, so also is the endosperm tissue increased in
amount as a direct manifestation of hybrid vigor.

Nemec^^2 j^^g sought to account for endosperm hy-
bridization as an adaptation which results in a better
adjustment of the composition of the reserve food supply
to the needs of a hybrid embryo. The cross between some-
what different types results in an embryo which presum-

204 INBEEEDING AND OUTBEEEDING

ably partakes of certain features of both parents. If
forced to depend upon food supplied by only one parent it
might be handicapped to some extent in comparison with
another embryo supplied with food which was intermedi-
ate with respect to the two parents. If endosperm hybrid-
ization does indeed supply such a need, the fact that the
endosperm is also increased in amount would have equal
importance. It may well be that to fill either purpose
endosperm hybridization has sufficient value to account
for its maintenance in the angiosperms. However this
may be, increased adaptability through recombination of
characters which is such an important factor in sexual
reproduction has no significance in this case, as the endo-
sperm does not perpetuate itself.

Additional light may be thrown on the importance of
heterosis in sex origin from the part it possibly has had in
a related series of events. In the algae and mosses, the
principal life processes are carried on in the haploid gen-
eration and the parts which result from fertilization and
produce the spores are relatively insignificant and are
dependent upon the gametophyte for maintenance. How
the sporophyte has gradually become more specialized,
taking up the manufacture of food for itself until finally
the relations are changed completely, are matters of
common knowledge. This series of events is usually
referred to as the rise of the sporophyte and decline of
the gametophyte.

Just why there has been this radical and complete
change in the plant kingdom is rather difficult to explain,
but it should be noted that the increased variability and
greater adaptability which seems reasonable in account-
ing in a large measure for the survival of sex, is not

EVOLUTION 205

applicable here. Recombinations occurring at the reduc-
tion division can be utilized by the gametophyte as well as
by the sporophyte, hence there seems to be no necessity
for plants to change from dominant gametophytes to
dominant sporophytes in order to secure the greater
adaptability offered by sexual reproduction. Every com-
bination of characters possible in the sporophyte occurs
in the haploid condition, if we leave out of considera-
tion heterozygous combinations which are the interaction
of two members of a contrasted pair of factors and can-
not be fixed. Haploid combinations even have certain
advantages over diploid combinations in that all the char-
acters are expressed and offer more material to natural
selection. Favorable combinations have a better chance
of survival and unfavorable combinations are more
quickly eliminated. If, then, variability is not a factor
in the rise of the sporophyte, and if we refuse to admit
any value in chromosome-doubling itself, and the evidence
certainly does not indicate that it has any significance, the
only factor which remains, as far as we can see now, is the
vigor derived from hybridization. Heterosis, of course,
can operate only in the sporophyte. In the lower plants
where the sporophyte is less important in the life cycle,
heterosis would be of value only in spore formation. Later
as the sporophyte became of more consequence, heterosis
would have had more and more value ; and it may well be
that it had considerable to do with this revolution in
plant life.

If sexual reproduction is so useful that it has been
adopted as the principal means of reproduction at a
sacrifice of speed of multiplication and economy of ma-
terial, why, then, has it been given up by the many species

206 INBEEEDING AND OUTBEEEDING

which have resorted to vegetative propagation or par-
thenogenesis? Even self-fertilization, which is the rule
with many plants, nullifies the advantages which were re-
sponsible for its development. As far as there is signifi-
cance in amphimixis in inducing variability, continuous
self-fertilization must for the most part be left out of
consideration. "Weismann ^^^ states the problem :

If amphimixis has been abandoned in the course of phylogeny by
isolated groups of organisms, this has happened beoause other advan-
tages accrued to them in consequence, which gave them greater security
in the struggle for existence ; but it must be admitted that they thereby
lost their perfect power of adaptation, and that they have thus bartered
their future for the temporary securing of their existence.

Let us see what it is for which these organisms ^ * barter
their future.'' According to the view of heterosis out-
lined previously, there is no advantage in the hetero-
zygous state in itself, but on account of linkage it is
difficult to obtain all the more favorable characters which
exist in a species combined in one individual in a pure
breeding, homozygous condition. There is always the
possibility of obtaining such combinations, however, and
the resulting individuals are well fitted for survival
as long as the environment remains the same. If the pro-
duction of these favored few is accompanied by any
change which renders cross-fertilization difficult, and if
there is nothing to prevent them from resorting to self-
fertilization, parthenogenesis or vegetative means of
propagation, there is no obvious reason why the plants
should not undergo the change. They would possess the
most efficient means of multiplication and would doubtless
be fitted for survival through long periods of time. They
would not be flexible, however, and if the environment

EVOLUTION 207

changed would probably lose in the race with more adapt-
able cross-fertilized forms. Their handicap is their lack
of chances for progress.

A seoondaiy advantage of sexual reproduction is the
division of labor made possible by secondary sexual char-
acters, using the term very generally and including even
such differences as those which separate the egg and the
sperm. It is not known just how these differences arose
or by what mechanism they are transmitted. The great-
est hope of reading the riddle lies in an investigation of
hermaphroditic plants, for there are technical difficulties
which till now have prevented its solution in animals. For
example, breaks in the linkage between sex-linked char-
acters occur only in the female in Drosophila, and as the
sex chromosome is double in the female, it cannot be de-
termined whether the differentiation between male and
female is due to the whole chromosome or not. But this
ignorance does not give reason for a denial of the
great advantage which sexes bearing different characters
hold over sexes alike in all characters except the primary
sex organs.

The only glimpse of the truth we have on the matter
comes from recent work on the effect of secretions of the
sex organs on secondary sexual characters. The effect of
removing the sex organs and the result of transplanting
them to abnormal positions in the body have sho\\Ti that
in vertebrates the secretions of these organs themselves
activate the production of the secondary sexual char-
acters. This does not seem to be the case in arthropods,
however; so one cannot say that primary sexual differ-
entiation and secondary sexual differentiation are one and
the same thing. Nevertheless, the generalization is not

208 INBEEEDING AND OUTBEEEDING

improbable. Surgical castrations of insects which have
not altected the secondary sexual characters even though
made in the early larval stages, are not conclusive because
of the possibility of the primary sex organs having a
marked influence on development very early in the life
cycle ; and parasitic castration brings in another variable
through the presence of the alien organism.

Again, there is a presumable advantage in bisexual
reproduction in having sex-linked characters. We say
presumable advantage, for all of the relationships be-
tween sex and sex-linked characters are not clear. The
facts are these : One sex is always heterozygous for the
sex determiner and the factors linked with it. Even if
there be no actual advantage in the heterozygous condi-
tion, if heterosis prove to be only an expression of the
meeting of dominant characters, a possible advantage still
accrues to this phenomenon because the mechanism con-
tributes toward mixing of germ plasms. As an example,
let us take the Drosophila type of sex determination.
There the sperm is of two kinds ; the one containing the
sex chromosome and its sex-linked factors, the other lack-
ing it. The eggs are all alike, each bearing the sex
chromosome. It follows, then, that the male always re-
ceives this chromosome from his mother who may have
received it from either her father or mother. Moreover,
further variability may be derived from the linkage
breaks which occur always in the female. This last phe-
nomenon is hardly worthy of special mention, however,
until it is shown to be typical of bisexual reproduction.

This short reconnaissance presents only the facts on
the role of reproduction in evolution as they are affected
directly or indirectly by inbreeding and outbreeding. A

EVOLUTION 209

very great number of interesting things connected with
reproduction during the course of evolution have not been
mentioned. This is because it is felt that the vital feature
in the whole affair, the persistence in both the animal and
the plant kingdoms of innumerable mechanisms providing
for cross-fertilizations, is to be explained solely on the
ground of offering selective agencies the greatest amount
of raw material. Mendelian recombination is thus
assigned a part in phylogenetic development second only
to inherent variability, and the whole history of repro-
ductive change becomes clear without the ill-advised
assumption that complex processes like autogamy are
harmful in themselves.

14

CHAPTER XI

THE VALUE OF INBEEEDING AND OUTBREED-
ING IN PLANT AND ANIMAL IMPROVEMENT

The origin of our more important domestic animals
and cultivated plants is a matter on wliich there is no
direct evidence. Among animals the ostrich is the only
example of modern domestication; among plants not a
single species of great economic worth has been brought
into cultivation within historic times. If one must have
a theory concerning their genesis, and what one of us
does not delight in theorizing, the weight of evidence is in
favor of a poly-phyletic origin in nearly every case. There
is more than one ivild species related to our modern dogs,
cattle, swine and sheep, our wheats, barleys, apples and
grapes; and these species will cross together and yield
partially fertile hybrids. The wild relatives of the do-
mestic forms were variable, so variable that many species
were differentiated by natural causes; yet these species
groups remained so well adapted to each other germinally
that their hybrids are not completely sterile. What seems
more reasonable than to suppose the original domestic
races to have been produced by uniting two or more wild
types and following this union of diverse germ plasms
with more or less close inbreeding and selection?

Such procedure, at least, has been the method whereby
the clearly distinct and highly valuable breeds of the pres-
ent day have originated. Take the draft horses as an
example. In the early days of Europe native breeds were
developed in every country for military purposes. Just

210

PLANT AND ANIMAL IMPROVEMENT 211

how tliey originated we cannot say. The obvious fact is
that none of them developed outstanding merits except
the Flemish horse. Then improvement became rapid and
steady. With an infusion of Flemish blood came the
remarkable development of the Clydesdale in Scothand,
the Shire in England, and the Belgian in the low countries.
Adding the Arabian blood which came in with the defeat
of the Saracens in 732, and the wonderful Percheron of
France came into being.'^^

Similarly the origin of all modem breeds of coach,
light harness and saddle horses may be traced. To the
native breeds of Europe were added the blood of tlie
iBarb or its derivatives, the Turk and the Arabian. In
France, in Spain, in England and in Russia the history is
the same — hybridization, then close breeding and selection.

If one turns to cattle, the story varies but little. The
basis of our modern strains is the cross between domesti-
cated progeny of wild European cattle and their Asiatic
relatives. From this stock numerous breeds grew up dif-
fering in contour, size and color. Some were horned,
others were hornless. Some were developed for meat pro-
duction, large at maturity and quick in attaining it ; others
were selected for the dairy, a great milk production and a
high percentage of butter fat. As time went on and com-
mercial channels became better established crosses were
made between the better animals of the different beef
breeds and between those of the various dairy breeds.
Crossing followed by inbreeding has been the touchstone
of success.

Similar more or less useless generalities could be given
about swine, sheep, dogs, cats, the cereals, the perennial
fruits, the numerous floricultural novelties, but this would

212 INBEEEDING AND OUTBEEEDING

serve no purpose. We have seen from our consideration
of the facts of heredity that both inbreeding and out-
breeding must be used if one would succeed in improving
the products of domestication. There must be cross-breed-
ing to furnish a variety of character combinations from
which to select; there must be inbreeding to provide the
opportunity to isolate the combinations desired. What
we want to know now is the manner of their use, the degree
of inbreeding permissible under given conditions, the effi-
cacy of crossing for particular purposes.

While there has always been a certain amount of in-
breeding as a necessary adjunct in building up breeds of
livestock because of the necessity of mating near relatives
in order to establish uniformity, the opinions of breeders
have differed and still differ as to how long or how close
intermating can be practiced with safety. Yet some of
the most noted modern livestock strains owe their excel-
lence to a close and continuous inbreeding that would be
looked upon with misgivings by the majority of animal
raisers. In fact, some of the inbreeding actually prac-
ticed was due more to enforced isolation, or the expense
or difiiculty of securing unrelated animals with desirable
characteristics, than to a firm belief in the desirability of
the method. This might be said of the Shetland pony, the
Angora goat, the Merino sheep in America, and of many
breeds of dogs.

Notwithstanding these facts, it would be a mistake not
to recognize how great an amount of continuous and ex-
tended inbreeding has been practiced intentionally with
the best of results after the general characteristics of a
breed have been established. This is true as a generalized
statement for the modern trotting horse and saddle horse

Fig. 43. — First generation cross resulting from a pure bred Slirojisliiic ram mainl
with grade Delaine-Merino ewes. First prize cross-bred yearling ewes at tin- hitcrnatiniial
Livestock Exposition in 1917. (From Severson.)

PLANT AND ANIMAL IMPROVEMENT 213

which have shown so much speed; for the Shorthorn and
Hereford, the most famous English breeds of beef cattle;
for the Southdown and the other famous sheep breeds, the
Shropshire, the Oxford and the Hampshire, to which it
has given rise; and for almost all of the more famous
breeds of dogs, not even excepting the large types, the
mastiff, the St. Bernard, and the Newfoundland, which
are derived from the Tibetan dog, Canis niger, as a foun-
dation stock.

Perhaps the most notable examples of conscious use
of intense inbreeding in developing breeds of marked ex-
cellence are the dairy cattle of the channel islands, the
Jersey and the Guernsey. One does not need to describe
or to eulogize these strains. What they are and what they
have accomplished in producing milk and butter fat are
known throughout the world. Starting with the cattle of
Normandy and Brittany as foundation stock, these two
breeds have been built up by persistent use of a more
intense system of inbreeding than is recorded in the his-
tory of any other strain of livestock. In fact, since 1763
rigidly enforced laws have prevented landing any live
cattle whatsoever on either island except for slaughter.
When one realizes that the larger of these two islands,
that of Jersey, is but eleven miles long by six miles wide,
he can appreciate the amount of inbreeding these laws
have promoted.

With swine, one gathers that injurious results from
close mating may be somewhat more pronounced than
with some other animals ; in other words, that swine carry
a large number of deleterious recessive characters. But
many of the famous breeds of swine have been rather
closely inbred. Mr. N. H. Grentry of Sedalia, Missouri,

\

214 INBREEDING AND OUTBREEDING

who has achieved quite a remarkable success with Berk-
shires, rarely went outside of his own drove for breeding
stock. He is quoted as saying (Mumford ^^^) : "If it is
true that inbreeding intensifies weakness of constitution,
lack of vigor, or too great fineness of bone, as we all be-
lieve, is it not as reasonable and as certain that you can
intensify strength of constitution, heavy bones, or vigor,
if you have these traits w^ell developed in the blood of the
animals you are inbreeding? I think I have continued to
improve my herd, being now able to produce a larger per-
centage of really superior animals than at any time in
the pasf

This quotation exemplifies the opinion of the best in-
formed of the practical breeders of the present day in re-
gard to the practice of inbreeding. In general they recog-
nize that the results obtained depend largely upon the
character and constitution of the animals, and the care
and skill with which they are selected for mating. They
have learned by experience what matings are the most
successful and how far it is advisable to carry close breed-
ing with a particular stock. Rarely is inbreeding as close
as brother and sister or parent and offspring mating con-
tinued for many successive generations, however ; for they
are apprehensive at all times that inbreeding may reduce
the fertility and lessen the constitutional vigor of their
animals, and they frequently introduce stock from outside
to counteract any tendency in this direction whether
fancied or real.

In plants the problem is different. No systematic in-
dividual mating system is practiced, as is the case with
animals, so that whether plants are inbred or outbred is a
matter w^hich is left to regulate itself automatically.

PLANT AND ANIMAL IMPROVEMENT 215

Among those plants which are largely self-pollinated by
nature, chance crossing, or, in some cases, systematic
hybridization, has originated new types. Self-pollination
has brought these types to uniformity, and by isolation
new varieties have been established. Among naturally
crossed plants genetical variations are continually being
produced and selection for certain of the more conspicu-
ous features has led to the creation of well-marked varie-
ties. Indian com is one of the best examples in this class.
There are many distinct types, and the less distinct but
fairly well recognized varieties are almost innumeral)lo,
adapting the plant to a range of conditions from the edge
of the Arctics to the Tropics, throughout the world.

In eveiy locality where com is grown the usual habit
is to prevent inbreeding as much as possible. Many com
growers make a regular practice of bringing in seed from
other localities, and often two or more somewhat different
varieties are planted together and allowed to mix. The
reason why this practice is followed is easily apparent
from the controlled experiments on the effects of inbreed-
ing and cross-breeding upon this plant. But even keeping
in mind the injurious results of inbreeding, indiscriminate
crossing is not desirable. Many of the well-known varie-
ties in the Com Belt States, such as Eeid^s Yellow Dent,
Learning, and Boone County White, are the results of
long-continued selection for certain standards without
crossing with other varieties. Inbreeding, therefore, has
secured individuality for varieties of cultivated plants as
well as for breeds of animals.

