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Monographs on Experhvental biology 


JACQUES LOEB, Rockefeller Institute 

T. H. MORGAN, Columbia University 

W. J. V. OSTERHOUT, Harvard University 









By JACQUES LOEB, Rockefeller Institute 

By G. H. PARKER, Harvard University 

By T. H. MORGAN, Columbia University 



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


By H. S. JENNINGS, Johns Hopkins University 


By R. PEARL, Johns Hopkins University 


By E. G. CONKLIN, Princeton University 

By R. G. HARRISON, Yale University 



By W. J. V. OSTERHOUT. Harvard University 

By L. J. HENDERSON, Harvard University 


By T. B. ROBERTSON, University of Toronto 


By A. R. MOORE, Rutgers College 

By E. N. HARVEY, Princeton University 


Monographs on Experimental Biology 











.^- •) 



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

J I ^ fi ri vi ^ 


The rapidly increasing specialization makes it im- 
possible for one author to cover satisfactorily tlie 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 
Physics. 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 Greneral Physiology are one 
and the same science, by method as well as by contents, 
siace both aim at explaining life from the physico-^jhemical 
constitution of living matter. The series of monographs 
on Experimental Biology wUl therefore include the field 
of traditional General Physiology. 

Jacques Loeb, 
T. H. Morgan, 



It is inevitable that each work planned as a member of 
a series of biologioal 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 it." 
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 appHoation 
of the results to a,griculture 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 
himian 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 iu 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. 



The manuscript has, been the product, as it purports 
to be, of a very intimate collaboration, and the authors 
join ia acknowledging their indebtedness to their fellow 
biologists for the privilege of copying several illustrations 
as noted iu 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 



I. Introduction 13 ^ 

II. Reprodtjction Among Animals and Plants 20 

III. The Mechanism op Reproduction '. 36 

IV. The Mechanism of Heredity 50 • 

V. Mathematical Considerations op Inbreeding 80 

VI. Inbreeding Experiments with Animals and Plants 100 

VII. Htbrid Vigor or Hjbterosis 141 • 

VIII. Conceptions as to the Cause op Hybrid Vigor 164 

IX. STEMLiry AND Its Relation to Inbreeding and Cross-brbbdinq 188 

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

XI. The Value op Inbreeding and Outbreeding in Plant and 

Animal Improvement 210 ■ 

XII. Inbreeding and Outbreeding in Man; Their Effect on 

THE Individual 226 • 


XIII. The Intermingling op Races and National Stamina 245 

Literature 266 



1. Asexual Reproduction, an Amceba 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 

5. 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-linked 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 Allelomorphio Pairs in Successive Generations of 

Self-fertilization 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 Difler- 

ent Series of Male Albino Rats 108 



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

Generations of Inbreeding Albino Rats by Brother and Sister 
MatingB 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 SeK-fertihzation 130 

30. Graphs Showing the Reduction of VariabiUty and Segregation of 

Ear Row Number in Selfed Strains of Maize ' 132 

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

Their Fi Hybrid 150 

32. Ears of Maize after Six Generations of Self-fertilization 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 



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 Darwin 
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 totemio 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 



the origm 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 
case^ 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- 


terns have been common, and the logical outcome of a 
caste system is marriage between near relatives. Pride 
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 


body politic due to differential fecundity, birth control, 
and otber 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' 

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 


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 


inbreedihg was established. But this system appears to 
have been deficient. Some evolutionary 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 


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. 


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 Avith 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 



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

There 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 flat 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 (Trochelminthes) 
and the molluscs have no asexual 
reproduction in the usual sense of 
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 unknown. 
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 

Fig. 1. — Asexual reproduc- 
tion. An amceba in division. 
cv. contractile vacuole; ek, ecto- 
sarc; en, entosarc; n, nucleus. 
(Kingsley after Schulze. Cour- 
tesy Henry Holt & Co.). 


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 propagatiooi. 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 dicecism). 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 
Fio. 2.— Asexual repro- Arthropodu it is rare. An extended 

duction by means of runners . , . ■, i • j /• i 

in the hawkweed. (After experiment ou the suDjcct 01 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.^^ 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 


by group, for example unisexual land or fresli-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 speci^ization 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^Htfcess. Only a few degenerate forms retained self- 
"rertUization 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 developed 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- 



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- 


Fig. 3. — Hermaphroditism in the tapeworm proglottid. K, genital pore; ou, ovary; ra, 
receptaculum seminalis; (, testes: u, uterus; ud, Vas deferens. (Kingsley after Sommer). 

ing the sexual orifices. The isopods of the family Cymo- 
tJioidce, 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 laiowledge of zoology; but 
when the animal passes over into the female stage it be- 
comes merely a great egg sac many times the previous 
size. One would hardly suppose the two stages belonged 


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 hermaphreditic, 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 intestinalis. 

In what appear to be the essential features, the 
vicissitudes of reproduction have been similar in the 
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 Icingdom 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 



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 

Fig. 4. 

Fia. 5. 

^- -V, 

Fig. 4, — Rhopalura, an example of extreme sexual dimorphism. _ (After Caullery.) 
Fig, 5. — Sexual reproduction in Fucuts, giving some idea of the dilTerence 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. 


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 Peredmeee, the Conjugatce and the Diatomece — the 
ConjugatcB 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 ConjugatcB, 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, the 
contents of one cell pass over to the other. We may 



think 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, zo- 
ospores; Z), germination of zoospore; E, 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 algffi. 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 


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 gamete pro- 
tection and provisions for assuj-ing fertilization, so also 
the final step in each, the mammals and the seed plants, 


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- 

Fia. 7. — Adaptation for self-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 


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 
permit 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 


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 unquestionablj" 
the list is very incomplete. Examples from Polypodiacece, 
Banunculacece and Rosaceos, are not uncommon, but in 
particular it is the Compositce, 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 modern 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 


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, iu relinquishing the 
possibility of self-fertilization. 

Some of the various cross-fertilization mechanisms 
utilized are very reminiscent of those of animals. Monoe- 
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 com m on 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- 




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

But by far the most numerous and most interesting 
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 

FlQ. 8. — Adaptation for crosB-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 MTiUter,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 


provisions are made to use them as pollen carriers 
throngli 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. • 


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 ea,ch 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 



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 kind 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 abc, regarding A, B, and 
C as of maternal and a, h, and c as of paternal origin, at 
synapsis A only pairs with a, B with h and C with c. 
This procedure, however, will yield eight types of gam- 
etes, ABC, ABc, AhC, aBC,\Abc, aBc, abC, and abc, since 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 



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


Spermato- 0^ 

Moltiplicatien ^ 
r pcnorf A 


Growth period -< 



/ \ 

r ® ^ 


Sp!?ffiS?yte \^j I P*i'"i''« "^ Chro™oson,e= J ^^^-j Primary oocyte 

f \ > deducing division J A 

Secondary /^ 
spermato- \C^o 

,^-/ V ^J\ Secondary oocyte 
yV # \ (ovum and f iret 
^ ' polar bodyj 

/' , Y Mature, ovum ,. 
/ j and polar bodies 

/% 1 Mature ovum 

Zygote. of , l\ f\ fpU Jiomber of 
fertilized [ ^ \ \ chromosomes 
ovum witii V ° ^ / restored 

FiQ. 9. — Diagram of gametogeneeis 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 &^^. 

Fertilization consists in the fusion of one Q^'g with one 
sperm, thus bringing back the double number of chromo- 


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. 

Fig. 10. — Diagram to illustrate fertilization; cT, male pronucleus; 9 , female pro- 
nucleus; observe that the chromosomes of maternal and paternal origin, respectively, 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. 



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 

Fia. 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 with a function to perform. ' One fuses with the egg 
and the other with the so-called endosperm nucleus, com- 


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

In the meantime, the egg and the endosperm nucleus 
have been prepared by the 
necessary 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 3x: 

Fig. 12. — Fertilization in the embryo sac 
of the lily, e, egg; /s, first sperm; pp, 
fused polar nuclei ; ss, second sperm. 
(After Bergen and Davis.) 



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 



FiQ. 13. — Entrance of the spermatozoon through the membrane of the egg of a star- 
fish giving an idea of the dlfi^erence in size between the sperm and the egg. ("Wilson's " The 
Cell." Courteay 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 


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 fertiliz9,tion through the chromosome 
complex is from several very different sources. 

First, there is the phenomenon of multiple hirths 

Protenor cf 


FiQ. 14. — Diagram Bhowing the distribution of the sex chromosome in PTOtenor. 


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 known as 
identical twins. Such is the case with the four young in 
each litter of the nine-banded armadillo (Fig. 15). Now 



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. 

Fia. 15. — Identical quadruplets in the nlne-bandecf armadillo. (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 


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 have 
but one and become males. 

Diagrammatically it is this : 

Ovum with x fertilized by sperm with 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 aUke, but the result 
is the same ; the sex distribution follows the chromosome 

In dioecious plants there is some evidence of a similar 
condition. Strasburger ^"^ found in one of the liverworts, 
Sphserocarpus, 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 SpJicero- 
carpus Donnellii, but it has been corroborated by one of 
his students working with SphcBrocarpus texanus. 


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- 
fluence, 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 cAridence from very 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 fact, 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 


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-bliadness in man, 
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 normal 
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 iu marriage between cousins, 
on the average one-half of her daughters as well as one- 
half of her sons will be abnormal. 

This iateresting and apparently complicated inheri- 
tance is really very simple if we merely assume that the 
sex chromosomes of the color-blind individuals also caxry 
the determiner for color-blindness. Fig. 16 shows what 
would be expected. Representing 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 



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

Female Line 

of grand- 

Fig. 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, 


and for this reason will transmit color-blindness te one- 
half of their sons by a normal man, as wiU 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-blind. 

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. 