The value of inbreeding in plant and animal improve-
ment in the past may be summed up in the statement that
it is the greatest single agency in bringing about uni-

216 INBEEEDING AND OUTBKEEDING

formity and the concentration of desired qualities. So
valuable have been the results, particularly with animals,
that it has often been continued even though concentra-
tion of characters which made for lessened constitutional
vigor and fertility accompanied the accumulation of de-
sirable features, for the good outweighed the evil. To
overcome anticipated calamities, animal breeders have
from time to time introduced fresh stock. In doing this
they certainly were wise, since a rather high probability
always exists that such a procedure will introduce the
dominant complements of the detrimental characters.
But even granting the good sense at the base of both prac-
tices, it may be doubted whether inbreeding and cross-
breeding have been used in the best possible manner as
means of improvement. There are precise uses to which
each may be put which hitherto have not been considered.
Experiments with maize show that undesirable quali-
ties are brought to light by self-fertilization which either
eliminate themselves or can be rejected by selection. The
final result is a number of distinct types which are con-
stant and uniform and able to persist indefinitely. They
have gone through a process of purification such that only
those individuals which possess much of the best that was
in the variety at the beginning can survive. Although
these resultant, purified t^^es have little value in them-
selves, they have possibilities. The characters which they
have can now be estimated more nearly at their true
worth. By crossing, the best qualities which have been
distributed to the several inbred strains can be gath-
ered together again and a new variety re-created. Size,
vigor and fertility can be fully restored with the advan-

•■>i>T-,'CC.*S

Fig. 44. — "California Favorite," finst generation cross between a Hereford and Sliort-
horn; grand champion steer at the International Livestock Exposition in 1910; considered
to be one of the finest steers ever exhibited. (.From True.)

i

PLANT AND ANIMAL IMPROVEMENT 217

tage of real improvement through the elimination of
certain undesirable characters.

At present, this application of inbreeding to the im-
provement of cross-bred animals and plants is somewhat
of an unknown quantity. It has not been as thoroughly
tested as might be desired, but the basic principle is
sound. Although it is a drastic procedure, it is merely
utilizing to the fullest extent what practical breeders
have recognized as one of the most valuable benefits of
close mating. Accepting the doctrine that consanguinity
in itself is not in any way injurious and that good or evil
results from it solely through the inheritance received,
we can attack the century-old problem of inbreeding with
a clarity of vision heretofore impossible. Breeds of ani-
mals, and naturally crossed varieties of plants, which are
necessarily more or less heterogeneous in their hereditary
constitution, can be split up into their component parts
by this means. The pure types obtained can then be
selected wdth far more surety than is ever possible with
organisms in a continuously hybrid condition, thereby
presenting basic stock of tested value for further hybrid-
ization and recombination.

With plants the application of this method would be
simpler than with animals. Most naturally crossed plants
can be artificially self -fertilized and constancy and uni-
formity reached in about eight generations if there are
no complicating factors such as self -sterility. The ex-
pense would not be prohibitive, although many pure lines
must be tested in order to have a high probability of
obtaining all that is best in a variety. After the most
desirable combinations are isolated, their recombination
into a new and better varietv, which could be maintained

218 INBEEEDING AND OUTBREEDING

by seed propagation, would be a comparatively easy
undertaking.

"With plants which are propagated vegetatively, the
matter is even less difficult. Nearly all varieties of fruits,
flowers and vegetables propagated in this way are notori-
ously unstable when grown from seed. The excellent
varieties that we now have midoubtedly owe their supe-
riority in large measure to a fortunate combination of
manv different characters so made as to obtain the maxi-
mum effect from hybrid vigor. Attempting to obtain
further improvement by crossing these already widely
crossed varieties is like trying to solve a picture puzzle
in the dark. First analyze the material to be used by
systematic and rigorous inbreeding, let the consequences
be what they may. Then cross the different constant
types which may be ultimately obtained and test one com-
bination after another until a real improvement is effected.
When that is done the individuals can be propagated in-
definitely by the same means utilized before. Of course,
this method has the objection that many of the plants
propagated asexually require several years for each
sexual generation. Results would be slow for that reason,
it is true, but they would be sure.

With animals the application of this method would be
quite a different proposition. Inbreeding closer than
brother and sister mating could not be practiced, and the
time required to obtain purity and constancy would be
much greater than is the case with self-fertilization.
Moreover, the number of individuals which could be ob-
tained would be so small that selection could not be made
advantageously. Finally, the cost of raising most ani-
mals is so great that the maintenance of animals of little

PLANT AND ANIMAL IMPROVEMENT 219

or no value in themselves solely for a possible ultimate
improvement miglit well be too discouraging an under-
taking. But what could be done is to use animals from
some of the intensively inbred herds of the present day
as basic stock for building up new strains through cross-
breeding and selection. The point which we particularly
wish to make here is that the apparently disastrous
effects of inbreeding need not be so greatly feared as is
usually the case ; because if anything is lost by inbreeding
it is usually something undesirable. Inbreeding, there-
fore, may prove to be a very great gain if used as a
method of purifying and analyzing a cross-bred stock.

Wliile the full value of inbreeding in plant and animal
improvement has not as yet been fully recognized, the
advantages derived from outbreeding are more generally
known. Outbreeding as a means of improvement may be
considered under two heads : First, the immediate value to
be derived from crossing related types and thus securing
the maximum benefit from hybrid vigor ; second, the more
complex problem of crossing radically different forms to
create variability out of which new breeds or new varie-
ties may be constructed by a process of selection.

In some cases the first generation cross, although
vigorous, is sterile. An example is the mule, which,
though having the disadvantage of not being able to re-
produce, has held a place in agriculture and industry
throughout historic times. According to Mumford ^^'^
there were nearly five millions of these animals in the
United States in 1915. Of it he says :

This was more than one-fifth of the total number of horses in the
comitry at the time. The |)roduotion of mules has increased at a more
rapid rate than hoi-ses, and the use of mules is becoming more exten-

220 INBEEEDING AND OUTBREEDING

sive. The mule hybrid is a remarkable example of the practical ad-
vantagfes which follow a particular cross. This animal is more hardy
and enduring than either parent. As compared with the horse, the mule
is longer lived, less subject to disease or injury, and more efficient in
the use of food. The mule can be safely put to work at a younger
age, will thrive on coarser feed, and seems to be much better able to
avoid many dangere which menace the usefulness of the horse. The
mule will perform more arduous labor on less food. The mule will
endure the heat of southern latitudes more successfully than the horse
and is therefore a more popular draft animal in the South.

Other first generation crosses among animals, which
are not sterile like the mule, have good qualities and are
well known. Youatt, early in the nineteenth centuiy,
stated that crosses between the English and Chinese
breeds of s\vine were frequently made, and that in Ger-
many the native breeds were often crossed with the Eng-
lish breeds. To-day the first generation cross between the
Duroc-Jersey and the Poland-China, and between the
Poland-China and Chester White are popular animals
among the feeders. No attempt is made to breed from
them as it is well known that the later generations are
variable in color, size and conformation, and usually
possess less vigor than the animals of the original cross.

First generation crosses between many of the stand-
ard breeds of beef cattle are raised, and frequently they
whi the first prizes at the stock shows. The Shorthorn
and Aberdeen-Angus combination is popular.

The Mediterranean breeds of poultry are sometimes
crossed with the heavier types. First crosses of Leg-
horns and Plymouth Eocks give birds which are not so apt
to become over-fat and yet are more valuable for meat
than the smaller Leghorns.

The opi>ortunities for improvement in this way

Fig. 45. — "Big Jim," the product uf a pure hvcd Percheron stallion mated witli a
grade mare of the same breed, showing the value of coneentrating desirable (lualities liy
close breeding in pure bred livestock. (From Sanders and Dinsmore in "A History of
the Percheron Horse.")

PLANT AND ANIMAL IMPROVEMENT 221

through the utilization of hybrid vigor are no less great
in plants. The increased cost of seed is an item and the
practice can only be followed with those plants which are
easily crossed and which produce a large amount of seed.
Many plants in which production might be increased in
this way have such low economic value, however, that it
would not be profitable to utilize the method. Cases in
point are squashes and pumpkins. Tomatoes and cucum-
bers in certain crosses, on the other hand, have been
found to give appreciable increases in yield and other
desirable qualities, advantages which are readily secured
every time the particular cross is made.

Maize is the plant which is most suitable for use in this
way, a notable fact since it is the most valuable farm
crop in the Western Hemisphere. The reason it merits
this statement is because it is easily crossed on a large
scale by sowing the two types to be crossed in alternate
rows in an isolated plot and detasseling all of one kind
before pollen is shed. As early as 1876 Beal ^ reported
that com could be increased in yield in this way. Since
that time numerous tests have been made and the fact is
established that crosses between varieties of com of some-
what different type may be expected to outyield either
parent in many cases, and when the parental varieties
differ in time of maturing may be expected to ripen
earlier than the later parent. Thus out of fifty first gen-
eration crosses between varieties of com grown in Con-
necticut, eighty-eight per cent, yielded more than the
average, and sixty-six per cent, yielded more than either
parent. The average increase in all the crosses above the
average of their parents was about ten per cent., includ-
ing the crosses which gave no indication of hybrid vigor.

222 INBREEDING AND OUTBEEEDING

The greatest increases occurred in the crosses between
flint and dent varieties, and often there was a really note-
worthy hastening of the time of ripening, which is of con-
siderable importance in those regions where early fall
frosts are a limiting factor.

The greatest improvement to be made in this way
comes from crossing varieties which have previously been
put through a process of self-pollination. When certain
inbred strains are crossed the increase in growth is re-
markable, as previously noted. This comes partly from
the fact that following inbreeding the maximum effect of
hybrid vigor is obtained while in ordinary varieties seg-
regation brings about partial homozygosity in many
plants. It is also due to the elimination of many unde-
sirable characters during the process of inbreeding. The
crossed plants are remarkably uniform. One plant is a
replica of another. Given proper conditions the^^ all pro-
duce good ears which form a remarkable contrast to ordi-
nary varieties in their similarity to each other. There
are no barren stalks, and the abnormalities and mon-
strosities which commonly occur in every field of corn are
almost entirely absent. In those cases in which one or
both of the parent strains is resistant to parasitic infec-
tion, such as smut, the cross is also resistant and this is a
factor for greater r)roduction.

There are, on the other hand, certain serious disad-
vantages in the practical utilization of first generation
crosses between inbred strains. In the first place the
yields of the inbred plants are low, which makes the cost
of the crossed seed high. What is more serious, the seeds
produced on inbred plants are small and less well devel-
oped than seeds of ordinary corn, and the seedlings com-

PLANT AND ANIMAL IMPROVEMENT 223

ing from these seeds are less vigorous and are thereby
greatly handicapped at the start. The plants at first are
smaller and have a less healthy color than plants of ordi-
nary varieties, and although they usually overcome this
handicap, they may not always do so if the conditions in
the earlier part of the season are particularly unfavorable.
A method which overcomes these objections is now
being tested at the Connecticut Agricultural Experiment
Station, and promises excellent results. This method is
as follows : Four inbred strains are selected which when
tested by crossing in all the six different combinations
give an increased yield. Two of these strains are crossed
to make one first generation hybrid and the other two are
crossed to give another. These two different crosses,
which are large vigorous plants, are again crossed and the
seed obtained used for general field planting. This pro-
cedure may be diagrammed as follows:

Original variety

Inbred strains

First generation crosses

(AXB)

(CXI))

Double first generation cross

{AXB)XiCXD)

In this way large yields of well-developed seed are
obtained, and the young plants are not handicapped in any
way. The beautiful uniformity of the first cross is sacri-

224 INBEEEDING AND OUTBREEDING

ficed, but the advantages gained promise to counterbal-
ance any loss in this respect. Theoretically there is little
reduction in heterozygosity and presumably little reduc-
tion in the incentive towards increased size and produc-
tiveness. A great many different possibilities are
involved in such double crossing and they have not been
sufficiently tested to warrant extravagant claims, but
judging by their appearance such doubly-crossed plants
are clearly the finest specimens of corn so far obtained
under the conditions in which they have been tested.

The first impression probably gained from the outline
of this method of crossing corn is that it is a rather com-
plex proposition. It is somewhat involved, but it is more
simple than it seems at first sight. It is not a method that
will interest most farmers, but it is something that may
easily be taken up by seedsmen; in fact, it is the first time
in agricultural history that a seedsman is enabled to gain
the full benefit from a desirable origination of his own or
something that he has purchased. The man who origi-
nates devices to open our boxes of shoe polish or to
autograph our camera negatives, is able to patent his
product and gain the full reward for his inventiveness.
The man who originates a new plant which may be of
incalculable benefit to the whole country gets nothing —
not even fame — for his pains, as the plants can be propa-
gated by anyone. There is correspondingly less incentive
for the production of improved types. The utilization of
first generation hybrids enables the originator to keep the
parental types and give out only the crossed seeds, which
are less valuable for continued propagation.

The second phase of the subject of outbreeding in its
relation to plant and animal improvement — that of wide

Fic. 4G. — First generation cross of Chester Wliite and Poland China. (From Detlefsen.1

PLANT AND ANIMAL IMPEOVEMENT 225

crossing between distinct varieties, species or even genera
— is so large a topic it cannot be more than touched upon
here. Each particular cross presents technical problems
of its own. All one can say as a generality is that the
principle in every case is the same. Crossing brings to-
gether germ plasms having various attributes. These
attributes, the hereditary factors, recombine with regu-
larity and precision. They Mendelize. Prom Mendelian
segregation and recombination come the possibilities of
new and improved races. Except in those rare instances
when new variations previously unknown to the species
occur, nothing can come out of the cross that did not go
in. But the number of combinations possible when the
two parents differ by many hereditary factors is so great
that practically speaking many character complexes may
appear which have never before had the chance of showing
their merits or defects. In them lie our hopes.

It was noted earlier that many species crosses are par-
tially sterile, that there is often a degeneration of many
of the germ cells and embryos, and that certain extreme
types are thereby produced more frequently than is usu-
ally to be expected. The extreme variability induced by
such wide crossing offers the best field in which to look for
the beginnings of new and valuable types of animals and
plants. This is not a theory; it is a general fact born of
long experience, for when we look into the origin of many
of our most valuable domesticated animals and plants we
find unmistakable evidence of their hybrid ancestry.

15

CHAPTER XII

INBEEEDING AND OUTBREEDINQ IN MAN:
THEIR EFFECT ON THE INDIVIDUAL

The world has entered an age of reason. The leaven
of education is working rapidly, and all relations of man
to his fellow-man, all connections of man with his environ-
ment, are being subjected to thorough scrutiny. To ac-
company the current changes in the arts due to new
advances in science, enlightened democracy demands
progress in religion and philosophy, in government and
social policy. It has set upon its program the task of
establishing a broad scheme of social hygiene, and more
than one might suspect has been accomplished.

Although there is still room for improvement, general
communistic sanitation has reached a degree of efficiency
which a few years ago would hardly have been deemed
possible. The civilized world has gone through a clean-
ing-up period which has provided reasonably hygienic
buildings, tidy streets and excellent waste disposal ; Avhich
has bettered the condition of the people and lowered their
death rate by quarantine regulations, public hospitals,
and free medical attention in the schools ; and has passed
on to preventive work, vaccination, pure food legislation,
and the like. There has been marked progress in
ameliorating conditions of work. Sanitation has been
made the subject of many laws ; hours of toil shortened —
particularly for women and children. Factory super-
vision and wage regulation are accepted facts ; industrial
insurance is in the air. Public education has made strides

226

MAN 227

with seven league boots. That dignilied monument, the
free school, is not the only evidence. There are normal
schools and universities, museums and research institu-
tions, public collections of books and public printing
of books in numbers sufficient to form libraries
by themselves.

Is it realized just what this means — why social policy
has developed in precisely this manner? It is because
this is the mental line of least resistance, the order of
social reform needing the least foresight. The first efforts
were to clear up obvious filth, the accumulated debris of
human activity — the record of the past ; the step f oi^^ard
was an appreciation of the efficiency in production result-
ing from comfort and satisfaction in conditions of work —
the present; and then came the spread of educational
facilities — a preparation for work, an insurance on the
immediate future.

But change, progress, reform, whatever one may call
it, ought not and will not stop here. The program of social
hygiene is not complete if there is failure to provide for a
future still more distant. And this is the real thought in
the minds of a few clear thinkers of Europe and America
whose names are connected with the spread of eugenic
policies. It was thought for the care of the coming gen-
eration that led Budin to establish the Infant Consulta-
tions and MilJc Depots in Paris, that led Miele to start his
School for Mothers in Ghent. It was thought for the
future of the race as a whole that gave the impulse to
Galton's work.

We have no eugenic system of conduct to lay down
here, for we believe the acquisition and diffusion of knowl-
edge are needed more than widespread dogma and ill-

228 INBREEDING AND OUTBREEDING

advised legislation at the present day. The recommenda-
tions of the sympathetic altruist with a little learning have
done more than anything else to hinder a healthy growth
of eugenic ideas. All we would ask is that the physician,
the clergyman, the social worker, the penologist, the
statesman, give conscientious consideration to the facts
of heredity as a guiding principle in the solution of the
problems of the family with which they have to do. No
questions are so hedged about with superstition, with
irrational tradition, with religious dogma, as those which
concern sex and reproduction; no problems are more
delicate, more difficult, than those which seek the direction
of human evolution ; yet after all man is an animal and
must be dealt with as such. Civic law he may escape, to
natural law there is no immunity.

We have seen how characters are transmitted in
sexual reproduction in the lower animals and in plants,
how hereditary differences carried as potentialities in the
germ cells ^re shuffled and divided when these are formed,
by a law as definite and precise as one of chemistry or
physics. "We have seen how the operation of this law
brings about the outstanding phenomena of inbreeding
and outbreeding. Man is just another sexually reproduc-
ing mammal and a priori his heredity is guided by this
law. Being a thoroughgoing egotist, he doesn^t like to
realize this. It takes time for the truth to filter in. Com-
parative anatomy and physiology and the doctrine of evo-
lution have been the greatest agents in this familiariza-
tion process. The veriest schoolboy now recognizes the
homologies between the bones and muscles of the lower
mammals and those of man, and sees nothing out of the
ordinary that their digestive metabolisms are the same.

MAN 229

Those with no biological training have now no difficulty
in accepting as fact the idea that man came into being by
the same process of evolution as the rest of the organic
world. But even in these cases it has been a long struggle
against prejudice, and the scientific study of heredity is
too recent to have outgrown it. We will, therefore, not
confine our argument strictly to the logic of the question.
Inheritance in man has actually been studied by the same
general methods as have brought such wonderful results
in other organisms, and corroboration of every detail has
been the outcome.

When one says the fundamentals of Mendelism have
been supported in detail by investigations on the human
race, he does not mean to unply that the critical investiga-
tions needed to establish the Mendelian hypothesis in the
beginning were supplied by such data. This is obviously
impossible because of inability to control matings. The
only records which can be analyzed are pedigrees of
families carrying some striking hereditary phenomenon.
Such a method is unsatisfactory because the data must be
gathered second-hand through several generations —
often by untrained workers. It is necessary to work
backward instead of forward, to be content with frag-
mentary information, to realize the high percentage of ex-
perimental error. What is meant by corroboration of
Mendelism in human heredity is simply that starting with
the assumption of the truth of the law, all human data
have been found to fit. But it is often very difficult to say
whether the inheritance of a particular human trait is
dominant or recessive, whether it is controlled by one or
by several factors, whether it is sex-linked or independent.

Several skeletal abnormalities unquestionably show a

230 INBEEEDING AND OUTBREEDING

high degree of dominance. Among them ma,j be men-
tioned the peculiar type of dwarfing known as achondro-
plasty, and the various digital malformations termed
medically brachydactyly, Polydactyly and syndactyly.
Evidence of complete dominance is probably better in
these than in any other cases, but in view of the many
instances where subsidiary factors enhance or diminish
the expression of a primary factor, it seems decidedly
unwise to follow Davenport and to recommend marriage
with unaffected members of such families with the assur-
ance that the latter cannot transmit the trouble which
afflicts their relatives. If this advice could be accepted in
good faith, the inheritance of dominant traits, whether
disagreeable or desirable, would have little interest. They
would stand revealed in those possessing them; they alone
could transmit them. But this is not the whole truth.
Some of the so-called dominant characters in man are ab-
normalities which no one cares to see expressed in his or
her children, and their dominance is imperfect or uncer-
tain. In many instances the records have been analyzed
hastily and carelessly; for example, hare-lip and cleft
palate, which is clearly a recessive condition in face of
the data, though passing as dominant in the various text-
books of heredity. In all cases there is no guarantee that
the unaffected member of the stricken family is germinally
a pure normal. The family is one whose alliance is not to
be sought by those who have a proper pride in a normal
healthy posterity. Let us enumerate some of the troubles
that come in this category: Hereditary cataract, ichy-
thyosis or scaly skin, defective hair and teeth, diabetes
insipidus, Huntington ^s chorea — an affection of the
nervous system, imperfectly developed sex organs.

MAN 231

Does any one desire the establishment of sub-races
thus characterized?

Other undesirable traits are more certainly recessive
and the heterozygous carriers of the factors which control
them cannot be distinguished by any differentiating char-
acters of their own. Some of these abnormalities are
extremely rare and for various reasons are not likely to
increase. Among them may be mentioned pigmentary de-
generation of the retina, Friedrich's ataxia, and xero-
derma pigTuentosum. But there are others which well may
give some cause for dismal forebodings — hereditary
feeble-mindedness and some forms of epilepsy and in-
sanity. These characters may be put do^vn as largely
hereditary, and probably transmitted as single Mendelian
units, but it must not be supposed that each manifestation
of them is of similar kind. From the graduated character
of feeble-mindedness and from the frequency with which
epilepsy and other forms of neurosis appear in feeble-
minded families, it is reasonable to suppose that minor
factors of several types play a part. Nevertheless, for the
deductions we wish to make here, they may be accepted as
true examples of Mendelian recessiveness.

Other characters are not so simple in their inheritance.
The Davenports^^'^^'^^'^^ have collected a large amount
of data on the inheritance of skin color in negro-white
crosses, the inheritance of hair color in Caucasian mix-
tures, and the inheritance of normal differences in stature.
These characters are all complex. They are transmitted
just as are the differences in height in plants — more or
less of a blend in the first hybrid generation, and the
appearance of such second generation types as would be
expected if the differences were controlled by the segre-

232 INBEEEDING AND OUTBKEEDING

gation and recombination of several factor pairs. This in
general is the interpretation given the inheritance of gen-
eral mental ability or inherent ability in music, literature,
art, or mathematics. We simply know that such abilities
are inherited in some complex way, which, it is logical to
assmne, is Mendelian. We know the fact from pedigrees
of families in which ability of a particular kind is very
marked; we make the assumption from such circumstan-
tial evidence of the generality of Mendelian phenomena
as has been presented in abstract in this volume.

Having this basis, what shall be said of the effect of
inbreeding and crossing on the individual! It would be
easy to point to the conclusions reached when discussing
domestic animals and plants, and say: **The same line of
reasoning holds for man; draw your own conclusions. ' '
But this is hardly satisfactory. It is true enough, as a
generality, to point to the desirability of some mating out-
side a particular line in order to assure physical vigor by
complementary hereditary factors meeting each other, or
to mention the possibility of undesirable characters being
brought to light in some strains and of desirable char-
acters being added in others by inbreeding. One would
hardly feel this to be an answer to the question. If
the study of heredity has resulted in an advance in knowl-
edge having some practical value, it ought to be possible
to make a more definite analysis of the facts as applied to
the human race.

Let us ask first, What is ability in the human race, and
what the evidence that it is inherited ? A fair definition of
ability may be given in the phrase, * * skill in accomplish-
ment, ' ' and this puts considerable emphasis on mentality.
We all desire a healthy mind in a healthy body, but a

MAN 233

feeble-minded Goliath is hardly of much use in the world,
while a Eobert Louis Stevenson struggling through life
with the handicap of a delicate constitution leaves an im-
perishable monument. At any rate, there are few who
deny the inheritance of physical differences. Pedigrees
showing the exact method of inheritance of physical traits
are too numerous.

The first real study of the inheritance of mental
capacity was Galton's *' Hereditary Genius,'' jDublished in
1869.'^^ By comparing the attainments of the relatives of
eminent men from the United Kingdom with the attain-
ments of its population as a whole he proved beyond a
reasonable doubt the inheritance of potential capacity,
though he had no inkling of how this capacity was trans-
mitted. His conclusions have been corroborated by the
works of Havelock Ellis ®^ on ^^British Men of Genius";
of Woods 22^ on ** Heredity in Eoyalty," where lack of
opportunity did not play such a disturbing role ; and of
Cattell,^^ Nearing, ^^^' ^^^ and Davenport**' on eminent
Americans. Perhaps the most striking feature of Gal-
ton's researches is the evidence of rarity of genius among
a people who have contributed the greatest amount of
creative work of the first magnitude in modern times (see
Merz ^'*^). Only two hundred and fifty men per million
of the British population became eminent. Though un-
questionably this proportion must be increased several
times because of the lack of opportunity of those similarly
endowed to give full rein to their capacity, and because
eminence as measured by history is fallacious in the ex-
treme, nevertheless, when translated into the terms of
modem genetics this ratio has a definite meaning. The
hereditary factors which contribute toward the possibility

234 INBREEDING AND OUTBREEDING-

of genius are numerous. Only occasionally is the proper
combination brought together. The factors exist in the
population at large, distributed in part to one individual,
in part to another, but in the main the combinations make
but for mediocrity. Only on a rare occasion is a favored
one so showered with these gifts that he stands out
supreme among his fellow-men.

No one knows what the component parts of these desir-
able qualities are, or can distinguish by external traits the
individual who carries them, but a knowledge of the opera-
tion of Mendelian heredity enables one to say in a general
way what ought to occur under given conditions with the
same confidence as when dealing with similar indefinite
qualities in the lower animals. Close selection, inbreeding,
tends toward the production of gametic purity with mathe-
matical precision. Does any one doubt but that close
breeding in families which have shown superior civic value
tends to concentrate, to purif}^, in genetic terms to render
homozygous, the particular factorial combinations which
cause this superior endowment! Will any one deny there
is a real privilege in being allowed to marry into a family
of proved worth, or a real reason for that family to scru-
tinize carefully the ancestry of one who asks to become
allied with it?

We have seen how when certain hereditary factors
have been brought together by the proper breaks in link-
age, they tend to be transmitted together in the same man-
ner as did the previous set of coupled factors. This
same idea, followed to its logical conclusion, is a great
help in visualizing the inheritance of capacity. The indi-
vidual who owes his capacity to a complex which mav be
represented gi^aphically by the linked series AhCd-aBcD

MAN 235

where the capital letters represent the desirable factors,
must be much more common than the individual who by
linkage breaks receives the inheritance ABC D -abed;
yet the latter is the one who has the greatest power of
transmitting his endowments. Of the influence of the
individual in heredity, much has been w^ritten, particu-
larly by those who, great themselves, have founded great
families in American history. Elizabeth Tuttle through
Jonathan Edwards, William Fitzhugh, the three Lees —
sons of Eiehard Lee, and the various lines established by
John Preston — Venable, Payne, Wooley and Breckin-
ridge— are' examples. It seems most reasonable to sup-
pose that such prepotency for good comes about by the
gathering together of groups of significant factors in the
manner outlined above. Can one object to the concentra-
tion of such worth by relatively strict inbreeding despite
its possibilities for ill! In fact, if we examine carefully
the geneological records of such families, marriage of
near relatives is found to be a common occurrence. AVould
it not be wise to do away with statutes against the mar-
riage of first cousins such as are laid doAvn in the laws of
nearly half our States, even though the argument on the
other side, as we shall show, is just as great ? If such laws
had been followed in every mating the world would have
lost an Abraham Lincoln and have been compelled to
punish a Charles Darwin.

The mention of the name of the great Civil War Presi-
dent doubtless brings the question : How does one account
for his capacity on this hypothesis? The question is well
placed. Such geniuses as Lincoln, Dalton, Faraday,
Franklin, Pasteur — scions of the coimiion people, simple
examples of greatness standing among the commonplace —

236 INBREEDING AND OUTBREEDING

have been the stock arguments of those sociologists who
beheve every man a star of the hrst magnitude darkened
by lack of opportunity. Let us consider such cases in
some detail.

The worst trouble with the euthenic idealists is that
superhcially they are not so far wrong, although funda-
mentally they miss the point entirely. Greatness in this
world is governed by many factors. Appreciating Lincoln
and Franklin for all they were, it is still allowable to
question whether there were not some thousands of others
of the same periods who would have entered the Hall of
Fame had they been given the same political environ-
ments. Many of these contemporaries doubtless were
great in the callings to which they were turned by force of
circumstances ; but the world seldom makes a very wide
path to the door of him who makes a better mouse-trap
than his neighbor. Castle ^^ has called attention to the
notable astronomers, the eminent biologists, inspired by
the teachings of Briinnow and of Agassiz. If Briinnow
had found a home at Princeton instead of Michigan, and
Agassiz a place at Yale instead of Harvard, new names
would be found among American astronomers and biolo-
gists, but their number would not be less. In short, one
may admit with the euthenist the role of chance, of oppor-
tunity, of personal influence, of political preferment, of
economic stress, in the moulding of men; one may ac-
knowledge the difficulties attending the ranking of the
great and the near-great; and yet abate not a jot or tittle
of the position that inherent capacity, inherited potenti-
ality, lies at the base of all. It is the one solid foundation
on which to build.

Could one gauge the ability of the progenitors of

MAN 237

Franklin and Pasteur in some other way than by the bio-
graphical dictionary, they would probably be found to
have a fair share of natural gifts. Their family lines are
not to be compared with those of the notorious Zeros,
Jukes and Nams, where the individuals through their bad
heredity simply lacked even moderate capacity and were
unable to rise when given normal opportunity. They be-
longed to the good solid bourgeoisie stock which fonns
the balance-wheel of modem democracy. But even grant-
ing to historical rank a justice which it does not have,
admitting no personal superiority in any of the relatives
of these two men and others like them, their very exist-
ence is a link in the proof of Mendelian com.bination in
the making of mentality. The great bulk of the population
inherits certain factors contributing toward ability. They
do not have any one of the numerous inherited complexes
which make a genius in the rough, but they have a random
sample of the constituent parts. As a mere fulfillment of
the laws of chance, matings between these individuals
must occasionally bring together the happy combination
of which we speak.

"Whether these talents lie wrapped in a napkin or not,
depends, of course, on a variety of circumstances. One
can keep a good man down, though with some difficulty.
It was probably the general attitude of society in those
particular epochs that gave a Golden Age to Greece, a
Eenaissance to Italy, an Elizabethan period to England,
al Napoleonic era to France, rather than a concentrated
production of high mentality. Galton concluded the ablest
race in history was that built up in Attica between 530 and
430 B.C., when from 45,000 free-bom males surviving the
age of fifty there came fourteen of the most illustrious

238 INBREEDING AND OUTBREEDINa

men of all time. He was hardly justified in this. Men-
tality was then the order of the day in Attica. Accom-
plishment was appreciated. Minds of high order were
drawn from surrounding countries. The great poet,
strange as it may seem, was valued more highly than
the wealthy merchant. Such conditions must account for
much of this apparent racial superiority. Further, there
can be little question but that in this little settlement there
was much selective breeding. Had we the data, would we
not find the Athenians all more or less related to one
another? Had they not built up somewhat of a super-
stock by inbreeding? Endogamy was their custom (West-
ermarck, 1901). Marriage with half-sisters was allowable,
and if an Athenian lived as husband or wife with an
alien, he or she was liable to be sold as a slave and have all
property confiscated. Such inbreeding, given the posses-
sion of desirable characteristics on which to base selection,
could hardly fail to bring results.

In a sense this is the obverse of the picture ; the reverse
is not as pleasing. Dreary histories have been written of
consistently degenerate families, families with such a
monotonously infamous record they are known through-
out the world. There are the Jukes,^^ an inbred family
whose record of pauperism, prostitution and crime has
been traced for six generations. There is the *' Tribe of
Ishmael/' a race of indigent vagrants since 1790, con-
sistent in their wavs of life no matter what their sur-
roundings.^^2 There is the Nam family, descendants of a
racial mixture, Indian-white, less uniform than the first
two in their anti-social traits, but characterized on the
whole by vagabondage, stupidity and lack of ambition.^^
There is the family of which Poellman recorded 709 life-

MAN 239

histories, a distressing chronicle of illegitimacy and
pauperism. There is the Zero clan, a name fixing well
their value to the world, ne'er-do-w^ells since the seven-
teenth century.^ ^^

We have no intention of going into further detailed
descriptions of these people. They are but examples of an
heredity which the world does not desire. He who does
not believe their characteristics to be due to their ger-
minal constitution allows his sympathies to run riot with
his reason. Let anyone examine the published pedigrees
of these tribes of unregenerates, in the light of the facts
discussed in these pages. He will find degenerate mating
with degenerate generation after generation, incest of the
highest degree, inbreeding of the most intense kind. He
will see segregation in different strains characterized by
distinct kinds of degeneracy, as in the Jukes, where the
descendants of Ada are prevailingly criminal, the off-
spring of Bell without sexual inhibitions, the progeny of
Effie paupers. He will find that outbred lines have some-
times separated from the main stock, have had ambition,
and have gone out and made respectable names for them-
selves. He will find that these people have been given
chances, have been removed from old associations, taken
to reputable homes, clothed, fed, educated according to
their capabilities, and have remained — degenerate.