,The scientific era in the investigation of heredity be- 
gan in the latter half of the nineteenth century with the 
vrork 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 



discuss here only the broader relationships of Mendeliau 
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 studjdng 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 Fo 
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 the 
plants bearing intermediate spikes again produce the 
ratio exhibited by the F.^ generation. Diagrammatically 
the result of the cross is as follows : 

P, Long spikes x Short spikes 

Fj Intermediate spikes 

F Long spikes Intermediate spikes Short spikes 

F Long spikes Long spikes Inter- Short spikes Short spikes 




If the description of tlie 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 F^ 
generation is plain. Furthermore, it is clear why the 
types like the grandparents breed true and the type like 
the hybrid Fj 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 hotli of its parents ; there- 

Fetnale ganets 

Male gamete 

Fig. 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 I A gamete bear- 
ing L fuses with a gamete bearing ;S' and a zygote L8 is 
formed. The interaction of the factors L and 8 produces 
an F^ plant bearing intermediate ears., When this hybrid 


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 8. When the 
F-^ generation is selfed, therefore, it is a matter of 
chance which of these germ cells meet to form zygotes. 
If a large progeny is produced, there will be a ratio of 

1\L\L :2 L S -.18 8\, and since the formulae L\L and 

\8\8\ 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\8\ like the F^ generation, will behave in the same man- 
ner when selfed. 

That the ratio wiU be approximately 1|L|L : 2 L\8\ 

1 8 8\ 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 8 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 8. The second member of the pair 
represents the male cell and the chances are likewise l^ 
that it will be L or 8. 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\8\ . Likewise 

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

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

these possibilities, the ratio 1\L\L\ : 2 \L\8\ : 1 \8\8\ is 



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

^ gametes 

S zygote 

L S 

Eggs iSperjis 

Fa zygote 


Fig. is. — Diagram to illustrate Mendelism in a cross between long-spiked and short-spiked 


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 raage 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 


an L sperm is 1/2 x y^ = V-k, and the chance of an L egg 
meeting an 8 sperm is % x 1/2 = %. Similarly, the chance 
of an 8 G:gg meeting an L sperm is % and of an L egg 
meeting an 8 sperm %. The sum of the possibilities 
is then 

L+L = l/4 
8 + L = l/2 
S + 8 =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. When 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 %, 
yet in a small number 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 
Mendel's Laws of Inheritance, the first of which may be 
stated as follows : Unit factors contributed by tivo parents 
having 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 aiid 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 


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 Fj generation is 
3 dominant : 1 recessive. But since only one-third of these 
dominants breed true" and two-thirds behave as did the 
Fj 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 kaown as the Law of 
Recombination. This Imv states that tivo or more allelo- 
morphic pairs of factors may segregate independently 
and may recombime in all the combinations possible gov- 
erned by chance only. 

Though thousands of characters have been investi- 
gated since Mendel's 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 h. 
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 ov hybrid for it. 


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 Fj 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 grpen 108 

db wrinkled and green 32 

These figures stand approximately in the relation of 
9AB to 3aB to 3Ab to lah. The forms appeared to 
belong to but four homogeneous classes (phenotypes). 
This was due to the phenomenpn 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 obtain&d by 
self-fertilization : 

(1) A ABB see& 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, yeUow 

and green 138 

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

(5) A Abb seeds all round and green 35 

(6) Aabb seeds all green, round and wrinkled 67 
From the wrinkled 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 seedis (apparently ab) were obtained: 

(9) aabb seeds all wrinkled and green 30 


Tlie total number of plants in the 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 : ZaB : 3Ab : lab obtained in the Fg generation 
is made up of the following actual classes : 

9 apparently AB made up of 

3 apparently Ah made up of 


2 AaBB 
2 AABh 
4 AaBh. 

1 AAhh 

2 Aabh 

3 apparently aB made up of jo aaBfe 

1 apparently ab made up of 1 aabh 

The four classes AABB, aaBB, AAhh and aahh, 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 
F4 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 h 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, Ah, aB and ah will be formed in 



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

I n 



Fig. 19i — Diagram to illustrate gamete formation in a dihybrid,in independent inheritance. 

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 


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 
a b 


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 F^ 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, ZAB : ZaB : lab, 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 


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

It is evident, if one runs over these examples and works 
out all the possibilities involved, he will find that two 
white races of sweet pea, when crossed, will give purples 
in the Fj 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 he 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 


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 Fj 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 F2 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- 



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. 




will givs 









will give 






Fio. 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 F^ 
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 offspring 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-croseovers Crossovers 

Black vestigial Gray long Black long Gray vestigial 

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




Gametes of Pi 

f'' Is^-.^^ 't 

Gametes of Male 
All direct s^regates 

■black Gray 

I/ong vestigial 

50.0^ ■ 50.o)S 

black Gray black Gray 

Long vestigial vestigial Long 

41.532 41.S3S a. 5% 6.4% 

Fia. 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 peculiar fact that 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 ^1* or a,s "^1^, crossing over is the same in each 
ease; that is, the tendency is just as great for Ab 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 au- exception, furnished striking evi- 
dence in support of th^ 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 ta 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 th.e 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- 


ditions because we know what 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 controUed 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 


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. When pure or homozygous races are crossed, the 
Fi populations are similar to the parental races in uni- 
formity. This conclusion devolves from observations 
that if any particular factors AA and aa are homozygous 
in the parental races, they can only form Aa individuals 
in the F-i generation. 


2. If the parental races are pure, F^, 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 F^ populations produced 
from such crosses should be much greater than that of the 
Fj 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 F^. 

4. In certain cases F^ individuals should be produced 
showing 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 F^ genera- 
tion should produce populations differing in type. The 
idea on which this statement is based is, of course, that all 
Fj 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 F^ generation should give Fg 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- 


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 true 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 with 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 F2 populations which it produced 
are much more variable ; and one can easily calculate that 
if several thousand plants had been grown instead of 










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about 200, the grandparental sizes probaWy would have 
been obtained. Ftirthennore, if one studies the results 
obtained in the F3, 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 allelomorphio 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 Fj 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 tall. 

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 



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 



f 1 AABB 
2 AaBB 
2 AABh 
4 AaBb 

1 AAhh 

2 Adbh 

1 aaBB 

2 aaBh 

1 adbh 

28 inches 
24 inches 
24 inches 
20 inches 

20 inches 
16 inches 

20 laches 
16 inches 

12 inches 

If three independent size characters instead of two 
were involved ta this cross, the F-^ individuals would fall 
in the same class as before, but the F^ 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 types 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 Fa Mendelian expression for N allelomorpMo pairs 
when dominance is complete is the expanded bionominal: 

(3 + 1)" or (3/4 + 1/4)" 
N = l (3 + 1)1 = 3 + 1 

JV = 2 (3 + 1)2 = 32+3 + 3 + 1 = 9+3 + 3 + 1 
N = 3 (3 + 1)3 = 33 + 3(32)2+3 (3) + 1 = 27 + 9 + 9 + 9 
+ 3+3+3 + 1 

Likewise, the expanded bionomial (1/2 + %)^" gives the 
numerical relationships when dominance is absent and N 
represents the number of aUelomorphic pairs. The ex- 
pression is (% + %)*" instead of (% + %)" because it is 
supposed that the presence of any aUelomorphic 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 Fg when 
parents of different sizes differing in N aUelomorphic 
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 


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

N=l 12 1 

N=2 14 6 4 1 

Ar = 3 1 6 15 20 15 6 1 

N = 4: 1 8 28 56 70 56 28 8 1 

iV = 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 Fa 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 
§ize, but it would be foolish to expect that a plant capable 


Oi only a LLmited amouut of development could bear as 
large an ear as if it were as a wkole 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 


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 MendeHan 
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- 


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 comparatively 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. Wheeler ^^^ 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- 


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 Difflugia coronata and Stylonychia 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 wMch might be given, 
could further space be devoted to the subject, we believe 
there should be no hesitation in identifying the hypotheti- 


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 effects on 
the characters of the organism, do occur ; but their fre- 
quency is not such as to make necessary 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. 



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 aTgiven 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 concej^tion 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 presv/mably are members of one inbred 
line. 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, 



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 redwctio 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, iii 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 systems 
of mating have been devised by Pearl by the use of a 
Coefficient of Inbreeding and a Coefficient of Eelation- 
ship.*- The first is a measure of the actual number of 

a Pearl has made a somewhat more precise analysis of the Inbreeding 
and Relationship Coefficients in later papers, 175, 176, and has suggested 
a Partial Inbre^ing Index, in a percentage which one-half of the Relation- 
ship Coefficient is of the Inbreeding Coefficient. This constant is a measure 
of the amount of inibreeding due to relationship between the sire and dam. 
Further, he. has described a single numerical measure of inbreeding for 
bisexual organisms, in the ratio of the area of the inbreeding curve in any- 
pedigree to the area of the maximum (brother X sister) curve. 

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


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

lOO/p -q ) 
V n + l n+1/ 

n p 

n + l 

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

x-^-^d) 2-«— ?- (2) 4-t-> (3) 8^— ^(4) 16<— > (5) 32-«— > (re)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, ia 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 

*-^-^ (1)2 -«-^ (2)4-2/1 <-^ (3)8-2/2 -t-* (4)16-2/3<-^ (5)32-2/4. . ., 

where «/i = 2, y.^ 6, y^ = 14, /y^ = 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- 


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

Z(, = 100 (2-2): 



Z^ = 100 (4-2) : 

= 50 

Z^ = 100 (8-2) : 

= 75 


^3 = 100 (16-2) = 

= 87.5 


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, untU 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. 



Two individuals may have the same Coeffioients 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 



O 40 







// 5/ 









« 8 10 



FiQ. 22. — Curvea 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 id) continued first cousin X first 
cousin mating where the cousinship is single (C^ XCO- 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 Relationship, 
K^ which is essentially the per cent, of the individuals in 


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 
froin 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) " Coefficients 

King Melia Rioter Mth and Blossom's Glorene 


^0 (K,) 




z, (X.) 





z^ (£,) 






z. (^J 






z^ (^J 






Z, (K,) 






Z, {K,) 






Z, (K,) 





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 inbreediag 
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 



reaoli the possible number, and by specifying tbe location 
in the series of the generation under discussion." King 
Melia Eioter 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 Grlorene had nearly 
60 per cent. From these figures it is evident that King 
Meha Rioter 14th is a much more inbred animal than 
























Fig. 23. — Graphs showing (a) the total inbreeding (heavy solid line) and (&) 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 Bister and parent X offspring breeding. (After Pearl.) 

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 


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 coeflScients 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 Pearl's 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 with a Jersey 
bull, the product of a similar system of inbreeding {K = 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 1^=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 applying 
the laws of probability to Mendelian formulae. 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 


inbreeding, and some idea of the composition of the indi- 
vidual may often be bad 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 allelomorphic 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, 


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 Mendelian phe- 
nomena, and its expression is roughly proportional to the 
number of heterozygous factors present and disappears 
when homozygosity is brought about. 