Social workers have been troubled because more repre-
sentatives of these blood lines have not been removed from
their isolation and the evil example it entails, and been
given the stimulation of association mth people of more
stamina. Let us be glad that these natural experiments
were carried on as they were. The disadvantage of
neglect, isolation and bad example to the individual must

240 INBREEDING AND OUTBREEDING

be admitted, but does anyone believe that these families
would have been a credit to the communities harboring
them if the environment were changed. It was tried many
times and failed. No ! What happened in these cases was
the establishment of near-homozygous races having a bad
heredity. The result of inbreeding where the germ plasm
is bad stands forth as a terrible example. What would
have happened had there been no isolation would have
been the contamination of good blood lines. In fact, cer-
tain illegitimate sub-strains in these clans did stand out
above their relatives. Was this not due to a better endow-
ment being brought in from the alien male?

The traits just discussed, at best mere uselessness or
lack of capacity, seem to be somewhat less complex in
their heredity than those leading to marked superiority,
if one may judge by the seeming ease with which they are
concentrated. Other undesirable mental characteristics
appear to be still less complex. These are f eeble-minded-
ness and the conditions related to it. Goddard®^ has
studied 327 family histories in which feeble-mindedness
entered. Somewhere between 50 per cent, and 75 per cent,
of these family trees show distinct evidence of the heredi-
tary nature of the defect. There were 144 matings of
feeble-minded with feeble-minded producing 482 children
of which all but 6 were feeble-minded. These few excep-
tions to the expectation for the union of two Mendelian
recessives may reasonably be explained by assuming there
is a paternity other than that assigned. Other types of
mating come just as close to Mendelian expectancy, al-
though Goddard himself has failed to analyze them prop-
erly by not correcting for the necessary omission of
heterozygous matings having no feeble-minded children.
We may, therefore, conclude that feeble-mindedness is

MAN 241

due to a single principal unit factor, recessive to what we
may call normal mentality.

There is evidence, however, of other minor factors
which modify the grade of feeble-mindedness, and con-
siderable reason for feeling that in some similar way cer-
tain forms of insanity, epilepsy and other neuroses are in
some way related. At any rate, all of these abnormalities
are in many cases inherited as recessive traits.

Here, then, is something wholly undesirable which
may be the result whenever the proper unions occur ; and
as we have seen how inbreeding tends to bring out reces-
sive characters, in feeeble-mindedness lies a potential
danger. Let us see what this danger is as regards the
United States.

It appears that in our present population of over 100,-
000,000 there are something like 300,000 persons who are
feeble-minded, epileptic or insane through an hereditary
defect, a ratio of 3 per 1000. How many of these detec-
tives resulted from a mating wherein at least one of the
parents was of the same type is a difficult question and can
only be answered with a rough approximation. The sta-
tistics at present available are meagre, but from their
examination 200,000 may be considered to be above the
mark. This leaves 100,000 defectives, then, which have
been produced in a single generation by the mating of two
transmitters of defective mentality who did not exhibit
such defects in themselves. ^^

These 100,000 defectives were produced during a
period when there were rather less than 20,000,000 mar-
ried couples of reproductive age, by parents who were
both heterozygous. But since only one-fourth of the
progeny of such matings will be defective, at least 100,000
couples of this type were reproducing throughout this

16

242 INBREEDING AND OUTBREEDING

time. This low estimate would presuppose the survival
of four children per couple long enough to have their
mental status determined, an assumption which would
require a total reproductivity of six or seven children per
married pair.

If, then, out of 20,000,000 pairs of married persons,
100,000 were heterozygous for feeble-mindedness or
attendant ills on both sides of the house, what would be
the number of such persons in the general population!
The problem may be stated a little more clearly : A cer-
tain number of persons out of a marriageable population
of 40,000,000 carry defective germ cells. If two of them
marry, one-quarter of their children will be feeble-minded.
If 100,000 such marriages did occur, what is the ratio of
these defect carriers to normals in the general population?

Pairing among defect carriers has occurred in the
ratio of 1 to 200 marriages; then these individuals must
be present in the general population in the ratio of 1 to 14,''
if no disturbing factors exist.

The thought that one person out of every fourteen
carries the basis of serious mental defectiveness in over
half of his or her reproductive cells is enough to make the
stoutest heart quake. The problem of cutting off defec-
tive germ plasm is not the theoretically simple one of
preventing the multiplication of the afflicted; it is the
almost hopeless task of reducing the birth rate among
the personally unaffected transmitters where there is

\ 200 =

approximately ^S. The probability of normal mating normal =

14

(13y =tl69, the probability of normal mating carrier is 2 / 13 J_\
14/ 196 \i4^14/

26 the probability of two carriers mating is/_l_\'== 1 .

196, \14/ 196

MAN 243

little prudential restraint and consequently a high repro-
ductive rate.

The problem exists in just the form we have stated it,
but perhaps the picture has been overdrawn. Although
the ratio is extremely conservative from one point of view
because of the low estimate of defectives and the tre-
mendous birth rate used, it may be considerably too high
by reason of our inability to allow for a proper selective
marriage rate between the carriers. Goddard, who has
made a more intensive study of these persons than any-
one else, is of the opinion that the heterozygotes are not
in the same class with pure normals. They are more or
less dull, stupid, lacking in real ability. For this reason
they are unquestionably thrown together more than would
otherwise be the case. They tend to form a class of the
population which weds within itself. This is not a whole-
some thing, but it is much better than having it corrupt
the good germ plasm of the country. Although one can
make no true estimate, such a selective marriage rate may
be high enough to revise our ratio two or three hundred
per cent. Instead of 1 out of 14 being of this type, it may
be only 1 out of 28 or 42. At best it is food for thought.

Enough has been said about the effect of inbreeding in
man to show why the numerous statistical investigations
on marriages of near kin have reached no concordant re-
sults. Hundreds of such investigations have been made.
The earlier ones were compiled by Huth,^"^ who came very
near the truth considering the state of knowledge of his
time. The later data have been brought together by
Westermarck,2i* but Westermarck was so imbued with
preconceived ideas of what ought to be true that he made
matters more chaotic than ever. The impossibility of a

244 INBEEEBING AND OUTBREEDING

correct statistical answer to tlie problem is clear if one
works back from the answer given by the research on
heredity: '* Inbreeding is not in itself harmful; whatever
effect it may have is due wholly to the inheritance re-
ceived.'^ It is not to be wondered, therefore, that exami-
nation of the pedigree record of one family led to one
conclusion, and of another family to exactly the opposite.

Nothing has been mentioned about the effect of cross-
ing in the human race, whether such crosses be narrow or
wide. Such a discussion belongs more properly in the
concluding chapter as it concerns the race more than the
individual. Physically outcrossing may often be of value
to the individual, but there are reasons to be discussed
later for not generalizing hastily in the matter. What we
wish to say in conclusion here refers still to inbreeding.
It is this :

Owing to the existence of serious recessive traits there
is objection to indiscriminate, irrational, intensive inbreed-
ing in man ; yet inbreeding is the surest means of estab-
lishing families which as a whole are of high value to the
community. On the other hand, owing to the complex
nature of the mental traits of the highest type, the bright-
est examples of inherent ability have come and will come
from chance mating in the general population, the com-
mon people so-called, because of the variability there
existent. There can be no permanent aristocracy of
brains, because families, no matter how inbred, will re-
main variable while in existence and will persist but a
comparatively short time as close-bred strains. But he is
a trifler with little thought of his duty to the state or to
himself, who, having ability as a personal endowment,
does not scan with care the genealogical record of the
family into which he enters.

CHAPTER XIII

THE INTEBMINGLING OF RACES AND NATIONAL

STAMINA

A PROPOSAL to discuss racial mixtures as a final topic
in such a condensed treatment of inbreeding and outbreed-
ing as is presented here, may be deemed somewhat pre-
sumptuous, both because of the intricacy and difficulty of
the subject itself and because of the immense amount of
partially codified knowledge relating to such matters
which has been gathered by anthropologists. On the con-
trary, these very facts are the logical reasons for ventur-
ing to indicate how and where the conclusions of
experimental genetics can be applied to the problems
knoA\Ti collectively as race problems.

The data of anthropology are largely those of the his-
torical type in which the control of variables is always
uncertain and often impossible. It is obvious that the
nature of the material to a certain extent limits direct
investigation in this field to the historical and the statisti-
cal methods of research. Generally speaking, one cannot
use man as the subject of quantitative laboratory experi-
ments. Yet the difficulties involved demand varied
methods of attack; and since genetics has furnished a
satisfactory interpretation of heredity by dealing with the
lower organisms and has proved that the same mode of
inheritance prevails in man, it is inexcusable if the broad
ethnological application of the results is neglected. Of
course, one must not expect the impossible. Problems so
complex can have no genetical solutions permitting
predictions in individual cases, but the principles do

245

246 INBREEDING AND OUTBREEDINO

give information based upon the law of averages which
has some importance.

Man is a single species if one may judge from the
interf ertility and the blood chemistry of existing peoples,
but mankind are not all brothers in spite of this oft-
repeated laoonicism of idealists and radicals; through
some 300,000 years of evolution the relationship between
the extremes is rather vague. During this period the
black race, the yellow race, the white race — three well-
marked varieties of the species — have come into existence ;
and the total number of heritable variations differentiat-
ing sub-races and individuals is almost incalculable.
Naturally, the selective agents concerned in this process
of segregation were numerous ; but isolation in a broad
sense, necessitating as it did a variety of group struggles
for existence amid different enviroimients, probably may
be regarded as the factor chiefly responsible. In the im-
mediate past, however, a short period as evolutionary
time is marked, there has been an increasingly swift
reversal of this process of racial separation. History, in
fact, has been hardly more than a record of successive
race migrations with the inevitable mingling of the con-
queror vnth the conquered.

With the twentieth century the world enters a new
phase of development. Witliin a single generation man
has reached out and grasped the mastery of his environ-
ment. Space has been annihilated with the telegraph and
telephone, the railway, the steamboat, the submarine, the
aeroplane. As a result of this freedom of communication
there will be even more colonization until the limit of the
food supply is reached ; and then, a stationary population,
through an increased death rate or a decreased birth rate.

INTERMINGLING OF RACES 247

It is unfortunate, in view of the facts in the case, that
many should still scoff at the conclusions of Malthus on
the subject of population, reached a century ago. The
impossibility of the food supply keeping pace with an un-
checked natural increase of population is a truism which
cannot be glossed over by pointing to the ingenuity of
man in applying mechanics to agriculture. The truth is
that the world is approaching a population limit faster
even than Malthus supposed, and the result of applying
new methods to field culture is merely to exploit the
natural fertility of the soil at a higher rate. The sup-
posed increase in the amount of food is illusorj^ In the
United States, naturally the richest country on the globe,
the per capita production of all the important meat
animals and some of the great agricultural crops
is decreasing.

At present the situation is this : China, having reached
the limit of her food supply, and having little or no
foreign trade, has become stationary in population. Large
portions of Europe and the country of Japan have reached
the limit of sustenance within themselves, but are increas-
ing at a rate of from ten to fifteen per thousand annually
because their commerce is such as to permit importation
to supply the deficit. Australia and New Zealand and
other parts of Asia and Europe are increasing at a rate
which neither their agriculture nor their commerce can
long sustain. The Americas and Africa are left as the
great centres of colonization. Each will support a large
additional number of people ; but when they have reached
their limit, and that limit will come within a very few
centuries — three at most — each country, or at least each
continent, must support its own population.

248 INBREEDING AND OUTBEEEDING

As an outcome of these conditions, the world faces in-
creasing amounts of race amalgamation, and there is
naturally an acute interest in race problems. The greater
part of this interest is due to prejudice arising from
racial and national arrogance. Normally each sub-race
believes implicitly in its own superiority and hopes for
continued increase and ultimate survival. Perhaps such
prejudice prevents any wholly objective discussion of the
matter. But apart from desires and hopes concerning
racial domination, it ought to be possible to set forth the
facts as they are and to determine roughly what ought to
occur under given conditions.

In order that there shall be no misunderstanding in
regard to the premises taken, let us first consider the
classification of man from the anthropological and from
the genetic viewpoints.

Anthropologists have been confronted with the very
difficult task of classifying existent peoples both with the
view of furnishing a useful nomenclature and for the pur-
pose of solving problems of descent. They have recog-
nized the insubstantial character of a language or a
nationality basis and have founded their systems on
physical traits. Even these, head form, hair shape, skin
color, stature, and so on, have been freely acknowledged
to be less satisfactory than might be desired. Neverthe-
less the systems in vogue have been serviceable in many
ways, and it is only when they are used as quantitative
measures of ancestry that the geneticist is inclined to raise
certain objections.

The difficulties of the anthropologist are relatively
much greater than those of other systematists. Inter-
group sterility is a great aid to botanists and zoologists.

INTERMINGLING OF RACES 249

In general their taxonomists have had only to differen-
tiate strains which do not interbreed. The mission of the
ethnologist may be compared rather to that of the agri-
culturist who is called upon to produce a usable classifica-
tion of the numerous strains of a variable domesticated
species, such as cattle and swine, or even wheat and
maize. He must for the sake of convenience make a mor-
phological grouping that is non-existent in physical fact.
He does this by taking advantage of isolation; without
isolation it is impossible.

An, appreciation of Mendelian inheritance shows the
fallacy involved in making such a system a basis for trac-
ing ethnic relationships. An immense number of heredi-
tary variations have occurred in man. Some can be
described by the main structural changes they effect, some
cannot. Some distinct changes, such as eye color, have a
very simple method of inheritance. They are the mark
of single-factor differences in the germ plasm. Other
ichanges, such as those expressed in stature and skull
form, appear to be controlled by numerous factors. There
are even numerous factor changes which seem to produce
no visible effect on the individual and whose existence can
be shown only by crossing. For example, it may be
assumed with considerable confidence that individuals can
have the same cephalic index and yet differ by several
hereditary factors whose chief functions are the control
of this character. At least such cases have been found in
other species and there is no reason for supposing they do
not occur in man.

Now these various physical, physiological and psychic
characters are controlled by factors transmitted alter-
natively. They may be linked in various manners, it is

250 INBEEEDING AND OUTBEEEDING

true, but they are presumably Mendelian. Consequently
one must be very cautious about drawing genetic conclu-
sions in the human race based upon the possession of
particular traits, in the absence of proof of a long-con-
tinued isolation. Long isolation, it must be assumed,
aided in segregating some well-marked human sub-
species. It may serv^e a purpose to continue to accept cer-
tain of these types as implied in the terms, white, yellow
and black races. Yet one must not forget that real isola-
tion belongs to past epochs. There has been no small
amount of interbreeding between even these main types,
and the magnitude of the interbreeding between sub-races
is largely a matter of historical record. Traits originally
characteristic of certain peoples because of isolation and
the consequent inbreeding have been shifted back and
forth, combined and recombined. It is positively mis-
leading, therefore, to classify Englishmen as resembliiig
Danish, Norman^ Pictic, Celtic or Bronze Age types, as is
done in more than one work of authority. Even if it were
known what the average values of the various characters
of these early strains were, there is little reason for be^
lieving that a present-day individual bearing one or two
particularly striking traits should be felt to hold any
closer relationship to the strain in which these traits are
supposed to have arisen than his neighbors who are with-
out them. He may have outstanding characters which
were once peculiar to a comparatively pure race ; but he
probably carries these characters as a mere matter of
Mendelian recombination. It is wholly possible, for ex-
ample, that a tall, blue-eyed, dolichocephalic Frenchman
really possesses less of the so-called Nordic factors than a
short, dark-eyed round-head.

INTERMINGLING OF RACES 251

One other matter to be kept in mind in this connection
is the impossibility of knowing what factors have survived
and what have perished. The great differences between
individuals in inherent traits both physical and mental,
make it probable that even within a race the average
capacity of some strains is greater than others. This
seems a fair deduction after making all due allowances
for changes in the spirit of the times which accelerate or
retard the development of natural ability. Now we do not
know and cannot know how the hereditary factors existent
to-day compare with those existent 2000 years ago. Selec-
tion within the population is an invariable concomitant of
human existence. There is a selective death rate, selec-
tive mating, selective fertility, each influenced by many
conditions. These selective agencies do not remain the
same, nor does the material upon which they work. We
do not know, for example, whether the most desirable
germ plasm of Greece, of Rome, of Mediasval Europe, has
been passed on or has ceased to exist. The world has
received a great legacy in the creative production of the
master men of the past ; that it is their heir in physical
fact is not so certain. In the strictly biological sense, i.e.,
in the material basis of heredity, the world may be better
or it may be worse than it was in the time of Pericles.
This phase of the subject is mentioned because one often
hears comments on the degeneracy of certain nations who
have had periods of enviable greatness. Social condi-
tions may be the cause, but it should not be forgotten
that they are not the sole possible cause. Essential
hereditary factors may have been cut off, may have been
wholly eliminated.

These genetic ideas of race heredity among mankind

252 INBREEDING AND OUTBREEDING

are, we believe, fundamental. They give a clue as to what
has happened in the racial mixtures of the past, and
enable one to visualize more clearly the probable result of
the intense race amalgamation to be expected in the
twentieth century.