The reduction of the number of heterozygous allelo- 
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, heterozygotes 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 heterozygotes, the ratio is 2" -1 : 1. Of 
course, the matter is not quite so simple when several 



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


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

Segregating Generations 

Fig. 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- 


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 teinn 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[P (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 aUelo- 
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 aU 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- 


lation as a whole where any niiinber 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 nuniber of heterozygous fac- 
tors at the commencement of inbreeding iacreases the 
more nearly will the reduction to homozygosity follow the 
curve shown, because the chance of choosing a completely 

6 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 Remiek (213), Robbing 
(186, 187) which are useful in predicting the character of inbred genera- 
tions when certain conditions are fulfilled. 



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


The Theoretical Ntimbeb and Ratio op Individtjals in the Ciasses of 

DiFPEKENT Degrees op Het'erozygosity, After Recombination, 

WHEN Fifteen Mendbijzing Units Are iNVOLviaj. 

The total number of 

Ratio of indi- 
viduals in the 

The number of 
factOFB in re- 

The total number of heterozy- 

individuals in all 

classes with 

Bpect to which 

gous and homozygous factor 


thepoasible Mende- 

different num- 



pairs in all the individuals in 


lian recombinations 
in F2 when 15^fac- 

ber of hetero- 
z y g u s and 



each class: 

tors are involved. 

h m z ygpus 

factors - co- 





efficients (a-(- 



factor pairs 

factor pairs 
























































































" 1,365 














' 15 






















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 with 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- 


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 grovm 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 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 



generations it is 99.9' per cent, ajid after 19 generations 
99.999 per cent. 

Althougli nearly complete homozygosis is theoretically 
brought about by 7 generations of self-fertilization the 
attainment of absolute homozygosity is a difficult matter 
and in practice it may never be reached. The fact that 
hereditary factors are distributed by the ehromosomes, 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, 33 V{ 
per cent, "crossovers") in both sexes ; and also when the 
linkage is complete in one sex, as in Drosophila 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 

Per cent, of complete homozygotes . 
Per cent, of complete heterozygotes 
Per cent, homozygous in one pair but 

not in other 

Per cent, homozygous factors 

Per cent, heterozygous factors 

Two factors 



Linkage ratio 
2 in both sexes 




complete in 

one sex, 2 in 





It will 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 iu 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 wUl vary more widely 
between dfferent lines if the factors involved are par- 
tially linked than if they are all iadependent. 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. Bnt 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, 


since the heterozygous individuals are more vigorous than 
the homozygous, selection either unconsdous 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 AA 1/4 3/8 7/16 1/2 AA^ =.492 

Parent type aa Q 1/4 3/8 7/16 1/2 aOg=.492 

Hybrid type 4a 1 1/2 1/4 1/8.. Aa^=M8 

In brother and sister mating the procedure is 
as follows : 

Generation F^ F^ F^ F^ F^ 

Parent type 4-1 1/4 2/8 5/16 11/32 1/2 44„ =.490+ 

Parent type (W 1/4 2/8 5/16 11/32 1/2 ao„=.490-H 

HybridtypeAa 1 1/2 2/4 3/8 5/16 Aa^^=.W9,+ 

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 matiugs in bringing about homozygosis. 

Of parent by offspring matings there are several 


methods ■which may be carried on with different results. 
If the individuals are mated at random to all the diif er- 
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 maintain 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 inbreeding 
is thus due to a reduction of the genetical variability. 
Inbreeding affects physiological or developmental vari- 


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

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 wiU also follow 
the same course. Some cases are to be expected in which 
stability is reached earlier, and some cases in which it is 
reached later; or, theoretically it may never be reached. 
With these points ia mind, let us see what are the actual 
results of long-continued inbreeding. 



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 stiU 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 


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 naay 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- 
•win '^' ^* was the first to 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 were 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."»' "«- ^^i The first 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 


young, oMained by crossing an albino female with a white 
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 in litter 7.5 7.1 7.1 6.5 4.2 3.2 

Per cent, infertile 

matings 0.0 2.6 5.6 17.4 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 


feel that both of these experiments sHould be disregarded. 
Each was started with hybrid stock, and such experi- 
ments with hybrid stock bring in an additional 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 difficulties 
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 Fj 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 Detlef sen *'' 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 
East'^ on the genus Nicotiana show conclusively that 
various hereditary factors are involved in the partial 
sterility exhibited in many species crosses, and that these 
factors may be expected to recombine in the usual manner. 


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-year 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 
jhis 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 Fj 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 


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 modem 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."9- i^"' 121 It was 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 B, were carried on for twenty- 
five generations by matihg 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 


attributed partly to selective eUmiiiatioii 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 



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

Fia. 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 generations 
inclusive; B, graph for the males of the tenth to the twelfth generations inclusive; C, graph 
for the males of the thirteenth to the fifteenth generations inclusive; D, graph for the males 
of the first 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 


curve ^ if 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 

d 20 40 00 80 ioO \ZD 140 160 ISO 200 220 .240 260 280 300 320 340 360 380 400 420 440 460' 480 

Fig. 26. — Graphs"" showing the increase in the weight of the bodyf'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. (After King.) 

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 la fertility as is evident if one plots the theoreti- 
cal curve which fits the experimental curve for litter size 



througliout 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 6.7. 
Further there was a notable increase in longevity in line 
A and a marked increase in line B. 

Average Size of Utter 



T^n t n I I I I I I I I J I r 111 III 

10 II 12 13 14 IS IS .17 IB le 20 11 22 23 2* 2S 

Fia. 27. — Graph showing the average size of litters produced in successive generations of 
inbreeding albino rats by brother and sister 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 


fertile, and longer lived than many strains of stock rats. 
It is clear that this was the result of Mendelian recom- 
hination 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 Sewall Wright, who has made a very illumi- 
nating 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- 

S a 

1-1 c» 



ably with the original stock; others degenerated so rap- 
idly that they soon became extinct in spite of every effort 
to prevent such a catastrophe. Among the families still 
in existence, there is even evidence that vigor as a general 
term may be divided iato various causative factors and 
that these factors may be combined in various ways. By 
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 arei many hereditary factors which affect 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, Drosophila melanogaster, 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 


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 

Geaierations 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 


per day, rate of growth, and variability. The proper in- 
terpretation, of their results is somewhat obscure, unless 
one hypothecates the origin of frequent mutations. The 
number of generations bred and the number of families 
under observation were not sufficient to demonstrate the 
segregation of differences in these characteristics, though 
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 parthenogenetio ifiultiplication 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 inBre^uig depend more upon 
the genetic composition of the individuals subjected to 
inbreeding rather than upon any pernicious influence 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 inheri- 
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 



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 imcertainty 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 
nothtag in vigor, productiveness or ability to survive. 
Among wild plants many species of the family Legumi- 
nossB, 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-feriilized 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." Darwiu, 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." 


Darwia compared self-fertilized plants with inter- 
crossed plants in many different species. In the 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 inbreeding kept on accumulating until 
eventually a plant or animal continuously reproducing in 
that manner was doomed to extinction. His own experi- 
mental results came far short of proving such an assump- 
tion, however. The two plants with which inbreeding was 
practiced the longest — Ipomea and Mimulus — 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 their 
ability to grow as well as in minor, visible, hereditary 
characters. In both species plants appeared which 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, Darwin 
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 the fact 
that the plants were grown for several generations under 


the same conditions, but it is easy to see why lie 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 Dirnithus, 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 

HeigM, coimipared 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 
ia 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 


uniform conditions, does not benefit the offspring in tlie 
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 plants 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 solu- 


tion of the problem of inbreeding was lacking. Mendel's 
work was yet unrecognized ; the principles of inheritanoe 
of separate cbaracters, 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 Gt. H. ShuH and of East, 
undertaken independently in 1905 with maize, an ideal 
cross-fertilized species, as the subject. 

Shull's 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 
self -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 Marrow and Gardner ^^^' ^^^ and 


Sliamel ^®=* ; but the conclusion wMcli lie drew was new. 
The universality of this decrease in vigor was to ShuU 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, ShuU noted that this decrease in size 
and vigor accompanying self-fertilization, instead of 
proceeding at a steady or even at an increasing rate as 
might be expected from this older view, actually became 
less and less in succeeding generations — ^presumably in- 
dicating an approach to stability. The neatness with 
which these observations fit a Mendelian 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 niay be 
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, morphologi- 
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. Obviously crosses between plants 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 



between distinct self -fertilized lines should often result 
in high-yielding Fj 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 Fi generations, should become 
much more variable and much less vigorous in the F^ 
generation. These general propositions Shull tested in 
a limited way in 1910 after his families had been self -fer- 
tilized for five 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 : 



Coefficient of variation 



8.60 per cent. ± 

.47 per cent. 



9.66 per cent. ± 

. 74 per cent. 

■ AXB (Fi) 


10.00 per cent. ± 

.87 per cent. 



8.13 per cent. ± 

. 42 per cent. 

AXB (Fz) 


14.64 per cent. ± 

.67 per cent. 



10.62 percent. ± 

. 56 per cent. 

Clearly the F^ generation, made with either type as the 
mother, is as uniform as the parent strains, but the Fo 
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 



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

Average height . . 
Average No. rows 
Average yield 



X Sibs 








Fi Sib 






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 ShuU states, of some heterozygosis still remaining in 
the selfed families ; in other particulars Mendelian 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 



there has been no exception, the several inhred 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 : 

Coliored and colorless pericarps, cobs, silks and iglumes. 
Profusely branched tassels and ecantily branched or unbranched 

Long ears and short ears. 
Bound cobs and flat cobs. 
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 


production — ^though with some difficulty. Some plants 
are obtained with ear malformations and thus produce 
but a minimum amount of seed. Other plants lack brace 
roots and are unable to stand upright. Still others show 
various 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 uniform 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 noticeable. 

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, height of plant 
and other maize characters. 

The original experiment began with four individual 


plants obtained from seed of a oommercial variety grown 
in Illinois known as Learning Dent. This variety was 
given the number 1, and four plants which were self- 
poUinated 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 Learning 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. 