The world faces two types of racial combination : one
in which the races are so far apart as to make hybridiza-
tion a real breaking-dowTi of the inherent characteristics
of each ; the other, where fewer differences present only
the possibility of a somewhat greater variability as a
desirable basis for selection. Roughly, the former is the
color-line problem; the latter is that of the White Melt-
ing Pot, faced particularly by Europe, North America
and Australia.

The genetics of these two kinds of racial intermixture
is as follows: Consider first a cross between two ex-
tremes, typical members of the white and of the black race.
In the first generation the individuals show a notable
amount of heterosis, indicating differences in a large
number of hereditary factors. They are intermediate in
hair form, skin color, head shape, and various other
physical attributes, in mental capacity, and in psychical
characters in general ; although they show extraordinary
physical vigor. In later generations segregation and re-
combination in many of these characters can be traced
with little difficulty ; but if one describes the descendants
of the cross as a population, or even the total character-
istics of a single individual, fluctuation around the aver-
age of the two original races is still the rule. There may
be an approach to the head form of one race combined
with the skin color of the other, an approximation of the
hair of the one coupled with the other's stature; never-

INTERMINGLING OF RACES 253

theless, there is little likelihood of an individual return to
the pure type of either race. The difficulties involved are
those described in Chapter VII. The races differ by so
many transmissible factors, factors which are probably
linked in varied ways, that there is, practically speaking,
no reasonable chance of such breaks in linkage occurring
as would bring together only the most desirable features,
even supposing conscious selection could be made. And
selection is not conscious. Breeding for the most part is
at random. The real result of such a wide racial cross,
therefore, is to break apart those compatible physical and
mental qualities which have established a smoothly oper-
ating whole in each race by hundreds of generations of
natural selection.

If the two races possessed equivalent physical char-
acteristics and mental capacities, there would still be
this valid genetical objection to crossing, as one may
readily see. Butin reality the_negro is inferior toJ;he
white.^®^ This is not hypothesis or supposition; it is a
crude statement of actual fact. The negro has given the
world no original contribution of high merit. By his own
initiative in his original habitat, he has never risen.
Transplanted to a new environment, as in the case of
Haiti, he has done no better. In competition with the
white race, he has failed to approach its standard. But
because he has failed to equal the white man's ability, his
natural increase is low in comparison. The native popu-
lation of Africa is increasing very slowly, if at all. In
the best environment to which he has been subjected, the
United States, his ratio in the general population is
decreasing. His only chance for an extended survival
is amalgamation.

254 INBREEDING AND OUTBREEDING

The United States has been confronted by this grave
question for some time. In Africa it has hardly yet come
to the fore, but within three generations it will be recog-
nized as the political and economic problem. What the
solution will be, no one knows. It seems an unnecessary
accompaniment to humane treatment, an illogical exten-
sion of altruism, however, to seek to elevate the black
race at the cost of lowering the white. And the state-
ment is made with all due regard to the fact that there
are certain desirable characteristics existent in the black
race, and that unquestionably the two races overlap in
general inlierent capacity. The white race as a whole is
not equal to the black race in resistance to several serious
diseases, as the medical records of the United States
army show. The two strains have built up disease resist-
ance along different lines, and the addition of both
sets of immunity factors would be desirable. But the
practical attainment of such a benefit, given the genetic
premises, is so improbable as to be negligible, apart from
other considerations.

What would be the result of racial intermixture be-
tween the yellow and the white is not so certain. Both
races have produced high types. Can one say that either
is on the whole the better ? The Chinese, the development
of very early tribal mixtures, have had a great productive
period. In a sense their productivity has decreased, yet
their germ plasm is unquestionably good. The Japanese,
the result of a much later racial amalgamation, have de-
veloped into a wonderful people. Whether it is fair to
say the white race is the greater because in the past two
centuries they have made such wonderful contributions
to civilization is a question. The contributions of the

INTERMINGLING OF RACES 255

yellow race 4000 years ago were as marvellous in their
time. Yellow-white amalgamation may not be fraught
with the evil consequences in the wake of the yeUow-
blaek and the white-black crosses. At the same time it
should be pointed out that the Caucasian and the Mon-
golian are far apart in descent, and the advantages to be
gained by either in thus breaking up superior hereditary
complexes developed during an extended past are not
clear. At any rate, there seems to be no prospective bene-
fit to the superior yellow peoples in mating with some
of the inferior existent whites, and no presumable good
to the superior white in intermingling with the poorer
yellow offshoots, as has been done to the south of the
United States.

Our first conclusion may be said to be a decision
against the union of races having markedly different
characteristics — particularly when one is decidedly the
inferior. Through the operation of the laws of heredity
such unions tend to break apart series of character com-
plexes which through years of selection have proved to
be compatible with each other and with the persistence
of the race under the environment to which it has been
subjected. Because of the transmission of factors in
linked groups, the low probability of obtaining a single
recombination equal or superior to the average of the
better race does not warrant the production of multitudes
of racial mediocrities which such a mixture entails.

Our second thesis is seemingly paradoxical. It asserts
that the foundation stocks of races which have impressed
civilization most deeply have been produced by inter-
mingling peoples who through one cause or other became
genetically somewhat unlike. Theoretically, this theorem

256 INBKEEDING AND OUTBREEDING

is not difficult to develop. Whatever the causes of racial
separation under the isolation characteristic of former
times, peoples did come to have a rather narrow vari-
ability. They were, one might say, homozygous for cer-
tain traits. These traits naturally differed in their value.
There were great peoples, mediocre peoples, and wretched
peoples. But each was more or less standardized. "When
there came occasion for these standardized peoples dii-
fering in their transmissible characters to intermingle,
great variability was produced; and if the differences
were not too great, the chances were high that valuable
character combinations would come to light. Later
through the close breeding due to the marriage selection
which always develops within human society, the ten-
dency was again to produce purer strains characterized
differently ; but without the chance of repeated Mendelian
recombinations the probability of establishing superior
strains was small.

*• This hypothesis, developed wholly from a considera-
tion of the genetic facts, is not refuted by ethnological
data. Thus, if one considers the peoples of Europe, he
finds high civilizations, invariably following the migra-
tion of that ancient race or mixture of races termed
Aryan, a people of whom there is now only circumstantial
evidence. It was manifestly not mere hybridization
which brought results of outstanding value, however, but
hybridization of good strains not too widely differen-
tiated, followed by periods of more or less intensive in-
breeding. This is a reasonable deduction from the
rapidity with which European and particularly North
European culture has outstripped that of Central and
Southern Asia.

INTERMINGLING OF RACES 257

The difficulty with using these data as actual support
of the hypothesis under consideration comes from the
fact that the amount of hybridization appears to be about
the same in various peoples who differ greatly in their
contributions to civilization. This may mean that the
strains supposedly lower in ability have potentialities not
yet realized, or it may mean inherent differences in the
original constituent parts. This much seems to be true,
however. The great individuals of Europe, the leaders
in thought, have come in greater numbers from peoples
having very large amounts of ethnic mixture. Even the
Scandinavians, a relatively pure strain of the stock to
which much of the greatness of Germany, England and
even France is supposed to be due, have been somewhat
behind these peoples in the production of constructive
leaders. Is it not a fair assumption that the backward-
ness of Spain and Ireland is due to their relative isola-
tion! Is it not because the waves of migration were
nearly spent before they reached these lands' ends?

Contrast the people in the United Kingdom, more
particularly the natives of the south and west of Ireland,
with those of Scotland and England. In proportion to
their numbers no modern people has approached the Eng-
lish and Scotch in number of illustrious men or in height
of creative ability except the French ; the true Irish have
hardly a single individual meriting a rank among the
great names of history, or a contribution to literature,
art, or science of first magnitude.

The Irish are supposed to have arisen somewhat as
follows (Ripley,^^^ MacNamara ^^^). In the early quater-
nary period, western Europe and northern Africa were
occupied by an extremely low type of being of Mongolian
17

258 INBREEDING AND OUTBREEDING

antecedents, the Iberian race. Until the neolithic period
these tribes were the only inhabitants of the British Isles.
During the Stone Age, however, there is evidence of the
presence of a second Mongoloid race, the Turanian. Just
before the Bronze Age an Aryan stock, the Celts, invaded
Britain and Ireland. These people came from the south
— France or Spain. Probably they were originally close
relatives of the Aryans who migrated from, Asia to the
northwest and by intermingling with the natives and de-
veloping as they went, formed the vigorous Teutonic
Aryan or Nordic. But the southern migration of Aryans
met very different tribes on their journey, producing in
the Celts a somewhat inferior stock. However this may
be, the original Celtic horde probably did not make a great
impression on the racial character of the Irish ; something
which also may be said of the second Celtic strain, more
highly civUized and warlike than the original visitors,
which entered Ireland during the Bronze Age. This later
stream of invasion continued over a long period for the
island was not completely subjugated until well into the
fifth century; but the intruders came as conquerors of a
higher social order whose social ideal was to keep their
stock uncontaminated with the blood of the native race.

The Norsemen, Nordic Aryans, attempted many times
to gain possession of Ireland between the ninth and the
fourteenth centuries, but were unsuccessful, and as the
Romans and the Saxons never attempted to invade Ire-
land, the land won by the Celtic chiefs remained in the
hands of their direct descendants until 1654, when Crom-
well confiscated it, and either killed or reduced them to
the condition of laborers.

The present inhabitants of Ireland, then, with the ex-

INTERMmGLINa OF RACES 259

ception of the northern counties, where there is a consid-
erable proportion of English and Scotch, are in the main
descended from two savage tribes, the Iberian and Turan-
ian, both probably Mongolian admixtures, with the addi-
tion of some blood of the conquering and ruling Celtic
Aryans, who genetically must have been more or less in-
tercrossed with Iberian and Turanian tribes by the time
they reached the island. Comparatively close inter-
breeding for at least ten centuries has produced the
Irish of to-day.

The original population of Britain, as of Ireland, was
Iberian overlaid with Turanian in the north and some
other Mongoloid tribes in the south. These races and the
types they produced by intermarriage formed the bulk of
the population even up to the time of Caesar *s invasion,
though the ruling classes were probably Celtic Aryans.
The Romans, anthropologists tell us, made little change
in the racial character of the inhabitants ; but this state-
ment must be taken with some reservation. It is hardly
likely that the large garrisons kept by the Roman Empire
for 500 years left no descendants. As a population, per-
haps, the racial characteristics were not changed to a
noticeable degree; nevertheless, a comparatively few
thousand persons with Roman blood may have had some
considerable effect on the nation as individuals, and the
probable presence of this germ plasm must not be counted
as negligible. However this may be, the matter is per-
haps of little importance as far as the Scotch and Eng-
lish of to-day are concerned, for the greater part of these
early peoples, as well as the descendants of the Jutes who
entered the country in the fifth century, were extermi-
nated by the Nordic Aryans that invaded the country

260 INBREEDING AND OUTBREEDING

under the various names of Saxons, Angles and Franks
between the sixth and tenth centuries. There was no
great racial change made, then, when the Danes con-
quered the country in the early part of the eleventh
century, or when William the Conqueror brought over
his Normans of the same stock in the latter part.

The main point we wish to bring out is that England
and Scotland are to-day inhabited by an extremely vari-
able people, made so by innumerable crosses into which
entered the blood of many Nordic Aryans who differed
from each other in some degree. It makes no difference
whether there is some variance among ethnologists as to
the exactitude of the racial history. That is not essential
and one need not quibble about it. The fact remains that
the English and Scotch have a generally high civic value
and are extremely variable. They produce genius and
they produce wretchedness as the natural result of the
recombination of these variations. Selections made from
the best of these segregates have given the United States
names of which one may well be proud ; selections made
from the other extreme have furnished several of the
undesirable strains described previously under the
pseudonyms Nam, Juke, etc.

The Irish, on the other hand, and the same might be
said of some other isolated types, are, much purer
from the genetic standpoint. Is there not some reason
for attributing to this comparative purity, to this lack
of flexibility, their present position as a race and
as individuals?

A case similar to that of England and Scotland might
be made out of France and for Germany, though France
has perhaps a greater proportion of the blood of the

INTERMINGLING OF RACES 261

Alpine and Mediterranean peoples than even the south of
England and Wales. But even so, the racial differences
have not been so great but that France has become one
people, with all the chances for good held by a compara-
tively small united nation, when an amount of close
breeding has taken place sufficient to bring out the in-
herent possibilities.

Another people, great in their influence on the civil-
ization of western Europe, are the Jews. They should
not be overlooked in this connection, because of the mis-
taken idea that they form a pure race of narrow vari-
ability characterized by fixed traits. Nothing is further
from the truth. If it were the truth it might be questioned
whether the Jew would have produced the great number
of illustrious men who must in all fairness be credited
to them.

The very term race applied to the Jew is a misnomer.
There is no more a Jewish race than there is an English
race. The fiction has been kept up because of a cult of
racial purity in their religion. As a matter of fact, the
early Jewish people arose from complex crosses in whicli
at least three different stocks entered: the Arabs, the
Assyrioides or Hittites and the Aryan Amorites. More
or less inbreeding did follow before their dispersal, but
that great racial variability must have remained at the
most nationalistic period of their history any student of
history knows. After their dispersal there was a period
of proselyting which broadened their possibilities. Later,
moving into every part of Europe, they mixed with tlie
people with whom they sojourned to a very considerable
extent, though keeping up the while the religious ideal of
racial purity. In actual fact the Spanish Jew, the German

262 INBEEEDING AND OUTBEEEDING

Jew and the English Jew are to-day scarcely more alike
than the Spaniard, the German or the Englishman ; but the
practical results of their religious beliefs, since they were
attained but partially, have been good in the genetic sense,
for a sufficient amount of inbreeding has prevailed to
bring out the possibilities inherent in the combinations
made. Civically this conventional isolation has worked to
the disadvantage both of the people themselves and of the
State in which they held citizenship, and at present it
would unquestionably be better from all points of view for
them to follow the advice of some of their broader minded
leaders in the United States and abandon it, since their
own variability has become so great as to make even a
theoretically fixed policy of intraracial marriage undesir-
able. An alliance of a Jew of high capacity and proved
worth with a ^^Nam*^ or a ''Juke'' of his own religion is
no more to be commended than a similar alliance among
those of other faiths.

These three illustrations must suffice as anthropologi-
cal support of the point we have endeavored to emphasize.
In themselves they are not particularly convincing, it
must be admitted. Such data can never be used as critical
tests of biological theory. At the same time, when con-
sidered carefully in the light of the purely genetic facts
presented, it seems to us one must assent to the general
truth of the theses laid down. Man, like other organic
species, has varied markedly in hereditary characters.
Eaces have arisen which are as distinct in mental capacity
as in physical traits. These transmissible qualities are
governed by germinal factors and these factors are
passed on to succeeding generations by the same precise
laws that have been discussed in the preceeding pages.

INTERMINGLING OF RACES 263

This being true, racial crossing may be desirable or un-
desirable, depending first on whether the stocks concerned
possess a preponderance of desirable characteristics, and
second, on whether they are extremely differentiated or
not. It may be questioned whether all existing peoples
do not possess some desirable traits and hence hold out
the possibility of the production of some superior indi-
viduals when crossed with presumably superior stock.
Nevertheless, even as in breeding for quality in domestic
animals, the frequency with which the superior individual
is obtained by such a procedure is so low that economi-
cally radical experiments are unwise. Given some pre-
sumption of equally desirable contribution in the union,
the wisdom of a particular racial cross is governed by the
number of hereditary differences brought together. The
hybridization of extremes is undesirable because of the
improbability of regaining the merits of the originals, yet
hybridization of somewhat nearly related races is almost a
prerequisite to rapid progress, for from such hybridiza-
tion comes that moderate amount of variability which pre-
sents the possibility of the super-individual, the genius.
To produce greatness a nation must have some
wretchedness, for such is the law of Mcndelian recom-
bination; but the nation that produces wretchedness
is not necessarily in the way of producing greatness.
There must be racial mixture to induce variability, but
these racial crosses must not be too wide else the chances
are too few and the time required is too great for the
proper recombinations making for inherent capacity to
occur. Further, there must be periods of more or less
inbreeding following racial mixtures, if there is to be any
high probability of isolating desirable extremes. A third

264 INBEEEDING AND OUTBREEDING

essential in the production of racial stamina is that the
ingredients in the Melting Pot be sound at the beginning,
for one does not improve the amalgam by putting in dross.

May we consider, in conclusion, the bearing of these
facts upon the problem of this particular country, the
United States of America? The United States at one
time was the Mecca of the politically oppressed. Free-
dom-loving people of good lineage and worthy attain-
ments came to its shores. Now, except for temporary
abatement of immigration due to the world war, the
stream, though swelling in volume, has changed both its
source and the impelling cause of its flow. The early
settlers came from stock which had made notable contri-
butions to civilization. They were drawn by a desire
from within to carve out great names and fortunes. And
they have not disgraced their kin across the seas.