The Effect op Inbreeding on the Yield and Height of Maize 

No. of 

Four inbred strains derived from a variety of Learning dent corn 







bu. per 




bu. per 



bu. per 




bu. per 


i9i 445.4 









' 86.7 








' 'si.'i 





■ '96.5 





' 96.0 

' 97.7 







78 7 







In Table III the yield of grain and height of plant of 
the fonr inbred Learning strains are given in the suc- 
cessive generations of self-fertilization. In 1916 seed of 
the original variety, which had been grown, 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 sheUed 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 intei-^ening 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 diflBcult 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 haying been used for hand pollinations. 


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. 

AH along the several Leaming strains have shown 
considerable differences 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 corn-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 


view of the further reduction iu other strains, it would 
have been extremely difficult. 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 amoimt of poUen 
produced. These infertile types are dependent on other 
plants for poUen 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. 

From 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 that 
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 signifieant 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 that the reduction in 
vegetative vigor and productiveness is very 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 


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 original 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 coefficdents 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 readUy 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- 



1— » h-* V-* H* 

03 OS 05 03 

1 (Original 
1 non-inbred 
1 Learning 
1 variety) 









bo to to to 


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pletely homozygous types by seK-fertilization is greatest 
in the generations from the third to the sixth if su large 
numbet 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 imable 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 ori^nal 
conditions. The greatest segregation has taken place 
between the first and the eighth generations. In the eighth 
genera,tion the lines were again split up, but show no 
marked change after this point. Differences in the ears 
of these iiibreii strains of com are shown in Fig. 29. 

The rate of reduction in variability aud 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 lines 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 

Fig. 29. — Representative samples of inbred strains of maize after 11 generations of self- 
fprtilization ghowing characteristic diffprpncep but uniformity within e&ch strain, 



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I I 



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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- 


d C o 
> UK 

Ave . No . Rowa 
of 1-7-1-1 

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


— ) 1 1 1 

8 9 10 11 

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 


apparent in details of plant and ear structure which are 
difficult of statistical expression. The beautiful uni- 
formity 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 characters in accompany- 
ing illustrations (Fig. 29). In the minutiae 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 adequately, but 
are perhaps the most noticeable feature about them. 
The tassels or the ears of all these four Leaming 
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 einbryos 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 lines have 
been obtained which produce these peculiar seeds as a 
common feature. One of the straias shows 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 observed to bear 
them. Here, then, is a character which does not appear 
except at rare intervals when the plants are crossed 


and in full vigor. Wlien 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 aU. 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 single 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 shewed 
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 into 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-inbred varieties the inbred plants 
are more difficult 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 


constant hereditary factors. Some of the characters 
which appear after long-contiaued 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 characters which 
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 practiced 
throughout these inbreeding experiments, a great deal of 
selection upon many characters has been unavoidable as 
is the case in any inbreeding experiment. lii 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 which are weak, sterile, dis- 
eased or in any way abnormal, tend to become eliminated 
wherever these causes reduce the chance of obtaining seed. 
This unconscious selection becomes more rigid in the later 
generations of inbreeding as reduction in vigor and pro- 
ductiveness becomes more pronounced. Again, the small 


amount of seed produced by liand 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 Learning com which have 
been continuously self-fertilized for twelve generations 
are now very nearly, if not completely, homozygous in aU 
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 stUl 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 


futile to maintaiii that there is every reason to suppose 
wild species should behave exactly as tlieir domestic 
cousins. Wild types, in general, might not present such 
an appearance of injury under inhreeding as is often 
shown 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 the individuals 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 been made 
this is the case. Castle's experiments on the fruit fly gave 
no markedly unfavorable results. Collins states that self- 
fertilizing teosinte, a semi-wild relative of maize, causes 
no loss of vigor. Tet Darwin compared self -fertilized 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 argument. 

From, the precedmff observation it can he said that 
inhreeding has hut one demonstrable effect on organisms 
subjected to its action — the isolation of homozygous 
types. The diversity of the resulting types depends di- 
rectly upon number of heterozygous hereditary factors 
present in the individuals with which the process is he- 


gun; it is lihely, therefore, to vary directly with the 
a/mount 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 thisi 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 ohameters which become homozygous will be 


recessives or combinations of recessives which seldom are 
seen under ordinary circxmistances, because they are hid- 
den by their dominant allelomorphs. These recessives 
are the "corrupt fruit" 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 iubreeding 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 eAdl effects are apparent. 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 we undertook to inquire 
was whether inbreeding' is injurious merely hy reason of 
the consanguinity. "We answer, No! The only injury 
proceeding from inbreeding comes from the inheritance 
received. The constitution of the individuals resulting 
from a process of inbreeding depends upon the chance 
allotment of characters preexisting in the stock before in- 


breeding was commenced. If undesirable characters 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 condenmed 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 inunediately regained by crossing. Not only 
is the full vigor of the original stock 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. 


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 related forms 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), BerthoUet (1827), 
Wiegmann (1828), Herbert (1837), Lecoq (1845), Gart- 
ner (1849). In fact, in Focke's compilation of this early 
work, "Die Pflanzen-Mi&chlinge" (1881), cases of heter- 
osis worthy of special mention were found in fifty-nine 
families of the flowering plants as well as in the conifers 
and the ferns. Animal husbandmen were somewhat less 



inclined to acknowledge and discuss the matter, althougli 
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/25 ^jjg gj-st 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,^'''' a 
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 powere of the male and female in their effects on iiie 
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 


experiment, a striking instance of the sitimjulating effieots of crossing the 
breeds; for the smallest variety, whose height rarely exceeded two feet, 
was increased to six feet, whilst the height of the large and luxuriant 
kind was very little diminished. 

It is evident that La this particular case Knight was 
dealing with dwarf and standard peas, and dominance of 
the tall 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 sufficient number 
of really striking manifestations of the phenomenon were 
found to give adequate foundation for his anti-inbreeding 
principle, elaborated by Darwin fifty years later. 

Probably the most extensive series of early experi- 
ments on hybridization were those of G-artner.''* 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 250 
different hybrids. Many of his attempted crosses either 
f aUed 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 conspicuous and common characteristics of plant 
hybrids is the luxuriance of all their parts, a luxuriance that is shown in 
the rankness of their growth and a prodigal development of root shoots, 
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 planted in the open, as 


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 steans. In vartooos 
hybrids of the genus Verbaseran, for example lychnitis-fhapsus, the stem 
shoots up 12 to 15 feet high, with a panicle 7 to 9 feet, the six highest 
side branches 2 to 3 feeit, and the stem 1 1/4 inches in diameter at the 
base: in Althaea cannabino-offieinalis the stem is 10 to 12 feet; in 
Malva mauritano-sylvestris 9 to 11 feet; in Digitalis purpureo-ochroleuca 
8 to 10 feet, with panicles 4 to 5 feet; and in Petunia nyctagimfloro- 
phcenicea and Lobelia cardinali-syphilitiea 3 to 4 feet each. Prof. 
Wiegmann lalso 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, Lycium, Lychnis, Lobelia, Geum, and 
Pentstemon hybrids, put forth stalks easily and therefore are readily 
propagated by layers, stolons, or cuttings. The observajtions 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-fios 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 branohes, particularly their length, 
nevertheless the leaves take part in it by 'becoming larger. Hybrids in 
the genera Datura, Nicotiana, Tropseolum, Verbascum, and Pentstemon 
are examples. 

Naudin/^" the contemporary of Mendel, whose ideas 
very nearly resembled modem 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. 


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

The longer of the two parental stems is usually exceeded by the 
hybrid, a fact which is possibly only attributable to the greater luxuriance 
which appears in aU parts of the plants when stems of very different 
lengths are crossed. Thus, for instance, in repeated experiments, stenus 
of 1 foot and of 6 feet in length yielded without exception hybrids which 
varied in length between 6 feet and 7 1/2 feet. 

Focke,''" in the book already cited, gives the results of 
a series of experiments nearly as extensive as those of 
Gartner and catalogues his own results along with those 
of his predecessors. The compilation is so careful, so 
painstaking, and so complete that one may turn to the 
final 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 shown 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, but they also noted 
a great number of instances in which crosses between 
closely related forms, such as varieties or sub-varieties 
of cultivated plants, gave remarkable increments in 



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 hst 
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 floral structures as adaptations for cross-poUination. 
So zealously was this luie 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-poUination. 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 
oases out of 37, cross-fertilization increased the height 


of plant; in 5 out of 7 experiments, the -weight was in- 
creased. Moreover, the crossed plants 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 different 
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 are 
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 evidently considered the 
ill effects of inbreeding and the good effects of crossing to 
be two different things. He was right in stressi'ng 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 


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 
invariably self -fertilized. Crossing within a pure line of 
such a species shows no heterosis; but if the parents 
united in the cross differ more or less in minor external 
features an increase in growth is usually to be ex- 
pected. This has been shown to be true for peas, toma- 
toes, tobacco and many other normally self-felrtilized 
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, whUe occasionally the increased development pf 
sterile hybrids may be due in part to their having ex- 
pended rio 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 hrasilia Comes and A^. rustica scdbra Comes. 


The frequency distributions of height of plant of the two 
parents and the reciprocal hybrids are given in Table VI. 


Hhiqht of Cbosses Between Nicotiana Rustica Scabra (352) and 

N. Rustica Bbazilia (349) 

Variety or 

Class canters ia inches 


27 30 33 36 39 42 45 48 51 54 57 60 








352X349, Fi 
349X352, F, 


10 22 14 7 

2 1 5 11 16 17 6 

13 5 

3 5 










In both of the first hybrid generations the average height 
is above the major extreme of either parent. Similar 
iucreases 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 different 
parts of the world — that is, exposure to different environ- 
mental conditions — ^has no necessary relation to the 
phenomenon of hybrid vigor, for such individuals may 
be alike in constitution. Darwin'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 that it is differences 
in germinal construction which bring about hybrid vigor ; 
and this is the principal point at issue. 


The- 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 intemodes. 
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 ceUs, as well as by a multi- 
plication in the number of cells. 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 ia 
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 


« £. 

2 3 



a return to the condition of the origiaal stock before in- 
breeding was commenoed. 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. 1-6 has been used as the 
female parent have regularly given the highest yields, and 
this straia 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 (Fig. 
33), but soon catches up and passes the second generation. 
At maturity the second generation is shorter and less 
productive, although it has a much greater variability. 
The third generation from self ed 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 genera- 
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 homozygosity was again 






Growth Curves of 
Two Inbred Stra-ins 
of Maize and Their 
and F, Hybrids. 

30 40 50 60 70 80 

number of Bays from Planting 



FzG. 33. — Graphs showing growth curves of two inbred strains of maize and their first 
and second generation hybrids. 


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 different 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 transitory 
effect, for the most part, impossible of fixation. 

Increases in yield of grain are also frequently 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 type 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 yield by hybridizing types of com from different 
geographical regions. Three different varieties of com 
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 plant's 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 outcrossed seeds 
could be distinguished by differences in endosperm color. 


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 self ed 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 efficiency 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. 

Fig. 34. — James River Walnut, a famous tree considered to be a natural hybrid be- 
tween the Persian walnut 'and the common butternut. According to C. S. Sargent, director 
of the Arnold Arboretum, this is the most remarkable hardwood tree in the United States; 
height 166 feet, spread of branches 134 feet, diameter of trunk 31 feet. (From Bisset) 


and G-artner ''* gives them his especial attention. Under 
the heading, "Ansdauer und Lebenstenacitat der Bas- 
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 of the same species have 
a longer life at times as, for example, Draba verna which has both 
annual and; biennial forms. The longevity of a plant thereby furnishes 
no specific difference but at most only signifies a variability. However, 
in hybrids this difference deserves special consideration. In most 
hybrids an increased longevity and greater endurance caoi be observed 
as compared to their parental races even if they come into bloom a 
year earlier. The iinion of an annual, herbaceous female plant with a 
perennial, shrubby species does not shorten the life cycle of the forth- 
coming hybrid, as the union of Hyoscyamus agrestis with niger, Nicotiana 
rustica with perennis, Calceolaria plantaginea with rugosa shows. So 
also in reciprocal crosses when the perennial species furnishes the seed 
and the annual species srapplies the pollen, as Nicotiana glauca with 
Langsdorffiij Dianthus cdryophyllus with chinensis; Malva sylvestris 
with ma/uritiana or biennials with perennials and reciprocally, aa 
Digitalis purpurea with ochroleiica or lutea, and Iwtea with pitrpurea, or 
oohroleuea with purpurea. From the union of two races of different 
longevity a hybrid usually results into which 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 Kolreuter that the 
longer Me 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. Gernert '^'' states that teo- 
sinte and the first generation cross of teosinte and maize 


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. Eadish seedlings 
which were naturally cross-fertilized were much less dam- 
aged by damping-off fungus than uncrossed seedlings 
from the same plants. Eesistance is not shown by all 
first generation hybrids when the parents differ in suscep- 
tibility. Some cases are known in which the hybrids are 
fuUy 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 self ed 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. 


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 above the average of the parents in both 
height and leaf size. The length of the corollas, on the 
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. Grenerally speaking, favorable growing conditions 
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 parts and a de- 
pressing effect on others. This is shown in many species 
crosses in which reproductive ability is greatly reduced 
or even totally eliminated, while at the same time vegeta- 
tive growth is enormously increased. Freeman and Sax 
have independently obtained se'eds from crosses between 
common bread wheats and macaroni wheats which were 
shrunken in appearance and snaall in size, owing 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 beneficial to 
animals ; but animals do not furnish as desirable research 


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 2<") . In the fruit fly, however, upon 
which the greatest amount of genetic work has been done, 
Castle,2i Moenkhaus,"* Hyde^^ and MuUer"* 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 2^® and A. F. ShuU ^®^ 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 


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

The greatest amount of data on this subject, just as 
there is the greatest amount of interest, has been obtained 
from the mammals. In the meat breeds of cattle, swine 
and sheep, as ia 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 obtained 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 specific 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 and the zebu also 
give animals of considerable merit, and one can hardly 
refrain from thinking that within a few years some con- 
siderable use will be made of them. 

For precise data on the effect of crossing different 
races, however, we must turn to the small mammals used 
so constantly in experimental work, the mouse, the rat, the 



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 without question that the 
effect of crossing on animals is the same as upon plants. 
























Age in Days 40 


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


smaller than the first generation, hybrids from birth on, 
showing that some of the growth impetus produced by the 
hybrid-ization had not been retained. But 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 iucrease 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 expected, 
for ordinary races of mammals are continually being 
crossed within the variety and, therefore, hybridization 
would not be expected to increase heterozygosis to any 
marked degree. But results similar to those obtained in 
plants may be expected if the genetic conditions are sim- 
ilar. This is proved by the data Wright obtained when he 
crossed guinea-pigs bom of unrelated inbred mothers and 
fathers. The cross-breds were distinctly superior to their 
inbred relatives in. nearly all characters connected 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 birth 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. Briefly, in 
crosses which are fertile the effects are such as to con- 
tribute to a greatly increased reproductive abUity, making 


possible a larger nuinber 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 genmnal consti- 
tution whether the organisms crbssed 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. 


A somewhat similar series can be arranged with animals, 
although usually in wide crosses if the hybrids can be 
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 they may not be the result of the 
operation of one single law. 




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 



proved beyond question the great advantage to be gained 
by crossing even when the individuals themselves were 
weak. These facts, taken together with the many mar- 
velous and intricate contrivances of plants to insure 
cross-pollination, led him to believe that self-fertilization 
was inherently harmful and something, to be avoided if 
possible. The benefits accruing from crossing he ascribed, 
as we have seen, to the meeting of sexual elements having 
diverse constitutions. 

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

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

Shortly after Mendel's experiments became known, 
Bateson crossed two pure white flowered varieties of the 
sweet pea. Instead of having the white flowers charac- 
teristic of two parental races, the hybrid flowers were 
purple. The wild progenitor of the sweet pea has purple 
flowers. Here was a case in which crossing brought back 
previously existing conditions, a return to the wild type 
characters. This phenomenon had been observed long 
before this time; in fact, it was so well known it had been 


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 Mendel's 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- 
naUy 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 imit factor, as there is to the degree of stability of 


the atom, the principle of the pure Une has heen firmly 
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 that 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 sterile 
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 shown in small numbers in 
a cross-pollinated field of maize, but not in suflScient fre- 
quency to reduce productiveness seriously. As the result 
of self-fertilization some of the strains obtained 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 effi- 
ciency of the plants which possess them. But 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. 


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. Thisi hypothesis 
(East and Hayes, ^^ ShuU '**) 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 plg,sm al- 
lowed them to appear. Inbreeding was in ieffect 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- 


ing and outbreeding with the phenomena of Mendelian 
heredity was a great step forward. It went 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. Bnt it held 
some disadvantages. The assumption of a physiological 
stimul'ation arising from the interaction of different 
hereditary factors was not altogether satisfactory, for it 
looked 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 superseded. 

The basis for this hypothetical stimulation was seen in 
the fact that fertilization is usually necessary to start the 
development of the egg. In most oases, without the union 
with the sperm, the egg cannot divide and development 
is prevented. The reaction of the different substances 
brought together at fertilization stimulates cell division 
and starts development. This made it reasonable to as- 
sume that when the egg and sperm differed in hereditary 
factors stimulus to development was increased and con- 
tinued throughout the growth of the resulting organism. 
According to the view of G-. H. ShuU and of East, 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 sperm 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 wonld still remain to react with the cytoplasmic 
matter. Moreover, it was further assumed that the stim- 
ulation might decrease in long-continued asexual propa- 


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 intemodes 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 
intemodes. These factors they designated T and L. One 


of the parental varieties was medium ia height, because 
it possessed one of these factors ; e.g., that for thick stems, 
but lacked the other. Such a plant had the formula TTll. 
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 tt LL. Both of these factors 
showed dominance over the aUelomorphio 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 aU 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 


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 is 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 accuniulated 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 symmetrical 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 


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 chronaosome 
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 deteiTnined, 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 


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 maximwm 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 
v'hy the effects of heterozygosis result in an increase 
in deivelopment, 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- 



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 









* B B C C a' if 8" tf C (/ 

































X X Y : 12 







A / 8 8' C (^ 



















Via. 36. — To show hoir f aotors contributed by eaoh parent may enable the first generation 
of a croBB 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 i aJihe haplo id as in the diploid co n:, 
dition ; in other words, they show perfect TJominance over 
their absence. It will be seen from the diagram that when 
these individuals are crossed together the progeny de- 


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


Composition of a Mendblian Thi-Htbeid in Fj Where the Development 
Which Each Individual Attains Depends upon the Number op 
Heterozygous Chromosomes Contained and Thereby upon the 
Total Number op Different Factors Present. 

Number of individ- 

Contributions of 


uals in each 


each chromoBome 












A A' 

B B 

















A A' 












A A' 






A A' 


















A A' 






A A' 


















A A' 





4 ■ 

A A' 




























































A A' 











64 Total 


Distribution of the 

F: individuals 


to the 

development attained 



.. 6 
.. 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 


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, 
respeotively. In other words, the distribution is sym- 
metrical, and this sjonmetry 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, 



rarely occurs, and even wlien it does occur, it may 
be merely an appearance rather than a reality. The gen- 
eral consensus of opinion 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 eharacter 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- 
jond 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 aU degrees, true albinism is recessive to the 
normal condition. In maize, the heterozygous green plants 
cannot 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. 


Certain factors have even been recognized, and in tlie 
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, iJie 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 difficult 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 


favorable to tlie 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. Eecessive 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 pooling of 


3 S, 



(Tl t» 

fts 5 


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

This complementary action can be illustra.ted by as- 
suming three linked factors, aU of which are essential for 
best development. In one organism there is AbC; in an- 
other there is oBc. Domitiance, either partial or complete, 
is charaoteristic of each. Now in the former iaterpretation 
of heterosis, where a physiological stimulation was as- 
sumed, the heterozygous combiaation, 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 
wiU 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 
inbreediug, and the extreme rarity of homozygous com- 
binations approaching those of the first hybrid generation 
in vigor. With a knowledge of independent Mendelian 
heredity only it was necessary. But if in our illustration 
the individual AhC-aBc is vigorous because of heredity 


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 stiU 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, wiU 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 iu 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 aU 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. 


Our hypothetical illustration 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 weU-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- 


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 inherita^nce 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 melanog aster, 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 id), gray body {B) dominant to black body 
(&), red eye (P) dominant to purple eye ip), and normal 
wings (F) 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 6 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. 


Now if a female fly with dachs legs, gray body, purple 
eyes and normal wings (dBpV), be crossed witb a male 
baving long legs, black body, red eyes and vestigial wings 
(DbPv), the resulting progeny will have the usual wUd- 
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 e:spect to find one gamete out of every sixteen to be 
of the constitution DBPV, and would obtain one F^ 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 F2, 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 Fg, 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 small 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 


loosely linked factors, except when long periods of time 
are available and when natural elimination 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- 


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. 