This tide has ebbed, and has been succeeded by a
greater. Fifteen million foreign-born now live within
the boundaries of the nation, though nearly half have
never sought its citizenship. They come in increasing
numbers from peoples who have impressed modern civil-
ization but lightly. They come, not so much from inborn
ambition of their own, but because attracted by the in-
ducements of those who would exploit them for their
own convenience. "Whether any considerable part oP
these people are genetically undesirable, whether real
capacity will be discovered under the new environment,
no one can say. Time alone will tell. But there is a
thought in this connection that cannot be emphasized too
strongly or too often. To make this a united nation,
there must be an enormous amount of open racial inter-
mixture. The publicist and sociologist should realize

INTERMINGLING OF RACES 265

that if they do not give their children in marriage with
the immigrant, they must with the immigrant's children.
Invidious comparisons are, therefore, unnecessary ; ques-
tions of what this or that race has done or may do need
not be settled. It is quite within the province, it is indeed
the duty of the native citizen, to require a pause in this
mad rush for mere population, until there is a diffusion
of education and a healthy growth of a nationalistic
spirit. By the time this has been accomplished, the result
of the previous policy of the Open Door can be estimated
more justly, and any necessary adjustments made ^vith
better regard for the good of all the people.

LITEEATURE ^

1 Allen_, C. E. : a Chromosome Difference Correlated with Sex Dif-

ferences. Science, N. S., 1917, xlvi, 466, 467.

2 Arner^ G. B. L. : Consanguineous Man^iages in the American Popu-

lation. Studies in Hist., Econ. and Pub. Law, 1909, xxxi. No. 3.

3 Beal^ W. J. : Eeports, Michigan Board of AgTiculture, 1876, 1877,

1881 and 1882.

4 Bell^ a. G. : Memoir upon the Formation of a Deaf Variety of the

Human Race. Mem. Nat. Acad. Sci., 1884, pp. 86.

5 BemisS; S. M. : Report on Influence of Marriages of Consanguinity

upon Offspring. Trans. Amer. Med. Assn., 1858, xi, 321—425.

6 Berthollet^ S. : Phenomenes de I'acte mysterieux de la fecondation.

Mem. Soc. Linneenne de Paris, 1827, i, 81-83.
■^ Blyth^ E. : On the Physiological Distinctions between Man and all

other Animals. Mag. Nat. His., N. S., 1837, i, 1-9, 77-85, 131-141.
s BoNHOTE, J. L. : Vigour and Heredity. London, 1915, pp. 263.
® BouDiN^ M. : Dangers des unions consanguines et necessite des croise-

ments dans I'espece humaine et parmi les animaux. Ann. d'Hygiene

pub. et de Med. legale, 1862, xviii, 5-82.

10 Bridges, C. B. : Non-disjunction as a Proof of the Chromosome

Theory of Heredity. Genetics, 1916, i, 1-51, 107-163.

11 Bridges, C. B.: Deficiency. Genetics, 1917, ii, 445-465.
12BRITT0N, E., G.: A Hybrid Moss. Plant World, 1898, i, 138.

13 Bruce, A. B. : A Mendelian Theory of Heredity and the Augmenta-

tion of Vigor. Science, N. S., 1910, xxxii, 627, 628.

14 Bruce, A. B. : Inbreeding. Jour. Gen., 1917, vi, 195-200.

15 BuRGEOis, A. : Quelle est I'influence des mariages consangnines sur

les generations? Theses L'Ecole de Bled., 1859, ii. No. 91.

16 Carriej?., L. : The Immediate Effect of Crossing Strains of Corn.

Virginia Agr. Exp. Sta. Bull. 202, 1911, pp. 11.

a This list of literature makes no pretension of citing other than a few
of the most important books and papers on inbreeding published in pre-
Mendelian days. Those interested in the subject from the standpoint of
marriages of near kin can obtain access to the literature by following up the
citations of Huth and of Westermarck. Tlie real development of the subject
has come from the investigations on heredity completed since the year 1900.
Since it is impracticable and unnecessary to cite* all the genetic work of
this period, only those titles which are in some way directly connected with
the subjects discussed, have been given.

266

LITERATURE 267

17 Castle^ W. E.: The Early Embryology of Ciotia intestinalis Flem-

ming (L.). Mus. Com. Zool. Bull. 27, 1890, 201-280.
IS Castle^ W. E. : Genetics and Eugenics. Cambridge, 1910, pp. 353,
1^ Castle, W. E., and Little, C. C: On a Modified Mendelian Ratio
Among Yellow Mice. Science, N. S., 1910, xxxii, 808-870.

20 Castle, W. E., and Wright, S.: Studies of Inheritance in Guinea-

pigs and Rabbits. Carnegie Inst. Pub. 211, Washington, 1910,
pp. 192.

21 Castle, W. E., Carpenter, F. W. et ah: The Effects of Inbreeding,

Cross-breeding, and Selection upon the Fertility and Vaiiability
of Drosophila. Proc. Amer. Acad. Arts and Sci., 1900, xli, 731-780.

22 Cattell, J. M. : A Statistical Study of American Men of Science.

Scie7ice, N. S., 1900, xxiv, 058-005, 099-707, 732-742.

23 Caullery, M. : Les problemes de la sexualite. Paris, 1917, pp. 332.
2^ Chamberlain, H. S.: The Foundations of the Nineteenth Century.

Trans. J. Lees. 2 vol. New York, 1910, pp. 578-580.

25 Chapeaurouge^ A. de : Einiges liber Inzucht und ihre Leistung auf

verschiedenen Zuchtgebieten. Hamburg, 1909.

26 Collins, G. N. : Increased Yields of Corn from Hybrid Seed. Year-

book U. S. Dept. AgT., 1910, 319-328.

27 Collins, G. N. : The Value of First Generation Hybrids in Corn.

Bull. 191, Bur. Plant Ind., U. S. Dept. Agr., 1910, pp. 45.

28 Collins, G. N., and Kempton_, J. H. : Effects of Cross-pollination on

the Size of Seed in Maize. Cir. 124 U. S. Dept. Agr. 1913, 9-15.

29 Collins, G. N. : A More Accurate Method of Comparing First Gen-

eration Hybrids with Their Parents. Jour. Agr. Res., 1914, iii,
85-91.

30 Collins, G. N. : Maize : Its Origin and Relationships. Notes of the

123d Regular Meeting Bot. Soc. Wash., Jour. Wash. Acad. Sci.,
1918, viii, 42, 43.

31 Cook, 0. F. ; The Superiority of Line Breeding Over Narrow Breed-

ing. Bull. 140, U. S. Dept. Agr., Bur. Plant Ind., 1909, pp. 45.

32 Coulter, J. M. : The Evolution of Sex in Plants. Chicago, 1914,

pp. 140.

33 Cramer, P. J. S. : Kritische Uebei^icht der bekannten Fiille von Knos-

penvariation. Haarlem, 1907, pp. 474.

34 Crampe, H. : Zuchtversuehe niit zahmen Wanderratten. Landw.

Jahrh., 1883, xii, 389-458.

35 Cull, S. W. : Rejuvenescence as the Result of Conjugation. Jour.

Exp. Zool., 1907, iv, 85-89.

36 Daffner^ F. : Das Wachstmn des Menschen. Leipzig, 1902, pp. 475.

268 INBREEDING AND OUTBREEDING

37 Danielson, F. H., and Davenport, C. B. : The Hill Folk. Mem. 1,

Eugenics Record Offitce, Cold Spring Harbor, 1912, pp. 56.

38 Darwin, C. : The Variation of Animals and Plant* Under Domesti-

cation. 2ud Ed., London, 1875, 2 vols., pp. 461-478.

39 Darwin, C. : The Effects of Cross- and Self -Fertilization in the

Vegetable Kingdom. London, 1876, pp. 482.

40 Davenport, C. B. : Heredity in Relation to Eugenics. New York,

1911, pp. 298.

41 Da\^nport, C. B. : Inheritance of Stature. Genetics, 1917, ii,

313-389.
4 2 Davenport, C. B., and Davenport, G. : Heredity of Eye-color in

Man. Science, N. S., 1907, xxvi, 589-592.
4 3 Davenport, C. B., and Davenport, G. : Heredity of Hair-form in

Man. Amer. Nat., 1908, xlii, 341-349.

44 Davenport, C. B., and Davenport, G. : Heredity of Hair-color in

Man. Amer. Nat., 1909, xliii, 193-211.

45 Davenport, C. B., and Davenport, G. : Heredity of Skin-pigmenta-

tion in Man. Amer. Nat., 1910, xliv, 641-672; 705-731.

46 Deniker, J.: The Races of Man. New York, 1906, pp. 611.

47 Detlefsen, J. A. : Genetic Studies on a Cavy Species Cross. Car-

negie Pub. 205, Washington, 1914, pp. 134.

48 DuGDALE, R. L. : The Jukes. New York, 1877, 4th Ed., 1910, pp. 121.

49 DtJsiNG, K. : Die Factoren welehe die Sexualitat entscheiden. Jena,

1883. Inaug. Dissertation.

50 East, E. M. : Inbreeding in Com. Connecticut AgT. Exp. Sta. Rpt.

for 1907, 1908, 419-428.

51 East, E. M. : A Study of the Factors Influencing the Improvement

of the Potato. Illinois Agr. Exp. Sta. Bull. 127, 1908, 375-456.

52 East, E. M. : The Distinction Between Development and Heredity

in Inbreeding. Amer. Nat., 1909, xliii, 173-181.

53 East, E. M. ; An Interpretation of Sterility in Certain Plants.

Proc. Amer. Phil. Soc, 1915, liv, 70-72.

54 East, E. M. : Studies on Size Inheritance in Nicotiana. Genetics,

1916, i, 164-176.

55 East, E. M. : The Bearing of Some General Biological Facts on

Bud-variation. Amer. Nat., 1917, li, 129-143.

56 East, E. M. : Hidden Feeble-miiidedness. Jour. Her., 1917, viii,

215-217.

57 East, E. M. : The Role of Reproduction in Evolution. Amer. Nat.,

1918, lii, 273-289.

58 East, E. M., and Hayes, H. K. : Inheritance in Maize. Conn. Agr.

Exp. Sta. Bull. 167, 1911, pp. 141.

LITERATURE 269

59 East^ E. M., and Hayes^ H. K. : Heterozygosis in Evolution and in

Plant Breeding. Bull. 243, U. S. Dept. Agr., Bur. Plant Ind.,

1912, pp. 58.

60 Ellis, H.: A Study of British Genius. London, 1904, pp. 300.

61 Emerson, R. A. : Inheritance of Sizes and Shapes in Plants. Amer.

Nat., 1910, xliv, 739-746.

62 Emerson, R. A. : The Inheritance of Certain Abnormalities in

Maize. Amer. Breed. Assn. Rpt. 1912, viii, 385-399.

63 Emerson, R. A., and East, E. M. : The Inheritance of Quantitative

Characters in Maize. Nebraska Agr. Exp. S'ta., Research Bull. 2,

1913, pp. 120.

64 Enriques, p. : La conjugazione e il differenziamento nogli Infusoria.

Arch. f. Protistenkiinde, 1907, ix, 195-296.

65 Estabrook, a. H., and Davenport, C. B. : The Nam Family. Mem.

2, Eugenics Record Office. Cold Spring Harbor, 1912, pp. 85.

66 Fabre-Domengue, p. : Unions consanguines chez les colombins.

Ulntermediare des Biol., 1898, i, pp. 203.

67 Fay, E. a. : Marriages of the Deaf in America. Washington, 1898,

pp. 527.
6S Fischer, E. : Die Rehobother Bastards und das Bastardierungs-

problem beim Menschen. Jena. Review. Jour. Her., 1914, v,

465-468.
69 Fish, H. D. : On the Progi-essive Increase of Homozygosis in

Brother-Sist-er Matings. Amer. Nat., 1914, xlviii, 759-761.
70FOCKE, W. 0.: Die Pflanzen-Mischlinge. Berlin, 1881, pp. 569.
71 Frazer, J. S. : Totemism and Exogamy. 4 vol., London, 1910.
7 2 Freud, S. : Totem and Taboo. Trans. A. A. Brill. New York, 1918,

pp. 265.
73GALTON, F.: Hereditary Genius. 2nd Ed., London, 1892, pp. 379.
7* Gartner, C. F. : Versuche nnd Beobachtungen liber die Bastarder-

zeugung im Pflanzenreich. Stuttgart, 1849, pp. 790.

75 Gay, C. W.: The Breeds of Livestock. New York, 1916, pp. 483.

76 Gentry, N. W. : Inbreeding Berkshires. Amer. Breed. Assn. Ann.

Rpt., 1905, i, 168-171.

77 Gernert, W. B.: Aphis Immunity of Teosinte-Com Hybrids. iS'ct-

ence, N. S., 1917, xlvi, 390-392.

78 Gerschleu, M. W. : Ueber alternative Vererbiing bei Kreuzung von

Cyprinodontiden-Gattungen. Zeitschr. f. ind. Ahst. u. Vererh.,

1914, xii, 73-96.

79 GOBINEAU, Le Compte de : Essai sur I'inegalite des races humaines.

2 vol. Paris, 1884, pp. 561-566.

80 GODDARD, H. H. : The Kallikak Family. New York, 1913, pp. 121.

270 INBREEDING AND OUTBREEDING

81 GODDARD^ H. H. : Feeble-mindedness : Its Causes and Consequences.

New York, 1914, pp. 599.

82 GoLDSCHMiDT, R. I Zuclitversuche mit Enten, I. Ztschr. f. ind. Ahst.

u. Vererb., 1913, ix, 161-191.

83 GouRDON, J.: Consangiiinite chez les animaux domestiques. Ann.

d; Hygiene inib. et de Med. legale, 1862, xviii, 463, 464.

84 Grant, M, : The Passing of the Great Race. 2nd Ed. New York,

1918, pp. 295.

85 Gravatt, F. : A Radish-cabbage Hybrid. Jour. Her., 1914, v, 269-

272.

86 GuAiTA, G. von : Versuche mit Kreuzungen von verschiedenen Rassen

der Hausmaus. Ber. d. Naturforsch. Gesell. zu Freiburg, 1898,
X, 317-332.

87 GuAiTA^ G. von : Zweite Mittheilung liber Versuche mit Kreuzungen

von verschiedenen Hausmausrassen. Ber. d. Naturforsch. Gesell.
zu Freiburg, 1900, xi, 131-143.

88 Haddon^ a. C. : Races of Man. London, 1909, pp. 126.

89 Hammond^ J. : On Some Factors Controlling Fertility in Domestic

Animals. Jour. Agr. Sci., 1914, vi, 263-277.
so Hardy, G. H. : Mendelian Proportions in a Mixed Population. Sci-
ence, N. S., 1908, xxviii, 49, 50.

91 Hartley, C. P., et al.: Cross-breeding Corn. Bull. 218, U. S. Dept.

Agr., Bur. Plant Ind., 1912, pp. 72.

92 Hayes, H. K. : Com Improvement in Connecticut. Connecticut

Agr. Exp. Sta., Rpt. for 1913, 1914, 353-384.

93 Hayes, H. K., and East, E. M. : Improve-ment in Corn. Connecticut

Agr. Exp. Sta. Bull. 168, 1911, pp. 21.

94 Hayes, H. K., and East, E. M. : Further Experiments on Inheri-

tance in Maize. Connecticut Agr. Exp. Sta. Bull. 188, 1915, pp. 31.

95 Hayes, H. K., and Jones, D. F. : The Effects of Cross- and Self-

fertilization on Tomatoes. Connecticut Agr. Exp. Sta. Rpt. for
1916, 1917, 305-318.

96 Herbert, W. : Amaryllidaceas. London, 1837, pp. 428.

97 HuTH, A. H. : The Marriage of Near Kin. London, 1875, pp. 359.

98 Hyde, R. H. : Fertility and Sterility in Drosophila ampelophila.

Jour. Exp. Zool., 1914, xvii, 141-171, 173-212.
09 Jacoby, p. : Etudes sur la selection chez rhomme. 2nd Ed., Paris,

1904.
i<^o Janssens, F. a.: La theorie de la ehiasmatypie. La Cellule, 1909,

XXV, 389-414.

LITERATURE 271

101 Jennings, H. S.: Heredity, Variation and Evolution in Protozoa.

II. Heredity and Variation of Size and Form in Paramecium, with

Studies of Growth, Environmental Action, and Selection. Vroc.

Amer. Phil. Soc, 1908, xlvii, 393-546.
10 2 Jennings, H. S.: Production of Pure Homozygotic Or^^anisras

from Heterozygotes by Self-Fertilization. Amer. Nat., 1912, xlvi,

487-491.

103 Jennings, H. S. : The Effect of Conjugation in Paramecium. Jour.

Exp. Zool., 1913, xiv, 279-391.

104 Jennings, H. S. : Formula} for the Results of Inbreeding. Amer.

Nat., 1914, xlviii, 693-696.