Peobably 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 ^^^ with guinea-pigs are all thus characterized. 

Miss King/^* on the other hand, has inbred albiuo 
rats for twenty-five successive generations by brother and 
sister mating without any appreciable reduction in. fer- 
tility. Similarly, Oastle^^ and his students maintained 
the fertility of Drosophila for fifty-nine generations of 
brother and sister mating by breeding 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 



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 shrunken, some- 
times shedding no poUen 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 
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 influencing 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 when 
it occurs gradually increases upon inbreeding until homo- 
zygosity is attained, but for the most part it does not show 


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. Eecessive 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- 


ring. In other words, there is here a concrete illustration 
of the primary 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. Eepro- 
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. 
Lcmgsdorfjiij for example, are distinct species having dif- 


ferences in many charaoters, 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 
rampaat 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 
americanus; the yak, Bibos gruniens; the gayal, Bihos 
frontalis; the gaur, Bihos gaurus, and the domestic cow, 
Bos taunts. 

(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 
Nicotia/na tabacum and N. panicidata, 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, accom- 
panied by sterility, sometimes obtained in wide crosses. (Gravatt, in Jour. Heredity.) 


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 first 
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 sterility. 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 wide 
crosses, however, there is an excuse for meddling. The 
peculiar resemblance of the effect of inbreeding to the 



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 
individurals thus characterised 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. 



Ik 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 reproduc- 



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 physiological 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- 
zygous characters possible. We have examined manv 
populations of this character in the genus Nicotiana and 
have been astounded at the extremely narrow variability 

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

(From Emerson.) 


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 coeffi- 
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. Biit 
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 


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 spedes, 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 the 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 parthenogenetic 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 


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 improhable. 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 diaracter 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 wiU 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 sexusd 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- 


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 '^® suggests that the cyto- 
plasm of the egg is roughly the potential embryo and that 


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 
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 
as functional bisexuality was restored by self -sterility, 
protandry, protogyny or mechanical devices which pro- 
moted cross-fertilization. The prime reason for the suc- 
cess of sexual reproduction, then, as Weismann ^"9 first 
maintained, though he knew not the exact reason, is the 
opportunity it gives for mingling germ plasms of different 
constitutions, and thereby furnishing selective agencies 
many timesthe 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 


qualities unquestionably having surAdval 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 fiEict, 
nothing similar to heterosis has been found in unicellular 
organisms. The lowest type where distinct evidence of 
the phenomenon has been discovered is in Trochelminthes, 
cross-fertilization increasing size, vigor, viability and re- 
productive raite in rotifers. B^ut it would be strange 
indeed if no such effect did occur in low forms when it is 

Fig. 40. 

Fig. 41. 

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

unif ormi ty. 

Fig. 41. — Plants of the first generation cross between the same two inbred strains of maize. 

Note the uniformity in height. 


Fig. 42. — Diagram showing a method of double crossing maize to secure maximum yields, 
illustrated by actual field results. 


sa widespread in all the higher plants and animals. Herit- 
able variations are constantly arising in simple organisms, 
as has been demonstrated by Jennings ^°'' 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 sam,e chance of bringing together the greatest 
number of favorable growth factors, and the progeny 
would thus be benefited, even though the mechanism for 
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 endosperm 
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 the 
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 embryo 
and the resulting plant may be greatly benefited by cross- 
fertilization, so also is the endosperm tissue increased in 
amount as a direct manifestation of hybrid vigor. 

Nemec^*'^ has 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- 


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 equial 
importance. It may well be that to fill either purpose 
endosperm hybridization has sufficient value to account 
for its maintenance in the anglosperms. 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 


applicable here. Eecombinations 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 


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 because 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 aU 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 


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

A secondary 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 diffioulties 
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 shown 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 


improbable. Surgical castrations of insects which have 
not affected the secondary sexual characters even though 
rnade in the early larval stages, are not conclusive because 
of the possibility of the primary sex organs having a 
marjied 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 presomiable advantage in bisexual 
reproduction in having sex-linked characters. We say 
presumable advantage, for all of the relationships be- 
tween sex and sex-linkedi 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 ah 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 othe^r lack- 
ing it. The eggs are all alike, each bearing the sax 
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 


very great number of interesting tilings 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. 




The origin of our more important domestic animals 
and cultivated plants is a matter on which, there is no 
direct evidence. Among animals the ostrich is the only 
example of modem 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 them one wild 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 germinaUy 
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 dr aft hors es as an 
example. In the early days of Europe native breeds were 
developed in every country for military purposes. Just 
210 ~~" 


how they originated we oaiuiot 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 Scotland, 
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 the 
JBarb 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 modem 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 homed, 
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 oould be given 
about swine, sheep, dogs, cats, the cereals, the perennial 
fruits, the numerous floricultural novelties, but this would 


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 efS- 
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 difficulty 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 modem trotting horse and saddle horse 

"' ■'■'^■^'' 

Fig. 43. — First generation cross resulting from a pure bred Shropshire ram mated 
with grade Delaine-Merino ewes. First prize cross-bred yearling ewes at the International 
Livestock Exposition in 1917, (From Severson.) 


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 f oun^ 
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. Gentry of Sedalia, Missouri, 


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 well 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 past." 

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, Earely 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 which is left to regulate itself automatically. 


Among those plants which are largely self -pollinated by 
nature, chance crossing, or, in some oases, 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 innumerable, 
adapting the plant to a range of conditions from the edge 
of the Arctics to the Tropics, throughout the world. 

In every 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, 
Leaming, 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- 


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 types 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- 

Fig. 44. — "California Favorite," first generation cross between a Hereford and Short- 
horn; grand champion steer at the International Livestock Exposition in 1916; considered 
to be one of the finest steers ever exhibited. (.From True.) 


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 with 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 recombiaation 
into a new and better variety, which could be maintained 


by seed propagation, -would be a comparatively easy 

"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 undoubtedly owe their supe- 
riority in large measure to a fortunate combination of 
many different characters so made as to obtain the maxi- 
mum effect from hybrid vigor. Attempting to obtain 
further improvement by crossing these already widelj^- 
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. Eesults 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 greiater 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 


or no value in themselves solely for a possible ultimate 
improvement might well be too discouraging an under- 
taldng. 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. 

While 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 
country ait the time. The production of mules has increased at a more 
rapid rate than horses, and the use of mules is becoming more exten- 


sive. The mule hybrid is a remarkable example of the practical ad- 
vantages 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 mtde 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 dangers 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 century, 
stated that crosses between the English and Chinese 
breeds of swine 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 
win 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 Rocks give birds which are not so apt 
to become over-fat and yet are more valuable for meat 
than the smaller Leghorns. 

The opportunities for improvement in this way 

Fig. 45. — "Big Jim," the product of a pure bred Percheron stallion mated with a 
grade mare of the same breed, showing the value of concentrating desirable qualities by 
close breeding in pure bred livestock. (From Sanders and Dinsmore in "A History of 
the Percheron Horse.") 


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. 


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 they 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 com 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 production. 

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 com, and the seedlings com- 



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 thes6 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 

Double first generation cross (^X^) X (^X^) 

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- 


liced, 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 outHae 
of this method of crossing com 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 

Fig. 46. — First generation cross of Chester White and Poland China. (From Detlefsen.) 


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 factoifs, recombine with regu- 
larity and precision. They Mendelize. From 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. 




The world lias 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 eflSciency 
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 ; which 
has bettered the condition of the people and lowered their 
death rate by quarantiae 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 


MAN 227 

with seven league boots. That dignified 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 forward 
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 Milli 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 
Gralton'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- 


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 iigy 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 aU 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 are 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 
Lq 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 imply 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 


high degree of dominance. Among them may be men- 
tioned the peculiar type of dwarfing known as achondro- 
plasty, and the various digital malformations termed 
medically brachydaetyly, 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; ithey 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 aU cases there is no guarantee that 
the unaffected member of the stricken family is germinaUy 
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, iehy- 
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 tbese abnormalities are 
extremely rare and for various reasons are not likely to 
increase. Among them may be mentioned pigmentary de- 
generation of the retina, Friedricb's ataxia, and xero- 
derma pigmentosum. 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 down as largely 
hereditary, and probably transmitted as siiigle 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- 


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 
assume, 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 Robert 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 iaheritance of physical traits 
are too numerous. 

The first real study of the inheritance of mental 
capacity was Galton's "Hereditary Genius," published 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 2^^* 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 
eminehee 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 


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 purify, 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 graphically by the linked series AbCd-aBcD 

MAN 235 

where the capital letters represent the desirable factors, 
must be much more common than the individual who by 
hnkage breaks receives the inheritance ABCD-abcd; 
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 written, 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 Eichard Lee, and the various lines established by 
John Preston — ^Venable, Payne, Wooley and Breckin- 
ridge — ar0 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. Would 
it not be wise to do away with statutes against the mar- 
riage of first cousins such as are laid down 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 common people, simple 
examples of greatness standing among the commonplace — 


kave been the stock arguments of. those sociologists who 
^ believe every man a star of the first magnitude darkened 
by lack of opportunity. Let us consider such cases in 
some detail. 

The worst trouble with the euthenic idealists is that 
I superficially 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 siinply lacked even moderate capacity and were 
unable to rise when given normal opportunity. They be- 
longed to the good solid bourgeoiisie stock which forms 
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 combination 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 Grolden Age to Greece, a 
Eenaissance to Italy, an Elizabethan period to England, 
ai Napoleonic era to France, rather than a concentrated 
production of high mentality. G'alton 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 


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, 
s.trange 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 ways of life no matter what their sur- 
roundings. ^^^ There is the Nam familv, 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 chroniole of illegitimacy and 
pauperism. There is the Zero clan, a name fixing well 
their value to the world, ne'er-do-wells 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 ndt 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 
Efifie 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 with 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 


, be admitted, but does anyone believe tbat 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 thesQ 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 oontamiaation 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 distract 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 defec- 
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 



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 ntimber 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 ^3. The probability of normal mating normal = 

(13y =1169, the probability of normal mating carrier is 2 /13 _1 \ 
14/ 196 \14*14/ 

: 26 the probability of two carriers mating is /_l_y=_l_. 

196, Vl4/ 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 revisei 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,^" 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 


correct statistical answer to tlie problem is clear if one 
works back from tbe 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. uDn 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 
^flm chance ma^ jig in the general population, the oom- 
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. 




A PEOPOSAL to discuss racial mixtures as a final topic 
in such a condensed treatment ofinbreeding 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 b,ecause of the immense amount of 
partially codified knowledge relating to such matters 
which has been gathered by ajithropologists. 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 
known collectively as race problems. 

The data of anthropology are largely those of the his- 
torical type in which the control of variables is always 
luncertain and often impossible. It is obvious that the 
inature of the material to a certain extent limits direct 
/investigation in this field to the historical and the statisti- 
/ cal methods of research. Generally speakin.g, one cannot 
j use man as the subject of quantitative laboratory experi- 
I ments. Yet the difiiculties involved demand varied 
I 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 



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 weU- 
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 enviromnents, 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 with the conquered. 

With the twentieth century the world enters a new 
phase of development. Within 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. 


It is unfortunate, in view of the f axsts in the case, that 
many should still scoff at the conclusions of Malthus on 
the subject of population, reached a century ago. The 
impossibiEty 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 agficulture. The truth is 
that the world is approaching a population limit faster 
even than Malthus supposed, and the resulTof 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 illusory. 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 dedreasing. 

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 — thi^ee at most — each country, or at least each 
continent, must support its own population. 


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. 


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 isi non-existent in physical fact. 
He does this by taMng 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 
changes, 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 oases 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 



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 serve 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 resemblilig 
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 rec ombination. J 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. 


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 


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-down of the inherent characteristios 
of each ; the other, where fewer differences present only 
the possibility of a somewhat greater variability as a 
desirable basis for selection. Eoughly, 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- 


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. Xnd 
selection is not conscious. Breeding for the most part is 
at random. The reaVresult 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. But in reality the negro^ is inferior to the 
white." ^ This is notTypothesis~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. JJ 
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 j 
decreasing. His only chance for an extended survivalj 
is amalgamation. 


The United States has heen confronted by this grave 
question for some time. In Africa it has hardly yet come 
to the fore, hut within three generations it will be recog- 
^ nized as the political and economic problem. "What the 
solution wiU be, no one knows. It seems an unnecessary 
accompaniment to humane treatment, an illogical exten- 
t sion of altruism, however, to seek to elevate the black 
! ratoe 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 inherent 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 o£ 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 


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- 
black 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, anid 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 interming'ling with the poorer 
yellow offshoots, as has been done to the south of the 
United States. 

Our fi rst con clusion may be said to be a decision ' 
against tlie union pf~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 t ransmis sion 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 thesisjs 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 


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. Wh.en 
there cade 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 charaicterized 
differently ; but without the chance of repeated Mendelian 
recombinations the probability of establishing superior 
strains was small. 

This hypothesis, developed whoUy 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. 


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 I 
having very large amounts of ethnic mixture. Even thejl 
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 ^ j, x, 
of creative ability except the French ; the true Irish have ^^^ ", 
hardly a single individual meriting a rank among the VsP^ 
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 



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 rajoe, 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 civilized 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 
Eomans 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 Idlled or reduced them to 
the condition of laborers. 

The present inhabitants of Ireland, then, with the ex- 


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 


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 


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 aU the chances for good held by a compara- 
tively smaU 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 which 
at least three different stocks entered: the Arabs, the 
Assyrioides or Hittites and the Aryan Amorites. More 
or less inbreeding did foUow before their dispersal, but 
that great racial va,riability must have reraained 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 the 
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 


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 inbreeduig 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. 
Races 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 
lawsi that have been discussed in the preceeding pages. 


This being true, racial orossing 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. 
Nevei-theless, 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 Mendelian 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 Itess 
inbreeding following racial mixtures, if there is to be any 
high probability of isolating desirable extremes. A third 


essential in the production of racial stamina is that the 
ingredients in the Melting Pot be sonnd 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-bom. 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 modem 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 of 
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 realizQ 


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, uptil 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 with 
better regard for the good of all the people.' 



1 Allen, C. E. : A Chromosome Difference Correlated with Sex Dif- 

ferences. Science, N. S., 1917, xlvi, 466, 467. 

2 Arnbr, G. B. L. : ConsangTiineous Marriages in the American Popu- 

lation. Studies in Hist., Econ. and Pub. Law, 1909, xxxi, No. 3. 

3 Beal, W. J. : Reports, Michigan Board of Agriculture, 1876, 1877, 

1881 and 1882. 

* 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. 

* Bebthollet, S. : Phenomenes de I'acte mysterieux de la fecondation. 

Mem. Soc. Linneenne de Paris, 1827, i, 81-83. 
■^ Bltth, E. : On the Physiological Distinctions between Man and all 
other Animals. Mag. Nat. His., N. S., 1837, i, 1-9, 77-85, 131-141. 

* BoNHOTE, J. L. : Vigour and Heredity. London, 1915, pp. 263. 

* BoTJDiN, M. : Dangers des unions consanguines et necessite des croise- 

ments dans I'espeee humaine et parmi les animaux. Ann. d'Hygiene 
pub. et de Med. legale, 1862, xviii, 5-82. 
1* 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. 

12 Bbitton, 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. 
1* Bruce, A. B. : Inbreeding. Jour. Gen., 1917, vi, 195-200. 
15 BuRGEOis, A. : Quelle est I'influence des manages consanguines sur 

les generations? Theses L'j^cole de Med., 1859, ii, No. 91. 
18 Carrier, L. : The Imanediate Effect of Crossing Strains of Com. 

Virginia Agr. Exp. Sta. Bull. 202, 1911, pp. 11. 

^ 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. The real development of the subject 
has come from the investigations on heredity completed since the year 1900. 
Since it is impracticable and unnecessary to citel all the genetic work of 
this period, only those titles which are in some way directly connected with 
the subjects discussed, have been given. 


" Castle, W. E. : TJie Early Eimbryology of Ciona intestinalis Flem- 
1 ming (L.). Mus. Com. Zool. Bull. 27, 1896, 201-280. 
"^8 Castle, W. E. : Genetics and Eugenics. Camabridge, 1916, pp. 353. 
1" Castle, W. E., and Little, C. C. : On a Modified Mendelian Ratio 
Among Yellow Mice. Science, N. S., 1910, xxxii, 868-870. 

20 Castle, W. E., and Wright, S. : Studies of Inheritance in Guinea- 

pigs and Rabbits. Carnegie Inst. Pub. 241, Washington, 1916, 
pp. 192. 

21 Castle, W. E., Caepentbb, F. W. et al.: The Effects of Inbreeding, 

Cross-breeding, and Selection upon the Fertility and Variability 
of Drosophila. Prdc. Amer. Acad. Arts and Sci., 1906, xli, 731-786. 

22 Cattell, J. M. : A Statistical Study of American Men of Science. 

Science, N. S., 1906, xxiv, 658-665, 699-707, 732-742. 

23 Caullebt, M. : Les problemes de la sexualite. Paris, 1917, pp. 332. 

24 Chamberlain, H. S. : The Foundations of the Nineteenth Century. 

Trans. J. Lees. 2 vol. New York, 1910, pp. 578-580. 

25 Chapeaubodge, a. de : Einiges iiber Inzucht und ihre Leistung auf 

verschiedenen Zuchtgebieten. Hamburg, 1909. 

26 Collins, G. N. : Increased Yields of Com from ITybrid Seed. Year- 
book U. S. Dept. Agr., 1910, 319-328. 

2T Collins, G. N. : The Value of First Generation Hybrids in Corn. 
Bull. 191, Bur. Plant Ind., U. S. Dept. Agr., 1910, pp. 45. 

2« Collins, G. N., and Kempton, J. H. : Effeots of Cross-pollination on 
the Size of Seed in Maizje. 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, 

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. 146, 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. : Kritisehe UebeKsicht der bekannten Faile von Knos- 

penvariation. Haarlem, 1907, pp. 474. 

34 Crampe, H. : Zuchtversuche mlt zahmen Wanderratten. Landw. 

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Hydatina senta. Jour. Exp. Zool., 1912, xii, 337-362. 

217 Whitney, D. D. : " Strains " in Hydatina. Bial. Bull, 1912, xxii, 


218 Whitney, D. D. : Weak Parthenogenetic Races of Hydatina senta 

Subjected to a Varied Environment. Biol. Bull., 1912, xxiii, 321- 

ai9 WiEGMANN, A. F. : Ueber die Bastarderzeugung im. Pflanzenreich. 

Braunschweig, 1828, pp. 40. 
220 WiTHiNGTON, C. F. : Consanguincous Marriages: Their Effect upon 

Offepring. Mass. Med. Sac, 1885, xiii, 453-484. 
821 Wolfe, T. K. : Ftother Evidence of the Immediate Effect of Cross- 
ing Varieties of Corn on the Size of the Seed Produced. Jour. 

Amer. Soc. Agr., 1915, vii, 265-272. 
282 WoODEtriT, L. L. : Two Thousand Generaitions 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 Monarehs. New York, 1913, 

pp. 421. 

225 Weight, S. : The Effects of. Inbreeding on Guinea-pigs. I. Decline 

in Vigor. II. Differentiation among Inbred Families. III. Crosses 
between Different Highly Inbred Families. (Doctor Wright kindly 
permitted the authors to read these valuable unpublished papers 
in manuscript.) 