105 Jennings, H. S. : The Numerical Results of Diverse Systems of

Breeding. Genetics, 1916, i, 53-89.
loe Jennings, H. S. : The Numerical Results of Diverse Systems of
Breeding, with Respect to Two Pairs of Characters, Linked or In-
dependent, with Special Relation to the Effects of Linkage. Gen-
etics, 1917, ii, 97-154.

107 Jennings, H. S.: Heredity, Variation and the Results of Selection

in the Uniparental Reproduction of Bifflugia corona. Genetics,
1916, i, 407-534.

108 Jennings, H. S., and Lashley, K. S. : Biparental Inheritance and

the Question of Sexuality in Paramecium. Jour. Exp. Zool., 1913,
xiv, 393-466.

109 Johannsen, W. : Ueber Erblichkeit in Populationen und in reinen

Linien. Jena, 1903, pp. 68.

110 Johannsen, W. : Elemente der exakten Erbliehkeitslehre. Jena,

1909, pp. 515.

111 Jones, D. F. : Dominance of Linked Factors as a Means of Account-

ing for Heterosis. Proc. Nat. Acad. Sci., 1917, iii, 310-312. Also
Genetics, 1917, ii, 466-479.

112 Jones, D. F. : Bearing of Heterosis upon Double Fertilization. Bot.

Gaz., 1918, Ixv, 324-333.
112 Jones, D. F. : Tlie Effects of Inbreeding and Cross-liroediiig upon

Development. Conn. AgT. Exp. Sta. Bull. 207, 1918, pp. 100.
114 Jones, D. F., and Hayes, H. K. : Increasing the Yield of Corn by

Crossing. Connecticut Agr. Exp. Sta. Rpt. for 1916, 1917, 323-347.
iisjoiiGER, J.: Die Familie Zero. Arch. f. Bass. u. Gesellschafts-

biologie, 1905, ii, 494-559.
116 Keeble, F., and Pellew, C. : The Mode of Inheritance of Stature

and of Time of Flowering in Peas {Pisum sativum). Jour. Gen.,

1910, i, 47-56.

272 INBEEEDING AND OUTBREEDING

117 King, H. D.: On the Normal Sex Ratio and the Size of the Litter

in the Albino Rat {Mus norvegicus aThinus). Anat. Bee, 1915,
ix, 403-419.

118 King, H. D.: The Relation of Age to Fertility in the Rat. Anat,

Bee, 1916, xi, 269-287.

1 19 King, H. D. : Studies on Inbreeding. I. The Effects of Inbreeding

on the Growth and Variability in Body Weight of the Albino Rat.
Jour. Exp. Zool., 1918, xxvi, 1-54.

120 King, H. D. : Studies on Inbreeding. II. The Effects of Inbreeding

on the Fei-tility and on the Constitutional Vigor of the Albino Rat.
Jour. Exp. Zool., 1918, xxvi, 55-98.

121 King, H. D.: Studies on Inbreeding. III. The Effects of Inbreed-

ing with Selection, on the Sex Ratio of the Albino Rat. Jour.
Exp. Zool., 1918, xxvii, 1-35.

122 Knight, T. A. : An Account of Some Experiments on the Fecunda-

tion of Vegetables. Phil. Trans. Boy. Soc, Lon., 1799, Ixxxix,
195-204.
123 Knight, T. A.; Physiological and Horticultural Papers. London,
1841, pp. 389.

124 Knuth, p. : Handbuch der Bliitenbiologie. Leipzig, 1898-1905.

3 vol.

125 KbLREUTER, J. G. : Dritte Fortsetzung der vorlaufigen Nachricht

von einigen das Geschleoht der Pflanzen betreffenden Versuchen
und Beobactungen. Leipzig, 1766, pp. 156. (Reprinted in Ost-
wald's Klassiker der exakten Wissenschaften, No. 41, Leipzig,
1893.)

126 Kraemer, H. : Ueber die ungiinstigen Wirkungen naher Inzucht.

Mitt. d. deut. landw. GeseU., 6 and 13, 1913. Trans. Jour. Her.,
1913, v, 226-234.

127 Lecoq, H. : De la fecondation naturelle et artificielle de vegetaux et

de rhybridation. Pai-is, 1845, pp. 287.
128LINDLEY, J.: The Theory of Horticulture. 2nd Ed., New York,
1852, pp. 364.

129 LoEB, J. : The Organism as a Whole. From a Physico-chemical

Viewpoint. New York, 1916, pp. 379.

130 Marchal, el., and Marchal em.: Aposporie et sexuaUte chez les

Mousses. I, II, III. Bull. Acad. Boy. Belg., CI. S'ci., 1907, 765-
789; 1909, 1249-1288; 1911, 750-778.

131 McCluer, G. W.: Com Crossing. Illinois Agr. Exp. Sta. Bull. 21,

1892, 82-101.

132 McCuLLocH, 0. C. ! The Tribe of Ishmael: A Study in Social
Degradation. Proc. 15th Natl. Conf. Char, and Cor., 1888.

LITERATURE 273

138 MacNamara, N. C: Origin and Cliaracter of the British People.

London, 1900, pp. 242.
134]VIarshall, F. H. a.: The Physiolo^- of Reproduction. London,

1910, pp. 706.

135 jyi^BTjj^^ R, : Lehrbuch der Antliropologie in systematischer Dar-

stellung. Jena, 1914, pp. 1181.

136 ]yxAUPAS_, E. : Reeherches experimentales sur la multiplication des

infusoires cilies. Arch. d. Zool. Exp. et Gen., 'II, LSS9, vi, 1(15-277.

137 Maupas, E.: La rajeunissement karyogamique chez les cili6s.

Arch. d. Zool Exp. et Gen., II, 1889, vii, 149-517.

138 Mauz, E. : In Correspondenzblatt des Wiirttemburgischen Landw.

Ver. 1825.

139 Mendel, G. J.: Versnche iiber Pflanzen-Hybriden. Verh. Naturf.

Ver. in Briinn, 1865. Trans, in Castle's Genetics and Eugenics,

Cambridge, 1916, pp. 281-321.
i^oMerz, J. T. : A Histoi-y of European Thought in the Nineteenth

Century. 3rd Ed., 3 vol., London, 1907.
141Metz^ C. W. : The Linkage of Eight Sex-linked Characters in

Drosophila virilis. Genetics^ 1918, 107-134.

142 MiDDLETON, A. R. I Heritable Variations and the Results of Selec-

tion in the Fission Rate of Stylonychia pustulata. Jour. Exp.
Zool, 1915, xix, 451-503.

143 Mitchell^ A. : Blood-relationship in Marriage, Considered in Its

Influence upon the Offspring. Mem. Anthropol Soc, Lon., 1SG5,
ii, 402-456.
1144 MOENKHAUS, W. J. I The Effects of Inbreeding and Selection on
Fertility, Vigor and Sex-ratio of Drosophila ampelophila. Jour.
Morph., 1911, xxii, 123-154.

145 Montgomery, E. G. : Preliminaiy Report on Effect of Close and

Broad Breeding on Productiveness in Maize. Nebraska Agr. Exp.
Sta., 25th Ann. Rpt., 1912, 181-192.

146 Morgan, T. H. : Sex-limited Inheritance in Dix)sophila. Science,

N. S., 1910, xxxii, 120-122.

147 Morgan, T. H. : Chromosomes and Associative Inheritance. Sci-

ence, N. S., 1911, xxxiv, 636-638.

148 Morgan, T. H. : An Attempt to Analyze the Constitution of the

Chromosomes on the Basis of Sex-limited Inheritance in Droso-
phila. Jour. Exp. Zool., 1911, xi, 365-413.

149 Morgan, T. H. ; Heredity and Sex. New York, 1913, pp. 282.

150 Morgan, T. H., and Bridges, C. B. : Sox-linked Inheritanoe in

Drosophila. Carnegie Ins. Pub. 237, Washington, 1916, pp. 87.
18

274 INBREEDING AND OUTBREEDING

151 Morgan, T. H., Sturtevant, A. H., Muller, H. J., and Bridges,

C. B. : The Meclianism of Mendelian Heredity. New York, 1915,
pp. 262.

152 Morrow, G. E., and Gardner, F. D. : Field Experiments witli Com.

Illinois Agr. Exp. Sta. Bull. 25, 1893, 173-203.

153 Morrow, G. E., and Gardner, F. D. : Experiments with Corn. Illi-

nois Agr. Exp. Sta. BuU. 31, 1894, 359, 360.
i^^MuLLER, H. J.: The Mechanism of Crossing Over. I, II, III, IV.
Amer. Nat., 1916, 1, 193-221, 2^1-305, 350-366, 421-434.'

1 55 MuLLER, H. J. : An (Enothera-like Case in Drosophila. Proc. Nat.

Acad. ScL, 1917, iii, 619-626.

156 MuLLER, H. J. : Genetic Variability, Twin Hybrids and Constant

Hybrids, in a Case of Balanced Lethal Factors. Genetics, 1918,
iii, 422-499.

157 MtJLLER, H. : Die Befruchtung der Blumen durch Insekten und die

gegenseitigen Anpassungen beider. Leipzig, 1873, pp. 478.

158 MuMFORD, F. B. I The Breeding of Animals. New York, 1917,

pp. 303.

159 Naudin, C. : NouveUes recherches sur I'hj^bridite dans les vegetaux.

Nouv. Arch. du. Mus. d'Hist. Nat. de Paris, 1865, i, 25-174.

160 Bearing, S.: Geographical Distribution of American Genius. Pop.

Sci. 3Ion., 1914, Ixxxv, 189-199.

161 Nearing, S. : The Younger Generation of American Genius. Sci.

Mon., 1916, ii, 48-61.

162 Nemec, B. : Das Problem der Befruchtmigsvorgange. Berlin, 1910,

pp. 532.

163 Odin, A. : Genese des grandes hommes gens de lettres fran^ais

modernes. 2 vol., Paris, 1895.
16-1 Parker, G. H., and Bullard, C. : On the Size of Littei^ and tJie

Nmnber of Nipples in Swine. Proc. Amer. Acad. Arts and Sci.,

1913, xlix, 397-426.
i65Pearl^ R. : The Mode of Inheritance of Fecundity in the Domestic

Fowl. Jour. Exp. Zool., 1912, xiii, 153-268.

166 Pearl, R. : On the Con-elation Between the Number of Mammas of

the Dam and Size of Litter in Mammals. I. Interracial Correla-
tion. Proc. Soc. Exp. Biol, and Med., 1913, xi, 27-30.

167 Pearl, R. : On the Correlation Between the Number of Mammge of

the Dam and Size of Litter in IVIammals. II. Intraracial Correla-
tion in Swine. Proc. Soc. Exp. Biol, and Med., 1913, xi, 31, 32.

168 Pearl, R. : A Contribution Towards an Analysis of the Problem of

Inbreeding. Amer. Nat., 1913, xlvii, 577-614.

LITERATUEE 275

169 Pearl, R., and Miner, J. R. : Tables for Calculating CoefTicicnts of
Inbreeding. Maine Agi-. Exp. Sta. Kept, for lOi;], 11)1-202.

I'^o Pearl, R. : On the Results of Inbreeding a Mendelian Population;
a Correction and Extension of Previous Conclusions. Amer. Nat
1914, xlviii, 57-62.

171 Pearl, R. : On a General Formula for the Constitution of the N'ih

Generation of a Mendelian Population in which all Matings are of
Brother X Sister. Amer. Nat., 1914, xlviii, 491-494.

172 Pearl, R. : Inbreeding and Relationship Coefhcients. Amer. Nat.,

1914, xlviii, 513-523.

178 Pearl, R.: Modes of Research in Genetics. New York, 1915,

pp. 182.

174 Pearl, R. : Some Further Considerations Regarding Coasin and

Related Kinds of Mating. Amer. Nat., 1915, xlix, 570-575.

175 Pearl, R. : Some ruither Considerations Regarding the Measure-

ment and Numerical Expression of Degrees of Kinship. Amer.
Nat., 1917, li, 545-559.

176 Pearl, R. : A Single Numerical Measure of the Total Amount of

Inbreeding. Amer. Nat., 1917, li, 636-639.

177 Pearson, K. : On a Generalized Theory of Alternative Inheritance,

with Special References to Mendel's Laws. Phil. Trans. Roy. Soc.
(A), 1904, cciii, 53-86.
17« Phillips^ J. C. : Size Inheritance in Ducks. Jour. Exp. Zool.,
1912, xii, 369-380.

179 Phillips, J. C: A Further Study of Size Inheritance in Ducks,

with Observations on the Sex Ratio of Hybrid Birds. Jour. Exp.

Zool., 1914, x^d, 131-148.
160 Plate, L. : Vererbungslehre. Leipzig, 1913, pp. 519.
181 popENOE, P., and Johnson, R. H. : Applied Eugenics. New York,

1918, pp. 459.
i82PxjNNETT, R. C, and Bailey, P. G. : On Inheritance of Weight in

Poultry. Jour. Gen., 1914, iv, 23-39.
183 Ripley, W. Z.: The Races of Europe. New York, 1899, pp. 624.
1)84 Ritzema-Bos, J. : Untersuchungen iiber die Folgen der Zucht in

engster Blutverwandtschaft. Biol. Cent., 1894, xiv, 75-81.

185 Roberts, H. F. : First Generation Hybrids of American and Chi-

nese Com. Amer. Breed. Assn. Rpt. 1912, viii, 367-384.

186 RoBBiNS,, R. B. : Some Applications of Mathematics to Breeding

Problems. I, II, III. Genetics, 1917, ii, 489-504; 1918, iii, 73-
92; 1918, iii, 375-389.

187 RoBBiNS, R. B. : Random Mating with the Exception of Sister by

Brother Mating. Genetics, 1918, iii, 390-39G.

276 INBREEDING AND OUTBREEDING

188 RoMMELL^ G. M. : The Fecundity of Poland-China aiid Duroc-Jersey
Sows. Cir. 95, U. S. Dept. Agr., Bur. An. Ind., 1906, pp. 12.

ii89 RoMMELL^ G. M. : The Inheritance of Size of Litter in Poland-
China Sows. Amer. Breed. Assn. Rpt., 1907, v, 201-208.

190 RoMMELL^ G. M., and Phillips^ E. F. : Inheritance in the Female

Line of Size of Litter in Poland-China Sows. Proc. Amer. Phdl.
Soc, 1906, xlv, 245-254.

191 Sageret, a. : Considerations snr la production des hyb rides, des

variantes et des varietes en general, et sur celles de la famille de
Cucurbitacees en particulier Ann. des Sci. Nat., 1826, viii, 294-
314.

192 Shamel^ a. D. : The Effects of Inbreeding in Plants. Yearbook

U. S. Dept. Agr., Washing1)on, 1905, pp. 377-392.

193 Shull_, G. H. : The Composition of a Field of Maize. Amer. Breed.

Assn. Rpt., 1908, iv, 296-301.

194 Shull_, G. H. : A Pure Line Method of Com Breeding. Amer.

Breed. Assn. Rpt., 1909, v, 51-59.

195 Shull^ G. H. : Hybridization Methods in Corn Breeding. Amer.

Breed. Mag., 1910, i, 98-107.

196 Shull, G. H. : The Genotypes of Maize. Amer. Nat., 1911, xlv,

234-252.

197 Shull, G. H. : Duplicate Genes for Capsule Form in Bursa hursa-

pastoris. Zeitschr. f. ind. Ahst. u. Vererh., 1914, xii, 97-149.

198 Shull, a. F. : The Influence of Inbreeding on Vigor in Hydatina

senta. Biol. Bull., 1912, xxiv, 1-13.

199 Shull^ a. F. : Studies in the Life Cycle of Hydatina senta. III.

Jour. Exp. Zool., 1912, xii, 283-317.

200 Strasburger^ E. : Versuche mit diocischen Pflanzen in Riicksioht

auf Geschlechtsverteilung. Biol. Centralbl., 1900, xx, 657.

201 Strasburger^ E. : Ueber geschlechtbestimmende Ursachen. Jahrb.

Wiss. Bot., 1910, xlviii, 427-520.

202 Sturtevant^ A. H. : The Linear Arrangement of Six Sex-linked

Factors in Drosophila, as Shown by Their Mode of Association.
Jour. Exp. Zool, 1913, xiv, 43-59.

203 Sturtevant, A. H. : The Behavior of the Chromosomes as Studied

through Linkage. Zeitschr. f. ind. Ahstam. u. Vererh., 1915, xiii,
234-287.

204 Surface, F. M. : Fecundity in Swine. Biometrika, 1909, vi, 433-436.

205 Toyama, K. : Mendel's Law of Heredity as Applied to Silkworm

Crosses. Biol. Chi, 1906, xxvi, 321-334.

206 VoisiN, A. : Contribution a Fhistoire des manages entre oonsangnins.

Compt. rend. Acad. Sci., 1865, Ixv, 105-108.