Achondroplaay, 230 

Adaptation, for cross-pollination, 34 

for self-pollination, 30 
Africa, 247, 254, 257 

native population of, 253 
Agassiz, 236 
Agriculture, 18 
Algffi, 204 
Allelomorphs, 55 
Allen, 45 

Alternation of generations, 29, 46 
Althwa, 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, 21f., 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 
Basidiomyeetes, 31 

Bateson, 165 
Beal, 221 
Beans, 114 
Bees, 158 
Berthollet, 141 
Bibos, frontalis, 192 

gaurus, 192 

gruniens, 192 
Birds, 158 

Bison a/merica/nus, 192 
Blossom's G-lorene, 85ff. 
Bos taunts, 192 
Brachydactyly, 230 
Brassica oleracea, 192 
Breckenridge, 235 
Bridges, 184 
Britain, 258f. 
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, 

Cat, 211 
Gatalpa, 150 
Cataract, 230 
Cattell, 233 
Cattle, 210, 213 
Caucasian, 255 
Caullery, 22 
Gavia, 103 

(ywtleri, 160 

species, hybrids, 192 
Celts, 258 
CereaJs, 211 
Cephalic index, 249 




Chemotropism, 154 

China, 247 

Chinese, 254 

Chordates, 21 

Chromosomes, 36 

Ciona imtestinalis, 25 

Cirripedes, 25, 34 

Cleft palate, 230 

Cocoinea, 144 

Coefficient of cross-relationship, 86 

of heredity, 196, 199 

of inbreeding, Slff. 

of relationship, 81, 84^. 
Coelenterates, 21 
CoUinsy 137, 153 
Coloration, protective, 146 
Color-blindness, 47ff. 
Complemental males, 25 
Compositas, 32 
Conjugatce, 27 
Connate seeds of maize, 133 
Consanguinity, 113, 139 
Corn, varieties of, 215 
Coulter, 26 
Cow, 192 
Cramer, 198 
Crampe, 10 Iff. 
Cromwell, 258 
Orosaing-over, diagram to illustrate, 

Cucumber, 221 
Cucumis, 144 
Cymothoidw, 24 
Cytoplasm of egg, 200 

Dalton, 235 
Danes, 260 
Darwin, 13, 25, 32, 34, 101, 114ff., 

137ff., 143, 146ff., 154, 164/., 186, 

Datura, 142, 144 
Davenport, 230f., 233 
Delphino, 34 
Detlefsen, 103, 192 
Diabetes insipidus, 230 
Dianthus, 116f., 142, 144, 155 

DiatomecB, 27 

Dichogamy, 33 

DifHugia, 203 
coronata, 78 

Digitalis, 144, 155 

Dioecism, 22 

Disease, stisceptibility and resis- 
tance to, 134 

Dog, 210ff. 

Dominance, 57, 72, 177ff. 

Dominant factors, complementary 
action of, 171 

Double cross, 223 

Double fertilization, 203. 

Draba, 155 

Drosophila, 179, 188, 198, 208 
melcmogaster, 61, 111, 184 
sterility in, 112 

Dusing, 106 

Echinodermata, 22 
Echinoderms, 21 
Edwards, 235 
Ellis, 233 
Emerson, 183 
Endogamy, 238 
Endosperm fertilization, 153 
England, 257ff., 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, 
Exogamy, 13, 15, 201 

Factors, stability of, 76ff. 
Faraday, 235 
Feeble-mindedness, 231 
Ferns, 29 

Fertilization, diagram to illustrate, 

double, 41 

in embryo sac of the lily, 41 



Fish, 92, 158 

Fitzhugh, 235 

Flat worms, 21 

Foeke, 141, 145 

Fossil ants in amber of Oligooene 

period, 77 
France, 257f., 260/^. 
Franklin, 2Zoff. 
Franks, 260 
Frazer, 13 
Freeman, 157 
French, 257 
Frenchman, 250 
Frequency distribution of corolla 

length in tobacco, 70 
Freud, 13 
Fuous, 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 
Gonoehorism, 22, 33 
Grape, 210 
Gravatt, 192 
Greece, 251 
Green Algjer, 26 
Guaita, von, lOlff. 
Guinea-pig, 188 
growing curves of, 160 

Guinea-pig, inbreeding experiments 
with, llOff. 

Haiti, 253 

Hammurabi, code of, 14 

Hare-lip, 230 

Hawkweed, 22 

Hayes, 89, 148, 168, 192 

Herbert, 141 

Heredity coefficient, 196, 199 

Hermaphroditism, 22f., 26, 30, 33, 

Hero, 117 

Heterosis, 16, 96, 141, 144, 157, 172, 

importance of in sex origin, 204ff. 

manifestations of, 150 

selective effect of, 154 
Heterozygosis, 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, 210ff. 
Huntington's chorea, 230 
Huth, 243 
Hybrid vigor, 16, 88, 96, 141 

benefit from, 219 

cause of, 164 
Hydatina, 158 

senta, 112f. 
Hyde, 112, 158 
Hyoiscywmus, 155 

Iberian, 259 

race, 258 
Ichythyosis, 230 
Inbreeding, curves of, 84 

effect of, on yield and height of 
maize, 124^. 

effect on organisms, 137 



Inbreeding, experiments with ani- 
mals and plants, 100 

index, 81 

intensity of, 85 

mathematical oonfiiderations of, 

problem, phases of, 81 

reduction in vigor resulting from, 

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, 257ff. 
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, 237ff., 260, 262 
Jutes, 259 

Keeble, 170, 172 

Kempton, 153 

Kerner, 34 

King, lOlff., 105, 107, 160, 188 

King Melia Rioter 14th, 85ff. 

Knight, 114, 141f. 

Knight-Darwin law, 33 

Knuth, 34 

Kolreuter, 141/., 144, 154 

Lang, 13 
Lavatera, 144 
Learning strains, 124 
Lecoq, 141 

Lee, 235 

LeguminosvB, 1 14 
Lethal factors, 179 
Liruma, 144 
Lincoln, 235/. 
Lindley, 143 
Liverworts, 29, 45 
Lobelia, 144 
Loeb, 200 
Lychnis, 144 
Lyovum, 144 
Luffa, 144 

MacLennan, 13 

MacNamara, 257 

Maize, connate seeds of, 133 

distribution of rows of grain 
of, 129 

fertility of, 188ff. 

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^. 
\Man, inbreeding and outbreeding 
\ in, 226 
'v interfertility of, 246 
Majchala, 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 



Mice, 188 

Middleton, 78 

Miela, 227 

Milk depots, 227 

Mimicry, 146 

Mimulus, 115, 117 

Mirabilis, 142 

Moenkhaua, 112, 158 

Mollusooids, 21 

Molluscs, 21 

Mongolian, 255, 257 

MonoBcism, 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 
Naiidin, 144 
Nearing, 233 
Negro, 252ff. 
Nemathelminthes, 22 
Nematode, 21 
Nemec, 203 
Neolithic period, 258 
New Zealand, 247 
Nicotiema, 103, 139, 142, 144, 148, 
155, 157, 192, 196f. 

alata, 19 If. 

height of species and crosses, 149 

LrnigsdorfjU, 191 

longiflora, 69 

pa/tiMulata, 192 

rustica, 192 

tabacvim, 192 
Nordic factors, 250 
Normans, 260 
Norsemen, 258 
Nucleus, 36 

Oats, 114 

Oogenesis, 37 
Open door, 265 
Ostrich, 210 

Papaver, 144 

Paramecium, 202 

Parasitism, 22 

Parthenogenesis, 206 

Pasteur, 235, 237 

Payne, ^35 

Pearl, 80/=., 83ff., 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 
PolypodAacew, 32 
Preston, 235 
Primula, 144 
Protandry, 23, 135, 201 
Protogyny, 23, 201 
Protozoa, 21 
Pumpkin, 221 
Punnett, 159 

Qtiautitative characters, inheritance 
of, 66f . 

Races, intermingling of and national 

stamina, 2i5ff. 
Racial types, 250 
Radish, 192 
BoMwnculacecB, 32 
Raphanus satimis, 192 
Rat, 188 

curves showing body weight of, 

inbreeding experiments with, 
lOlff., 105 



Eat, size of litters of, 109 
Reduction of heterozygous individ- 

uaJa and allelomorphic pairs, 90 
Remick, 92 

Reproduction, among animals and 
plants, 20 

asexual, 17, 21, 22, 30, 78 • 

sexual, 17, 20f., 26, 201, 205 

eexual, origin of, 26^. 
Retina, pigmentary degeneration of, 

Bhopalura, 2G 
Rice, 114 
Ripley, 257 

Ritzema-Boa, lOlff., 188 
Roberts, 153 
Robbins, 92 
Roman Empire, 259 
Romans, 259 
Rome, 251 
Rommel, 110 
Rosaoew, 32 
Rotifer, 158, 170, 202 

SaocuUna, 23 ^ 

Sageret, 141, 144, 156, 192 

Sax, 157 

Saxons, 258, 260 

Scandinavians, 257 

Schizophytes, 26 

School for mothers, 227 

Scotch, 259f. 

Scotland, 257, 260 

Sex, determination of, iSff. 

Sexual dimorphism, 26 

Self-fertilization as means of obtain- 
ing homozygosity, 97 

Self-sterility, 25, 33 

Sex-linked characters, 47^. 
inheritance of, iSf. 

Sex, origin of, 201 

Sex ratio, 106 

Shamel, 119 

Sheep, 210ft. 

Shull, A. F., U2f., 158, 169 

Shull, G. H., nsff., 168/. 

Silkworms, 158 
Spain, 257/. 
Spaniard, 262 
Spermatogenesis, 37 
Spermatozoon, entrance of, through 

membrane of egg, 42 
Sphwroearpus, 45 

DonnelUi, 45 

texanus, 45 
Spirogyra, 27 
Sponges, 21^. 
Spores, 29 
Sporophyte, 29 
Squashes, 221 
Sterility, 188ff. 
Stevenson, 233 
Stone Age, 258 
Strasburger, 45 
Sturtevant, 184 
Btylonyohia pustulata, 78 
Sv?ine, 210/., 213 
Synapsis, 37 
Syndactyly, 230 

Talmud, Hebraic, 14 
Tapeworm, 23f. 
Tobacco, 114, 148, 156 
corolla length of, 70 
Tomatoes, 114, 148, 221 
Toyama, 158 
Trochelminthes, 21/., 202 
Tropseolum, 144 
Tunicates, 23 
Turanian, 259; race, 258 
Turbellarians, 23 
Tuttle, 235 

Ulothrix, 26, 28 

Unit of heredity, 77 

United Kingdom, 257 

United States^ 253ff., 260, 262, 264 

XJstilago maydis, 118 

Venable, 235 
Verbascum, 142, 144 



Wales, 261 

Weismaim, 101fl=., 188, 201, 206 
Wentworth, 92, 112 
Westermarck, 238, 243 
Wheat, 114, 197, 210 
Wheeler, 77 
Whitney, 112, 158 
Wiegmann, 141, 144, 154 
William the Conqueror, 260 
Woods, 233 

Wooley, 235 

Wright, llOf., 160f., 188 

Xeroderma pigmentosum, 231 

Yak, 192 

Yellow mouse, 179 

Youattj 220 

Zero, 2^7, 239