LITERATURE 277

207 Warren, H. C. : Numerical Effects of Natural Selection Acting

Upon Mendelian Characters. Genetics, 1917, ii, 305-312.

208 Weinstein, a. : Coincidence of Crossing Over in Drosophila mel-

anogaster [ampelophila) . Genetics, 1918, iii, 135-159.

209 Weismann, a. The Evolution Theory. (Trans. J. A. Thomson

and M. R. Thomson.) London, 1904, 2 vol.

210 Wellington, R. : Influence of Crossing in Increasing the Yield of

the Tomato. New York Agr. Exp. Sta. Bull. 34G, 1912, 57-76.

21 1 Wentworth, E. N. : The Segi^egation of Fecundity Factors in

Drosophila. Jour. Gen., 1913, iii, 113-120.

212 Wentworth, E. N., and Aubel, C. E. : Inheritance of Fertility in

Swine. Jour. Agr. Bes., 1916, v, 1145-1160.

21 3 Wentworth, E. N., and Remick, B. L. : Some Breeding Properties

of the Generalized Mendelian Population. Genetics, 1916, i, 608-
616.

214 Westermarck, E. : The History of Human Marriage. 3rd Ed.,

London, 1903, pp. 644.

215 Wheeler, W. M. : The Ants of the Baltic Amber. Schrlft. Phijsik-

okonom. GeseU. Kordgsherg, 1914, Iv, pp. 142.

216 Whitney, D. D. : Reinvigoration Produced by Cross-fertilization in

Hydatina senta. Jour. Exp. Zool., 1912, xii, 337-362.

217 Whitney, D. D. : " Strains " in Hydatina. Biol. Bull., 1912, xxii,

205^218.

218 Whitney, D. D. : Weak Parthenogenetic Races of Hydatina senta

Subjected to a Varied Environment. Biol. Bull., 1912, xxiii, 321-

330.

219 WiEGMANN, A. F. : Ueber die Bastarderzeugung im Pllanzenreich.

Braunschweig, 1828, pp. 40.

220 Withington, C. F. : Consanguineous Marriagas : Their Effect upon

Offspring. Mass. Med. Soc, 1885, xiii, 453-484.

221 Wolfe, T. K.; Further Evidence of tlie Immediate Effect of Cross-

ing Varieties of Corn on the Size of the Seed Produced. Jour.
Amer. Soc. Agr., 1915, vii, 265-272.

222 Woodruff, L. L.: Two Thousand Generations of Paramecium.

Arch. f. Protistenkunde, 1911, xxi, 263-266.

223 Woods, F. A. : Heredity in Royalty. New York, 1906, pp. 312.

224 Woods, F. A.: The Influence of Monarchs. New Y^ork, 1013,

pp. 421. T T^ r

225 Wright, S.: The Effects of Inbreeding on Guinea-pigs. T. Dei>lino

in Vigor. II. Differentiation among Inbred Families. III. Crosses
between Different Highly Inbred Families. (Doctor Wright kindly
pemiitted the authors to read these valuable unpublished papers
in manuscript. )

PROPERTY UBlUjir

fj[. C. StaU ColKS^

INDEX

Achondroplasy, 230

Adaptation, for cross-pollination, 34

for self-pollination, 30
Africa, 247, 254, 257

native population of, 253
Agassiz, 236
Agriculture, 18
Algae, 204
Allelomorphs, 55
Allen, 45

Alternation of generations, 29, 46
Althcea, 144
America, 247, 252
Amoeba in division, 21
Amorites, 261
Amphimixis, 206
Angles, 260
Annelids, 21
Anthropology, 18, 245
Apomixis, 32
Apple, 210
Arabs, 261
Armadillo, 42

nine-banded, identical quadru-
plets in, 44
Arthropoda, 22
Arthropods, 2 If., 207
Aryan, 258

Celtic, 259

Nordic, 259f.

races, 256

Teutonic, 258
Asia, 247, 258

culture of, 256
Assyrioides, 261
Atavism, 166
Ataxia, Friedrich's, 231
Australia, 247, 252
Autogamy, 30

Barley, 114, 210
Basidiomycetes, 31

Bateson, 105
Bcal, 221
Beans, 114
Bees, 158
Berthollet, Ml
BihoSy frontalis, 192

gaums, 192

(jr aniens, 102
Birds, 158

Bison anicricnnns, 192
Blossom's Glorcne, S5ff.
Bos taurus, 192
Brachydactyly, 230
Brassica olcracea, 192
Breckenridge, 235
Bridges, 184
Britain, 2o8f.
British Isles, 258
Bronze Age, 258
Briinnow, 236
Bryozoans, 23
Budin, 227
Buffalo, 192

Cabbage, 192

Caesar's invasion of Britain, 259

Calceolaria, 155

Castle, 25, 111, 137, 158, 160, 188,

236
Cat, 211
Catalpa, 150
Cataract, 230
CsittiiW, 233
Cattle, 210, 213
Caucasian, 255
Caullery, 22
Cavia, 103

cutlcri, 160

species, hybrids, 192
Celts, 258
Cereals, 211
Cephalic index, 249

279

280

INDEX

Cliemotropism, 154

China, 247

Chinese, 254

Chordates, 21

Chromosomes, 36

Ciona intestinalis, 25

Cirriped.es, 25, 34

Cleft palate, 230

Coccinea, 144

Coefficient of cross-relationship, 86
of heredity, 196, 199
of inbreeding, 81/f.
of relationship, 81, 84^.

Coelenterates, 21

Collins, 137, 153

Coloration, protective, 146

Color-blindness, 47^.

Complemental males, 25

Composite^, 32

Conjugatce, 27

Connate seeds of maize, 133

Consanguinity, 113, 139

Corn, varieties of, 215

Coulter, 26

Cow, 192

Cramer, 198

Crampe, 101^.

Cromwell, 258

Crossing-over, diagram to illustrate,

64
Cucumber, 221
Cucumis, 144
Cymotlioidce, 24
Cytoplasm of egg, 200

Dalton, 235

Danes, 260

Darwin, 13, 25, 32, 34, 101, 114^.,

137^., 143, 146^., 154, 164/., 186,

235
Datura, 142, 144
Davenport, 230/., 233
Delphino, 34
Detlefsen, 103, 192
Diabetes insipidus, 230
Dianthus, 116/., 142, 144, 155

Diatotnece, 27

Dichogamy, 33

Difflugia, 203
coronata, 78

Digitalis, 144, 155

Dioecism, 22

Disease, susceptibility and resis-
tance to, 134

Dog, 210f.

Dominance, 57, 72, 177^.

Dominant factors, complementary
action of, 171

Double cross, 223

Double fertilization, 203.

Draha, 155

Drosophila, 179, 188, 198, 208
melanogaster, 61, 111, 184
sterility in, 112

Diising, 106

Echinodermata, 22
Echinoderms, 21
Edwards, 235
Ellis, 233
Emerson, 183-
Endogamy, 238
Endosperm fertilization, 153
England, 257^., 260/.
Englishman, 250, 262
Epilepsy, 231, 241
Eschscholtzia, 117
Europe, 247, 251/., 257, 261

leaders of, 257
European culture, 256
Evolution, 13

inbreeding and outbreeding in,
195^.
Exogamy, 13, 15, 201

Factors, stability of, 76f.
Faraday, 235
Feeble-mindedness, 231
Ferns, 29

Fertilization, diagram to illustrate,
39

double, 41

in embryo sac of the lily, 41

INDEX

281

Fish, 92, 158

Fitzhugh, 235

Flat worms, 21

Focke, 141, 145

Fossil ants in amber of Oligocene

period, 77
France, 257f., 260/=.
Franklin, 235^.
Franks, 260
Frazer, 13
Freeman, 157
French, 257
Frenchman, 250
Frequency distribution of corolla

length in tobacco, 70
Freud, 13
Fucus, 26, 28
Funaria, 46

Galton, 50, 227, 233, 237
Gamete formation in dihybrid, 59, 63
Gametogenesis, 38, 55
Gametophyte, 29
Gardner, 118

Gartner, 141, 143, 145, 155
Gaur, 192
Gayal, 192
Genetics, 50

Genotype hypothesis, 166
Gentry, 213

Germany, 257, 260, 262
Germplasm, mixture of, 201
Gernert, 155
Gerschler, 158
Geum, 144
Goat, 212
Goddard, 240, 243
Goliath, 110 (fig. 28), 233
Gonochorism, 22, 33
Grape, 210
Gravatt, 192
Greece, 251
Green Algser, 26
Guaita, von, 101^.
Guinea-pig, 188
growing curves of, 160

Guinea-pig, inbreeding experiments
with, 110^.

Haiti, 253

Hammurabi, code of, 14

Hare- lip, 230

Hawkweed, 22

Hayes, 89, 148, 168, 192

Herbert, 141

Heredity coefficient, 196, 199

Hermaphroditism, 22^., 26, 30, 33,

201
Hero, 117

Heterosis, 16, 96, 141, 144, 157, 172,
202

importance of in sex origin, 20i ff.

manifestations of, 150

selective effect of, 154
Heterozj^gosis, 121, 138

degrees of, 93

similarity of effect of, with en-
vironment, 157
Hittites, 261
Homozygosis, 113
Homozygosity, 134

affected by linkage, 95

attainment of, 95
Horse, 212

origin of, 2l0ff.
Huntington's chorea, 230
Huth, 243
Hybrid vigor, 16, 88, 96, 141

benefit from, 219

cause of, 164
Bydatina, 158

senta, ll2f.
Hyde, 112, 158
nyoscyamus, 155

Iberian, 259

race, 258
Ichythyosis, 230
Inbreeding, curves of, 84

effect of, on yield and height of
maize, 124^.

effect on organisms, 137

282

INDEX

Inbreeding, experiments with ani-
mals and plants, 100

index, 81

intensity of, 85

mathematical considerations of,
80

problem, phases of, 81

reduction in vigor resulting from,
96

and outbreeding in plant and
animal improvement, 210
Infant consultations, 227
Infusoria, 78

Inheritance, Mendelian, 72
Insanity, 231, 241
Insects in Oligocene amber, 197
Ipomea, 115, 117
Ireland, 257^.
Irish, 260
Isopods, 24

Japan, 247

Japanese, 254f.

Jennings, 78, 92, 95, 97, 202/.

Jew, 261/.

English, 262

German, 261

Spanish, 261
Johannsen, 78, 166
Jukes, 237^., 260, 262
Jutes, 259

Keeble, 170, 172

Kempton, 153

Kerner, 34

King, 101/f., 105, 107, 160, 188

King Melia Rioter 14th, 85^.

Knight, 114, 141^.

Knight-Darwin law, 33

Knuth, 34

Kolreuter, 141/., 144, 154

Lang, 13
Lavatera, 144
Learning strains, 124
Lecoq, 141

Lee, 235

LeguminoscB, 114
Lethal factors, 179
Linaria, 144
Lincoln, 235/.
Lindley, 143
Liverworts, 29, 45
Lobelia, 144
Loeb, 200
Lychnis, 144
Lycium, 144
Liiffa, 144

MacLennan, 13

MacNamara, 257

Maize, connate seeds of, 133

distribution of rows of grain
of, 129

fertility of, 188^.

growth curves of, 152

inbred strains and hybrids of,
150 (Fig. 31)

inbred strains of, 130 (Fig. 29)

number of nodes of, 150

segregation of ear row number
of, 131, 132

variety crosses of, 153
Malthus, 247
Malva, 144, 155
Mammals, crossing of, 159^.
JNIan, inbreeding and outbreeding
in, 226

inter fertility of, 246
Marchals, 46

Marriage of near relatives, 100
Matings, brother and sister, 97

parent and offspring, 97
Mauz, 141

Mecca of politically oppressed, 264
Mechanism of heredity, 50

of reproduction, 36
Mendel, 50, 118, 144/., 165
Mendelian segregation, 88
Mendelism, 51^.

Mendel's laws of inheritance, 55
Merz, 233

INDEX

283

Mice, 188

Middleton, 78

Miela, 227

Milk depots, 227

Mimicry, 146

Mimulus, 115, 117

Mirabilis, 142

Moenkhaus, 112, 158

Molluscoids, 21

Molluscs, 21

Mongolian, 255, 257

Monoecism, 33

Morgan, 62, 198

Morphology, comparative, 18

Morrow, 118

Moss, 29, 46, 204

Mule, 142, 219/.

Miiller, 34

Muller, 158

Mumford, 214

Myxomycetes, 26

Nam, 237f., 260, 262
ISTaudin, 144
Nearing, 233
Negro, 252^.
Nemathelminthes, 22
Nematode, 21
Nemec, 203
Neolithic period, 258
New Zealand, 247
Nicotiana, 103, 139, 142, 144,
155, 157, 192, 196/=.

alata, 19lf.

height of species and crosses,

Langsdorfjli, 191

longifiora, 69

paniculata, 192

rustica, 192

tahacum, 192
Nordic factors, 250
Normans, 260
Norsemen, 258
Nucleus, 36

Oats, 114

148,

149

Oogenesis, 37
Open door, 265
Ostrich, 210

Papaver, 144

Paramecium, 202

Parasitism, 22

Parthenogenesis, 206

Pasteur, 235, 237

Payne, 235

Pearl, 80/., 83^., 87

Pearson, 92

Peas, 148

Pellew, 170, 172

Pentstemon, 144

Peredinece, 27

Petunia, 117, 144

Phaseolus, 139

Phillips, 159

Pimm, 139

Pcellman, 238

Pollen grains, formation of in the

lily, 40
Polydactyly, 230
Polypodiacece, 32
Preston, 235
Primula, 144
Protandry, 23, 135, 201
Protogyny, 23, 201
Protozoa, 21
Pumpkin, 221
Punnett, 159

Quantitative characters, inheritance
of, 60^.

Races, intermingling of and national

stamina, 245^.
Racial types, 250
Radish, 192
RaivwnculacecB, 32
Raphanus sativus, 192
Rat, 188

curves showing body weight of,
107/.

inbreeding experiments with,
101^., 105

284

INDEX

Rat, size of litters of, 109

Eeduction of heterozygous individ-
uals and allelomorphic pairs, 90

Remick, 92

Reproduction, among animals and
plants, 20
asexual, 17, 21, 22, 30, 78
sexual, 17, 20^., 26, 201, 205
sexual, origin of, 2()ff.

Retina, pigmentary degeneration of,
231

Rhopalura, 26

Rice, 114

Ripley, 257

Ritzema-Bos, 101^., 188

Roberts, 153

Robbins, 92

Roman Empire, 259

Romans, 259

Rome, 251

Rommel, 110

Rosacece, 32

Rotifer, 158, 170, 202

SaccuUna, 23

Sageret, 141, 144, 15G, 192

Sax, 157

Saxons, 258, 2G0

Scandinavians, 257

Schizopliytes, 26

School for mothers, 227

Scotch, 259f.

Scotland, 257, 260

Sex, determination of, 43^.

Sexual dimorphism, 26

Self-fertilization as means of obtain-
ing homozygosity, 97

Self- sterility, 25, 33

Sex-linked characters, 47^.
inheritance of, 48/".

Sex, origin of, 201

Sex ratio, 106

Shamel, 119

Sheep, 210^.

Shull, A. F., 112/=., 158, 169

Shull, G. IL, 118f., 168/.

Silkworms, 158
Spain, 257/.
Spaniard, 262
Spermatogenesis, 37
Spermatozoon, entrance of, through

membrane of egg, 42
Spliwrocarpus, 45

Donnellii, 45

texanus, 45
Spirogyra, 27
Sponges, 21^.
Spores, 29
Sporophyte, 29
Squashes, 221
Sterility, 188^.
Stevenson, 233
Stone Age, 258
Strasburger, 45
Sturtevant, 184
Htylonychia pustulata, 78
Swine, 210/., 213
Synapsis, 37
Syndactyly, 230

Talmud, Hebraic, 14
Tapeworm, 23/.
Tobacco, 114, 148, 156
corolla length of, 70
Tomatoes, 114, 148, 221
Toyama, 158
Trochelminthes, 21/., 202
Tropa^olum, 144
Tunicates, 23
Turanian, 259; race, 258
Turbellarians, 23
Tuttle, 235

Ulothrix, 26, 28

Unit of heredity, 77

United Kingdom, 257

United States, 253^., 260, 262, 264

Ustilago maydis, 118

Venable, 235
Verhascum, 142, 144

INDEX

285

Wales, 261

Weismann, 101^., 188, 201, 206
Wentworth, 92, 112
Westemiarck, 238, 243
Wheat, 114, 197, 210
Wheeler, 77
Whitney, 112, 158
Wiegmann, 141, 144, 154
William the Conqueror, 260
Woods, 233

Wooli'v, 235

Wrigli't, 110/-., 100/., 188

Xeroderma pigmentoaum, 231

Yak, 192

Yellow mouse, 179

Youatt, 220

Zero, 237, 239

(

(